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In2O3 COVID-19 biosensors and two-dimensional materials: synthesis and applications
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In2O3 COVID-19 biosensors and two-dimensional materials: synthesis and applications
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Content
In2O3 COVID-19 BIOSENSORS AND TWO-DIMENSIONAL
MATERIALS: SYNTHESIS AND APPLICATIONS
by
Mingrui 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
(MATERIALS SCIENCE)
May 2024
Copyright 2024 Mingrui Chen
ii
Dedication
I want to dedicate this thesis to my mentor, parents, friends, and everyone else who offered
me help through the amazing adventure of my Ph.D. career.
Particularly, in memory of my best friend Xin Wan, we spent the best time of our lives
together and I hope you rest in peace relieved from all the pain and sadness unfairly imposed on
you. You will be in my heart forever.
iii
Acknowledgments
I would like to first express my sincere gratitude to my advisor and lifetime mentor Dr.
Chongwu Zhou, for offering me the ticket to the Ph.D. journey in his splendid group. You held my
hand to show me the road of scientific research, and your invaluable guidance and meticulous
care with patience have allowed me to fulfill my potential in the research field with freedom. It
has been an honor to work with you for the past 5 years.
I also want to thank members of my dissertation committee and qualifying committee, Prof.
Han Wang, Prof. Wei Wu, Prof. Aiichiro Nakano, and Prof. Priya Darshan Vashishta. Your
expertise and the rigorous examination of my work are treasured, and your advice has
significantly improved the quality of my dissertation.
I am profoundly grateful for the help from previous and current members of our research
group. Dr. Anyi Zhang, Dr. Qingzhou Liu, Dr. Yihang Liu, Dr. Fanqi Wu, and Dr. Li Zhen, I could
never have built up the momentum for my Ph.D. study so smoothly without your selfless help
and advice. I am also happy to have been working with Zhiyuan Zhao(Dajie), Dingzhou Cui(YeYe),
and Fugu Tian. My friendship with you is a treasure of my whole life.
To my family, your unconditional love and support across the giant Pacific Ocean have been
the driving force for me throughout my entire Ph.D. career. We haven’t met for 5 years due to
the pandemic, I miss you so much and my love for you will never waver.
To my friends, thank you for the brainstorming sessions, the moral support, and the fun
moments shared with you. Kang Di, I would not have finished my biosensor projects without your
professional suggestions, patient help, and countless discussions with you. Bofan Zhao, although
iv
you have a soft belly and are only a little cute, I still love you just like I would love my own son,
hope you don’t work so hard in gym so that I can tease you next time I see you. Yingchao Peng,
although I am jealous of your saintlike personality, I have learned a lot of good qualities from you,
hope you can farm a lot of fortune every Sunday. I value the friendships with Nan Wang, Sizhe
Wen, Zhi Cai, Haonan Wang, Zerui Liu, Chi Xu, Chao Cao, Ruoxi Li, and Yunxiang Wang, thank you
for making my life so colorful and enjoyable.
My dissertation is a collective effort of all those mentioned above, I am truly fortunate to
have had your support.
With heartfelt appreciation,
Mingrui
v
Table of Contents
Dedication..................................................................................................................................... ii
Acknowledgments........................................................................................................................ iii
List of Figures.............................................................................................................................. vii
Abstract........................................................................................................................................ ix
Chapter 1: Gold-vapor-assisted chemical vapor deposition of aligned monolayer WSe2 with
large domain size and fast growth rate.........................................................................................1
1.1 Introduction to aligned WSe2 synthesis...............................................................................1
1.2 Experimental CVD growth of WSe2 and transfer of WSe2 ...................................................3
1.2.1 Gold assisted CVD growth of WSe2 on sapphire .......................................................3
1.2.2 Transfer of WSe2.......................................................................................................4
1.3 Gold assisted CVD growth of WSe2 on sapphire and its characterizations..........................5
1.3.1 characterizations......................................................................................................5
1.3.2 Optical and Raman/PL results...................................................................................5
1.3.3 In-depth study of location dependence and the catalytic mechanism in the
WSe2 growth .....................................................................................................................7
1.3.4 Continuous WSe2 film form by merging of WSe2 flakes..........................................12
1.3.5 TEM study of the synthesized WSe2 .......................................................................14
1.3.6 Electronic device performance...............................................................................15
1.4 Conclusion of the gold assisted align growth of WSe2 research........................................17
Chapter 2: Highly Sensitive, Scalable, and Rapid SARS-CoV-2 Biosensor Based on In2O3
Nanoribbon Transistors and Phosphatase ..................................................................................19
2.1 Introduction to COVID-19 and SARS‑CoV-2 biosensor.......................................................19
2.2 Biosensors based on In2O3 nanoribbon FET devices..........................................................22
2.3 Electronic biosensing realized by enzymatic reaction of phosphatase..............................24
2.4 Detection of SARS-CoV-2 spike protein in PBS and UTM...................................................27
2.5 Detection of spike protein IgG antibody in PBS and human whole blood .........................30
2.6 Conclusion to COVID-19 In2O3 biosensors.........................................................................34
Chapter 3: High-performance and flexible inkjet-printed MoS2 field-effect transistors and
their gas sensing applications .....................................................................................................35
3.1 Introduction to 2D materials ink and inkjet printing technique ........................................35
3.2 Inkjet-printed MoS2 films using MoS2 ink and their characterizations ..............................38
3.2 Electronic properties of the printed MoS2 FETs.................................................................41
vi
3.2.1 Back-gate printed MoS2 FETs..................................................................................41
3.2.2 Ion gel-gated printed MoS2 FETs ............................................................................43
3.3 Flexibility of the printed MoS2 FETs...................................................................................44
3.4 NO2 and NH3 gas sensors using the printed MoS2 FETs.....................................................46
3.5 Methods............................................................................................................................49
3.5.1 Solution-processable MoS2 ink preparation: ..........................................................49
3.5.2 Inkjet printing of MoS2 film and post-treatment of printed MoS2 FETs:.................50
3.5.3 Preparation of ion gel and ion gel gate devices:.....................................................50
3.5.4 Characterization: ....................................................................................................51
3.6 Conclusion of inkjet-printed MoS2 field-effect transistors and their gas sensing
applications.............................................................................................................................51
Chapter 4: Stretchable printed MoS2 field effect transistor and it’s wearable biosensors
applications.................................................................................................................................53
4.1 Introduction to stretchable biosensors based on inkjet-printed 2D materials..................53
4.2 Fabrication of stretchable MoS2 FETs and functionalization process for biosensors.........55
4.2.1 Fabrication of stretchable MoS2 FETs.....................................................................55
4.2.2 Stretchability of MoS2 devices................................................................................56
4.3 Stretchable MoS2 devices for biosensor application .........................................................57
4.3.1 Method to functionalize MoS2 devices...................................................................57
4.3.2 Potential of the stretchable MoS2 devices in biosensor application.......................59
Conclusion...................................................................................................................................62
Bibliography ................................................................................................................................64
Appendix.....................................................................................................................................73
vii
List of Figures
Figure 1.1 Schematic setup of the WSe2 CVD synthesis; optical and Raman/PL
characterizations of the synthesized WSe2 flakes. ........................................................................7
Figure 1.2 WSe2 flakes obtained in the downstream and upstream regions...............................10
Figure 1.3 Optical and Raman/PL characterizations of the continuous monolayer WSe2 film. ...13
Figure 1.4 TEM characterizations of the synthesized WSe2 flakes...............................................15
Figure 1.5 Electrical characteristics of the back-gate FETs fabricated on continuous
monolayer WSe2 film ..................................................................................................................17
Figure 2.1 Schematic structure of SARS-CoV-2; optical, SEM and AFM charaterizations of
In2O3 FETs; electroinc performance of back-gated In2O3 FETs; and schematic illustration of
the electronic biosensing setup for S1 protein detection. ..........................................................24
Figure 2.2 Mechanism of the electronic ELSIA. ...........................................................................27
Figure 2.3 Detection of SARS-CoV-2 spike protein ......................................................................30
Figure 2.4 Detection of SARS-CoV-2 spike protein IgG antibody.................................................33
Figure 3.1 Schematic diagram of the preparation of the MoS2 ink..............................................38
Figure 3.2. Characterizations of exfoliated MoS2 nanoplates and printed MoS2 films................41
Figure 3.3 Electronic properties of the back-gated printed MoS2 FETs. ......................................43
Figure 3.4 Electronic properties of the ion gel-gated printed MoS2 FETs....................................44
Figure 3.5 Flexibility of ion gel-gated printed MoS2 FETs on polyimide substrates.....................46
Figure 3.6 Sensing performance of printed MoS2 FETs to NO2 and NH3......................................49
Figure 4.1 Table of content showing the FET biosensors based on exfoliated MoS2………………... 54
Figure 4.2 Procedure of fabrication of stretchable MoS2 FETs...................................................55
Figure 4.3 Stretchable MoS2 devices with P3 nanotube on PDMS substrate...............................56
Figure 4.4 Stretchability and cyclability of MoS2 devices on SEBS substrate.............................57
Figure 4.5 Schematic of functionalization of MoS2 biosensors, related APTES/glutaraldehyde
chemistry, and evidence of CA-125 antibody functionalization..................................................58
Figure 4.6 Stability of printed MoS2 FETs in 1XPBS .................................................................59
Figure 4.7 ID-VD curves of MoS2 devices before and after incubation of DNA molecules..........61
viii
Figure A1.1 Photograph of the gold-vapor-assisted CVD set up.................................................73
Figure A1.2 Optical micrograph images of the control experimental results using the same
growth parameters but without gold foil after (a) 30 s and (b) 5 min growth times...................74
Figure A1.3 An Optical micrograph image showing the intermediate state of the formation of
continuous monolayer WSe2 film................................................................................................74
Figure A1.4 X-ray photoelectron spectroscopy (XPS) study. (a), (b) are the survey profiles,
Au 4f, respectively.......................................................................................................................75
Figure A1.5 Energy dispersive X-ray spectroscopy (EDX) study, (a) SEM image of a selected
area (b), (c), (d) Element mapping of Se, W and Au, respectively ...............................................75
Figure A1.6 Characterization of Au on the surface of the substrate after 48 hours. ..................76
Figure A1.7 An optical micrograph image showing electrodes fabricated on continuous
WSe2 using photolithography .....................................................................................................76
Figure A2.1 Photograph of the liquid gate measurement setup. ...............................................77
Figure A2.2 Drain current versus back gate voltage of nine randomly picked In2O3
nanoribbon FET devices. .............................................................................................................77
Figure A2.3 Mobilities of the nine devices labeled in figure 2.1b...............................................78
Figure A2.4. Family curves of IDS−VDS measured in 0.01× phosphate-buffered saline with the
liquid gate. ..................................................................................................................................78
Figure A2.5 Baseline of our electrical measurement, with changed volume of MgCl2 and
NaOH solution, pH=9.7 ...............................................................................................................79
Figure A2.6 Current vs time curve for S1 antigen detection. Current increased after adding
substrate solution, and it dropped after bubbling to mix the solution thoroughly.....................79
Figure A3.1 AFM images of multiple MoS2 nanoplates and channel region of MoS2 FETs.........80
Figure A3.2 Optical images of a printed single drop of MoS2 inks with (a) 90% IPA + 10% 2-
butanol binary solvent and (b) pure IPA solvent. ........................................................................80
Figure A3.3 SEM image and EDS result of the continuous MoS2 film.........................................81
Figure A3.4 Raman spectra corresponding to the Raman mapping in Figure 3.2d.....................81
Figure A3.5 (a,b)Double sweep transfer (ID-VG) characteristics in both linear and logarithm
scale of back-gated and ion gel-gated MoS2 FETs. (c) Output (ID-VD) characteristics of a backgated MoS2 FET in the saturation region.....................................................................................82
Figure A3.6 An optical image of the channel region of the interdigitated inkjet-printed MoS2
FET, which is used as NO2 and NH3 gas sensors ..........................................................................82
ix
Abstract
Orientation-controlled growth of two-dimensional (2D) transition metal
dichalcogenides (TMDCs) may enable many new electronic and optical applications. However,
previous studies reporting aligned growth of WSe2 usually yielded very small domain sizes.
Herein, we introduced gold vapor into the chemical vapor deposition (CVD) process as a
catalyst to assist the growth of WSe2 and successfully achieved highly aligned monolayer WSe2
triangular flakes grown on c-plane sapphire with large domain sizes (130 μm) and fast growth
rate (4.3 μm·s−1). When the aligned WSe2 domains merged together, a continuous monolayer
WSe2 was formed with good uniformity. After transferring to Si/SiO2 substrates, field effect
transistors were fabricated on the continuous monolayer WSe2, and an average mobility of 12
cm2·V−1·s−1 was achieved, demonstrating the good quality of the material. This report paves
the way to study the effect of catalytic metal vapor in the CVD process of TMDCs and
contributes a novel approach to realize the growth of aligned TMDC flakes. In Chapter 1, the
aligned growth of WSe2 will be introduced.
Developing convenient and accurate SARS-CoV-2 antigen test and serology test is
crucial in curbing the global COVID-19 pandemic. In this work, we report an improved Indium
Oxide (In2O3) nanoribbon field-effect transistor (FET) biosensor platform detecting both
SARS-CoV-2 antigen and antibody. Our FET biosensors, which were fabricated using a scalable
and cost-efficient lithography-free process utilizing shadow masks, consist of an Indium Oxide
(In2O3) channel and a newly developed stable enzyme reporter. During the biosensing process,
the phosphatase enzymatic reaction generated pH change of the solution, which was then
detected and converted to electrical signal by our In2O3 FETs. The biosensors applied
x
phosphatase as enzyme reporter, which has a much better stability than the widely used
urease in FET based biosensors. As proof-of-principle studies, we demonstrate the detection
of SARS-CoV-2 spike protein in both phosphate-buffered saline (PBS) buffer and universal
transport medium (UTM) (limit of detection [LoD]: 100 fg/mL). Following the SARS-CoV-2
antigen tests, we developed and characterized additional sensors aimed at SARS-CoV-2 IgG
antibodies, which is important to trace past infection and vaccination. Our spike protein IgG
antibody tests exhibit excellent detection limits in both PBS and human whole blood (LoD: 1
pg/mL). Our biosensors display similar detection performance in different mediums,
demonstrating that our biosensor approach is not limited by Debye screening from salts and
can selectively detect biomarkers in physiological fluids. The newly selected enzyme for our
platform performs much better performance and longer shelf life which will lead our biosensor
platform to be capable for real clinical diagnosis usage. In Chapter 2, the SARS-CoV-2
biosensors based on In2O3 FETs will be discussed.
Owing to the simplicity, scalability, and cost-efficiency, solution-processable twodimensional (2D) semiconductors have attracted great interest in electronic applications,
especially as the channel material for field-effect transistors (FETs). Inkjet printing is a
lithography-free technique to achieve drop-on-demand patterning of solution-processable 2D ink.
However, thus far, inkjet-printed 2D FETs exhibit limited performance due to the coffee-ring
effect and consequent discontinuity of the printed 2D material films. Here, we report highperformance and flexible inkjet-printed MoS2 FETs with high mobilities and high on/off ratios,
and their gas sensing applications. By preparing high-quality MoS2 ink comprised of MoS2
nanoplates using electrochemical exfoliation and then applying a binary solvent comprised of 2-
xi
butanol and isopropanol, the obtained ink was printed to form a continuous and relatively
uniform MoS2 film, and high-performance printed MoS2 FETs were demonstrated, with mobilities
of 11 cm2V-1S-1 and on/off ratios of 106. Furthermore, low voltage gate modulation was achieved
by applying an ion gel gate, and robust electrical performance under tensile strain was observed
for the ion gel-gated MoS2 FETs printed on flexible substrates. As the printed MoS2 film is
abundant in edge sites and sulfur vacancies, we further demonstrated our MoS2 FETs as highperformance gas sensors with a limit of detection (LoD) of 10 ppb for NO2 and 0.5 ppm for NH3,
together with a fast recovery rate. Chapter 3 of this dissertation will focus on the preparation of
MoS2 ink and printed MoS2 FETs.
Due to the nature of chemical bonds in MoS2, MoS2 is not a stretchable material.
Therefore, although flexible MoS2 devices have been demonstrated in many works, very few
papers about stretchable MoS2 devices were published so far. However, for our printed MoS2
devices, the interlayer sliding between MoS2 nanoflake can effectively accommodate the strain
applied to the device, making them a promising candidate to meet the challenge of stretchable
MoS2 devices. On the other hand, field effect transistors based on 2D materials are potential
candidates for sensing applications because any perturbation of such atomically thin 2D channels
can significantly affect their electronic properties. Besides, our inkjet-printed MoS2 films, with
abundant edge sites and sulfur vacancies as active sites for sensing, also exhibit great potential
in biosensing field. Combining the stretchability and advantage for biosensors, we will
demonstrate the potential of our printed MoS2 devices in wearable electronic biosensors.
Chapter 4 will include the introduction of stretchable MoS2 biosensors and some preliminary data
we got so far.
1
Chapter 1: Gold-vapor-assisted chemical vapor deposition of aligned
monolayer WSe2 with large domain size and fast growth rate
1.1 Introduction to aligned WSe2 synthesis
Two-dimensional (2D) layered materials beyond graphene, especially transition metal
dichalcogenides (TMDCs), have attracted a lot of attention in recent years due to their intriguing
properties 1, 2. For example, tungsten diselenide (WSe2), which is one of the well-studied species
among the large family of TMDCs, has been demonstrated to be a promising candidate for various
applications including field-effect transistors 3–6
, sensors 7, 8
, and printed and flexible electronics
9–12. Compared with the mechanical exfoliation method, chemical vapor deposition (CVD) is a
better solution to achieve large-scale, good-uniformity, and high-yield TMDCs 13–16.
The orientation of TMDCs synthesized by the traditional CVD method is usually random with
small size 13, 17–28. When the growth conditions are precisely controlled, TMDC flakes can merge
together to form a continuous film 17, 29. If the orientation of the TMDC flakes is random, the grain
boundaries between two flakes can significantly degrade the electronic properties of the
obtained film, and thus the wafer-scale electronic applications of TMDCs could be greatly limited.
However, when flakes in the same orientation merge together, seamless coalescence may lead
to single-crystalline films 29, 30. Therefore, TMDC flakes grown in the same orientation are highly
preferred. In addition, aligned growth of TMDCs with large domain size is highly desired, because
large flake size can reduce the formation of grain boundaries in subsequent coalescence.
In order to achieve aligned growth of TMDCs, the selection of growth substrate is very
important. Since thermally grown SiO2 is amorphous, it is not suitable for aligned growth of
2
TMDCs. As a result, many researchers have explored various substrates such as BN 18, GaN 19, 20,
mica 21, graphene 22–24, and graphite 25. The deposited TMDC domains can be aligned by the
substrate lattice or step edges. Recently, c-plane sapphire has been widely studied as a substrate
to grow aligned TMDC flakes. Dumcenco et al. achieved aligned growth of MoS2 on sapphire
substrates and obtained continuous films by coalescence of MoS2 flakes 26, which was a
significant accomplishment. However, in their work, only ~ 91.5% of the MoS2 flakes are well
aligned, and the relatively small flake size (~ 20 μm) and slow growth rate (~ 0.04 μm·s−1) are
still limitations of their MoS2 growth. Furthermore, it is known that WSe2 is more challenging to
synthesize than MoS2, because selenium precursor is less reactive than sulfur. In addition, WO3
precursor is more difficult to sublimate than MoO3 (boiling points of WO3 and MoO3 are 1,700
and 1,155 ℃, respectively), and therefore it is more difficult to control the tungsten supply than
the molybdenum supply during the CVD process. Previously, we found that the WSe2 flakes with
a trapezoid shape can nucleate along the sapphire step edges and grow in an aligned fashion
when a high-temperature treatment was applied for the c-plane sapphire substrates, but the
typical grain size was less than 10 μm 27. It was reported that atomic step-terrace structures can
be formed on the surface of c-plane sapphire after high temperature (> 950 °C) annealing due to
high-temperature-triggered surface reconstruction 31, 32. Besides, some other researchers also
demonstrated that aligned growth of TMDC flakes in triangle shapes is also possible using
traditional CVD or metalorganic CVD on sapphire without the high temperature (> 950 °C)
treatment, owing to the lattice match between c-plane sapphire and TMDCs. Huang et al.
reported aligned growth of WSe2 on sapphire and the average domain size for the aligned WSe2
3
was about 5 μm 13. Eichfeld et al. 23 and Zhang et al. 17 used metal-organic CVD to achieve aligned
WSe2 on sapphire, but their domain sizes were less than 2 μm and the flakes were not strictly
monolayer. In the work mentioned above, large randomly oriented WSe2 flakes up to 100 μm can
be synthesized;however, the reported flake size is much smaller when the aligned growth was
achieved. There are still several problems with the state-of-art TMDC aligned growth techniques:
(1) the domain size is usually very small (< 10 μm) due to high nucleation density; (2) the thickness
is not uniform in some reports, which would limit the electronic applications because TMDC
flakes with different thickness have different bandgaps; (3) the growth rate is usually low (< 2
μm·min−1).
By introducing Au vapor in WSe2 CVD and using c-plane sapphire as substrates, we have
successfully achieved highly aligned monolayer WSe2 with large domain size (~ 100 μm), uniform
thickness (monolayer), and fast growth rate (~ 200 μm·min−1). At the edge of the sapphire
substrate, aligned WSe2 can also merge together to form a continuous monolayer WSe2 film. The
high quality and good uniformity of the WSe2 film were confirmed using Raman spectroscopy,
photoluminescence (PL), and transmission electron microscopy (TEM) characterizations. In
addition, field-effect transistors (FETs) fabricated on continuous monolayer WSe2 showed good
performance, indicating the potential for practical applications.
1.2 Experimental CVD growth of WSe2 and transfer of WSe2
1.2.1 Gold assisted CVD growth of WSe2 on sapphire
The gold-vapor-assisted CVD process was performed in a horizontal quartz tube. The system
set up is schematically illustrated in Fig. 1.1(a), and a photograph of the setup before the CVD
process is shown in Figure A1.1 . In a typical CVD process, a piece of Au foil with 0.1 g WO3 was
4
placed in a quartz boat (Boat A) and a piece of c-plane sapphire (~2 cm × 1 cm in size) was placed
on top of the WO3 powder with the polished side facing WO3, the position of WO3 powder is right
below the upstream edge of the substrate. The vertical distance between the sapphire substrate
and the gold foil is 3 mm. The whole quartz boat was put in the center of the tube furnace, where
the highest temperature could be achieved. Another quartz boat (Boat B) with 1 g Se powders
was placed upstream outside the heating zone of the furnace initially, with a stainless-steel rod
used to control the position of the quartz boat during the heating process. The whole system was
first evacuated to 450 Torr with a vacuum pump, and then 100 sccm Ar was introduced to the
system as the carrier gas. Later on, the center of the furnace was gradually heated from room
temperature to 900 °C in 15 min and maintained at the temperature for another 2 min to ensure
sufficient amount of Au vapor was present in the chamber and the substrate was preheated to
the desired temperature. It’s worth mentioning that there was still no Se vapor in the chamber
at this time since Boat B with Se powder was still outside the heating zone. Then a magnet was
used to control the stainless-steel rod from outside the quartz tube and to push Boat B to the
region with a temperature about 250 °C inside the heat zone. At the same time, 4 sccm H2 was
introduced into the chamber to reduce the WO3 to WO3−x. In this way, the chamber was filled by
Se vapor and WO3−x vapor to start the deposition of WSe2. After 30 s growth time, the reaction
was terminated by moving both boats quickly to the low-temperature zone. Using this method,
we are able to synthesize aligned monolayer WSe2 with large domain size (> 100 μm) and fast
growth rate (~ 200 μm·min−1).
1.2.2 Transfer of WSe2
To fabricate field-effect transistors on as-grown WSe2 and perform TEM characterization,
5
a modified transfer process was developed. First, the sapphire substrate with grown WSe2 was
spin-coated (3,500 rpm, 60 s, baking at 90 ℃ for 15min) with polystyrene (PS) solution, which
was prepared by dissolving 0.9 g PS (280,000 g·mol−1) in 10 mL toluene. Then, we scratched the
PS film from the edge and immersed the substrate in hot water (80 ℃) for 20 min. Water could
penetrate between the PS layer and sapphire, delaminating the PS film together with WSe2 from
the substrate. The WSe2-PS assembly was picked up by a target substrate. Subsequently, the
substrate was baked at 80 ℃ for 1 h and 150 ℃ for 30 min to remove any water residue and
eliminate possible wrinkles. Finally, the PS layer was removed by rinsing with toluene.
1.3 Gold assisted CVD growth of WSe2 on sapphire and its characterizations
1.3.1 characterizations
Raman and PL spectra were measured using a Renishaw Raman system, with 532 nm laser.
The TEM characterization was conducted on a JEOL JEM-2100F at 200 kV. Back-gated WSe2 fieldeffect transistors were fabricated on Si/SiO2 substrates with 285 nm SiO2 following standard
photolithography, development and e-beam evaporation of Ti/Au (1 nm/50 nm) source/drain
electrodes. An Agilent 4156B Semiconductor Parameter Analyzer was used to measure device
performance.
1.3.2 Optical and Raman/PL results
A typical optical microscope (OM) image of WSe2 flakes grown using the gold-vapor-assisted
CVD approach is shown in Fig. 1.1(b). All the flakes have only two orientations (0° and 60°) and
their thickness is uniform judging from the optical microscope image. In addition, the domain size
6
can be larger than 100 μm. In Fig. 1.1(c), we compared the largest domain size in our work with
other aligned TMDC work 13, 17–28. Among all of the aligned TMDC growth work, our domain size
is 4 times larger than the previous record, which reported MoS2 slightly smaller than 20 μm 18–22,
24–26, 28. More specifically, among all of the aligned WSe2 growth work, our domain size is 9 times
larger than the largest domain size reported previously 13, 17, 23, 27.
Raman and PL spectroscopy were collected to characterize the WSe2 grown using our goldvapor-assisted CVD. The Raman spectrum excited by a 532 nm laser is shown in Fig. 1.1(d). The
two characteristic peaks at 248 and 259 cm−1 can be attributed to E1
2g mode and A1g mode,
respectively, indicating that the WSe2 is monolayer 33. In addition, the two high-energy peaks at
360 and 375 cm−1 can be assigned to 2E1g and A1g+LA modes, but no B1
2g peak at ~ 307 cm−1 can
be observed, consistent with typical Raman spectra of monolayer WSe2 13. The PL spectrum
shown in Fig. 1.1(e) was also excited using a 532 nm laser. A single strong peak can be observed
at 767 nm and the full width at half-maximum (FWHM) is about 34 nm. The sharp peak can be
attributed to the direct bandgap emission, which is another evidence of the monolayer nature of
the synthesized WSe2 34, 35.
7
Figure 1.1 (a) Schematic diagram showing the gold-vapor-assisted CVD set up. (b) An OM image showing the aligned WSe2
flakes with large domain size we can achieve. (c) Comparison study of the largest domain size between this work and
previously reported aligned TMDC growth works. (d) Raman and (e) PL spectra of a typical WSe2 flake grown by the goldvapor-assisted CVD on a sapphire substrate.
1.3.3 In-depth study of location dependence and the catalytic mechanism in the WSe2 growth
We also found that the WSe2 growth behavior had location dependence. The optical
microscope images together with a statistical morphology study of WSe2 flakes grown in the
downstream region (defined as ~1 cm region from the downstream edge of the substrate) and
the upstream region (defined as ~1 cm region from the upstream edge) on the same sapphire
8
substrate are shown in Fig. 1.2. In the downstream region (Fig. 1.2(a)), the WSe2 flakes were
strictly monolayer. The edge orientation of the flakes was dominated by 0° and 60° with a ratio
as high as 97%, as illustrated in the statistical analysis in Figure 1.2(b). The largest flake size we
observed was about 130 μm and the average size was up to 95 μm, and the corresponding size
distribution is depicted in Fig. 1.2(c). In this way, the average growth rate in this region was as
high as 3.2 μm·s−1 due to the catalytic effect of Au. In comparison, the morphology of WSe2 flakes
grown in the upstream region of the same sapphire substrate is shown in Fig. 1.2(d). Obviously,
the upstream region showed inferior alignment behavior and smaller domain sizes than the
downstream region. Aligned behavior and 0° and 60° edge orientations were found in most of
the flakes (~ 63%), as shown in Fig. 1.2(e). The edge sizes of the WSe2 domains were usually less
than 60 μm with an average size of about 37 μm, which was much smaller than the size of WSe2
flakes in the downstream region. The detailed distribution of edge sizes in the upstream region
is shown in Fig. 1.2(f). Since the Se precursor was not in the hot zone of the furnace until the
starting point of the growth, we can assume that the Se vapor supply was sufficient with a
constant rate. When the Se supply is limited, the WSe2 flakes usually have hexagonal morphology.
However, the WSe2 flakes grown by our method are always in triangle shape, suggesting that Se
supply is excess. Aljarb et al. also reported that the alignment of MoS2 had location dependency,
and the downstream region behaved better than the upstream region 28. We hypothesize that
this phenomenon should be highly related to the Se and WO3−x concentration in the gas phase.
In the upstream region, the WO3−x concentration was higher because WO3 precursor was placed
right below the upstream end of the substrate. Due to the high WO3−x concentration, the growth
of WSe2 is under kinetic control and it was easier for WO3−x to nucleate on the sapphire substrate
9
to form WSe2 seeds. In this way, WSe2 flakes are not always formed in low energy lattice matching
orientation, leading to inferior alignment behavior in upstream region. In contrast, the WO3−x
concentration in the downstream region was lower, the growth is under thermodynamic control,
so that only WSe2 seeds with good lattice matching with the substrate can be formed. Therefore,
the WSe2 flakes only showed 0° and 60° orientations and the incoming precursors preferred to
contribute to the lateral growth of the existing flakes, resulting in larger domain size and lower
nucleation density.
In order to understand the role of gold foil in the CVD process, we performed a series of
control experiments following the same growth procedure but without using a gold foil. The
results of control experiments with the 30-second and 5-minute growth times are shown in
Figure A1.2. When no gold foil was used, with 30 seconds growth time, which is the typical growth
time we used for our gold-vapor-assisted CVD, only small nucleation seeds and small but thick
WSe2 flakes ( ~ 1 μm in size ) could be found on the substrate (Figure. A1.2a), and no large
monolayer WSe2 flake was observed, in contrast to Fig. 1.1(b). When the growth time increased
to 5 min, we were able to observe WSe2 monolayer flakes with thick nucleation seeds at the
centers of the monolayer flakes (Figure A1.2b). The domain size of the monolayer flakes was
around 10 μm, and an irregular polygon grew from a single nucleation seed because it was
difficult for nucleation to happen in lack of gold vapor. In addition, in our typical gold-vaporassisted growth, the vertical distance between the sapphire substrate and the gold foil is 3 mm.
We observed that when the gap distance was increased to 5 mm, thicker and smaller WSe2 flakes
were obtained, because less Au vapor could reach the substrate. If the distance was further
increased to 10 mm, we obtained WSe2 flakes with size and thickness similar to WSe2 flakes
10
obtained without Au foil. This also demonstrates the key role of gold foil in the CVD process: only
thick, small and randomly orientated flakes can be obtained if there is inadequate Au vapor
reaching the substrate. Based on the above facts, we can confirm that the gold vapor can not
only improve the growth rate dramatically, but also increase the size of obtained WSe2 flakes.
Figure 1.2 (a) The OM image of the WSe2 flakes grown in the downstream region, and the corresponding histograms of (b)
the orientation distribution and (c) edge size distribution. (d) The OM image of the WSe2 flakes grown in the upstream region,
and the corresponding histograms of (e) the orientation distribution and (f) edge size distribution
11
The catalytic effect of metals was reported in several previous papers, albeit with
different experimental settings and results 36–44. For example, Gao et al. used gold foils as
substrates for WS2 42 and WSe2 43 synthesis and observed millimeter-sized domains. Due to the
catalytic effect on the Au foil surface, the attachment of W and Se on the edges of WSe2 flakes is
highly preferred. In addition, Liu et al. also reported that Cu vapor can assist WSe2 CVD to prepare
strictly monolayer WSe2 with a fast growth rate 44. However, neither of the above work showed
orientation control. In our gold-vapor-assisted CVD system, the vapor pressure of Au is about 5 ×
10−5 Pa, and the calculation is shown in the ESM. It’s reasonable to suggest that the vaporized Au
atoms tend to attach onto the edges of WSe2, because of the high activity W and Se atoms with
dangling bonds on the edges. Then, these Au atoms perform as active sites for subsequent lateral
growth, which can accelerate the rate of growth and reduce the probability of nucleation and
vertical growth. In combination with a lattice-matching substrate like sapphire, well-aligned WSe2
with large size can be achieved. After the growth, XPS and EDX were used to examine the Au
residue on the surface of the substrate. As shown in Figure A1.4 and A1.5, no discernible gold
signal was detected. This is because only a trace amount of Au could be vaporized and then
participated in the growth process. To demonstrate the existence of gold vapor in the system,
we prolonged the growth time to 48 hours while keeping all the other conditions unchanged.
After 48 hours, it was clear that the gold foil became tattered and gold signal was detected on
the substrate through EDS (as shown in Figure A1.6).
12
1.3.4 Continuous WSe2 film form by merging of WSe2 flakes
It’s also worth mentioning that the aligned monolayer WSe2 flakes can merge to form a
millimeter-sized continuous monolayer WSe2 film at locations close to the downstream edge of
the sapphire substrates. The continuous WSe2 film size is typically about 0.5 cm × 1 cm. Figure
1.3(a) shows the optical microscope image of a typical continuous monolayer WSe2 region. Due
to the uniformity of the WSe2 film. Figure 1.3(a) just shows a uniform image without any
discernable pattern. The reason why continuous films are more likely to be formed close to edges
of the sapphire substrate can be attributed to the higher gold vapor concentration at the edges,
as the edges of the sapphire substrate are located very close to the gold foil. The Raman and PL
spectra of a typical location in the continuous region is shown in Figs. 1.3(b) and 1.3(c), which are
very similar to the Raman and PL of WSe2 single flakes. In the photoluminescence spectrum
shown in Fig. 1.3(c), the single strong peak at 763 nm shows a FWHM of about 32 nm (marked in
Fig. 1.3(c)), demonstrating the monolayer nature of the obtained continuous film. In order to
confirm the uniformity, we performed Raman characterization in 5 selected positions (labeled as
“a” to “e” in Fig. 1.3(a)), and the corresponding Raman spectra are exhibited in Fig. 1.3(d). All the
Raman spectra collected in the 5 positions had strong E1
2g and A1g peaks and no B1
2g peak,
indicating that the aligned monolayer WSe2 flakes can merge to form a continuous monolayer
WSe2 film with good uniformity. It’s also worth mentioning that we observed the intermediate
state of the merging process in some locations. In Figure A1.3, the brighter part is a continuous
monolayer WSe2 area, and the darker part is the exposed sapphire substrate which has not been
covered by WSe2 yet. We can clearly see that most of the edges of the WSe2 are parallel to each
other, indicating the perfect alignment of the WSe2 flakes before they merged together. Because
13
of the pre-aligned lattice orientation of the individual WSe2 domains, in principle, there should
be much fewer grain boundaries than a continuous film formed by WSe2 domains with random
orientations 29, 30. The reduction of grain boundary can effectively improve the electronic
performance of the continuous WSe2 and is very important for large-scale electronic device
fabrication.
Figure 1.3 (a) An OM image of a continuous monolayer WSe2 region. (b) Raman and (c) PL spectra of a typical position in the
continuous region. (d) Raman spectra in the five locations a–e in (a).
In our CVD method, the Au foil can be repeatedly used without any extra treatment. Gao
et al. used Au foil under similar conditions as our CVD growth, observed that Au foil didn’t show
any apparent change after being reused for 100 times 43. Furthermore, by combining wafer-scale
14
Au foil and more precise control of selenium and tungsten precursor supply (e.g., by using
gaseous precursors), our gold assisted CVD growth with fast growth rate has potential for scalable
production of WSe2.
1.3.5 TEM study of the synthesized WSe2
In order to further evaluate the quality of the WSe2 grown by our method, TEM
experiments were conducted. Figure 1.4(a) shows a low-magnification TEM image of the straight
edge of a monolayer WSe2 flake. The edge was slightly folded due to the transfer process. The
selected area electron diffraction (SAED) pattern shown in Fig. 1.4(b) was collected in the area
shown in Fig. 1.4(a). The SAED pattern presents only one set of six-fold symmetry diffraction spots,
indicating that the as-grown WSe2 flake in Fig. 1.4(a) is single crystalline and has a hexagonal
lattice structure. Indices of the diffraction spots are labeled in Figure 1.4(b). Furthermore, the
high-resolution TEM image of our monolayer WSe2 is shown in Fig. 1.4(c), with (100) and (110)
crystal planes highlighted. The periodic honeycomb-like structure represents the atomic
structure of the single crystalline WSe2. The measured lattice distances of the (100) and (110)
plane are 0.28 nm and 0.31 nm, which are very close to the ideal value (shown below), only 6 and
24 pm off, respectively.
√3
2 2 = 0.2855
15
Figure 1.4 (a) Low-magnification TEM image showing the straight edge of a WSe2 flake. (b) Typical SAED pattern of the WSe2
sample in (a). (c) High-magnification TEM image of the WSe2 sample in (a) showing the uniform hexagonal lattice structure.
1.3.6 Electronic device performance
In principle, the high quality of WSe2 grown by our method can lead to excellent electronic
device performance. In order to explore the potential electronics applications of our aligned
WSe2, we modified a wet transfer method reported in Ref. [45] to transfer our continuous
monolayer WSe2 to Si/SiO2 substrates with 285 nm SiO2 dielectric. Then, FETs were fabricated on
the continuous WSe2 film using photolithography followed by metal evaporation to deposit 1 nm
Ti and 50 nm Au as electrodes (as shown in Figure A1.7). The transfer characteristic of a typical
back-gated FET is depicted in Fig. 1.5(a), indicating that the WSe2 grown by our gold-vaporassisted CVD has a p-type behavior. In addition, the on current of the device was as high as 1.7 ×
10−5 A and when a constant voltage of −1 V was applied between the source and the drain, and
the corresponding ON/OFF ratio is about 6 × 105
. The field effect mobility of charge carriers can
be calculated using the equation below
μ = [dI/dV] × [L/(WCV)]
where L is the channel length, W is the channel width, and Ci is the gate oxide capacitance.
In this way, the mobility extracted from the FET in Figure 1.5(a) is around 21 cm2 V-1 s-1
. The IDS-
16
VDS curves of the same device are plotted in Figure 1.5(b). The linear output characteristic
suggests that ohmic contacts were formed between the Ti/Au electrodes and the underneath
WSe2. In order to investigate the quality of our WSe2, we measured 26 FETs fabricated by
photolithography and plotted a histogram of mobility distribution in Fig. 1.5(c). The FETs showed
decent mobility of 12 cm2·V−1·s−1 on average. The good FET performance demonstrates the
uniformity of our continuous monolayer WSe2 film formed by merging of aligned WSe2 flakes,
which is ready for potential large-scale nanofabrication processes.
17
Figure 1.5 Electrical characteristics of the back-gate FETs fabricated on continuous monolayer WSe2 film. (a) IDS–VG and (b)
IDS–VDS curves of a typical WSe2 FET. (c) Histogram exhibiting the mobility distribution of 26 FETs fabricated on continuous
monolayer WSe2 film
1.4 Conclusion of the gold assisted align growth of WSe2 research
In summary, we have successfully developed an Au-vapor-assisted CVD method to
synthesize aligned monolayer WSe2. The advantages of this approach can be concluded as follows:
(1) 97% of WSe2 flakes in the downstream region have 0° and 60° edge orientations; (2) the edge
size of the aligned monolayer WSe2 flakes can be as large as 130 μm; (3) thanks to the catalytic
18
effect of Au, our CVD set up can achieve a fast growth rate of 4.3 μm·s−1 and the growth time can
be as short as 30 s. With Raman, PL, and TEM characterization and FET device study, we have
demonstrated that the aligned monolayer WSe2 performed good quality. The continuous
monolayer WSe2 film formed by individual aligned domains showed outstanding uniformity and
electronic properties, and it is suitable for large-scale and low-cost nanofabrication processes like
photolithography. We believe that the introduction of catalytic metal vapor should be able to
impact the CVD process of other TMDCs with various substrates.
19
Chapter 2: Highly Sensitive, Scalable, and Rapid SARS-CoV-2 Biosensor
Based on In2O3 Nanoribbon Transistors and Phosphatase
2.1 Introduction to COVID-19 and SARS‑CoV-2 biosensor
On the 11th of March 2020 the World Health Organization (WHO) declared Coronavirus
disease 2019 (COVID-19) outbreak, caused by the severe acute respiratory syndrome coronavirus
(SARS-CoV-2), as a pandemic 46. SARS-CoV-2 can cause severe respiratory distress, and as of
November 2021, cumulative number of infected people is over 250 million, resulting in over 5
million death all over the world 47,48. Owing to the high infectivity of the virus and lack of specific
treatments against it, early diagnosis of COVID-19 is essential in pandemic prevention and control,
which mandates a highly sensitive and yet more scalable, cost-efficient, and rapid detection
method of SARS-CoV-2, particularly for developing countries carrying a heavy economic burden
caused by COVID-19 pandemic 49
.
SARS-CoV-2 mainly contains 4 types of proteins: spike(S), membrane(M), nucleocapsid(N),
and envelop(E) proteins (Figure 2.1a). Among these proteins, the S protein is on the viral
envelope and its S1 subunit is the outermost component of the virus. During SARS-CoV-2
infection, S1 proteins recognize and bind with the cellular receptor on the surface of the target
cells, facilitating the virus particles to enter the target cells and triggering the infection 50-53.
Therefore, S1 protein is an important target for SARS-CoV-2 diagnostic detection. Moreover,
detecting S1 protein specific antibody in human blood is also important for COVID-19 diagnosis
in terms of determining previous viral exposure and immunity status. Antibody tests (also known
as serology test) can be nearly 100% accurate for blood samples collected 20 days after infection
or the onset of symptoms 54-56. In this work, we developed a versatile electronic biosensing
20
platform detecting both SARS-CoV-2 S1 protein and S1 protein IgG antibody, whose detection
target can be easily switched between the two biomarkers.
Thus far, based on previously established laboratory protocols, diagnosis of COVID-19
depends mainly on nucleic acid amplification tests (NAATs), specifically real-time reverse
transcription-polymerase chain reaction (RT-PCR) 57-59. Although this method is sensitive and
shows good specificity for COVID-19 detection, this technique has a long processing time (usually
4-5 hours) and depends on advanced instruments, expensive reagents, and skilled technicians.
Among recent advances in rapid screening methods, field-effect transistor (FET)-based
biosensors have many advantages such as small size, real-time detection, high-sensitivity, and
capability for integrated multiplexing 60-65. Seo et al. reported graphene-based FET biosensors for
spike protein of SARS-CoV-2 66, and Shao et al. demonstrated single-walled carbon nanotube
(SWCNT)-based FETs to probe spike and nucleocapsid proteins of SARS-CoV-2 67. As it is widely
known, graphene is semi-metallic while semiconducting carbon nanotubes usually exhibit p-type
conductivity. To reduce the probability of getting false positive and false negative diagnosis, it is
desirable to have complementary sensing capability by using both p-type and n-type
semiconductors 68. In2O3 is a well-known n-type semiconductor and has been successfully applied
as the channel material for a variety of biosensors 69-74. In our work, we fabricated In2O3
transistors using a lithography-free process. Our simplified fabrication method made In2O3
biosensors cost-efficient and suitable for large-scale COVID-19 clinical diagnosis.
FET-based biosensors can be combined with enzymatic reactions because many enzymatic
reactions can release acidic or basic products, in which process the pH change of the environment
21
will generate electronic signal of the FETs 75, 76. Urease is the most common enzyme incorporated
with FET-based biosensors 77-82. However, urease displays poor stability 83-87, which would lose
most of the enzymatic activity after being stored at 4℃ for 10 days 87. In our work, we applied
alkaline phosphatase as the enzyme reporter for our electronic biosensors. During the sensing
process, alkaline phosphatase produces proton by enzymatically removing a phosphate group
from the substrate compound, and the decreased pH value of the medium will then lead to an
increase in the current of our In2O3 FETs. Phosphatase is widely used as the enzyme in commercial
colorimetric ELISA kits (LoD: 16 pg/mL 88, 0.55 mg/mL 89). Compared to urease, phosphatase is
much more stable and suitable for long-term storage while maintaining high activity.
Consequently, our biosensing platform with phosphatase overcomes the limit of electronic
biosensing technique, whose application used to be constrained by the poor stability of enzyme
reporter. Our biosensors display a prolonged shelf life and is advantageous for practical COVID19 diagnosis.
In this work, Our In2O3 biosensors were able to detect S1 antigen of SARS-CoV-2 in both
phosphate-buffered saline (PBS) buffer and universal transport medium (UTM) with a reliable
performance and similar LoD of 100 fg/mL. Besides antigen detection, S1 protein IgG antibody
detection in both PBS and human whole blood (WB) were also carried out. The detection
performance in both mediums displayed LoD of 1 pg/mL. The similar detection results in different
mediums indicate the capability of our electronic biosensors to eliminate the interference of salts,
cells, and other particles in detected specimens. Taken together, our biosensors are highly
sensitive and scalable, showing good potential for clinical SARS-CoV-2 diagnosis. Our findings
22
shed light on a new generation of biosensors for rapid and highly sensitive COVID-19 screening,
getting us closer to winning the fight against COVID-19 pandemic.
2.2 Biosensors based on In2O3 nanoribbon FET devices
Figure 2.1b shows the photograph of the In2O3 nanoribbon FET devices on a Si/SiO2 (500 nm)
substrate. The lithography-free fabrication of the FETs includes two steps. First, In2O3
nanoribbons were defined by the first shadow mask and deposited using radio frequency (RF)
sputtering technique. The channel length and width of the In2O3 nanoribbons were 500 µm and
25 µm, respectively (Figure 2.1c), and the thickness was 18 nm (Figure 2.1d). Next, the source
and drain electrodes were defined by the second shadow mask. After alignment of the second
mask, 1 nm Ti and 50 nm Au were deposited by electron beam evaporation. Twenty-eight
biosensing chips can be fabricated on a 3-inch substrate, and each chip contains an array of 4
separate In2O3 FET devices.
The electrical performance of the devices was first characterized in ambient environment
using an Agilent Semiconductor Analyzer 1500B. Nine different In2O3 FET devices were randomly
picked over the substrate (labelled in Figure 2.1b), and their drain current − back gate voltage (IDS
− VGS) curves with the drain voltage fixed at 1V are shown in Figure 2.1e. To further demonstrate
the uniformity and stability of our devices, IDS − VGS curves were measured for more randomly
picked devices with drain voltage fixed at 150 mV and 5 V (Figure A2.2a, b), and we measured
one device for 20 times over a period of 1 week (Figure A2.2c). The IDS−VGS curves of the FET
devices illustrate n-type transistor behavior with an average mobility of 108±4.3 cm2
/ V·s (Figure
A2.3) and on-off ratio of ∼1×105. Owing to the thick SiO2 dielectric, the devices can be turned on
23
only at high back gate voltage. Figure 2.1e demonstrates the electrical uniformity of In2O3
nanoribbon devices over the entire wafer.
Figure 2.1f shows the schematic diagram depicting the functionalization of our In2O3 devices
for detection of S1 protein. The photograph of the experimental set up is shown in Figure A2.1,
a Teflon cell with an opening at the bottom was used to contain the medium in contact with the
In2O3 biosensors. Prior to the electronic biosensing process, the In2O3 devices were first
immersed in boiling acetone and isopropyl alcohol for 5 minutes each to clean the surface,
followed by an O2 plasma treatment to generate hydroxyl groups on the surface of In2O3. 3-
Phosphonopropioninc acid, as the linker molecules, was then applied to bind with the hydroxyl
groups on the surface. Subsequently, devices were functionalized with N-(3-
dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS),
creating activated carboxyl groups to bind with amino groups from proteins. Next, capture
antibodies that can specifically target S1 antigens of SARS-CoV-2 were applied and immobilized
via reaction with the EDC/NHS groups on the surface. This immobilization process is followed by
a washing step to rinse off unbounded antibodies as well as a blocking step by 10% bovine serum
albumin (BSA) solution to deactivate the surface of In2O3 in order to avoid non-specific binding
during consecutive processes. Target biomarkers were then introduced to our biosensors. As
shown in Figure 2.1f, S1 antigen solutions with known concentrations were dissolved in either
PBS or UTM followed by a 1-hour incubation period. Next, biotinylated secondary antibody were
introduced and incubated for 1 hour followed by a 40-minute incubation of streptavidin
conjugated phosphatase. Finally, phosphatase substrate solution was applied and an
amperometric signal was recorded as the result of the detection. The total time required since
24
applying biomarkers until obtaining final sensing result is nearly 3 hours, with actual detection
time ranges between 10 minutes to 15 minutes.
Figure 2.1 (a) Schematic structure of SARS-CoV-2. (b) Optical image of a 3-inch wafer with an In2O3 nanoribbon biosensors.
(c) Scanning electron microscope image of the channel region of one device (L = 500 μm, W = 25 μm). The scale bar is 200
μm (d) Atomic force microscopy image of a ∼18 nm thick In2O3 nanoribbon. The scale bar is 20 μm. (e) Drain current versus
back gate voltage of nine In2O3 nanoribbon FET devices with locations labelled in (b) by red dots, with drain voltage fixed at
1 V, plotted in linear and logarithmic scale. (f) Schematic illustration of the electronic biosensing setup for S1 protein
detection.
2.3 Electronic biosensing realized by enzymatic reaction of phosphatase
Our electronic detection of SARS-CoV-2 is based on enzymatic reaction of alkaline
phosphatase. As shown in Figure 2.2a, when catalytic alkaline phosphatase is present, the
phosphate group of the substrate compound would get removed during the reaction and protons
would get produced, leading to a change in the solution’s pH value. To prepare the substrate
solution, 10 mg of 4-Nitrophenyl phosphate disodium salt hexahydrate, which is the commercial
phosphatase substrate for colorimetric ELISA, was dissolved in 10 mL of 0.5 M MgCl2 solution.
The pH value of the substrate solution was then adjusted to 9.7 by NaOH solution. We monitored
25
the pH change after manually mixing 5 µL of 1.5 mg/mL alkaline phosphatase solution with 3 mL
of the substrate solution, and the pH value decreased by 1.3 in 5 minutes (Figure 2.2b).
Our In2O3 nanoribbon FET devices served as a pH sensor in response to the pH change
caused by the phosphatase enzymatic reaction. Hence, devices are required to operate in a wet
environment (Figure 2.1f). An Ag/AgCl reference electrode was used to apply liquid gate bias.
Figure 2.2c shows the drain current − liquid gate voltage (IDS − VGS) curve of the devices immersed
in 0.01xPBS buffer with the drain voltage fixed at 200 mV. Figure A2.4 shows the family curves of
drain current− drain voltage (IDS−VDS) where In2O3-FETs display good current saturation at high
VDS. Based on these measurements, we can deduce that our In2O3-FETs can be reliably and
efficiently controlled using the liquid gate with on/off ratios (Ion/Ioff) of 104.
When the enzymatic reaction starts, the deprotonated hydroxyl groups on the surface of
In2O3 nanoribbons become protonated as the pH decreases. Therefore, the negative charges on
the surface of In2O3 nanoribbons decrease and the conduction of the material increases. Figure
2.2d shows the current change of the FET devices in response to pH change, with a fixed liquid
gate voltage at 150 mV and a fixed drain voltage at 200 mV, and the IDS of the device increased
by a factor of 14 as the pH changed from 10 to 4.
In order to demonstrate that In2O3 nanoribbon FET devices and phosphatase enzymatic
reaction can be integrated for electronic biosensing, we performed a positive control experiment.
Figure 2.2e depicts the schematic diagram of the sequence of molecule binding. The streptavidinconjugated phosphatase was directly attached to the FET device by EDC/NHS chemistry, while
the detailed incubation process was similar to the S1 antigen detection described above.
26
To ensure the detection of the final amperometric signal, prior to adding substrate solution,
the channel region of the In2O3 FET was immersed in MgCl2 and NaOH solution with the same pH
value as the substrate solution. A liquid gate voltage of 150 mV was applied while the drain
voltage was fixed at 200 mV to generate the observed baseline. Substrate solution was then
added to trigger the enzymatic reaction and IDS of the device was recorded as a function of time.
As shown in Figure 2.2f, the IDS started to increase as soon as the substrate was introduced with
a current enhancement of 110% after 8 minutes.
Furthermore, to examine the interference resulting from potential electrochemical reaction
in the system and the change in volume of the solution during the measurement, we performed
a negative control experiment. With fixed liquid gate voltage at 150 mV and the same substrate
solution (MgCl2 and NaOH,pH=9.7) as our biosensing condition, we measured IDS to create a
baseline. As shown in figure A2.5, we observed a very stable baseline, illustrating that potential
electrochemical reactions in the system won’t cause any observable current change and
therefore won’t disturb the result of our biosensing. Besides, we changed the volume of the
substrate solution during this process. As a result, no current change was observed,
demonstrating that the volume change won’t affect the result of our biosensing either.
27
Figure 2.2 Mechanism of the electronic ELSIA. (a) Chemical equation of the enzymatic reaction catalyzed by phosphatase.
(b) Change in pH value after mixing phosphatase with the substrate solution. The inset shows the simultaneous color change.
(c) Drain current versus liquid gate voltage with drain voltage fixed at 1 V, plotted in linear and logarithmic scale. (d) Realtime responses obtained from an In2O3 nanoribbon device exposed to commercial buffer solutions with pH from 10 to 4. (e)
Schematic illustration of the electronic biosensing setup for positive control, which demonstrates phosphatase is compatible
with the electronic biosensing technique. (f) Real-time responses monitored from the positive control setup
2.4 Detection of SARS-CoV-2 spike protein in PBS and UTM
We performed electronic biosensing targeting at S1 antigen of SARS-CoV-2 spike protein in
1x PBS with known concentrations ranging from 100 fg/mL to 1 ng/mL. Figure 2.3a shows the
normalized responses for each concentration. After taking average of the sensing results from 5
biosensors and subtracting the negative control, we obtained responses of 5%, 11%, 28%, 54%
and 78% of current increase for S1 antigen concentrations of 100 fg/mL, 1 pg/mL, 10 pg/mL, 100
pg/mL and 1 ng/mL, respectively (Figure 2.3c). The negative control was performed by merely
replacing the antigen biomarker with BSA solution during the functionalization process. The slight
28
increase in the signal of the negative control can be caused by the non-specific binding of the
phosphatase to the In2O3 nanoribbons.
The detection of S1 protein dissolved in UTM with the same range of concentrations was
carried out as well. Universal transport medium is a common solvent for collection, transport,
maintenance, and long-term freeze storage of clinical specimens containing viruses. It’s widely
used for cell culture, rapid antigen detection, PCR, and nucleic acid amplification assays. Hence,
antigen detection in UTM helps to assess our biosensor to be used for clinical diagnosis of SARSCoV-2. In order to simulate a real clinical setting, we have adopted a nasopharyngeal swab sample
taken from a healthy person in our UTM-based detection setup. The swab was suspended in the
UTM along with S1 protein. Figure 2.3b shows the obtained sensing signal. 4%, 10%, 25%, 53%,
and 78% of current changes (average of 5 trials) have been generated in response to antigen
concentrations of 100 fg/mL, 1 pg/mL, 10 pg/mL, 100 pg/mL and 1 ng/mL, respectively (Figure
2.3c). The results of detections in PBS (black squares) and UTM (red dots) are plotted in Figure
2.3c. The results illustrated that the ingredients of UTM, such as Hank’s balanced salts, BSA and
sucrose, would not interfere with the sensing results. This finding reveals that our electronic
biosensing approach can circumvent Debye screening from salts in the fluid, indicating that our
biosensors can be used for clinical diagnostic testing without any special sample preparation or
pretreatment.
The LoD of our biosensor reached 100 fg/mL, which is 2 orders of magnitude lower than
conventional colorimetric ELISA 88, 89. This could be caused by the local reaction nature of our
sensing technique. We expected the local pH changes of the solution surrounding the In2O3
nanoribbon to generate the amperometric signal instead of changing the pH of the entire
29
solution, which would require greater number of phosphatase molecules anchored to the surface
of In2O3 and take a prolonged period of time for detection. This hypothesis can be supported by
the measurement shown in Figure A2.6, as bubbling the solution extensively to mix it thoroughly,
the current dropped back to a point close to the baseline, indicating the pH of the entire solution
didn’t change much. Once the bubbling was stopped, the current increased again as the
enzymatic reaction changed the local pH of the solution.
To investigate the reproducibility of our detection, we performed biosensing of 10 pg/mL S1
antigen in different mediums using devices from 4 different batches of fabrication. As shown in
Figure 2.3d, 5 rounds of detection in PBS and 5 rounds of detection in UTM generated similar
sensing responses. Hence, it is safe to say that our fabricated biosensors exhibit a remarkable
reproducibility for large scale clinical diagnostic testing.
30
Figure 2.3 Detection of SARS-CoV-2 spike protein. (a) Real-time responses monitored at 100 fg/mL, 1 pg/mL, 10 pg/mL, 100
pg/mL and 1 ng/mL of S1 proteins in PBS. (b) Real-time responses monitored at 100 fg/mL, 1 pg/mL, 10 pg/mL, 100 pg/mL
and 1 ng/mL of S1 proteins in UTM. (c) Plot of normalized responses at each S1 protein concentration, in nanograms per
milliliter, n=5, error bars represent standard deviation. (d) Real-time responses monitored using ten biosensors from four
batches of fabrication at 10 pg/mL of S1 protein, showing the reproducibility of our technique.
2.5 Detection of spike protein IgG antibody in PBS and human whole blood
To demonstrate the versatility of our electronic biosensing platform, we applied our In2O3
devices to detect SARS-CoV-2 spike protein IgG antibody. Figure 2.4a shows schematics of the
structure of surfaced functionalized In2O3 nanoribbon for S1 protein specific antibody detection.
In this structure, the sequence of molecule binding along with the conjugated antibody and S1
protein serve as the capturing component. After forming this capturing component, target
antibody was incubated for 1 hour. Biotin conjugated anti-Human IgG antibody was then
31
introduced as the secondary antibody followed by the introduction of streptavidin-conjugated
phosphatase. Streptavidin can link to biotin to anchor phosphatase to our devices, which will
generate pH change of the solution during the biosensing process. Finally, amperometric signal
was recorded while the device is immersed in liquid gate setup.
Figure 2.4b shows the amperometric signals generated as the result of detecting antibodies
dissolved in PBS buffer solution. 1 pg/mL, 10 pg/mL, 100 pg/mL and 1 ng/mL concentrations of
antibodies generated 18%, 37%, 65%, and 82% of current enhancement (average of 5 trials),
respectively (Figure 2.4d). It’s worth noting that the anti-Human IgG antibody could potentially
bind with the primary antibody. Therefore, to investigate the effect of this undesired binding, we
performed a negative control experiment, where BSA solution was used to replace the target
antibody. Figure 2.4b shows the result of this negative control (green curve) where a 4% current
increase was generated. This indicates that the anti-Human IgG antibody could hardly bind with
the primary antibody, which is already anchored to the surface of In2O3, and hence, would not
affect the sensing results.
Furthermore, our biosensors exhibit high selectivity when detecting S1 protein specific
antibody in physiological solutions, as demonstrated in figure 2.4c. Here, we spiked target
antibodies with known concentrations into human whole blood (WB) and carried out the
detection. We varied S1 protein specific antibody concentration to include 1 pg/mL, 10 pg/mL,
100 pg/mL and 1 ng/mL, which led to current enhancement (average of 5 trials) of 16%, 29%, 58%
and 77%, respectively (Figure 2.4d). The results of the detection in PBS (black squares) and human
whole blood (red dots) are plotted in figure 2.4d. As depicted in the figure, antibodies spiked in
PBS and WB generated analogous responses while the magnitude of sensing responses detecting
32
antibody in blood is slightly lower than that in PBS. These miniscule differences could stem from
the high viscosity of the whole blood, where precipitated blood cells can attach to the surface of
the biosensor during the incubation period, and thus could induce interference with the sensing
results. Nevertheless, we can safely say that the sensing behaviors in both mediums are mainly
attributed to the binding of the target antibody.
The results of detecting SARS-CoV-2 S1 antigen in PBS and UTM, and S1 protein specific
antibody in PBS and human whole blood reveal that our In2O3 devices and electronic biosensing
approach can be highly sensitive, dual-functional biosensors for COVID-19 diagnosis. The ability
of our biosensors to switch from antigen detection to antibody detection by simply adding an
additional step of antigen incubation and changing the secondary antibody, along with their high
immunity to interference caused by ions, cells, and other particles from complex fluids, makes
our cost-efficient biosensors desirable for worldwide rapid screening of SARS-COV-2 virus with
high sensitivity.
The demonstrated electronic biosensing platform based on phosphatase and shadow mask
fabrication process is not limited to In2O3 material and SARS-CoV-2 biosensing. This technology
has the potential to be applied to other semiconducting materials such as silicon nanowires 61, 90,
metal oxide nanowires/nanoribbons 91, carbon nanotubes 92, graphene 93, and other twodimensional materials 94 for biosensing applications. In addition, the electronic biosensing
technique based on phosphatase has the potential to be generalized as a standard protocol of
electronic biosensors for highly sensitive detection of various proteins with prolonged shelf life.
33
Since the enzyme and other reagents used in the functionalization of our biosensors are
also used for commercial colorimetric ELISA kits, it is possible to integrate our biosensors with
conventional colorimetric ELISA for a dual-readout biosensor platform. Such integration would
improve the sensing characteristics with high reliability. In this platform, colorimetric signal can
be used for quick and preliminary detection, while the electrical signal can subsequently be used
for a more sensitive diagnosis.
Figure 2.4 Detection of SARS-CoV-2 spike protein IgG antibody. (a) Schematic illustration of the electronic biosensing setup
for antibody detection, anti-Human IgG antibody was introduced to the setup to switch the target biomarker to SARS-CoV2 spike protein IgG antibody (b) Real-time responses monitored at 1 pg/mL, 10 pg/mL, 100 pg/mL and 1 ng/mL of antibody
in PBS. (c) Real-time responses monitored at 1 pg/mL, 10 pg/mL, 100 pg/mL and 1 ng/mL of antibody in human whole blood.
(d) Plot of normalized responses at each antibody concentration, in nanograms per milliliter, n=5, error bars represent
standard deviation.
34
2.6 Conclusion to COVID-19 In2O3 biosensors
We have demonstrated sensitive, scalable, and cost-efficient COVID-19 biosensors using
electronic biosensing platform based on In2O3 FET devices functionalized with phosphatase. The
devices were fabricated by a simple and cheap shadow mask method. Our biosensors were able
to detect SARS-CoV-2 spike protein in UTM (LoD: 100 fg/mL) and S1 protein specific IgG antibody
in human whole blood (LoD: 1 pg/mL), indicating its potential for clinical diagnostic testing. Our
results can be instrumental for the management and control of the current pandemic and can
possibly prevent further community transfer through early, rapid, and cost-efficient screening of
COVID-19, giving us the upper hand to win the fight against this pandemic.
35
Chapter 3: High-performance and flexible inkjet-printed MoS2 field-effect
transistors and their gas sensing applications
3.1 Introduction to 2D materials ink and inkjet printing technique
Two-dimensional (2D) materials, arising from their near-atomic thickness, have
demonstrated their potential in a broad spectrum of next-generation devices and systems,
including electronics95-99, optoelectronics100, 101, and sensors102, 103. In particular, high-quality
solution-processable 2D materials are rapidly gaining vast attention due to their simplicity,
scalability, and cost-efficiency. Significant progress has been made to implement 2D material inks
for different applications including photodetectors 104-107, electronic devices104, 108-112, lightemitting diodes 113, 114, solar cells115, 116, energy storage devices 117, 118, supercapacitors119, and
sensors120. In this work, we applied a liquid-phase electrochemical exfoliation method reported
by Lin et. al108 to prepare MoS2 ink, which offers advantages in producing uniform and pure
semiconducting 2H phase MoS2 nanoplates. Additionally, the dangling-bond-free flake surfaces
and subsequent pinning-free flake-to-flake interfaces make it possible to be applied for highperformance electronics. Using our high-quality semiconducting MoS2 ink prepared by
electrochemical exfoliation and a binary solvent of 2-butanol and isopropanol, we obtained
continuous and rather uniform inkjet-printed MoS2 films. We also demonstrated highperformance and flexible inkjet-printed MoS2 field-effect transistors with carrier mobilities of 11
cm2V-1S-1 and on/off ratios (Ion/Ioff) of 106, and their gas sensing applications.
Although there are many advances in the preparation of 2D material ink108, 121-122, depositing
the ink to specific locations is a basic requirement to utilize the ink and is still challenging. A
typical thin film deposition method is spin coating, as reported in several previous papers.108, 109
36
As reported by Lin et. al.108, spin-coated MoS2 film was used for field-effect transistors with a
mobility of 10 cm2V-1S-1. However, this spin-coating method or other common deposition
methods requires additional patterning and etching steps to fabricate desired devices, which
increases the complexity and cost of device fabrication. In comparison, inkjet printing is a
lithography-free, drop-on-demand deposition method, and 2D material inks can be directly
“written” to the desired location with the aid of computers, which makes this technique perfectly
compatible with 2D inks.123 However, thus far, very few inkjet-printed 2D FET devices were
demonstrated in the literature, and the reported printed 2D FETs usually possessed very limited
performance. For example, three groups reported inkjet-printed MoS2 FETs110-112, however, the
reported on-off ratios were only in the range of 102-103
, and mobilities were very low (0.1 cm2
V1
S-1 in Ref. 110, 4 cm2V-1S-1 in Ref. 111 with graphene electrodes and HfO2 dielectric, and between
0.1 and 0.2 cm2
V-1S-1 in Ref. 112).
This poor device performance stems from two major challenges. The first one is the coffeering effect, namely, the MoS2 nanoplates would be driven toward the droplet edge, and therefore
a ring-shaped MoS2 profile with a large thickness would form on the substrate.124, 125 The other
challenge is the continuity and uniformity of the deposited thin film, which can hardly be
obtained with a single or even several prints. As a result, inkjet-printed MoS2 FETs usually suffer
from low on/off ratio and poor mobility, and require large and unrealistic width-to-length ratios
to produce reasonably high current densities. In this work, we mixed 2-butanol and isopropyl
alcohol as a binary solvent126, which was previously demonstrated to suppress the coffee ring
effect, to dissolve our MoS2 ink prepared by electrochemical exfoliation. With the binary solvent,
37
we demonstrated continuous and relatively uniform inkjet-printed MoS2 films and achieved highperformance FETs with on/off ratios up to 106 and mobilities of around 11 cm2V-1S-1.
In addition, the solution-processable 2D ink and drop-on-demand inkjet printing technique
are compatible with flexible substrates because the MoS2 channel comprised of overlapped MoS2
nanoplates can effectively release the tensile strain during the deformation of the substrate.104,
105 Therefore, we also demonstrated the potential of our printed MoS2 devices for wearable
electronic applications by making flexible devices on polyimide substrates. To achieve fully
printed flexible FETs, we inkjet printed ion gel as the dielectric. Our flexible devices showed great
stability after 200 cycles of bending and under different bending curvatures.
Furthermore, in recent years, 2D MoS2 has gained vast attention in the gas sensing field due
to its high surface-to-volume ratio, position-dependent gas molecules adsorption, and good
stability in presence of oxygen. Several efforts have been made on MoS2 gas sensors aiming at
NO2 and NH3, which are the two most common air pollutants.102-103, 127-129 According to the
primary standard set by the U.S. Department of Environmental Protection Agency (EPA),
exposure to NO2 of a concentration beyond 53 parts per billion (ppb) may lead to respiratory
diseases, such as chronic bronchitis, asthma, and pneumonia.130 Exposure to NH3 may cause
temporary blindness, pulmonary edema, and respiratory irritation.130 Our printed MoS2 film,
which is comprised of stacked MoS2 nanoplates, is abundant in active sites for gas sensing, such
as edge sites and sulfur vacancies. Therefore, it is a promising candidate for NO2 and NH3
detection. In this work, we demonstrate our MoS2 FETs as NO2 and NH3 gas sensors, with a limit
of detection (LoD) of 10 ppb for NO2 and 0.5 ppm for NH3. Moreover, MoS2 NO2 sensors usually
suffer from slow desorption of gas at room temperature, and subsequent low recovery rate and
38
incomplete recovery. In this work, we facilitate NO2 desorption by applying ultraviolet (UV) light,
achieving a fast recovery rate of 23 seconds.
3.2 Inkjet-printed MoS2 films using MoS2 ink and their characterizations
Our MoS2 ink was prepared by liquid phase exfoliation108, where electrochemical
intercalation of tetraheptylammonium cation (THA+
) and sonication were applied successively. A
schematic diagram of the MoS2 ink preparation is shown in Figure 3.1, and details of the
preparation are described in the Methods section. Figure 3.1c shows the obtained ink and the
SEM image of a typical MoS2 plate. A typical AFM image of multiple MoS2 nanoplates is presented
in Figure A3.1a showing lateral dimensions of 0.5–3 μm, and Figure 3.2a shows the extracted
statistical thickness of 325 MoS2 nanoplates, which reveals that the exfoliated MoS2 nanoplates
have a narrow thickness distribution of 2.8±1.5 nm.
Figure 3.1. Schematic diagram of the preparation of the MoS2 ink. (a) Electrochemical intercalation of crystal MoS2 with
THAB. (b) Sonication of obtained bulk MoS2 after electrochemical intercalation to produce MoS2 nanoplates. (c) Photograph
of the obtained MoS2 ink and SEM image of typical individual plates in the MoS2 ink. Scale bar, 2 µm. (d) Inkjet printing of an
array of MoS2 FETs.
(a) (b)
(d) (c)
39
Figure 3.1d shows the schematic diagram depicting an array of our printed field-effect
transistors (FETs) based on inkjet-printed MoS2 film as the channel material. The source/drain
electrodes were patterned by lithography and then 5 nm of Ti and 50 nm of Au were deposited
using electron-beam evaporation. We note that the metal electrodes can be easily printed even
though here we used fabrication for convenience. It was followed by inkjet printing MoS2 ink to
the channel region. The concentration of the ink was tuned to be 0.4 mg/ml, and the ink was
then loaded into a micropipette with an opening of 50 µm. Unlike traditional inkjet printing, we
applied SonoPlot's Microplotter system for the inkjet printing, which works by first lowering the
printing nozzle to let the MoS2 ink get in contact with and wet the substrate, which would form
a circular film of MoS2 with a diameter of 400 µm (Figure A3.2), and then moving the nozzle
horizontally like writing with a pen to pattern the MoS2 film. Two to three printing passes are
needed to form a continuous and uniform film.
It is noteworthy that, to suppress the coffee-ring effect during inkjet printing and improve
the continuity of the printed film, we used a mixture of 10 vol% 2-butanol and 90 vol% isopropyl
alcohol (IPA) as the binary solvent for the ink. We note that adding 2-butanol to our MoS2 ink did
not affect the stability of the ink, as no perceptible change was observed in the ink after adding
the 2-butanol for more than 3 months. A control experiment was performed by printing MoS2 ink
with pure IPA solvent and our binary solvent, as shown in Figure A3.2. It can be seen that a single
drop of the MoS2 ink with the binary solvent can form a relatively uniform film showing light
green color (Fig. A3.2a). In contrast, when pure IPA solvent was printed, the MoS2 nanoplates
concentrated in the edge region, and a large region near the center showed the color of the
substrate, indicating few MoS2 nanoplates deposited there (Fig. A3.2b). The FETs printed with
40
MoS2 ink with pure IPA solvent exhibited poor performance with on/off ratios ranging from 10
to 1000. The thickness of the printed MoS2 film is determined to be around 10 nm by AFM
measurement over a scratch on the film, as shown in Figure A3.1c. Figure 3.2b, 3.2c, and 3.2d
show the characterization of the printed channel region comprised of MoS2 nanoplates. Figure
3.2b presents the scanning electron microscopy (SEM) image of the channel region. Figure 3.2c
shows an optical image of the channel region, and Figure 3.2d shows the corresponding Raman
mapping, which shows the intensity of the A1g peak of MoS2 at 408 cm-1, over the region labeled
in Figure 3.2c. Notably, no evident pinhole is present in the channel region of our devices in
Figures 3.2b, 3.2c, and 3.2d. Accordingly, this demonstrates the good continuity of our printed
MoS2 film.
In Figure 3.2b, we can see some large MoS2 flakes with higher brightness, and in Figure 3.2c,
we can also see some yellow thick MoS2 flakes over the channel region, which cause slight nonuniformity in the thickness of the printed film. We note that these large and thick flakes would
not significantly affect the performance of the FETs as they are not continuous over the entire
channel region. The transport of our FETs is dominated by the more uniform and continuous
MoS2 films in the background of the large and thick flakes. The continuity of the printed MoS2
film was further confirmed by the energy-dispersive spectroscopy (EDS) results in Figure A3.3 and
the AFM measurement over the channel region in Figure A3.1b. In Figure 3.2d, the intensity of
the A1g peak of MoS2 varies within a relatively large range, which can also be ascribed to those
large and thick MoS2 flakes. The Raman spectra corresponding to the mapping result were shown
in Figure A3.4. In order to further improve the thickness uniformity of the printed films, more
41
precise centrifugation can be used to narrow down the thickness distribution of the MoS2
nanoplates in the ink.
Figure 3.2. Characterizations of exfoliated MoS2 nanoplates and printed MoS2 films. (a) Thickness distribution of 325 MoS2
nanoplates, measured using AFM. (b) SEM image of the channel region of a printed MoS2 FET. Scale bar, 10 µm. (c)
Microscopic image of the channel region of the MoS2 FETs. Scale bar, 25 µm. (d) Raman mapping over the labeled region in
(c), showing the intensity of the A1g peak of MoS2 at 408 cm-1.
.
3.2 Electronic properties of the printed MoS2 FETs
3.2.1 Back-gate printed MoS2 FETs
We carried out a systematic study of the electronic performance of our printed MoS2 FET
devices. The electrical measurements were performed in a glove bag filled with Ar atmosphere.
Figure 3.3a shows the schematic diagram of a back-gated FET built on an inkjet-printed MoS2
channel with 300 nm SiO2 as dielectric and 50 nm gold as contacts. Figure 3.3b shows the transfer
42
(ID-VG) characteristics of a representative printed MoS2 device (L=50 µm, W=150 µm) with VD =
1V. The mobility is calculated to be 11 cm2V-1s-1, the on/off ratio is about 106, and the threshold
voltage is about 35 V. Figure 3.3c presents the output (ID-VD) characteristics measured in the
triode region, in which the ID-VD curves appear to be very linear for VD between -0.5 and 2 V,
indicating the Ohmic contacts between the printed MoS2 film and the deposited gold electrodes.
The device exhibited saturation behavior under higher VD, as shown in Figure A3.5c. Moreover,
due to our improved MoS2 ink composition with 2-butanol, our printed devices are expected to
perform uniformly. Figure 3.3d shows the calculated mobilities and on/off ratios for 10 individual
FETs (L=50 µm, W=150 µm). The narrow distributions of the mobility and on/off ratio
demonstrate the uniformity and reproducibility of our printed FETs. To get a more
comprehensive understanding, we fabricated FETs with different channel lengths, ranging from
10 µm to 100 µm. Figure 3.3e exhibits the ID-VG curves for devices with various channel lengths
and Figure 3.3f summarizes the corresponding resistance averaged over 10 devices when VG was
set at 100 V. It can be illustrated that the resistance is proportional to the channel length, which
further demonstrates the uniformity and reliability of our printed MoS2 films. According to the
transmission line method (TLM), by extracting the intercept from Figure 3.3f, we can also
determine that the contact resistance is about 436 kΩ·µm.
43
Figure 3.3. Electronic properties of the back-gated printed MoS2 FETs. (a) Schematic diagram of a back-gated FET consisting
of printed MoS2 channel, 300 nm SiO2 as dielectric, and 50 nm Au as electrodes. (b, c, d) Transfer (ID-VG) characteristics with
VD = 1V (b), output (ID-VD) characteristics in triode region (c), and mobility and on/off ratio (d) of ten typical printed MoS2
FETs (L=50, W=150 µm). (e, f) Transfer (ID-VG) characteristics with VD = 1V (e) and averaged resistance, n=10 (f) of the printed
MoS2 FETs with 5 different channel lengths.
3.2.2 Ion gel-gated printed MoS2 FETs
As shown in Figure 3.3, with 300 nm SiO2 as back-gate dielectric, the devices can only be
turned on/off at a large gate voltage (up to 100 V). To achieve gate modulation at a low gate
voltage, and to demonstrate the versatility of our inkjet printed FETs, such as on flexible
substrates, we introduced ion gel gate to our printed MoS2 devices. Figure 3.4a is the schematic
diagram of our ion gel-gated MoS2, the ion gel solution (1-ethyl-3-methylimidazolium in ethyl
acetate) was also inkjet printed to the FETs after printing MoS2 film to the channel region, and
the gate voltage was applied by a side gate. The details of the ion gel preparation process are
described in the Methods section. Figure 3.4b shows the transfer (ID-VG) characteristics of the
printed MoS2 FET with an ion gel gate (L=50 µm, W=150 µm) with VD = 1V. Owing to the large
capacitance of the ion gel gate dielectric, the on-current reaches 30 µA at a very low gate bias
44
(VG=2V), with an on/off ratio exceeding 106
. Notably, little hysteresis was observed, as shown in
the inset of Figure 3.4b, indicating the adsorbed moisture and oxygen are removed by the
introduction of ion gel dielectric. The hysteresis of both back-gated and ion gel-gated MoS2 FETs
in linear and logarithm scales are shown in Figure A3.5a, b. We notice that there is a Schottky
barrier (SB) existing in our ion gel-gated MoS2 FETs, as evidenced by the output (ID-VD)
characteristics in Figure 3.4c.
Figure 3.4. Electronic properties of the ion gel-gated printed MoS2 FETs. (a) Schematic diagram of an ion gel-gated printed 2
FET, a side gate electrode is used to apply voltage. (b, c) Transfer (ID-VG) characteristics, with VD = 1V. Inset shows a double
sweep ID-VG curve (b) and output (ID-VD) characteristics (c), of a typical ion gel-gated printed MoS2 FET (L=50, W=150 µm).
3.3 Flexibility of the printed MoS2 FETs
We further characterized the flexibility of our inkjet-printed MoS2 FETs as demonstrated in
Figure 3.5. The printed MoS2 devices with ion gel gate were patterned on a polyimide (PI)
substrate, following the same protocol as on Si/SiO2 substrates. After that, the bending test was
carried out where we tightly wrapped our printed MoS2 devices around a cylinder, as shown in
Figure 3.5a, and electrical measurements were performed. Figure 3.5b compares the transfer (IDVG) characteristics of a typical printed MoS2 FET (L=50 µm, W=150 µm) in relaxed (unstrained)
status, bent with a radius of curvature of ∼3.5 mm, and after 200 bending cycles. No perceptible
(a) (b) (c)
45
change in ID-VG curves was observed, and the mobilities under these three conditions were all
calculated to be about 11.5 cm2V-1S-1. When the PI substrate was bent with a radius of curvature
of 3.5mm, a tensile strain of ~1.2% parallel to the current flow direction was applied to the MoS2
FETs. Furthermore, Figure 3.5c exhibits the effect of the radius of curvature on the on-current
and on/off ratio of our printed MoS2 FETs. Five different bending conditions with radii of 3.5 mm,
7 mm, 10 mm, 21 mm, and infinity (relaxed) were tested, and 3 FETs were measured and
averaged under each bending condition. The measured on-current varies within a narrow range
from 170 µA to 190 µA, and the on-off ratio stays between 105 and 106 for all different bending
conditions. Figure 3.5d shows the on-current and the on/off ratio averaged over 3 FETs after 0
(before bending), 2, 20, and 200 bending cycles. There was only a very small variation in the oncurrent and the on/off ratio after 200 cycles of bending. Therefore, we deduce that our printed
MoS2 devices with ion gel gate displayed good stability under different bending conditions and
after 200 bending cycles, demonstrating their potential in wearable electronics applications.
Notably, the performance of the printed MoS2 devices on PI substrate is enhanced compared to
those on Si/SiO2 with respect to the current density. This is probably because the two acyl groups
(C=O) bonded to nitrogen (N) of polyimide help IPA and 2-butanol to wet the surface and improve
the adhesion between the MoS2 nanoplates and the substrates, which helps to further suppress
the coffee-ring effect and form a uniform MoS2 film. Moreover, the two acyl groups (C=O) bonded
to nitrogen (N) of polyimide are good electron donors, they can have an n-doping effect on ntype semiconductor MoS2, which is beneficial to the electrical performance.
46
Figure 3.5. Flexibility of ion gel-gated printed MoS2 FETs on polyimide substrates. (a) Photograph of printed MoS2 FETs on a
polyimide substrate wrapping around a glass cylinder, whose radius is 3.5 mm. (b) Transfer (ID-VG) characteristics (VD =1 V)
of a printed MoS2 FET (L=50, W=150 µm) in three conditions: relaxed, after bending for 200 cycles and bent with a bending
radius of 3.5 mm. (c, d) Averaged on-state currents and on/off ratios of ion gel-gated printed MoS2 FETs under different
bending radii (c) and after different bending cycles (radius =3.5 mm) (d).
3.4 NO2 and NH3 gas sensors using the printed MoS2 FETs
Our printed MoS2 film consists of stockpiled MoS2 nanoplates, providing abundant edge
sites and sulfur vacancies as active sites for gas sensing. We prepared printed MoS2 FETs with
interdigitated source/drain electrodes on Si/SiO2 substrates for NO2 and NH3 sensing, as shown
in Figure A3.6. The distance between two adjacent electrodes is 50 µm and the length of each
finger is 200 µm. Upon adsorption, NO2 and NH3 can decrease and increase the conductance of
the MoS2 FETs due to their p-type and n-type doping effect, respectively. Figure 3.6a shows the
transfer (ID-VG) characteristics of the printed MoS2 FETs, which were measured after exposing the
47
MoS2 devices to NO2 flows of various concentrations for 3 minutes, ranging from 10 ppb to 250
ppb. Based on our measurements, the limit of detection (LoD) for NO2 can reach 10 ppb, which
yields a 20.7% conductance decrease. Meanwhile, as NO2 concentration increased to 250 ppb, a
remarkable 81.7% conductance decrease was observed (at VG=100V). The demonstrated LoD of
10 ppb satisfies the sensing standard set by U.S. Environmental Protection Agency (53 ppb).
Figure 3.6b plots the real-time current change of the MoS2 FET in response to 100 seconds of
exposure of 10 ppb, 20 ppb, 50 ppb, 100 ppb, and 250 ppb NO2 gas, causing current decreases of
12.9%, 42.1%, 62.0%, 72.0%, and 80.5%, respectively (VG = 30 V, VD = 1 V).
The zoom-in plot of real-time sensing of 100 ppb NO2 is presented in Figure 3.6c as an
example to illustrate the response and recovery rates. Our sensors showed an initially rapid
conductance decrease upon exposure to 100 ppb NO2 and finally decreased by 72% within 100 s.
After 100s, the NO2 flow was turned off, and a 254 nm ultraviolet light was used to facilitate the
recovery. As shown in Figure 3.6c, our sensor can be fully recovered within a recovery time of 23
seconds (defined as the time the sensor takes to recover to 10% of the maximum response after
turning off the gas). This rapid recovery overcomes the shortcoming of the slow desorption rate
of NO2 at room temperature observed with typical NO2 sensors based on MoS2. Moreover, we
compared our results to other NO2 sensors based on MoS2 operating at room temperature, as
shown in Figure 3.6d. 102-103, 127-128, 131-145 We point out that even though many of those reported
sensors took advantage of unusual morphologies of MoS2, the assistance of various light, or
decoration of additional materials, our printed sensors based on bare MoS2 still exhibited
excellent performance, especially in terms of recovery rate and limit of detection. The advantages
of our NO2 sensors can stem from the rich edge sites and sulfur vacancies of our printed MoS2
48
film. At the same time, the thin, relatively uniform, and continuous film allows the adsorbed NO2
to cause a fast and significant response in conductance. Furthermore, to clarify the assistance of
UV light to the recovery, we want to point out that UV light can increase the conductance of MoS2
FETs by generating hot charge carriers by photoexcitation and desorption of NO2.
127 As we turned
off the UV light to end the photoexcitation, it can be noticed that the current gradually returned
to the baseline, which indicates the complete desorption of NO2. Therefore, we can conclude
that overall recovery was facilitated by the desorption of NO2.
NH3, another common toxic pollutant, was also detected by our printed MoS2 FETs. Figure
3.6e presents the transfer (ID-VG) characteristics and Figure 3.6f presents the real-time current
change of the MoS2 FET in response to various concentrations of NH3 flow. The limit of detection
to NH3 is 500 ppb, and as revealed in Figure 3.6f, current increases of 4.8%, 9.3%, 17.4%, 28.0%,
41.7%, and 55.8% were caused by 0.5 ppm, 2 ppm, 8 ppm, 20ppm, 50 ppm, and 100 ppm NH3 in
100 s, respectively (VG = 30 V, VD = 1 V). As for NH3 sensing, UV light was not used to assist
recovery, since the observed effect of NH3 is an increase in conductance of the MoS2 FET, and UV
light will induce a similar increase in conductance, making subsequent sensing of NH3 hard to
observe.
49
Figure 3.6. Sensing performance of printed MoS2 FETs to NO2 and NH3. (a,b) Transfer (ID-VG) characteristics (a), and Realtime current change (b) of the printed MoS2 FET upon exposure to 10, 20, 50, 100, and 250 ppb of NO2. (c) Zoom-in plot of
current change of the MoS2 FET in response to 100 ppb of NO2, extracted from (b). (d) Comparison study between our NO2
sensors and previously reported NO2 sensors based on MoS2 operating at room temperature, concerning recovery time, and
the lowest concentrations of NO2 which were detected in the references, (Ref. number labeled in the figure(d) +94 = Ref.
number in text). (e,f) Transfer (ID-VG) characteristics (e), and Real-time current change (f) of the printed MoS2 FETs upon
exposure to 0.5, 2, 8, 20, 50, and 100 ppm of NH3.
3.5 Methods
3.5.1 Solution-processable MoS2 ink preparation:
The electrochemical exfoliation in the liquid phase was carried out using a 2-electrode cell.
The cathode was a thin piece of MoS2 crystal (SPI supplies) fixed by a copper clip, and a graphite
rod served as the anode. Tetraheptylammonium bromide (THAB) (99% from Sigma-Aldrich) was
dissolved in acetonitrile (10 mg/mL) as the electrolyte. During the electrochemical process,
tetraheptylammonium (THA+) cations were intercalated between MoS2 layers, which was driven
by an applied voltage of 8V for one hour.
After the intercalation of THA+ cations, the obtained fluffy MoS2 was rinsed with ethanol to
get rid of the acetonitrile and THAB residuals and transferred to
50
polyvinylpyrrolidone/dimethylformamide (PVP: M.W. 58,000, Alfa Aesar) (25 mg/mL) solution. It
was followed by the pool sonication for 2 hours to obtain a greenish MoS2 dispersion.
In order to remove excessive PVP, the MoS2 dispersion was washed with isopropanol (IPA)
for 4 times. The MoS2 nanoplates can be collected by centrifugation at 9000 r.p.m. and
redispersed into IPA to form the desired MoS2 ink. Before the final MoS2 was obtained, an
additional centrifugation was performed at 1500 r.p.m for 5 mins and the precipitates, comprised
of large MoS2 bulks, were discarded. Furthermore, 10 vol % 2-butanol can be added to the ink to
improve the spreading and drying properties on substrates.
3.5.2 Inkjet printing of MoS2 film and post-treatment of printed MoS2 FETs:
The MoS2 ink was printed to form a film as the channel material of FETs on Si/SiO2 (300nm)
or polyimide substrate using an inkjet printer (Microplotter Desktop, SonoPlot, Inc.). The MoS2
film can be directly printed onto prefabricated Ti/Au (5/50 nm) source/drain electrodes, which
are patterned using lithography and deposited by electron beam evaporation.
After the inkjet printing, to get rid of the PVP and other compounds used during the ink
preparation, the obtained MoS2 FETs were immersed in 10 mg/mL
bis(trifluoromethane)sulfonimide (TFSI, > 95.0%, Sigma-Aldrich) in 1,2-dichloroethane (Sigma
Aldrich) at 80℃ for 1h. Finally, an annealing treatment was performed in an Ar atmosphere at
200℃ for 1h.
3.5.3 Preparation of ion gel and ion gel gate devices:
The ion gel was prepared by mixing the triblock copolymer poly(Styrene-bmethylmethacrylate-b-Styrene) (PS-PMMA-PS, Polymer Source Inc. MPS = 4 kg/mol, MPMMA =
51
11kg/mol), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI, Sigma
Aldrich) and ethyl acetate (Sigma Aldrich), in a weight ratio of 3.5:46.5:100, respectively. It was
followed by stirring the mixture at 900 rpm overnight.
The prepared ion gel was inkjet printed over the channel region of the MoS2 FETs as the gate
dielectric. After that, the ion gel gate MoS2 devices were annealed in an Ar atmosphere at 120℃
for 1 h before electrical measurements.
3.5.4 Characterization:
The SEM studies and EDS analyses were performed using a Nova NanoSEM 450 fieldemission scanning electron microscope. Atomic force microscopy imaging was conducted using
a Digital Instruments DI 3100. The optical microscopy image and Raman mapping spectra were
collected using a Renishaw inVia micro-Raman spectrometer.
Electrical characteristics of the printed MoS2 FETs and gas sensing results were measured
with a Keithley 4200-SCS semiconductor characterization system.
3.6 Conclusion of inkjet-printed MoS2 field-effect transistors and their gas sensing
applications
In conclusion, we have demonstrated high-quality solution-processable MoS2 ink comprised
of MoS2 nanoplates and high-performance FETs based on inkjet-printed MoS2 channels. By
applying binary solvent of IPA and 2-butanol to our MoS2 ink, we obtained continuous and rather
uniform printed MoS2 film, which enabled our devices to exhibit carrier mobility of 11 cm2V-1s-1
and an on/off ratio of 106. We also demonstrated low gate voltage modulation using ion gel-
52
gated FETs based on the printed MoS2 film. The ion gel-gated FETs were further prepared on
flexible polyimide substrates, showing great stability under tensile strain. Lastly, we applied our
printed MoS2 devices for NO2 and NH3 gas sensing, with a LoD of 10 ppb for NO2 and 500 ppb for
NH3. Owing to the rich edge sites and sulfur vacancies, our printed gas sensors displayed high
sensitivity, fast response, and rapid recovery with the assistance of UV light, making them ideal
for integrated gas sensing applications.
53
Chapter 4: Stretchable printed MoS2 field effect transistor and it’s
wearable biosensors applications
4.1 Introduction to stretchable biosensors based on inkjet-printed 2D materials
Field effect transistors based on 2D materials are potential candidates for sensing
applications because any perturbation of such atomically thin 2D channels can significantly affect
their electronic properties. MoS2 materials, synthesized by various methods, with various
morphologies, have been applied for biosensors, targeting various biomarkers 146-153. Among
various types of sensors, the most promising category of biosensors based on MoS2 are FET-based
sensors creating electronic signals, because they are more suitable to be integrated into modern
medical equipment and be used for wearable devices and real-time detection. To achieve
electrochemical sensors based on MoS2, high-performance MoS2 devices are needed, which
previously required exfoliated or CVD MoS2. For example, Sarkar et. al. applied MoS2 field-effect
transistors as biosensors using exfoliated MoS2 152, as shown in Figure 4.1. In this chapter, with
high-performance FETs based on our printed MoS2 films demonstrated in chapter 3, we will start
to investigate the potential them as the biosensors in the future research. Besides, our inkjet
printed MoS2 films, with abundant edge sites and sulfur vacancies as active sites for sensing, have
been demonstrated to be high-performance NO2 and NH3 in chapter 3.
Moreover, most of the existing MoS2 biosensors require a grafting layer such as Al2O3 or
HfO2, as shown in Figure 4.1, over MoS2 due to the difficulty directly functionalize the surface of
MoS2 and instability of MoS2 devices in liquid phase. The grafting layer will increase the physical
distance between the channel and the charged biomolecules and screen the signaling charges
caused by interaction between the receptor molecules and the target analytes. In this chapter,
54
we also want to demonstrate the potential of our MoS2 FETs as direct biosensors without the
grafting layer.
Figure 4.1 Table of content showing the FET biosensors based on exfoliated MoS2, reported in Ref 152.
In addition, to fulfill the potential of our MoS2 device in clinical and point-of-care diagnosis,
we also want to pave the way to wearable biosensors. As we demonstrated in Chapter 3, our
printed devices work well on flexible substrates. The flexible MoS2 devices have also been
demonstrated in several papers 154-158. However, to achieve high-performance wearable
electronic, flexibility alone is not enough, stretchability is the next goal. Unfortunately, due to the
nature of chemical bonds in MoS2, the MoS2 itself is not stretchable. The crack will occur in
monolayer MoS2 under strain below 10% strain, thus stretchable MoS2 devices is still a big
challenge. However, for our printed MoS2 devices, the interlayer sliding between MoS2 nanoflake
can effectively accommodate the strain applied to the device. Therefore, our printed MoS2 device
55
is a promising candidate for next generation stretchable devices, and we will try to demonstrate
this point in this chapter.
4.2 Fabrication of stretchable MoS2 FETs and functionalization process for
biosensors
4.2.1 Fabrication of stretchable MoS2 FETs
Figure 4.2 Procedure of fabrication of stretchable MoS2 FETs
The fabrication process of the stretchable MoS2 devices is shown in Figure 4.2. The MoS2
channel was first printed on Si/SiO2 (300 nm SiO2) substrate following the recipe described in
chapter 3.5.2. It worth mentioning that the uniformity can be further improved by using O2
plasma to treat Si/SiO2 substrate before printing. Then the Au (50 nm) electrodes were patterned
and deposition by photolithography and e-beam metal evaporator. After that, PDMS or SEBS
solution was spin-coated or drop cast on the prepared device, PDMS was cured at 70℃ for 4
hours and SEBS was cured at room temperature overnight. It was followed by immersing the
substrate in 2M KOH solution to etch SiO2 sacrifice layer so that the MoS2 device would be
transferred to the stretchable substrate and peeled off from Si/SiO2 substrate.
56
Furthermore, to achieve intrinsic stretchable MoS2 devices, P3-CNTs can be used as
electrodes material. P3 nanotubes are mixture of metallic and semiconducting CNTs, they were
first drop cast on Si/SiO2 substrate, and then annealed at 100℃ for 20 min. The electrodes were
patterned by photolithography and the unwanted region can be etched by O2 plasma. Some
stretchable MoS2 devices with P3-CNT electrodes after being transferred to PDMS substrate are
shown in Figure 4.3.
Figure 4.3 Optical image of stretchable MoS2 devices with P3 nanotube on PDMS substrate
4.2.2 Stretchability of MoS2 devices
We prepared MoS2 devices on both PDMS and SEBS substrate. Although SEBS can be
dissolved by most of organic solvent which limits the fabrication methods, it is a more stretchable
material suitable for higher strain. We tested the conductance of our 2 devices on SEBS under
different strain and after different cycles of stretchable, as shown in Figure 4.4. As shown in
Figure 4.4a, our device can bear strain up to 50%, which is the best result reported so far. In
Figure 4.4b, after stretching our device to 15% for more than 100 times, negligible change in
conductance was observed. However, we have to point out that the ability of our stretchable
devices to “recover” from high strain to low strain is not ideal. For example, after being stretched
to 35% strain and relaxed to 15% strain, the device can never recover its previous conductance
57
at 15% strain even when we relax the device to 15%. This is an important point we need to
improve in future study by optimizing the fabrication process and device structure.
We also tried to apply ion gel as the gate dielectric, which is frequently used in stretchable
devices, with similar method described in chapter 3. However, unfortunately, the gate leakage
of ion gel gate MoS2 devices on SEBS is 10 ten times larger than that on Si/SiO2, while the current
density decreases a lot, which makes it hard to obtain good FET performance. We would like to
explore new stretchable dielectric materials and further optimize the fabrication process of our
stretchable MoS2 devices to eliminate the negative effect of the large leakage.
Figure 4.4 (a) ID-VD curve of the MoS2 devices on SEBS under different strain (b) Cyclability of the MoS2 devices on SEBS
under 15% strain
4.3 Stretchable MoS2 devices for biosensor application
4.3.1 Method to functionalize MoS2 devices
The functionalization process of MoS2 devices is shown in Figure 4.5a. A mild O2 plasma
was first carried out on the printed MoS2 devices to create some oxygen vacancies, so that in the
58
next step APTES (Structure shown in Figure 4.5b) can link to these sites on the edge of the MoS2
flakes. After that, glutaraldehyde linker was applied to the devices, the related chemistry was
shown in Figure 4.5c. The desired antibody or probe DNA can then be introduced to our devices
and be linked to the surface. A similar functionalization method was reported in a published
paper159, we modified it and applied it to our biosensor. To test this functionalization method,
we applied CA-125 antibody with biotin and streptavidin with Au nanoparticles in turn. As shown
in the SEM image in Figure 4.5d, the high density of gold nanoparticles confirmed the successful
linkage of CA-125 antibody of the printed MoS2 devices.
Figure 4.5 (a) procedure of functionalization of the MoS2 devices (b) Chemical structure of APTES
(c) APTES/glutaraldehyde chemistry (d) SEM image of the surface of MoS2 channel after functionalizing with CA-125 antibody
59
4.3.2 Potential of the stretchable MoS2 devices in biosensor application
I want to first clarify that we only got some preliminary data to indicate the potential of
our stretchable MoS2 devices in biosensor application, more in depth study need to be carried
out to accomplish the stretchable MoS2 devices in biosensor.
To achieve MoS2 biosensors without grafting layer, we first need to ensure the stability of
devices in liquid phase. We measured the ID-VG curves of our printed MoS2 FETs on Si/SiO2
substrate in 1X PBS over 5 days, which is a commonly used solvent in biosensing field to mimic
the ion strength and pH value of human and body fluids. The liquid-gate voltage is applied by an
Ag/AgCl standard electrode and VD is fixed at 0.1V. The result is shown in Figure 4.6, we can claim
that our printed MoS2 devices exhibit excellent stability to fulfill the working condition of
biosensors.
Figure 4.6 Stability of printed MoS2 FETs in 1XPBS
60
Unfortunately, our MoS2 biosensors only showed very little current change in response
to either CA-125 antigen and DNA targets. We believe this is due to the fact that the printed MoS2
films are too thick (about 10 nm as described in Chapter 3) and most of the biomarkers are linked
to the top layer of the MoS2 film, which is the non-continuous part of our printed film, thus they
can only have very limited effect on the conductance of the MoS2 devices.
On the other hand, our stretchable devices can solve this problem because, during the
transfer of devices from Si/SiO2 substrate to PDMS/SEBS substrate, the previous bottom
continuous MoS2 layers are now the top layer, which is to be functionalized and conjugated with
biomarkers. We measured the conductance change of the stretchable devices on SEBS in
response to DNA molecules. As shown in Figure 4.7, in 3 trials of measurements, we can see
obvious conductance change after adding DNA biomarkers, thus demonstrating potential of our
devices as wearable biosensor. I want to point out that here the DNA is only anchored to our
MoS2 devices by the Van der Waals force without a linker, which has also been demonstrated in
several papers 160-162. It’s also worth mentioning that we encountered many challenges
applying our stretchable devices to biosensors, for example, it’s hard to stabilize the stretchable
61
device in PBS, current can change by 10-20% by applying the probe on electrodes, and device
variation is large. We hope to solve these problems in our future studies.
Figure 4.7 ID-VD curves of MoS2 devices before and after incubation of DNA molecules
62
Conclusion
In this dissertation, we first reported an Au-vapor-assisted CVD method to synthesize
aligned monolayer WSe2. The advantages of this approach can be concluded as follows: (1) 97%
of WSe2 flakes in the downstream region have 0° and 60° edge orientations; (2) the edge size of
the aligned monolayer WSe2 flakes can be as large as 130 μm; (3) thanks to the catalytic effect of
Au, our CVD set up can achieve a fast growth rate of 4.3 μm·s−1 and the growth time can be as
short as 30 s. With Raman, PL, and TEM characterization and FET device study, we have
demonstrated that the aligned monolayer WSe2 performed good quality. The continuous
monolayer WSe2 film formed by individual aligned domains showed outstanding uniformity and
electronic properties, and it is suitable for large-scale and low-cost nanofabrication processes like
photolithography. We believe that the introduction of catalytic metal vapor should be able to
impact the CVD process of other TMDCs with various substrates.
Secondly, we have demonstrated sensitive, scalable, and cost-efficient COVID-19
biosensors using electronic biosensing platform based on In2O3 FET devices functionalized with
phosphatase. The devices were fabricated by a simple and cheap shadow mask method. Our
biosensors were able to detect SARS-CoV-2 spike protein in UTM (LoD: 100 fg/mL) and S1 protein
specific IgG antibody in human whole blood (LoD: 1 pg/mL), indicating its potential for clinical
diagnostic testing. Our results can be instrumental for the management and control of the
current pandemic and can possibly prevent further community transfer through early, rapid, and
cost-efficient screening of COVID-19, giving us the upper hand to win the fight against this
pandemic.
63
Furthermore, we have demonstrated high-quality solution-processable MoS2 ink and
high-performance FETs based on inkjet-printed MoS2 channels. By applying binary solvent of IPA
and 2-butanol to our MoS2 ink, we obtained continuous and uniform printed MoS2 film, which
enabled our devices to exhibit mobility of 11 cm2
V-1s-1 and an on/off ratio exceeding 106. We also
demonstrated ion gel-gated FETs based on the printed MoS2 film, so that the gate modulation at
a low gate voltage of 2 V was achieved. The ion gel-gated FETs were further prepared on flexible
polyimide substrates, showing great stability under tensile strain. Lastly, we applied our printed
MoS2 devices as NO2 and NH3 gas sensors, with a LoD of 10 ppb for NO2 and 500 ppb for NH3.
Owing to the rich edge sites and sulfur vacancies, our gas sensors displayed high sensitivity, fast
response, and rapid recovery with the assistance of UV light.
At last, we demonstrated the potential of our printed MoS2 FETs in stretchable electronics
and wearable biosensors. However, there are still many challenges to solve in our future studies.
We need to optimize the structure of our devices and the process of fabrication to further
improve the stretchability, and explore new stretchable dielectric to obtain great FET
performance. We also need to improve the biosensing performance by decreasing the thickness
of MoS2 films and designing better sensing structure.
64
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Appendix
Figure A1.1. Photograph of the gold-vapor-assisted CVD set up.
74
Figure A1.2. Optical micrograph images of the control experimental results using the same growth parameters but
without gold foil after (a) 30 s and (b) 5 min growth times.
Figure A1.3. An Optical micrograph image showing the intermediate state of the formation of continuous monolayer
WSe2 film. Most of the area in the image is covered by monolayer WSe2 together with a small part of the exposed sapphire
substrate. The parallel edges of WSe2 can be clearly observed.
75
Figure A1.4. X-ray photoelectron spectroscopy (XPS) study. (a), (b) are the survey profiles, Au 4f, respectively. No
discernible gold signal is detected.
Figure A1.5. Energy dispersive X-ray spectroscopy (EDX) study, (a) SEM image of a selected area (b), (c), (d) Element
mapping of Se, W and Au, respectively. The scale bars are 5 μm. No discernible gold signal was detected.
76
Figure A1.6. Characterization of Au on the surface of the substrate after 48 hours. (a) Optical image of the Au foil. (b)
Optical image of the substrate. (c), (d) SEM and EDX of the region marked in (b).
Figure A1.7. An optical micrograph image showing electrodes fabricated on continuous WSe2 using photolithography.
77
Figure A2.1. (a) Photograph of the liquid gate measurement setup. (b) Magnified image of the channel region of the
In2O3 device, a Teflon cell with an opening at the bottom was used to contain the solution.
Figure A2.2. Drain current versus back gate voltage of nine randomly picked In2O3 nanoribbon FET devices with drain
voltage fixed at (a) 150 mV. (b) at 5 V. (c) IDS − VGS of the same device measured for 20 times over a period of 1 week
with Vd fixed at 1V.
78
Figure A2.3. Mobilities of the nine devices labeled in figure 2.1b at Vsd = 1 V.
Figure A2.4. Family curves of IDS−VDS measured in 0.01× phosphate-buffered saline with the liquid gate varying starting
from 800mV in steps of 100 mV.
79
Figure A2.5. Baseline of our electrical measurement, with changed volume of MgCl2 and NaOH solution, pH=9.7.
Figure A2.6. Current vs time curve for S1 antigen detection. Current increased after adding substrate solution, and it
dropped after bubbling to mix the solution thoroughly. Antigen concentration = 100pg/mL.
80
Figure A3.1. (a) A typical AFM image of multiple MoS2 nanoplates. Scale bar, 5μm. (b) AFM image of the channel region
of the MoS2 FETs and the height profile along the green dash line. Scale bar, 20 μm. (c) Optical image of a printed MoS2
film and the height profile over a scratch on the printed MoS2 film to determine the thickness. The AFM scan was taken
in the location labeled by the red dot. Scale bar, 250 μm.
Figure A3.2. Optical images of a printed single drop of MoS2 inks with (a) 90% IPA + 10% 2-butanol binary solvent (Scale
bar, 200 μm) and (b) pure IPA solvent (Scale bar, 200 μm).
81
Figure A3.3. (a) SEM image of the continuous MoS2 film. Scale bar, 10 μm. (b,c) EDS results of Molybdenum (b) and
sulfur (c), over the region shown in (a)
Figure A3.4. (a) Typical Raman spectra and (b) all Raman spectra corresponding to the Raman mapping in Figure 3.2d.
82
Figure A3.5. (a, b) Double sweep transfer (ID-VG) characteristics in both linear and logarithm scale of (a) back-gated
and (b) ion gel-gated MoS2 FETs, VD = 1V. (c) Output (ID-VD) characteristics of a back-gated MoS2 FET in the saturation
region.
Figure A3.6. An optical image of the channel region of the interdigitated inkjet-printed MoS2 FET, which is used as NO2
and NH3 gas sensors. Scale bar, 100 μm.
Abstract (if available)
Abstract
Orientation-controlled growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) may enable many new electronic and optical applications. However, previous studies reporting aligned growth of WSe2 usually yielded very small domain sizes. Herein, we introduced gold vapor into the chemical vapor deposition (CVD) process as a catalyst to assist the growth of WSe2 and successfully achieved highly aligned monolayer WSe2 triangular flakes grown on c-plane sapphire with large domain sizes (130 μm) and fast growth rate (4.3 μm·s−1). When the aligned WSe2 domains merged together, a continuous monolayer WSe2 was formed with good uniformity. After transferring to Si/SiO2 substrates, field effect transistors were fabricated on the continuous monolayer WSe2, and an average mobility of 12 cm2·V−1·s−1 was achieved, demonstrating the good quality of the material. This report paves the way to study the effect of catalytic metal vapor in the CVD process of TMDCs and contributes a novel approach to realize the growth of aligned TMDC flakes. In Chapter 1, the aligned growth of WSe2 will be introduced.
Developing convenient and accurate SARS‑CoV-2 antigen test and serology test is crucial in curbing the global COVID-19 pandemic. In this work, we report an improved Indium Oxide (In2O3) nanoribbon field-effect transistor (FET) biosensor platform detecting both SARS‑CoV-2 antigen and antibody. Our FET biosensors, which were fabricated using a scalable and cost-efficient lithography-free process utilizing shadow masks, consist of an Indium Oxide (In2O3) channel and a newly developed stable enzyme reporter. During the biosensing process, the phosphatase enzymatic reaction generated pH change of the solution, which was then detected and converted to electrical signal by our In2O3 FETs. The biosensors applied phosphatase as enzyme reporter, which has a much better stability than the widely used urease in FET based biosensors. As proof-of-principle studies, we demonstrate the detection of SARS‑CoV-2 spike protein in both phosphate-buffered saline (PBS) buffer and universal transport medium (UTM) (limit of detection [LoD]: 100 fg/mL). Following the SARS-CoV-2 antigen tests, we developed and characterized additional sensors aimed at SARS-CoV-2 IgG antibodies, which is important to trace past infection and vaccination. Our spike protein IgG antibody tests exhibit excellent detection limits in both PBS and human whole blood (LoD: 1 pg/mL). Our biosensors display similar detection performance in different mediums, demonstrating that our biosensor approach is not limited by Debye screening from salts and can selectively detect biomarkers in physiological fluids. The newly selected enzyme for our platform performs much better performance and longer shelf life which will lead our biosensor platform to be capable for real clinical diagnosis usage. In Chapter 2, the SARS‑CoV-2 biosensors based on In2O3 FETs will be discussed.
Owing to the simplicity, scalability, and cost-efficiency, solution-processable two-dimensional (2D) semiconductors have attracted great interest in electronic applications, especially as the channel material for field-effect transistors (FETs). Inkjet printing is a lithography-free technique to achieve drop-on-demand patterning of solution-processable 2D ink. However, thus far, inkjet-printed 2D FETs exhibit limited performance due to the coffee-ring effect and consequent discontinuity of the printed 2D material films. Here, we report high-performance and flexible inkjet-printed MoS2 FETs with high mobilities and high on/off ratios, and their gas sensing applications. By preparing high-quality MoS2 ink comprised of MoS2 nanoplates using electrochemical exfoliation and then applying a binary solvent comprised of 2-butanol and isopropanol, the obtained ink was printed to form a continuous and relatively uniform MoS2 film, and high-performance printed MoS2 FETs were demonstrated, with mobilities of 11 cm2V-1S-1 and on/off ratios of 106. Furthermore, low voltage gate modulation was achieved by applying an ion gel gate, and robust electrical performance under tensile strain was observed for the ion gel-gated MoS2 FETs printed on flexible substrates. As the printed MoS2 film is abundant in edge sites and sulfur vacancies, we further demonstrated our MoS2 FETs as high-performance gas sensors with a limit of detection (LoD) of 10 ppb for NO2 and 0.5 ppm for NH3, together with a fast recovery rate. Chapter 3 of this dissertation will focus on the preparation of MoS2 ink and printed MoS2 FETs.
Due to the nature of chemical bonds in MoS2, MoS2 is not a stretchable material. Therefore, although flexible MoS2 devices have been demonstrated in many works, very few papers about stretchable MoS2 devices have been published so far. However, for our printed MoS2 devices, the interlayer sliding between MoS2 nanoflakes can effectively accommodate the strain applied to the device, making them a promising candidate to meet the challenge of stretchable MoS2 devices. On the other hand, field-effect transistors based on 2D materials are potential candidates for sensing applications because any perturbation of such atomically thin 2D channels can significantly affect their electronic properties. Besides, our inkjet-printed MoS2 films, with abundant edge sites and sulfur vacancies as active sites for sensing, also exhibit great potential in the biosensing field. Combining the stretchability and advantage for biosensors, we will demonstrate the potential of our printed MoS2 devices in wearable electronic biosensors. Chapter 4 will include the introduction of stretchable MoS2 biosensors and some preliminary data we have so far.
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Chen, Mingrui
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In2O3 COVID-19 biosensors and two-dimensional materials: synthesis and applications
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Doctor of Philosophy
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Materials Science
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2024-05
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