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Thermoelectric and transport studies of low dimensional materials & hot electron-driven photocatalysis on plasmon-resonant grating nanostructures

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Thermoelectric and transport studies of low dimensional materials & hot electron-driven photocatalysis on plasmon-resonant grating nanostructures

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Thermoelectric and Transport Studies of Low Dimensional Materials & Hot Electron-Driven
Photocatalysis on Plasmon-Resonant Grating Nanostructures
By
Yu Wang

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirement for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)

May 2022

Copyright 2022 Yu Wang
ii

EPIGRAPH

Challenge is Opportunity

iii

Acknowledgements
I want to thank my advisor Prof. Stephen B. Cronin for supporting me and guiding me
through my Ph.D. journey. He gave me a lot of help when conducting research on very different
topics. I am very fortunate to have become one of his students and I am and will be grateful through
the rest of my academic career.
I want to thank Prof. Jayakanth Ravichandran for being my thesis defense committee chair,
and I want to thank Prof. Brent Melot for serving on my defense committee. I also want to thank
Prof. Jahan Dawlaty, Prof. Edward Goo for serving on my Ph.D. candidate qualifying committee.
I also want to thank my collaborators Prof. Li Shi from University of Texas at Austin, Prof.
David Johnson from University of Oregon, and Prof. Jahan Dawlaty from University of Southern
California, and looking forward to more collaboration in the future.
I want to thank my group members and friends for helping me with my daily research.
Special thanks to Mrs. Indu Aravind for working diligently on finite difference time domain
(FDTD) simulation in my thesis. I would like to thank Dr. Lang Shen for encouraging me to
continue my Ph.D. research.
Finally, I want to thank my parents for supporting me through my Ph.D. journey. I want to
thank my beloved wife Lu Niu for giving me so much care and emotional support whenever I felt
down or discouraged. Thank you.

iv

Table of Contents
Acknowledgements ........................................................................................................................ iii
List of Figures ................................................................................................................................ vi
Abstract .......................................................................................................................................... ix
Chapter 1: Introduction ................................................................................................................... 1
1.1 Clean energy development .................................................................................................... 1
1.2 Thermal energy and thermoelectricity .................................................................................. 3
1.3 Solar energy and photoelectrocatalysis ................................................................................. 6
Chapter 2: Enhanced Low-Temperature Thermoelectric Performance in (PbSe)1+δ(VSe2)1
Heterostructures due to Highly Correlated Electron in Charge Density Waves ............................. 9
2.1 Abstract ................................................................................................................................. 9
2.2 Introduction ......................................................................................................................... 10
2.3 Results and Discussion ....................................................................................................... 12
2.4 Conclusion .......................................................................................................................... 27
Chapter 3: Hot Electron Driven Photocatalysis on Plasmon-resonant Grating Nanostructures ... 29
3.1 Abstract ............................................................................................................................... 29
3.2 Introduction ......................................................................................................................... 30
3.3 Results and Discussion ....................................................................................................... 31
3.4 Conclusion .......................................................................................................................... 49
Chapter 4: In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis
in Plasmon-Resonant Grating Structures ...................................................................................... 51
4.1 Abstract ............................................................................................................................... 51
4.2 Introduction ......................................................................................................................... 53
v

4.3 Experimental Methods ........................................................................................................ 57
4.4 Results and Discussion ....................................................................................................... 60
4.5 Conclusion .......................................................................................................................... 73
Chapter 5: Outlook and future work ............................................................................................. 76
5.1 Thermoelectric and Transport Study of (PbSe)1+δ(VSe2)1,2 Heterostructures .................... 76
5.2 Thermoelectric Study of Interlayer Excitons ...................................................................... 80
5.3 Photocatalysis Based on Plasmonic Nanostructures ........................................................... 82
Bibliography ................................................................................................................................. 83

vi

List of Figures
Figure 1.1. (a) Changes in global surface temperature recreated from paleoclimate archives
between AD 1-2000 and from direct measurements within 1850-2020. (b) Simulated and
observed changes in global surface temperature between 1850-2020. All the temperature
changes are relative to the average temperature between 1850-1900.
1
.......................................... 2
Figure 1.2. United States primary energy consumption by energy source in the year 2020.
2
........ 3
Figure 1.3. (a) State-of-art reports on thermoelectric materials from 2010-2020 with
benchmarked 𝑍𝑇 values, costs, and calculated cost-effectiveness, which are defined as
𝑍𝑇𝑚𝑎𝑥 /cost. (b) Various reports of thermoelectric materials showing plotted 𝑍𝑇 values with
respect to their discovered years. The pink and cyan dashed lines indicate 𝑍𝑇 values of 1.0 and
2.0, respectively.
4
............................................................................................................................ 5
Figure 1.4. Comparison of realized dual junction PEC cells with their theoretical limits. ............ 7
Figure 2.1. Optical microscope image of in-plane (a) Seebeck and (b) resistivity measurements
of the (PbSe)1+δ(VSe2)n films. (c) HAADF-STEM image of (PbSe)1+δ(VSe2)1 heterostructure
film. ............................................................................................................................................... 14
Figure 2.2. (a) Low angle X-ray reflectivity (XRR) patterns, (b) specular X-ray diffraction
patterns and (c) grazing incidence in-plane X-ray diffraction patterns for the samples
investigated in this study. The corresponding miller indices are also labeled for each material. . 16
Figure 2.3. (a) Electrical resistivity, (b) Seebeck coefficient and (c) power factor of
(PbSe)1+δ(VSe2)n films plotted as a function of temperature. The inset in 2.3a shows the change
in resistivity over the temperature range during which the Seebeck coefficient exhibits a sudden
jump for the (PbSe)1+δ(VSe2)1 material. ....................................................................................... 20
Figure 2.4. S/T vs. 1/σ of (PbSe)1+δ(VSe2)1,2 heterostructure materials. The dashed and dotted
black lines are the linear fitted regions of (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 films,
respectively. .................................................................................................................................. 22
Figure 2.5. Estimated electrical contribution of the thermal conductivities of (PbSe)1+δ(VSe2)1
and (PbSe)1+δ(VSe2)2 heterostructures based on Wiedemann-Franz (WF) law using temperature
dependent resistivities data. .......................................................................................................... 23
Figure 2.6. Basal plane area of (a) VSe2 layer and (b) PbSe layer as a function of temperature
for (PbSe)1+δ(VSe2)1 material, which are measured by in-plane X-ray diffraction (XRD). ......... 25
Figure 2.7. Raman spectra of (a) (PbSe)1+δ(VSe2)1 and (b) (PbSe)1+δ(VSe2)2 heterostructures
measured at room temperature (300K) and 77K. .......................................................................... 26
Figure 3.1. (a) SEM image of the plasmon resonant Ag grating structure. (b) UV-vis diffuse
reflectance measurement of Ag grating and Ag film. ................................................................... 33
vii

Figure 3.2. (a) Atomic force microscope (AFM) image of the surface of the Ag grating and (b)
the corresponding surface profile extracted from the AFM image. .............................................. 34
Figure 3.3. Schematic diagrams of the AC lock-in technique photocurrent measurement setup. 36
Figure 3.4. (a) Schematic diagram of the experimental measurement, (b) photoreflectance, and
(c) AC photoelectrochemical current measured as a function of incident angle. The red curve
corresponds to p-polarized light, and the black curve corresponds to s-polarized light. .............. 38
Figure 3.5. (a) Calculated angle-dependent photoreflectance of the Ag grating under 633 nm
light. Simulated electric field distributions with (b) resonant and (c) non-resonant polarizations
at 7.0
o
incidence with 633nm light. .............................................................................................. 40
Figure 3.6. (a) AC photocurrent measured as a function of incident angle and (b) simulated
absorptance spectra for 785 nm illumination. (c) electric field distributions at 7.6
o
incident angle
for (c) p-polarized and (d) s-polarized 785 nm wavelength irradiation. ....................................... 43
Figure 3.7. (a) AC photoelectrochemical current measured as a function of incident angle and
(b) simulated absorptance spectra for Ag grating with 532nm irradiation. .................................. 44
Figure 3.8. Calculated electric field distributions at (a) 18.4
o
, (b) 24.0
o
, and (c) 28.8
o
incident
angles for Ag grating with 532nm illumination with p-polarization. ........................................... 45
Figure 3.9. (a) Calculated resonant condition for different incident angle (0
o
-40
o
) and incident
light wavelength (400 nm-900 nm). (b) Simulated absorptance spectrum of Ag grating with
respect to incident light wavelength and incident angle. .............................................................. 48
Figure 4.1. (a) LSPR-induced hot electrons and holes are initially extended over a non-thermal
distribution and (b) quickly thermalized to a hot Fermi-Dirac distribution (red profile) through
electron-electron scattering in 50 fs and subsequently to a room-temperature Fermi-Dirac
distribution (blue profile) through electron-phonon scattering within 1 ps to 10 ps. ................... 55
Figure 4.2. (a) Scanning electron microscope (SEM) image of Au grating with 500 nm period
and 266 nm metal linewidth. Schematic diagrams of (b) transient absorption (TA) measurement
and (c) the hot electron driven photocatalytic water splitting process and (c) angle-dependent
photocurrent measurement setup. ................................................................................................. 59
Figure 4.3. Transient absorption (TA) measurements for Au grating with 266 nm metal line
width and 500 nm period (a) in solution and (b) in air environment with p-polarized probe pulse.
Selected broadband TA spectra of different delay time between pump and probe for (c) in
solution and (d) in air condition, respectively. (e) Time evolution of normalized ∆ 𝐴 at 631 nm
and 549 nm for Au grating in solution and in air, respectively. The solid lines are fitted curves
based on a monoexponential decay convoluted with 100 fs FWHM Gaussian pump pulse. (f)
Relative shift of the plasmon resonant bleaching peaks in TA spectra as delay time increases. .. 62
viii

Figure 4.4. (a) Transient absorption spectra of 266 nm linewidth Au grating when the probe is
s-polarized (i.e., electric field parallel to the grating lines). (b) Selected broadband TA spectra
of different delaying time between pump and probe. ................................................................... 68
Figure 4.5. Measured UV-Vis spectrum of 266 nm linewidth Au grating in air with unpolarized
light under normal incidence. ....................................................................................................... 69
Figure 4.6. (a) Measured AC photoelectrochemical current, (b) measured and calculated
absorptance plotted as a function of incident angle for the 266 nm linewidth grating. Electric
field intensity distributions near the grating surface at incident of (c) 4.0
o
, (d) 8.3
o
, and (e) 9.3
o

for p-polarized 633 nm irradiation. ............................................................................................... 71
Figure 4.7. (a) Measured AC photocurrent and calculated absorptance plotted as a function of
incident angle for the 300 nm linewidth grating. Electric field intensity profile near the grating
surface at (b) 7.5
o
, (c) 8.5
o
, and (d) 10.5
o
for p-polarized 633 nm light. ...................................... 73
Figure 5.1 Designed and synthesized cross-plane (PbSe)1+δ(VSe2)1,2 thermoelectric devices (a)
before and (b) after Al2O3 capsulation and top heater fabrication. ............................................... 77
Figure 5.2 Optical microscope images of (a) zoom out and (b) zoom in views of fabricated
device for anisotropic electrical transport measurement. ............................................................. 79
Figure 5.3 (a) In-plane and cross-plane resistivities, (b) resistivity ratio of cross-plane over in-
plane for (PbSe)1(VSe2)1 heterostructures as the function of temperature. .................................. 80
Figure 5.4 Schematics showing the excitonic thermoelectric device based on WSe2/h-
BN/MoSe2 heterostructure. ........................................................................................................... 81
ix

Abstract
This dissertation presents the research I conducted during my Ph.D. study in the University
of Southern California. It consists of two parts. The first part is about thermoelectricity, which
converts thermal energy into electrical energy. The second part is about plasmonic water splitting,
which converts photon energy into chemical energy. The overall goal of my thesis is to try to
address the climate change, the biggest challenge of the 21
st
century.
Chapter 1 is to give an introduction of the status of climate change issue, such as the high
correlation between the continuing rising of earth surface temperature and human activities. It also
introduces the concept of thermoelectricity as well as photocatalysis.
Chapter 2 talks about how we can use charge density wave (CDW) phase transition to
enhance the thermoelectric performance in (PbSe)1+ 
(VSe2)1 heterostructure thin films. It also
explains the fundamental mechanism of the observed enhancement.
Chapter 3 studies the hot electron-driven hydrogen evolution reaction (HER), which results
from the decay of surface plasmon polaritons (SPPs). In this study, wavelength dependent reaction
rate measurements are also conducted, showing better performance at longer wavelength.
Chapter 4 discusses the effect of chemical adsorbates on the relaxation dynamics of hot
carriers on localized surface plasmon resonance (LSPR) grating nanostructures through transient
absorption (TA) measurements. Reaction rates are also studied on those gratings with different
linewidths.
Chapter 5 presents the outlook and future works based on the research in previous chapters.

1

Chapter 1: Introduction
1.1 Clean energy development
The rising surface temperature on earth caused by well-mixed greenhouse gas (GHG) has
become the greatest challenge to be solved in the 21st century. Since 1750, the increases in GHG
concentrations in the atmosphere, ocean and land are mostly caused by human activities. The
average concentrations of GHG have been continuously rising since 2011 and reached 410 parts
per million (ppm), 1866 parts per billion (ppb), and 332 ppb for carbon dioxide, methane, and
nitrous oxide, respectively.
1
Global surface temperature between 2001-2020 was 0.99
o
C higher
than that between 1850-1900. Besides, the acidification of the surface open ocean and retreat of
glaciers mainly result from human-caused GHG. Figure 1.1a shows the changes in global surface
temperature from year 1 to 2020 compared to the average temperature between 1850-1900.
1
The
solid grey line depicts the reconstructed temperature rises from paleoclimate archives, and grey
shadings are their likely ranges. On the other hand, the solid black line indicates the observed
increases in global surface temperature between 1850-2020, and the most recent temperature is
already the highest in at least the last 100,000 years. Figure 1.1b presents the simulated temperature
rise caused by human/natural and natural only, which overlay with the observed temperature
increase within 1850-2020. It clearly shows that human activities are the main driver for the
increase in global temperature.
2

Figure 1.1. (a) Changes in global surface temperature recreated from paleoclimate archives
between AD 1-2000 and from direct measurements within 1850-2020. (b) Simulated and observed
changes in global surface temperature between 1850-2020. All the temperature changes are
relative to the average temperature between 1850-1900.
1

Right now, most of the energy sources still come from fossil fuels, including petroleum,
natural gas, and coal. Take the United States for an example, the consumed energy from these
three non-renewable sources accounts for 79% of the total energy consumption.
2
Figure 1.2 shows
the primary energy consumption sources in the United States in 2020, in which renewable energy
sources make up 12% of total consumed energy, and solar and wind occupy 37% of renewable
energy. In order to combat climate change and control the global average temperature rise to be
under 2
o
C, we need to decarbonize our energy sources at an unprecedented rate.
3

Figure 1.2. United States primary energy consumption by energy source in the year 2020.
2

1.2 Thermal energy and thermoelectricity
As about 90% of the energy consumption around the world involves heat generation or
manipulation over a broad range of temperatures, various technologies in thermal science need to
be further developed for deep decarbonization to mitigate climate change.
3-4
Thermoelectric
devices are used to scavenge waste heat during various industrial processes by directly converting
heat into electricity, and they are also used in solid-state cooling systems without any refrigerant
involved. Thermoelectric performance is usually characterized by a dimensionless figure of merit,
𝑍𝑇 , which is defined as

𝑍𝑇 =
𝑆 2
𝜎 𝜅 𝑇 =
𝑆 2
𝜎 𝜅 𝑒 + 𝜅 𝑙 𝑇
(1.1)
4

where 𝑆 is the Seebeck coefficient, 𝜎 is the electrical conductivity, 𝑇 is the temperature, and 𝜅 is
the total thermal conductivity. Furthermore, 𝜅 is further decomposed into 𝜅 𝑒 and 𝜅 𝑙 , which are
thermal conductivities contributed by electrons and phonons, respectively. 𝑆 2
𝜎 is also called
power factor, meaning the maximum power that can be extracted from the thermoelectric generator
through impedance matching. On the other hand, 𝜅 indicates the thermal power transferred from
the higher-temperature to lower-temperature sides, representing the power consumed by the
generator. So, a high 𝑍𝑇 value implied a high ratio between power output and power input, leading
to better performance in thermoelectricity.
Studies of thermoelectrics consist of three aspects, which are structural designs,
dimensional designs, and devices fabrications.
4
For structural designs, there are various efforts on
band engineering through doping and phase manipulations. Crystal imperfections are also under
intense research, such as point defects, dislocations, interfaces as well as porosity. For dimensional
designs, much attention has been drawn towards quantum confinement effects,
5
such as nanodots,
nanowires, superlattices, and ball-milled polycrystals. For device fabrications, research has been
moved from conventional thermoelectric generators to miniature modules and stretchable devices.
Figure 1.3a shows the state-of-art research on thermoelectric materials with measured 𝑍𝑇 values,
costs, and calculated cost-effectives, which are defined as 𝑍 𝑇 𝑚𝑎𝑥
/cost.
4
The material with highest
𝑍𝑇 value of 2.8 is the Sn1-xSe, and the most cost-effective material is the skutterudites. Figure 1.3b
presents those reported materials from different studies in their discovered years, and the insets
showcased data from selected literatures.
5

Figure 1.3. (a) State-of-art reports on thermoelectric materials from 2010-2020 with benchmarked
𝑍𝑇 values, costs, and calculated cost-effectiveness, which are defined as 𝑍 𝑇 𝑚𝑎𝑥
/cost. (b) Various
reports of thermoelectric materials showing plotted 𝑍𝑇 values with respect to their discovered
years. The pink and cyan dashed lines indicate 𝑍𝑇 values of 1.0 and 2.0, respectively.
4

6

1.3 Solar energy and photoelectrocatalysis
As the penetration of solar and wind electricity increases, its intermittency presents a
serious challenge to the grid. Hydrogen can be used as the energy source to compensate for this
intermittency either through direct combustion or fuel cells. Traditionally, hydrogen is produced
by stream water reforming, which generates large amounts of carbon dioxide. A greener way to
produce hydrogen is through electrolysis of water, but the electrolytic modules need to be
connected to photovoltaic panels and require optimal impedance matching. Peak efficiencies of
12.6% and 24.6% could be achieved theoretically by connecting electrolytic modules with Si and
III-V triple junction photovoltaic (PV) modules, respectively.
6
The high cost of PV-electrolyzer
system and low capacity factors undermines further commercialization of this technology.
On the other hand, direct solar to hydrogen conversion through photocatalysis or
photoelectrocatalysis (PEC) offers another route to make green hydrogen and can potentially be
more commercially viable.
7
There are three stages in a typical photocatalytic system, which are
light absorption, charge separation and diffusion, and surface reaction. Development of a
competitive PEC system requires good light absorbers and long charger carrier lifetimes. The first
demonstrated solar water splitting system is through n-type TiO2 connected with a Pt counter
electrode by Fujishima and Honda in 1972.
8
However, TiO2 is a wide band-gap material (3.2 eV)
and can only utilize the ultra-violet (UV) portion in solar energy spectrum, which only accounts
for about 4%. Later studies have been focused on materials with band-gaps in the visible light
range, such as GaAs, InP, and GaP.
9
Khaselev and Turner showcased 12.4% solar-to-hydrogen
(STH) conversion efficiency by using GaInP2 photocathode combined with a GaAs photovoltaic
layer.
10
However, those semiconductors are not stable in aqueous solution and often undergo
photodecomposition. By applying a thin layer of amorphous TiO2 (less than 5 nm) on top, their
7

performances in water splitting are stabilized. In addition to providing protection against photo-
corrosion, the n-type TiO2 forms a large band-edge offset with p-type III-V semiconductors,
leading to decreased recombination rates and increased lifetime of photo-generated electron-hole
pairs.
9
Figure 1.4 shows recent performances of PEC devices consisting of dual junctions, which
indicates the approaching theoretical limits of those cells.
11
Nevertheless, the fabrication costs are
still too high to be deployed on a large scale economically. More recently, BiVO4, Fe2O3, Cu2O
and C3N4 have been under intense study because of their low cost and promising performance in
water oxidation reaction, which is the rate determining half reaction in water splitting.
7, 12

Figure 1.4. Comparison of realized dual junction PEC cells with their theoretical limits.
8

In recent decade, plasmonic water splitting has been attracting more and more attention, in
which free electrons on the metal surfaces oscillate coherently with respect to the electrical field
of incident light.
13-14
There are three effects in plasmonic photocatalysis, which are photothermal
effect, concentrated electric field effect, and hot carrier effect. Photothermal effect induces higher
temperatures on the metal surfaces, resulting in better performances when coupled with thermal
catalysts.
15
As we mentioned before, many semiconductors are very expensive to fabricate, which
induces interests in reducing their thicknesses. The disadvantage of reducing thickness is the less
absorption of light. However, the light absorption of semiconductors can be increased largely when
depositing plasmonic nanoparticles on top, which is due to concentrated electric field effect near
the surfaces.
16-17
Furthermore, photo-induced minority carriers will be generated in shallower
regions of semiconductors and can diffuse to the surface more easily to drive chemical reactions.
18

Last but not least, nonthermalized hot carriers can be induced directly through nonradiative
damping of plasmon resonance and directly participate in the chemical reaction without any
semiconductor involved, which will be intensively discussed in later chapters.
13-14

9

Chapter 2: Enhanced Low-Temperature Thermoelectric
Performance in (PbSe)
1+ δ
(VSe
2
)
1
Heterostructures due to Highly
Correlated Electron in Charge Density Waves
This chapter is similar to Wang et al., published in Nano Letters.
19

2.1 Abstract
We explore the effect of charge density wave (CDW) on the in-plane thermoelectric
transport properties of (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructures. In
(PbSe)1+δ(VSe2)1, we observe an abrupt 86% increase in the Seebeck coefficient, 245% increase
in the power factor, and a slight decrease in resistivity over the CDW transition. This behavior is
not observed in (PbSe)1+δ(VSe2)2 and rather unusual compared to the general trend observed in
other materials. The abrupt transition causes a deviation from Mott relationship through correlated
electron states. Raman spectra of the (PbSe)1+δ(VSe2)1 material show the emergence of additional
peaks below CDW transition temperature associated with VSe2 material. Temperature dependent
in-plane X-ray diffraction (XRD) spectra show a change in the in-plane thermal expansion of VSe2
in (PbSe)1+δ(VSe2)1 due to lattice distortion. The increase in the power factor and decrease in the
resistivity due to CDW suggests a potential mechanism for enhancing the thermoelectric
performance at low temperature region.

10

2.2 Introduction
Since the 1950’s, many researchers have tried to improve the thermoelectric efficiency of
materials using various strategies including asymmetric density of states in low dimensional
materials,
5, 20
engineering materials and nanostructures with low lattice thermal conductivity,
21-22

and distortion of the density of states and creating resonant levels in the materials.
23-24
It should be
noted that, while there are relatively good thermoelectric materials (i.e., ZT ~ 1) at room
temperature and elevated temperatures (e.g., 800-1000
o
C), there are currently no efficient
thermoelectric materials at low temperatures. The phenomenon presented here can potentially
provide a new mechanism for increased thermoelectric performance at low temperatures that
deviates from the standard Mott relationship through the creation of correlated electron states.
Several transition metal dichalcogenides (TMDCs) exhibit charge density wave (CDW) transitions
at low temperatures.
25-26
Below the transition temperature, TCDW, the atomic lattice forms a
periodic structural distortion, which creates a condensed ground state of electrons and a gap in the
Fermi surface.
25-26
As a result, the electrical resistivity increases below TCDW. Under applied
electrostatic gate fields, the CDW can become unpinned from impurities and slide with respect to
the lattice. The sliding CDW can then contribute to the electron transport, adding to the single-
particle conductance contribution.
27-28
This collective conductance caused by the CDW sliding
motion can be modulated with an applied gate voltage up to two orders of magnitude higher than
the single-particle conductance.
29

In TiSe2 and VSe2, the resistivity increases due to the CDW phase transition, and the
transition temperature has been found to depend on both the applied pressure and layer thickness,
30-
33
which tune the interlayer interaction. VSe2 is composed of hexagonally stacked Se-V-Se sheets
with interlayer van der Waals bonding,
32
and has a CDW transition temperature near 100 K.
34

11

Recent work by the Johnson group has shown that the transition temperature can be tuned by
changing the PbSe separation thickness (m) or the VSe2 layer thickness (n) in [(PbSe)1+δ]m(VSe2)n
heterostructures.
35-36
The in-plane resistivity increase below TCDW is found to be most pronounced
in [(PbSe)1+δ]1(VSe2)1 structures, as shown in Figure 2.3. The in-plane resistivity increases below
TCDW in [(PbSe)1+δ]m(VSe2)1 structures are much more pronounced than observed in bulk VSe2.
35-
36
To our knowledge, no thermoelectric measurements have been performed on this interesting
new materials system. The cross-plane thermal conductivity of semiconducting layered
heterostructures has been found to be ultralow (  ~ 0.05 W/m∙K), making them promising
candidates for thermoelectric applications.
21
The in-plane lattice thermal conductivity of these
materials has also been found to be quite low ( l ~ 0.4 W/m∙K) according to the measurements
with the use of suspended micro-devices.
37-38
Both the in-plane and cross-plane thermal
conductivity measurements were done on semiconducting systems, where the electronic
contribution to the thermal conductivity ( e) is negligible. In comparison, e can be significant in
[(PbSe)1+δ]m(VSe2)n in the normal metallic state above TCDW and can be calculated from the
measured electrical conductivity using the Wiedemann-Franz law.
39-40
However, the Wiedemann-
Franz law does not necessarily hold true for CDW systems,
41
due to electron-electron interactions
as well as the collective conductance associated with the sliding motion of the CDW from electron-
phonon coupling.
42-46
Previous thermal conductivity measurements across the CDW transition
have shown an unusual peak in thermal conductivity at the CDW transition, which was explained
by an additional contribution from low frequency phasons.
47-48

Here, we explore the thermoelectric transport properties of (PbSe)1+δ(VSe2)1 and
(PbSe)1+δ(VSe2)2 heterostructures both above and below the charge density wave transition
temperature observed in the (PbSe)1+δ(VSe2)1 heterostructure. These measurements entail both in-
12

plane electrical conductivity and Seebeck coefficient measurements as a function of temperature.
We also measure the temperature dependence of the Raman spectra of these materials to
substantiate that a structural change occurs at the CDW transition. Besides, specular and in plane
X-ray diffraction patterns are measured, from which the room temperature structures of both
compounds and the temperature dependence of the lattice parameters of (PbSe)1+δ(VSe2)1 were
determined.

2.3 Results and Discussion
Figure 2.1 shows an optical microscope image of the sample used to measure the in-plane
Seebeck coefficient and electrical resistivity, via 4-point resistivity measurement which eliminates
the effect of contact resistance. Here, the Seebeck coefficient was measured by small micro-
fabricated heaters and thermometers, which are fabricated using electron beam lithography (EBL).
The [(PbSe)1+δ(VSe2)n] films, approximately 50 nm thick, were deposited in lithographically-
defined windows approximately 50 µm × 50 µm in size followed by a lift-off process.
22, 49
We
measured the temperature dependence of the Raman spectra in an optical cryostat. The
(PbSe)1+δ(VSe2)n films used in this study were formed via the self-assembly of various precursors
by physical vapor deposition (PVD) of elemental sources in a home-built high vacuum chamber
(< 5 × 10
-7
Torr). Pb and V were deposited by electron beam evaporation and Se was deposited
with a Knudsen effusion cell. Each constituent element was deposited sequentially to form the
layered precursors that matched the chemical composition ratios of each desired target structure.
The distance between the source and substrates was approximately 1 m and evaporation rates were
maintained below 1 Å/sec. Films were deposited on both unpatterned substrates (for structural
characterization) and patterned substrates (for in-plane transport measurements). Calibrated
13

deposition parameters were previously reported for the (PbSe)1+δ(VSe2)n compounds.
35
After the
initial deposition, the films were crystallized into the layered structures in an inert N2 environment
by heating to 250 °C for 60 min. The thickness of as-deposited films is 50 nm. Figure 2.1c shows
the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
measurement of (PbSe)1+δ(VSe2)1 heterostructure film. In the figure, alternating PbSe and VSe2
layers are clearly distinct from each other. Here, the PbSe layers are considerably brighter than the
VSe2 layers due to higher atomic number of Pb.
14

Figure 2.1. Optical microscope image of in-plane (a) Seebeck and (b) resistivity measurements of
the (PbSe)1+δ(VSe2)n films. (c) HAADF-STEM image of (PbSe)1+δ(VSe2)1 heterostructure film.
15

X-ray diffraction characterization of the (PbSe)1+δ(VSe2)n heterostructures were performed
after annealing and recrystallization. Figure 2.2 contains the X-ray reflectivity of two samples
investigated in this work. The spectra contain Kiessig fringes below the first order Bragg
reflections from the heterostructures and Laue oscillations after the first order Bragg reflection
from the finite number of unit cells in the films. The observation of fringes beyond 10° indicates
that the films have less than 1 Å of roughness within the coherence of the x-ray source. All the
maxima in the specular diffraction patterns can be indexed as 00l planes, indicating that the c-axes
are perpendicular to the substrates. The c-axis lattice parameters calculated from the patterns are
12.23(1) Å and 18.31(1) Å for (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 respectively, agreeing with
those previously reported. The maxima in the grazing incident in-plane diffraction patterns can all
be indexed as hk0 reflections of either PbSe or VSe2 planes, confirming the crystallographic
alignment of the crystals with the substrate. The in-plane lattice parameters are also consistent with
those previously reported. The lengths of a-axis of PbSe are 6.06(1) Å for both materials, and the
a-axis lengths of VSe2 are 3.43(1) Å and 3.40(1) Å for (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2,
respectively. Figure 2.2c shows that the PbSe pattern contains 110 and 310 reflections which are
forbidden in the rock salt structure, indicating that the crystal structure, while still cubic, has
distorted slightly from that of the bulk. Lattice parameters were determined from in-plane X-ray
diffraction (XRD) spectra of the (PbSe)1+δ(VSe2)1 compound collected as a function of temperature
at the Advanced Photon Source (APS) and are plotted as a function of temperature as shown in
Figure 2.6. The slope of the in-plane lattice parameter as a function of temperature of VSe2 changes
its sign at ~125K, indicating that VSe2 layers undergo a structural transition at low temperature
while the PbSe does not.
16

Figure 2.2. (a) Low angle X-ray reflectivity (XRR) patterns, (b) specular X-ray diffraction patterns
and (c) grazing incidence in-plane X-ray diffraction patterns for the samples investigated in this
study. The corresponding miller indices are also labeled for each material.

17

Figure 2.3 shows a comparison of the in-plane electrical resistivity and Seebeck coefficient
plotted as function of temperature over the range 77-300 K for both the (PbSe)1+δ(VSe2)1 and
(PbSe)1+δ(VSe2)2 heterostructures. For the (PbSe)1+δ(VSe2)1 material, the electrical resistivity
shows a minimum around 140 K, while the (PbSe)1+δ(VSe2)2 material shows a linear
monotonically increasing resistivity with temperature over the entire temperature range as
expected for a metal. These results are consistent with our previous measurement of charge density
waves in this material system, which reported a CDW transition around 140K for the 1:1 material
and not the 1:2 material.
35
The Seebeck coefficient plotted in Figure 2.3b shows a large abrupt
increase from 10.6 μV/K to 19.7 μV/K (increases by 85%) for the (PbSe)1+δ(VSe2)1 film when the
temperature is lowered from 150 K to 136 K, while the resistivity actually decreases from 3.768
μOhm∙m to 3.765 μOhm∙m over the same temperature range (shown in the inset in Figure 2.3a).
(PbSe)1+δ(VSe2)2 does not show this behavior and has relatively low Seebeck coefficient (3.4
μV/K). Tani et al. reported a similar abrupt change in the Seebeck coefficient of 1T-TaS2 around
210 K due to a CDW transition. However, this behavior was only observed in one out of a total of
six samples.
50-51
Bhatt et al. and Huang et al. reported Seebeck coefficient measurements through
the CDW transition of TiSe2-x, which resulted in gradual changes in the resistivity and Seebeck.
However, the suppression of the CDW transition actually improved the thermoelectric
performance in their samples.
52-53
Naik and Rastogi studied 2H-NbSe2 with and without Ga-
intercalation and found that resistivity shows a kink and Seebeck coefficient has broad maximum
around 35 K for single crystalline structure, and CDW effect is quenched with Ga-intercalation.
54

Compared to previous thermoelectric study on other CDW materials, the enhancement in the
Seebeck coefficient and the power factor observed in our (PbSe)1+δ(VSe2)1 heterostructures results
from two reasons. The first reason is that reducing the dimensions of VSe2 component to the
18

monolayer limit will enhance the CDW order and Fermi-surface nesting through correlated
electron states.
44-46
Research by Feng et al. shows that their density functional theory (DFT)
calculation overestimates the bandwidth of near-EF energy states compared to experimentally
observed value by a factor of 1.4. The discrepancy is attributed to electron-electron interaction
from V 3d orbitals that has not been included in the DFT.
44
Duvjir et al. demonstrate strong CDW
order and perfect Fermi-surface nesting due to enhanced electron-electron correlations in
monolayer VSe2.
45-46
Second, the heterointerface between the VSe2 and PbSe layers also plays an
important role in electronic reconstruction.
45
A metal-insulator transition was observed around
135K in a monolayer VSe2/bilayer graphene system, which is caused by heterointerfacial coupling
between VSe2 and graphene layers, such as interlayer charge transfer and hybridization.
45
We only
observe enhanced thermoelectric performance in (PbSe)1+δ(VSe2)1, because the CDW is only
observed when there is a monolayer VSe2 sandwiched by two PbSe layers. Above the CDW
transition temperature, the two materials (1:1 and 1:2 heterostructures) studied here show very
similar behaviors with the VSe2 in the metallic state. We observed the same Seebeck and resistivity
temperature-dependent profiles on several different samples. To characterize the thermoelectric
performance of those materials, power factors as a function of temperature have been calculated
and shown in Figure 2.3c. The power factor of the (PbSe)1+δ(VSe2)1 film changes from 29.8
μW/m∙K
2
above to 102.9 μW/m∙K
2
below the CDW transition, corresponding to a 245% increase
in the power factor. This large enhancement in the power factor of the (PbSe)1+δ(VSe2)1 film
originates from a sudden jump in the Seebeck coefficient and slightly decrease in resistivity. The
power factor of (PbSe)1+δ(VSe2)2 is 4.4 μW/m∙K
2
around 140 K. The ratio of power factors
between these two materials increases from 7 above to more than 23 below the CDW. At 80 K the
ratio of the power factors has increased to 90 (the (PbSe)1+δ(VSe2)2 film has a power factor 0.8
19

μW/m∙K
2
, while (PbSe)1+δ(VSe2)1 has a power factor of 72 μW/m∙K
2
). We would like to emphasis
that the 1:2 heterostructures (i.e., (PbSe)1+δ(VSe2)2) did not show this unique phenomenon, and we
believe it is unique to the VSe2 material in the monolayer limit. Hite et al. found that the
temperature-dependent mobility of the (PbSe)1+δ(VSe2)1 heterostructure decreases from 1.1
cm
2
/V∙s to 0.6 cm
2
/V∙s when temperature decreases from 300K to 120K. When temperature is
below CDW transition temperature, its mobility starts to increase and reaches 1.6 cm
2
/V∙s when T
= 10K.
35
A unique feature that we have observed is that the Seebeck coefficient increases abruptly
below the transition temperature while the resistivity decreases slightly. This behavior is rather
unusual compared to the general trend that has been observed in most material systems. This
unique trend has led to a large increase of the Seebeck coefficient and the power factor right below
the transition temperature. The unusual combination of increasing Seebeck and reduced resistivity
is apparently related to the presence of electron-electron correlation. Although the values of the
Seebeck coefficient and the power factor of our materials are low and lack commercial interest at
this point of development, the enhancement percentages reported here are quite high (86% in
Seebeck and 245% in power factor). Our findings suggest a potential enhancement mechanism for
thermoelectric performance, worthy of further investigation in future thermoelectric materials
design.
20

Figure 2.3. (a) Electrical resistivity, (b) Seebeck coefficient and (c) power factor of
(PbSe)1+δ(VSe2)n films plotted as a function of temperature. The inset in 2.3a shows the change in
resistivity over the temperature range during which the Seebeck coefficient exhibits a sudden jump
for the (PbSe)1+δ(VSe2)1 material.
21

Figure 2.4 shows the Seebeck coefficient over temperature S/T as a function of one over
conductivity 1/ 𝜎 for (PbSe)1+δ(VSe2)1,2 heterostructures. Under the assumption of weak electron-
electron interaction, the Mott relation gives the Seebeck coefficient of a metallic system as:

𝑆 = −
𝜋 2
3
𝑘 𝐵 2
𝑒 𝑇 𝜎 𝑑 𝜎 𝑑𝐸 |
𝐸 𝐹
(2.1)
where S is the Seebeck coefficient, 𝑘 𝐵 is the Boltzmann constant, T is the absolute temperature, e
is electron charge and 𝜎 is electrical conductivity. Equation (1) can be rewritten as:

𝑆 𝑇 = ( −
𝜋 2
3
𝑘 𝐵 2
𝑒 𝑑 𝜎 𝑑𝐸 |
𝐸 𝐹 )
1
𝜎
(2.2)

Figure 4 contains a plot of S/T vs. 1/ 𝜎 for both (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2, In the
(PbSe)1+δ(VSe2)2, the S/T versus 1/ 𝜎 relationship is linear over the entire measured temperature
range (80-300K) as indicated by black dotted line, indicating that
𝑑 𝜎 𝑑𝐸
|
𝐸 𝐹 is roughly constant over
this range. In addition, 𝑑𝜎 / 𝑑𝐸 can be written as ( 𝑑𝜎 𝑑𝑛 ) ( 𝑑𝑛 𝑑𝐸 ⁄ ) ⁄ = 𝑒𝜇 ( 𝑑𝑛 𝑑𝐸 ⁄ ) , where 𝑒 is
electron charge, 𝜇 is the mobility, and 𝑑𝑛 /𝑑𝐸 is proportional to the density of states. For
(PbSe)1+δ(VSe2)1, the S/T vs. 1/ 𝜎 is similarly linear between room temperature and CDW transition
temperature of ~140K, where the Mott relation is expected to be satisfied. Below 140K there is a
distinct change in behavior as the structure undergoes a structural distortion. The Mott relation is
no longer valid below the CDW transition temperature because the conduction electron
movements are highly correlated and no longer independent, and because the CDW transition is
accompanied by a gap opening in the electronic structure to make the system non-metallic.
22

Figure 2.4. S/T vs. 1/σ of (PbSe)1+δ(VSe2)1,2 heterostructure materials. The dashed and dotted black
lines are the linear fitted regions of (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 films, respectively.

Mavrokefalos et al. showed that the in-plane thermal conductivities of [(PbSe)0.99]x(WSe2)x
with x = 2, 3, 4 are around 0.5 W/m∙K at room temperature and are insensitive to interface
density.
37-38
For our (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructures, the thermal
conductivity is likely to be dominated by the electrical contribution since the VSe2 layers are
metallic. Figure 2.5 shows the estimated electronic contribution of the in-plane thermal
conductivities 𝜅 ∥
of these two heterostructures using temperature dependent resistivities data,
which is based on the Wiedemann-Franz (WF) law:

𝜅 𝜎 = 𝐿𝑇
(2.3)
23

Where 𝜅 is the electrical contribution of the thermal conductivities, 𝜎 is the electrical conductivity,
𝐿 is the proportionality constant called Lorenz number (2.44 × 10
-8
WΩ/Κ
2
), and 𝑇 is the
temperature. Both heterostructures exhibit monotonically decreasing behavior as temperature
decreases. At each temperature, the 𝜅 ∥
of (PbSe)1+δ(VSe2)2 is higher than that of (PbSe)1+δ(VSe2)1,
due to the higher percentage of VSe2 layers in the (PbSe)1+δ(VSe2)2 material. However, it should
be noted that, for (PbSe)1+δ(VSe2)1 material, the WF law may not hold true when the temperature
is below the CDW transition temperature.

Figure 2.5. Estimated electrical contribution of the thermal conductivities of (PbSe)1+δ(VSe2)1 and
(PbSe)1+δ(VSe2)2 heterostructures based on Wiedemann-Franz (WF) law using temperature
dependent resistivities data.

In order to further probe the presence of structural change with respect to temperature, the
in-plane diffraction patterns of (PbSe)1+δ(VSe2)1 heterostructure have been measured at several
24

different temperatures (Advanced Photon Source, Beamline 33-BM). Figure 2.6 plots the in-plane
lattice parameters of VSe2 and PbSe as a function of temperature. A local minimum of lattice
parameter of VSe2 is observed when temperature is around CDW phase transition. The thermal
expansion coefficient of VSe2 in (PbSe)1+δ(VSe2)1is about 10 x 10
-6
/K between 140K to 300K.
This is approximately a factor of two smaller than the linear thermal expansion coefficient of bulk
VSe2 (18.6 x 10
-6
/K). However, below 140K the thermal expansion coefficient is negative,
approximately -30 x 10
-6
/K. The thermal expansion coefficient of the PbSe layer remains constant
over the entire temperature range and is within a factor of two found for bulk PbSe. The change in
the thermal expansion coefficient of the VSe2 constituent is consistent with a structural change
occurring at ~140K. Raman spectra of the (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructures
taken at 77K and 300K, shown in Figure 2.7, also suggest a structural change in the VSe2 layer of
(PbSe)1+δ(VSe2)1 as temperature is decreased. For the 1:1 heterostructure material, we see the
emergence of several additional peaks in the Raman spectra at temperatures at 77K, which is
consistent with a transition to a lower symmetry lattice structure associated with the CDW
transition of the VSe2 constituent. On the other hand, the temperature dependence of the Raman
spectra of the (PbSe)1+δ(VSe2)2 heterostructure does not show any temperature dependence in the
Raman spectra. Previously, Balandin and coworkers reported similar changes in the temperature
dependence of the Raman spectra of TiSe2 corresponding to a CDW transition.
30
It should be noted
that bulk VSe2 exhibits a CDW transition with a much smaller change in resistivity above and
below the CDW temperature. In (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 the VSe2 layers have two
or one adjacent PbSe layers rather than adjacent VSe2 layers and there will be charge transfer
between the PbSe and VSe2 layers. Both differences will change the CDW transition behavior of
25

the VSe2 layers relative to bulk VSe2. The striking difference in behavior between the
(PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 material is consistent with our previous measurements.
35

Figure 2.6. Basal plane area of (a) VSe2 layer and (b) PbSe layer as a function of temperature for
(PbSe)1+δ(VSe2)1 material, which are measured by in-plane X-ray diffraction (XRD).

26

Figure 2.7. Raman spectra of (a) (PbSe)1+δ(VSe2)1 and (b) (PbSe)1+δ(VSe2)2 heterostructures
measured at room temperature (300K) and 77K.

27

2.4 Conclusion
In conclusion, we report the temperature dependent thermoelectric properties of
(PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructure materials. The (PbSe)1+δ(VSe2)1 material
shows an increased Seebeck coefficient from 10.6 μV/K to 19.7 μV/K (85% increase) and power
factor from 29.8 μW/m∙K
2
to 102.9 μW/m∙K
2
(245% increase) before and after CDW phase
transition, while the change in resistivity over the same temperature range is negligible. The
(PbSe)1+δ(VSe2)2 material shows monotonically increasing values of the transport properties with
temperature and lower values than those of the (PbSe)1+δ(VSe2)1 film. Above the CDW transition
temperature, the S/T vs. 1/ 𝜎 relationship is linear for both heterostructures in accordance with the
Mott equation, which is valid for metals with weak electron-electron interactions. However, non-
Mott behavior appears below the CDW transition for the (PbSe)1+δ(VSe2)1 material resulting from
the highly correlated conduction of electrons. Furthermore, temperature dependent in-plane XRD
and Raman spectra indicate unique structural changes for the (PbSe)1+δ(VSe2)1 heterostructure film.
The mechanism underlying these results may be used to provide a new approach for enhancing the
thermoelectric performances of materials at low temperatures.

28

Acknowledgements
This research was supported by the Department of Energy (DOE) Award Nos. DE-FG02-
07ER46376 (Y.W.), DE-FG02−07ER46377 (L.S.), NSF award No. CBET-2012845 (Z.C.), and
NSF Award No. CBET-1905357 (J.C.). This material is based upon work supported by the
National Science Foundation Graduate Research Fellowship Program under Grant No. 1309047.
Any opinions, findings, and conclusions or recommendations expressed in this material are those
of the author(s) and do not necessarily reflect the views of the National Science Foundation. The
authors acknowledge support from the National Science Foundation under grant DMR-1710214.

29

Chapter 3: Hot Electron Driven Photocatalysis on Plasmon-
resonant Grating Nanostructures
This chapter is similar to Wang et al., published in ACS Applied Materials & Interfaces.
13

3.1 Abstract
We demonstrate hot electron injection of photoexcited carriers in Ag-based plasmon resonant
grating structure. By varying the incident angle of irradiation, sharp dips are observed in the
reflectance with p-polarized light (electric field perpendicular to grating lines) when there is
wavevector matching between the incident light and the plasmon resonant modes of the grating,
and no angle dependence is observed with s-polarized light. This configuration enables us to
compare photoelectrochemical current produced by plasmon resonant excitation with that of bulk
metal interband absorption simply by rotating the polarization of the incident light while keeping
all other parameters of the measurement fixed. With 633 nm light, we observe a 12-fold
enhancement in the photocurrent (i.e., reaction rate) between resonant and non-resonant
polarizations at incident angles of ±7.6
o
from normal. At 785 nm irradiation, we observe similar
resonant profiles to those obtained with 633 nm wavelength light but with a 44-fold enhancement
factor. Using 532 nm light, we observe two resonant peaks (with approximately 10X enhancement)
in the photocurrent at 19.4
o
and 28.0
o
incident angles, each corresponding to different order of
resonant modes in the grating. The lower enhancement factors observed at shorter wavelengths are
attributed to more interband transitions and less absorption of light. Finite difference time domain
(FDTD) simulations of these grating structures confirm the resonant profiles observed in the angle-
dependent spectra of these gratings and provide a detailed picture of the electric field intensity
profiles on and off resonance.
30

3.2 Introduction
Plasmon resonant enhancement of photocatalytic processes has become a topic of great
research interest over the past 10 years. Early studies reported mostly photothermal processes.
55-58

Later studies investigated plasmon enhancement of photoelectrochemical redox processes
59
, such
as water splitting, carbon dioxide reduction, and chemical decompsition
16-17, 60-68
. Liu et al.
demonstrated plasmonic enhancement factors of 5X at 532nm and 66X at 633nm using Au
nanoparticles deposited on TiO2.
61
Here, Liu et al. integrated strongly plasmonic nanoparticles
with strongly catalytic materials (e.g., TiO2) to drive photocatalytic reactions at an accelerated rate.
Since the TiO2 used in these prior studies had relatively high defect concentrations, the mechanism
of enhancement was thought to be local field enhancement of sub-band gap defect states.
More recently, there have been several reports of hot electrons in plasmon resonant
nanostructures playing an important role in photochemical
69-70
and photoelectrochemical
71-72

processes. Here, photoexcitation of metal nanostructures is used to create populations of hot
electrons and hot holes that lie substantially above and below the Fermi energy, presenting the
exciting possibility of driving high barrier reactions with visible light. For example, Mukherjee et
al. demonstrated dissociation of H2 by irradiating Au nanoparticles with visible light through a
Feshbach resonance with the anti-bonding state of the hydrogen molecule.
69-70
The Govorov,
Atwater, and Nordlander groups have performed detailed calculations of the time evolution of
these hot electron distributions in various plasmon resonant nanostructures.
73-77
It is now generally
accepted that the relatively narrow distributions of hot electrons and hot holes (separated by ~2-3
eV) that are initially created with visible light in metal nanostructures decay into a hot Fermi
distribution over the time scale of 50-100 fs through electron-electron scattering.
78
Ultrafast pump-
probe experiments have since revealed that this hot Fermi distribution decays back to equilibrium
31

with the lattice temperature over time scales of several ps.
79-80
In the experiments reported here,
we believe that the electrons in this hot Fermi distribution are primarily responsible for producing
the photocurrents measured in our photoelectrochemical cell.
Previously, we demonstrated plasmonic amplification with Au gratings and obtained an
enhancement factor of 2.3X.
71
This provided a proof-of-principle demonstration that the hot
electron can be generated by surface plasmon resonance decay. However, finite difference time
domain (FDTD) simulations have predicted that Ag gratings should have sharper and narrower
resonances than Au, as a result of there being fewer interband transitions in the visible wavelength
range in Ag as compared with Au. And Atwater and co-workers showed that hot electrons
generated from Au have lower energy than that of hot holes by 1-2 eV, while hot electrons and
holes in Ag have equally distributed energy
74
. Besides, Ag is commercially much cheaper
compared to Au, which has a huge impact for large scale application. In the work presented here,
we investigate hot electron-driven photochemistry in Ag-based plasmon resonant grating
structures at several different wavelengths of excitation. In this study, we present the wavelength
dependence of surface plasmon polariton (SPP), which shows different resonance modes and
diffraction coupling orders at various wavelengths. Additionally, a detailed model mapping the
resonant condition and intensity as a function of incident angle and wavelength is also provided.
This can be used as a guideline for future SPP grating design.

3.3 Results and Discussion
Figure 3.1a shows a scanning electron microscope (SEM) image of the plasmon resonant grating
structure used in this work. Here, elemental silicon is covered with 100 nm silicon oxide, and
patterned by photolithography and reaction ion etching (RIE) to form a fine corrugated structure
32

with a 500 nm period. A 50 nm Ag film is subsequently deposited on the silicon oxide.
71, 81
Figure
3.2 shows the structure topography characterized by atomic force microscopy (AFM), indicating
a peak-to-valley height of 50 nm. Since this is a continuous film grating, the fill factor is 1. A UV-
vis reflectance spectrum of this grating structure is shown in Figure 1b. The spectrum was
measured by exciting the grating at normal incident (in air) with unpolarized light and collecting
light integrated over the full solid angle. Here, we observe two sharp dips in the spectra
corresponding to surface plasmon resonant modes at 492 nm and 604 nm.
33

Figure 3.1. (a) SEM image of the plasmon resonant Ag grating structure. (b) UV-vis diffuse
reflectance measurement of Ag grating and Ag film.
34

Figure 3.2. (a) Atomic force microscope (AFM) image of the surface of the Ag grating and (b) the
corresponding surface profile extracted from the AFM image.

In our photoelectrochemical setup, we use an AC lock-in technique in which the incident
light is modulated by an optical chopper wheel (Stanford Research Systems SR-540)
71-72, 82
, and
35

the AC photocurrent is measured using a lock-in amplifier (Stanford Research Systems SR-830),
as illustrated in Figure 3.3. This enables us to measure the small photocurrents produced by the
relatively short-lived hot electrons in the metal. This AC technique also enables us to separate the
hot electron photocurrent from the DC electrochemical current produced by equilibrated (i.e., cold)
electrons. The photoelectrochemical measurement was conducted with standard 3-teminal
potentiostat (Gamry Inc.) in 0.5 M Na2SO4 aqueous solution with the Ag grating as the working
electrode, Ag/AgCl (3 M NaCl) as reference electrode, and a Pt wire as counter electrode (BASI
Inc.). The applied bias with respect to Ag/AgCl is -0.5 V. Copper wires are connected to the top
of the Ag gratings by silver paint (SPI Supplies Inc.) to exclude the underlying Si substrate from
the circuit during the photoelectrochemical measurements. Epoxy resin (DEVCON Inc.) is used
to seal the grating on top of a glass slide, exposing only the grating area to the electrolyte
71, 81
.
36

Figure 3.3. Schematic diagrams of the AC lock-in technique photocurrent measurement setup.

The angle-dependent reflectivity of each grating is calculated using the FDTD method
(Lumerical Inc.). The cross-sectional SEM image of the corrugated grating structure (Figure 3.1a)
37

was imported and fitted to establish the surface profile. The gratings were modeled as custom
structure objects with the fitting equations describing the boundaries of the object in Lumerical
FDTD solutions:
𝑦 = 50 { exp ⁡[− 𝑎 ( − 𝑥 + 250 )
2
] + exp ⁡ [− 𝑎 ( − 𝑥 − 250 )
2
] } (3.1)
Where 𝑎 = ( 2 /125 )
2
. All the simulations were performed with a mesh size of 0.5 nm, and the
background medium was water. The gratings were excited by a plane wave source with various
incident angles and polarization directions. Bloch boundary conditions were applied on the sides
of the gratings to account for the phase change across each period. Perfectly matched layer (PML)
boundaries were used along the direction perpendicular to the grating structure. A planar power
monitor placed behind the source was used to monitor the reflected power from the gratings, and
a two-dimensional (2D) field monitor in the plane of the grating was used to record the electric
field profile.
Figure 3.4a shows a schematic diagram illustrating our basic measurement configuration.
We achieve resonance by scanning the incident angle of monochromatic light (532 nm, 633 nm
and 785 nm). For light polarized perpendicular to the lines on the grating (i.e., p-polarized), we
observe sharp dips in the photoreflectance (Figure 3.4b) when there is wavevector matching
between the incident photons and the plasmon resonant mode. For the experimental Ag-coated
grating structure with 633 nm light, this resonance occurs at ±7.3
o
from normal incidence. For
light polarized parallel to the grating lines (i.e., s-polarization), there is no coupling to the SPP,
and consequently no angle dependence in the photoreflectance. Figure 3.4c shows the AC
photoelectrochemical current under 633 nm illumination. We observe sharp peaks in the AC
photocurrent at similar incident angles with p-polarized light and no angular dependence with s-
38

polarized light. Here, we are able to achieve a 12-fold increase in photocurrent when exciting on
the plasmon resonance, as shown in Figure 3.4c.

Figure 3.4. (a) Schematic diagram of the experimental measurement, (b) photoreflectance, and (c)
AC photoelectrochemical current measured as a function of incident angle. The red curve
corresponds to p-polarized light, and the black curve corresponds to s-polarized light.
39

FDTD simulations of these structures were performed to further define the underlying
nature of this plasmonic effect. Figure 3.5a shows the photoreflectance plotted as a function of
incident angle for the Ag gratings under 633 nm wavelength light. We observe sharp dips in the
photoreflectance at ±7.0
o
, consistent with our experimental measurements shown in Figure 3.4b.
The electric field intensity is plotted along the cross-section of the grating structure in Figure 3.5b
and Figure 3.5c. The data in Figure 3.5b corresponds to resonant excitation at +7.0
o
incidence with
p-polarization, while the data in Figure 3.5c corresponds to non-resonant excitation at +7.0
o

incidence with s-polarization. On resonance, we see a plasmon resonant mode with a peak electric
field intensity enhancement factor (|E|
2
/|E0|
2
) of 40X at the metal surface, where E is the electric
field at the water-metal interface, and E0 is the electric field of the incident light.
40

Figure 3.5. (a) Calculated angle-dependent photoreflectance of the Ag grating under 633 nm light.
Simulated electric field distributions with (b) resonant and (c) non-resonant polarizations at 7.0
o

incidence with 633nm light.

41

Figure 3.6a shows the AC photocurrent measured with 785 nm wavelength light
illumination. Again, sharp peaks are observed around ±7.1
o
incidence with p-polarization, and no
angular dependence is observed with s-polarized light. Here, a plasmonic enhancement factor of
44X is observed in the photocurrent (i.e., hydrogen evolution reaction rate). The corresponding
FDTD simulations show similar angle-dependent profiles, as plotted in Figure 3.6b. It should also
be noted that the resonant profiles are narrower at 785 nm than at 633 nm. Larger enhancement
factors and narrower resonances observed at longer wavelengths are a result of less interband
transitions and higher photoabsorption. This also explains why smaller enhancement factors are
observed with similar gratings made with Au, which has substantially more interband transitions
in the visible wavelength range than Ag. 2D plots of the electric field intensity (i.e., |E|
2
) profiles
are plotted in Figure 3.6c and Figure 3.6d. These figures show excitation of a highly symmetric
mode with p-polarization at 785 nm, as compared with the less symmetric mode of Figure 3.6b.
Here, the hottest spot at the water-metal interface has a 200X electric field intensity enhancement
factor.
42

43

Figure 3.6. (a) AC photocurrent measured as a function of incident angle and (b) simulated
absorptance spectra for 785 nm illumination. (c) electric field distributions at 7.6
o
incident angle
for (c) p-polarized and (d) s-polarized 785 nm wavelength irradiation.

Figure 3.7a shows the AC photocurrent measured as a function of the incident angle for
the same Ag grating taken with 532 nm wavelength light. Here, we observe two resonant peaks at
incident angles of ±19.4
o
and ±28.0
o
from normal incidence with p-polarized light and no angular
dependence with s-polarized light. The calculated absorptance (100% - reflectance) based on
FDTD simulations is plotted in Figure 3.7b and exhibits the same double peak behavior as that
plotted in Figure 3.7a but with resonant angles of ±18.4
o
and ±28.8
o
. The electric field profile
calculated on resonance at 18.4
o
plotted in Figure 3.8a exhibits higher electric field intensity
compared to that at 28.8
o
(Figure 3.8b).
44

Figure 3.7. (a) AC photoelectrochemical current measured as a function of incident angle and (b)
simulated absorptance spectra for Ag grating with 532nm irradiation.

45

Figure 3.8. Calculated electric field distributions at (a) 18.4
o
, (b) 24.0
o
, and (c) 28.8
o
incident angles
for Ag grating with 532nm illumination with p-polarization.

46

In order to optically excite surface plasmons on diffraction gratings with light polarized
perpendicular to the grating lines (i.e., p-polarized), the following condition has to be met
83-84
:

2 𝜋 𝑛 𝑑 𝜆 𝑠𝑖 𝑛𝜃 + 𝑚 2 𝜋 𝑃 = ⁡ − 𝑅𝑒 { 𝛽 𝑆𝑃
} ,
(3.2)

where 𝑛 𝑑 is the dielectric constant of the surrounding water environment, 𝜆 the wavelength of the
incident light in vacuum space, 𝑃 the period of grating, m an integer (±1, ±2, …) representing the
order of diffraction, and 𝛽 𝑆𝑃
represents the surface plasmon wavevector.

𝛽 𝑆𝑃
= ⁡ 𝛽 𝑆𝑃
0
+ ⁡ ∆ 𝛽 = ⁡
𝜔 𝑐 ⁡
√
𝜀 𝑑 𝜀 𝐴𝑔
𝜀 𝑑 + 𝜀 𝐴𝑔
+ ⁡ ∆ 𝛽 ,
(3.3)

where 𝛽 𝑆𝑃
0
accounts for the surface plasmon wavevector along the smooth interface between
dielectric and Ag metal, ∆ 𝛽 represents the grating perturbation, 𝜔 stands for the angular frequency
of incident light, 𝜀 𝑑 and 𝜀 𝐴𝑔
means the dielectric constant of water environment and Ag metal
respectively. For the shallow surface grating like what we used in our experiment, we can neglect
∆ 𝛽 and assume it to be zero. Then from Equation (3.2) and Equation (3.3), we can get:

2 𝜋 𝑛 𝑑 𝜆 𝑠𝑖 𝑛𝜃 + 𝑚 2 𝜋 𝑃 = ⁡ −𝑅𝑒 {
𝜔 𝑐 ⁡
√
𝜀 𝑑 𝜀 𝐴𝑔
𝜀 𝑑 + 𝜀 𝐴𝑔
} ,
(3.4)

Figure 3.9a shows the calculated resonance condition of the Ag grating for different incident light
angles (0
o
– 40
o
) and wavelengths (400 nm – 900 nm) based on Equation (3.4), Figure 3.9b shows
the simulated absorptance for various incident angles and wavelengths within the same range, and
all the calculated resonant angles have been flipped to the positive range. As Figure 3.9a shows,
when the incident light wavelength is from 575 nm – 900 nm and incident light angle is between
47

0 and 40 degrees, only 1
st
diffraction order coupled SPP (m = 1) can be excited, and absorptance
in Figure 7b at resonant angle increased from about 60% to nearly 100% with increasing incident
light wavelength. Between 400 nm – 575 nm, besides 1
st
diffraction order coupled SPP, there is
another 2
nd
diffraction order coupling (m = 2). The absorptance for 2
nd
diffraction order coupling
is about 10% – 20% lower than that of 1
st
diffraction order for the same incident light wavelength.
Here, both calculated and simulated results agree very well with our experimental data where we
observed a single peak in the absorption for 785 nm incident light having highest absorptance at
the resonant angle, which also shows highest photocurrent enhancement factor (44X). Besides,
633 nm light incidence also shows a single resonant peak but absorbing a lower percentage of light,
thus having a lower photocurrent enhancement factor (12X). At 523 nm, two resonant peaks are
seen with lower angle peak having higher enhancement factor than that of the higher angle peak.
48

Figure 3.9. (a) Calculated resonant condition for different incident angle (0
o
-40
o
) and incident light
wavelength (400 nm-900 nm). (b) Simulated absorptance spectrum of Ag grating with respect to
incident light wavelength and incident angle.

49

3.4 Conclusion
We have presented a spectroscopic approach for studying hot electron-driven
photocatalysis using a sensitive AC lock-in technique with plasmon resonant grating structures.
The incident angle and polarization dependence of these grating structures provide a unique
platform for comparing plasmon resonant excitation and bulk interband absorption of metals in
photocatalytic processes. As the angle of incident light is tuned through the plasmon resonance
with p-polarized light, sharp dips are observed in the photoreflectance and sharp peaks are
observed in the photocurrent (i.e., reaction rate). We observe plasmon resonant enhancement
factors of 10X, 12X, and 44X at incident wavelengths of 532 nm, 633 nm, and 785 nm respectively.
The lower enhancement factors observed at shorter wavelengths are attributed to more interband
transitions and less photoabsorption. FDTD simulations confirm the angular dependence of the
resonant profiles observed experimentally and provide a detailed picture of the electric field
profiles of these grating structures for both on and off resonance.

50

Acknowledgements
This research was supported by Army Research Office (ARO) Award No. W911NF-17-1-0325
(Y.W.), National Science Foundation (NSF) Award No. CBET-1512505 (L.S.), Air Force Office
of Scientific Research Grant No. FA9550-15-1-0184 (I.A), Department of Energy (DOE) Award
No. DE-FG02-07ER46376 (Z.C), and ACS-PRF Grant #55993-ND5 (J.C.).

51

Chapter 4: In Situ Investigation of Ultrafast Dynamics of Hot
Electron-Driven Photocatalysis in Plasmon-Resonant Grating
Structures
This chapter is similar to Wang et al., published in Journal of the American Chemical Society.
14

4.1 Abstract
Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them
in photocatalytic applications. While most studies have focused on hot carrier dynamics at
metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from
metal to adsorbates. Here, we report hot electron-driven hydrogen evolution reaction (HER) by
exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ
ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot
electrons. When the sample is immersed in solution under -1 V applied potential, the extracted
electron-phonon interaction time decreases from 0.94 ps to 0.67 ps due to additional energy
dissipation channels. The LSPR TA signal is red-shifted with delay time due to charge transfer
and subsequent change of dielectric constant of nearby solution. Plateau-like photocurrent peaks
appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light,
accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent
measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor
(i.e., reaction rate) is 15.6X between p-polarized and s-polarized light for the 300 nm linewidth
grating and 4.4X for the 266 nm linewidth grating. Finite difference time domain (FDTD)
simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR
modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work
52

in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier
dynamics are studied on the same plasmon resonance structure with and without adsorbates.
53

4.2 Introduction
Since the mid 2000’s, many researchers have tried to improve the photocatalytic
performance of metal surfaces and metal/semiconductor interfaces by exploiting the localized
surface plasmon resonance (LSPR),
85-91
in which the coherent oscillation of free electrons is
coupled strongly to electromagnetic fields from incident light.
65-66, 83, 92-96
Early studies focused on
photothermal effects.
15, 59
Later, LSPR-induced local electric field enhancement was established
as an important mechanism in enhancing many photochemical and photoelectrochemical
processes.
16, 97
Under LSPR, the absorption cross-section of a metallic nanoparticle (NP) can be 3-
5 orders of magnitude higher compared to that of a typical dye-sensitized molecule.
98
Using this
approach, Liu et al. reported a 66-fold enhancement in the reaction rate by adding Au nanoparticles
(NPs) on a catalytic TiO2 substrate under 633 nm light irradiation. Electromagnetic simulations
showed that the highly improved photocatalytic activity is caused by electric field concentration
at AuNP/TiO2 interfaces.
97
Christopher et al. also showed that by introducing optically active Ag
nanostructures onto TiO2 photocatalysts in the methylene blue decomposition reaction, the
conversion efficiency of incident photons into e
-
/h
+
pairs in TiO2 was highly increased. They also
found that the size and shape of Ag nanostructures have strong effects on the photocatalytic
performance of TiO2.
99

More recently, hot electron-driven photocatalysis and electrophotocatalysis have drawn
considerable interest.
13, 78, 100-106
Here, non-thermalized hot carriers are induced through
nonradiative dephasing (i.e., damping) of plasmon resonance and used to drive high activation
energy barrier chemical reactions, such as hydrogen dissociation.
71, 79-80, 92, 100, 107-110
These
plasmon-induced hot electrons can be excited near metal surfaces and subsequently transferred to
the lowest unoccupied molecular orbitals (LUMOs) of adsorbates.
92, 100
Alternatively, this charge
54

transfer can occur through energy state hybridization between metal surfaces and adsorbates,
leaving hot holes at electron-donating states on the metal surfaces.
100, 111-112
The first demonstration
of a chemical reaction driven by hot electrons was the charge transfer resonance study of p-
aminothiophenol (p-ATP) adsorbed on Ag nanoparticles through the surface-enhanced Raman
scattering (SERS) phenomenon in 1994.
113
After that, various photochemical reactions driven by
LSPR-generated hot electrons have been demonstrated, such as water splitting,
79-80
CO2
reduction,
114
and chemical decomposition.
108-109, 115
Mukherjee et al. showcased hot electron-
induced H2 dissociation on Au nanoparticles with visible light irradiation on SiO2 and TiO2
substrates. In those works, hot electrons transfer into H2 antibonding orbitals followed by returning
to their ground states with elongated bond lengths, ultimately resulting in complete dissociation.
108-
109
Recently, Sytwu et al. discovered new nucleation sites for phase transformation of palladium
hydride, which are activated by LSPR-induced hot spots.
116

The timescales of LSPR-induced hot carrier generation and relaxation have been
theoretically calculated and experimentally studied.
74-77, 79-80, 92, 117-118
Upon LSPR excitation,
generation of non-thermal hot electrons and holes near metal surfaces occurs from non-radiative
LSPR dephasing (within 20 fs) as shown in Figure 4.1a, which is followed by internal
thermalization through electron-electron scattering (Figure 4.1b). Electron-electron interaction
time varies from tens of fs to several hundred fs, depending on individual nanostructure as well as
optical excitation wavelength and intensity.
119-120
During and after the thermalization, electrons
dissipate energy to the nearby lattice through electron-phonon scattering within 1 ps to 10 ps.
80, 92

Finally, heat is further distributed to the environment through phonon-phonon scattering (100 ps
to 10 ns). However, in the presence of adsorbates, chemical interface damping (direct electron
transfer) and chemical interface scattering (indirect electron transfer) provide two additional
55

channels for hot carrier energy dissipation, which are both expected to decrease hot carrier
relaxation time.
100

Figure 4.1. (a) LSPR-induced hot electrons and holes are initially extended over a non-thermal
distribution and (b) quickly thermalized to a hot Fermi-Dirac distribution (red profile) through
electron-electron scattering in 50 fs and subsequently to a room-temperature Fermi-Dirac
distribution (blue profile) through electron-phonon scattering within 1 ps to 10 ps.

In the work presented here, in situ TA measurements are performed under both dry
condition and in solution to compare the hot carrier excitation and relaxation processes with and
56

without adsorbates on the same grating nanostructure photoelectrode. While there are previous
studies focusing on ultrafast hot carrier dynamics on plasmonic nanostructures, most of these either
focus on colloidal nanoparticles or metal/semiconductor interfaces instead of electrochemically-
tunable purely metallic surfaces.
121-128
Within the plasmonics and photocatalysis research
communities, it is important to quantify the effect of adsorbates on the electron-phonon interaction
times during plasmon-induced hot carrier excitation while driving reactions. However, because
hot carrier dynamics depend on the metal composition, nanostructure morphology, irradiation
wavelength and intensity, the extracted electron-phonon interaction times vary widely from group
to group, making it difficult to compare relaxation time constants between different works.
129-131

As such, a direct comparative study on the same plasmonic nanostructure with and without
adsorbates is still lacking. To our knowledge, our results are the first to quantitatively examine the
effect of adsorbates on hot carrier dynamics in metallic nanostructured photoelectrodes. Another
focus in this work is on different spectral shifts observed in transient absorption (TA)
measurements, which were not studied in our previous research.
80
Blue shifting with delay time in
air results from the cooling of hot electrons, and red shifting with delay time in solution is attributed
to charge transfer during the reaction and subsequent change of dielectric constant at the nearby
environment. For many researchers in the community, this red shifting trend can be used as a quick
assessment of whether there is charge transfer. Previously, we drove HER through hot electrons
induced by surface plasmon polaritons (SPPs) on continuous film gratings and studied the effect
of different incident wavelengths on reaction rates.
13
Here, we present different reaction rates of
HER utilizing hot carriers originating from localized surface plasmon resonance (LSPR) instead
of SPPs. In addition, we focus on different grating linewidths instead of irradiation wavelengths.
Finite difference time domain (FDTD) simulations are used to study the local electric field
57

intensity distribution (i.e., |E|
2
/|E0|
2
) near the surface of the metal lines on and off resonance. To
our knowledge, this is the first work in which in situ photoexcited carrier dynamics and LSPR-
induced hot electron driven photochemistry are studied on the same plasmon resonance structure
with and without adsorbates.
4.3 Experimental Methods
Figure 4.2 shows a scanning electron microscope (SEM) image and a schematic diagram
of the grating structure, as well as the photoelectrochemical measurement setup. Here, a periodic
array of 50 nm thick Au metal lines with different widths are patterned by photolithography
(ASML Holding) and reactive ion etching with a 500 nm pitch on top of BK-7 fused silica
substrates. Copper wires are attached by silver paint (SPI Supplies Inc.) to establish electrical
contact for photocurrent measurements using 3-terminal potentiostat (Gamry Inc.). The as-
fabricated grating is connected to the working electrode with Ag/AgCl (3 M NaCl) as the reference
electrode and a Pt wire (BASI Inc.) as the counter electrode. Applied potentials of -0.3 V vs.
normal hydrogen electrode (NHE) in a 0.5 M Na2SO4 (Anhydrous ACS, VWR) solution are used
to assist in the hydrogen evolution reaction (HER). In order to detect the relatively small
photocurrents produced by the non-thermalized, short-lived hot electrons, the incident light (633
nm) is modulated by an optical chopper (Stanford Research System SR-540) at a frequency of
200 Hz and lock-in amplifier (Stanford Research System SR-830) is used to measure the signal
from potentiostat (current monitor port) at the specific frequency of the chopper as shown in Figure
4.2d.
16, 71, 82, 132
The grating sample is mount on a motorized rotational stage (Thorlabs, Inc.) in
order to sweep the incident angle during photoelectrochemical current measurements, and a half
waveplate is used to adjust the polarization direction of the incident light in order to tune on and
off of resonance while keeping all the other parameters of the measurement constant.
58

To probe the ultrafast dynamics of hot electrons during photoelectrochemical hydrogen
evolution, in situ TA measurements are conducted. Here, a 680 nm pump and a broadband 400 –
670 nm probe femtosecond lasers are used in the experiment at normal incident angle, and both
pump and probe pulses are generated by a 1 kHz Ti:sapphire amplifier (Legend Elite HE+,
Coherent). Pump pulses are generated using a doubling stage of the OPA (OPerA Solo, Coherent)
output and modulated at 250 Hz. The broadband white light probe is generated by focusing the
Ti:sapphire onto a CaF2 crystal and modulated by an optical chopper at 500 Hz. A balanced
detection scheme is employed to eliminate noise due to fluctuations in the probe spectrum. The
probe signal is detected using a CCD camera (Princeton Instruments Pixis) passing through a
spectrometer (Horiba iHR320) with a 320 mm focal length and a 150 g/mm grating. The focal spot
diameters for the pump and probe are 140 and 180 μm, respectively. The TA signals are taken as
the difference between the absorption of probe pulse before and after the pump with certain time
delay, which is shown in Figure 4.2b.
59

Figure 4.2. (a) Scanning electron microscope (SEM) image of Au grating with 500 nm period and
266 nm metal linewidth. Schematic diagrams of (b) transient absorption (TA) measurement and
(c) the hot electron driven photocatalytic water splitting process and (c) angle-dependent
photocurrent measurement setup.

The angle-dependent absorptance and electric field intensity profiles are calculated using
the finite difference time domain (FDTD) method (Lumerical Inc.).
71, 133
50 nm thick Au metal
lines with a period of 500 nm and different linewidths of 266 nm and 300 nm deposited on top of
fused silica substrates are created with a surrounding environment refractive index of 1.33 (for the
aqueous environment). A mesh size of 0.5 nm is used, and a plane wave source is placed above
the grating with various incident angles from 0
o
to 20
o
and different polarization directions. Bloch
60

boundary conditions are applied at the sides of the grating, and the direction perpendicular to the
grating surface is defined by perfectly matched layer boundary conditions. Power monitors above
and below the grating surface are used to capture the reflected and transmitted light for calculating
the absorptance. Another power monitor is placed normal to the grating substrate to simulate the
field intensity distribution near the surface of metal lines. Spurious effects of total internally
reflected light from the bottom surface of the glass substrate are excluded.
4.4 Results and Discussion
In our pump-probe femtosecond pulsed laser system, we selectively pump electrons in the
metal, then broadband probe their relaxation back to equilibrium in both wavelength and time.
80

The measured signal is the change in absorbance of the probe pulse before and after pump
excitation (i.e., ∆ 𝐴 = 𝐴 𝑎𝑓𝑡 𝑒𝑟 − 𝐴 𝑏𝑒𝑓𝑜𝑟𝑒 ). Figure 4.3a shows two-dimensional map of time
evolution of in situ transient absorption (TA) measurement of a 266 nm linewidth Au grating with
a 500 nm period. The sample is emersed in a 0.5 M Na2SO4 solution under an applied voltage of -
1 V in a two-terminal home-built electrochemical cell with transparent CaF2 side windows. Figure
4.3b shows TA mapping of the same sample in dry conditions without any water adsorbate. Figure
4.3c and Figure 4.3d present horizontal cuts of the measured TA mapping at various time delays
for samples in solution and in air, respectively. Pump pulses with 680 nm wavelength excite
electrons with energies of up to 1.82 eV below the Fermi level in the Au 6s band to states above
the Fermi level with maximum energy equal to 1.82 eV, leaving hot holes generated. The 680 nm
wavelength is selected to be lower than the Au interband transitions (around 2.4 eV) so that the
pulse does not disturb the electron energy distribution in the lower Au d band. On the other hand,
the 400 – 670 nm broadband probe pulse covers both interband transitions and LSPR wavelengths.
The positive absorption peak in Figure 4.3a around 500 nm is caused by excess interband
61

transitions excited by the probe pulse from the Au 5d band to the 6s band associated with pulse-
induced hot holes. In TA measurements on the same sample in air (shown in Figure 4.3b), the
positive modulated signal at 500 nm does not shift with changing environment, which further
confirms its origin from interband transitions.
62

Figure 4.3. Transient absorption (TA) measurements for Au grating with 266 nm metal line width
and 500 nm period (a) in solution and (b) in air environment with p-polarized probe pulse. Selected
broadband TA spectra of different delay time between pump and probe for (c) in solution and (d)
in air condition, respectively. (e) Time evolution of normalized ∆ 𝐴 at 631 nm and 549 nm for Au
grating in solution and in air, respectively. The solid lines are fitted curves based on a
63

monoexponential decay convoluted with 100 fs FWHM Gaussian pump pulse. (f) Relative shift of
the plasmon resonant bleaching peaks in TA spectra as delay time increases.

In contrast, the depletion peak (i.e., less absorption) around 630 nm in Figure 4.3a is caused
by LSPR-induced hot electrons from the p-polarized probe pulse. The electric field direction in the
probe pulse is perpendicular to the grating lines so that the transverse dipolar resonance mode is
excited. However, the LSPR signal for the grating in air (Figure 4.3b) shifts from 630 nm to 550
nm due to the different dielectric constant of the environment, which changes from 1.33 (aqueous)
to 1 (air). When the probe pulse is s-polarized, there is no depletion peak because no LSPR-
generated hot electrons are involved, as shown in Figure 4.4. After pump laser irradiation, the
heating of electrons will cause the probe-generated LSPR absorbance signal to be broadened and
decrease in intensity, thus, creating a dip around 630 nm.
80, 112
This broadening is related to the
changes in real ( 𝜖 1
) and imaginary ( 𝜖 2
) parts of the metal dielectric function resulting from the
pump disturbance. The pump excitation changes the electron distribution based on Fermi-Dirac
statistics at higher electronic temperatures, which results in the changes in the dielectric
function.
134
This broadening also corresponds to a faster plasmon oscillation dephasing time. At
higher temperatures, more electrons occupy higher energy states, which leads to higher electron
scattering rate and increased damping of the plasmon oscillation.
135
The measured UV-Vis
transmittance spectrum of one of our 266 nm linewidth Au gratings is shown in Figure 4.5. Here,
the dip observed near 560 nm corresponds to the localized surface plasmon resonance (LSPR),
which appears as a depletion peak in transient absorption (TA) measurements shown in Figure
4.3b and Figure 4.3d. In addition, the decrease in transmittance for wavelengths below 500 nm
observed in Figure 4.5 is due to interband transitions, which is detected as a positive modulation
in the TA measurements (Figure 4.3b and Figure 4.3d).
64

To quantify and compare the electron-phonon interaction time constants 𝜏 𝑒𝑝
in air and in
solution, we use a simple two-temperature model (TTM), in which electrons with temperature 𝑇 𝑒
exchange energy with the lattice at temperature 𝑇 𝑙 .
120, 136-138
After pump excitation, a broad non-
Fermi distribution with electron energies up to the pump photon energy above the Fermi level is
created. Energy is redistributed within electrons through electron-electron interactions, leading to
a hot Fermi distribution with a definite temperature 𝑇 𝑒 . Since the electron-electron interaction time
constant is faster than the pump pulse used in our system, we only consider the process after this
initial thermalization of electrons is completed. As soon as the electron temperature is well
established and assuming electron-electron and phonon-phonon interactions are quick enough to
maintain local temperature of each subsystem, the temperature time evolution can be modeled
through the following two coupled differential equations:

𝐶 𝑒 ( 𝑇 𝑒 )
𝜕 𝑇 𝑒 𝜕𝑡
= ∇ ( 𝜅 ∇ 𝑇 𝑒 ) − 𝐺 ( 𝑇 𝑒 − 𝑇 𝑙 ) + 𝐻 ( 𝑧 , 𝑡 )
(4.1)

𝐶 𝑙 𝜕 𝑇 𝑙 𝜕𝑡
= 𝐺 ( 𝑇 𝑒 − 𝑇 𝑙 )
(4.2)

Where 𝐶 𝑒 and 𝐶 𝑙 are heat capacities of electrons and lattices, respectively, 𝜅 stands for the
electronic thermal conductivity describing the heat transferred away from the laser spot, 𝐺 is the
electron-phonon coupling constant, and 𝐻 ( 𝑧 , 𝑡 ) defines the spatial and temporal evolution of
exciting pump energy source. Since the thickness of grating lines studied in this work is only 50
nm thick and 266 nm wide, the initial excitation is homogeneous over the thickness of the sample
and heat diffusion effects are neglected. 𝐶 𝑒 usually depends on electronic temperature but for small
changes in temperature, it can be treated as a constant. The rise in temperature of electrons changes
their occupation states near the Fermi level, leading to absorption modification near the plasmon
65

bleach band and consequently affects the metal dielectric function. When ∆ 𝑇 𝑒 is small, it leads to
linear change in 𝜖 1
and 𝜖 2
. The optical properties such as changes in absorbance ( ∆ 𝐴 ) in Figure
4.3a and Figure 4.3b can be treated as linear combinations of ∆ 𝜖 1
and ∆ 𝜖 2
, which are used to
monitor the temperature dynamics and are predicted to decay exponentially based on Equations
(4.1) and (4.2). Under the assumption of instantaneous thermalization of the electron gas and
neglecting heat diffusion as we mentioned previously, the impulse response function in our system
has the following form:

𝑆 ( 𝑡 ) = 𝑢 ( 𝑡 ) ( 𝑒 − 𝑡 / 𝜏 𝑒𝑝
+ 𝛼 )
(4.3)

Where 𝑢 ( 𝑡 ) is a Heaviside step function, and 𝛼 accounts for finite increase of local electron-
phonon temperature compared to the initial temperature. 𝛼 is expected to decay exponentially with
time constant on the order of 100 ps mainly through phonon-phonon interactions.
120
In our system,
only signals in the first several ps are collected so it can be assumed to be a constant.
In order to extract 𝜏 𝑒𝑝
from our measurements, we did the vertical cut of the TA mapping
in Figure 4.3a and Figure 4.3b, and the cutting wavelengths are selected for the plasmon bleaching
bands to show the highest sensitivity. Figure 3e shows the time evolutions of normalized ∆ 𝐴 at
631 nm and 549 nm for the 266 nm Au gratings in solution under -1 V potential and in air,
respectively. The collected signals are fitted with a monoexponential decay convoluted with a
Gaussian pulse with full width at half maximum (FWHM) of 100 fs, which corresponds to the
finite time duration of the pump pulse. The fitted 𝜏 𝑒𝑝
for gratings in air is 0.94 ps, which agrees
well with previous reports.
120, 129-131, 134
Groeneveld et al. showed from 0.25 ps to 1 ps electron-
phonon relaxation time in Au and Ag thin films when inducing surface plasmon polariton
66

resonance.
129
Schoenlein et al. reported 2-3 ps relaxation time in Au metal through transient
reflectivity measurements, which is on the same order of magnitude as we report here.
131
However,
𝜏 𝑒𝑝
decreases from 0.94 ps to 0.67 ps when the sample is emersed in 0.5 M Na2SO4 solution under
-1 V applied potential. This decrease in 𝜏 𝑒𝑝
clearly indicates extra relaxation channels for
dissipation of electron energy other than electron-phonon interactions. There are two extra
relaxation channels. In one channel, after LSPR excitation, the dephasing of the LSPR directly
populates electrons from the metal surface to water adsorbates (i.e., chemical interface damping),
so the temperature of hot electrons after internal thermalization is less than that of sample in air.
This leads to a smaller temperature difference between electron and phonon subsystems and faster
𝜏 𝑒𝑝
, as observed in Figure 4.3e. Wu et al. first observed chemical interface damping in cadmium
selenide nanorods with gold tips in 2015.
128
In their study, the quantum efficiency of this process
is larger than 24%, independent of excitation photon energy over a 1 eV range. In the other channel,
after LSPR dephasing and a hot Fermi-Dirac electron distribution is established, the electron
subsystem not only transfers energy to the phonon subsystem, but also dissipates energy into water
adsorbates through hot electron injection (i.e., chemical interface scattering) and drives HER under
applied potential, resulting in faster 𝜏 𝑒𝑝
.
100

Another feature of this LSPR related depletion peak is that it is red shifted by 8 nm as the
time delay between pump and pulse is increased from 0.4 ps to 3 ps (Figure 4.3c and Figure 4.3f).
As hot electrons generated by the pump are transferred from the metal surface to adsorbed water
molecules’ LUMO states, the effective dielectric constant of the water environment is decreased.
The decrease in effective environmental dielectric constant increases the restoring force of free
electrons on metal surfaces, leading to the observed increase of LSPR resonance frequency.
139-141

This is why we observe shorter wavelength depletion peaks when the delay time is small. As the
67

delay time increases from 0.4 to 3 ps, there is less charge transfer as hot electrons relax back to
their original ground states through electron-phonon coupling, which results in a red shift of the
depletion peak. There are two mechanisms, as we previously mentioned, which cause charge
transfer from Au electrode to water adsorbates. One mechanism is direct charge transfer (i.e.,
chemical interface damping), in which higher energy electrons are generated directly in LUMO
states of surrounding adsorbates, leaving holes in Au electrode. The other mechanism is indirect
charge transfer (i.e., chemical interface scattering). The hot electrons are first generated in the Au
photocathode then migrate into water LUMO states driven by applied external electric field. In
contrast, Figure 4.3b and Figure 4.3d show TA signals for the same grating structure in air without
any applied voltage. Here, the interband transition signal appears at the same wavelength near 500
nm since the interband transitions correspond to the intrinsic properties of Au and do not depend
on the surrounding environment or geometry of the grating. Figure 4.3f shows the relative shift
produced by the plasmon-resonant absorption plotted as a function of time. Here, we can see more
clearly the blue shift with delay time in air, which is due to cooling of the electrons. Yeshchenko
et al. studied the temperature dependence of the LSPR in gold nanoparticles and found a quasi-
linear monotonic blue shift of LSPR wavelength when sample temperature decreases from 1000
o
C to 20
o
C.
142
They attributed this trend mainly to the thermal expansion of the nanoparticles. In
our work, as the time delay increases from 0.4 to 3 ps, the electron temperature decreases due to
electron-phonon scattering, leading to shorter LSPR wavelengths. In summary, a redshift with
delay time is observed for Au gratings in solution under -1 V potential originating from charge
transfer, while a blueshift is observed for the same sample in air due to the decrease of electron
temperature.
68

Figure 4.4. (a) Transient absorption spectra of 266 nm linewidth Au grating when the probe is s-
polarized (i.e., electric field parallel to the grating lines). (b) Selected broadband TA spectra of
different delaying time between pump and probe.
69

Figure 4.5. Measured UV-Vis spectrum of 266 nm linewidth Au grating in air with unpolarized
light under normal incidence.
Figure 4.6a shows the AC photoelectrochemical current measurement of the Au grating
with a 266 nm linewidth and 500 nm period under 633 nm light irradiation. For p-polarized light
(electric field perpendicular to the grating lines), LSPR is induced when the incident light angle is
between 3.6
o
to 9
o
with respect to the grating surface normal. Hot electrons generated from LSPR
dephasing are transferred to the LUMO states of water molecules adsorbed on the metal surfaces
through chemical interface damping (i.e., direct electron transfer) or chemical interface scattering
(i.e., indirect electron transfer),
100
thus, driving the hydrogen evolution reaction (2H2O + 2e
-
→
H2 ⁡+ 2OH
-
), as indicated in Figure 4.2c. Here, no plasmon-resonant hot electrons are generated
when the incident light is s-polarized (electric field parallel to the grating lines). The photocurrent
70

generated from s-polarized light is mainly caused by interband transitions and is 4.4X smaller than
the photocurrent observed when irradiated with p-polarized light on resonance. That is, the
enhancement factor in the reaction rates between p and s polarization is 4.4X. The experimentally
measured absorptance in Figure 4.6b also exhibits a similar angle-dependent profile for p-polarized
light, as seen in the photocurrent measurement. The FDTD simulated absorptance spectrum in
Figure 4.6b exhibits resonant features spanning a range of angles from 4.0
o
to 9.3
o
, similar to these
observed experimentally. However, the dip around 8.3
o
is too sharp to be measured experimentally
due to inhomogeneity of the device morphology, leading to a plateau-like profile in the measured
photocurrent and absorptance. The simulated electric field intensity distributions (i.e., |E|
2
/|E0|
2
) at
4.0
o
, 8.3
o
, and 9.3
o
are shown in Figure 4.6c, Figure 4.6d, and Figure 4.6e, respectively, and two
different dipolar resonance modes are observed. One mode corresponds to the field confinement
on metal surfaces near water environment (Figure 4.6c) and the other results from a resonant mode
with field confinement adjacent to the fused silica substrate (Figure 4.6e). Sherry et al. reported
similar phenomenon for silver nanocubes on fused silica substrate.
140
In their work, when the
nanocube is far away from the substrate, the scattering spectra only has one peak associated with
dipolar LSPR mode, and as the nanocube is approaching the substrate, this single peak begins to
split into two peaks. This is because the underlying fused silica substrate breaks the symmetry of
the Au metal lines structure and causes an anisotropic dipole oscillation.
141

71

Figure 4.6. (a) Measured AC photoelectrochemical current, (b) measured and calculated
absorptance plotted as a function of incident angle for the 266 nm linewidth grating. Electric field
intensity distributions near the grating surface at incident of (c) 4.0
o
, (d) 8.3
o
, and (e) 9.3
o
for p-
polarized 633 nm irradiation.

Figure 4.7a shows the photocurrent measurement as well as simulated absorptance with
respect to incident angle of the Au grating with a 300 nm linewidth and 500 nm period. The
simulated absorptance spectrum in Figure 5a shows double peaks feature at 7.5
o
and 10.5
o
with p-
polarized light corresponding to different dipolar LSPR modes. When the incident light is s-
72

polarized, neither photocurrent nor absorptance shows angle dependence. The electric field
intensity profiles at 7.5
o
, 8.5
o
, and 10.5
o
for p-polarized light are shown in Figure 4.7b, Figure 4.7c
and Figure 4.7d, respectively. The dip in simulated absorptance shown in Figure 4.7a is not as
sharp as that for 266 nm linewidth grating, and the measured photocurrent in Figure 4.7a shows
the double-peak profile. A higher enhancement factor (15.6X) of reaction rates is observed when
the incident light angle is at 6.7
o
, which corresponds to the upper surface dipolar LSPR mode and
higher absorptance. When light is irradiated at 8.4
o
, the enhancement factor is somewhat lower
(9.5X) because of lower absorptance and there are fewer adsorbed water molecules in regions
where the hot electrons are generated.

73

Figure 4.7. (a) Measured AC photocurrent and calculated absorptance plotted as a function of
incident angle for the 300 nm linewidth grating. Electric field intensity profile near the grating
surface at (b) 7.5
o
, (c) 8.5
o
, and (d) 10.5
o
for p-polarized 633 nm light.

4.5 Conclusion
In conclusion, in situ TA measurements probe the dynamics of these hot carriers with both
in solution and in air. The extracted 𝜏 𝑒𝑝
decreases from 0.94 ps to 0.67 ps when the Au grating is
emersed in 0.5 M Na2SO4 solution under an applied potential of -1 V, which is mainly due to extra
energy dissipation channels between the metal surface and water adsorbates. Localized surface
plasmon resonance (LSPR) induced depletion peak observed in the TA spectra is red shifted with
time in solution caused by charge transfer and blue shifted under air resulting from hot electron
74

cooling. Besides, s and p-polarized angle dependent photocurrent measurements on 500 nm period
Au gratings with different linewidths are conducted to differentiate current driven by hot electrons
from interband transitions. A double-peak profile with enhancement factor of 15.6X, as well as
plateau like photocurrent profiles with enhancement factor of 4.4X are observed for 300 nm and
266 nm linewidth gratings, respectively. FDTD simulations show two peaks in the absorptance
spectra for both gratings, corresponding to different dipolar LSPR modes near and far away from
the grating/substrate interfaces.

75

Acknowledgements
This research was supported by Army Research Office (ARO) Award No. W911NF-17-1-0325
(Y.W.), National Science Foundation (NSF) Award No. CHE-1708581 (L.S.), Air Force Office of
Scientific Research Grant No. FA9550-19-1-0115 (I.A), Department of Energy (DOE) Award No.
DE-SC0019322 (Z.C), and ACS-PRF Grant #55993-ND5 (J.C.). A portion of this work was
carried out at the University of California Santa Barbara (UCSB) nanofabrication facility.

76

Chapter 5: Outlook and future work
5.1 Thermoelectric and Transport Study of (PbSe)
1+ δ
(VSe
2
)
1,2
Heterostructures
Besides the in-plane transport study of (PbSe)1+δ(VSe2)1,2 heterostructures, we are also
interested in the cross-plane thermoelectric properties of those materials. In the cross-plane
direction, there is no long-range order since each layer has a random rotation with respect to its
adjacent layers. This lack of long-range order may further disturb the phono propagation and
decrease the cross-plane thermal conductivity, which is beneficial to the thermoelectric
performance. Figure 5.1 shows the designed and fabricated device for cross-plane thermoelectric
measurements. Here, bottom Au resistor temperature detector (RTD) is first deposited onto the
Si/SiO2 substrate, which is followed by window opening and material deposition. After that, a top
Au RTD is fabricated to sandwich the material under study in between as shown in Figure 5.1a.
These two RTDs are used to calibrate the temperature difference between the top and bottom of
the material, and they also serve as the metallic contacts to measure the Seebeck voltage. Then, 50
nm Al2O3 film is deposited to insulate the whole structure underneath, and a serpentine heater is
synthesized on top to create a temperature gradient in the cross-plane direction. Previously, we
were able create more than 3 K temperature difference across 50 nm thick SnSe and SnSe2 thin
films.
49
In the (PbSe)1+δ(VSe2)1 heterostructures where VSe2 layer is in the monolayer limit, we
observe negative Seebeck coefficient, indicating mainly n-type charge carriers. However, when
the VSe2 material increases to double layers in (PbSe)1+δ(VSe2)2 heterostructure, the Seebeck
coefficient changes into positive sign, showing the charge carrier type flipping. Since VSe2
material is metallic, while PbSe material is a small band gap semiconductor, the transport
properties in the cross-plane direction are expected to be largely determined by PbSe component.
This change of charge carrier type is mainly caused by charge transfer from VSe2 to PbSe layers.
77

When the VSe2 component is in double-layer configuration, more charge carriers around the Fermi
surface of the VSe2 material are available to diffuse into the PbSe material and eventually flip the
main carrier type in the PbSe layers, which is manifested by the change of sign in measured
Seebeck coefficient.

Figure 5.1 Designed and synthesized cross-plane (PbSe)1+δ(VSe2)1,2 thermoelectric devices (a)
before and (b) after Al2O3 capsulation and top heater fabrication.

Another direction currently under pursuit is to study the anisotropic resistivity in
(PbSe)1+δ(VSe2)1 heterostructure. While in-plane resistivity is relatively straightforward to
measure using standard 4-point technique as long as the contact metal is Ohmic, measuring the
electrical resistivity in the cross-plane direction is a non-trivial challenge because of the small film
thickness (~ nm) and large contact resistance. Most of the cross-plane measurements in previous
78

studies involve an extra reference sample with thinner thickness, and a 2-point differential
measurement method is used to eliminate the contact resistance and extract the resistivity
property.
49, 143-144
However, this method requires the variation of contact resistance to be much
smaller than the sample resistance, and many thin heterostructure materials do not meet this
standard. So, to be able to characterize the resistivity in both directions on the same sample is
highly desirable. As shown below, we developed a technique which can be used to measure both
in-plane and cross-plane electrical resistivities simultaneously without ever needing a reference
sample.
Figure 5.2 shows optical images of device used in measuring anisotropic resistivities. Here,
four 5/50 nm Ti/Au electrodes serving as bottom contacts are first patterned via electron beam
lithography (Raith EBPG 5200) and deposited through electron beam evaporator (Temescal
Systems) on the Si substrate (Graphene Supermarket) with 285 nm SiO2 layer. Then a lithograph
defined 40 µm × 300 µm window is developed for (PbSe)1+δ(VSe2)1 material deposition with
nominal thickness of 50 nm followed by lift-off in acetone. Finally, another four electrical contacts
are fabricated on top of the material. All the contacts are labeled in Figure 5.2b. The in-plane
resistances of the material between the inner contacts are first measured through standard 4-point
technique through top and bottom 4 contacts. We use 𝑅 1
and 𝑅 2
to indicate the material’s in-plane
resistances measured using top and bottom 4 electrodes, respectively. The in-plane resistance of
the material 𝑅 𝑖𝑛
is estimated to be the average value of 𝑅 1
+ 𝑅 2
. Then, 2-point technique through
inner contacts is used to measure the resistances 𝑅 3
and 𝑅 4
, where 𝑅 3
and 𝑅 4
are resistances
measured using top and bottom inner leads, respectively. In this way, the exact values of contact
resistances of inner leads can be extracted. After that, resistance 𝑅 5
is accessed using 2-point
method through top-left and bottom-left inner electrodes. The same thing is done for top-right and
79

bottom-right inner electrodes to get 𝑅 6
. Finally, the cross-plane resistance of the material 𝑅 𝑐𝑟
is
extracted through the Equation 1:
𝑅 𝑐𝑟
= ⁡ ( 𝑅 5
+ 𝑅 6
− 𝑅 3
− 𝑅 4
+ 𝑅 1
+ 𝑅 2
) / 2 (1)

In this way, only one device is needed for anisotropic electrical transport measurement without
ever needing reference samples. Because the actual contact resistances are accurately evaluated in
the device, the extracted cross-plane resistivity is expected to be much more reliable compared to
traditional 2-point differential methods.

Figure 5.2 Optical microscope images of (a) zoom out and (b) zoom in views of fabricated device
for anisotropic electrical transport measurement.

Figure 5.3a shows the measured in-plane and cross-plane resistivities of (PbSe)1+δ(VSe2)1
with respect to temperature ranging from 80 – 300 K. Here, the resistivity in the cross-plane
direction shows the value of around 100000 μ *m through the entire temperature range, while in-
plane resistivity is around 10 μ *m. The resistivity ratio of cross-plane over in-plane is shown in
80

Figure 5.3b, which demonstrates a 4 orders of magnitude difference. This high anisotropy is mainly
caused by different conducting mechanisms in in-plane and cross-plane directions. In in-plane
direction, the main contribution of conduction is from metallic VSe2 component, while in cross-
plane direction, the determining conducting medium should be PbSe, which is a small bandgap
semiconductor. Besides, the interfaces between random rotated layers further increases the
resistivity in the cross-plane direction. Another phenomenon we observed is the local minimum in
the resistivity ratio around 140 K, which we believe is due to the charge density wave (CDW)
transition.

Figure 5.3 (a) In-plane and cross-plane resistivities, (b) resistivity ratio of cross-plane over in-
plane for (PbSe)1(VSe2)1 heterostructures as the function of temperature.

5.2 Thermoelectric Study of Interlayer Excitons
An exciton consists of an electron and a hole bounded by Coulomb attraction. Previous
theoretical study showed that bilayer excitons would lead to an exceptionally high Seebeck
coefficient as well as a small Lorenz number.
145
This is due to the increase of entropy in excitons,
which results from orbital and spin degeneracies. In addition, in exciton transport, the weak
81

interaction between excitons and phonons increases the excitonic electric conductivity, which is
also beneficial to the thermoelectric performance.
Figure 5.4 shows the schematics of counterflow thermoelectric device we are currently
working on, which is driven by exciton transport. Here, WSe2 and MoSe2 layer are tuned to be p-
type and n-type through top and bottom gate, respectively. At the cold side, two layers are
connected to form the counterflow geometry. While at the hot side, holes in the WSe2 component
are bounded to electrons in MoSe2 material and diffuse to cold side as excitons. Currently, we
demonstrated the tunability of charge carrier concentration through gating in WSe2 and MoSe2
materials. We will conduct counterflow thermoelectric measurement in the future.

Figure 5.4 Schematics showing the excitonic thermoelectric device based on WSe2/h-BN/MoSe2
heterostructure.

82

5.3 Photocatalysis Based on Plasmonic Nanostructures
Previously, we demonstrated hot electron driven water splitting on plasmonic grating
nanostructures, but the solar to hydrogen conversion rate is still very low.
13
One topic of research
we are currently working on is to speed up the reaction rate. There are several approaches we are
trying. One method is to deposit co-catalysts on the surface of Ag gratings, such as Pt or Ir. Another
method is to put scavenger molecules like ethanol in the solution to decrease energy barrier to
drive the oxidation potential.
Another topic of research we are pursuing is to get more understanding on the transient hot
carrier dynamics on plasmonic gratings. We previously extracted electron phonon interaction time
𝜏 𝑒𝑝
during photoelectrochemical water splitting process and found that 𝜏 𝑒𝑝
decreased from 0.94 to
0.67 ps when compared to in air condition.
14
However, we only conducted the experiment under
the applied potential of -1 V. One future topic is to do the potential dependent transient absorption
(TA) measurements to compare 𝜏 𝑒𝑝
under different applied voltages. Another topic is to do the
pump power dependent TA probing to extract hot carrier injection percentages under different
conditions.

83

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Abstract (if available)

Abstract This dissertation presents the research I conducted during my Ph.D. study in the University of Southern California. It consists of two parts. The first part is about thermoelectricity, which converts thermal energy into electrical energy. The second part is about plasmonic water splitting, which converts photon energy into chemical energy. The overall goal of my thesis is to try to address the climate change, the biggest challenge of the 21st century.
Chapter 1 is to give an introduction of the status of climate change issue, such as the high correlation between the continuing rising of earth surface temperature and human activities. It also introduces the concept of thermoelectricity as well as photocatalysis.
Chapter 2 talks about how we can use charge density wave (CDW) phase transition to enhance the thermoelectric performance in (PbSe)1+(VSe2)1 heterostructure thin films. It also explains the fundamental mechanism of the observed enhancement.
Chapter 3 studies the hot electron-driven hydrogen evolution reaction (HER), which results from the decay of surface plasmon polaritons (SPPs). In this study, wavelength dependent reaction rate measurements are also conducted, showing better performance at longer wavelength.
Chapter 4 discusses the effect of chemical adsorbates on the relaxation dynamics of hot carriers on localized surface plasmon resonance (LSPR) grating nanostructures through transient absorption (TA) measurements. Reaction rates are also studied on those gratings with different linewidths.
Chapter 5 presents the outlook and future works based on the research in previous chapters.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Wang, Yu (author)

Core Title Thermoelectric and transport studies of low dimensional materials & hot electron-driven photocatalysis on plasmon-resonant grating nanostructures

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Degree Conferral Date 2022-05

Publication Date 05/02/2022

Defense Date 02/25/2022

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

Tag charge density wave,hydrogen evolution reaction,low dimensional materials,OAI-PMH Harvest,photocatalysis,plasmonics,thermoelectricity

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Cronin, Stephen B. (committee chair), Ravichandran, Jayakanth (committee chair), Melot, Brent C. (committee member)

Creator Email wang233@usc.edu,wangyu08.lzu@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC111159643

Unique identifier UC111159643

Document Type Dissertation

Format application/pdf (imt)

Rights Wang, Yu

Type texts

Source 20220502-usctheses-batch-935 (batch), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

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