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Hot carrier enhanced photocatalysis in plasmon resonant metal grating systems
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Hot carrier enhanced photocatalysis in plasmon resonant metal grating systems
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Content
HOT CARRIER ENHANCED PHOTOCATALYSIS IN PLASMON RESONANT METAL
GRATING SYSTEMS
By
Lang Shen
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)
August 2018
Copyright 2018 Lang Shen
i
Acknowledgments
I want to thank my research advisor Prof. Stephen B. Cronin for his invaluable guidance,
support and inspiration. I have been fortunate to have his continuous help throughout my PhD
study. He spent days and nights teaching me how to research, write and think critically. I am a
better researcher because of him.
I want to thank my dissertation committees Prof. Steve R. Nutt, Prof. Chongwu Zhou for
their invaluable suggestions, as well as Prof. Andrew Armani and Prof. Wei Wu for serving on
my qualifying exam committee.
I would also like to thank Dr. Matthew Mecklenburg from the Center for Electron
Microscopy and Microanalysis at USC for introducing me to the world of electron microscopy
and letting me play with TEM. I would also like to thank my friends and colleagues from Cronin
group, Zhou group and Yoon group. I am grateful to Xuan, Nirakar and Yu for encouraging me
and motivating me to graduate.
Finally I would like to thank my family and my girlfriend Ruqin for all the love and
support through life.
.
.
ii
Contents
Acknowledgments ............................................................................................................................ i
List of Tables ................................................................................................................................. iv
List of Figures ................................................................................................................................. v
Abstract ........................................................................................................................................ xiii
Chapter 1 : Introduction .................................................................................................................. 1
1.1 Artificial photosynthesis ..................................................................................................................... 1
1.2 Photoelectrolysis ................................................................................................................................. 3
1.3 Plasmonic enhancement in photoelectrolysis.................................................................................... 13
Chapter 2 : Nanostructured silicon photocathodes for solar water splitting ................................. 20
2.1 Abstract ............................................................................................................................................. 20
2.2 Introduction ....................................................................................................................................... 21
2.3 Experimental details .......................................................................................................................... 22
2.4 Results and discussion ...................................................................................................................... 24
2.5 Conclusion ........................................................................................................................................ 40
Chapter 3 Plasmon resonant amplification of hot electrons in a grating based photodiode ......... 43
3.1 Abstract ............................................................................................................................................. 43
3.2 Introduction ....................................................................................................................................... 44
3.3 Experimental details .......................................................................................................................... 46
3.4 Results and discussion ...................................................................................................................... 49
3.5 Conclusion ........................................................................................................................................ 54
Chapter 4 Plasmon resonant amplification of hot electrons in water splitting ............................. 56
4.1 Abstract ............................................................................................................................................. 56
4.2 Introduction ....................................................................................................................................... 57
4.3 Experimental details .......................................................................................................................... 59
4.4 Results and discussion ...................................................................................................................... 62
4.5 Conclusion ........................................................................................................................................ 68
Chapter 5 Nanoscale thermometry using electron energy loss spectroscopy (EELS) in
suspended MoS2 ............................................................................................................................ 70
5.1 Abstract ............................................................................................................................................. 70
5.2 Introduction ....................................................................................................................................... 71
5.3 Experimental details .......................................................................................................................... 72
5.4 Results and discussion ...................................................................................................................... 76
iii
5.5 Conclusion ........................................................................................................................................ 84
Chapter 6 Outlook and future work .............................................................................................. 86
6.1 Hot electron photocatalysis ............................................................................................................... 86
6.2 Hot electron injection into 2D materials ........................................................................................... 93
Bibliography ................................................................................................................................ 100
Appendix A: Plasmonic silver nanostructures prepared by electron beam evaporation ............. 107
iv
List of Tables
Table 1-1 U.S. electricity generation by source, amount, and share of total in 2017 ..................... 3
Table 2-1 Measured Photoelectrochemical Characteristics of Bare and Nanostructured Silicon
Photocathodes in Water Splitting.................................................................................................. 40
v
List of Figures
Figure 1-1: Pourbaix diagram of water.
3
......................................................................................... 4
Figure 1-2: Schematic demonstration of alkaline water electrolysis.
4
............................................ 6
Figure 1-3:Unipolar design of alkaline electrolyzer. ...................................................................... 7
Figure 1-4: Bipolar design of alkaline electrolyzer......................................................................... 7
Figure 1-5: Schematic demonstration of PEM electrolysis.
6
.......................................................... 8
Figure 1-6: The band structures of an n-type semiconductor in contact with a solution
containing a redox couple.
1
............................................................................................................. 9
Figure 1-7 Schematic diagram of a photoelectrochemical cell. .................................................... 10
Figure 1-8: Single semiconductor for unassisted water splitting under illumination. .................. 11
Figure 1-9: Band edge positions of common semiconductors used in photocatalysis.
1
............... 11
Figure 1-10: Schematic illustration of a localized surface plasmon.
12
.......................................... 14
Figure 1-11: (a) Shape and (b) size tuning of plasmon resonance wavelength of silver
nanoparticles.
11
.............................................................................................................................. 15
Figure 1-12: Injection of hot electrons from plasmonic structures into a semiconductor.
15
......... 17
Figure 1-13: Schematic diagrams illustrating the (a) hot electron injection (HEI) of
photoexcited carriers and (b) local field enhancement (LFE) of sub-bandgap absorption (i.e.,
defect states).................................................................................................................................. 18
Figure 1-14: Schematic diagrams of simple plasmonic metal gratings irradiated with (a)
transverse electric (TE) and (c) transverse magnetic (TM) polarized light. Cross-sectional
distribution of the electric field intensity (E
2
) of this grating when irradiated with (b) TE and
(d) TM polarized light. .................................................................................................................. 19
vi
Figure 2-1: (a) Schematic illustration of the processing steps to fabricate nanostructured silicon
photocathodes incorporating one-dimensionally periodic lamellar morphology derived from
the self-assembly of symmetric poly(styrene-b-methyl methacrylate) diblock copolymers. (b)
Schematic energy band diagrams for pp
+
(left) and n
+
pp
+
(right) Si photocathodes under
illumination, where q, Ecb, Evb, Ef,n, Ef,p, Voc, and Eº(H
+
/H2) are electronic charge, energy levels
at the conduction and valence band edge, electron and hole quasi-Fermi levels, photovoltage,
and reduction potential for the hydrogen evolution reaction, respectively. (c) Tilt-view
scanning electron microscope (SEM) image of a representative silicon photocathode with
lamellar surface nanopatterns after the reactive ion etching and removal of Cr. Inset shows a
photographic image of the completed nanostructured silicon photocathodes mounted on a slide
glass............................................................................................................................................... 26
Figure 2-2: (a) Top-view scanning electron microscope (SEM) images of the nanostructured
surface of the pp
+
-silicon at various etching times of inductively-coupled plasma reactive ion
etching (ICP-RIE) using a thin (~10 nm) layer of Cr patterned through the block copolymer
lithography as an etch mask. The insets show cross-sectional-view SEM images. (b) Plot of
the measured etch depth (red) and corresponding calculated ratio of the surface area (blue) for
the nanostructured silicon (ABCP) at various etching times with respect to that of a bare silicon
substrate (Abare). (c) Measured total (specular + diffuse) reflectance spectra of nanostructured
and bare silicon at various etching times, measured on a spectrophotometer equipped with an
integrating sphere at near-normal incidence (θ = 8º) using Spectralon
as a 100% reflectance
standard. ........................................................................................................................................ 28
vii
Figure 2-3: Top-view SEM images of pp
+
-silicon used for the construction of model
nanostructured silicon for FDTD calculations, at the etching times of (a) 30, (b) 45, (c) 60, and
(d) 90 s, respectively. .................................................................................................................... 31
Figure 2-4: Top-view SEM images of n
+
pp
+
-silicon used for the construction of model
nanostructured silicon for FDTD calculations, at the etching times of (a) 30, (b) 45, (c) 60, and
(d) 90 s, respectively. .................................................................................................................... 32
Figure 2-5: Schematic illustration of constructed model 3D nanostructures for pp
+
-silicon with
the characteristic surface morphologies of self-assembled BCP lamellae, at the etching times
of (a) 30, (b) 45, (c) 60, and (d) 90 s, respectively. ...................................................................... 33
Figure 2-6: (a) Simulated reflectance spectra (solid lines) of pp
+
-silicon at normal incidence
from semi-infinite silicon with model nanostructured surfaces of BCP lamellae at various
etching times, overlaid with experimentally measured spectra (dotted lines) for comparison.
To construct the model 3D nanostructured surface, top-view SEM micrographs of etched
silicon over the area of ~3.0 x 5.0 μm
2
were imported, where the height of the nanostructured
region was matched with the etch depth in experiments. (b) Simulated reflectance spectra from
the same nanostructured silicon as in (a) yet in water as a superstrate medium. (c) The
corresponding integrated solar flux absorption (S_abs) calculated by Eq. (1) over AM1.5G
standard solar illumination. (d) Comparison of S_abs of semi-infinite silicon incorporating the
BCP surface nanostructure (red) and a perfectly aligned 1D periodic grating (blue) as a
function of the height of the nanostructures, both at normal incidence. In the case of the 1D
grating (see the inset image), the periodicity (50 nm) and duty (50%) were assumed
approximately same as those of BCP lamellae, and the calculation was averaged over
transverse electric (i.e. electric field parallel to the direction of the trench) and transverse
viii
magnetic (i.e. electric field perpendicular to the direction of the trench) polarizations (Figure
S3). (e) Corresponding reflectance (R(λ)) and absorption (A(λ)) spectra for BCP nanostructure
and 1D periodic grating in water at the optimum heights that maximize S_abs for each case
(80 nm for 1D grating, 90 nm for BCP)........................................................................................ 34
Figure 2-7: (a) Simulated reflectance spectra (solid lines) of n
+
pp
+
silicon at normal incidence
from semi-infinite silicon with model nanostructured surfaces of BCP lamellae at various
etching times, overlaid with experimentally measured spectra (dotted lines) for comparison.
(b) Simulated reflectance spectra from the same nanostructured silicon as in (a) yet in water
as a superstrate medium. The inset shows the corresponding integrated solar flux absorption
(S_abs) calculated by Eq. (1) over AM1.5G standard solar illumination. .................................... 35
Figure 2-8: Schematic illustration of the model nanostructured silicon with 1D periodic
gratings for the reflectance calculation by a FDTD method, where the periodicity and duty
cycle of gratings are 50 nm and 50%, respectively. The calculation was averaged over both
transverse electric (i.e. electric field parallel to the y-axis, θ = 90º) and transverse magnetic
(i.e. electric field parallel to the x-axis, θ = 0º) polarizations. ...................................................... 36
Figure 2-9: Current density (J)-potential (E, with respect to the reversible hydrogen
electrode(RHE)) plot of nanostructured silicon photocathodes (a) without and (c) with a buried
pn-junction, measured under AM1.5G simulated standard solar spectrum in 0.5 M aqueous
sulfuric acid. (b-d) Comparison of corresponding PEC characteristics including open circuit
density (Voc), potential at the current density (J) of 20 mA/cm
2
, and saturation current density
(Jsat). .............................................................................................................................................. 39
Figure 3-1: Schematic illustration of the experimental design. .................................................... 45
ix
Figure 3-2: (a) Cross-sectional scanning electron microscope (SEM) image and (b) schematic
diagram of the plasmon resonant grating structure. (c) Energy band diagram with respect to
vacuum illustrating the mechanism of hot electron injection. ...................................................... 47
Figure 3-3: (a) Photocurrent and (b) photoreflectance plotted as a function incident angle for
633 nm light polarized parallel and perpendicular to the grating structure. (c) Schematic
diagram of the experimental measurement configuration. ........................................................... 48
Figure 3-4: (a) Finite difference time domain (FDTD) simulations of the photoreflectance as a
function of the incident angle for p- and s-polarized light. Cross-sectional electric field
intensity profiles (E
2
) for illumination at (b,d) normal and (c,e) 10
o
incidence. .......................... 51
Figure 3-5: FDTD simulations of reflection of 633 nm light for different metals and pitches at
different incident angles. ............................................................................................................... 53
Figure 4-1: Schematic illustration of the experimental design. .................................................... 58
Figure 4-2: (a) Cross-sectional scanning electron microscope (SEM) image and (b) schematic
diagram of the plasmon resonant grating structure. (c) Energy band diagram with respect to
NHE illustrating the mechanism of hot electron injection mechanism. ....................................... 60
Figure 4-3: (a) AC photocurrent and (b) photoreflectance plotted as a function incident angle
for 633nm light with an intensity of 106mW/cm
2
polarized parallel and perpendicular to the
grating structure. (c) DC photocurrent plotted as a function of reference potential when
illuminated at -10
o
with respect to normal incidence. (d), (e) Schematic diagrams of the
experimental measurement configuration. .................................................................................... 61
Figure 4-4: Comparison of diffuse reflectance spectra of flat Au and Grating surface ................ 64
Figure 4-5: AC photocurrent after applying constant potential for different time periods. .......... 64
Figure 4-6: Angle dependence of photocurrent of a grating with 5 nm Al2O3 coating. ............... 65
x
Figure 4-7: (a) Finite difference time domain (FDTD) simulations of the photoreflectance as a
function of the incident angle for p- and s-polarized 633nm light. Cross-sectional electric field
intensity profiles for illumination at (b) p-polarized and normal incidence; (c) p-polarized and
10
o
incidence; (d) s-polarized and normal incidence; (e) s-polarized and 10
o
incidence. ............ 67
Figure 5-1: (a) Schematic illustration of the suspended MoS 2 sheet on TEM chip. (b) STEM
(c) optical and (d) SEM images of MoS2 sheet............................................................................. 74
Figure 5-2: Optical images of (a), (b) MEMS microheater chip for NanoEx
TM
-i/v. (c)
microheater with MoS2 flakes. (d) Raman spectra of MoS2 at different temperatures. (e)
Comparison of theoretical and experimental Raman peak position as a function of temperature.
....................................................................................................................................................... 75
Figure 5-3: (a) Raman spectra of MoS2 at different stage temperatures in the cryostat. (b)
Raman spectra of MoS2 showing local heating effects from incident laser. (c) Raman peak
positions at different incident laser powers measured at a stage temperature of 400 K. .............. 77
Figure 5-4: Raman peak positions as a function of incident laser power at different stage
temperatures for (a) E
1
2g and (b) A1g modes. ................................................................................ 78
Figure 5-5: (a) Dark field image and plasmon energy maps of MoS2 from EELS measurement.
(b) Comparison of theoretical and experimental plasmon energy shifts as a function of
temperature. .................................................................................................................................. 80
Figure 5-6: Measured temperatures obtained from PEET and Raman studies plotted as a
function of chip temperature. ........................................................................................................ 82
Figure 5-7: (a) 2D temperature map and (b) 1D temperature profile plotted along the suspended
length of a MoS2 sheet from EELS mapping. ............................................................................... 83
xi
Figure 6-1: (a) Schematic diagram of the plasmon resonant grating structure. (b) Energy band
diagram with respect to NHE illustrating the mechanism of hot electron injection mechanism.
....................................................................................................................................................... 87
Figure 6-2: (a) AC photocurrent and (b) photoreflectance plotted as a function incident angle
for 633 nm light with an intensity of 106 mW/cm
2
polarized parallel and perpendicular to the
grating structure. (c) DC photocurrent plotted as a function of reference potential when
illuminated at -10
o
with respect to normal incidence. ................................................................... 89
Figure 6-3: k vector of incident light in medium and estimate of resonant k vector as a function
of incident angle. ........................................................................................................................... 90
Figure 6-4: (a) FDTD simulations of the photoreflectance as a function of the incident angle
for p- and s-polarized 633 nm light. Cross-sectional electric field intensity profiles for
illumination at (b) p-polarized and normal incidence; (c) p-polarized and 10
o
incidence; (d) s-
polarized and normal incidence; (e) s-polarized and 10
o
incidence. ............................................ 92
Figure 6-5: Energy band diagram illustrating the injection of hot electrons from the metal to
the MoS2........................................................................................................................................ 96
Figure 6-6: (a) Schematic diagram and (b) SEM image of the metal/MoS2 structure. ................. 97
Figure 6-7: Photoluminescence spectra of (a) MoS2 on the plasmonic nanostripe arrays (PNAs)
and (b) bulk Au film. .................................................................................................................... 98
Figure 6-8: FDTD simulation of the plasmonic structure and Au film in TE and TM mode. ...... 99
Figure A-1: TEM images of plasmonic silver nano-island with different nominal thicknesses:
(a), (b) 10 nm. (c) (d) 15 nm. ...................................................................................................... 108
xii
Figure A-2: (a) TEM image of the simulated region (500 X 500 nm). (b) Index and (c) Electric
field profile from the top view. (d) Index and (e) Electric field profile from the cross-section
view. ............................................................................................................................................ 109
xiii
Abstract
This dissertation presents our efforts in developing and improving artificial
photosynthesis processes based on silicon photocathodes and metal grating structures. These
experiments shed light on some fundamental physics in such devices, and could potentially
help in designing more efficient artificial photosynthesis processes.
Chapter 1 provides introduction and background materials to aid in understanding the
experiments presented in the dissertation. The chapter starts with a brief overview of the
utilization of solar energy and artificial photosynthesis. Next we briefly discuss about solar
water splitting as an example of general photoelectrolysis. We also introduce the concept of
plasmon resonance and discuss our motivations to pursue hot electrons for photocatalysis.
More detailed background materials for each topic are covered in the relevant chapter.
Chapter 2 presents some water splitting experiments with silicon based photocathodes.
These experiments show how the photocatalytic behavior of a semiconductor photocathode
can be tuned by improving the light absorption and increasing electrode surface area with
nanostructured surface.
Chapter 3 shows our investigations of hot electrons injection in a diode structure. We
show that with the Au grating/oxide/graphene structure, photocurrent is generated under
irradiation and corresponds well with the plasmon resonance. These results help us to
distinguish between local field enhancement mechanism and hot electron injection mechanism,
which is critical in understanding our enhanced photocatalytic performance in various works.
In Chapter 4 we discuss the extension of the results in Chapter 3 to direct water splitting
experiments. The same grating structures are used in aqueous solution and photocurrent are
xiv
observed as a sign of reversing the water oxidation reaction with photon-generated hot
electrons.
In Chapter 5 focuses on the study of few layer MoS 2 sheets in TEM. Both Raman and
EELS based methods are used to monitor the temperate changes in MoS2. Temperature maps
with nanometer-scale spatial resolution are generated via PEET.
1
Chapter 1 : Introduction
1.1 Artificial photosynthesis
Harvesting energy from the sunlight and converting it to clean, dense and transportable
forms, for example energy stored in H2 and hydrocarbon fuels in the form of chemical bonds,
has been one of the ultimate goals for scientists in the field of energy generation. Solar energy
offers a total power equal to 130 million 500 MW power plants hitting the earth’s surface in
the form of sunlight
1
. Due to the diffuse and intermittent nature of sunlight, utilization of solar
energy has limited means and is currently far from meeting the global energy demand. At
present the main routes to exploit the solar resource are solar electricity and solar-derived fuel
from biomass. While the latter provides the primary energy source for over a billion people,
solar electricity is only 1.3% percent of the total electricity generated in the United States in
2017, as can be seen from Table 1-1
2
. The huge gap between the enormous demand in
renewable energy and our present utilization of solar energy brings a great opportunity and
challenge in developing terawatt-scale, cost-effective and efficient processes to harvest solar
energy.
Solar electricity generation has grown dramatically in the past decade, but still requires
further reduction in cost to compete in the market with fossil energy and nuclear electricity.
Until further development in the field of energy storage, the daily and seasonal oscillations of
solar radiation still significantly limit use of solar electricity. As an alternative way to utilize
solar energy, sunlight can be collected, converted and stored in the form of chemical bonds
and at last released whenever and wherever needed. This has been done successfully in nature
for billions of years, via photosynthesis in bacteria and plants. With artificial photosynthesis,
we now want to mimic this oldest solar energy harvesting process, use solar energy more
2
efficiently and selectively produce the products in need. Artificial photosynthesis starts with
light absorbing materials like semiconductors, which can capture the photon energy and
produce electron-hole pairs, like what chlorophyll is doing in plants. Then energy-storage
molecules ATP and NADPH in plants are then used together with carbon dioxide to produce
organic molecules. Similarly, in artificial photosynthesis, the carriers generated with higher
energies, are then transferred from the semiconductor to certain interfaces and used to drive
desired reactions. Examples of artificial photosynthesis include solar water splitting and solar
carbon dioxide reduction. Splitting water into hydrogen produces a clean fuel whose waste
product upon utilization is only water. Solar carbon dioxide reduction not only provides a way
to harvest solar energy and produce hydrocarbon fuels, but also a sustainable way to recycle
the waste product of all the hydrocarbon fuels including fossil fuels. Both routes are promising
in the pursue of renewable, clean energy generation, but still require further development of
efficient, economically viable photoelectrolysis processes.
3
Table 1-1 U.S. electricity generation by source, amount, and share of total in 2017
Energy source Billion kWh Share of total
Total - all sources 4,015
Fossil fuels (total) 2,495 62.70%
Nuclear 805 20.0%
Renewables (total) 687 17.1%
Hydropower 300 7.5%
Wind 254 6.3%
Biomass (total) 64 1.6%
Wood 43 1.1%
Landfill gas 11 0.3%
Municipal solid waste (biogenic) 7 0.2%
Other biomass waste 3 0.1%
Solar (total) 53 1.3%
Photovoltaic 50 1.2%
Solar thermal 3 0.1%
Geothermal 16 0.4%
Pumped storage hydropower -6 -0.2%
Other sources 13 0.3%
1.2 Photoelectrolysis
In this section we mainly discuss about water splitting as an example of electrolysis and
photoelectrolysis.
Electrolysis is the process that drives a non-spontaneous chemical reaction with
electricity. The most common case, water electrolysis is an important technology for high
quality hydrogen fuel production in small-scale applications. In water electrolysis, electrical
energy is used to split water into hydrogen and oxygen, as shown below:
H
2
O ( l)→ H
2
( g)+
1
2
O
2
( g) ∆G
0
= 237.2 kJ/mol
To split one mole water molecule and produce one mole of H2 and half mole of O2 under
standard conditions, a free energy change of 237.2 kJ is required. Based on the Nernst equation,
4
this corresponds to 1.23 V per electron transferred. To drive reactions at the surface of
electrodes, electrons need to be transferred across the semiconductor/liquid interface. This
requires extra energy to overcome the concentration and kinetic overpotentials, along with the
Ohmic resistance losses resulting in a more general requirement of ~1.5 V in practice to run
steady-state electrolysis of water. Figure 1-1 shows a Pourbaix diagram for water, in which
the predominant species at a given pH and electrode potential are shown in different zones.
The decomposition reaction varies depending on solution pH, and the threshold potentials for
different pathways change with pH at a Nernstian rate of -0.059 V/pH.
2H
2
O ( l)→ 4H
+
( aq)+ O
2
( g)+ 4e
−
(low pH)
2H
+
( aq)+ 4e
−
→ H
2
( 𝑔 ) (low pH)
4𝑂𝐻
−
( aq)→ 2H
2
O ( l)+ O
2
( g)+ 4e
−
(high pH)
2H
2
O ( l)+ 2e
−
→ H
2
( 𝑔 )+ 2𝑂𝐻
−
( aq) (high pH)
Figure 1-1: Pourbaix diagram of water.
3
5
Among water electrolysis technologies, alkaline electrolysis is well-established and can
scale up to megawatt range at commercial level
4
. Figure 1-2 shows the cell for standard
alkaline electrolysis. Typically, an aqueous solution of water with ~30 wt% potassium
hydroxide (KOH) is used. There are two electrodes immersed in electrolytes used for two
different half reactions, anode on the left for oxygen production and cathode on the right for
hydrogen production. To minimize recombination of hydrogen and oxygen, these electrodes
are separated by a diaphragm, as shown at the center of the cell. The diaphragm should have
high ionic conduction activity to allow ion transportation between the two separated
electrolytes. In practice these systems can be configured as unipolar (Figure 1-3) or bipolar
(Figure 1-4). The unipolar design has simple structures and requires fewer parts, but usually
operates at low current densities and low temperatures
5
. The bipolar design offers higher
current density and high-pressure gas output, but is harder to repair with complicated
structures. The efficiency of this process is often limited by the gas bubbles generated at the
electrode surface, reducing contact between electrode and electrolyte and increasing cell
resistance.
Another technology widely used at smaller scales is proton exchange membrane (PEM)
water electrolysis, which is based on a solid proton-conducting membrane. An example is of
PEM electrolyzer is shown in Figure 1-5. Water is oxidized at the anode generating oxygen
while proton is reduced generating hydrogen at the cathode. The thin, ion-conducting film
allows transfer of proton from the anode side to the cathode side, and separates the two gas
products. The bipolar configuration is usually used for PEM polarizers. PEM offers low gas
crossover (higher purity of hydrogen product) and differential pressure configuration allowing
delivering hydrogen at a higher pressure.
6
Figure 1-2: Schematic demonstration of alkaline water electrolysis.
4
7
Figure 1-3:Unipolar design of alkaline electrolyzer.
Figure 1-4: Bipolar design of alkaline electrolyzer.
8
Figure 1-5: Schematic demonstration of PEM electrolysis.
6
Without changing the chemistry in electrolysis, photoelectrolysis can be done by
connecting electrolyzers or catalytic electrodes to photovoltaic modules and supply all the
energy required from sunlight. A more attractive version of this is making water splitting cells
with direct semiconductor/liquid contacts and avoid using separate electrolyzers wired to solar
cells, thus significantly reducing the fabrication and system costs
7
. This is attractive because
an electric field can be easily created at semiconductor/liquid junctions
8
. As shown in Figure
1-6, when inserting a semiconductor electrode into a solution containing a redox couple,
bending of the conduction band edge and the valence band edge are introduced resulting from
electrons flow between the semiconductor and the solution. The charge transfer process stops
when the Fermi level in the semiconductor is the same as the electrochemical potential in the
solution, reaching an equilibrium with the established local field. For an n-type semiconductor
electrode, the redox couple of interest is the O2/H2O couple. As a result of the charge transfer
9
at the interface, the electrode will have an excess positive charge spread out over the depletion
width and the solution will have an excess negative charge. The built-in electric field in an n-
type semiconductor is attractive for a photoanode because it can help separate and drive the
photogenerated holes to the solution. P-type semiconductors can contribute in an analogous
way as a photocathode. The initial difference between the Fermi level of the semiconductor
and the electrochemical potential in solution determines the strength of built-in electric field
near the solid/liquid interface. The local field can be on the order of 10
5
V/cm considering ~1
eV initial energy difference and ~100 nm depletion width in the semiconductor. This field is
generally very effective in separating the photogenerated electron-hole pairs in crystalline
inorganic semiconductors.
Figure 1-6: The band structures of an n-type semiconductor in contact with a solution
containing a redox couple.
1
Solar water splitting with a wide band gap semiconductor TiO2 was first reported by
Fujishima
9
. Figure 1-7 shows an example of a photoelectrochemical cell with 3-terminal
configuration, similar to the setup Fujishima used for TiO2 experiments. The cell contains an
electrode (photocathode or photocathode) that absorbs sunlight and drives the desired reaction,
10
and a counter electrode made of inert materials like Pt or carbon. The reference electrode is
used for measurement of standard electrochemical potentials and is not necessary for the solar
water splitting cells in practice. Solar water splitting with semiconductor electrodes can be
divided into 4 steps: 1) Absorption of sunlight in the semiconductor and excitation of electron-
hole pairs; 2) Separation of the carriers in the semiconductor and transporting them to the
solid/liquid interfaces for the desired half reactions; 3) Reduction of water molecules with
electrons for H2 generation; 4) Oxidation of water with holes for O2 evolution. The electrodes
can be wired externally, or in a wireless configuration connected and made into one piece. In
a simplified case, if a single semiconductor material is used for unassisted solar water splitting,
the band gap of the material should be large enough to count for thermodynamic requirement
and overpotentials, and the band-edge energies should straddle the standard reduction
potentials of H
+
/H2 and O2/H2O, as shown in Figure 1-8.
1
Figure 1-7 Schematic diagram of a photoelectrochemical cell.
Potentiostat
semiconductor Ref Counter
Electrolyte
11
Figure 1-8: Single semiconductor for unassisted water splitting under illumination.
Figure 1-9: Band edge positions of common semiconductors used in photocatalysis.
1
Semiconductor
H
+
/H
2
O
2
/H
2
O
h ν
e
-
h
+
E
c
E
v
1.23 eV
Liquid
12
Figure 1-9 shows the conduction band edge (left) and the valence band edge (right)
versus the normal hydrogen electrode for some semiconductors used in photocatalysis. Some
n-type semiconductors like TiO2 have large enough band gap and suitable band edge positions
for water splitting, but their efficiency under sunlight is significantly limited by the large band
gap and poor utilization of solar spectrum. Smaller band gap p-type semiconductors harvest
more energy from sunlight and are good candidates for photocathodes to drive H2 evolution
reaction, but they need to be used in combination with a photoanode to provide enough energy
to drive the water splitting reaction. Combination of two semiconductor materials for the
photoelectrodes is also attractive because this can capture a larger portion of the solar spectrum.
In optimization of solar water systems, main efforts are in the following directions:
1) Light absorption: selection of materials (bandgap energies), reducing reflection
losses and introducing extra light absorbers.
2) Charge separation: material improvement and local field enhancements.
3) Reaction kinetics: increase reaction cites, incorporating catalysts and plasmonic
enhancements.
In Chapter 2 we discuss about our efforts in optimization of light absorption and reaction
kinetics of silicon based photocathodes.
13
1.3 Plasmonic enhancement in photoelectrolysis
In plasmon-enhanced photoelectrolysis systems, the most common device geometries
include nanostructured metals in combination with semiconductors. These structures can
improve the photoelectrolysis via catalytic effects and improved charge separation. Besides,
plasmonic effects from strong interactions with light can provide further enhancement of the
efficiency, which we will describe in this section.
The central concept of localized surface plasmon is that, with an applied oscillating
electric field, electrons in metals can act as a harmonic oscillator. The electron densities near
the metal surface or at different parts of metal nanostructures can vary with the applied field.
This leads to oscillations of the charge density and electric field and the coherent oscillations
are called the localized surface plasmon, as shown in Figure 1-10. In metal nanostructures, the
electron oscillation frequency with minimal dissipation is referred to as the plasmon resonance
frequency, which can be tuned by varying the nanostructure material, geometry and local
dielectric environment
10
. Gold and silver nanostructures are common in solar water splitting
studies because their resonance can be easily tuned to the visible range of solar spectrum and
they also have relatively large optical absorption cross section. For example, by varying the
size and shape of silver nanoparticles, the plasmon resonance frequency of silver can be tuned
from UV range to a large visible range, as shown in Figure 1-11
11
.
14
Figure 1-10: Schematic illustration of a localized surface plasmon.
12
15
Figure 1-11: (a) Shape and (b) size tuning of plasmon resonance wavelength of silver
nanoparticles.
11
(a)
(b)
16
In solar water splitting, the photo-generated carriers need to travel to the solid/liquid
interface to drive the desired reaction. In this process, bulk recombination in semiconductors
can significantly reduce the overall efficiency. Plasmonic structures help minimize the bulk
recombination by localizing and concentrating the optical energy to regions near the
interface. Estimation of the local electric field by electromagnetic simulations show that at
local hot spots, the field intensity can be enhanced as much as 1000 times that of the incident
field
13-14
. Since generation of electron-hole pairs is directly proportional to square of the
local electric field intensity E
2
, majority of the light absorption can be confined within ~10
nm range of the semiconductor, suppressing undesired bulk recombination and improving
the overall efficiency. This enhancement mechanism is usually referred to as the local field
enhancement.
Beside the local field enhancement, there is another mechanism usually referred to as
the charge transfer mechanism. A plasmon can decay, either to a phonon and dissipate the
energy as heat, or generate hot carriers with high energies, i.e. hot electrons or hot holes. Hot
electrons produced near a wide gap n-type semiconductor with sufficient energy can be
injected into the conduction band of the semiconductor, as shown in Figure 1-12
15
. In this
scenario, the plasmonic nanostructures can act as a sensitizer similar to that of a dye-sensitized
solar cell, utilize the light below band gap of the semiconductor and further improve the
efficiency.
17
Figure 1-12: Injection of hot electrons from plasmonic structures into a semiconductor.
15
Experimentally, when plasmonic structures are used in combination with semiconductor
materials, it is difficult to distinguish between the effects of hot electron injection (HEI) and
local field enhancement (LFE). These two processes are illustrated in Figure 1-13. In the case of
hot electron injection (Figure 1-13 (a)), the photon is absorbed in the metal, and the excited
electron is injected into the semiconductor. In the case of local field enhancement (Figure 1-13
(b)), the photon is absorbed in the semiconductor and the minority carrier is then swept out to the
metal by the built-in field of the Schottky junction. Here, an electron is excited from a defect
state within the bandgap to the conduction band (i.e., sub-band gap absorption). While both
mechanisms produce a photocurrent in the same direction, the processes are quite different. LFE
results purely from classical electromagnetism, while HEI is strictly a quantum mechanical
18
phenomenon. High energy barrier electrochemical reactions, such as CO2 reduction, typically
require large external voltages to drive them in the desired direction.
Figure 1-13: Schematic diagrams illustrating the (a) hot electron injection (HEI) of
photoexcited carriers and (b) local field enhancement (LFE) of sub-bandgap absorption (i.e.,
defect states).
In Chapter 3 and 4, our focus of work is to provide a detailed understanding of hot electron
processes in chemical reactions, improved chemical selectivity, and enable photoelectrochemical
reactions to be driven at smaller applied voltages. We take advantage of a unique platform of
plasmon resonant photocatalytic nanostructures that enable us to distinguish between hot electron
injection and plasmonic local field enhancement. These nanostructures are based on plasmonic
gratings that can be excited plasmon-resonantly or non-resonantly (i.e., bulk metal absorption) by
simply varying the polarization of the incident light. Figure 1-14 (a) and (c) show schematic
diagrams of a plasmonic grating structure irradiated with transverse electric (TE) and transverse
magnetic (TM) polarized light, respectively. Figure 1-14 (b) and (d) plots the electric field intensity
(E
2
) distribution in one period of a simple plasmonic grating, which show bright hot spots when
irradiated in the TE polarization and a minimal response (i.e., comparable to bulk Au) when
irradiated in the TM polarization. These nanostructures enable us to distinguish between plasmon-
(a) (b)
≠
LFE
19
resonant excitations (TE polarization) and non-resonant bulk metal absorption (TM polarization),
while maintaining all other variables in the experiment constant (i.e., sample morphology, photon
energy, etc.). These plasmonic grating structures typically have a pitch of 400-500nm, and will be
fabricated using electron beam and photo-lithography. The defect concentration in the
semiconductor provides another degree of freedom with which to explore and separate these two
mechanisms of photocatalytic enhancement. The defect concentration directly affects the
photocurrent produced by local field enhancement (LFE) (through sub-band gap absorption) but
not that produced by hot electron injection (HEI), which does not depend on the presence of defects
(to first order).
Figure 1-14: Schematic diagrams of simple plasmonic metal gratings irradiated with (a)
transverse electric (TE) and (c) transverse magnetic (TM) polarized light. Cross-sectional
distribution of the electric field intensity (E
2
) of this grating when irradiated with (b) TE and
(d) TM polarized light.
E H
H E
(a)
(c)
TE
TM
(b)
(d)
Au
Au
100
80
60
40
20
0
100
80
60
40
20
0
20
Chapter 2 : Nanostructured silicon photocathodes for solar water
splitting
This chapter is similar to Shen et al., published in ACS Applied Materials & Interfaces.
16
2.1 Abstract
One-dimensionally (1D) periodic lamellar nanopatterns derived from the self-assembly of
symmetric poly(styrene-block-methyl methacrylate) block copolymers (BCPs) were incorporated
on the surface of single-crystalline silicon for utilization in solar photoelectrochemical (PEC)
water splitting in two distinct doping configurations with and without a buried pn-junction. The
resulting nanostructured silicon with the characteristic BCP morphology provided enhanced
photocatalytic performance for the hydrogen evolution reaction compared to bare silicon. The pp
+
-
silicon photocathodes with a liquid/silicon junction exhibited more pronounced improvement of
the PEC performance upon the introduction of the nanostructured surface compared to the n
+
pp
+
-
silicon with a solid-state junction owing to the corresponding increase in the liquid/silicon junction
area. Systematic studies on the morphological, optical properties, and PEC performance of
nanostructured silicon photocathodes, in conjunction with optical modeling based on a finite-
difference time-domain (FDTD) method, revealed that the anti-reflection and increased surface
area of etched silicon photocathodes collectively contributed to the enhanced PEC characteristics
in solar-driven water reduction including the increased saturation current density as well as the
reduced potential at a constant level of driving current.
21
2.2 Introduction
Photoelectrochemical (PEC) solar water splitting has gained much attention due to its
potential to convert solar energy into storable forms of chemical fuels.
17-19
A variety of inorganic
semiconductor materials have been investigated for photoelectrodes in solar-driven water splitting.
In particular, silicon represents one of the most attractive materials candidates owing to its natural
abundance, environmental safety, appropriate bandgap energy and band-edge alignment, as well
as well-established research and manufacturing infrastructures for large-scale production.
20-23
With recent advances in materials growth and processing technologies, micro- and nanostructured
forms of silicon prepared by a variety of bottom-up
24-27
or top-down
28-31
fabrication techniques
have been extensively studied for solar water splitting, with potential advantages in reduced
materials cost, enhanced light absorption, as well as efficient charge-carrier collection and
transfer.
32-34
Among various methods for creating nanoscale features on silicon, self-assembled
block copolymers has already shown significant promise for advanced lithography due to the
relative ease of attaining characteristic dimensions less than 50 nm, the diverse range of one- (1D),
two- (2D), and three-dimensionally (3D) periodic morphologies that can be readily achieved via
tunable molecular parameters, and simple, high throughput processing schemes.
35-37
In the work
presented in this chapter, we investigated a type of nanostructured single-crystalline silicon as a
photocathode in solar water splitting, in which one-dimensionally periodic nanoscale surface
morphologies derived from the self-assembly of lamellar forming diblock copolymers were
implemented onto the silicon with and without a buried metallurgical junction. To elucidate the
effect of etching depth upon the optical and photoelectrochemical characteristics of nanostructured
silicon photocathodes, this study intentionally focused on the system of pure silicon without
additional passivation layers (e.g. TiO2) and metal co-catalysts, both of which are expected to
22
boost the efficiency of silicon photocathodes in water splitting significantly. The silicon
photocathode with randomly oriented lamellar nanopatterns exhibited improved
photoelectrochemical (PEC) performance in solar water splitting compared to the bare silicon
owing to combined effects of reduced surface reflectance and enlarged surface area. Systematic
studies on morphological and optical properties, and photoelectrochemical performance of
nanostructured silicon photocathodes in the hydrogen half reaction, together with optical modeling
based on a finite-difference time-domain method (FDTD), quantitatively describe the reported
system and provide optimal design rules of lamellar-patterned silicon photocathodes.
2.3 Experimental details
The fabrication of nanostructured silicon photocathodes began with doping of the rear and
front surfaces of p-type (100) silicon (1-20 cm, WRS Materials) wafers by thermal diffusion
(1000℃ in N2 atmosphere) of boron (BN-1250, Saint Gobain) and phosphorous (PH-1000, Saint
Gobain) solid-state doping sources, respectively, where silicon nitride (~600 nm) deposited by
plasma enhanced chemical vapor deposition (PECVD, Plasmalab) was used as a selective doping
mask. The doped substrate was then cleaned in a piranha solution (H2O2:H2SO4 =3:7 by volume at
70°C for 20 min) and pretreated with 10 nm of a random copolymer brush (poly(styrene0.58-
random-methyl methacrylate0.31-random-glycidyl methacrylate0.11), Mn = 15 kg mol
-1
), followed
by spin-coating of a symmetric poly(styrene-block-methyl methacrylate) diblock copolymer (Mn
= 100 kg mol
-1
, PDI = 1.12, and 50.0 vol% PS, Polymer Source). Self-assembly of the lamellar
morphology was achieved by thermal annealing at 190°C for 24 h under vacuum. To transfer the
resulting block copolymer nanopattern to the silicon wafer, the PMMA domain was selectively
removed by UV exposure (UVP CL-1000 with λ = 254 nm and 1 J/cm
2
) and development in acetic
acid, followed by thermal evaporation of Cr (10 nm, CVC 3-boat thermal evaporator).
38-39
23
Sonication in warm toluene (Fisher Scientific FS30H, 70-80 h) lifted off the PS template
40
to yield
nanostructured Cr wires that were used as an etch mask for inductively coupled plasma reactive
ion etching (ICP-RIE, SF6/C4F8 = 10/25 sccm, 10 mTorr, 50W RF and 500 W ICP). After the ICP-
RIE, the remaining Cr and organic residue (e.g. C4F8) were removed by wet chemical etching (CE-
8040P, Transene) and oxygen reactive ion etching (O2-RIE, 27.4 sccm, 100 mTorr, 10 W, 3 min),
respectively. A metal ohmic contact was formed on the back-surface of the silicon by electron
beam evaporation of metal (Cr/Au =20 nm/100 nm, Temescal), to which a copper wire was
connected using silver paint (SPI Supplies). The reflectance spectra of etched silicon samples were
obtained on a spectrophotometer (Perkin Elmer Lambda 950) equipped with an integrating sphere
using Spectralon
TM
(SRM-99) as a 100% reflectance standard. The completed silicon
photoelectrode was then mounted and sealed on a slide glass (VWR) using thermally curable
epoxy (Hysol 9460, Loctite). All PEC measurements were performed in an aqueous solution (0.5
M, pH = 0.45~0.25, T = 20~31ºC) of H2SO4 (EMD Chemicals Inc., GR ACS, 95-98%) in a glass
container under AM 1.5G standard solar spectrum (1000 W/m
2
) provided by a full-spectrum solar
simulator (94042A, Oriel), where the optical path length of light in water was around ~2 cm. Linear
sweep voltammetry data were collected by a potentiostat (Reference 600, Gamry) using a three-
electrode configuration at a scan rate of 50 mV/s, with Ag/AgCl (RE-5B, Bioanalytical Systems
Inc.) and platinum wire (MW-1032, Bioanalytical Systems Inc.) as a reference and counter
electrode, respectively. A small amount of surfactant (Triton X-100, SPI Supplies) was added to
the electrolyte to facilitate the release of generated gas bubbles from the sample surface. For the
conversion of electrode potential from Ag/AgCl to reversible hydrogen electrode (RHE), a linear
sweep voltammetry scan was performed using a platinum electrode (MF-2013, Bioanalytical
Systems Inc.) as a cathode under otherwise same conditions described above. The PEC data were
24
taken after the same number (e.g. 20) of linear voltammetry scans to eliminate the effect of natural
oxide layers.
2.4 Results and discussion
Figure 2-1 (a) illustrates the processing steps to fabricate nanostructured silicon
electrodes for solar water splitting via block copolymer self-assembly. For photocathodes with
a buried metallurgical junction (referred to as n
+
pp
+
-photocathode), the front- and rear-
surfaces of a p-type (100) silicon substrate (10-20 cm) were doped by thermal diffusion of
solid-state phosphorous (1000°C for 15 min in N2) and boron (1000°C for 20 min in N2)
sources to incorporate a pn-junction and a back surface field (BSF), respectively. For
photocathodes without a buried junction (referred to as the pp
+
-photocathode), only the rear-
surface of the silicon was doped with boron (1000°C for 20 min in N2) to form a BSF and a
heavily doped region for the deposition of a metal ohmic contact (i.e. Cr/Au). Doping
configurations and corresponding energy band diagrams of pp
+
and n
+
pp
+
silicon electrodes
are schematically illustrated in Figure 2-1 (b). The front-surface of the doped silicon substrate
was pretreated with 10 nm of a random copolymer brush (poly(styrene0.58-random-methyl
methacrylate0.31-random-glycidyl methacrylate0.11), PS-r-PMMA-r-PGMA), Mn = 15 kg mol
-
1
) to implement a ‘neutral-wetting’ surface, on which a 40-nm-thick poly(styrene-block-
methyl methacrylate) block copolymer (PS-b-PMMA, Mn = 100 kg mol
-1
, PDI = 1.12, 50.0
vol% PS) thin film was spin-coated and annealed under vacuum (190°C for 24 h) to form a
perpendicularly oriented lamellar morphology.
41-44
The self-assembled 1D periodic
nanopatterns were transferred to the Si wafer by the selective removal of the PMMA domains,
metal (i.e. Cr, ~10 nm) deposition and liftoff, and inductively coupled plasma reactive ion
etching (ICP-RIE, SF6/C4F8 = 10/25 sccm, 10 mTorr, 50W RF and 500 W ICP). Subsequently,
25
the remaining Cr mask was removed by wet chemical etchant (CE-8040P, Transene), followed
by an oxygen RIE (O2, 100 mTorr, 10W, 3 min) to remove organic residues. Figure 2-1 (c)
depicts a representative tilt-view scanning electron microscope (SEM) image of the etched
silicon surface with the characteristic lamellar morphology of BCPs. The fabrication of the
silicon photocathodes was completed by connecting a copper wire on the metal-deposited
(Cr/Au = 100 nm/20 nm) back surface of silicon using a silver paint, and mounting the entire
unit on a glass substrate, and sealing with thermally curable epoxy (inset in Figure 2-1 (c)).
26
Figure 2-1: (a) Schematic illustration of the processing steps to fabricate nanostructured silicon
photocathodes incorporating one-dimensionally periodic lamellar morphology derived from
the self-assembly of symmetric poly(styrene-b-methyl methacrylate) diblock copolymers. (b)
Schematic energy band diagrams for pp
+
(left) and n
+
pp
+
(right) Si photocathodes under
illumination, where q, Ecb, Evb, Ef,n, Ef,p, Voc, and Eº(H
+
/H2) are electronic charge, energy levels
at the conduction and valence band edge, electron and hole quasi-Fermi levels, photovoltage,
and reduction potential for the hydrogen evolution reaction, respectively. (c) Tilt-view
scanning electron microscope (SEM) image of a representative silicon photocathode with
lamellar surface nanopatterns after the reactive ion etching and removal of Cr. Inset shows a
photographic image of the completed nanostructured silicon photocathodes mounted on a slide
glass.
spin-coat & anneal
PS-b-PMMA BCP
dope by
thermal diffusion
remove PMMA phase
evaporate
Cr & liftoff
ICP-RIE &
Cr etch
O
2
RIE &
form
metal
contact
PS-r-PMMA-r-PGMA
PS domain PMMA
domain
Cr (10 nm)
(a)
(c)
100 nm
5 mm
BCP-patterned
Si photocathodes
-qEº
(H
+
/H
2
)
(b)
p-Si p
+
-Si
E
cb
E
vb
E
F
E
cb
E
vb
E
F
-qEº
(H
+
/H
2
)
p-Si p
+
-Si n
+
-Si
pp
+
Si
photocathode
n
+
pp
+
Si
photocathode
etching etching
27
Figure 2-2(a) shows top-down and cross-sectional view SEM images of the nanostructured
pp
+
-silicon with BCP lamellar morphologies obtained at various etching times of 30, 45, 60, and
90 s, in which the bright and dark regions of the width of ~20-25 nm correspond to the PMMA
(unetched) and PS (etched) nanodomains, respectively. The average heights of silicon
nanostructures measured from the cross-sectional SEM images are approximately 45, 65, 85, and
110 nm, respectively, at the etching times of 30, 45, 60, and 90 s, respectively. The introduction
of BCP morphologies on the front surface of the silicon was accompanied by the apparent
modification of the surface area, which was increased by up to ~6.2 times the area of bare silicon
as summarized in Figure 2-2 (b). Another important advantage of nanostructured silicon in solar
water splitting is the lowering of the reflectance of incident light due to the reduced mismatch of
refractive index between the surrounding medium (i.e. water) and silicon. The total (i.e. specular
and diffuse) reflectance (R( )) of nanostructured silicon at near-normal incidence ( = 8º) was
measured on a spectrophotometer equipped with an integrating sphere using fluoropolymer-based
reflectance standard (Spectralon
TM
). As expected, the reflectance monotonously decreased as the
height of the lamellar nanostructure increased with etching time. Comparable morphological and
optical characteristics were also obtained from n
+
pp
+
-silicon processed under the same etching
conditions as with the pp
+
samples (Figure S1, SI). It is notable that the band bending for charge-
carrier separation is provided by the silicon/liquid junction in pp
+
-silicon, in which the junction
area varies with the height of the etched nanostructures. By contrast, in n
+
pp
+
-silicon, the depth (>
~300 nm) of the solid-state pn-junction defined by the thermal diffusion of a doping source is
deeper than the etching depth employed in this study, and therefore the junction area is nearly
invariant as schematically illustrated in Figure 2-1 (b).
28
Figure 2-2: (a) Top-view scanning electron microscope (SEM) images of the nanostructured
surface of the pp
+
-silicon at various etching times of inductively-coupled plasma reactive ion
etching (ICP-RIE) using a thin (~10 nm) layer of Cr patterned through the block copolymer
lithography as an etch mask. The insets show cross-sectional-view SEM images. (b) Plot of
the measured etch depth (red) and corresponding calculated ratio of the surface area (blue) for
the nanostructured silicon (ABCP) at various etching times with respect to that of a bare silicon
substrate (Abare). (c) Measured total (specular + diffuse) reflectance spectra of nanostructured
and bare silicon at various etching times, measured on a spectrophotometer equipped with an
integrating sphere at near-normal incidence (θ = 8º) using Spectralon
as a 100% reflectance
standard.
30 45 60 75 90
0
30
60
90
120
2
4
6
8
(a)
60 nm
200 nm
30 s 45 s
60 s 90 s
(c)
Reflectance (%)
etch time
0 s (bare Si)
30 s
45 s
60 s
90 s
(b)
Etch time (s)
Etch depth (nm)
A
BCP
/A
bare
etch depth
surface area
400 600 800 1000
0
20
40
60
Wavelength (nm)
29
To further elucidate underlying optical effects of BCP nanostructures upon the absorption
of incident solar light in water, we performed semi-empirical numerical modeling based on FDTD
methods, where the top-view SEM micrographs (~3 x 5 m
2
) of nanostructured silicon (Figure 2-3
and Figure 2-4) were imported to generate a model silicon photocathode with BCP lamellar
morphologies at a semi-infinite substrate thickness (Figure 2-5).
45
The imported 2D images were
then extended vertically to form 3D structures, with heights same as those determined from cross-
sectional SEM images at each etching time. FDTD calculations were performed for the semi-
infinite silicon with a model nanostructured surface using periodic boundary conditions for x- and
y-directions. A continuous plane-wave source with a broad Gaussian frequency spectrum
(270~750 THz) was incident normally to the front surface of nanostructured silicon. Figure 2-6 (a)
(Figure 2-7 (a)) shows simulated total reflectance spectra (solid lines) from the pp
+
(n
+
pp
+
) model
nanostructure as a function of etching time at normal incidence, which matched well with
experimental data (dotted lines). A small discrepancy is attributed to the slightly non-vertical
profile of etched nanostructures especially for samples with longer etching time (e.g. > 60 s), the
limited area of simulation, as well as optical losses in the calibration standard. Based on the
quantitative agreement between the simulated and measured data, the calculation was extended to
the case where the medium is water (n = 1.33) to evaluate the corresponding absorption of BCP-
patterned silicon photocathodes in the electrolyte. Owing to the reduced contrast in refractive index,
the overall magnitude of reflectance spectra decreased in water as shown in Figure 2-6 (Figure 2-7
(b) for n
+
pp
+
silicon). The calculated absorption spectra (A( ) = 1 – R( )) were then used to obtain
a total absorbed photon flux integrated over the AM1.5G standard solar spectrum, S_abs, given by
30
1100
1.5G
400
1100
1.5G
400
_
A I d
hc
S abs
Id
hc
where h, c, and I1.5G are Planck’s constant, speed of light, and the standard solar irradiance (AM
1.5G; ASTM G-173), respectively
45-46
. The S_abs for bare silicon in air was 65%, while it
increased to 76% in water due to the reduced index difference. The S_abs of silicon in water further
increased upon etching to 87, 91, 93, and 93% for the pp
+
-silicon (Figure 2-6(c)), and 90, 92, 93,
and 91% for the n
+
pp
+
-silicon (Figure 2-7) at etching times of 30, 45, 60, and 90 s, respectively.
31
Figure 2-3: Top-view SEM images of pp
+
-silicon used for the construction of model
nanostructured silicon for FDTD calculations, at the etching times of (a) 30, (b) 45, (c) 60,
and (d) 90 s, respectively.
1 μm
(a) (b)
(c) (d)
1 μm
1 μm 1 μm
32
Figure 2-4: Top-view SEM images of n
+
pp
+
-silicon used for the construction of model
nanostructured silicon for FDTD calculations, at the etching times of (a) 30, (b) 45, (c) 60,
and (d) 90 s, respectively.
1 μm 1 μm
1 μm 1 μm
(a) (b)
(c)
(d)
33
Figure 2-5: Schematic illustration of constructed model 3D nanostructures for pp
+
-silicon with
the characteristic surface morphologies of self-assembled BCP lamellae, at the etching times
of (a) 30, (b) 45, (c) 60, and (d) 90 s, respectively.
(a) (b)
(c)
(d)
500 nm 500 nm
500 nm 500 nm
34
Figure 2-6: (a) Simulated reflectance spectra (solid lines) of pp
+
-silicon at normal incidence
from semi-infinite silicon with model nanostructured surfaces of BCP lamellae at various
etching times, overlaid with experimentally measured spectra (dotted lines) for comparison.
To construct the model 3D nanostructured surface, top-view SEM micrographs of etched
400 600 800 1000
0
20
40
60
400 600 800 1000
0
20
40
60
(b)
(a)
Reflectance (%)
Wavelength (nm)
0 s (bare Si)
30s
45s
90s
60s
etch time
Wavelength (nm)
Reflectance (%)
etch time
0 s (bare Si)
30 s
45 s
60 s
90 s
cal. exp.
(d)
400 600 800 1000
0
20
40
60
80
100
R( ) or A ( ) (%)
Wavelength (nm)
BCP 1D grating
A( )
R( )
(d)
0 50 100 150 200 250
60
70
80
90
100
0.8
0.9
1.0
1.1
1.2
1.3
Etch depth (nm)
S_abs (%)
S_abs/S_abs,bare
BCP
1D grating
50 nm
25 nm
30 60 90
75
80
85
90
95
S_abs (%)
(c)
Etch time (s)
bare Si (76%)
In air In water
In water
87%
91%
93% 93%
35
silicon over the area of ~3.0 x 5.0 μm
2
were imported, where the height of the nanostructured
region was matched with the etch depth in experiments. (b) Simulated reflectance spectra from
the same nanostructured silicon as in (a) yet in water as a superstrate medium. (c) The
corresponding integrated solar flux absorption (S_abs) calculated by Eq. (1) over AM1.5G
standard solar illumination. (d) Comparison of S_abs of semi-infinite silicon incorporating the
BCP surface nanostructure (red) and a perfectly aligned 1D periodic grating (blue) as a
function of the height of the nanostructures, both at normal incidence. In the case of the 1D
grating (see the inset image), the periodicity (50 nm) and duty (50%) were assumed
approximately same as those of BCP lamellae, and the calculation was averaged over
transverse electric (i.e. electric field parallel to the direction of the trench) and transverse
magnetic (i.e. electric field perpendicular to the direction of the trench) polarizations (Figure
S3). (e) Corresponding reflectance (R(λ)) and absorption (A(λ)) spectra for BCP nanostructure
and 1D periodic grating in water at the optimum heights that maximize S_abs for each case
(80 nm for 1D grating, 90 nm for BCP).
Figure 2-7: (a) Simulated reflectance spectra (solid lines) of n
+
pp
+
silicon at normal incidence
from semi-infinite silicon with model nanostructured surfaces of BCP lamellae at various
etching times, overlaid with experimentally measured spectra (dotted lines) for comparison.
(b) Simulated reflectance spectra from the same nanostructured silicon as in (a) yet in water
as a superstrate medium. The inset shows the corresponding integrated solar flux absorption
(S_abs) calculated by Eq. (1) over AM1.5G standard solar illumination.
We also calculated and compared the absorption of silicon having BCP lamellar
morphologies with silicon that incorporates a 1D periodic grating, in which geometrical parameters
such as the periodicity and duty cycle of the grating were identical to those in BCPs. The absorption
spectra for the 1D grating were averaged over transverse electric and transverse magnetic
400 600 800 1000
0
20
40
60
30 60 90
75
80
85
90
95
400 600 800 1000
0
20
40
60
0 s (bare Si)
30s
45s
90s
60s
etch time
(a)
Wavelength (nm)
Reflectance (%)
etch time
0 s (bare Si)
30 s
45 s
60 s
90 s
cal. exp.
(b)
Reflectance (%)
Wavelength (nm)
Etch time (s)
S_abs (%)
bare Si
36
polarizations (Figure 2-8). As summarized in Figure 2-6 (d), silicon with the randomly oriented
BCP lamellar nanostructures exhibited higher absorption in water than the perfectly ordered 1D
gratings under the simulated standard solar radiation, with the maximum integrated absorption of
~93% at the etching time of 90 s. Figure 2-6 (e) depicts the corresponding reflectance and
absorption spectra of the BCP nanostructures and 1D gratings at the optimum height (i.e. ~80 nm
for the 1D grating and ~90 nm for the BCP) that maximizes the integrated solar absorption (S_abs).
For calculations of reflectance spectra with 1D photonic crystals at normal incidence as shown in
Figure 2-6 (c) and (d), 1D periodic gratings were implemented on the surface of semi-infinite
silicon with periodicity and duty cycle identical to those obtained from experiments. The
calculated spectra were averaged over transverse electric and transverse magnetic polarizations as
schematically illustrated in Figure 2-8.
Figure 2-8: Schematic illustration of the model nanostructured silicon with 1D periodic
gratings for the reflectance calculation by a FDTD method, where the periodicity and duty
cycle of gratings are 50 nm and 50%, respectively. The calculation was averaged over both
transverse electric (i.e. electric field parallel to the y-axis, θ = 90º) and transverse magnetic
(i.e. electric field parallel to the x-axis, θ = 0º) polarizations.
E
k
in
50 nm
etch depth
25 nm
Si (semi-infinite)
x
y
z
37
The photoelectrochemical performance of nanostructured silicon photocathodes were
measured on a potentiostat (Reference 600, Gamry) with a three-electrode configuration using an
aqueous solution of sulfuric acid (0.5M) under AM1.5G standard solar illumination (1000
mW/cm
2
), in which a Pt wire and Ag/AgCl were used as a counter and reference electrode,
respectively. Figure 2-9(a) and (c) show current density (J) - potential (E) curves obtained from
linear sweep voltammetry with bare and BCP-patterned silicon photocathodes with pp
+
- and n
+
pp
+
-
doping configurations, respectively. In both cases, the silicon photocathodes implemented with
surface nanostructures exhibited higher saturation current density (Jsat) than the bare silicon mainly
due to the reduced front-surface reflectance and increased solar flux absorption. The increase in
Jsat with nanostructured silicon electrodes over the bare silicon at the etching times of 30, 60, 90 s
were 11, 11, and 13% for the pp
+
-silicon and 9, 14, and 14% for the n
+
pp
+
-silicon, respectively,
consistent with the trend of the calculated absorption (S_abs) enhancement (14, 21, and 22% for
the pp
+
-silicon, 18, 22, 19% for the n
+
pp
+
-silicon). The comparatively smaller increase in Jsat in
experiments over calculation is attributed to the associated parasitic losses in the experimental
system including carrier recombination, series resistance in metal wires, and/or the reflection of
light at the water/air interface. In particular, the effect of surface recombination becomes more
pronounced at a higher etching depth due to the increasing density of surface dangling bonds,
which is also evidenced by the gradual decrease of open circuit voltage (Voc) at a longer etching
time. In both configurations with and without a buried metallurgical junction, the nanostructured
surface of silicon photocathodes resulted in the translation of the JE curves to the right along the
x-axis (i.e. V vs. RHE) with little change in the curve shape as the surface area increased compared
to the bare silicon, and the local current density and over-potential to drive the hydrogen half-
reaction decreased.
30
Here the shift of the JE curve was quantified by the electrode potential (V20)
38
at a fixed current density of 20 mA/cm
2
, which monotonously increased with etching time as
summarized in Figure 2-9(b) and (d). As expected, the open circuit voltage (Voc) of n
+
pp
+
-silicon
photocathodes was higher than pp
+
-silicon owing to the built-in metallurgical junction. It is also
noteworthy that the tendency of JE curve shift at various etching depths is different between the
two electrode configurations. The JE curves of nanostructured pp
+
-silicon exhibited a larger shift
from the bare silicon possibly due to the increase in the electrolyte/silicon junction area and
decrease in the local current density. On the other hand, the planar junction area in nanostructured
n
+
pp
+
-silicon remained nearly invariant to the bare silicon as the junction is located below the
etched region, which is also supported by the preservation of Voc. Consequently, the magnitude of
the JE curve shift under the cathodic potential was smaller than in pp
+
-silicon, where the main
effect of nanostructured silicon surface was to enhance the light absorption by suppressing the
front-surface reflectance. More detailed PEC characteristics extracted from Figure 2-9(a) and (b)
are summarized in Table 2-1. The saturation current density was obtained by averaging J between
−1.75 and −1.0 V (vs RHE). The system efficiency (η) was calculated by η(%) = ((−J max × Emax(vs
RHE))/(Pin(100 mW/cm
2
))) × 100, where Jmax and Emax (vs RHE) are the current density and the
potential at a maximum power point.
39
Figure 2-9: Current density (J)-potential (E, with respect to the reversible hydrogen
electrode(RHE)) plot of nanostructured silicon photocathodes (a) without and (c) with a buried
pn-junction, measured under AM1.5G simulated standard solar spectrum in 0.5 M aqueous
sulfuric acid. (b-d) Comparison of corresponding PEC characteristics including open circuit
density (Voc), potential at the current density (J) of 20 mA/cm
2
, and saturation current density
(Jsat).
(b)
-1.5 -1.0 -0.5 0.0
-40
-30
-20
-10
0
J (mA/cm
2
)
E (V vs. RHE)
(a)
(c) (d)
J (mA/cm
2
)
0 s (bare Si)
30s
90s
60s
etch time
E (V vs. RHE)
0 s (bare Si)
30s
90s
60s
etch time
-1.5 -1.0 -0.5 0.0
-40
-30
-20
-10
0
0.0
0.2
0.4
0.6
0.8
1.0
0
7
14
21
28
35
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
V
oc
(V vs. RHE)
J
sat
(mA/cm
2
)
J
sat
V
20
0 s (bare Si)
30s
90s
60s
etch time
V
oc
V
20
(V vs. RHE)
0.0
0.2
0.4
0.6
0.8
1.0
0
7
14
21
28
35
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
V
oc
(V vs. RHE)
J
sat
(mA/cm
2
)
J
sat
V
20
V
oc
V
20
(V vs. RHE)
0 s (bare Si)
30s
90s
60s
etch time
40
Table 2-1 Measured Photoelectrochemical Characteristics of Bare and Nanostructured Silicon
Photocathodes in Water Splitting
etching time (s) Jsat
(mA/cm
2
)
Vonset (V vs
RHE)
V20 (V vs
RHE)
Jsc
(mA/cm
2
)
fill
factor
η (%)
pp
+
0 −28.0 0.13 −0.701 −0.0195 0.174 0.00048
30 −31.2 0.11 −0.472 −0.148 0.084 0.00149
60 −31.2 0.07 −0.429 −0.100 0.101 0.00081
90 −31.6 0.07 −0.398 −0.075 0.123 0.00074
n
+
pp
+
0 −28.0 0.42 −0.574 −0.099 0.128 0.00531
30 −29.3 0.38 −0.500 −0.206 0.091 0.00716
60 −30.6 0.38 −0.473 −0.337 0.083 0.01058
90 −30.5 0.42 −0.455 −0.684 0.067 0.01931
2.5 Conclusion
In summary, we studied the effect of the 1D periodic BCP nano-patterns on optical and
photoelectrochemical properties of silicon photocathodes for solar water splitting without
additional surface passivation and metal co-catalysts. The characteristic surface morphology of
silicon derived from randomly oriented BCP lamellae provided higher absorption under
unpolarized solar illumination than bare silicon as well as a perfectly ordered 1D periodic grating
owing to the reduced refractive index contrast and front-surface anti-reflection. The
nanostructured surface also effectively increased the interfacial area of silicon with liquid
electrolytes to reduce the local current density and the over-potential required to drive the hydrogen
evolution reaction in solar water splitting. The pp+-silicon, upon the introduction of the
nanostructured surface, exhibited a comparatively larger shift of the JE curves than n+pp+-silicon
with a buried metallurgical junction owing to the corresponding increase of the liquid/silicon
junction area. Further studies are currently underway for the implementation of additional metal
co-catalysts (e.g. Pt) and passivation layer (e.g. TiO2), as well as implementation of BCP lamellae
with higher etching depth. Materials and fabrication concepts presented in this work will be readily
41
applicable to other materials systems (e.g. III-V or oxides) that can take advantages of
nanostructured morphologies in light absorption and photocatalysis, thereby contributing to the
continuing progress towards the development of high performance, low cost semiconductor
photoelectrodes in solar-driven water splitting through scalable and cost-effective fabrication
routes based on block copolymer lithography.
42
The parts of the work involving the fabrication of nanostructured photocathodes, and optical
modeling were supported by the National Science Foundation under Grant No (ECCS-1202522)
and DARPA YFA program (N66001-12-1-4244), respectively. The block copolymer self-
assembly and templating was supported in part by the National Science Foundation under Grant
No. ECS-0335765 through the Colorado Nanofabrication Laboratory and University of Colorado’s
Nanomaterials Characterization Facility.
43
Chapter 3 Plasmon resonant amplification of hot electrons in a grating
based photodiode
This chapter is similar to Shen et al., published in Nano Research.
47
3.1 Abstract
We report plasmon resonant excitation of hot electrons in a photodetector based on a
metal/oxide/metal (Au/Al2O3/graphene) heterostructure. In this device, hot electrons, excited
optically in the gold layer, jump over the oxide barrier and are injected into the graphene layer,
producing a photocurrent. To amplify this process, the bottom gold electrode is patterned into
a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent produced in
this device is measured using 633-nm-wavelength light as a function of incident angle. We
observe the maximum photocurrent at ±10° from normal incidence under irradiation with light
polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the
grating, and a constant (angle-independent) photocurrent under irradiation with light polarized
perpendicular to the incident plane (s-polarization) and parallel to the grating. These data show
an amplification factor of 4.6× under resonant conditions. At the same angle (±10°), we also
observe sharp dips in the photoreflectance corresponding to wavevector matching between the
incident light and the plasmon mode in the grating. In addition, finite-difference time-domain
simulations predict sharp dips in the photoreflectance at ±10°, and the electric field intensity
profiles show clear excitation of a plasmon resonant mode when illuminated with p-polarized
light at this angle.
44
3.2 Introduction
Plasmon resonance has been utilized in many applications, including biosensing,
48
surface enhanced Raman spectroscopy (SERS),
49
and photocatalysis,
50-51
mainly through the
effect of local field enhancement. More recently, however, the idea of hot electrons excited in
metals has been utilized in photocatlaysis
52-56
and solid state devices.
57-58
Brongersma’s group
reported hot-electron photodetection with a plasmonic nanostripe antenna.
57
In this work, they
used a metal/oxide/metal stack, and photocurrent was only generated when the incident photon
energy was larger than the oxide barrier energy. In 2015, Halas’ group reported similar
measurements, which compared the polarization dependence of plasmon-resonant devices
with Ohmic and Schottky contacts with defect-rich TiO2.
58
Several recent theoretical studies
have concluded that plasmon resonant excitations can decay into hot electrons in metal
nanostructures,
59-63
which presents the possiblity of engineering useful devices and structures
utilizing this effect despite the extremely short lifetimes of hot electrons in metals (~10 fsec).
64-
65
Plasmonic grating structures provide a useful and unique platform for studying plasmon
resonant phenomena. These nanostructures can be excited plasmon-resonantly or non-
resonantly (i.e., bulk metal absorption) by simply varying the polarization of the incident light.
These nanostructures enable us to distinguish between plasmon-resonant excitations (p-
polarization) and non-resonant bulk metal absorption (s-polarization), while maintaining all
other variables in the experiment constant (i.e., sample morphology, photon energy, etc.). In
the work presented in this chapter, we study the amplification of a hot electron-driven
photodetector using a plasmon resonant grating, as shown in Figure 3-1. The angle dependence
of the photocurrent is correlated with the photoreflection to verify the conditions for resonantly
45
exciting the plasmon mode. Electromagnetic simulations are used to further verify the nature
of this amplification and make predictions of further enhancement in alternative grating
configurations.
Figure 3-1: Schematic illustration of the experimental design.
Au
Al
2
O
3
Graphene
θ
Hot e
-
A
I
E
p
E
s
k
-20 -10 0 10 20
0
2
4
6
8
10
12
14
16
18
I
AC
(pA)
Incident Angle (Degrees)
s polarization
p polarization
46
3.3 Experimental details
Metal gratings are fabricated by first etching a silicon wafer using reactive ion etching.
This creates a corrugated surface with a pitch of 500 nm. Then, a 50 nm gold film is deposited
on top of this structure with a 3 nm Ti adhesion layer between the silicon and the gold.
66
A
scanning electron microscope (SEM) cross-sectional image of one of these gratings is shown
in Figure 3-2(a). Next, a 5 nm film of Al2O3 is deposited using atomic layer deposition (ALD).
Monolayer graphene is grown by chemical vapor deposition (CVD) on copper foil at 1000 ℃
in methane gas. After growth, the copper foil is spin-coated with PMMA-A6 at 2000 rpm for
45 s and then baked at 150 ℃ for 5 minutes. The copper foil is then etched away in copper
etchant and the graphene with PMMA is “scooped” out and rinsed in 10% HCl and DI water.
Next, the monolayer graphene is transferred to the target substrate with the same scooping
method and baked at 120 ℃ for 5 minutes to improve adhesion. After this the PMMA layer is
removed with a 5-minute acetone dip
20, 67
. Lastly, the sample is mounted on a rotational stage
and illuminated with collimated 633 nm wavelength light, as illustrated in Figure 3-3(c). A
chopper wheel is used to modulate the light at 200 Hz, and the AC photocurrent is measured
using a lock-in amplifier.
47
Figure 3-2: (a) Cross-sectional scanning electron microscope (SEM) image and (b) schematic
diagram of the plasmon resonant grating structure. (c) Energy band diagram with respect to
vacuum illustrating the mechanism of hot electron injection.
Figure 1
Au
-5.1 eV
-4.8 eV
Al
2
O
3
-3.3 eV
1.96 eV
e-
Gr
Au
Si
(a)
(b)
(c)
Au grating
Graphene
Al
2
O
3
48
Figure 3-3: (a) Photocurrent and (b) photoreflectance plotted as a function incident angle for
633 nm light polarized parallel and perpendicular to the grating structure. (c) Schematic
diagram of the experimental measurement configuration.
-20 -10 0 10 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
s polarization
p polarization
(a)
(b)
(c)
Rotation stage
Chopper
Laser
Waveplate
-20 -10 0 10 20
0
2
4
6
8
10
12
14
16
18
I
AC
(pA)
Incident Angle (Degrees)
s polarization
p polarization
49
3.4 Results and discussion
Figure 3-3 (a) shows the AC photocurrent plotted as a function of the incident angle for
light with an intensity of 106 mW/cm
2
polarized both parallel and perpendicular to the lines
on the grating. Here, we see two peaks appearing at ±10
o
from normal incidence when the
light is polarized parallel to the plane of incidence (p-polarization) and perpendicular to the
grating, but a constant (angle independent) photocurrent when the light is polarized
perpendicular to the incidence plane (s-polarization) and parallel to the grating. This data
shows that the hot electrons are, in fact, amplified by a factor 4.6X in the metal under resonant
conditions. Figure 3-3 (b) shows the photoreflectance plotted as a function of the incident
angle, exhibiting sharp dips at ±10
o
from normal. This is a clear signature of the plasmon
resonance, which is achieved when there is wavevector matching between the incident light
and the plasmon resonant modes in the grating.
In order to further understand the amplification observed in these plasmon resonant
devices, we performed electromagnetic simulations using the finite-difference time-domain
(FDTD) method. Figure 3-4 (a) shows the calculated reflectance plotted as a function of the
incident angle for both s- and p-polarized light. As in our experimental measurements, the
simulated data exhibits sharp dips at ±10
o
from normal incidence for p-polarized light and
nearly constant reflection for s-polarized light. Figure 3-4 (b) and (d) show the electric field
intensity distribution along the cross section of these gratings when illuminated at normal
incidence. Here, we see relatively low electric field intensities corresponding to the case where
plasmonic modes are not being coupled to. At 10
o
incident angle, however, we can clearly see
the plasmon resonant mode excited by p-polarized light, which produces an electric field
enhancement of approximately 66X at the surface of the metal. Figure 3-4 (c) shows the
50
electric field intensity distribution along the cross-section of this grating irradiated on
resonance (at 10
o
with p-polarized light). Here, intense electric field strengths can be seen
indicating the clear excitation of the plasmon resonance. With s-polarized light, however, the
field profile looks almost the same as the case of the 0
o
incident light. The plasmon-enhanced
absorption of light in the gold layer leads to generation of more hot electrons and, hence, a
higher photocurrent.
68
Due to limited mean free path of the hot electrons in the gold layer,
only a fraction of the generated electrons can contribute to the photocurrent. Based on a
literature value of 20 nm for the mean free path of electrons with energy 2 eV above the Fermi
level, we estimate the amplified photoresponse from enhanced light absorption by taking the
integral of E
2
within the topmost 20 nm of the gold, as described by the following equation:
69
𝐸𝐹 =
∫ |𝐸 𝑝 |
2
𝑑𝑥𝑑𝑦𝑑𝑧 𝑓 ( 𝑥 ,𝑦 )
𝑓 ( 𝑥 ,𝑦 ) −20𝑛𝑚
∫
|𝐸 𝑠 |
2
𝑑𝑥𝑑𝑦𝑑𝑧 𝑓 ( 𝑥 ,𝑦 )
𝑓 ( 𝑥 ,𝑦 ) −20𝑛𝑚
where f(x,y) is the profile of the Au top surface. This model predicts a relative p/s-polarization
ratio of 5.4X under resonant conditions, which is in good agreement with our experimental
findings.
51
Figure 3-4: (a) Finite difference time domain (FDTD) simulations of the photoreflectance as
a function of the incident angle for p- and s-polarized light. Cross-sectional electric field
intensity profiles (E
2
) for illumination at (b), (d) normal and (c), (e) 10
o
incidence.
-20 -10 0 10 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
s-polarization
p-polarization
(a)
(b)
(c)
(e)
(d)
(b) 100
50
0
-50
-100
-200 -100 0 100 200
0
3
6
9
12
15
y (nm )
x (nm )
100
50
0
-50
-100
-200 -100 0 100 200
0
3
6
9
12
15
y (nm )
x (nm )
(c)
100
50
0
-50
-100
-200 -100 0 100 200
0
3
6
9
12
15
y (nm )
x (nm )
(d)
100
50
0
-50
-100
-200 -100 0 100 200
0
3
6
9
12
15
y (nm )
(e)
x (nm )
52
In the work reported here, no attempt was made to optimize grating structure for
maximum amplification, however, simple modifications of the grating such as pitch or
material can greatly improve the resonant behavior. As a demonstration, we performed
simulations of several alternative grating configurations (i.e., pitch and metal composition).
As shown in Figure 3-5, using the same configuration and incident light, the resonant angle
can be moved from 4
o
to 18
o
for gold gratings by changing the pitch from 400 nm to 600 nm.
For 400 nm pitch gratings, the coupling can be made significantly stronger if Au is replaced
with Al, corresponding to the much sharper dip and almost zero reflection at resonance.
53
Figure 3-5: FDTD simulations of reflection of 633 nm light for different metals and pitches at
different incident angles.
0 5 10 15 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
Au
Ag
Cu
Al
0 5 10 15 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
Au
Ag
Cu
Al
400 nm pitch
500 nm pitch
0 5 10 15 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
Au
Ag
Cu
Al
600 nm pitch
54
3.5 Conclusion
In summary, we observe plasmon resonant amplification of hot electrons in a
Au/Al2O3/graphene photodetector. Here, optically-excited hot electrons jump over the oxide
barrier, thus, producing a photocurrent. In this device configuration, the bottom gold electrode
contains a plasmon resonant grating structure, in order to increase the photodetection
efficiency. Using 633 nm wavelength light, we observe clear peaks in the photocurrent at an
angle of ±10
o
from normal incidence for light polarized perpendicular to the gating, while no
modulation of the photocurrent is observed with light polarized parallel to the grating. This
data shows an amplification factor of 4.6X for hot carrier injection under resonant conditions.
We also see sharp dips in the photoreflectance at these same angles, which corresponds to
good wavevector matching between the plasmon mode in the grating and the incident light.
Electromagnetic (FDTD) simulations predict the same reflectance profiles observed
experimentally, and show the clear excitation of a plasmon resonant mode when irradiated
under resonant conditions (i.e., p-polarization at ±10
o
).
55
This work was supported by NSF Award No. CBET-1512505 (L.S.), Air Force Office
of Scientific Research Grant No. FA9550-15-1-0184 (B.H.), Army Research Office (ARO)
Award No. W911NF-14-1-0228 (H.S.), Department of Energy (DOE) Award No. DE-FG02-
07ER46376 (N.P.), and ACS-PRF grant #55993-ND5 (J.C.).
56
Chapter 4 Plasmon resonant amplification of hot electrons in water
splitting
4.1 Abstract
We report plasmon resonant excitation of hot electrons in a metal-based photocatalyst
in the half reaction of water oxidation in aqueous solution. Here the photocatalyst is a 100 nm
thick Au film deposited on a silicon substrate. In this configuration, hot electrons, photo-
excited in the metal, inject into the solution ultimately reversing the water oxidation reaction
(O2 + 4H
+
+ 4e
-
⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the
gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500nm.
The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using
633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±16
o
from normal when the light is polarized parallel to the incident plane (p-polarization) and
perpendicular to the lines on the grating. Based on these peaks, we estimate an overall
plasmonic gain (or amplification) factor of 6.6X in the charge transfer rate. At these same
angles, we also observe sharp dips in the photo-reflectance, corresponding to the condition
when there is wavevector matching between the incident light and the plasmon mode in the
grating. No angle dependence is observed in the photocurrent or photoreflectance when the
incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to
the lines on the grating. Finite difference time domain (FDTD) simulations also predict sharp
dips in the photoreflectance at ±8
o
, and the electric field intensity profiles show clear excitation
of a plasmon-resonant mode when illuminated at those angles with p-polarized light.
57
4.2 Introduction
Plasmon-enhanced photocatalytic water splitting was first reported in 2011, and was
attributed to the local field enhancement of sub-band gap (i.e., defect) absorption in the
underlying TiO2.
70-72
More recently, however, the concept of using hot electrons in metals to
drive photocatalytic processes was put forth by Mukherjee et al., who reported photo-
dissociation of H2 on Au nanoparticles deposited on SiO2 and TiO2 supports.
52-53
DuChene et
al. reported the use of hot electrons in a plasmonic-metal/semiconductor heterostructure for
photocatalytic water splitting.
54
Brongersma’s and Halas’s groups have both reported hot
electron processes in solid state devices enhanced by plasmonic grating structures.
57-58
In the
work of Robatjazi et al., plasmon resonant nanoparticles were deposited on top of NiOx
semiconducting films, and the resulting photocurrent generated was attributed to the direct
injection of electrons excited in the metal nanoparticles to ions in the solution.
55
However, in
this sample configuration, there are several possible mechanisms capable of producing a
photocurrent, which are difficult to separate. In a more direct observation, Hou et al. measured
photocatalytic water splitting on a bulk metal film with no semiconductor present.
73
Here, an
AC lock-in technique was developed to detect the relatively small photocurrents produced by
the short-lived hot electrons in the metal (~10 fsec). In addition, there have been several recent
theoretical studies concluding that plasmon resonant excitations decay into hot electrons in
metal nanostructures,
59-63, 74
which opens up the exciting possibility of amplifying the
relatively small photocurrents (i.e., charge transfer rates and efficiencies) that are produced by
hot electrons using plasmon resonant nanostructures.
64-65, 75-78
We selected plasmonic grating structures in this work as the platform to study plasmon
resonant excitations and hot electrons for the reasons as followed. While the incident light can
58
be absorbed in metal nanostructures non-resonantly (bulk metal absorption) and resonantly
(plasmon resonance), these gratings enable us to easily tune between these two cases. Without
changing other variables in the experiment, by switching the polarization of the incident light
between p-polarization and s-polarization we could distinguish any response from plasmon
resonant excitations and bulk metal absorption. A demonstration of the experimental design is
shown in Figure 4-1. The resonance conditions for the plasmon mode can be verified simply
be sweeping the incident angle with p-polarized light and comparing the angle dependence of
photoreflection and photocurrent. More details about the nature of the plasmon resonance can
be revealed by running electromagnetic simulations based on imported nanostructures in finite
difference time domain analysis (FDTD, Lumerical).
Figure 4-1: Schematic illustration of the experimental design.
Au
θ
Hot e
-
E
p
E
s
k
+ e
-
- e
-
H
2
O O
2
+ 4H
+
59
4.3 Experimental details
Metal gratings used in this work (a 100 nm gold film with a 10 nm Ti adhesion layer on
top of a corrugated Si wafer with a pitch of 500 nm pitch and a grating area of 1×1 cm
2
) were
fabricated using the same method as reported by Rice et al
66
. Figure 4-2(a) shows the cross-
sectional image of the grating from a scanning electron microscope (SEM). Figure 4-2 (b)
shows an illustration of the sample geometry, where electrical connection was made directly
to the top Au surface, thus keeping the bottom Si substrate out of the circuit in all the
electrochemical measurements. Copper wire with insulating coating and Silver paint (SPI
Supplies.) were used for electrical connection and a 5-min Epoxy was used to encapsulate the
sample. The sample was mounted on a rotational stage and illuminated with collimated light,
as shown in Figure 4-3 (d). Figure 4-3 (e) shows the setup for photoelectrochemical
measurement with a three-terminal configuration. The grating sample was measured as the
working electrode. And a Ag/AgCl electrode and a Pt wire (BASi, Corp) were used as the
reference and counter electrode, respectively. The measurements were conducted in a 0.5 M
Na2SO4 (Anhydrous ACS, VWR) solution. DC bias was applied and monitored between the
sample (working electrode) and reference electrode through a potentiostat (Gamry, Inc.). To
detect small photocurrents around a few micro-amperes in this study, we adopted a modified
AC lock-in technique as reported in our previous work
73
. An optical chopper (SR540, Stanford
Research Systems, Inc.) was used in combination with a lock-in amplifier (SRS830, Stanford
Research Systems, Inc.). The optical chopper can be used to modulate incident laser (633 nm
HeNe laser, maximum power ~35 mW, beam spot ~ 0.3 cm
2
) with frequency 4~400 Hz. Under
the chopped illumination, the current flowing through the sample at a given bias was measured
by the potentiostat (Gamry, Inc.) and supplied to the lock-in amplifier. Any photo-response
60
from the working electrode would generate an AC component in the current output, with the
same frequency of the chopper, which then can be detected and measured by the lock-in
amplifier.
Figure 4-2: (a) Cross-sectional scanning electron microscope (SEM) image and (b) schematic
diagram of the plasmon resonant grating structure. (c) Energy band diagram with respect to
NHE illustrating the mechanism of hot electron injection mechanism.
Au
Si
Au
0.66 eV
1.23 eV
1.96 eV
e-
O
2
/H
2
O
(b)
(c)
Au
Si
(a)
61
Figure 4-3: (a) AC photocurrent and (b) photoreflectance plotted as a function incident angle
for 633nm light with an intensity of 106mW/cm
2
polarized parallel and perpendicular to the
grating structure. (c) DC photocurrent plotted as a function of reference potential when
illuminated at -10
o
with respect to normal incidence. (d), (e) Schematic diagrams of the
experimental measurement configuration.
I-monitor output
(a)
(b)
(c)
Laser
Chopper
Waveplate
Rotation Stage
(d)
(e)
-20 -10 0 10 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
s-polarization
p-polarization
1.0 1.2 1.4 1.6 1.8 2.0
0
1
2
3
I
DC
(mA)
V vs. Ref(V)
I
DC
0.0
0.1
0.2
I
AC
( A)
I
AC
-20 -10 0 10 20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
AC Photocurrent ( A)
Incident Angle (Degrees)
s-polarization
p-polarization
Potentiostat
Au Si Ag/AgCl Pt Chopper
Lock-in Amplifier
ω monitor
output
62
4.4 Results and discussion
The photo-response of the plasmonic gratings as a function of the incident angle in the
electrolyte is shown in Figure 4-3(a). When the incident light is s-polarized (parallel to the
grating lines and perpendicular to the incident plane), an angle-independent photocurrent is
observed. The finite current observed under s-polarized light is possibly due to surface
roughness. A clear angle-dependent photocurrent is observed under p-polarized light
(perpendicular to the grating lines and parallel to the incident plane). Due to the symmetry in
our grating structures, two symmetric maxima in the photocurrent under p-polarization can be
seen at ±16º from normal incidence. By comparing the photocurrent at ±16º for two
polarizations, we estimate an amplification factor of 660% for hot electron generation in the
metal grating by exploiting plasmon resonance conditions. We estimate the quantum
efficiency, here, to be on the order of 0.007% electrons per incident photons under resonant
conditions. However, the system is far from optimized. For a given frequency of light,
plasmonic behavior of the grating can be greatly changed by altering the material (for example
silver and aluminum) and grating profile. The photoreflectance of the grating after Au
deposition was also measured in the same solution as we swept the incident angle. The dip at
±8º from normal incidence is a clear signature of wavevector matching between the incident
light and the plasmon resonant mode in the grating. Figure 4-4 shows the diffuse reflection
spectra of the grating compared with flat Au surface, from UV-Vis measurement with ~8°
incident angle. A clear dip in reflection can be seen around ~610 nm wavelength in the spectra
showing wavevector matching in this measurement. Note that the matching angle not only
depends on the grating structure and the surrounding medium, but also varies with wavelength.
We believe here the deviation of resonance angles between photocurrent and photoreflection
63
is a result of surface roughing in the process of constantly driving the oxidation reaction. As
the surface of the Au grating structure is roughened, the resonance condition would vary since
it is closely related to the surface condition. Figure 4-5 shows how the angle dependence of
photocurrent changes as we drive the oxidation reaction for a longer time. We observe that the
resonance angle would gradually increase, while photocurrent at resonance angles and normal
incidence would drop and increase respectively, as we run the reaction longer. We also
explored addition of a very thin oxide layer (5nm Al2O3) to protect the grating structure. As
shown in Figure 4-6, we observe similar photo-response with the Au/Al2O3 structure and the
stability of the device is greatly enhanced. Figure 4-3 (c) shows the DC and AC current plotted
as a function of reference potential, which exhibits the clear and abrupt onset of photocurrent
at +1.7 V vs reference electrode. Here, it is important to note that the DC current is driving
water oxidation (i.e., 2H2O ⇋ O2 + 4H
+
+ 4e
-
), while photocurrent reduces the products
generated by the oxidation reaction resulting in a reverse reaction. the reverse reaction here is
producing the AC photocurrent.
64
Figure 4-4: Comparison of diffuse reflectance spectra of flat Au and Grating surface
Figure 4-5: AC photocurrent after applying constant potential for different time periods.
400 500 600 700 800 900 1000 1100
0
20
40
60
80
100
Reflection (%)
Wavelength (nm)
Bare Au
Grating
-20 -10 0 10 20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
AC Photocurrent ( A)
Incident Angle (Degrees)
Original test
+10 min
+30 min
65
Figure 4-6: Angle dependence of photocurrent of a grating with 5 nm Al2O3 coating.
To reveal more details of the plasmon resonance in these gratings, we performed
electromagnetic simulations using FDTD. The grating structure used in FDTD (infinite Si
substrate, 100 nm-thick Au, with 500 nm pitch) was imported from the cross-sectional SEM
image, and all simulations were done using a background refractive index of 1.3317 (from
H2O at 633 nm). The calculated reflectance for both polarizations is shown in Figure 4-7 (a).
The simulated data for both polarizations exhibits similar behaviors as in our experimental
measurements: p-polarization produces sharp dips in reflectance at ±8º incidence while s-
polarization shows no angle dependence and nearly constant reflection. The electric field
intensity distributions are shown in Figure 4-7 (b) and (c) for p-polarization and Figure 4-7 (d)
and (e) for s-polarization. When the incident light is not coupled to the plasmon modes (Figure
4-7 (b), (d), (e)), similar low electric fields are observed for all three cases. With p-polarized
light at 8º incidence, we can see clear excitation of the plasmon resonance. The ratio of the
-15 -10 -5 0 5 10 15
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
I
AC
( A)
Incident Angle (Degree)
Au grating/5nm Al
2
O
3
66
electric field intensity of the brightest point on the metal surface is 20 larger under p-polarized
than under s-polarized illumination (i.e., |Ep|
2
/|Es|
2
= 20). However, in order to more accurately
predict the enhancement factor observed experimentally, we must integrate over the metal
surface. Based on these simulations, we can calculate the relative density of hot electrons by
integrating the electric field intensity in the metal (|𝐸 |
2
) , which is proportional to the hot
electron generation rate, under both s- and p-polarization conditions. In the z-dimension, we
integrate from the top metal surface (z = f(x,y)) to one mean free path length below the surface
(z = 𝑓 ( 𝑥 , 𝑦 )− 𝜆 Au
), as described by equation
79
𝐸𝐹 =
∫ |𝐸 𝑝 |
2
𝑑𝑥𝑑𝑦𝑑𝑧 𝑓 ( 𝑥 ,𝑦 )
𝑓 ( 𝑥 ,𝑦 ) −20𝑛𝑚
∫
|𝐸 𝑠 |
2
𝑑𝑥𝑑𝑦𝑑𝑧 𝑓 ( 𝑥 ,𝑦 )
𝑓 ( 𝑥 ,𝑦 ) −20𝑛𝑚
Based on literature values of the mean free path of electrons with different energies above the
Fermi level, we use 20 nm for electrons with 1.96eV energy for our estimation here.
69
Performing this integral yields an average integrated enhancement of 16X, which is on the
same order as the enhancement observed experimentally.
67
Figure 4-7: (a) Finite difference time domain (FDTD) simulations of the photoreflectance as
a function of the incident angle for p- and s-polarized 633nm light. Cross-sectional electric
field intensity profiles for illumination at (b) p-polarized and normal incidence; (c) p-polarized
and 10
o
incidence; (d) s-polarized and normal incidence; (e) s-polarized and 10
o
incidence.
-15 -10 -5 0 5 10 15
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
s-polarization
p-polarization
(a)
(d)
(e)
(c)
(e)
(d)
(b)
(c)
(b)
-200 -100 0 100 200
200
225
250
275
300
x (nm)
y (nm)
0
2.0
4.0
6.0
8.0
-200 -100 0 100 200
200
225
250
275
300
x (nm)
y (nm)
0
2.0
4.0
6.0
8.0
-200 -100 0 100 200
200
225
250
275
300
x (nm)
y (nm)
0
2.0
4.0
6.0
8.0
-200 -100 0 100 200
200
225
250
275
300
x (nm)
y (nm)
0
2.0
4.0
6.0
8.0
68
4.5 Conclusion
In summary, plasmon resonant excitation of hot electrons is observed in a metal-based
photocatalyst, which is used to drive the reverse reaction of water oxidation. In this
configuration, hot electrons, photoexcited in the metal, inject into the aqueous solution and
reverse the water oxidation half reaction (O2 + 4H
+
+ 4e
-
⇋ H2O), thus, producing a
photocurrent. By utilizing a plasmonic grating structure of under resonant conditions, we
demonstrate amplification of this process. The photocurrent, and hence charge transfer rate, is
measured with respect to the angle of incidence with 633nm wavelength light. Sharp peaks in
the photocurrent are observed at incident angles of ±16
o
when the light is polarized
perpendicular to the grating (p-polarization). We also observe sharp dips in the
photoreflectance with similar angle dependence, which correspond to the condition when there
is good wavevector matching between the incident light and the plasmon mode in the grating.
When illuminated with light polarized parallel to the grating (s-polarization), we observe no
angle dependence in the photocurrent or photoreflectance. Based on this data, we estimate a
plasmonic gain factor of 6.6X in the charge transfer rate when irradiated under resonant
conditions. Electromagnetic (FDTD) simulations corroborate the sharp dips observed in the
photoreflectance at ±8
o
, and provide a detailed picture of the electric field intensity profiles,
which show the excitation of a plasmon-resonant mode, when illuminated under resonant
conditions (i.e., 8
o
and p-polarization).
69
This research was supported by NSF Award No. CBET-1512505 (L.S.), Air Force
Office of Scientific Research Grant No. FA9550-15-1-0184 (B.H.), ARO Award No.
W911NF-14-1-0228 (H.S.), Department of Energy (DOE) Award Nos. DE-FG02-07ER46376
(N.P.), and ACS-PRF Grant #55993-ND5 (J.C.).
70
Chapter 5 Nanoscale thermometry using electron energy loss
spectroscopy (EELS) in suspended MoS2
5.1 Abstract
In this work, we demonstrate the use of electron energy loss spectroscopy (EELS) to
spatially map the temperature profile across electrically-heated, suspended MoS2. Here, few-
layer MoS2 flakes were transferred onto pre-pattered electrodes using a home-built contact
aligner. The electrodes are patterned on TEM-compatible SiN membranes by
photolithography and slits are cut by focused ion beam milling, in order to create a suspended
region for the subsequently transferred MoS2. The suspended nature of the MoS2 eliminates
undesired heat transfer and EELS signal from the underlying substrate. The plasmon mode in
the EELS spectra of MoS2 is observed around 22 eV and is correlated with temperature using
a temperature controlled stage. The suspended MoS2 flake is heated by local microheaters on
TEM chips, and the temperatures were measured in situ by Raman spectroscopy and EELS.
The electrically-heated temperatures in the suspended MoS2 were corroborated using Raman
spectroscopy, which provide a robust measure of the temperature but have much spatial lower
spatial resolution than those obtained with the focused electron beam. These results
demonstrate our ability to use an electron beam just 1 nm in diameter to measure the
temperature of an atomically-thin material using EELS. These results push the limits of
temperature spatial resolution and material volume that can be used in the experimental
determination of temperature.
71
5.2 Introduction
Transition metal dichalcogenides (TMDCs) have drawn more and more attention
following the great success of atomically thin 2D graphene studies. The weak out-of-plane
bonding of layered TMDC materials enables easy isolation of these materials into atomically
thin forms. Their versatile and extraordinary electrical and optical properties in combination
with mechanical flexibility make TMDCs excellent candidates for novel nanoscale electronic
and optoelectronic devices
80
. In such applications, the device features are rapidly approaching
nanometer scale, and in some cases reaching the atomic limit
81
. At these small length scales
and reduced dimensions, nanoscale thermometry techniques with special resolution below
1000 nm are required to better understand thermal transport and heat management. Currently
available high-spatial-resolution techniques are generally optical
82-83
or scanning probe
84-85
.
The scanning probe technique requires the probe in contact and in thermal equilibrium with
the system, which brings disturbance in small systems. While some optical techniques require
inserting local probes like luminescent nanoparticles, noncontact optical techniques have
limited spatial resolution from optical diffraction limit. In pursue of a noncontact, nanoscale
temperature-mapping technique, Mecklenburg et al. reported a thermometric technique,
plasmon energy expansion thermometry (PEET), based on electron energy loss spectroscopy
(EELS) in a transmission electron microscope (TEM)
86-87
. In this approach, shifts of the bulk
plasmon energy in the material are linked to temperature changes via thermal expansion. The
expansion of lattice at elevated temperatures results in a change in the valence electron density,
which in turn shifts the bulk plasmon energy. For materials with sufficiently sharp plasmon
resonances, this technique provides an in situ method for obtaining temperature gradients with
nanometer-scale resolution. In the work presented here, we apply PEET to a widely studied
72
member of the TMDCs group, molybdenum disulfide (MoS2), and compare the measurements
to those of a conventional Raman-based thermometry technique. Both measurements are
calibrated using temperature reads from heater resistance on a TEM-compatible microheater
chip, and then used to estimate the bulk thermal expansion coefficient (TEC) of MoS2. PEET
also enables us to generate a temperature map with nanometer spatial resolution for suspended
few-layer MoS2 sheets and study the in-plane temperature gradients across the sheets in TEM.
The temperature gradient is induced by Joule heating in a microheater on one side of the
suspended sheet while keeping the other side at room temperature. The temperature map is
constructed by acquiring electron energy loss spectra across the MoS2 and carefully measuring
the bulk plasmon energy shifts and converting them to temperatures.
5.3 Experimental details
Few layer MoS2 sheets used in this work are dry-transferred via the scotch tape method
(i.e., mechanical exfoliation) to a customized TEM chip (Figure 5-1) or a commercially
available chip (Figure 5-2 (a), FEI NanoEx
TM
-i/v)
88
. The customized chip has a 2 μm-wide
slit and microheaters patterned on both sides of the slit. The dry transfer method prevents
contamination like chemical residues from wet transfer processes
89
, and studying the MoS2
sheets in suspended region eliminates any potential thermal effects from underlying substrates.
MoS2 is first exfoliated from bulk material (obtained from SPI Supplies, Inc.) onto a
transparent polydimethylsiloxane (PDMS) substrate, and sheets with the desired thicknesses
are located under an optical microscope
90
. The target sheet is then aligned and transferred to
the slit on the TEM chip (or the windows on the NanoEx chip) using a home-built contact
aligner
91
. The chips are mounted and wire bonded to chip carriers that are customized for a
TEM biasing holder (Hummingbird, Inc.), which can provide electrical power to the
73
microheaters on chip with a sourcemeter (2400 Sourcemeter, Keithley). Desired temperatures
or heating for measurements are obtained by controlling the applied power with a customized
LabVIEW program. Raman spectra are collected at <10
-5
Torr with a Cryostat (Cryo Industries,
Inc.) excited by 532 nm laser source (100 μW) in a Renishaw InVia spectrometer. EELS data
is acquired in a JOEL JEM-2100F STEM with a Gatan Quantum 963 GIF. Standard imaging
conditions have an accelerating potential of 80 kV, a 40 μm diameter condenser aperture, a
beam current of approximately 0.5 nA, a beam convergence semi-angle α of 12-14 mrad, and
a spectrometer collection semi-angle β of 19-22 mrad.
The commercially available microheater chip (FEI NanoEx
TM
-i/v) provides high
temperature uniformity over the electron-transparent windows with high stability during
operation. The microheater chip contains electron-transparent windows surrounded by a
meander-shaped heater with four contact pads, as shown in Figure 5-2 (a). Heat generated by
Joule heating produces a temperature change in the window region (∆𝑇 𝐹𝐸𝐼
), which can be
determined by the 4-probe resistance from the following equation:
∆𝑇 𝐹𝐸𝐼
= 𝑇 − 𝑇 0
=
𝑅 − 𝑅 0
𝑇𝐶𝑅 ∙ 𝑅 0
Here, 𝑅 0
is the resistance at the reference temperature 𝑇 0
and 𝑇𝐶𝑅 is the temperature
coefficient of resistance (dR/dT). And these values are calibrated and provided by the
manufacturer.
74
Figure 5-1: (a) Schematic illustration of the suspended MoS2 sheet on TEM chip. (b) STEM
(c) optical and (d) SEM images of MoS2 sheet.
EELS
Raman
E-beam LASER
(a)
(b) (c)
Heater 1
Heater 2
MoS
2
flake
Slit
MoS
2
flake
20 μm 20 μm
(c) (d)
10 μm
(b)
Suspended MoS
2
flake
Slit
75
Figure 5-2: Optical images of (a), (b) MEMS microheater chip for NanoEx
TM
-i/v. (c)
microheater with MoS2 flakes. (d) Raman spectra of MoS2 at different temperatures. (e)
Comparison of theoretical and experimental Raman peak position as a function of temperature.
200 300 400 500 600 700 800 900 1000 1100
375
380
385
390
395
400
405
410
Raman Shift (cm
-1
)
T_FEI (K)
E
1
2g
A
1g
Ref_E
1
2g
Ref_A
1g
(a)
(b) (c)
50 μm 50 μm
200 μm
(d)
(e)
360 370 380 390 400 410 420 430
Intensity (a.u.)
Raman Shift (cm
-1
)
298 K
401 K
501 K
597 K
701 K
798 K
898 K
997 K
76
5.4 Results and discussion
Raman spectroscopy is a noncontact, nondestructive technique widely used to study
mechanical and thermal properties of graphene and TMDCs, and extensive temperature
dependent Raman studies of few layer TMDCs (MoS2, WS2, etc.) in literature reveal detailed
properties of such materials at various temperatures. For example, the Raman peak position
shifts and peak broadening as temperature elevates are studied for vapor-phase-grown,
exfoliated and hydrothermal-synthesized MoS2 sheets, showing linear variation of the E
1
2g and
A1g modes frequencies with temperature
92-93
. Although limited in spatial resolution as an
optical technique, the well-established Raman-temperature dependence of few-layer MoS2
sheets provides a reliable route to evaluate the temperature changes in an averaged region on
~1 μm scale. Figure 5-3 (a) shows an example of Raman shifts of MoS2 calibrated with the
stage temperature controller in the cryostat. Depending on the excite laser power, local heating
might play a role and change the Raman spectra of MoS2. To rule out this effect, we conduct
a laser power dependence study in the same setup. As can be seen from Figure 5-3 (b) and (c),
there is a substantial change of Raman peak position at laser powers larger than 300 μW.
Similar trends are shown in Figure 5-4 for different stage temperatures. We select ~100 μW
in the followed studies to maximize Raman signals while keeping the local heating effect from
laser at negligible level.
77
Figure 5-3: (a) Raman spectra of MoS2 at different stage temperatures in the cryostat. (b)
Raman spectra of MoS2 showing local heating effects from incident laser. (c) Raman peak
positions at different incident laser powers measured at a stage temperature of 400 K.
360 370 380 390 400 410 420 430
0
500
1000
1500
2000
2500
3000
Counts
Raman Shift (cm
-1
)
400K
300K
360 370 380 390 400 410 420 430
10
100
1000
10000
100000
Counts
Raman shift (cm
-1
)
41.5 uW
123 uW
342 uW
1140 uW
400K
0.0 0.2 0.4 0.6 0.8 1.0 1.2
382.0
382.4
382.8
383.2
383.6
384.0
384.4
384.8
Raman Shift (cm
-1
)
E
1
2g
Linear Fit
Raman Shift (cm
-1
)
Incident Laser Power (mW)
Equation y = a + b*x
Adj. R-Square 0.94104
Value Standard Error
F Intercept 383.10796 0.04121
F Slope -0.73086 0.06883
406.0
406.4
406.8
407.2
407.6
408.0
408.4
408.8
A
1g
Linear Fit
Equation y = a + b*x
Adj. R-Square 0.79056
Value Standard Error
G Intercept 408.57935 0.08043
G Slope -0.70355 0.13435
(a)
(b)
(c)
78
Figure 5-4: Raman peak positions as a function of incident laser power at different stage
temperatures for (a) E
1
2g and (b) A1g modes.
Figure 5-2 (d) shows the substantial red-shifts of both the E
1
2g and A1g Raman modes at
elevated temperatures from MoS2 sheets on a NanoEx chip. The temperature reads 𝑇 𝑁𝑎𝑛𝑜𝐸𝑥
are determined via a four-wire measurement of the chip heater resistance, as described in the
supplemental documents. Figure 5-2 (e) shows the Raman peak position shifts in E
1
2g and A1g
modes as a function of temperature. Both modes show linear variation with temperature, which
agrees well with linear dependence by Sahoo (solid lines in Figure 5-2 (e)). In Raman spectra
of few layer MoS2, the in-plane mode E
1
2g and out-of-plane mode A1g frequencies vary with
temperature mainly due to contribution from thermal expansion and anharmonicity caused
0.0 0.2 0.4 0.6 0.8 1.0 1.2
382.0
382.5
383.0
383.5
384.0
384.5
Raman Shift (cm
-1
)
Incident Laser Power (mW)
400K
375K
350K
325K
300K
0.0 0.2 0.4 0.6 0.8 1.0 1.2
407.5
408.0
408.5
409.0
409.5
410.0
Raman Shift (cm
-1
)
Incident Laser Power (mW)
400K
375K
350K
325K
300K
(a)
(b)
79
temperature contribution
92
. The underlying nature of the Raman-temperature dependence is
very similar to the bulk plasmon energy shifts with temperature, which makes it a perfect
candidate for calibrating temperature measurements in MoS2 via PEET.
As a first step towards applying PEET to determine temperature gradients in MoS2 sheets,
we measure the temperature dependence of MoS2 bulk plasmon energy on a NanoEx chip.
EELS spectrum images of MoS2 are acquired at different temperatures (𝑇 𝑁𝑎𝑛𝑜𝐸𝑥 ) and the
plasmon energy shifts are obtained from the images as described in supplemental documents.
An example dark field (DF) image and plasmon energy maps at different temperatures are
shown in Figure 5-5 (a). The bulk plasmon energy of MoS2 from the free-electron model is
given by:
𝐸 = ħ𝜔 𝑝 = ħ
√
4𝜋𝑛 𝑒 2
𝑚
At elevated temperatures, the number density of valence electrons in the MoS 2 decreases due
to thermal expansion by the relation
𝑛 ( 𝑇 )≈ 𝑛 ( 𝑇 0
) [1 − 𝑓 ( 𝑇 ) ]
where 𝑓 ( 𝑇 )= ∫ 𝛼 𝑉 ( 𝑇 ′) 𝑑𝑇 ′
𝑇 𝑇 0
≈ 𝛼 1
∆𝑇 + 𝛼 2
∆𝑇 2
and 𝛼 1
and 𝛼 2
are the first and second order
linear thermal expansion coefficient, respectively
94-96
. Combining these equations we can
write the bulk plasmon energy as a function of temperature:
𝐸 = 𝐸 0
[1 −
1
2
𝑓 ( 𝑇 ) ] = 𝐸 0
[1 −
1
2
( 𝛼 1
∆𝑇 + 𝛼 2
∆𝑇 2
) ]
The bulk plasmon energy from our EELS measurements are plotted versus temperature as
shown in Figure 5-5 (b). In comparison, theoretical predictions of E based on this equation
and reported 𝛼 𝑉 values are shown in the same plot as black line
97
and green line
98
in the same
80
plot. Using different TECs would lead to variations in the plasmon energy prediction. Our
experimental results agree best with predictions based on values reported by Ding.
Figure 5-5: (a) Dark field image and plasmon energy maps of MoS2 from EELS measurement.
(b) Comparison of theoretical and experimental plasmon energy shifts as a function of
temperature.
(b)
(a)
200 400 600 800 1000 1200
22.9
23.0
23.1
23.2
23.3
23.4
Plasmon Energy (eV)
Temperature (K)
Experiments
EL-Mahalawy
Ding
DF Image
1076 K 969 K 869 K 771 K 671 K
298 K 369 K 470 K 570 K
200 nm
23.30 23.10 23.15 23.20 23.25
Plasmon Energy (eV)
81
In order to use PEET to determine temperatures in MoS2 sheets, we first rewrite the
temperature as a function of relative plasmon energy change
T = 𝑇 0
+ ∆𝑇 = 𝑇 0
+
𝛼 1
2𝛼 2
(√1 −
8𝛿 𝛼 2
𝛼 1
2
− 1)
where 𝛿 is the normalized change in the plasmon energy 𝛿 ≡
𝐸 ( 𝑇 ) −𝐸 ( 𝑇 0
)
𝐸 ( 𝑇 0
)
. 𝛿 at a given
temperature is obtained by aligning the maps at T and 𝑇 0
and comparing plasmon energies at
each pixel. Using TEC values 𝛼 1
= 2.16732 × 10
−5
𝐾 −1
and 𝛼 2
= 2.0210× 10
−9
𝐾 −2
from literature
97
, we can now obtain the temperatures in MoS2 via PEET (TPEET), as plotted in
Figure 5-6. Following a similar route to PEET, we can relate Raman shifts to the temperature
change by a linear equation
𝜔 ( 𝑇 )= 𝜔 0
+ 𝛾 ( 𝑇 − 𝑇 0
)
where 𝜔 0
is the frequency of Raman mode at 𝑇 0
and 𝛾 is the first order temperature coefficient
of the Raman mode
92
. Using coefficients 𝛾 ( 𝐸 2𝑔 1
)= −1.47 × 10
−2
𝑐𝑚 −1
/𝐾 and 𝛾 ( 𝐴 1𝑔 )=
−1.23 × 10
−2
𝑐𝑚 −1
/𝐾 reported by Wilson
99
, temperatures in MoS2 sheets can be determined
from Raman (TRaman) by the equation:
𝑇 = 𝑇 0
+
𝜔 − 𝜔 0
𝛾
Figure 5-6 shows both TPEET and TRaman plotted as a function of the chip temperature, which
shows good agreement for both methods over the range from 300 to 1000 K.
82
Figure 5-6: Measured temperatures obtained from PEET and Raman studies plotted as a
function of chip temperature.
To construct a temperature gradient map using PEET with the customized chip, we vary
the power applied to the heaters on one side of the slit, while keeping the other heater short-
circuited (zero power, room temperature), thus creating different temperature profiles across
the MoS2 sheet under different heating powers. We can also reverse the direction of the
temperature gradient by switching the hot and cold heaters. To generate a temperature map for
a certain heating power temperature, two EELS maps, one with and one without heating, are
first acquired by focusing the STEM electron beam into a nanometer-sized probe and rastering
it over the suspended MoS2 sheet. Those maps are then aligned to remove special drifts, and
are compared to obtain the normalized change in plasmon energy (𝛿 ) for each pixel. After this
step, the 𝛿 map can be plotted and converted to a temperature map. An example temperature
200 400 600 800 1000
200
400
600
800
1000
E1_2g
A_1g
El-Mahalawy
Ding
Slope=1
Temperature (K)
T
FEI
(K)
83
map is shown in Figure 5-7 (a). By averaging the area of interest along the y direction, we can
plot out the temperature as a function of position along the flake, as shown in Figure 5-7 (b).
Figure 5-7: (a) 2D temperature map and (b) 1D temperature profile plotted along the
suspended length of a MoS2 sheet from EELS mapping.
0.0 0.5 1.0 1.5 2.0
0.00
0.25
0.50
X ( m)
Y ( m)
0.0 0.5 1.0 1.5 2.0
300
400
500
600
700
Temperature (K)
X (um)
Right 100mW
Left 100mW
(b)
(a)
200 nm
400 500 450
Temperature (K)
84
5.5 Conclusion
In conclusion, we have studied the temperature dependence of Raman modes as well as
bulk plasmon energies in few layer MoS2 flakes. The linear variation of Raman frequencies
versus temperature provides a convenient noncontact technique to determine temperature in
MoS2, but can only reach micrometer spatial resolution due to optical diffraction limit. The
bulk plasmon energy shifts in MoS2, which originate from a change in the number density of
valence electrons from lattice thermal expansion, can also be used to determine the
temperature change in the MoS2 sheets via PEET and achieve nanometer-scale spatial
resolution. The temperatures obtained with both methods are calibrated and in good agreement
with the carrier chip temperature. Temperature gradient maps across suspended MoS 2 sheets
are obtained via PEET and reversible gradient profiles are shown with a customized TEM chip.
The methods reported here can be applied to other metals and semiconductors with sufficiently
sharp plasmon resonances and large thermal expansion coefficients. These methods can be
also applied to noncontact nanoscale thermal studies of modern microelectronic devices
without introduction of any thermometric materials.
85
This research was supported by NSF Award No. 1402906 (L.S.), Department of Energy
(DOE) Award No. DE-FG02–07ER46376 (R.D.)
86
Chapter 6 Outlook and future work
6.1 Hot electron photocatalysis
As extension of the hot electron photocatalysis work and a step toward our ultimate goal
of achieving CO2 reduction with hot electrons, we first explore application of the same grating
structures in a non-aqueous solution with a one electron reaction, Fe(C5H5)2
+
+ e
-
=> Fe(C5H5)2.
An oxide layer,5nm thick Al2O3 was deposited on the grating by atomic layer deposition (ALD)
at 200
o
C using TMA as the Al source and water vapor as the O source. Figure 6-1 (a) shows
an illustration of the sample geometry, for which electrical connection is made directly to the
exposed top Au surface, while keeping the bottom Si substrate out of the circuit. Given the
5nm oxide layer thickness, here we can rule out tunneling as a potential means of photocurrent
generation. Instead, we believe the photoexcited hot electrons jump over the oxide barrier and
propagates in the Al2O3 to the ions in solution. as illustrated in Figure 6-1 (b). The samples
are measured in the same setup as described in previous chapters. The only changed
experimental parameter is the electrolyte, from the aqueous solution to an acetonitrile solution
containing 0.001 M ferrocene (98%, Sigma-Aldrich), with 0.1 M (EMIM)BF4 (99.0%, Sigma-
Aldrich) as the supporting electrolyte.
87
Figure 6-1: (a) Schematic diagram of the plasmon resonant grating structure. (b) Energy band
diagram with respect to NHE illustrating the mechanism of hot electron injection mechanism.
Au
0.66 eV
0.62 eV
Al
2
O
3
-1.14 eV
1.96 eV
e-
Fc/Fc
+
(a)
(b)
88
We observe similar photo-response of the plasmonic gratings as a function of the
incident angle in the electrolyte is shown in Figure 6-2 (a). When the incident light is s-
polarized, an angle-independent photocurrent is observed. We believe the finite current
observed under s-polarized light is mostly from surface roughness. A clear angle-dependent
photocurrent is observed under p-polarized. Two symmetric maxima in the photocurrent under
p-polarization can be seen at ±10º from normal incidence. By comparing the photocurrent at
±10º for two polarizations, we estimate an amplification factor of 230% for hot electron
generation in the metal grating by exploiting plasmon resonance conditions. The
photoreflectance of the grating was also measured as we swept the incident angle. The dip at
±10º from normal incidence is a clear signature of wavevector matching between the incident
light and the plasmon resonant mode in the grating. A plot of the plasmon resonant wavevector
as a function of incident angle is plotted in Figure 6-3, which predicts a resonance at an
incident angle of 10 degrees. Figure 6-2 (c) shows the DC photocurrent plotted as a function
of reference potential, which exhibits the clear and abrupt onset of photocurrent at +0.45 V vs
NHE. Here, it is important to note that the DC current is driving ferrocene oxidation (i.e.,
Fe(C5H5)2 + e
+
=> Fe(C5H5)2
+
), which depletes all the neutral Fe(C5H5)2 species, while
photocurrent reduces the Fe(C5H5)2
+
(i.e., Fe(C5H5)2
+
+ e
-
=> Fe(C5H5)2), and this is the
reaction that is producing the AC photocurrent. Based purely on the AC photocurrent, it is
hard to distinguish between electron and hole charge transfer. However, from the energy band
diagram, we believe that hot holes will have a much higher barrier to jump over, and therefore,
hot electrons dominate this AC photocurrent.
89
Figure 6-2: (a) AC photocurrent and (b) photoreflectance plotted as a function incident angle
for 633 nm light with an intensity of 106 mW/cm
2
polarized parallel and perpendicular to the
grating structure. (c) DC photocurrent plotted as a function of reference potential when
illuminated at -10
o
with respect to normal incidence.
(a)
(b)
(c)
Laser
Chopper
Waveplate
Rotation Stage
(d)
(e)
-20 -10 0 10 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
s-polarization
p-polarization
Lock-in
Potentiostat
Au Si Al
2
O
3
Ag/AgCl Pt
Chopper
0.0 0.2 0.4 0.6 0.8
0.0
0.1
0.2
0.3
0.4
DC Current (mA)
V vs. NHE (V)
s-polarization
p-polarization
-20 -10 0 10 20
0
1
2
3
4
5
6
AC Photocurrent (uA)
Incident Angle (Degrees)
s-polarization
p-polarization
90
Figure 6-3: k vector of incident light in medium and estimate of resonant k vector as a function
of incident angle.
We also performed similar FDTD simulations in acetonitrile. The calculated reflectance
for both polarizations is shown in Figure 6-4 (a). The simulated data for both polarizations
exhibits similar behaviors as in our experimental measurements: P-polarization produces sharp
dips in reflectance at ±10º incidence while s-polarization shows no angle dependence and
nearly constant reflection. The electric field intensity distributions are shown in Figure 6-4 (b),
(c) for p-polarization and Figure 6-4 (d), (e) for s-polarization. When the incident light is not
coupled to the plasmon modes Figure 6-4 (b), (d) and (e), similar low electric fields are
observed for all three cases. With p-polarized light at 10º incidence, we can see clear excitation
of the plasmon resonance. The ratio of the electric field intensity of the brightest point on the
metal surface is 63X larger under p-polarized than under s-polarized illumination (i.e.,
|Ep|
2
/|Es|
2
= 63X). Based on these simulations, we also estimate the relative density of hot
0 5 10 15 20
0
50
100
k
light
or k
resonant
(um
-1
)
Incident Angle (Degrees)
k
light
k
resonant
91
electrons by integrating the electric field intensity in the metal (|E|
2
) , which is proportional to
the hot electron generation rate, under both s- and p-polarization conditions. In the z-dimension,
we integrate from the top metal surface (z = f(x,y)) to one mean free path length below the
surface (z = 𝑓 ( 𝑥 , 𝑦 )− 𝜆 Au
), as described by
79
𝐸𝐹 =
∫ |𝐸 𝑝 |
2
𝑑𝑥𝑑𝑦𝑑𝑧 𝑓 ( 𝑥 ,𝑦 )
𝑓 ( 𝑥 ,𝑦 ) −20𝑛𝑚
∫
|𝐸 𝑠 |
2
𝑑𝑥𝑑𝑦𝑑𝑧 𝑓 ( 𝑥 ,𝑦 )
𝑓 ( 𝑥 ,𝑦 ) −20𝑛𝑚
This integral gives an average integrated enhancement of 5.3X, on the same order as the
enhancement observed experimentally.
92
Figure 6-4: (a) FDTD simulations of the photoreflectance as a function of the incident angle
for p- and s-polarized 633 nm light. Cross-sectional electric field intensity profiles for
illumination at (b) p-polarized and normal incidence; (c) p-polarized and 10
o
incidence; (d) s-
polarized and normal incidence; (e) s-polarized and 10
o
incidence.
-20 -10 0 10 20
0
20
40
60
80
100
Reflection (%)
Incident Angle (Degrees)
s-polarization
p-polarization
(a)
(d)
(e)
(c)
(e)
(d)
(b)
(c)
(b)
93
6.2 Hot electron injection into 2D materials
There have been several recent theoretical reports claiming that plasmon resonant
excitations decay into hot electrons in metal nanostructures. Experimentally, however, it is
difficult to distinguish between the effects of local field enhancement and hot electron
generation. Recently, we demonstrated that optically excited hot electrons in various bulk
metals can be preferentially injected into the Σ-valley of MoS2 (and WSe2) giving rise to
enhanced indirect band gap luminescence. This process of hot electron injection is facilitated
by the layered structure of transition metal dichalcogenides (TMDCs), which are free of
dangling bonds and surface reconstruction and, thereby, provide a clean interface between the
pz orbitals of the sulfur atoms (i.e., Σ-valley) of MoS2 and the electronic wavefunctions in the
bulk metal. This metal/TMDC system thereby provides a unique platform for distinguishing
between hot electron injection and plasmonic local field enhancement. In measuring the
photoluminescence spectra of TMDC/metal heterojunctions comprised of plasmonic
nanostripe antennas, we observe a substantial increase in the indirect band gap (i.e., Σ-valley)
emission, when irradiated with light polarized along the axis of the stripes (i.e., bulk
excitation). When irradiating with light polarized perpendicular to the stripes (i.e., plasmonic
excitation), however, only a small increase in the indirect band gap emission is observed. In
both cases, no enhancement (2X = reflection) of the of the direct band gap emission is observed,
indicating that these effects are not simply due to local field enhancement, which would affect
both direct and indirect emission. These results can show experimental evidence that plasmon
resonant nanostructures do, in fact, generate hot carriers (relative to bulk excitation).
The prospect of generating hot carriers from plasmon resonant nanostructures presents
the exciting possibility of effectively reducing the energy barrier heights in electronic devices
94
and photocatalytic reactions, enabling new mechanisms of energy conversion. Sundararaman
et al. calculated the energy distributions of hot carriers generated by the decay of surface
plasmons based on a quantized plasmon model coupled together with detailed electronic
structure calculations.
100
Manjavacas et al. also reported calculations of plasmon-induced hot
carrier generation in spherical nanoparticles and nanoshells.
101
Here, the energy distribution
of the hot carriers was calculated assuming various hot carrier lifetimes τ ranging from 0.05
to 1 ps. However, they found that substantial hot carrier populations were only obtained for
lifetimes above 500 fsec, which is considerably larger than the carrier lifetimes in bulk metals,
~10 fsec. Zheng et al. measured photocurrent generation in plasmonic nanowire arrays
deposited on heavily doped TiO2.
15
In a similar study, Chalabi et al. measured photocurrent
generation in a Au/Al2O3/Au structure, in which the top Au layer was comprised of a
plasmonic nanostripe antenna array, enabling plasmon excitation (with light polarized parallel
to the nanostrips) to be compared with bulk excitation (with light polarize perpendicular to the
nanostrips).
68
While these results are intriguing, it is still hard to rule out the possibility that
the photocurrent is simply due to local field enhancement of sub-band gap defect states. For
Zheng’s work, the TiO2 was heavily doped => a large defect concentration. For Al 2O3,
however, this is less likely although, not zero. In addition to photocurrent generation, hot
electrons have also been reported for use in photocatalytic reactions.
102-103
Since the local field
enhancement can be very large, this effect can be seen even if there are a trace amount of sub-
band gap states.
In 2015, Li et al. reported hot electron injection in metal/MoS2 and metal/WSe2
heterojunctions. Here, an enhancement in the indirect band gap photoluminescence emission
in observed with respect to that of the bare TMDC. This enhancement arises because of the
95
preferential injection of hot electrons from the bulk metal over the Schottky barrier into the Σ-
valley of MoS2 (and WSe2), as illustrated in Figure 6-5. The Σ-valley consists primarily of the
pz orbitals of the sulfur atoms, which stick up out of the MoS2 plane and can readily interact
(i.e., overlap) with the electronic wavefunctions of the metal. The K-valley (which
corresponds to direct bandgap emission) originates from the in-plane d orbitals of the Mo
atoms that do not reside on the surface of the S-Mo-S stack and, therefore, do not interact with
the electrons in the metal. These orbitals are illustrated in Figure 6-5, based on DFT
calculations. While this previous work only looked at hot carrier injection from a bulk metal,
this metal/TMDC system provides a unique platform that can distinguish between plasmon-
induced hot electron injection and plasmonic local field enhancement. In the case of plasmonic
local field enhancement, we expect to see enhancement of both indirect and direct bandgap
emission. Whereas, hot electron injection will only enhance the Σ-valley/indirect emission.
96
Figure 6-5: Energy band diagram illustrating the injection of hot electrons from the metal to
the MoS2.
MoS2 flakes are exfoliated from bulk material (SPI Supplies, Inc.) onto a
polydimethylsiloxane (PDMS)-coated glass slide using the “Scotch tape” method.
104
Relatively thick (>100 nm) flakes are identified optically and transferred onto a separate chip
containing a Au pad and a plasmonic nanostripe antenna array using a home built contact
aligner. These Au nanostripes consist of 50 nm thick, 250 nm wide lines separated by 250 nm,
as patterned by electron beam lithography. The flakes are then transferred such that they lie
partially across the Au pad and the nanostripe array, as shown in Figure 6-6. We then collect
PL spectra from regions of the flake on the metal pad, and on the nanostripe array and on SiO2
by irradiating with 532 nm light (0.1 mW).
X
Metal
K valley Σ valley
E
F
Σ point has a Lower
Energy, and is atomic
orbital favored
0.5eV
e
-
Laser
d orbital of
Mo atom
p
z
orbital of sulfur
atom, more overlap
with metal
S
Mo
S
97
Figure 6-6: (a) Schematic diagram and (b) SEM image of the metal/MoS2 structure.
Figure 6-7 shows the photoluminescence spectra taken in various locations on the
sample. The spectrum taken from bare MoS2 shows indirect (1.35 eV) and direct (1.8 eV)
band gap peaks of roughly equal intensity. For MoS2 on the plasmonic nanostructure, there is
a clear difference in the spectra taken with light polarized parallel and perpendicular to the
nanostrips. While the direct bandgap emission is nearly the same for both polarizations (due
to local field enhancement and reflection), the indirect emission is substantially pronounced
for the parallel polarization in which strong excitation of surface plasmons occurs. As a
comparison, spectra taken from MoS2 on the Au pad shows no polarization dependence. Here,
the indirect emission is enhanced by a factor of 4 due to hot electron injection while the direct
band gap peak is enhanced by a factor of 2X, due to reflection. One of the interesting
characteristics in these spectra is that the TE oriented spectra on the plasmonic nanostripe
array are blue-shifted by 50 meV. This is not currently understood. Also, the linewidth of the
plasmon-enhanced peak is considerably narrower (83 meV) than the bare MoS2 spectrum (150
meV), and the non-plasmon excited spectra.
Au
MoS2
98
Figure 6-7: Photoluminescence spectra of (a) MoS2 on the plasmonic nanostripe arrays (PNAs)
and (b) bulk Au film.
Figure 6-8 shows the results of FDTD calculations for the Au pad and the Au plasmonic
nanostripe arrays under parallel and perpendicular polarization. Under TM illumination, the
field is uniformly distributed along the top surface of the stripe, similar to the field distribution
in a bulk metal film (i.e. Au pad). Under TE illumination, however, a plasmon resonance is
excited and a clear redistribution of the fields towards the edges can be seen.
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
0
10000
20000
30000
40000
Counts
Energy (eV)
MoS2 only, TM
MoS2 on Au, TM
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
0
10000
20000
30000
40000
Counts
Energy (eV)
MoS2 only, TM
MoS2 on PLs, TM
MoS2 on PLs,TE
99
Figure 6-8: FDTD simulation of the plasmonic structure and Au film in TE and TM mode.
E
2
, TE mode, PLs Au E
2
, TE mode, bulk Au
E
2
, TM mode, PLs Au E
2
, TM mode, bulk Au
Simulation region
Au
TiO2
100
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Appendix A: Plasmonic silver nanostructures prepared by electron
beam evaporation
Figure A-1 shows an example of silver plasmonic nano-island structure prepared by
electron beam evaporation. The gap width plays in important role in the plasmonic
enhancement, and can be easily tuned by varying the nominal thickness during the deposition
process. Figure A-2 shows an example where the nanostructure is used in combination with a
2D material, WSe2, to promote its light absorption and photocatalytic performance.
108
Figure A-1: TEM images of plasmonic silver nano-island with different nominal thicknesses:
(a), (b) 10 nm. (c) (d) 15 nm.
(a) (b)
(c) (d)
109
Figure A-2: (a) TEM image of the simulated region (500 X 500 nm). (b) Index and (c) Electric
field profile from the top view. (d) Index and (e) Electric field profile from the cross-section
view.
-200 -100 0 100 200
-200
-100
0
100
200
X (nm)
Y (nm)
-200 -100 0 100 200
-200
-100
0
100
200
X (nm)
Y (nm)
0
0.5
1.0
1.5
2.0
-150 -140 -130 -120 -110
-20
-10
0
10
20
Y (nm)
Z (nm)
-150 -140 -130 -120 -110
-20
-10
0
10
20
Y (nm)
Z (nm)
0
0.5
1.0
1.5
2.0
(a)
(b) (c)
(d) (e)
Ag
WSe
2
H
2
O
SiO
2
Abstract (if available)
Abstract
This dissertation presents our efforts in developing and improving artificial photosynthesis processes based on silicon photocathodes and metal grating structures. These experiments shed light on some fundamental physics in such devices, and could potentially help in designing more efficient artificial photosynthesis processes. ❧ Chapter 1 provides introduction and background materials to aid in understanding the experiments presented in the dissertation. The chapter starts with a brief overview of the utilization of solar energy and artificial photosynthesis. Next we briefly discuss about solar water splitting as an example of general photoelectrolysis. We also introduce the concept of plasmon resonance and discuss our motivations to pursue hot electrons for photocatalysis. More detailed background materials for each topic are covered in the relevant chapter. ❧ Chapter 2 presents some water splitting experiments with silicon based photocathodes. These experiments show how the photocatalytic behavior of a semiconductor photocathode can be tuned by improving the light absorption and increasing electrode surface area with nanostructured surface. ❧ Chapter 3 shows our investigations of hot electrons injection in a diode structure. We show that with the Au grating/oxide/graphene structure, photocurrent is generated under irradiation and corresponds well with the plasmon resonance. These results help us to distinguish between local field enhancement mechanism and hot electron injection mechanism, which is critical in understanding our enhanced photocatalytic performance in various works. ❧ In Chapter 4 we discuss the extension of the results in Chapter 3 to direct water splitting experiments. The same grating structures are used in aqueous solution and photocurrent are observed as a sign of reversing the water oxidation reaction with photon-generated hot electrons. ❧ In Chapter 5 focuses on the study of few layer MoS₂ sheets in TEM. Both Raman and EELS based methods are used to monitor the temperate changes in MoS₂. Temperature maps with nanometer-scale spatial resolution are generated via PEET.
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Shen, Lang
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Hot carrier enhanced photocatalysis in plasmon resonant metal grating systems
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Viterbi School of Engineering
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Doctor of Philosophy
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Materials Science and Engineering
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07/27/2018
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