1 Reduced Graphene Oxide Based Transparent Electrodes for Organic Electronic Devices Tarun Ramesh Chari Electrical and Computer Engineering McGill University, Montreal February 2011 A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Master of Engineering
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1
Reduced Graphene Oxide Based Transparent
Electrodes for Organic Electronic Devices
Tarun Ramesh Chari
Electrical and Computer Engineering
McGill University, Montreal
February 2011
A thesis submitted to McGill University in partial fulfillment of the requirements of the
degree of Master of Engineering
i
Abstract
This thesis explores the utility of reduced graphene oxide and hybrid reduced graphene
oxide/single walled carbon nanotubes as a transparent electrode. Graphene oxide was fabricated
using the modified Hummers method, transferred to arbitrary substrates by a vacuum filtration
method, and reduced chemically and thermally thus creating thin, large area reduced graphene
oxide films. Films were characterized electrically, optically, spectroscopically, and topographically.
Raman and X-ray photoelectron spectroscopy techniques were utilized to ensure successful
fabrication of reduced graphene oxide. The reduced graphene oxide electrodes exhibit sheet
resistances on the order of 10 – 100 kΩ/sq with transparencies between 60 – 90 %. To ameliorate
these electronic properties, single walled nanotubes were introduced during the filtration process
to separate the graphene oxide nanoplatelets and prevent structural deformation during reduction.
This nanotube doping yielded a two-fold decrease in sheet resistance for low nanotube to graphene
oxide ratios, but increased sheet resistance for higher nanotube to graphene oxide ratios. Reduced
graphene oxide electrodes and nanotube/reduced graphene oxide hybrid electrodes were used in
organic light emitting diode and organic solar cell applications. Organic light emitting diodes
exhibited current efficiencies of about 1 cd/A and organic solar cells exhibited power conversion
efficiencies less than 1 % for both reduced graphene oxide and hybrid electrodes.
ii
Résumé
Cette thèse examine l’utilité de l’oxyde de graphène réduit et de l’hybride oxyde de graphène réduit
et nanotubes carbone en fonction d’une utilisation comme électrode transparente. L’oxyde de
graphène a été fabriqué par la méthode de Hummers modifié puis a été transféré sur un substrat
arbitraire par la méthode de filtration avec suction à vide, et a été réduit chimiquement et
thermiquement pour créer des feuilles d’oxyde de graphène réduit qui sont minces et qui couvrent
une grande surface. Les feuilles ont été caractérisées par des mesures électriques, optiques,
spectroscopiques, et topographiques. Les spectroscopies Raman et par photoélectron induits par
rayons-X ont été utilisées pour s’assurer que la fabrication de l’oxyde de graphène reduit a été
obtenue. Les électrodes d’oxyde de graphène reduit montrent des résistances de feuille de 10– 100
kΩ/sq avec des transparences entre 60 – 90 %. Pour améliorer ces propriétés, des nanotube de
carbone monoparois ont été introduits pendant le processus de filtration pour séparer les
nanoplatelets d’oxyde de graphène et pour éviter la déformation structurelle pendant la processus
de réduction. Ce dopage de nanotubes a diminué la résistance de feuille par un facteur deux pour
des proportion faibles de nanotubes avec l’oxyde de graphène, mais a augmenté la resistance pour
les hautes proportions. Les électrodes d’oxyde de graphène reduit et les électrodes hybrides
nanotubes/oxyde de graphène reduit ont été utilisées dans des dispositifs optoélectroniques
organiques; spécialement des diodes électroluminescentes et des cellules solaires. Les diodes
électroluminescentes organiques ont des rendements de courant inferieurs à 1 cd/A et les cellules
solaire ont des rendements de puissance inferieurs à 1 % pour les deux types d’életrodes: oxyde de
graphène réduit et hybrides.
iii
Acknowledgements
I would like to thanks all my colleagues from the Izquierdo Lab and the Siaj Lab. In particular, Yu-
Mo Chien and Jayantha for building and testing the OLED and solar cell devices, respectively. I
would also like to thank Abdeladim Geurmoune for his aid in SEM, Alexandre Rodrigue-Witchel for
his help with Raman microscopy, and Suzie Poulin for the XPS measurements. Thank you to Dr.
Izquierdo, Dr. Shih and NSERC for their financial support. Finally, thank you to Dr. Mohamed Siaj,
Dr. Ricardo Izquierdo, Dr. Ishiang Shih, and Dr. Thomas Szkopek for their help and support
throughout my Master’s program.
iv
Table of Contents
Abstract ......................................................................................................................................................................................... i
Résumé ........................................................................................................................................................................................ ii
Acknowledgements ............................................................................................................................................................... iii
Nanotube Background Information ....................................................................................................................... 5
Characterization of Common CNT Electrode Fabrication Techniques .................................................... 6
Chemical Vapor Deposition ....................................................................................................................................... 8
Transfer Techniques ....................................................................................................................................................... 14
Spin-Coating GO ............................................................................................................................................................ 14
Organic Solar Cells ....................................................................................................................................................... 20
Four Point Probe .......................................................................................................................................................... 22
v
Atomic Force Microscopy ......................................................................................................................................... 23
Characterization of Hybrid Electrodes ................................................................................................................ 39
Intermediate Substrate Transfer Method ............................................................................................................... 45
Conclusion of Electrode Fabrication and Characterization ............................................................................. 45
Chapter 7 Fabrication of OEDs using graphene based electrodes .................................................................... 46
Organic Solar Cells ............................................................................................................................................................ 46
Reduced GO OLEDs ..................................................................................................................................................... 47
Hybrid electrodes for OLEDs .................................................................................................................................. 50
Conclusion of OEDs .......................................................................................................................................................... 52
Chapter 8 Discussion and Conclusion ........................................................................................................................... 53
Sheet Resistance of a four point probe .................................................................................................................... 57
Spin-coating suspended GO on substrates was first demonstrated by Gomez-Navarro et al. in 2007,
revealing promising electrical characteristics [47]. This work was furthered by Becerril et al. who
used this method to pattern electrodes for organic electronic devices [44]. In this method, GO
solution is spin-coated according to the recipe developed by Becerril et al.; GO is deposited and left
to wet the surface for 1 minute followed by spinning at 600, 800, and 1600 RPM for 1 minute at
each speed. Films were left to dry in an oven for several hours before reducing. Some substrates,
such as glass and quartz, require surface functionalization to enhance surface-GO adhesion while
other substrates, such as PET, do not require such functionalization. In a dry glovebox, substrates
15
were functionalized with 3-aminopropyltriethoxysilane (ATPES) by soaking the substrates in a 3%
solution of APTES in anhydrous toluene for 1 hour.
Vacuum Filtration
Using small sized cellulose acetate filters (0.05 µm from Millipore Inc.), GO is filtered via vacuum
filtration. Firstly, ~50-100 mL of deionized water is filtered to fully wet the cellulose filter. Then the
desired volume of GO dispersed in solution is filtered. Increasing the volume of filtered GO will only
contribute to the overall film thickness. This is because the cellulose acetate filters used to fabricate
the GO films are the same size and shape, thus the filtered area is the same for all GO films. Due to
this correlation, the relative thickness of a film can be discussed without quantifying its thickness.
After all the solution has been filtered, the vacuum pump is left on for ~5-10 minutes in order to
ensure no solution on the surface of the filter remains and to partially dry the filter. Finally, filters
are stored in a plastic, covered Petri and left to dry overnight or dried in a desiccator under vacuum
for a few hours. Figure 6 depicts the vacuum filtration process (A) and the subsequent cellulose
acetate filters (B) left to dry in plastic Petri dishes.
Figure 6 - A) Vacuum filtration process B) Several dried cellulose acetate filters
Once the filters are dry, they can be cut into arbitrary shapes. This is a powerful benefit to the
vacuum filtration method since the electrodes can be pre-patterned, thus photolithography is
unnecessary. Target substrates (glass, PET, quartz, SiO2 on Si, copper films, etc.) are then cleaned
using a Piranha solution (if permissible) followed by successive sonication in acetone, isopropyl
alcohol, and water. The cut filters are then soaked in ortho-dichlorobenzene for 1 minute and then
16
placed on the substrate with the GO in contact with the substrate. Mild pressure is applied to
ameliorate film adhesion to the substrate. This step is shown in Figure 7 A and B. This method was
first demonstrated for CNTs by Wu et al. in 2004 and then for GO by Eda et al. in 2008 [17, 58].
Figure 7 - Vacuum filtration film transfer process: A) filtered GO films are immersed in ortho-dichlorobenzene for 1
minute. B) Filtered GO films are then placed (GO side down) directly on the target substrate. C) Samples are placed in an acetone vapour system to initialize the removal of the cellulose acetate filter.
Transferred films are placed in an acetone vapour system for at least six hours to enhance the
adhesion of the films to the substrate. Figure 8 shows a photograph of the acetone vapour system.
Afterwards, the films are immersed in an acetone bath to fully dissolve the cellulose acetate filter
leaving the GO film adhered to the substrate. The films are left in acetone overnight to ensure the
complete removal of the filter. The successfully transferred films are then soaked in IPA and
deionized water to remove the acetone, then dried under a nitrogen stream, and stored in an oven.
Figure 8 - Acetone vapour system with various samples being transferred to arbitrary substrates. PET, glass, and Si target
substrates are shown in this photograph.
17
Aquatic Method
The aquatic transfer method is similar to the vacuum filtration transfer method. GO is filtered,
dried, and cut in the same fashion but instead of transferring the films to another substrate, they
are reduced in a hydrazine vapour system for 3.5 hours. Afterwards, they are gently placed in a
basin of water. Initially, the films float on the surface of the water and after a few seconds the
cellulose filter separates from the reduced graphene oxide film. The cellulose filter sinks to the
bottom of the basin while the reduced GO films floats on the surface of the water as shown in Figure
9. The reduced GO film can then be lifted out of the water using the target substrate.
Figure 9 - Aquatic transfer method. Left: graphene is attached to the filter. Right: graphene has separated from the filter
Reduction Methods
In this work, two reduction schemes were investigated. These reduction schemes, which can be
used individually or in tandem, are adapted from the work done by Becerril et al. [44].
Hydrazine Reduction: Transferred GO films are placed in a Parafilm sealed desiccator system with 1
mL of hydrazine monohydrate. The system is heated to 40 °C to vaporize the hydrazine and the
films are exposed to the hydrazine vapour for 18 hours. Afterwards, the samples are rinsed with de-
ionized water, dried with nitrogen, and further dried in an oven for several hours. Figure 10 shows
a GO sample on glass reducing by hydrazine vapor.
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Figure 10 - Hydrazine reduction vapour system with a GO film under reduction
Thermal annealing: Transferred films are loaded into a quartz tube furnace and Argon is passed
through the system for several minutes. After which, the Argon is cut off and a vacuum of 10-5 is
made. Once the vacuum is made, the temperature is increased to at least 400 °C. More robust
substrates, like silicon and quartz, can endure higher temperatures (1000 °C) permitting a more
thorough reduction of the system; however substrates such as glass cannot withstand such high
temperatures and are annealed no higher than 500 °C. The vacuum and argon are used to minimize
deleterious atmospheric molecules (such as oxygen) which, at high temperatures, would damage
the films.
Nanotube Electrodes
The method used to fabricate SWNT electrodes is very similar to the vacuum filtration method for
reduced GO electrodes. With nanotubes, SWNTs are dispersed in a surfractant solution by
sonication and subsequently isolated by ultracentrifugation. In this case, a solution of 0.1 g of
sodium dodecyl sulfate (SDS) is mixed with 0.01 g of SWNTs (P2 from Carbon Solutions Inc.) and 10
mL of de-ionized water is sonicated for 24 hours. The sonicated solution is then ultracentrifuged for
1 hour at 30,000 RPM and the resulting supernatant is removed and stored. This solution can then
be vacuum filtered similarly to GO however it is imperative to wash the surfractant from the filter
by continuously filtering water after filtering the nanotube until the surfractant induced bubbles
are no longer present. After filtration, the films are transferred in a manner identical to GO.
19
Hybrid Structures
There are two methods employed to fabricate hybrid electrodes. The first is the bulk heterojunction
approach wherein the GO and SWNT solution are uniformly mixed prior to filtration. Thus the
resulting film is a uniform blend of both GO and SWNT. This type of electrode is referred to as a
blended electrode. The second method is to create a typical heterojunction structure by completely
filtering one solution and then filtering the other solution. This type of electrode is referred to as a
composite electrode. In order for the GO to be fully reduced, it must be exposed to atmosphere
when the film is transferred to an arbitrary substrate. If the GO layer is sandwiched between the
SWNT layer and the substrate, the hydrazine vapour reduction method will be impeded. Therefore,
when fabricated composite electrodes, GO must be filtered first.
Intermediate Substrate Transfer Method
Flexible electronic device fabrication is a critically important application for carbon based
transparent electrodes. Given that typical flexible substrates (such as PET) cannot withstand the
high temperatures experienced during the thermal reduction of GO an intermediate substrate must
be used. To accomplish this task, a method adapted from Kim et al. is used [52]. Figure 11 is a
schematic flow diagram of the transfer process and the following steps describe in detail the
procedure.
1. Cut and dried filters are soaked in ortho-chlorobenzene for 1 minute and placed on top of
the cleaned, sacrificial aluminum substrate.
2. Sample is first placed in an acetone vapour system, then an acetone bath for several hours
to remove cellulose filter.
3. GO is reduced in tube furnace at 500 °C for 3 hours as per the aforementioned reduction
steps.
4. PMMA is drop cast unto the reduced GO/Al sample and hard baked at 100 °C until PMMA
solidifies.
5. Sample is floated on a ferric chloride bath which etches the aluminum leaving the PMMA
supported reduced GO film.
6. Sample is gently removed from the ferric chloride bath left to soak in a deionized water bath
to remove residual ferric chloride. PET is then used to lift the sample out of the water bath,
contacting the PET and the reduced GO film.
7. Finally, hot acetone gently deposited onto the sample using an eye dropper to remove the
PMMA.
20
Figure 11 - Flow diagram for reduced GO film transfer method
Organic Optoelectronics Device Fabrication
Organic Light Emitting Diodes
Green emission organic material was used to fabricate OLEDs on the transparent electrodes. The
organic material consists of a blend of poly(vinylcarbazole) (PVK), 2-(4-biphenylyl)-5-(4-tert-
butylphenyl)-1,3,4 oxadiazole (PBD), tris(2-phenyl-pyridinato) iridium (Ir(ppy)3), and N,N’-
diphenyl-N,N’-bis(3-methylphenyl)-1, 1’-biphenyl-4 ,4’-diamine (TPD) into mixed solvent of 1,2-
dichloroethane and chloroform. This solution of green emitting organic material was prepared as
per Park et al. and processed in air [59]. A Poly(3,4-ethylenedioxythiophene):poly(4-
Styrenesulphonate) (PEDOT:PSS) layer was deposited as a hole transport layer between the active
layer and the transparent, hole injection electrode while LiF was deposited as an electron transport
layer between the active layer and the electron injection electrode (aluminum in this case). To
fabricate the device, the organic materials are spin-coated; first PEDOT:PSS and then the active
layer. Finally, LiF and Al are thermally evaporated (in that order) to form the electron transport
layer and metal electrode, respectively.
Organic Solar Cells
After successful fabrication, characterization, and cleaning of the transparent electrode, a 30 nm
layer of PEDOT:PSS (from CleviosTM) was spin-coated and baked at 120 °C for one hour and then
transferred to a dry, nitrogen glove box where the active layer is deposited. The P3HT (from Rieke)
and phenyl-C61-butyric acid methyl ester (PCBM) (from Sigma-Aldrich) were separately dissolved
in ortho-chlorobenzene in ratios of 10 mg/ml and 8 mg/ml for P3HT and PCBM, respectively.
21
Individual solutions were stirred at 40 °C for one hour and then subsequently mixed (a weight ratio
of P3HT:PCBM of 1:0.8) and stirred for 16 hours at 40 °C; thus completing the active layer
preparation. The active layer blend was spin-coated at 1000 RPM for 60 s, transferred to a Petri
dish, left to dry for one hour, and then baked at 140 °C for one hour. Samples were taken from the
glove box, briefly exposed to atmosphere, and placed in a thermal evaporator. Under a 10-6 mbar
vacuum, a 1 nm lithium fluoride (LiF from Sigma-Aldrich) layer and a 100 nm aluminum electrode
were evaporated completing the 0.2 cm2 device structure. Finally, solar cell characterization was
carried out using a 150 W Oriel Xenon lamp solar simulator with an AM 1.5G filter; thus exhibiting
an input power of 100 mW/cm2.
Experimental Characterization
UV-visible Spectrometer
A UV-visible spectrometer is used to characterize materials optically. A known quantity of light of a
specific wavelength is emitted from a light source, passes through the material, and the resulting
light is collected. The difference between the quantity of light emitted with the quantity of light
collected is the amount of light absorbed by the material at each specified wavelength of light. This
data can be used to calculate the absorption (in arbitrary units) and optical transparency (as a
percentage) spectrum of the material. Optical transparency is an import metric for gauging the
quality of a transparent electrode. Typically the optical transparency of a transparent electrode is
given as a percentage at 550 nm. Figure 12 shows a schematic depiction of the device operation. All
measurements are made with respect to a reference or baseline measurement.
For this work, only the absorption optical transparency spectra are measured. When measuring the
absorption spectrum, GO solution is put in a quartz container (1 x 1 x 4 cm3) and the beam of light
passes through the width of the container (i.e. it traverses a 1 cm distance of the solution). The
absorption spectrum baseline measurement is taken using deionized water since the GO is
suspended in deionized water. Thus all absorption spectra of GO are in solution and referenced to
deionized water.
Transparency measurements are taken after the GO or reduced GO film has been produced on a
transparent substrate (i.e. glass, quartz, or PET). In this case, the film-on-substrate is fixed to a
support and placed in the pathway of the beam of light such that the light passes through the film.
Furthermore, the transparency is referenced to the substrate; thus a baseline measurement is taken
for a pristine substrate. For all measurements a Cary 300 UV-visible spectrophotometer was used.
Measurements were taken from 900 nm to 300 nm for both absorption and transparency spectra.
Four Point Probe
The four point probe is a technique used to measure resistivity per unit thickness (or sheet
resistance) of a thin film material. A typical four point probe system (schematically represented in
Figure 13) has four evenly spaced probes where the outer two probes drive current into the
material and the inner two probes measure the voltage. From the measured voltage, the sheet
resistance can be calculated from Equation 4 whose derivation can be found in the appendix.
Figure 13 - Four Point Probe Schematic
Equation 4
This equation is valid for film whose lateral boundaries are 3.25 probe spacing units away from the
inter-probe spacing distance away from any probe and whose thickness is less than half the inter-
probe spacing distance.
23
The specific four point probe used is from Lucas Labs (model S-304) with 1.016 mm probe spacing
(denoted by the variable s in Figure 13) and 40.64 µm probe radius. Thus to satisfy the boundary
conditions sample sizes must be on the order of cm2. The precise geometry of the minimum
measurable area (based on 3.25 probe spacing units) is shown to scale in Figure 14, gives rise to a
minimum area of 0.55 cm2.
Figure 14 – To scale minimum sample size for accurate four point probe measurements using Lucas Labs probe head
Atomic Force Microscopy
Atomic force microscopy (AFM) is a topographical measurement of a surface with nanometer
resolution. Whereas conventional microscopes rely on resolving and focusing photons or electrons
to view a surface, AFM quantifiably feels the surface. A typical AFM design uses a pointed cantilever
with tip dimensions on the order of microns, as shown in Figure 15. The tip is brought into such
proximity (by piezoelectric material) of the surface that the inter-atomic forces (Van der Waals,
electrostatic, etc.) of the surface interact with the tip causing it to deflect (dF). A laser, pointed on
the cantilever, is used to detect this deflection. When the inter-atomic forces cause the cantilever to
deflect, the angle of reflection of the laser changes accordingly (dθ) and these angular changes of
the laser are detected by a photodetector.
24
Figure 15 – Schematic operation of a typical AFM device
Figure 16 - AFM Probe tip from APPNano
AFM has the advantage of measuring in three dimensions whereas optical and electronic
microscopies yield two dimensional figurations. Thus, surface roughness and other topographical
analysis are permissible. Whereas scanning electron microscopy (SEM) requires a conductive
surface, AFM has no such requirement. Furthermore, AFM can be done in ambient conditions and
does not require the vacuum conditions of SEM. Conversely, AFM has the disadvantage of longer
scan times (several minutes at least) and smaller scan areas (100 x 100 µm2). Under some
conditions, AFM can achieve atomic resolution; however, in general and in this work the AFM
resolution is of several nanometers.
For this work the NSCRIPTOR DPN from NanoInk was used in close contact (tapping) mode to take
the AFM measurements. ACL silicon tips were used from APPNano (shown in Figure 16). Finally, all
AFM images were taken with maximum resolution, 1024 x 1024 pixels.
25
Raman Spectroscopy
Raman spectroscopy is a measurement of the Raman scattering of photons or equivalently the
vibrational modes of a system. It can be used to identify molecules since vibrational modes are
specific to chemical bonds. The scattering of photons can occur elastically or inelastically. In elastic
scattering, or Rayleigh scattering, incident photons and scattered photons have the same energy,
frequency, and wavelength and thus the change in energy is zero. Two forms of inelastic photon
scattering include fluorescence and Raman scattering. Fluorescence completely absorbs the photon
and over a specific decay life time the photon will be re-radiated. In Raman scattering, the electron
becomes excited to a virtual energy state and is immediately re-radiated. Furthermore, Raman
scattering occurs for all frequencies whereas fluorescence has specific resonant frequencies.
Figure 17 - Energy diagram comparing Rayleigh and Raman scattering mechanisms
To exploit Raman scattering as a spectroscopic tool, a monochromatic laser is used to excite
electrons which will re-radiate elastically and inelastically. Elastically re-radiated photons can be
filtered and a photodetector is used to measure the wavelength of the Raman scattered photons.
Typical Raman spectrographs are plotted in counts (i.e. number of photons) against the difference
in wavelength of the monochromatic laser and the scatter photons (in units of cm-1). For this work,
a confocal Raman microscope was used (Renishaw inVia) with a 514.5 nm pumping laser. For each
sample, several measurements were taken, at various points on the surface of the film, and
averaged. Using various software programs (OMNIC, MatLab, and Excel) the data was smoothed
and the background (i.e. substrate signature) was removed.
26
X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) can be used to measure the elemental composition and
chemical bonding of a system. In XPS, a material is irradiated with x-rays in an effort to liberate
bound electrons from their material and the quantity of liberated electrons and their kinetic energy
are measured. Since different elements and bonding configuration give rise to electrons with
differing binding energies, one can quantifiably determine the element and bonding configuration
based on the kinetic energy of a free electron by calculating the difference in kinetic energy and
input energy. Finally, XPS is a surface measurement, yielding information regarding 1 – 10 nm of
the material.
The binding energy of a system can be calculated from XPS measurements given a known material
work function. This is because incident x-ray photons excite core shell (such as the s orbital)
electrons to vacuum and measures the resulting kinetic energy. The photon energy less the kinetic
energy of the photoelectron and the work function of the XPS photoelectron detector material is the
binding energy of the material Equation 5. In XPS measurements, photon energy and detector work
function are known and the kinetic energy is measured. This equation is depicted as an energy
diagram in Figure 18.
Equation 5
In a focused, narrow band scan one is able to determine the elemental composition and bonding
structure and this is achieved by fitting known responses or peaks (such as the response of a C-O
bond or C-C bond) to reconstruct the signal. These fitted peaks represent the contribution of
specific bonding structures to the overall signal; thus the peak location (in eV) identifies the specific
bonding structure and the relative intensity identifies percent contribution to the composition of
the material. For this work, an ESCALAB MKII from VG Scientific was used. An Mg Ka (1253.6 eV) X-
ray source at 300 W (15 kV, 20 mA) power was used to take the measurements. The measured
surface area was 2 mm x 3 mm with a surface penetration depth of 50 – 100 Å.
27
Figure 18 - XPS Diagram: Since the sample and the spectrometer share the same Fermi level, the relative vacuum levels of
the sample and the spectrometer differ. Thus, the measured kinetic energy of the photoelectron is the photon energy minus the sum of the spectrometer work function and the binding energy. Since photon energy and the spectrometer work function are controllable and the electron’s kinetic energy is measureable, the binding energy can be calculated.
28
Chapter 5 Spectroscopic Characterization of GO and reduced GO films
In the process of fabricating hybrid CNT/reduced GO transparent electrodes, reduced graphene
oxide must first be made. Thus, it is imperative to validate and verify the quality of the produced
reduced GO material. Graphene is most conclusively identified by its room temperature quantum
hall effect [60], however this identification technique limits its post-identification applications due
to the required Hall bar contact geometry. Also, this technique has been verified only for pristine
graphene and not for reduced GO. Other, noninvasive techniques to verify the quality and
fabrication of GO and reduced GO are to measure the Raman spectrum and the x-ray photoelectron
spectrum (XPS). Thus, the Raman spectra and XPS of the GO and reduced GO films are measured
and compared to other work to verify the quality of the films. Upon comparison it is found that
these spectra compare favorably with other work thus indicating the successful fabrication of GO
and reduced GO.
Raman Spectroscopy
Given the topography and elemental structure of GO and reduced GO it is plausible to expect a
Raman spectra similar to graphene, but not identical. The Raman spectra of pristine graphene was
measured by Andrea Ferrari in which he did a comparative study of the Raman shift in single to few
layer graphene, graphite, and nanotubes. It was shown that graphene exhibits strong peaks at 1600
cm-1 (G peak) and 2700 cm-1 (2D peak) with the 2700 cm-1 peak being several times larger than the
1600 cm-1 peak. Graphite also exhibits these same peaks except the 1600 cm-1 peak is larger than
the 2700 cm-1 peak. Furthermore, measuring the Raman spectrum at the edge of a graphene flake
revealed a third peak at 1350 cm-1 (D peak) [61].
Since a reduced graphene oxide film is a randomly distributed cluster of stacked graphene, the
expected Raman spectrum should show a large 1600 cm-1 to 2700 cm-1 peak ratio like in graphite,
however, it should also have a strong 1350 cm-1 peak like in graphene. This is because reduced
graphene oxide is like graphite in that it is stacks of graphene however, it is unlike graphite since it
is not a crystal structure; it is a disordered graphitic stack with numerous, discontinuous edges.
While the work done by Andrea Ferrari does not include graphene oxide or other oxygenated
carbon structures, a response from the oxygen groups is expected which can be identified by the
relative change in peak intensity between the oxygen response and the other peaks. Thus the
expected result is to have large 1600 cm-1 and 1350 cm-1 peaks relative to the 2700 cm-1 peak which
are unchanging with respect to reduction and to have an response due to oxygenation which
reduces in relative magnitude after reduction.
29
Figure 19 shows the Raman spectra of GO and reduced GO. The raw data was smoothed and the
baseline (i.e. the response from the substrate) was subtracted. Table 2 – Table 4 summarize the key
findings of the Raman spectra. Both samples indicate peaks at ~1360 and ~1600 cm- 1 (D and G
peaks, respectively) as well as a broad response between 2500 and 3300 cm-1 centered around
2950 cm-1 (Ox band). The absolute Raman shift of the D and G peaks remains relatively constant
between GO and reduced GO samples: 0.3% change for G and -0.1% change for D. Furthermore, the
relative peak intensity between the G and D peak exhibits little change between GO and reduced
GO: 4.6% change. However, the Ox band exhibits an increase in relative peak intensity by 50%
(relative to the G peak) which implies that the Ox band is due to the oxygenation of graphene.
Figure 19 - Raman spectrum comparison of GO and reduced GO
Table 2 - Peak positions of reduced GO and GO Raman spectra
G
(cm-1) D
(cm-1) Plateau (L)
(cm-1) Ox
(cm-1) Plateau (R)
(cm-1) GO 1602 1358 2701 2941 3178
Reduced GO 1597 1360 2707 2953 3201
0
0.2
0.4
0.6
0.8
1
1000 1500 2000 2500 3000 3500
Inte
nsi
ty (
arb
itra
ry u
nit
s)
Wavenumber (cm-1)
GO
Reduced GOD
G
Ox
0
30
Table 3 – Left and right width half maximum (LWFM and RWHM, respectively) of D, G, and Ox peaks in reduced GO and GO Raman spectra
Reduced GO
(cm-1) GO
(cm-1) D G Ox D G Ox
LWHM 58 54 60 60 52 54 RHWM 62 46 62 70 32 70
Table 4 - Peak to peak ratios of Raman peaks in reduced GO and GO
G:D G:Ox GO 1.29 2.91
Reduced GO 1.23 4.39
In pristine graphene, there are relatively fewer unique electron transitions which can occur
compared to graphene oxide. Consequently, the Raman spectrum of pristine graphene reveals only
three intense peaks. However, the increased chemical complexity of graphene oxide gives rise to
many more Raman responses. As expected, there is little change in the D and G peaks before and
after reduction since these peaks are due to the honeycomb, carbon lattice structure of graphene
and the edge states in disordered graphene; two characteristics which are unchanged by reduction.
However, the Ox band is not found in graphite or graphene and changes after reduction. From this
data, it is not unreasonable to suppose the Ox band is due to the various oxide groups in GO. The
2700 cm-1 peak found in graphene is still present here (as the left plateau) and does not shift after
reduction.
The constancy of the D and G peaks between GO and reduced GO is consistent with other work [62-
65]. However, the Ox band surrounding 2950 cm-1 peak is inconsistent with the two studies
showing the Raman spectrum of GO above 2000 cm-1. Cuong et al. shows a 2D peak at 2655 cm-1
and a 3S peak at 2906 cm-1. These peaks are a 55 cm-1 shift from the Ox band and the left plateau in
the presented Raman data. Furthermore, Cuong et al. does not show any data above 3000 cm-1 and
thus it cannot be concluded whether a peak corresponding to the right plateau is present. Yang et
al. also show a 2D peak at 2700 cm-1, but the 3S peak found by Cuong et al. is not present nor is
there data above 3000 cm-1. While the data surrounding the Ox band may not correlate well with
other studies, there is a general inconsistency among the literature in the Raman spectra of GO
around the 2700 cm-1 peak. Furthermore, other Raman studies indicate that the Ox band signature
is a result of aromatic and aliphatic C-H stretching (i.e. C-H contamination) [66, 67]. Thus, the
regime labeled Ox band may not, in fact, be a result of oxidation. With limited data and a lack of
31
consensus among other studies it is difficult to draw any conclusions regarding the nature of the Ox
band, other than the reduced peak-to-peak ratios observed in this report. Despite the inconclusive
data surrounding the Ox band, the D and G peaks correspond well with literature and with the
expected result, thus indicating successful GO and reduced GO fabrication.
XPS
In this XPS study, GO was spin-coated on adhesion treated glass substrates and subsequently
reduced as per the methods described in Chapter 4 Experimental Methods. Since this chemical
reduction removes oxygen from the GO film, we expect a decrease in the oxygen content of the
overall system. Furthermore, the majority of the oxide groups in GO involve oxygen atoms bonded
to carbon atoms. Therefore, we focus our XPS spectrum to the C1S regime. Figure 20 and Figure 21
are the figuration of the XPS measurements. The XPS of the GO film reveals peaks at 284.9, 286.0,
287.5, and 288.9 eV which correspond with C-C, C-O, C=O, and O-C=O, respectively. C=O is the most
dominant peak with C-C and C-O peaks exhibiting similar magnitudes. This implies that C=O
bonding is the most common bond in GO followed by C-C and C-O (and to a lesser extent O-C=O).
The XPS of the reduced GO films reveals a drastic change in the relative dominance of the peaks.
Here, C-C is much more dominant while the other oxygenated carbon bonds exhibit smaller
magnitudes. The decrease in the oxygen content from GO to reduced GO can be quantified; 26.6% to
14.7% for C-O bonds and 26.2% to 11.0% for C=O. The addition of a fifth peak, 285.6 eV is due to C-
N bonding which forms during hydrazine reduction.
32
Figure 20 – X-ray photoelectron spectrum of GO
Figure 21 - X-ray photoelectron spectrum of reduced GO
By and large, the XPS data presented here correlated well with other work. While all other works
have varying peak amplitudes, the C-C peak presented here is much less pronounced in comparison
indicating the graphene is heavily oxidized. Conversely, the reduced GO spectrum correlates
extremely well with other work [43, 44, 58, 68-70]. Deoxygenated carbon rings (284.9 eV), C-O
bonds (286.0 eV), carboxyl groups (C=O, 287.5 eV), and carboxylate groups (O-C=O, 288.9) are all
aromaticity for graphene oxide films. Reduced graphene oxide films presented diminished oxide
bonds with strong C-C bonds as well as a new C-N bond peak induced by hydrazine reduction.
These results and the results from the Raman spectroscopic analysis are consistent with other,
published spectra. Ultimately, the purpose of the spectroscopic analysis was to ensure accurate
fabrication of reduced graphene oxide. Given the results of this analysis and its comparison with
other studies, it is plausible to conclude that reduced graphene oxide was successfully fabricated.
Next reduced graphene oxide transparent electrodes were fully characterized topographically,
electrically, and optically. Typical sheet resistance measurements were on the order of 10 – 100
kΩ/sq (depending on film thickness) with optical transparencies greater than 60%; data which
correlates very well with literature. However, compared to ITO which achieves a sheet resistance of
10 Ω/sq at 80% transparency these results are still inferior. Published work and experimental
evidence has shown that upon reduction, graphene oxide nanoplatelets crumple into more one or
three dimensional structures (i.e. physically stable structures). However, this crumpling will have
adverse effects on the electronic properties of the electrode thus (in part) accounting for the high
sheet resistance. Thus, preventing the structural deformation of graphene oxide upon reduction
54
will lead to improved electrical characteristics of the individual reduced graphene oxide
nanoplatelets.
To ameliorate the electrical characteristics, single-walled carbon nanotubes were mixed with the
graphene oxide nanoplatelets during the vacuum filtration process. The idea was that separating
the nanoplatelets would limit the crumpling effect upon reduction thus improving the sheet
resistance. This hypothesis was partially supported. When a relatively low ratio of nanotubes to
graphene was employed sheet resistances consistently improved by a factor of two (relative to the
parallel resistive calculation). However, when larger nanotube to graphene ratios were used an
unforeseen bulking of the nanotubes occurred wherein large quantities of nanotubes existed next
to areas relatively devoid of them. The nature of this bulking is unknown; however, the impact on
the electrical characteristic is evident. This bulking reduces the effective pathways of electrical
conduction; instead of electrons experiencing the same resistance in all directions, they experience
a lower resistance within the bulked nanotubes and thus non-uniform electrical conduction occurs.
This is evident by the increase in the sheet resistance (relative to the parallel resistance calculation)
as opposed to the decrease seen with the lower ratio electrode.
In an effort to realize flexible organic electronics, a technique to deposit fully reduced hybrid
electrodes onto a flexible substrate (PET) was used. After filtration, the hybrid film was transferred
to an aluminum substrate and then chemically and thermally reduced. PMMA was then drop cast on
the reduced hybrid film and the aluminum was etched leaving behind the hybrid film on PMMA. The
hybrid film can then be transferred to PET and the PMMA dissolved in acetone leaving behind a
fully reduced hybrid electrode on a flexible substrate. Attempts to accomplish this feat were
unsuccessful primarily because the hybrid film would not adhere well to the PET substrate and
would lift off during the PMMA removal. However, using this technique, one sample was
successfully transferred to a glass substrate and shows sheet resistances two fold lower than
identical films transferred and reduced to glass conventionally. This sheet resistance improvement
is likely due to doping from aluminum atoms and it was from this sample that the hybrid organic
solar cell was made.
Organic solar cells fabricated with hybrid electrodes showed resistive characteristics for voltages
greater than the open circuit voltage yet yielded typical an exponential trend for lower voltages.
Unfortunately, whether produced with only reduced graphene oxide or with hybrid electrodes,
power conversion efficiencies were less than 1%. OLEDs based on blended hybrid electrodes were
also fabricated with mixed results. Current density measurements were much improved over
55
reduced GO and ITO based devices, however device luminance was orders of magnitudes smaller.
This result is likely due to the relatively low optical transparency of the hybrid electrodes.
Consequently, device efficiencies were substantially reduced. Apart from the transparent electrodes
themselves, material choices in the organic solar cell and OLED could be improved. For example,
the work function mismatch between reduced GO and PEDOT:PSS impairs device performance, this
and other materials in the devices could be tailored to improve device efficiencies. Furthermore, the
OED fabrication procedure requires the organic material to be briefly exposed to air thus degrading
the organic layer and further limiting the device efficiency. Fully encapsulated devices would also
lead to improved device efficiencies.
In conclusion, a significant volume of work was done to develop and fabricate reduced graphene
oxide based transparent electrodes. Despite the relatively modest performance of the electrodes, it
was found that doping reduced graphene oxide with nanotubes leads to ameliorated results. Yet, to
fully exploit this fact, it is necessary to understand the nature of this improvement. How do the
nanotubes separate the nanoplatelets and preserve their cysrtalinity? How can this process be
engineered and controlled? Can other materials, such as nanoparticles, be used instead of
nanotubes? Answering these questions will lead to further improvement in reduced graphene oxide
based transparent electrodes. Ultimately, the full benefit of reduced graphene oxide electrodes is
found in flexible electronics. Using the unique transfer process initially developed in this thesis, one
can produce fully reduced graphene oxide based transparent electrodes on flexible substrates by
improving the procedure for PMMA removal.
While reduced graphene oxide is unlikely to become a dominant material for transparent
electrodes, it is an interesting material which demands further investigation. Because graphene
oxide is a solution processable nanostructure it lends itself to interesting electrochemical
applications. For example, graphene oxide nanoplatelets could be used in the active layer of an
organic electronic device. The fabrication procedure used in this process disperses graphene oxide
in water which is unsuitable for organic materials, but graphene oxide could be dispersed (to
varying degrees) in other solvents more suitable for organic materials. Thus, it is possible that
reduced graphene oxide could be used as a high electron affinity material in the active layer of an
organic solar cell instead of fullerenes. Furthermore, graphene oxide could be used as a semi
conducting material. Physical confinement and free electron occupation in graphene has been
shown to split the graphene band gap and graphene oxide possesses both physical confinement by
nanoplatelet structure and free electron occupation with the various bonded oxide groups.
56
Furthermore, graphene oxide absorption measurements presented herein indicate that graphene
oxide does not absorb in the high wavelength, visible spectrum but absorbs (at exponentially
increasing rates) in the low wavelength, visible spectrum to UV spectrum. However, development of
these applications requires a deeper study of the material itself. Yet the study of graphene oxide is
still very much in its infancy as is the field of graphene in general. Undoubtedly further, profound
discoveries are coming which will open new and interesting avenues for both graphene and
graphene oxide.
57
Appendix
Sheet Resistance of a four point probe
Resistance, R, can be defined as the resistivity, ρ, multiplied by the length of a material, x, and
divided by the materials surface area, A. In sheet resistance measurements, x is the spacing between
the probes. Therefore, changes in x will lead to changes in R.
Assuming the probes make contact with the material at a singular finite point, the area, A, is a
function of x. In bulk materials, this dependence is proportional to distance squared (assuming
spherical emanation of current from the probes). However, in thin film materials, this dependence
is linear with distance. Thus, the effective area seen by the current in a bulk material is the surface
area of a hemisphere of radius x while in a thin film it is the surface area of a cylinder of radius x
and thickness t.
Therefore, the thin film resistance is given by the following integral.
From the topography of the measured voltage and the resulting superposition of the currents, the
resistance can be expressed by the following:
Consequently, resistivity per unit thickness or sheet resistance is given by the following:
58
In the case of non-infinite surfaces, boundary conditions must be taken into account. Valdes
calculated several boundary conditions for four point probe measurements [88]. Two such
conditions apply to this work, where the probe tips are perpendicular to the boundary where they
are parallel to the boundary as shown in Figure 39 and Figure 40. The error induced by a boundary
was analytically solved by Valdes and the functions are plotted in Figure 41. The plots reveal that
even when measurements are taken at distances on the order of two probe spacing’s away from a
boundary, the error is less than 5%. In order to ensure a less than 1% error in any measurement
the measurement should be taken at least 3.25 probe spacing’s away from any boundary.
Figure 39 – Perpendicular (Ⱶ) Four Point Probe Boundary
Condition
Figure 40 - Parallel (ǁ) Four Point Probe Boundary
Condition
Figure 41 - Percent Error induced by the boundaries of surface on four point probe measurements
59
Organic Light-Emitting Diodes
For comparison, four OLEDs were fabricated, two using blended hybrid electrodes and two using
reduced GO electrodes. The hybrid electrodes were fabricated by filtering the same volume of GO
used to create the reduced GO electrodes with an additional 100 µl of the nanotube solution, as was
done in Figure 32. All electrodes were fabricated by vacuum filtration and deposited on glass
substrates. Current density and luminance curves for these OLEDs are shown in Figure 42 and
Figure 43, respectively. In both figures, like color indicates the same volume of GO was filtered. The
current density curves reveal the expected trend where the lower sheet resistance yielded the
higher current density. Conversely, the luminance was very low for all devices and followed the
opposite trend.
Figure 42 - Current Density vs. Voltage for various reduced GO and blended hybrid electrode based OLEDs
0
50
100
150
200
250
300
350
400
450
0 5 10 15 20 25 30
Cu
rre
nt
De
nsi
ty (
mA
/cm
2)
Voltage (V)
Hybrid 3.8 kΩ/sq 66%
Hybrid 7.30 kΩ/sq 83%
RGO 76 kΩ/sq 77%
RGO 300 kΩ/sq 87%
60
Figure 43 - Luminance vs. Voltage for various reduced GO and blended hybrid electrode based OLEDs
From comparing Figure 35 with Figure 43, it is evident that the luminance is substantially lower in
Figure 43, irrespective of the type of transparent electrode. This is likely due to damage incurred
during fabrication. During the photolithographic process, the electrodes began to flake off the
substrate resulting in physically non-uniform electrodes. Consequently, device luminance
decreased. The expected trend seen in the current density (despite the damage) is maintained
because the degree of electrode damage is constant with respect to all electrodes.
0
1
2
3
0 5 10 15 20 25 30
Lu
min
an
ce (
cd/
m2)
Voltage (V)
Hybrid 3.8 kΩ/sq 66%
Hybrid 7.30 kΩ/sq 83%
RGO 76 kΩ/sq 77%
RGO 300 kΩ/sq 87%
61
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