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Synthesis, characterization, and device application of two-dimensional materials beyond graphene

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Synthesis, characterization, and device application of two-dimensional materials beyond graphene

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Content SYNTHESIS, CHARACTERIZATION, AND DEVICE APPLICATION OF TWO DIMENSIONAL MATERIALS BEYOND GRAPHENE Copyright 2017 By Liang Chen A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Electrical Engineering) August 2017 Liang Chen Acknowledgement. Firstly, I would like to thank my PhD advisor, Dr. Chongwu Zhou, for offering me such a great opportunity to do serious researches in the nanotechnology field. Dr. Zhou is a talented and respected professor who provides me numerous help over the past five years of my PhD study. Under his guidance, I've learned a lot of important professional skills as well as the way of thinking and solving problems. Most importantly, his emphasis on independent and scientific thinking arouses me from trivial technical details to looking for creative ideas, which is the most essential part of PhD study. I also would like to thank my dissertation committee, Dr. Wei Wu and Dr. Aiichiro Nakano for generous advising and helping with both my academic activities and thesis writing. Meanwhile, I would like to thank Dr. Stephen Cronin, Dr. Han Wang, and Dr. Edward Goo for serving my qualification exam as well. Moreover, it's a great honor to have so many awesome colleagues during my PhD study, Dr. Bilu Liu, Dr. Gang Liu, Dr. Mingyuan Ge, Dr. Chuan Wang, Dr. Jialu Zhang, Dr. Yi Zhang, Dr. Haitian Chen, Dr. Yuchi Che, Dr. Jia Liu, Dr. Luyao Zhang, Dr. Xiaoli Wang, Dr. Ahmad N. Abbas, Dr. Hui Gui, Dr. Xin Fang, Dr. Noppadol Arronyadet, Dr. Maoqiang Yao, Dr. Jiepeng Rong, Dr. Yuqiang Ma, Dr. Yu Cao, Dr. Xuan Cao, Anyi Zhang, Chenfei Shen, Fanqi Wu, Qingzhou Liu, Sen Cong, Yihang Liu, Zhen Li, Chi Xu, Nai-yun Shih, Pattaramon Vuttipittayamongkol, Pyojae Kim, Youngkyun Na, Yilin Huang, Yue Fu, and A Xue Lin. Especially, I would like to thank Dr. Bilu Liu for sharing his research experience with me and helping me on all kinds of difficulties. Finally, I'm really grateful to have my lovely family and friends in China. I would like to thank my parents and girlfriend for being so supportive along the pass years. B Table of Contents. Acknowledgement ...................................................................................................... A L . ifF. . .. ist o igures . .......................................................................................................... 111 List of tables . ............................................................................................................. xi Abstract . ................................................................................................................... xii 1. Introduction to two-dimensional (2D) materials beyond graphene ........................... 1 1.1 Introduction to graphene ........................................................................................ 1 1.2 Exploration of 2D materials beyond graphene ........................................................ 3 1.3 The family of2D transition metal dichalcogenides (TMDCs) .................................. 5 1.4 Preparation of 2D TMDCs using chemical vapor deposition (CVD) ..................... 10 Chapter 1. References ..................................................................................................... 12 2. High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors ........................................................ 16 2.1 Introduction .......................................................................................................... 16 2.2 CVD synthesis and characterization of monolayer MoS2 ...................................... 19 2.3 Device characteristics of MoS2 FETs ..................................................................... 22 2.4 Sensing performance of MoS2 FETs to N02 and NH3 ............................................ 26 2.5 MoS2 sensing mechanism study ............................................................................. 32 2.6 Additional information ......................................................................................... 38 2.7 Conclusion ............................................................................................................ 40 Chapter 2. References ................................................................................................. .... 41 3. Screw-Dislocation-Driven Growth of Two-Dimensional Few-Layer and Pyramid- Like WSe2 by Sulfur-Assisted Chemical Vapor Deposition ......................................... 46 3.1 Introduction .......................................................................................................... 46 3.2 CVD setup of sulfur-assisted growth ..................................................................... 47 3.3 AFM, Raman, and TEM characterization of thin WSe2 flakes .............................. 51 3.4 Detailed AFM characterization of pyramid-like WSe2 .......................................... 55 3.5 Proposed models of screw-dislocation initialization and propagation .................... 60 3.6 Device characteristics of FETs built on pyramid-like WSe2 flakes ......................... 63 3.7 Conclusion ............................................................................................................ 64 Chapter 3. References ..................................................................................................... 65 4. Step-Edge-Guided Nucleation and Growth of Aligned WSe2 on Sapphire via a Layer-Over-Layer Growth Mode ............................................................................... 69 4.1 Introduction .......................................................................................................... 69 4.2 Aligned growth on sapphire vs random growth on Si/SiO2 .................................... 71 4.3 Atomic-step-edge effect on sapphire surface ......................................................... 74 4.4 Raman, AFM and TEM characterization of aligned WSe2 flakes .......................... 79 4.5 Layer-over-layer formation of few-layer structures .............................................. 82 4.6 Conclusion ............................................................................................................ 85 Chapter 4. References ..................................................................................................... 85 5. High-Performance Sub-Micron WSe2 Field-Effect Transistors Prepared Using A Flood-Dike Printing Method with High On-State Current Density and High On/Off Current Ratio ........................................................................................................... 90 5.1 Introduction .......................................................................................................... 90 5.2 Advantages of sub-micron printing on 2D TMDCs ............................................... 93 5.3 Synthesis ofmonolayer WSe2 with controllable morphology ................................. 94 5.4 Printing on TMDC surface ................................................................................... 97 5.5 Sub-micron printing using a flood-dike method .................................................... 99 5.6 Characteristics of the printed sub-micron channel devices ................................. 100 5.7 Statistic study of the printed ultrashort channel devices ..................................... 103 5.8 Conclusion .......................................................................................................... 104 Chapter 5. References ................................................................................................... 105 6. Summary and future work .................................................................................. 109 6.1 Summary ............................................................................................................ 109 6.2 Future work ........................................................................................................ 110 6.2.1 Wafer-scale synthesis of high-quality and uniform TMDC films .......................................... 110 6.2.2 Flexible electronics with TMDC materials ............................................................................ 111 6.2.3 Printed electronics with TMDC inks ..................................................................................... 112 Chapter 6. References ................................................................................................... 113 Bibliography ........................................................................................................... 114 ii List of Figures. Figure 1.1.1 Lattice structure of graphene. (a) Isolated graphene sheets from layered graphite. (b) Real space lattice of graphene with a honeycomb structure, where al and a2 are two primitive vectors. ( c) Reciprocal lattice of graphene, where b 1 and b2 are reciprocal vectors. K, M, and r are high symmetric points. Figure 1.1.2. Graphene devices and potential applications. ( a) FET arrays made of graphene. (b) Graphene strain sensors. ( c) Graphene phones. ( d)( e) Potential applications for flexible electronics and energy storage. Figure 1.1.3. Issues of graphene with zero bandgap. ( a) Band structure of graphene showing a zero bandgap property. (b) Graphene FE Ts showing an low on/off current ratio. Figure 1.3.1. The family of TMDCs in the periodic table. There can be massive combinations to form a TMDC material. Figure 1.3.2. Crystal structures ofTMDCs. (a) TMDCs are layered materials with a weak van der Waals bond between adjacent layers. (b )( c) Two polymorph structures of TMDCs, 2H phase (b) and 1 T phase ( c ). Figure 1.3.3. van der Waals stacking of layered 2D materials. Figure 1.3.4. List ofTMDCs with corresponding compositions and properties. Figure 1.3. 5. Band structure of MoS2. ( a) Bandgap transition from bulk MoS2 to mono layer MoS2. (b) Band structure of mono layer MoS2 with opposite spin-orbit splitting of the valence band at Kand K' points. Figure 1.4.1. CVD setup of mono layer MoS2 growth. ( a) Schematic diagram showing the CVD process. (b) As-grown monolayer MoS2 samples on substrate. Figure 1.4.2. CVD setup ofmonolayer WSe2 growth. (a) Schematic diagram of the CVD setup. (b) As-grown mono layer WSe2 samples on Si/SiO2 substrate. Figure 2.2.1. Growth and characterization of monolayer MoS2 using a three-zone CVD furnace. (a) Schematic of the three-zone CVD furnace used for MoS2 monolayer growth. Sulfur was put in zone 1, and the MoO3 precursor and the growth substrates were put in zone 2 or zone 3. The temperatures of the three zones can be controlled separately. (b) Optical microscopy image of the as-grown MoS2 monolayers on 300 nm SiOi/Si substrates. iii (c) AFM image of the as-grown MoS2. The height scale bar is 12 nm. (d) High-resolution TEM image of a MoS2 mono layer. ( e,f) Raman and normalized PL spectra of mono layer MoS2 from different samples. Inset of (f) shows the normalized Raman peak of the silicon substrates (2.266 e V) and two Raman peaks from MoS2 (2.283 and 2.280 e V). The Raman and PL measurements were performed using a 532 nm laser. Figure 2.2.2. Optical microscopy images of as-grown MoS2 monolayers on SiOi/Si substrates, showing the evolution from (a), (b) individual triangulars to (c), (d) quasi continuous films. Figure 2.3.1. Device characteristics of MoS2 FETs. (a) Schematic of the back-gate MoS2 FET in this study. (b) Optical image of two devices. (c,d) Typical output (fos-VDS) and transfer characteristics (fos-VBa) of the MoS2 FETs. Figure 2.3.2. Temperature-dependent transport measurements of MoS2 FETs. (a) fos-VDS characteristics of the devices at different temperatures from 77.4 to 300 K. The VDS spans from negative to the positive range and VBa ~ 40 V (b) Zoom-in plot of the fos-VDS characteristics at low VDs regime. (c) Linear fit of the Arrehenius plot, ln(fos/T 312 ) versus 1000/T (at temperatures of 160,200,250, and 300 K), and a slope can be obtained at each VDS. (d) Slopes extracted from (c) as a function of VDS. Linear fitting of the slope versus VDS gives a y-intercept of -0.839. Figure 2.4.1. Transfer characteristics of two MoS2-FETs measured in air (black) and in Ar (red). A small hysteresis and large on-state current were observed for both devices measured in Ar. Figure 2.4.2. Sensing performance of MoS2 FETs to NO2 and NH3. (a,b) Conductance change ofMoS2 FETs upon exposure to 400 ppb ofNO2 (a) and 500 ppm ofNH3 (b). The devices were initially turned on at VBa ~ 30 Vin (a) and turned off at VBa ~ 0 Vin (b ). A conductance decrease by a factor of 174 was achieved in (a) and increase by a factor of 1218 was achieved in (b). (c,d) Real time conductance change of MoS2 FETs with time after exposure to NO2 ( c) and NH3 ( d) under different concentrations. The arrows in ( c) and ( d) indicate the time when the corresponding concentrations of gas were introduced. Inset in (d) is a zoom-in plot of the sensor response to low concentrations ofNH3 of 1, 5, and 10 ppm. ( e,f) Conductance change versus NO2 concentration ( e) and NH3 concentration (f) based on MoS2 FET sensors. Figure 2.4.3. Raman spectra ofMoS2 sensors after exposure to 400 ppb ofNO2 (a) and 500 ppm ofNH3 (b ). The spectra were vertically shifted for clarify. The peaks denoted by* at 520.7 cm· 1 come from SiO2/Si substrates and were used for calibration of MoS2 Raman peaks. The excitation laser wavelength is 532 nm and the laser power is the same for all iv measurements. Figure 2.5.1. MoS2 sensing mechanism study. (a,b) Transfer characteristics (fos-VBa) of MoS2 FETs upon exposure to NO2 (a) and NH3 (b) with different concentrations. (c,d) Threshold voltage (V th) versus gas concentrations for NO2 ( c) and NH3 ( d). Figure 2.5.2. Plot of inverse gas concentration (Conc)- 1 versus inverse sensor response ( 6 G)_ 1 for (a) NO2 and (b) NH3. Go Figure 2.5.3. Proposed band alignment diagrams at the MoS2-metal junctions. (a) Energy diagram of the Ti/ Au and MoS2 before contact. (b) Band realignment and energy diagram of the Ti/ Au and MoS2 after contact and the formation of Schottky barrier. Blue, green, and red lines indicate the energy band of the pristine MoS2 (solid blue), after exposure to NO2 (dashed green), and after exposure to NH3 (dashed red), respectively. NO2 adsorption increases the SB by /1.SBl while NH3 adsorption decreases the SB by /I.SB2. In addition, the built-in voltage (Vbi) will decrease (increase) upon NO2 (NH3) exposure, resulting in a widening (thinning) of the SB width and a decrease (increase) of the device current. Figure 2.6.1. Output characteristics of the annealed MoS2 devices where a saturation behavior is observed in (a) and the fos-VDS show linear relationship at small bias (b). Figure 2.6.2. Real time conductance change of an ohmic-contacted MoS2 FET with NO2 concentrations from 20 ppb to 400 ppb. The arrows indicate the time when certain concentrations of NO2 were introduced. The conductance changes were smaller than 5% for 400 ppb NO2. Figure 2.6.3. Effective mobility changes of the MoS2 transistors upon exposure to NO2 (a) and NH3 (b) with different concentrations. Figure 2.6.4. Transfer characteristic of a MoS2-FET after exposure to 500 ppm NH3 for 1 hr in linear scale (a) and log scale (b). The device shows an effective mobility up to 26.9 cm 2 /Vs under a back gate configuration. The measurement was performed from 50 V to - 50 V for VBa under a VDs of 5V. Figure 3.2.1. CVD set up and optical microscopy characterization of CVD-grown few layer and pyramid-like WSe2 flakes. (a) A schematic diagram shows the CVD setup for the sulfur-assisted WSe2 growth. (b, c, d) Optical microscopy images of thin WSe2 flakes with different shapes. (e-m) Optical microscopy images of thick WSe2 flakes with different stacking morphologies. The color contrasts reflect the thickness variations both from sample to sample and from center to edge within each sample. The white dotted lines V indicate the stacking angles between top and bottom features in thick flakes. The scale bars are 3 µm for all images. Figure 3.2.2. Optical images of samples grown at different temperatures. (a) Samples grown at 900 °C. Both thin triangular and thick triangular flakes are observed. (b) Samples grown at 925 °C. All the flakes are thick ones, and hexagonal flakes can be found at this growth temperature. Figure 3.3.1. AFM and Raman characterization of thin WSe2 flakes. (a, b) AFM images along with cross section height profiles of two thin WSe2 flakes with ribbon-like features on top. The bottom layers are 2.5 nm and 3 nm in height, corresponding to around tri-layer WSe2. The heights of ribbon layers are -0.9 nm or -1. 7 nm, corresponding to one or two layers of WSe2. ( c, d) Raman spectra of several thin WSe2 flakes. Characteristic peaks of fog 1 , A1g, and B2g 1 modes of WSe2 were detected while the A1g peak of WS2 was not observed. ( e, f) Raman intensity mapping of A 1 g (259 cm· 1 ) and B 2 g 1 peaks (309 cm· 1 ) of the same WSe2 flake shown in Figure 2(b ). The excitation wavelength of laser is 532 nm during Raman measurements. Figure 3.3.2. AFM images of thin WSe2 flakes along with their cross-section height profiles. (a) A triangular WSe2 flake with a height of-3.5 nm, corresponding to four-layers. (b) Another hexagonal WSe2 flake with a height of -3.5 nm, corresponding to four-layers as well. Figure 3.3.3. TEM-EDX characterization of as-grown WSe2 flakes. (a) TEM image of a thin triangular WSe2 flake transferred from silicon substrate to TEM grid. (b) High resolution TEM image along one edge of the flake. ( c) A typical EDX spectrum taken from as-grown WSe2 flakes. The sulfur peak (2.307 ke V) is negligibly small indicating the concentration of sulfur is quite low, if any, in the as-grown WSe2 samples. The peaks of Cu and C are from TEM grid. Figure 3.4.1. AFM studies of thick WSe2 flakes with different stacking morphologies. (a) AFM phase image of a thick triangular flake. Helical fringes and herring-bone contours are clearly observed. (b) A3D AFM image of the flake in (a), showing a pyramid-like structure vividly with a height of -40 nm from base to the summit. ( c) AFM height profile of the pyramid-like flake along the red line in (b ). ( d, e, f) AFM phase images of three other thick flakes with different morphologies. Figure 3.4.2. Systematic AFM studies on thick triangular flakes. Most of these triangular flakes have a 0° stacking angle. Figure 3.4.3. Systematic AFM studies on thick hexagonal flakes. The most common vi morphology is hexagon-triangle stacking with a 30° stacking angle. Figure 3.5.1. Proposed models for SDD growth ofWSe2. (a) Two adjacent WSe2 domains before intersecting. (b) The boundary uplifting occurs when the two domains intersect. ( c) A screw dislocation generated after the second layer extension on the uplifted edge. ( d) Schematic diagrams showing the process of screw dislocation propagation. ( e) AFM image of a spiral grown flake at early stage. A zoom-in image with the height profile shown in the right part. Figure 3.6.1. Device performance ofCVD-grown WSe2 flakes. (a) AFM image ofa back gated WSe2 FET on a 5-nm-thick sample along with its Icts-V g family curves at different Vcts (c). (b) Another back-gated WSe2 FETon a 20-nm-thick sample with its Icts-Vg family curves (d). Insets in the right plots (c) and (d) are transfer curves plotted in a log scale at Vcts~O.l V Figure 4.2.1. Optical microscopic observations of WSe2 flakes grown at different temperatures on C-plane sapphire and Si/Si02 substrates. (a) and (b) Optical microscopy images of aligned WSe 2 flakes grown on C-plane sapphire at 900 °C (a) and 950 °C (b). Inset in image (b) shows an SEM image of the sample grown at 950 °C. The scale bar is 10 µm. ( c) Optical microscopy image of the WSe2 flakes grown on Si/Si02 substrates at 950 °C. (d), (e ), and (f) Histograms of the orientation distributions based on images (a), (b), and ( c ), respectively. Growth on C-plane sapphire exhibits better orientation control than growth on Si/Si02 substrates (plots d and e versus plot f). Meanwhile, high growth temperature improves the alignment of WSe2 growth on sapphire (plot d versus plot e ). Figure 4.3.1. Characterization of C-plane sapphires and observations of the step-edge guided nucleation and aligned growth phenomena. (a) A photo image of commercial C plane sapphire used in the experiments. (b) AFM height image of the sapphire surface after a high temperature treatment (950 °C), revealing the formation of periodical step patterns. ( c) Cross-section height profile of the steps along the blue line in (b ). ( d) and ( e) Optical microscopy images of the as-grown WSe2 flakes when the gas flow direction is perpendicular ( d) or parallel ( e) to the step direction during the growth. 0 is defined as the angle between the step direction and the base edge of trapezoid flakes. (f) Statistical analysis of the alignment in (d) and (e) based on the 0 values, showing good alignment in both experiments. (g) AFM image showing initial WSe2 nuclei aligned at the sapphire step edges. (h) Atomic models show the aligned nucleation ofWSe2 on C-plane sapphire. Figure 4.3.2. AFM characterization of pristine C-plane sapphire substrates. (a) An AFM height image shows isolated and irregular atomic steps on pristine sapphire surface before high temperature treatment. (b) Cross-section height profile of the atomic steps along the white line in (a). vii Figure 4.4.1. Raman and AFM characterization on a typical aligned WSe2 flake grown on C-plane sapphire substrates. (a) Optical image of an aligned WSe2 flake with a trapezoid shape. (b) Raman spectrum of the flake in (a) shows it is a few-layer sample. The intensity is multiplied by a factor of two from region 300 to 400 cm· 1 . (c) Raman intensity mapping of the flake in (a) at 247 cm· 1 shows non-uniform intensity over the whole flake, indicating a change of sample thickness at different locations. (d) AFM height image of the whole flake. The lateral size of this flake is about IO µm. The areas e, g, and h are further zoomed in as shown in ( e ), (g), and (h) respectively. ( e) Zoom-in image of the area e. The periodical sapphire step pattern is observed with a consistent direction. (f) Cross-section height profile showing the front domain of the flake is a bilayer. (g) Zoom-in image of the area g showing the detailed thickness variation along the base edge. The white dot line indicates the base edge of Layer B, which is covered by Layer A. (h) Zoom-in image of the area h reveals thatthe core is a screw dislocation hillock as well as the layer-over-layer thickness variation. (i) Cross-section height profile along the blue line in (h). Figure 4.4.2. TEM characterization ofWSe2 flakes. (a) TEM image of a WSe2 flake after being transferred onto TEM grid using PMMA-mediated transfer method. The flake becomes crumpled after transferring. (b )-( e) Diffraction patterns taken at locations 1-4 as indicated in image (a). These results show that the flake is a single crystal. (f) HRTEM image shows high crystalline WSe2 flake. The spacing is measured to be 0.285 nm for WSe2 (100) lattice planes. Figure 4.5.1. Studies of the layer-over-layer features along with proposed growth models. (a) Schematic diagrams showing layer-by-layer growth mode and layer-over-layer growth mode. (b) Optical image of as-grown WSe 2 flakes on C-plane sapphire at 990 °C. The domains near the base edges of these flakes evolve into a wing-like structure. ( c), (d) Two situations of forming WSe2 few-layers by layer-over-layer overlapping of individual WSe2 layers. ( c) Overlapping occurs between the layers grown from adjacent steps. ( d) Overlapping occurs between the layers grown from non-adjacent steps. The insets correspond to the height profiles along the blue lines in ( c) and ( d). The propagation across the uphill step is suppressed at high growth temperature for both situations leaving straight and aligned base edges. Figure 5.2.1. Schematic diagrams comparing different printed devices on CVD-grown TMDCs. (a) Traditional printing with resolution> 25 µm on isolated TMDC flakes would cause open channel devices. (b) Traditional printing on continuous TMDC films with dense grain boundaries and junctions inside the channel would lead to compromised device performance. ( c) Ultra-short channel devices with a sub-micron channel length can efficiently bridge a single TMDC flake without grain boundaries or flake-to-flake junctions. viii Figure 5.3.1 Synthesis and characterization of CVD monolayer WSe2. (a) Schematic diagrams of CVD setups for triangle-shaped WSe2 growth and hexagram-shaped WSe2 growth. For hexagram-shaped growth, the substrate is enclosed in a special inner tube with a small opening facing the upstream supplies. (b )( c) OM images showing the as-grown WSe2 flakes with triangular shapes (b) and hexagram shapes ( c ). The lateral size of an individual WSe2 flake is -10 µm for triangular-shaped samples and -20 µm for hexagram shaped samples. ( d) AFM image of a hexagram-shaped WSe2 flake along with the cross section height profile of the white dash line. The height of the sample is measured to be -0.8 nm corresponding to a monolayer TMDC. (e) Raman spectrum of the as-grown CVD WSe2 showing the two characteristic peaks of E.f. 9 mode and A 19 mode. (f) PL spectrum of the CVD WSe2 sample showing a strong PL peak at-770 nm, confirmed the monolayer status. Figure 5.3.2. Comparison of CVD results from different method. ( a) Optical image of CVD samples using WO3 and Se precursors show a mixed result with monolayer flakes and bulk flakes. (b) Triangle-shaped flakes grown with WSe2 precursors. (c) Hexagram-shaped flakes grown with combination ofWSe2 precursors and inner tube structure. Figure 5.4.1. Studies of the wetting property of Au ink on SiO2 and WSe2. (a) Statistic analysis showing the diameter distribution of printed electrodes on SiO2. Inset shows a device printed on SiO2 with a 300 µm printing distance. (b) Statistic analysis showing the broad diameter distribution of printed electrodes on CVD WSe2. Inset shows a shorted device printed on WSe2 with a 300 µm printing distance. (c)(d) Both OM image (c) and SEM image ( d) show the over-wetting of Au inks on WSe2 due to the differences of surface energy between WSe2 and Si 02. (e)(f) OM image (e) and SEM image (f) of a shorted device printed with traditional printing method. Figure 5.5.1. Printed ultra-short channel devices with a sub-micron channel length. (a) Schematic diagrams of the three-step printing technique. After the first electrodes were printed, SAM functionalization was performed to form a shielding layer which can block the ink flow of the second printed electrodes. (b) OM image of a printed device using the three-step printing process. A clear observation of the blocking effect was displayed. ( c) SEM image of the same device in (b ). An extremely narrow gap can be observed with a length less than 1 µm. Figure 5.6.1. Transport study of the printed ultra-short channel devices on monolayer WSe2. (a) fos-Vas transfer curve measured at a positive VDS of2V. A clear p-type behavior can be observed. (b) fos-Vas transfer curve measured at a negative V DS of -2V on the same device in (a). A strong increasing of the on-current was shown. Inset displays the transfer curve plotted in a logarithmic scale showing an Ion/Ioff ratio of 10 5 . ( c) A family of fos-VDS output ix curves of the printed devices showing the existence of Schottky barrier. (d) fos-VDS output curves after zoom-in at the low-bias region. Figure 5.6.2. Printed ultra-short channel devices with few-layer CVD WSe2. (a) fos-Vas transfer curve measured at a positive VDS of 2 V (b) fos-V as transfer curve measured at a negative VDS of -2 VA clear p-type behavior can be observed for few-layer WSe2 samples. Compared to monolayer WSe2 devices, few-layer WSe2 devices show higher on-current but lower Ionlloff ratio. (c)(d) A family of fos-VDS output curves measured at different Vas. A clear Schottky behavior can be observed from the output curves. Figure 5.7.1. Statistic studies of the performance of printed ultra-short channel devices. (a) Ionlloff ratio distribution of the devices printed on monolayer WSe2. (b) Effective mobility distribution of the printed devices on mono layer WSe2. ( c) Comparison of the device performance of printed TMDC devices. Ultra-short channel devices with CVD grown WSe2 show the highest on-state current density together with a high Ionlloff ratio compare to the long channel devices printed on CVD MoS2 and solution-processed TMDCs. X List of tables. Table 1.2.1. List of 2D materials. Three major groups all included as elemental 2D materials, 2D chalcogenides, and 2D oxides. xi Abstract. In this dissertation, I present a series of systematic work on a newly developing research direction called two-dimensional (2D) materials beyond graphene. As it is named, the major focus of this research field is to explore the novel 2D materials other than graphene. Generally speaking, the initial works started around 2010 after several years of the discovery of graphene. And now this field is expanding dramatically. Back to my research work, it covers from fundamental materials synthesis and characterization to advanced device study and applications. Details about each work will be introduced in this thesis. Chapter I is an introduction to the general information of 2D materials including history, definitions, material properties, current focus, etc. This chapter will first discuss the background of 2D materials beyond graphene and the motivation of related research. Then it will introduce some representative materials, especially transition metal dichalcogenides (TMDCs ), that are highly involved in current research field along with their fundamental knowledge such as material structures and physical properties. At last, a brief introduction about the preparation method of 2D TMDCs will be presented. Chapter 2 reports the first use of Schottky-contacted chemical vapor deposition (CVD) grown monolayer MoS2 as high-performance room temperature chemical sensors. The Schottky-contacted MoS2 transistors show current changes by 2-3 orders of magnitude upon exposure to very low concentrations ofNO2 and NH3. Specifically, the MoS2 sensors xii show clear detection ofNO2 and NH3 down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS2 sensors. This study shows the potential of2D semiconductors as high performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors. Chapter 3 reports an observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe2 flakes and pyramid-like structures using a sulfur-assisted CVD method. Few layer and pyramid-like WSe2 flakes instead ofmonolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization ofWO3, which is the precursor of tungsten. Clear observations of steps, helical fringes, and herring-bone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe2. The generation and propagation mechanisms of screw dislocations during the growth ofWSe2 were discussed. Back-gated field-effect transistors were made on these 2D WSe 2 materials, which show on/off current ratios of 10 6 and mobility up to 44 cm 2 /V·s. In material research area, controlled growth is always desired. In Chapter 4, we report a brand new mechanism, step-edge-guided nucleation and growth, for the aligned growth of 2D WSe2 by a chemical vapor deposition method using C-plane sapphire as substrates. This mechanism is different to commonly reported epitaxial growth via a substrate-flake interaction. We found that at temperatures above 950 °C, the growth is strongly guided by xiii the atomic steps on the sapphire surface, which lead to the aligned growth of WSe2 along the step edges on sapphire substrate. In addition, such atomic steps facilitate a layer-over layer overlapping process to form few-layer WSe2 structures, which is different from the classical layer-by-layer mode for thin film growth. This work opens up new ways to achieve oriented growth of2D WSe2 and adds fresh knowledge on the growth mechanism ofWSe2 and potentially other 2D TMDCs. Chapter 5 discusses a "flood-dike" printing technique which is used to produce high performance TMDC transistors with a sub-micron channel length based on CVD grown monolayer WSe2 materials. Our printing approach mainly involves three steps: (1) printing of the first electrodes on CVD WSe2 flakes, (2) functionalization of the first electrodes with a self-assembled monolayer (SAM) covering the entire surface, and (3) printing of the second electrodes close to the first ones. During the third step, the fresh ink-flow will spread to the first electrodes but be stopped by the SAM, which acts as a flood-dike, leaving an ultra-short channel with a sub-micron length. The devices produced using this flood- dike printing technique possess high on-state current densities (>0.2 µA/µm) and high Ionlloff ratios (> 10 5 ), which are superior to other reported values of printed devices on 2D TMDCs. xiv 1. Introduction to two-dimensional (2D) materials beyond graphene 1.1 Introduction to graphene The term, "graphene", typically refers to a single sheet of graphite with one layer of carbon atoms. Although the first observation of such monolayer graphite can be traced back to 1962, it was not until 2004 that graphene attracted a tremendous attention after its re-discovery by two scientists in University of Manchester, Andre Geim and Konstantin Novoselov. 1 That was the first time that people experimentally demonstrated the existence of free-standing 2D crystals. 2 By using a mechanical exfoliation method, they were able to isolate a mono layer graphene sheet simply with scotch tapes (Figure 1.1. la). 1 • 3 In addition, the exfoliated graphene sheet shows intriguing properties such as high mechanical strength, good transparency, and good electronic conductivity. 2 • 4 • 5 Later on, graphene becomes one of the hottest topics in the scientific research world over the past decade. As previously introduced, graphene consists only one atomic layer of carbon atoms. In another word, the thickness of a graphene sheet is in an atomic scale, which is also the key feature of all other two-dimensional (2D) materials. On the other hand, the lateral structure of graphene can be found in Figure 1.1.1 b, where a honeycomb lattice pattern can be observed. 2 Each carbon atom is covalent bonded with three other carbon atoms in a flat plane, where two neighbor atoms are 1.42 A apart. 2 Since 2004, there have been intensive efforts devoted to graphene research including synthesis, 6 • 7 fundamental physics study, 3 • 5 and potential applications. 4 • 7 - 9 A significant 1 progress has been made towards real technologies as shown in Figure 1.1.2, including sensors, flexible devices, displays, etc. 10 - 12 Nevertheless, graphene is a semi-metal material with zero bandgap (Figure 1.1. 3a),3 which becomes the bottleneck of its wide applications in semiconductor field. One example is the extremely low on-off cun·entratio of graphene field-effect transistors (FETs) (Figure 1.1.3b) As the traditional silicon industry is approaching the limit of Moore's Law, one solution is to replace silicon with new semiconductor materials that can break the scaling limit. 13 2D materials are believed to be promising candidates since the electrons can only move freely in the planar direction. However, transistors built on graphene owns a rather low on-off current ratio (<JO) which is not satisfying for most of the device applications, for example, digital circuit Even after years of research, the problem of absent bandgap still cannot be well solved, which significantly draws back the whole field. ( a) Graphite Graphene (b) I n \ \ \ _ , Figure 1. I. I Lattice strucb.ire of graphene. (a) Isolated graphene sheets from layered graphite. (b) Real space lattice of graphene with a honeycomb structure, where al and a2 2 are two primitive vectors. ( c) Reciprocal lattice of graphene, where b 1 and b 2 are reciprocal vectors. K, M, and r are high symmetric points. (b) Figure 1.1.2. Graphene devices and potential applications. (a) FEf affays made of graphene. (b) Graphene strain sensors. (c) Graphene phones. ( d)( e) Potential applications for flexible electronics and energy storage. (a) Zero bandgap (b} <0.038 .S,. 0.036 c 0.034 ~ 0.032 3 0.030 U 0.028 .5 O.D26 a 0.024.......,..~-~~~-~ ............ . 20 -10 0 10 20 30 40 Gate Voltage (V) Figure 1.1.3. Issues of graphene with zero bandgap. (a) Band stmcture of graphene showing a zero bandgap property. (b) Graphene FETs showing an low on/off cuffent ratio. 1.2 Exploration of2D materials beyond graphene Though people have developed multiple ways trying to create a bandgap within graphene, the results are still not promising enough to fulfill rigid requirements. 14 • 15 On 3 the other hand, an alternative approach was brought out by researchers that whether there exists some other 2D materials with a bandgap. Following this clue, people start to put efforts on looking for new 2D materials other than graphene. 16 • 17 A new research direction is therefore arisen named as 2D materials beyond graphene. Not surprisingly, there were a plenty of2D materials discovered during the past several years. 18 - 20 Moreover, those newly discovered 2D materials not only include semiconductors, but also they cover metals and insulators, even superconductors. 21 Such discoveries immediately generate a broad interest among researchers from different disciplines including physics, materials research, semiconductor device etc 18 • 22 • 23 , . Up to now, there are three major groups of 2D materials as listed in Table 1.2.1, (1) elemental 2D materials (including graphene ), 20 • 24 • 25 (2) 2D chalcogenides, 18 • 19 • 26 and (3) 2D oxides. 17 For elemental 2D materials, they typically consist only one elemental composition for each and usually have an analogous crystal structure to graphene. Such materials including silicene, 25 germanene, 27 and phosphorene 20 have already been demonstrated the successful existence. Among those elemental 2D materials, black phosphorous is a semiconductor with high mobility and relative stability, which makes it quite promising for numerous future researches. 20 • 28 On the other hand, 2D chalcogenides mostly refer to layered transition metal dichalcogenides (TMDCs ). This is a huge family of materials that are composed of transition metal and chalcogen elements. With a different combination of metal elements and chalcogen elements, the properties of TMDCs can widely span among metals, superconductors, semiconductors, and insulators. 21 Currently, 4 TMDC is a major research direction for 2D materials beyond graphene. Finally, for 2D oxides, the compositions are rather diverse as well as the structures and properties. Common 2D oxides include TiO2, MoO3, mica, and perovskite-like crystals like LaNb2O7. 29 · 30 Overall, this 2D oxides group still needs more investigation. Throughout the entire 2D research history, people are always amazed by every new discovery of these materials. In most cases, the physical property of bulk materials will undergo a dramatic change after thinning them down to atomic layers. 31 Especially for TMDCs, a great interest has been arisen among researchers and a significant progress has been made in all kinds of directions. Elemental 2D Graphene Silicene, Germanene Black phosphorous 2D Chalcogenides TMDCs: MoS 2 , WSe 2 , WS 2 ZrS 2 , N bSe 2 , etc. Gase, lnSe, Bi 2 Se 3 2D Oxides MoO 3 , WO 3 TiO 2 , MnO 2 , V 2 O 5 , etc. Perovskite: LaNb 2 O 7 , Bi 4 Ti 3 O 121 Ca 2 Ta 2 TiO 10 Table 1.2.1. List of 2D materials. Three maJ or groups all included as elemental 2D materials, 2D chalcogenides, and 2D oxides. 1.3 The family of 2D transition metal dichalcogenides (TMDCs) TMDCs have been widely used as solid lubricants in the past due to its layered features. However, not until 2011, these materials arouse people's interest after a paper published by A. Kis et al. which revealed that the single layer M0S2 is also a semiconductor material. 18 In that paper, the monolayer M0S2 shows an interesting bandgap property and a good 5 transistor behavior which are exactly what graphene is missing. Later on, other similar TMDCs such as M0Se2 and WSe2 were investigated as well. 19 · 32 Most of the materials in TMDC family can be expressed as MX2, where M stands for transition metal elements such as Mo and W, and X stands for chalcogen elements such as Sand Se (Figure 1.3.1). Based on the periodic table, there are abundant choices of the combination between transition metal elements (M) and chalcogen elements (X). After a systematic study, people found that most TMDCs from group IVB to VIIB own a stable 2D layered status while those in group VIIIB are likely non-layered materials. 21 The structures of layered TMDCs are similar to graphene where adjacent layers are coupled by a weak van der Waals force (Figure l.3.2a). 21 As a result, bulk TMDC crystals can be isolated into 2D sheets following a similar way of graphene exfoliation. In addition to the loosely bonded interlayer stacking, each TMDC layer contains three covalent bonded X-M-X atomic layers. From Figure l.3.2a, we can see that the metal layer is sandwiched between two chalcogen layers. The thickness of a typical mono layer is -0. 7 nm, which is slightly larger than a monolayer graphene sheet. To be noticed, some TMDCs have two polymorph structures, one is called 2H phase and the other is called 1 T phase. 21 The difference between 2H structure and 1 T structure depends on the coordination of the metal atom in the unit cell (Figure l.3.2b and l.3.2c). To be specific, the unit cell in 2H phase is trigonal prismatic while the unit cell in 1 T phase is octahedral or trigonal anti prismatic. One important feature about the TMDC family that attracts so many researchers' attention is the rich physics behind each material. First, depending on the actual 6 composition, the properties of TMDCs can range from metals, semiconductors to insulators. 26 Such diversity allows numerous studies especially for complicated heterostructure research. Because of the thin layered feature, those TMDCs can be easily stacked together as LEGO blocks (Figure 1.3.3). 17 By stacking different TMDC layers, it would be possible to achieve all kinds of junctions with distinctive properties. Figure 1.3.4 lists some typical TMDCs along with their compositions and properties. 21 Meanwhile, some TMDCs such as M0S2 and WSe2 would have a bandgap transition, from indirect bandgap to direct bandgap, if the material is thinned down to mono layer status (Figure 1.3.Sa). 31 As a result, it provides great opportunities for thin-film optoelectronics such as solar cells and LEDs where a direct bandgap is desired. 33 - 35 In addition, 2D TMDCs are also promising candidates for a lot of other research and applications. For example, semiconductor TMDCs are suitable for flexible electronics because of the natural flexibility. 36 At the same time, such 2D TMDCs are good materials for sensing applications since they have a high surface-to-volume ratio. 37 Recently, researchers found an even more interesting property that the valance band of monolayer M0S2 and WSe2 can split with opposite spin-orbit splitting at K and K' points, which is qualified for spintronics and valleytronics (Figure 1.3. Sb). 38 7 MX 2 M = Transition metal Li Be X = Chalcogen B C N 0 Na Mg 3 4 5 6 7 8 9 10 11 12 Al Si p s K Ca Sc TI V Cr M n Fe Co NI Cu Zn Ga Ge As Se Rb Sr y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te Cs Ba La-Lu Hf Ta w Re Os Ir Pt Au H g Tl Pb Bi Po Fr Ra Ac-Lr Rf Db Sg B h Hs M t Os Rg Cn Uut Fl Uup Lv Figure 1.3.1. The family of TMDCs m the periodic table. There combinations to form a TMDC material. (a) ~ . (b)<¢«~ ~ n~A 2H phase • Mo lT phase Few-layer MoS 2 Trigonal prismatic (D3n) Octahedral (Ot,) or trigonal antiprismalic point group Ci o! O,, (D,.J Monolayer MoS 2 H e F Ne Cl Ar Br K r Xe At R n Uus Uuo can be massive Figure 1.3.2. Crystal structures of TMDCs. (a) TMDCs are layered materials with a weak van der Waals bond between adjacent layers. (b )( c) Two polymorph structures of TMDCs, 2H phase (b) and 1 T phase ( c ). 8 Figure 1.3.3. van derWaals stacking oflayered 2D materials. G roup M 4 Ti, Hf. Zr 5 V. Nb, Ta 6 M o, VV 7 10 Tc, R e Pd, Pl X S, S e, Te S. Se, Te S, S e, Te Properties Semiconducting (E ,= 0.2· 2 eV). Diamagnetic. Narrow band metals (p • 1 0· 4 {l.cm) or semimelals. Superconducting. Charge density wave (CDW ) . Paramagnelic, antiferromagnetic, or diamagnetic. Sulfides and sclcnidcs are semiconducting (E. -le\/). Tellurides are semimeta llic (p-1 0-•n cm). Diamagnetic. S, Se, Te Small-gap semiconductors. Diamagnetic. S. Se, Te Sulfides and 5€lenides are 5€miconducling (E, = 04cV) and diamaenetic Tellurides are metallic and paramagnetic. PdTe, is superconducting. Figure 1.3.4. List ofTMDCs with corresponding compositions and properties. (a) MoS,bulk 0.2 31 ~ 0.0 "' c ,.;;- - 0.2 i- M K r r Mo5 2 munolaycr (b) M K r ~ - K, - ·~ •[ -K • Figure 1.3.5. Band st:rncture ofMoS2. (a) Bandgap transition from bulk MoS2 to monolayer MoS2. (b) Band stmcture of monolayer MoS2 with opposite spin-orbit splitting of the valence band at K and K' points. 9 1.4 Preparation of 2D TMDCs using chemical vapor deposition (CVD) Currently, the most widely adopted method to prepare monolayer TMDC samples is the mechanical exfoliation approach, which uses scotch tapes to peel off layers repeatedly from a bulk crystaJ. 1 , 18 Though this method is easy and fast, the issue of yield and uniformity is the major drawback since it can only produce isolated flakes with random shapes and thicknesses, In order to produce uniform and large-area monolayer samples, chemical vapor deposition (CVD) could be one possible solution, So far, several groups have demonstrated using CVD to achieve large-area monolayer M0S2 and WSe2 crystals, which are the two most representative TMDC species, 22 , 39 - 44 And in this thesis, I will focus on these two materials. First, for M0S2 growth, two separated ceramic boats containing MoO3 precursor and S precursor are placed in a quartz tube furnace as shown in Figure 1.4.la. 22 A Si/SiO2 substrate with the polished side facing down is put on top of the boat which holds the MoO3 powders. Then the furnace will be heated up to 650 °C along with a continuous Ar gas supply which acts as the carrying gas to bring the precursors together and react on the surface of substrate. After maintaining the whole setup at the growth temperature (650 °C) for a certain period (-15 mins ), triangular flakes of mono layer M0S2 can be found on the substrate. The as-grown samples mostly preserve a triangular shape, which is the thermodynamically stable morphology of most TMDCs. Based on the above process, using M0S2 as an example, a commonly adopted growth theory is then developed: (1) The S vapor will reduce MoO3 into volatile MoOx species, 10 which can be easily vaporized into gas phase, (2) Gas phase MoOx will then diffuse to the substrate and form the initial nuclei, and (3) The S vapor will continuously react with MoOx and produce MoS2 crystals on the substrate. 45 Then, for WSe2, which has the same crystal structure as MoS2, its monolayer samples can be synthesized using a CVD method like the MoS2 synthesis as well. However, people discovered that H2 is needed to assist the WSe2 growth because of the low reactivity of Se. 44 Unlike the combination of S and Mo0 3 , where S is strong enough to reduce Mo0 3 into MoOx, Se is not a strong reducer to reduce W03 into WOx, resulting in a compromised growth result. Therefore, H2 gas is introduced to the system in order to efficiently reduce the W03 into volatile WOx species. Then the vapor-phase WOx will diffuse to the substrate and react with the Se vapor there. Figure l.4.2a illustrates the whole CVD setup of monolayer WSe2 growth. Generally, Se powders are put at the upstream while W03 powders are put at the mid of a 1-inch tube furnace. Meanwhile, a Si/Si02 substrate is set at the downstream to collect the final product. During the growth at 1000 °C, the carrier gas mixed of Ar and H2 ( 4: 1) will be flowed into the system to facilitate the CVD process. 11 (a) Previous work (b) ••••••• 20 Figure 1.4.1. CVD setup of monolayer MoS2 growth. (a) Schematic diagram showing the CVD process. (b) As-grown mono layer MoS2 samples on substrate. (a) Monolayer WSe 2 synthesis CVD Figure 1.4.2. CVD setup of monolayer WSe2 growth. (a) Schematic diagram of the CVD setup. (b) As-groV\ill monolayer WSe2 samples on Si/SiO2 substrate. Chapter 1. Refermces 1. Novoselov, K. S.; Geim, A. K.; Moroz.ov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric field effect in atomically thin carbon films. Science 2004, 306, 666-669. 2. Geim, A. K.; Novoselov, K. S., The rise of graphene. Nit Mater 2007, 6, 183-191. 3. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-dimensional gas of massless Dirac fennions in graphene. Nature 2005, 438, 197-200. 4. Bae, S.; Kim, H; Lee, Y.; Xu, X. F.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y.1.; Kim, Y. J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J. H.; Hong, B. H.; Iijima, S., Roll to-roll production of30-inch graphene films for transparent electrodes. Nat Nanotechnol 1010, 5, 574-578. 5. Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P., Expe1imental observation of the quantum 12 Hall effect and Berry's phase in graphene. Nature 2005, 438, 201-204. 6. Li, X. S.; Cai, W.W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velarnakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S., Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312-1314. 7. Kirn, K. S.; Zhao, Y; Jang, H.; Lee, S. Y; Kirn, J.M.; Kirn, K. S.; Ahn, J. H.; Kirn, P; Choi, J. Y; Hong, B. H., Large-scale pattern growth of graphene fihns for stretchable transparent electrodes. Nature 2009, 457, 706-710. 8. Schedin, F.; Geirn, A. K.; Morozov, S. V.; Hill, E.W.; Blake, P; Katsnelson, M. I.; Novoselov, K. S., Detection of individual gas molecules adsorbed on graphene. Nat Mater 2007, 6, 652- 655. 9. Lin, Y M.; Dimitrakopoulos, C.; Jenkins, K. A.; Farmer, D. B.; Chiu, H. Y; Grill, A.; Avouris, P, 100-GHz Transistors from Wafer-Scale Epitaxial Graphene. Science 2010, 327, 662-662. 10. Lee, Y; Bae, S.; Jang, H.; Jang, S.; Zhu, S. E.; Sim, S. H.; Song, YI.; Hong, B. H.; Ahn, J. H., Wafer-Scale Synthesis and Transfer of Graphene Films. Nano Lett 2010, 10, 490-493. 11. Ahn, J. H.; Hong, B. H., Graphene for displays that bend. Nat Nanotechnol 2014, 9, 737-738. 12. Ren, W. C.; Cheng, H. M., The global growth of graphene. Nat Nanotechnol 2014, 9, 726-730. 13. Fiori, G.; Bonaccorso, F.; Iannaccone, G.; Palacios, T.; Neumaier, D.; Seabaugh, A.; Banerjee, S. K.; Colombo, L., Electronics based on two-dimensional materials. Nat Nanotechnol 2014, 9, 768-779. 14. Ohta, T.; Bostwick, A.; Seyller, T.; Hom, K.; Rotenberg, E., Controlling the electronic structure of bilayer graphene. Science 2006, 313, 951-954. 15. Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J., Chemically derived, ultrasrnooth graphene nanoribbon semiconductors. Science 2008, 319, 1229-1232. 16. Bhirnanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y; Cha, J.; Das, S.; Xiao, D.; Son, Y; Strano, M. S.; Cooper, V. R.; Liang, L.B.; Louie, S. G.; Ringe, E.; Zhou, W.; Kim, S. S.; Naik, R.R.; Sumpter, B. G.; Terrones, H.; Xia, F. N.; Wang, Y L.; Zhu, J.; Akinwande, D.; Alern, N.; Schuller, J. A.; Schaak, R. E.; Terrones, M.; Robinson, J. A., Recent Advances in Two Dimensional Materials beyond Graphene. ACS Nano 2015, 9, 11509-11539. 17. Geirn, A. K.; Grigorieva, I. V., Van der Waals heterostructures. Nature 2013, 499, 419-425. 18. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS, transistors. Nat Nanotechnol 2011, 6, 147-150. 19. Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A., High-Performance Single Layered WSe2 p-FETs with Chemically Doped Contacts. Nano Lett 2012, 12, 3788- 3792. 20. Li, L. K.; Yu, Y J.; Ye, G. J.; Ge, Q. Q.; Ou, X. D.; Wu, H.; Feng, D. L.; Chen, X. H.; Zhang, YB., Black phosphorus field-effect transistors. Nat Nanotechnol 2014, 9, 372-377. 21. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P; Zhang, H., The chemistry oftwo dirnensional layered transition metal dichalcogenide nanosheets. Nat Chem 2013, 5, 263-275. 22. Lee, Y H.; Zhang, X. Q.; Zhang, W. J.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y C.; Wang, J. T. W.; Chang, C. S.; Li, L. J.; Lin, T. W., Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv Mater 2012, 24, 2320-2325. 13 23. Yun, W. S.; Han, S. W.; Hong, S. C.; Kirn, I. G.; Lee, J. D., Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-M X-2 semiconductors (M ~ Mo, W; X ~ S, Se, Te). Phys Rev B 2012, 85. 24. Wang, X. M.; Jones, A. M.; Seyler, K. L.; Tran, V.; Jia, Y C.; Zhao, H.; Wang, H.; Yang, L.; Xu, X. D.; Xia, F. N., Highly anisotropic and robust excitons in mono layer black phosphorus. Nat Nanotechnol 2015, 10, 517-521. 25. Tao, L.; Cinquanta, E.; Chiappe, D.; Grazianetti, C.; Fanciulli, M.; Dubey, M.; Molle, A.; Akinwande, D., Silicene field-effect transistors operating at room temperature. Nat Nanotechnol 2015, 10, 227-231. 26. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 2012, 7, 699-712. 27. Ni, Z. Y; Liu, Q. H.; Tang, K. C.; Zheng, J. X.; Zhou, J.; Qin, R.; Gao, Z. X.; Yu, D. P.; Lu, J., Tunable Bandgap in Silicene and Germanene. Nano Lett 2012, 12, 113-118. 28. Wang, H.; Wang, X. M.; Xia, F. N.; Wang, L. H.; Jiang, H.; Xia, Q. F.; Chin, M. L.; Dubey, M.; Han, S. J., Black Phosphorus Radio-Frequency Transistors. Nano Lett 2014, 14, 6424-6429. 29. Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-+. 30. Kodenkandath, T. A.; Lalena, J. N.; Zhou, W. L. L.; Carpenter, E. E.; Sangregorio, C.; Falster, A. U.; Simmons, W. B.; O'Connor, C. J.; Wiley, J.B., Assembly of metal-anion arrays within a perovskite host. Low-temperature synthesis of new layered copper-oxyhalides, (Cux)LaNb,O,, X ~ Cl, Br. J Am Chem Soc 1999, 121, 10743-10746. 31. Splendiani, A.; Sun, L.; Zhang, Y B.; Li, T. S.; Kirn, J.; Chim, C. Y; Galli, G.; Wang, F., Emerging Photoluminescence in Monolayer MoS,. Nano Lett 2010, 10, 1271-1275. 32. Kong, D.S.; Wang, H. T.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y, Synthesis ofMoS, and M0Se2 Films with Vertically Aligned Layers. Nano Lett 2013, 13, 1341-1347. 33. Baugher, B. W. H.; Churchill, H. 0. H.; Yang, Y F.; Jarillo-Herrero, P., Optoelectronic devices based on electrically tunable p-n diodes in a mono layer dichalcogenide. Nat Nanotechnol 2014, 9, 262-267. 34. Withers, F.; Del Pozo-Zamudio, O.; Schwarz, S.; Dufferwiel, S.; Walker, P. M.; Godde, T.; Rooney, A. P.; Gholinia, A.; Woods, C.R.; Blake, P.; Haigh, S. J.; Watanabe, K.; Taniguchi, T.; Aleiner, I. L.; Geirn, A. K.; Fal'ko, V. I.; Tartakovskii, A. I.; Novoselov, K. S., WSe, Light Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature. Nano Lett 2015, 15, 8223-8228. 35. Pospischil, A.; Furchi, M. M.; Mueller, T., Solar-energy conversion and light emission in an atomic rnonolayer p-n diode. Nat Nanotechnol 2014, 9, 257-261. 36. Pu, J.; Yornogida, Y; Liu, K. K.; Li, L. J.; Iwasa, Y; Takenobu, T., Highly Flexible MoS, Thin Film Transistors with Ion Gel Dielectrics. Nano Lett 2012, 12, 4013-4017. 37. Liu, B. L.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C. W., High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Mono layer 14 MoS2 Transistors. ACS Nano 2014, 8, 5304-5314. 38. Xiao, D.; Liu, G. B.; Feng, W. X.; Xu, X. D.; Yao, W., Coupled Spin and Valley Physics in Monolayers ofMoS, and Other Group-VI Dichalcogenides. Phys Rev Lett 2012, 108. 39. Najrnaei, S.; Liu, Z.; Zhou, W.; Zou, X. L.; Shi, G.; Lei, S. D.; Yakobson, B. I.; Idrobo, J. C.; Ajayan, P. M.; Lou, J., Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers. Nat Mater 2013, 12, 754-759. 40. van der Zande, A. M.; Huang, P. Y; Chenet, D. A.; Berkelbach, T. C.; You, Y M.; Lee, G. H.; Heinz, T. F.; Reichman, D.R.; Muller, D. A.; Hone, J.C., Grains and grain boundaries in highly crystalline rnonolayer molybdenum disulphide. Nat Mater 2013, 12, 554-561. 41. Zhang, Y; Zhang, Y F.; Ji, Q. Q.; Ju, J.; Yuan, H. T.; Shi, J.P.; Gao, T.; Ma, D. L.; Liu, M. X.; Chen, YB.; Song, X. J.; Hwang, H. Y; Cui, Y; Liu, Z. F., Controlled Growth of High-Quality Monolayer WS, Layers on Sapphire and Imaging Its Grain Boundary. ACS Nano 2013, 7, 8963- 8971. 42. Kang, K.; Xie, S. E.; Huang, L. J.; Han, Y M.; Huang, P. Y; Mak, K. F.; Kirn, C. J.; Muller, D.; Park, J., High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity. Nature 2015, 520, 656-660. 43. Eichfeld, S. M.; Hossain, L.; Lin, Y C.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kirn, M. J.; Mayer, T. S.; Redwing, J.M.; Robinson, J. A., Highly Scalable, Atomically Thin WSe, Grown via Metal Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080-2087. 44. Huang, J. K.; Pu, J.; Hsu, C. L.; Chiu, M. H.; Juang, Z. Y; Chang, Y H.; Chang, W. H.; Iwasa, Y; Takenobu, T.; Li, L. J., Large-Area Synthesis of Highly Crystalline WSe, Mono layers and Device Applications. ACS Nano 2014, 8, 923-930. 45. Ji, Q. Q.; Zhang, Y F.; Gao, T.; Zhang, Y; Ma, D. L.; Liu, M. X.; Chen, YB.; Qiao, X. F.; Tan, P. H.; Kan, M.; Feng, J.; Sun, Q.; Liu, Z. F., Epitaxial Monolayer MoS2 on Mica with Novel Photoluminescence. Nano Lett 2013, 13, 3870-3877. 15 2. High-Performance Chemical Sensing Using Schottky- Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors 2.1 Introduction Environmental pollutant monitoring and trace chemical detection are important issues for both military and civilian purposes. Among air pollutants, toxic gaseous species such as nitrogen oxide (N02) and ammonia (NH3) are two of the most common ones 1 which can be generated from emissions of vehicles, power plants, and off-road equipment. N02 in ambient conditions contributes to the formation of ground-level ozone and acid rain and leads to fine particle pollution. Exposure to N02 may cause chronic bronchitis, emphysema, and respiratory irritation. On the other hand, exposure to NH3 may lead to temporary blindness, pulmonary edema, and respiratory irritation. 2 Due to these environmental and health concerns, taking N02 as an example, the U.S. Department of Environmental Protection Agency (EPA) has set a primary standard of 53 ppb (parts per billion) for N0 2 , 2 above which may cause possible health problems especially for those sensitive populations including children, elderly, and people with asthma. Therefore, it is crucially important to develop high-performance sensors that are capable of detecting such toxic gases quickly and reliably even at very low concentrations, for example, ppb level. Nanomaterials hold promising potential toward these requirements due to their large surface-to-volume ratio and intrinsic small dimension, which enable the fabrication of ultrasensitive chemical and biological sensors with minimized dimensions and 16 consequently high packing densities. 3 - 9 In this connection, various nanomaterials including carbon nanotubes, 3 · 5 · 1° silicon nanowires, 6 · 9 and metal oxide nanowires and nanobe1ts 11 · 15 have been demonstrated to show high sensitivity to a large variety of gas molecules and chemical and biological species. Two-dimensional materials (2D materials) such as graphene, in the mono layer state, offer the highest surface-to-volume ratio and can provide ultimate sensitivity down to the single-molecule level. 16 · 17 Abundant effort has been made in the fabrication of graphene based chemical and biosensors. Previous studies have also shown that proper gating of the sensors may offer them with optimal sensitivity. 6 Noticeably, layered transition metal dichalcogenides (TMDCs ), such as M0S2, share similar 2D structures with graphene, which render them very high surface areas. Moreover, recent theoretical and experimental studies have confirmed that monolayer M0S2 is a direct band gap semiconductor with impressive device performance. 18 · 20 This offers gate-tunable conductance for MoS 2 field effect transistors (FE Ts). 18 · 20 - 23 In addition, the contact property between MoS 2 and metal electrodes sensitively depends on not only the types and work functions of contact materials 24 · 28 but also the annealing recipes and contact area of metal/MoS 2 , 18 · 26 · 29 · 30 offering a way to tune the nature of contacts in M0S2 devices. These features render M0S2 a potential chemical sensing material. Mechanically exfoliated monolayer to few-layer M0S2 have been demonstrated as promising sensing materials for chemical and biological species including NO, NO 2 , NH 3 , nerve gases, proteins, etc. 31 - 35 For example, Li et al. presented a detection limit of -800 ppb ofNO, 32 and Late et al. showed a detection limit 17 of a few hundred parts per million (ppm) for both NH3 and N02 using mechanically exfoliated few-layer MoS2 FETs. 33 Interestingly, in those studies, 32 · 33 the authors found that monolayer MoS2 devices were not stable for sensing applications. Very recently, Perkins et al. have reported highly sensitive detection of nerves gases using mechanically exfoliated mono layer MoS2-based FE Ts. 34 It is therefore interesting to study whether mono layer MoS2 can be a good sensing material or not since the mono layer is the ultimate form of these 2D materials. On the other hand, taking N02 as an example, the reported detection limit of the current MoS2 sensors (e.g., a few ppm to a few hundred ppm) is still moderate and higher than the primary standard set by the EPA (i.e., 53 ppb ). Clearly, more effort should be devoted to MoS2-based sensing devices to understand their working principle as well as to develop devices with higher sensitivity to fulfill the application needs. Another consideration is that mechanical exfoliation, which is used to prepare MoS2 materials for sensing in most of the recent studies, is not suitable for large-scale fabrication of sensors, and more scalable methods should be developed. Since MoS2 is a layered material with most of its atoms being directly exposed to ambient conditions, it is also of fundamental importance to study the interaction and chemical reactions between gaseous species and the surface of the monolayer 2D semiconducting materials, about which little is currently known. In this article, we report chemical vapor deposition (CVD) growth of mono layer MoS2 in a three-zone tube furnace. We demonstrate the first use of CVD-grown mono layer MoS2 transistors with Schottky contacts for the ultrasensitive detection of N02 down to a few 18 ppb level and NH3 down to 1 ppm and potentially even lower concentration. Considering their atomic thickness and good mechanical robustness, 36 this work shows that the 2D M0S2 monolayer stands as a competitive candidate for high-performance room temperature gas sensors. 2.2 CVD synthesis and characterization of monolayer M0S2 We followed recent work for the CVD growth of monolayer Mos? 7 - 49 with some modifications. Here, a three-zone CVD furnace (Figure 2.2. la) was used to grow M0S2 monolayers, and the temperature of each zone can be controlled separately. Sulfur powder was used as solid sulfur source and was placed at the first zone. MoO3 powder was used as the Mo source which was placed at the second or the third zone. The growth substrates, SiO2/Si, were placed facing downward on top of the quartz boat hosting the MoO3 powders. The furnace was ramped up to the growth temperature of 625 °C rapidly in 7 min, and M0S2 growth lasted for 5-15 min, which resulted in either isolated triangular monolayers or quasi-continuous films (Figure 2.2.2). Figure 2.2. lb shows a typical optical microscopic image of the as-grown triangle-shaped materials, which are light blue under the optical microscope. The lateral dimensions of these triangles are found to be 5-30 µm. In addition to the large triangles, detailed atomic force microscopy (AFM) characterization also shows the existence of some small triangles, with lateral size of -1 µm or smaller, located around the large sheets and can be barely seen under an optical microscope. Interestingly, bright dots or triangles are frequently found at the center of the large triangles (Figure 2.2. lc and 19 Figure 2.2.2), which we speculate acted as "seeds" for initial nucleation of M0S2 monolayers. 41 The effectiveness of seeds in promoting the CVD growth of monolayer MoS 2 has also been reported recently by Ling et al. 50 Our Raman characterization shows that these dots are multilayer M0S2 in our samples (Figure 2.2.2 e, f). The degree of surface coverage can be tuned by the growth time, ranging from isolated individual domains (Figures 2.2. lb and Figure 2.2.2a, b) to quasi-continuous films (Figure 2.2.2c, d). The crystalline nature of the M0S2 was also confirmed by high-resolution transmission electron microscopy (TEM) characterization with a spacing of -0.27 mn for the (100) crystal planes (Figure 2.2. ld). Raman spectroscopic and photoluminescence (PL) spectroscopic studies were conducted to evaluate the number oflayers and the optical quality of the grown materials. Shown in Figure 2.2. le are representative Raman spectra of the large triangles, where the distance between the in-plane E} 9 mode and out-of-plane A 19 mode is 18-21 cm· 1 , indicating the formation of predominately monolayer M0S2, consistent with early reports. 37 - 39 . 41 - 43 One interesting property of mono layer M0S2 is that it is a direct band gap semiconductor with strong PL, showing sharp contrast with its bulk counterpart, which is an indirect band gap semiconductor with negligible quantum yield. 51 · 52 Our PL studies show that the samples show a dominated single PL peak at -1.85 to 1.86 e V (Figure 2.2. lf), which has an exceptionally high intensity (for example, >70 times higher than the Raman peaks from the SiOi/Si substrate located at -2.266 e V; see the blue spectrum in Figure 2.2. lf). We have measured tens of different samples and found that they possess very 20 similar Raman and PL characteristics, indicating the high unifonnity of the products. 360 380 400 420 Ram an shift (cm ·') Figure 2.2.1. Growth and characterization of monolayer MoS2 using a three-zone CVD furnace. (a) Schematic of the three-zone CVD furnace used for MoS2 monolayer growth. Sulfur was put in zone 1, and the Mo03 precursor and the growth substrates were put in zone 2 or zone 3. The temperatures of the three zones can be controlled separately. (b) Optical microscopy image of the as-grownMoS2 monolayers on 300 nm Si02/Si substrntes. (c) AFM image of the as-grown MoS2. The height scale bar is 12 nm. (d) High-resolution TEM image of a MoS2 monolayer. ( e,f) Raman and nonnalized PL spectra of monolayer MoS2 from different samples. Inset of ( f) shows the normalized Raman peak of the silicon substrates (2.266 eV) and two Raman peaks from MoS2 (2.283 and 2.280 eV). The Raman and PL measurements were pe1formed using a 532 nm laser. 21 Figure 2.2.2. Optical microscopy images of as-grown MoS2 monolayers on SiO2/Si substrates, showing the evolution from (a), (b) individual triangulars to (c), (d) quasi continuous films. 2.3 Device characteristics ofMoS2 FETs Layered materials like graphene and MoS2 possess the highest possible surface-to- volume ratio when they are in monolayer states, and it is expected that these materials may offer ultrahigh sensitive detection of various chemical species. The semiconducting nature of MoS2 may render efficient gate modulation of the conductance in FETs, which adds another freedom to manipulate the properties of devices.6, 15 Such studies may also benefit the fundamental understanding of the gas-solid interactions and interfacial phenomena. We have fabricated bottom -gated FE Ts direct! y on the Si O2/Si substrates where MoS2 were grown. The devices were fabricated using e-beam lithography, and 5 nm Ti/50 nm Au was deposited as source and drain electrodes. Figure 2.3. la shows a schematic diagram of the 22 device configuration in this study. An optical image of two fabricated devices is shown in Figure 2.3.lb with channel length of-I µm and channel width being determined by the size of the MoS2 sheets and the positions of the electrodes (dotted white lines indicate the position ofMoS2 in one device). Typical output (fos-VDS) and transfer characteristics (IDS- VBa) of a MoS2 FET are shown in Figure 2.3. lc,d. Figure 2.3. ld shows n-type transistor behavior for MoS2 FETs, consistent with an n-type semiconducting nature of MoS2 and recent electrical transport measurements on mechanically exfoliated or CVD-grown MoS 2 . 21 · 22 · 41 · 42 The effective mobility and on/off current ratios of our CVD-grown MoS2 in a bottom-gate FET configuration range from 0.2 to 3 cm 2 v- 1 s· 1 and 10 4 to 10 6 , respectively, which are comparable with the mechanically exfoliated or CVD-grown mono layer MoS2 under similar device configuration. 21 · 37 - 38 - 41 - 42 - 53 We notice that there is considerable level of Schottky barrier (SB) existing in our devices with Ti/ Au electrodes, as evidenced by the output characteristics (IDS-VDS) ofMoS2 devices from negative to positive VDS range, which show the rectifying characteristic of as-fabricated MoS2 devices (Figure 2.3. lc and Figure 2.3.2a,b ). The shape of fos-VDS originates from MoS2 devices with Schottky contacts at both source and drain sides, consistent with a very recent study using Co contact. 54 All of the as-fabricated MoS 2 devices (without annealing) show similar Schottky contact behavior. Such devices were used for sensing studies reported later. We have performed systematical low-temperature measurements to quantitatively determine the height of SB (Figure 2.3.2). We used the 2D thermionic emission equation 23 to describe the electrical transport behavior of Schottky-contacted MoS2 devices following Kawakami's recent report: 54 (1) Here, A is the contact area of the MoS2-electrode junction, A; 0 is the two-dimensional equivalent Richardson constant, T is the absolute temperature, q is the magnitude of the electron charge, kB is the Boltzmann constant, <DB is the height of the Schottky barrier, and n is the ideality factor. In addition, a reduced power law of T 312 instead of T 2 is used for the two-dimensional transport system. Figure 2.3.2c plots the linear fit of the Arrehenius plot, In (Ios/T 312 ) versus 1000/T (at temperatures of 160, 200, 250, and 300 K), and a slope can be obtained at each Vos. Figure 2.3.2d plots the slopes extracted from Figure 2.3.2c as a function of Vos. Linear fitting of the slope versus Vos gives a y-intercept of -0.839. Based on Equation (1), the height of the SB can be deduced as follows: (2) Therefore, the height of the SB was determined to be 72.4 me V for this device, with a standard error of 6. 5 me V We have measured a few devices, and the SB heights are in the range of 52.6 to 82.0 me V We note this shows reasonably good agreement with a recent study where a SB height of 50 me V was obtained for Ti-contacted MoS2 devices. 27 24 (a) (b) (c) (d) 2.5 - SOV 1.6 - 40V Vos 2.0 - 30V 1.2 - 20V - 0V ~ 1.5 _ , V ,.-... -1V :::t - OV 10.8 - 2v ~1 .0 - -10V ~ 3V - -20V 1/) 0 _o 0.4 - 4V 0.5 V sG - - f'>V 0.0 0.0 0 1 2 3 4 5 -40 -20 0 20 40 Vos (V) VsG (V) Figure 2.3.1. Device characteristics ofM0S2 FETs. (a) Schematic of the back-gate M0S2 FET in this study. (b) Optical image of two devices. (c,d) Typical output (Ios-Vos) and transfer characteristics (Ios-Vao) of the M0S2 FETs. 25 (a) 60 30 ~ <( .s 0 (/) _0-30 -60 -2 - 77.4K - 90 K - 1osK - 130 K - 160K - 200K - 250 K - 300K -1 0 Vos (V) 1 2 (c) -28~--------~ ~- -29 t:: <ll -30 0 C: ....J -31 3 4 5 6 1000/T (K- 1 ) (b) 4 2 (/) ..9 -2 ..._ 77.4K ..... 90 K..._ 105K ....,.... 130 K~ 160K -+- 200K (d) Q) a. -4 ..... 250 K ..... 300 K -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 Vos(V) -0.5 -0.6 ~ -0.7 , , -0.8 1- ,, " '~ -0.839 0.0 0.2 0.4 0.6 Vos (V) 0.8 Figure 2.3.2. Temperature-dependent transport measurements ofM0S2 FETs. (a) Ios-Vos characteiistics of the devices at different temperatures from 77.4 to 300 K. The Vos spans from negative to the positive range and Vao = 40 V. (b) Zoom-in plot of the IDs-Vos characteristics at low Vos regime. (c) Linear fit of the Arrehenius plot, ln(Ios/T 312 ) vei·sus 1000/T(at temperatures of 160,200,250, and300 K), and a slope can be obtained at each Vos. ( cl) Slopes extracted from ( c) as a function of Vos. Linear fitting of the slope vei·sus Vos gives a y-intercept of -0.839. 2.4 Sensing performance of M0S2 FETs to N02 and NH3 The high surface area of monolayer M0S2 renders it veiy susceptible to variation in ambient conditions. For example, the devices exhibit different on-state current and hystei·esis in air and in argon, indicating the effect of environment on the transp01t propei·ties of the M0S2 (Figure 2.4.1). 55 To study the gas-solid interaction and to get a qualitative picture on the conchJctance mochJlation ofMoSi monolayers by gas exposure, 26 we chose NO2 and NH3, the two representative toxic gases with rather different electron affinity, for illustration. Figure 2.4.2a plots the fos-VDS of a M0S2 FET upon exposure to 400 ppb ofNO2. The device was gated at VBa ~ 30 V and was highly conductive before NO2 exposure. A conductance decrease by a factor of 174 was achieved under such a low concentration of NO2, indicating the very high sensitivity of our M0S2 sensor to NO2. Furthermore, we monitored the conductance change of the M0S2 FET with exposure to NH3. The device was gated at VBa~ 0 V and showed little conduction before NH3 exposure, as shown in Figure 2.4.2b. We observed a conductance increase of -1218 times after exposure to 500 ppm ofNH3. We chose different VBa for NO2 and NH3 sensing since the threshold regime of the devices may provide better sensitivity. 6 For example, at V 8 a ~ 30 V, the device is in on-state before NO2 exposure. The introduction of NO2 may make current change more significant than a device initially in the off-state. The same consideration was taken for NH3 sensing. Collectively, the above results show the high sensitivity and large modulation of conductance of M0S2 transistors by exposure to NO2 and NH3. Moreover, it also reveals the different interaction processes between M0S2 and those two gaseous species. Later, we performed systematic chemical sensing studies of M0S2 FE Ts toward NO2 and NH3 molecules under different concentrations, as shown in Figure 2.4.2c, d. Figure 2.4.2c shows the real time conductance changes of a representative M0S2 FET toward exposure to NO2 at concentrations of 20, 40, 100, 200, and 400 ppb. Clear response ( conductance decrease) was observed for all of these concentrations among multiple 27 devices we tested. The sensor response (S) is defined as follows: 5 = Gs-Go X 100% = 6 G X 100% Go Go (3) Here, Gs and Go are the conductance of the MoS2 FETs under certain gas exposure and at an initial state, respectively. Significantly, it can be seen from Figure 2.4.2c that even for 20 ppb of NO2, a sufficiently large response of >20% was achieved, which is superior to recent reports using exfoliated few-layer MoS2 for sensing NOx under a few ppm to even hundreds of ppm concentrations. 31 · 33 We point out that this is the record detection limit (20 ppb) for NO2 using 2D TMDCs so far, and it shows several orders of magnitude improvement compared to recent studies. In addition, the clear detection of a few ppb level of NO2 shares comparable performance with the best nanowire and nanotube-based NO2 sensing devices. 5 • 11 Since the response at 20 ppb ofNO2 is sufficiently large, we think that the detection limit can be further pushed to an even lower value by optimizing the device performance. Figure 2.4.2d shows the conductance change (increase) of a MoS2 FET upon exposure to NH3 with different concentrations of I, 5, 10, 50, and 500 ppm. Similar to NO2, clear conductance modulations were observed for each NH3 concentration. Once again, we noticed that a sufficiently large sensitivity of >40% was achieved with low concentration (I ppm) of NH3 exposure, indicating that detection below ppm level of NH3 is highly possible. The response time of the sensors, which was defined as the time required to achieve a 90% change of the conductance of the overall range at a specific gas concentration, was found to be between 5 and 9 min. This is a direct reflection of the adsorption rate of the sensors, and a response time of several minutes is comparable to 28 sensors based on metal oxides, conducting polymers, and other nanomaterials like carbon nanotubes, 3 ' 11 as well as recent reports on M0S2 sensors, 32 , 33 Figure 2,4,2e, f show the sensor response versus the concentrations ofNO2 and NH3, respectively, The sensors show initially rapid conductance change upon exposure to low concentrations of NO2 and NH3, and then quasi-saturation behavior was observed for high gas concentrations, For both NO2 and NH3 sensing, we have fabricated multiple devices and conducted sensing experiments with different M0S2 sensors and repeated measurements on the same M0S2 sensor after its recovery, Here we found that the M0S2 sensors can be fully recovered by putting the devices in air for -12 h at room temperature, Alternatively, ultraviolet irradiation is found to be very efficient to fully recover the devices within a few seconds (data not shown), Among these series of sensing experiments (more than 15 sensing experiments in total), we found that the detection limits of different devices are close to each other (e,g,, -10-20 ppb for NO2 and 1 ppm for NH3), The sensor response under certain gas concentration varies by a factor of 2 among different devices, Under repeated measurements of the same device, the sensitivity remains largely unchanged, It is important to understand the gas-M0S2 interaction and sensing mechanism in the process, One important question is whether there are chemical reactions that take place and new compounds formed between analyte and M0S2 sensors, To probe this, we first collected Raman spectra of M0S2 on five different sheets, Then, the M0S2 sensor was put in 400 ppb ofNO2 ( or 500 ppm ofNH3) for 10 min (the highest gas concentrations and the same amount of time exposure in the sensing experiments), Lastly, we collected Raman 29 spectra at exactly the same positions as collected before, under assistance of markers and electrodes on the substrates. Figure 2.4.3 shows representative Raman spectra of pristine and gas exposed MoS2for NOi (Figure 2.4.3a) andNib (Figure 2.4.3b). We do not observe the formation of Mo03 after NOi exposure, which is expected to show several strong Raman peaks at 821, 666, and 284 cm· 1 , etc., as reported by Kalantar-zadeh et al. 5 6-.5ll Similarly, we do not see any noticeable changes for MoS2 after exposure to NH3 either. These results reveal that there are negligible irreversible chemical reactions taking place between NOi ( or NH3) andMoS2 sensors, which is in agreement with our observation that the sensors are fully recovered after long time air storage. (a) (b) • 4 • In A ir • 1 n A ir • In A r • • n Ar • ~ . .;; ~ l "' "' u _ u , - 2 0 0 ~O -40 -!O O 20 40 60 -61 .t() -20 (1 20 4 (1 50 VEG (V) v .. (\i) Figure 2.4.1. Transfer characteristics of two MoS2-FETs measured in air (black) and in Ar (red). A small hysteresis and large on-state current were observed for both devices measured in Ar. (e) (a)0.9~------~ ~ ------~ 100 1~------~ -Before NO 2 exposure ~ 0.6 - In <00 ppb NO 2 1 -~ 0.3 0.0~ !!!!!!!1!;; ...... -!!!!!!!!!!!!!!!!!!!!!!-eee!I 0 2 3 4 5 Vos M (!) 0 -40 =:=~;~~: cj - ·- •Oppb 1 -60 = · =~~~ ::: -·- • OO ppb -80 NO 2 0 30 60 Time (min) 90 (b) (d) 0.3~------~ - Before NH 3 exposure ~ 0.2 - In 500 ppm NH 3 1 Jo.1 = t~T = lE/ - ·- $0pl)«I ~ J 200 , --· ---;~~ o 20 "° eo NH Q 800 3 120 ~ 400 ' ' ' ' ' 0.0 1!!!!!!!!!!!1!!;;;,,,,""""'"""""!!!!!!!!!!!l!!!!!!!!!!!!!!I 0 l::::::::::::::i::::::::::.=...~-~_J 0 2 3 4 5 0 30 60 90 120 V 05 (V) Time (min) 80 60 ~ 40 .. Qo ~ ~ 20 ,. (f) 1000 0 100 200 300 400 500 NO 2 concentration (ppb) .. .. Q 100 : ~ .. 0 100 200 300 400 500 NH 3 concentration {ppm) Figme 2.4.2. Sensing performance of M0S2 FETs to N02 and NH3. (a,b) Conductance change ofM0S2 FE Ts upon exposme to 400 ppb ofN02 (a) and 500 ppm of NH3 (b ). The devices were initially turned on at VBG = 30 Vin (a) and tmned off at VBG = 0 Vin (b). A conductance decrease by a factor of 174 was achieved in (a) and increase by a factor of 1218 was achieved in (b). (c,d) Real time conductance change of M0S2 FETs with time after exposure to N02 (c) and NH3 (d) under different concentrntions. The mTows in (c) and (d) indicate the time when the coITesponding concentrations of gas were introduced. Inset in ( d) is a zoom-in plot of the sensor response to low concentrations of NH 3 of 1, 5, and 10 ppm. (e,f) Conductance change versus N02 concentrntion (e) and NH3 concentration (f) based on M0S2 FET sensors. (a)~-----=------. (b) - Selor e - Alter 400 ppb N0 2 exposure 200 300 400 500 600 700 800 900 -1 Raman shift (cm ) z ·u; C Q) c - Be fore -After 500 ppm NH 3 ~,cposu e 200 300 400 500 600 700 800 900 Raman shift (cm- 1 ) Figme 2.4.3. Raman spectra ofM0S2 sensors after exposme to 400 ppb ofN02 (a) and 500 31 ppm ofNH3 (b ). The spectra were vertically shifted for clarify. The peaks denoted by* at 520.7 cm· 1 come from SiO2/Si substrates and were used for calibration of M0S2 Raman peaks. The excitation laser wavelength is 532 nm and the laser power is the same for all measurements. 2.5 M0S2 sensing mechanism study Later, we studied how the device characteristics change upon exposure of gases and tried to shed some light on the understanding of sensing mechanism of M0S2 sensors (Figure 2.5.1). Figure 2.5.la shows the fos-VBa curves of a M0S2 FET in different concentrations of NO2. For this set of experiments, the devices were exposed to corresponding gas concentrations for 10 min and then the fos-VBa curves were measured. The fos-VBa curve taken at the initial state (before 20 ppb NO2 exposure) was also plotted as a reference. A clear monotonic shift of the curves toward positive gate voltage direction is observed, indicating a continuous increase of the threshold gate voltage (V th), as shown quantitatively in Figure 2.5. lc. In contrast, a monotonic shift of the fos-VBa curves, but in the negative gate voltage direction, was observed for NH3 sensing, as shown in Figure 2.5.lb and quantitatively plotted in Figure 2.5.ld. The M0S2 FETsensor relies on the conductance (resistance) change of the devices upon gas exposure. The resistance of a M0S2 FET can be expressed as follows. R = Rchannel + Rcontact ( 4) Here, R, Rchannel, and Rcontact are the total device resistance, the channel resistance from M0S2, and the contact resistance at metal electrode/M0S2 junctions, respectively. The channel resistance Rchannel is inversely proportional to the carrier concentration in M0S2, 32 while the contact resistance, Rcontact, relates to both electron concentration and the height of the SB and can be expressed as follows: 59 1 Rchannel OC - (5) n 1 (PSB) R Contact CC - e kT ( 6) n where n is the electron concentration in MoS2, <psB is the height of SB formed at the MoS2- electrode junctions, k is the Boltzmann constant, and T is the absolute temperature. Obviously, a change of either electron concentration or SB height can lead to a change in the total resistance (conductance) of the MoS2 FET and consequently can be reflected in the sensing experiments in Figure 2.4.2. Charge transfer between gaseous species and nanomaterials serves as an important work principle for sensing devices based on nanomaterials. NO2 is a well-known strong oxidizer due to an unpaired electron from nitrogen atom, which tends to withdraw electrons from sensing materials like MoS2 studied here. This leads to a decreased electron concentration in the conduction band of the MoS2, and thus more positive gate voltage is required to turn on the transistor, indicating a positive shift of the V th upon exposure to NO2 (Figure 2.5.lc). NH3, on the other hand, having a lone electron pair tends to donate electrons to the conduction band of MoS2, and this would lead to an increased electron concentration in MoS2, and thus a low gate voltage is required to operate the transistor (Figure 2. 5. ld). Such a charge transfer mechanism was also used recently to understand the sensing behavior of MoS2 toward NH3, NO, and nerve gas by other researchers and showed good agreement with our experimental results. 31 · 33 · 34 · 60 In our experiments, we 33 observed that the detection limit for N02 is around 2 orders of magnitude lower than that ofNH3 (e.g., 20 ppb versus 1 ppm), which is a common phenomenon observed in many other nanomaterial-based sensors including carbon nanotubes and nanowires. 61 Density functional theory calculations show that N02 has much stronger interaction with carbon nanotubes than NH3 does, which may be responsible for the detection of much lower concentration ofN02 than NH3. A very recent calculation on the gas-M0S2 interaction also shows that N02 has a much larger charge transfer ability (10 times larger) than NH3 with bilayer M0S2. 33 This, consequently, will lead to a very sensitive detection ofN02 because a small amount ofN02 adsorption can cause a substantial change of carrier concentration, which can be reflected through electrical transport measurements. The charge transfer mechanism suggests that carrier concentration change is the work principle for sensing. A reasonable approximation is that the conductance change of the device should be related to the surface occupancy (0) of the gas molecules on sensing materials based on a site-binding hypothesis, 61 which assumes that atoms on the surface of the sensing materials can act as binding sites for molecule adsorption. That is 6G 0 -cc Go (7) The surface coverage of the adsorbed molecules follows a Langmuir isotherm, and this leads to a linear relationship between inverse sensitivity (1/S) and the inverse gas concentration (1/C), as shown previously for ohmic-contacted In 2 0 3 nanowire 11 and graphene nanoribbon 62 chemical sensors. However, the Schottky-contacted MoS 2 sensors do not follow such a linear relationship between 1/S and 1/C (Figure 2.5.2). This suggests 34 that there should be other reasons in addition to charge transfer that also contribute to the observed sensing behavior. As there is clear SB in our devices (Figure 2.3.2) and according to Equation 6, the property of the SB can significantly influence the resistance of the Schottky-contacted devices due to their exponential relationship. 59 Figure 2.5.3 shows the schematic of an energy diagram of a Ti/ Au electrode and MoS2 mono layer, where the work functions of Ti (<pTi) and Au (<pAu) are 4.3 and 5.1 eV, respectively, 59 and the electron affinity (x) ofMoS 2 is -4.2 e V. 63 • 64 As we deposited 5 nm Ti/50 nm Au onto MoS2 to form source/drain electrodes, we may have Ti contacting MoS2 for most area of the source/drain contacts and Au contacting MoS2 at the edges of source/drain contacts because the metal evaporation cannot be completely vertical. Therefore, the actual metal contact may be an average effect of both Ti and Au. In addition, we further note that whether there are surface states between metals and 2D materials like MoS2 has not been well understood, and the presence or absence of surface states may affect the band diagram. Nevertheless, it is still reasonable to assume that, before contact, the Fermi level of Ti/ Au lies between the conduction band and valence band of MoS2 (Figure 2.5.3a). After contact, the energy bands bend for MoS2 and a SB forms with a "nominal" height of (flss = (f)Au - X (8) When MoS2 devices are exposed to different gases, the conductance change can usually result from a coplay of two factors: charge transfer and SB modulation. 61 The absorption of gas will modify the built-in potential (Vbi) and the width of the SB. Specifically, NO2 35 absorption can move the Fermi level ofMoS2 toward the valence band. This will increase the width of the SB and decrease the Vbi and device current. The opposite is true for NH3 absorption (Figure 2.5.3b). On the other hand, it has been reported that absorbed gas species can change the work function of both metal electrodes and semiconducting materials due to surface dipole layer formation. 61 · 65 - 67 In the case of NO2, it can withdraw electrons and form negatively charged NOi°- species, which may increase the SB height. For example, oxidative species such as oxygen was reported to increase the SB height of a ZnO nanowire transistor. 12 Collectively, both charge transfer and SB modulation are believed to have the same trend to decrease the conductance of MoS2 devices in the case ofNO2 exposure. On the other hand, NH3 may hold the opposite trend compared with NO2. Figure 2.5.3b shows the band realignment and energy diagram of the metal-MoS2junction after exposure to NO2 and NH3, depicting the effects of change of SB height and width ( and V bi) upon gas absorption. 36 (a) 0.4 Vos='SV Before exposure 0.3 20 ppb 10.2 40ppb 100 ppb r/) 200 ppb 0 400 ppb - 0.1 N02 0.0 -40 -20 0 (b) 5 V oo (V) v 05 = 5V 4 Before exposure ~3 1 ppm 5 ppm -2, 10ppm ~2 50ppm 200 ppm 1 SOOppm 0 NH 3 -40 -20 0 V 6G 0/) 20 40 20 40 (c) 25 20 ~15 a .t::. >10 a 0 100 200 300 400 NO 2 concentration (ppb) (d) 30 a 20 a -10 , 0 100 200 300 400 500 NH 3 concentration (ppm) Figure 2.5.l. MoS2 sensing mechanism study. (a,b) Transfer characteristics (los-VaG) of MoS2 FETs upon exposure to N02 (a) and NH3 (b) with different concentrations. (c,d) Threshold voltage (Vth) versus gas concentrations for N02 (c) and NH3 (d). (a) -4 (!) -3 > ~ (.9 I -2 -1 '---'---'-----''-----"'-----"'-----' 0.00 0.01 0.02 0.03 0.04 0.05 0. 06 1 /NO 2 concentration (1/ppb) (b) 3~-----~ 0.0 0.2 0.4 0.6 0.8 1.0 1/NH 3 concentration (1 /ppm ) Figure 2.5.2. Plot of inverse gas concentration (Conc)- 1 versus inverse sensor response (7,:)- 1 for (a) N02 and (b) NH3. 37 a) --- . ------------------ Vacuum e (!>Metal ex ~-~-- Ee ------ _ E ____ EF g TI/Au (b) NO •· - ~~:er ~-:_-"7 - - - - - - • .1S82 SB "----"--- Ev .... ------------- TI/Au MoS 2 Figure 2.5.3. Proposed band alignment diagrams at the MoS2-metal junctions. (a) Energy diagram of the Ti/Au and MoS2 before contact. (b) Band realignment and energy diagram of the Ti/Au and MoS2 after contact and the formation of Schottky bani er. Blue, green, and red lines indicate the energy band of the pristine MoS2 (solid blue), after exposure to NO2 (dashed green), and after exposure to NH3 (dashed red), respectively. NO2 adsorption increases the SB by t,.SB I while NH3 adsorption decreases the SB by 11SB2 In addition, the built-in voltage (Vbi) will decrease (increase) upon NO2 (NH3) exposure, resulting in a widening (thinning) of the SB width and a decrease (increase) of the device current 2.6 Additional information In addition, we found that some MoS2 devices show more Ohmic contact after vacuum annealing (Figure 2.6. !), and the sensing results show that the ohmic-contacted device exhibits little conductance change (<5%) upon exposure to NO2 at concentrations up to 400ppb (Figure 2.6 2) This suggests that SB modulation may play a key role in our sensors. Further study is needed to obtain quantitative information about the relative contribution 38 from each mechanism. Interestingly, we also obsetved that the effective mobility ofMoS2 FETs changed upon N02 and NH:i exposure, and NH3 exposure increased the effective mobility of MoS2 (Figure 2.6.3 and Figure 2.6.4). This finding may serve as an effective strategy to engineer the effective mobility ofMoS2-based electronic devices. (a) r==-------::; 8 VBG ~ -•- OV -,. 6 - .. 25V ,I("""~ <{ ~ 50V .,,,, ,,,,,,,,....,.,,,,,.,,,... :, 4 :,...;. ... ..9 ? .,,.f,., 0 0 4 5 (b) v •• .,.,., .. 2 --ov ~ ,.... ,..,...,., <{ C - 1 (/) 0 0 ~ 2~V + 50V / .. ,,. ,..A "' .---• ~ / ,,.,,, ~ / ---· ,.,, .. ... ~ / .4 ---•-•-•-•-•-•-•-• 0.0 0.2 0.4 0.6 0.8 1.0 Vos (V) Figure 2.6.1. Output characteristics of the annealed MoS2 devices where a saturation behavior is obsetved in (a) and the II:6-Vos show linear relation ship at small bias (b ). 30 20 - 10 ~ 0 0 - 0 -10 (9 -- -20 (9 2 o ppb I 20 ppb 100 ppb 400 ppb <l -30 -40 -50 20 40 60 80 Time (minute) Figure 2.6.2. Real time conductance change of an ohmic-contacted MoS2 FET with N02 concentrations from 20 ppb to 400 ppb. The arrows indicate the time when certain 39 concentrations of NO2 were introduced. The conductance changes were smaller than 5% for 400 ppb NO2. (a) (b) U) • .a Device 1 U) • Device 1 <:3 • Device 2 < 8 g Device 2 N N E • Device 3 E • Device 3 ~ • ~ () • £2 • Device 4 ~6 £ 15 :g 4 ~· 0 g;, E • E • 0 (I) 1 0 • (I) I -~ ~ Q • :6 2 t5 ~ t g • • () 0 • ffi 0 • (I) • • ' ~ 0 •• • • 0 100 200 300 400 0 100 200 300 400 500 NO 2 concentration (ppb) N H 3 concentration (ppm) Figure 2.6.3. Effective mobility changes of the M0S2 transistors upon exposure to NO2 (a) and NH3 (b) with different concentrations. ( a) 40 . (b) 10 30 500 ppm NH 3 1 hr exposure ! i i 0.1 10 -50 -40 -30 -20 -10 0 10 20 30 40 50 VBG (V) ~ 0.01 1E-3 1E-4 500 ppm NH 3 1 hr exposure -50 -40 -30 -20 -10 0 10 20 30 40 50 VBG (V) Figure 2.6.4. Transfer characteristic of a M0S2-FET after exposure to 500 ppm NH3 for 1 hr in linear scale (a) and log scale (b). The device shows an effective mobility up to 26.9 cm 2 /Vs under a back gate configuration. The measurement was performed from 50 V to - 50 V for VBG under a Vos of 5V. 2. 7 Conclusion In conclusion, we have demonstrated room temperature highly sensitive detection of NO2 and NH3 using CVD-grown monolayer M0S2 with Schottky contacts. In particular, 40 20 ppb of N02 and 1 ppm of NH3 were clearly detected with a conductance change larger than 20 and 40%, respectively, which are much superior to recently reported MoS2 sensors. Both charge transfer between gaseous species and MoS2 monolayers and SB modulation at the MoS2-metal electrode junctions are suggested to be responsible for the observed sensing behavior, while Schottky barrier modulation is believed to be the key factor for the significantly improved sensitivity. The detection limit can be further pushed to sub-ppb level by optimizing the device performance like the features of the Schottky barrier. Our work demonstrates the potential of these 2D layered semiconductors as strong candidates for gas and chemical sensing applications. Chapter 2. References 1. Yamazoe, N. Toward Innovations of Gas Sensor Technology, Sens. Actuators, B 2005, 108, 2- 14. 2. http://www.epa.gov/ Air/Nitrogenoxides/. 3. Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Nanotube Molecular Wires as Chemical Sensors, Science 2000, 287, 622-625. 4. Snow, E. S.; Perkins, F. K.; Robinson, J. A. Chemical Vapor Detection Using Single-Walled Carbon Nanotubes, Chem. Soc. Rev. 2006, 35, 790-798. 5. Pengfei, Q. F.; Verrnesh, O.; Grecu, M.; Javey, A.; Wang, O.; Dai, H. J.; Peng, S.; Cho, K. J. Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection, Nano Lett. 2003, 3, 347-351. 6. Gao, X. P A.; Zheng, G. F.; Lieber, C. M. Subthreshold Regime Has the Optimal Sensitivity for Nanowire FET Biosensors, Nano Lett. 2010, 10, 547-552. 7. Hahm, J.; Lieber, C. M. Direct Ultrasensitive Electrical Detection of DNA and DNA Sequence Variations Using Nanowire Nanosensors, Nano Lett. 2004, 4, 51-54. 8. Cui, Y; Wei, Q. Q.; Park, H.K.; Lieber, C. M. Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species, Science 2001, 293, 1289-1292. 9. Zheng, G. F.; Patolsky, F.; Cui, Y; Wang, W. U.; Lieber, C. M. Multiplexed Electrical Detection of Cancer Markers with Nanowire Sensor Arrays, Nat. Biotechnol. 2005, 23, 1294-1301. 10. Ganzhom, M.; Vijayaraghavan, A.; Dehm, S.; Hennrich, F.; Green, A. A.; Fichtner, M.; Voigt, A.; Rapp, M.;von Lohneysen, H.; Hersam, M. C. et al. Hydrogen Sensing with Diameter- and 41 Chirality-Sorted Carbon Nanotubes, ACS Nano 2011, 5, 1670-1676 11. Zhang, D. H.; Liu, Z. Q.; Li, C.; Tang, T.; Liu, X. L.; Han, S.; Lei, B.; Zhou, C. W. Detection of NO, down to ppb Levels Using Individual and Multiple In,O, Nanowire Devices, Nano Lett. 2004, 4, 1919-1924. 12. Wei, T.-Y; Yeh, P.-H.; Lu, S.-Y; Wang, Z. L. Gigantic Enhancement in Sensitivity Using Schottky Contacted Nanowire Nanosensor, J. Am. Chem. Soc. 2009, 131, 17690-17695. 13. Yeh, P. H.; Li, Z.; Wang, Z. L. Schottky-Gated Probe-Free ZnO Nanowire Biosensor, Adv. Mater. 2009, 21, 4975-4978. 14. Kolmakov, A.; Klenov, D. O.; Lilach, Y; Stemmer, S.; Moskovits, M. Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles, Nano Lett. 2005, 5, 667--673. 15. Zou, X. M.; Wang, J. L.; Liu, X. Q.; Wang, C. L.; Jiang, Y; Wang, Y; Xiao, X. H.; Ho, J. C.; Li, J. C.; Jiang, C. Z.et al. Rational Design of Sub-Parts Per Million Specific Gas Sensors Array Based on Metal Nanoparticles Decorated Nanowire Enhancement-Mode Transistors, Nano Lett. 2013, 13, 3287-3292. 16. Schedin, F.; Geirn, A. K.; Morozov, S. V.; Hill, E.W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection oflndividual Gas Molecules Adsorbed on Graphene, Nat. Mater. 2007, 6, 652- 655. 17. Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katsnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Molecular Doping of Graphene, Nano Lett. 2008, 8, 173-177. 18. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS, Transistors, Nat. Nanotechnol. 2011, 6, 147-150. 19. Jariwala, D.; Sangwan, V. K.; Late, D. J.; Johns, J.E.; Dravid, V. P.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Band-like Transport in High Mobility Unencapsulated Single-Layer MoS, Transistors,Appl. Phys. Lett. 2013, 102, 173107. 20. Jariwala, D.; Sangwan, V. K.; Wu, C. C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Gate-Tunable Carbon Nanotube-MoS, Heterojunction p-n Diode, Proc. Natl. Acad. Sci. USA. 2013, ll0, 18076-18080. 21. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2, Nat. Nanotechnol. 2013, 8, 497-501. 22. Radisavljevic, B.; Kis, A. Mobility Engineering and a Metal-Insulator Transition in Monolayer MoS,, Nat. Mater. 2013, 12, 815-820. 23. Sangwan, V. K.; Arnold, H. N.; Jariwala, D.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Low Frequency Electronic Noise in Single-Layer MoS, Transistors, Nano Lett. 2013, 13, 4351-4355. 24. Chuang, S.; Battaglia, C.; Azcatl, A.; McDonell, S.; Kang, J. S.; Yin, X.; Tosun, M.; Kapadia, R.; Fang, H.; Wallace, R. M.et al. MoS, p-Type Transistors and Diodes Enabled by High Work Function MoOxContacts, Nano Lett. 2014, 14, 1337- 1342. 25. Walia, S.; Balendhran, S.; Wang, Y C.; Ab Kadir, R.; Zoolfakar, A. S.; Atkin, P.; Ou, J. Z.; Sriram, S.; Kalantar-zadeh, K.; Bhaskaran, M. Characterization of Metal Contacts for Two Dimensional MoS, Nanoflakes, Appl. Phys. Lett. 2013, 103, 232105. 26. Late, D. J.; Liu, B.; Matte, H. S. S. R.; Dravid, V. P.; Rao, C. N. R. Hysteresis in Single-Layer 42 MoS2 Field Effect Transistors,ACS Nano 2012, 6, 5635-5641. 27. Das, S.; Chen, H. Y; Penurnatcha, A. V.; Appenzeller, J. High Performance Multilayer MoS2 Transistors with Scandium Contacts Nano Lett. 2013, 13, 100-105. 28. Popov, I.; Seifert, G.; Tomanek, D. Designing Electrical Contacts to MoS, Monolayers: A Computational Study, Phys. Rev. Lett. 2012, 108, 156802. 29. Lee, G. H.; Yu, Y Y; Cui, X.; Petrone, M.; Lee, C.H.; Choi, M. S.; Lee, D. Y; Lee, C.; Yoo, W. J.; Watanabe, K. et al. Flexible and TransparentMoS, Field-Effect Transistors on Hexagonal Boron Nitride-Graphene Heterostructures, ACS Nano 2013, 7, 7931-7936. 30. Liu, H.; Si, M.; Deng, Y; Neal, A. T.; Du, Y; Najrnaei, S.; Ajayan, PM.; Lou, J.; Ye, PD. D. Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: An Insight into Current Flow across Schottky Barriers, ACS Nano 2014, 8, 1031-1038. 31. He, Q. Y; Zeng, Z. Y; Yin, Z. Y; Li, H.; Wu, S. X.; Huang, X.; Zhang, H. Fabrication of Flexible MoS2 Thin-Film Transistor Arrays for Practical Gas-Sensing Applications, Small 2012, 8, 2994-2999. 32. Li, H.; Yin, Z. Y; He, Q. Y; Li, H.; Huang, X.; Lu, G.; Fam, D. W. H.; Tok, A I. Y; Zhang, Q.; Zhang, H. Fabrication of Single- and Multilayer MoS, Film-Based Field-Effect Transistors for Sensing NO at Room Temperature, Small 2012, 8, 63-67. 33. Late, D. J.; Huang, Y K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J. J.; Yan, A.; Charles, D.; Waghrnare, U.; Dravid, V. Pet al. Sensing Behavior of Atomically Thin-Layered MoS, Transistors,ACS Nano 2013, 7, 4879-4891. 34. Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P M.; Jernigan, G. G.; Jonker, B. T. Chemical Vapor Sensing with Monolayer MoS2, Nano Lett. 2013, 13, 668-673. 35. Sarkar, D.; Liu, W.; Xie, X.; Anselmo, A. C.; Mitragotri, S.; Banerjee, K. MoS2 Field-Effect Transistor for Next-Generation Label-Free Biosensors, ACS Nano 2014, DOI: 10.1021/nn5009148. 36. Castellanos-Gomez, A.; Poot, M.; Steele, G. A.; van der Zant, H. S. J.; Agrait, N.; Rubio Bollinger, G. Elastic Properties of Freely Suspended MoS2 Nanosheets, Adv. Mater. 2012, 24, 772-775. 37. Lee, Y H.; Zhang, X. Q.; Zhang, W. J.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y C.; Wang, J. T. W.; Chang, C. S.; Li, L. I.et al. Synthesis of Large-Area MoS, Atomic Layers with Chemical Vapor Deposition, Adv. Mater. 2012, 24, 2320-2325. 38. Lee, Y H.; Yu, L. L.; Wang, H.; Fang, W. J.; Ling, X.; Shi, Y M.; Lin, C. T.; Huang, J. K.; Chang, M. T.; Chang, C. S.et al. Synthesis and Transfer of Single-Layer Transition Metal Disulfides on Diverse Surfaces, Nano Lett. 2013, 13, 1852-1857. 39. Liu, K. K.; Zhang, W. J.; Lee, Y H.; Lin, Y C.; Chang, M. T.; Su, C.; Chang, C. S.; Li, H.; Shi, Y M.; Zhang, H.et al. Growth of Large-Area and Highly Crystalline MoS, Thin Layers on Insulating Substrates, Nano Lett. 2012, 12, 1538-1544. 40. Shi, Y M.; Zhou, W.; Lu, A. Y; Fang, W. J.; Lee, Y H.; Hsu, A. L.; Kirn, S. M.; Kirn, K. K.; Yang, H. Y;Li, L. let al. van der Waals Epitaxy ofMoS, Layers Using Graphene as Growth Templates, Nano Lett. 2012, 12, 2784-2791. 41. Najrnaei, S.; Liu, Z.; Zhou, W.; Zou, X. L.; Shi, G.; Lei, S. D.; Yakobson, B. I.; Idrobo, J. C.; 43 Ajayan, P. M.;Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers, Nat. Mater. 2013, 12, 754-759. 42. van der Zande, A. M.; Huang, P. Y; Chenet, D. A.; Berkelbach, T. C.; You, Y M.; Lee, G. H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and Grain Boundaries in Highly Crystalline Monolayer Molybdenum Disulphide, Nat. Mater. 2013, 12, 554-561. 43. Zhan, Y J.; Liu, Z.; Najmaei, S.; Ajayan, P. M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization ofMoS2 Atomic Layers on a SiO2 Substrate, Small 2012, 8, 966-971. 44. Yu, Y F.; Li, C.; Liu, Y; Su, L. Q.; Zhang, Y; Cao, L. Y Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-Layer MoS, Films, Sci. Rep. 2013, 3, 1866. 45. Lin, Y C.; Zhang, W. J.; Huang, J. K.; Liu, K. K.; Lee, Y H.; Liang, C. T.; Chu, C. W.; Li, L. J. Wafer-Scale MoS, Thin Layers Prepared by MoO, Sulfurization, Nanoscale 2012, 4, 6637- 6641. 46. Wang, H. T.; Lu, Z. Y; Xu, S. C.; Kong, D. S.; Cha, J. J.; Zheng, G. Y; Hsu, P. C.; Yan, K.; Bradshaw, D.;P.rinz, F. B.et al. Electrochemical Tuning of Vertically Aligned MoS2 Nanofilms and Its Application in Improving Hydrogen Evolution Reaction, Proc. Natl. Acad. Sci. USA. 2013, ll0, 19701-19706. 47. Kong, D. S.; Wang, H. T.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y Synthesis of MoS, and MoSe, Films with Vertically Aligned Layers, Nano Lett. 2013, 13, 1341-1347. 48. Ji, Q. Q.; Zhang, Y F.; Gao, T.; Zhang, Y; Ma, D. L.; Liu, M. X.; Chen, YB.; Qiao, X. F.; Tan, P.H.; Kan, M.et al. Epitaxial Monolayer MoS2 on Mica with Novel Photoluminescence, Nano Lett. 2013, 13, 3870-3877. 49. Wu, S. F.; Huang, C. M.; Aivazian, G.; Ross, J. S.; Cobden, D. H.; Xu, X. D. Vapor-Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization, ACS Nano 2013, 7, 2768-2772. 50. Ling, X.; Lee, Y H.; Lin, Y X.; Fang, W. J.; Yu, L. L.; Dresselhaus, M.; Kong, J. Role of the Seeding Promoter in MoS, Growth by Chemical Vapor Deposition, Nano Lett. 2014, 14, 464- 472. 51. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS,: A New Direct-Gap Semiconductor, Phys. Rev. Lett. 2010, 105, 136805. 52. Splendiani, A.; Sun, L.; Zhang, Y B.; Li, T. S.; Kirn, J.; Chim, C. Y; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS,, Nano Lett. 2010, 10, 1271-1275. 53. Wang, X. S.; Feng, H. B.; Wu, Y M.; Jiao, L. Y Controlled Synthesis of Highly Crystalline MoS2 Flakes by Chemical Vapor Deposition, J. Am. Chem. Soc. 2013, 135, 5304-5307. 54. Chen, J. R.; Odenthal, P. M.; Swartz, A.G.; Floyd, G. C.; Wen, H.; Luo, K. Y; Kawakami, R. K. Control of Schottky Barriers in Single Layer MoS, Transistors with Ferromagnetic Contacts,NanoLett. 2013, 13, 3106-3ll0. 55. Qiu, H.; Pan, L. J.; Yao, Z. N.; Li, J. J.; Shi, Y; Wang, X. R. Electrical Characterization of Back-Gated Bi-layer MoS, Field-Effect Transistors and the Effect of Ambient on Their Perforrnances,App/. Phys. Lett.2012, 100, 123104. 56. Ou, J. Z.; Campbell, J. L.; Yao, D.; Wlodarski, W.; Kalantar-zadeh, K. In Situ Raman Spectroscopy ofH2 Gas Interaction with Layered MoO,, J. Phys. Chem. C 2011, ll5, 10757- 44 10763 57. Balendhran, S.; Deng, J. K.; Ou, J. Z.; Walia, S.; Scott, J.; Tang, J. S.; Wang, K. L.; Field, M. R.; Russo, S.; Zhuiykov, S. et al. Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide,Adv. Mater. 2013, 25, 109-114. 58. Wang, Y C.; Ou, J. Z.; Balendhran, S.; Chrimes, A. F.; Mortazavi, M.; Yao, D. D.; Field, M. R.; Latham, K.; Bansal, V.; Friend, J. R.et al. Electrochemical Control of Photoluminescence in Two-Dimensional MoS2 Nanoflakes,ACS Nano 2013, 7, 10083-10093. 59. Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; John Wiley & Sons, Inc.: New York, 2007. 60. Lee, K.; Gatensby, R.; McEvoy, N.; Hallam, T.; Duesberg, G. S. High Performance Sensors Based on Molybdenum Disulfide Thin Fihns, Adv. Mater. 2013, 25, 6699-6702. 61. Javey, A.; Kong, J. Carbon Nanotube Electronics; Springer Science+Business Media: New York, 2008. 62. Abbas, A.; Liu, G.; Liu, B. L.; Zhang, L.; Liu, H.; Ohlberg, D.; Wu, W.; Zhou, C. W. Patterning, Characterization and Chemical Sensing Applications of Graphene Nanoribbon Arrays down to 5 nm Using Helium Ion Beam Lithography,ACS Nano 2014, 8, 1538-1546. 63. Gong, C.; Zhang, H. J.; Wang, W. H.; Colombo, L.; Wallace, R. M.; Cho, K. J. Band Alignment of Two-Dimensional Transition Metal Dichalcogenides: Application in Tunnel FieldEffectTransistors,App/. Phys. Lett. 2013, 103, 053513. 64. Fontana, M.; Deppe, T.; Boyd, A. K.; Rinzan, M.; Liu, A. Y; Paranjape, M.; Barbara, P Electron-Hole Transport and Photovoltaic Effect in Gated MoS, Schottky Junctions, Sci. Rep. 2013, 3, 1634 65. Nah, J.; Kumar, S. B.; Fang, H.; Chen, Y Z.; Plis, E.; Chueh, Y L.; Krishna, S.; Guo, J.; Javey, A. Quantum Size Effects on the Chemical Sensing Performance of Two-Dimensional Semiconductors J. Phys. Chem. C 2012, 116, 9750-9754. 66. Cui, X. D.; Freitag, M.; Martel, R.; Brus, L.; Avouris, P Controlling Energy-Level Alignments at Carbon Nanotube/ Au Contacts, Nano Lett. 2003, 3, 783-787. 67. Gui, E. L.; Li, L. J.; Zhang, K. K.; Xu, Y P; Dong, X. C.; Ho, X. N.; Lee, P S.; Kasitn, J.; Shen, Z. X.;Rogers, J. A.et al. DNA Sensing by Field-Effect Transistors Based on Networks of Carbon Nanotubes J. Am. Chem. Soc. 2007, 129, 14427-14432. 45 3. Screw-Dislocation-Driven Growth of Two- Dimensional Few-Layer and Pyramid-Like WSe2 by Sulfur-Assisted Chemical Vapor Deposition 3.1 Introduction Recently, two-dimensional (2D) layered materials beyond graphene have attracted huge amounts of attention especially for those transition metal dichalcogenides (TMDCs) with formula of MX 2 (M ~ Mo, W; X ~ S, Se). 1 - 4 Such TMDCs are also layered materials like graphene coupled by weak van der Waals forces between adjacent layers. But more than that, each TMDC layer contains three covalent bonded X-M-X atomic layers. Besides the unique structure, these materials exhibit interesting properties when the thickness goes down to monolayer or few-layers,2· 5 - 7 making them as good candidates for advanced electronics, 3 - 11 optoelectronic devices, 12 - 15 energy storage devices, 16 and electrocatalysts. 17 So far, a lot of research efforts have been devoted to these materials focusing on material synthesis,2· 13 - 23 characterization, 24 · 25 fundamental property studies, 4 · 26 - 28 and applications. 11 · 15 · 29 · 30 Among those four materials (MoS2, M0Se2, WS2, and WSe2), MoS2 is the most heavily studied one. On the other hand, WSe2 is not well studied until some recent papers. 13 - 15 Compared to MoS 2 , WSe 2 possesses a smaller bandgap and it exhibits ambipolar transport phenomenon_ 13- 15 Chemical vapor deposition (CVD) is a widely used method for TMDC synthesis. 13 - 21 Early in 2012, Balendhran and Kalantar-zadeh et al. have shown the preparation of thin MoS2 flakes by evaporation of sulfur and MoO3, following by exfoliation. 31 Although CVD 46 growth ofMoS2 has been well developed, only a few papers reported the successful growth of 2D WSe2 structures. 32 - 34 The difficulty of CVD growth of WSe2 is believed to originate from the low reactivity of selenium. 20 · 32 For example, Huang et al. have reported the synthesis of large area monolayer WSe2 on sapphire under low-pressure CVD, and they found that adding H2 as an additional reducing reactant is a must for successful WSe2 synthesis. 32 Few-layer WSe 2 synthesis was also reported recently by Lin et al. using graphene as an epitaxial substrate. 34 Nevertheless, the growth mechanism ofWSe 2 is still not clear. And more efforts should be devoted to further explore the controlled synthesis of WSe2. Here we report a sulfur-assisted CVD method to grow few-layer and pyramid-like WSe2 flakes following a screw-dislocation-driven (SDD) growth fashion. Such SDD growth is a universal growth mechanism in nanomaterials including 1D nanotubes, 35 · 36 1D nanowires, 37 - 39 and 2D nanoplates. 40 · 41 However, it has not been reported in CVD synthesis of2D layered materials. We believe this unique growth process in our method is due to low concentration of the reactants, and accordingly, relevant models are proposed to understand the SDD growth process in the CVD synthesis of WSe2 flakes. 3.2 CVD setup of sulfur-assisted growth Figure 3.2. la illustrates the CVD setup we used for the synthesis of few-layer and pyramid-like WSe2 flakes in this study. Similar to recent CVD methods for TMDC synthesis, 30 .3 2 a quartz boat containing WO 3 powders with a Si/SiO 2 substrate sitting on top was placed in the middle of a 1-inch tube furnace. The temperature of WO3 varied from 47 875 °C to 925 °C. Selenium and sulfur powders were loaded in two separated quartz boats and put at the upstream region. Note that only very small amounts of sulfur powders were used and they were intentionally put at a position with temperature below the melting point of sulfur to minimize their sublimation. After the growth, the SDD grown flakes can be found everywhere on the substrate. But the nucleation density is higher at positions closer to the WO3 powders. By adjusting the growth temperature and time, a variety of WSe2 flakes with different thicknesses and morphologies were synthesized (Figures 3.2. lb-3.2. lm), including few layer triangles, few-layer hexagons, thick triangles, and thick hexagons. Figures 3.2.lb, 3.2. lc, and 3.2. ld are optical microscopy images of thin flakes synthesized at temperatures ranging from 875 °C to 900 °C. Besides the triangular and merged triangular flakes that are commonly observed in CVD-grown TMDCs, 25 hexagonal flakes are also occasionally found here. The lateral sizes of these flakes are ranging from 3 µm to 5 µm. When the growth temperature increased to 925 °C, we found that the flakes were predominantly thick ones based on optical microscopy observations. This is consistent with recent results showing that additional layers would grow at high temperature during CVD synthesis of MoS 2 and WSe 2 . 32 · 42 In our experiments, we found that the growth window of WSe 2 is narrow, and temperature plays the most important role in determining the thickness and shape of the materials. In general, both thin and thick flakes coexist at a temperature of 900 °C (Figure 3.2.2a), while at relative high temperatures (above 925 °C), all the flakes are thick ones with heights over 10 nm (Figure 3.2.2b ). We did a statistical analysis of the 48 samples grown at 900 °C based on Figure 3.2.2a, and the results show that 78% are thin flakes and 22% are thick flakes (> 10 nm). Temperature also plays a crucial role in affecting the shapes of as-grown materials. Most flakes are triangular when the growth temperature is below 900 °C. After the growth temperature rises to 925 °C, both triangular flakes and hexagonal flakes exist. Statistical studies on the samples grown at 925 °C shows that 96% are triangular and 4% are hexagonal. We note that this ratio may vary from location to location on a substrate. Very interestingly, most thick WSe2 flakes in our products have intriguing terrace-like morphologies with different stacking angles and shapes, as exhibited from Figures 3.2. le to 3.2. lm. The color contrasts of these images in Figure 3.2.1 reflect the differences of thickness among the WSe2 samples. Based on the optical microscope inspections, the thicknesses of these thick flakes decrease from centers to edges, indicating the formation of pyramid-like structures. To be noticed, although Figure 3.2.1 shows all possible morphologies observed during the experiments, some stacking types do appear more frequently than others. Detailed results will be discussed later. 49 (a) Ar -+ -+ s Se wo. - - • 3µm - . : 60° ' ,.,,,.-• I ,:,",. -~ I --=--..-1 (h) - (i) - (g) • Figure 3.2.1. CVD set up and optical microscopy characterization of CVD-grown few layer and pyramid-like WSe2 flakes. (a) A schematic diagram shows the CVD setup for the sulfur-assisted WSe2growth. (b, c, d) Optical microscopy images of thin WSe2flakes with different shapes. (e-m) Optical microscopy images of thick WSe2 flakes with different stacking morphologies. The color contrasts reflect the thickness variations both from sample to sample and from center to edge within each sample. The white dotted lines indicate the stacking angles between top and bottom features in thick flakes. The scale bars are 3 µm for all images. Figure 3.2.2. Optical images of samples grown at different temperatures. (a) Samples grown at 900 °C. Both thin triangular and thick triangular flakes are observed. (b) Samples grown at 925 °C. All the flakes are thick ones, and hexagonal flakes can be found at this growth temperature. 50 3.3 AFM, Raman, and TEM characterization of thin WSe2 flakes To further explore the detailed structures of thin and stacked thick WSe2 flakes, we performed systematical atomic force microscopy (AFM) and Raman studies. AFM characterization shows that most of the thin flakes (Figures 3.2. lb, 3.2. lc, and 3.2. ld) are few-layer materials, such as tri-layers (Figures 3.3.la and 3.3.lb) and four-layers (Figure 3.3.2), as evident from the cross-section height profiles (bottom parts of Figures 3.3. la and 3.3.lb). More importantly, we frequently observed that there are ribbon-like features lying on top of these thin flakes. The ribbons usually have heights equal to one or two-layer thickness of WSe2 (red and black height profiles in Figures 3.3.la and 3.3.lb). Figures 3.3. lc and 3.3. ld are Raman spectra taken from several such thin flakes. Two characteristic peaks were observed in the region from 245 cm· 1 to 260 cm· 1 , which can be assigned to E 2 g 1 and A 1 g modes of WSe 2 , respectively_ 43 - 45 For few-layer WSe 2 , these two peaks are very close to each other and thickness-dependent shift of Raman peak position is relatively small comparing to M0S2. 43 • 46 Therefore, it is difficult to use the peak-to-peak distance to precisely determine the layer numbers. Nevertheless, the existence of B 2 g 1 peaks at 309 cm· 1 reveals that they are few-layer flakes, 44 • 45 which is consistent with AFM measurements shown above. Moreover, there is no peak found at -420 cm· 1 , which would correspond to A1g mode of WS2, indicating the absence of sulfur doping or the sulfur concentration is negligible in the as-grown WSe2 flakes. Moreover, we performed transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) studies. The as- 51 grown WSe2 flakes were first transferred onto TEM grid, usmg a method reported previously. 47 Figure 3.3.3 are the TEM images and a typical EDX spectrum, showing no obvious sulfur peak was found. We have acquired a few spectra from different flakes, and the results are very similar. Quantitative analysis shows that if we only count the three elements of W, Se, and S, the atomic ratio of S is below 0.5%, which is the limit of EDX technique. However, we cannot exclude the possibility thatthere are trace amount of sulfur doping in as-grown WSe2 flakes. Compared with recent papers on the growth of TMDC alloys, 43 - 51 the amount of sulfur as well as its temperature were much lower in our study, which lead to the growth of WSe2 flakes with negligible sulfur doping, if any. We also performed Raman mapping on the same flake shown in Figure 3.3.lb. Figures 3.3. le and 3.3. lf are Raman intensity mapping images for the A1g and B2g 1 modes ofWSe2, respectively. As it can be clearly discerned, the areas covered by ribbons are darker than other parts. The same mapping study was performed on another flake with ribbons on top, and the results are consistent with Figures 3.3. le and 3.3. lf. This suggests that the ribbon covered parts are thicker than other areas since both A1g and B2g 1 peak intensities will decrease with increasing the layer numbers of WSe 2 . 44 · 45 The intensity of Raman peaks are determined by the intensity of incident light and the amount of materials involved during the scattering process. For thick WSe2 materials, or more generally high reflective index TMDCs, the local electrical field will be much weaker than the incident electrical field, the so-called local field effect. Therefore, thick WSe2 materials will exhibit weak Raman signal. On the other hand, for very thin WSe2 layers, the local field effect is relatively small, and 52 the Raman intensity will be related to the amount of materials involved during Raman scattering process. 5 2 Therefore, the overall Raman intensitywill be jointed determined by the local filed effect and the amount of materials, and there might be a peak at cettain height. In our study, we observed that the intensity of Raman peaks decreased in few-layer samples when increasing the layer numbers. This is consistent with a recent study where they showed that bilayer WSe2 exhibit the strongest Raman intensity, and decrease in the order of 3L, 4L, and 5L. 44 (a ) 2Dnm (b) 0 5000 (f) (c) i )1' i i E2g 1 ! A1g ~ ~ H-.. ~ ~ ._....._ ...;. ! IS II\' .:11 200 250 300 Raman shift (cm- 1 ) 300 350 400 Raman Shift (cm- 1 ) 0 1000 Figure 3 .3 .1. AFM and Raman characterization of thin WSe2 flakes. (a, b) AFM images along with cross section height profiles of two thin WSe2 flakes with ribbon-like features on top. The bottom layers are 2.5 nm and 3 nm in height, corresponding to around tri-layer WSe2. The heights of ribbon layers are----0.9 nm or - 1.7 nm, corresponding to one or two layers of WSe2. ( c, d) Raman spectra of several thin WSe2 flakes. Oiaracteristic peaks of fa g 1 , A1g, and B2 g 1 modes of WSe2 were detected while the A1 g peak of WS2 was not 53 observed. (e, f) Raman intensity mapping of A1, (259 cm- 1 ) and B2i peaks (309 cm- 1 ) of the same WSe2 flake shown in Figure 2(b). The excitation wavelength of laser is 532 nm during Raman measurements. (o) (b) • . , ' J 'V"\,\f' . ) • -~~" l . El ,...,,, _,_tV~~ ~. ,, .$ ' .$ ' " i~ r· NM'i"vNv: ., :,: . , 1/~'v.,. l.\,.,,rv . , -2 ., .3 Figure 3.3.2. AFMimages of thin WSe2flakes along with their cross-section height profiles. (a) A triangular WSe2 flake with a height of -3.5 nm, corresponding to four-layers. (b) Another hexagonal WSe2 flake with a height of -3.5 nm, corresponding to four-layers as well. 54 (c) S e - w Cu w S e Cu c\ !s w <ru l S e .ll .l . . 0 2 4 6 8 10 12 14 16 18 Energy (keV) Figure 3.3.3. TEM-EDX characterization of as-grown WSe2 flakes. (a) TEM image of a thin triangular WSe2 flake transferred from silicon substrate to TEM grid. (b) High resolution TEM image along one edge of the flake. ( c) A typical EDX spectrum taken from as-grown WSe2 flakes. The sulfur peak (2.307 ke V) is negligibly small indicating the concentration of sulfur is quite low, if any, in the as-grown WSe2 samples. The peaks of Cu and C are from TEM grid. 3.4 Detailed AFM characterization of pyramid-like WSe2 AFM characterization reveals even more interesting features for stacked thick WSe2 flakes. Figures 3.4. la, 3.4. ld, 3.4. le, and 3.4. lf are AFM phase images of four typical thick flakes with different stacking morphologies. In a more systematical AFM analysis (Figures 55 3.4.2 and 3.4.3), we found that flakes with different stacking morphologies appear at different frequencies. Among the ten triangular flakes we examined, six of them have a 0° stacking angle, two have a 60° stacking angle, and one has a 15° stacking angle. We also checked ten hexagonal flakes, eight of them are hexagon-triangle stacks with a 30° stacking angle, and two flakes are hexagon-hexagon stacks. These results of stacking angles and shapes are consistent with what been observed under optical microscopy (Figure 3.2.1). Moreover, steps and helical fringes were clearly observed, which strongly support the existence of screw dislocations in these WSe2 samples. Additional evidences like herring bone contours were also observed. Taking all the optical microscopy, AFM, and Raman observations together (Figures 3.2.1, 3.3.1, and 3.4.1), we proposed that these WSe2 flakes followed a SDD spiral growth fashion. 40 • 41 In classical crystal growth theory, there are three basic growth types:4°· 41 SDD growth (BCF theory), 53 • 54 layer-by-layer (LBL) growth, 53 and dendritic growth. The growth preference depends on the degree of supersaturation as expressed as a ~ In( cl Co), where a is the degree of supersaturation, c is the precursor concentration, and Co is the equilibrium concentration. 40 • 41 • 53 At a low supersaturation ( a) condition, SDD growth is much more favorable than the other two because screw dislocations can provide active edges as nucleation sites, while LBL growth and dendritic growth require nuclei formations that occur only at certain high supersaturation conditions. To describe the screw dislocations more quantitatively, we measured the key parameters of the screw dislocations in as-grown WSe2 flakes, including step height (h), 56 terrace width (A), and slope (p ~ h!A). These values can reflect the growth conditions at certain degree. In most cases, low supersaturation ( a) would result a small p and large A. 40 Figure 3.4.lb is a three-dimensional (3D) image of the same flake shown in Figure 3.4.la. A pyramid-like structure with a height of 40 nm (from base to summit) is clearly observed. The height profile along the red dash line in Figure 3 .4.1 b is shown in Figure 3.4. lc. The step height (h) of this particular dislocation hillock is measured to be -0.86 nm, which is very close to the thickness of an individual WSe2 layer ( composed to Se-W-Se layer and has a height of-0.7 nm). This value is equal to one elemental Burgers vector. 41 Along with the single helical pattern, we conclude that only a single screw dislocation with one elemental Burgers vector is involved in this particular flake. Other spiral growth modes with different Burgers vectors or multiple screw dislocations also exist. For example, the sample in Figure 3.4. lf has two helical patterns, which can be a result of simultaneously growth from two screw dislocations. Another important parameter, terrace width (A), of the sample in Figures 3.4.la and 3.4.lb is -68 nm, which is smaller comparing to the other flakes in Figures 34. ld, 3.4. le, and 3.4. lf. From screw dislocation growth theory, the terrace width is mostly affected by the reactant concentrations. Specifically, the value of A is determined by the lateral step velocity (vs) and growth rate normal to the surface (Rm)- A sample grown with a relative higher Rm and smaller Vs would display a smaller A. And usually, a smaller A facilitates a better observation of herring-bone contours than a larger A does. This is the reason why Figure 3.4. la has sharp herring-bone contours while the others do not. With the measured terrace width (A) and step height (h ), we can calculate that the 57 flake shown in Figures 3.4. la and 3.4. lb has a slope of p ~ h/1o, ~ 0.0126. Typically, a small terrace width will lead to a high slope of the pyramid. Moreover, along this study, we have examined tens of such spiral grown flakes and found that both clockwise and counterclockwise grown samples with varied stacking angles exist. The different spiral features are likely to originate from the spiral growth curvature, and the spiral curvature depends much on the reactant concentrations as well. However, such concentration- curvature relation is rather complicated. In other words, the terrace width and curvature do not have direct relations. For example, in our experiments, we find flakes with the same stacking angle, but different terrace widths as shown in Figure 3.4.2 and Figure 3.4.3. As the concentrations of reactants in a CVD tube furnace varies from location to location due to the use of solid precursors, this leads to the observation of many kinds of stacking morphologies on a same substrate. 58 (d) (e) 30° (c) 12 ~ 10 1= 8 (fl 200 400 600 Distance (nm) Figure 3.4.1. AFM studies of thick WSe2 flakes with different stacking morphologies. (a) AFM phase image of a thick triangular flake. Helical fringes and herring-bone contours are clearly observed. (b) A3D AFM image of the flake in (a), showing a pyramid-like structure vividly with a height of --40 nm from base to the summit. ( c) AFM height profile of the pyramid-like flake along the red line in (b ). ( d, e, f) AFM phase images of three other thick flakes with different morphologies. Figure 3.4.2. Systematic AFM studies on thick triangular flakes. Most of these triangular flakes have a 0° stacking angle. 59 Figure 3.4.3. Systematic AFM studies on thick hexagonal flakes. The most common morphology is hexagon-triangle stacking with a 30° stacking angle. 3.5 Proposed models of screw-dislocation initialization and propagation To illustrate how a screw dislocation may generate, taking the situation with only one elemental Burgers vector as an example, we drew schematic diagrams to illustrate this process (Figures 3.5.la, 3.5.lb, and 3.5.lc). When two WSez domains intersect, it can cause an uplifting of one grain boundary (Figure 3.5.lb). This process will leave some unsaturated Se atoms hanging on this uplifted edge. At this moment, latter sources can be either added to the lateral edges forming a lateral growth or to the uplifted edge forming a second layer growth. Once the second layer is extended, a screw dislocation will be created, which facilitates further spiral growth of WSez following a SDD model (Figure 3.5 .1 c ). This stage corresponds to what we have observed on those thin flakes in Figures 3.3.1 a and 3. 3.1 b. Later on, among those three growth modes of LBL growth, continuous lateral growth, and SDD spiral growth, which growth type is preferred depends on the 60 concentration of reactants. As mentioned above, according to classical crystal growth theory, SDD spiral growth is the most favorable type at low supersaturation conditions. 40 · 41 For CVD growth of MoS 2 , MoO 3 and S react with each other easily, thus the concentrations of active reactants may be high enough to facilitate a large area lateral growth and few-layer LBL growth. Nevertheless, some similar uplifted second layer features also exist under certain conditions. 42 On the other hand, it is rather different for CVD growth of WSe2. Due to the low reactivity of Se, Huang et al. found that H2 has to be introduced to help reducing WO3 into WO3-x and to obtain monolayer dominated WSe2 flakes. 32 Here in our case, we discovered that sulfur can play a similar role as H 2 does, and it is not a must to have H2 involved. Since sulfur is not a strong reducer as H2 does, and the amount of sulfur is quite little in our case, the concentration of WO3-x active source is still not high enough for other types of growth except SDD growth in our case. So in this situation, lateral growth and LBL growth are likely to be prohibited while spiral growth on screw dislocations is preferred. We also performed CVD experiments without the addition of sulfur powders, and no such WSe2 growth was found. This is consistent with Huang et al. 's recent results. 32 Figure 3.5. ld illustrates how a screw dislocation propagates and eventually leads to the pyramid-like WSe2 flake. At first with a low supersaturation environment, the step propagation velocities at the dislocation core (vc) and outer edges (vo) are approximately the same. This is evidenced by the uniformity of terrace width (A) in AFM images. Thus, the steps can continuously spread without piling up. Moreover, the growth rate normal to 61 the surface (Rm) is much lower than lateral step velocities (vs), which leads to a 2D flake instead of a lD stmcture. 40 A product at the early stage with only a few te1Tace steps was shown in Figure 3.5.le, which clearly reveals the early structures of screw dislocations in the samples. During the CVD growth period, the concentration of Se will decrease gradually along with the Ar flow due to the amount of Se powder left will decrease. Based on SDD growth theory, the terrace width will increase as the reactant concentration decreases. This may be the main reason that we frequently observe on most flakes that the te1Tace width increases after certain periods. (c) (e) Figure 3.5.1. Proposed models for SDD growth of WSe2. (a) Two adjacent WSe2 domains before intersecting. (b) The boundary uplifting occurs when the two domains intersect. (c) A screw dislocation generated after the second layer extension on the uplifted edge. (d) Schematic diagrams showing the process of screw dislocation propagation. ( e) AFM image of a spiral grown flake at early stage. A zoom-in image with the height profile shown in the right part. 62 3.6 Device characteristics of FE Ts built on pyramid-like WSe2 flakes We further performed electrical transport measurements to study the electronic quality of the as-grown WSe2 flakes. Compared to a recent study of CVD growth of WSe2 on sapphire, 32 our samples were grown directly on Si/SiO2 substrates, which facilitates the fabrication of back-gated field-effect transistors (FETs). The devices were fabricated using standard e-beam lithography and the electrodes were 1 nm/75 nm of Ti/Pd. Figure 3.6. la shows a device with a 2 µm channel length on a 5-nm-thick flake and Figure 3.6. lb shows another device with a 1 µm channel length on a 20-nm-thick flake. The Icts-V g family curves are shown in each corresponding figures (Figures 3.6. lc and 3.6. ld). Interestingly, since Pd was used as the contacts, these devices show unipolar p-type behavior. This is different with recent results of ambipolar transport behavior of WSe2 flakes when using Au as contact. 14 • 15 The results suggest that the transport behavior ofWSe 2 can be tuned by careful selection of metal contacts, as also demonstrated in MoS 2 devices. 55 The on/off current ratios of the devices are around 10 4 to 10 6 , and effective hole mobility is about 40cm 2 /V·s. These values compared favorably with recent reported values for few-layer WSe 2 . 10 • 56 63 (a) 100nm (c) 30 vds -o.1v - 0.3V - o.sv 20 - 0.7V - - 0.9V ~ - 1.1v :::J.. .._., (/) 10 _-o 0 -100 -50 0 50 100 V 9 (V) (b) (d) 80 100nm vds 10 1 - -o.1v ~10-1 - 0.3V 60 - o.sv ";;;10-3 - 0.7V - -cJ - 0.9V ~ 40 10-5 - 1.1v - -100 -50 0 50 100 Ill Vg (V) _-o 20 0 -100 -50 0 50 100 V 9 (V) Figure 3.6.1. Device petformance ofCVD-grown WSe2 flakes. (a) AFM image ofa back gated WSe2 FET on a 5-nm-thick sample along with its lds-V g family curves at different Vds (c). (b) Another back-gated WSe2 FET on a 20-nm-thick sample with its lds-Vg family curves ( d). Insets in the right plots ( c) and ( d) are transfer cmves plotted in a log scale at Vds = 0.1 V. 3.7 Conclusion In conclusion. few-layer and pyramid-like WSe2 flakes were synthesized using asulfur assisted CVD method. The WSe2 growth was proposed to follow a SDD growth process due to the low supersaturation of reactants. Sulfur was found to play an important role here as it can paitially reduce the WO3 while keeping the WOJ-x concentrntion low. Key screw dislocation features including steps, helical fringes, and herring-bone contours were 64 observed to support the spiral growth. Schematic models were drawn to illustrate how the screw dislocations generate and propagate. In addition, transistors fabricated using these WSe2 flakes show that they possess decent device performance with on/off current ratio up to 10 6 and hole mobility up to 44 cm 2 /V·s. This work sheds new light on the understanding of growth mechanism of layered WSe2, which might be also adoptable in other 2D materials. Chapter 3. References 1. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. 2. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263- 275. 3. Geirn, A. K.; Grigorieva, I. V. Van der Waals Heterostructures. Nature 2013, 499, 419-425. 4. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS, Transistors. Nat. Nanotechnol. 2011, 6, 147-150. 5. Kuc, A.; Zibouche, N.; Heine, T. Influence of Quantum Confinement on the Electronic Structure of the Transition Metal Sulfide TS,. Phys. Rev. B 2011, 83, 245213. 6. Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS, by Optical Helicity. Nat. Nanotechnol. 2012, 7, 494-498. 7. Zeng, H.; Dai, J.; Yao, W.; Xiao, D.; Cui, X. Valley PolarizationinMoS, Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490-493. 8. Pu, J.; Yomogida, Y; Liu, K. K.; Li, L. J.; Iwasa, Y; Takenobu, T. Highly Flexible MoS2 Thin film Transistors with Ion Gel Dielectrics. Nano Lett. 2012, 12, 4013-4017. 9. Chang, H.-Y; Yang, S.; Lee, J.; Tao, L.; Hwang, W.-S.; Jena, D.; Lu, N.; Akinwande, D. High Performance, Highly Bendable MoS2 Transistors with High-K Dielectrics for Flexible Low Power Systems. ACS Nano 2013, 7, 5446-5452. 10. Tosun, M.; Chuang, S.; Fang, H.; Sachid, A. B.; Hettick, M.; Lin, Y; Zeng, Y; Javey, A. High Gain Inverters Based on WSe, Complementary Field-Effect Transistors. ACS Nano 2014, 8, 4948-4953. 11. Wang, H.; Yu, L.; Lee, Y H.; Shi, Y; Hsu, A.; Chin, M. L.; Li, L. J.; Dubey, M.; Kong, J.; 65 Palacios, T. Integrated Circuits Based on Bilayer MoS2 Transistors. Nano Lett. 2012, 12, 4674- 4680. 12. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS,. Nat. Nanotechnol. 2013, 8, 497-501. 13. Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; et al. Electrically Tunable Excitonic Light-emitting Diodes Based on Monolayer WSe2 p-n Junctions. Nat. Nanotechnol. 2014, 9, 268-272. 14. Pospischil, A.; Furchi, M. M.; Mueller, T. Solar-energy Conversion and Light Emission in an Atomic Monolayer p-n Diode. Nat. Nanotechnol. 2014, 9, 257-261. 15. Baugher, B. W. H.; Churchill, H. 0. H.; Yang, Y; Jarillo-Herrero, P. Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide. Nat. Nanotechnol. 2014, 9, 262-267. 16. Chang, K.; Chen, W. L-Cysteine-Assisted Synthesis of Layered MoS,/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 5, 4720-4728. 17. Hinnemann, B.; Moses, P. G.; Bonde, J.; Jorgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; N0rskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. 18. Lee, Y H.; Zhang, X. Q.; Zhang, W.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y C.; Wang, J. T.; Chang, C. S.; Li, L. J.; et al. Synthesis of Large-area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320-2325. 19. Ling, X.; Lee, Y H.; Lin, Y; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J. Role of the Seeding Promoter in MoS, Growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464-472. 20. Wang, X.; Gong, Y; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G.; Vajtai, R.; Lou, J.; Liu, Z.; Ringe, E.; et al. Chemical Vapor Deposition Growth of Crystalline Monolayer M0Se2. ACS Nano 2014, 8, 5125-5131. 21. Zhang, Y; Zhang, Y; Ji, Q.; Ju, J.; Yuan, H.; Shi, J.; Gao, T.; Ma, D.; Liu, M.; Chen, Y; et al. Controlled Growth of High-Quality Monolayer WS2 Layers on Sapphire and Imaging Its Grain Boundary. ACS Nano 2013, 7, 8963-8971. 22. Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. Two-dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571. 23. Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y Synthesis ofMoS, and Mo Se, Films with Vertically Aligned Layers. Nano Lett. 2013, 13, 1341-1347. 24. Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J.C.; Ajayan, P. M.; Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater. 2013, 12, 754-759. 25. van der Zande, A. M.; Huang, P. Y; Chenet, D. A.; Berkelbach, T. C.; You, Y; Lee, G. H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and Grain Boundaries in Highly Crystalline Monolayer Molybdenum Disulphide. Nat. Mater. 2013, 12, 554-561. 26. Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsoe, H.; Clausen, B. S.; Laegsgaard, E.; 66 Besenbacher, F. Size-dependent Structure of MoS2 Nanocrystals. Nat. Nanotechnol. 2007, 2, 53-58. 27. Zhu, W.; Low, T.; Lee, Y H.; Wang, H.; Farmer, D. B.; Kong, J.; Xia, F.; Avouris, P Electronic Transport and Device Prospects of Monolayer Molybdenum Disulphide Grown by Chemical Vapour Deposition. Nat. Commun. 2014, 5, 3087. 28. Duerloo, K. A.; Li, Y; Reed, E. J. Structural Phase Transitions in Two-dimensional Mo- and W-dichalcogenide Monolayers. Nat. Commun. 2014, 5, 4214. 29. Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P M.; Jernigan, G. G.; Jonker, B. T. Chemical Vapor Sensing with Monolayer MoS,. Nano Lett. 2013, 13, 668-673. 30. Liu, B.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C. High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors. ACS Nano 2014, 8, 5304-5314. 31. Balendhran, S.; Ou, J. Z.; Bhaskaran, M.; Sriram, S.; Ippolito, S.; Vasic, Z.; Kats, E.; Bhargava, S.; Zhuiykovd, S.; Kalantar-zadeh, K. Atomically Thin Layers ofMoS2 via a Two Step Thermal Evaporation-Exfoliation Method. Nanoscale 2012, 4, 461-466. 32. Huang, J.-K.; Pu, J.; Hsu, C.-L.; Chiu, M.-H.; Juang, Z.-Y; Chang, Y-H.; Chang, W.-H.; Iwasa, Y; Takenobu, T.; Li, L.-J. Large-Area Synthesis of Highly Crystalline WSe2 Monolayers and Device Applications. ACS Nano 2014, 8, 923-930. 33. Xu, K.; Wang, Z.; Du, X.; Safdar, M.; Jiang, C.; He, J. Atomic-layer Triangular WSe, Sheets: Synthesis and Layer-dependent Photoluminescence Property. Nanotechnology 2013, 24, 465705. 34. Lin, Y-C.; Lu, N.; Perea-Lopez, N.; Lin, J. L. Z.; Peng, X.; Lee, C.H.; Sun, C.; Calderin, L.; Browning, P N.; Bresnehan, M. S.; et al. Direct Synthesis of van der Waals Solids. ACS Nano 2014, 8, 3715-3723. 35. Meng, F.; Jin, S. The Solution Growth of Copper Nanowires and Nanotubes Is Driven by Screw Dislocations. Nano Lett. 2012, 12, 234-239. 36. Morin, S. A.; Bierman, M. J.; Tong, J.; Jin, S. Mechanism and Kinetics of Spontaneous Nanotube Growth Driven by Screw Dislocations. Science 2010, 328, 476-480. 37. Morin, S. A.; Jin, S. Screw Dislocation-driven Epitaxial Solution Growth of ZnO Nanowires Seeded by Dislocations in GaN Substrates. Nano Lett. 2010, 10, 3459-3463. 38. Zhu, J.; Peng, H.; Marshall, A. F.; Barnett, D. M.; Nix, W. D.; Cui, Y Formation of Chiral Branched Nanowires by the Eshelby Twist. Nat. Nanotechnol. 2008, 3, 477-481. 39. Bierman, M. J.; Lau, Y K.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Dislocation-driven Nanowire Growth and Eshelby Twist. Science 2008, 320, 1060-1063. 40. Morin, S. A.; Forticaux, A.; Bierman, M. J.; Jin, S. Screw Dislocation-driven Growth ofTwo dimensional Nanoplates. Nano Lett. 2011, 11, 4449-4455. 41. Zhuang, A.; Li, J. J.; Wang, Y C.; Wen, X.; Lin, Y; Xiang, B.; Wang, X.; Zeng, J. Screw dislocation-driven Bidirectional Spiral Growth ofBi2S"3 Nanoplates. Angew. Chem. 2014, 53, 6425-6429. 42. Ji, Q.; Zhang, Y; Gao, T.; Zhang, Y; Ma, D.; Liu, M.; Chen, Y; Qiao, X.; Tan, PH.; Kan, M.; et al. Epitaxial Monolayer MoS, on Mica with Novel Photoluminescence. Nano Lett. 2013, 13, 67 3870-3877. 43. Luo, X.; Zhao, Y; Zhang, J.; Toh, M.; Kloc, C.; Xiong, Q.; Quek, S. Y Effects of Lower Symmetry and Dimensionality on Raman Spectra in Two-dimensional WSe,. Phys. Rev. B 2013, 88, 195313. 44. Li, H.; Lu, G.; Wang, Y; Yin, Z.; Cong, C.; He, Q.; Wang, L.; Ding, F.; Yu, T.; Zhang, H. Mechanical Exfoliation and Characterization of Single- and Few-layer Nanosheets of WSe, , TaS2, and TaSe,. Small 2013, 9, 1974-81. 45. Tonndorf, P; Schmidt, R.; Bottger, P; Zhang, X.; Bomer, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R.; et al. Photoluminescence Emission and Raman Response of Monolayer MoS2, M0Se2, and WSe,. Opt. Express 2013, 21, 4908-4916. 46. Sahin, H.; Tongay, S.; Horzurn, S.; Fan, W.; Zhou, J.; Li, J.; Wu, J.; Peeters, F. Anomalous Raman Spectra and Thickness-dependent Electronic Properties ofWSe,. Phys. Rev. B 2013, 87, 165409. 47. Liu, B.; Ren, W.; Gao, L.; Li, S.; Pei, S.; Liu, C.; Jiang, C.; Cheng, H.M. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 2082-2083. 48. Li, H.; Duan, X.; Wu, X.; Zhuang, X.; Zhou, H.; Zhang, Q.; Zhu, X.; Hu, W.; Ren, P; Guo, P; et al. Growth of Alloy MoS2xSe2(1-x) Nanosheets with Fully Tunable Chemical Compositions and Optical Properties. J. Am. Chem. Soc. 2014, 136, 3756-3759. 49. Mann, J.; Ma, Q.; Odenthal, PM.; Isarraraz, M.; Le, D.; Preciado, E.; Barroso, D.; Yamaguchi, K.; von Son Palacio, G.; Nguyen, A.; et al. 2-dirnensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2c1-x) Sezx Monolayers. Adv. Mater. 2014, 26, 1399-1404. 50. Gong, Y; Liu, Z.; Lupini, A. R.; Shi, G.; Lin, J.; Najrnaei, S.; Lin, Z.; Elias, A. L.; Berkdernir, A.; You, G.; et al. Band Gap Engineering and Layer-by-Layer Mapping of Selenium-Doped Molybdenum Disulfide. Nano lett. 2014, 14, 442-449. 51. Chen, Y; Xi. J.; Durncenco, D. O.; Liu, Z.; Suenaga, K.; Wang, D.; Shuai, Z.; Huang, Y; Xie, L. Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys. ACS Nano 2013, 7, 4610-4616. 52. Splendiani, A.; Sun, L.; Zhang, Y; Li, T.; Kirn, J.; Chim, C. Y; Galli, G.; Wang, F. Emerging Photoluminescence in Mono layer MoS,. Nano lett. 2010, 10, 1271-1275. 53. Jin, S.; Bierman, M. J.; Morin, S. A. A New Twist on Nanowire Formation: Screw-Dislocation Driven Growth ofNanowires and Nanotubes. J. Phys. Chem. Lett. 2010, 1, 1472-1480. 54. Burton, W. K.; Cabrera, N.; Frank, F. C. The Growth of Crystals and the Equilibrium Structure of their Surfaces. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 1951, 243, 299-358. 55. Chuang, S.; Battaglia, C.; Azcatl, A.; McDonnell, S.; Kang, J. S.; Yin, X.; Tosun, M.; Kapadia, R.; Fang, H.; Wallace, R. M.; et al. MoS, P-type transistors and diodes enabled by high work function MoOx contacts. Nano Lett. 2014, 14, 1337-1342. 56. Allain, A.; Kis, A. Electron and Hole Mobilities in Single-Layer WSe,. ACS Nano 2014, 8, 7180-7185. 68 4. Step-Edge-Guided Nucleation and Growth of Aligned WSe2 on Sapphire via a Layer-Over-Layer Growth Mode 4.1 Introduction Layered two-dimensional (2D) materials beyond graphene such as transitional metal dichalcogenides (TMDCs), 1 - 3 black phosphorus (BP), 4 - 6 and silicene 7 recently have attracted significant attentions due to their unique properties. A common feature shared by all 2D materials is that they possess weak interlayer interaction and strong intra-layer covalent bonds, which makes them inherently flexible and stands as good candidates for flexible electronics, 3 - 10 optoelectronic, 11 - 15 and some other applications. 2 · 3 · 16 - 21 So far, over tens of materials have been distinguished in this large family of 2D materials. 22 Among them, layered TMDCs are quite attractive because they have very diverse properties spanning metals, semiconductors, superconductors, etc. 2 Up to now, researchers have developed several methods including mechanical exfoliation, 23 · 24 liquid exfoliation, 25 - 27 chemical vapor deposition (CVD), 28 - 36 and physical vapor deposition (PVD) 37 to prepare monolayer or few-layer TMDC materials. Among these methods, vapor phase based growth approaches such as CVD and PVD are particularly interesting since they can produce materials with high quality and have the potential to be scaled up. Controlled growth oflarge single crystals and continuous thin films ofM0S2, M0Se2, WS2, and WSe2 have been achieved recently. 28 · 31 · 38 - 41 Location- and orientation-controlled growth of 2D TMDCs 1s another important 69 direction to pursuit in order to extract more potential of these materials. Currently, in many synthesis approaches, TMDC materials nucleate randomly on substrates and their orientation cannot be well controlled. Mis-oriented growth can lead to the formation of grain boundaries and defects during the merging of adjacent domains, which may have negative effects on the electrical, optical, thermal, and mechanical properties of as-grown materials. For graphene growth, it has been shown that wafer scale single crystalline graphene can be grown if the initial graphene nuclei have the same orientation, demonstrating the importance of alignment of graphene domains on large single crystal growth. 42 Recently, there have been a few attempts on the location- and orientation- controlled growth of TMDCs especially M0S2. For example, Han et al. have achieved location-controlled growth of M0S2 via selective positioning of seeds. 43 In other studies, Dumcenco et al. and Ji et al. also demonstrated orientation-controlled growth ofM0S2 via lattice epitaxial process on certain substrates. 44 · 45 Meanwhile, epitaxial growth of WSe 2 with alignment was also reported by Eichfeld et al. and Huang et al. using sapphire as substrates. 28 · 30 It was concluded that the interaction between TMDCs and crystalline sapphire substrates was the origin for the aligned growth. We noted that in these experiments, the temperatures of the growth substrates were relatively low (700-800 °C). Here we report a new mechanism, step-edge-guided growth, for the aligned growth of WSe 2 on crystalline C-plane sapphire substrates at high temperatures above 950 °C. The idea was inspired by early studies on one-dimensional (ID) carbon nanotube 46 - 48 and nanowire growth 49 , where crystalline substrates with lattice-potential-guided or step-edge- 70 guided growth are two major principles responsible for the aligned growth of these 1D systems. In this study, we found the steps on C-plane sapphire surface play a crucial role in oriented nucleation, directed propagation, and few-layer formation of WSe2. We observed that such step-edge-guided aligned growth of WSe2 flakes becomes prominent only at high growth temperatures due to the remarkable surface reconstruction of sapphire steps, which can only be trigged at high temperatures. 50 · 51 Moreover, we found that the growth of few-layer WSe2 follows a novel layer-over-layer (LOL) growth mode, which is different from the classical layer-by-layer (LBL) growth mode as widely adopted in literature for thin film growth. 52 Our finding opens up a possible way for location controlled nucleation and orientation-controlled growth of 2D WSe2 and potentially other TMDCs. 4.2 Aligned growth on sapphire vs random growth on Si/Si02 We used a CVD method to grow WSe2 flakes, where selenium powders and WO3 powders were used as source materials. Other procedures are similar to traditional CVD method. We have tested different substrates and growth temperatures, and found that they have significant influence on the features of as-grown WSe2 flakes. Figure 4.2. la, 4.2.1 b, and 4.2. lc are optical microscopy images showing critical difference of the morphology of WSe2 flakes after CVD growth at different temperatures on C-plane sapphire (Figure 4.2. la and 4.2. lb) and Si/SiO2 (silicon substrate with a 300 nm thick thermally grown SiO2 layer, Figure 4.2. lc) substrates. The flakes in Figure 4.2. la were synthesized on C-plane sapphire 71 at 900 °C, which show quasi-hexagonal shapes with certain preferred orientations, indicating a possible epitaxial growth process. Similar results have also been reported in CVD growth of graphene. 53 When the growth temperature increased to 950 °C, the as- grown flakes turned out to be trapezoids ( which can also be described as truncated triangles) with a clear alignment of the base edges, as reveled by optical microscopy (Figure 4.2. lb) and scanning electron microscopy (SEM) (inset of Figure 4.2. lb). Meanwhile, these flakes grown on C-plane sapphire are few-layer WSe2 based on color contrast, as also revealed by atomic force microscopy (AFM) and Raman measurements shown later. In contrast, the flakes grown on Si/Si02 substrate, under identical growth conditions, are mostly thick ones with yellow color (Figure 4.2. lc). More importantly, a quick examination of the WSe2 flakes grown on Si/Si02 indicates a random distribution of orientations of flakes compared to the samples grown on C-plane sapphire. We defined 0, the smallest angle between the vertical direction in the images (which is close to [ 11 2 OJ direction for C-plane sapphire) and the diagonal (Figure 4.2.la), the base edge (Figure 4.2.lb), and the midperpendicular line (Figure 4.2. lc) respectively, to quantitatively describe the degree of alignment of each flake, as indicated in Figures 4.2. la-4.2. lc. The results were plotted in Figure 4.2. ld, 4.2. le, and 4.2. lf respectively, and several trends can be clearly observed. First, C-plane sapphire exhibits much better orientation control of as-grown WSe2 flakes than Si/Si02 substrates do. Second, high growth temperature improves the alignment ofWSe2 flakes. For example, the flakes grown at 950 °C on C-plane sapphire show the best alignment with 82% of the flakes having a 0 less than 10° comparing with the sample grown at 900 °C on sapphire. 72 We also noted that the WSe2 flakes grm,vn at 950 °C in this study show one preferred alignment direction, which is parallel to the step edges of sapphire substrates as will discuss later. In contrast, the MoS2 flakes grown at 700 °C via lattice epitaxial process in Kis et al. 's recent study show three pref erred alignment directions . 44 (a) 900 °C C -plane sapphire 10µm (d) 15 210 C ::::i 0 u 5 Q -60-50-40-30-20-10 0 10 20 30 40 50 60 0 (Degree) (e) 40 Sapphire 950 •c Q -60-50-40-30-20-10 0 10 20 30 40 50 60 0 (Degree) ( f )1 Q .-,--,-.,........,.........,.--,---,--,- = - ~ =S'=:i/S::'.:i0,=95~0 = .C':;j 8 26 C 64 u 2 Q -60-50-40-30-20-10 0 10 20 30 40 50 60 0 (Degree) Figure 4.2. 1. Optical microscopic observations of WSe2 flakes grown at different temperatures on C-plane sapphire and Si/SiO2 substrates. (a) and (b) Optical microscopy images of aligned WSe2 flakes grown on C-plane sapphire at 900 °C (a) and 950 °C (b). Inset in image (b) shows an SEM image of the sample grown at 950 °C. The scale bar is 73 10 µm. ( c) Optical microscopy image of the WSe2 flakes grown on Si/Si02 substrates at 950 °C. (d), (e ), and (f) Histograms of the orientation distributions based on images (a), (b), and ( c ), respectively. Growth on C-plane sapphire exhibits better orientation control than growth on Si/Si02 substrates (plots d and e versus plot f). Meanwhile, high growth temperature improves the alignment of WSe2 growth on sapphire (plot d versus plot e ). 4.3 Atomic-step-edge effect on sapphire surface The aligned growth ofWSe 2 flakes at 950 °C is likely to be originated from atomic step edges on the C-plane sapphire substrates. Commercial C-plane sapphire wafers would usually develop atomic step-terrace structures on the surface when heated to high temperatures. Such atomic steps have been reported to have significant effects on the aligned growth of carbon nanotubes, 46 - 48 GaN nanowires, 49 and graphene. 54 A photo image of the sapphire substrates used in this study is shown in Figure 4.3. la, with a primary cutting edge of (11 2 0) plane. Thus the direction parallel to the cutting edge is [l 1 00] and the direction perpendicular to the edge is [112 OJ. Careful AFM characterization (Figure 4.3. lb and 4.3. lc) clearly shows the formation of atomic steps with a periodically distributed pattern appearing on the sapphire surface after the substrate went through a 950 °C pretreatment in Ar/H 2 atmosphere (but without W0 3 and Se source adding during the pretreatment). Interestingly, such sharp steps are formed only at high temperatures above 950 °C due to the high temperature trigged surface reconstruction of sapphire substrates. so. 51 Without high temperature treatment, there are only irregular and isolated small steps (Figure 4.3.2), which is not sufficient to conduct step-edge-guided aligned growth of WSe2 flakes. Extracted from the height profile (Figure 4.3.lc), the average 74 terrace width is about 60 nm and the typical step height is around 0.2 nm (c/6, where c ~ 12.99 A is the lattice constant in the z-direction of C-plane sapphire). Those measured values are consistent with what were reported in previous literature. so. 51 Based on the terrace width and step height, the corresponding miscut angle was calculated to be 0.2°. This number is consistent with the datasheet from the vendor (University Wafers, USA). Moreover, these atomic steps are along the [11 2 OJ direction of the sapphire substrates. For convenience, we define it as the step direction in this study. In 1D carbon nanotube growth, it has been well documented that both gas flow and substrate have effects on the morphology of final products. And by careful control of these two factors, complicated nanotube structures like serpentine nanotubes have been grown. 55 • 56 It would be interesting to study whether such effects would happen in 2D material growth as well. To further study the alignment mechanism and particularly the effect of sapphire steps on the aligned growth of WSe2 flakes, controlled experiments were conducted by changing the gas flow direction with respect to sapphire step direction. The gas flow direction is perpendicular to the step direction in Figure 4.3. ld, while the gas flow direction is parallel to the step direction in Figure 4.3. le. From optical microscopic observations, the as-grown flakes on both samples are all aligned with the step direction in regardless of the change of gas flow directions. These results confirm that the alignment is indeed originated from the substrate steps, not from gas flow. A statistical analysis of the degree of alignment based on Figure 4.3. ld and 4.3. le is also performed and presented in Figure 4.3. lf. The angle between the step direction and the base edge of the trapezoid flakes were measured 75 to verify the alignment. Among 48 flakes we examined in Figure 4.3.ld, 44 of them are well aligned with an angle less than 10°. The result on the other sample in Figure 4.3. le is similar to the sample in Figure 4.3. ld, specifically, 40 flakes out of a total of 48 flakes are aligned with an angle less than 10°. Based on the above experiments, we conclude that sapphire steps are responsible for the aligned growth of WSe2 flakes and good alignment can be achieved through careful engineering of those atomic steps. In-depth AFM studies further reveal that the aligned growth of WSe2 flakes originates from the preferred nucleation of WSe2 nuclei along the atomic steps on sapphire surface. In our experiments, we observed the initial nuclei are formed along the step edges with the same orientation as shown in Figure 4.3. lg. We speculate that the step edges on sapphire substrates are more attractive to reactants than other portions of the substrate at initial WSe2 nucleation stage because of the existence of dangling bonds and defects at these step edges. 57 In chemical reactions, atoms or molecules of reactants would choose sites of the highest bonding energy and preferentially absorb at those positions. Usually, defect sites such as steps on a flat surface have the highest binding energy, and therefore, the atomic steps of materials are generally more reactive than atomically flat areas. We speculate that this might be the major reason for the preferential nucleation of WSe2 nuclei close to the atomic steps of sapphire, as observed in this study. In addition, nucleation of WSe2 at atomic steps of sapphire can increase the contact areas and consequently, the strength of van der Waals interactions, between WSe2 and substrate. This will help stabilize the small nuclei ofWSe2 especially at the initial nucleation stage. Consequently, step edges can serve 76 as active sites for WSe2 nucleation and subsequent growth. Similar effect has also been reported in carbon nanotube growth. 46 Meanwhile, as Wang et al. mentioned in previous MoS2 growth work, Mo atoms and S atoms have different chemical activities, which would be similar for the different binding energies here between sapphire step edges and W or Se atoms or species. 58 Thus, it is possible that one kind of element between W and Se may preferentially adsorb at the step edges of sapphire. Figure 4.3. lh shows relevant atomic models to describe such step-edge-guided nucleation and aligned growth of WSe2 flakes. The gas phase reactants during the CVD WSe2 growth would preferentially attach to the step edges, form initial aligned WSe2 nuclei, and further merge into flakes which would be also aligned along the step edges. 77 I Step direction 1 ~ (h) " (c) 0.4 ---- E E,02 E -~oo ::c -0.2 (f) 50 40 !! 30 C ::J c320 0 100 200 300 Distance (nm) 10 I.I O .eo.50.40.30.20-10 o 10 20 30 40 50 eo a (Degree) t. o • Al .. 0 o w O Se Figure 4.3.1. Characte1ization of C-plane sapphires and obse1vations of the step-edge guided nucleation and aligned growth phenomena. (a) A photo image of commercial C plane sapphire used in the experiments. (b) AFM height image of the sapphire surface after a high temperature treatment (9 50 °C), revealing the formation of periodical step patterns. ( c) Cross-section height profile of the steps along the blue line in (b ). ( d) and ( e) Optical microscopy images of the as-grown WSe2 flakes when the gas flow direction is perpendicular ( d) or parallel ( e) to the step direction dUiing the growth. 0 is defined as the angle between the step direction and the base edge of trapezoid flakes. (f) Statistical analysis of the alignment in ( d) and ( e) based on the 0 values, showing good alignment in both experiments. (g) AFM image showing initial WSe2 nuclei aligned at the sapphire step edges. (h) Atomic models show the aligned nucleation of WSe2 on C-plane sapphire. 78 (a) (b) 0nm Sectior . .. ! I I I I½ i j I I i i J '' / \ i 1 1 :/ 1 / "J\ I \ \: ''ir\ \/V I I J \)j ' ·1 1 1 I I' \ / G ! ' ¥ ~, V r, : ; ·1 \ = ' Figure 4.3.2. AFM characterization of pristine C-plane sapphire substrates. (a) An AFM height image shows isolated and irregular atomic steps on pristine sapphire surface before high temperature treatment. (b) Cross-section height profile of the atomic steps along the white line in (a). 4.4 Raman, AFM and TEM characterization of aligned WSe2 flakes The as-grown flakes on sapphire were characterized to be few-layer WSe2 by Raman andAFM. Figure 4.4. la shows a typical WSe2 flake with a trapezoid shape, and the size of this flake is around 10 µm. Two characteristic Raman peaks of WSe2 at 247 cm· 1 and 256 cm·1, corresponding to E}g andA 1 g modes, were obsenred in the Raman spectrum (Figure 4.4. lb). Meanwhile, another sharp peak at around 306 cm· 1 assigned to be B~g mode also exists, suggesting the formation of few-layer WSe 2 as this B}g peak is quite sensitive to the layer numbers ofWSe2. 23 Later, we performed Raman intensity mapping (Figure 4.4. lc) on the flake to study the structural uniformity. Interestingly, the mapping results reveal that the Raman intensity at 247 cm· 1 has some variation across the flake. Specifically, in the mapping image, the intensity in the front area and the area along the base edge are different. Such variation in Raman signal comes from the variation of thickness, which is also revealed by AFM studies later. 79 Detailed AFM measurements are shown in Figure 4.4. ld, 4.4. le, 4.4. lg, and 4.4. lh, revealing the fine structures of the same trapezoid flake as well as its presence on stepped sapphire substrate. Figure 4.4. ld is an AFM height image of the entire flake, which clearly illustrates a non-uniform feature with a triangular core and varied layers along the base edge domain. Three representative areas ( e, g, and h) were then zoomed in to reveal the detailed information as shown in Figure 4.4. le, 4.4. lg, and 4.4. lh respectively. After zoom in, the periodical sapphire steps were observed simultaneously with the WSe2 flake (Figure 4.4. le and 4.4. lg). And the direction of those steps is the same as the direction of the WSe2 flake base edge, which is consistent with the optical microscopic results. Thus, both AFM and optical observations confirm that the step direction determines the growth direction. Moreover, Figure 4.4. lg and 4.4. lh show that the thickness of the flake varies from the front domain to the base edge domain. The front domain is rather uniform and it has a thickness of 1. 7 nm, which can be characterized as a bilayer WSe2 region. Interestingly, the sapphire step topography is well duplicated onto this bilayer region. The extra height at the flake edge is presumably due to the edge rolling effect as also reported in graphene study. 59 Meanwhile, Figure 4.4.lg shows more details about the thickness variation along the base edge. As we can see, the origin of the thickness variation comes from the layer over-layer overlapping. Similar features can be found in Figure 4.4. lh as well: one layer (Layer A) grows over another layer (Layer B) forming a stack. However, due to the layer mismatch, such overlapping growth generates high probability to initiate screw dislocations, which leads to a screw dislocation hillock. 60 So those 'cores' along the based 80 edges are initialized from layer-over-layer growth and formed by fmther screw-dislocation- driven growth. Similar screw dislocation hillocks exist in almost all the flakes. We also transferred aligned flakes onto transmission electron microscopy (fEM) grids by standard PMMA transfer method for high resolution TEM (HRTEM) and diffraction studies (Figme 4.4.2). 61 HRTEM images show high crystalline WSe2 samples with clear hexagonal symmetry. And diffraction results suggest that the as-grown WSe2 flake is a single crystal with a regular AB stacking structme. (a) (b) (c) E}g x2 :::, ~ ~ .iii C Q) 'c 250 300 350 400 Raman Shift (cm- 1 ) (e) (f) 3 I 2 1: 1 0) _ [1~7 nm ~o -1 200 400 Distance (nm) (g) (h) (i) 3 t1 -1 0 400 800 Distance (nm) Figme 4.4.1. Raman andAFM characterization on a typical aligned WSe2 flake grown on C-plane sapphire substrates. (a) Optical image of an aligned WSe2 flake with a trapezoid shape. (b) Raman spectmm of the flake in (a) shows it is a few-layer sample. The intensity 81 is multiplied by a factor of two from region 300 to 400 cm- 1 • (c) Raman intensity mapping of the flake in (a) at 247 cm- 1 shows non-uniform intensity over the Vvhole flake, indicating a change of sample thickness at different locations. ( d) AFM height image of the Vvhole flake. The lateral size of this flake is about 10 µm. The areas e, g, and h are fmther zoomed in as shown in (e), (g), and (h)respectively. (e) Zoom-in image of the areae. The periodical sapphire step pattern is observed with a consistent direction. (f) Cross-section height profile showing the front domain of the flake is a bilayer. (g) Zoom-in image of the areag showing the detailed thickness variation along the base edge. The Vvhite dot line indicates the base edge of Layer B, which is covered by Layer A (h) Zoom-in image of the area h reveals that the core is a screw dislocation hillock as well as the layer-over-layer thickness variation. (i) Cross-section height profile along the blue line in (h). (a) le) (ff.' ·:.,-,.·.-.-.-.~ ·-·1~,·,-r ...... . ~ v .. ,, .. , . •• , ••••••.• , .r• -· • ... -~. - ~ · . ... ,~.., .. ' • • • • • I ,---. 1J. ·'I' ,Ju • • • • • • • ••• , 1':f . . • •• • •• • •• *A~~ .' ....... ~ . • cr.zBi1'nn/tr .. ·>:-"•:«·M~ o}i>!~ ~- . ..... --.: ...... - . . . ' ...••..... . ' . .-::.·::::, Ii • • . • . • •Jti ..••• ~ .. . ·. :.· : .. • :;: .,., •. 1 nM ' • , • • •. . . , . . . .. Figure 4.4.2. IBM characterization of WSe2 flakes. (a) IBM image of a WSe2 flake after being trnnsfen·ed onto IBM grid using PMMA-mediated transfer method. The flake becomes cmmpled after transfen-ing. (b)-(e) Diffraction patterns taken at locations 1-4 as indicated in image (a). These results show that the flake is a single crystal. (f) HRTEM image shows high crystalline WSe2 flake. The spacing is measured to be 0.285 nm for WSe2 (100) lattice planes. 4.5 Layer-over-layer formation of few-layer structures Besides the aligned nucleation and growth phenomena we described above, the atomic steps on sapphire also have a significant effect on the few-layer WSe2 formation based on 82 our observations (Figure 4.4. lg and 4.4. lh) and previous studies on step-edge-guided growth of other materials. 46 • 49 The existence of periodic sapphire steps facilitates an aligned layer-over-layer growth during the propagation of each individual WSe2 layer. Previously, the growth of few-layer TMDCs is reported to follow a traditional layer-by- layer growth mode, that is, nucleation is first formed at the pre-existing bottom layer and then grown to become an additional layer (Figure 4.5. la). In our case, the few-layer features are grown from the overlapping of individual layers. A typical model can be described as the uphill layer will propagate to overlap with the downhill layer while the growth across the uphill sapphire step is suppressed leaving a straight boundary as also shown in Figure 4.5. la. This phenomenon becomes more essential at higher temperature (990 °C) as the domains near the base edge become wing-like structures (Figure 4.5.lb). With further AFM measurements, two kinds of overlapping were identified as shown in Figure 4.5. lc and 4.5. ld. In Figure 4.5. lc, the overlapped few-layer regions have a waved structure. Since the height of a single sapphire step (c/6, -0.2 nm) is less than the thickness of a monolayer WSe2 (0.7 nm), the uphill layer has to bend when it propagates to overlap with the downhill layer generated from the adjacent step. In the other situation as shown in Figure 4.5. ld, the uphill layer will naturally fall onto the downhill layer if they are grown from non-adjacent steps showing a flat overlapped region. In addition, most of the aligned WSe2 flakes in this study possess a truncated shape, which would come from the modulations of atomic steps as well. The periodic atomic steps have effects on both the front and back sides of WSe2 flakes during growth. The growth front of flakes is a line 83 instead of a point, which is originated from the straight atomic steps on sapphire substrates. For the back side, WSe2 flakes cannot climb over an uphill step, which gives a line shape base edge. For the front side, WSe2 can grow downhill, but the speed of growth front will be modulated by the atomic steps. Therefore, we observed truncated WSe 2 flakes with the front and base edges parallel to the direction of atomic steps on substrates. Similar phenomenon, i.e., the growth of truncated graphene (or can be described as lens-shaped) on Ru substrate, was reported previously. 62 The authors stated that the formation of such lens-shaped graphene is due to the atomic steps on Ru (0001) surface. (a) A B A B (b) - - ! ' C B - A A B ~ ' C A B A B Layer-by-layer (LBL) mode Layer-over-layer (LOL) mode C-plane sapphire 10µ1.'n - (c) (d) Figure 4.5.1. Studies of the layer-over-layer features along with proposed growth models. (a) Schematic diagrams showing layer-by-layer growth mode and layer-over-layer growth mode. (b) Optical image of as-grown WSe 2 flakes on C-plane sapphire at 990 °C. The domains near the base edges of these flakes evolve into a wing-like structure. (c), (d) Two situations of forming WSe2 few-layers by layer-over-layer overlapping of individual WSe2 84 layers. ( c) Overlapping occurs between the layers grown from adjacent steps. ( d) Overlapping occurs between the layers grown from non-adjacent steps. The insets correspond to the height profiles along the blue lines in ( c) and ( d). The propagation across the uphill step is suppressed at high growth temperature for both situations leaving straight and aligned base edges. 4.6 Conclusion In summary, we have developed a new method for aligned growth of few-layer WSe2 flakes by CVD method using C-plane sapphire substrates. We observed that at high growth temperatures, the aligned features ofWSe2 flakes originate from the periodical atomic steps on the sapphire surface as supported by detailed AFM examinations. Such atomic steps on sapphire have two significant effects on the growth behavior of WSe2. First, the atomic steps provide active nucleation sites to generate aligned WSe2 nuclei. Second, the periodical steps lead to a layer-over-layer overlapping fashion to form aligned few-layer WSe2 flakes, which is distinct to the classical layer-by-layer mode in thin film formation. This study not only introduces a new method for the alignment controlled growth of 2D WSe2, but also sheds lights on the nucleation and growth mechanism of WSe2. Chapter 4. References 1. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS, Transistors. Nat. Nanotechnol. 2011, 6, 147-150. 2. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. 3. Wu, W.; Wang, L.; Li, Y; Zhang, F.; Lin, L.; Niu, S.; Chenet, D.; Zhang, X.; Hao, Y; Heinz,T. F.; et al. Piezoelectricity of Single-Atomic-Layer MoS, for Energy Conversion and 85 Piezotronics. Nature 2014, 514, 470-474. 4. Li, L. K.; Yu, Y J.; Ye, G. J.; Ge, Q. Q.; Ou, X. D.; Wu, H.; Feng, D. L.; Chen, X. H.; Zhang, YB. Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9, 372-377. 5. Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X. F.; Tomanek, D.; Ye, P. D. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033-4041. 6. Xia, F. N.; Wang, H.; Jia, Y C. Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 2014, 5, 4458. 7. Tao, L.; Cinquanta, E.; Chiappe, D.; Grazianetti, C.; Fanciulli, M.; Dubey, M.; Molle, A.; Akinwande, D. Silicene Field-Effect Transistors Operating at Room Temperature. Nat. Nanotechnol. 2015, JO, 227-231. 8. Pu, J.; Yomogida, Y; Liu, K. K.; Li, L. J.; Iwasa, Y; Takenobu, T. Highly Flexible MoS2 Thin Fihn Transistors with Ion Gel Dielectrics. Nano Lett. 2012, 12, 4013-4017. 9. Chang, H. Y; Yang, S. X.; Lee, J. H.; Tao, L.; Hwang, W. S.; Jena, D.; Lu, N. S.; Akinwande, D. High-Performance, Highly Bendable MoS2 Transistors with High-K Dielectrics for Flexible Low-Power Systems. ACS Nano 2013, 7, 5446-5452. 10. Akinwande, D.; Petrone, N.; Hone, J. Two-Dimensional Flexible Nanoelectronics. Nat. Commun. 2014, 5, 5678. II. Baugher, B. W. H.; Churchill, H. 0. H.; Yang, Y F.; Jarillo-Herrero, P. Optoelectronic Devices Based on Electrically Tunable P-N Diodes in a Monolayer Dichalcogenide. Nat. Nanotechnol. 2014, 9, 262-267. 12. Pospischil, A.; Furchi, M. M.; Mueller, T. Solar-Energy Conversion and Light Emission in an Atomic Monolayer P-N Diode. Nat. Nanotechnol. 2014, 9, 257-261. 13. Ross, J. S.; Klement, P.; Jones, A. M.; Ghirnire, N. J.; Yan, J.; Mandrus, D. G.; Taniguchi,T.; Watanabe, K.; Kitamura, K.; Yao, W.; et al. Electrically Tunable Excitonic Light-Emitting Diodes Based on Monolayer WSe, P-N Junctions. Nat. Nanotechnol. 2014, 9, 268-272. 14. Desai, S. B.; Seo!, G.; Kang, J. S.; Fang, H.; Battaglia, C.; Kapadia, R.; Ager, J. W.; Guo, J.; Javey, A. Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe,. Nano Lett. 2014, 14, 4592-4597. 15. Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y; Sun, Y; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74-80. 16. Mak, K. F.; He, K. L.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nat. Nanotechnol. 2012, 7, 494-498. 17. Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P. M.; Jernigan, G. G.; Jonker, B. T. Chemical Vapor Sensing with Monolayer MoS,. Nano Lett. 2013, 13, 668-673. 18. Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; et al. Enhanced Catalytic Activity in Strained Chemically Exfoliated WS, Nanosheets for Hydrogen Evolution. Nat. Mater. 2013, 12, 850-855. 19. Liu, B.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C., High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS, Transistors. ACS Nano 2014, 8, 5304-5314. 20. Wang, H.; Yu, L.; Lee, Y; Shi, Y; Hsu, A.; Chin, M. L.; Li, L.; Dubey, M.; Kong, J.; Palacios, 86 T. Integrated Circuits Based on Bilayer MoS2 Transistors. Nano Lett. 2012, 12, 4674-4680. 21. Tosun, M.; Chuang, S.; Fang, H.; Sachid, A. B.; Hettick, M.; Lin, Y J.; Zeng, Y P.; Javey, A. High-Gain Inverters Based on WSe, Complementary Field-Effect Transistors. ACS Nano 2014, 8, 4948-4953. 22. Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; et al. Progress, Challenges, and Opportunities in Two Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898-2926. 23. Li, H.; Lu, G.; Wang, Y; Yin, Z.; Cong, C.; He, Q.; Wang, L.; Ding, F.; Yu, T.; Zhang, H. Mechanical Exfoliation and Characterization of Single- and Few-Layer Nanosheets ofWSe,, TaS2, and TaSe,. Small 2013, 9, 1974-1981. 24. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geirn, A. K. Two-Dimensional Atomic Crystals. Proceedings of the National Academy of Sciences of the United States of America 2005, 102, 10451-10453. 25. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H., The Chemistry ofTwo Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263- 275. 26. Green, A. A.; Hersam, M. C., Solution Phase Production of Graphene with Controlled Thickness Via Density Differentiation. Nano Lett. 2009, 9, 4031-4036. 27. Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571. 28. Huang, J. K.; Pu, J.; Hsu, C.; Chiu, M.; Juang, Z.; Chang, Y; Chang, W.; Iwasa, Y; Takenobu, T.; Li, L. Large-Area Synthesis of Highly Crystalline WSe2 Mono Layers and Device Applications. ACS Nano 2014, 8, 923-930. 29. Zhou, H. L.; Wang, C.; Shaw, J.C.; Cheng, R.; Chen, Y; Huang, X.; Liu, Y; Weiss, N. O.; Lin, Z.; Huang, Y; et al. Large Area Growth and Electrical Properties of P-Type WSe, Atomic Layers. Nano Lett. 2015, 15, 709-713. 30. Eichfeld, S. M.; Hossain, L.; Lin, Y; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; et al. Highly Scalable, Atomically Thin WSe, Grown Via Metal Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080-2087. 31. Wang, X.; Gong, Y; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G.; Vajtai, R.; Lou, J.; Liu, Z.; Ringe, E.; et al. Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe,. ACS Nano 2014, 8, 5125-5131. 32. Zhang, Y; Ji, Q.; Han, G.; Ju, J.; Shi, J.; Ma, D.; Sun, J.; Zhang, Y; Li, M.; Lang, X.; et al. Dendritic, Transferable, Strictly Monolayer MoS, Flakes Synthesized on SrTiO, Single Crystals for Efficient Electrocatalytic Applications. ACS Nano 2014, 8, 8617-8624. 33. Ling, X.; Lee, Y H.; Lin, Y; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J., Role of the Seeding Promoter in MoS, Growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464-472. 34. Chen, Y Z.; Medina, H.; Su, T. Y; Li, J. G.; Cheng, K. Y; Chiu, P. W.; Chueh, Y L. Ultrafast and Low Temperature Synthesis of Highly Crystalline and Patternable Few-Layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process. ACS Nano 2015, 9, 4346-4353. 87 35. Liu, B.; Fathi, M.; Chen, L.; Abbas, A.; Ma, Y; Zhou, C. W. Chemical Vapor Deposition Growth of Monolayer WSe2 with Tunable Device Characteristics and Growth Mechanism Study. ACS Nano 2015, 9, 6119-6127. 36. Ma, Y; Liu, B.; Zhang, A.; Chen, L.; Fathi, M.; Shen, C.; Abbas, A.; Ge, M.; Mecklenburg, M.; Zhou, C. W. Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe, and Applications for Devices. ACS Nano 2015, DOI: 10.1021/ acsnano.5b02399. 37. Clark, G.; Wu, S. F.; Rivera, P; Finney, J.; Nguyen, P; Cobden, D. H.; Xu, X. D. Vapor Transport Growth of High Optical Quality WSe, Monolayers. APL Mater. 2014, 2. 38. van der Zande, A. M.; Huang, PY; Chenet, D. A.; Berkelbach, T. C.; You, Y; Lee, G.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hong, J. C. Grains and Grain Boundaries in Highly Crystalline Monolayer Molybdenum Disulphide. Nat. Mater. 2013, 12, 554-561. 39. Lu, X.; Utarna, M. I. B.; Lin, J.; Gong, X.; Zhang, J.; Zhao, Y; Pantelides, S. T.; Wang, J.; Dong, Z.; Liu, Z.; et al. Large-Area Synthesis of Monolayer and Few-Layer M0Se2 Fihns on Si 0, Substrates. Nano Lett. 2014, 14, 2419-2425. 40. Cong, C. X.; Shang, J. Z.; Wu, X.; Cao, B. C.; Peirnyoo, N.; Qiu, C.; Sun, L. T.; Yu, T. Synthesis and Optical Properties of Large-Area Single-Crystalline 2D Semiconductor WS2 Monolayer from Chemical Vapor Deposition. Adv. Opt. Mater. 2014, 2, 131-136. 41. Kang, K.; Xie, S.; Huang, L.; Han, Y; Huang, PY; Mak, K. F.; Kirn, C. J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Fihns with Wafer-Scale Homogeneity. Nature 2015, 520, 656-660. 42. Lee, J. H.; Lee, E. K.; Joo, W. J.; Jang, Y; Kirn, B. S.; Lim, J. Y; Choi, S. H.; Ahn, S. J.; Ahn, J. R.; Park, M. H.; et al. Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium. Science 2014, 344, 286-289. 43. Han, G. H.; Kybert, N. J.; Naylor, C.H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y; Lee, Y H.; Agarwal, R.; et al. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. 44. Durncenco, D.; Ovchinnikov, D.; Marinov, K.; Lopez-Sanchez, O.; Krasnozhon, D.; Chen, M.; Gillet, P; Morral, A. F. I; Radenovic, A.; Kis, A.; et al. Large-Area Epitaxial Monolayer MoS,. ACS Nano 2015, 9, 4611-4620. 45. Ji, Q. Q.; Zhang, Y; Gao, T.; Zhang, Y; Ma, D.; Liu, M.; Chen, Y; Qiao, X.; Tan, P; Kan, M.; et al. Epitaxial Monolayer MoS, on Mica with Novel Photoluminescence. Nano Lett. 2013, 13, 3870-3877. 46. Isrnach, A.; Segev, L.; Wachtel, E.; Joselevich, E. Atornic-Step-Templated Formation of Single Wall Carbon Nanotube Patterns. Angew. Chem. Int. Ed. 2004, 43, 6140-6143. 47. Han, S.; Liu, X. L.; Zhou, C. W. Template-Free Directional Growth of Single-Walled Carbon Nanotubes on a- and R-Plane Sapphire. J. Am. Chem. Soc. 2005, 127, 5294-5295. 48. Kocabas, C.; Hur, S. H.; Gaur, A.; Meitl, M.A.; Shim, M.; Rogers, J. A. Guided Growth of Large-Scale, Horizontally Aligned Arrays of Single-Walled Carbon Nanotubes and Their Use in Thin-Film Transistors. Small 2005, 1, 1110-1116. 49. Tsivion, D.; Schvartzrnan, M.; Popovitz-Biro, R.; von Huth, P; Joselevich, E. Guided Growth 88 of Millimeter-Long Horizontal Nanowires with Controlled Orientations. Science 2011, 333, 1003-1007. 50. Kumosikov, O.; Van, L. P.; Cousty, J. About Anisotropy of Atomic-Scale Height Step on (0001) Sapphire Surface. Swf Sci. 2000, 459, 256-264. 51. Heffelfrnger, J. R.; Bench, M. W.; Carter, C. B. Steps and the Structure of the (0001) Alpha Alumina Surface. Surf Sci. 1997, 370, L168-L172. 52. Yan, K.; Peng, H. L.; Zhou, Y; Li, H.; Liu, Z. F. Formation of Bilayer Bernal Graphene: Layer by-Layer Epitaxy via Chemical Vapor Deposition. Nano Lett. 2011, 11, 1106-1110. 53. Zhang, Y; Zhang, L. Y; Kirn, P.; Ge, M. Y; Li, Z.; Zhou, C. W. Vapor Trapping Growth of Single-Crystalline Graphene Flowers: Synthesis, Morphology, and Electronic Properties. Nano Lett. 2012, 12, 2810-2816. 54. Saadi, S.; Abild-Pedersen, F.; Helveg, S.; Sehested, J.; Hinnernann, B.; Appel, C. C.; Norskov, J. K. On the Role of Metal Step-Edges in Graphene Growth. J. Phys. Chem. C 2010, 114, 11221-11227. 55. Geblinger, N.; Isrnach, A.; Joselevich, E. Self-Organized Nanotube Serpentines. Nat. Nanotechnol. 2008, 3, 195-200. 56. Liu, B. L.; Wang, C.; Liu, J.; Che, Y C.; Zhou, C. W. Aligned Carbon Nanotubes: From Controlled Synthesis to Electronic Applications. Nanoscale 2013, 5, 9483-9502. 57. Sornorjai, G. A. Modem Surface Science and Surface Technologies: An Introduction. Chem. Rev. 1996, 96, 1223-1235. 58. Wang, S.; Rong, Y; Fan, Y; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution ofMonolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371-6379. 59. Gao, L.B.; Ren, W. C.; Liu, B. L.; Saito, R.; Wu, Z. S.; Li, S. S.; Jiang, C. B.; Li, F.; Cheng, H. M. Surface and Interference Coenhanced Raman Scattering of Graphene. ACS Nano 2009, 3, 933-939. 60. Chen, L.; Liu, B. L.; Abbas, A. N.; Ma, Y Q.; Fang, X.; Liu, Y H.; Zhou, C. W. Screw Dislocation-Driven Growth of Two-Dimensional Few-Layer and Pyramid-Like WSe2 by Sulfur-Assisted Chemical Vapor Deposition. ACS Nano 2014, 8, 11543-11551. 61. Liu, B.; Ren, W.; Gao, L.; Li, S.; Pei, S.; Liu, C.; Jiang, C.; Cheng, H. M. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 2082-2083. 62. Sutter, P. W.; Flege, J. I.; Sutter, E. A. Epitaxial Graphene on Ruthenium. Nature Mater. 2008, 7, 406-411. 89 5. High-Performance Sub-Micron WSe2 Field-Effect Transistors Prepared Using A Flood-Dike Printing Method with High On-State Current Density and High On/Off Current Ratio 5.1 Introduction Transition metal dichalcogenides (TMDCs) which are two-dimensional (2D) layered materials generally have a formula of MX2, where M stands for a transition metal and X stands for chalcogen elements. 1 - 3 Depending on the choice of elemental combinations, the TMDCs can span from superconductors to semiconductors and even insulators 1 · 4 · 5 Meanwhile, other factors including thickness, defect, and strain can also have a significant effect on the physical properties such as bandgap and field-effect mobility. 6 - 10 Among the large family of TMDCs, M0S2 and WSe2 are the two most widely studied species with interesting semiconductor features_ll- 13 In particular, monolayer WSe 2 which possesses a direct bandgap and ambipolar transport behavior, 14 · 15 has been demonstrated as a good candidate for optoelectronics, 16 - 19 spintronics, 20 and valleytronics.13· 21 At the same time, this TMDC material is also suitable for other electronic applications such as chemical sensing and flexible electronics because of the 2D layered nature. 22 - 25 In another aspect, the current semiconductor manufacturing highly relies on complicated fabrication processes such as lithography, vacuum deposition, etc., which would dramatically increase the cost in terms of both time and money. Fortunately, the electronic printing technique designed for solution-based low-temperature processmg 90 could be a possible solution to avoid the high-cost processes. 26 Up to now, a number of printing work have been reported including field-effect transistors (FE Ts) and solar cells using carbon nanotubes (CNT), 27 - 31 organics films, 32 · 33 or inorganic nanoparticles, 34 which demonstrate the great potential of the printing technique for low-cost and large-scale electronic applications. 35 - 37 At the same time, a significant progress has been made on printed electronics of 2D TMDCs as well. 38 - 41 For example, Kim et al. reported printed FETs on CVD MoS 2 with inkjet-printed contacts. 39 Later on, solution-processed TMDC inks have been also applied for all-printed FETs by Kelly et al. 41 and all-printed photodetectors and memory devices by McManus et al., 42 respectively. Nevertheless, most of the printed 2D TMDC devices usually possess a compromised performance such as low current density, low Ionlloffratio, and low mobility. One critical reason is that the traditional printing technique would produce rather long channels (>25 µm) while the typical flake size of TMDCs is about several to tens of micrometers, which would cause massive grain boundaries or flake-to-flake junctions inside the channel. It is therefore of great importance to produce printed devices with a sub-micron channel length, which could efficiently contact with a single TMDC crystal. Here we reported a reliable workflow from a monolayer WSe2 CVD synthesis with tunable morphologies to a three-step printing technique using a flood-dike method, producing low-cost, high-performance printed TMDC FETs with a sub-micron channel length. Firstly, monolayer WSe2 samples were grown onto a Si/Si02 substrate using a CVD method. This CVD process is highly reproducible, producing mono layer WSe2 flakes with 91 either triangular shapes or hexagram shapes depending on the growth conditions. Then the special printing technique was performed on the as-grown WSe2 materials to achieve ultra short channel devices. 32 · 43 · 44 Other sub-micron printing work using a print-slide mode has been reported with organic films or CNT networks, 32 · 45 however the same process does not fit well with 2D TMDC materials due to the special wetting properties. The metal nanoparticle inks can spread on TMDC surface rather easily, which would cause shorted channels when following the print-slide method. In an alternative approach, we developed a flood-dike printing technique which is especially suitable for such 2D materials. The entire printing process involves three main steps: (1) printing of the first electrodes on CVD WSe2, (2) surface functionalization with a self-assembled monolayer (SAM) covering the first printed electrodes, and (3) printing of the second electrodes close to the first ones with a certain distance. During the second step, the SAM attaches to the surface of the first electrodes and forms a protecting layer which acts as a flood-dike. This screening layer is quite essential for the later process. Then in the last step, the fresh inks of the newly printed electrodes will spread to the first electrodes but be stopped by the SAM flood-dike, leaving an ultrashort channel with sub-micron lengths. This flood-dike printing method is highly reliable with -85% yields. Compared to printed devices with a large channel length, these ultra-short channel devices possess a high on-state current density (>0.2 µA/µm) and a high Ionlloff ratio (> 10 5 ), which would be beneficial for numerous applications. 92 5.2 Advantages of sub-micron printing on 2D TlVIDCs Traditional CVD methods for Tlv:IDC growth usually produce samples with either isolated triangular flakes or continuous films with merged Tlv:IDC flakes. 3, 46 On the other hand, the resolutions of current printing techniques are limited to tens of micrometers which is problematic to efficiently bridge a single Tlv:IDC flake as shown in Figure 5. 2.1 a. 45 Meanwhile, the device mobility and on-state current density will be significantly reduced by the dense grain boundaries or flake-to-flake junctions inside the long channel, which is inevitable due to the limitation of current printing resolution (Figure 5.2. lb) For such reasons, a short channel printing technique for 2D Tlv:IDCs with a channel length below one micron would be important to improve both the printing efficiency and device performance such as on-stage current density and mobility (Figure 5.2. lc) (a) Figure 5.2.1. Schematic diagrams comparing different printed devices on CVD-grown Tlv:IDCs. (a) Traditional printing with resolution> 25 µm on isolated Tlv:IDC flakes would cause open channel devices. (b) Traditional printing on continuous Tlv:IDC films with dense grain boundaries and junctions inside the channel would lead to compromised device performance. (c) Ultra-short channel devices with a sub-micron channel length can efficiently bridge a single Tlv:IDC flake without grain boundaries or flake-to-flake junctions. 93 5.3 Synthesis of monolayer WSei with controllable morphology Figure 5.3.1 shows the typical as-grown WSe2 samples synthesized on Si/SiO2 substrates by our CVD method. In this study, we used WSe2 powder and Se powder as precursors instead of a combination of WO3 and Se powders. We found that this PVD-like approach gave us a better uniformity and controllability than the common CVD methods using metal oxides and selenium (Figure 5.3.2). To be noticed, we successfully achieved two kinds ofWSe2 samples with either triangular shapes or hexagram shapes by using two different sets of CVD setups as schematically illustrated in Figure 5.3. la. Specifically, we added a specially designed inner tube for the hexagram-shaped growth, where the inner tube has two openings with a small window facing the upstream and a large window facing the downstream. We discovered that an inner tube with such a structure can greatly facilitate the hexagram-shaped growth. Figure 5.3.lb and 5.3.lc are two representative optical microscopic (OM) images of the as-grown triangle-shaped and hexagram-shaped WSe2 flakes. The lateral size of an individual flake is around 10 µm for triangle-shaped samples and 20 µm for hexagram-shaped samples, respectively. Atomic force microscope (AFM) inspection was performed on the hexagram-shaped WSe2 flakes as shown in Figure 5.3. ld. The cross-section height profile along the white dashed line presents a clear step with a height of -0.8 nm which is close to the thickness of a monolayer WSe2. Raman and photoluminescence (PL) spectra were further conducted to verify the properties of the CVD-grown WSe2. Figure 5.3. le is a Raman spectrum taken under a 516 nm incident laser, 94 where both in-plane E.f. 9 and out-plane A 19 characteristic peaks can be clearly observed. Meanwhile, no Raman peak for B} 9 mode was found, indicating the monolayer status of the CVD-grown WSe2 samples. In addition, the monolayer feature was further confirm by PL measurements where a sharp PL emission peak appeared at -770 nm, corresponding to the direct bandgap transition of monolayer WSe2 (Figure 5.3. lf). According to previous studies, a possible reason for the hexagram-shaped growth may be due to the unique local environment created by the special inner tube with a reduced reactant concentrations and a quasi-static reactant distribution. First, the small front window facing the upstream will significantly reduce the amount of reactants that were introduced into the inner tube, therefore reducing the density of nucleation sites. This is clearly evidenced in Figure 5.3. lb and Figure 5.3. lc. Then, the reduced nucleation sites would result less active growth edges compare to the normal triangle-shaped growth with massive growth edges. At last, the reduced number of growth edges along with the quasi-static environment ensure a continuous and sufficient reactant supply to all growth edges, resulting in a uniform growth rate on all directions. 47 Similar work has been reported in CVD graphene growth where six-lobe graphene flowers were synthesized using a gas trap setup. 48 95 {c) , L=.I WSe lOµm - ::::, E~g =! (IJ nl c -- ·;;; >- C :t:! Q) A1g (J) c C I Cl) C /B~g - (IJ C E ...J (IJ 0:: Q. 200 250 300 350 400 450 680 720 760 800 840 Raman Shift (cm- 1 ) Wavelength (nm) Figure 5.3.1 Synthesis and characterization of CVD monolayer WSe2. (a) Schematic diagrams of CVD setups for triangle-shaped WSe2 growth and hexagram-shaped \iVSe2 growth. For hexagram-shaped growth. the substrate is enclosed in a special inner tube with a small opening facing the upstJ:eam supplies. (b)(c) OM images showing the as-grown \iVSe2 flak.es with triangular shapes (b) and hexagram shapes (c). The lateral size of an individual \iVSe2 flake is -10 µm for triangular-shaped samples and-20 µm for hexagram shaped samples. ( d) AFM image of a hexagram-shaped WSe2 flake along with the cross section height profile of the white dash line. The height of the sample is measured to be -0.8 nm corresponding to amonolayerTMDC. (e) Raman spectrnm of the as-grown CVD \iVSe2 showing the two characteiisti.c peaks of E} 9 mode and A 19 mode. (f) PL spectmm of the CVD WS e2 sample showing a strnng PL peak at -770 nm, confirmed the monolayer status. 96 Precursor: WSe 2 20µm - Figure 5.3.2. Comparison of CVD results from different method. (a) Optical image of CVD samples using WO3 and Se precursors show a mixed result with monolayer flakes and bulk flakes. (b) Triangle-shaped flakes grown with WSe2 precursors. (c) Hexagram-shaped flakes grown with combination ofWSe2 precursors and inner tube structure. 5.4 Printing on TMDC surface For the printing of source/drain (S/D) electrodes, Au nanoparticle ink (UT Dots, Inc.) was selected as the electrode material in order to have suitable contacts following previous studies on TMDC-metal contacts. 49 Due to the difference of surface energies between SiO2 and WSe2, the Au ink has different wetting properties after contacting each substrates. 39 Figure 5.4. la and 5.4. lb show the diameter distributions of the printed electrodes formed from a single ink droplet on SiO2 (Figure 5.4. la) and CVD WSe2 (Figure 5.4. lb). In comparison to the electrodes that were printed on bare SiO2 surface which had an average size of-200 µm along with a very narrow diameter distribution, the electrodes printed on CVD WSe2 showed larger lateral sizes and a much broader diameter variation which may cause a critical problem of shorted channel during a standard printing process as illustrated in the insets where two pairs of printed SID electrodes on SiO2 and CVD WSe2 with the 97 same printing distance of 300 µm were presented. Figme 5.4.lc is an optical microscope image showing the wetting of Au ink on WSe2 surface with an excessive spreading distance compared to the ink spread on Si02 surface. This phenomenon is revealed by the SEM image in Figme 5.4. ld as well. For this reason, standard p1inti.ng technique can be hardly working with the CVD TMDC materials in order to reach a channel length below one micrometer. This is evidenced in Figure 5.4.le (OM) and Figure 5.4.1 f (SEM) where two p1inted electrodes immediately merged together due to the wetting effect. {a) - On Si/Si0 2 10 ~ 4 C ~ ~ C 0 ~ 05 o 0 2 0 160 200 240 280 320 360 200 240 280 320 360 Diameter (µm) Diameter (µm) Figure 5.4.1. Studies of the wetting prope1ty of Au ink on Si02 and WSe2. (a) Statistic analysis showing the diameter distiibution of p1inted electi·odes on Si 02. Inset shows a device p1inted on Si02 with a 300 µm printing distance. (b) Statistic analysis showing the 98 broad diameter distribution of printed electrodes on CVD WSe2. Inset shows a shorted device printed on WSe2 with a 300 µm printing distance. (c)(d) Both OM image (c) and SEM image ( d) show the over-wetting of Au inks on WSe2 due to the differences of surface energy between WSe2 and Si 02. (e)(f) OM image (e) and SEM image (f) of a shorted device printed with traditional printing method. 5.5 Sub-micron printing using a flood-dike method In order to achieve printed TMDC devices with a sub-micron channel length, a three- step printing strategy is adopted as illustrated in Figure 5.5. la. In the first step, Au electrodes were directly printed onto the CVD WSe2 material followed by sintering at 220 °C to remove the solvent. Then the sample was functionalized with SAM (lH, lH, 2H, 2H-perfluorodecanethiol (PFDT)) to modify the surface of the previously printed electrodes as the second step. Right after the SAM treatment, the second gold electrodes were closely printed next to the first electrodes as the last step. Due to the existence of SAM layer which surrounds the entire first electrodes, the ink flow of the newly printed electrodes will be blocked by this protecting layer, resulting in a sub-micron channel. Figure 5.5.lb shows the result after such three-step printing process, where two printed electrodes were separated by an extremely narrow gap. Compared to Figure 5.4. le, the merging of electrodes due to the wetting effect can be clearly prevented with the SAM functionalization, resulting in an arc-shaped channel. Among a total of 21 devices printed in this method, 18 devices worked, illustrating a good reliability of this technique. After a follow-up SEM inspection, most of the printed devices retained a channel length less than one micrometer as shown in Figure 5.5. lc. 99 . - 20 µm Au Figure 5.5.1. Printed ultra-short channel devices with a sub-micron channel length. (a) Schematic diagrams of the three-step printing technique. After the first electrodes were printed, SAM functionalization was performed to form a shielding layer which can block the ink flow of the second printed electrodes. (b) OM image of a printed device using the three-step printing process. A clear observation of the blocking effect was displayed. (c) SEM image of the same device in (b). An extremely narrow gap can be observed with a length less than 1 µm. 5.6 Characteristics of the printed sub-micron channel devices Figure 5.6.1 presents the electrical properties of the printed ultra-short channel FETs based on the CVD monolayer WSe2. The measurements were conducted under ambient condition with a back-gated structure. Figure 5.6.la and Figure 5.6.lb are the transfer characteristics of a typical printed device on monolayer WSe2 with a channel length of -0.15 µm and a channel width of -8 µm. At first, the device was measured at a positive Vos of 2 V (Figure 5.6.la). A strong unipolar p-type behavior was observed, which is different :from the reported ambipolar behavior when using fabricated Au metal as contacts. 14 • 15 The suppression of then-branch is widely observed among the printed ultra- 100 short channel WSe2 devices. In comparison, a negative VDS at -2 V was also applied to the same device in Figure 5.6. la for fos-Vas measurements (Figure 5.6. lb), which showed an enhanced on-state current compared to the result under a positive VDS. The inset in Figure 5.6. lb shows the transfer curve plotted in logarithmic scale with an I 0 n/I 0 ff ratio of 10 5 . The effective mobility (µ,ff) was also calculated with a value of 0.85 cm 2 /V·s which is comparable to the reported value for CVD monolayer WSe 2 . 14 The results of printed ultra short channel FETs on few-layer WSe2 were presented in Figure 5.6.2 with a reduced Ionlloff ratio of 10 3 and an increased µ,ff of 10.4 cm 2 /V·s. Figure 5.6. lc shows a family of output curves (fos-VDS) at various Vas from -100 V to -50 V The non-linear behavior indicates the existence of Schottky barriers. Figure 5.6. ld is the same output curve after zoom-in at the low VDS bias region. 101 (a) Monolayer wse, {b) 0 10' 1.0 10° ~ ,....._ ::? -1 - 10' 1 1. 10' :::t - ~10' _, 0.5 --- 10• (/) ~ -2 0 10' V 0 s = 2 V -100 -50 0 50 100 VG S (V) 0.0 V 0 s = -2 V -100 0 100 -3 -100 -50 0 50 100 VGS (\/) VGsM (c)15 {d) 4 10 5 2 ,-... ,......_ 1 0 1 0 -- -- 8 -5 (/) 0 -10 VGS -2 VGS - -100V - -SOV - -1oov - ..sov -15 -aov - -1ov - -aov - -70V -20 - -60V - -60V -4 - -60V - -oOV -10 -5 0 5 10 -2 -1 0 1 2 VD S (V) Vos (V) Figure 5.6.1. Transport study of the printed ultra-short channel devices on monolayer WSe2. ( a) IDs-V GS transfer curve measured at a positive V DS of 2V. A clear p-type behavior can be observed. (b) fos-V Gs transfer curve measured at a negative V DS of -2V on the same device in (a). A strong increasing of the on-current was shown. Inset displays the transfer curve plotted in a logarithmic scale showing an Ionlloff ratio of 10 5 . (c) A family ofIDs-VDs output curves of the printed devices showing the existence of Schottky barrier. ( d) fos-VDs output curves after zoom-in at the low-bias region. 102 (a)~~ (b) 0 1 - v.,=-2v1 20 ...-..-10 ...-.. 'i 15 -10 'i --20 (/) (/) _o 5 0 0 -30 -5 -100 0 100 -100 -50 0 50 100 ( c)100 VGsM (d)50 VGsM 50 i i 0 .._., 0 .._., (/) (/) _o v. _o v. - -•mv - ·•IIIV - av - av - •v - -v -50 - -•v - .,av - •v -50 - •v - •v - •v ~v - ~v -10 -5 0 5 10 -3 -2 -1 0 1 2 3 Vos(V) Vos(V) Figure 5.6.2. Printed ultra-short channel devices with few-layer CVD WSe2. (a) IDs-VGs transfer curve measured at a positive VDs of 2 V. (b) IDs-V Gs transfer cu1ve measured at a negative Vos of -2 V. A clear p-type behavior can be observed for few-layer WSe2 samples. Compared to monolayer WSe2 devices, few-layer WSe2 devices show higher on-cun-ent but lowerlonlloff ratio. ( c )( d) A family ofIDs-VDs output curves measured at different V GS A clear Schottky behavior can be observed from the output curves. 5. 7 Statistic study of the printed ultrashort channel devices Further, a systematic statistic study was performed on 15 printed monolayer WSe2 devices to verify the reliability and reproducibility of the three-step printing process. All the devices achieved through this SAM-assisted printing technique possess an ultra-short channel of less than one micrometer -while still sustain a high Ionlloff ratio around 10 4 to 10 5 as shoVIID in Figure 5. 7.1 a Figure 5. 7. lb presents the distribution of µelf ranging from 0.01 to 10 cm 2 N ·s, -which are among the range ofreported values of as-groV\111 CVD monolayer TMDCs with a back-gated structure. 14 In addition, a significantly improved on-state cun-ent 103 density was demonstrated in this work comparing to other printing works on 2D TMDC materials as shown in Figure 5.7.lc. Using solution-processed TMDC inks as the channel material reported a relatively low Io nlloff ratio due to the flake-to-flake junctions and defects. 40 · 41 On the other hand, printed CVD TMDC devices with a channel length above 100 µm would result compromised device performance, for example, low on-state current density, due to the location-to-location variation of CVD materials and excessive grain boundaries inside the channel. 39 (a) • l00/l 0 ff ratio (b) - Mobility (c) o.3o A Ref _ 41 , L = 120 µm with WS 2 inks 8 10 E 0.25 T Ref . 41, L = 200 µm with WSe 2 inks • Ref. 39, L= 100 µmwith CVD M0S2 Cf) ;f 0.20 • Ref . 40, L = 13 µm with MoS 2 inks * 6 Cf) c * This work , L < 1 µm w ith CVD WSe 2 c :J 6 0.15 54 0 ~ 05 a 0.10 0 2 - .. 0.05 0.00 • • • 0 10· 1 10° 10 1 10 2 10 3 10 4 10 5 10 6 0 10·2 10-1 1 o 0 10 1 10° 10 1 10 2 10 3 1 O' 10 5 l 0 /l 0 rr Ratio Mobility (cm 2 V 1 s· 1 ) l 0 /l 0 ff Ratio Figure 5.7.1. Statistic studies of the performance of printed ultra-short channel devices. (a) Ion l loff ratio distribution of the devices printed on monolayer WSe2. (b) Effective mobility distribution of the printed devices on mono layer WSe2. ( c) Comparison of the device performance of printed TMDC devices. Ultra-short channel devices with CVD grown WSe2 show the highest on-state current density together with a high Ionllo ff ratio compare to the long channel devices printed on CVD M0S2 and solution-processed TMDCs. 5.8 Conclusion In summary, we developed a highly reproducible strategy to print low-cost, high- efficiency, and high-performance monolayer WSe2 devices with a sub-micron channel length. Firstly by using a CVD approach, we successfully synthesized large-area 104 monolayer WSe2 samples, which can greatly facilitate the follow-up manufacturing processes. Then by adopting the special three-step printing technique, we managed to achieve printed FETs with a sub-micron channel length while keeping a good device performance. The on-current density and Ion/Ioff ratio are superior compared to other reported TMDC printed devices. The combination of CVD synthesis and ultra-short channel printing technique sheds a light on the future development of low-cost and high performance electronics with 2D TMDCs. Chapter 5. References 1. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H., The chemistry oftwo dirnensional layered transition metal dichalcogenide nanosheets. Nat Chem 2013, 5, 263-275. 2. Geirn, A. K.; Grigorieva, I. V., Van der Waals heterostructures. Nature 2013, 499, 419-425. 3. Kang, K.; Xie, S. E.; Huang, L. J.; Han, Y M.; Huang, P. Y; Mak, K. F.; Kirn, C. J.; Muller, D.; Park, J., High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity. Nature 2015, 520, 656-660. 4. Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Neto, A. H. C., 2D materials and van der Waals heterostructures. Science 2016, 353. 5. Costanzo, D.; Jo, S.; Berger, H.; Morpurgo, A. F., Gate-induced superconductivity in atomically thin MoS2 crystals. Nat Nanotechnol 2016, 11, 339-+. 6. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 transistors. Nat Nanotechnol 2011, 6, 147-150. 7. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS,: A New Direct-Gap Semiconductor. Phys Rev Lett 2010, 105. 8. Yun, W. S.; Han, S. W.; Hong, S. C.; Kirn, I. G.; Lee, J. D., Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-M X-2 semiconductors (M ~ Mo, W; X ~ S, Se, Te). Phys Rev B 2012, 85. 9. Desai, S. B.; Seo!, G.; Kang, J. S.; Fang, H.; Battaglia, C.; Kapadia, R.; Ager, J. W.; Guo, J.; Javey, A., Strain-Induced Indirect to Direct Bandgap Transition in Multi layer WSe,. Nano Lett 2014, 14, 4592-4597. 10. Tongay, S.; Suh, J.; Ataca, C.; Fan, W.; Luce, A.; Kang, J. S.; Liu, J.; Ko, C.; Raghunathanan, R.; Zhou, J.; Ogletree, F.; Li, J. B.; Grossman, J. C.; Wu, J. Q., Defects activated 105 photoluminescence in two-dimensional semiconductors: interplay between bound, charged, and free excitons. Sci Rep-Uk 2013, 3. II. Splendiani, A.; Sun, L.; Zhang, Y B.; Li, T. S.; Kirn, J.; Chim, C. Y; Galli, G.; Wang, F., Emerging Photoluminescence in Monolayer MoS,. Nano Lett 2010, 10, 1271-1275. 12. Wu, S. F.; Ross, J. S.; Liu, G. B.; Aivazian, G.; Jones, A.; Fei, Z. Y; Zhu, W. G.; Xiao, D.; Yao, W.; Cobden, D.; Xu, X. D., Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS,. Nat Phys 2013, 9, 149-153. 13. Jones, A. M.; Yu, H. Y; Ghimire, N. J.; Wu, S. F.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J. Q.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu, X. D., Optical generation of excitonic valley coherence in monolayer WSe,. Nat Nanotechnol 2013, 8, 634-638. 14. Liu, B. L.; Fathi, M.; Chen, L.; Abbas, A.; Ma, Y Q.; Zhou, C. W., Chemical Vapor Deposition Growth of Monolayer WSe, with Tunable Device Characteristics and Growth Mechanism Study. ACS Nano 2015, 9, 6119-6127. 15. Das, S.; Appenzeller, J., WSe2 field effect transistors with enhanced ambipolar characteristics. Appl Phys Lett 2013, 103. 16. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 2012, 7, 699-712. 17. Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J. Q.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; Cobden, D. H.; Xu, X. D., Electrically tunable excitonic light-emitting diodes based on mono layer WSe, p-njunctions. Nat Nanotechnol 2014, 9, 268-272. 18. Pospischil, A.; Furchi, M. M.; Mueller, T., Solar-energy conversion and light emission in an atomic monolayer p-n diode. Nat Nanotechnol 2014, 9, 257-261. 19. Baugher, B. W. H.; Churchill, H. 0. H.; Yang, Y F.; Jarillo-Herrero, P., Optoelectronic devices based on electrically tunable p-n diodes in a mono layer dichalcogenide. Nat Nanotechnol 2014, 9, 262-267. 20. Yuan, H. T.; Bahramy, M. S.; Morimoto, K.; Wu, S. F.; Nomura, K.; Yang, B. J.; Shimotani, H.; Suzuki, R.; Toh, M.; Kloc, C.; Xu, X. D.; Arita, R.; Nagaosa, N.; Iwasa, Y, Zeeman-type spin splitting controlled by an electric field. Nat Phys 2013, 9, 563-569. 21. Yuan, H. T.; Wang, X. Q.; Lian, B.; Zhang, H.J.; Fang, X. F.; Shen, B.; Xu, G.; Xu, Y; Zhang, S. C.; Hwang, H. Y; Cui, Y, Generation and electric control of spin-valley-coupled circular photo galvanic current in WSe,. Nat Nanotechnol 2014, 9, 851-857. 22. Akinwande, D.; Petrone, N.; Hone, J., Two-dimensional flexible nanoelectronics. Nat Commun 2014, 5. 23. Liu, B. L.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C. W., High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Mono layer MoS, Transistors. Acs Nano 2014, 8, 5304-5314. 24. Das, S.; Gulotty, R.; Sumant, A. V.; Roelofs, A., All Two-Dimensional, Flexible, Transparent, and Thinnest Thin Film Transistor. Nano Lett 2014, 14, 2861-2866. 25. Pu, J.; Funahashi, K.; Chen, C.H.; Li, M. Y; Li, L. J.; Takenobu, T., Highly Flexible and High- 106 Performance Complementary Inverters of Large-Area Transition Metal Dichalcogenide Monolayers.Adv Mater 2016, 28, 4111-4119. 26. Park, J. U.; Hardy, M.; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. K.; Lee, C. Y; Strano, M. S.; Alleyne, A.G.; Georgiadis, J. G.; Ferreira, P. M.; Rogers, J. A., High-resolution electrohydrodynarnic jet printing. Nat Mater 2007, 6, 782-789. 27. Cao, X.; Lau, C.; Liu, Y H.; Wu, F. Q.; Gui, H.; Liu, Q. Z.; Ma, Y Q.; Wan, H. C.; Arner, M. R.; Zhou, C. W., Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochrornic Displays Using Carbon Nanotube Thin-Fihn Transistors. ACS Nano 2016, 10, 9816-9822. 28. Cai, L.; Zhang, S. M.; Miao, J. S.; Yu, Z. B.; Wang, C., Fully Printed Foldable Integrated Logic Gates with Tunable Performance Using Semiconducting Carbon Nanotubes. Adv Funct Mater 2015, 25, 5698-5705. 29. Chen, P. C.; Fu, Y; Arninirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.; Zhou, C. W., Fully Printed Separated Carbon Nanotube Thin Fihn Transistor Circuits and Its Application in Organic Light Emitting Diode Control. Nano Lett 2011, 11, 5301-5308. 30. Ha, M. J.; Seo, J. W. T.; Prabhurnirashi, P. L.; Zhang, W.; Geier, M. L.; Renn, M. J.; Kirn, C. H.; Hersarn, M. C.; Frisbie, C. D., Aerosol Jet Printed, Low Voltage, Electrolyte Gated Carbon Nanotube Ring Oscillators with Sub-5 µs Stage Delays. Nano Lett 2013, 13, 954-960. 31. Lau, P.H.; Takei, K.; Wang, C.; Ju, Y; Kirn, J.; Yu, Z. B.; Takahashi, T.; Cho, G.; Javey, A., Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on Flexible Substrates. Nano Lett 2013, 13, 3864-3869. 32. Noh, Y Y; Zhao, N.; Caironi, M.; Sirringhaus, H., Downscaling of self-aligned, all-printed polymer thin-fihn transistors. Nat Nanotechnol 2007, 2, 784-789. 33. Sirringhaus, H.; Brown, P. J.; Friend, R.H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M., Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401, 685-688. 34. Yang, J.; Choi, M. K.; Kirn, D. H.; Hyeon, T., Designed Assembly and Integration of Colloidal Nanocrystals for Device Applications. Adv Mater 2016, 28, 1176-1207. 35. Yeorn, C.; Chen, K.; Kiriya, D.; Yu, Z. B.; Cho, G.; Javey, A., Large-Area Compliant Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv Mater 2015, 27, 1561. 36. Gu, X.; Zhou, Y; Gu, K.; Kurosawa, T.; Guo, Y; Li, Y; Lin, H.; Schroeder, B. C.; Yan, H.; Molina-Lopez, F.; Tassone, C. J.; Wang, C.; Mannsfeld, S. C. B.; Yan, H.; Zhao, D.; Toney, M. F.; Bao, Z., Roll-to-Roll Printed Large-Area All-Polymer Solar Cells with 5% Efficiency Based on a Low Crystallinity Conjugated Polymer Blend Advanced Energy Materials Early View. Advanced Energy Materials 2017, DOI: 10.1002/aernn.201602742. 37. Xu, W. Y; Zhao, J. W.; Qian, L.; Han, X. Y; Wu, L. Z.; Wu, W. C.; Song, M. S.; Zhou, L.; Su, W. M.; Wang, C.; Nie, S. H.; Cui, Z., Sorting of large-diameter semiconducting carbon nanotube and printed flexible driving circuit for organic light emitting diode (OLED). Nanoscale 2014, 6, 1589-1595. 38. Carey, B. J.; Ou, J. Z.; Clark, R. M.; Berean, K. J.; Zavabeti, A.; Chesrnan, A. S.; Russo, S. P.; Lau, D. W.; Xu, Z. Q.; Bao, Q.; Kevehei, O.; Gibson, B. C.; Dickey, M. D.; Kaner, R. B.; 107 Daeneke, T.; Kalantar-Zadeh, K., Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals. Nat Commun 2017, 8, 14482. 39. Kirn, T. Y; Arnani, M.; Ahn, G. H.; Song, Y; Javey, A.; Chung, S.; Lee, T., Electrical Properties of Synthesized Large-Area MoS, Field-Effect Transistors Fabricated with Inkjet-Printed Contacts. ACS Nano 2016, 10, 2819-2826. 40. Li, J. T.; Naiini, M. M.; Vaziri, S.; Lemme, M. C.; Ostling, M., Inkjet Printing ofMoS,. Adv FunctMater 2014, 24, 6524-6531. 41. Kelly, A. G.; Hallam, T.; Backes, C.; Harvey, A.; Esrnaeily, A. S.; Godwin, I.; Coelho, J.; Nicolosi, V.; Lauth, J.; Kulkarni, A.; Kinge, S.; Siebbeles, L. D.; Duesberg, G. S.; Coleman, J. N., All-printed thin-film transistors from networks of liquid-exfoliated nanosheets. Science 2017, 356, 69-73. 42. McManus, D.; Vranic, S.; Withers, F.; Sanchez-Rornaguera, V.; Macucci, M.; Yang, H. F.; Sorrentino, R.; Panrez, K.; Son, S. K.; Iannaccone, G.; Kostarelos, K.; Fiori, G.; Casiraghi, C., Water-based and biocornpatible 2D crystal inks for all-inkjet-printed heterostructures. Nat Nanotechnol 2017, 12, 343-350. 43. Sele, C. W.; von Werne, T.; Friend, R.H.; Sirringhaus, H., Lithography-free, self-aligned inkjet printing with sub-hundred-nanometer resolution. Adv Mater 2005, 17, 997. 44. Zhao, N.; Chiesa, M.; Sirringhausa, H.; Li, Y N.; Wu, Y L., Self-aligned inlget printing of highly conducting gold electrodes with subrnicron resolution. J Appl Phys 2007, 101. 45. Cao, X.; Wu, F. Q.; Lau, C.; Liu, Y H.; Liu, Q. Z.; Zhou, C. W., Top-Contact Self-Aligned Printing for High Performance Carbon Nanotube Thin-Film Transistors with Sub-Micron Channel Length. ACS Nano 2017, 11, 2008-2014. 46. van der Zande, A. M.; Huang, P. Y; Chenet, D. A.; Berkelbach, T. C.; You, Y M.; Lee, G. H.; Heinz, T. F.; Reichman, D.R.; Muller, D. A.; Hone, J.C., Grains and grain boundaries in highly crystalline rnonolayer molybdenum disulphide. Nat Mater 2013, 12, 554-561. 47. Chen, J. Y; Liu, B.; Liu, Y P.; Tang, W.; Nai, C. T.; Li, L. J.; Zheng, J.; Gao, L.B.; Zheng, Y; Shin, H. S.; Jeong, H. Y; Loh, K. P., Chemical Vapor Deposition of Large-Sized Hexagonal WSez Crystals on Dielectric Substrates. Adv Mater 2015, 27, 6722. 48. Zhang, Y; Zhang, L. Y; Kirn, P.; Ge, M. Y; Li, Z.; Zhou, C. W., Vapor Trapping Growth of Single-Crystalline Graphene Flowers: Synthesis, Morphology, and Electronic Properties. Nano Lett 2012, 12, 2810-2816. 49. Xu, Y; Cheng, C.; Du, S. C.; Yang, J. Y; Yu, B.; Luo, J.; Yin, W. Y; Li, E. P.; Dong, S. R.; Ye, P. D.; Duan, X. F., Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions. ACS Nano 2016, 10, 4895-4919. 108 6. Summary and future work 6.1 Summary The research of 2D materials is an important direction in the nanotechnology field. It has tight connections to many disciplines including fundamental physics, materials chemistry, device physics, etc. There have been a huge amount of 2D materials discovered over the past years. Among them, the family of TMDCs is especially interesting, as it consists various materials with distinctive properties. Current findings have demonstrated that TMDCs own unique advantages and can be promising candidates for a lot of exciting applications such as spintronics and valleytronics. Further research effort is required to fully extract the potential of TMDCs. In this thesis, I first introduced the background of TMDCs with detailed information about history, lattice structure, compositions, properties, and preparing methods. Then I discussed several works which are closely related to TMDCs that I have accomplished during my PhD study. First, we synthesized monolayer MoS2 using a modified CVD strategy to achieve large-size crystals. And we utilized such MoS2 monolayer for high- performance chemical sensors, which can detect NO2 with a superior record under to 20 ppb. The outstanding performance can be contributed to the high surface-to-volume ratio of the monolayer MoS2 and the existence of Schottky-barrier within the MoS2 FETs. Second, we developed a unique CVD method which uses sulfur as additional reducer to control the growth mode of WSe2 synthesis. By tuning the concentration of reactants, we successfully alternated the commonly observed layer-by-layer growth to a screw- 109 dislocation-driven growth. The resulting WSe2 can grow into a pyramid-like 3D structure instead of 2D layers. Third, we adopted sapphire as the growth substrate for CVD WSe2 synthesis. The periodic atomic steps on sapphire surface had a significant effect on the growth behavior of WSe2. We discovered that the reactants will selectively attach to the atomic steps and form aligned nuclei along the steps. In this way, the growth orientation of WSe2 flakes can be well controlled. Fourth, we developed a special printing technique to achieve ultrashort channel WSe2 FETs with high on-state current density and high on-off current ratio. This printing method uses a SAM as a flood-dike to create an ultrashort channel with a sub-micron length. It can significantly ease the device fabrication process and provide a new way to achieve high-performance TMDC devices. 6.2 Future work 6.2.1 Wafer-scale synthesis of high-quality and uniform TMDC films Compared to mechanical exfoliation method, CVD approaches have demonstrated their advantages of large-scale capability and good controllability in terms of flake shapes and uniformity. Nevertheless, current CVD results are still not satisfying enough for challenging applications. First, the mobility and quality of the CVD produced TMDCs are rather low. 1 One possible reason is the existence of high-density defects like S/Se vacancies associated with the CVD materials. 1 • 2 Second, most of the current CVD methods only produce either isolated flakes or crystals with a rather small size, which generates unexpected grain boundaries and junctions that may hamper the device performance. For 110 these reasons, an advanced CVD method that can produce wafer-scale uniform material with high quality is highly desired. So far, gas phase growth using a metal-organic CVD (MOCVD) approach has been developed to produce wafer-scale monolayer TMDCs. 3 By using gas phase precursors, the growth process becomes more controllable than normal CVD setups, resulting in a uniform monolayer growth. Nevertheless, the cost of such process is still too high as the growth period is extremely long. Meanwhile, the as-grown materials are polycrystalline composed of small crystals around several micrometer sizes. Starting from here, the next goal would be develop a highly efficient CVD method which can produce wafer-scale uniform TMDC films with high quality and good controllability. 6.2.2 Flexible electronics with TMDC materials The market for flexible electronics is rapidly growing. Wearable devices and foldable electronics are attracting more and more people's attention. 4 For all 2D layered materials, the atomic thin feature and weak interlayer force make them naturally good candidates for flexible devices. In specific, MoS2 and WSe2, unlike graphene, have bandgap which can be suitable for flexible FE Ts and optoelectronics. On the other hand, there are some issues need to be solved before TMDCs can be widely applied in flexible electronics. First of all, the material issue is the biggest obstacle. As we discussed before, wafer scale uniform layer is needed for scalable applications. Second, the mobility of CVD TMDCs are low at this moment. Typical mobility is around 0.1-10 cm 2 v- 1 s- 1 for both CVD MoS 2 and WSe 2 . 1 Third, the contact issue is another problem for TMDC devices. So far, the traditional metal 111 contacts do not work well because of the large contact resistance. 1 • 5 Some researchers believe that graphene may be an optimistic contact material for TMDCs as it is flexible and its Fermi level can be tunable with electrostatic gating. 6 6.2.3 Printed electronics with TMDC inks Electronic printing method is a low-temperature, solution-based process. Compared to traditional fabrication method, printing approach provides a convenient way to produce devices with significantly reduced cost. 7 People are putting efforts on applying this printing technique to thin film transistor applications like displays. Most of the current works about printed electronics are based on carbon nanotubes or organic thin films. 8 • 9 More investigations with 2D TMDCs are required for further development. At this moment, the most important issue with TMDCs is the choice of the starting materials. Although, solution-processed TMDC inks can generate large-area films, the resulting devices only show an inapplicable performance with low on-off current ratio and low mobility. 10 • 11 A possible reason is the high-density defects and flake-to-flake junctions among the ink produced films. CVD materials could be a good choice which provides large-area materials with reasonable quality. Another possible approach would be using special ligands on solution-processed films to weld the junctions and form a better connection. 12 112 Chapter 6. References 1. Liu, B. L.; Fathi, M.; Chen, L.; Abbas, A.; Ma, Y Q.; Zhou, C. W., Chemical Vapor Deposition Growth of Monolayer WSe, with Tunable Device Characteristics and Growth Mechanism Study. ACS Nano 2015, 9, 6119-6127. 2. Kirn, I. S.; Sangwan, V. K.; Jariwala, D.; Wood, J. D.; Park, S.; Chen, K. S.; Shi, F. Y; Ruiz Zepeda, F.; Ponce, A.; Jose-Yacarnan, M.; Dravid, V. P; Marks, T. J.; Hersarn, M. C.; Lauhon, L. J., Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition Derived MoS,. ACS Nano 2014, 8, 10551-10558. 3. Kang, K.; Xie, S. E.; Huang, L. J.; Han, Y M.; Huang, PY; Mak, K. F.; Kirn, C. J.; Muller, D.; Park, J., High-mobility three-atom-thick semiconducting fihns with wafer-scale homogeneity. Nature 2015, 520, 656-660. 4. Akinwande, D.; Petrone, N.; Hone, J., Two-dimensional flexible nanoelectronics. Nat Commun 2014, 5. 5. Liu, B. L.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C. W., High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Mono layer MoS2 Transistors. ACS Nano 2014, 8, 5304-5314. 6. Liu, Y; Wu, H.; Cheng, H. C.; Yang, S.; Zhu, E. B.; He, Q. Y; Ding, M. N.; Li, D. H.; Guo, J.; Weiss, N. O.; Huang, Y; Duan, X. F., Toward Barrier Free Contact to Molybdenum Disulfide Using Graphene Electrodes. Nano Lett 2015, 15, 3030-3034. 7. Chen, PC.; Fu, Y; Arninirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.; Zhou, C. W., Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and Its Application in Organic Light Emitting Diode Control. Nano Lett 2011, 11, 5301-5308. 8. Cao, X.; Lau, C.; Liu, Y H.; Wu, F. Q.; Gui, H.; Liu, Q. Z.; Ma, Y Q.; Wan, H. C.; Arner, M. R.; Zhou, C. W., Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochrornic Displays Using Carbon Nanotube Thin-Film Transistors. ACS Nano 2016, 10, 9816-9822. 9. Noh, Y Y; Zhao, N.; Caironi, M.; Sirringhaus, H., Downscaling of self-aligned, all-printed polymer thin-film transistors. Nat Nanotechnol 2007, 2, 784-789. 10. McManus, D.; Vranic, S.; Withers, F.; Sanchez-Rornaguera, V.; Macucci, M.; Yang, H. F.; Sorrentino, R.; Parvez, K.; Son, S. K.; Iannaccone, G.; Kostarelos, K.; Fiori, G.; Casiraghi, C., Water-based and biocornpatible 2D crystal inks for all-inkjet-printed heterostructures. Nat Nanotechnol 2017, 12, 343-350. 11. Kelly, A. G.; Hallam, T.; Backes, C.; Harvey, A.; Esrnaeily, A. S.; Godwin, I.; Coelho, J.; Nicolosi, V.; Lauth, J.; Kulkarni, A.; Kinge, S.; Siebbeles, L. D.; Duesberg, G. S.; Coleman, J. N., All-printed thin-film transistors from networks of liquid-exfoliated nanosheets. Science 2017, 356, 69-73. 12. Jang, J.; Dolzhnikov, D. S.; Liu, W. Y; Nam, S.; Shim, M.; Talapin, D. V., Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular "Solders": Approaching Single Crystal Mobility. Nano Lett 2015, 15, 6309-6317. 113 Bibliography Novoselov, K. S.; Geirn, AK.; Morozov, S. V.; Jiang, D.; Zhang, Y; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric field effect in atomically thin carbon fihns. Science 2004, 306, 666- 669. Geirn, A. K.; Novoselov, K. S., The rise of graphene. Nat Mater 2007, 6, 183-191. Novoselov, K. S.; Geirn, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438, 197-200. Bae, S.; Kirn, H.; Lee, Y; Xu, X. F.; Park, J. S.; Zheng, Y; Balakrishnan, J.; Lei, T.; Kirn, H. R.; Song, Y I.; Kirn, Y J.; Kirn, K. S.; Ozyihnaz, B.; Ahn, J. H.; Hong, B. H.; Iijirna, S., Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nat Nanotechnol 2010, 5, 574- 578. Zhang, Y B.; Tan, Y W.; Stormer, H. L.; Kirn, P, Experimental observation of the quantum Hall effect and Berry's phase in graphene. Nature 2005, 438, 201-204. Li, X. S.; Cai, W.W.; An, J. H.; Kirn, S.; Nah, J.; Yang, D. X.; Piner, R.; Velarnakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S., Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312-1314. Kirn, K. S.; Zhao, Y; Jang, H.; Lee, S. Y; Kirn, J.M.; Kirn, K. S.; Ahn, J. H.; Kirn, P; Choi, J. Y; Hong, B. H., Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457, 706-710. Schedin, F.; Geirn, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P; Katsnelson, M. I.; Novoselov, K. S., Detection of individual gas molecules adsorbed on graphene. Nat Mater 2007, 6, 652-655. Lin, Y M.; Dirnitrakopoulos, C.; Jenkins, K. A.; Farmer, D. B.; Chiu, H. Y; Grill, A.; Avouris, P, 100-GHz Transistors from Wafer-Scale Epitaxial Graphene. Science 2010, 327, 662-662. Lee, Y; Bae, S.; Jang, H.; Jang, S.; Zhu, S. E.; Sim, S. H.; Song, Y I.; Hong, B. H.; Ahn, J. H., Wafer-Scale Synthesis and Transfer of Graphene Fihns. Nano Lett 2010, 10, 490-493. Ahn, J. H.; Hong, B. H., Graphene for displays that bend. Nat Nanotechnol 2014, 9, 737-738. Ren, W. C.; Cheng, H. M., The global growth of graphene. Nat Nanotechnol 2014, 9, 726-730. 114 Fiori, G.; Bonaccorso, F.; Iannaccone, G.; Palacios, T.; Neumaier, D.; Seabaugh, A.; Banerjee, S. K.; Colombo, L., Electronics based on two-dimensional materials. Nat Nanotechnol 2014, 9, 768- 779. Ohta, T.; Bostwick, A.; Seyller, T.; Hom, K.; Rotenberg, E., Controlling the electronic structure of bilayer graphene. Science 2006, 313, 951-954. Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H.J., Chemically derived, ultrasrnooth graphene nanoribbon semiconductors. Science 2008, 319, 1229-1232. Bhirnanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y; Cha, J.; Das, S.; Xiao, D.; Son, Y; Strano, M. S.; Cooper, V. R.; Liang, L.B.; Louie, S. G.; Ringe, E.; Zhou, W.; Kirn, S.S.; Naik, R.R.; Sumpter, B. G.; Terrones, H.; Xia, F. N.; Wang, Y L.; Zhu, J.; Akinwande, D.; Alem, N.; Schuller, J. A.; Schaak, R. E.; Terrones, M.; Robinson, J. A., Recent Advances in Two-Dimensional Materials beyond Graphene. ACS Nano 2015, 9, 11509-11539. Geirn, A. K.; Grigorieva, I. V., Van der Waals heterostructures. Nature 2013, 499, 419-425. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 transistors. Nat Nanotechnol 2011, 6, 147-150. Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A., High-Performance Single Layered WSe2 p-FETs with Chemically Doped Contacts. Nano Lett 2012, 12, 3788-3792. Li, L. K.; Yu, Y J.; Ye, G. J.; Ge, Q. Q.; Ou, X. D.; Wu, H.; Feng, D. L.; Chen, X. H.; Zhang, YB., Black phosphorus field-effect transistors. Nat Nanotechnol 2014, 9, 372-377. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P; Zhang, H., The chemistry of two dimensional layered transition metal dichalcogenide nanosheets. Nat Chem 2013, 5, 263-275. Lee, Y H.; Zhang, X. Q.; Zhang, W. J.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y C.; Wang, J. T. W.; Chang, C. S.; Li, L. J.; Lin, T. W., Synthesis of Large-Area MoS, Atomic Layers with Chemical Vapor Deposition. Adv Mater 2012, 24, 2320-2325. Yun, W. S.; Han, S. W.; Hong, S. C.; Kirn, I. G.; Lee, J. D., Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-M X-2 semiconductors (M ~ Mo, W; X ~ S, Se, Te). Phys Rev B 2012, 85. Wang, X. M.; Jones, A. M.; Seyler, K. L.; Tran, V.; Jia, Y C.; Zhao, H.; Wang, H.; Yang, L.; Xu, X. D.; Xia, F. N., Highly anisotropic and robust excitons in rnonolayer black phosphorus. Nat 115 Nanotechnol 2015, 10, 517-521. Tao, L.; Cinquanta, E.; Chiappe, D.; Grazianetti, C.; Fanciulli, M.; Dubey, M.; Molle, A.; Akinwande, D., Silicene field-effect transistors operating at room temperature. Nat Nanotechnol 2015, 10, 227-231. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 2012, 7, 699-712. Ni, Z. Y; Liu, Q. H.; Tang, K. C.; Zheng, J. X.; Zhou, J.; Qin, R.; Gao, Z. X.; Yu, D. P.; Lu, J., Tunable Bandgap in Silicene and Gerrnanene. Nano Lett 2012, 12, 113-118. Wang, H.; Wang, X. M.; Xia, F. N.; Wang, L. H.; Jiang, H.; Xia, Q. F.; Chin, M. L.; Dubey, M.; Han, S. J., Black Phosphorus Radio-Frequency Transistors. Nano Lett 2014, 14, 6424-6429. Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-+. Kodenkandath, T. A.; Lalena, J. N.; Zhou, W. L. L.; Carpenter, E. E.; Sangregorio, C.; Falster, A. U.; Simmons, W. B.; O'Connor, C. J.; Wiley, J. B., Assembly of metal-anion arrays within a perovskite host. Low-temperature synthesis of new layered copper-oxyhalides, (Cux)LaNb,O,, X ~ Cl, Br. J Am Chem Soc 1999, 121, 10743-10746. Splendiani, A.; Sun, L.; Zhang, YB.; Li, T. S.; Kirn, J.; Chim, C. Y; Galli, G.; Wang, F., Emerging Photoluminescence in Mono layer MoS,. Nano Lett 2010, 10, 1271-1275. Kong, D. S.; Wang, H. T.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y, Synthesis ofMoS, and Mo Se, Fihns with Vertically Aligned Layers. Nano Lett 2013, 13, 1341-1347. Baugher, B. W. H.; Churchill, H. 0. H.; Yang, Y F.; Jarillo-Herrero, P., Optoelectronic devices based on electrically tunable p-n diodes in a rnonolayer dichalcogenide. Nat Nanotechnol 2014, 9, 262-267. Withers, F.; Del Pozo-Zamudio, O.; Schwarz, S.; Dufferwiel, S.; Walker, P. M.; Godde, T.; Rooney, A. P.; Gholinia, A.; Woods, C. R.; Blake, P.; Haigh, S. J.; Watanabe, K.; Taniguchi, T.; Aleiner, I. L.; Geirn, A. K.; Fal'ko, V. I.; Tartakovskii, A. I.; Novoselov, K. S., WSe, Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature. Nano Lett 2015, 15, 8223-8228. Pospischil, A.; Furchi, M. M.; Mueller, T., Solar-energy conversion and light emission in an atomic 116 rnonolayer p-n diode. Nat Nanotechnol 2014, 9, 257-261. Pu, J.; Yornogida, Y; Liu, K. K.; Li, L. J.; Iwasa, Y; Takenobu, T., Highly Flexible MoS, Thin Fihn Transistors with Ion Gel Dielectrics. Nano Lett 2012, 12, 4013-4017. Liu, B. L.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C. W., High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Mono layer MoS, Transistors. ACS Nano 2014, 8, 5304-5314.Xiao, D.; Liu, G. B.; Feng, W. X.; Xu, X. D.; Yao, W., Coupled Spin and Valley Physics in Monolayers of MoS, and Other Group-VI Dichalcogenides. Phys Rev Lett 2012, 108. Najrnaei, S.; Liu, Z.; Zhou, W.; Zou, X. L.; Shi, G.; Lei, S. D.; Yakobson, B. I.; Idrobo, J.C.; Ajayan, P. M.; Lou, J., Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers. Nat Mater 2013, 12, 754-759. van der Zande, A. M.; Huang, P. Y; Chenet, D. A.; Berkelbach, T. C.; You, Y M.; Lee, G. H.; Heinz, T. F.; Reichman, D.R.; Muller, D. A.; Hone, J.C., Grains and grain boundaries in highly crystalline rnonolayer molybdenum disulphide. Nat Mater 2013, 12, 554-561. Zhang, Y; Zhang, Y F.; Ji, Q. Q.; Ju, J.; Yuan, H. T.; Shi, J. P.; Gao, T.; Ma, D. L.; Liu, M. X.; Chen, Y B.; Song, X. J.; Hwang, H. Y; Cui, Y; Liu, Z. F., Controlled Growth of High-Quality Monolayer WS2 Layers on Sapphire and Imaging Its Grain Boundary. ACS Nano 2013, 7, 8963- 8971. Kang, K.; Xie, S. E.; Huang, L. J.; Han, Y M.; Huang, P. Y; Mak, K. F.; Kirn, C. J.; Muller, D.; Park, J., High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity. Nature 2015, 520, 656-660. Eichfeld, S. M.; Hossain, L.; Lin, Y C.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kirn, M. J.; Mayer, T. S.; Redwing, J.M.; Robinson, J. A., Highly Scalable, Atomically Thin WSe2 Grown via Metal-Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080-2087. Huang, J. K.; Pu, J.; Hsu, C. L.; Chiu, M. H.; Juang, Z. Y; Chang, Y H.; Chang, W. H.; Iwasa, Y; Takenobu, T.; Li, L. J., Large-Area Synthesis of Highly Crystalline WSe, Mono layers and Device Applications. ACS Nano 2014, 8, 923-930. Ji, Q. Q.; Zhang, Y F.; Gao, T.; Zhang, Y; Ma, D. L.; Liu, M. X.; Chen, YB.; Qiao, X. F.; Tan, P. H.; Kan, M.; Feng, J.; Sun, Q.; Liu, Z. F., Epitaxial Monolayer MoS, on Mica with Novel Photoluminescence. Nano Lett 2013, 13, 3870-3877. 117 Yamazoe, N. Toward Innovations of Gas Sensor Technology, Sens. Actuators, B 2005, 108, 2-14. Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H.J. Nanotube Molecular Wires as Chemical Sensors, Science 2000, 287, 622-625. Snow, E. S.; Perkins, F. K.; Robinson, J. A. Chemical Vapor Detection Using Single-Walled Carbon Nanotubes, Chem. Soc. Rev. 2006, 35, 790-798. Pengfei, Q. F.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, O.; Dai, H.J.; Peng, S.; Cho, K. J. Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection, Nano Lett. 2003, 3, 347-351. Gao, X. P. A.; Zheng, G. F.; Lieber, C. M. Subthreshold Regime Has the Optimal Sensitivity for Nanowire FET Biosensors, Nano Lett. 2010, 10, 547-552. Hahm, J.; Lieber, C. M. Direct Ultrasensitive Electrical Detection of DNA and DNA Sequence Variations Using Nanowire Nanosensors, Nano Lett. 2004, 4, 51-54. Cui, Y; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species, Science 2001, 293, 1289-1292. Zheng, G. F.; Patolsky, F.; Cui, Y; Wang, W. U.; Lieber, C. M. Multiplexed Electrical Detection of Cancer Markers with Nanowire Sensor Arrays, Nat. Biotechnol. 2005, 23, 1294-1301. Ganzhom, M.; Vijayaraghavan, A.; Dehm, S.; Hennrich, F.; Green, A. A.; Fichtner, M.; Voigt, A.; Rapp, M.;von Lohneysen, H.; Hersam, M. C. et al. Hydrogen Sensing with Diameter- and Chirality Sorted Carbon Nanotubes,ACS Nano 2011, 5, 1670-1676 Zhang, D. H.; Liu, Z. Q.; Li, C.; Tang, T.; Liu, X. L.; Han, S.; Lei, B.; Zhou, C. W. Detection of NO, down to ppb Levels Using Individual and Multiple In,O, Nanowire Devices, Nano Lett. 2004, 4, 1919-1924. Wei, T.-Y; Yeh, P.-H.; Lu, S.-Y; Wang, Z. L. Gigantic Enhancement in Sensitivity Using Schottky Contacted Nanowire Nanosensor, J. Am. Chem. Soc. 2009, 131, 17690-17695. Yeh, P. H.; Li, Z.; Wang, Z. L. Schottky-Gated Probe-Free ZnO Nanowire Biosensor, Adv. Mater. 2009, 21, 4975-4978. Kohnakov, A.; Klenov, D. O.; Lilach, Y; Stemmer, S.; Moskovits, M. Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles, Nano Lett. 2005, 5, 667--673. 118 Zou, X. M.; Wang, J. L.; Liu, X. Q.; Wang, C. L.; Jiang, Y; Wang, Y; Xiao, X. H.; Ho, J.C.; Li, J. C.; Jiang, C. Z.et al. Rational Design of Sub-Parts Per Million Specific Gas Sensors Array Based on Metal Nanoparticles Decorated Nanowire Enhancement-Mode Transistors, Nano Lett 2013, 13,3287-3292. Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katsnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Molecular Doping of Graphene, Nano Lett. 2008, 8, 173-177. Jariwala, D.; Sangwan, V. K.; Late, D. J.; Johns, J. E.; Dravid, V. P.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Band-like Transport in High Mobility Unencapsulated Single-Layer MoS2 Transistors, Appl. Phys. Lett. 2013, 102, 173107. Jariwala, D.; Sangwan, V. K.; Wu, C. C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Gate-Tunable Carbon Nanotube-MoS, Heterojunction p-n Diode, Proc. Natl. A cad. Sci. U.S.A. 2013, ll0, 18076-18080. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2, Nat. Nanotechnol. 2013, 8, 497-501. Radisavljevic, B.; Kis, A. Mobility Engineering and a Metal-Insulator Transition in Monolayer MoS2, Nat. Mater. 2013, 12, 815-820. Sangwan, V. K.; Arnold, H. N.; Jariwala, D.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Low Frequency Electronic Noise in Single-Layer MoS, Transistors, Nano Lett. 2013, 13, 4351-4355. Chuang, S.; Battaglia, C.; Azcatl, A.; McDonell, S.; Kang, J. S.; Yin, X.; Tosun, M.; Kapadia, R.; Fang, H.; Wallace, R. M.et al. MoS2 p-Type Transistors and Diodes Enabled by High Work Function MoOxContacts, Nano Lett. 2014, 14, 1337- 1342. Walia, S.; Balendhran, S.; Wang, Y C.; Ab Kadir, R.; Zoolfakar, A. S.; Atkin, P.; Ou, J. Z.; Sriram, S.; Kalantar-zadeh, K.; Bhaskaran, M. Characterization of Metal Contacts for Two-Dimensional MoS2 Nanoflakes, Appl. Phys. Lett. 2013, 103, 232105. Late, D. J.; Liu, B.; Matte, H. S. S. R.; Dravid, V. P.; Rao, C. N. R. Hysteresis in Single-Layer MoS2 Field Effect Transistors, ACS Nano 2012, 6, 5635-5641. Das, S.; Chen, H. Y; Penumatcha, A. V.; Appenzeller, J. High Performance Multilayer MoS, Transistors with Scandium Contacts Nano Lett. 2013, 13, 100-105. Popov, I.; Seifert, G.; Tomanek, D. Designing Electrical Contacts to MoS, Monolayers: A 119 Computational Study, Phys. Rev. Lett. 2012, 108, 156802. Lee, G. H.; Yu, Y Y; Cui, X.; Petrone, M.; Lee, C.H.; Choi, M. S.; Lee, D. Y; Lee, C.; Yoo, W. J.; Watanabe, K. et al. Flexible and Transparent MoS, Field-Effect Transistors on Hexagonal Boron Nitride-Graphene Heterostructures, ACS Nano 2013, 7, 7931-7936. Liu, H.; Si, M.; Deng, Y; Neal, A. T.; Du, Y; Najrnaei, S.; Ajayan, P M.; Lou, J.; Ye, P D. D. Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: An Insight into Current Flow across Schottky Barriers,ACS Nano 2014, 8, 1031-1038. He, Q. Y; Zeng, Z. Y; Yin, Z. Y; Li, H.; Wu, S. X.; Huang, X.; Zhang, H. Fabrication of Flexible MoS, Thin-Film Transistor Arrays for Practical Gas-Sensing Applications, Small 2012, 8, 2994- 2999. Li, H.; Yin, Z. Y; He, Q. Y; Li, H.; Huang, X.; Lu, G.; Fam, D. W. H.; Tok, A. I. Y; Zhang, Q.; Zhang, H. Fabrication of Single- and Multilayer MoS, Film-Based Field-Effect Transistors for Sensing NO at Room Temperature, Small 2012, 8, 63-67. Late, D. J.; Huang, Y K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J. J.; Yan, A.; Charles, D.; Waghrnare, U.; Dravid, V. Pet al. Sensing Behavior of Atomically Thin-Layered MoS2 Transistors, ACS Nano 2013, 7, 4879-4891. Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, PM.; Jernigan, G. G.; Jonker, B. T. Chemical Vapor Sensing with Monolayer MoS2, Nano Lett. 2013, 13, 668-673. Sarkar, D.; Liu, W.; Xie, X.; Ansehno, A. C.; Mitragotri, S.; Banerjee, K. MoS, Field-Effect Transistor for Next-Generation Label-Free Biosensors,ACS Nano 2014, DOI: 10.1021/nn5009148. Castellanos-Gomez, A.; Poot, M.; Steele, G. A.; van der Zant, H. S. J.; Agrait, N.; Rubio-Bollinger, G. Elastic Properties of Freely Suspended MoS2 Nanosheets,Adv. Mater. 2012, 24, 772-775. Lee, Y H.; Yu, L. L.; Wang, H.; Fang, W. J.; Ling, X.; Shi, Y M.; Lin, C. T.; Huang, J. K.; Chang, M. T.; Chang, C. S.et al. Synthesis and Transfer of Single-Layer Transition Metal Disulfides on Diverse Surfaces, Nano Lett. 2013, 13, 1852-1857. Liu, K. K.; Zhang, W. J.; Lee, Y H.; Lin, Y C.; Chang, M. T.; Su, C.; Chang, C. S.; Li, H.; Shi, Y M.; Zhang, H.et al. Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates, Nano Lett. 2012, 12, 1538-1544. Shi, Y M.; Zhou, W.; Lu, A. Y; Fang, W. J.; Lee, Y H.; Hsu, A. L.; Kirn, S. M.; Kirn, K. K.; Yang, H. Y;Li, L. let al. van der Waals Epitaxy of MoS, Layers Using Graphene as Growth 120 Templates, Nano Lett. 2012, 12, 2784-2791. Zhan, Y J.; Liu, Z.; Najmaei, S.; Ajayan, P. M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization ofMoS2 Atomic Layers on a SiO2 Substrate, Small 2012, 8, 966-971. Yu, Y F.; Li, C.; Liu, Y; Su, L. Q.; Zhang, Y; Cao, L. Y Controlled Scalable Synthesis ofUniform, High-Quality Monolayer and Few-Layer MoS2 Films, Sci. Rep. 2013, 3, 1866. Lin, Y C.; Zhang, W. J.; Huang, J. K.; Liu, K. K.; Lee, Y H.; Liang, C. T.; Chu, C. W.; Li, L. J. Wafer-Scale MoS2 Thin Layers Prepared by MoO, Sulfurization, Nanoscale 2012, 4, 6637-6641. Wang, H. T.; Lu, Z. Y; Xu, S. C.; Kong, D. S.; Cha, J. J.; Zheng, G. Y; Hsu, P. C.; Yan, K.; Bradshaw, D.;Prinz, F. B.et al. Electrochemical Tuning of Vertically Aligned MoS, Nanofihns and Its Application in Improving Hydrogen Evolution Reaction, Proc. Natl. Acad. Sci. U.S.A. 2013, ll0, 19701-19706. Wu, S. F.; Huang, C. M.; Aivazian, G.; Ross, J. S.; Cobden, D. H.; Xu, X. D. Vapor-Solid Growth of High Optical Quality MoS, Monolayers with Near-Unity Valley Polarization, ACS Nano 2013, 7, 2768-2772. Ling, X.; Lee, Y H.; Lin, Y X.; Fang, W. J.; Yu, L. L.; Dresselhaus, M.; Kong, J. Role of the Seeding Promoter in MoS2 Growth by Chemical Vapor Deposition, Nano Lett. 2014, 14, 464-472. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS,: A New Direct-Gap Semiconductor, Phys. Rev. Lett. 2010, 105, 136805. Wang, X. S.; Feng, H. B.; Wu, Y M.; Jiao, L. Y Controlled Synthesis of Highly Crystalline MoS, Flakes by Chemical Vapor Deposition, J. Am. Chem. Soc. 2013, 135, 5304-5307. Chen, J. R.; Odenthal, P. M.; Swartz, A. G.; Floyd, G. C.; Wen, H.; Luo, K. Y; Kawakami, R. K. Control of Schottky Barriers in Single Layer MoS2 Transistors with Ferromagnetic Contacts, Nano Lett. 2013, 13, 3106-3 ll0. Qiu, H.; Pan, L. J.; Yao, Z. N.; Li, J. J.; Shi, Y; Wang, X. R. Electrical Characterization ofBack Gated Bi-layer MoS, Field-Effect Transistors and the Effect of Ambient on Their Performances, Appl. Phys. Lett.2012, 100, 123104. Ou, J. Z.; Campbell, J. L.; Yao, D.; Wlodarski, W.; Kalantar-zadeh, K. In Situ Raman Spectroscopy ofH2 Gas Interaction withLayeredMoO,,J. Phys. Chem. C2011, ll5, 10757-10763 Balendhran, S.; Deng, J. K.; Ou, J. Z.; Walia, S.; Scott, J.; Tang, J. S.; Wang, K. L.; Field, M. R.; 121 Russo, S.; Zhuiykov, S. et al. Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide, Adv. Mater. 2013, 25, 109-114. Wang, Y C.; Ou, J. Z.; Balendhran, S.; Chrirnes, A. F.; Mortazavi, M.; Yao, D. D.; Field, M. R.; Latham, K.; Bansal, V.; Friend, J. R.et al. Electrochemical Control of Photoluminescence in Two Dimensional MoS, Nanoflakes,ACS Nano 2013, 7, 10083-10093. Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; John Wiley & Sons, Inc.: New York, 2007. Lee, K.; Gatensby, R.; McEvoy, N.; Hallam, T.; Duesberg, G. S. High Performance Sensors Based on Molybdenum Disulfide Thin Films, Adv. Mater. 2013, 25, 6699-6702. Javey, A.; Kong, J. Carbon Nanotube Electronics; Springer Science+Business Media: New York, 2008. Abbas, A.; Liu, G.; Liu, B. L.; Zhang, L.; Liu, H.; Ohlberg, D.; Wu, W.; Zhou, C. W. Patterning, Characterization and Chemical Sensing Applications of Graphene Nanoribbon Arrays down to 5 rnn Using Helium Ion Beam Lithography,ACS Nano 2014, 8, 1538-1546. Gong, C.; Zhang, H. J.; Wang, W. H.; Colombo, L.; Wallace, R. M.; Cho, K. J. Band Aligrnnent of Two-Dimensional Transition Metal Dichalcogenides: Application in Tunnel Field Effect Transistors,App/. Phys. Lett. 2013, 103, 053513. Fontana, M.; Deppe, T.; Boyd, A. K.; Rinzan, M.; Liu, A. Y; Paranjape, M.; Barbara, P Electron Hole Transport and Photovoltaic Effect in Gated MoS, Schottky Junctions, Sci. Rep. 2013, 3, 1634 Nah, J.; Kumar, S. B.; Fang, H.; Chen, Y Z.; Plis, E.; Chueh, Y L.; Krishna, S.; Guo, J.; Javey, A. Quantum Size Effects on the Chemical Sensing Performance of Two-Dimensional Semiconductors J. Phys. Chem. C 2012, 116, 9750-9754. Cui, X. D.; Freitag, M.; Martel, R.; Brus, L.; Avouris, P Controlling Energy-Level Alignments at Carbon Nanotube/ Au Contacts, Nano Lett. 2003, 3, 783-787. Gui, E. L.; Li, L. J.; Zhang, K. K.; Xu, Y P; Dong, X. C.; Ho, X. N.; Lee, P S.; Kasim, J.; Shen, Z. X.;Rogers, J. A.et al. DNA Sensing by Field-Effect Transistors Based on Networks of Carbon Nanotubes J. Am. Chem. Soc. 2007, 129, 14427-14432. Kuc, A.; Zibouche, N.; Heine, T. Influence of Quantum Confmernent on the Electronic Structure of the Transition Metal Sulfide TS,. Phys. Rev. B 2011, 83, 245213. Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS, by 122 Optical Helicity. Nat. Nanotechnol. 2012, 7, 494-498. Zeng, H.; Dai, J.; Yao, W.; Xiao, D.; Cui, X. Valley Polarization in MoS, Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490-493. Pu, J.; Yomogida, Y; Liu, K. K.; Li, L. J.; Iwasa, Y; Takenobu, T. Highly Flexible MoS, Thin-film Transistors with Ion Gel Dielectrics. Nano Lett. 2012, 12, 4013-4017. Chang, H.-Y; Yang, S.; Lee, J.; Tao, L.; Hwang, W.-S.; Jena, D.; Lu, N.; Akinwande, D. High Performance, Highly Bendable MoS, Transistors with High-K Dielectrics for Flexible Low-Power Systems. ACS Nano 2013, 7, 5446-5452. Tosun, M.; Chuang, S.; Fang, H.; Sachid, A. B.; Hettick, M.; Lin, Y; Zeng, Y; Javey, A. High-Gain Inverters Based on WSe, Complementary Field-Effect Transistors. ACS Nano 2014, 8, 4948-4953. Wang, H.; Yu, L.; Lee, Y H.; Shi, Y; Hsu, A.; Chin, M. L.; Li, L. J.; Dubey, M.; Kong, J.; Palacios, T. Integrated Circuits Based on Bilayer MoS, Transistors. Nano Lett. 2012, 12, 4674-4680. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS,. Nat. Nanotechnol. 2013, 8, 497-501. Chang, K.; Chen, W. L-Cysteine-Assisted Synthesis of Layered MoS,/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 5, 4720-4728. Hinnemann, B.; Moses, P. G.; Bonde, J.; forgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; N0rskov, J. K. Biomimetic Hydrogen Evolution: MoS, Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. Wang, X.; Gong, Y; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G.; Vajtai, R.; Lou, J.; Liu, Z.; Ringe, E.; et al. Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe,. ACS Nano 2014, 8, 5125-5131. Zhang, Y; Zhang, Y; Ji, Q.; Ju, J.; Yuan, H.; Shi, J.; Gao, T.; Ma, D.; Liu, M.; Chen, Y; et al. Controlled Growth of High-Quality Monolayer WS, Layers on Sapphire and Imaging Its Grain Boundary. ACS Nano 2013, 7, 8963-8971. Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. Two-dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571. 123 Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsoe, H.; Clausen, B. S.; Laegsgaard, E.; Besenbacher, F. Size-dependent Structure ofMoS, Nanocrystals. Nat. Nanotechnol. 2007, 2, 53-58. Zhu, W.; Low, T.; Lee, Y H.; Wang, H.; Farmer, D. B.; Kong, J.; Xia, F.; Avouris, P Electronic Transport and Device Prospects of Monolayer Molybdenum Disulphide Grown by Chemical Vapour Deposition. Nat. Commun. 2014, 5, 3087. Duerloo, K. A.; Li, Y; Reed, E. J. Structural Phase Transitions in Two-dimensional Mo- and W dichalcogenide Monolayers. Nat. Commun. 2014, 5, 4214. Xu, K.; Wang, Z.; Du, X.; Safdar, M.; Jiang, C.; He, J. Atomic-layer Triangular WSe2 Sheets: Synthesis and Layer-dependent Photoluminescence Property. Nanotechnology 2013, 24, 465705. Lin, Y-C.; Lu, N.; Perea-Lopez, N.; Lin, J. L. Z.; Peng, X.; Lee, C. H.; Sun, C.; Calderin, L.; Browning, P N.; Bresnehan, M. S.; et al. Direct Synthesis of van der Waals Solids.ACS Nano 2014, 8, 3715-3723. Meng, F.; Jin, S. The Solution Growth of Copper Nanowires and Nanotubes Is Driven by Screw Dislocations. Nano Lett. 2012, 12, 234-239. Morin, S. A.; Bierman, M. J.; Tong, J.; Jin, S. Mechanism and Kinetics of Spontaneous Nanotube Growth Driven by Screw Dislocations. Science 2010, 328, 476-480. Morin, S. A.; Jin, S. Screw Dislocation-driven Epitaxial Solution Growth of ZnO Nanowires Seeded by Dislocations in GaN Substrates. Nano Lett. 2010, 10, 3459-3463. Zhu, J.; Peng, H.; Marshall, A. F.; Barnett, D. M.; Nix, W. D.; Cui, Y Formation of Chiral Branched Nanowires by the Eshelby Twist. Nat. Nanotechnol. 2008, 3, 477-481. Bierman, M. J.; Lau, Y K.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Dislocation-driven Nanowire Growth and Eshelby Twist. Science 2008, 320, 1060-1063. Morin, S. A.; Forticaux, A.; Bierman, M. J.; Jin, S. Screw Dislocation-driven Growth of Two dimensional Nanoplates. Nano Lett. 2011, 11, 4449-4455. Zhuang, A.; Li, J. J.; Wang, Y C.; Wen, X.; Lin, Y; Xiang, B.; Wang, X.; Zeng, J. Screw dislocation-driven Bidirectional Spiral Growth of Bi2S"3 Nanoplates. Angew. Chem. 2014, 53, 6425-6429. Luo, X.; Zhao, Y; Zhang, J.; Toh, M.; Kloc, C.; Xiong, Q.; Quek, S. Y Effects of Lower Symmetry and Dimensionality on Raman Spectra in Two-dimensional WSe,. Phys. Rev. B 2013, 88, 195313. 124 Tonndorf, P.; Schmidt, R.; Bottger, P.; Zhang, X.; Bomer, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D.R.; et al. Photoluminescence Emission and Raman Response ofMonolayer MoS2, M0Se2, and WSe,. Opt. Express 2013, 21, 4908-4916. Sahin, H.; Tongay, S.; Horzum, S.; Fan, W.; Zhou, J.; Li, J.; Wu, J.; Peeters, F. Anomalous Raman Spectra and Thickness-dependent Electronic Properties ofWSe,. Phys. Rev. B 2013, 87, 165409. Liu, B.; Ren, W.; Gao, L.; Li, S.; Pei, S.; Liu, C.; Jiang, C.; Cheng, H.M. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 2082-2083. Li, H.; Duan, X.; Wu, X.; Zhuang, X.; Zhou, H.; Zhang, Q.; Zhu, X.; Hu, W.; Ren, P.; Guo, P.; et al. Growth of Alloy MoS2xSe2c1-x) Nanosheets with Fully Tunable Chemical Compositions and Optical Properties. J. Am. Chem. Soc. 2014, 136, 3756-3759. Mann, J.; Ma, Q.; Odenthal, P. M.; Isarraraz, M.; Le, D.; Preciado, E.; Barroso, D.; Yamaguchi, K.; von Son Palacio, G.; Nguyen, A.; et al. 2-dimensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2c1-x) Se2x Monolayers. Adv. Mater. 2014, 26, 1399-1404. Gong, Y; Liu, Z.; Lupini, A. R.; Shi, G.; Lin, J.; Najmaei, S.; Lin, Z.; Elias, A. L.; Berkdemir, A.; You, G.; et al. Band Gap Engineering and Layer-by-Layer Mapping of Selenium-Doped Molybdenum Disulfide. Nano lett. 2014, 14, 442-449. Chen, Y; Xi. J.; Dumcenco, D. O.; Liu, Z.; Suenaga, K.; Wang, D.; Shuai, Z.; Huang, Y; Xie, L. Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys. ACS Nano 2013, 7, 4610-4616. Jin, S.; Bierman, M. J.; Morin, S. A. A New Twist on Nanowire Formation: Screw-Dislocation Driven Growth ofNanowires and Nanotubes. J. Phys. Chem. Lett. 2010, 1, 1472-1480. Burton, W. K.; Cabrera, N.; Frank, F. C. The Growth of Crystals and the Equilibrium Structure of their Surfaces. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 1951, 243, 299-358. Chuang, S.; Battaglia, C.; Azcatl, A.; McDonnell, S.; Kang, J. S.; Yin, X.; Tosun, M.; Kapadia, R.; Fang, H.; Wallace, R. M.; et al. MoS, P-type transistors and diodes enabled by high work function MoOx contacts. Nano Lett. 2014, 14, 1337-1342. Allain, A.; Kis, A. Electron and Hole Mobilities in Single-Layer WSe,. ACS Nano 2014, 8, 7180- 7185. 125 Wu, W.; Wang, L.; Li, Y; Zhang, F.; Lin, L.; Niu, S.; Chenet, D.; Zhang, X.; Hao, Y; Heinz,T. F.; et al. Piezoelectricity of Single-Atomic-Layer MoS2 for Energy Conversion and Piezotronics. Nature 2014, 514, 470-474. Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X. F.; Tomanek, D.; Ye, P. D. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033-4041. Xia, F. N.; Wang, H.; Jia, Y C. Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 2014, 5, 4458. Chang, H. Y; Yang, S. X.; Lee, J. H.; Tao, L.; Hwang, W. S.; Jena, D.; Lu, N. S.; Akinwande, D. High-Performance, Highly Bendable MoS, Transistors with High-K Dielectrics for Flexible Low Power Systems. ACS Nano 2013, 7, 5446-5452. Desai, S. B.; Seo!, G.; Kang, J. S.; Fang, H.; Battaglia, C.; Kapadia, R.; Ager, J. W.; Guo, J.; Javey, A. Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe,. Nano Lett. 2014, 14, 4592-4597. Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y; Sun, Y; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single Layer MoS, Phototransistors. ACS Nano 2012, 6, 74-80. Mak, K. F.; He, K. L.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nat. Nanotechnol. 2012, 7, 494-498. Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; et al. Enhanced Catalytic Activity in Strained Chemically Exfoliated WS, Nanosheets for Hydrogen Evolution. Nat. Mater. 2013, 12, 850-855. Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; et al. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898-2926. Li, H.; Lu, G.; Wang, Y; Yin, Z.; Cong, C.; He, Q.; Wang, L.; Ding, F.; Yu, T.; Zhang, H. Mechanical Exfoliation and Characterization of Single- and Few-Layer Nanosheets ofWSe,, TaS,, and TaSe,. Small 2013, 9, 1974-1981. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proceedings of the National Academy of Sciences of the United States of America 2005, 102, 10451-10453. 126 Green, A. A.; Hersarn, M. C., Solution Phase Production of Graphene with Controlled Thickness Via Density Differentiation. Nano Lett. 2009, 9, 4031-4036. Zhou, H. L.; Wang, C.; Shaw, J.C.; Cheng, R.; Chen, Y; Huang, X.; Liu, Y; Weiss, N. O.; Lin, Z.; Huang, Y; et al. Large Area Growth and Electrical Properties of P-Type WSe2 Atomic Layers. Nano Lett. 2015, 1 5, 709-713. Wang, X.; Gong, Y; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G.; Vajtai, R.; Lou, J.; Liu, Z.; Ringe, E.; et al. Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe,. ACS Nano 2014, 8, 5125-5131. Zhang, Y; Ji, Q.; Han, G.; Ju, J.; Shi, J.; Ma, D.; Sun, J.; Zhang, Y; Li, M.; Lang, X.; et al. Dendritic, Transferable, Strictly Monolayer MoS, Flakes Synthesized on SrTiO, Single Crystals for Efficient Electrocatalytic Applications. A CS Nano 2014, 8, 8617-8624. Chen, Y Z.; Medina, H.; Su, T. Y; Li, J. G.; Cheng, K. Y; Chiu, PW.; Chueh, Y L. Ultrafast and Low Temperature Synthesis of Highly Crystalline and Pattemable Few-Layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process. ACS Nano 2015, 9, 4346-4353. Ma, Y; Liu, B.; Zhang, A.; Chen, L.; Fathi, M.; Shen, C.; Abbas, A.; Ge, M.; Mecklenburg, M.; Zhou, C. W. Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices. ACS Nano 2015, DOI: 10.1021/ acsnano.5b02399. Clark, G.; Wu, S. F.; Rivera, P; Finney, J.; Nguyen, P; Cobden, D. H.; Xu, X. D. Vapor-Transport Growth of High Optical Quality WSe, Monolayers. APL Mater. 2014, 2. Cong, C. X.; Shang, J. Z.; Wu, X.; Cao, B. C.; Peirnyoo, N.; Qiu, C.; Sun, L. T.; Yu, T. Synthesis and Optical Properties of Large-Area Single-Crystalline 2D Semiconductor WS, Mono layer from Chemical Vapor Deposition. Adv. Opt. Mater. 2014, 2, 131-136. Lee, J. H.; Lee, E. K.; Joo, W. J.; Jang, Y; Kirn, B. S.; Lim, J. Y; Choi, S. H.; Ahn, S. J.; Ahn, J. R.; Park, M. H.; et al. Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium. Science 2014, 344, 286-289. Han, G. H.; Kybert, N. J.; Naylor, C.H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y; Lee, Y H.; Agarwal, R.; et al. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. Durncenco, D.; Ovchinnikov, D.; Marinov, K.; Lopez-Sanchez, O.; Krasnozhon, D.; Chen, M.; Gillet, P; Morral, A. F. I; Radenovic, A.; Kis, A.; et al. Large-Area Epitaxial Monolayer MoS,. 127 ACS Nano 2015, 9, 4611-4620. Isrnach, A.; Segev, L.; Wachtel, E.; Joselevich, E. Atornic-Step-Ternplated Formation of Single Wall Carbon Nanotube Patterns. Angew. Chem. Int. Ed. 2004, 43, 6140-6143. Han, S.; Liu, X. L.; Zhou, C. W. Template-Free Directional Growth of Single-Walled Carbon Nanotubes on a- and R-Plane Sapphire. J. Am. Chem. Soc. 2005, 127, 5294-5295. Kocabas, C.; Hur, S. H.; Gaur, A.; Meitl, M.A.; Shim, M.; Rogers, J. A. Guided Growth ofLarge Scale, Horizontally Aligned Arrays of Single-Walled Carbon Nanotubes and Their Use in Thin Fihn Transistors. Small 2005, 1, 1110-1116. Tsivion, D.; Schvartzrnan, M.; Popovitz-Biro, R.; von Huth, P.; Joselevich, E. Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations. Science 2011, 333, 1003- 1007. Kumosikov, O.; Van, L. P.; Cousty, J. About Anisotropy of Atomic-Scale Height Step on (0001) Sapphire Surface. Swf Sci. 2000, 459, 256-264. Heffelfinger, J. R.; Bench, M. W.; Carter, C. B. Steps and the Structure of the (0001) Alpha Alumina Surface. Surf Sci. 1997, 370, L168-L172. Yan, K.; Peng, H. L.; Zhou, Y; Li, H.; Liu, Z. F. Formation of Bilayer Bernal Graphene: Layer-by Layer Epitaxy via Chemical Vapor Deposition. Nano Lett. 2011, 11, 1106-1110. Zhang, Y; Zhang, L. Y; Kirn, P.; Ge, M. Y; Li, Z.; Zhou, C. W. Vapor Trapping Growth ofSingle Crystalline Graphene Flowers: Synthesis, Morphology, and Electronic Properties. Nano Lett. 2012, 12, 2810-2816. Saadi, S.; Abild-Pedersen, F.; Helveg, S.; Sehested, J.; Hinnernann, B.; Appel, C. C.; Norskov, J. K. On the Role of Metal Step-Edges in Graphene Growth. J. Phys. Chem. C 2010, 114, 11221- 11227. Geblinger, N.; Isrnach, A.; Joselevich, E. Self-Organized Nanotube Serpentines. Nat. Nanotechnol. 2008, 3, 195-200. Liu, B. L.; Wang, C.; Liu, J.; Che, Y C.; Zhou, C. W. Aligned Carbon Nanotubes: From Controlled Synthesis to Electronic Applications. Nanoscale 2013, 5, 9483-9502. Sornorjai, G. A. Modern Surface Science and Surface Technologies: An Introduction. Chem. Rev. 1996, 96, 1223-1235. 128 Wang, S.; Rong, Y; Fan, Y; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371- 6379. Gao, L.B.; Ren, W. C.; Liu, B. L.; Saito, R.; Wu, Z. S.; Li, S. S.; Jiang, C. B.; Li, F.; Cheng, H. M. Surface and Interference Coenhanced Raman Scattering of Graphene. ACS Nano 2009, 3, 933-939. Chen, L.; Liu, B. L.; Abbas, A. N.; Ma, Y Q.; Fang, X.; Liu, Y H.; Zhou, C. W. Screw-Dislocation Driven Growth of Two-Dimensional Few-Layer and Pyramid-Like WSe, by Sulfur-Assisted Chemical Vapor Deposition. ACS Nano 2014, 8, 11543-11551. Sutter, P W.; Flege, J. I.; Sutter, E. A. Epitaxial Graphene on Ruthenium. Nature Mater. 2008, 7, 406-411. Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Neto, A. H. C., 2D materials and van der Waals heterostructures. Science 2016, 353. Costanzo, D.; Jo, S.; Berger, H.; Morpurgo, A. F., Gate-induced superconductivity in atomically thin MoS, crystals. Nat Nanotechnol 2016, 11, 339-+. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS,: A New Direct-Gap Semiconductor. Phys Rev Lett 2010, 105. Yun, W. S.; Han, S. W.; Hong, S. C.; Kirn, I. G.; Lee, J. D., Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-M X-2 semiconductors (M ~ Mo, W; X ~ S, Se, Te). Phys Rev B 2012, 85. Desai, S. B.; Seo!, G.; Kang, J. S.; Fang, H.; Battaglia, C.; Kapadia, R.; Ager, J. W.; Guo, J.; Javey, A., Strain-Induced Indirect to Direct Bandgap Transition in Multi layer WSe,. Nano Lett 2014, 14, 4592-4597. Tongay, S.; Suh, J.; Ataca, C.; Fan, W.; Luce, A.; Kang, J. S.; Liu, J.; Ko, C.; Raghunathanan, R.; Zhou, J.; Ogletree, F.; Li, J.B.; Grossman, J. C.; Wu, J. Q., Defects activated photoluminescence in two-dimensional semiconductors: interplay between bound, charged, and free excitons. Sci Rep Uk 2013, 3. Wu, S. F.; Ross, J. S.; Liu, G. B.; Aivazian, G.; Jones, A.; Fei, Z. Y; Zhu, W. G.; Xiao, D.; Yao, W.; Cobden, D.; Xu, X. D., Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS,. Nat Phys 2013, 9, 149-153. 129 Jones, A. M.; Yu, H. Y; Ghirnire, N. J.; Wu, S. F.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J. Q.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu, X. D., Optical generation of excitonic valley coherence in rnonolayer WSe,. Nat Nanotechnol 2013, 8, 634-638. Das, S.; Appenzeller, J., WSe2 field effect transistors with enhanced arnbipolar characteristics. Appl Phys Lett 2013, 103. Yuan, H. T.; Bahrarny, M. S.; Morimoto, K.; Wu, S. F.; Nomura, K.; Yang, B. J.; Shirnotani, H.; Suzuki, R.; Toh, M.; Kloc, C.; Xu, X. D.; Arita, R.; Nagaosa, N.; Iwasa, Y, Zeeman-type spin splitting controlled by an electric field. Nat Phys 2013, 9, 563-569. Yuan, H. T.; Wang, X. Q.; Lian, B.; Zhang, H.J.; Fang, X. F.; Shen, B.; Xu, G.; Xu, Y; Zhang, S. C.; Hwang, H. Y; Cui, Y, Generation and electric control of spin-valley-coupled circular photogalvanic current in WSe,. Nat Nanotechnol 2014, 9, 851-857. Das, S.; Gulotty, R.; Surnant, A. V.; Roelofs, A., All Two-Dimensional, Flexible, Transparent, and Thinnest Thin Film Transistor. Nano Lett 2014, 14, 2861-2866. Pu, J.; Funahashi, K.; Chen, C. H.; Li, M. Y; Li, L. J.; Takenobu, T., Highly Flexible and High Performance Complementary Inverters of Large-Area Transition Metal Dichalcogenide Monolayers. Adv Mater 2016, 28, 4111-4119. Park, J. U.; Hardy, M.; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. K.; Lee, C. Y; Strano, M. S.; Alleyne, A. G.; Georgiadis, J. G.; Ferreira, P. M.; Rogers, J. A., High-resolution electrohydrodynarnic jet printing. Nat Mater 2007, 6, 782-789. Cao, X.; Lau, C.; Liu, Y H.; Wu, F. Q.; Gui, H.; Liu, Q. Z.; Ma, Y Q.; Wan, H. C.; Arner, M. R.; Zhou, C. W., Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochrornic Displays Using Carbon Nanotube Thin-Film Transistors. ACS Nano 2016, 10, 9816-9822. Cai, L.; Zhang, S. M.; Miao, J. S.; Yu, Z. B.; Wang, C., Fully Printed Foldable Integrated Logic Gates with Tunable Performance Using Semiconducting Carbon Nanotubes. Adv FunctMater 2015, 25, 5698-5705. Chen, P. C.; Fu, Y; Arninirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.; Zhou, C. W., Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and Its Application in Organic Light Emitting Diode Control. Nano Lett 2011, 11, 5301-5308. Ha, M. J.; Seo, J. W. T.; Prabhurnirashi, P. L.; Zhang, W.; Geier, M. L.; Renn, M. J.; Kirn, C. H.; Hersarn, M. C.; Frisbie, C. D., Aerosol Jet Printed, Low Voltage, Electrolyte Gated Carbon Nanotube Ring Oscillators with Sub-5 µs Stage Delays. Nano Lett 2013, 13, 954-960. 130 Lau, P.H.; Takei, K.; Wang, C.; Ju, Y; Kim, J.; Yu, Z. B.; Takahashi, T.; Cho, G.; Javey, A., Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on Flexible Substrates. Nano Lett 2013, 13, 3864-3869. Noh, Y Y; Zhao, N.; Caironi, M.; Sirringhaus, H., Downscaling of self-aligned, all-printed polymer thin-fihn transistors. Nat Nanotechnol 2007, 2, 784-789. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M., Two dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401, 685-688. Yang, J.; Choi, M. K.; Kim, D. H.; Hyeon, T., Designed Assembly and Integration of Colloidal Nanocrystals for Device Applications. Adv Mater 2016, 28, 1176-1207. Yeorn, C.; Chen, K.; Kiriya, D.; Yu, Z. B.; Cho, G.; Javey, A., Large-Area Compliant Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv Mater 2015, 27, 1561. Gu, X.; Zhou, Y; Gu, K.; Kurosawa, T.; Guo, Y; Li, Y; Lin, H.; Schroeder, B. C.; Yan, H.; Molina Lopez, F.; Tassone, C. J.; Wang, C.; Mannsfeld, S. C. B.; Yan, H.; Zhao, D.; Toney, M. F.; Bao, Z., Roll-to-Roll Printed Large-Area All-Polymer Solar Cells with 5% Efficiency Based on a Low Crystallinity Conjugated Polymer Blend Advanced Energy Materials Early View. Advanced Energy Materials 2017, DOI: 10.1002/aenrn.201602742. Xu, W. Y; Zhao, J. W.; Qian, L.; Han, X. Y; Wu, L. Z.; Wu, W. C.; Song, M. S.; Zhou, L.; Su, W. M.; Wang, C.; Nie, S. H.; Cui, Z., Sorting of large-diameter semiconducting carbon nanotube and printed flexible driving circuit for organic light emitting diode (OLED). Nanoscale 2014, 6, 1589- 1595. Carey, B. J.; Ou, J. Z.; Clark, R. M.; Berean, K. J.; Zavabeti, A.; Chesrnan, A. S.; Russo, S. P.; Lau, D. W.; Xu, Z. Q.; Bao, Q.; Kevehei, O.; Gibson, B. C.; Dickey, M. D.; Kaner, R. B.; Daeneke, T.; Kalantar-Zadeh, K., Wafer-scale two-dimensional semiconductors from printed oxide skin ofliquid metals. Nat Commun 2017, 8, 14482. Kirn, T. Y; Arnani, M.; Ahn, G. H.; Song, Y; Javey, A.; Chung, S.; Lee, T., Electrical Properties of Synthesized Large-Area MoS2 Field-Effect Transistors Fabricated with Inkjet-Printed Contacts. ACS Nano 2016, 10, 2819-2826. Li, J. T.; Naiini, M. M.; Vaziri, S.; Lemme, M. C.; Ostling, M., Inkjet Printing ofMoS,. Adv Funct Mater 2014, 24, 6524-6531. 131 Kelly, A.G.; Hallam, T.; Backes, C.; Harvey, A.; Esrnaeily, A. S.; Godwin, I.; Coelho, J.; Nicolosi, V.; Lauth, J.; Kulkarni, A.; Kinge, S.; Siebbeles, L. D.; Dues berg, G. S.; Coleman, J. N., All-printed thin-fihn transistors from networks ofliquid-exfoliated nanosheets. Science 2017, 356, 69-73. McManus, D.; Vranic, S.; Withers, F.; Sanchez-Rornaguera, V.; Macucci, M.; Yang, H. F.; Sorrentino, R.; Parvez, K.; Son, S. K.; Iannaccone, G.; Kostarelos, K.; Fiori, G.; Casiraghi, C., Water-based and biocornpatible 2D crystal inks for all-inkjet-printed heterostructures. Nat Nanotechnol 2017, 12, 343-350. Sele, C. W.; von Werne, T.; Friend, R. H.; Sirringhaus, H., Lithography-free, self-aligned inkjet printing with sub-hundred-nanometer resolution. Adv Mater 2005, 17, 997. Zhao, N.; Chiesa, M.; Sirringhausa, H.; Li, Y N.; Wu, Y L., Self-aligned inkjet printing of highly conducting gold electrodes with subrnicron resolution. J Appl Phys 2007, 101. Cao, X.; Wu, F. Q.; Lau, C.; Liu, Y H.; Liu, Q. Z.; Zhou, C. W., Top-Contact Self-Aligned Printing for High Performance Carbon Nanotube Thin-Film Transistors with Sub-Micron Channel Length. ACS Nano 2017, 11, 2008-2014. Chen, J. Y; Liu, B.; Liu, Y P.; Tang, W.; Nai, C. T.; Li, L. J.; Zheng, J.; Gao, L.B.; Zheng, Y; Shin, H. S.; Jeong, H. Y; Loh, K. P., Chemical Vapor Deposition of Large-Sized Hexagonal WSe2 Crystals on Dielectric Substrates. Adv Mater 2015, 27, 6722. Zhang, Y; Zhang, L. Y; Kirn, P.; Ge, M. Y; Li, Z.; Zhou, C. W., Vapor Trapping Growth ofSingle Crystalline Graphene Flowers: Synthesis, Morphology, and Electronic Properties. Nano Lett 2012, 12, 2810-2816. Xu, Y; Cheng, C.; Du, S. C.; Yang, J. Y; Yu, B.; Luo, J.; Yin, W. Y; Li, E. P.; Dong, S. R.; Ye, P. D.; Duan, X. F., Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions. ACS Nano 2016, 10, 4895-4919. Kirn, I. S.; Sangwan, V. K.; Jariwala, D.; Wood, J. D.; Park, S.; Chen, K. S.; Shi, F. Y; Ruiz-Zepeda, F.; Ponce, A.; Jose-Yacarnan, M.; Dravid, V. P.; Marks, T. J.; Hersarn, M. C.; Lauhon, L. J., Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition DerivedMoS,.ACSNano2014, 8, 10551-10558. Liu, Y; Wu, H.; Cheng, H. C.; Yang, S.; Zhu, E. B.; He, Q. Y; Ding, M. N.; Li, D. H.; Guo, J.; Weiss, N. O.; Huang, Y; Duan, X. F., Toward Barrier Free Contact to Molybdenum Disulfide Using Graphene Electrodes. Nano Lett 2015, 15, 3030-3034. 132 Jang, J.; Dolzhnikov, D. S.; Liu, W. Y; Nam, S.; Shim, M.; Talapin, D. V., Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular "Solders": Approaching Single Crystal Mobility. Nano Lett 2015, 15, 6309-6317. 133

Abstract (if available)

Abstract In this dissertation, I present a series of systematic work on a newly developing research direction called two-dimensional (2D) materials beyond graphene. As it is named, the major focus of this research field is to explore the novel 2D materials other than graphene. Generally speaking, the initial works started around 2010 after several years of the discovery of graphene. And now this field is expanding dramatically. Back to my research work, it covers from fundamental materials synthesis and characterization to advanced device study and applications. Details about each work will be introduced in this thesis. ❧ Chapter 1 is an introduction to the general information of 2D materials including history, definitions, material properties, current focus, etc. This chapter will first discuss the background of 2D materials beyond graphene and the motivation of related research. Then it will introduce some representative materials, especially transition metal dichalcogenides (TMDCs), that are highly involved in current research field along with their fundamental knowledge such as material structures and physical properties. At last, a brief introduction about the preparation method of 2D TMDCs will be presented. ❧ Chapter 2 reports the first use of Schottky-contacted chemical vapor deposition (CVD) grown monolayer MoS₂ as high-performance room temperature chemical sensors. The Schottky-contacted MoS₂ transistors show current changes by 2–3 orders of magnitude upon exposure to very low concentrations of NO₂ and NH₃. Specifically, the MoS₂ sensors show clear detection of NO₂ and NH₃ down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS₂ sensors. This study shows the potential of 2D semiconductors as high-performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors. ❧ Chapter 3 reports an observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe₂ flakes and pyramid-like structures using a sulfur-assisted CVD method. Few-layer and pyramid-like WSe₂ flakes instead of monolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization of WO₃, which is the precursor of tungsten. Clear observations of steps, helical fringes, and herring-bone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe₂. The generation and propagation mechanisms of screw dislocations during the growth of WSe₂ were discussed. Back-gated field-effect transistors were made on these 2D WSe₂ materials, which show on/off current ratios of 10⁶ and mobility up to 44 cm²/V∙s. ❧ In material research area, controlled growth is always desired. In Chapter 4, we report a brand new mechanism, step-edge-guided nucleation and growth, for the aligned growth of 2D WSe₂ by a chemical vapor deposition method using C-plane sapphire as substrates. This mechanism is different to commonly reported epitaxial growth via a substrate-flake interaction. We found that at temperatures above 950 ℃, the growth is strongly guided by the atomic steps on the sapphire surface, which lead to the aligned growth of WSe₂ along the step edges on sapphire substrate. In addition, such atomic steps facilitate a layer-over-layer overlapping process to form few-layer WSe₂ structures, which is different from the classical layer-by-layer mode for thin film growth. This work opens up new ways to achieve oriented growth of 2D WSe₂ and adds fresh knowledge on the growth mechanism of WSe₂ and potentially other 2D TMDCs. ❧ Chapter 5 discusses a “flood-dike” printing technique which is used to produce high-performance TMDC transistors with a sub-micron channel length based on CVD grown monolayer WSe₂ materials. Our printing approach mainly involves three steps: (1) printing of the first electrodes on CVD WSe₂ flakes, (2) functionalization of the first electrodes with a self-assembled monolayer (SAM) covering the entire surface, and (3) printing of the second electrodes close to the first ones. During the third step, the fresh ink-flow will spread to the first electrodes but be stopped by the SAM, which acts as a flood-dike, leaving an ultra-short channel with a sub-micron length. The devices produced using this flood-dike printing technique possess high on-state current densities (>0.2 µA/µm) and high Ion/Ioff ratios (>10⁵), which are superior to other reported values of printed devices on 2D TMDCs.

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Asset Metadata

Creator Chen, Liang (author)

Core Title Synthesis, characterization, and device application of two-dimensional materials beyond graphene

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Electrical Engineering

Publication Date 06/19/2017

Defense Date 06/05/2017

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemical sensing,device physics,Electronics,materials synthesis,nanomaterials,nanotechnology,OAI-PMH Harvest,two-dimensional materials

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Nakano, Aiichiro (committee member), Wu, Wei (committee member)

Creator Email chen83@usc.edu,chenliangxs@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-386543

Unique identifier UC11256026

Identifier etd-ChenLiang-5422.pdf (filename),usctheses-c40-386543 (legacy record id)

Legacy Identifier etd-ChenLiang-5422.pdf

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Document Type Dissertation

Rights Chen, Liang

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA