Chemical growth routes to graphene and graphene applications 1


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CHEMICAL GROWTH ROUTES TO GRAPHENE AND
GRAPHENE APPLICATIONS








ANG KAILIAN PRISCILLA










NATIONAL UNIVERSITY OF SINGAPORE
2012
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
CHEMICAL GROWTH ROUTES TO GRAPHENE AND
GRAPHENE APPLICATIONS

ANG KAILIAN PRISCILLA
B.Sc (Hons)
National University of Singapore






A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
NUS GRADUATE SCHOOL FOR INTEGRATIVE
SCIENCES AND ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2012
I

Declaration

I hereby declare that this thesis is my original work and it has been written by
me in its entirety. I have duly acknowledged all the sources of information which
have been used in the thesis.
This thesis has also not been submitted for any degree in any university
previously.

_____________________
Ang Kailian Priscilla
17 September 2012
II
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Acknowledgements

This dissertation would not have been possible without the opportunity given
to me by the NUS Graduate School for Integrative Sciences and Engineering (NGS)
and the constant support and inspiration bestowed to me by the following people:
My principle supervisor, Professor Loh Kian Ping, has always been an earnest
motivator and educator ever since I joined his research group in 2006 during my
undergraduate studies in NUS. His philosophy in doing good science, critical thinking
skills and passion for research are inspiring. He never fails to provoke our thoughts to
think deeper and think outside the box. He exudes the true spirit of a scientist, a
fighter - constantly asking questions, constantly seeking answers, constantly
displaying a never-say-die attitude. His accolades in the research and education sector
are widely recognised and it is truly a privilege to be under his supervision.
My co-supervisor, Associate Professor John Thong, never fails to provide
useful solutions and insights to problems pertaining to semiconductor device
fabrication and characterisation. He always avails himself to students on a weekly
basis to provide timely advice on our work and to brainstorm for plausible solutions.
My collaborator, Associate Professor Thorsten Wohland, has always been
patient in explaining the principles behind state-of-the-art fluorescence techniques
with great enthusiasm. His enthusiasm for research rubs off on me when he co-
supervised me for my undergraduate final year project in 2007/2008. Collaborating
with him again for my Ph.D research work has been an enjoyable journey.
My collaborator, Professor Lim Chwee Teck, is an epitome of an admirable
and amicable educator, one who listens to students and provides useful advice, not

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just in their research endeavours but also in their personal lives – a holistic approach
to the development of an individual. His passion in the biomedical research and
translation of its fruits to useful devices for clinical applications inspires me to strive
towards the goals of real life application for my research
My heartfelt thanks extend to the colleagues in Graphene Research Centre,
Centre for Integrated Circuit Failure Analysis & Reliability (CICFAR), Biophysical
Fluorescence Laboratory, Infectious Diseases unit and BioSystems and
Micromechanics unit under the Singapore-MIT Alliance for Research & Technology
(SMART). I would like to give special thanks to Miss Goh Bee Min, Dr. Lu Jiong,
Miss Lena Tang, Miss Candy Lim, Dr. Wang Shuai, Dr. Bao Qiaoliang, Dr. Wang Yu,
Dr. Zhang Huijuan, Dr. Hao Yu Feng, Dr. Wang Ziqian, Dr. Wang Rui, Miss Liu Dan,
Miss Meng Lei, Mrs Ho Chiow Mooi, Mr. Koo Chee Keong, Mr. Jagadish Sankaran,
Dr. Li Ang, Dr. Hou Han Wei and Miss Xu Xiaofeng.
I am eternally grateful for the unending love of my husband, Brandon Koh.
Despite having a workaholic wife like me, he never fails to surprise me with little acts
of love and acts of service. I have the best parents ever. Without their steadfast love, I
would never become who I am today. I have a great sister, who always gives me
refreshment courses on integration and partial differentiation, matrices and statistics. I
am thankful for my parent-in-laws for their constant support. I can never do without
my best friends. This is not an oxymoron but I have two extremely wonderful and
God-sent angels, Annie Chan and Chia Poju. Their hugs, advice and words of wisdom
are carved in my heart for keep. Finally, I am blessed to have two wonderful mini
schnauzers, one cute poodle and four lovely Siberian hamsters which provide some
form of welcome distraction and relief from my work.
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List of Publications

1. Ang, P. K.; Li, A.; Jaiswal, M.; Wang, Y.; Hou, H. W.; Thong, J. T.; Lim, C.
T.; Loh, K. P., Flow sensing of single cell by graphene transistor in a
microfluidic channel. Nano Lett 2011, 11, 5240-5246.
2. Ang, P. K.; Jaiswal, M.; Lim, C.; Wang, Y.; Sankaran, J.; Li, A.; Lim, C. T.;
Wohland, T.; Barbaros, O.; Loh, K. P., A Bioelectronic Platform Using a
Graphene-Lipid Bilayer Interface. Acs Nano 2010, 4, 7387-7394.
3. Wang, S.; Ang, P. K.; Wang, Z. Q.; Tang, A. L. L.; Thong, J. T. L.; Loh, K. P.,
High Mobility, Printable, and Solution-Processed Graphene Electronics. Nano
Letters 2010, 10, 92-98.
4. Ang, P. K.; Wang, S.; Bao, Q. L.; Thong, J. T. L.; Loh, K. P., High-
Throughput Synthesis of Graphene by Intercalation - Exfoliation of Graphite
Oxide and Study of Ionic Screening in Graphene Transistor. Acs Nano 2009, 3,
3587-3594.
5. Ang, P. K.; Loh, K. P.; Wohland, T.; Nesladek, M.; Van Hove, E., Supported
Lipid Bilayer on Nanocrystalline Diamond: Dual Optical and Field-Effect
Sensor for Membrane Disruption. Advanced Functional Materials 2009, 19,
109-116.
6. Ang, P. K.; Chen, W.; Wee, A. T. S.; Loh, K. P., Solution-Gated Epitaxial
Graphene as pH Sensor. Journal of the American Chemical Society 2008, 130,
14392-14393.
7. Wang, Y.; Lee, W. C.; Manga, K. K.; Ang, P. K.; Liu,Y. P.; Lim, C. T.; Loh,
K. P., Fluorinated Graphene for Promoting Neuro-Induction of Stem Cells.
Advanced Materials 2012, 24, 4285-4290.
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8. Cole, D. J.; Ang, P. K.; Loh, K. P., Ion Adsorption at the
Graphene/Electrolyte Interface. The Journal of Physical Chemistry Letters
2011, 1799-1803.
9. Polavarapu, L.; Manga, K. K.; Yu, K.; Ang, P. K.; Cao, H. D.; Balapanuru, J.;
Loh, K. P.; Xu, Q. H., Alkylamine capped metal nanoparticle "inks" for

printable SERS substrates, electronics and broadband photodetectors.
Nanoscale 2011, 3, 2268-2274.
10. Hu, M S.; Kuo, C C.; Wu, C T.; Chen, C W.; Ang, P. K.; Loh, K. P.; Chen,
K H.; Chen, L C., The production of SiC nanowalls sheathed with a few
layers of strained graphene and their use in heterogeneous catalysis and
sensing applications. Carbon 2011, 49, 4911-4919.
11. Zhao, M.; Wang, S.; Bao, Q. L.; Wang, Y.; Ang, P. K.; Loh, K. P., A simple,
high yield method for the synthesis of organic wires from aromatic molecules
using nitric acid as the solvent. Chemical Communications 2011, 47, 4153-
4155.
12. Lim, C. X.; Hoh, H. Y.; Ang, P. K.; Loh, K. P., Direct Voltammetric
Detection of DNA and pH Sensing on Epitaxial Graphene: An Insight into the
Role of Oxygenated Defects. Analytical Chemistry 2010, 82, 7387-7393.
13. Loh, K. P.; Bao, Q. L.; Ang, P. K.; Yang, J. X., The chemistry of graphene.
Journal of Materials Chemistry 2010, 20, 2277-2289.
14. Pachoud, A.; Jaiswal, M.; Ang, P. K.; Loh, K. P.; Ozyilmaz, B., Graphene
transport at high carrier densities using a polymer electrolyte gate. Epl 2010,
92.
15. Midya, A.; Mamidala, V.; Yang, J. X.; Ang, P. K. L.; Chen, Z. K.; Ji, W.; Loh,
K. P., Synthesis and Superior Optical-Limiting Properties of Fluorene-
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Thiophene-Benzothiadazole Polymer-Functionalised Graphene Sheets. Small
2010, 6, 2292-2300.
16. Chong, K. F.; Loh, K. P.; Ang, K.; Ting, Y. P., Whole cell environmental
biosensor on diamond. Analyst 2008, 133, 739-743.


VII
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Table of Contents

Declaration I
Acknowledgements II
List of Publications IV
Table of Contents VII
Summary XII
List of Tables XIV
List of Figures XV
List of Abbreviations and Symbols XXV

PART I BASIC ASPECTS

Chapter 1: Introduction
1.1. An introduction to carbon materials 1
1.2. Background on graphene 1
1.2.1. Unique structure and properties of graphene 2
1.2.2. Synthesis routes to graphene 5
1.3. Graphene as a transducer in field-effect transistor (FET) sensor 11
1.4. Overview of objectives and work scope 13
1.5. References 15

Chapter 2: Literature Review
2.1. Principles of FET sensors 23
2.1.1. Basic operation of FET sensors 23
2.1.2. Working principles of back-gated and electrolytically top-gated
FET sensors 26
2.1.3. Comparisons between back-gated and electrolytically top-gated
FET sensors 29
2.2. Bioelectronic applications of graphene FET (GFET) 31

2.2.1. Deoxyribonucleic acid (DNA)-based GFET 32
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2.2.2. Protein-based GFET 36
2.2.3. Cell-based GFET 37
2.3. Summary and outlook 39
2.3.1. Key challenges for the development of GFET sensors 39
2.3.2. Outlook 41
2.4. References 42

Chapter 3: Experimental Techniques
3.1. Introduction 48
3.2. Spectroscopy 48
3.2.1. Raman spectroscopy 49
3.2.2. Ultraviolet-visible spectroscopy 51
3.2.3. X-ray photoelectron spectroscopy (XPS) 54
3.2.4. Fluorescence correlation spectroscopy (FCS) 56
3.3. Microscopy 58
3.3.1. Atomic force microscopy (AFM) 58
3.3.2. Scanning electron microscopy (SEM) 60
3.3.3. Total internal reflection fluorescence microscopy (TIRFM) 61
3.3.4. Differential interference contrast (DIC) microscopy 63
3.4. Lithography 65
3.4.1. Optical lithography 66
3.4.2. Electron beam lithography (EBL) 68
3.5. Microfluidic flow cytometry 70
3.6. References 73

PART II CHEMICAL ROUTES TO EXFOLIATED GRAPHENE SHEETS

Chapter 4: High-Throughput Synthesis of Graphene by Intercalation-Exfoliation
of Graphite Oxide and Study of Ionic Screening in Graphene Transistor
4.1. Introduction 76
4.2. Materials and methods 78
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4.2.1. Oxidation of graphite and intercalation by tetrabutylammonium
ions………………………………………………………………… 78
4.2.2. Fabrication and electrical measurements of GFET 79
4.2.3. Raman and optical contrast spectroscopy 80
4.3. Results and discussion 81
4.3.1. Reaction monitoring of intercalation-exfoliation of graphite oxide 81
4.3.2. Characterisation of graphene thin film FET……………………… 87
4.3.3. Ionic screening effect on graphene transistor performance 90
4.4. Conclusion 96
4.5. References 97

Chapter 5: High Mobility, Printable and Solution-Processed Graphene
Electronics
5.1. Introduction 102
5.2. Materials and methods 105
5.2.1. Synthesis of big sized graphene oxide (BSGO) sheets 105
5.2.2. Fabrication and electrical measurements of GFET 105
5.2.3. Fabrication of all-carbon FET 106
5.2.4. Raman and optical contrast spectroscopy 107
5.3. Results and discussion 107
5.3.1. Characterisation of big sized GO (BSGO) sheets 107
5.3.2. Characterisation of graphene thin film FET 110
5.3.3. Fabrication and characterisation of all-carbon FET 114
5.3.4. Factors influencing carrier mobility 115

5.3.4.1. Graphene/metal and graphene/graphene interface 115
5.3.4.2. Ionic screening effect on all-carbon transistor performance 117
5.4. Conclusion 120
5.5. References 121



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PART III GRAPHENE BIOHYBRID DEVICES

Chapter 6: A Bioelectronic Platform Using Graphene-Lipid Bilayer Interface
6.1. Introduction 124
6.2. Materials and methods 125
6.2.1. Fabrication of CVD graphene film 125
6.2.2. Fabrication and measurement of CVD GFET 126
6.2.3. Formation of biomimetic membranes on graphene 127
6.2.4. Imaging total internal reflection-fluorescence correlation
spectroscopy (ITIR- FCS) instrumentation 128
6.2.5. Capacitance-voltage (C-V) measurements 129
6.2.6. AFM and epifluorescence measurements 129
6.3. Results and discussion 130
6.3.1. Characterisation of GFET 130
6.3.2. Quality of biomimetic membranes on graphene 132
6.3.3. Influence of charged lipids on graphene 134
6.3.3.1. Graphene Dirac point and charge carrier density 134
6.3.3.2. Graphene charge carrier mobility 138
6.3.3.3. Effect of interfacial hydrated layer on Coulomb
potential of impurity charges 139
6.3.4. Electrical detection of membrane disruption 141

6.3.5. Biosensing mechanism for membrane disruption by GFET 145
6.4. Conclusion 146
6.5. References 147

Chapter 7: Flow Sensing of Single Cell by Graphene Transistor in a Microfluidic
Channel
7.1. Introduction 154
7.2. Materials and methods 157
7.2.1. Fabrication of GFET array and microfluidic flow cytometric
assay 157
7.2.2. Electrical measurements of GFET 158
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7.2.3. AFM and epifluorescence measurements 158
7.2.4. Plasmodium falciparum parasite culture preparation 159
7.2.5. Optimisation of microfluidic flow cytometric assay 159
7.2.6. DIC microscopy and video analysis 160
7.2.7. AFM tip functionalisation and force curves analysis 160
7.3. Results and discussion 161
7.3.1. Biosensing mechanism for single malaria infected cell by GFET 161
7.3.2. Characterisation of CD36-functionalised graphene 163
7.3.3. Simultaneous optical and electrical detection of malaria infected
cells by GFET 166
7.3.3.1. Characterisation of bare GFET and CD36-functionalised
GFET 166
7.3.3.2. Static sensing of malaria infected cells 168
7.3.3.3. Flow-catch-release sensing of single malaria infected cell 170
7.3.3.4. Adhesive dynamics of malaria infected cells 172
7.3.4. Effect of charge impurity density on cell-graphene interface 174
7.4. Conclusion 178

7.5. References 178

Chapter 8: Conclusions
8.1. Uniqueness of graphene 182
8.2. Summary and outlook 182
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Summary

Graphene presents boundless potential in transforming the way conventional
electronics are made. Prior to its full-fledged implementation in industries such as
semiconductor and biomedical, several bottlenecks have to be overcome: (1) the
controlled synthesis of graphene to achieve high yield production of high quality
graphene sheets and (2) the fundamental understanding of the interface between
biological entities and graphene. The work involved in this dissertation therefore
relates to these two points and is broadly divided into two sections, namely the
chemical routes to exfoliated graphene sheets and the fabrication of graphene
biohybrid devices for protein and cellular sensing.
Several key factors that affect the transistor performance of chemically
processed graphene, such as the destruction of π-conjugation network by oxygen
functional groups, sheet-to-sheet junctions in reduced graphene oxide thin films,
impurity scattering effect and electron-hole asymmetry were identified. Therefore, we
designed a rational route to produce big-sized chemically processed graphene sheets
through intercalation-exfoliation of graphite derivatives and modification of the
oxidation conditions of graphite. By careful gradient separation, we isolated a
homogenous suspension of monolayer chemically processed graphene sheets with
minimal oxygen groups. With thermal reduction, ionic screening and replacement of
metal drain/source contacts with multilayer graphene sheets as interconnects, we
achieved high film conductivity and carrier mobility.
To understand the electronic interaction between living cells and graphene,

biomimetic membranes or artificial lipid bilayers were deposited atop the graphene
transistor channel. Since cell membranes are frequently charged, their presence on
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graphene exerts spatially inhomogeneous Coulomb potential, leading to changes in
the minimum conductivity, carrier concentration and carrier mobility of graphene.
Therefore, by monitoring changes in transistor characteristics, the lipid bilayer-
graphene bioelectronic platform can be employed to detect membrane disrupting
agents. The biosensing mechanism involves a complex interplay of biomolecular
doping by charged lipids and ionic screening by mobile ions in solution, which allows
the elucidation of the mechanism behind bacterial membrane disruption by
antimicrobial peptides, Magainin 2. Besides detecting lipid charges, we demonstrated
the charge-based detection of receptor-ligand binding for disease detection and
diagnosis. Malaria infected red blood cells are morphologically different from healthy
red blood cells; the former possessing positively charged membrane knobs for the
cytoadherence to epithelial cells. Therefore, by functionalising graphene with
biorecognition proteins such as CD36 receptors, we successfully constructed a
graphene biohybrid device capable of detecting malaria infected red blood cells with
single-cell resolution. By integrating graphene transistor array with microfluidic flow
cytometric assay, a high throughput analysis of the percentage of infected cells in a
diseased blood stream and statistical differentiation of the stage of infection by
characteristics conductance changes in graphene were made possible.
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List of Tables


Table 1.1. Properties of graphene, CNT and hydrogen-terminated diamond which are
relevant to biosensing.

Table 6.1. The field effect of lipid charges on graphene surface potential, charge
carrier density and charge carrier mobility (where hole and electron mobility are
nearly identical). ∆V
g,min
corresponds to Dirac point shift with respective to V
g
= 0 V.
∆n
g,min
represents the change in charged carrier density for the corresponding change
in Dirac point position. n
0
is the residual carrier density obtained from the width of
voltage plateau. µ
FE
is the field-effect mobility.


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List of Figures


Figure 1.1. (Left) Honeycomb lattice of single-layer graphene showing C atoms in A
and B sublattices. (Right) Band dispersion of graphene showing the π bands touching
one another at the K point, and the Dirac cone approximation to E(k) relation for
small k around Dirac point (K). Reproduced with permission from ref.3. Copyright
(2010) IEEE.

Figure 1.2. (a) Structural model and (b) 3D view of graphene showing carboxylic
groups at the edge, phenol hydroxyl and epoxide groups mainly at the basal plane.
Reprinted (adapted) with permission from ref. 42. Copyright (2009) American
Chemical Society.


Figure 1.3. Chemical exfoliation of graphite intercalation compound. (a) Fluorinated
graphite intercalation compound (FGIC). (b) Easily soluble expanded graphite
(ESEG). (c) Scheme of process for the formation of ESEG dispersed in solution,
showing pure natural graphite (top), the FGIC (middle), and ESEG with surfactant
molecules surrounding it (bottom). Reproduced with permission from ref.47.
Copyright (2009) John Wiley and Sons.


Figure 1.4. Methodology for transfer of graphene film. (“Gr” = graphene) The top-
right and bottom-left insets are the optical micrographs of graphene transferred on
SiO
2
/Si wafers (285 nm thick SiO
2
layer) with “bad” and “good” transfer, respectively.
The bottom-right is a photograph of a 4.5 × 4.5 cm
2
graphene on quartz substrate.
Reprinted (adapted) with permission from ref. 42. Copyright (2009) American
Chemical Society.


Figure 2.1. Cross-section of an n-channel Si MOSFET. (a) When the voltage applied

between the source and gate electrodes exceeds a threshold voltage, V
th
, a conducting
channel is formed and a drain current, I
ds
, flows. The length of the channel is defined
by the length of the gate electrode; the thickness of the gate-controlled channel region
is the depth to which the electronic properties of the semiconductor (p-doped Si in this
case) are influenced by the gate. (b) FET transfer characteristics showing I
ds
(on a
logarithmic scale on the left and a linear scale on the right) versus the gate–source
voltage, V
g
. The transistor is considered to be switched on when V
g
is equal to the
maximum voltage supplied to the device, V
dd
. The higher the slope in the subthreshold
region (V
g
< V
th
), the better the transistor switch-on characteristics become. Above
threshold, the change in I
ds
for a given change in V
g
is called the terminal

transconductance, g
mt
. Reproduced with permission from ref.1. Copyright (2010)
Nature Publishing Group.


Figure 2.2. Conical-shaped band structure and distribution of charge carriers in the
channel at different operation regions of GFETs. Assuming that V
ds
> 0, in region I,
the device has n-type carriers everywhere in the channel. In region II, the device has
XVI

n-type carriers at the source side of the channel and p-type carriers at the drain side of
the channel. This is the ambipolar region. Point X is the recombination point. In
region III, the device has p-type carriers everywhere in the channel. Reproduced with
permission from ref.3. Copyright (2011) IEEE.


Figure 2.3. (a) Schematic illustration of electrolyte-gated GFETs. Reprinted (adapted)
with permission from ref. 4. Copyright (2009) American Chemical Society. (b)
Schematic drawing representing the modulation of the carrier density in the graphene
film: the applied gate voltage (with respect to the reference electrode) shifts the Fermi
level in graphene above (shown) or below the Dirac point, thus modulating the
number of free carriers. Reproduced with permission from ref.2. Copyright (2010)
John Wiley and Sons.


Figure 2.4. Structure of double stranded DNA.

Figure 2.5. DNA sequencing by graphene. (a) The individual bases of a ssDNA
molecule (backbone in green, bases in alternating colours) sequentially occupy a gap
in graphene (hexagonal lattice) while translocating through it. Their conductance is
read, revealing the sequence of the molecule. The contacting electrodes to the
graphene nanogap (Au, yellow) are on the far left and right sides of this image. (b)
Schematic representation of the transverse conductance technique. Reprinted (adapted)
with permission from ref. 16. Copyright (2010) American Chemical Society.


Figure 2.6. (a) Time course of I
ds
for an aptamer-modified GFET. At 10 min intervals,
various concentrations of IgE were injected. (b) Change in drain current vs IgE
concentration. The red dashed curve shows a fit to the Langmuir adsorption isotherm
with dissociation constant of 47 nM. Reprinted (adapted) with permission from ref. 21.
Copyright (2010) American Chemical Society.


Figure 2.7. (a) Combination of an optical microscopy image of a transistor array and
a fluorescence image of the calcein-stained cell layer on the same array. The scale bar
is 100 μm. Simultaneous current recordings of eight transistors in one FET array over
tens of seconds (b) and hundreds of milliseconds (c). (d) Exemplary single spikes.
The current response has been converted to an extracellular voltage signal. The upper
spike resembles a capacitive coupling followed by the opening of voltage-gated
sodium channels whereas in the bottom one the ion channels dominate over the
capacitive coupling. Reproduced with permission from ref.25. Copyright (2011) John
Wiley and Sons.

Figure 3.1. The electromagnetic spectrum.


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
Figure 3.2. (a) Energy levels involved in Raman spectroscopy. (b) Raman spectrum
showing Rayleigh, Raman Stokes and anti-Stokes. Stokes line is in general more
intense that anti-Stokes line.
2



Figure 3.3. Electronic transitions involving π, σ and n electrons.


Figure 3.4. (a) Diagram of energy versus interatomic distance, showing electronic
transitions between different vibrational states. (b) Well-resolved vibrational fine
structure observed for permanganate ion.
2



Figure 3.5. X-ray excitation of a 1s core electron.


Figure 3.6. Principles of fluorescence correlation spectroscopy. A tightly focused
laser (blue beam) excites the fluorescence of molecules diffusing across the
observation volume. The fluorescence light (green beam) is collected by a high-
numerical aperture objective, spectrally filtered by a bandpass filter and detected onto
a single-photon counting detector. The fluctuating fluorescence intensity is auto-

correlated in order to determine the number and the mean residence time of molecules
in the observation volume. Reproduced with permission from ref.6. Copyright (2008)
Springer.


Figure 3.7. Experimental configuration used to detect forces acting on the AFM tip.


Figure 3.8. Schematic representation of the instrumentation of scanning electron
microscopy.


Figure 3.9. (a) Schematic representation of the concept of total internal reflection
fluorescence microscopy depicting the selective excitation of fluorescent molecules in
a cell membrane of a living cell resting on a glass slide. (b) Exponential intensity
decay of the evanescence field at the glass-buffer interface. Copyright (2012)
Olympus America Inc.


Figure 3.10. Schematic illustration of the microscope configuration for differential
interference contrast. Light is polarised in a single vibration plane by the polariser
before entering the lower Wollaston prism that acts as a beam splitter. Next, light
passes through the condenser and sample before the image is reconstructed by the
objective. Above the objective, a second Wollaston prism acts as a beam-combiner
and passes the light into the analyser where it interferes constructively and
destructively


Figure 3.11. Standard operating procedure for optical lithography.
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
Figure 3.12. Schematic representation of electron beam lithography using a positive
resist.


Figure 3.13. Monte Carlo simulation of electron trajectories of 100 electrons at (a) 10
kV and (b) 20 kV in PMMA film. Reproduced with permission from ref.9. Copyright
(1975) American Institute of Physics


Figure 3.14. A microfluidc flow cytometer based on impedometric detection.
Reprinted (adapted) with permission from ref.13. Copyright (2005) American
Chemical Society.


Figure 4.1. Schematic representation depicting intercalation of tetrabutylammonium
ions in large graphite oxide sediments and unreacted graphite particles to obtain
monolayer mildly oxidised graphene sheets dispersed in DMF. These exfoliated
graphene sheets were deposited onto SiO
2
/Si substrate to form graphene thin film FET.
Effect of ionic screening and chemical doping effect of NaF and KCl were
investigated with electrical transport and in situ Raman measurements.


Figure 4.2. Reaction monitoring of TBA intercalation in large graphite oxide particles.
(a) Colour change of reaction mixture in DMF monitored over 2 days. Suspension
was centrifuged at 10 000 rpm for 10 minutes to remove unreacted particles. (b)
Precipitation of relatively hydrophobic mildly oxidised graphene sheets in deionised
water after reaction for 1 day (left) and 2 days (centre) and re-dispersion in DMF

(right). (c) UV-visible absorption spectra of GO dispersions as reaction proceeded for
over 2 days. (d) C 1s XPS spectra of GO dispersion with reaction time show gradual
increase in the C-C bonding component from 55% to 81%.


Figure 4.3. Comparison of XPS spectra revealed a greater percentage of C-O groups
(~20%) for highly oxidised graphene sheets in the supernatant as compared to those
mildly oxidized ones in the precipitate. Inset shows the deconvolution of precipitate
with 66% C–C group.


Figure 4.4. Optical micrographs and tapping mode AFM characterisation of mildly
oxidised graphene sheets. (a) Optical image of large-sized mildly oxidised graphene
sheets. (b) Size distribution of monolayer mildly oxidized graphene sheets (total
counts = 1435) with mean sheet area of 330 ± 10 µm
2
. Inset shows a good spread of
monolayer graphene sheets with some overlapping regions; scale bar is 50 µm. The
total number of sheets counted was ~1600, of which 1435 sheets were monolayer. (c)
Tapping mode AFM image of a mildly oxidised graphene sheet. (d) Topographical
height for mildly oxidised graphene sheet was ~0.93 nm, which was larger than RGO
sheet due to the presence of protruding oxygen functionalities.

Figure 4.5. (a) Transport characteristics of mildly oxidised graphene sheet prior to
thermal annealing. V
ds
was kept constant at 1 V. Hole and electron mobility extracted
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
was 0.0005 cm

2
/(V s) and 0.0001 cm
2
/(V s), respectively with sheet conductivity of 2
± 1 S/m. Although carrier mobilities were negligibly small, the sheet conductivity
obtained was 1 order of magnitude higher than conventional GO synthesised by the
modified Hummer’s oxidation method (0.5 S/m).
3
(b) Effective removal of TBA upon
thermal annealing at 1000
o
C (red line). This presents an effective intercalation-
exfoliation procedure to obtain monolayer mildly oxidised graphene sheets with
appreciable conductivity as compared to Hummer’s GO method. This method is also
surfactant-free. (c) Transport characteristics of RGO sheet annealed at 1000
o
C.
Drastic improvements in hole mobility (7.23 cm
2
/(V s)), electron mobility (1.45
cm
2
/(V s)) and sheet conductivity (3210 S/m) were obtained. Inset shows the four-
point probe configuration defined by electron beam lithography. Channel length and
width was 5.01 µm and 5.78 µm, respectively.


Figure 4.6. Characterisation of graphene film thickness and morphology. (a) Tapping
mode AFM characterisation of graphene film with film thickness of ~3 nm prior to
annealing. (b) Contrast spectra for 1 to 4 layers of graphene sheets. Inset shows

calibration curve for contrast as a function of the number of graphene layers (c)
Integrated intensity of Raman G peak increased as the number of graphene layers
increased. (d) Optical images of graphene film across the active channel.


Figure 4.7. (a) Transport characteristics of graphene thin film (approximately 1 – 4
layers) with sheet conductivity of 15 000 S/m and hole and electron mobility of 59
cm
2
/(V s) and 17 cm
2
/(V s), respectively. Inset shows two-point probe configuration
of graphene thin film FET with PMMA insulation of contacts and active channel area
exposed by electron beam lithography. Channel length and width was 29.3 µm and
37.8 µm, respectively. (b) Transport characteristics of graphene film with 1 to 4 layers
of graphene sheets. Hole and electron mobility obtained were 10.1 cm
2
/(V s) and 4.9
cm
2
/(V s), respectively. As the channel length increased from 29.3 µm to 100 µm,
carrier mobilities decreased by 4 to 5 times. This can be attributed to an increasing
effect of sheet-to-sheet junctions across the graphene active channel as channel length
increases.


Figure 4.8. (a) Transfer characteristics of graphene thin film FET device in different
NaF concentration. V
ds
was kept constant at 10 mV. Inset shows mobility (indicated

by the slope of σ-V
g
curve) increased with NaF concentration, which was indicative of
effective ionic screening of charged impurities on SiO
2
substrate. (b) Hole carrier
concentration decreased from 1.5 × 10
12
cm
-2
in dry condition to 2.8 × 10
11
cm
-2
in 1
M NaF.


Figure 4.9. (a) Raman spectra of G peak response fitted with Lorentzian component
of the Voigt profile in different NaF concentrations. (b) G peak down-shifted by ~8
cm
-1
and line-width increased by ~12 cm
-1
. (c) I
2D
/I
G
as a function of NaF
concentration. (d) Integrated 2D peak intensity increased with NaF concentration.

Note that besides 2D peak, there was also a combination of D and G peaks resulting
in a S3 peak at ~2900 cm
-1
which was commonly observed for chemically processed
graphene.
5
Inset shows the decrease in G peak asymmetry in 1M NaF.
XX

Figure 4.10. (a) Transfer characteristics of graphene film in different KCl
concentration. V
ds
is kept constant at 10 mV. Inset shows carrier mobility increased
initially and remained constant as KCl concentration increased beyond 10 mM. (b)
Raman spectra show G peak response fitted with Lorentzian component of the Voigt
profile in different KCl concentrations. (c) G peak up-shifts by ~12 cm
-1
and line-
width decreases by ~10 cm
-1
. (d) Initial slight decrease in hole carrier concentration
was observed when the graphene thin film FET was exposed to 10 mM KCl, but it
gradually increased to 2.3 × 10
12
cm
-2
in 1 M KCl.


Figure 5.1. (a) AFM image of BSGO sheets. (b) The narrow size distribution of

BSGO sheets with Gaussian mean at 250 ± 10 µm
2
. (c) AFM image of BSGO films
showing good film morphology. Inset shows that GO solution remained stable after 5
months with a concentration as high as 50 mg/mL. (d) Deconvolution of the C 1s XPS
spectra and raman spectra for BSGO before and after thermal annealing at 1000
o
C.


Figure 5.2. (a) Optical micrograph of 4-point probe measurement on one thermally
reduced BSGO sheet with channel length of 3.40

m and width of 2.99

m. (b) This
reduced BSGO sheet showed linear I-V characteristics and returned a sheet
conductivity of 760 S/cm. Inset shows the transfer characteristics. Hole and electron
mobility was 5.2 cm
2
/(V s) and 1.3 cm
2
/(V s), respectively.


Figure 5.3. Optical contrast spectra of different number of graphene layers. (a)
Contrast spectra of 1 to 5 layers. Inset shows the calibration curve of contrast as a
function of the number of layers. The contrast values obtained were 0.076 ± 0.005 (1
layer), 0.153 ± 0.010 (2 layers), 0.230 ± 0.013 (3 layers), 0.311 ± 0.012 (4 layers),
0.405 ± 0.015 (5 layers) which increased approximately linearly and showed

saturation after 10 layers. (b) Contrast spectra for graphene active channels with an
average of ~1 (left), ~2 (centre) and ~ 3 layers (right). (c) Optical micrographs of
representative 1, 2, 3, 4 and 5 layer(s) of reduced BSGO deposited onto 285 nm SiO
2
.


Figure 5.4. AFM section analyses of graphene film thickness and their corresponding
surface area coverage. (a) Monolayer graphene film (b) Bilayer graphene film (c)
Trilayer graphene film.


Figure 5.5. Single sheet graphene and graphene thin film FET. (a) Optical image of
about monolayer reduced BSGO thin film with overlapping edges. (b) The transfer
characteristics of monolayer reduced BSGO thin film FET with channel width of
31.8 µm, channel length of 26.92 µm, and hole and electron mobility of 5.4 cm
2
/(V
s) and 1.1 cm
2
/(V s), respectively at a fixed V
ds
of 1V. Inset shows the two-point
probe configuration of the monolayer reduced BSGO thin film FET. Scale bar
corresponds to 20 µm. (c) Optical image of bilayer reduced BSGO thin film. (d) The
transfer characteristics of bilayer reduced BSGO thin film FET with channel width of
32.55 µm, channel length of 30.00 µm and hole and electron mobility of 50 cm
2
/(V
s) and 10 cm

2
/(V s), respectively at a fixed V
ds
of 10mV. Inset shows the schematic
representation of the reduced BSGO thin film FET. (e) Optical image of about trilayer
XXI

reduced BSGO thin film. (f) The transfer characteristics of trilayer reduced BSGO
thin film FET with channel width of 29.87 µm, channel length of 32.11 µm and
hole and electron mobility of 92 cm
2
/(V s) and 51 cm
2
/(V s), respectively at constant
V
ds
of 10mV. Inset shows the schematic representation of the reduced BSGO thin film
FET. The thickness of gate dielectric SiO
2
for all FETs is 285 nm.


Figure 5.6. Schematic representation of the microfabrication of all-carbon FET. (a)-
(d), Layer-by-layer approach where both BSGO active channel and electrodes were
deposited onto SiO
2
/Si substrate by drop-casting or printing. (e)-(h), Combination of
photolithography and conventional device fabrication techniques employed in the
semiconductor industry for microfabrication of multilayer BSGO electrodes with a
narrow channel width of 2 µm. (i) Schematic representation of the multilayer BSGO

electrodes made by photolithography. Optical micrograph of a pair of multilayer
BSGO electrodes separated by a few-layer BSGO film with channel width of 2 µm. (j)
Schematic illustration of the device made from (a) to (d). (k) Optical micrograph of
the all-carbon FET made of few-layer graphene active channel, and multilayer
graphene source and drain.


Figure 5.7. (a) Transfer characteristics of all-carbon FET with trilayer graphene
layers, channel width of

2 mm, length of

1 mm, multilayer graphene source-drain
electrodes and Si backgate at a fixed V
ds
of 10 mV. (b) Mobility enhancement of all-
carbon FET in different NaF concentrations. (c) Carrier mobility and I
on
/I
off
ratio
increased by ~10 times as concentration of NaF electrolyte increased.


Figure 5.8 Carrier mobility of different RGO thin film FETs. I, The mobility of
previously reported small-sized GO thin film FET was approximately 0.1 cm
2
/(V s),
6

measured in N
2
. II-V, The carrier mobility of reduced BSGO FETs, which were
measured in air and at room temperature. II, monolayer reduced BSGO thin film
FETs. III, Trilayer reduced BSGO thin film FETs with gold electrodes. IV, Printable
trilayer reduced BSGO thin film FETs with multilayer graphene electrodes. V,
Printable trilayer reduced BSGO thin film FETs with multilayer graphene electrodes
exposed to 1M NaF.


Figure 6.1. (a) Schematic representation of biomimetic membrane-graphene FET. (b)
Raman spectrum of single layer CVD graphene. Inset shows the optical image of
CVD graphene film. (c) Water-gated ambipolar FET response of device at V
ds
= 100
mV. (d) Resistance, R, as a function of carrier density, n, where n is obtained by
integrating over the C-V curve (inset). The red fitted curve allows the extraction of
field-effect mobility and contact resistance.


Figure 6.2. Autocorrelation function curves (ACFs) captured by ITIR-FCS for Rho-
PE labeled POPC bilayers. (a) ACFs of membranes on glass. (b) ACFs of membranes
on graphene. The figures showed all 441 correlation curves captured in a 21 × 21
pixel region of interest on the EMCCD camera. The labeling ratio of Rho-PE/POPC
was 0.01% for glass and 0.02% for graphene. The sample was excited with 2 mW at a
XXII

wavelength of 514 nm. The recording time was 5.6 s for 10 000 frames. The diffusion
coefficient images (with the corresponding colour scale) which confer spatial
uniformity are shown for (c) glass and (d) graphene. The average fluorescent intensity

image of membranes prepared on glass and graphene are presented in (e) and (f)
respectively.


Figure 6.3. Electrical characterisation of graphene coated with charged and neutral
lipid membranes. (a) σ-V
g
measurements of graphene coated with negatively charged
(POPC/POPG = 2/1), positively charged (DOPC/DOTAP = 2/1) and neutral POPC
membranes. (b) C-V measurements of different charged and neutral membranes on
graphene in water. (c) Corresponding R-n graphs. The red fitted curves allow the
extraction of parameters shown in Table 6.1.



Figure 6.4. Simulated graphene conductivity versus carrier density resulting from
charged lipid scatterers (The influence of other scatterers is not included in this plot).
The lines correspond to different values of impurity separation, d = 0.5 nm (blue), 1
nm (red), 1.5 nm (green), 2 nm (yellow) and 2.5 nm (black). Remote scatterers (d ≥ 2
nm) also induce non-linear deviations.
38



Figure 6.5. Membrane thinning effect of Magainin 2 peptides on gram-negative
bacteria biomimetic membrane. AFM and epifluorescence (inset) images of
membrane (a) before and (b) after addition of 1 µM Magainin 2 peptides. Inset shows
the epifluorescence image in which adsorption of peptides (represented by bright
spots) was clearly observed at the periphery of the membranes. These bright spots are
speculated to be due to the onset of membrane thinning in which adsorbed peptides

push apart the lipid headgroups on the top leaflet, causing these fluorescent lipids to
be dislodged from the surface. (c) Conductivity curves of biomimetic membrane-
GFET in 10 mM NaF with increasing Magainin 2 concentration and (d) the
corresponding shifts in Dirac points.


Figure 6.6. Control experiment where cationic peptides were directly added to bare
graphene. A very small negative shift in Dirac point coupled with a small increase in
conductivity minimum were observed. This negative shift in Dirac point was
significantly smaller (∆V
g,min
~-0.03 V) as compared to that observed by peptide-
induced perturbation of membrane electric charges (∆V
g,min
~-0.18 V) when 1 µM
Magainin 2 peptides were added. Another careful consideration encompasses the fact
that cationic peptides can partially neutralise negatively charged lipids, similar in
effect to ionic screening.


Figure 6.7. A schematic diagram illustrating the sensing concept of membrane
thinning and perforation. Brown ovals represent peptides.

Figure 7.1. Graphene-based detection of single Plasmodium falciparum-parasitised
erythrocyte (PE). (a) (Left) Schematic illustration of an array of graphene transistors
on quartz. The electrodes are protected by SU-8 photoresist which conveniently acts
XXIII

as the side wall for the microfluidic channel through which cells flow. (Right)
Specific binding between ligands located on positively charged membrane knobs of

parasitised erythrocyte and CD36 receptors on graphene channel produces a distinct
conductance change. Conductance returns to baseline value when parasitised
erythrocyte exits the graphene channel. (b) DIC image of independent graphene
transistors with SU-8/PDMS microfluidic channel. Inset shows the etched graphene
strip between source and drain electrodes. Scale bar is 30 µm. (c) Three-dimensional
AFM images of (left) PE
18
(Scale bar is 1 µm) and (right) three-dimensional height
plot of the surface of PE revealing protruding knobs which overlaid with adhesion
maps of knob ligands using CD36-functionalised AFM tip (yellow regions).


Figure 7.2. Characterisation of CD36-functionalised graphene. (a) CD36 coupled
with FITC-conjugated CD36 antibody on graphene (inset) showed significant
fluorescence quenching as compared to that on quartz. Scale bar is 100 µm. (b) AFM
image and height profile of CD36 coupled with FITC-conjugated CD36 antibody on
graphene showed a combined protein height of ~3 nm.


Figure 7.3. Protein adhesion force on graphene and quartz. (a) AFM phase image of
GFET on quartz. (b) Box plots of adhesion force on graphene and quartz and
representative force curves obtained over the (c) graphene and (d) quartz area with a
CD36-functionalised AFM tip. The top and bottom of the box denote 75
th
and 25
th

percentiles of the population respectively, while the top and bottom whiskers denote
90
th

and 10
th
percentiles respectively. Maximum and minimum values are denoted by
open squares. Gaussian distribution of raw data points is shown.


Figure 7.4. Viability test of CD36 receptors adsorbed on (a) graphene and (b) quartz
over 10 days


Figure 7.5. (a) Device performance for bare GFET and CD36-functionalised GFET
electrolytically gated in culture medium. Inset shows the corresponding C-V
measurements. Device channel length and width were 10 µm and 20 µm, respectively.
(b) I
ds
-V
g
curves shows distinct shifts in charge neutrality point when PE adhered onto
CD36-functionalised graphene channel. The increase in the width of the minimum
conductivity plateau upon PE adhesion is also evident.


Figure 7.6. I
ds
-V
g
curves showed distinct shifts in charge neutrality point when
schizont-PE adhered onto CD36-functionalised graphene channel. The increase in the
width of the minimum conductivity plateau upon PE adhesion was more pronounced
when transistor length and width was 8 µm and 15 µm, respectively.

Figure 7.7. Parasite differentiation by graphene channel conductance. (a)
Conductance-time plots for (early to mid) trophozoite-PE and schizont-PE measured
at V
g
= 0.1 V and corresponding DIC images on the right. Device channel length and
width was 8 µm and 15 µm, respectively. (b) Box plots of percentage conductance