raman spectroscopy and z-scan based third order nonlinear ... · “measurement of third order...
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Raman Spectroscopy and Z-Scan
Based Third Order Nonlinear
Study of Graphene & Plasmonic
Metal Nanohybrids
A thesis submitted for the degree of
Doctor of Philosophy
by
Syed Salmaan Rashid
Centre for Micro-Photonics
Faculty of Science, Engineering and Technology
Swinburne University of Technology
Melbourne, Australia
2016
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Dedicated to my grandparents, parents, siblings, relatives and my wife.
This is the result of your prayers, unwavering support & belief you had in me.
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And we have not sent you [O Muhammad], except as a mercy to the worlds.
– Quran 21:107
Good and Evil deeds cannot be equal. Repel evil with what is better. Then you will
see that the one who was once your enemy has become your dearest friend.
– Quran 41:34
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Declaration
I, Syed Salmaan Rashid, declare that this thesis entitled :
“Raman Spectroscopy and Z-scan Based Third Order
Nonlinear Study of Graphene & Plasmonic Metal
Nanohybrids”
is my own work and has not been submitted previously, in whole or in part, in
respect of any other academic award.
Syed Salmaan Rashid
Centre for Micro-Photonics
Faculty of Science, Engineering and Technology
Swinburne University of Technology
Australia
Dated this day, July 04, 2016
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Abstract
Graphene is first ever exfoliated two-dimensional (2D) thin film consisting of single
layer of carbon atoms bonded together with in-plane sp2 hybridization bonds.
Discovery of graphene and its amazing properties revolutionized the field of
nanotechnology and opened the gates for miniaturizing related application to
atomic level. This led to the renewed interest in scientific community to extract
similar materials from its bulk three-dimensional (3D) structure. For this to
materialize, the layer between the materials needs to have weak van der Waals
forces and strong in-plane covalent or ionic bonding. Once separated these 2D
layers will have large surface areas and atomic layer thickness. Many 2D materials
similar to graphene have already been discovered. They are grouped under different
categories like layered metal oxides, layered double hydroxides, layered metal
chalcogenides, atomic layer metal films etc. This further led to hybridization of 2D
materials of different elements which was unseen and unheard of before. As a
result these hybridized materials became subjects of further investigation into their
intrinsic optical, electronic, mechanical, thermal, electrical, plasmonic properties etc.
This has led to revolutionizing applications in energy storage, fuel cells, electronics,
environmental, biomedical technologies etc. In this thesis I will investigate the
intrinsic properties of hybrid materials made from single layer graphene (SLG),
multi-layer graphene (MLG), silver nanoplates (AgNP), single crystalline gold
nanosheets (SC-AuNS) and polycrystalline thin gold films (PC-AuF). The
hybridized materials are AgNP-SLG, AuNS-SLG, (SC-AuNS)-MLG and (PC-
AuNS)-MLG. There are two critical aspects of this thesis, the first of which is to
study the extent of hybridization occurring in these materials using Raman
spectroscopy. The second purpose is to study their third order nonlinear
absorption (NLA) coefficient using Z-Scan for different wavelengths and repetition
rates.
In this thesis I report the Raman spectroscopy study of AgNP-SLG hybrid using
Raman spectroscopy for five different wavelengths. The dispersion and shift in the
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D-band, G-band and 2D-band is analysed in detail. The charge doping effect due to
hybridization between silver metal and graphene film is reported and explained in
terms of its work function. The nature of doping between these hybrid materials is
accurately identified as either n-doping or p-doping based on the spacing distance
between these 2D materials. Apart from doping there is enhancement in the Raman
signal when the silver nanosheets are photo-thermally melted using laser light. This
is accurately attributed to localized surface plasmon resonance (LSPR) scattering as
AgNP has sharp features, which are photo-thermally unstable and ablates, giving
rise to more stable structures like rods and spheres. It is for the first time that I was
able to distinguish the interplay between charge doping effect and LSPR effect in
AgNP-SLG hybrid materials due to photo-thermal melting. When this sample is
stored and analysed after few months I found that the effect of hybridization
disappears altogether because of the oxidization of silver nanoplates. This indirectly
confirms the process of hybridization of AgNS with SLG before the process of
oxidation sets in.
The Raman spectroscopy study is further extended to study the hybridization of
(SC-AuNS) with SLG. Study of Raman spectroscopy for (SC-AuNS) with MLG is
also reported. The effect of hybridization is compared for both the materials as a
function of graphene layer thickness and a detail analysis is performed to study this
effect on D, G and 2D band of hybrid materials. The splitting in G band due to
hybridization is analysed in detail. The Fermi energy shift is calculated to ascertain
the type of doping taking placed due to hybridization. It has been found that SLG
is better candidate for hybridization than MLG.
Third order nonlinear absorption coefficient of MLG, (SC-AuNS) and (PC-AuF)
are studied individually using Z-Scan technique for femtosecond laser pulse
excitation. The effect of pulse width is removed from the measurements by
experimentally calculating the pulse width for each wavelength and pulse repetition
rate. The observed variation in nonlinearity as I change pulse repetition rate is
explained as heat accumulation effect in metal films. MLG exhibited saturable
absorption (SA) phenomenon whereas (SC-AuNS) and (PC-AuF) were found to
exhibit two-photon absorption (TPA) phenomenon while measuring the open
aperture reading. There was no effect of repetition rate on NLA measurement in
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MLG while the metal films exhibited higher NLA coefficients for decrease in
repetition rates. The NLA values for (PC-AuF) are slightly greater than (SC-AuNS)
and I attribute this to the field enhancement effect due to rough surface wherein
the field is increased around the tip of the conical protrusions thereby increasing
nonlinear absorption co-efficient. The measured NLS of MLG was found to
increase from 3.11×10-5 to 3.65×10-5 cmW-1 for intensity variation from 100
GW/cm2 - 210 GW/cm2 for various wavelength range (700 ~ 900 nm) indicating it
usefulness as heat sink for thermally volatile materials.
I finally proceed to measure NLA coefficient of (SC-AuNS)-MLG and (PC-AuNS)-
MLG hybrid material. This is a unique measurement because the individual study of
metal films exhibited two-photon absorption phenomenon and MLG exhibited
saturable absorption phenomenon. Hybridizing such diverse materials opens new
opportunities in developing better materials with control over their intrinsic
characteristics. It is reported here that the hybrid materials behaved like saturable
absorbers and had better NLA values than individual MLG. This further highlights
the fact that these hybrid materials have better thermal dissipation than MLG alone.
The work presented herein is a step towards understanding of the intrinsic
properties (such as doping and LSPR effect) and nonlinear behaviour of hybrid
materials. This study further underlines the importance in recent rise in research of
hybrid materials and the strong desire for engineering better optical, chemical and
thermal properties using hybridization phenomenon.
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Author's Publications
Journal Articles
1 Salmaan R. Syed, Guh-Hwan Lim, Stuart J. Flanders, Adam B. Taylor,
Byungkwon Lim and James W. M. Chon, “Single Layer Graphene
Hybridization with Silver Nanoplates: Interplay Between Doping and
Plasmonic Enhancement” Appl. Phys. Lett. 109(10), 103103 (2016)
2 Salmaan R. Syed, Guh-Hwan Lim, Byungkwon Lim and James W. M.
Chon, "Measurement of the Third Order Non-Linearity of Gold-Graphene
Hybrid Nanocomposite for Near-Infrared Wavelengths", Proc. SPIE 9894,
Nonlinear Optics and its Applications IV, 98941G (2016)
3 Arif M. Siddiquee, Adam B. Taylor, Salmaan R. Syed, Guh-Hwan Lim,
Byungkwon Lim, and James W.M. Chon, “Measurement of Plasmon
Mediated Two-Photon Luminescence Action Cross-Sections of Single Gold
Bipyramids, Dumbbells and Hemispherically-Capped Cylindrical
Nanorods” J. Phys. Chem. C, 119(51), 28536-28543 (2015)
4 Salmaan R. Syed, Guh-Hwan Lim, Byungkwon Lim and James W. M.
Chon, “Z-Scan Based Measurement of the Third Order Non-Linearity of
Single Crystalline and Polycrystalline Gold-Graphene Hybrid
Nanocomposite for Near-Infrared Wavelengths and Different Pulse
Repetition Rate” (Will be submitting to APL)
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Selected Conference Papers
1 Salmaan R. Syed, Guh-hwan Lim, Byoungkwon Lim & James W.M Chon,
“Measurement of the Third Order Nonlinearity of Gold-Graphene Hybrid
Nanocomposite for Near-Infrared Wavelengths” SPIE Photonics Europe,
Brussels, Belgium, April 3-7th, 2016
2 Salmaan R. Syed, Guh-Hwan Lim, Byungkwon Lim, Adam B. Taylor &
James W.M. Chon, “Raman Spectroscopy Study of Graphene Hybridization
with Silver Nanoplates” (Poster presentation) International Conference on
Nanoscience and Nanotechnology (ICONN),Canberra, Australia, February 7 –
11th, 2016
3 Salmaan R. Syed, Guh-Hwan Lim, Byungkwon Lim & James W.M. Chon,
“Measurement of the Third Order Non-Linearity of Single Crystalline Gold
Nanosheets for Near-Infrared Wavelengths” (Oral presentation) International
Conference on Nanoscience and Nanotechnology (ICONN),Canberra, Australia,
February 7 – 11th, 2016
4 Salmaan R. Syed & James W. M. Chon, “Measurement of the Third Order
Non-Linearity of Gold Nanosheets” (Oral presentation) IONS KOALA,
Auckland, New Zealand, November 23-27th, 2015
5 Salmaan R. Syed, Byungkwon Lim, Guh-Hwan & James W.M Chon,
“Measurement of Third Order Nonlinearity of Gold Nanorods and Gold
Nanofilms” (Poster presentation) 21st Australian Institute of Physics, Canberra,
Australia, December 7-12th, 2014
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Acknowledgments
In the name of Allah, the most beneficent, the most merciful.
I am highly indebted and thankful to my lord for all the bounties that he
showered on me throughout my life. I consider myself fortunate to have travelled
this path of pursuing highest educational degree in one of the most prestigious
institutions, when there are many in this world that earnestly wish to be in my
shoes, but were prevented due to many unknown circumstances.
After I finished my Master degree from the field of nano-plasmonics, I
became very eager to learn the practical aspects of conducting an experiment using
lasers and optical devices as my master’s thesis was based only on computer
simulations. This desire to gain practical knowledge and the noble and wise advice
of my previous supervisor Dr. Mohammed A. Alsunaidi pushed me to pursue PhD
program in western countries.
I had an offer to pursue PhD from University of Victoria, Canada and
Swinburne University, Australia. It was because of the persistent impetus from my
elder brother Mr. Nouman Arshad to come to Australia that I chose Swinburne
University over others. I am glad that I never regretted this decision and I am
indebted to his advice as I found the research facilities at Swinburne to be world-
class. I would like to express my gratitude to Swinburne University, for offering me
their highest scholarship and for providing me with the environment that really
favours research.
I am highly grateful to my supervisor Prof. James W.M Chon for giving me
the freedom to pursue research in my field of interest. Had it not been for this
freedom and the faith that you had in me, I would never be able to come this far in
my journey of PhD. I would like to specially commend your patience and far-
sightedness in handling me when I was becoming disillusioned with my research
topic. I am thankful for all the wise advices you gave me as my mentor and I hope
that I will fulfil my goal of being an excellent educator in near future. I am thankful
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to Prof. B.K. Lim and his group for collaborating with us and for timely supply of
Ag-nanoplate and Au-nanosheet samples.
I would like to thank all my colleagues in CMP for their help and training
me to learn how to setup and carryout experiments in lab. I would like to thank
Pierrette Michaux, Adam Taylor, Tim Chow, Rakesh, Mohsin and Arif for their
support in labs. I would like to specifically thank Stuart Flanders for setting-up and
carrying out the COMSOL experiments for me. I would also like to express my
gratitude to Azim Ullah, Amit, Zubaidah, Khattab, Priya, Chiara Paviola and many
other friends with whom I spent memorable time during my research.
Last but not the least; I would like to thank my grandparents, parents and
siblings for their love and support throughout my life. I would like to specially
acknowledge the support, advice, warmth and love of my mom and dad for being
the doting parents that they are; I would never be able to accomplish this task
without their support.
This acknowledgement will be incomplete without the special appreciation
for my wife who took care of all my domestic chores, helped me with my tutoring
and lab assignments, accompanied me during my late stays in lab and at my desk,
motivated and corrected me when I was going off-track. I owe this to you. I had
the luxury of spending some beautiful and loving moments every day with my
niece, Ayesha. I express my gratitude to all my previous teachers, mentors and my
lifelong friends for making me what I am today. Life until now has been a beautiful
learning lesson with such wonderful people around me.
Salmaan
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Abstract ix
Author's Publications xiii
Acknowledgments xv
Chapter 1 1
Introduction 1
1.1 Graphene 5
1.2 Developing ultra-thin graphene film 8
1.3 Properties of graphene 10
1.4 Graphene based photonic devices 13
1.5 Multilayer graphene 14
1.6 Silver and gold nanosheets 16
1.7 Hybridization of materials in general and their latest applications and
development 17
1.8 Raman spectroscopy as tool 18
1.9 Third order Nonlinearity 19
1.10 Z-Scan Theory 20
Outline 22
Chapter 2 25
Experimental Setup and Characterization of Nanomaterials 25
2.1 Introduction 25
2.2 Principle of Raman spectroscopy 26
2.3 Raman spectroscopy experimental setup 29
2.4 Z-Scan setup 32
2.5 Measurement of pulse width 34
2.6 Pulse selection system 38
2.7 Scanning Electron Microscopy 40
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2.8 Atomic Force Microscopy 41
2.9 Ellipsometry 42
2.10 Sputtering 43
2.11 Single crystalline gold nanosheets 44
2.12 Polycrystalline thin gold metal film 46
2.13 Single and multilayer graphene 46
2.14 Single crystalline silver nanoplates 49
Chapter 3 53
Raman Spectroscopy Study of Single Layer Graphene Hybridized
with Silver Nanoplates 53
3.1 Introduction 53
3.2 Literature review 54
3.3 Characterization of silver nanoplates through experimental and simulation
method 58
3.4 Hybridization of Ag nanoplates with SLG 61
3.5 Raman band broadening after hybridization 64
3.6 Dispersion relation of Raman bands with respect to wavelengths 66
3.7 Analysis of Raman peak shift and enhancement after hybridization 68
3.8 Surface plasmon induced Raman enhancement in Ag nanoplates-SLG
hybrid 75
3.9 Effect of oxidation on Raman spectra of hybridized Ag nanoplate-SLG
sample 77
3.10 Summary and conclusion 77
Chapter 4 80
Raman Spectroscopy Study of Au-Graphene Hybrid
Nanocomposite 80
Introduction 80 4.1
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Materials needed for preparing single crystalline gold-graphene hybrid 4.2
nanocomposite 81
Raman spectroscopy study of gold nanosheet-SLG hybrid 82 4.3
Raman spectroscopy study of gold nanosheet-MLG hybrid 87 4.4
Analysis and comparison of gold nanosheet on SLG and MLG hybrid 92 4.5
Conclusion 94 4.6
Chapter 5 96
Z-Scan Based Nonlinear Optical Study of Gold-Graphene Hybrid
Materials 96
5.1 Introduction 96
5.2 Materials needed for preparing gold-graphene hybrid nanocomposite 97
5.3 Z-Scan equations for fitting experimental data 99
5.4 Measuring nonlinear two-photon absorption (β) of single crystalline
AuNSs 100
5.5 Measuring nonlinear two-photon absorption (β) of polycrystalline gold
thin film 102
5.6 Measuring nonlinear saturable absorption (α) of multilayer graphene 105
5.7 Measuring nonlinear saturable absorption (α) of multilayer graphene-gold
hybrid nanocomposite 108
5.8 Conclusion 110
Chapter 6 112
Conclusion and Future Work 112
Future Research 116
6.1 Plasmonic switch based on plasmonic metal-graphene hybrid structure 117
6.2 Synthesizing and characterizing new hybrid materials 120
Bibliography 123
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List of Figures
1 Figure 1 - 1: (a) Hexagonal honeycomb lattice of graphene with two atoms
(A and B) per unit cell. (b) Graphical representation of 4 layer graphene
with each layer stacked on top of each other (c) The 3D band structure of
graphene (adapted from Ref. [66]). (d) Dispersion of the states of graphene.
Approximation of the low energy band structure, as two cones touches at
the Dirac point. The position of the Fermi level determines the nature of
the doping and the transport carrier (adopted from Ref. [67]). ....................... 6
2 Figure 1 - 2: (a) The unit cell of a bilayer graphene with x and y denoting
the unit vectors of carbon atoms (b) The unit cell of a trilayer graphene
which is nothing but the stacking or bilayer graphene with a single layer
graphene on top (adapted from Ref. [98]) (c) Calculated phonon dispersion
relation of six phonon branches, namely iLO, iTO, oTO, iLA, iTA and
oTA of single layer graphene (Adapted from Ref. [81]). ................................ 14
3 Figure 1 - 3: Schematic diagram of Z-Scan set-up [177] ................................ 20
4 Figure 1 - 4: (a) Narrowing and (b) Broadening of the beam as sample
passes through the focal plane............................................................................ 21
5 Figure 1 - 5: (a) Typical signature of a saturable absorption and (b) Two
Photon absorption curves. .................................................................................. 22
6 Figure 2 - 1: Energy-level diagram showing different types of states
involved in Raman spectral ................................................................................. 27
7 Figure 2 - 2: Block diagram of a Renishaw Raman spectroscopy set up ..... 30
8 Figure 2 - 3: Block diagram of the dispersion of inelastically scattered light
in a Renishaw InVia Micro-Raman .................................................................... 32
9 Figure 2 - 4: Detailed block diagram of a home built Z-Scan system .......... 33
10 Figure 2 - 5: (a) A schematic diagram of GRENOUILLE setup which is
insensitive to alignment parameters contains a Fresnel biprism that replaces
the beam splitter, delay line, and beam-recombining optics of
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autocorrelator. It also shows a thick SHG crystal which acts as both the
nonlinear-optical time-gating element and the spectrometer. (b) Crossing
beams that are automatically aligned both spatially and temporally at an
angle using a Fresnel biprism (different colours are used to distinguish the
beams. (c) The very thick SHG crystal substitutes for thin crystal and
spectrometer and generates second harmonics of all colours in the forward
direction after being illuminated with a broadband light. .............................. 36
11 Figure 2 - 6: Intensity autocorrelation, typical FROG trace along with both
temporal and spatial profile of Ti:Sapphire femtosecond laser at 780 nm
wavelength acquired by Grenouille. .................................................................. 37
12 Figure 2 - 7: Block diagram depicting the detail operation of a pulse picker.
The fundamental building component of a pulse picker is a Pockels cell that
changes the polarization of incoming light through the application of
extremely high voltage across it. ........................................................................ 39
13 Figure 2 - 8: (a) SEM Image of chemically synthesized single crystalline gold
nanosheet (b) AFM image of single crystalline AuNS and the 20nm
thickness measurement taken across a cross section is shown (c) SEM
Image showing single crystalline AuNS edge having thickness of 20 nm (d)
Ellipsometry measurement for refractive index of AuNSs and comparing it
with Rakic et al[190] ............................................................................................ 45
14 Figure 2 - 9: (a) polycrystalline thin gold metal film with a portion peeled
off to measure its thickness. Scale bar is of 2 µm. (b) Thickness
measurement of polycrystalline thin gold metal film using AFM which was
~20nm approximately. ........................................................................................ 46
15 Figure 2 - 10: (a) Raman spectrum of SLG at 514nm excitation wavelength
(b) Optical reflection microscope image of Multi-layer graphene with a 20
µm scale bar. Patches of different thickness and dimensions are visible. (c)
Raman spectrum of MLG at 514nm excitation wavelength with G-peak at
~1585 cm-1 and broadened 2D peak at ~2700 cm-1. (d) The 2D band of
MLG is fitted with 3 components of Lorentzian curve (green color) which
indicates that it is a 4-Layer graphene. Blue color is the envelop of fitted
curve and red color curve indicates Raman data. ............................................ 47
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16 Figure 2 - 11: (a) Overview of Heat-up synthesized silver nanoplates. (b)
SEM image of the Ag particles and plates separated after centrifugation of
the as-prepared solution. (c) Extinction spectra of the Ag particles and
plates obtained after centrifugation along with the original as-prepared
species are shown. Extinction spectra of the nanoplates are shown
exhibiting strong absorption at NIR range due to distributed plasmon peak
wavelengths (provided by Guh-Hwan et al [208]). .......................................... 51
17 Figure 2 - 12: (a) SEM image depicting silver nanoplates of different shapes
and sizes that are formed during chemical synthesis. (b) Single silver
nanoplate is shown, with thickness ~ 20 nm. (c) Typical dark-field
scattering spectrum of five individual nanoplates is shown. Peak scattering
at 540 ~ 600 nm is from the higher mode of surface plasmon resonance. . 52
18 Figure 3 - 1: (a) SEM image of silver nanoplates, showing mixture of
triangular, hexagonal plates and spherical particles. (b) Single nanoplate
dark-field scattering spectra is shown to exhibit higher mode of surface
plasmon resonance peaks at 600 and 750 nm. (c) |Eplate|2 simulation
image of a silver nanoplate of side length 1100 nm, thickness 25 nm, radius
of curvature 60 nm, excited at 750 nm laser wavelengths (Plane wave,
vertical polarization). (d) Simulation of cross section spectra of nanoplates
with aspect ratio 5 ~ 25 (width 10 nm) in the steps of 5 shown. (e)
Configuration of tetrahedron mesh employed to simulate the
electromagnetic field surrounding a 1µm x 25 nm silver nanoplate. Mesh
element sizes within nanoplate range between 1 and 25 nm (provided by
Stuart). .................................................................................................................... 60
19 Figure 3 - 2: (a) SEM image of SLG on top of a Ag nanoplate on silica
substrate. Patches of darker regions are multiple layer region. (b) SEM
image of Ag nanoplates on top of SLG on silica substrate. (c) Typical
Raman spectrum for SLG only and for SLG on Ag nanoplate structure of
(a) showing that G peak at ~1580 cm-1 is enhanced and 2D peak at 2700
cm-1 is reduced for Ag nanoplate hybridization. Inset shows an optical
microscope image of where Raman spectra were taken. Scale bar is 2 µm.
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(d) Similar Raman observation for Ag nanoplate on top of SLG on glass
slide, (b). Wavelength of Raman laser is 533 nm. Numerical aperture of the
objective lens is 0.85. ........................................................................................... 62
20 Figure 3 - 3: Raman spectrums of Ag nanoplates on SLG (blue colour) and
SLG alone (red colour) for laser wavelength excitation of 457, 488, 533, 633
and 785 nm. Numerical aperture of the objective lens used was 0.85. ....... 63
21 Figure 3 - 4: (a) & (c) Raw Raman data (red colour) and fitted Raman
spectra (blue colour) of SLG at 488, 514, 633 and 785 nm laser
wavelengths. Black line is guide to an eye. (b) & (d) The deconvolved
spectra of G and 2D bands after Ag nanoplate deposition. The average split
observed in G band is about ~8 cm-1 while in 2D band the split is about
~15 cm-1 ................................................................................................................ 64
22 Figure 3 - 5: Dispersion relation of G and 2D bands of single layer
graphene obtained through Raman spectroscopy for four different laser
excitation energies. ............................................................................................... 67
23 Figure 3 - 6: Summary of G and 2D peak changes from SLG to SLG - Ag
nanoplate, SLG – laser modified Ag nanoplate, SLG – oxidized Ag
nanoplate hybrids. (a). I(2D)/I(G) ratio change. SLG – Ag nanoplate shows
the ratio lower than 1, meaning that there is charge doping on SLG. (b)
Total enhancement of the G and 2D peaks. Only laser modified hybrid
shows plasmon enhancement. (c) G peak position shift upon hybridization.
SLG – Ag nanoplate shows stiffening ~ 10 cm-1 due to charging, but the
others show no change with that of SLG. (d) 2D peak position shift upon
hybridization. Again, only SLG-Ag nanoplate shows stiffening from SLG.
2D peak shows dispersion with the excitation photon energy...................... 69
24 Figure 3 - 7: D peak enhancement and shift of SLG with Ag nanoplate and
oxidized Ag nanoplate. ........................................................................................ 70
25 Figure 3 - 8: |Eplate|2 pattern of a silver nanoplate of side length 900 nm,
thickness 25 nm, radius of curvature 60 nm, excited at 9 different laser
wavelengths (Plane wave, vertical polarization is used. With help from
Stuart) .................................................................................................................... 72
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26 Figure 3 - 9: Optical microscope images and SEM image of laser modified
Ag nanoplates on SLG and corresponding Raman spectrum change. (a) &
(b) show optical images of Ag nanoplates on SLG before laser irradiation.
Scale bar is 10 µm long. (c) Optical image of a nanoplate after laser
irradiation, showing the lifting of the tip of triangles. (d) Optical image of a
nanoplate after laser irradiation, showing laser ablated edge of the tip of
triangles. (e) Corresponding Raman peak enhancements. Both (c) and (d)
nanoplates show enhancements, without change in I(2D)/I(G), indicating
only plasmonic enhancement is in play. (f) & (g) Enlarged optical image of
Ag nanoplate before and after laser irradiation. (h) SEM image of Ag
nanoplate after laser irradiation. Scale bar is 800 nm long. ............................ 75
27 Figure 3 - 10: Summary of Raman spectrum evolution of SLG, hybridized
with various Ag nanoplates. When Ag nanoplates are hybridized with SLG,
unlike in the case of silver nanoparticles, p-doping on SLG was observed,
reducing I(2D)/I(G) below 1 and stiffening G and 2D bands without any
plasmon enhancement. When the nanoplates were modified in shape with
laser irradiation, either by plasmon hot printing or laser-induced photo-
oxidation, the charge doping was lifted and strong plasmonic enhancement
of Raman signals was observed. These two effects could be turned off by
oxidizing the whole plate, where the Raman signals were returned back to
the original SLG state. ......................................................................................... 78
28 Figure 4 - 1: (a) & (b) Side view and Top view of Au nanosheet on SLG
hybrid sample on glass substrate. (c) Raman spectra for SLG (blue colour)
and Au nanosheets hybridized with SLG (red colour) for five different laser
wavelengths. .......................................................................................................... 84
29 Figure 4 - 2: (a) Optical reflection microscope image of Au nanosheet on
SLG with different points showing the spots where Raman spectra were
acquired. The white coloured scale bar shown is 8 µm long. (b) Raman
spectra of SLG hybridized with Au nanosheets for different positions
shown in Figure (a) for 514 nm laser wavelength. .......................................... 85
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30 Figure 4 - 3: (a) & (b) Crumpled gold nanosheets during centrifugation
process with lift-off in edges clearly shown by black rounded circles. (c)
Gold nanosheet having thick edges and small lift-off along with a small
triangular gold nanosheet hidden beneath it at the centre. (d) Magnified
image of highlighted edge of Figure (c). ........................................................... 86
31 Figure 4 - 4: (a). Raman spectra of SLG hybridized with Au nanosheets for
different laser exposure times of 514 nm laser wavelength at position 3 of
Figure 4 - 2(a). The intensity of Raman spectra increases almost linearly
with increasing laser exposure duration. (b) Raman spectra of SLG
hybridized with Au nanosheets for different laser powers of 514 nm laser
wavelength at position 3 of Figure 4 - 2(a). The average laser power
measured at the back end of the numerical objective was 3.5 mW. It can
clearly be seen that the Raman spectra for varying laser power also increases
linearly with increasing laser power. .................................................................. 86
32 Figure 4 - 5: (a) & (b) Side view and Top view of Au nanosheet on MLG
hybrid sample on glass substrate. (C) Raman spectra for SLG (blue colour)
and Au nanosheets hybridized with MLG (red colour) for five different
laser wavelengths. ................................................................................................. 88
33 Figure 4 - 6: (a) & (b) The deconvolved spectra of G bands after Au
nanosheet deposition on SLG and MLG for 514 nm laser wavelength. The
split observed in G band of SLG after hybridization is about ~22 cm-1, on
the other hand there was no split observed in MLG after hybridization with
Au nanosheets. ..................................................................................................... 89
34 Figure 4 - 7: (a) The deconvolved spectra of G bands after Au nanosheet
deposition on SLG. (b) The deconvolved spectra of G bands after Au
nanosheet deposition on MLG for five different laser wavelengths. The
average split observed in G band is about ~22 cm-1. .................................... 91
35 Figure 4 - 8: Summary of G and 2D peak changes from SLG to SLG - Au
nanosheets (a) G peak position shift of SLG and MLG upon hybridization.
SLG and MLG shows stiffening of ~3 cm-1 after hybridization indicating
the occurrence of charge doping effect (b) G band enhancement of SLG
and MLG after hybridization. Enhancement is dominant when Au
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nanosheets are hybridized with SLG (c) 2D peak position shift of SLG and
MLG upon hybridization by ~4 cm-1 to higher wavenumber. Dispersion
can be seen before and after hybridization of SLG and MLG. (d) 2D band
enhancement of SLG and MLG after hybridization. Both show reduction in
Raman intensity. (e) D peak enhancement and shift of SLG and MLG
before and after hybridization. (f) D band enhancement of SLG and MLG
after hybridization. D band shows similar trend as G band. (g) I(2D)/I(G)
ratio change. SLG – Au nanosheets shows drastic change in the I(2D)/I(G)
ratio to less than 1 after hybridization compared to MLG sample, meaning
strongest interaction occurs between noble metals and single-layer
graphene................................................................................................................. 92
36 Figure 5 - 1: Pictorial representation of five different samples used to
measure third order nonlinearity using Z-Scan (a) Multilayer graphene
sample (b) Single crystalline gold nanosheet sample (d) sputter coated
polycrystalline gold film sample (d) Single crystalline gold-graphene
nanohybrid sample (e) Polycrystalline gold-graphene nanohybrid sample .. 98
37 Figure 5 - 2: (a) & (b) Open Aperture and closed/open aperture Z-Scan
Experimental data and fitting at 780 nm, ~ 82 MHz repetition rate and
0.0165 Jcm-2 power density at the focus for single crystalline AuNSs (c)
Nonlinear absorption (TPA) coefficient β values of single crystalline AuNSs
for 700-900nm wavelength range from 0.82-82MHz repetition rate. ........ 101
38 Figure 5 - 3: (a) Nonlinear absorption coefficient β of polycrystalline thin
gold metal film values for 700-900 nm wavelength range from 0.82-82 MHz
repetition rate. (b) Comparison of nonlinear coefficient β for single
crystalline and polycrystalline thin gold metal film for 8.2MHz repetition.
............................................................................................................................... 103
39 Figure 5 - 4: (a) Relation between tip radius of curvature and the local field
enhancement around the tips (b) AFM image of ultra-smooth single
crystalline AuNSs having average surface roughness of 0.143 nm (c) AFM
image of polycrystalline thin gold metal film having average surface
roughness of ~4.3 nm (d) 3D view of a section of polycrystalline thin gold
xxvii
metal film surface of 5 -3-(c) with sharp conical tips and black background.
.............................................................................................................................. 105
40 Figure 5 - 5: (a) & (b) Open Aperture and closed/open aperture Z-Scan
Experimental data and fitting at 780 nm, ~ 1 MHz repetition rate and 210
GW/cm2 power density at the focus for MLG (c) Absolute nonlinear
coefficient α values of MLG for 700-900 nm wavelength range from 0.82-
82MHz repetition rate. ...................................................................................... 106
41 Figure 5 - 6: (a) Absolute nonlinear saturable absorption coefficient ‘α’
values of single crystalline AuNSs-MLG hybrid composite for 700-900nm
wavelength range from 0.82-82MHz repetition rate. (b) Absolute nonlinear
saturable absorption coefficient ‘α’ values of polycrystalline thin gold metal
film -MLG hybrid composite for 700-900 nm wavelength range from 0.82-
82 MHz repetition rate. ..................................................................................... 109
42 Figure 6 - 1: Block diagram of proposed plasmonic switch. ....................... 117
43 Figure 6 - 2: (a) SEM image of the FIB milled Bragg gratings on the gold
structure. (a) SEM image of one batch of FIB milled Bragg grating structure
congaing six sets of gratings of varying separations from 2-12 μm. .......... 118
44 Figure 6 - 3: (a) SEM image of four sets of FIB milled Bragg grating
structure each congaing six sets of gratings of varying separations (b)
Microscopy image of the same FIB milled Bragg grating structure clearly
visible as small bright spots as highlighted. The highlighted grating with
yellow circle is magnified and shown in Figure-(c). (c) Graphical view for
implementing proposed plasmonic switch for the bottommost grating. .. 119
45 Figure 6 - 4: Artistic view of single layer gold on top of single layer
graphene. ............................................................................................................. 121
xxviii
List of Tables
46 Table 2 - 1: Summary of G and 2D band peaks of SLG before and after
hybridization with silver nanoplates along with the information of fitted
curve peaks for G band denoted as G1, G2 and for 2D band as D1, D2. ..... 66
47 Table 4 - 1: Summary of splitting in G band peaks of SLG after
hybridization with Au nanosheets along with the information of fitted curve
peaks for G band denoted as G1 and G2. ........................................................ 90
48 Table 5 - 1: Comparison of nonlinear saturable absorption coefficients (α)
............................................................................................................................... 107
1
Chapter 1
Introduction
The quest of mankind for finding new materials is since his very beginning on this
planet earth. As a result early researchers were discovering, recording and studying
the properties of new materials and their allotropes for half a millennium. One of
the most dramatic events was the discovery of modern periodic table by Dmitri
Mendeleev in 1869 [1, 2]. He left few places vacant in his period table predicting
the existence of still undiscovered elements in nature. One of the elements in this
periodic table is Carbon, which is abundantly available in this universe. Carbon
allotropes such as diamond and graphite are three-dimensional (3D) structures
having sp3 and sp2 networks and are commonly known since time immemorial [3].
But, what was not expected was the existence of two-dimensional (2D) allotrope of
carbon or for that matter any other 2D element in free state. The origin of this
assumption dates back 70 years when Landau and Peierls [4-6] contended that 2D
lattice structure were thermodynamically unstable and could not exist. This
argument was buttressed by Mermin and reinforced by experimental observations
[7-9]. The zero-dimensional (0D) curved structures of graphene such as soot,
fullerenes and nanotubes were also not expected to exist as they were supposed to
be unstable. However, the 2D allotrope of carbon was studied theoretically for sixty
years [10-12] and was named as graphene in 1994 by Boehm, Setton and Stumpp
[13].
Graphene was accidentally discovered by Novoslov and Geim in 2004 [14, 15]. It
was a single atomic plane of carbon atoms. Later experiments further confirmed
that its charge carriers were truly massless Dirac fermions, leading to windfall of
new 2D material discoveries and inventions [16, 17]. This in turn led to opening up
of new science related to 2D materials and its properties. In single-layer graphene
(SLG) the carbon atoms are tightly arranged in a 2D honeycomb crystal lattice.
2
SLG in its suspended state was measured to have high mobility and near-ballistic
transport at room temperature [18, 19]. It has incredible optical opacity as it can be
seen despite having single atomic layer thickness [20, 21]. SLG reflects <10% of
visible light. It linearly absorbs 2.3% of incident light with addition of each layer
over the visible spectrum, which means 4-layer graphene can absorb nearly 10% of
light [22]. It possesses the highest saturable absorption for any given material due to
Pauli blocking [23, 24]. Luminescence in graphene is achieved through non-
equilibrium carriers and through physical and chemical treatments [25-31].
Graphene is a promising plasmonic candidate for guiding plasmons at terahertz
frequencies [32]. It was observed that the surface plasmons in graphene can be
easily coupled to the incoming light at terahertz frequency. Moreover, attractive
properties such as the tunability of SPs using chemical doping and bias gating, close
confinement and long propagating ranges of surface plasmons were observed [33].
Graphene also possesses excellent mechanical strength, extreme electronic and
thermal conductivities, stiffness, elasticity, robustness and environmental stability
[34]. Graphene is used in wide range of applications such as transparent
conductors, sensors (photovoltaic devices, light-emitting devices, photodetectors,
touch screens, flexible smart window, bistable display), photonics (saturable
absorbers, ultrafast lasers, optical limiters, optical frequency converters), terahertz
devices, energy storage, nanoelectronics and bioapplications [34, 35].
Graphite is nothing but the 3D stacking up of graphene sheets on one another.
Now the question is how many layers of graphene are needed to make it truly 3D.
Recently it has become clear that 10 layer of graphene approaches the 3D threshold
to form graphite [36]. Single layer graphene (SLG) and Bi-layer graphene (BLG) are
regarded as zero-gap semiconductors and has simple electronic spectra with only
one type of electron and hole respectively and with no overlapping band structures.
Several charge carriers starts appearing from three or more layers onwards with
increasingly complicated electronic spectra with overlapping conduction and
valence bands [37]. This helps in characterizing graphene based on number of
layers as single layer, double layer and few layer (3-10) graphene. Anything thicker
than this should be regarded as graphitic thin films. Multi-layer graphene (MLG) are
also used in wide range of applications. BLG can be used to make tunable band gap
3
semiconductiors [38], tri-layer graphene (TLG) has unique electronic band structure
and can be approximated as a combination of massless SLG and massive BLG
subbands [39-41]. Similarly MLG is being increasingly investigated for their
application in useful devices such as displays, touch screens and solar cells. As
mentioned earlier that 4-layer MLG has transmission of T ≈ 90% of incident light
whereas indium tin oxide [ITO] which is widely used in transparent conductive
films has transmission of T ≈ 80%. This better transmission accompanied by high
sheet resistance Rs = 20 Ω/□ of 4-layer MLG compared to that of ITO (Rs=10
Ω/□) was used to build one of the best transparent conductive films [42].
Unique physical, chemical, optical, mechanical and electronic properties of
graphene made researchers explore further 2D nanostructures of other traditional
materials such as metals. Like graphene, gold (Au) and silver (Ag) are being actively
looked into regarding the possibility of either extracting or fabricating single and
few atomic layer thick films. Noble metal thin films are interesting candidates as
they support plasmon propagation across its surface [43] and have high nonlinear
properties [44]. The plasmonic and optical properties of these noble metal
nanomaterials are closely interrelated with their size, shape, structure and
composition. Nanoparticles of Au such as nanospheres, nanorods, nanoplates and
nanowires show plasmon bands in visible and near-infrared (NIR) wavelengths
depending upon their size, shape and aspect ratios [45, 46]. Moreover, the sharp
features such as corners and edges of noble metal nanoparticles displays
predominantly strong localized surface plasmon resonance (LSPR). This is of
specific interest as it favours surface enhanced Raman scattering (SERS) across
edges [47]. Moreover, ultra smooth thin Au films support the propagation of
surface plasmon polaritons across its surface for long distances possible leading to
the development of plasmonic circuitry [48]. Noble metal nanoplates and
nanosheets find applications in the field of catalysis, SERS, LSPR-based sensing,
NIR photothermal therapy, plasmonic circuitry and all optical switching etc. [49].
Transition metal dichalcogenides (TMDs) are the latest addition to the family of 2D
materials having MX2 configuration with X-M-X layer where, M = Bi, Hf, Mo, Nb,
Ta, Ti, V, W, Zr and X = S, Se, Te are thoroughly investigated systems. The most
4
widely reported among them are MoS2, MoSe2 and WS2 apart from nearly three
dozen different TMDs [15, 50-55]. They have potential in various application such
as semiconductors [16], hydrogen production [56, 57], thermo-electric devices [58]
etc. The other important 2D crystal is hexagonal Boron nitride (h-BN) having
similar honey comb structure like graphene with sp2 hybridized hexagonal rings of
alternating three boron and nitrogen atoms with almost the same spacing distance
between two layers as graphene [59, 60].
But, the real potential of atomic layer thin materials comes from their hybridization
with other 2D atomic crystals and their heterostructures. As outlined before, there
are other extractible 2D crystals like h-BN and MoS2 apart from graphene that have
their own distinctive properties and can possible offer the freedom to fine-tune
material properties to suit a specific technology better or to be used in combination
with other 2D structures [15, 61, 62]. These 2D materials possess diverse properties
such as extreme insulation, extraordinary carrier mobility and conduction, being
structurally strongest to the softest etc. The simplest way to achieve such hybrid
structures is by placing or sandwiching one material with the other or by carefully
tailoring multiple 2D materials with atomic precision on top of each other. This
gives scope to produce artificial materials with wide ranging properties and
potential future applications. One such early application was encapsulating
graphene between hexagonal boron nitride layers to fabricate ultrathin top gate
dielectric [60]. Another such application was the graphene heterostructure with
atomically thin h-BN or MoS2 as vertical transport barrier leading to the
development of field-effect tunnelling (vertical) transistor [63].
In order to achieve different type of hybrid materials and such splendid
applications, one needs to identify and fabricate such heterostructure and then
study the intrinsic properties of material such as electric and thermal conductivity,
nonlinear behaviour, mechanical strength etc. Hence, the purpose of this thesis is
fabricate and study such hybrid materials made up of single layer graphene, multi-
layer graphene, noble metal nanosheets and thin films using Raman spectroscopy. I
will then investigate the third order nonlinear absorption co-efficient of MLGs, Au
5
thin metal films and their hybrids using Z-Scan to determine if there is any
significant enhancement in the properties of hybrid materials.
In the present chapter, I will give a broad introduction about single layer graphene,
multilayer graphene, silver and gold nanosheets and hybridization of the materials
in general and their latest applications and development. I will then explore Raman
spectroscopy as a tool to classify and study the properties of hybrid materials. I will
then give brief introduction of third order nonlinearity of materials and the Z-scan
technique needed to measure it. Finally, I will sketch the summary of each chapter
outlined in this thesis.
1.1 Graphene
Carbon forms the principal composition of all existing organic life on earth. Due to
its unique tetravalent atomic structure it gives rise to one of the most mystifying
elements known to mankind like diamond, graphite, amorphous carbon etc.
Among the known elements in the world, these are one of the strongest, hardest
and thermally stable constituents. The dimensionality of these structures plays an
important role in giving them these remarkable physical properties. Among them, a
two-dimensional allotrope of carbon known as Graphene forms the basis for
understanding various fundamental properties of all other allotropes. The carbon
atoms in graphene are arranged in a hexagonal manner resembling exactly like a
honeycomb structure as shown in Figure 1 - 1(a). It is similar to a benzene ring
structure without any of their hydrogen atoms [64]. Graphene forms the
fundamental building block for other dimensional structures like Fullerenes, which
are considered as zero dimensional entities from physical point of view, because of
their spherically arranged carbon atoms cut out from the graphene sheet. To
produce Fullerenes, positive curvature defects such as pentagons need to be
introduced and hence, fullerenes can be understood as wrapped-up graphene.
Similarly if we roll up the graphene sheet in a particular direction reconnecting the
carbon bonds, then the resulting structure resembles Carbon nanotubes. Hence
carbon nanotubes are one dimensional entities consisting of pure honeycomb
graphene sheet. But the oldest know allotrope of carbon to mankind is Graphite,
6
which became even more popular after its usefulness in writing instruments such as
pencil which was invented in 1564 [65]. Graphite is a three dimensional carbon
allotrope made up of stacks of graphene layers bounded weakly by van der Waals
forces as shown in Figure 1 - 1(b). PERSPECTIVE
Figure 1 - 1: (a) Hexagonal honeycomb lattice of graphene with two atoms (A
and B) per unit cell. (b) Graphical representation of 4 layer graphene with each
layer stacked on top of each other (c) The 3D band structure of graphene
(adapted from Ref. [66]). (d) Dispersion of the states of graphene.
Approximation of the low energy band structure, as two cones touches at the
Dirac point. The position of the Fermi level determines the nature of the doping
and the transport carrier (adopted from Ref. [67]).
For centuries people were inadvertently producing graphene stacks or even
individual graphene layer during writing process by pressing pencil nib against a
sheet of paper. Hence graphene can be considered as the mother of all carbon
allotropes. But it was invented by Novoselov et al. [68], only in 2004 i.e., roughly
after 440 years of invention of pencil [69]. This was because the graphene was least
7
expected to exist in free state. Moreover the technology was not so sophisticated to
extract or confirm the existence of single layer graphene flakes from the pencil
debris or graphite powder. It was finally discovered because of the delicate optical
effect it creates when coated on top of a chosen SiO2 substrate which felicitate its
observation using an ordinary optical microscope. Hence manufacturing and
analysing essential properties of single layer graphene is a big challenge.
Single layer graphene has hexagonal honeycomb shaped lattice structure, having
two carbon atoms per unit cell as shown in Figure 1 - 1(a). This 2D graphene layer
is unswervingly responsive to any chemical and physical modifications to it. Wallace
in 1947 was the first to calculate its unique band structure [10]. π electrons in
graphene form π and π* states which are non-interacting and form the valence and
the conduction band respectively touching each other symmetrically at six points
known as Dirac or neutrality points which are mutually independent. These six
points are further reduced to a pair of K and K’ bands as shown in Figure 1 -
1(c).The bands have linear dispersion at low energies which is quite relevant in
electron transport. Figure 1 - 1(d) shows the band structure as two cones touching
at EDirac without crossing into each other as the orthogonal π and π* states do not
interact. Hence, it can be inferred from this figure that the graphene has zero band
gap as the band structure just touches at EDirac point. Therefore, graphene is usually
labelled as a zero-gap semiconductor. Holes and electrons in pristine graphene have
similar properties as their band structure is symmetric about the Dirac point. The
most important observation made during the experimental study of graphene is that
the electrons in graphene behave as relativistic Dirac Fermions having zero rest
mass due to its linear dispersion. Apart from the chemical doping method, Fermi
level of graphene can be varied from valence band to conduction band by applying
gate voltage across it, resulting in a pronounced ambipolar electric field effect [68].
The conductivity ‘σ’ of graphene increases with increase in the concentration of
electrons induced by positive gate voltages as the Fermi level is driven inside the
conduction band. Similarly the converse is also true. Whenever the Fermi level goes
from the conduction band to the valence band or vice-versa, it crosses the zero
density of states point i.e., the Dirac point, but, the conductivity remains finite even
though the carrier density vanishes [70].
8
1.2 Developing ultra-thin graphene film
There are several methods for manufacturing graphene. These methods can be
broadly classified based on their quality. The quality of graphene decides the
production process to be used. Like for example, the graphene (or reduced
graphene oxide) that is needed for preparing conductive paints, hybridized materials
etc., are classified as low quality graphene, whereas the ones used in the
manufacture of low-performance photonic devices falls under mediocre quality
graphene and the ones used for the manufacture of electronics is graded as high
quality graphene [34]. Here I will discuss few important techniques such as
micromechanical cleavage, liquid phase exfoliation, chemical vapour deposition,
carbon segregation and chemical synthesis.
Micromechanical cleavage: The pioneering technique used for the extraction of
single layered graphene was through micromechanical exfoliation of graphite. Till
date this technique produces the highest quality pristine graphene in terms of
mobility, defects, purity, electronic and optical properties. This technique peels
graphene layer from the graphite chunk by means of adhesive tape [15]. This
method is basically suited for fundamental research as it produces single layer
graphene (SLG) of up to millimetre size with excellent structural quality and
outstanding electronic and optical properties. This technique is not suitable for
large-scale production as its yield and throughput is extremely low along with its
tedious and cumbersome process.
Liquid-phase exfoliation: In this method individual pellets of graphite are made
and exposed to solvents possessing surface tension that helps increase the total
surface area of these pellets [71, 72]. This solvent can be both an aqueous and non-
aqueous solution. With the help of sonication these pellets yield single layer
graphene film in the suspension which can be further improved by centrifugation.
Sodium deoxycholate is used for mild sonication in aqueous solution followed by
extreme centrifugation in order to produce approximately 70% of single layer
graphene sedimentation. These graphene films of controlled thickness can be
segregated out by the use of bile salt surfactants [73]. This technique is economical
9
for mass production of SLG films and composites. It can be used to manufacture
nano-ribbons with less than 10 nm dimensions [74].
Chemical vapour deposition: In this technique transitional metals such as Nickel
(Ni) or Copper (Cu) is first deposited onto the silicon dioxide (SiO2) having Silicon
wafer base using electron beam evaporation method. These thin transitional metal
films act as catalyst. The substrate is now placed in the furnace which is heated to
around 1000 oC. Gaseous carbon source is sprayed on the substrate using gas
delivery system embedded inside the furnace. As a result of high temperature on
the metal surface the carbon atoms gets adsorbed and then absorbed onto it. This
carbon is then precipitated into graphene film by cooling down the substrate to
room temperature at the rate of 10-25 oC/min. The sample is then coated with
polymethyl methacrylate (PMMA) and dipped in appropriate etchant solutions to
remove SiO2 and metal films successively. The substrate is then transferred to the
water and rinsed. After slight vibrations the visible thin film can be seen floating in
water without the Silicon wafer base. After taking this film on the desired substrate,
film is allowed to dry and then rinsed with acetone to remove PMMA revealing
pristine single layered graphene [75]. This technique can be used to produce
graphene on mass scale. But the problem with this process is that it consumes huge
amounts of energy and involves the tedious process of removing underlying metal
layer. Nevertheless, production of square meter graphene using this technique has
already been reported [42].
Carbon segregation: By separating carbon atoms from Silicon carbide (SiC) it has
been illustrated that graphene layers can be produced. In the argon gas atmosphere
pristine few layer graphene can be produced on SiC substrate by electronically
decoupling Carbon atoms by hydrogen treatment followed by high-temperature
annealing [76]. As a result the Si atoms in the SiC are rerouted thus leaving only the
graphitic layer. But the major drawback of this method is that it needs high
temperatures for annealing and huge costs associated with SiC wafers.
Chemical synthesis: This technique is the latest among the existing methods for
producing Graphene. Graphene like poly-aromatic hydrocarbons can be produced
by total organic synthesis [77, 78]. In order to produce large layers of single layered
graphene like nanoribbons, these synthetic nano-graphene are assembled in order
10
to achieve atomically accurate patters for device fabrication or circuitry from the
grassroot levels. Nano-graphene form ordered layers, with precise control of
orientation and spacing [79]
1.3 Properties of graphene
Transport properties: Graphene displays exceptional transport properties [67, 80,
81]. Carriers travel with a Fermi velocity of VF ≈ 106 ms-1 in the ballistic transport
regime. Transport of carriers becomes diffusive due to inelastic and elastic collision
in extended graphene strips. Irregularities such as defects, impurities, surface
roughness and adsorbates in the graphene lead to elastic scattering whereas
phonons are responsible for inelastic scattering [69]. In graphene, the densities of
carriers play a prominent role in determining the efficacy of carrier mobility.
Decrease in carrier mobility is observed in single layer graphene due to the increase
in carrier density. The nature of the leading scatterers decides the specific carrier
mobility behaviour. When the substrate free interactions is achieved through
fabricating graphene using liquid exfoliation technique, charge mobilities greater
than 200000 cm2 V-1 s-1 is achieved which is an order of magnitude higher than that
of an Indium Phosphide (InP) [80, 81]. When this graphene is deposited on
dielectrics such as amorphous SiO2, the carrier mobility is drastically reduced,
subject to pureness of dielectric. Furthermore decrease in the mobility of carriers is
observed in chemical vapour deposition (CVD) and eptaxially grown graphene due
to its innate nature of adding defects and impurities in graphene during the
fabrication process.
Electrical properties: Similar to its transport properties, the graphene has
exceptional electric properties as well [67]. But, the problem is that it needs to be
injected with carriers and then collected through metal contacts. The metal contacts
creates potential barrier similar to the p-n junction diode in semiconductors. This
strongly effects the performance of graphene diode and it need to be bypassed. The
generation of potential barrier at the graphene-metal contact is due to their
dissimilar work functions, causing the transfer of charges between them. The
resulting polarization of charges induces doping in graphene along the metal-
graphene interface and causing the bending of graphene bands extending into the
11
graphene channel. This polarization of charges is severe in reactive metals such as
Platinum (Pt) and Nickel (Ni) leading to significant hybridization of graphene. The
consequence of inserting the carriers into graphene is that, it will have to pass
through two barriers, namely, dipole and doped-undoped (graphene) barrier. The
method for calculating contact resistance of graphene is by engineering different
channel length devices and then inferring to zero channel length. It was found that
contact resistance Rc, is reliant upon the gate bias, with Rc values ranging from 100
Ω μm to a few kΩ μm is measured [67]. The maximum value of Rc occurs near the
Dirac point. The most promising part of graphene is that transport of carriers after
being inserted into the graphene channel can be controlled by gate. It is to be noted
that the Fermi level of graphene is raised by Negative gate bias and similarly it is
lowered by positive gate bias. Hence it can be understood that when the gate bias is
lower than the Dirac voltage (Vg < VDirac) the holes are the majority carriers and
when the gate bias is greater than the Dirac voltage (Vg >VDirac) the electrons are
the majority carriers. The gate bias alters the Fermi energy causing the density of
states and the carrier densities to change. This is the underlying concept of
“switching” in graphene. But, the basic difference between the usual semiconductor
switch having band gap and a graphene switch is that the latter does not turns off
completely even when the density of states (DOS) equals 0 at EDirac. This is due to
the absence of band gap in graphene along with the zero rest mass Dirac Fermion
behaviour of carriers. This has wide reaching consequences in confining the quasi-
particles under gate bias.
Optical properties: From the Figure 1 - 1(d) it is can be seen that at the Dirac
point the tips of conical-shaped valence and conduction bands intersect. Thus pure
single layer graphene has distinctive electronic structure which contributes to its
frequency independent optical conductance over wide range of wavelengths. As a
result, the optical conductance of pure single layer graphene is also wavelength
independent and is exclusively determined by the fine structure constant α [22].
The above equation indirectly suggests that the broadband absorption per unit mass
of graphene is extremely strong. Though it is strange to say that monolayer
12
graphene has absorbance of which is orders of magnitude higher than
many single atomic layer elements specifically water, GaAs etc., [82]. It has
extremely weak reflectance under the normal light illumination R=1.3×10-4 which is
many order smaller than transmittance [22]. Similarly the absorbance of few layer
graphene can be inferred by just scaling the number of layers. Hence the
transparency of graphene can be retained while simultaneously possessing low sheet
resistance. Consequently, if the graphene is appropriately doped, it can be used as a
translucent conductor in touch screens and solar cells, ultimately replacing Indium
Tin oxide (ITO) which is very brittle in nature [42, 83]. As stated in the previous
sections, the fundamental properties of graphene can be controlled by changing the
Fermi level within the material. Hence by applying gate bias or by chemically
doping the graphene, its behaviour can be altered from dielectric to metallic in
optical sense [82].
Unlike metals, the graphene can support selective polarization coupling of
plasmons due to the influences from both interband and intraband conductivities
[32, 84, 85]. Similarly the optical response of graphene can also be tuned over wide
range of frequencies.
Plasmonic properties: When the incident photons or electrons hit the metal-
dielectric interface, collective oscillation of electrons at the surface of conductors
happens. This is generally referred to as surface plasmons. Among the metals gold
(Au) and silver (Ag) are considered to be the finest plasmonic materials. The
problem with these metals is that they suffer with huge resistive losses and the lack
of tunability of these metals to control the surface plasmons. Since the discovery of
Graphene, it has become a promising plasmonic candidate for guiding plasmons at
terahertz frequencies [32]. It was observed that the surface plasmons in graphene
can be easily coupled to the incoming light at terahertz frequency. Moreover,
attractive properties such as the tunability of SPs using chemical doping and bias
gating, close confinement and long propagating ranges of surface plasmons were
observed [33]. Hence there is every possibility of metal plasmonics being replaced
by graphene plasmonics with wide range of proven applications [86-88]. Lightly
doped graphene has exceedingly low chemical potential with dynamic conductivity
over a large photon energies making the graphene behave like a semiconductor.
13
This semiconductor like behaviour of graphene facilitates the propagation of
transverse-electric (TE) SPP waves [89]. For highly doped graphene with giant
chemical potential the graphene behaves like metal similar to that of noble metals.
This behaviour in turn facilitates the propagation of transverse magnetic (TM) SPP
wave. Moreover, recent technological breakthrough has made the excitation and
detection of SPP waves in graphene a practical possibility. Though it is possible to
excite surface plasmons on graphene, it is a demanding and challenging task to
excite and detect it on graphene films. Because the wavevector mismatch between
free space photons and graphene plasmons is enormous. The latest breakthrough in
exciting the graphene plasmons was achieved by illuminating infrared light using
scattering-type scanning near-field optical microscope (s-SNOM) on graphene [90].
This provided researchers with real time pictures of graphene plasmons which
helped in calculating its confinement factor. It was reported that the confinement
factor of graphene plasmons is around 40. This research was done on a Si based
semiconductor device using graphene oxide. Hence, there is every possibility that
the future research will pave way for the development of hybrid devices involving
metal, semiconductor and graphene.
1.4 Graphene based photonic devices
The excellent optical, electrical and thermal property of graphene has motivated
researchers for developing optoelectronic and photonics based state-of-the-art
applications such as graphene waveguide, broadband polarizer [89, 91], graphene
modulator [92], graphene photodetector [93], surface plasmon enhanced
photodetector, saturable absorber for mode-locked and Q-switched pulse lasers
[94], broadband optical limiter [95] etc. The distinctive electronic structure of
graphene is somehow responsible for the broadband performance of these
photonic devices. The operation range of these devices is from visible to near-
infrared (NIR) wavelength.
Apart from these there are other applications such as Ultrafast transistors,
photovoltaic devices, Light emitting devices, Touch screens, Flexible smart
windows and bistable displays, optical limiters, optical frequency converters [96],
14
composite materials paints and coating, energy generation, storage and bio-
applications [97].
1.5 Multilayer graphene
Bi-layer graphene (BLG), three-layer graphene (TLG) and 4-layer up until 10-layers
are referred to as multilayer graphene (MLG). Beyond 10-layers it is treated as thin
graphite sheet and is usually considered as 3D material. As mentioned in section
1.1, the SLG contains two carbon atoms A and B per unit cell as shown in Figure 1
- 1(a). By assembling SLG layers on top of one another (c-axis or crystal-axis) in an
AB (or Bernal) stacking arrangement in which the carbon atoms A1 and A2 are on
top of each other and the atoms B1 and B2 respectively sits in the vacant centres of
hexagons of adjacent graphene layers, one can build a bi-layer graphene (BLG) as
shown in Figure 1 - 2(a). Similarly, if SLG is stacked on top of BLG where the
alignation of A3 atom is over A1 and B3 over B1, it forms a three-layer graphene
(TLG) as shown in Figure 1 - 2(b). Four layer graphene (4-LG) is nothing but the
superimposition of two unit cells of BLG on top of each other. Therefore we can
easily see that the four carbon atoms A1, A2, B1 and B2 will form the unit cell of
graphite in AB stacking. First Brillouin zone of SLG displays great symmetry as
shown in Figure 1 - 2(c) where K and K’ points are at the corners of the hexagons,
M points in the middle of the hexagonal sides and point at the zone center.
Figure 1 - 2: (a) The unit cell of a bilayer graphene with x and y denoting the unit
vectors of carbon atoms (b) The unit cell of a trilayer graphene which is nothing
but the stacking or bilayer graphene with a single layer graphene on top (adapted
15
from Ref. [98]) (c) Calculated phonon dispersion relation of six phonon
branches, namely iLO, iTO, oTO, iLA, iTA and oTA of single layer graphene
(Adapted from Ref. [81]).
In order to understand the Raman spectra of graphene it is very essential to
comprehend its phonon dispersion. There are six phonon dispersion bands in SLG
as it has two carbon atoms A and B per unit cell. These six bands consists of three
optic (O) phonon branches and three acoustic (A) phonon branches. The atomic
vibration for one acoustic (A) branch and one optic (O) phonon branch is
categorized as out-of-plane (o) phonon modes as it is transverse to the graphene
plane. For the remaining two acoustic and optic phonon branches the vibrations
are in-plane (i). Conventionally speaking, if the direction of vibrations in graphene
are along the direction of its nearest carbon-carbon atom alignment then it is
considered to be longitudinal (L) vibrations and if it the vibrations is at right angle
to the carbon-carbon atom direction then it is classified as transverse (T) vibration.
Hence the six phonon dispersion curves along the M and K directions are LA,
LO, iTA, oTA, iTO, and oTO phonon modes as shown in Figure 1 - 2(c).
Sometimes in literature MLG is denoted as few-layer graphene (FLG). If the MLG
sample is obtained through mechanical exfoliation of natural graphite or highly
oriented pyrolytic graphite (HOPG) then the exfoliated film is usually in Bernal AB
stacking order. But, if the graphene is obtained through other growth techniques
then the orientation of graphene is rotationally random with respect to each other
along the c-axis. Crystallographic stacking in MLG allows for many different types
of configuration [40, 69, 99-101].
For example, the three layer graphene (TLG) has 2 different types of stacking order
namely, Bernal (ABA) stacking and rhombohedral (ABC) stacking. While the ABA
stacked TLG acts like semimetals with electrically tunable band overlap [40, 102-
104], ABC TLG acts like a semiconductor with electrically tunable band gap. As the
stacking order varies in MLG its electronic properties [40, 99, 101-117], band
structure [40, 99, 101-104, 110-113], spin-orbit coupling [115], magnetic state [116,
117] and interlayer screening [114] varies accordingly. There is an increasing
demand for MLG as it has amazing fundamental properties that can be tailored
based on stacking order and has many applications. The existence of stable poly
16
types of FLG provides a new tool to tailor the electronic structure for both
fundamental studies and applications [40, 69, 102, 105, 110, 111, 118].
1.6 Silver and gold nanosheets
The science that deals with the behaviour and treatment of electromagnetic signals
by coherent coupling of photons to unbound electron oscillations at the conductor-
dielectric interface is referred to as 'Plasmonics'. Ritchie [119] predicted that, at
metal surfaces there could exist self-sustained collective excitations during his
investigation of the characteristic energy losses of fast electrons passing through
thin metal films. In the course of his further investigation about the impact of the
film boundaries on the production of collective excitations he found that the
boundary effect is the main cause for the appearance of a new lowered loss due to
the excitation of collective surface oscillations. Powell and Swan [120] in a series of
electron energy-loss experiments demonstrated the existence of these collective
excitations at metal surfaces, generally referred to as Surface Plasmon Polaritons
(SPPs) that propagates along the surface of a conductor, the quanta of which Stern
and Ferrell [121] called ‘the surface plasmon’. If these collective electronic
oscillations are confined to the material then they are referred to as localized
surface plasmon resonance (LSPR) [122]. The LSPR is determined by the geometry
and composition of the material [123].
Among all the metals that support plasmon resonance, Au and Ag were the most
widely studied for the past 20 years because of the fact that they support SPP and
LSPR in visible and near-infrared (NIR) regime [122]. Many of the metal
nanosheets and nanoplates are so thin that they appear semi-transparent when
viewed under microscope. Most of the research interest was directed at Au [124-
126] and Ag nanoplates and nanosheets as they had very strong NIR plasmon
absorption peak compared to other particle geometries [127-130]. Stability of these
metal films is an issue because of the high surface energy at the sharp corners and
rounded edges [131]. They are sensitive when exposed to high temperature and
caustic surrounding environment. Because of their large surface areas and sharp
geometric features noble metal nanosheets and nanoplates are used as catalysis
17
agents [123, 132], surface enhanced Raman scattering (SERS) [122, 133, 134], LSPR
based sensing [135, 136] and many more applications.
1.7 Hybridization of materials in general and
their latest applications and development
Engineering hybrid 2D material employing different chemical species is the latest
revolution the field of nanotechnology is going through. This is due to the fact that
astonishing functionalities can be attained through synergetic coupling between the
materials [137-140]. This gives us the freedom required to tailor band gaps of
hybrid materials keeping a specific application in view which might not be possible
for a material made-up of single component. 2D nanosheets obtained through
mechanical or chemical exfoliation have high uniformity in its crystal structure and
morphology [141-147]. As the 2D materials have large surface areas, chemical
interactions between two layers made of different materials each having sub-
nanometre thickness can be altered through interaction with guest species giving
unique properties to the resulting hybrid material. The hybridization can occur
between organic, inorganic, metal, semiconductor, polymers, biomolecule layers
etc., giving rise to materials with diverse chemical composition, crystal structure,
surface properties and so on. Moreover the electronic states of hybrid materials can
be controlled which in turn facilitates the optimization of physiochemical
properties.
For the material to be exfoliated into a 2D material it should possess certain
characteristics like high anisotropy, strong in-plane bonds between atoms and very
weak van der Waals forces between out-of-plane atoms. A variety of material
satisfies these conditions in nature and hence has been recently exfoliated into 2D
materials. Some of them are then classified as layered metal oxides [148-154],
layered double hydroxides [155, 156], layered metal chalcogenides [16, 157-165] etc.
Some of the areas where 2D hybrid materials find applications are in field of energy
storage such as fuel cells, supercapacitors, Li-ion batteries and environmental
technologies such as photocatalysts, carbon dioxide capture etc [137, 166-169].
18
1.8 Raman spectroscopy as tool
Raman spectroscopy is a technique that is extremely sensitive to subtle changes in
the geometry and bonding within molecules. Even small deviations in crystal
structures and their alignments lead to marked difference in its Raman spectrum.
As the carbon allotropes like diamond, graphite, graphene, fullerenes, nanoribbons
and nanotubes only vary in their geometric structure and the alignment and
bonding between atoms within the molecule, Raman spectroscopy comes to the aid
of researcher in studying their properties. Moreover, Raman spectroscopy is
extremely sensitive to changes in external factors such as doping, temperature,
pressure, surface impurity and environmental effects. It is usually performed in
room temperature under normal atmospheric pressure with minimum operational
skills. Since Raman spectroscopy uses photon (which is a chargeless and massless
particle) as a probe, it is classified as a non-invasive and non-destructive
spectroscopy technique.
The basic working principle of Raman spectroscopy depend upon the phenomenon
called as inelastic scattering of light where the kinetic energy of photons are not
conserved. Adolf Smekal was the one who first predicted the existence of inelastic
scattering [170] and it was later practically observed by Sir C. V. Raman in 1928
[171] for which he was awarded Noble prize.
When a sample is illuminated with a monochromatic light (usually from lasers),
certain portion of it is absorbed and then reemitted inelastically with a different
frequency. This inelastic scattering can be of two types. If the final vibrational state
of material has higher energy than its initial state then the scattered photons will
have lower frequency to maintain total energy balance. This is called as Stokes
scattering. But, if the final vibrational state of material has lower frequency than its
initial state then the scattered photons will have higher frequency, this is called anti-
stokes scattering. This inelastic scattering contains the information of vibrational
modes which are unique to every molecule and works as materials fingerprints.
Raman spectroscopy was historically used for structural characterization of
19
graphitic materials [98]. It is the most versatile technique for understanding the
behaviour of electrons and phonons in graphene [69, 96, 97, 172-174].
1.9 Third order Nonlinearity
In conventional optics, the electric polarization vector is assumed to be linearly
proportional to the Electric field strength of an applied optical wave as shown,
(1.1)
where, is the susceptibility of a given medium and is the free space
permittivity.
Based on this linear assumption it was believed that there can be no coupling
between different light beams or different monochromatic components when they
pass through the same medium. Similarly it was assumed that the attenuation of an
optical beam propagating in an absorbing medium is linearly proportional to the
local intensity itself. Moreover it was supposed that the transmitted intensity of
light through a medium was linearly proportional to the incident intensity, based on
the belief that refractive index of a medium is constant and independent of the
incident beam intensity for a given wavelength. This notion of linearity in optics
changed after the invention of the first laser. The above mentioned equation was
unable to explain second harmonic frequency generation at an optical frequency
observed in the piezoelectric crystal sample pumped with laser beam. Moreover, it
was unable to explain optical sum-frequency generation, optical difference-
frequency generation and third-harmonic generation discovered subsequently. It
was then realized that the new effects could be realistically explained by replacing
the linear term on the right hand side of the Eq. 1.1 with a power series as shown,
[ ( ) ( ) ( ) ] (1.2)
where, ( ), ( ) and ( ) are first order (linear), second order (non-linear), and
third-order (non-linear) susceptibilities and so on, eventually giving rise to the birth
of a new science called ‘Non-linear Optics’ [175]. In non-linear optics the principle
of superposition is no longer valid. As a result the interaction between the spectral
20
components of the electromagnetic field occurs through the nonlinear interaction
with the matter. Optical Kerr effect, stimulated Raman scattering, self-focusing and
Optical Solitons are few more examples which fall under non-linear optics. The
first order (linear) susceptibility ( ) in Eq. 1.2 describes the effect of the incident
light in creating net dipole moment within an atom. The real part of this
susceptibility corresponds to real part of the refractive index and the imaginary part
corresponds to optical gain or loss. The third-order (non-linear) susceptibility can
be determined using Z-Scan technique as explained in next section.
1.10 Z-Scan Theory
In a Z-Scan experiment [176], sample under test is moved along the propagation
path (z) of a focused Gaussian beam. The transmittance at the other end is
measured using a photo detector in the far field through finite aperture as shown in
the Figure 1 - 3. It is reported that the sensitivity of Z-Scan technique is better than
λ/300 wavefront distortion. Many materials possess both nonlinear refraction and
nonlinear absorption properties and it is possible to separately evaluate their effect
by performing Z-Scan with and without the aperture respectively. The Figure 1 - 3
shows a sample made of nonlinear medium is moved along the focused Gaussian
beam and the transmittance through it is measured in the far-field as a function of
the sample position z measured with respect to the focal plane.
Figure 1 - 3: Schematic diagram of Z-Scan set-up [177]
I performed Z-Scan measurements by scanning the sample from the far end of the
focussed beam i.e., negative z end. Initially the beam irradiance is low and negligible
nonlinear refraction is recorded. Hence, the transmittance (D2 /D1 in Figure 1 - 3)
21
remains relatively constant. But as the samples moves from negative z direction to
positive z direction it crosses through the point where beam is sharply focused, as a
result the beam irradiance increases due to collimation, leading to negative self-
lensing in the sample as shown in Figure 1 - 4(a). The measured transmittance at
the aperture increases as a result of beam narrowing at the aperture. Once the
sample crosses the beam’s focal plane and continue to traverse towards the positive
z direction the opposite phenomenon prior to focus occurs i.e., the beam tends to
become self-defocused leading to the increases in beam divergence, which causes
the beam broadening at the aperture, and thus a decrease in transmittance as shown
in Figure 1 - 4(b).
Figure 1 - 4: (a) Narrowing and (b) Broadening of the beam as sample passes
through the focal plane
If the recorded transmittance at the detector has a pre-focal transmittance
maximum (peak) followed by a post-focal transmittance minimum (valley), it
indicates the Z-scan signature of a nonlinear saturable absorption phenomenon and
the reverse phenomenon indicates Z-scan signature of a two-photon absorption
phenomenon as shown in Figure 1 - 5(a) and (b) respectively. Through these
observations it can be inferred that a multi-photon absorption suppresses the peak
and enhances the valley, while saturation absorption process produces the opposite
effect.
22
Figure 1 - 5: (a) Typical signature of a saturable absorption and (b) Two Photon
absorption curves.
The robustness of the Z-Scan technique is that the removal of the aperture
completely removes its sensitivity to nonlinear refraction effect. However, in this
case, the Z-scan will still be sensitive to nonlinear absorption. As a result the
nonlinear absorption coefficients can be extracted from such “open” aperture
experiments.
It was S. Mansoor et al., [176] who first reported this sensitive single-beam
technique for measuring both the nonlinear refractive index and nonlinear
absorption coefficient for a wide variety of materials. In nonlinear optics, Non-
linear absorption coefficient β and non-linear refractive index ‘n2’ (also known as
Kerr nonlinearity) are measured respectively using "open" and "closed" methods
via Z-scan measurement technique. It can be inferred that the nonlinear absorption
can affect the measurement of the non-linear refractive index; hence the open
aperture method is typically used in conjunction with the closed aperture method.
Outline For the fabrication of novel hybrid nanostructures it is essential that the properties
of all its components are thoroughly understood. One of the best tools to study any
new material is Raman spectroscopy. Hence I undertook Raman study to
characterize the hybrid materials and reported astounding results concerning its
hybridization at five different excitation wavelengths. As hybrid materials are
usually targeted for application like manufacturing semiconductors, saturable
absorbers etc., it is essential that I study their non-linear properties. To that end,
this thesis will present both theoretical and practical techniques needed to study the
23
third order non-linear properties of hybrid materials using femtosecond lasers for
near-infrared (NIR) frequencies. Hence, it can be broadly stated that there are two
main objectives of this thesis. The first objective is to study the effect of
hybridization of 2D materials such as single layer graphene and multi-layer
graphene with single crystalline silver nanoplates and single crystalline gold
nanosheets. The second objective is to study the third order nonlinear behaviour of
multi-layer graphene, single crystalline gold nanosheets and polycrystalline thin gold
films individually along with their combined hybrids. This thesis has been presented
in chapters with each chapter highlighting precise methodology and results towards
characterizing and studying properties of hybrid material. The current chapter
outlines the theoretical concepts and experimental techniques that will be applied
for explaining the rest of this thesis.
In chapter 2, I will explain in detail the experimental setup of Raman spectroscopy
and femtosecond Z-Scan system. As I performed Z-scan experiments for varying
pulse repetition rates and different wavelengths, I noticed that the pulse width of
femtosecond laser slightly fluctuates with the change of operational frequency.
Hence, I will explain the pulse width measurements carried out using frequency-
resolved optical grating (FROG). I will then describe the equipment and technique
used for picking femtosecond pulses to change the repetition rate. I will proceed to
explain the material characterization of single layer graphene, multi-layer graphene,
single crystalline silver nanoplates and single crystalline gold nanosheets using
plethora of different techniques like reflection mode optical microscopy,
ellipsometry, scanning electron microscopy, atomic force microscopy, Raman
spectroscopy etc.
In chapter 3, I will demonstrate the Raman spectroscopy study of silver nanoplates
hybridized with single layer graphene for five different wavelengths. I will analyse
the evolution of different band structures of this hybridized material in detail. I will
demonstrate that the material has indeed been hybridized. To cross confirm it I will
analyse the evolution of Raman spectrum for single layer graphene hybridized with
oxidized silver nanoplates and show that there is no hybridization phenomenon
taking place because of the oxidation barrier which prevents the transfer of charges
from one material to another. I will theoretically calculate the work function of
24
graphene after hybridization with silver nanoplates to confirm the type of charge
transfer taking place between the two materials. I will also demonstrate that on
photo-thermal reshaping of silver nanoplates the Raman spectrum is enhanced
which helps in separating the effect of doping due to hybridization from LSPR
effect due to sharp tip geometry of plates.
In chapter 4, I will begin by explaining the procedure for fabricating hybrid films
using single crystalline gold nanosheets multilayer graphene. I will show the Raman
spectroscopy study of single crystalline gold nanosheets hybridized with single layer
graphene and multilayer graphene for five different wavelengths and confirm the
effect of hybridization in it. I will compare the effect on hybridization due to
change in graphene layers.
In chapter 5, I will show the procedure for fabricating polycrystalline thin gold film
on multilayer graphene sample. I will then begin with Z-Scan experiments of single
crystalline gold nanosheets and polycrystalline thin gold film. I will compare the
nonlinear effect of these two thin films by removing any traces of pulse width
variation from the measurements. I will show that the difference in their nonlinear
coefficients is due to the surface roughness which produces cone shaped island in
polycrystalline films which enhances the tip-effect factor thereby causing the
nonlinearities to differ. I will also explain of heating effect due to increased pulse
repetition rate on nonlinear coefficients of gold films. I will proceed to study the
third order nonlinear properties of multi-layer graphene with Z-Scan technique. I
will compare our results with the values reported in literature and highlight our
contribution to this field of research. I will then proceed to study the third order
nonlinear properties of these hybrid materials for different pulse repetition rate and
compare their results. I will then explain the importance of our findings to the
current research occurring in 2D hybrid materials.
In chapter 6, I will summarize the key achievements of this thesis and propose
possible future research work that can be undertaken to advance this field.
25
Chapter 2
Experimental Setup and
Characterization of Nanomaterials
2.1 Introduction
There are numerous techniques for characterizing different aspects of graphene,
such as microscopy and spectroscopy, each with their own advantages and
drawbacks. Using microscopy techniques such as optical microscope, atomic force
microscopy (AFM), scanning tunnel microscopy (STM), scanning electron
microscopy (SEM), transmission electron microscopy (TEM) etc., the graphene
samples can be visualized at different scales, from micrometres to angstroms. For
example, the SEM and TEM can resolve the graphene sample to atomic resolution.
Any defects or missing atoms can be located and analysed. It was through
microscopic techniques that graphene was accidentally discovered [15, 68]. Optical
microscope and AFM require certain substrate for visualizing graphene on top of it.
SEM and TEM can damage graphene samples as they operate at energies beyond
the damage threshold of graphene for very high resolution. Spectroscopy
techniques are suitable for studying the energy distribution of electron and phonon
systems. Moreover, through spectroscopy technique the number of layers, defects,
doping, stain levels, disorders, electronic structure and impurity in graphene can be
analysed in a non-invasive fashion without causing any damage to the sample. Some
of widely used spectroscopy techniques for studying graphene are Raman
spectroscopy, Angle-Resolved Ultraviolet Photoelectron Spectroscopy, X-Ray
Photoemission Spectroscopy etc. The most versatile tool for spectroscopy among
all other techniques is Raman spectroscopy [98]. Using this technique the
measurements can be obtained for different laser excitation in short duration in
ambient environment without causing any significant damage to the material under
test.
26
Similarly there are numerous techniques to measure nonlinear absorption (NLA)
coefficient of a material reported in literature. Some of them are transmission
measurements, three wave mixing, two-photon fluorescence, degenerate four-wave
mixing, photothermal techniques, chirped-pulse pump-probe technique etc. All
these techniques have some advantages and disadvantages, but the majority of them
have one drawback in common and that is the complex optical alignment and wave
propagation analysis which make them very cumbersome to use. The most
commonly used technique amongst them is the Z-Scan technique [176] which falls
under the sub-category of transmission techniques. This is one of the simplest
experimental analysis for measuring material nonlinearity. The open and close
aperture measurement of Z-Scan is extremely sensitive to nonlinear absorption
(NLA) and nonlinear refraction (NLR) effects. The advantage of using this
technique is that the materials exhibiting both NLA and NLR effects can be
separated out by merely adjusting the width of the aperture present just before the
detector.
In this chapter I will describe the principle of Raman spectroscopy, experimental
setup of Renishaw in-via micro-Raman system, Z-Scan experimental setup, pulse
width measurement system, pulse picker, scanning electron microscopy (SEM),
atomic force microscopy (AFM), ellipsometry and characterization of SC-AuNS,
AgNP, SLG, MLG using reflection optical microscopy, SEM, AFM, ellipsometry,
Raman spectroscopy etc.
2.2 Principle of Raman spectroscopy
Further expanding the theory of Raman spectroscopy from previous chapter it can
be underlined that Raman scattering occupies central position in molecular
spectroscopy. Raman spectroscopy is nothing but the study of inelastic scattering
occurring from a molecule after exciting it with monochromatic light using a laser
in near ultraviolet, visible or near infrared wavelengths. The laser light on impinging
with a molecule leads to shift in the energy either due to molecular vibrations,
phonons or supplementary excitations within the system. This shift in energy
occurs only to few incident photons, but this miniscule number of photons with
27
shifted energy gives vital information about the vibrational modes in the system.
Hence this inelastically scattered light is usually referred to as Raman scattering.
Usually in a Raman spectroscopy setup, the sample is illuminated with a laser of
certain wavelength using a focusing lens and the reflected photons are collected
through another lens and sent through a monochromator. The Rayleigh scattering
consisting of elastically scattered radiation of the incident laser wavelength is
filtered out using either a notch filter, edge filter or band pass filter and the rest of
the inelastically scattered light is spread across a detector (usually charged couple
cameras) using holographic gratings or diffraction gratings. The Raman scattering is
exceptionally weak and hence it is very difficult to separate this inelastically
scattered light from the strong elastically scattered laser light. Advanced techniques
have been invented to characterize molecules based upon Raman scattering
principles like the surface-enhanced Raman spectroscopy (SERS), resonance
Raman spectroscopy, tip-enhanced Raman spectroscopy, polarised Raman
spectroscopy, stimulated Raman spectroscopy, transmission Raman spectroscopy,
spatially offset Raman spectroscopy, hyper Raman spectroscopy etc.
Figure 2 - 1: Energy-level diagram showing different types of states involved in
Raman spectral
28
Whenever a molecule is irradiated with a laser light, the molecule can remain either
within its vibrational energy states or it can be excited to some undefined virtual
state as can be seen from the above Figure. This molecular energy state transition
can happen only when the electron cloud within the molecules is polarized with the
incoming laser light. There are four different possibilities. The first possibility
amongst them is that the incident photon (of IR wavelength) excites the molecule
from its ground state to a higher vibrational energy state as consequence of which
photon disappears due to infrared absorption by the molecules. The second
scenario is the one in which the molecule on illumination with a photon is excited
to higher virtual energy state where it stays for short period of time before
returning to its ground state by elastically scattering the photon. This elastic
scattering referred to as Rayleigh scattering. It forms ~99.99% of the scattered light
and has the same frequency as that of the incident laser light source. The third
possibility is that the molecule which is in ground vibration energy state is excited
to a virtual energy state and returns after certain delay to an excited vibrational
energy state different from its original vibration state upon incident with a photon.
This phenomenon is referred to as Stokes Raman scattering. This causes the
photons to scatter inelastically and has lower energy than the incident photon. The
fourth scenario is the one in which the molecule is in an already excited vibrational
state and upon incident with a photon excites to a virtual energy state and returns
after certain time period to ground vibrational state. This also causes the photon to
scatter inelastically and has higher energy than the incident photon. This
phenomenon is referred to as anti-Stokes Raman scattering. There is common
occurrence in both Stokes Raman scattering and anti-Stokes Raman scattering and
that is the final vibration state/energy state of the molecule is different from the
initial energy state. This difference in the energy between initial state and final state
causes the shift in frequency of scattered photon from its original laser excitation
frequency. Stokes Raman scattering causes the scattered photon to have lower
frequency while the opposite is true for anti-Stokes Raman scattering. This shift in
frequency is unique to all molecules and hence is generally referred as finger print
of molecules as it can uniquely identify it from others.
29
In short, we can restate the whole Raman spectroscopy as a phenomenon where
the ability of molecule to get polarized is initiated by the photons of laser light to
induce dipole moment in the molecule and this dipole moment in turn emits
photons while returning back to ground state. This emitted photon containing
different frequency and energy levels are classified as Raman spectra.
2.3 Raman spectroscopy experimental setup
Raman spectroscopy system is a relatively simple device consisting of laser,
microscope with objective lens, notch or band pass filters, Rayleigh filter, coupling
mirrors, holographic notch filter, diffraction grating, optical lens, detector and a
software to operate and process data. The laser systems usually employed in a
Raman system are green (514 or 532 nm), red (633 or 660 nm) and near-infrared
(785 or 860 nm) laser wavelengths. Choosing the right laser wavelength for a
sample to excite its Raman scattering spectra is critical because the inelastically
scattered photons have frequency shift relative to excitation frequency. As per the
scattering theory the excitation laser wavelengths should be below the first
electronic transition of the molecule under test. Molecules that have inherent
fluorescence should be excited with longer wavelengths like the near-infrared
regime so as to avoid interference of fluorescence with Raman scattering. But this
method has a drawback, as it reduces the scattering efficiency which in turn leads to
an increase in the excitation power needed and simultaneous increase in integration
time to process Raman signal. Reduction of scattering efficiency is not a good
omen as only one photon scatters inelastically containing Raman signature for 1
million photons incident on the sample. Hence, Raman spectroscopy is always
performed using lasers as they can pump monochromatic coherent photons at high
input powers with a stable collimated beam for long distances. Visible wavelength
lasers are best to analyse inorganic materials like graphene, carbon nanotubes,
fullerenes etc., metal oxide, minerals and surface enhanced Raman scattering. UV
lasers are used to analyse bio-molecular samples such as RNA, DNA, proteins and
for fluorescence suppression. By using lasers with smaller wavelengths, higher
sensitivity and better spatial resolution can be achieved.
30
Figure 2 - 2: Block diagram of a Renishaw Raman spectroscopy set up
The Renishaw in-via Raman system at our lab has 457 nm, 487 nm, 514 nm, 633
nm and 785 nm laser wavelengths. Laser light is focussed into the system through
neutral density (ND) filters which helps in controlling the intensity of the incident
laser light as shown in Figure 2 - 2. The laser after passing through the ND filters
are focussed onto a spatial filter in order to remove high order spatial modes and
thereby reducing laser-induced noise into the system. This spatial filtering help in
better focusing of the laser light onto the desired sample placed under the
microscope after traversing through two lenses to form a fully expanded beam to
fill the back-end of the objective lens. This ensures that the smallest focal spot is
produced while focussing on the sample after passing through series of mirrors.
The microscope system used in our experiment is manufactured by Leica and it has
five numerical objective lenses with the highest resolution provided by a 0.85
numerical aperture (NA) lens having 100 × magnification. It has a motorized
rotational stage that can be moved with sub-micrometre precision along the X, Y
and Z axis either manually or through the inbuilt software program.
The microscope shown in Figure 2 - 2 can house one extra optical component at a
time, such as a half wave plate, quarter wave plate, linear or a circular polarizer
along the beam path. Laser light scattered from the sample is collected by the
31
objective lens and directed towards the detector through a series of filters and
gratings. The reflected laser light is first incident on the holographic filter (or notch
filter) that removes all the elastically scattered light having same frequency as laser
light. This filtered light is known as Rayleigh scattering as explained in the previous
section. This holographic filter also serves as a mirror to focus the beam on
objective lens. The light that passes the holographic filter has the Raman signature
of the sample under test. The back scattered light then passes through another
holographic filter to remove any traces of Rayleigh scattering. Raman light after
passing through the second holographic filter is dispersed into different
wavelengths using a diffraction grating which is then simultaneously converted and
measured as separate frequencies. Finally the Raman signals are focussed onto the
thermoelectrically cooled charge couple device (CCD) camera after passing through
the diffraction grating. The scattered light after impinging upon the CCD camera
generates photoelectrons. Inside the CCD camera the scattered light is distributed
vertically across rows of pixels which are clubbed (or summed) together to
add/integrate each signal. The resulting electric signal is displayed on the computer
screen after signal processing by the inbuilt software system.
The Laser, holographic filter and diffraction grating are the manually variable
features of this Raman spectroscopy setup. The notch filter is selected based upon
the incident laser wavelength. Diffraction grating comes in three different densities
(1200, 1800 and 2400 grooves/mm). The greater the groove density better is the
spatial resolution of the system. By increasing the focal length of the spectrometer
one can increase the spectral dispersion of the system.
The optical block diagram of the dispersion of inelastically scattered light in a
Renishaw InVia Micro-Raman is as shown in Figure 2 - 3. The slit located just
before the dove mirror in Figure 2 - 2 is further simplified and shown at the centre
of Figure 2 - 3 which functions as pinhole rendering the whole system to be a
confocal Raman system.
32
Figure 2 - 3: Block diagram of the dispersion of inelastically scattered light in a
Renishaw InVia Micro-Raman
A horizontal slit coupled with a CCD camera removes the requirement of pinhole
essential for developing any confocal system. The slit can be manually or
automatically aligned using the inbuilt software before measuring any signal in the
confocal setup.
2.4 Z-Scan setup
I setup a home-built Z-Scan system in order to measure third order nonlinear
(NLA) coefficient of multilayer graphene, single crystalline gold nanosheets and
polycrystalline gold thin film and their hybrids. The detailed block diagram of Z-
Scan is as shown in the Figure 2 - 4. Ti-Sapphire femtosecond laser (Tsunami) is
used for the measurement of nonlinear absorption coefficients as nonlinearity in
materials can be excited only at higher excitation laser power. Ti-Sapphire
femtosecond laser setup consists of Ti-Sapphire oscillator, which is pumped
through a 532 nm continuous wave diode-pumped solid-state laser (Millennia eV
with 10W maximum power). As a result of this a full tuning range of wavelengths
stretching from 700 nm to 1100 nm is generated out of the Ti-Sapphire oscillator.
The Ti-Sapphire oscillator is connected to regenerative mode-locking mechanism
which sustains pulses of 82 MHz repetition rate. The advantage of Ti-Sapphire
33
femtosecond laser is that it has operational simplicity and can be tuned between
continuous wave mode and pulsing mode with extreme ease. Ti-Sapphire
femtosecond laser system is sensitive to change in pulse width caused due to the
change in excitation wavelength. Hence I used a modified version of frequency-
resolved optical-grating (known as FROG from Swamp optics Inc.) to measure
ultrashort pulse width our system. The details about the FROG measurements and
its setup are given in next section. The laser wavelength spectrum was monitored
by installing a spectrometer (Ocean optics) that can detect electromagnetic radiation
from 10 nm to 2000 nm. The spectrometer is followed by a shutter control and
then by a pulse picking system as shown in Figure 2 - 4.
Figure 2 - 4: Detailed block diagram of a home built Z-Scan system
The pulse picking system picks individual pulses or pulses with repetition rate up to
10’s of megahertz from the pulse train of the femtosecond laser. The pulse picking
system consists of 3 vital components, they are an optical modulator, a modulator
driver and synchronization electronics. The working principle and the measurement
techniques are explained section 2.6 of this chapter. The laser beam is reflected
perpendicularly from an ultrafast mirror and then passes through polarizers (Glan
Thompson), which basically acts as neutral density filters. In order to linearly
34
polarize the laser beam half-wave plate is used. Similarly a quarter wave-plate is
used for circular polarization in the whole setup. The laser beam again reflects from
another ultrafast mirror and enters into a beam expansion system as shown in
Figure 2 - 4. The expanded beam then traverses through an Iris to remove any
noise and higher order modes from entering into system. The noise filtered beam
then fills the back aperture of the objective lens producing a focal spot of Gaussian
characteristics. This objective lens is also used for visualizing the single crystalline
gold nanosheets by focusing the backscattered light onto a charged couple device
(Hamamatsu) through a one directional reflecting prism. The beam from the
objective lens in the forward direction is then focused on the sample which is
mounted on a motorized linear stage (Newport UTS190PP) having approximate
range of 190 mm. This motorized stage is controlled using a unidirectional
controller (Newport ESP301-1N) which comes with inbuilt software. The laser
beam is then incident upon a detector (Newport 918D) after passing through an
aperture. The detector is connected to a bench top two-channel optical power
meter (Newport 2936-R). The aperture can be closed or opened to measure NLA
and NLR effects in the sample under intense laser beam intensity. The power
measurement obtained through the power detector and the motorized linear stage
control is achieved through meticulously incorporating their respective software on
the computer without any delay between their operations.
For all the Z-Scan measurement mentioned throughout this thesis I used
femtosecond laser pulses with pulse width ranging from 115 fs to 130 fs having
wavelength range tuned between 700 nm to 900 nm with repetition rates of 0.82
MHz to 82 MHz. The objective lens used has a numerical aperture 0.25 (Olympus)
to produce an airy focal spot with ~4µm, measured for 780 nm laser wavelength.
The average power at the back aperture of objective lens is varied from few
microwatts to tens of milliwatts.
2.5 Measurement of pulse width
It has always been a challenge to measure ultrashort pulses specially the
femtosecond pulses as they are one of the shortest events ever generated. Since
35
many years ultrashort pulses were generated with ease due to the industrial scale
production of femtosecond lasers, but were equally difficult to measure. Many
techniques such as autocorrelation and spectroscopy were tried but it produced
ambiguous results. The matter of fact is that autocorrelation is a very difficult
technique to measure ultrashort pulses as they use second harmonic-generation
(SHG) crystals, wherein the pulse under test is split into two pulses and then
focussed and recombined both in space and time onto the SHG crystal. This
require painstaking efforts to align the pulse in three sensitive degree of freedom
i.e., two spatial and one temporal and to maintain this alignment even when the
delay changes. Moreover, the drawback of using SHG crystal is that it is required to
be thin which yields a very weak signal and reduces measurement sensitivity.
Since few years advances has been made in measuring ultrashort pulses at the cost
of increasing complexity. One such popular technique is Frequency-Resolved
Optical-Grating (FROG) [178] that gives full information of intensity and phase of
the ultrashort pulse with respect to time. Frog has the same complex underlying
principle of an autocorrelator with a spectrometer as an additional component.
In recent years an extraordinarily simple FROG device has been invented and
named as Grenouille (Swamp Optics Pty Ltd, USA) [179] which removes all the
drawbacks and complexities of previous devices as shown in Figure 2 - 5(a). In this
setup a simple element known as Fresnel biprism replaces beam splitter, delay line
and beam combining optics. Moreover, it breaks the convention by introducing
thick SHG crystal in place of thin crystal (as used in autocorrelators and FROG)
thereby simplifying the phase-matching-bandwidth requirement. The thick SHG
crystal improves the signal strength of ultrashort pulses under measurement and
simultaneously replaces the spectrometer.
A Fresnel biprism [180] is like any other ordinary prism, but with an obtuse apex
angle (179○) close to 180○. Fresnel biprism on illumination with a wide beam
produces interference fringes by splitting the incoming beam into two beamlets and
crossing them across each other as shown in Figure 2 - 5(b).
36
Figure 2 - 5: (a) A schematic diagram of GRENOUILLE setup which is
insensitive to alignment parameters contains a Fresnel biprism that replaces the
beam splitter, delay line, and beam-recombining optics of autocorrelator. It also
shows a thick SHG crystal which acts as both the nonlinear-optical time-gating
element and the spectrometer. (b) Crossing beams that are automatically aligned
both spatially and temporally at an angle using a Fresnel biprism (different
colours are used to distinguish the beams. (c) The very thick SHG crystal
substitutes for thin crystal and spectrometer and generates second harmonics of
all colours in the forward direction after being illuminated with a broadband
light.
Generating fringes is not of any importance in measuring ultrashort pulses, but the
very fact that the Fresnel biprism generates crossing beams at an angle is of vital
significance as this is required in traditional autocorrelators and FROG devices.
This crossing of beam generates relative beam delay which is mapped onto the
horizontal position of crystal. The advantage of using Fresnel biprism is that it
automatically aligns the split beams spatially and temporally leading to significant
simplification of the device. But, unlike conventional single-shot geometries, beams
that are split and crossed by a Fresnel biprism are automatically aligned in space and
in time, which is a significant simplification.
37
The beam then is incident on a thick SHG crystal of Grenouille which has two
purposes. The first is the generation of variable delay between the two identical
beams coming from biprism as it crosses the SHG crystal, which is nothing but a
self-grating process. Secondly it acts as a spectrometer by converting the incidence
angle into wavelength. As the crystal has relatively small phase matching bandwidth
it produces wavelength varying with incident angle as shown in Figure 2 - 5(c).
Grenouille device is completed by installing two additional cylindrical lenses into
the setup as shown in Figure 2 - 5(a). The purpose of the first lens is to focus the
beam through Fresnel prism onto the SHG crystal in such a manner that it
produces array of crystal incidence angles large enough to cover the complete
spectrum of the pulse. The second cylindrical lens positioned after the SHG crystal
maps the beam exiting from the crystal onto the vertical position at the camera.
The individual wavelength of this beam corresponds to near-linear function of
vertical position in the camera.
Figure 2 - 6: Intensity autocorrelation, typical FROG trace along with both
temporal and spatial profile of Ti:Sapphire femtosecond laser at 780 nm
wavelength acquired by Grenouille.
38
Figure 2 - 6 shows the intensity autocorrelation, typical retrieved FROG trace along
with both spatial and temporal profile of Ti:Sapphire femtosecond laser at 780 nm
acquired by Grenouille as explained above. In this thesis I measured the pulse
width of our femtosecond laser pulses and I found that the pulse width varied from
115 fs to 130 fs for wavelength range tuned between 700 nm to 900 nm
respectively. This is important while measuring NLR and NLA of our samples as I
can remove the effect of pulse width from our calculations.
2.6 Pulse selection system
As outlined in the section 2.4 of this chapter, a pulse selection system consist of an
optical modulator, a modulator driver and synchronization electronics. The optical
modulator is also sometimes referred to as pulse picker as it picks individual pulses
from a train of picosecond or femtosecond pulses. Pulse picker also controls
femtosecond multi-pass amplifier and regenerative amplifier.
The basic working principle of pulse picker relies upon Pockels effect. Pockels
effects also known as a linear electro-optic effect produces change in the refractive
index of the medium by a constant or varying electric field. The Pockels effect is
directly proportional to the applied electric field. Pockels effect is generally
exhibited by Pockels cells which are nothing but voltage controlled wave plates.
The change in the birefringence of Pockels cells sometimes results in the rotation
of polarization of abeam passing through it. The Deuterated potassium dihydrogen
phosphate (KD2PO4) abbreviated as DKDP is an electro-optical crystal placed
inside the pulse picker which acts as Pockels cell. This Pockels cells is placed
between two different polarizers that are oriented at right angles with one another
as shown in Figure 2 - 7. As can be seen the femtosecond pulse train passes
through the first polarizer and gets linearly polarized. Extremely high voltage (~10
kV) is applied across the DKDP electro-optic crystal functioning as Pockels cell to
induce birefringence in it. On application of enough electric filed the birefringent
phase difference across the Pockels cells reaches λ/2. As a result of this the
incoming polarized femtosecond beam rotates 90○ and easily passes through the
subsequent polarizer. But, when no electric field is applied across the Pockels cell
39
the polarization of the femtosecond pulse train doesn’t rotates and gets reflected by
the second polarizer.
Figure 2 - 7: Block diagram depicting the detail operation of a pulse picker. The
fundamental building component of a pulse picker is a Pockels cell that changes
the polarization of incoming light through the application of extremely high
voltage across it.
The modulator driver is nothing but a series of high voltage pulse (~8 ns duration)
produced by a high voltage power unit governed by a control unit. The control unit
is in need of four specific signals in order to generate high voltage signals; they are
femtosecond optical pulse train, external trigger signal to indicate the arrival of
pulse train so as to lock it with the opening of Pockels cell. The cell opening is
determined by two delay signals Delay-0 and Delay-1 also referred to as channel A
and B.
In our thesis I picked the pulses from inbuilt femtosecond laser pulse train of 82
MHz. For NLR and NLA coefficient measurement using Z-Scan measurement, I
used the original pulse train of 82 MHz, picked trains of 8.2 MHz and 0.82 MHz
using the above system. I maintained a constant ratio of 100 between the picked
pulse and unpicked pulses so as to obtain enough power at the back aperture of our
objective lens.
40
2.7 Scanning Electron Microscopy
The best tool for studying the surface morphology of a specimen among many
available tools is scanning electron microscopy (SEM). In SEM raster scanning of
focussed beam of electrons along with the knowledge of beam position generates
images of the sample under test with sub-nanometre resolution. The electron beam
on interaction with the sample produces several different types of signals which
encompass information about the surface topography and material configuration of
the sample. The depth at which the electron beam interacts with atoms results in
the generation of variety of signals. Different types of signals that are generated
while performing SEM are classified as secondary electrons (SE), reflected or back-
scattered electrons (BSE), photons of characteristic X-rays and
cathodoluminescence (CL), absorbed and transmitted electrons. SEM can be
performed in very high or low vacuum, cryogenic conditions, high temperatures
and in wet surroundings.
The electron beam on interaction with the surface atoms emits secondary electrons
from among many other emitted signals as outlined above. Secondary electrons are
inelastically scattered from the k-shell of the surface atoms on excitation with beam
electrons. Secondary electrons possess low energy as they spurt-out from within a
few nanometres of the specimen surface. This close emission of secondary electron
from the specimen surface gives the tool the sub-nanometre resolution for which it
is highly renowned. The most prevalent technique for producing SEM images is to
record and analyse these secondary electrons. The successful detection of secondary
electrons at the receiver depends upon the angle of incident electron beam and the
surface composition of sample. Secondary electron detection is present in every
single SEM machine as a default technique.
There is a drawback to this technique such that the sample needs to have surface
electrical conductivity and the sample needs to be grounded to prevent the build-up
of electrostatic charges at the surface. As a result metal specimens are easy to image
while the non-conducting samples such as organic materials, polymers and
insulators accumulate static charges leading to image distortion. Hence such
41
nonconductive samples need to be coated with a thin layer of conducting materials
such as platinum, gold and other metals.
2.8 Atomic Force Microscopy
Atomic-force microscopy (AFM) is an ultra-high-resolution microscopy which can
reveal features on sub nanometre scale and even up to angstrom scale. It
outperforms optical microscopy by order of magnitude three times better; this is
because the microscope probes the sample through light which has a diffraction
limit. It is referred to as atomic-force because in AFM the information is gathered
by feeling or toughing the surface with a cantilever probe which experiences the
atomic forces between the cantilever tip and sample surface. This atomic force
leads to oscillation in the cantilever’s tip position as postulated in Hooke’s law. As a
result the AFM needs ultra-precision in controlling the movements of cantilever tip
with atomic accuracy which can be achieved through a piezoelectric device and
electronic circuitry.
Most AFM device consists of cantilever with a very sharp tip. The radius of
curvature of tip has thickness in the order of few nanometres. Such a tip can only
be fabricated by using silicon or silicon nitride. The other end of this cantilever tip
is firmly fixed to the device which has a piezoelectric element attached to it. The
piezoelectric element helps to precisely control the cantilever oscillations. AFM has
a sample stage that can be moved in all the three XYZ directions. The deflections
in the cantilever tip position due to the atomic forces present at the surface is
recorded by a detector and converted into electric signal proportional to the tip
displacement from equilibrium position.
The three primary functions of AFM are to measure the force between cantilever
tip and surface, imaging and manipulation. AFM measures the force between
cantilever tip and surface as a function of their mutual separation thereby making
the distance between tip and surface a source for future imaging. When the tip is
brought extremely close to the surface atom it experiences reactionary forces on the
tip which can be used to visualize the three dimensional topography of specimen’s
surface at angstrom resolution. For this to happen, the tip needs to be scanned
42
across the surface. The raster scanning of the surface is followed by recording the
variation is separation of tip from surface as peak or valley feature on the surface.
Usually such topography is presented in the form of false colours by the in-built
software of AFM. The force between the Cantilever tip and specimen surface can
be used to alter the properties of the material in an orderly fashion. AFM is not just
confined to measure the topography of a sample, but it can also measure material
conductivity, surface potential, adhesion strength, stiffness, surface roughness,
material thickness etc.,
2.9 Ellipsometry
In ellipsometry dielectric properties such as complex refractive index of thin film
materials is determined using optical source. It helps in determining material
thickness, surface roughness, electric conduction etc. It is highly sensitive to any
changes such as phase and polarization of incident light upon the specimen. It is
non-invasive and non-destructive technique in measuring thickness and optical
constant of thin films. The principle behind ellipsometry is that the material under
test is irradiated with a linearly polarized light and the change is polarization of the
light upon reflection or transmission is recorded and fitted with a suitable model to
determine the refractive index of the material. The alteration in polarization state of
incident light is quantized in the form of amplitude ratio and phase difference. The
change in polarization is strongly related to intrinsic properties of materials.
It is worth mentioning that the biggest drawback of using light source is its
diffraction limit, but this is overcome by employing the phase information of the
light. The use of phase information of light in ellipsometry gives this technique sub-
nanometre resolution. This indicates that films having thickness range from few
microns to less than one nanometre can be characterized using this technique.
Single atomic layers and one micrometre wide thin films have also been
characterized using this technique. But, the ellipsometry measurements assume that
the sample is homogeneous with discrete and well-defined layers and any deviation
from this assumption may lead to spurious results. Modelling the experimentally
obtained data in ellipsometry is a complex and tedious work.
43
Setting up of an Ellipsometry experiments is a relatively simple task. It consists of
one light source which is usually a laser. The laser light passes through a linear
polarizer which is then incident upon the sample and depending upon the sample
characteristics the polarization of laser light changes. The reflected light is collected
through a detector which houses a phase modulator inside it. The collected light is
then analysed for any change in its phase and amplitude which is then fitted using
the already established models to characterize the specimen under test.
2.10 Sputtering
Sputtering is a process in which high energy gas ions are bombarded onto a solid
target material specimen such as gold, platinum, titanium, aluminium etc., as a result
these high energy ions eject particles such as atoms from the surface of the target
material. For the particles to be ejected from the surface of a target material the
incoming particles need to have higher kinetic energy than the thermal energy of
sputter material itself. Usually such heavy bombardment of high energy particles for
long duration erodes the target surface and can cause permanent damage to
materials. But, this process finds application in variety of techniques such as sputter
coating of thin films, etching and other nanofabrication procedures.
The basic principle behind the working of sputtering mechanism is that when high
energy particles such as ions collide with the target material it starts a series of
collisions within the material. When these ionic collisions recoil and reach an energy
level greater than the surface binding energy, they knock an atom off the target
surface. This is true for thick target materials, but if the target is thin film then these
collisions can eject an atom in transmission mode from the back surface of the
target material provided they possess energy greater than the atom surface binding
energy. Usually these ion sources are selected from inert gases such as argon so as
to avoid any chemical reaction between sputtering source and target material.
Sputter yield is defined as the average number of atoms that are ejected from the
surface of target material for every single ion bombarded onto it. This sputter yield
depends on a lot of factors such as the mass of the ions used, the mass of target
atoms, the energy of incident ion, the angle of incidence of ion, surface binding
44
energy of the atoms to the target, the alignment of crystal axis with respect to target
surface etc. The particles needed for sputtering of atoms can come from various
sources such as plasma, an accelerator, alpha particle emitting radioactive material
or an ion source itself.
2.11 Single crystalline gold nanosheets
Flat two dimensional materials made of metal and possessing excellent unaffected
conductivity under large strains has attracted enormous curiosity for their potential
applications in display devices [181], semiconductor electronics [182], energy
storage device [183] and actuators [184]. Recently such thin metal sheets are sought
after for continuous printing of electric circuit patterns so as to miniaturize
electronic circuitry. Gold nanosheets (AuNSs) are the best candidates for such
applications as they possess long term stability, high metallic conductivity and
electrical stability under high strains. Moreover, Au electrodes are preferred while
making organic devices as their work function is compatible with p-type organic
semiconductors. Numerous techniques such as seed-mediated growth [185],
electrochemical synthesis [186], polyol synthesis [187], biological synthesis [188]
have been used to produce shape and size controlled nanoplates.
The synthesis of gold nanosheets (AuNSs) that I used in our experimental was
done by B. K. Lim’s group according to Ref. [189], and here, I briefly recapture the
synthesis method. For AuNSs synthesis, initially 5 mL of water is taken in a beaker
(which acts as a reactor) and to it 1.7 mg of L-arginine is added. The solution in the
beaker is heated until it reaches 95°C. In the meantime, 13.5 mg of hydrogen
tetrachloroaurate trihydrate (HAuCl4•3H2O, Aldrich) is dissolved into a 2 mL
aqueous solution and it is then rapidly injected into the beaker using a pipette. The
beaker is maintained at 95°C for 2 hours and then allowed to gradually cool to
room temperature. It was reported that L -Arginine served as a mild reductant and
a capping agent, similar to the mechanism of the amino acid-based synthesis of Au
nanoparticles. The resulting wet chemically synthesized AuNSs were found to
possess high resistivity and excellent electrical stretchability. This was the first
procedure ever reported for wet chemical synthesis of single crystalline gold
nanosheets.
45
Figure 2 - 8(a) shows scanning electron microscopy (SEM) image of a chemically
synthesized gold nanosheet (taken using Raith 150Two with 10 nm resolution). The
average dimension of such single crystalline AuNSs was found to be 40 µm × 40µm
in dimension.
Figure 2 - 8: (a) SEM Image of chemically synthesized single crystalline gold
nanosheet (b) AFM image of single crystalline AuNS and the 20nm thickness
measurement taken across a cross section is shown (c) SEM Image showing
single crystalline AuNS edge having thickness of 20 nm (d) Ellipsometry
measurement for refractive index of AuNSs and comparing it with Rakic et
al[190]
Atomic force microscopy (AFM) [Bruker Dimension Icon] was used to determine
the thickness and surface roughness of single crystalline AuNSs. Figure 2 - 8(b)
shows AFM image of single crystalline AuNSs with white line indicating the section
across which the thickness was measured. The thickness was found to be ~20 nm.
The measured average surface roughness of these sheets was 0.143 nm. The
thickness of single crystalline AuNSs was cross-verified by measuring the edge of
AuNSs as shown in Figure 2 - 8(c) and it was found to be 20.10 nm confirming
with the thickness measured through AFM. I measured the refractive index ‘n’ and
extinction coefficient ‘k’ of the AuNSs using the spectroscopic ellipsometer (J.A.
Woollam M-2000DI). As can be seen from the following Figure 2 - 8(d) for
wavelengths ranging from 200 nm to 1600 nm the experimental values exactly
matches to the theoretical values[190]. It signifies that the single crystalline AuNSs
46
grown through chemical synthesis has exactly the same characteristics as that of
bulk gold found in nature.
2.12 Polycrystalline thin gold metal film
Gold thin film of 20nm thickness was deposited on top of SiO2 glass substrate
using sputtering technique as explained in section 2.10 of this chapter. Sputtering of
thin gold film was accomplished using a high vacuum (HV) thin film deposition
system (CMS-18 Kurt J. Lesker, USA). The sputtering process was performed at
room temperature. A 20 nm Au thin film was deposited at a working pressure of
6.6 x 10-4 Pa and a deposition rate of 2 nm/sec. Sputtering power was set to 90 W.
The distance between the substrate and sputtering target sample inside the
deposition system is kept at 30 cm. In order to achieve uniform coating during
deposition the substrate holder rotates around its axis thereby fabricating a
relatively smooth thin film.
Figure 2 - 9: (a) polycrystalline thin gold metal film with a portion peeled off to
measure its thickness. Scale bar is of 2 µm. (b) Thickness measurement of
polycrystalline thin gold metal film using AFM which was ~20nm approximately.
A section of sputter coated polycrystalline thin gold metal film was peeled off to
measure its thickness using atomic force microscopy (AFM). Figure 2 - 9(a) shows
microscope image of polycrystalline thin gold metal film with peeled off region
appearing as black rectangular patch. The measured thickness using AFM
microscopy was found to be ~20 nm approximately as shown in Figure 2 - 9(b).
2.13 Single and multilayer graphene
Single and multilayer graphene (abbreviated as SLG and MLG respectively) was
purchased from Graphene Supermarket (Graphene Laboratories Inc. Calverton,
47
NY). At the fabrication plant, the SLG was grown on copper metal film of
thickness 18 µm and MLG was grown on nickel metal film of thickness 25 µm
using chemical vapour deposition (CVD) and then transferred onto a quartz
substrate through a series of wet chemical synthesis process. SLG and MLG was
characterized using Raman spectroscopy (Renishaw Raman Microscope) using
514nm laser with exposure time of 10 seconds as shown in Figure 2 - 10(a) and (c).
Figure 2 - 10(b) shows the optical reflection microscope image of MLG with
patches of different brightness. As mentioned in chapter 1 of this thesis, the single
layer has 2.3% absorption[22] and this keeps increasing as the number of graphene
layers increases. Hence, the brighter the patch the more the number of layers
present in it. The size of the patches is about 3-10 microns. I could not easily
observe SLG under microscope as it requires appropriate substrate beneath it in
order to obtain sufficient contrast for viewing.
Figure 2 - 10: (a) Raman spectrum of SLG at 514nm excitation wavelength (b)
Optical reflection microscope image of Multi-layer graphene with a 20 µm scale
bar. Patches of different thickness and dimensions are visible. (c) Raman
spectrum of MLG at 514nm excitation wavelength with G-peak at ~1585 cm-1
and broadened 2D peak at ~2700 cm-1. (d) The 2D band of MLG is fitted with 3
components of Lorentzian curve (green color) which indicates that it is a 4-Layer
48
graphene. Blue color is the envelop of fitted curve and red color curve indicates
Raman data.
The salient underlying features of the Raman spectrum of graphene (both SLG and
MLG) that are to be monitored are: a) the D-line b) G-line and c) 2D-line (or G’-
line). The first band appearing around 1350 cm-1 is called as D-band as it
corresponds to the defects [191] present in the graphene structure. Hence the small
protruding D band indicates that SLG and MLG have minute defects and
impurities. Another band appears at twice the frequency of D band i.e., 2700 cm-1
and is referred to as 2D-band. In order to avoid giving false interpretation that this
is also due to defect (like the D-band) in graphene and to avoid mixing it up with
the abbreviation of two-dimensionality (which is 2D) researchers generally refer to
it as G’-band in literature. This band arises due to the double resonance [98, 174,
192, 193] in electronic process and is sensitive to its vibrational characteristics and
electronic structure. Another prominent band that appears at 1582 cm-1 is called as
G-band to indicate the strength of graphitic nature in the sample. This shape and
position of G-band indicates the extent of doping [174, 191] and strain levels [194,
195]. The G-band comes from the first order Raman scattering phenomenon in
graphene and is related to iTO and LO doubly degenerate phonon modes.
Conversely, the second-order process is involved in the origin of D and 2D-band.
D-band is due to the iTO phonon and one defect whereas 2D-band involves two
iTO phonons near the K point as shown in Figure 1- 2(c) of Chapter 1. Another
band that usually appears at 1620 cm-1 is denoted as D’ band and it is due to the
disorder within the graphene crystal structure.
The classical way of distinguishing between SLG and few layer graphene is through
the study of I2D/IG ratio of the obtained Raman spectrum [98, 172, 196, 197]. In the
Raman spectrum as shown in Figure 2 - 10(a), the I2D/IG ratio varies from 3.0 ~ 3.5
for 514.5 nm (2.41 eV) which matches other experimental observations [97, 172,
198]. This indicates that the material under test is definitely a SLG sheet. Relatively
weak G peak in comparison to 2D peak in SLG is due to the destructive quantum
interference of the states contributing to the I(G) [199], and can be lifted by
doping or phonon coupling to out-of-plane layers or materials [196].
49
The intensity of G band is greater than 2D band in Figure 2 - 10(c) indicating the
presence of multiple layers of Graphene unlike what is measured for SLG as seen in
Figure 2 - 10(a). Moreover the slight shift in 2D band and its broadening is another
indication that the sample under test in Figure 2 - 10(b) is a MLG. After analyzing
Raman spectra for 120 independent measurements it was found that our MLG
sample has few monolayers of graphene, usually between 1-7 layers with an average
of 4 monolayer thickness. Figure 2 - 10(d) shows 3 components of Lorentzian
curve fitted into the 2D band of Figure 2 - 10(c) which clearly indicated that it is 4-
layer graphene.
2.14 Single crystalline silver nanoplates
Similar to single layer graphene and single crystalline gold nanosheets, single
crystalline silver nanoplates find immense applications in catalysis, sensing, surface
enhance Raman scattering [200-205] etc. These single crystalline silver nanoplates
are two-dimensional and anisotropic in nature. The anisotropic property is clearly
evident from the fact that any variation in the lateral dimensions of silver
nanoplates affects the optical properties, plasmonic properties, electromagnetic
properties etc. As a result it is very important to choose the right technique for
synthesizing silver nanoplates having dimensions larger than 1 µm.
Previous techniques such as photochemical reduction employed by Jin et al [127]
and chemical reduction employed by Xiong et al [206] produced silver nanoplates
with dimensions below 1 µm. There is another technique based on consecutive
depositions in conjunction with seeded growth [207] to produce silver nanoplates
with extended lateral sizes above 1 µm. But, this technique is very tedious and time
consuming. The silver nanoplates that I investigated are produced by B. K. Lim’s
group using technique from Ref. [208], which produces nanoplates of dimensions
greater than 1 µm, and here, I briefly recapture the synthesis method.. Unlike hot-
injection method, this technique synthesizes silver nanoplates by slowly heating the
reaction mixture which contains precursors and surfactants to which the stabilizers
prepared at low temperature are occasionally added until they reach the desired
reaction temperature. This results in the nucleation and growth of nanocrystals.
50
Silver nanoplates were synthesized in an aqueous solution. In this aqueous solution
Poly vinyl pyrrolidone (PVP) was used to reduce silver nitride (AgNO3) to produce
single crystalline silver nanoplates through a slow heating process. In this heat-up
synthesis process for creating silver nanoplates, 1.88 g of reducing agent PVP (MW
= 29000, Aldrich) and 96 mg of AgNO3 (Aldrich) was dissolved in 11 mL of
deionized water held in a 20-mL vial at room temperature. The vial was closed with
a lid and was heated gradually at a rate of 0.42○C/min using a magnetic stirrer until
the temperature of the vial reached 95○C. The temperature of the vial was
maintained 95○C for 12 hours using a magnetic stirrer and then cooled down to
room temperature. At initial stages when the temperature is low the concentration
of silver atoms are sparse due to slow reduction process of AgNO3 by PVP. Hence,
the number of silver crystals formed is meagre in number. But, as the temperature
is gradually increased, the number of silver atoms generated through reduction also
increases dramatically. As a result the nuclei of silver atoms metamorphosize into
plate-like seeds and the newly generated silver atoms would cluster onto an already
existing silver seed to grow into silver nanoplates in a lateral fashion. Thus the
aqueous phase heat-up synthesis technique can produce silver nanoplates with
lateral dimension larger than 1 µm.
The as-prepared solution contains both the silver nanoplates of dimension larger
than 1 µm and the silver particles that are formed as by-product during the
chemical synthesis. It is essential to separate them so as to have a higher yield per
ml of silver nanoplate concentration compared to its by-products. Hence, a very
familiar technique of separating silver nanoplates by adding Ethanol solution into
the as prepared solution was followed as shown in Figure 2 - 11(a). Then the
ethanol mixed solution was put into a centrifuge and the vial was spun at 2000 rpm
to separate lighter particles from dense and heavier particles. The resulting solution
are then separated and collected in two different bottles with clear distinction in
their colours and composition as shown in Figure 2 - 11(a). SEM images of these
centrifugally separated particles are shown in Figure 2 - 11(b) with a scale bar of 1
µm.
51
Figure 2 - 11: (a) Overview of Heat-up synthesized silver nanoplates. (b) SEM
image of the Ag particles and plates separated after centrifugation of the as-
prepared solution. (c) Extinction spectra of the Ag particles and plates obtained
after centrifugation along with the original as-prepared species are shown.
Extinction spectra of the nanoplates are shown exhibiting strong absorption at
NIR range due to distributed plasmon peak wavelengths (provided by Guh-
Hwan et al [208]).
Figure 2 - 11(c) shows the extinction spectra of 3 different samples namely 1) the
original as-prepared solution and this original solution separated through centrifuge
process into (2) silver nanoparticles and (3) silver nanoplates. The extinction spectra
of nanoplate solution shows strong absorption in NIR range due to distributed
plasmon peak wavelengths. This is due to the variation in lateral size and shape of
the plates, causing the spread in the resonant plasmon modes in the visible and
NIR.
52
Figure 2 - 12(a) shows the enlarge SEM image of synthesised silver nanoplates of
different shapes and sizes like triangular nanoplates, hexagons, octagons and
truncated triangles etc. The majority of these nanoplates have triangular shapes as
can be seen clearly in the figure. Figure 2 - 12(b) shows the SEM image of single
nanoparticle with aspect ratio, defined as side length divided by the width, to be
approximately 1100 nm ± 100 / 20 nm ± 3 with a scale bar of 500 nm and whose
fundamental peak extends well into the THz region. The silver nanoplates has
round edge radius of curvature as can be seen in figure.
Figure 2 - 12: (a) SEM image depicting silver nanoplates of different shapes and
sizes that are formed during chemical synthesis. (b) Single silver nanoplate is
shown, with thickness ~ 20 nm. (c) Typical dark-field scattering spectrum of five
individual nanoplates is shown. Peak scattering at 540 ~ 600 nm is from the
higher mode of surface plasmon resonance.
On the other hand, single plate dark-field scattering spectrum shown in Figure 2 -
12(c), can resolve plasmon peaks in the visible wavelength region. The dark field
scattering spectrum was acquired using a dark field microscope (Eclipse Ti – S,
Nikon, AU) with a 1.2 - 1.3 NA dark - field condenser (Nikon, AU) and a 0.6 - 1.3
NA oil immersion objective lens (with 100x magnification). White light was used as
excitation source during the experiments. The scattering images were acquired with
a coloured cooled digital camera (Nikon DS - Fi1c - U3 5Mb). Typical visible to
NIR spectrum (400 nm ~ 1000 nm) of 5 individual plates show peak at 540 nm ~
600 nm. These peaks correspond to high-order surface plasmon modes.
53
Chapter 3
Raman Spectroscopy Study of
Single Layer Graphene Hybridized
with Silver Nanoplates
3.1 Introduction
Material hybridization between two-dimensional sheets made up of different
compounds is a fast growing field for it can open frontiers never explored and
imagined before. This is true for material hybridization between graphene and
plasmonic metal nanostructures as it encompasses interesting characteristics that
can find prospective application for graphene in photonics [35, 92, 209-211]. Some
of these potential applications are in the field of photo-catalysis [200, 201], sensors
[212-214], surface enhanced Raman scattering (SERS)[201, 205, 215-224],
photovoltaics [225], LED [226] electrodes [227-229], memory storage etc.
Historically rough metal surfaces were exploited in SERS application as these rough
spots usually confines high energy fields and plasmon resonances around it. This
made metallic particles possessing properties such as non-crumpled surfaces,
extended lateral dimensions and supporting plasmonic oscillations suitable
candidates for studying Raman spectroscopy with graphene [216, 219, 223, 224,
229, 230]. Hybridization between these two materials is extremely advantageous
due to the fact that in visible and NIR regime graphene properties can be tuned
with numerous optical functionalities of surface plasmon resonance. The vice versa
is also true as graphene possess excellent transport, thermal, electrical and
mechanical properties which can contribute to SPR based devices. Even though the
Graphene supports SPR in THz region while the metals support SPR in visible and
NIR region, the fundamental studies to understand their properties are still in early
stages.
54
Some of the examples for anisotropic plasmonic nanoparticles are triangular gold or
silver nanoplates [127, 128, 208], which are applied to logic gates [231], optical
antenna [232] and SERS [233]. 2D structure of large aspect ratio nanoplates greatly
enhances the interaction area and excites wide variety of plasmon mode shapes
[234, 235], allowing the bandgap hybridization with graphene possible. Laser
irradiation on plasmonic nanostructures can cause photothermal melting and
reshaping [236], plasmon hot printing [237], photo-oxidation [238] or laser ablation
[239]. Such effects could induce change in contact distance between the SLG and
plasmonic nanostructure, inducing field enhancement dominating over charge
doping on the SLG. This is particularly true as the charge transfer between metal
and SLG happens through contact between them (< 5 Å [240]), which cause Fermi
level shift and subsequent charge migration, whereas the field enhancement can
influence SERS from up to tens of nanometres distance apart.
3.2 Literature review
There is enormous interest in carrying out research to understand the fundamental
optical, physical, chemical and electronic properties when gold and silver
nanoparticles of various morphologies are deposited on graphene. There are already
many papers reported in literature which talk about interactions between gold and
silver nanoparticles deposited on graphene by electro-deposition, mechanical
transfer, spin coating and other numerous different techniques. Lee et al [229]
demonstrated the massive increment in G and 2D peaks of single layer graphene
(SLG) sprinkled with silver nanoparticles. They saw clear evidence of charge doping
effect along with G band splitting. The splitting of G band was due to the lift off of
the degeneracy at point in phonon dispersion. The splitting of G band was
reported only for single, bilayer and tri-layer graphene. There was no splitting of G
band reported for multiple layer graphene. As the Raman shift positions varied the
degree of enhancement also varied, signifying the presence of charge doping effect
in the silver nanoparticle decorated SLG sample. Lee et al [230] demonstrated even
higher enhancement in the Raman spectrum of graphene decorated with gold
nanoparticles using thermal evaporation. The enhancement for gold nanoparticles
of thickness 4 nm was 120 times for 633 nm laser excitation compared to 24 times
55
for silver nanoparticle of 4 nm deposition [229] for 532 nm laser excitation. The
enhancement in G band is greater than 2D band for both silver and gold
nanoparticle deposition on graphene. Moreover, it was reported that SLG provides
higher SERS enhancement than multilayer graphene. Schedin et al [216] reported
SERS enhancement at 633 nm laser excitation for graphene Raman spectra by
depositing arrays of gold particles of well-defined dimensions on a graphene/SiO2
(300 nm)/Si system.
Khorasaninejad et al [219] demonstrated exceptionally high enhancements in the
Raman scattering of graphene on a plasmonic nanostructure platform consisting of
two silver nanostructures made up of rings and crescents arranged in a periodic
array on top of a gold mirror. The reported G band enhancement in graphene with
crescent arrayed structures was nearly three orders of magnitude higher compared
to graphene on a silicon dioxide surface. The gap between nanostructures and
graphene layer was found to play critical role in determining the enhancement
factor. Smaller the separation between graphene and nano-structures, greater is the
SERS enhancement. Congwen Yi et al [224] demonstrated the coupling between
SPR of Gallium nanoparticles with graphene on SiC/support system. Ga-
nanoparticle is specifically chosen because their SPR can be tuned from UV light
into the near-IR region. SPR in Ga-nanoparticles was found to evolve with
increasing nanoparticle size. Moreover, the SPR of graphene/SiC with Ga
nanoparticles on top were found to have damped resonances compared to Ga
nanoparticles on SiC, indicating the presence of theoretically predicted screening
effect with small change in energy shifts. Hence, they were able to tailor the SPR
resonances of Ga nanoparticle heterojunction according to the desired use in
application. The enhancements in the Raman modes of graphene were contributed
by the doping and SPR phenomenon of Ga NPs with graphene.
Zhou et al [241] studied the effect of silver film and gold film deposition on
different graphene layer thickness. Dependence of Ag film morphologies for
various thermal deposition temperatures on graphene was also studied. It was
found that the increase in sample temperature caused the Ag island size to increase
considerably. Moreover, by keeping the temperature constant, silver metal thickness
was varied from 2 nm to 5 nm and it was found the increase in Ag film thickness
56
led to increase in Ag particle size due to reduced Ag nanoparticle density. The
resulting increase in silver particle size led to decrease in Raman spectra of Ag-
graphene hybrid structure. It was also reported that the Raman enhancement was
highest for monolayer graphene and least for thickest layered graphene. Moreover,
as the wavelength increased from 514 nm to 633 nm the Raman enhancement
decreased indicating that enhancement was mostly due to coupling of Ag SPR
which in turn decreases with increasing wavelength. Fu et al [242] synthesised
graphene region at nanoscales by reducing graphene oxide (GO) and verified the
end product using Raman spectroscopy. They reported SERS enhancement of two
orders of magnitude in D and G bands of graphene hybridized with gold
nanoparticles for 632.8 nm laser wavelength excitation. The gold particle was
thought to quench fluorescence excitation of graphene and hence aided the Raman
spectroscopic characterization by quantifying G and D bands. The resonant Raman
response for both the D and G bands were found to occur at 593 nm (2.09 eV)
laser wavelength approximately. As this resonant response was far away from the
plasmon resonance of gold nanoparticles reported at 548 nm, they attributed this
shift in peak resonance to be coming from charge transfer between gold and
graphene in addition to the plasmon resonance of gold nanoparticles. They
observed the stiffening in the G band of gold-graphene hybrid structure compared
to that of pristine graphene. Hence the electron-phonon coupling in G band
spectrum was said to play a prominent role in the charge transfer of hybrid
materials.
Li et al [243] demonstrated that by covering silver nanoparticles with a chemical
vapour deposition (CVD) grown monolayer graphene the oxidation process of
silver nanoparticles can be completely avoided along with the prevention of particle
aggregation and deformation. They coated Rhodamine 6G (R6G) molecules on top
of silver nanoparticles. The SERS from the R6G molecules with and without
graphene cover are studied for 28 days. It was found that the uncovered substrate
led to rapid fall of Raman signals. Similarly the sample covered with graphene oxide
was less effective in preventing oxidation of silver nanoparticles. On the other hand
the substrate covered with graphene showed no decay or degradation in the SERS
signal coming for R6G coated on silver nanoparticles for 28 days. This showed that
graphene can be utilized as a cost effective way of stabilizing silver based substrates
57
for SERS application. Sidrov et al [244] attached gold and silver nanoparticles
using locally developed chemical methods onto single layer graphene deposited on
arbitrary substrates. They reported 45 and 150-fold enhancement in Raman spectra
of silver and gold substrates respectively. Zhou et al [245] show enhanced catalytic
reaction in vertical graphene-nanosheet sandwiched by Ag-nanoparticles on the
silicon nanocone array even at a very low concentration of 10−11 M which is four
orders of magnitude lower than anything reported previously. Pashaee et al [246]
demonstrated tip-enhanced Raman spectroscopy (TERS) to characterize graphene-
like and graphitic platelets composed of a few layers of graphene. The near field
measurements of gap-mode TERS using gold coated AFM provides a larger
enhancement of the local electromagnetic field at the junction formed by a gold
sharp tip and a gold substrate.
In summary, most of these reports showed huge enhancement in G and 2D peaks
of SLG decorated with plasmonic nanostructures and simultaneously observed
some degree of charge doping effect represented by the reduction of I(2D) / I(G)
[191, 240]. However, this ratio can also be influenced by the relative field
enhancement at G and 2D band detuning from the SPR peak of the plasmonic
structure [216]. Interplay and separation of these two effects on Raman signals of
hybridized SLG have not been studied in detail until now. To what degree does the
effect due to SPR assisted enhancement or the doping phenomenon play is not
clearly understood yet. Unravelling this intertwining effect is very essential for
gathering fundamental knowledge about this nascent field and also for the
development of specifically engineered applications such as SERS and data storage.
This can be done only if the experimental setup can excite SPR in silver
nanoparticles and simultaneously gather Raman scattering information on
excitation with the hybridized sample.
In this chapter I will present the Raman spectroscopy study of SLG hybridized with
silver nanoplates and clearly demonstrate the manifestation of hybridization
phenomenon. I will validate our claim by systematically studying the evolution of
the different band structures namely D, G and 2D bands of the given hybridized
material for five different wavelengths. I will show the deconvolved spectra of G
and 2D bands after Ag nanoplate deposition and highlight the split observed in two
58
Lorentizian modes. I will also explain the effect of doping on hybridization by
calculating the work function of nanoplates and graphene. I will then compare the
evolution of bands in this hybrid material with the Raman spectroscopy
measurements of SLG hybridized with oxidized silver nanoplates. I will show that
the hybridization effect is vanished by comparing the band structures of oxidized
and non-oxidized silver nanoplates hybridized with SLG. The disappearance of
hybridization is ascribed to fact that doping cannot take place when the silver
nanoplates are oxidized, which essentially acts as a barrier for charge transfer and
indirectly confirm the charge doping effect explanation attributed for hybridization.
I will show the multi-physics COMSOL simulation employed to figure out the
expected theoretical values for band shift and band enhancement due to
hybridization and compare them with our experimental results.
In order to distinguish between the hybridization arising due to charge doping
effect from LSPR effect, I performed Raman spectroscopy of SLG-non-oxidized
silver nanoparticle hybrid with laser operated at high energy and tightly focussed by
the objective lens. It will become evident that the sharp tip geometry of nanoplates
ablates/melts away due to LSPR and as a consequence of this I observed manifold
increase in the intensity of band structures. Hence, unlike the previous studies, the
distinction between the SPR induced field enhancement effect and the charge
doping effect can be clearly distinguished through photothermal reshaping of the
silver nanoplates. I will also demonstrate the Raman spectroscopy study of single
crystalline gold nanosheets hybridized with single layer graphene for five different
wavelengths and calculate their work function to prove that the hybridization
phenomenon at this juncture is purely due to charge doping effect only.
3.3 Characterization of silver nanoplates through
experimental and simulation method
Silver plates were synthesized as explained in the section 2.14 of chapter 2. Figure
3 - 1(a) shows SEM image of chemically synthesized silver nanoplates of various
shapes. The most common silver nanoplates shape is the triangular plate. As
explained in the previous section, the dimension of silver nanoplates exceeds 1 µm
in lateral dimension and has an average thickness of ~20 nm. SLG – Ag nanoplate
59
– glass slide and Ag nanoplate – SLG – glass slide arrangement was used to prepare
two different samples for hybridization with graphene. The first sample, SLG – Ag
nanoplate – glass slide was prepared by dispersing silver nanoplates on top of glass
slide. SLG was grown using a series of steps and then transferred on top of silver
nanoplates. Chemical vapour deposition technique was used to deposit SLG on top
of Cu foil. Then it was coated with poly(methyl methacrylate) (PMMA) to
completely cover it. Ammonium persulfate (Aldrich) aqueous solution was then
used to etch Cu from the sample. The resulting SLG – PMMA film was transferred
on top of silver nanoplates. Finally the PMMA was removed by chemically etching
it with acetone to give SLG – Ag nanoplate – glass slide sample. The second
sample, Ag nanoplate – SLG – glass was prepared by dispersing silver nanoplates
on top of single layer graphene deposited on glass substrate, where the SLG was
purchased from Graphene Supermarket (Graphene-Supermarket.com.). Figure 3 -
1(b) shows dark field scattering spectrum of an individual silver nanosheet with
plasmon peaks, which can be resolved around the visible range with a dominant
peek at 600 nm and a secondary peak at 750 nm. The shift in the peak wavelengths
along with additional peaks that we see in the dark field scattering spectrum using a
photomultiplier tube (PMT, Princeton Instruments) is indicative of higher order
modes, as the fundamental mode of surface plasmon resonance is extended well
into the near infra-red region [234, 235]. Extinction spectrum shows a featureless
broad spectrum covering visible and NIR region as shown in section 2.14 of
chapter 2. This electromagnetic response of the silver nanoplates under plane wave
irradiation has been simulated using finite element method (COMSOL
Multiphysics) to ascertain the validity of our observed experimental results. Field
calculations are performed at their respective peak SPR wavelengths, and the
scattering spectra were obtained from field integration at 4 micron distance from
the centre of the nanoplate. Laser polarization was along the vertical axis, and the
input laser irradiation was continuous wave with average electric field of 1 V/m.
Tetrahedral meshing was employed in the simulations. Typically minimal and
maximal mesh element sizes were 1 nm and 5 nm respectively. Outside the
nanoplate volume, maximum mesh size was 30 nm. Figure 3 - 1(e) shows the mesh
configuration around the geometry which was used to simulate the triangular
nanoplate.
60
Figure 3 - 1: (a) SEM image of silver nanoplates, showing mixture of triangular,
hexagonal plates and spherical particles. (b) Single nanoplate dark-field scattering
spectra is shown to exhibit higher mode of surface plasmon resonance peaks at
600 and 750 nm. (c) |Eplate|2 simulation image of a silver nanoplate of side
length 1100 nm, thickness 25 nm, radius of curvature 60 nm, excited at 750 nm
(e)
61
laser wavelengths (Plane wave, vertical polarization). (d) Simulation of cross
section spectra of nanoplates with aspect ratio 5 ~ 25 (width 10 nm) in the steps
of 5 shown. (e) Configuration of tetrahedron mesh employed to simulate the
electromagnetic field surrounding a 1µm x 25 nm silver nanoplate. Mesh element
sizes within nanoplate range between 1 and 25 nm (provided by Stuart).
The simulation was carried out for dimensions of 1100 nm side length and 25 nm
thickness (aspect ratio of 44) with a rounded edge (radius of curvature ~ 50 nm) on
silica substrate of thickness 40 nm [234] to look at the field pattern at the resonance
conditions ( ~ 750 nm, Figure 3 - 1(c)), and their scattering and extinction spectrum
(Figure 3 - 1(d)). As the lateral dimension of silver nanoplates varies, the resonant
plasmon modes also show variations in their peak wavelengths along with the
appearance of higher order modes. As the dimension varies the fundamental modes
and higher order modes experience shift in their peak wavelengths. The field
pattern at 750 nm shows that there are strong fields at the tips and 3 nodes at the
sides. This is a 5th order SPR mode. Near 600 nm, 7th mode of SPR was detected.
Scattering spectra of varying aspect ratio shows fundamental dipole mode
extending into near infra-red region, and high order resonant mode appearing in
the visible.
3.4 Hybridization of Ag nanoplates with SLG
Dramatic changes in the Raman intensities of G, 2D and D peaks are observed for
silver nanoplates deposited on top of SLG. Similarly silver nanoplates coated with
SLG also show strong fluctuations in peak intensities of these bands. Figure 3 -
2(a) shows the SEM image of SLG on nanoplate on glass, and Figure 3 - 2(b)
shows nanoplate on SLG on glass. Dark region around the edges of nanoplates are
visible in Figure 3 - 2(a) after the SLG deposition on silver nanoplates, this is due to
the stretching of SLG around the edge of the nanoplate.
62
Figure 3 - 2: (a) SEM image of SLG on top of a Ag nanoplate on silica substrate.
Patches of darker regions are multiple layer region. (b) SEM image of Ag
nanoplates on top of SLG on silica substrate. (c) Typical Raman spectrum for
SLG only and for SLG on Ag nanoplate structure of (a) showing that G peak at
~1580 cm-1 is enhanced and 2D peak at 2700 cm-1 is reduced for Ag nanoplate
hybridization. Inset shows an optical microscope image of where Raman spectra
were taken. Scale bar is 2 µm. (d) Similar Raman observation for Ag nanoplate on
top of SLG on glass slide, (b). Wavelength of Raman laser is 533 nm. Numerical
aperture of the objective lens is 0.85.
An enhancement 1.2~ 3 times is seen in G peak and D peaks, while a significant
reduction of ~ 0.6 times is observed in 2D peak, making I(2D)/ I(G) ratio below 1
as shown in Figure 3 - 2(c) and (d). This effect is seen irrespective of the
geometrical arrangement of SLG on top or under the nanoplate as shown in Figure
3 - 2 (a) and (b).
Detailed Raman spectroscopy measurements are shown in Figure 3 - 3 for five
different wavelengths varying from 457 nm (2.71 eV), 488 nm (2.54 eV), 514 nm
(2.41 eV), 633 nm (1.96 eV), and 785 nm (1.58 eV) using Renishaw Raman
spectrometer for Ag nanoplate-SLG hybrid (architecture shown in Figure 3 - 2(b)).
63
Figure 3 - 3: Raman spectrums of Ag nanoplates on SLG (blue colour) and SLG
alone (red colour) for laser wavelength excitation of 457, 488, 533, 633 and 785
nm. Numerical aperture of the objective lens used was 0.85.
An objective lens having 0.85 numerical aperture with 100× magnification (Leica)
was used to acquire Raman spectrum. The laser power used was less than 1 mW to
ensure no photothermal heating occurs on the sample during spectroscopy. Figure
3 - 3 shows two different coloured Raman spectra for 457, 488, 514, 633 and 785
64
nm wavelengths, red colour for SLG alone and the blue colour for Ag nanoplates
on SLG locations. It can be clearly seen that there is notable enhancement in the
intensities of G peak and D peak while the intensity of 2D peak is reduced for 457,
488 and 514nm wavelengths. On the other hand there is no significant
enhancement in D and G peaks for 633 and 785 nm wavelengths. Detailed analysis
of band shifts and band enhancements before and after hybridization is discussed
in detail in the following section 3.7.
3.5 Raman band broadening after hybridization
Apart from dramatic changes in D, G and 2D bands of Ag nanoplates on
hybridization with SLG, there is broadening effect observed in D, G and 2D bands.
It can be clearly seen from the Figure 3 - 4(b) and (d) the G and 2D bands are
broadened and this broadening effect is due to the change in graphene electronic
structure induced due to the silver nanoplate deposition on graphene.
Figure 3 - 4: (a) & (c) Raw Raman data (red colour) and fitted Raman spectra
(blue colour) of SLG at 488, 514, 633 and 785 nm laser wavelengths. Black line is
guide to an eye. (b) & (d) The deconvolved spectra of G and 2D bands after Ag
65
nanoplate deposition. The average split observed in G band is about ~8 cm-1
while in 2D band the split is about ~15 cm-1
On the other hand, pure SLG Raman spectra can be fit with one Lorentzian curve
as shown in Figure 3 - 4(a) and (c). It has already been reported by Lee et al [229]
that the G band is split upon the silver metal deposition on single layer graphene
for all laser wavelengths. They have found that the splitting in G band decreases
with the increase in the number of graphene layers with no splitting observed for
very thick multilayered graphite. It was also reported by Dong et al. [247] that
aromatic molecules upon interaction with monolayer graphene results in splitting of
G band by breaking the phonon symmetry at the point by altering the electron
density distribution of monolayer graphene by lifting the two-fold degeneracy of
the LO and TO phonons. They observed this splitting in G band to be
independent of excited laser wavelengths indicating that it is a first order Raman
process. I also found that the splitting of G band is observed for silver nanoplate
deposited graphene for all laser wavelengths as can be seen in Figure 3 - 4(b) and
(c) indicating that the Raman signal of silver nanoplate deposited graphene is due to
a first-order process. Similarly the splitting of 2D band into two peaks was
observed in the Raman spectrum of silver nanoplate deposited SLG.
Lee et al [229] also observed the split in 2D band for SLG, but they also noticed
that the splitting in 2D band disappeared as the layer thickness increased. This is
because the 2D band could already be deconvolved to four peaks for bilayer and
two peaks for tri-layer graphene even before any silver deposition. I have already
shown that multilayer graphene with four layer thickness can be fitted with 3
Lorentzian curves in Figure 2 – 10(d) of chapter 2. The
Table 2 - 1 shown below summarizes the G and 2D band peaks of SLG before and
after hybridization with silver nanoplates along with the information of Lorentzian
fitted peak values for G band denoted as G1, G2 and for 2D band as D1, D2. One
can notice that the average split in G band is about ~8 cm-1 while in 2D band the
split is about ~15 cm-1. The split in G and 2D band indicates the shift in Fermi
level of SLG due to the deposition of silver nanoplates on top of it.
66
Table 2 - 1: Summary of G and 2D band peaks of SLG before and after
hybridization with silver nanoplates along with the information of fitted curve
peaks for G band denoted as G1, G2 and for 2D band as D1, D2.
Laser
specifications SLG band
Ag nanoplate on
SLG
Ag nanoplate on
SLG
λ (nm) Energy
(eV)
G 2D G G1 G2 D D1 D2
785 1.58 1587 2603 1588 1584 1591 2607 2595 2610
633 1.96 1590 2643 1602 1595 1603 2653 2644 2659
514 2.41 1586 2691 1595 1589 1598 2698 2690 2704
488 2.54 1584 2698 1595 1588 1597 2707 2697 2712
3.6 Dispersion relation of Raman bands with
respect to wavelengths
Single layer graphene on excitation with different laser wavelengths show
dispersion in the G and 2D band peaks as shown in the Figure 3 - 4(a) and (c) along
with D band. SLG is an ideal candidate for calculating the phonon dispersion
relation which represents variety of other sp2 carbon nanostructures such as carbon
nanotubes, graphite etc. Due to the Kohn anomaly [248] there is wide discrepancy
in the values of dispersion slopes and K point positions reported in literature [248-
255]. Although a wide variety of techniques are used to calculate phonon
dispersion, Raman spectroscopy experiment can easily determine the longitudinal
acoustic (LA) and the in-plane transverse optical (iTO) phonon dispersions of
67
monolayer graphene near the Dirac point [256]. As shown in the Figure 3 - 4(a) and
(c) G and 2D bands of SLG undergo dispersion when excited with 488, 514, 633
and 785 nm laser wavelengths, with a black dotted line acting as guide to the eye.
The 2D band exhibits a highly dispersive behaviour with a slope of 95 cm-1/eV for
monolayer graphene as shown in Figure 3 - 5 and this is in good agreement with
the value reported in literature, 88 cm-1/eV[257].
Figure 3 - 5: Dispersion relation of G and 2D bands of single layer graphene
obtained through Raman spectroscopy for four different laser excitation energies.
Compared to 2D band, G band has comparatively very less dispersion of about 8
cm-1/eV for SLG. Moreover on careful observation one can see that the dispersion
for G and 2D bands are in relatively opposite direction as can be seen both in the
Figure 3 - 5 and Figure 3 - 4(a) & (c).
While calculating the phonon dispersion slopes of SLG, the G and 2D band peaks
obtained using 785 nm laser are omitted. Its value is shown in Figure 3 - 5 only as a
matter of representation of our complete experimental analysis. As can be seen
from the Figure 3 - 4, G and 2D modes for 785 nm laser are slightly offset from the
other visible laser Raman spectra. This is because the frequencies of G and 2D
bands show different peak modes with the change in incident laser energy. As a
result, the D band peak is more enhanced for 785 nm laser excitation with its
primary peak at 1299 cm-1 [258] unlike what is observed at visible laser wavelengths,
68
which is around 1350 cm-1 as seen in Figure 3 - 3. The D band broadening and
enhancement is explained using the nearest neighbour tight bonding theory [257].
According to this theory the Dirac cones get deformed due to stronger trigonal
warping effect with the increase in laser excitation energy. Furthermore, there is no
enhancement in the intensity of 2D band of SLG compared to G band upon 785
nm laser excitation as shown in Figure 3 - 3, but the effect of dispersion is
profound in 2D band. Moreover the Si/SiO2 substrate under the SLG exhibits
fluorescence for near infrared wavelength like 785 nm. As a consequence of all the
above factors, the most commonly reported wavelengths for Raman spectroscopy
study of SLG are visible laser wavelengths and not the near-Infrared laser
wavelengths.
3.7 Analysis of Raman peak shift and
enhancement after hybridization
Figure 3 - 6(shown below) summarizes the peak enhancement and peak position
shits of Raman spectra after silver nanoplate deposition on SLG for all the
aforementioned five laser excitation energies. This was the best way forward to
capture and explain the effect of both oxidized and non-oxidized nanoplate
deposition, illuminated with different excitation energy, on Peak enhancements and
shifts of obtained Raman spectrum. I(2D)/ I(G) for SLG are represented in black
points and SLG-Ag nanoplate hybrid are represented in red points as shown in
Figure 3 - 6(a), and red points are seen to be reduced to less than 1 for photon
energies larger than 2 eV. In Figure 3 - 6(b), the total enhancement of G peak and
2D peak is presented, where total enhancement is calculated as (Ihybrid (G) + Ihybrid
(2D)) / (I SLG (G) + I SLG (2D)). For photon energies larger than 2eV, total
enhancement is ~2 times, mainly due to the enhancement in I(G). For other
wavelengths, it is less than 2 (red points in Figure 3 - 6(b). When the laser excitation
energy exceeds 2 eV, G and 2D band for non-oxidized silver nanoplates deposition
shows average peak shift of ~ 10 cm-1 to higher wavenumber compared to SLG
alone as shown in Figure 3 - 6(c) & (d) respectively. Figure 3 - 6(d) also shows the
dispersive behaviour of 2D peak of SLG having slope in the range of 95 ± 10 cm-
1/eve as previously outlined in Figure 3 - 5.
69
Figure 3 - 6: Summary of G and 2D peak changes from SLG to SLG - Ag
nanoplate, SLG – laser modified Ag nanoplate, SLG – oxidized Ag nanoplate
hybrids. (a). I(2D)/I(G) ratio change. SLG – Ag nanoplate shows the ratio lower
than 1, meaning that there is charge doping on SLG. (b) Total enhancement of
the G and 2D peaks. Only laser modified hybrid shows plasmon enhancement.
(c) G peak position shift upon hybridization. SLG – Ag nanoplate shows
stiffening ~ 10 cm-1 due to charging, but the others show no change with that of
SLG. (d) 2D peak position shift upon hybridization. Again, only SLG-Ag
nanoplate shows stiffening from SLG. 2D peak shows dispersion with the
excitation photon energy.
After the deposition of nanoplates on SLG the slope of 2D band is marginally
increased to 98.3 ± 7.7 cm-1/eV, signifying that there is alteration in the phonon
band structure. Interesting feature apart from the peak shift higher wavenumbers
after nanoplate deposition is the fact that there is a decrease in the intensity of 2D
peak upon hybridization. For 514.5 nm excitation wavelength the decrease of about
40% of 2D peak intensity is observed. Apart from G and 2D band, the D band also
shows dispersive behaviour with a slope of 47.0 ± 5.2 cm-1/eV for SLG alone, and
44.6 ± 3.9 cm-1/eV for silver nanoplate deposition similar to what was observed
with respect to 2D band peak shift. As anticipated, the dispersion values of D band
slope are approximately half of the 2D slopes as shown in appendix Figure 3 - 7.
70
Figure 3 - 7: D peak enhancement and shift of SLG with Ag nanoplate and
oxidized Ag nanoplate.
D band represent defects and misalignment of carbon structure in graphene and are
found to have been greatly enhanced upon the nanoplate deposition. The increase
in D band peak is reported to be more than 4 times the original height of pristine
graphene sample.
In literature the enhancement in graphene Raman emission due to the field
enhancement driven by surface plasmons is widely reported [216, 219, 223, 224,
229, 230, 241, 242, 259, 260]. This is specifically true for graphene sample sputter
coated with silver nanoparticles which were reported to exhibit 24 times
enhancement for G band and approximately 16 times for 2D band [229]. This is
true for other noble metal nanoparticles as well. For instance, the G and 2D band
for sputter coated gold nanoparticles are reported to exhibit an enhancement of
approximately 120 and 66 times respectively as mentioned earlier [230]. Taking this
research further, Raman spectroscopy study for well-designed and tailor made
structures were studied. Of late the plasmonic nanostructures fabricated through
electron beam lithography showed an enhancement of 890 times in the G band of
fabricated hybrid nanostructure [219]. These findings are clear indications that the
hot spots of coupled nanoparticles and surface plasmon induced enhancement
along the sharp tips and edges are primarily responsible for the enhanced Raman
bands.
Schedin et al [216] developed quantitative analytical and numerical theory to explain
the physics behind the SERS enhancement in Raman spectra of graphene upon Au
particle deposition. This analysis is possible due to the two dimensional nature of
graphene and it is reported to be in agreement with experiments. The SERS
1.6 1.8 2.0 2.2 2.4 2.6 2.80
1
2
3
4
5
6
7I D
(SLG
+ A
g N
anop
late
) / I D
(SLG
)
Photon Energy (eV)
SLG - oxidized Ag nanoplate SLG - Ag nanoplate Simulation
D peak enhancement
1.6 1.8 2.0 2.2 2.4 2.6 2.8
1300
1320
1340
1360
1380
1400D peak shift
W
aven
umbe
r (cm
-1)
Photon Energy (eV)
SLG SLG - Ag nanoplate SLG - oxidized Ag nanopate
71
increases with the increase in nanoparticle cross section as a fourth power of the
Mie enhancement and is inversely proportional to the tenth power of the separation
between graphene and the centre of the nanoparticle. Hence, they selected metallic
nanodisks for numerical simulations as it is a perfect representation for SERS in
two-dimension. This analytical model that assumes two different sizes of circular
gold nanodisks superimposed on graphene to simulate a prototype of gold
nanosphere antenna for investigating the enhanced absorption and emission of G
and 2D bands of graphene. The gold nanosphere antenna model was validated
using finite difference time domain numerical simulation. No analytical theory is
available for silver plates. However, the field Eplate at excitation and emission
wavelengths can be extracted from the following expression,
∫| ( )| | ( )|
(3.1)
The Eq. 3.1 is used to estimate the I(2D)/I(G) for various excitation energies
assuming that the nominal ratio is 3.25 and is plotted in Fig. 3-6(a) as black line.
The total enhancement factor for SERS signal at various excitation energies for G,
2D and D band emissions was calculated using Finite element analysis (Comsol
Multiphysics) method as shown in Figure 3 - 6(a) and (b) as a theoretical simulation
line in black colour.
The numerical simulation of the nanoplate near-field pattern Eplate at varying
excitation wavelengths is shown in Figure 3 - 8. It can be seen that the nanoplates
display strong field pattern at the edges. It is to be noted that the dark modes
excited due to laser irradiation can only contribute to the excitation, but not
emission. The expected values for I(2D)/I(G) ratio resulting from plasmon field
enhancements is shown in Figure 3 - 6(a). It can be simply noticed that the
experimentally observed enhancements miserably fail to match with the simulation
data. One important point to be noted here is that the ratio of I(2D)/I(G) below 1
indicates that the G band is much larger than 2D band. Whereas this I(2D)/I(G)
ratio for SLG is observed to be greater than 1, which indicates that there is clear
enhancement in band structure due to nanoplate deposition. Further, the decrease
in emission is not predicted by any simulation, while the 2D band obtained in our
72
experimental observation after nanoplate deposition shows clear decrease in its
strength.
Figure 3 - 8: |Eplate|2 pattern of a silver nanoplate of side length 900 nm,
thickness 25 nm, radius of curvature 60 nm, excited at 9 different laser
wavelengths (Plane wave, vertical polarization is used. With help from Stuart)
One possible reason for this decrease in 2D band can be that the Raman dipoles
covered by nanoplate are easily re-absorbed by its own metal surface and lost [216],
therefore causing the decrease in its signal strength. But through our experimental
observations, this explanation appears to be false, as the arrangement of graphene
on top or on bottom of the silver nanoplates makes no difference in G and 2D
band change as shown in Figure 3 - 2(c) and (d). Even if we consider this to be true
for the sake of argument, one cannot explain selective loss in only 2D while G is
enhanced. As stacking order between graphene and silver nanoplate makes no
difference in emission coupling, it can be concluded that this coupling
73
phenomenon between graphene dipoles and silver nanoplate antenna is an
extremely efficient process.
Hence this selective enhancement and reduction in the G and 2D band respectively
in our experimental observations can be attributed to the transfer of charges
between SLG and silver nanoplates. As a result of this electron doping process
between nanoplate and graphene, one can observe an enhancement contrast. It was
known earlier that the shift in G and 2D bands along with the change in
I(2D)/I(G) ratio can occur due to doping of electrons or holes from silver
nanoparticles or films [191, 229, 230, 261-265]. The change of I(2D)/I(G) ratio in
SLG is well documented in [223, 229]. If one calculates the work function of silver
for 111 facet and SLG, it turns out to be 4.9 eV and 4.48 eV [240]. This specifies
that n-doping has taken place in silver film after depositing it on graphene [240].
Both n- and p- doping were reported to occur in graphene under controlled doping
experiments by applying external voltage across the graphene layer [261]. This
caused an increase in G peak intensity while simultaneously decreasing the 2D peak.
However, it has been observed that the charge doping effect has pronounced effect
on 2D peak as it decreased almost linearly with the applied voltage, which would
then contribute more to determining the I(2D)/I(G) ratio. On applying a voltage of
1V the G peak merely increases 50 ~ 100% of its original strength, while the 2D
peak on the other hand decreases to about 10% for 1.5V of applied voltage. This
signifies that the original I(2D)/I(G) for SLG ratio of ~ 3.25 can easily be
decreased to less than 1 on nanoplate deposition, which is what is reported in
Figure 3 - 6(a). Of late it has been highlighted that with respect to doping, the
suppression of the phonon decay path into electron-hole pair creation due to states
being occupied (n-doping) or empty (p-doping) causes the G band intensity to
increase, which in turn cause the decrease in FWHM [191]. As a result of this
phenomenon, I too observed slight sharpening of G band, from original 21cm-1 to
15 cm-1. Further, the perceived stiffening of the G mode as shown in Figure 3 - 6(c)
is again in agreement with charge doping effect, which is expected to change the
equilibrium lattice parameter and phonon dispersion near the Kohn Anomaly [191].
However, it is to be noted that both n- and p-type doping causes the G band to
shift to higher wavenumbers. But, the main differentiation comes from the 2D
74
band shift. Normally, the band softening is observed due to n-doping while
stiffening in band structure is attributed to p-doping. Nevertheless, there is an
exception to this phenomenon that small stiffening can occur in n-doping as well as
reported in experimental results for low values of doping (< 2 × 1013 cm-2, ~ 5 cm-
1) [191]. However, in our experimental observation we have seen a stiffening of
around 10 cm-1 (Figure 3 - 6(c)), which cannot be accounted for. One reason for
this could be due to the surface distance between SLG-metal surfaces causing a
shift in Fermi level. G and 2D band stiffening can also be caused due to
compressive stress [266], but the magnitude of the shift is generally much larger (~
40 cm-1). Hence, I conclude in the case of these observations that the graphene has
undergone p-doping.
The calculated work function for graphene is 4.48 eV [240] and for silver
nanoplates whose surface structure is (111) [208] is found to be 4.9 eV. This is a
clear indication that graphene coated with silver nanoplates is undergoing p-doping.
However, when the separation between graphene and metal surface is altered from
its equilibrium postion (~ 3.3 Å for silver), causing n-doping to occur instead of p-
doping, then the Fermi level in graphene undergoes a definite shift from its Dirac
conical point as shown by Giovannetti et al [240]. This shift in the Fermi energy of
graphene,
( ) is expressed analytically as
( ) √ ( )| ( )|
( ) (3.2)
Where D0 is the graphene density of state constant (~ 0.09 eV2 per unit cell), α is
the plate capacitance per single graphene unit cell (~ 34.93 eV/ Å), d is the
separation between the two surfaces, d0 is a constant determining the effective
distance zd due to charge displacement, WM, WG, Δc(d) refer to work functions of
metal, graphene and work function change due to chemical interaction, respectively.
During the chemical synthesis of silver nanoplate its facets are shaped by PVP
molecules acting as surface ligands having considerable molecular weight (~ 29000).
The surface ligands get attached to the surface of silver nanoplates and acts as a
stabilizer. Therefore the surface of silver nanoplate and graphene will have a
separation larger than equilibrium separation (~ 3.3 Å) by at least the atomic radius
75
of carbon, ~ 0.7 Å. Considering the total distance d = 4.0 Å and applying it to the
Eq. 3.2 gives shift in the Fermi energy as ΔEF ~ 0.2 eV, which indicates the
lowering of Fermi level below the Dirac point by 0.2 eV. This allows p-doping to
occur from silver nanoplate confirming our Raman observations in Figure 3 - 6(c).
3.8 Surface plasmon induced Raman
enhancement in Ag nanoplates-SLG hybrid
One can easily change the outcome in graphene Raman enhancement in silver
nanoplates from charge doping to surface plasmon resonance by inducing shape
modification of silver nanoplates using lasers [235]. The silver nanoplate shape
modification can come in the form of photothermal melting and reshaping [235],
plasmon hot printing [237], photo-oxidation [238] or laser ablation [239]. Figure 3 -
9 shows silver nanoplate optical microscopy and SEM images along with their
corresponding Raman signals taken before and after laser irradiation (532 nm 20
mW exposure for 20 s). The silver nanoplate shapes are seen to be clearly deformed
after laser irradiation, either due to plasmonic hot printing effect in Figure 3 - 9(c)
or due to ablation in Figure 3 - 9(d). Figure 3 - 9(f-h) shows reflection microscopy
images and SEM images of silver nanoplates before and after the Laser treatment.
Figure 3 - 9: Optical microscope images and SEM image of laser modified Ag
nanoplates on SLG and corresponding Raman spectrum change. (a) & (b) show
optical images of Ag nanoplates on SLG before laser irradiation. Scale bar is 10
µm long. (c) Optical image of a nanoplate after laser irradiation, showing the
lifting of the tip of triangles. (d) Optical image of a nanoplate after laser
irradiation, showing laser ablated edge of the tip of triangles. (e) Corresponding
76
Raman peak enhancements. Both (c) and (d) nanoplates show enhancements,
without change in I(2D)/I(G), indicating only plasmonic enhancement is in play.
(f) & (g) Enlarged optical image of Ag nanoplate before and after laser
irradiation. (h) SEM image of Ag nanoplate after laser irradiation. Scale bar is 800
nm long.
Laser-induced photothermal melting experiment was performed on SLG-silver
nanoplate sample so as to study the change in Raman peak of various band
structure. Through this Laser-induced photothermal melting process I wanted to
control the interplay between the surface plasmon to the charge doping on the
silver nanoplate-graphene hybrids [237]. This is shown in Figure 3 - 9(e). The
simultaneous increase in both the G and 2D bands indicates that plasmon effect is
dominant over charge doping effect when the nanoplates are melted. It must be
noted here that the enhancement in D band is not due to silver deposition-induced
defect, but it is mainly due to the enhancement due to charge doping effect and
SERS effect which was responsible for G band enhancement also. I also confirmed
that the enhancement is consistent with the theory, demonstrating the control over
the two effects on the Raman.
One can clearly see that in Figure 3 - 9(c) the edges of triangular nanoplate are
lifted, this phenomenon has previously been reported in gold nanoplates due
plasmon hot printing [238]. This lift-off of the edge is due to the build-up of strong
plasmon near-fields around the tip, which leads to heat accumulation and local
bending of the nanoplate tip. This causes remarkable enhancement in the Raman
signal of the hybrid material. This indicates that the SPR induced enhancement is
dominant over the charge doping effect. The measured plasmon induced
enhancement compared with the calculated enhancement and they are found to be
in agreement as shown in Figure 3 - 6(b) (shown as green star points). One of the
reasons for this could be due the separation in surface contacts between the
graphene and nanoplates as a result of laser-induced deformation, but are still
within the reach of the near-field distance of the nanoplates. This causes the Raman
dipole to couple with the nanoplate antenna for both excitation and emission. Due
to laser irradiation it is sometimes observed that tip of nanoplates are ablated and
the ejected metal from the tip is redeposited as small nanoparticles nearby as was
observed previously in Ag spheres [267], and nanorods [268]. Raman enhancement
due to plasmon signals is still observed as shown in Figure 3 - 9(e) but not as
77
noticeable as the enhancement from the intact nanoplates. The wavenumber of G
and 2D peaks are measured for Raman spectrum of nanoplates acting as nano-
antennas after laser irradiation so as confirm whether the charging of SLG is
removed or not due to the hot printing process, which is represented in Figure 3 -
6(c) and (d) as a star point. It was observed that the G and 2D band stiffening has
vanished and their wavenumbers exactly matches with that of SLG. Hence, I can
conclude that the laser based hot printing removes the charge doping from SLG.
This is probably due to the reduction of contact area between the two materials.
3.9 Effect of oxidation on Raman spectra of
hybridized Ag nanoplate-SLG sample
The nanoplate-SLG hybrid sample was stored in cool and dry place for one month
at 2o centigrade, in order to study the effect of silver nanoplate oxidation on Raman
spectrum. Silver nanoparticles and surfaces used in SERS are severely affected by
oxidation and can be clearly seen in their Raman spectrum [269]. The result is
shown in Figure 3 - 10 for Raman spectrum of oxidized nanoplates on SLG. The
Raman spectrum of SLG and oxidized nanoplate-SLG hybrid looks the same
except for slight broadening of G peak which could enclose G` peak with in it.
Figure 3 - 6 shows the detailed analysis of G, 2D peak positions, total enhancement
and I(2D)/I(G) results. The enhancement of G and 2D bands have totally
disappeared and there is an enhancement of D peak clearly indicating that there is
neither any charge transfer taking place nor is there any plasmon enhancement
effect after the silver nanoplates have been oxidized. This shows a complete
switching between various effects of SERS of silver nanoplate – SLG hybrid.
3.10 Summary and conclusion
Figure 3 - 10 summarizes the various scenarios of silver nanoplates hybridized with
SLG and are shown as distinct Raman spectrum observations. Four different
scenarios can be seen.
78
Figure 3 - 10: Summary of Raman spectrum evolution of SLG, hybridized with
various Ag nanoplates. When Ag nanoplates are hybridized with SLG, unlike in
the case of silver nanoparticles, p-doping on SLG was observed, reducing
I(2D)/I(G) below 1 and stiffening G and 2D bands without any plasmon
enhancement. When the nanoplates were modified in shape with laser irradiation,
either by plasmon hot printing or laser-induced photo-oxidation, the charge
doping was lifted and strong plasmonic enhancement of Raman signals was
observed. These two effects could be turned off by oxidizing the whole plate,
where the Raman signals were returned back to the original SLG state.
The first phase is depicting the Raman spectra of pure SLG, the second phase
shows the effect of hybridization on Raman spectra due to p-doping, the third
phase shows shape change induced plasmon enhancement and the last phase shows
the disappearance of hybridization effect due to Ag nanoplate oxidation.
To conclude, I have successfully demonstrated that the silver nanoplates act as a p-
doping source for graphene unlike what is observed with silver nanoparticles by
reducing I(2D)/I(G) ratio below 1 and stiffening G and 2D bands, while its
plasmonic effect remains suppressed. This phenomenon can be reversed in the
laser-induced plasmon hot printing or laser-induced photo-oxidation scenario,
where the metamorphosis of the nanoplate shape into more thermodynamically
stable shape causes the plasmonic effect to increase, by suppresses the charge
79
doping effect. The charge doping effect has been indirectly confirmed through
different methods such as G and 2D band splitting indicating the occurrence of
First order Raman process along with the calculation of work function of Ag
nanoplates and graphene to decisively confirm the manifestation of p-doping
effect due to hybridization. By oxidizing silver nanoplates the charge doping effect
and plasmon enhancement effect are switched off and the Raman signal resembles
the original SLG state with slightly enhanced D band and inclusion of G` band into
the G band. Hence the hybridization between SLG and plasmonic nanostructures
display band gap hybridization and charge doping along with plasmon field
enhancement. In the future, this demonstration can be utilised in charge sensing, or
distance sensing.
80
Chapter 4
Raman Spectroscopy Study of Au-
Graphene Hybrid Nanocomposite
Introduction 4.1
The current field is moving towards hybridization of novel materials. As a result
material hybridization between graphene and plasmonic metals nanostructures and
the study of their nonlinear properties has recently received much attention for
expanding potential applications of graphene in photonics[35, 216]. A number of
such potential applications are in the field of fabricating different nanocomposite
such as high performance electrolyte materials [270, 271], optical and conducting
materials [272], heavy-duty polymer nanocomposites [273], fillers [274] etc.
Major advantage of the hybridization between these two materials is the fact that
properties of graphene can be tuned to include the rich optical functionalities of
surface plasmon resonance in the visible and NIR range. As outlined in the section
1.8 of Chapter 1, the best tool to study material hybridization of graphene is
through Raman spectroscopy. I saw wonderful and dramatic changes in the D, G
and 2D band peaks of Raman spectrum when silver nanoplates were hybridized
with graphene layer. Moreover, I saw very distinct influence of charge transfer
effect and plasmon enhancement effect due to nanoplate shape modification.
Therefore, I wish to continue the Raman spectroscopy study of single crystalline
gold nanosheets (single crystalline-AuNSs) hybridized with single and multilayer
graphene to see if the optical, physical, chemical and electronic properties are
altered due to hybridization. By doing so I will also be able to observe and
distinguish between the effect of hybridization of single and multilayer graphene
with single crystalline-AuNSs.
In this chapter I began with the summary of preparation and characterization of
single crystalline-AuNSs, sputter coated polycrystalline thin gold metal film and
81
multilayer graphene using different techniques explained in chapter 2. I will present
the Raman spectroscopy study of single layer graphene hybridized with single
crystalline AuNSs to demonstrate the appearance of hybridization phenomenon. I
will validate this claim by systematically studying the evolution of the different band
structures namely D, G and 2D bands of the given hybridized material for five
different wavelengths. I will explain the effect of doping on hybridization by
calculating the work function of single crystalline-AuNSs and graphene. I will then
compare the evolution of bands in this hybrid material with the Raman
spectroscopy measurements of multilayer graphene hybridized with single
crystalline-AuNSs. I will show that the hybridization effect is reduced by comparing
the enhancement in band structures of single crystalline-AuNSs hybridized
separately with single and multilayer graphene. The reduction in the extent of the D
and G band enhancement after hybridization with multilayer graphene will be
analysed as function of graphene layer thickness.
Materials needed for preparing single 4.2
crystalline gold-graphene hybrid nanocomposite
Single crystalline gold nanosheets
The synthesis of AuNSs that I used in our experimental was done according to ref
[189] by B. K. Lim’s group and is explained in section 2.11 of chapter 2 in detail. It
is important to highlight while summarizing the preparation technique and
synthesis of single crystalline AuNSs that the synthesis needs hot aqueous solution
to be maintained at a temperature of 95°C containing mild reductant and a capping
agent like L–arginine which is then rapidly injected with hydrogen tetrachloroaurate
trihydrate (HAuCl4·3H2O) solution. The resulting wet chemical synthesized AuNSs
were found to possess high resistivity and excellent electrical stretchability. The
average dimension of such single crystalline AuNSs is much larger than Ag-
Nanoplates and was found to be 40 µm × 40 µm in dimension with average
thickness of 20 nm.
82
Multi-layer graphene
The third material used for preparing gold-graphene hybrid nanocomposite is
Multi-layer graphene (Graphene Laboratories Inc. Calverton, NY). Multi-layer
graphene was grown on nickel using chemical vapour deposition (CVD) and then
transferred onto a quartz substrate. As explained in section 2.13 of chapter 2 of this
thesis, the multilayer graphene consists of an average of 4 layers single layer
graphene stacked on top of each other. Apart from the above mentioned materials
I used single layer graphene to prepare the hybrid nanocomposites for Raman
spectroscopy study. Hence the Raman spectroscopy study is performed on both
single and multilayer graphene-gold hybrid nanocomposite (here the gold referred
to is wet chemical synthesized single crystalline gold nanosheets).
Hybrid nanocomposites
The hybrid composite of single crystalline-AuNSs and MLG was prepared by drop
casting single crystalline-AuNSs on MLG and then dried in a thermal evaporator
for 15 minutes to remove any traces of water molecules. Using this same procedure
the SLG and single crystalline AuNS hybrid nanocomposite was also prepared. On
the other hand, the hybrid composite of polycrystalline thin gold metal film and
MLG was prepared by sputter coating 20 nm of thin gold film on MLG using high
vacuum (HV) thin film deposition system without the need for any adhesive. This
is because the gold film firmly adhered to MLG layer deposition and was found to
be uniformly coated on top of MLG.
Raman spectroscopy study of gold nanosheet-4.3
SLG hybrid
Similar to silver nanoplate-graphene hybrid sample studied in chapter 3, wet-
chemical synthesized single crystalline gold nanosheets (AuNS) can potentially be
used to hybridize graphene sheets. There are a few reports available in literature
which talks about the Raman spectroscopy study of polycrystalline gold-graphene
hybrid films.
83
For example Kim et al [260] demonstrated the variation in the Raman spectra of a
single layer graphene sheet when placed in five different gold substrate
arrangements. They were analysed in the context of surface enhanced Raman
scattering. SLG was transferred on top of (1) silicon substrate, (2) thermally
deposited gold film and (3) a closely-packed gold nanosphere layer. SLG was also
sandwiched between (4) two gold nanosphere layers and between (5) gold
nanosphere and thin film. Upon 514 nm laser illumination it was reported that the
SERS was negligible, but for 633 nm laser the enhancement was in the range of 3 to
50 folds depending upon the sample type from the five different samples
mentioned above. It is reported that the SERS enhancement can be classified into
chemical mechanism and electromagnetic mechanism. The enhancement in the
SERS of graphene deposited on gold film was predominantly due to chemical
mechanism (i.e., doping) and for graphene deposited on nanosphere was due to
electromagnetic mechanism (i.e., SPR). There were several unidentified band peak
appearances along with spectral distortion of G and 2D peaks. They also observed
enhancement of a broadened D peak. This enhancement is attributed to local field
enhancement of gold nanospheres and is not due to the chemical mechanism. The
spectrum distortion is due to high local field induced by the laser. They developed a
model to explain the difference in the enhancement factors among the various gold
substrates. The prominent factors for this difference are reported to be due to the
orientation of the inserted graphene sheet in the hybrid structure, polarization and
spatial distribution of the local field.
There are already a few papers reported in literature which demonstrate potential
applications in the field of medicine and cancer treatment through hybridization of
gold nanosheets with graphene layers. Manikandan et al [275] synthesized gold
nano-hexagons on graphene using locally developed techniques. These enhanced
Raman scattering from gold nano-hexagons were used to differentiate between
human breast cancer stem cells (BCSCs) and breast cancer cells (BCCs) from
healthy cells. They reported a Raman enhancement of 5.4 folds in detecting BCCs
and 4.8 folds in detecting (BCSCs) from healthy cells.
Taking cognisance of such potential application in medicine and bio-sciences, I
performed Raman spectroscopy experiments to study the effect of hybridization on
gold nanosheets deposited on top of SLG as shown in the Figure 4 - 1(a) and (b).
84
The schematic diagram shows the side view and top view of gold nanosheet
deposited on top of glass slide covered with SLG. Figure 4 - 1(c) shows the Raman
spectra for SLG (blue colour) and hybridized material (red colour) for five different
laser wavelengths.
Figure 4 - 1: (a) & (b) Side view and Top view of Au nanosheet on SLG hybrid
sample on glass substrate. (c) Raman spectra for SLG (blue colour) and Au
nanosheets hybridized with SLG (red colour) for five different laser wavelengths.
Dramatic changes in the Raman intensities of G, 2D and D peaks are observed for
gold nanosheets deposited on top of SLG. The observed enhancement in G band
of hybrid material compared to SLG alone for 457 nm (2.71 eV), 488 nm (2.54 eV),
514 nm (2.41 eV), 633 nm (1.96 eV), and 785 nm (1.58 eV) is 4.1, 4.74, 6.88, 4.21
and 1.26 respectively. Both the D and G bands are broadened and enhanced upon
hybridization along with the reduction in 2D band. The decrease in 2D band
intensity after hybridization is 10%, 50%, 70% and 65% approximately for 457,
488, 514 and 785 nm laser wavelengths respectively.
85
I did preliminary investigation to study the effect of tip and edges on the Raman
spectra of SLG hybridized with gold nanosheets. I acquired Raman spectrum at five
different positions as shown in Figure 4 - 2(a). The position 1 corresponds to
Raman spectra of SLG alone while the position at 2, 4 and 5 are closer to
nanosheet edges. Position 3 represents approximate centre of nanosheet.
Figure 4 - 2: (a) Optical reflection microscope image of Au nanosheet on SLG
with different points showing the spots where Raman spectra were acquired. The
white coloured scale bar shown is 8 µm long. (b) Raman spectra of SLG
hybridized with Au nanosheets for different positions shown in Figure (a) for
514 nm laser wavelength.
It can be clear seen from Figure 4 - 2(b) that the Raman spectra displays classical
signature of doped SLG, where the G band is enhanced and 2D band is reduced in
intensity. There is not much enhancement due to plasmon induced tip effect as was
observed in Figure 3 - 9(e) where both the G and 2D bands are enhanced due to
increased SPR effect. The enhancement at position 2, 3, 4 and 5 are approximately
1.3, 3.8, 1.8 and 1 times respectively compared to SLG at position 1. Hence all the
Raman spectra outlined in Figure 4 - 1(c) and Figure 4 - 5(c) are taken at the centre
of nanosheets. The variations in the G band increase at different positions shown
in Figure 4 - 2 can be due to various factors such as slight bending of the edges,
folding and crumpling of nanosheets during centrifuge process and uneven
thickness at the edges during growth process as shown in SEM images of Figure 4 -
3.
86
Figure 4 - 3: (a) & (b) Crumpled gold nanosheets during centrifugation process
with lift-off in edges clearly shown by black rounded circles. (c) Gold nanosheet
having thick edges and small lift-off along with a small triangular gold nanosheet
hidden beneath it at the centre. (d) Magnified image of highlighted edge of Figure
(c).
I similarly studied the effect of laser power variation by keeping exposure time
constant and likewise measuring Raman spectra for different laser exposure times
by keeping laser power constant as shown in Figure 4 - 4 (a) & (b) respectively.
Figure 4 - 4: (a). Raman spectra of SLG hybridized with Au nanosheets for
different laser exposure times of 514 nm laser wavelength at position 3 of Figure
4 - 2(a). The intensity of Raman spectra increases almost linearly with increasing
laser exposure duration. (b) Raman spectra of SLG hybridized with Au
(a) (b)
(c) (d)
(a) (b)
87
nanosheets for different laser powers of 514 nm laser wavelength at position 3 of
Figure 4 - 2(a). The average laser power measured at the back end of the
numerical objective was 3.5 mW. It can clearly be seen that the Raman spectra
for varying laser power also increases linearly with increasing laser power.
I noticed that as the laser power and exposure times were increased gradually, the
Raman spectrum intensity was found to increase linearly as expected.
Raman spectroscopy study of gold nanosheet-4.4
MLG hybrid
In addition to the Raman spectroscopy study of single crystalline-AuNSs on SLG, I
will present Raman spectroscopy study after hybridization of gold nanosheets with
multilayer graphene (MLG). The sample is made by drop casting gold nanosheets
on MLG. There is hardly any report available in literature which talks about the
Raman spectroscopy study of single crystalline gold-graphene hybrid films. One
report similar to our experiment investigates the effect of hybridization between
silver nanoparticles and multilayer graphene. Zhang et al [223] performed Raman
experiments for multilayer graphene transferred directly on top of silver
nanoparticles in order to investigate the effect of coupling between graphene and
localized surface plasmons (LSPs) of Ag nanoparticles. They found that the SERS
of multilayer graphene has increased approximately by 7-fold by near-fields of
plasmonic Ag nanoparticles. This is relatively small compared to what was reported
by Lee et al [229] for SLG and they also predicted that the enhancement will
decrease with increasing number of graphene layers. The increase in particle size
further enhanced the graphene G peak as was shown by Schedin et al [216]. They
also observed broadening and redshift in the LSP resonances of Ag nanoparticles in
the presence of graphene which was attributed to the coupling between the Ag
LSPs and the graphene layer.
The schematic diagram in Figure 4 - 5(a) and (b) shows the side view and top view
of the gold nanosheets deposited on top of glass slide covered with MLG. Figure 4
- 5(c) shows the Raman spectra for MLG (blue colour) and hybridized gold-
graphene material (red colour) for five different wavelengths. Contrary to what was
observed when noble metal was deposited on SLG, no dramatic changes in the
88
Raman intensities of G, 2D and D peaks are observed for gold nanosheets
deposited on top of MLG.
Figure 4 - 5: (a) & (b) Side view and Top view of Au nanosheet on MLG hybrid
sample on glass substrate. (C) Raman spectra for SLG (blue colour) and Au
nanosheets hybridized with MLG (red colour) for five different laser
wavelengths.
The observed enhancement in G band of hybrid material compared to SLG alone
for 457 nm (2.71 eV), 488 nm (2.54 eV), 514 nm (2.41 eV), 633 nm (1.96 eV), and
785 nm (1.58 eV) is 1.68, 1.56, 1.45, 1.2 and 1.1 respectively. There is approximately
20%, 5%, 30% decrease in the intensity of 2D band for 488, 514 and 633 nm lasers
respectively. There is no remarkable change in the width of D and G band except
for 457 and 488 nm laser wavelengths where the increase in the widths of D and G
band was observed.
Apart from dramatic changes in D, G and 2D bands of Au nanosheets on
hybridization with SLG, there is broadening effect observed in D, G and 2D bands
89
as shown in Figure 4 - 1(c) as mentioned earlier. Similar broadening was observed
in G and 2D bands in Figure 3 - 4(b) and (d) of previous chapter upon
hybridization of Ag nanoplates with SLG. But, the difference between Au
nanosheet hybridization with SLG and MLG is that there is no broadening effect
seen in Au nanosheet-MLG hybrid sample as shown in Figure 4 - 5(c). Figure 4 -
6(a) clearly depicts the broadening in G band and this broadening effect is due to
the change in graphene electronic structure induced due to the gold nanosheet
deposition on graphene. Figure 4 - 6(a) is fitted with two Lorentzian curves with
peaks at 1587 cm-1 and 1609 cm-1. On the other hand, the G band Raman spectra of
Au nanosheet-MLG hybrid sample can be fitted with only one Lorentzian curve
with peak at 1584 cm-1 as shown in Figure 4 - 6(b) upon 514 nm laser wavelength
excitation.
Figure 4 - 6: (a) & (b) The deconvolved spectra of G bands after Au nanosheet
deposition on SLG and MLG for 514 nm laser wavelength. The split observed in
G band of SLG after hybridization is about ~22 cm-1, on the other hand there
was no split observed in MLG after hybridization with Au nanosheets.
As described earlier, Lee et al [229] have reported the spilt in G band upon silver
metal deposition on single layer graphene for all laser wavelengths while the
splitting in G band disappeared upon hybridization of silver with multilayered
graphite. Dong et al. [247] explained that the split in G band observed by aromatic
molecules upon interaction with monolayer graphene was caused due to the
breaking the phonon symmetry at the point by altering the electron density
distribution of monolayer graphene by lifting the two-fold degeneracy of the LO
and TO phonons, which was seen for all laser wavelength excitations as outlined in
SLG MLG
90
previous chapter. This indicates that the hybridization of Au nanosheet with SLG
also induces first order Raman process inside the sample causing the transfer of
charges to occur between metal and single layer graphene as will be explained in
next section. Such electron density distribution doesn’t occur when Au nanosheet
is deposited on top of multilayer graphene and hence no G band splitting is
observed. This is true for all five laser wavelengths outlined in Table 4 - 1. The
pictorial depiction of splitting in G band for five different laser excitation
wavelengths for Au nanosheet hybridization with both SLG and MLG are shown
in Figure 4 - 7 for all laser wavelengths.
Table 4 - 1: Summary of splitting in G band peaks of SLG after hybridization with
Au nanosheets along with the information of fitted curve peaks for G band
denoted as G1 and G2.
Laser
specifications
Au nanosheet on
SLG
λ (nm) Energy (eV)
G G1 G2
785 1.58 1592 1578 1594
633 1.96 1586 1583 1608
514 2.41 1589 1587 1609
488 2.54 1589 1577 1598
457 2.71 1591 1584 1611
Similarly, the split in 2D band for SLG upon hybridization with Au nanosheets can
be studied, but it cannot be compared with the splitting in 2D band of Au
nanosheets hybridized with multilayer graphene. This is because the multilayer
graphene is already split into multiple modes as shown in Figure 2 - 10(d) of
chapter 2 and hence difficult to study the effect of splitting in 2D band if at all any
split exists.
91
Figure 4 - 7: (a) The deconvolved spectra of G bands after Au nanosheet
deposition on SLG. (b) The deconvolved spectra of G bands after Au nanosheet
deposition on MLG for five different laser wavelengths. The average split
observed in G band is about ~22 cm-1.
SLG MLG
92
Analysis and comparison of gold nanosheet 4.5
on SLG and MLG hybrid
Complete analysis of D, G and 2D band for single crystalline gold nanosheets
hybridized with single and multilayer graphene is shown in Figure 4 - 8.
Figure 4 - 8: Summary of G and 2D peak changes from SLG to SLG - Au
nanosheets (a) G peak position shift of SLG and MLG upon hybridization. SLG
and MLG shows stiffening of ~3 cm-1 after hybridization indicating the
occurrence of charge doping effect (b) G band enhancement of SLG and MLG
93
after hybridization. Enhancement is dominant when Au nanosheets are
hybridized with SLG (c) 2D peak position shift of SLG and MLG upon
hybridization by ~4 cm-1 to higher wavenumber. Dispersion can be seen before
and after hybridization of SLG and MLG. (d) 2D band enhancement of SLG and
MLG after hybridization. Both show reduction in Raman intensity. (e) D peak
enhancement and shift of SLG and MLG before and after hybridization. (f) D
band enhancement of SLG and MLG after hybridization. D band shows similar
trend as G band. (g) I(2D)/I(G) ratio change. SLG – Au nanosheets shows
drastic change in the I(2D)/I(G) ratio to less than 1 after hybridization compared
to MLG sample, meaning strongest interaction occurs between noble metals and
single-layer graphene.
The G band of MLG is shifted to lower wavenumber and the 2D band is shifted to
higher wavenumbers compared to SLG upon laser excitation as is widely reported
in literature. G band shows an average peak shift of ~3 cm-1 respectively for both
SLG and MLG hybridized with gold nanosheets to higher wavenumber compared
to SLG and MLG alone as shown in Figure 4 - 8(a). Similarly, 2D showed slope
enhancement of ~4 cm-1/eV after hybridization of graphene with gold nanosheets.
As can be seen from Figure 4 - 8(c) the 2D band shows dispersive behaviour and
its slope increases slightly from 103 ± 2 cm-1/eve to 107 ± 4 after gold nanosheets
hybridization with SLG and increases from 106 ± 3.1 cm-1/eve to 110 ± 2.7 cm-
1/eve after hybridization of MLG with gold nanosheets. This enhancement in 2D
slope after hybridization expresses the manifestation of alteration in the phonon
band structure. D band also shows dispersive behaviour but with a slope half that
of the 2D band of 45.45 ± 4.2 and 50 ± 4.9 cm-1/eV after hybridization of SLG
and MLG with gold nanosheets respectively. Figure 4 - 8(b) & (e) shows
enhancement summary of G and D band peaks. It can be seen that nanosheets on
hybridization with SLG (Red coloured squares) shows higher enhancement
compared to hybridization with MLG (Blue coloured circles). The decrease in 2D
band peak was greater when gold nanosheets were hybridized with SLG as shown
in Figure 4 - 8(d). The D, G and 2D band peak variations are pictorially
summarized in Figure 4 - 1(c) and Figure 4 - 5(c) before and after hybridization
with gold nanosheets. Figure 4 - 8(g) is another way of demonstrating the drastic
changes that occurred in band peaks when gold nanosheets was hybridizes with
SLG as the I(2D)/I(G) values dips from above 1 to values below 1. This means the
transfer of charge is prominent in Au nanosheet-SLG hybrid compared to Au
94
nanosheet-MLG hybrid. This difference in Raman spectra of SLG and MLG can be
used for exact characterization of graphene.
In order to explain the type of doping occurring in gold-graphene hybrid
nanocomposites, I calculated the shift in the Fermi energy level of graphene. The
calculated work function for graphene ( ), as explained in Eq. 3.2 of previous
chapter. The calculated work function for graphene is 4.48 eV [240] and for gold
nanosheets [189] is found to be 5.54 eV. This points towards p-doping in gold
plates as its work function is greater than graphene and hence the flow of electrons
is from gold to graphene. In Eq. 3.2, D0 (~ 0.09 eV2 per unit cell) and α (~ 34.93
eV/ Å) remains the same as mention earlier. The equilibrium separation of gold d0
is given as ~ 3.1 Å [240]. Considering the general equilibrium separation between
graphene and clean metal surfaces as d ~ 3.3 Å, I got the shift in Fermi energy as
ΔEF ~ 0.15 eV. This permits p-doping to take place from gold nanosheets and
thereby endorsing the Raman observation depicted in Figure 4 - 8.
There is no oxidization effect observed while carrying out the Raman spectroscopy
over prolonged duration of time. This is because gold is historically known to be
inert to surrounding atmosphere. Similarly no surface plasmon effect due to the
shape modification of gold nanoplates was observed. This is due to the fact that the
gold nanoplates are extremely large in dimensions compared to silver nanoplates
used in Chapter 3 and hence possesses the ability to dissipate heat effectively for
continuous mode low power lasers.
From the above results for noble metals deposited on SLG and MLG, I found a
remarkable correlation between the enhancement of the D, G and I(2D)/I(G) band
with the graphene layer thickness. The order of the extent of the D and G band
enhancement is inversely proportional to the graphene layer thickness. Therefore, it
can be said that the strongest interaction occurs between noble metals and single-
layer graphene.
Conclusion 4.6
In summary, Raman spectroscopy study of single crystalline gold nanosheet on
SLG and MLG were studied and their band enhancements and band shifts were
95
compared and summarized. I also studied the splitting in G band after hybridizing
SLG and MLG with gold nanosheets. I calculated the shift in Fermi lever after
hybridization to confirm the presence of p-doping due to hybridization of SLG
with Au nanosheets. The study clearly showed that the gold nanosheets on SLG
had higher hybridization effect compared to gold nanosheets on MLG as the
average G enhancement of former sample was ~4.25 times greater and D band
enhancement was ~35 times greater compared to SLG alone. Moreover the
I(2D)/I(G) ratio of >1 for SLG falls to <1 on hybridization with gold nanosheets.
On the other hand gold nanosheet on MLG shows no such drastic effect on
I(2D)/I(G) which remains below 1 before and after hybridization along with
meagre enhancement in D and G bands. Hence I can conclude that SLG is a better
candidate for hybridization than MLG.
96
Chapter 5
Z-Scan Based Nonlinear Optical
Study of Gold-Graphene Hybrid
Materials
5.1 Introduction
Research on plasmonic materials such as gold, silver, aluminium and their
nanostructures such as nanorods, nanoparticles and nanosheets has been
thoroughly studied over last decade because of the tunability of surface plasmon
resonance (SPR) bands over wide optical wavelength ranges [276-278]. Similarly,
since the discovery of Graphene in 2004 [68], all kinds of studies related to
nonlinear characteristics and device application for graphene has been thoroughly
undertaken till date [35].
As the plasmonic material are known to support surface plasmons across their
surface and bulk plasmon within their bulk, it will be very interesting to study how
the hybridization with graphene might change its intrinsic properties. Hence, in
order to fully utilize the potential of these composite nano-materials, their linear
and nonlinear properties need to be thoroughly investigated. The nonlinear
behaviour of single crystalline-AuNSs and sputter coated polycrystalline thin gold
metal films hybridized with multilayer grapheme (MLG) at high intensity, short-
pulse laser illumination at near-infrared wavelengths has not been thoroughly
investigated yet. Unlike graphene, plasmonic materials are known to exhibit
nonlinear optical (NLO) responses under high intensity laser illumination due to
surface plasmons[279]. Hence, I also intend to study the nonlinear behaviour of
these composite materials at near-infrared wavelengths which is far away from their
plasmon resonance.
Metal nanoparticles and metal thin sheets are known to display two photon
absorption and saturable absorption depending upon the material thickness,
97
concentration, excitation laser wavelength, dissolving medium, surrounding
dielectric environment etc. Hence, it will be interesting to see which type of
nonlinearity graphene and gold metal films will display at near-infrared wavelengths.
Most reports showed that the graphene displayed nonlinear saturable absorption
effect (NLA-SA) and metal films and thin metal sheets displayed nonlinear two
photon absorption (NLA-TPA) effect. Studying the interaction of these two effects
during graphene hybridization with metal films might lead to development of
specific application with deep insight into the fundamental knowledge of this
emerging field. This can be done through Z-Scan experimental setup with high
power laser excitation on hybridized sample.
In this chapter I will also present precise measurements of NLA-SA of multilayer
graphene and NLA-TPA of single crystalline gold nanosheets (single crystalline-
AuNSs) and sputter coated polycrystalline thin gold metal film using Z-Scan
technique for near-infrared wavelengths (NIR) ranging from 700 nm to 900 nm,
laser pulse width ranging from 115 fs to 130 fs and for repetition rates of 0.82 MHz
to 82 MHz. I will show that intensity dependence on open aperture Z-scan was
studied in detail for all materials. I will also compare the NLA-SA values of
multilayer graphene with the reported values in literature. I will then analyse the
NLA-TPA values of single crystalline-AuNSs with polycrystalline gold metal films
and explain the reason for the differences observed in the values of these two metal
films. I will attempt to explain the rationale behind the reduction in NLA-TPA
values with the change in repetition rate of laser pulses. Finally Z-Scan results for
single crystalline and poly crystalline gold-MLG nanocomposite will be presented
which clearly demonstrates the effect of hybridization and potential use for future
applications.
5.2 Materials needed for preparing gold-graphene
hybrid nanocomposite
I studied five different materials using Z-Scan technique to determine their third
order nonlinearity. The pictorial diagram of all the five materials used for measuring
third order nonlinearity using Z-Scan is shown in Figure 5 - 1. The preparation and
98
characterization of first two materials consisting of MLG and single crystalline-
AuNSs is already explained in detail in section 2.11 and 2.13 of chapter 2 and
section 4.2 of Chapter 4 and is shown in Figure 5 - 1 (a) & (b) respectively.
Figure 5 - 1: Pictorial representation of five different samples used to measure
third order nonlinearity using Z-Scan (a) Multilayer graphene sample (b) Single
crystalline gold nanosheet sample (d) sputter coated polycrystalline gold film
sample (d) Single crystalline gold-graphene nanohybrid sample (e) Polycrystalline
gold-graphene nanohybrid sample
Polycrystalline thin gold metal film
The third material that I used in the study of third order nonlinearity using Z-Scan
is polycrystalline gold thin films and is shown in Figure 5 - 1(c). The details of
sputter coating technique, sample preparation and characterization are explained in
detail in section 2.12 of Chapter 2. I used high vacuum thin film deposition system
to sputter 20 nm thin gold film at room temperature at sputtering power of 90W
and deposition rate of 2 nm/sec. Usually the deposition of polycrystalline thin gold
film requires an adhesive layer like Ti, Cr of minimum 2-3 nm thickness to properly
stick to glass surface. Since the materials that I use have thickness in the ranges of
few nanometres it is highly undesirable that I sputter coat I films on top of such
adhesive materials. The reason for taking this line of approach is that these adhesive
materials might interfere with the measurement of nonlinear properties of
polycrystalline thin gold film and hybrid nanocomposites.
99
Hybrid nanocomposites
The fourth material is a hybrid composite of single crystalline-AuNSs and MLG,
which is already explained in the section 4.2 of Chapter 4 and shown in Figure 5 -
1(d). The final hybrid nanocomposite sample shown in Figure 5 - 1(e) is made of
polycrystalline thin gold metal film and MLG, which was prepared by sputter
coating 20 nm of thin gold film on MLG using high vacuum (HV) thin film
deposition system without the need for any adhesive. This is because the gold film
firmly adhered to MLG layer deposition and was found to be uniformly coated on
top of MLG.
5.3 Z-Scan equations for fitting experimental data
Now I will measure the nonlinearity of single crystalline and polycrystalline gold
and MLG samples along with their hybrids using theoretical and experimental Z-
Scan technique. As mentioned earlier in Chapter 1, Z-Scan is used to measure Non
Linear Refraction γ (NLR) and Non Linear Absorption β (NLA) of nonlinear
materials. The detail experimental Z-Scan setup and its related accessories like pulse
width measurement, pulse repetition rate control is explained in Chapter 2 in detail.
The open aperture readings of Z-Scan gives NLA values and closed aperture
readings is used to calculate NLR. The NLR coefficient γ is obtained by fitting the
experimental data of closed aperture divided by Open aperture with expressions
[176].
(5.1)
where is the laser power density in the focal point, is the effective sample
thickness, ( ) , is the difference of fraction
transmitted, ( ( ⁄ ) , is the radius of the aperture, and is
the beam radius in the aperture. The nonlinear optical absorption coefficient β can
be obtained by fitting the data of open aperture with the following expressions
( ) ∑ ( )
( ) ⁄ (5.2)
where, ( ) ( ) and is the Rayleigh length.
100
5.4 Measuring nonlinear two-photon absorption
(β) of single crystalline AuNSs
Z-Scan experiment was carried out using Ti:sapphire femtosecond pulsed laser
(Tsunami, SpectraPhysics, 700 nm - 900 nm wavelength, 115-130 fs pulse width
and 0.82 MHz-82 MHz repetition rate). The laser was focused using an objective
lens with numerical aperture 0.25, which produces an airy focal spot with 4.0 µm
full-width half maximum (FWHM). The open aperture and closed aperture reading
of Z-Scan reading were fitted according to ref [176]. These parameters remain same
while calculating β values for polycrystalline thin gold metal film, MLG and hybrid
nanocomposites of single and polycrystalline thin gold metal film with MLG.
Enhanced optical effects observed in noble metal nanoparticles and nanosheets is
due to the fact that these materials supports surface plasmon resonance across its
surfaces which causes the local electric fields to be enhanced, specifically at sharp
features such as edges, tips and corners which leads to enhanced luminescence
[280-283], enhanced third-order optical nonlinearity and enhanced Raman
scattering (SERS) [284-286]. Similar to what was observed in two-dimensional silver
nanoplates as shown in Chapter 3, micrometre scale metal nanosheets are also
reported to exhibit multiple SPR modes from visible to near-infrared region [287].
Potential application of chemically synthesized nanoplates include laser-induced
heat based cancer treatment [185], sensors [288] and as SERS substrates [286]. Liu
et al [289] have recently reported synthesizing micrometre scale gold nanoplates of
hexagonal, triangular and truncated triangular shapes using polyol process, which
exhibited strong SPR extinction in visible and near-infrared region. They reported
nonlinear absorption and nonlinear refraction coefficient values of 1.18×10-7 cm/W
and −1.04×10−12 cm2/W at laser pulse width of 2.5 ps and a repetition rate of 76
MHz using Z-Scan technique.
In our Z-Scan measurements, wet chemical synthesized single crystalline gold
nanosheets having approximate dimensions of 50 µm were found to exhibit two-
photon absorption (TPA) phenomenon (dip/valley signature) while measuring the
open aperture reading and closed/open aperture showed peak followed by valley
signature as shown in Figure 5 - 2(a) and (b). The NLR and NLA-TPA coefficients
101
for 780 nm wavelength at 0.0165 Jcm-2 were calculated as γ = 5.95 ×10-12 cm2W-1
and β = 1.08×10-6 cmW-1.
Figure 5 - 2: (a) & (b) Open Aperture and closed/open aperture Z-Scan
Experimental data and fitting at 780 nm, ~ 82 MHz repetition rate and 0.0165
Jcm-2 power density at the focus for single crystalline AuNSs (c) Nonlinear
absorption (TPA) coefficient β values of single crystalline AuNSs for 700-900nm
wavelength range from 0.82-82MHz repetition rate.
These measurements were repeated for various wavelength range (700 - 900 nm)
and repetition rates (0.82MHz to 82MHz) with different color markings as shown
in Figure 5 - 2(c). The effect of change in pulse width on each wavelength was
removed by individually calculating the pulse width for each wavelength and
repetition rate (i.e., 0.82, 8.2 and 82 MHz) using frequency –resolved optical-grating
(FROG) instrument. Figure 5 - 2(c) shows that the nonlinear coefficient β values
decreases with increase in wavelength. Moreover at higher repetition rates NLA-
TPA phenomenon is diminished due to the temperature accumulation effect.
Hence it can be said that as the repetition rate increases the nonlinear effect is
reduced.
It has already been reported by Wickremasinghe et al [290] that increasing
repetition rate causes increase in two-photon absorption coefficients due to the
accumulation of heat in the organic material film. The reason for heat accumulation
was reported due to the non-radiative recombination of Frenkel excitons, excimers
and charge transfer excitons. To support this idea they calculate the thermal
diffusion time of organic materials using the formula,
102
(5.3)
where, td represents thermal diffusion time, D is the heat diffusion constant and ω0
is the focal radius of objective lens. They have found that the heat built-up inside
their sample is faster than the thermal diffusion time of the organic material.
Using Eq. 5.3 I can approximately estimate the thermal diffusion time ‘td’ of gold
nanosheet and gold nanofilms. The heat diffusion constant of gold at room
temperature (300K) is given as D = 1.27×10-4 m2/s [291]. The focal radius of the
objective lens is ω0 = 2 µm at 780 nm laser wavelength. Substituting these values in
Eq. 5.3 gives approximate thermal diffusion time of td ≈ 0.8×10-8 seconds. On the
other hand, the pulse repetition frequencies used in our Z-Scan setup are 0.82, 8.2
and 82 MHz which corresponds to time intervals of 1.2×10-6, 1.2×10-7 and 1.2×10-8
seconds between two consecutive pulses. It can be seen that the thermal diffusion
time of gold and time interval taken by 82 MHz pulse train is almost of the same
order of magnitude causing the heat to build-up in the metal film, while 8.2 MHz
and 0.82 MHz pulse train takes longer time to arrive and allows the metal to
dissipate heat before the onset of next pulse.
Hence I can conclude that the difference in the measured nonlinear absorption
(TPA) values for gold nanosheet and gold nanofilms for different repetition rates of
femtosecond lasers is due to negligible difference between thermal diffusion time
and pulse arrival time for 82 MHz train. This causes high heat build-up in our
samples leading to decrease in two-photon absorption coefficients with increasing
repetition rate as shown in the above calculations. Moreover the difference in the
two-photon absorption coefficients of 8.2 MHz and 0.82 MHz is not affected due
to shorter diffusion time compared to the relatively longer arrival time of
consecutive pulses.
5.5 Measuring nonlinear two-photon absorption
(β) of polycrystalline gold thin film
Similarly I performed Z-Scan experiment for polycrystalline thin gold metal film for
the same aforementioned wavelengths and repetition rates. Moreover the third
103
order nonlinear measurements of sputter coated and electron beam deposited
polycrystalline gold film is abundantly reported in literature [44, 292-296] for film
thicknesses as small as 2 nm to near bulk gold thickness of 52 nm for wide range of
laser wavelengths and pulse widths using Z-Scan technique. For e.g., Rotenberg et
al [296] measured nonlinear optical absorption coefficient of 20 nm thick gold film
at 630 nm for varying laser pulse widths. They reported that the nonlinear optical
absorption coefficient is dependent on laser pulse fluence and laser pulse width.
For the pulse width ranging from 0.1 to 5.8 picoseconds they observed nonlinear
optical absorption coefficient to vary from 6.8×10−7 to 6.7×10−5 cmW−1
respectively. This was due to the thermal smearing of d-bands and electron heating
inside the gold film.
In our Z-Scan measurements, Sputter coated polycrystalline gold thin films were
also found to exhibit two-photon absorption (TPA) phenomenon similar to that of
single crystalline AuNSs. Figure 5 - 3(a) shows β of polycrystalline thin gold metal
film values for 700-900 nm wavelength range from 0.82-82 MHz repetition rate
which looks quite similar in characteristics to that of single crystalline AuNSs
shown in Figure 5 - 2(c).
Figure 5 - 3: (a) Nonlinear absorption coefficient β of polycrystalline thin gold
metal film values for 700-900 nm wavelength range from 0.82-82 MHz repetition
rate. (b) Comparison of nonlinear coefficient β for single crystalline and
polycrystalline thin gold metal film for 8.2MHz repetition.
It can be observed from Figure 5 - 3(a) that the nonlinear two-photon absorption
coefficient ‘β’ values decreases slightly with increase in wavelength and increases
with decreasing repetition rate. But, one interesting factor to note is that the
104
nonlinear two-photon coefficient ‘β’ values for polycrystalline thin gold metal film
are marginally greater than single crystalline AuNSs as shown in Figure 5 - 3(b) for
8.2MHz repetition rate for 700-900 nm wavelength range. I attribute this to the
field enhancement effect due to rough surface wherein the field is increased around
the tip of the spheroidal protrusions thereby increasing nonlinear absorption co-
efficient.
It has already been reported by Siddiquee et al [297] that the nonlinear effect such
as two photon luminescence and nonlinear absorption is affected due to the tip
geometry of nanoparticles such as nanorods, bipyramids, dumbells and spheroids
etc. They calculate the two photon action cross section of nanoparticles at
excitation frequency ωex using the below formula,
( ) ∫ ( )| ( )|
| ( )|
( )
--- (5.4)
where η(ωem) is intrinsic luminescence spectrum. σ2pbulk(ωex) = β(ωex)/C where
σ2pbulk(ωex) is the two-photon absorption coefficient of bulk gold, β is nonlinear
absorption coefficient and C is the number density of the particles in a unit volume.
ω2 and ω1 are the upper and lower frequency limits of a detection system, and L(ω)
= ∫ sENR dA/∫ sE0 dA is the surface averaged local field enhancement factor around
a nanorod.
They showed that the bipyramid tips confine strong electromagnetic tips compared
to nanorods and dumbells. This is because the bipyramid tips are much sharper as
shown in Figure 5 - 4(a). Whereas the tips of nanorods and dumbells are reported
to be blunt and rounded and hence have lower field strengths. Moreover the
surface average local field enhancement factor around nanorods in Eq. 5.4 increases
as fourth power of tip sharpness. Eq. 5.4 inherently indicates that as the tip radius
of curvature decreases the enhancement factor increases as shown in Figure 5 -
4(a).
105
Figure 5 - 4: (a) Relation between tip radius of curvature and the local field
enhancement around the tips (b) AFM image of ultra-smooth single crystalline
AuNSs having average surface roughness of 0.143 nm (c) AFM image of
polycrystalline thin gold metal film having average surface roughness of ~4.3 nm
(d) 3D view of a section of polycrystalline thin gold metal film surface of 5 -3-(c)
with sharp conical tips and black background.
Figure 5 - 4(b) shows AFM image of ultra-smooth single crystalline AuNSs having
average surface roughness of 0.143 nm with a 1 µm scale as inlet. Whereas the
Figure 5 - 4(c) shows AFM image of polycrystalline thin gold metal film have
average surface roughness of ~4.3 nm with a 2 µm scale as inlet. Looking at the 3-
dimensional AFM image of polycrystalline thin gold metal film shown in Figure 5 -
4(d), it can be clearly seen that it has very rough surface with sharp cone line
projections inducing tip-effect [291]. This emphasizes the fact that polycrystalline
thin gold metal film possesses higher two-photon absorption values than single
crystalline AuNSs. AFM section of polycrystalline gold film is also shown in Figure
2 - 9(b) of Chapter 2.
5.6 Measuring nonlinear saturable absorption (α)
of multilayer graphene
Despite single layer graphene being a massless band gap structure and multilayer
graphene being 1-3 nm thin, they are known to demonstrate strong nonlinear
optical effects such as frequency mixing, frequency multiplication, four-wave
mixing and nonlinear saturable absorption properties, which is usually studied for
bulk materials [298]. Zhang et al [299] showed that few layer graphene exhibited
giant nonlinear refractive index n2 ≃ 10−7 cm2 W−1 which is 9 orders of magnitude
larger than bulk dielectrics at 1550 nm using picosecond laser. Yang et al [300]
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showed giant two-photon absorption in single layer graphene and bilayer graphene
at 780 and 1100 nm using a femtosecond laser. They showed that the two photon
absorption in single and bilayer graphene is dependent upon ω-3 and ω-4 of light
frequency. Kamaraju et al [301] showed that carbon nanotubes also exhibited large
nonlinear absorption coefficients at 790 nm using femtosecond laser. Huang et al
[302] showed saturable and reverse saturable absorption for 1, 8 and 16 layer
graphene at 532 nm using picosecond laser by employing Z-Scan technique.
Continuing with our Z-Scan experiments for MLG, I found that MLG exhibited
saturable absorption (SA) phenomenon i.e., peak/crest signature while measuring
open aperture and peak followed by valley in its close/open aperture readings as
shown in Figure 5 - 5 (a) and (b) unlike what was observed in metals as shown in
Figure 5 - 2(a) and (b). The NLR coefficient value and nonlinear saturable
absorption coefficient value for 780 nm wavelength at 0.0171 Jcm-2 were calculated
as γ = 2.60 ×10-10 cm2W-1 and α = -4.0×10-5 cmW-1. The measured absolute
nonlinear saturable absorption (NLA-SA) coefficient ‘α’ was found to increase from
3.11×10-5 to 3.65×10-5 cmW-1 for intensity variation from 100 GW - 210 GW for
various wavelength range (700 ~ 900 nm) as shown in Figure 5 - 5(c).
Figure 5 - 5: (a) & (b) Open Aperture and closed/open aperture Z-Scan
Experimental data and fitting at 780 nm, ~ 1 MHz repetition rate and 210
GW/cm2 power density at the focus for MLG (c) Absolute nonlinear coefficient
α values of MLG for 700-900 nm wavelength range from 0.82-82MHz repetition
rate.
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Interestingly there was no effect of pulse repetition rate (0.82MHz to 82MHz) seen
on NLA-SA coefficient for MLG indicating that there is no temperature
accumulation effect. This indicates that few layer graphene is an excellent source of
thermal dissipation. It can be used as heat sink for thermally volatile materials. I
have estimated the approximate thermal diffusion time ‘td’ of ~4-layer graphene on
glass substrate using Eq. 5.3. The heat diffusion constant of ~4-layer graphene on
glass substrate at room temperature (300K) is given as D = 6.5×10-4 m2/s [297] and
the focal radius of the objective lens is ω0 = 2 µm. Substituting these values in Eq.
5.3 gives approximate thermal diffusion time of td ≈ 0.15×10-8 seconds which is
one order of magnitude smaller than the fastest pulse time intervals of 1.2×10-8
seconds for 82 MHz and many orders of magnitude smaller than 1.2×10-6, 1.2×10-7
for 0.82 and 8.2 MHz repetition rate respectively. Moreover, the heat dissipation in
4-layer graphene is ~6 faster than gold value mentioned before. Hence there is
virtually no heat built-up seen in multilayer graphene even for femtosecond lasers
confirming its usefulness for heat sink based material applications.
The measured nonlinear saturable absorption coefficient of multilayer graphene is
shown in Table 5 - 1 compared with other materials such as single walled carbon
nanotubes (SWNTs), chemical vapor deposited (CVD) monolayer graphene, bilayer
graphene and multilayer graphene.
Table 5 - 1: Comparison of nonlinear saturable absorption coefficients (α)
Material Laser specifications α (10-9
cm/W) Method Reference
SWNTs 140 fs, 1 kHz, 780 nm 1,400 Z-Scan Kamaraju et al
[301]
Bilayer Graphene
400 fs, 1 kHz, 780 nm 10,000
Z-Scan Hongzhi Yang
et al [300] 200 fs, 1 kHz, 1100 nm 20,000
Monolayer Graphene
17 ps, 10 Hz, 532 nm 1,700-54,000
Z-Scan Pi Ling Huang
[302]
7-layer graphene
3.8 ps, 10 MHz, 1550 nm
~50,000 Z-Scan Han Zhang et
al [299]
Multi-layer graphene
(~4 layers)
150 fs, 0.82-82 MHz,
700-900 nm
31,100-36,500
Z-Scan Present study
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As shown in Table 5 - 1, the value of nonlinear saturable absorption coefficient of
multilayer graphene of 31,100-36,500 ×10-9 cm/W is between bilayer graphene and
7-layer graphene in the infrared region. This implies that the multilayer graphene
has greater potential for application in infrared technology such as heat sink, neutral
density filters to block excess light and as saturable absorbers.
5.7 Measuring nonlinear saturable absorption (α)
of multilayer graphene-gold hybrid nanocomposite
Homogeneous samples made of single materials can exhibit saturable absorption,
reverse saturable absorption, multi-photon absorption from among the many
nonlinear phenomenon upon excitation with high intensity pulsed lasers. The
prominent property that is desired in any nonlinear materials is its fast nonlinear
optical response which is highly sought after in the field of communication, data
processing, data storage etc. Other such desirable properties needed in a nonlinear
material are its low cost of production, large nonlinear response, high mechanical
strength, ease of fabrication, low losses at desired operation wavelength etc. No
material has individually ever displayed these characteristic with itself. Hence early
researchers were trying to hybridize materials so that their performance can be
enhanced. For e.g., researchers have tried physical mixing materials with opposing
nonlinear mechanism such as blending saturable absorptive materials with reverse
saturable absorption materials, stacking different material geometries such as dyes
and carbon nanotubes on top on one another to fulfill the requirements of good
nonlinear material.
For instance Liu et al have shown that graphene on hybridization with Porphyrin
and Fullerene exhibits reverse saturable absorption for laser pulse width of 5 ns and
laser wavelength of 532 nm using Z-Scan [303]. Zhang et al have demonstrated that
graphene-oligotheophene hybrid also exhibits Z-Scan based reverse saturable
absorption using 5 ns Q-switched pulsed laser at 532 nm wavelength [304]. Zheng
et al have shown that graphene materials with hybrid glasses show reverse saturable
absorption using 8 ns pulsed laser at 532 nm wavelength and saturable absorption
using 21 ps pulsed laser at 532 nm wavelength. The switch in nonlinearity was due
to ground-state bleaching in the sp2 regime of graphene atoms [305].
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I performed Z-Scan measurement to determine the nonlinearity of single and
polycrystalline thin gold metal film -MLG hybrid nanocomposite. This was a novel
measurements being carried out to determine the nonlinear properties when
materials are hybridized with totally different characteristics as was seen in above
measurements. It is to be noted that single and polycrystalline thin gold metal film
exhibited two-photon absorption nonlinear phenomenon whereas the MLG
exhibited nonlinear saturable absorption phenomenon. Hybridizing such diverse
materials opens new opportunities in developing better materials with control over
their intrinsic characteristics. Figure 5 - 6 (a) shows the absolute nonlinear saturable
absorption ‘α’ values of single crystalline AuNSs-MLG hybrid composite for
repetition rate 0.82-82 MHz repetition rate and 700-900 nm wavelength range. The
measured absolute NLA-SA coefficient ‘α’ was found to be approximately from
≈1.8×10-5 to 4.5×10-5 cmW-1. Similarly the absolute nonlinear saturable absorption
coefficient ‘α’ values of polycrystalline thin gold metal film-MLG hybrid composite
for 700-900 nm wavelength range from 0.82-82 MHz repetition rate was found to
be approximately from ≈1.7×10-5 to 4.5×10-5 cmW-1 as shown in Figure 5 - 6 (b).
Figure 5 - 6: (a) Absolute nonlinear saturable absorption coefficient ‘α’ values of
single crystalline AuNSs-MLG hybrid composite for 700-900nm wavelength
range from 0.82-82MHz repetition rate. (b) Absolute nonlinear saturable
absorption coefficient ‘α’ values of polycrystalline thin gold metal film -MLG
hybrid composite for 700-900 nm wavelength range from 0.82-82 MHz
repetition rate.
There is clear effect of hybridization being reflected in the nonlinearity of this
nanocomposite material. The nonlinear saturable absorption ‘α’ values of hybrid
nanocomposite materials are greater than MLG. These results demonstrate that
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single and polycrystalline gold-MLG nanocomposite is a promising and better
candidate for optical limiting applications than MLG alone.
5.8 Conclusion
Precise measurements of nonlinear refraction and nonlinear absorption (NLA)
coefficient of single crystalline gold nanosheets, polycrystalline thin gold metal film,
multilayer graphene , single crystalline and polycrystalline gold-multilayer graphene
nanocomposites were presented for near-infrared wavelengths using Z-Scan for
different repetition rate. Single crystalline AuNSs of 20 nm thickness were prepared
through chemical synthesis. Multilayer graphene was found to have few monolayers
of graphene, usually between 1-7 layers with an average of 4 monolayer thickness.
The composite of AuNSs and MLG was prepared by drop casting AuNSs on MLG
and the composite of polycrystalline thin gold metal film and MLG was prepared
by sputter coating gold on top of MLG. Z-Scan experimental was carried out using
Ti:sapphire femtosecond pulsed laser (700 nm - 900 nm wavelength, 115-130 fs
pulse width and 0.82 MHz-82 MHz repetition rate). Intensity dependence on open
aperture Z-scan was studied in detail for all materials.
It was found that single and polycrystalline gold film exhibit two-photon
absorption. The nonlinear absorption of polycrystalline thin gold metal film was
found to be fractionally higher than that of single crystalline-AuNSs. This is
thought to be due to field enhancement effect caused due to the large surface
roughness of polycrystalline thin gold metal film caused due to the formation of
gold islands during sputtering process. At higher repetition rates NLA
phenomenon is diminished due to the temperature accumulation effect. As the
repetition rate decreases the nonlinear effect is enhanced. Unlike gold metal films,
MLG exhibited nonlinear saturable absorption (NLA-SA) effect indicating its
usefulness as heat sink in photonic applications. Z-Scan results for single crystalline
and poly crystalline gold-MLG nanocomposite exhibit NLA-SA characteristics. The
measured absolute NLA-SA coefficient ‘α’ for hybrid nanocomposites was found to
be approximately ≈1.7×10-5-4.5×10-5 cmW-1 which is lower than that of MLG,
clearly demonstrating the effect of hybridization. The single and polycrystalline
gold-MLG nanocomposite was found to have better thermal dissipation property
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than MLG alone due to its large NSA effect. This underlines the importance in
recent rise in research of hybrid materials and the strong desire for engineering
better optical, chemical and thermal properties using hybridization phenomenon as
they are found to possess better intrinsic properties than individual materials of
similar thickness and dimensions.
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Chapter 6
Conclusion and Future Work
In this thesis Raman spectroscopy study of single layer graphene, multilayer
graphene and their hybridization with noble metals for five different laser
wavelengths was studied in detail. This study finds application in wide variety of
scientific application such as SERS based sensing, antennas, catalysis etc. I also
studied third order nonlinear absorption coefficients of single crystalline and
polycrystalline gold-graphene hybrid nanocomposite for near infrared wavelengths
and different repetition rates. The study of nanocomposite hybrid materials helps
us to unravel potential useful properties that do not exist in homogeneous natural
materials. Z-Scan technique was used to measure third order nonlinearity because
of its simplicity in building, operating and analysing the data.
1. Single layer graphene upon hybridization with plasmonic silver metal
nanoplate exhibited strong variation in the D, G and 2D Raman spectrum
peaks. Theoretical calculations using COMSOL predicted the
enhancements to be 3-4 times the original intensities observed in pure SLG
alone.
2. I studied the enhancement, dispersion, broadening and band splitting in
silver nanoplate-SLG hybrid using Raman spectroscopy. I calculated the
shift in Fermi energy of graphene due to silver nanoplate deposition using
analytical theory and it showed that charge transfer is taking place due to
hybridization. I studied that the enhancement in Raman peaks and the
charge transfer effect of hybrid material after the silver plates are oxidized
after 1 month duration.
3. I also studied the nanoplate shape modification induced plasmon enhanced
Raman spectrum of silver-graphene hybrid composite using laser beam. The
enhancements due to charge transfer effect and surface plasmons were
compared with the theoretical simulations. This helped in unwinding the
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enhancement due to charge transfer effect from the surface plasmon effect
in hybrid materials for the first time.
4. I also carried out Raman spectroscopy study on gold nanosheet and
graphene hybrid structure by varying the thickness of graphene from single
layer to multilayer graphene. This helped us to understand the effect of
layer thickness on plasmonic metal-graphene hybrid structures.
5. I then studied the third order nonlinear absorption coefficient of multilayer
graphene, wet chemically synthesized single crystalline gold nanosheets and
sputter coated polycrystalline gold nanofilms using Z-Scan technique. I then
extended this to study the third order nonlinear absorption coefficient of
multilayer graphene hybridized with single and polycrystalline gold films.
This enabled us to explore the effect of crystallinity on the nonlinear
properties of hybrid materials.
Two dimensional (2D) materials which are atomic layer thin or having few atomic
layers can be stacked on top of one another to fabricate artificial materials of
astonishing functionalities that has already revolutionized the field of
nanotechnology. Graphene and few nanometre thin plasmonic metal nanomaterials
are among such highly explored 2D materials. By hybridizing these materials, the
band gaps of the resulting hybrid materials can be tailored to suit specific
engineering applications such as LEDs, memory storage, photovoltaics etc. In
order to explore these features more effectively and to understand their material
properties upon hybridization I performed Raman spectroscopy study and Z-Scan
experiments to calculate their nonlinear properties.
In this work, I attempted to measure and compare Raman spectrum of single layer
graphene (SLG) and silver nanoplate-SLG hybrid for five different wavelengths. I
noticed strong D, G and 2D band variation in Raman spectra of hybrid material. I
analysed the enhancement in D, G band peaks and reduction in 2D band peak of
SLG and hybrid material. D and G band showed enhancement factor of 4 and 3.5
times upon hybridization compared to pure SLG spectrum. The I(2D)/I(G) ratio
for hybrid materials was found to be below 1 with the stiffening of G and 2D band.
Dispersive behaviour in the D, G and 2D band of hybrid material was observed
with D band having half the dispersion slope of 2D band. I calculated the shift in
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the Fermi energy of graphene
)(dEF after the silver nanoplate deposition on
SLG and found that the Dirac point is lowered by 0.2 eV. I concluded from the
Fermi energy shift calculations that the enhancements are due to the silver
nanoplate acting as p-type doping source for graphene.
I also performed Raman spectroscopy study of the plasmon enhancement of D, G
and 2D bands of silver nanoplate-SLG hybrid material induced due to the laser
based nanoplate shape modification. I observed that upon high power laser
illumination the edges of triangular nanoplates are lifted due to build-up of strong
plasmon field around the tip, which in turn leads to heat accumulation and bending
of nanoplate edges. This gives a remarkably enhanced Raman signal of the hybrid
material with all D, G and 2D bands enhanced unlike what is seen in charge doping
effect where the G band is enhanced and 2D band reduced. I also found that the
experimental values for surface plasmon enhanced Raman spectra match with
calculated values. I observed that some of the silver nanoplate edges are ablated and
redeposits as small nanoparticles showing overall weak plasmon enhancement
compared to the intact nanoplates with lifted edges. I found that the G and 2D
band stiffening has vanished in the surface plasmon based enhancement and
matches with SLG, indicating the disappearance of charge transfer effect due to
reduction of contact area. I was able to conclude that the plasmon based
enhancement is due to the fact that the modified silver nanoplates were acting as
antenna structure.
I also measured the effect of oxidation of silver nanoplates on the charge transfer
and SERS phenomenon by placing our nanoplate-graphene hybrid sample in cold
storage for one month at 2o centigrade. I found that the SERS and charge transfer
effect has totally disappeared as the oxide layer around the silver nanoplate acts as
barrier from any free transfer of charges between silver nanoplate and graphene. In
our study of analysing silver nanoplate-graphene hybrid nanocomposite I was able
to unwind and differentiate the effect of charge transfer effect from plasmon
enhancement and I was able to switch off these effects by oxidizing the silver
nanosheets.
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I extended our Raman spectroscopy study to gold nanosheet-graphene hybrid
nanostructures for five different laser wavelengths. I found that the gold
nanosheets hybridized with single layer graphene show dramatic enhancements in
D and G bands while displaying reduced intensities in 2D band peak of the Raman
spectrum. Similar to what was observed in silver nanoplate-graphene hybrid
nanostructure, D and G bands had enhancement factors of 3 and 4.5 respectively.
The I(2D)/I(G) ratio for hybrid materials was found to be below 1. But, the
interesting observations came from the study of hybridization between multilayer
graphene and gold nanosheets. I did not observe any significant variations in the
peak intensities of G and D band except for a small average enhancement factor of
~1.4 for G band. Moreover band splitting was observed only in Au nanosheet
hybridized with SLG and was not seen when gold was hybridized with MLG. This
spilt in G band is indication of transfer of electron density of graphene due to the
presence of other substrate. The calculations of Fermi energy shift also indicated
that p-doping effect was taking place Au nanosheet-SLG hybrid structure. Through
this observation I was able to come to a conclusion that the plasmonic
nanomaterials on hybridization with SLG has higher SERS and charge transfer
based enhancement effects than what is observed with multilayer graphene based
hybridization.
In this thesis I did not restrict my research to merely study the Raman characteristic
properties of hybridized silver and gold nanofilms with graphene. I extended this
study further to understand the changes that might occur to the nonlinear
properties of hybrid materials on excitation with femtosecond laser in the near-
infrared wavelength region. This is because the hybrid materials with nonlinear
properties are highly sought in photonic applications such as broadband modulator,
transistors, novel nonlinear materials etc. Hence I began my research by first
studying the Z-Scan based nonlinear absorption coefficients of multilayer graphene,
single crystalline gold nanosheets and polycrystalline thin gold films. I observed that
multilayer graphene displayed saturable absorption indicating its efficacy as heat
sink, while single and polycrystalline gold films displayed two-photon absorption.
The α value for multilayer graphene was found to be within the range specified in
literature. On the other hand the β value of polycrystalline thin gold film was
slightly greater than single crystalline AuNSs. I determined that the reason for this
116
discrepancy was due to the high field enhancement effect in polycrystalline gold
films because of its high surface roughness. For metals the β was changing with the
change in pulse repetition rate and I ascribed the decrease in β with the increase in
repetition rate to heat accumulation.
I also measured the nonlinear absorption coefficients ‘α’ of single crystalline gold
nanosheets and polycrystalline thin gold films hybridized with multilayer graphene
separately. This was a very interesting study as the metals and graphene displayed
two opposite nonlinear properties namely two photon absorption and saturable
absorption respectively. I found that the hybrid material also displayed greater
saturable absorption phenomenon than multilayer graphene alone. This indicates
that the hybrid material will possess better thermal dissipation characteristics
compared to individual multilayer graphene sample.
This thesis highlighted the unique and significant contribution that hybrid materials
can make to the field of nanophotonics, nanoplasmonics, nonlinear photonic
applications etc. Upon hybridization of chemically diverse 2D materials, the optical,
mechanical, physical and chemical properties can be altered in a controlled fashion
to suit engineering applications. Hybridization through stacking of chemically inert
materials which do not naturally develop a chemical bond with each other like
carbon, metals, and dielectrics has the potential to push frontiers of nanoscales
science to never imagined heights of human scientific experience.
Future Research
The research carried out in this thesis can be further extended toward more
comprehensive understanding of new hybrid materials made up of diverse periodic
table elements. This will help to develop applications in the field of energy storage
such as fuel cells, supercapacitors, Li-ion batteries and environmental technologies
such as photocatalysts, carbon dioxide capture and also for plasmonic
nanoantennas, memory storage and thin film applications.
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6.1 Plasmonic switch based on plasmonic metal-
graphene hybrid structure
In future single layer graphene and ultrathin noble metal films can be used to
develop hybrid plasmonic nanostructures to facilitate active functionalities in
plasmonic circuitry through active switching of surface plasmon polaritons (SPPs)
which can act as information carriers. This can unravel the plasmonic’s potential for
enhancement of real-world optoelectronic and photonic devices thereby catering to
the need of optical physics and photonic technologies. Plasmonics is a field where
active control for switching of plasmons is achieved either through light
compression or amplification in the nanostructures through the change in the
intrinsic properties of the dielectric medium by applying electric, thermal, optical,
and magnetic field externally. Graphene comes to the aid of plasmonics as it is
shown to possess excellent transport, electrical, thermal, mechanical, optical,
magnetic and higher order non-linear properties rivalling silicon electronics.
Moreover as graphene has atomic layer thickness with electrical control over its
properties, it can be used to build graphene based plasmonic switch.
The proposed plasmonic switch consists of thin noble metal film on top of glass
substrate as shown in Figure 6 - 1.
Figure 6 - 1: Block diagram of proposed plasmonic switch.
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Noble metal film of 120 nm thickness is deposited using sputter coating technique
on top of glass side coated with 2 nm thin Ti layer acting as adhesive layer for noble
metal film. Focused ion beam (FIB) lithography technique was used to fabricate
gratings so as to excite continuous wave plasmons on metal surface using p-
polarized laser of suitable frequency. Rhodamin 6G molecules can be used for
visualization of these surface waves by spin coating it over thin SiO2 spacer layer
separating rhodamin from metal surface. Single layer graphene film will be
developed using CVD technique as outlined in the section 2.13 of Chapter 2 and
then transferred to the center of proposed switch as shown in Figure 6 - 1. Metal
contacts are deposited either sides of graphene film to provide electric circuitry.
I have already made some preliminary attempts to build the proposed plasmonic
switch in nanofabrication laboratory using numerous fabrication devices and
techniques. Figure 6 - 2(a) and (b) shows SEM image of the Bragg grating
structures fabricated using FIB milling of gold film coated on glass substrate by
applying 24 keV of Ion beam voltage. Figure 6 - 2(a) shows a pair of in and out-
coupling gratings. The depth of in- and out-coupling gratings were chosen to be
120 nm, etching all the way down to the glass substrate to allow an effective in-
coupling of laser beam with the Bragg gratings and also to eliminate any device
variations in the out-coupled intensity due to uneven etching of the metal.
Figure 6 - 2: (a) SEM image of the FIB milled Bragg gratings on the gold
structure. (a) SEM image of one batch of FIB milled Bragg grating structure
congaing six sets of gratings of varying separations from 2-12 μm.
Each line of Bragg gratings in Figure 6 - 2(a) has a uniform width of 200 nm (V1)
and a length of 4 μm dimensions (H1), each separated by a distance of 550 nm (V2)
119
as shown in Black square boxes against blue line markers. The separation between
the in-coupling grating (on left) and out-coupling grating (on right) is approximately
2 μm (V3). Figure 6 - 2(b) shows six different set of inter-grating spacings with the
grating separation increasing uniformly at a rate of 2 μm until the first and last set
of grating have an effective separation of approximately 2 and 12 μm respectively as
shown in black square boxes with separations labelled as H1-H6. By measuring the
out-coupled intensity as a function of grating spacing, the decay length of SPP can
be determined. Single layer can now be transferred on top of these Bragg grating
structures to complete the plasmonic switch.
Next step toward achieving the proposed plasmonic switch is to thoroughly
understand and master the excitation, detection, imaging and measuring of vital
plasmonic parameter techniques. Hence in order to excite and visualize the SPPs, I
use a similar kind of setup built by Ditlbacher et al [306]. The Bragg gratings shown
as SEM image in Figure 6 - 2(a) appears as shown in Figure 6 - 3(b) when viewed
with CCD camera under the white light illumination using a 0.95 NA objective lens.
Figure 6 - 3: (a) SEM image of four sets of FIB milled Bragg grating structure
each congaing six sets of gratings of varying separations (b) Microscopy image of
the same FIB milled Bragg grating structure clearly visible as small bright spots as
120
highlighted. The highlighted grating with yellow circle is magnified and shown in
Figure-(c). (c) Graphical view for implementing proposed plasmonic switch for
the bottommost grating.
It can be clearly seen that under the white light illumination the structures are
clearly visible and easy to locate and perform experiments upon. The scattering
intensity, propagation loss over various path-lengths of SPPs propagating along the
Bragg gratings can now be measured by exciting Laser beam on in-coupling grating
and collected via the out-coupling grating. High intensity laser light can now be
incident onto the graphene film to excite third order non-linear Kerr effect in order
to change the refractive index of graphene film. Hence I propose that the intensity
of the SPPs travelling on the noble metal surface can be modulated by inducing
( ) related Kerr effect phenomenon which in turn induces change in refractive
index of graphene. Graphical view of the in- and out- coupling of plasmons
(indicated by white and yellow circles respectively as viewed under CCD camera
lens) along with control beam for switching the non-linear refractive index of
graphene ‘on’ and ‘off’ (indicated by striped red oval spot). The graphene properties
can also be controlled by applying voltage across the metal contacts as depicted in
the Figure 6 - 1.
6.2 Synthesizing and characterizing new hybrid
materials
The recently extracted single atomic thin layer gold [132] through chemical
synthesis can be used instead of 20 nm thin gold nanosheets or silver nanoplates to
hybridize single layer graphene. This is more fascinating as atomically thin gold
films will possess properties unseen in bulk metal as has already been demonstrated
in single layer graphene [68], where the charge carriers go ballistic unlike what is
observed in thick graphite material. It might be very challenging to stack single
atomic thin layer gold film on top of single layer graphene or vice versa. But, the
successful stacking of single atomic layer gold on top of graphene as shown in
Figure 6 - 4 would cause quantum confinement of two artificially stacked elements
that can cause dramatic shifts in Fermi level giving us with deep insight into
interesting quantum physical properties of such hybrid materials.
121
Figure 6 - 4: Artistic view of single layer gold on top of single layer graphene.
Moreover, the research carried out in our thesis can be further extended towards
more comprehensive understanding of new hybrid materials made up of diverse
periodic table elements with single atomic layer materials such as graphene, layered
metal oxides, layered double hydroxides, layered metal chalcogenides crystals (BN,
NbSe2, TaS2, MoS2) etc. These were produced by applying similar strategies like
mechanical exfoliation, liquid-phase exfoliation, CVD growth as was used to obtain
graphene. Hence various heterostructure such as a BN sandwiched between
graphene or graphene sandwiched between MoS2 can be prepared and studied to
reveal new interesting and unexplored properties of such materials. These tasks can
be undertaken with full assurance keeping in mind the excellent properties of
individual 2D materials which will be greatly enhanced when they are sandwiched
on top of one another. Moreover, I will have full control over the precise alignment
of each layer so that I can tailor their energy band gaps according to our needs.
Raman spectroscopy along with AFM, STEM, SEM,TEM etc can be used to
characterize such heterostructure with great details. Similarly the nonlinear
properties of these materials can be studied using Z-Scan technique and new areas
of application such as reinforcement fragile materials to absorb or dissipate more
heat can be achieved by coating, mixing and through adulteration of the target
sample with these new hybrid materials.
Apart from the above mentioned future works, 2D materials such as graphene can
be used to build a transistor, replacing the existing silicon based technology with
graphene as has already been demonstrated as proof of concept [63]. New
transistors are being built by hybridizing graphene with other atomic layer thin
122
materials like WS2 [307]. These atomic layer thin hybrid structures are being
researched ardently as a key to future electronic applications.
Furthermore, graphene acts as impermeable membrane to any molecule, but it can
be designed in such a fashion by carefully plucking carbon atoms from graphene,
that it only allows certain molecules to permeate and hence can be used as
biological membrane. Similarly the other 2D heterostructure materials can be used
in application such as biotechnology, life sciences and medicine.
123
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