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The reversible ionic liquid anion intercalation/deintercalation of multilayer graphene
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The reversible ionic liquid anion intercalation/deintercalation of multilayer graphene
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1
The Reversible Ionic Liquid Anion Intercalation/Deintercalation of Multilayer Graphene
by
Zhi Cai
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIAL SCIENCE)
August 2024
i
Dedication
I want to dedicate this thesis to my mentor, parents, wife, and friends, who have encouraged me
to overcome the difficulties I met during my PhD career.
ii
Acknowledgments
I extend my heartfelt thanks to my advisor and lifelong mentor, Dr. Stephen B. Cronin,
for allowing me to embark on the Ph.D. journey within his remarkable team. Your guidance has
paved the way for me in scientific research, and your invaluable support and meticulous
attention have enabled me to realize my full potential in the field of study with autonomy.
Collaborating with you over the past five years has been a privilege.
I would also like to express my gratitude to the members of my dissertation committee
and qualifying committee, including Prof. Jayakanth Ravichandran, Prof. Aiichiro Nakano, Prof.
Priya Darshan Vashishta, and Prof. Zhenglu Li. Your expertise and thorough evaluation of my
work are highly valued, and your guidance has dramatically enhanced the quality of my
dissertation.
I am deeply thankful for the assistance provided by both past and present members of
our research group, including Dr. Haotian Shi, Dr. Bingya Hou, Dr. Jihan Chen, Dr. Bo Wang,
Dr. Sisi Yang, Dr. Bofan Zhao, Dr. Yu Wang, and Dr. Indu A. Aravind. Your selfless support
and guidance have facilitated my Ph.D. journey seamlessly. I am also grateful for the
opportunity to collaborate with Ruoxi Li, Sizhe Weng, Boxin Zhao, Caleb Medchill, Curtis
Hauck, Yuyun Wang, Rifat Shahriar, and Mehedi Hasan Himel. The friendships I have formed
with you are cherished treasures in my life.
I owe a debt of gratitude to my family for their unwavering love and support from across
the vast Pacific Ocean, which has been the cornerstone of my entire Ph.D. journey. Despite
iii
being unable to meet for five years due to the pandemic, I deeply miss you all, and my love for
you remains steadfast and unwavering.
I want to thank my friends for the brainstorming sessions, the moral support, and the
enjoyable moments we shared. I cherish the friendships with Nan Wang, Haonan Wang, Zerui
Liu, Chi Xu, Chao Cao, Zhiyuan Zhao, Dingzhou Cui, and Yunxiang Wang. Thank you for
adding color and joy to my life.
My dissertation is a collective effort of all those mentioned above; I am truly fortunate
to have had your support.
iv
Table of Contents
Dedication ................................................................................................................................i
Acknowledgments.................................................................................................................ii
Table of Figures:..................................................................................................................vii
Abstract..................................................................................................................................xii
Chapter 1: Introduction ..................................................................................................... 1
Chapter 2: Gate-tunable Modulation of the Optical Properties of Multilayer
Graphene by the Reversible Intercalation of Ionic Liquid Anions............................... 5
2.1. Introduction ..................................................................................................5
2.2. Experiments and Results...............................................................................6
2.2.1. CVD growth of multi-layer graphene and sample fabrication .................6
2.2.2. Characterization......................................................................................9
2.2.3. Experiment ...........................................................................................12
2.3. Conclusion..................................................................................................21
v
Chapter 3: A Dynamic Study of Intercalation and Deintercalation of Ionic
Liquids in Multilayer Graphene Using an Alternating Current Raman
Spectroscopy Technique..................................................................................................... 23
3.1. Introduction ................................................................................................23
3.2. Experiments................................................................................................25
3.2.1. Fabrication............................................................................................25
3.2.2. Experiments..........................................................................................27
3.2.3. Results..................................................................................................29
3.3. Conclusions .............................................................................................37
Chapter 4: Scalable Spray-on Fabrication Approach for Voltage-Tunable
Thermal Emissivity Surfaces............................................................................................. 38
4.1. Introduction ................................................................................................38
4.2. Experiments and Results.............................................................................39
4.2.1 Fabrication.............................................................................................39
4.2.2 Experiment setup ...................................................................................41
4.2.3 Characterization.....................................................................................43
4.2.4 Experiments...........................................................................................47
4.3 Conclusion...................................................................................................56
vi
Future work........................................................................................................................... 57
Chapter 5: High temperature voltage-modulate thermal emissivity surface........57
5.1. Introduction ................................................................................................57
5.2. Experiments................................................................................................59
5.2.1. Sample fabrication ................................................................................59
5.2.2. Experiment setup ..................................................................................61
5.2.3. Result....................................................................................................63
5.2.4. Future work ..........................................................................................64
Bibliology.............................................................................................................................. 65
Appendix ............................................................................................................................... 72
vii
List of Figures:
Figure 2.1 (a) Schematic diagram and (b) cross-sectional diagram of the
sample consisting of a multiple layer graphene top electrode, porous PES
membrane filled with ionic liquid ([DEME][TFSI]), and the gold electrode
bottom contact. .......................................................................................................8
Figure 2.2 Raman spectra of an oxidized silicon wafer with and without MLG.
...............................................................................................................................9
Figure 2.3 (a) AFM image of multi-layer graphene and (b) corresponding
cross-sectional plots..............................................................................................11
Figure 2.4 (a) Waterfall plot of the voltage-dependent Raman spectra
(background subtracted). (b) Differential infrared reflectance spectra under
various applied voltages........................................................................................13
Figure 2.5 (a) Energy band diagram of the Pauli blocking process for the 2D
band Raman mode. (b) 2D:G band Raman intensity ratio plotted as a function
of voltage..............................................................................................................15
Figure 2.6 (a) Raman intensity ratio between the intercalated Gint band (1620
cm-1
) and the uncharged G band (1585 cm-1
) under various applied voltages. (b)
Luminescent background intensity observed in the Raman spectra at 2250 cm1 under various applied voltages. (c) Differential FTIR reflectance observed at
viii
2.22 µm under various applied voltages relative to the reflectance at zero
applied voltage......................................................................................................17
Figure 2.7 (a) Schematic diagram illustrating the time-resolved temperature
measurement. (b) Time-resolved temperature measurement with bi-exponential
fits and corresponding time constants. ..................................................................20
Figure 2.8 Linear fit to the background Raman intensity (at 2550 cm-1
) and
FTIR reflection (at 2.22 µm).................................................................................21
Figure 3.1 (a) Schematic diagram of the chemical vapor deposition (CVD setup
for multilayer graphene growth on a Ni substrate. (b) Drawing, (c) crosssectional diagram, and (d) photograph of the sample consisting of a multiple
layer graphene (MLG) top electrode, porous Al2O3 membrane filled with ionic
liquid ([DEME][TFSI]), and the copper bottom electrode.....................................26
Figure 3.2 (a) Schematic diagram of the in-situ AC Raman spectroscopy setup.
(b) Voltage pulse sequence and (c) laser pulse sequence for measuring the
intercalated and deintercalated phases of the MLG electrode................................28
Figure 3.3 Raman spectra taken during the intercalation and deintercalation
phases from (a) the top side and (b) the bottom side of the MLG electrode...........30
ix
Figure 3.4 (a) Schematic diagram of the thermal imaging camera setup. (b-d)
Apparent temperature observed at 10 Hz, 0.2 Hz, and 10 Hz pulse voltage
frequency, applied successively. ...........................................................................32
Figure 3.5 Apparent temperature change measured as a function of frequency.
.............................................................................................................................34
Figure 4.1 (a) Air brush and graphene ink. (b) Photograph of the completed
MLG device. (c,d) Cross-sectional SEM images of the graphene ink layer on
the alumina membrane before and after the roll compression process...................40
Figure 4.2 (a) Cross-sectional diagram of the MLG device and schematic
diagram of the thermal imaging camera measurement setup. (b) Voltage pulse
sequence applied to the sample (illustrated here for a period of 2 sec). .................42
Figure 4.3 (a) Schematic figure of the rolling compression. (b-e) SEM images
of the rolling compression under various pressure. (f) The average thickness of
the MLG layer under various pressure. (g) Temperature difference of thermal
test of MLG under various pressure. .....................................................................44
Figure 4.4 (a,c,e,g,i) SEM images of 0.1g,0.2g,0.3g,0.4g and 0.5g graphene
ink spray on alumina membrane without rolling compression process. (b,d,f,h,j)
SEM images of 0.1g,0.2g,0.3g,0.4g and 0.5g graphene ink spray on alumina
membrane with rolling compression process. (k) Average thickness of MLG
x
with various ink density. (l) Temperature difference in thermal test with various
ink density. ...........................................................................................................46
Figure 4.5 The apparent temperature measured during a 5-second period
voltage pulse sequence..........................................................................................48
Figure 4.6 The apparent temperature of the MLG device under a constant
applied voltage of +4V. ........................................................................................50
Figure 4.7 The apparent temperature difference of MLG on various hotplate
temperature...........................................................................................................51
Figure 4.8 Raman spectra of the MLG device at 0V and 4V. ...............................53
Figure 4.9 Durability test results under (a) a 20 sec-period voltage pulse
sequence and (b) a constant voltage of +4V..........................................................55
Figure 5.1 Schematic figure of the structure of the device....................................60
Figure 5.2 Schematic figure of the thermal test setup...........................................62
Figure 5.3 The temperature difference between the apparent temperature of
MLG and the kinetic temperature of the background hot plate..............................63
Figure A1 Photograph of the device with multilayer graphene, PES membrane
with ionic liquid, gold electrode, and glass substrate.............................................72
xi
Figure A2 Raw data plot of the voltage-dependent Raman spectra from
1000cm-1 to 3100cm-1
. (Same data from Figure 2a.) .............................................73
Figure A3 (a) and (b) SEM image of multilayer graphene and porous Al2O3
membrane. (c) and (d) AFM image of multilayer graphene and corresponding
cross-sectional plot. ..............................................................................................74
Figure A4 Schematic diagram of Raman setup of ITO bottom electrode sample
measurement.........................................................................................................75
Figure A5 Voltage-dependent Raman spectra of the MLG device........................76
xii
Abstract
Graphene ion intercalation has many electronic and optical applications, so we introduce
graphene-based optoelectronic devices that can control and manipulate electromagnetic waves
across a broad spectrum. Researchers are exploring methods to modulate graphene's charge density
and Fermi energy to tune its optical properties, including surface charge transfer, doping, carrier
injection/depletion, and electrolyte gating. Among these, electrostatic gating and ion intercalation
stand out as electrically tunable and reversible processes to control the infrared properties of host
materials. Ion intercalation can offer higher doping concentrations for thick materials like
multilayer graphene (MLG) films. Due to graphene's linear band structure, doping can shift the
Fermi level to enable tunable optical absorption and emission across a wide spectral range.
Increasing graphene layers can enhance the optical modulation depth.
In Chapter 2, we demonstrate a substantial modulation of the optical properties of
multilayer graphene (~100 layers) using a simple device consisting of a multilayer
graphene/polymer electrolyte membrane/gold film (MLG/PEM/Au) stack. Applying a voltage of
3-4V drives the intercalation of anion [TFSI]- (ion liquid diethylmethyl(2-
methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]) resulting in the
reversible modulation of the properties of this optically dense material. Upon intercalation, we
observe an abrupt shift of 35 cm-1 in the G band Raman mode, an abrupt increase in the FTIR
reflectance over the wavelength range from 1.67 to 5 µm (2000 – 6000 cm-1
), and an abrupt rise
in the luminescent background observed in the Raman spectra of the graphene. All of these abrupt
changes in the optical properties of this material arise from the intercalation of the TFSI- ion and
xiii
the associated change in the free carrier density (Dn = 1020 cm-3). Suppression of the D band Raman
mode observed around 3V corresponds to Pauli blocking of the double resonance Raman process
and indicates a modulation of the Fermi energy of DEF = 1.1eV.
Chapter 3 reports Raman spectra and IR imaging collected during the
intercalation/deintercalation half-cycles in a multilayer graphene (MLG) device (~100 layers)
operating at 0.2-10 Hz. The device consists of an MLG/alumina-membrane/copper stack, in which
the alumina membrane is filled with ionic liquid [DEME][TFSI], forming an electrochemical cell.
Upon applying a positive voltage, the TFSI- anions intercalate into the interstitial spaces in the
MLG. The incident laser light is modulated using an optical chopper wheel synchronized with (and
delayed concerning) a 0.2-10 Hz AC voltage signal. Raman spectra taken just 200 ms apart show
the emergence and disappearance of the intercalated G band mode around 1610 cm-1. A significant
Raman signal can be obtained by integrating over hundreds of cycles. The
intercalation/deintercalation is also monitored with thermal imaging via voltage-induced changes
in the carrier density, complex dielectric function ε(ω), and thermal emissivity of the device.
In Chapter 4, we report a new approach for fabricating gate-tunable thermal emissivity
surfaces by spraying on graphene ink. The devices consist of a multi-layer graphene (MLG)/porous
alumina membrane/gold stack, in which the MLG is deposited by spraying the graphene ink onto
the porous membrane using an airbrush. The graphene ink consists of µm-sized MLG flakes
suspended in a polyvinylpyrrolidone (PVP) solution and ethylene glycol. The alumina porous
membrane is filled with ionic liquid [DEME][TFSI], forming an electrochemical cell. When a
positive voltage is applied to the device, the intercalation of [TFSI]- anions causes significant
changes in the thermal emissivity of the MLG graphene. This, in turn, gives rise to a substantial
xiv
change in the apparent temperature (i.e., thermal camouflage) as measured by a thermal imaging
(i.e., FLIR) camera sensitive over the 7.5 - 14µm wavelength range. In this work, the apparent
temperature change reaches ΔT = 14o
C. This method of spray coating offers a scalable solution
that is compatible with roll-to-roll printing and manufacturing techniques.
In Chapter 5, we introduce the future work of high-temperature tunable surface emissivity
devices. The effectiveness of tuning the thermal emission properties in these devices, particularly
the emissivity, depends not only on the characteristics of the multilayer graphene (MLG) but also
on the properties of the ionic liquids (ILs) used. The ionic liquids serve as the source of dopants
that enable the electrostatic control of the MLG's emission characteristics. Therefore, the
performance and tunability of the thermal radiation output are inherently linked to both the MLG
and the ionic liquid components of the device. To develop the high-temperature thermal emissivity
modulation performance, we should study the ILs properties more. Therefore, in this project, a set
of ILs with different cations and anions combinations will be investigated to provide a high
modulation temperature range.
1
Chapter 1: Introduction
All objects above absolute zero temperature naturally emit infrared radiation, with the
intensity dependent on both the object's temperature (T) and its infrared emissivity (ε) 1
. This
property has found widespread use in applications such as infrared sources2, 3, night vision4
, and
temperature measurement5.
Graphene-based optoelectronic devices have the remarkable ability to control and
manipulate electromagnetic waves across a broad spectrum6. Some of the emerging devices that
leverage the tunable optical properties of graphene include radar absorbers7
, tunable infrared
surfaces8, 9, and electrochromic devices10-12 operating in the visible regime. This versatility is
enabled by the unique hexagonal crystal structure and linear electronic band structure of graphene,
as well as its suitability for diverse processing and fabrication techniques. As a result, graphenebased materials are extensively investigated as tunable materials for optoelectronic applications,
such as photodetectors13 and optoelectronic modulators11. Researchers are exploring various
approaches to modulate the charge density and Fermi energy of graphene to control its optical
properties14-17. Methods like surface charge transfer or substitutional doping can alter the material's
intrinsic structure and leave it with a fixed charge state. Alternative approaches that have been
reported include injection or depletion of substrate carriers, photoexcitation of surface carriers and
electrolyte gating. Among these tuning methods, electrostatic gating and ion intercalation stand
out as electrically tunable and reversible processes that can control the physical properties of host
materials in the infrared wavelength range. Unlike electrostatic gating, which only has charged
species at the surface electrodes, ion intercalation can offer significantly higher doping
2
concentrations for thick materials, as the ions can migrate into the interlayers of the host18, 19. For
effective ion intercalation, the host material should have a thin conductive surface, making
multilayer graphene (MLG) films a promising choice due to their high electrical conductivity and
broad tunability.
Due to the linear band dispersion in graphene, as shown in Figure 1.1, it exhibits very
broad-band optical absorption and emissivity characteristics, as described by Kirchhoff's radiation
law, which states that the infrared absorption and emissivity are equal at thermodynamic
equilibrium20-23. However, the interband transitions below the Fermi level are blocked in pristine
graphene because of the Pauli exclusion principle. In pristine graphene, the interband electronic
transitions below the Fermi level are blocked due to the Pauli exclusion principle. However,
doping can be used to shift the Fermi level to higher energy levels, which in turn enables tunable
optical absorbance or light emission in graphene across a very broad spectral range, from visible
to far-infrared frequencies24, 25. The optical absorption and emissivity of monolayer graphene is
fundamentally limited to around 2.3% due to the fine structure constant22. This relatively low
modulation depth is insufficient for many practical applications. However, increasing the number
of graphene layers can effectively enhance optical absorption and emissivity. By using multilayer
graphene structures, the modulation depth of the optical properties can be increased, providing a
viable approach to overcome the limitations of monolayer graphene10, 26, 27. While electrostatic
doping is an effective method for tuning the optical properties of monolayer graphene, this
approach is not as effective for multilayer graphene. This is due to the shielding effect of the
surface layers, which limits the ability to modulate the interband transitions in the multilayer
structure. To address this challenge, researchers have recently explored the intercalation process
as an alternative doping technique for multilayer two-dimensional (2D) materials, including
3
graphene11, 18, 28-33. This intercalation-based doping method has emerged as a promising solution,
as it is a reversible and semiconductor-compatible process28, 32.
Figure 1.1 Schematic figure of graphene band structure
The observation of these staging effects and the tunability of the physical characteristics in
intercalated graphite compounds were key findings from the early research in this field, which was
primarily driven by the potential applications in energy storage technologies.9, 15, 18. The highly
reactive nature of the metal ions, especially lithium, and the presence of the electrolyte medium
introduced critical limitations to the operating conditions of these systems. These factors
necessitated the use of an inert environment and proper sealing during the fabrication process.
Unlike the reactive metal ions and associated electrolyte media used in earlier research, ionic
liquids provide a more versatile and potentially more stable intercalation approach. The ability to
tailor the composition and properties of ionic liquids allows researchers to fine-tune the
4
characteristics of the intercalated graphitic materials, expanding the potential applications of these
systems26, 34, 35.
In this thesis, we explore the applied voltage driving the intercalation of ions into the
graphene films, which in turn modulates the thermal radiation emitted by the device. By leveraging
this voltage-controlled intercalation process, we can dynamically tune the emissivity
characteristics of the graphene-based infrared devices. The ability to precisely control and
modulate the emissivity characteristics, as well as the Fermi energy of the graphene films, is crucial
for the optimal performance of these infrared devices. In this study, we have investigated several
key parameters that can potentially influence the performance of tunable emissivity devices,
including the characteristics of the MLG films, such as the number of layers, defect density, and
structural integrity, which can play a crucial role in determining the overall device performance.
And appropriate voltage range for controlling device operation, in which the applied voltage is the
primary means of controlling the intercalation of ions and, consequently, modulating the thermal
radiation emitted by the devices.
5
Chapter 2: Gate-tunable Modulation of the Optical
Properties of Multilayer Graphene by the Reversible
Intercalation of Ionic Liquid Anions
2.1. Introduction
Over the past 15 years, there has been a large amount of research on various forms of
graphene.36, 37 Highlights of this research include the observation of the quantum hall effect,38 high
thermal conductivity,39, 40 ultrahigh electron mobilities,41 transparent conducting electrodes,22
tunable plasmon-polaritons,42 and superconducting phases of graphene.43 In addition to these
fundamental phenomena, the wide tunability of graphene’s properties with electrostatic and
electrochemical doping gives rise to many practical applications. 44, 45
The properties of bulk graphite have been exploited since the 1980s as anode materials for
Li-ion batteries46, high strength-to-weight ratio composite materials,47 and a large amount of
research has been conducted on graphite intercalation compounds (GICs).48, 49 Currently, graphite
is used in rechargeable lithium ion batteries. For example, electric vehicles typically contain 83 kg
of graphite as the anode material, in which lithium ions intercalate between the layers, enabling
extremely high charge storage capacities up to 372 mAh/g. This property of graphite is a direct
result of its layered structure, which enables the intercalation between the weakly bound layers
that are held together by van der Waals forces.
6
Bao et al. studied intercalation-induced transparency in graphitic materials using Li-ion
intercalation in the visible range.50 They introduced a Drude model with interband transitions to
explain the basic mechanism of Pauli blocking. The Raman spectra of graphite intercalation
compounds were reported by Chacon-Torres.51 These spectra reveal the staging of potassium
intercalation and the associated strain induced in the lattice that is associated with the intercalation.
More recently, in the works of Salihoglu et al.52 and Ergoktas et al.53 Multilayer graphene (MLG)
grown by chemical vapor deposition (CVD) was used in conjunction with an ionic liquid
electrolyte to demonstrate controllable infrared and thermal signatures.
2.2. Experiments and Results
2.2.1. CVD growth of multi-layer graphene and sample fabrication
Figure 2.1 shows an illustration and cross-sectional diagram of the multilayer graphene
device. Multilayer graphene is grown by chemical vapor deposition (CVD) on 0.125mm thick
nickel foil (Sigma-Aldrich #7440-02-0). The growth temperature is 1000℃, the flow rates are 100
sccm of H2 and 30 sccm of CH4 for 20min, and the pressure in the chamber was approximately 10
Torr. After growth, we etch the nickel foil with nickel etchant (Transene Company, Inc. Nickel
Etchant Type I). After the etching process, the multilayer graphene (MLG), which is hydrophobic,
floats on top of the nickel etchant. We then transfer the MLG to a solution of DI water to remove
any residual ions from the nickel etchant. Then, we immerse the porous PES (polyethersulfone)
7
membrane (Membrane Solutions (Nantong) Co., Ltd., #MSPES260120) into the DI water solution
and transfer the MLG onto the membrane.
Our device, consisting of multilayer graphene (MLG) (approximately 1.5 cm x 1.5 cm,
30nm thick) deposited on the PES membrane (approximately 2 cm x 2 cm, 1µm thick), which is
then deposited onto a gold electrode (2.54 cm x 2.54 cm, 100nm thick), as shown in Figure A1.
8
Figure 2.1 (a) Schematic diagram and (b) cross-sectional diagram of the sample consisting
of a multiple layer graphene top electrode, porous PES membrane filled with ionic liquid
([DEME][TFSI]), and the gold electrode bottom contact.
9
2.2.2. Characterization
The thickness of the MLG is determined by Raman spectroscopy of SiO2/Si wafer with and
without MLG, as shown in Figure 2.2. The normalized Si intensity is 0.05133. According to � =
�(�&' − 1)+, Where A=58.72, B=0.9894, C=0.2380, y is normalized Si intensity, x is the number
of layers, yields that our graphene is around 74 layers54.
Figure 2.2 Raman spectra of an oxidized silicon wafer with and without MLG.
400 500 600 700
0
10000
20000
30000
40000
50000
60000
70000
80000
Raman Intensity
Raman Shift (cm-1)
SiO2
/Si with MLG
SiO2
/Si without MLG
I
normalized=0.03247
10
An atomic force microscope (AFM) image of the MLG device is shown in Figure 2.3 of the
Supplemental Document, which has a roughness of approximately 10nm, as can be seen in the
cross-sectional plots. However, before depositing on the Au electrode, an ionic liquid
diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]) is
used to fill the interstitial spaces in the polyelectrolyte membrane enabling a voltage to be applied
between the bottom Au electrode and the multilayer graphene top electrode, which results in the
reversible intercalation of [TFSI]- anions.
11
Figure 2.3 (a) AFM image of multi-layer graphene and (b) corresponding crosssectional plots.
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
400
420
440
460
480
500
Height (nm)
Distance (µm)
(a)
(b)
12
2.2.3. Experiment
We measured the Raman spectra of MLG devices at various voltages between 0V and 4V,
as plotted in Figure 2.4a. Here, the incident laser wavelength is 532nm, and the laser spot size is
approximately one µm in diameter. Over the range from 1200 cm-1 to 1700 cm-1
, we observe the
D band (1350 cm-1
), G band (1582 cm-1
), and intercalated G band (i.e., Gint band) (1620 cm-1
).
After performing Raman spectroscopy, a comparable device is tested using a mirror-based
Hyperion microscope coupled to a Bruker FTIR spectrometer. The reflectance of the device is
measured under the same range of voltages as in the Raman spectroscopy measurements. Here,
the spot size of the infrared light is 100 µm × 100 µm, and the wavelength range extends from 1.67
µm to 5 µm (i.e., 2000 cm-1 to 6000 cm-1).
Figure 2.4a shows a waterfall plot of the Raman spectra taken at various voltages between
0 and 4 Volts. Here, we see the abrupt onset of the intercalated G band (i.e., Gint) at 1620 cm-1 at
3.5 V, corresponding to the intercalated state of the graphene. It is somewhat surprising that these
large anions (i.e., [TFSI]-
) can intercalate between the layers of the graphene so effectively.
Nevertheless, this abrupt shift in the G band is a well-known signature of intercalation and stands
in contrast to electrochemical doping, wherein the frequency of the G band increases
monotonically as a function of the applied voltage. Figure 2.4b shows a corresponding series of
FTIR spectra taken from a comparable device. Here, we see a large increase in the reflectance by
a factor of 100% over the 2000-6000 cm-1 range. This change in reflectance occurs because of the
13
large increase in the free carrier concentration from approximately 1018 cm-3 to 1020 cm-3 over this
applied voltage range.
Figure 2.4 (a) Waterfall plot of the voltage-dependent Raman spectra (background subtracted).
(b) Differential infrared reflectance spectra under various applied voltages.
14
Here, the device becomes more reflective (i.e., less absorption) under the applied potential,
as the heavily doped graphite undergoes Pauli blocking at these wavelengths due to the large
increase in Fermi energy. At large applied potentials, we begin to see suppression of the 2D-band
Raman mode due to Pauli blocking of the double resonance Raman process (illustrated in Figure
2.5), which occurs at |Δ�/| > 1/2�34, where Eex=2.33eV. For graphene, we can calculate the
carrier density by Δ� = 6 78
ℏ:8
;
<
/π , where �/ is the graphene Fermi velocity and ℏ is the reduced
Planck constant, yielding Δ� = 2.2 × 10 10 o
C. After this, the frequency response of the
intercalation/deintercalation is much faster, showing a substantial ΔT ≈ 2 o
C at 10 Hz, as plotted
in Figure 3.4d. For these AC measurements, we find that modulating the device first at a low
frequency (i.e., 0.2 Hz) is important for achieving better intercalation at higher frequencies (i.e.,
10 Hz).
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
46.70
46.75
46.80
46.85
46.90
Temperature (oC)
Time (s)
10Hz
0 5 10 15 20
28
30
32
34
36
38
40
42
Temperature (oC)
Time (s)
0.2Hz
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
37.5
38.0
38.5
39.0
39.5
40.0
40.5
41.0
Temperature (oC)
Time (s)
10Hz
(b)
(c) (d)
Copper
Al
2O3 Membrane/IL
Multi-layer Graphene
Hot Plate
Arduino
(a)
32
Figure 3.4 (a) Schematic diagram of the thermal imaging camera setup. (b-d) Apparent
temperature was observed at 10 Hz, 0.2 Hz, and 10 Hz pulse voltage frequency, and the
voltage was applied successively.
In order to provide a basic understanding of the intercalation-induced changes in the MLG
material and connect the thermal emissivity measurements with the Raman spectroscopy
measurement results. As such, this partial intercalation/deintercalation can have a significant
impact on the complex dielectric function of the material, which can be described using a Drude
model with interband transitions as follows 76, 77:
where
and
(a) (b)
�(�) = �∞ + ���� + �
��
2�
�(1 − ���) (2) (a) (b)
33
NMLG is the number of MLG layers, and d is the thickness of MLG. The voltage-induced
modulation of the dielectric function (Δε(ω)), in turn, affects the thermal emissivity �(ω, �) of
the material, as follows:
�(ω, �) = [1 − �(ω,�)][1 − exp[−β(ω,�)�]]
1 − �(ω,�) exp[−β(ω,�)�]
where
�(ω,�) = [�(ω, �) − 1]< + �(ω, �)<
[�(ω, �) + 1]< + �(ω, �)<
and
β(ω, �) = 4πω�(ω, �)
�
where �(ω, �) is the refractive index, and �(ω, �) is the extinction coefficient. While the
multilayer graphene does not fully deintercalate, ΔN = 3×1020 cm-3, which is substantial compared
to MLG’s intrinsic carrier concentration of 3×1018 cm-3. Based on this, we predict an DT of 10 o
C,
which is consistent with our experimental observations.
Figure 3.5 shows a linear-log plot of the apparent temperature change as a function of drive
frequency. Here, we observe that the apparent temperature change increases from 0.25 o
C at 10 Hz
to 14 o
C when the frequency decreases to 0.2 Hz, where the temperature change reach maximum
saturation. Based on this data, these devices exhibit a cutoff frequency of ~ 1 Hz, although
appreciable modulation can still be observed up to 10 Hz.
34
Figure 3.5 Apparent temperature change measured as a function of frequency.
0.1 1 10 100
0
2
4
6
8
10
12
14
16
△T (
oC)
Frequency (Hz)
35
Figure 3.6 shows a linear-log plot of the emissivity change as a function of drive frequency.
From the Stefan-Boltzmann law, we have a formula that connects the kinetic temperature and the
apparent temperature with the emissivity. �]^^]_3`a = �
K
b �cd`3ade. Where E is the emissivity of
the material, with this formula, we can extract the emissivity value from the temperature difference.
36
Figure 3.6 Emissivity change as a function of drive frequency.
0.1 1 10 100
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
D
E
Frequency (Hz)
37
3.3. Conclusions
In conclusion, we have demonstrated a strategy for investigating the dynamic process of
ionic liquid intercalation and deintercalation in multilayer graphene (MLG) through the utilization
of Raman spectroscopy combined with applied AC voltages. By analyzing the Raman spectra, we
observe changes in the spectra that occur during the intercalation and deintercalation processes.
One significant finding of our study is the notable difference in carrier density between the
intercalated and deintercalated states, which is approximately 3×1020 cm-3. This substantial
difference in carrier density results in a significant change in the thermal emissivity of the MLG,
which gives rise to a significant change in the apparent temperature variation of approximately
10 °C. This apparent temperature change, caused by the carrier density difference, highlights the
strong correlation between the intercalation/deintercalation processes and the dielectric function
ε(ω) of the MLG material. We believe that this dynamical spectroscopy approach can be used to
investigate a wide range of rechargeable battery electrodes in-situ.
38
Chapter 4: Scalable Spray-on Fabrication Approach
for Voltage-Tunable Thermal Emissivity Surfaces
4.1. Introduction
The process of intercalating small ions, predominantly from the alkali metal group, into
the layered structure of bulk graphite has received considerable attention and is now a key
mechanism for energy storage in lithium-ion batteries.48, 49, 59-61 More recently, ionic liquids that
consist of large ions (e.g. [DEME][TFSI]) have been reported to intercalate into multilayer
graphene (MLG) grown by chemical vapor deposition (CVD).52, 53 This MLG intercalation can be
utilized for broadband modulators, thermal camouflage, and covert communications.44, 45 As such,
MLG shows widely tunable optical properties.63, 64 However, the CVD growth method has several
limitations, including the size of the CVD growth material is constrained by furnace size, and CVD
growth of MLG requires high temperatures (~1000o
C). Wei et al. used graphene ink to fabricate
electrodes for lithium-ion batteries.78, 79 Ding et al. utilized graphene ink as a humidity sensor.80
He et al. developed a screen-printing method to utilize graphene ink to fabricate flexible printed
electronics.81 Here, we present a new strategy using graphene ink as a paint and an airbrush to
spray coat a porous alumina membrane substrate that ultimately forms a scalable MLG gatetunable thermal emissivity device.
39
4.2. Experiments and Results
4.2.1 Fabrication
Figure 4.1 illustrates the multilayer graphene (MLG) device configuration. As mentioned
above, the MLG layer is spray-deposited using an airbrush (Rhino Wisdom, 48 PSI rechargeable
cordless airbrush) with graphene ink. To prepare the graphene ink, 0.5 g polyvinylpyrrolidone
(PVP) (Sigma-Aldrich # PVP10-100G) is added into 50 ml ethylene glycol (Sigma-Aldrich #
324558-1L) and stirred for 2 hr to fully dissolve. Then, 1 g graphene powder (Sigma-Aldrich #
806625-25G) is added to the solution and stirred for 1 hr. Figure 1a shows the graphene ink and
the airbrush. The graphene ink is then sprayed onto the porous alumina membrane (Sigma-Aldrich
#WHA68095522) using the airbrush. The membrane is then baked at 80o
C for 1 hour to evaporate
the ethylene glycol, followed by a roll compression process to form a more continuous MLG layer
(see Figure 4.1d). Figures 4.1c and 4.1d show SEM images of the graphene ink layer and porous
alumina membrane before and after the roll compression process. After the roll compression
process, the membrane is heat-treated on a hotplate at 350o
C for 30 mins. Once the sample has
cooled back to room temperature, ionic liquid diethylmethyl(2-methoxyethyl)ammonium
bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]) is injected into the porous alumina membrane
from the back side before the MLG/Al2O3 is deposited onto the bottom gold electrode, as shown
in Figure 4.1b.
40
Figure 4.1 (a) Air brush and graphene ink. (b) Photograph of the completed MLG device.
(c,d) Cross-sectional SEM images of the graphene ink layer on the alumina membrane before
and after the roll compression process.
41
4.2.2 Experiment setup
Figure 4.2a shows a schematic diagram of the thermal imaging camera setup and a crosssectional diagram of the MLG-based device. The hot plate is set to 70o
C, and a FLIR
camera(A655sc), whose spectral range is 7.5 - 14µm, is used to measure the apparent temperature.
An Arduino microprocessor is used to control the voltage signal that is applied to the device, which
is programmed to trigger a three-state multiplexer (MUX) between +5V, -5V, and open circuit
conditions, as described previously.
62, 82 Figure 4.2b shows the voltage pulse sequence applied to
the sample. By adjusting the parameters of the Arduino microprocessor, we can control the period
of the voltage pulse from 0.1s to 5s.
42
Figure 4.2 (a) Cross-sectional diagram of the MLG device and schematic diagram of the
thermal imaging camera measurement setup. (b) A voltage pulse sequence was applied to the
sample (illustrated here for a period of 2 sec).
43
4.2.3 Characterization
Figure 4.3(a) shows the schematic figure of rolling compression. The alumina membrane
with graphene ink on top is put between two 1.5mm thickness Teflon plates, and then this sandwich
structure is inserted into a rolling machine. By adjusting the distance between the rolling rod, we
can control the rolling pressure on the graphene ink and alumina membrane. Figure 4.3(b-e) shows
the SEM image of the graphene ink microstructure under various compressed pressure. Figure
4.3(f) shows the average thickness of the MLG under various compressing pressures, and Figure
4.3(g) shows the temperature difference of these MLG. Note that the alumina membrane is broken
under 39.4 MPa.
44
Figure 4.3 (a) Schematic figure of the rolling compression. (b-e) SEM images of the rolling
compression under various pressures. (f) The average thickness of the MLG layer under
various pressures. (g) Temperature difference of thermal test of MLG under various
pressures.
45
The weight of the graphene ink will also affect the topography of the MLG layer on alumina
membranes before and after the rolling compression. Here, we take 0.1g,0.2g,0.3g,0.4g, and 0.5g
graphene ink spry on a 41mm diameter circle region, and then do the rolling compression process.
The SEM image is illustrated in Figure 4.4(a-j). Figure(k) shows the average thickness of the MLG
layer with various ink densities with and without the rolling compression process; figure (l) shows
the temperature difference in thermal tests with various ink densities.
46
Figure 4.4 (a,c,e,g,i) SEM images of 0.1g,0.2g,0.3g,0.4g and 0.5g graphene ink spray on
alumina membrane without rolling compression process. (b,d,f,h,j) SEM images of
0.1g,0.2g,0.3g,0.4g, and 0.5g graphene ink spray on alumina membrane with a rolling
compression process. (k) The average thickness of MLG with various ink densities. (l)
Temperature difference in thermal test with various ink density.
47
4.2.4 Experiments
Figure 4.5 shows the device’s apparent temperature measured under a five second-period
voltage pulse sequence. During the positive voltage phase, the apparent temperature exhibits a 3o
C
decrease. This apparent temperature drop follows an exponential decay with a time constant of τ1
= 1s. During the open circuit phase, the apparent temperature remains constant, and during the
negative voltage phase, the temperature increases back to the original temperature with the same
time constant τ2 = 1s. Fundamentally, this phenomenon is due to the voltage-induced carrier
density change. More specifically, during the positive voltage phase, the anion [TFSI]- is
intercalated into the MLG interlayer regions, resulting in a free carrier density increase and a
change in emissivity. As a result, the apparent temperature decreases. During the open circuit phase,
the intercalated ions cannot be deintercalated, and so the apparent temperature remains constant.
During the negative voltage phase, the [TFSI]- anions are deintercalated, resulting in a decrease in
free carrier density and an apparent temperature increase.
48
Figure 4.5 The apparent temperature and its corresponding emissivity measured during a 5-
second period voltage pulse sequence.
(a)
(b)
49
Figure 4.6 shows the device’s apparent temperature under a constant applied voltage of
+4V. From 0 to 30s, the device is turned off, and the apparent temperature is stable at 76o
C. After
30 seconds, the device is turned on, and the voltage is held constant at +4V. 20s after turning on
the device, the apparent temperature decreases to 62o
C, resulting in a change of the apparent
temperature of 14o
C. The temperature decrease follows an exponential decay with a time constant
of τ = 7s.
50
Figure 4.6 The apparent temperature and its corresponding emissivity of the MLG device
under a constant applied voltage of +4V.
(a)
(b)
51
Figure 4.7 shows the device’s apparent temperature difference under a constant applied
voltage of +4V at various hotplate temperatures. The temperature difference has a linear
relationship with the hotplate temperature.
Figure 4.7 The apparent temperature difference of MLG on various hotplate temperatures.
52
Figure 4.8 shows the Raman spectra of the MLG device under applied voltages of 0V and
4V. As the voltage reaches 4V, the G band peak (1580 cm-1
) disappears and a new peak, which we
refer to as Gint (G intercalated peak at 1616 cm-1). The existence of the Gint peak and the absence
of the G band peak, at 4V, indicate that the [TFSI]- ions have been fully intercalated into the
interlayer regions of the graphene ink. In addition, at 4V, the D band peak (at 1350 cm-1
)increases
because of the lattice distortion created by the [TFSI]-
ions (0.79 nm), which is greater than the
graphene lattice constant (0.24 nm) and the interlayer spacing (0.355 nm). The Raman spectra at
+4V also show a luminescent background, which is due to the increase in free carrier density. A
series of voltage-dependent Raman spectra of the MLG device is plotted in Figure A5 of the
Appendix, showing the reversibility of this voltage-induced intercalation process. Figure A6 shows
the device's original Raman spectrum, the intercalated and deintercalated spectra at the first cycle,
and the intercalated and deintercalated spectra after 20 cycles.
53
Figure 4.8 Raman spectra of the MLG device at 0V and 4V.
54
Figure 4.9a shows the results of a durability test of the MLG device. Here, a 100-sec-period
voltage pulse sequence, as illustrated in Figure 2b, is applied to the sample for 25 mins. After 25
mins, the device continues to exhibit a modulation of ΔT = 10o
C in the apparent temperature, with
an exponential decay time constant of τ = 6s. Figure 3.9b shows the results of a device durability
test under a constant applied voltage of +4V. During the test, the voltage is turned on at 2 min, and
the voltage is held constant at +4V for 23 mins. The apparent temperature jump at 8 min and 20
min is due to the auto-calibration of the FLIR camera.
55
Figure 4.9 Durability test results under (a) a 20 sec-period voltage pulse sequence and (b) a
constant voltage of +4V.
56
4.3 Conclusion
In conclusion, we have demonstrated a scalable method to fabricate gate-tunable thermal
emissivity MLG devices. The MLG deposited by the spray coating method can be intercalated by
ionic liquid [DEME][TFSI], resulting in a notable difference in the free carrier density between
the intercalated and unintercalated states. This difference in carrier density change leads to a
significant thermal emissivity change of the MLG, which shows a DT = 14o
C apparent temperature
decrease when a positive voltage is applied. This spray coating method represents a scalable
approach that is compatible with roll-to-roll printing and manufacturing methods.
57
Future work
Chapter 5: High temperature voltage-modulated
thermal emissivity surface
5.1. Introduction
The amount of thermal radiation emitted by an object is determined by its temperature and
its surface's emissivity, as described by the Stefan-Boltzmann law. The ability to control and tune
this thermal emission, particularly in the infrared range, is crucial for various applications,
including thermal management63, 83, infrared sensing and emission devices3, 84. The capacity to
adjust the thermal emission properties of materials is precious for these applications. Researchers
have investigated a wide range of materials and mechanisms to achieve tunable thermal emission
capabilities. The materials that have been studied include phase change materials85 and
electrochromic materials86. The mechanisms that have been explored to tune the thermal emission
properties of these materials include electrostatic gating and chemical doping23. This extensive
research effort aims to develop materials and techniques that can effectively control and modulate
the thermal radiation emitted by objects, particularly in the infrared range. Despite the extensive
research into materials and mechanisms for tunable thermal emission, many challenges remain
that have hindered the practical implementation of these technologies. The first and most important
one is low tunability, which is the ability to adjust the thermal emission properties significantly
and reliably. The second one is the low working temperature range, which is limited in room
temperature to 70 ℃. The slow response that these devices often exhibit slow response times and
58
limitstheir usefulness in applications. These challenges continue to pose barriers to the widespread
adoption and practical use of tunable thermal emission technologies across various applications.
Graphene, with its unique linear band structure, exhibits great potential for tuning its
optical absorption and emission properties across a wide range of wavelengths, from the visible to
the infrared spectrum 87. Recent research has demonstrated the capability to tune the thermal
radiation properties of devices that utilize multilayer graphene (MLG) as the top electrode and
ionic liquids (IL) as a source of dopants. In these devices, the emissivity of the MLG can be
controlled through the application of electrostatic gating, allowing for the tuning of the thermal
radiation emitted by the system8
. Extensive research efforts have been made to optimize the
performance of devices that utilize multilayer graphene (MLG) for tunable thermal radiation.
These studies have explored ways to tune the properties of the MLG, such as adjusting the number
of layers8
, as well as investigating the selection of different materials for the back electrodes16. The
goal of these optimization efforts is to enhance the capabilities and performance of the devices that
leverage MLG and ionic liquids for the electrostatic control of thermal radiation emission. While
significant research has focused on optimizing the properties of multilayer graphene and the back
electrodes in these tunable thermal radiation devices, relatively little attention has been devoted to
the study of the ionic liquids (ILs) used in these systems. Specifically, the stability and durability
of these devices over time have not been extensively investigated. It has been suggested that the
degradation of the devices may be attributed to two potential issues: the hydration of the ionic
liquid in ambient environments8, 16, or the occurrence of chemical reactions between the ionic
liquid and the graphene material88. These stability and reliability concerns related to the ionic
liquid component have not received the same level of scrutiny as other aspects of the device design
and optimization.
59
The effectiveness of tuning the thermal emission properties in these devices, particularly
the emissivity, depends not only on the characteristics of the multilayer graphene (MLG) but also
on the properties of the ionic liquids (ILs) used. The ionic liquids serve as the source of dopants
that enable the electrostatic control of the MLG's emission characteristics. Therefore, the
performance and tunability of the thermal radiation output are inherently linked to both the MLG
and the ionic liquid components of the device. To develop the high-temperature thermal emissivity
modulation performance, we should study the ILs properties more. Therefore, in this project, a set
of ILs with different cations and anions combinations will be investigated to provide a high
modulation temperature range.
5.2. Experiments
5.2.1. Sample fabrication
The device structure shown in Figure 5.1 consists of multilayer graphene (MLG) grown by
chemical vapor deposition (CVD) on a nickel foil substrate. The growth process involves heating
the nickel foil to 1050°C and flowing a mixture of hydrogen and methane gases for 20 minutes at
a pressure of approximately 10 Torr. After the growth, the nickel foil is etched away using a nickel
etchant, which causes the hydrophobic MLG to float on top of the etchant solution. The MLG is
then transferred to a deionized water bath to remove any residual ions from the etching process.
Next, a porous alumina membrane is immersed in the DI water, and the MLG is transferred onto
the membrane surface. This process results in the final device structure, which consists of the MLG
layer deposited on the porous alumina membrane. Then, a set of ILs with various combinations of
cations and anions will be injected from the backside of the porous alumina membrane. To
60
fabricate the device, we put the alumina membrane, which was filled with the various ILs, on a
piece of glass, which deposited a layer of gold as the bottom electrode.
Figure 5.1 Schematic figure of the structure of the device.
61
5.2.2. Experiment setup
Figure 5.2 shows the schematic figure of the thermal test setup. A FLIR thermal camera
(#FLIR A622sc) is used to measure the apparent temperature of the device. The spectrum range of
this thermal camera is from 7 to 14 µm, and the temperature range of this thermal camera is from
-40 ℃to 150℃or 100℃to 650℃. The device is put on a hot plate with a thermal couple to measure
the kinetic temperature of the device. This kinetic temperature is used to calibrate the emissivity
of the MLG. An industrial strategy to measure emissivity is to use a thermal camera. We can set
the built-in emissivity parameter in the thermal camera until the apparent temperature of the MLG
shown in the thermal camera is equal to the kinetic temperature shown in the thermal couple. Then,
this emissivity parameter that is set in the thermal camera is the emissivity of the MLG.
62
Figure 5.2 Schematic figure of the thermal test setup.
63
5.2.3. Result
Figure 5.3 shows a temperature difference change between the MLG apparent temperature
and the kinetic temperature of the background hot plate during the hot plate increasing temperature
process. The temperature difference has a linear relationship with the background temperature
increase with the slope equal to 0.26. From the Stefan-Boltzmann law, we can get that �]^^]_3`a =
�
K
b �cd`3ade. Here, E is the emissivity. So, we can see that the Emissivity of the MLG is around 0.3.
Figure 5.3 The temperature difference between the apparent temperature of MLG and the
kinetic temperature of the background hot plate.
64
5.2.4. Future work
The IL that we are using is [DEME][TFSI], whose boiling point is 180 ℃ (calculated value),
which theoretically means that we can observe the temperature until the hot plate reaches 180 ℃.
However, we observe that after the kinetic temperature reaches 130 ℃, the device is broken, and
the temperature difference no longer changes. The possible reason is that at high temperatures with
voltage induced, the IL has a chemical reaction with the oxygen, or the IL itself occurs in a
decomposition reaction. A possible solution is decreasing the voltage applied at high temperatures.
Another possible solution is to try other combinations of the cations and anions to get a more stable
ionic liquid.
65
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72
Appendix
Figure A1 Photograph of the device with multilayer graphene, PES membrane with ionic
liquid, gold electrode, and glass substrate.
73
Figure A2 Raw data plot of the voltage-dependent Raman spectra from 1000cm-1 to
3100cm-1. (Same data from Figure 2a.)
74
Figure A3 (a) and (b) SEM image of multilayer graphene and porous Al2O3 membrane. (c)
and (d) AFM image of multilayer graphene and corresponding cross-sectional plot.
-1 0 1 2 3 4 5 6 7
1.25
1.30
1.35
1.40
1.45
Height (μm)
Position (μm)
(a)
(c)
(b)
(d)
75
Figure A4 Schematic diagram of Raman setup of ITO bottom electrode sample
measurement.
532nm
Laser
Chopper
Wheel
1Hz
1:3 duty cycle
R:T=10:90
splitter
C
C
D
Photodiode
Arduino
Trigger ITO coated glass
40X
Al2O3 with IL
MLG
76
Figure A5 Voltage-dependent Raman spectra of the MLG device.
77
Figure A6. (a) The original MLG ink Raman Spectrum. (b) The first cycle of intercalation
and deintercalation Raman spectra in intercalated phase and deintercalated phase. (c)
Raman Spectra after 20 cycles of intercalation/deintercalation.
1000 1500 2000 2500 3000 3500
0
1000
2000
3000
4000
5000
6000
7000
8000
Intensity (a.u.)
Wavenumber (cm-1)
1000 1500 2000 2500 3000 3500
0
5000
10000
15000
20000
25000
30000
35000
40000
Intensity (a.u.)
Wavenumber (cm-1)
Intercalated
Deintercalated
1000 1500 2000 2500 3000 3500
0
5000
10000
15000
20000
25000
30000
35000
40000
Intensity (a.u.)
Wavenumber (cm-1)
Intercalated
Deintercalated
(a)
(b)
(c)
Abstract (if available)
Abstract
Graphene ion intercalation has many electronic and optical applications, so we introduce graphene-based optoelectronic devices that can control and manipulate electromagnetic waves across a broad spectrum. Researchers are exploring methods to modulate graphene's charge density and Fermi energy to tune its optical properties, including surface charge transfer, doping, carrier injection/depletion, and electrolyte gating. Among these, electrostatic gating and ion intercalation stand out as electrically tunable and reversible processes to control the infrared properties of host materials. Ion intercalation can offer higher doping concentrations for thick materials like multilayer graphene (MLG) films. Due to graphene's linear band structure, doping can shift the Fermi level to enable tunable optical absorption and emission across a wide spectral range. Increasing graphene layers can enhance the optical modulation depth.
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Cai, Zhi
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The reversible ionic liquid anion intercalation/deintercalation of multilayer graphene
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2024-08
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