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CHAPTER 4
4. Synthesis of graphene from methane, acetonitrile, xylene and
ethanol
4.1 Introduction
In this chapter, the synthesis of graphene from three different carbon precursors include gases
(methane, ethylene), liquid (ethanol, methanol) and solid (PMMA and polystyrene) using
CVD method is reported. Srivastava et al.[89] demonstrated the substrate-selective growth of
centimetre size (∼3.5 cm x1.5 cm), uniform and continuous single and few-layer graphene
films employing vacuum-assisted chemical vapor deposition on polycrystalline Cu foils using
liquid hexane as the carbon precursor and it exhibits better FET properties as compared to
exfoliated graphene. Guermoune et al. reported the synthesis of the graphene from methanol,
ethanol and propanol using CVD at 850°C. Low-purity carbon sources were used to grow
high purity, large area graphene films. Many researchers reported the growth of graphene
from solid carbon sources included PMMA [90], silicon carbide [91],amorphous carbon [92]
and highly oriented pyrolytic graphite (HOPG) [93] The solid precursors have large and
complex molecular structures. Synthesis of graphene from these precursors involves
complicated chemical reactions and processes.
Zhancheng Li et al. reported synthesis of graphene from liquid benzene precursor at low
temperature (300°C) compared to conventional high temperature. The benzene ring
resembles the basic unit of graphene. In contrast to the large molecules such as PMMA or
polystyrene, benzene molecules just need to dehydrogenate and connect to each other to form
the graphene structure. The activation energy of benzene dehydrogenation on Cu [111] is
(1.47 eV), which is lower than conventional carbon precursor [methane 1.77 eV]. Hence,
growing high-quality graphene using benzene as the carbon precursor might require lower
temperatures as compared using gas precursors such as methane, and acetylene [94].
Thus, there is value in further investigating the synthesis of high quality graphene (including
single layer) from new liquid organic precursors. This is also significant since graphene
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synthesis becomes possible without the use of expensive high purity gaseous hydrocarbons
(methane, acetylene and ethylene) [95, 96].
The modulation of electronic/electrical properties of graphene is highly important for the
development of electronic devices. Theoretical studies have revealed that substitutional
doping can alter the Fermi level and induce the metal-to-semiconductor transition in
graphene. Elements such as nitrogen and boron are ideal candidates as dopants in graphene.
Nitrogen-doped graphene (NG) has been successfully developed by direct synthesis and post
treatments. The promising applications of N-doped graphene in electronic devices, oxygen
reduction reaction, biosensing, and energy storage devices have attracted great attention [97].
Graphene synthesis using organic liquid precursors is feasible since most organic compounds
and their derivatives readily vaporize below 200°C. Doping of graphene with different
elements can be conducted by using organic precursors containing those elements. E.g.
Nitrogen and boron containing compounds such as pyridine and triethylborane for N-doped
and B-doped graphene respectively[89]. The liquid precursors can be more easily stored and
handled as compared to hydrocarbon gases. The use of inductive heating in the synthesis of
carbon nanotubes and graphene by CCVD can significantly reduce the energy consumption
and overall reaction time [64, 98]. This can lead to significant reduction in price, even while
maintaining the quality (purity and crystallinity) of carbon nanotubes and graphene.
In this chapter, we report the synthesis of large area (1 cm2 x 4 cm2), continuous graphene
films using atmospheric pressure radio frequency chemical vapor deposition (RF-CVD). The
catalyst used was polycrystalline copper foil, which is shown to have advantage of allowing
the synthesis of single to few layer graphene[60] at a reasonable cost. The different
precursors used were methane, xylene, ethanol and acetonitrile at temperatures ranging from
700–1000°C (Table 4-1).
4.2 Experimental method The two major steps involved in the synthesis of graphene are as follows
• Pre-treatment of the copper foils
• Growth of graphene on copper by CVD [method]
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4.2.1 Pretreatment of copper foil The catalyst used for graphene synthesis is copper.
Figure 4-1: Copper foils (Alfa Aesar 25μ thick) substrates for the synthesis of graphene.
The copper foil is cut with a flange and a hole in the centre to the dimension [1×1cm2] shown
in Figure 4–1 and Figure 4–2 shows the flange helps in holding the foil without causing
damage to the surface of copper while transferring and it is maintained as a reference for
positioning the foil. Also, for viewing the sample in optical microscopy the punched hole acts
as a reference point.
Figure 4-2: Schematic of the copper foil with dimension in mm The foil is placed between two glass slides to make it wrinkle free. The foil is taken from the
glass slide and it is rinsed in iso propyl alcohol (IPA) and acetone for 10 seconds respectively
to decrease the surface of organic impurities. Then the foil is dipped in acetic acid at 35°C for
10 min to remove if any oxide layer was formed and it is weighed. Before weighing, the
traces of acetic acid are cleaned using lint-free tissue paper. The cleaned copper foil is placed
in the graphite boat immediately, to avoid further formation of oxide layer, according to the
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arrangement shown in Figure 4-3 to study the deposition gradient. The graphite susceptor is
then carefully placed in the quartz tube. A summary of the procedure is shown in Figure 4-4
Figure 4-3: Arrangements of copper foils
Figure 4-4: Pretreatment steps in synthesis of graphene.
4.2.2 Graphene growth in CVD
A pressure gauge is placed in between the flow control rotameter and quartz tube as shown in
Figure 4-5. Argon gas is introduced into the one end of quartz tube and the other end of the
quartz tube is closed. So the pressure in the tube builds up and it will be indicated in pressure
gauge. After this process, the argon flow is stopped and a leak test is performed using soap
solution at every joint to identify any leakage and this process is repeated till the system is
leak proof.
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Figure 4-5: Pressure gauge for leak test If the pressure remains stable then the CVD system is considered to be leak proof and the
quartz tube is purged using argon gas with a flow rate of 600 mL/min for 20 minutes. Figure
4-6 shows the removal of oxygen and the increase in the concentration of Ar as a function of
time. These curves were obtained from a simple model considering the flow of argon and air
in the quartz tube as a plug flow (with no axial or radial counter-diffusion). It is clear from
the figure that after about 10 min, the level of oxygen becomes low enough to be undetected.
Yet the purging time for all experiments is kept at 20 min as a factor of safety before
introducing the hydrocarbon precursor. This is to prevent possible ignition or explosion of the
hydrocarbon gas in the presence of oxygen, as well as possible defects that might arise in the
graphene during its synthesis.
Table 4-1: Reaction condition of graphene synthesis Precursor Flow rate Temperature (°C) Reaction Time (min)
Methane 15 mL/min 1000 15
Acetonitrile 1 mL/hr 1000 15
Ethanol 2 mL/ min 1000 15
Xylene 1 mL/hr 1000-700 15
The temperature is then increased to 1000°C and the argon flow rate is reduced to 200
mL/min. Once the temperature reaches 1000°C, H2 at 100mL/min is introduced for 30min to
reduce the copper oxide layer from the copper foil. Then carbon precursors are introduced
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according to Table 4-1. The samples are then cooled down to room temperature in Ar
atmosphere.
Figure 4-6: Purging time chart
4.2 2 Transfer of Graphene The fabrication of samples and devices using other types of substrates is relevant for
fundamental studies or the optimization of graphene’s performance. The identification of
graphene sheets, down to one layer in thickness, with optical microscopy is possible via
colour contrast caused by the light interference effect on the SiO2 which is modulated by the
graphene layer. This makes the preparation of graphene samples and devices not only
possible but also efficient. However, the observation of a clear colour contrast requires Si
substrates with a specific SiO2 thickness (280-300nm), thus limiting the fabrication of
graphene devices to these substrates. Therefore, it is necessary to develop transfer methods
which can integrate graphene sheets with a wider variety of substrate materials [PMMA]
while also allowing an efficient identification of the graphene.
The graphene films synthesized from the Cu foils were removed by etching in an aqueous
solution of ferrous nitrate. The etching time was found to be a function of the etchant
concentration, the area, and thickness of the Cu foils. Typically, a 1 cm2 by 25-μm thick Cu
foil can be dissolved by a 0.05 g/ml ferrous nitrate solution overnight. For a 2 cm2and 25-μm
thick Cu foil, the optimized concentration of iron nitrate used was 0.083g/ml. The blank
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experiment of copper etching in ferrous nitrate is shown in Figure 4-7 and the graphene
grown on copper is etched in ferrous nitrate as shown in Figure 4-8.
We used two methods to transfer the graphene from the Cu foils: (1) The copper film is
dissolved, a substrate is brought into contact with the graphene film and it is ‘pulled’ from the
solution, (2) the surface of the graphene-on-Cu is coated with poly-methyl methacrylate
(PMMA) and the Cu is dissolved in the PMMA-graphene which is lifted from the solution.
The first method is simple, but the graphene films experiences more tear damage. The
graphene films are easily transferred with the second method to other desired substrates such
as SiO2/Si, with significantly fewer holes or cracks (<5% of the film area).
Figure 4-7: Etching of copper (without graphene) by ferrous nitrate
Figure 4-8: Etching of graphene grown copper by ferrous nitrate
Figure: 4-9: Steps involved in the transfer of graphene
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The overall steps involved in the transfer of graphene are listed below and shown in Figure
4-8 and depicted in Figure: 4-9
Remove the graphite boat out of the quartz tube slowly.
Use tweezers to gently remove the copper foil with graphene on a glass slide as shown
in Figure: 4-9a
Use a micropipette to drop coat a known amount PMMA solution in acetone
(0.0467g/mL) so that it just covers the surface of graphene as shown in Figure: 4-9 b
Carefully place the glass slide containing PMMA-coated graphene in the oven at
180˚C for 1 min to dry the PMMA coating.
Immediately remove the glass slide from the oven and place the PMMA/
graphene/copper foil onto the ferrous nitrate solution using tweezers so that it floats as
shown in Figure: 4-9c
After the completion of etching the PMMA/graphene film will remain floating on
nitrate solution where as the copper foil may or may not completely dissolve but it
will detach from the PMMA/graphene film as shown in Figure: 4-9d.
Gently scoop the PMMA/graphene from the iron nitrate solution using the required
substrate like silicon wafer or Cu grid coated with Formvar™ or glass slide
Depending on the convenience for further characterization as shown in Figure: 4-9d e
Figure 4-10: Steps in the transfer of graphene.
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4.2.3 Characterization High-resolution scanning electron microscopy (HRSEM) observations of the nanoparticles
were performed with a HRSEM Hitachi S4800 with EDX at an acceleration voltage of 5.0 kV
(This SEM instrument was used in Chapter 6, 7 & 8). Raman spectra were obtained with a
WITec Alpha 300 Confocal Raman system equipped with a Nd:YAG laser (532 nm) as the
excitation source (for used methane, acetonitrile and xlyene). X-ray photoelectron
spectroscopy (XPS) utilizing a PHOIBOS 100 analyser with an Al Ka radiation (1486.6 eV)
as an excitation source.
4.3 Results and Discussions
4.3.1 Synthesis of graphene from methane and acetonitrile The second-order Raman spectrum (2D band) of graphene is quite sensitive in determining
the number of layers of graphene [99]. The graphene layers were estimated from the FWHM
of the 2D band and the intensity ratio of the 2D and G band (I2D/IG). According to literature,
FWHM values of the 2D band < 32 cm-1 indicate single layer graphene, 33–55 cm-1 indicate
bilayer graphene and >70 cm-1 indicate few layer-graphene. Also, when the ratio of intensity
of the G and 2D bands (IG/I2D ratio) is less 0.5, then it is an indication of single-layer
graphene. Similarly, an IG/I2D ratio ~ 1 indicates bilayer and IG/I2D ratio >1.5 indicates few-
layer graphene [100, 101, 102]. The IG/ID ratio indicates the crystalline nature of graphene
and a higher value indicates a better quality material with low defects [103, 104]. The
presence of disorder in the crystalline lattice is indicated by the D band, which arises from the
A1g mode breathing vibrations of six-membered sp2 carbon rings [105].
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Table 4-2: Raman features of graphene and N-doped graphene Sample Reaction
Temperature
°C
G- Band
cm-1
D- Band
cm-1
2D– band
cm-1
2D- FWHM
cm-1*
IG/ID
cm-1*
I2D/IG
cm-1*
Methane 1000 1575 1354 2713 90±2.12 2.42±0.166 0.536±0.014
Acetonitrile 1000 1593 1337 2649 95±15.55 0.82±0.038 0.64±0.057
* Standard deviation value
Figure 4-11 indicates the Raman spectra of the graphene obtained from methane at 1000°C.
We observed peak positions at 1354, 1575 and 2713 cm-1 for D, G and 2D bands
respectively. The 2D band is the second order of the D band and its shape as well as its
position is used to identify a single layer sheet of graphene. The ratio of I2D/IG value was
0.536 ± 0.014 and 2D band FWHM value was 90±2.12 cm-1. These results indicate the
presence of good quality few-layer graphene (IG/ID was 2.42±0.166) [94, 96]. Figure 4-12 (a-
b) show SEM images of graphene in which different layers of graphene with wrinkles and
folds can be seen clearly. Figure 4-12(c) shows graphene sheets and multilayer graphitic
flakes along with nanoparticulate impurities, presumably from the catalyst (Cu) and etchant
(Fe (NO3)2[106]. Figure 4-12(d) shows an HRTEM image of graphene, in which few layer to
multilayer graphene is clearly visible (5–10 layers).
Figure 4-13 indicates the Raman spectrum of N-doped graphene, with IG/ID value of
0.82±0.038. This value is indicative of higher defect concentration, which is expected
because of the doping of nitrogen in graphene leading to a more pronounced D band. In
comparison, the IG/ID of graphene from methane was 2.42±0.166 indicating fewer defects.
Further, the Raman spectrum of N-doped graphene also shows a pronounced D’ peak, which
is absent in the graphene synthesized from methane. Since nitrogen atoms on graphene lattice
positions act as defects, the appearance of a D’ peak in the Raman spectrum can be related to
an incorporation of nitrogen [107].
The Figure 4-14 (a-b) indicates X-ray photoelectron spectroscopy of the N- doped graphene,
which is synthesized from acetonitrile at 1000°C.Figure 4-14 (a) indicates the main peak at
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284.6eV corresponds to the graphite-like sp2 C, indicating that most of the carbon atoms are
arranged in honeycomb lattice. The small peaks (Figure 4-14a) at 285.09 and 286.06eV can
be attributed to C-N bonding. (Figure 4-14b) N1s line scan of NG sample, which confirms
the presence of substitutional (400.6eV) and pyridine-like (399.41eV) nitrogen dopants[97].
The morphology analysis of N-doped graphene is shown in Figure 4-15 (a-b). Figure 4-15(a)
shows a large area graphene layer and its one edge was folded. Figure 4-15 (b) shows
graphene sheets with a darker region possibly corresponding to multi or few layers of
graphene.
Figure 4-11: Raman Spectrum of graphene from methane carbon precursor
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Figure 4-12: (a-b) SEM, (c-d) TEM image of graphene from methane
Figure 4-13: Raman Spectrum of graphene from acetonitrile carbon precursor
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Figure 4-14: XPS spectra of N-doped graphene from acetonitrile at 1000°C
Figure 4-15: TEM images of N-doped graphene from Acetonitrile at 1000°C (a-b)
4.3.2 Synthesis of graphene from xylene at different temperature (700–1000°C) Graphene was synthesized using xylene precursor in the temperature range 700–1000°C.
From the Raman analysis, the formation of single to double layer graphene at lower
temperature, and multilayer graphene at higher temperature was observed [108]. Figure 4-16
(a-d) shows the Raman features of graphene synthesized from xylene at 700-1000°C. For
graphene obtained from xylene, with an increase in synthesis temperature from 700 to
1000°C, there was an increase in the FWHM of the 2D band from 53±2.5 to 100±5.1 cm-1.
These results reveal the presence of double-layer graphene at 700°C. As the reaction
temperature increases, few-layer graphene is formed, and the 2D peak position shifts to
higher wave number. Among the samples the highest IG/ID ratio of 2.14±0.25 was obtained at
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800°C, while for other samples, the ratio was in the range of 1.31 1.41. This indicates the
presence of graphene with the best sp2-hybridized graphitic nature at 800°C.
a b
dc
Figure 4-16: Raman Analysis of graphene from xylene at 700 -1000°C
Table 4 3: Raman features of graphene synthesized from xylene at 700 – 800°C
Sample
Reaction Temperature
°C
G- Band
cm-1
D- Band
cm-1
2D – band
cm-1
2D- FWHM
cm-1
IG/ID
cm-1
I2D/IG
cm-1
1 700 1586 1326 2653 53±2.5 1.41±0.21 0.63±0.03
2 800 1561 1332 2661 65±3.1 2.14±0.25 0.92±0.05
3 900 1582 1330 2661 96±4.5 1.37±0.17 0.59±0.02
4 1000 1582 1330 2663 100±5.1 1.31 ±0.15 0.21±0.02
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Figure 4-17: SEM (a) and TEM (b-c) images of the graphene from xylene at 700°C
Figure 4-18: SEM (a, c) and TEM (b, d) images of the graphene from xylene at 800°C and 900°C
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Figure 4-19: SEM (a) and TEM (b) images of the graphene from xylene at 1000°C
The morphology analysis of the graphene samples synthesized at 700°C, 800°C, 900°C and
1000°C are obtained by SEM and TEM analysis shows in Figure 4-17 to Figure 4-19. These
results indicate a very transparent single to few layer graphene obtained from growth
temperature at 700 and 800°C.
Figure 4-17(c) shows presence of 1–3 layers of graphene sheet (700°C). Figure 4-18 (a-b)
SEM (a) and TEM (b) images of graphene from 800°C, Figure 4-18(b) indicates more
wrinkles and folded graphene sheets with less transparent as compared to graphene
synthesized at 700°C. However, Figure 4-18 (c) and Figure 4-19 (a) shows the less
transparent and more wrinkles with light dark colour graphene sheet [synthesized at 900 and
1000°C]. The same result also observed from TEM [Figure 4-18 (d) and Figure 4-19 (b)]
analysis, higher temperatures leads to the formation of fast decomposition of carbon
precursor and carbon solubility which leads to the formation of multilayer graphene.
4.3.2.1 Mechanism of graphene formation using xylene
Here reported, graphite boat with length 90mm and it is connected to K-type thermocouple,
RF-induction coil is used as a heating element. Induction heating leads to the heating of the
graphite boat surface alone. As a result, the xylene decomposes, leaving carbon deposits on a
small area since the boat length is 90mm. The decomposition of the xylene molecules is
uniform due to this induction heating process.
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Shevel'kova et al has investigated the pyrolysis of p-xylene at temperatures of 830°-1000°C
with different hydrocarbon partial pressures (argon/p-xylene=10:1, 20:1, 30:1). The
pyrolysed product contained following free radicals namely (aromatic hydrocarbon),
naphthalene, methylnaphthalene, biphenyl, diphenylmethane, diphenylethane, phenanthrene,
methyl- and dimethylphenanthrene, anthracene, methyl- and dimethylanthracene, di-p-
xylene, and dimethylstilbene. Methane and ethylene molecules were obtained from pyrolysis
of xylene at higher temperature. The formations of methane and ethylene molecule increased
at higher temperatures (830-1000°C) from 22 to 25.0 mol % and 0.1 to 1.7 mol%
respectively. At higher temperatures, ring opening of xylene molecules is favored, which
leads to the formation of non-aromatic compounds [109]. Further, the pyrolysis of the xylene
also shows 5.3 mole % of toluene as a byproduct at lower temperature (830°C, andargon:
xylene = 10:1).
Based on the above discussion, it could be concluded that at lower temperatures, aromatic
free radicals are present in significant concentrations, which deposit on copper surface. They
further undergo surface diffusion on copper until they are stabilized by London dispersive
forces and then cyclization of aromatic radical occurs by dehydrogenation. Finally, graphene
is formed on copper foil. The temperature, for the decomposition of xylene is at 1000°C,
results produced more number of free radicals with aromatic and non-aromatic compounds.
So most of the carbon radicals are deposited on copper foil, which leads to the formation of
multilayer graphene sheet.
4.3.3 Synthesis of graphene from ethanol Raman characterization was carried out on a JobinYvon Horiba LABRAM-HR 800
instrument with a laser source of 488nm (E= 2.53ev) and optical lens with optical
magnification is 50x, a spot size of 0.59μm, a single monochromator and a Peltier cooled
charge-coupled device. Raman mapping images show the typical Raman spectra of graphene
films at selected locations, the graphene films grown from ethanol as carbon precursor. All
the CVD graphene samples are etched from copper foil using ferric nitrate solution and then
transfer to silicon wafer. Raman mapping was performed over an area of 20 x 16 μm on each
sample, with each map consisting of 30 points of measurement Figure 4-20 shows the Raman
mapping of 2D band features (a) FWHM (b) intensity (c) peak position of the 2D band. The
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Figure 4-20 (a) 2D FWHM mapping (a) where size of the mapping with 30 points of the
Raman measurement on 20 x16 μm area of graphene film. The 2D FWHM maps we see that
most of the sample area had values 68–73 cm-1 (red color region), corresponding to few layer
graphene. The yellow colour region has 74-75 cm-1 and green color region is above 76 cm-1
which indicates the presence of multilayer graphene. Figure 4-20 (c) mapping of the 2D peak
position shows that the peak position at various points on the sample lie in the range of 2716
to 2720 cm-1. The peak position of the 2D band shifting towards a higher wave number is
indicative of the presence of few layers to multilayer graphene. Figure 4-20 (b) shows the
intensity of the 2D band, which lies between 3000-5500 a.u (red to green colour) and a higher
intensity is generally indicative of a better quality of graphene.
Figure 4-21(a) shows the I2D/IG ratio of 2D and G bands, which is related to number of the
layer of graphene, where the ratio ranges between 0.55-0.80 indicating few to multilayer
graphene as also inferred from the FWHM of 2D band. Figure 4-22(a, b &c) shows the
mapping of the D band features, FWHM of the D band 44±0.8 cm-1 due to presence of the
structural defects and Figure 4-22 (b) shows the intensity of the D band, red colored area
indicates the less defect compared to green colored (more defect due the graphene transferred
from copper foil to silicon wafer). The quality of the graphene sheet measured by FWHM and
the intensity of G band. Figure 4-23 (a) indicates the FWHM of the G-band with 38-44 cm-1.
A sharp G band indicates good quality of graphene sheet (lower values of FWHM of G
band). The G band intensity map revealed that most of the sample area has higher intensity
shown in yellow to green color region in Figure 4-23 (b). The G-peak position shifted 1581-
1584 cm-1, which indicates the change of the electronic properties of graphene due to the
presence of oxygen atoms. The graphene layer quality and crystallinity are determined by
intensity ratio of the G band and D band, Figure 4-21 (b) shows the mapping of IG/ID ratio,
the ratio is 0.90 -1.10 with 20×16µM (Raman mapping area). The results show most of the
area having good quality graphene sheet (light yellow to dark green colour) and few areas D
band intensity higher compared to G band intensity.
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Figure 4-20: Raman mapping of 2D-band features of graphene synthesized from ethanol at 1000°C. (a) FWHM, (b) Intensity and (c) Peak position of the 2D-band
Figure 4-21: Raman mapping of features of graphene synthesized from ethanol at 1000°C (a) Intensity ratio of 2D/G and (b) intensity ratio of G/D
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Figure 4-22: Raman mapping of D-band features of graphene synthesized from ethanol at
1000°C. (a) FWHM, (b) Intensity and (c) Peak position of the D-band.
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Figure 4-23: Raman mapping of G-band features of graphene synthesized from ethanol at
1000°C. (a) FWHM, (b) Intensity and (c) Peak position of the G-band
s Figure 4-24: TEM images of graphene from ethanol precursor
Figure 4-24 (a-b) shows the TEM image of graphene samples, which is obtained from ethanol
carbon precursor at 1000°C. Figure 4-24 (b) shows high resolution TEM image, where
observed 5-10 layer of graphene and Figure 4-24 (a) shows lager area graphene folded with
wrinkles. These results are well supported for micro Raman mapping (No. of the graphene
layers).
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4.4 Conclusions
We have synthesized graphene and N-doped graphene from different carbon precursors at
different temperatures (700–1000°C) on copper foil using atmospheric RF-CVD method.
Raman analysis indicates that few layer graphenes are obtained from methane and acetonitrile
precursor at 1000°C. The quality of the graphene as measursed by the intensity ratio of IG/ID,
for the few-layer graphene sheets obtained from methane [IG/ID is 2.42] was better as
compared to N-doped graphene [IG/ID is 0.82] due to presence of nitrogen atoms in the latter.
This was also confirmed with XPS results. Graphene was also synthesized from aromatic
carbon precursor (xylene) for the growth of graphene in the temperature range 700-1000°C
using CVD. The results indicate that an increase in the synthesis temperature favors the
formation few layer graphene sheets, while lower temperatures are more favorable for the
synthesis of single layer graphene. Graphene was also synthesized from ethanol precursor on
copper foil at 1000C and the Raman features (FWHM, peak intensity, and peak position of
the G, D, and 2D bands as well as the IG/ID and I2D/IG ratios) were mapped.
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