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Hot electron driven photocatalysis and plasmonic sensing using metallic gratings
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Hot electron driven photocatalysis and plasmonic sensing using metallic gratings
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Content
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PHYSICS)
August 2024
HOT ELECTRON DRIVEN PHOTOCATALYSIS AND PLASMONIC SENSING
USING METALLIC GRATINGS
by
Indu Aravind
ii
ACKNOWLEDGMENTS
First and foremost, I would like to thank my advisor Prof. Stephen B. Cronin for his guidance
along this journey. I was fortunate to find him as my advisor. He has given me opportunities to
explore a variety of projects ranging from thermoelectricity in two-dimensional materials to
plasma physics for particle remediation in electrostatic precipitators. The experience has been
invaluable. His dedication and hard work have always inspired me to push my limits.
I would like to thank all my former and current lab mates for their help, support, and most
importantly, friendship. Nirakar Poudel was my mentor when I joined the lab. He introduced me
to nanofabrication and optical and electrical measurements. I will always be grateful to Nirakar
for his mentorship. Along this journey, I had the opportunity to work with many amazing people
in Dr. Cronin’s lab: Yu Wang, Haotian Shi, Bofan Zhao, Sisi Yang, Bo Wang, Zhi Cai, Ruoxi Li,
Boxin Zhang, Sizhe Weng, and the list goes on. They have always been there for me whenever I
needed a hand, and for that, I will be eternally grateful. I am also thankful to my juniors Caleb
Medchill, Rifat Shahriar, Yu Yun Wang, Mariano Rubio, Mehedi Himel, Ehsan Shamsi and Sina
Ilkhani for all their help. I wish all the very best for their research and am confident that all their
hard work will lead to great success...
I owe an immeasurable debt of gratitude to my dear husband, Aravind Krishnan, whose
unwavering motivation, support, and encouragement have been the bedrock of my entire PhD
journey. I can only imagine the challenges he faced in standing by me during the toughest moments
of my academic pursuit. To have such a pillar of support by my side is a blessing beyond measure,
and I am profoundly thankful for his enduring love and unwavering commitment. I dedicate this
thesis to him.
I'd like to extend my heartfelt appreciation to my friends, Mythili, Chethan, Aishwarya, Ragesh,
Manju, Amal,Jayan, and Manasi. They've transformed my time away from home into a comforting
haven. With their presence, I've never felt disconnected from my homeland or my family.
I would like to express my deepest gratitude to my parents Asokan Pillai and Ambika Amma and
my elder brothers who were always there for me to fulfill my dreams. Your love, patience, and
sacrifices have been the foundation of my achievements. Your belief in my abilities has given me
the strength to persevere, and your constant guidance has been invaluable.
iii
TABLE OF CONTENTS
Acknowledgments........................................................................................................................... ii
list of figures................................................................................................................................... v
Abstract........................................................................................................................................... x
Chapter 1. Introduction ................................................................................................................... 1
1.1 Light Interaction with Metals................................................................................................ 1
1.2 Surface Plasmon Excitation .................................................................................................. 2
1.3 Surface Plasmon Decay and Hot Electron Generation ......................................................... 4
1.4 Plasmonic Photocatalysis...................................................................................................... 5
1.5 Surface Plasmon Sensors...................................................................................................... 7
Chapter 2. Hot Electron Driven Photocatalysis in Plasmonic Gratings........................................ 10
2.1 Introduction......................................................................................................................... 10
2.2 Surface Plasmon Excitation Using Metallic Gratings ........................................................ 10
2.3 Plasmonic Field Enhancement and Absorption .................................................................. 13
2.4 Metallic Gratings with Different Material Systems............................................................ 15
2.5 Photocatalysis in Ag Gratings............................................................................................. 17
2.6 Optimizing Field Enhancement in Grating Structures........................................................ 18
2.7 Conclusion .......................................................................................................................... 22
Chapter 3. Photo-excited Hot Electron Catalysis in Hybrid Systems........................................... 23
3.1 Introduction......................................................................................................................... 23
3.2 Electronic Properties of Plasmonic-Catalytic and Electro-Catalytic Materials.................. 24
3.3 Catalytic Hydrogen Evolution Reaction ............................................................................. 25
3.4 Hybrid Catalyst from Combining Plasmonic and Electronic Catalysts.............................. 26
3.5 Measuring Photocurrent...................................................................................................... 28
iv
3.6 Results and Discussion ....................................................................................................... 29
3.7 Conclusion .......................................................................................................................... 40
Chapter 4. Voltage-induced Modulation of Interfacial Ionic Liquids Measured Using Surface
Plasmon Resonant Grating Nanostructures .................................................................................. 41
4.1 Introduction......................................................................................................................... 41
4.2 Ionic liquid Pockels Effect.................................................................................................. 43
4.3 Results and Discussion ....................................................................................................... 44
4.3.1 Experimental Section................................................................................................... 46
4.3.2 Sensing Depth vs Sensitivity Tradeoff ........................................................................ 49
4.3.3 Numerical Model ......................................................................................................... 50
4.4 Conclusion .......................................................................................................................... 57
Chapter 5. Future Directions......................................................................................................... 58
5.1 Future Directions for Plasmonic Photocatalysis................................................................. 58
5.2 Future Directions for Electro-Optic Modulation Study of Ionic Liquids........................... 61
5.2.1 Resonant Shift of IL with Polarity of Electrode Potential ........................................... 61
5.2.2 Temporal Dynamics of EDL with Positive and Negative Biases................................ 62
5.2.3 EDL Formation with Temperature............................................................................... 63
Bibliography ................................................................................................................................. 64
v
LIST OF FIGURES
Figure 1.1. Light interaction with metals. The free electron interaction results in light energy
getting reflected or absorbed in general. Under appropriate conditions, resonant interaction
leads to bound surface waves known as surface plasmons......................................................................... 1
Figure 1.2. (a) SPPs at the interface between a metal and a dielectric (b) The evanescent field
perpendicular to the interface. δd is the decay length of the field in the dielectric, of the order
of half the wavelength of light, δm is the decay length into the metal, determined by the skin
depth. (c) The dispersion curve for a SPP mode.[1]..................................................................................... 3
Figure 1.3. (a) Excitation of a localized surface plasmon enhances light absorption. (b)
Generation and multiplication of hot carriers through Landau damping. (c) Hot carriers
redistribute their energy into the Fermi–Dirac-like distribution due to the electron-electron
interaction. (d) Heat is released to the surroundings via thermal conduction The gray area
represents the population of the electronic states; hot electrons and hot holes are represented
by the red and blue areas, respectively.[4]....................................................................................................... 5
Figure 1.4. Schematic representation of the potential energy surfaces at the ground and excited
states for a generic catalytic reaction on a metal surface.[5]...................................................................... 6
Figure 1.5. (a) Indirect hot-electron transfer mechanism. Hot electrons (e−) generated via the
nonradiative decay of an LSP are transferred to the adsorbate molecule. (b) Direct
intramolecular excitation mechanism. The LSP induces direct excitation from the occupied
state to the unoccupied state of the adsorbate. (c) Charge transfer mechanism. The electrons
are resonantly transferred from the metal to the molecule. [6].................................................................. 7
Figure 1.6. Schematic diagram summarizing the methods with which change in surrounding
media can be detected by properties of light wave optimally coupling with the surface
plasmons.[11]........................................................................................................................................................... 9
Figure 2.1. a) Light diffraction by a 1D diffraction grating. b) Vector representation of the
momentum-matching condition. c) Dispersion relations of light and surface plasmons on a
planar metal surface. d) Dispersion relations of light and surface plasmons when a diffraction
grating with pitch a is present.[15]................................................................................................................... 12
Figure 2.2. Schematic illustration of Hot electron generation and transfer to adsorbed
molecules................................................................................................................................................................. 13
vi
Figure 2.3. (a) Cross-sectional SEM image of the fabricated corrugated grating. (b) The
refractive index profile of the structure modeled by Equation 2.10....................................................... 15
Figure 2.4. Simulated absorption of gratings plotted as a function of incident angle and
wavelength (top) and resonant wavelength for different incident angles for each mode using
Eq.2.4 (bottom)...................................................................................................................................................... 16
Figure 2.5. (top row) The absorption spectra numerically obtained from FDTD simulations
(markers) and fitted spectra from eqn. (6) (solid lines) (top row). Corresponding resonant field
enhancements at 633nm (middle row) and 785nm (bottom row)........................................................... 17
Figure 2.6. (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.[16].......... 18
Figure 2.7. Loss rates of resonant modes at 633nm and 785nm for the initial grating geometry
of different material systems. ............................................................................................................................ 19
Figure 2.8. Field enhancement at 633nm and 785 for different metal gratings under
consideration as a function of A and �. The white dot represents the initial grating structure
and yellow star represents the optimized geometry.................................................................................... 19
Figure 2.9. (top row) The absorption spectra numerically obtained from FDTD simulations
(dots) and fitted spectra from Equation. (2.8) (solid lines) for the optimum structure of each
metal. Corresponding resonant field enhancements at 633nm (middle row) and 785nm (bottom
row)........................................................................................................................................................................... 21
Figure 2.10. Loss rates of resonant modes at 633nm and 785nm for the optimized geometry
of different material systems ............................................................................................................................. 21
Figure 3.1. Electronic properties of plasmonic and catalytic metals. (a, b) Electronic band
structure of typical plasmonic metals (a) and catalytic metals (b). To the right of each diagram
is the density of states of a simple molecular adsorbate.[54]................................................................... 25
Figure 3.2 Volcano plot for the HER for various pure metals. [55]...................................................... 26
Figure 3.3. Schematic of the surface plasmon enhanced photocatalysis in Pt coated gratings. .... 27
Figure 3.4. (a) Atomic Force Microscope (AFM) image of the fabricated grating. (b) Crosssectional AFM profile of the grating. The red curve is the raw measurement data and black
curve is the smooth fit to surface corrugation............................................................................................... 28
vii
Figure 3.5. (a) Optical setup of the angle-dependent photocurrent measurement. (b)
Electrochemical setup illustrating the AC photocurrent measurement. ................................................ 29
Figure 3.6. (a) Schematic diagram illustrating the corrugated grating structure. b) Band
structure of the Au grating calculated analytically based on diffraction conditions. (c)
Simulated broadband absorption at normal incidence. (d) Absorption of Au grating simulated
numerically using the FDTD method.............................................................................................................. 30
Figure 3.7. (a) Cross-sectional diagram of the unit cell of the corrugated grating structure. (b)
Electric field enhancement of bare Au grating illuminated on resonance (angle, polarization,
and wavelength). Electric field enhancement of the same Au grating with (c) 1 nm Pt and (d)
2 nm Pt coatings.................................................................................................................................................... 31
Figure 3.8. (a) Scanning electron microscope (SEM) image of a Au grating with 520 nm period
and 233 nm metal linewidth. (b) Measured photo-reflection spectrum from a bare gold grating.
.................................................................................................................................................................................... 32
Figure 3.9. Schematic diagram of the reflection measurement setup.................................................. 33
Figure 3.10. Cross-sectional STEM image of the fabricated grating. .................................................. 34
Figure 3.11. Angle-dependent photocurrent spectra of (a) bare gold and (b) Pt-coated gold
grating nanostructures. (c) Normalized photoreflectance spectra taken with and without the
platinum coating.................................................................................................................................................... 35
Figure 3.12. (a) Photocurrent plotted as a function of incident angle and (b) peak photocurrent
for bare grating and gratings with different Pt thicknesses....................................................................... 36
Figure 3.13. (a) Angle-dependent photocurrent spectra of Ni-coated gold grating
nanostructure. (b) Photoreflectance spectra with and without nickel coating. (c) Angledependent photocurrent spectra of Ru-coated gold grating nanostructure. (d) Photoreflectance
spectra with and without ruthenium coating. ................................................................................................ 37
Figure 3.14. (a) Electrochemical potential dependence of the AC photocurrent measured on
resonance with a gold grating structure (a) bare, (b) Pt coating, (c) Ni coating, and (d) Ru
coating...................................................................................................................................................................... 38
Figure 4.1. (a) Plasmon dispersion change with perturbation in the medium index. Shift in
resonance in (b) wavelength, and (c) angular domains with surrounding medium index change.
[11]............................................................................................................................................................................ 42
viii
Figure 4.2. Schematic illustration of SPR sensor based on simultaneous excitation of surface
plasmons by a polychromatic light and the dispersion of light on a special grating coupler[12].. 42
Figure 4.3. (a) First-order plasmon resonant mode and (b) shift in plasmon resonant angle at
785 nm plotted as a function of the index of refraction of the surrounding media for a 500 nm
corrugated Au grating (k=0). (c) Change in band diagram, and (d) shift in plasmon resonant
angle at 785 nm as a function of the imaginary part of surrounding media index for a 500 nm
corrugated Au grating (n=1.4). ......................................................................................................................... 45
Figure 4.4. (a) Schematic diagram of the electrochemical cell used for the electro-optic
measurement. (b) Cross-sectional SEM image of the 500 nm pitch corrugated Au grating
structure. (c) Schematic diagram of the angle-dependent photo-reflection measurement setup.
.................................................................................................................................................................................... 47
Figure 4.5. (a) Angle-dependent reflection for different voltages applied across the ionic liquid
electrolyte. (b) Zoomed-in plot of the reflection profile in the range (a) -5.4o and -3.2o ................. 48
Figure 4.6. (a) Resonance shift and (b) resonance sensitivity with EDL thickness for different
surface plasmon penetration depth. ................................................................................................................. 50
Figure 4.7. Resonance shift with (a) thickness of EDL layer for constant local index change
(b) perturbed index of EDL layer with constant thickness. ...................................................................... 51
Figure 4.8. (a) Interface of Au grating and homogeneous ionic liquid medium. (b) Electric
field intensity distribution and (c) field decay away from the metal for grating with
homogenous ionic liquid medium. (d) Interface of Au grating and ionic liquid medium with a
conformal EDL. (e) Electric field intensity distribution and (f) field decay away from the metal
for grating with EDL interface.......................................................................................................................... 52
Figure 4.9. (a) Simulated electric field profile calculated on resonance. (b) Plasmonic electric
field intensity as it decays away from the metal surface. (c) Plasmon resonance shift as a
function of the change in refractive index and thickness of the ionic liquid double layer. The
contour tracks the combination of index change and thickness required to satisfy the
experimentally observed shift of -0.35o with applied potential. (b) Change in reflection from
the grating as the double layer forms as a function of the change in refractive index and
thickness of the ionic liquid double layer. The contours represent the combination of index
change and thickness required to obtain a resonance shift of Df = -0.35o resonance shift and a
10% decrease in reflection. The red star represents the intersection point. (c) Simulated and (d)
ix
measured reflection as a function of incident angle to the grating with and without ionic liquid
double layer ............................................................................................................................................................ 54
Figure 4.10. Voltage-induced changes in reflectance measured on resonance in (a) ionic liquid,
(b) D2O, and (c) non-polar benzene control sample................................................................................... 56
Figure 5.1. a) TA measurements for Au grating with 266 nm metal line width and 500 nm
period in air environment with the p-polarized probe pulse. Selected broadband TA spectra of
different delay times between the pump and probe. b) Relative shift of the plasmon resonant
bleaching peaks in TA spectra as delay time increases[101]................................................................... 59
Figure 5.2. Schematic illustration of adsorbate density in (a)bare, and (b) catalytic metalcoated grating......................................................................................................................................................... 60
Figure 5.3. Transient absorption spectrum (top row), and UV-VIS spectrum (bottom row) of
(a) Bare Au grating in Air, (b) Hybrid grating in Air, (c) Bare Au grating in solution, and (d)
Hybrid grating in solution. ................................................................................................................................. 60
Figure 5.4. Cross section of the transient absorption spectrum of (a) Bare Au grating in Air,
(b) Hybrid grating in Air, (c) Bare Au grating in solution, and (d) Hybrid grating in solution ..... 61
Figure 5.5. Resonance shift with (a) 0V and +3V applied bias cycles, and (b) 0V and -3V
applied bias cycles. Zoomed-in view of the resonance dips is shown in the inset............................. 62
Figure 5.6. The reflected optical power modulation from Au/IL interface with (a) 0 to 3V, and
(b) 0 to -3V modulation. Electrical current response through IL solution with (c) 0 to 3V, and
d) 0 to -3V modulation........................................................................................................................................ 63
x
ABSTRACT
This dissertation investigates the applications of surface plasmon resonance toward driving
chemical reactions and sensing interfacial changes. Light interaction with free electrons in metals
is a complex phenomenon that depends on the properties of the metal, the nature of incident light,
and the interaction conditions. Under appropriate excitation conditions, we can couple the modes
of the electromagnetic wave with the modes of the surface plasma oscillation to create surface
plasmons. One of the exciting properties of surface plasmons is that they can help concentrate light
into narrow regions of space and create large electric field enhancement. The surface plasmon
excitation and subsequent field concentration can be used to generate high-energy electrons and
drive chemical reactions. The plasmon-induced hot carrier generation offers immense potential in
renewable energy storage, bio, and chemical sensing, driving thermodynamically unfavorable
reactions, etc. Yet the field is still in a nascent stage. In the first part of the dissertation, I am
exploring the fundamentals of plasmonic excitation in metallic gratings and optimizing the
structure of these gratings using computational electromagnetic tools for photocatalytic
applications.
Plasmonic metals can boost optical absorption and brute-force chemical reactions. However,
compared to their electrocatalyst counterparts, they offer poor performance in terms of weakening
the molecular bonds by surface interactions. The electrocatalytic metals on the other hand have
weak interaction with light but offer superior performance when it comes to surface interactions
via chemisorption. Combining these two aspects is challenging due to the fundamental differences
in the d-band of plasmonic and catalytic metals. We have developed a hybrid system that decouples
and combines the optical and chemical activity of plasmonic and catalytic metals. The hybrid
system offers the potential to improve the efficiencies of light-driven chemical reactions further.
The surface plasmon excitation is highly sensitive to the environmental conditions. The energy
and momentum required to excite the resonance in a plasmonic system varies with any perturbation
in the dielectric media. If we can track the changes in resonance, it would be an indirect measure
of the dielectric environment changes. We use this sensitivity property of surface plasmons to
study an important class of organic compounds called Ionic Liquids. Ionic liquids are highly
valuable in applications ranging from energy generation and storage to electrocatalysis and show
rich dynamics in the presence of an external electric field. We have studied the field-induced
xi
interface changes in Ionic liquids using plasmonic gratings, which act as the electrode to bias the
ionic liquid solution and sense the optical changes. With the help of electromagnetic simulation
models, we are attempting to deduce the thickness of the ordered layer of ionic species forming at
the electrode interface.
1
CHAPTER 1. INTRODUCTION
1.1 Light Interaction with Metals
When light interacts with metals, it sets off a multitude of phenomena, influenced by the
unique electronic properties of the metal. The free electrons in the metal's surface can move freely,
allowing them to respond quickly to the electromagnetic field of the incoming light wave. These
electrons oscillate and re-radiate the light, leading to high reflection. Some of the light energy can
be absorbed by the metal, particularly at specific wavelengths. This absorbed energy elevates the
electrons to higher energy states, subsequently converting the energy into heat and raising the
metal's temperature. Metals typically impede the passage of electromagnetic waves due to their
dense electron population, which interacts with and absorbs incoming light. However, very thin
metal films may exhibit partial light transmission, displaying absorption and interference effects.
Moreover, when light of certain frequencies, typically ultraviolet or higher, strikes a metal surface,
it can also induce the Photoelectric Effect. In this process, photons' energy is absorbed by electrons,
providing sufficient energy to overcome their binding force to the metal and causing their ejection
from the surface.
These interactions are intricately influenced by factors like light wavelength, metal type, surface
structure, and temperature. Understanding these interactions underpins various fields, including
optics, photonics, electronics, and material science.
Figure 1.1. Light interaction with metals. The free electron interaction results in light energy
getting reflected or absorbed in general. Under appropriate conditions, resonant interaction leads
to bound surface waves known as surface plasmons
2
In addition to these interactions, when the frequency of light matches the natural frequency of
electrons oscillating against the restoring force of positive nuclei, it induces a resonance
phenomenon known as surface plasmon resonance. That is, when polarized light strikes a metaldielectric interface under specific conditions, the free electrons at the metal surface can collectively
oscillate in resonance with the incident light. These oscillations, termed surface plasmons, resonate
with the incident light at specific angles and frequencies, resulting in a significant amplification of
the electromagnetic field at the interface.
Surface plasmons are neither conventional light waves nor electrons but rather collective
oscillations of free electrons at a metal's surface. They embody a coupled state of the
electromagnetic field and electron density waves in the metal, exhibiting characteristics of both
light (such as propagation and wavelength) and electrons (such as charge density oscillations).
They represent a distinct energy state that defies a complete description solely as light waves or
electrons but rather as a blend of both. Through the propagation of surface plasmons along the
metal surface, they can convey energy and information over relatively short distances.
1.2 Surface Plasmon Excitation
When Maxwell's equations are applied to a metal-dielectric boundary, it becomes imperative
that the tangential components of both electric and magnetic fields remain continuous at their
interface. This prerequisite condition serves as the foundation for determining the dispersion
relation governing surface plasmon resonance (SPR) or Surface Plasmon Polariton (SPP). The
dispersion relation elucidates the correlation between the frequency (or wavelength) of Surface
Plasmons (SPs) and their corresponding wavevector. In the scenario of a flat interface between a
metal (characterized by its dielectric function, �!) and a dielectric (with dielectric constant, �"),
the dispersion relation for SPPs is mathematically expressed as follows:[1]
�#$ = �%% �"�!
�" + �!
; (1.1)
where �#$ is the frequency-dependent Surface Plasmon wave-vector, �% ω/c is the free space
wavevector of incoming photons. The frequency-dependent permittivity of the metal, �!, and the
3
dielectric material, �" must have opposite signs if SPs are to be possible at such an interface.[2]
This condition is satisfied for metals because �! is both negative and complex.
Figure 1.2. (a) SPPs at the interface between a metal and a dielectric (b) The evanescent field
perpendicular to the interface. δd is the decay length of the field in the dielectric, of the order of
half the wavelength of light, δm is the decay length into the metal, determined by the skin depth.
(c) The dispersion curve for a SPP mode.[1]
After light is converted into a surface SP mode on a flat metal surface, it will propagate but will
gradually attenuate due to absorption losses in the metal. This attenuation depends on the dielectric
function of the metal at the oscillation frequency of the SP. The propagation length, �&', can be
found by seeking the imaginary part, �&'
" , of the complex surface plasmon wavevector, k)* =
�&'
+ + ��&'
" , from the SPP dispersion equation as:[1, 3]
�&' = 1
2�&'
" = �
� 3
�" + �!
+
�"�!
+ 4
,
- �!
+ -
�!
" (1.2)
where �!
+ and �!
” are the real and imaginary parts of the dielectric function of the metal, that is,
� = �!
+ + ��!
" .
Unlike the propagating nature of SPs along the surface, the field perpendicular to the surface
decays exponentially with distance from the surface, as illustrated in Figure 1.2b. This
perpendicular field is described as evanescent or near-field, resulting from the bound, nonradiative nature of SPs, which prevents power from propagating away from the surface. SPR
occurs when the momentum of incident light photons matches that of surface plasmons. For a
given frequency, the surface plasmon wavevector is higher than that of the incoming photons,
4
creating a momentum mismatch that must be bridged to generate SPP using light. There are three
primary techniques to provide the necessary additional momentum: First, by using prism coupling
to enhance the momentum of the incident light, second by scattering from topological defects on
the surface, such as subwavelength protrusions or holes, to generate SPs locally, and the third
method employs periodic corrugation on the metal's surface. For the work presented here, we have
used grating structures to excite surface plasmons.[3]
Besides surface plasmon polaritons and surface plasmons at a planar dielectric–metal interface,
localized surface electromagnetic excitations can occur in confined geometries, such as metallic
particles or voids of various shapes. These excitations in bounded geometries are known as
localized surface plasmons (LSPs). Localized surface plasmons can be resonantly excited by light
of the appropriate frequency and polarization, independent of the wave vector of the incoming
light.[2]
1.3 Surface Plasmon Decay and Hot Electron Generation
The attenuation or decay of surface plasmon resonance in nanostructures can manifest either
through radiative emission of a photon or through non-radiative pathways. Non-radiative decay
takes place through the quantum mechanical process called Landau damping which occurs within
a timescale of 1–100 femtoseconds. During this process, a plasmon quantum is transferred into a
high-energy single-electron-hole pair excitation. These hot electrons produced from plasmon
decay cannot escape into the vacuum due to the large work functions of standard plasmonic metals.
Instead, they rapidly distribute their energy among numerous lower-energy electrons through
electron-electron scattering processes such as Auger transitions. For extended surfaces, the hot
carrier relaxation occurs within 100 femtoseconds to 1 picosecond (te). Within this time scale these
carriers reach a Fermi-Dirac-like distribution characterized by a large effective electron
temperature (τel). As these lower-energy electrons slow down, their interactions with phonons
intensify, leading to equilibration with the lattice, represented by a lattice temperature (τl), which
occurs over a longer timeframe (tph) of several picoseconds. The two-temperature model with
electron temperature, τel, and lattice temperature τl can be used to model the dynamics of this
equilibration process. Here τel and τl become time-dependent and eventually converge. Finally, the
heat is transferred to the surrounding environment of the metallic structure, which can occur over
a timeframe ranging from 100 picoseconds to 10 nanoseconds, depending on the material, particle
5
size, and thermal conductivity properties of the surroundings.[4] Figure 1.3 summarizes the
plasmon excitation and dephasing process.
Figure 1.3. (a) Excitation of a localized surface plasmon enhances light absorption. (b) Generation
and multiplication of hot carriers through Landau damping. (c) Hot carriers redistribute their
energy into the Fermi–Dirac-like distribution due to the electron-electron interaction. (d) Heat is
released to the surroundings via thermal conduction The gray area represents the population of the
electronic states; hot electrons and hot holes are represented by the red and blue areas,
respectively.[4]
1.4 Plasmonic Photocatalysis
The fundamental idea behind plasmonic photocatalysis is to take advantage of the energy in
the surface plasmons in a metal structure to drive or enhance some useful catalytic processes.
Figure 1.4 schematically illustrates the potential energy of a system in which the reactants in state
A are transformed into the products in state B. Plasmon dissipation can induce an electronic
transition from the ground state to the excited state of the reactants-catalyst system (A* in Figure
1.4). This transition necessitates an electron (or hole) transfer process upon photon absorption. The
transient state can then engage in the catalytic reaction, progressing towards the product state B*,
traversing a new Potential Energy Surface (PES). The activation energy Ea* associated with this
new state may be lower than that of the ground state. In scenarios with multiple reaction pathways,
distinct potential energy minima may emerge, leading to different products. (as illustrated by D*
6
in Figure 1.4). This characteristic is pivotal, as it could potentially enhance product selectivity
compared to traditional thermal catalysis. Eventually, the product in the excited state decays back
to the ground state by transferring the electron (or hole) back to the plasmonic catalyst (depicted
as B or D in Figure 1.4).[5]
Figure 1.4. Schematic representation of the potential energy surfaces at the ground and excited
states for a generic catalytic reaction on a metal surface.[5]
In plasmonic photocatalysis, the energy within SPs is harnessed to convert adsorbates or reactants
to other chemical forms. The process of harnessing from SP involves the generation of hot
electrons within the plasmonic metal, which is then transferred to molecular adsorbates via direct
or indirect pathways. They occur at different timescales during the plasmon decay and under
different conditions. In the indirect mechanism, hot electrons are initially produced within the
metal and subsequently transferred to the Lowest Unoccupied Molecular Orbitals (LUMOs) of the
adsorbate, traversing the interfacial barrier between metal and adsorbate as shown in Figure 1.5a.
While only a small fraction of high-energy hot electrons can overcome this barrier, the majority
undergo electron-electron scattering relaxation. Notably, higher photon energy corresponds to
greater efficiency in the indirect electron-transfer process, as it generates electrons more capable
of surmounting the interfacial barrier. This process occurs after the dephasing is completed
7
Figure 1.5. (a) Indirect hot-electron transfer mechanism. Hot electrons (e−) generated via the
nonradiative decay of an LSP are transferred to the adsorbate molecule. (b) Direct intramolecular
excitation mechanism. The LSP induces direct excitation from the occupied state to the unoccupied
state of the adsorbate. (c) Charge transfer mechanism. The electrons are resonantly transferred
from the metal to the molecule. [6]
Conversely, direct transfer occurs due to the coupling between unoccupied states in the adsorbate
and excited surface plasmons, forming hybridized surface states between the metal and adsorbates.
This phenomenon introduces an additional dephasing channel for the plasmons. The dephasing
plasmon can excite electrons from the HOMO to LUMO states within the molecule as shown in
Figure 1.5b known as direct intramolecular excitation, or facilitates electron transitions within the
hybridized states known as chemical interface damping (CID) as shown in Figure 1.5c.[6-8] Unlike
indirect transfer, direct transfer occurs concurrently with plasmon dephasing within a few
femtoseconds, leading to higher electron-transfer efficiency and reduced energy loss.
1.5 Surface Plasmon Sensors
The propagation constant of surface plasmon is sensitive to variations in the refractive index
at the metal surface, thanks to their ability to strongly confine and enhance the electromagnetic
field close to the surface. This phenomenon can be used to create sensing devices using plasmonic
structures. Index perturbations at the metal interface affect the coupling condition between a light
wave and the surface plasmon, resulting in observable changes in the characteristics of the
electromagnetic wave that interacts with the surface plasmon. Both SPR (Surface Plasmon
Resonance) and LSPR (Localized Surface Plasmon Resonance) structures are becoming prominent
(a) (b) (c)
8
platforms for optical sensors. Optical plasmonic sensors have demonstrated capabilities in realtime, label-free sensing with exceptional sensitivity. They have found successful applications in
diverse fields such as chemistry, biology, medicine, and food safety, among others. Generally, the
operating mechanism of these sensors relies on the single mode of SPR and LSPR or their hybrid
modes. Theoretically, both SPR and LSPR offer the benefits of tunable resonant responses in terms
of wavelength, intensity, and phase. Furthermore, the performance of plasmonic sensors can be
enhanced by tweaking the geometric structures and employing specific functional materials.[9]
These sensors offer several advantages over traditional methods, including real-time monitoring
for observing the dynamics of binding in various biological interactions between biomolecules,
label-free detection, high reusability, quick response times, straightforward sample preparation,
and the use of minimal electronic components.[10] The detection methods employed in SPR/LSPR
sensors involve analyzing alterations in wavelength, angular, phase, and polarization
characteristics of the plasmonic structures.[9]
SPR sensors utilize different modulation techniques based on how the light wave interacts
with the surface plasmon, each offering a unique method for measuring sensor output. SPR sensors
can be classified based on the optical property they monitor during this interaction: angular,
wavelength, intensity, phase, or polarization modulation.
• Angular Modulation: In this type of SPR sensor, a monochromatic light wave induces a
surface plasmon. The coupling between the incoming light wave and the surface plasmon
is examined at different angles of light incidence. The angle that results in the strongest
coupling is selected as the sensor's output, which can be calibrated to determine the
refractive index.
• Wavelength Modulation: Here, a surface plasmon is triggered by a collimated light wave
containing multiple wavelengths, with a consistent angle of incidence on the metal film.
The coupling strength is assessed across these wavelengths, and the wavelength with the
strongest coupling becomes the sensor's output.
• Intensity Modulation: This type of SPR sensor measures the coupling strength between
the incoming light wave and the surface plasmon at a specific angle and wavelength. The
output of the sensor is the intensity of the light wave.
9
• Phase Modulation: In phase-modulated SPR sensors, the phase shift of the light wave
interacting with the surface plasmon is gauged at a particular angle and wavelength. This
phase shift serves as the sensor's output.
• Polarization Modulation: SPR sensors of this kind measure changes in the polarization
of the light wave interacting with the surface plasmon.[11-13]
Figure 1.6. Schematic diagram summarizing the methods with which change in surrounding media
can be detected by properties of light wave optimally coupling with the surface plasmons.[11]
The sensitivity of optical sensors to changes in refractive index (RI) is generally defined as:
S = dA
��
Here, A represents the measured parameters, such as angular, wavelength, or intensity, while n
stands for the RI of the analytes. For plasmonic sensors, the sensitivity � can range from 50
nm/RIU (refractive index unit) to 30,000 nm/RIU, depending on the design of the structures and
the materials used.[9]
Polarization change
Intensity change
Phase change
Coupling angle
change
Surface plasmon
Propagation constant
change
Light wave
Characteristics
Coupling wavelength
change
Bulk refractive
index change
Surface refractive
index change
10
CHAPTER 2. HOT ELECTRON DRIVEN PHOTOCATALYSIS IN
PLASMONIC GRATINGS
This chapter is similar to Aravind, Indu et al. published in Crystals.[14]
2.1 Introduction
Hot electron-driven photocatalysis offers transformative potential for energy conversion and
environmental remediation. When plasmonic systems absorb photons, they can generate hot
electrons, which are high-energy electrons excited above the Fermi level through the absorption
of light. These electrons, due to their high energy, can participate in redox reactions on the
material's surface or be transferred to adjacent materials, breaking chemical bonds or forming new
ones. This process opens new pathways for the development of efficient photocatalytic systems
capable of operating under mild conditions, utilizing sunlight as the primary energy source. The
exploration of hot electron-driven photocatalysis not only promises to advance our understanding
of light-matter interactions at the quantum level but also holds the potential for significant
breakthroughs in various applications. These include solar fuel production, such as hydrogen
generation from water splitting, CO2 reduction to valuable chemicals, and the degradation of
pollutants in water and air. As research in this area progresses, it aims to address some of the most
pressing challenges of our time, including energy sustainability and environmental protection, by
harnessing the power of sunlight and innovative photocatalytic materials.
2.2 Surface Plasmon Excitation Using Metallic Gratings
In order to excite the surface plasmon polaritons (SPPs) in the grating structure, the
momentum and energy of the incident photons have to match that of the SPPs (i.e., wavevector
matching). The standard method for exciting surface plasmons involves using a prism coupler
combined with the attenuated total reflection (ATR) technique. The ATR method can be
implemented in two configurations: Kretschmann geometry and Otto geometry. Utilizing the
diffracted light in the diffraction grating is another method for optically exciting SPR (Figure 2.1).
In this approach, a light wave from a dielectric medium with a refractive index n/ is incident upon
a metal grating with a periodicity � and grating depth q, with a dielectric constant �!.
11
When a light wave with the wavevector � is incident on the grating, it produces a series of
diffracted waves. The wavevector of the diffracted light, k0 can be expressed as:
�! = � + �� ; (2.1)
where � is the diffraction order and G is the grating vector. The grating vector G is related to the
grating periodicity, � = -1
2 �% , and is perpendicular to the grating's grooves and in plane to the
grating. Thus, the component of the diffracted light, which is perpendicular to the �! plane
matches that of the incident wave, while the component within the grating plane, �3! is altered
due to diffraction given by:[13]
�3! = �3 +
�2�
a
; (2.2)
The diffracted waves can couple with a surface plasmon when the propagation constant of the
diffracted wave along the grating surface, �3!, matches that of the surface plasmon, �#$:[13, 15]
2��"
� ���� + �
2�
� = �3! = ±��K�#$L (2.3)
Where: �#$ = �#$% + Δ� = 4
5 N 6!6"
6!76"
+ Δ� (2.4)
We can ignore the Δ� factor for a shallow grating and remaining �#$% denotes the propagation
along the smooth interface of a semi-infinite metal and a semi-infinite dielectric. Thus, for the
grating structure the incident angles that satisfy the momentum-matching condition at each
wavelength can be found by solving the following equation:[16]
2��"
� ���� + �
2�
� = −�� P
�
� % �"�!
�" + �!
Q (2.5)
where � is the periodicity of the grating, � is the order of the surface plasmon mode, � is the
incidence angle, and �! and �" are the dielectric functions of the metal and surrounding medium,
respectively.
12
Figure 2.1. a) Light diffraction by a 1D diffraction grating. b) Vector representation of the
momentum-matching condition. c) Dispersion relations of light and surface plasmons on a planar
metal surface. d) Dispersion relations of light and surface plasmons when a diffraction grating with
pitch a is present.[15]
We can excite surface plasmons in a diffraction grating by shining monochromatic light at an angle
that matches the surface plasmon momentum. Figure 2.2 schematically illustrates the plasmonic
photocatalytic process in a resonant grating structure. Equation 2.5 provides a convenient
analytical method to determine the excitation conditions for surface plasmon polaritons (SPPs) in
diffraction grating structures. Once excited, the surface plasmons decay on ultra-fast timescales to
generate hot carriers. These hot carriers can be transferred to surface adsorbates through either
direct hot-electron transfer or indirect transfer methods, driving interesting chemical reactions such
as water splitting and CO2 reduction. In this chapter, we theoretically study the geometrical factors
affecting the generation of hot electrons from plasmon decay. By optimizing certain geometrical
parameters, we can enhance the field associated with the surface plasmon, thereby increasing
carrier generation.
a
a a
13
Figure 2.2. Schematic illustration of Hot electron generation and transfer to adsorbed molecules.
2.3 Plasmonic Field Enhancement and Absorption
The hot electron generation rate is proportional to the electric field intensity in the metallic
structure.[17-20] Field enhancement in plasmonic structures has been modeled and studied using
Coupled Mode Theory (CMT).[21, 22] Using CMT, the following relation connecting the field
enhancement in plasmonic structures and the quality factor of the resonant optical modes can be
derived as:[23, 24]
|�9:5|!;3
-
|�%|- = 2�5�<
��=>>
�;?#
- �<;"
(�<;" + �;?#)- (2.6)
where |�9:5|!;3 and |�%| are the maximum value of electric field intensity and the incident field
amplitude, respectively: λr is the resonance wavelength, and Veff is the effective mode volume. Ac
is the effective aperture of the microstructure which is determined by the radiation pattern of the
whole grating array.[24, 25] The total radiation pattern of the large area grating structure can be
regarded as a plane wave and therefore, Ac is equal to incident plane wave cross section. Qrad and
Qabs are the radiation and absorption quality factors of the structure, respectively, which are related
to the resonant wavelength and loss rates in the structure as follows:[24]
�;?# = �%
�;?#
, �<;" = �%
�<;"
���
1
�@:@;9
= 1
�;?#
+
1
�<;"
(2.7)
14
where ω% is the resonance frequency and �<;" and �;?# are the radiative and absorptive losses in
the metallic structure. Equation 2.6 can be modified to express the field enhancement in terms of
loss rates:
Equation 2.8 signifies that the field enhancement of the grating structure can be modified by
engineering the radiation and absorption loss rates. Radiation loss from the structure is a strong
function of its geometric parameters while absorption losses mainly depend on the material type
and wavelength.[26] The mode volume is proportional to the propagating length of the surface
plasmon polaritons and the field decay constant in the direction normal to the metal surface. Veff
is not expected to depend strongly on the geometry when the dimensions are small compared to
the wavelength.[26] The field enhancement increases with Γ<;" and reaches the maximum when
Γ<;" = Γ;?# and then decreases with further increase in Γ<;". This condition (Γ<;" = Γ;?#) is called
the critical coupling. The critical coupling could be understood as analogous to the impedance
matching principle. When an electromagnetic wave couples with the resonant structure, the
maximum energy is stored in the resonator when the loss rates are matched.[23] The maximum
field enhancement at critical coupling varies inversely proportional to the absorption losses (Γ;?#).
Therefore the maximum field enhancement is obtained when �<;" = �;?# and �@:@ = �<;" +
�;?# = 2�;?# is as minimum as possible.
For a given structure we can extract the loss rates from the spectra and tune them to improve the
field enhancement. Using CMT, the absorption spectra of these plasmonic gratings can be
expressed as:[26-28]
where �$ is the non-resonant background reflection from the structure and � is the phase factor of
the surface plasmon mode. The loss rates can be extracted by fitting the absorption spectra of the
structure to the analytical expression obtained from CMT.
|�9:5|!;3
-
|�%|- ∝
�%
�=>>
�<;"
(�<;" + �;?#)- (2.8) (
A(ω) = 1 − _r* +
ΓA2/eBC
�(ω − ω%) +
ΓA2/ + Γ2D)
2
_
-
(2.9)
15
2.4 Metallic Gratings with Different Material Systems
Our metallic gratings are fabricated by depositing metal onto a corrugated Si template
obtained from Ciencia Inc. The process begins with a silicon substrate covered by a 100 nm silicon
oxide layer. Photolithography and reaction ion etching are then employed to create a finely
corrugated structure with a 500 nm period. This templated fabrication approach provides us with
the flexibility to easily create metallic gratings using different metals of our choice. Theoretical
investigations have focused on understanding the similarities and differences in surface plasmon
resonance across these various metallic grating systems. In our experimental setup, we can
measure the photocurrent at wavelengths of 532 nm, 633 nm, and 785 nm. Consequently, we
meticulously study the excitation conditions and resonant field enhancement at these specific
wavelengths.
We investigate the surface plasmon resonance in corrugated grating structures using the Finite
Difference Time Domain (FDTD) method in the Lumerical FDTD solutions package. The
geometry of the corrugated grating surface can be specified by the following equation:
Figure 2.3. (a) Cross-sectional SEM image of the fabricated corrugated grating. (b) The refractive
index profile of the structure modeled by Equation 2.10
where � is the height difference between the peaks and valleys of the corrugation, herein refers to
the corrugation amplitude and � specifies the length over which the corrugation amplitude goes
down by 1/e2
, one word refers to the steepness factor. From the SEM image shown in Figure 2.3a,
we obtain the geometric factors of A=58.9nm and � =125nm. The gratings have a corrugation pitch
(a) of 500nm and metal thickness (t) of 50nm. Figure 2.3b shows the index profile of the geometry
y(x) = A de
E-F
GEG#
H I
$
+ e
-F
GEG#
H I
$
e (2.10)
16
used in the numerical simulation. Water is considered as the surrounding medium for the
calculations with an index of refraction of n=1.33.
A 2D simulation is carried out with a fine mesh of 1nm X 1nm and an oblique excitation with a
plane wave source. Bloch boundaries are used along the x-direction (in-plane of the grating) and
Perfectly Matched Layer (PML) boundaries are used in the y-direction (perpendicular to the
grating) for single wavelength simulations. For broadband simulations at a fixed incident angle,
Broadband Fixed Angle Source Technique (BFAST) is used within Lumerical FDTD solutions.
The polarization of the incident light is set to be p-polarization with respect to the gratings. A
power monitor is placed behind the source to record the reflected power (Pref). Absorbed power
(Pabs) is calculated as Pabs = 1-Pref. The electric field intensity is monitored with a 2D field monitor.
The broadband absorption spectra are calculated for different incident angles for each metal, as
shown in Figure 2.4 (top row). By solving Equation 2.5 for different metals, the spectral position
of the plasmon resonant modes for each incident angle can be calculated. Figure 2.4 (bottom row)
shows the analytical band structures of different metallic gratings calculated using Equation 2.5.
Figure 2.4. Simulated absorption of gratings plotted as a function of incident angle and wavelength
(top) and resonant wavelength for different incident angles for each mode using Eq.2.4 (bottom).
It is evident from Figure 2.4 that the absorption is a maximum at the resonance conditions obtained
from Equation 2.5, reaffirming that the absorption enhancement is due to the surface plasmon
polaritons. The resonant absorption is different for each material system. Metals like Ag, Au, Al,
and Cu have close to zero non-resonant absorption above 600nm while Pt has significant
background absorption throughout the visible range. We are interested in establishing whether the
17
performance of these gratings is material-limited or geometry-limited and determining how much
of an improvement is possible by adjusting the structural parameters.
We consider two incident wavelengths 633nm and 785nm for which the metals support a welldefined resonant mode at these wavelengths for incident angles ranging from 40 to 100
. The
absorption spectra obtained from the FDTD simulations for each metallic grating are then fitted to
the analytical Equation 2.5 using non-linear curve fitting with the NLopt library[29] in MATLAB
to deduce the fitting coefficients (�<;", �;?#, �%, �$,�). Figure 2.5 shows the simulated absorption
spectra (markers) and fitted absorption spectra using the analytical equation (solid lines).
Figure 2.5. (top row) The absorption spectra numerically obtained from FDTD simulations
(markers) and fitted spectra from eqn. (6) (solid lines) (top row). Corresponding resonant field
enhancements at 633nm (middle row) and 785nm (bottom row).
The field profiles corresponding to the resonant angles at wavelengths 633nm and 785nm are also
plotted. For fitting the spectra with multiple resonant modes, the linear superposition of
Lorentzian-like coupled surface plasmon polariton modes is considered.[30] It can be seen from
Figure 2.5 that the analytical equation agrees well with the simulated spectra.
2.5 Photocatalysis in Ag Gratings
Figure 2.5 shows that, for the given geometry of the corrugated template, the optimal
material system is either gold (Au) or silver (Ag). Additionally, the maximum field enhancement
18
is achieved at a longer wavelength of 785 nm. We chose to conduct photocatalytic experiments
using Ag gratings because their interband absorption is shifted further to the blue side, away from
the plasmonic band. The experimental results aligned with our expectations, as we successfully
drove the water-splitting reaction using Ag gratings.
Figure. 2.6 (b and c) illustrates the angular reflectance and corresponding photocurrent
resulting from the water-splitting reaction at a 633 nm wavelength of light. Sharp dips are observed
in the angular reflectance spectrum 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. No angle dependence is observed with s-polarized light, and it shows the
absence of the plasmon-driven reaction at this configuration. Here, the spectrum Figure 2.6b shows
a 12-fold enhancement in the photocurrent (i.e., reaction rate) between resonant and non-resonant
polarizations at incident angles of ±7.6° from the normal.[16]
Figure 2.6. (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.[16]
2.6 Optimizing Field Enhancement in Grating Structures
The corrugated structure for each metal was analyzed within the CMT framework to see
whether the structure is optimal for the specific material system. Figure 2.7 shows the radiative
and absorptive loss rates obtained from the fit for different metals at their resonant modes for
633nm (blue) and 785nm (red), respectively. The black dashed line represents the critical coupling
condition (�<;" = �;?#). The points that lie close to the center line towards the origin in the Gamma
plots will have maximum field enhancement as it satisfies �<;" = �;?# and �@:@ = �<;" + �;?# is
(a) (b) (c)
19
minimum. We find that the initial structure depicted in Figure 2.7 is far from optimum for all the
metals at 633nm. At 785nm, the structure is close to critical coupling for all the metals except Pt.
Figure 2.7. Loss rates of resonant modes at 633nm and 785nm for the initial grating geometry of
different material systems.
Absorptive and radiative decay rates of different metallic gratings depend on the geometric
parameters of the corrugation. Here, we vary the corrugation amplitude (�) and the steepness factor
(�) to determine the changes in field enhancement and peak absorption at 633nm and 785nm,
respectively, for each metal. The average field enhancement is calculated by:
I2JK = ∬|E(x, y)|-dxdy
∬|E%|-dxdy (2.11)
Figure 2.8. Field enhancement at 633nm and 785 for different metal gratings under consideration
as a function of A and σ. The white dot represents the initial grating structure and the yellow star
represents the optimized geometry.
20
Figure 2.8 shows the field enhancement at 633nm and 785nm for different metal gratings under
consideration as a function of A and �. We have used the resonant angles obtained from the
analytical expression in Equation 2.5 to excite the surface plasmons in each metal at both
wavelengths. A was varied from 10nm to 100nm while σ was varied from 50 to 400nm. These
parameters ranges were chosen to represent fabrication-feasible corrugation gratings.
Within this parameter space, Ag shows the highest field enhancement among the metals while Pt
has the least. The 785nm modes were found to have higher field enhancement compared to the
633nm modes for all the metals. Table 1 lists the A and � values of the optimum structure that
maximizes the field enhancement at 633nm and 785nm, as compared with that of the original
grating geometry. The optimum structures are shallower than the original gratings for all the metals
except Al and Pt. For Pt, the corrugation amplitude has to increase to get the maximum field
enhancement whereas the initial grating amplitude was closer to the optimum for Al. For all metals,
the steepness (1/ σ) was reduced by almost 1.5 times than the original gratings to increase the field.
The resulting average field enhancement for 633nm and 785nm are 1.8X and 3.8X for Ag, 1.4X,
and 3.6X for Au, 1.4X and 1.3X for Al, 1.2X and 2.6X for Cu, and 1.2X and 1.3X for Pt.
Table 2-1. Values of corrugation amplitude (A) and steepness factor (σ) that yield maximum field
enhancement at 633nm and 785nm.
Original gratings Optimized geometry
A (nm) σ (nm)
�;LM
633nm
�;LM
785nm
A (nm) σ (nm)
�;LM
633nm
�;LM
785nm
Ag 58.9 125 7.05 6.04 41.4 235 13.00 23.00
Au 58.9 125 4.37 4.85 46.2 198 5.90 17.60
Al 58.9 125 5.14 5.82 59.5 198 7.06 7.44
Cu 58.9 125 3.65 4.77 50.5 215 4.47 12.20
Pt 58.9 125 1.50 1.97 95.7 178 1.73 2.48
Figure 2.9 shows the absorption spectra corresponding to resonant angles at 633nm (blue)
and 785nm (red) for the optimum structures for each metal. The resonant field profiles
corresponding to the two wavelengths are also shown. The resonant field enhancement and
21
resonant absorption had improved in these structures compared to the initial geometry. The field
enhancement for 633nm modes is lower than that of 785nm. Ag shows maximum field
enhancement compared to other metals considered. The field was found to be more uniformly
distributed along the corrugated surface in the final structures.
Figure 2.9. (top row) The absorption spectra numerically obtained from FDTD simulations (dots)
and fitted spectra from Equation. (2.8) (solid lines) for the optimum structure of each metal.
Corresponding resonant field enhancements at 633nm (middle row) and 785nm (bottom row).
To compare the loss rates of the optimized structures with that of the initial gratings, we fitted the
absorption spectra with Equation 2.9. Figure 2.10 shows the radiative and absorptive losses at
resonant angles corresponding to 633nm and 785nm for the above-mentioned gratings.
Figure 2.10. Loss rates of resonant modes at 633nm and 785nm for the optimized geometry of
different material systems
22
Compared with Figure 2.7, we can see that tuning the geometrical parameters will alter the losses,
which subsequently influence the field enhancement. The optimum structures have similar
absorption and radiation loss rates and, thus, lie closer to the critical coupling condition (dashed
center line). However, this alone does not determine the maximum field enhancement condition.
The sum of these loss rates should be as low as possible as well. The geometrical optimization
tends to match the radiation losses with absorptive losses. The maximum field enhancement these
corrugated gratings can achieve is found to be limited by the metal type. The gamma values for
785nm modes are closer to the optimum conditions required for field enhancement than the 633nm
modes, likely due to fewer interband transitions at 785nm. Overall Ag works the best in terms of
plasmonic field enhancement while Pt is found to be the least optimal.
2.7 Conclusion
In conclusion, we have studied the surface plasmon resonance in corrugated gratings of
different metals at 633nm and 785nm. Strong material and geometry dependences have been
observed in the field enhancement of these corrugated grating modes. The corrugation depth and
steepness were tuned to find the maximum field enhancement possible by geometry optimization
for each material system. The absorption spectra of each grating structure have been fitted with
the analytical equation obtained from CMT and extracted the absorptive and radiative loss rates.
The maximum field enhancement for a metallic grating is achieved by matching the radiation
losses with its absorptive loss by tuning the geometric parameters. The optimum structures are
found to be shallower for Ag, Au, and Cu, and deeper for Pt. The gratings become flat for all the
metals to increase the average field enhancement. Ag and Au were found to be the best in terms
of overall field enhancement while Pt had the least performance.
23
CHAPTER 3. PHOTO-EXCITED HOT ELECTRON CATALYSIS IN
HYBRID SYSTEMS
This chapter is similar to Aravind, Indu et al. published in ACS Applied Material and
Interfaces.[31]
3.1 Introduction
The utilization of hot electrons photoexcited in plasmon resonant nanostructures has been
explored for various applications in chemistry by many research groups.[32-39] One of the great
promises of hot electrons in chemistry has been to overcome high barriers and drive difficult
reactions. Mukherjee et al. demonstrated H2 dissociation through the photoexcitation of a Feshbach
resonance.[40] Zhou et al. quantified the hot carrier and thermal contributions in plasmonic
photocatalysis, demonstrating accelerated reaction rates and light-dependent activation barriers
that outperformed heating under thermal equilibrium conditions.[41] Theoretical calculations by
Sundararaman et al. and Liu et al. predicted narrow distributions of hot electrons and hot holes on
early timescales (<50 nsec) upon photoexcitation.[42, 43] Ultra-fast time-resolved spectroscopy
later revealed that these photoexcited hot electrons decay rapidly into a hot Fermi distribution that
persists for 1-2 ps before equilibrating with the lattice temperature.[44] Al-Zubeidi et al. showed
that hot electrons generated by plasmons are injected into water to form solvated electrons and
their yield is significantly increased when nanoparticle-decorated electrodes are used instead of
smooth silver electrodes.[45] Angle-dependent photocurrents measured on plasmon resonant
grating structures demonstrated plasmon-resonant enhancement of photo-electrochemistry,
however, the electrochemical potential dependence of these gratings has not yet been
investigated.[16, 17] Furthermore, no photoelectrochemical measurements have been performed
on plasmon-resonant grating structures with a metal co-catalyst (e.g., Pt, Ru). Platinum (Pt) based
catalysts have been studied extensively and proven to be highly efficient for the hydrogen
evolution reaction (HER).[46-48] On the other hand, ruthenium (Ru) exhibits catalytic properties
that enhance the oxygen evolution reaction (OER).[49-52] Nickel (Ni) surfaces have been
demonstrated to serve as good catalysts for both OER and HER.[51, 53] Combining the catalytic
activity of Pt, Ni, and Ru with the plasmonic properties of the metallic grating could offer an
effective way to improve the efficiency of HER and OER reactions.
24
3.2 Electronic Properties of Plasmonic-Catalytic and Electro-Catalytic
Materials
Plasmonic and catalytic metals differ fundamentally in their electronic band structures
especially the position of the d-band relative to their Fermi level, which enhances one phenomenon
while diminishing the other. Plasmonic metals have their d-band far from the Fermi level (Figure
3.1a), whereas catalytic metals have d-band centers close to the Fermi level (Figure 3.1b). As a
result, plasmonic metals exhibit high-quality intra-band plasmons from s-s transitions at optical
frequencies until interband transitions (d to s) occur. In contrast, the optical response of catalytic
metals is dominated by interband transitions. This increased likelihood of interband transitions
dampens the surface plasmons across all frequencies, making them spectrally broad and lower in
intensity.[54]
Unlike plasmonic responses, a strong catalytic response generally requires a d-band close to the
Fermi level. As per the d-band model of chemisorption, a molecule interacting with a metal surface
interacts with both the sp- and d-bands. The broad sp-band renormalizes the frontier molecular
orbitals (HOMO and LUMO levels), causing their energy to broaden and shift. The narrower dband hybridizes the molecular orbital into metal-adsorbate bonding and anti-bonding modes. The
sp-band is similar for most transition metals, but differences in the d-band lead to varying
adsorption rates and catalytic performance. When molecules interact with plasmonic metals, their
anti-bonding mode can lie below the Fermi level due to the metal's deep-lying d-band (Figure
3.1a). Consequently, molecules are less likely to adsorb to the surface, resulting in less effective
catalysts. However, with catalytic metals, generally, only the bonding states are below the Fermi
level (Figure 3.1b), promoting chemisorption and making the metal an effective catalyst.[54, 55]
25
Figure 3.1. Electronic properties of plasmonic and catalytic metals. (a, b) Electronic band structure
of typical plasmonic metals (a) and catalytic metals (b). To the right of each diagram is the density
of states of a simple molecular adsorbate.[54]
3.3 Catalytic Hydrogen Evolution Reaction
The Hydrogen Evolution Reaction (HER) is important for a variety of electrochemical
processes and has a wide range of applications as diverse as hydrogen fuel cells, electrodeposition
and corrosion of metals in acids, and storage of energy via H2 production.[56] The generally
accepted mechanism of the (HER) consists of three elementary reaction steps: [55, 57]
�%� + �& ⇄ �'() + ��& (������ ����)
�'() + �%� + �& ⇄ �% + ��& (��������� ����)
�'() ⇄ �% (����� ����)
The Volmer step describes the splitting of a water molecule and the adsorption of hydrogen on a
free site of the electrode/catalyst surface. The second step is hydrogen production for which there
are two alternative pathways: electrochemical desorption (Heyrovsky step) and/or chemical
recombination (Tafel step). It has been reported that, at low overpotentials, the HER mechanism
consists of the Volmer step followed by a parallel Heyrovsky and Tafel step, while at high
overpotentials the Tafel step is negligible, and the reaction proceeds via the Volmer–Heyrovsky
mechanism.
26
Figure 3.2 Volcano plot for the HER for various pure metals. [55]
The strength of the Metal–Hydrogen(Me-H) bond influences the electrocatalyst activity towards
the HER.[55, 56] This behavior can be explained with the Sabatier principle which states that
optimal catalytic activity can be achieved on a catalytic surface with intermediate binding energies
(or free energies of adsorption) for reactive intermediates. If the intermediates bind too weakly, it
is difficult for the surface to activate them, but if they bind too strongly, they will occupy all
available surface sites and poison the reaction; intermediate binding energies permit a compromise
between these extremes.[56] The relation between the HER current density and Me–H bond
strength can be expressed by a “volcano” plot. Figure 3.2 illustrates the case of pure metals. Figure
3.2 clearly demonstrates that noble metals exhibit the highest HER performance of all pure metals.
These metals possess hydrogen binding energy close to optimum, allowing the thermodynamically
easiest transition from reactants through intermediates to the elements on the left side exhibit
weaker Me–H bonds, which limits the adsorption of H atoms on the free catalytic site of the
electrode. On the other hand, a strong Me–H bond prevents the creation of a H–H bond and
subsequent desorption of produced H2.
3.4 Hybrid Catalyst from Combining Plasmonic and Electronic Catalysts
In this Chapter, the electrochemical potential dependence of hot electrons photo-excited in
plasmon resonant grating structures is systematically measured both with and without Pt, Ru, and
Ni catalyst coatings. Conventional plasmonic metals, such as noble metals like gold (Au) and silver
(Ag), possess low chemical reactivity, which constrains the types of chemical conversion that can
happen on their surfaces. On the other hand, catalytic materials like Pt, Ru, and Ni have notable
optical losses in the visible spectrum, leading to a weak and broadband plasmonic resonance.
27
Nonetheless, by combining these excellent catalytic properties with those of highly plasmonic
metals, it is possible to create a more efficient catalyst with multiple components. The plasmonic
part can capture energy and transfer it to the catalytic component, thereby driving the desired
chemical reactions. Hybrid catalytic systems offer significant potential for incorporating optical
functionality, expanding the range of reactions that are only feasible with catalytic metals.[54, 58,
59] Several research groups have explored nanoparticles with plasmonic materials cores and
catalytic shells.[58, 60-62] These studies have showcased a substantial enhancement in
photocatalytic activities achieved by harnessing photons through the plasmonic core and directing
the absorbed energy towards the catalytically active reaction sites located on the shell of the
nanostructure.[54, 58] While these structures allow precise control of the shell thickness, their
optical response is polarization-independent, making it difficult to separate the plasmonic
excitation from the bulk metal absorption. Grating structures make it simple to compare
photocatalysis generated both on and off plasmon resonance by merely altering the polarization of
the incoming light, while keeping all other aspects of the reaction constant (including photon
energy, sample, electrolyte), and thus obtaining the plasmonic enhancement factor. Figure 3.3
shows the schematic of a hybrid metallic grating structure.
Figure 3.3. Schematic of the surface plasmon enhanced photocatalysis in Pt-coated gratings.
28
3.5 Measuring Photocurrent
We fabricated the corrugated grating structures with a nominal periodicity of 500 nm so that
the plasmon resonance can be excited in the Vis-NIR wavelength range. The actual periodicity of
the fabricated grating was measured using atomic force microscopy (AFM) to be around 520 nm,
and the etch depth was measured to be around 75nm as shown in Figure 3.4. Of our three available
laser lines (532 nm, 633 nm, and 785 nm), 785nm yielded the sharpest resonance in our experiment
in the aqueous medium. At this wavelength (785 nm, 1.85 eV), the surface plasmon mode is well
separated from the interband absorption of the metal, which tends to dampen the plasmon
resonance. This yields in a plasmon resonant peak with a higher Q-factor at 785 nm compared to
532 nm (2.48 eV) and 633 nm (1.96 eV).
Figure 3.4. (a) Atomic Force Microscope (AFM) image of the fabricated grating. (b) Crosssectional AFM profile of the grating. The red curve is the raw measurement data, and the black
curve is the smooth fit to surface corrugation
The 1cm x 1cm grating is mounted on a motorized rotational stage (Thorlabs CR1-Z7, and
KDC101) and immersed in a 0.5 M Na2SO4 aqueous solution. A linearly polarized free space
laser (Oxxius L4Cc) at 785 nm wavelength is used to illuminate the center of the grating structure.
The beam diameter is approximately 2 mm, and the optical power illuminating the grating is
around 10 mW. The incident angle is changed by rotating the grating with respect to the beam. A
half-wave plate is inserted in the beam path to rotate the incident polarization between spolarization (resonance off) and p-polarization (resonance on). A 3-terminal Gamry potentiostat is
used to measure the electrochemical current in the system with the grating as the working
electrode, Ag/AgCl (3 M NaCl) as the reference electrode (BASI Inc.), and a Pt wire as the counter
a) b)
1 μm
29
electrode. When the resonance is turned on, a small hot electron photocurrent is added to the
electrochemical current. To isolate this photocurrent from the DC electrochemical current, the
laser beam is modulated using a chopper wheel at 200Hz frequency. The small modulation in
Gamry electrochemical current at 200Hz frequency is separated using a lock-in amplifier (Stanford
Research Systems, model SR830).
Figure 3.5. (a) Optical setup of the angle-dependent photocurrent measurement. (b)
Electrochemical setup illustrating the AC photocurrent measurement.
Figure 3.5a shows a schematic diagram of the experimental setup in which linearly polarized laser
light is used to illuminate the plasmon-resonant grating structure mounted on a rotational stage,
enabling hot electron-generated photocurrents to be measured as a function of incident angle in a
pH = 7, 0.5M Na2SO4 solution. The polarization of the incident light is switched between ppolarization (i.e., perpendicular to the grating lines) and s-polarization (i.e., parallel to the grating
lines) using a half-wave plate. In order to detect the relatively small photocurrents generated by
the short-lived hot electrons/holes, we modulate the laser using an optical chopper wheel and
measure the AC photocurrent using a lock-in amplifier (Stanford Research Systems, model
SR830), as illustrated in Figure 3.5b. The photocurrent measurements are carried out using a threeterminal potentiostat (Gamry Inc.). The differential AC lock-in technique can measure the
relatively small photocurrents associated with the short-lived hot electrons.
3.6 Results and Discussion
We have designed a gold grating that supports surface plasmon resonance in the VIS-NIR
range. To excite the surface plasmon polaritons (SPPs) in the grating structure, the momentum and
energy of the incident photons must match that of the SPPs (i.e., wavevector matching).[14, 16]
30
Figure 3.6a shows a schematic diagram of the grating structure. The grating has a periodicity of
520 nm and a linewidth of 233 nm. The grating is on a glass substrate with 50 nm Au film and 50
nm thick Au lines.
Figure 3.6. (a) Schematic diagram illustrating the corrugated grating structure. b) Band structure
of the Au grating calculated analytically based on diffraction conditions. (c) Simulated broadband
absorption at normal incidence. (d) Absorption of Au grating simulated numerically using the
FDTD method.
Figure 3.6b shows the first and second-order surface plasmon modes of the grating, analytically
calculated by solving the plasmon wavevector matching condition, with water as the surrounding
medium (n=1.328). The plasmonic mode is excited at an oblique angle with respect to normal
incidence at a wavelength of 785 nm. The resonant excitation leads to enhanced absorption in the
structure as verified using electromagnetic simulations. Figure 3.6c shows the broadband
absorption spectrum of the grating at normal incidence. The 1st order plasmon mode appears as a
peak in absorption in the visible part of the spectrum. The second-order plasmon mode is buried
within the interband absorption and appears as flat in the absorption spectra at lower wavelengths.
31
The absorption as a function of angle and wavelength of excitation is simulated and is shown in
Figure 3.6d (The colormap shows normalized absorption). A commercial Finite Difference Time
Domain (FDTD) solver (Ansys Lumerical FDTD solutions) is used to solve the Maxwells
equations in discrete time and space for the grating geometries. For broadband simulations at a
fixed incident angle, the Broadband Fixed Angle Source Technique (BFAST) is used within
Lumerical FDTD solutions. A power monitor is placed behind the source to record the reflected
power (Pref). Absorbed power (Pabs) is calculated as Pabs = 1-Pref. The resonant enhanced
absorption traces the plasmon mode expected from the analytical calculations in Figure 3.6b.
Figure 3.7. (a) Cross-sectional diagram of the unit cell of the corrugated grating structure. (b)
Electric field enhancement of bare Au grating illuminated on resonance (angle, polarization, and
wavelength). Electric field enhancement of the same Au grating with (c) 1 nm Pt and (d) 2 nm Pt
coatings.
To investigate the effect of the thin catalytic film on the surface plasmon modes, we calculated the
resonant field profiles of the Au grating structures with different thicknesses of Pt surface coatings
using Finite Difference Time Domain (FDTD) simulations. The electric field intensity is
monitored with a 2D field monitor. Bloch boundaries are used along the x-direction (parallel to the
grating) and Perfectly Matched Layer (PML) boundaries are used in the y-direction (perpendicular
to the grating) for the field calculations. A fine mesh of 0.2 nm x 0.2 nm is used in the simulations.
The field enhancement at 785 nm for the bare Au grating as well as the Pt-coated gratings is plotted
in Figure 3.7, which shows that the field enhancement goes down as the thickness of Pt coating
32
increases. This damping/screening of the plasmon resonance by the thicker Pt layer poses a
tradeoff between the hot electron generation in the Au nanostructure and the catalytic activity of
the Pt coating. Both theoretically and experimentally, we find that the platinum catalyst thickness
is optimized for a thickness of 1 nm. Pt films thicker than this spoil the plasmon resonance and
thinner than this do not provide any significant catalytic benefit.
The plasmon resonant gratings are fabricated using photolithography and reactive ion etching at
the University of California Santa Barbara nanofabrication facility. Figure 3.8a shows a scanning
electron microscope (SEM) image of the grating structure with a 520 nm period. The reflection
from the grating is measured as a function of incident angle using a free-space power meter
(Thorlabs S121C), as shown in Figure 3.8b. The Schematic diagram of the reflection measurement
setup is shown in Figure 3.9.
Figure 3.8. (a) Scanning electron microscope (SEM) image of a Au grating with 520 nm period
and 233 nm metal linewidth. (b) Measured photo-reflection spectrum from a bare gold grating.
(a) (b)
33
Figure 3.9. Schematic diagram of the reflection measurement setup
The thin Pt and Ru catalytic coatings were deposited using atomic layer deposition (ALD)
and the Ni coating was carried out using sputtering deposition. The Pt film is deposited onto the
gratings via atomic layer deposition (ALD) with a Veeco Savannah S200 system. We calibrated
the Pt ALD deposition rate using 4-probe resistivity measurements. Thick layers of Pt (300 to 600
cycles) are deposited onto to silicon substrate and the sheet resistance is measured as a function of
the number of ALD cycles. From the measured resistivity, the Pt film thickness is calculated using
the bulk resistivity of Pt (105 nΩ-m). The number of cycles required to achieve a 1nm thick Pt
layer is estimated by linear extrapolation to 30 cycles.
Figure 3.10 shows cross-sectional scanning transmission electron microscope (STEM) images of
the plasmon resonant grating structure taken at the California Nano Systems Institute, University
of California Los Angeles. The cross-sectional samples were prepared using a Ga-based focused
ion beam (FIB) system.
34
Figure 3.10. Cross-sectional STEM image of the fabricated grating.
Figures 3.11a and 3.11b show the angle-dependent photocurrent spectra of Au grating
structures with and without a 1 nm-thick platinum coating. Spectra are plotted for both s-polarized
(non-resonant) and p-polarized (resonant) light. Here, sharp peaks can be seen in the AC
photocurrent at ±2.3o (from normal incidence) for p-polarized light. However, s-polarized light
shows no angle dependence and substantially lower photocurrents. The plasmon resonance in our
grating system is polarization-dependent, which enables us to isolate the plasmon-generated
photocurrent from other sources. When the polarization is tuned to the s-polarization, the excitation
is non-resonant and there will not be any plasmon-mediated photocurrent. On the other hand, when
the excitation is tuned to the p-polarization with respect to the grating plane, there will be plasmongenerated hot electron-mediated photocurrent, which gets modulated at the chopping frequency of
the laser. The AC photocurrent can thereby be isolated using the sensitive lock-in measurement
technique. This sort of distinction is characteristic of the polarization-sensitive system and the
measurement technique. Here, based on the p-to-s polarized photocurrent ratio, we establish that
the plasmon-resonant enhancement factor is 28X for the bare Au grating (Figure 3.11a) and 64X
for the Pt-coated Au grating (Figure 3.11b). In addition, the photocurrent associated with the Ptcoated grating is less noisy and produces higher photocurrents than the bare Au grating due to the
faster kinetics (i.e., charge transfer) associated with the Pt-coated surface. The peaks in the
a) b) c)
500 nm 100 nm 50 nm
35
photocurrent coincide nicely with the peaks observed in the photo-reflectance spectra plotted in
Figure 3.11c.
Figure 3.11. Angle-dependent photocurrent spectra of (a) bare gold and (b) Pt-coated gold grating
nanostructures. (c) Normalized photoreflectance spectra taken with and without the platinum
coating
36
Figure 3.12 shows a plot of the angle-dependent photocurrent for gratings with up to 8nm
thick films of Pt. While the optimum thickness of Pt is 1 nm, the photocurrent is enhanced
compared to the bare Au grating for thicker layers of Pt, even up to 8nm thick. The peak
photocurrent is quite similar for 1-4 nm thick Pt layers. From FDTD calculations with a uniform
Pt coating, the field enhancement decreases with Pt layer thickness. However, the measured
photocurrent does not reduce at the same rate as Pt thickness. This could be due to insufficient
surface coverage of Pt. That is, for ultra-thin layer ALD deposition, the Pt may not form a
continuous uniform layer, and it may require more ALD cycles to provide uniform coverage and
produce a substantial amount of plasmon damping. Another potential reason for this discrepancy
could be that the increased density of adsorbed molecules on the Pt surface increases the charge
transfer efficiency, thus, compensating for a reduction in hot carrier generation rate.
Figure 3.12. (a) Photocurrent plotted as a function of incident angle and (b) peak photocurrent for
bare grating and gratings with different Pt thicknesses
We have conducted similar experiments on Au gratings coated with Ru and Ni thin films,
measuring photocurrent as a function of incident angle, polarization, and electrochemical potential.
Figures 3.13a and 3.13c show the angle-dependent photocurrent measured at +1.4V vs. Ag/AgCl
37
using Ni and Ru-coated gratings, respectively. Figures 3.13b and 3.13d show the angle-dependent
photo-reflection from these gratings compared to that of the corresponding bare Au grating.
Figure 3.13. (a) Angle-dependent photocurrent spectra of Ni-coated gold grating nanostructure.
(b) Photoreflectance spectra with and without nickel coating. (c) Angle-dependent photocurrent
spectra of Ru-coated gold grating nanostructure. (d) Photoreflectance spectra with and without
ruthenium coating.
The dips in photo-reflection correspond to the plasmon excitation and are relatively undisturbed
due to the presence of the surface coatings. The Ni-coated and Ru-coated gratings show plasmonresonant enhancement factors (i.e., p-to-s polarized photocurrent ratio) of 36X and 15X,
respectively at +1.4V.
The plasmonic field enhancement of bare Au gratings diminish in the presence of even a very thin
layer (1nm) of Pt, Ni, or Ru surface coating as evident from the electromagnetic simulations
(Figure 3.7). Nevertheless, the Au gratings with these catalytic metal coatings show enhanced
38
photocurrent compared to the bare grating. The plasmonic nearfield is strongly confined to a very
small mode volume around the Au grating. Here, the photochemical reactions occur mostly by the
transfer of hot carriers to the molecular adsorbates at the interfacial barrier between metal and
adsorbate. During photocatalysis, when molecules interact with the metal surface, the narrower dband of the metal hybridizes the molecular orbital into metal-adsorbate bonding and anti-bonding
modes. In the case of bare Au gratings, the anti-bonding mode can be below the fermi level due to
the deep-lying d band of the Au. As a result, the molecule is less likely to adsorb onto the Au
surface thereby limiting the transfer of plasmon-generated hot electrons. On the other hand, when
catalytic metals such as Pt, and Ru interact with adsorbates, generally only the bonding states are
below the Fermi level, leading to chemisorption.[54] The increased number density of hybridized
surface states between metal and adsorbates increases the dephasing channels for the plasmons,
thus, overcoming the reduction in hot electron generation rate.
Figure 3.14. (a) Electrochemical potential dependence of the AC photocurrent measured on
resonance with a gold grating structure (a) bare, (b) Pt coating, (c) Ni coating, and (d) Ru coating
39
The sensitivity of plasmon-mediated photocatalysis to the surface-bound species makes it an
attractive tool for studying metal-molecular electrochemical interactions. Figure 3.14 shows the
electrochemical potential dependence of the AC photocurrent produced by illuminating the bare
Au gratings and catalyst-coated gratings on resonance (i.e., p-polarized light at approximately
�=2.3°) from -0.8 to +1.2 V versus NHE. Figure 3.14a shows the potential dependent photocurrent
produced with a bare Au grating. The photocurrent increases monotonically as the potential is
reduced down to -0.4 V and then saturates. On the oxidation side, the photocurrent is relatively
low compared to the reduction side. Overall, the potential dependent photocurrent in the bare Au
grating is relatively featureless reflecting the poor catalytic properties of the Au surface. In Figure
3.14b, however, we observe a clear resonant feature centered around -0.3 V versus NHE for the
Pt-coated gratings. This corresponds to the conditions under which the energy of the hot electrons
photoexcited in the gold passes through a resonance with the prevailing/dominant redox potential
in solution and/or reactant species bound to the catalyst surface. This resonant feature is observed
repeatably over many different cycles (i.e., potential sweeps) and different samples. The fact that
there is no corresponding peak observed under oxidizing potentials reflects the reducing nature of
the platinum catalyst surface, which favors HER over OER.[5, 63, 64] In addition, the peak
photocurrent measured on the Pt-coated grating is roughly 2.3X higher than that of the bare Au
grating. The nickel-coated grating (Figure 3.14c) shows an increase in the hot-electron
photocurrent compared to bare Au grating both above +0.8 V and below -0.6 V vs NHE indicating
slight catalytic enhancement in both oxidation and reduction half-reactions. Similarly, the Rucoated grating (Figure 3.14d) shows an increase in hot-electron photocurrent above +0.8 V and
below 0 V vs. NHE. Compared to bare Au grating, the photocurrent increases rapidly with potential
on both the reduction and oxidation sides for the Ru-coated grating. On the reduction side, the rate
of increase of photocurrent for the Ru-coated grating is similar to that of the Pt-coated grating until
the potential reaches -0.4 V. Below this potential, the photocurrent for the Ru-coated grating rises
steadily. These data show that the hot electron-driven photocurrent is quite sensitive to the catalyst
surface composition and the electrochemical potential, both of which affect the surface-bound
intermediates, which thereby increase/decrease the charge transfer rates of these short-lived hot
electrons.
This photoelectrochemical approach provides energetic spectroscopy of surface-bound
intermediate species. While there are many functional groups on the photoelectrode surface such
40
as OH, OOH, and H, for large negative potentials, we expect the dominant reduction half-reaction
to be HER. For high positive potentials, we expect the dominant half-reaction to be OER. In the
intermediate range, it is unclear which surface intermediates are dominating these AC photocurrent
spectra. That is, for potentials below -0.4V vs NHE, we believe the AC photocurrent is largely due
to surface-bound H+ ions, and for potentials above +0.8 vs NHE (for Ru and Ni), the AC
photocurrent is largely due to surface-bound OH- ions. Nevertheless, we observe characteristically
distinct voltage sweeps (i.e., spectra) for different catalytic surfaces (i.e., Pt, Ru, Au, and Ni). For
example, the peak-like nature of the features observed on Pt and Ru between -0.3 and -0.4 could
be due to the electrochemical desorption of surface species. That is, sweeping the potential through
the resonant energy results in an increase and then a drop in the AC photocurrent, as these species
are depleted. Future studies will be needed in order to unambiguously identify these surface species
possibly by varying the electrolyte composition and/or scan rate.
3.7 Conclusion
In conclusion, we have demonstrated a differential AC spectroscopy approach that provides
a very sensitive way to measure surface species/catalyst surface interactions as a function of
electrode potential. We observe that the ultra-thin catalyst coatings (~1 nm thick) do not affect the
plasmon-enhanced hot electron generation in Au gratings. These catalytic coatings enhance the
charge transfer rates associated with these short-lived hot electrons involved in the hydrogen
evolution and oxygen evolution reactions. Due to the plasmon resonance excitation of hot
electrons, a 64X enhancement in the ratio of p-to-s polarization ratio is observed for the Pt-coated
grating, and a 28X enhancement is observed for the bare Au grating. Ni-coated and Ru-coated
gratings show an enhancement in the plasmon-resonant photocurrent under both oxidative and
reducing potentials compared to the bare Au grating. The catalytic surface coatings significantly
alter the interaction of adsorbate molecules with the grating surface, and the potential-dependent
photocurrent traces show distinctive behavior corresponding to the electronic interaction of the
molecule/metal complex at this interface.
41
CHAPTER 4. VOLTAGE-INDUCED MODULATION OF INTERFACIAL
IONIC LIQUIDS MEASURED USING SURFACE PLASMON RESONANT
GRATING NANOSTRUCTURES
This chapter is similar to Aravind, Indu et al. published in the Journal of Chemical Physics.[65]
4.1 Introduction
SPR sensors function as thin-film refractometers that detect changes in the refractive index
at the surface of a metal film, where a surface plasmon is supported. When light excites the surface
plasmon, it travels along the metal film, and its evanescent field interacts with the medium in
contact with the film. Any change in the refractive index of the medium affects the surface
plasmon's propagation constant, which in turn alters the characteristics of the light wave coupled
to the surface plasmon, such asthe coupling angle, wavelength, intensity, or phase. Based on which
characteristic of the light wave is measured, SPR sensors can be classified into types that focus on
angular, wavelength, intensity, or phase modulation.[12]
In SPR sensors with angular modulation, a monochromatic light wave excites the SPR. The
coupling strength between the incident wave and the SPR is monitored across multiple incidence
angles, observed as a dip in the angular spectrum of reflected light. The angle with the strongest
coupling is measured and used as the sensor output as shown in Figure 4.1b. A collimated light
wave with multiple wavelengths, such as a beam of polychromatic light is used in wavelengthmodulated SPR sensors to excite the surface plasmon. The excitation is observed as a dip in the
wavelength spectrum of reflected light. The wavelength with the strongest coupling is measured
as the sensor output as shown in Figure 4.1c. In SPR sensors with intensity modulation, the
intensity of the light wave serves as the sensor output. Here, the strength of the coupling between
the light wave and the surface plasmon is measured at a single incidence angle and wavelength.[66-
68] In this chapter, we are studying the ionic liquid metal electrode interface using the angular
modulation of SPR. Our metallic grating system is used for this study.
42
Figure 4.1. (a) Plasmon dispersion change with perturbation in the medium index. Shift in
resonance in (b) wavelength, and (c) angular domains with surrounding medium index change.
[11]
Grating couplers are not as commonly used in SPR sensors as prism couplers. However, their
suitability for mass production, makes them an appealing option for creating low-cost SPR sensing
structures, especially through replication of these into plastic structures.[12]
Figure 4.2. Schematic illustration of SPR sensor based on simultaneous excitation of surface
plasmons by a polychromatic light and the dispersion of light on a special grating coupler[12]
In SPR sensors with angular modulation, the sensor output is the coupling angle θr, and therefore
the sensor sensitivity S for a grating coupled system is equal to,
� = ��<
�� (4.1)
(a) (b) (c)
43
4.2 Ionic liquid Pockels Effect
Recently, there has been considerable speculation surrounding the behavior of room
temperature ionic liquids at charged electrochemical (i.e., electrode) interfaces,[69-75] however,
the structure and spatial profile of such interfacial charge remains largely elusive. The behavior of
electrolytes at electrode interfaces is of great interest for a variety of chemical and biological
processes ranging from electrocatalysis to bio-membranes. Many research groups are studying
these interfaces using various techniques including atomic force microscopy,[69, 70] sum
frequency generation (SFG) spectroscopy,[76, 77] and anisotropic fluorescence depth profiling
measurements.[74] Wang et al. reported a large Pockels effect in room-temperature ionic liquids
with a charge-induced change in the index of refraction of Δn = 0.3.[73] However, the mechanism
underlying these strong electro-optic effects is still a topic of some debate and may arise from a
charge-induced long-range ordering of the ionic liquid that extends over length scales of 100 µm,
as observed by the anisotropic fluorescence depth profiling measurements of Ma et al.[74] Krishna
et al. reported charge profiling using three-dimensional (3D) atomic force microscopy (CP-3DAFM) to experimentally quantify the real-space charge distribution of the electrode surface and
electric double layers (EDLs) with angstrom depth resolution and found that these charged
interfacial layers extend over approximately 1 nm.[69, 70] In, yet, another study, Toda et al.
reported observation of the Pockels effect in ionic liquids through voltage-induced changes in the
Raman spectra.[78, 79] From this, they concluded that the length scale of the ordering in the ILs
is 10 – 100 nm, extending more than would be expected for the electrical double layer but not as
far as the μm scale reported by Ma et al.[74] As such, this 5 order of magnitude discrepancy (i.e.,
100 µm vs. 1 nm) has created somewhat of a controversy within the field. Several theory groups
have explored the interfacial structure of metal/ionic liquid (IL) systems using ab initio molecular
dynamics simulations, however, a large gap currently exists between these simulations and
experimental measurements, and improved techniques for exploring the structure of these
interfacial layers are much needed.[80-87] The importance of further exploring the electric field
effects on ILs to unravel the intricacies of interfacial structures and their control, pivotal for
advancing their functionalities.
Surface plasmon resonant gratings have been used to detect small changes in the index of refraction
for many years.[88-91] This phenomenon has largely been used for detecting the presence of
biomolecules and biomarkers through surface functionalization, enabling applications in non-
44
invasive cancer diagnosis and drug discovery.[92] Several research groups have explored the
Pockels effect using low-angle optical measurements.[78, 93, 94] The SPR approach is
advantageous because it selectively probes a small volume, within just a few nm of the electrode
surface, without being influenced by the dielectric constant of the bulk solution. SPR sensors have
been previously used to study the time relaxation associated with the IL interfacial layers.[95, 96]
However, combining the experimental observations with electromagnetic simulations to explore
the structure of the electric double layer and Pockels effect at electrochemical/electrode interfaces
in ionic liquids is still underexplored. Here, we use the sensitivity of surface plasmon sensors to
interfacial index changes to explore voltage-induced changes, explore potential ordering at
interfaces, and compare IL, water, and dielectric solvents. This method enables us to investigate
the dynamics linked to the ordering and rearrangement of the double layer.
4.3 Results and Discussion
In this work, we have used metallic grating structures that support surface plasmon
resonance in the visible light spectrum to monitor changes in the refractive index at the interface
between the ionic liquid (IL) and the electrode, due to the application of an electric potential. The
complex propagation constant of surface plasmons is sensitive to the refractive index profile of the
surrounding medium. When an electric field is applied across the IL, it alters the refractive index
profile, impacting the conditions required for surface plasmon excitation. By observing alterations
in the resonant excitation conditions (such as wavelength, angle, and polarization) of the optical
wave interacting with the surface plasmon, we can infer changes in the medium's refractive index.
We measure the strength of coupling between monochromatic light and the grating structure across
various angles of incidence and observe the change in the incidence angle yielding the strongest
coupling with surrounding index profile perturbation. To excite the surface plasmons in the grating
structure, the momentum and energy of the incident photons must match that of the surface
plasmons (i.e., wavevector matching). The incident angles that satisfy this condition at each
wavelength can be found by solving the following equation:[13, 14, 16]
2��"
� sin� + �
2�
� = ± mRe o
�
� p �"�!
�" + �!
q
N
-
rs (4.2)
where � is the periodicity of the grating, � is the order of the surface plasmon mode, � is the
incidence angle, �! and �" are the dielectric functions of the metal and surrounding medium.[13]
45
Figure 4.3 shows the first-order surface plasmon mode in our corrugated gold grating with a 500
nm period obtained analytically using Equation 4.2. Figure 4.3a shows the shift in resonance when
the real part of the refractive index of the surrounding medium changes by 0.05 RIU (Refractive
Index Unit) from a nominal medium index of n=1.4 and k=0. The resonant shift depends on the
wavelength of the surface plasmon excitation and the local slope of the band diagram at the
wavelength. Figure 4.3b shows the resonance angle at a wavelength of 785 nm plotted as a function
of the change in the real part of the refractive index of the surrounding medium (Dn). The slope of
this curve represents the sensitivity of our measurement technique with this grating design and is
estimated to be � = "O
"P
= 50 degrees/RIU. The change in the imaginary part of the medium index
(k) has a minimal effect on the resonance shift, as evident from Figures 4.3c and 4.3d. For a Δk of
0.1, the resonant angle shift is negligible.
Figure 4.3. (a) First-order plasmon resonant mode and (b) shift in plasmon resonant angle at 785
nm plotted as a function of the index of refraction of the surrounding media for a 500 nm
corrugated Au grating (k=0). (c) Change in band diagram, and (d) shift in plasmon resonant angle
at 785 nm as a function of the imaginary part of surrounding media index for a 500 nm corrugated
Au grating (n=1.4).
(a) (b)
(c) (d)
��
�� � = 50
46
4.3.1 Experimental Section
For our electro-optic measurements, we employed a microfluidic cell comprising an ionic
liquid encapsulated between a gold (Au) grating and a gold-coated calcium fluoride (CaF2)
window with an optically transparent aperture in the center. Illustrated in Figure 4.4a, the
schematic outlines the design of the sample cell, where a ~1mm thick polydimethylsiloxane
(PDMS) stamp with a rectangular opening secures the ionic liquid sandwiched in place between
the Au grating and the CaF2 window. We have used the ionic liquid [DEME+][TFSI-
]
(diethylmethyl(2-methoxyethyl)ammonium bis-(trifluromethylsulfonyl)imide) for this
experiment. An electric potential is applied across the IL using electrodes attached to the grating
and the gold-coated CaF2 window. Figure 4.4b shows the cross-sectional SEM image of the 500
nm pitch Au grating structure used in this experiment. These gratings are fabricated by depositing
100 nm Au on a corrugated Si structure, patterned by photolithography and reactive ion etching
(RIE), as reported previously.[16, 17, 97] We sweep through the plasmon resonance by varying
the incident angle of monochromatic light at a wavelength of 785 nm. Figure 4.4c shows the
schematic demonstrates our setup for reflection measurements, where the polarization of the
incoming light is oriented perpendicular to the grating lines (p-polarization) by means of a halfwave plate. The sample cell is attached to a motorized rotational stage and the reflection from the
grating is measured at different incident angles using a silicon photodetector (Thorlabs S121C).
We observe sharp dips in the photoreflectance around ±4.6o
, corresponding to the conditions under
which there is wavevector matching between the incident light and the spacing in the grating.[14,
16, 44, 73, 97, 98]
47
Figure 4.4. (a) Schematic diagram of the electrochemical cell used for the electro-optic
measurement. (b) Cross-sectional SEM image of the 500 nm pitch corrugated Au grating structure.
(c) Schematic diagram of the angle-dependent photo-reflection measurement setup.
Figure 4.5 illustrates the variation in reflected power as the incident angle is swept from -
10° to 10° under applied voltages of 0V and 3V. A notable shift in the plasmon resonance towards
smaller angles by Δϕ=0.35° is observed upon application of the electric potential. In addition to
the angle shift, the peak reflectivity reduces with the applied potential as well. Under an applied
potential of 3V, a reflection change of 18% is observed ((18.27mW-14.94mW)/18.27mW=18.2%).
This resonance shift, which vanishes upon removal of the applied potential, is consistently
repeatable across several experiments, as evidenced in Figure 4.5. When a uniform perturbation of
(b)
(c)
(a)
48
the refractive index permeates the entire ionic liquid (IL) medium, resulting in a bulk refractive
index change, this shift in the resonant angle equates to an effective refractive index alteration in
the IL of Δneff = 0.35/S = 0.007, where S denotes the sensitivity determined in Figure 4.3b. The
effective index change agrees well with the electro-optic index changes in ILs previously reported
in the literature.[74, 79]
Figure 4.5. (a) Angle-dependent reflection for different voltages applied across the ionic liquid
electrolyte. (b) Zoomed-in plot of the reflection profile in the range (a) -5.4o and -3.2o
Alterations in the refractive index of ionic liquids with an applied electric field can be
primarily ascribed to the formation of an electrical double layer (EDL) at the interface, a
consequence of the electro-migration of cationic and anionic species. Previous studies have shown
that cations and anions are alternately stacked to form an interfacial layer, with its structure
influenced by the sign of the surface potential, as well as the disparities in size, shape, and surface
interactions among the ionic species.[99] The molecular ordering induced in the ionic liquid near
(a)
(b)
49
the electrode interface breaks the inversion symmetry, enabling the manifestation of the Pockels
effect within the interfacial domain, which subsequently alters the refractive index as detected in
experimental outcomes. Traditional narratives that suggest a uniform medium index change
overlook the critical aspect of electric field-induced molecular ordering adjacent to the electrode
interface. It has been postulated that a conformal interfacial layer, extending a finite distance (h)
from the electrode surface, facilitates the analysis of the well-defined electrode-ionic liquid (IL)
interface through the application of the ray optics model.[79] Although these models do not fully
encapsulate the role of an intermediate layer, which acts as a transitional zone between the highly
organized interfacial layer and the bulk liquid, they provide a framework for delineating the
structural transformations occurring during molecular assembly.
4.3.2 Sensing Depth vs Sensitivity Tradeoff
The ability of the surface plasmon sensor to detect changes in the EDL thickness (h)
depends on the penetration depth (LPD) of the surface plasmon field into the ionic liquid. A change
in the surface plasmon wave vector induced by a surface refractive index change occurring within
a layer with a thickness h can be expressed as:
δ� = δ�?Q9R(1 − �E-S*+T) (4.3)
where δ�?Q9R is the plasmon wave vector change with the index perturbation of the bulk medium.
The grating equation including the surface plasmon perturbation relates the resonance shift as a
function of EDL thickness for different penetration depths.
2��"
� sin� + �
2�
� = ± mRe o
�
� p �"�!
�" + �!
q
N
-
+
�
�
δ�?Q9R(1 − �E-S*+T)rs (4.4)
where � is the periodicity of the grating, � is the order of the surface plasmon mode, � is the
incidence angle, �! and �" are the dielectric functions of the metal and surrounding medium, nd is
the index of IL before the perturbation.[13] We can plot the resonance shift and change in
sensitivity of the resonance as a function of EDL thickness for different LPD. A δ�?Q9Rvalues of
0.0085 is used to obtain the measured resonance shift of -0.35 degrees as observed in our system.
From Figure 4.6a, it is evident that the larger the LPD, the sensor is sensitive to thicker EDLs.
However, this comes with a compromise in sensitivity as shown in Figure 4.6b. For slight changes
50
in the EDL thickness variation within LPD, the system is more sensitive when the field decays at a
faster rate away from the surface.
Figure 4.6. (a) Resonance shift and (b) resonance sensitivity with EDL thickness for different
surface plasmon penetration depth.
4.3.3 Numerical Model
In our system, we have modeled the EDL as a thin conformal layer above the grating
corrugation with a characteristic thickness of h and dielectric constant of �" + ��" (or index of
n+Δn). The perturbation in surface index change affects the surface plasmon wave vector and
thereby the conditions of excitation required to excite the resonance. For a thickness much larger
than the penetration depth of the surface plasmon, the change in the surface plasmon wave vector
converges to the same as in the case of bulk refractive index change.[11] By employing
computational electromagnetic tools to simulate the Pockels effect in ionic liquids using a thin
conformal layer characterized by variable refractive indices and layer thicknesses, a more profound
understanding of the interfacial layer's characteristics can be achieved. This approach enables a
comparative analysis between theoretical predictions and experimental findings, thereby yielding
enhanced insights into the interfacial layer's dynamics.
A commercial Finite Difference Time Domain (FDTD) solver (Ansys Lumerical FDTD
solutions) is used to solve Maxwell’s equations discretely in time and space for the grating
structure. The geometry of the corrugated grating surface is fitted to a measured AFM (Atomic
Force Microscope) cross-sectional profile using by the following equation:
�(�) = � d 1
1 + �EU(3EW)/; +
1
1 + �U(37W)/;e (4.5)
a) b)
51
where a is the periodicity of the grating (a = 500nm), � is the height difference between the peaks
and valleys of the corrugation (corrugation amplitude) and � specifies the slope of the corrugation,
and w determines the length of the flat top of the grating. By fitting the surface geometry with
Equation 4.5, the fit parameters are obtained as A = 54 nm, w = 0.25a, �=25. Using the momentum
matching condition (Equation 4.4), and Snell’s law, the refractive index of the bulk IL is estimated
to be equal to 1.41 to satisfy the resonant angle(�o=4.6o
) at 785nm illumination.
Figure 4.7. Resonance shift with (a) thickness of EDL layer for constant local index change (b)
perturbed index of EDL layer with constant thickness.
We have simulated the reflection from the grating structure as a function of the angle of
incidence in the presence of an EDL. A 785nm plane wave source is used to illuminate the grating
structure at an oblique angle. The reflection from the grating is monitored using a power monitor
placed behind the source as a function of the angle of incidence. Figure 4.7a shows the simulated
angular reflection of the grating as a function of the EDL thickness (h) for a fixed index change
(Δnlocal) of the interfacial EDL, and 4.7b shows the same as a function of the EDL index
perturbation for a fixed thickness of the EDL. The reflection from the grating as a function of the
angle reaches a minimum value when the maximum coupling condition is satisfied (corresponds
to the resonant excitation condition). For positive index perturbation, the resonance shifts to lower
angles with the increase in both Δnlocal and h. The change in the index also affects the resonant
reflection from the structure. The reflection initially goes down with the increase in Δnlocal and then
goes up. The change in reflection is mainly from the change in the index of EDL and is in alignment
with the expectation from Fresnel’s equations. The sensitivity of the resonance shift to both the
thickness and index of EDL can be explained by the nearfield perturbation.
(a) (b)
52
Figure 4.8. (a) Interface of Au grating and homogeneous ionic liquid medium. (b) Electric field
intensity distribution and (c) field decay away from the metal for grating with homogenous ionic
liquid medium. (d) Interface of Au grating and ionic liquid medium with a conformal EDL. (e)
Electric field intensity distribution and (f) field decay away from the metal for grating with EDL
interface.
Figure 4.8 shows the resonant field profile simulated using FDTD with and without an interfacial
EDL layer. When the medium is a homogenous layer of IL as shown in Figure 4.8a, the plasmonic
field decays exponentially away from the Au surface. The field is concentrated at the peak-totrough transition region of the grating. The penetration depth (Lpd) of the surface plasmon
excitation into the ionic liquid medium is estimated to be 84 nm, as shown in Figure 4.8c. The
resonant field profile with an interfacial EDL layer of 10 nm thickness and 0.1 RIU index change
is plotted in Figure 4.8e. The cross-sectional field profile in Figure 4.8f clearly shows the
perturbation of a smoothly decaying field due to the introduction of the EDL layer. If the thickness
of the EDL is smaller than the surface plasmon penetration depth, it can perturb the surface
plasmon field and, consequently, affect the plasmon wave vector. The sensitivity of the surface
plasmons to the EDL thickness goes down with an increase in thickness. Beyond the penetration
depth, the plasmonic system cannot distinguish the EDL from a bulk index change.
The h and perturbed refractive Dn of the interfacial layer is varied and the corresponding
change in resonant excitation is simulated using FDTD. Figures 4.9a and 4.9b illustrate the
simulated shifts in resonant angle (Δϕ) and variations in resonant reflectivity (ΔR/R), respectively,
contingent upon h and Δnlocal. The white contour in Figure 6a represents the required refractive
53
index change and corresponding thickness necessary to induce a resonance shift of a Δϕ = -0.35o
,
as observed in measurements. This contour line establishes an empirical correlation between the
thickness and refractive index adjustment of the interfacial layer, enabling the estimation of the
EDL layer's thickness based on the refractive index change of the conformal layer, and vice versa.
In the bulk phase, the ionic liquid exhibits a random assortment of ionic species. However,
applying an electrical bias close to the interface encourages greater organization of ions, thereby
amplifying the refractive index alteration of the conformal layer. Remarkably, to achieve a given
resonant angle shift, the requisite thickness (h) increases exponentially with minor perturbations
in the index.
When the EDL forms near the grating with a local index perturbation, the reflectivity of the
structure changes compared to the bulk medium. The reflection change is highly sensitive to the
local index change, but relatively independent of the EDL thickness. When the EDL forms near
the grating with a local index perturbation, the reflectivity of the structure changes compared to
the bulk medium. The reflection change is highly sensitive to the local index change, but relatively
independent of the EDL thickness. The reflectivity changes remain almost constant as h increases.
The simulations have revealed that the reflection from the structure decreases for small local index
changes (small Δnlocal) and increases for sufficiently large local index changes (large Δnlocal). The
maximum reflectivity drop observed in the simulation with a conformal index perturbation is
around 11%, which is lower than what is observed in the measurements. This discrepancy could
be due to the abrupt boundary and uniform layer thickness assumption of the double layer in
simulation. From Figures 4.9a and 4.9b, we obtain an h that produces a resonant angle shift of Δ�
= -0.35o and a reflectivity change of ΔR/R = 11%. The h of 8 nm indicated by the red star in Figure
4.9b represents the approximate thickness derived from the simulations that correspond to the
changes in refractive index from the experiment. Figure 4.9c shows the shifts in the simulated
reflection spectra of the SPR grating structure using an EDL thickness of 8nm and a local change
in refractive index of Δnlocal = 0.1 with respect to the bulk IL medium without any EDL layer. The
measured response with and without an applied potential is shown in Figure 4.9d and qualitatively
agrees with the simulations.
54
Figure 4.9. (a) Simulated electric field profile calculated on resonance. (b) Plasmonic electric field
intensity as it decays away from the metal surface. (c) Plasmon resonance shift as a function of the
change in refractive index and thickness of the ionic liquid double layer. The contour tracks the
combination of index change and thickness required to satisfy the experimentally observed shift
of -0.35o with applied potential. (b) Change in reflection from the grating as the double layer forms
as a function of the change in refractive index and thickness of the ionic liquid double layer. The
contours represent the combination of index change and thickness required to obtain a resonance
shift of Df = -0.35o resonance shift and a 10% decrease in reflection. The red star represents the
intersection point. (c) Simulated and (d) measured reflection as a function of incident angle to the
grating with and without ionic liquid double layer
In order to ensure that the origin of the electro-optic shift is indeed due to the Pockels
effect, not from a spurious correlation in the measurement system such as an indirect thermo-optic
shift of the grating or electro-optic effect from the optical windows used, we have changed the
solution within the measurement cell and repeated the experiment under the same conditions. We
have used a polar solvent (D2O) with a reasonably high Pockels coefficient and a non-polar solvent
(benzene) with a weak Pockels coefficient and compared it against the IL. The change in reflection
Δ� = -0.350
11% 18%
0.36O 0.35O
wo DL
with DL
0 V
3 V
(a) (b)
(c) (d)
ΔR/R = -10%
Δ� = -0.350
ΔR/R = -10%
55
with applied bias is measured at the respective plasmon resonant angle of each medium. Figure
4.10 shows the percentage change in reflection observed when the potential is periodically
modulated between -3V and +3V in D2O, benzene, and the ionic liquid. For the case of the ionic
liquid, a voltage-induced modulation of 20% in reflection is observed for the ionic liquid. A similar
voltage-induced modulation magnitude is observed for D2O, as plotted in Figure 4.8b. However,
the time constant for this index modulation is considerably longer for the polar solvent (D2O)
compared to the ionic liquid. Moreover, it takes several cycles to reach the full modulation depth
in the case of D2O compared to the IL. In contrast to these measurements, no modulation effect is
observed in the non-polar solvent benzene, which has a negligible Pockels effect.
56
Figure 4.10. Voltage-induced changes in reflectance measured on resonance in (a) ionic liquid,
(b) D2O, and (c) non-polar benzene control sample
(a)
(b)
(c)
57
The plasmonic sensor offers the opportunity to probe into the dynamics of the EDL
formation. It was also observed that the ionic liquid exhibits a strong asymmetric response with
the polarity of the applied bias. It can be inferred that the index shift in the interfacial layer is
asymmetric with the polarity of the immediate layer. In addition, it was observed that the time
constant of the optical response is much higher than that of the current response.[96, 100]
4.4 Conclusion
In conclusion, we have used surface plasmon resonant Au gratings to measure the refractive
index change due to the Pockels effect created at the interface of polar liquid and metal gratings
under an applied electrochemical voltage. We observe a voltage-induced change of Δϕ=0.35o in
the plasmon resonant angle at an excitation wavelength of 785 nm, which corresponds to an
effective index change of Δ� = 0.007, which is of the same order reported in previous studies.[79,
94] The electrostatic accumulation of ions from IL induces local index changes to the gratings over
the extent of the electrical double layer (EDL). Finite difference time domain (FDTD) simulations
are used to relate the observed shifts in the plasmon resonance and change in reflection to the
change in the local index of refraction of the electrolyte. The wavelength and intensity shift of the
resonance enables us to determine the change in index Δn of the double layer. We believe that this
technique can be used more broadly, allowing the dynamics associated with the potential-induced
ordering and rearrangement of ionic species in electrode-solution interfaces to be measured,
provided that the EDL is within the surface plasmon penetration depth.
58
CHAPTER 5. FUTURE DIRECTIONS
5.1 Future Directions for Plasmonic Photocatalysis
Hybrid catalytic systems hold great promise for incorporating electrochemical functionality
in plasmonic metals, thereby expanding the range of reactions that can be accessed by
photocatalytic systems. The geometry and structure of the hybrid catalyst play a major role in
determining the photo response and overall catalytic behavior. A more quantitative understanding
of these systems is necessary moving forward. While light absorption can be high, the subsequent
conversion of photons into reaction products depends on the primary mechanisms driving a
plasmon photocatalytic reaction. These mechanisms include direct hot electron transfer, resonant
energy transfer to the adsorbed molecular orbitals, increased vibrational energy, or heating. Among
these, some energy transfer steps are known to be less efficient compared to others. For instance,
the extraction and direct transfer of hot carriers are inefficient due to both the small fraction of
high-energy hot carriers and their shorter lifetimes compared to chemical reactions (tens of
femtoseconds versus milliseconds).[54] Increasing the density of adsorbed reactants readily
available to receive the short-lived carriers can result in boosting the indirect charge transfer
efficiencies. From the experimental side, ultrafast techniques such as transient absorption
spectroscopy, ultrafast electron diffraction, and ultrafast Raman spectroscopy show great promise
in differentiating and quantifying the effects of hot carriers versus photothermal heating.
To understand hot carrier dynamics, one can use ultrafast spectroscopy to track hot carrier
generation and their lifetimes. It has been demonstrated using transient absorption spectroscopy
that the hot carrier decay pathways in Au@Pd nanorods were affected by the thickness of the
conformal shell.[48] In our group, we have previously measured the photoexcited carrier dynamics
of hot electrons using transient absorption (TA) spectroscopy.[101] Hot electrons generated in
plasmonic gratings were found to decay back to equilibrium over a timescale of 2-3psec, as shown
in Figure 5.1a.
59
Figure 5.1. a) TA measurements for Au grating with 266 nm metal line width and 500 nm period
in air environment with the p-polarized probe pulse. Selected broadband TA spectra of different
delay times between the pump and probe. b) Relative shift of the plasmon resonant bleaching peaks
in TA spectra as delay time increases[101]
In addition to the rapid decay of these photoexcited hot electrons, we observe a blue/red shift of
the plasmon-resonant feature with time depending on whether the grating is in an aqueous solution
(red) or in the air (black) (see Figure 5.1b). In air, this time-dependent blueshift reflects the cooling
of the hot electrons back into equilibrium with the lattice temperature. In aqueous electrolytes, we
believe the red-shift with time arises from charge transfer between the metal surface and
adsorbates’ lowest unoccupied molecular orbital (LUMO) states decreases with time, increasing
the effective dielectric constant of the water environment near the metal grating surface.[44, 98]
One other important observation was that electron−phonon interaction time (τe-p) decreases from
0.94 to 0.67ps with adsorbates due to additional energy dissipation channels. While plasmons
dephase, the adsorbate states are directly populated through the process of chemical interface
damping (CID). More available adsorbate channels for plasmon energy dissipation could lead to a
smaller temperature difference between electron and phonon subsystems after internal
thermalization and lead to lower τe-p.
In our Hybrid photocatalytic system, we postulate that the improved performance comes from the
enhanced adsorption and subsequent charge transfer due to the catalytic coatings. There are two
main reasons to believe so. Primarily, the field enhancement in Bare gratings goes down with
catalytic metal coating, yet the photocurrent increases. The hot electron generation rate is
proportional to the field enhancement. Even with the reduced generation of hot carriers, an
increased photocatalytic activity indicates the efficient charge transfer mechanism in a hybrid
catalytic system. Secondly, the photocurrent from Pt coatings is much less noisy than the Bare Au
60
grating. This also indicates faster kinetics associated with the charge transfer. However, a
quantification of the charge transfer dynamics in these systems is yet to be explored.
Figure 5.2. Schematic illustration of adsorbate density in (a)bare, and (b) catalytic metal-coated
grating
The photocurrent measurements only tell us the net effect of carrier generation and transfer at the
equilibrium. In our hybrid catalytic system, we are planning to incorporate the hot carrier lifetime
study using Transient absorption spectroscopy to further understand the temporal dynamics of the
hot electron generation and transfer. We can compare the surface plasmon decay of bare and
catalytic metal-coated gratings in an electrochemical environment. Figure 5.3 shows the
preliminary result of the transient absorption spectra of bare and Pt deposited grating both in air
and water medium. The top row shows the change in absorption and the bottom row shows the
UV-VIS spectrum of the corresponding grating/medium. The interband absorption appears as
medium invariant and as a positive change in absorption at 500nm for 400nm pump excitation.
Figure 5.3. Transient absorption spectrum (top row), and UV-VIS spectrum (bottom row) of (a)
Bare Au grating in Air, (b) Hybrid grating in Air, (c) Bare Au grating in solution, and (d) Hybrid
grating in solution.
Bare Grating Hybrid Grating (a) (b)
450 500 550 600 650 700 750
64
66
68
70
72
(100-T) (%)
Wavelength (nm)
450 500 550 600 650 700 750 65
66
67
68
69
70
(100-T) (%)
Wavelength (nm)
534nm
500nm
450 500 550 600 650 700 750 42
44
46
48
50
52
54
(100-T) (%)
Wavelength (nm)
670nm
500nm
450 500 550 600 650 700 750 54
56
58
60
62
100-T (%)
Wavelength (nm)
670nm
500nm
Au grating in air Au grating in solution Au/Pt grating in solution
534nm
500nm
Au/Pt grating in air (a) (b) (c) (d)
(e) (f) (g) (h)
61
The surface plasmon absorption change appears as a negative change in absorption. For air
medium, the bare and Pt deposited grating shows almost similar transient absorption spectra
whereas in solution the plasmon peak is attenuated faster than the measurement resolution limit in
our Pt/coated grating samples. This qualitatively indicates the fast decay of surface plasmons due
to an increased number of adsorbates. However, extracting the decay time constant has been
difficult in the initial trials. The resolution of the measurement system needs to be improved.
Figure 5.4 shows the cross-section of transient spectra at 0.2ps
Figure 5.4. Cross section of the transient absorption spectrum of (a) Bare Au grating in Air, (b)
Hybrid grating in Air, (c) Bare Au grating in solution, and (d) Hybrid grating in solution
Further studies are required to resolve the fast decay of Pt-coated gratings and compare the decay
time constants to establish the quantification of charge transfer dynamics in these systems. It would
be interesting to study the charge transfer dynamics as a function of applied potential across the
electrode solution interface. The photocurrent spectra as a function of applied potential are
different for Bare Au gratings and different catalytic material coated gratings. It would be
interesting to see whether we can observe a similar behavior in the plasmon decay time constant
as a function of applied bias.
5.2 Future Directions for Electro-Optic Modulation Study of Ionic Liquids
5.2.1 Resonant Shift of IL with Polarity of Electrode Potential
We have observed an asymmetric resonance shift behavior when applying bias at the
electrode-IL interface. Specifically, we measured the reflection modulation from the interface
500 600 700
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
Au air 100uW
Au sln 100uW
Au/Pt air 100uW
Au/Pt sln 100uW
D
A
l(nm)
62
under both positive and negative applied bias conditions, as depicted in Figure 5.5. Notably, the
electro-optical shift exhibits asymmetry for equal and opposite biasing conditions. Consequently,
the corresponding modulation amplitude also displays this asymmetry. These findings suggest that
the index shift within the IL interfacial layer is influenced by the polarity of the adjacent layer. To
gain deeper insights, a systematic study investigating the structural constituents of the interfacial
layer and their impact on index perturbations would be intriguing.
Figure 5.5. Resonance shift with (a) 0V and +3V applied bias cycles, and (b) 0V and -3V applied
bias cycles. Zoomed-in view of the resonance dips is shown in the inset.
5.2.2 Temporal Dynamics of EDL with Positive and Negative Biases
We have modulated the applied bias across the IL solution from 0 to 3V and 0 to -3V. The
reflection from the measurement cell and the current response of the IL solution are measured
simultaneously. The IL exhibits an asymmetric response for positive and negative applied bias
voltages. The time constant of the optical response is much higher than that of the current response.
Both these characteristics (the asymmetric response with potential and the difference in time
constants associated with the optical response and electrical current) have been previously reported
for ionic liquids under similar conditions. [96]
63
Figure 5.6. The reflected optical power modulation from Au/IL interface with (a) 0 to 3V, and (b)
0 to -3V modulation. Electrical current response through IL solution with (c) 0 to 3V, and d) 0 to
-3V modulation.
5.2.3 EDL Formation with Temperature
Using our plasmon resonant grating structures, we could study the refractive index change
of the Ionic Liquid (IL) medium under electrical bias. One intriguing avenue for future exploration
is the IL’s electro-optic behavior as a function of temperature. To facilitate this investigation, we
made modifications to our measurement setup. Specifically, we mounted the sample cell inside a
Linkam stage with liquid nitrogen flow, effectively cooling down the samples. Anticipating a more
ordered formation of the Electric Double Layer (EDL) at cryogenic temperatures, we aim to
quantify the resulting dynamics. By examining refractive index variations at these temperatures
and understanding the timescale of changes, we can gain valuable insights into the EDL formation
and its temperature dependence. Additionally, we plan to extend our study to the electrode-water
interface at lower temperatures. Leveraging this technique, we hope to explore electro-freezing
phenomena by measuring angular shifts at different temperatures under applied potential.
64
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Abstract (if available)
Abstract
This dissertation investigates the applications of surface plasmon resonance toward driving chemical reactions and sensing interfacial changes. Light interaction with free electrons in metals is a complex phenomenon that depends on the properties of the metal, the nature of incident light, and the interaction conditions. Under appropriate excitation conditions, we can couple the modes of the electromagnetic wave with the modes of the surface plasma oscillation to create surface plasmons. One of the exciting properties of surface plasmons is that they can help concentrate light into narrow regions of space and create large electric field enhancement. The surface plasmon excitation and subsequent field concentration can be used to generate high-energy electrons and drive chemical reactions. The plasmon-induced hot carrier generation offers immense potential in renewable energy storage, bio, and chemical sensing, driving thermodynamically unfavorable reactions, etc. Yet the field is still in a nascent stage. In the first part of the dissertation, I am exploring the fundamentals of plasmonic excitation in metallic gratings and optimizing the structure of these gratings using computational electromagnetic tools for photocatalytic applications.
Plasmonic metals can boost optical absorption and brute-force chemical reactions. However, compared to their electrocatalyst counterparts, they offer poor performance in terms of weakening the molecular bonds by surface interactions. The electrocatalytic metals on the other hand have weak interaction with light but offer superior performance when it comes to surface interactions via chemisorption. Combining these two aspects is challenging due to the fundamental differences in the d-band of plasmonic and catalytic metals. We have developed a hybrid system that decouples and combines the optical and chemical activity of plasmonic and catalytic metals. The hybrid system offers the potential to improve the efficiencies of light-driven chemical reactions further.
The surface plasmon excitation is highly sensitive to the environmental conditions. The energy and momentum required to excite the resonance in a plasmonic system varies with any perturbation in the dielectric media. If we can track the changes in resonance, it would be an indirect measure of the dielectric environment changes. We use this sensitivity property of surface plasmons to study an important class of organic compounds called Ionic Liquids. Ionic liquids are highly valuable in applications ranging from energy generation and storage to electrocatalysis and show rich dynamics in the presence of an external electric field. We have studied the field-induced interface changes in Ionic liquids using plasmonic gratings, which act as the electrode to bias the ionic liquid solution and sense the optical changes. With the help of electromagnetic simulation models, we are attempting to deduce the thickness of the ordered layer of ionic species forming at the electrode interface.
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Asset Metadata
Creator
Aravind, Indu
(author)
Core Title
Hot electron driven photocatalysis and plasmonic sensing using metallic gratings
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
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Physics
Degree Conferral Date
2024-08
Publication Date
07/26/2024
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04/30/2024
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electrocatalyst,hydrogen evolution reaction,ionic liquid,metallic gratings,OAI-PMH Harvest,photocatalysis,photocatalyst,Pockels Effect,surface plasmon resonance
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), Nakano, Aiichiro (
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), Ravichandran, Jayakanth (
committee member
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Tags
electrocatalyst
hydrogen evolution reaction
ionic liquid
metallic gratings
photocatalysis
photocatalyst
Pockels Effect
surface plasmon resonance