Plasma-assisted growth and nitrogen doping of graphene films

Article abstract of DOI:10.1063/1.4729823

Microwave plasmas were employed to synthesize single- or double-layer graphene sheets on copper foils using a solid carbon source, polymethylmetacrylate. The utilization of reactive plasmas enables the graphene growth at reduced temperatures as compared to conventional thermal chemical vapor deposition processes. The effects of substrate temperature on graphene quality were studied based on Raman analysis, and a reduction of defects at elevated temperature was observed. Moreover, a facile approach to incorporate nitrogen into graphene by plasma treatment in a nitrogen/hydrogen gas mixture was demonstrated, and most of the nitrogen atoms were verified to be pyridinelike in carbon network.

Full text of DOI:10.1063/1.4729823

Plasma-assisted growth and nitrogen doping of graphene films
C. D. Wang, M. F. Yuen, T. W. Ng, S. K. Jha, Z. Z. Lu, S. Y. Kwok, T. L. Wong, X. Yang, C. S. Lee, S. T. Lee,
and W. J. Zhang
Citation: Applied Physics Letters 100, 253107 (2012); doi: 10.1063/1.4729823
View online: http://dx.doi.org/10.1063/1.4729823
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APPLIED PHYSICS LETTERS 100, 253107 (2012)
Plasma-assisted growth and nitrogen doping of graphene films
C. D. Wang, M. F. Yuen, T. W. Ng, S. K. Jha, Z. Z. Lu, S. Y. Kwok, T. L. Wong, X. Yang,
C. S. Lee, S. T. Lee, and W. J. Zhanga)
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science,
City University of Hong Kong, Hong Kong, China
(Received 3 February 2012; accepted 4 June 2012; published online 20 June 2012)
Microwave plasmas were employed to synthesize single- or double-layer graphene sheets on copper
foils using a solid carbon source, polymethylmetacrylate. The utilization of reactive plasmas enables
the graphene growth at reduced temperatures as compared to conventional thermal chemical vapor
deposition processes. The effects of substrate temperature on graphene quality were studied based on
Raman analysis, and a reduction of defects at elevated temperature was observed. Moreover, a facile
approach to incorporate nitrogen into graphene by plasma treatment in a nitrogen/hydrogen gas
mixture was demonstrated, and most of the nitrogen atoms were verified to be pyridinelike in carbon
C
V
network. 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729823]
The exotic physical properties of graphene, such as
ultrahigh carrier mobility, tunable band gap, and quantum
confinement effect,1–3 have led to its great application poten-
tials in the devices such as field effect transistors (FETs),4
transparent electrodes,5 supercapacitors,6 and rechargeable
lithium ion batteries.7 Since graphene was first demonstrated
by Geim and Novoselov,8 various synthesis methods have
been developed, e.g., mechanical exfoliation of highly ori-
ented pyrolytic graphite,8 thermal exfoliation of purified nat-
ural graphite,9 and chemical vapor deposition (CVD).10–12
Among these approaches, CVD has been successfully
employed to grow uniformly single- or few-layer graphene
films of large-scale on copper and Ni foils.10,13,14 Epitaxial
growth of graphene on single crystal SiC and Ru sub-
strates,15,16 and graphene growth by carburization of stain-
less steel from carbon-containing gas sources such as CH4
by CVD have also been demonstrated.17
ysis. In addition, nitrogen-doping has been shown to be a
promising approach to tune the electronic properties of gra-
phene, which may lead to its potential applications in electro-
chemical biosensors, high-performance supercapacitors, and
nanoelectronic devices.21–23 A facile approach for nitrogen
doping of graphene by plasma treatment in N2/H2 gas mixture
was also demonstrated in this work, and the x-ray photoelec-
tron spectroscopic (XPS) results revealed that nitrogen
induced in the graphene lattice was dominantly in the pyri-
dinelike format.
25 lm-thick Cu foils (Alfa Aesar) of 1 ꢁ 1cm2 in size
were used as substrates for the synthesis of graphene. After
the foils were sequentially cleaned in acetone, isopropyl
alcohol, and deionized (DI) water by sonication, 120 lL
PMMA (Sigma Aldrich) solution (4% in toluene) was depos-
ited on Cu foils by spin coating at 4000 rpm for 1 min. The
PMMA/Cu films were put in a vacuum oven at 70 ꢀC for 2 h
to distill the solvent. The thickness of resulting PMMA films
was about 100 nm. The PMMA/Cu films were then subjected
to the 1.5 kW ASTeX MWCVD system. When the substrate
was heated to desired temperature (400–700 ꢀC measured by
a thermal couple just below the substrate holder), hydrogen
plasma was switched on at a microwave power of 1300 W.
The hydrogen flow rate was 100 SCCM (standard cubic cen-
timeter per minute at standard temperature and pressure),
and the total pressure was maintained at 23 Torr during the
plasma treatment. It should be noted the generation of
plasma further increased the foil temperature by about
150 ꢀC as measured by an infrared pyrometer. The duration
of plasma treatment was kept at 20 min for all samples. After
the plasma was switched off, samples were cooled down
slowly at a controlled rate of 25 ꢀC/min. To transfer the as-
grown graphene sheets, a PMMA layer was spin coated on
the graphene/Cu. The samples were baked at 100 ꢀC for
1 min, and then merged in FeCl3 solution (0.05 g/ml) at
40 ꢀC overnight to remove the Cu substrates. The graphene/
PMMA layers were transferred to SiO2/Si substrates, and the
PMMA layer was finally dissolved by acetone. For the nitro-
gen doping of graphene layers, the as-synthesized graphene/
Cu samples were treated in N2/H2 plasmas. The flow rate of
H2 and N2 were 200 SCCM and 10 SCCM, respectively. The
In contrast to gaseous carbon sources, solid carbon sour-
ces, e.g., C60,18 amorphous carbon,19 and polymethylmetacry-
late (PMMA),11,20 have also be utilized for the graphene
growth by CVD. For the PMMA source, PMMA-precoated
Cu foil was subjected to the hot zone around 1000 ꢀC of a
tube furnace fed with hydrogen and argon for 10-20 min, and
then the Cu foil with graphene was fast-cooled to room tem-
perature by removing it from the hot zone.11 It was, however,
reported by Byun et al. that no Raman signals of graphene
could be observed after repetition of the experiment for many
times, which was considered to be due to the rapid evapora-
tion of PMMA before it was decomposed and dissolved into
Cu.12 In the same work, it was further reported that graphene
could be grown by employing a Ni/PMMA/SiO2/Si sandwich
structure, where the Ni top layer functioned as a catalyst for
graphene growth and as well as a capping layer to protect
PMMA from evaporation.12 In this work, we report the syn-
thesis of graphene by microwave plasma CVD (MWCVD)
using PMMA precoated on Cu foils as a solid carbon source.
The influence of substrate temperature on the quality of gra-
phene layers was studied systematically based on Raman anal-
a)Author to whom correspondence should be addressed. Electronic mail:
apwjzh@cityu.edu.hk.
C
V
0003-6951/2012/100(25)/253107/5/$30.00
100, 253107-1
2012 American Institute of Physics
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Wang et al.
Appl. Phys. Lett. 100, 253107 (2012)
FIG. 1. (a) AFM image of a graphene sheet transferred
onto SiO2/Si wafer, (b) Raman spectrum of a graphene
sheet atop SiO2/Si substrate, (c) 2D (2662 cmꢂ1), (d) G
(1585 cmꢂ1), and (e) D band (1330 cmꢂ1) Raman mapping
images of a graphene sheet, respectively.
microwave power was 400 W, and the pressure was 12 Torr.
The duration of treatment was 2 min with the substrate only
heated by plasmas.
not be evenly distributed, which is believed to be due to the
polycrystalline nature of the Cu foil substrate. The carbon
atoms could have different diffusion rates in grain bulk and
boundaries. Scanning electron microscopy (SEM) observa-
tions revealed that the as-grown graphene layer was partly
delaminated from Cu foil substrate (image not shown here).
The boundaries between the graphene grains could be distin-
guished, and the average grain size of the as-grown polycrys-
talline graphene was estimated to be ꢃ20 lm.
Fig. 1(a) shows atomic force microscopy (AFM) image
of a graphene layer transfered onto SiO2/Si wafer (substrate
temperature set at 700 ꢀC during plasma treatment). The gra-
phene sheet was revealed to be smooth and uniform over the
observed area. The observation was performed at the edge
region of the graphene layer, and the thickness of graphene
was measured to about 1.3 nm, suggesting the as-grown gra-
phene was of single- or double-layers.8 Visible Raman spec-
troscopy (633 nm) was carried out to characterize the
graphene films. The laser spot size is about 1 lm in diameter.
A typical spectrum collected on the graphene film atop SiO2/
Si substrate is shown in Fig. 1(b) with the following bands
denoted. (i) G band at 1585 cmꢂ1 assigned to the sp2 in-
plane phonon vibrations. The integrated area of G peak (AG)
was illustrated to be proportional to the number of C-C sp2
bonds.24 (ii) 2D band at 2662 cmꢂ1 characteristic to the for-
mation of graphene.24 The intensity ratio A2D/AG was dem-
onstrated to be sensitive to the number of graphene layers
and defect density.12 The Raman spectrum in Fig. 1(b) with
A2D/AG > 1 and the full width at half maximum (FWHM) of
2D band (W2D ꢃ 31 cmꢂ1) further verifies that the graphene
film is of single- or double-layers, being consistent with
AFM observations. (iii) D and D0 bands centered at 1360 and
1620 cmꢂ1, respectively, are associated with defects in the
graphene films.25,26 The observation of relatively strong D
and D0 peaks indicates that the graphene sheets synthesized
by the plasma-assisted process are defective.
The formation mechanism of graphene on metal catalyst
substrates by thermal CVD has been widely studied.9 The
carbon species (either from gaseous or solid sources) dis-
solved into the metal bulk at an elevated substrate tempera-
ture, and then diffuse back to the metal surface and construct
graphene lattice due to surface aggregation and structure
relaxation at gradually reduced substrate temperature. In
comparison with the conventional thermal CVD processes
using polymeric solid source, hydrogen plasma, instead of
heat, was utilized to decompose the PMMA covered atop the
Cu substrate into carbon species. The decomposition could
occur in plasmas at a temperature lower than that of conven-
tional thermal CVD, thus no capping layer was required in
this case. During the plasma treatment, the Cu substrates
were subjected to a substrate temperature ranging from 500
to 850 ꢀC for 20 min that drove the decomposition of
PMMA, the recrystallization of copper foil, and dissolution
of carbon species into it. As a reference, initial preheating
and annealing of Cu substrates were generally required in a
thermal CVD process for the recrystallization of Cu foil and
the formation of Cu grains on which graphene domains were
nucleated and grown.10 As the plasma was turned off, the
carbon species dissolved in Cu diffused back to foil surface
and reconstructed to graphene structure at gradually
decreased temperatures. The graphene was suggested to
grow actually after the plasma was switched off.
Raman mapping was also performed in this work to
investigate the uniformity of the as-synthesized graphene
sheet. Figs. 1(c)–1(e) depict the 2D, G, and D band Raman
mapping obtained on the graphene sheet as-grown on Cu foil,
respectively, indicating that the graphene sheet is reasonably
uniform almost over the entire growth area. The variation of
D band intensity, nevertheless, suggests that the defects may
To support the discussion, the role of cooling process in
growing graphene was investigated. Two PMMA-coated Cu
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253107-3
Wang et al.
Appl. Phys. Lett. 100, 253107 (2012)
TABLE I. The FWHMs (W) of D, D0, G, and 2D bands and AG/A2D values
of samples grown at different temperatures.
400 ꢀC
500 ꢀC
600 ꢀC
700 ꢀC
WD
WD
WG
41.0
19.4
43.0
72.3
1.0
41.0
21.4
39.0
70.4
1.6
22.1
13.5
23.4
40.1
3.6
18.9
18.2
18.7
33.7
6.0
0
W2D
2D/AG
A
on Cu foils synthesized at substrate temperatures of 400,
500, 600, and 700 ꢀC (the temperature without switching on
the plasmas). The sample grown at 400 ꢀC showed obvious
D, D0, G peaks, and a faint and broad 2D peak, indicating
poor quality of the graphene sheet. However, it is noted that
utilizing plasmas enables growing graphene at a temperature
much lower than that of conventional CVD (400 ꢀC vs. over
1000 ꢀC). As the substrate temperature increased from 400 to
700 ꢀC, the intensities of both 2D and G peaks increased, and
their peak width reduced significantly. Meanwhile, the inten-
sity of D0 peak decreased. The FWHMs of D, G, and 2D
peaks, and A2D/AG values of the samples are summarized in
Table I. It is shown clearly that the peak width decreased and
A2D/AG ratio increased with the elevation of substrate tem-
perature. Since the peak broadening and the intensity ratio
A2D/AG have been demonstrated to be reversely proportional
to defect intensity, the above Raman observations suggest
that a higher temperature is favorable to the growth of gra-
phene sheets with reduced defect density.26–28
FIG. 2. Raman spectra of the graphene sheets grown with the cooling proc-
esses at a controlled rate of 25 ꢀC/min (black) and cooling naturally (red).
Two samples were subjected to the same plasma treatment process.
foils were treated with plasma under identical conditions,
and the substrate temperature was set at 700 ꢀC during
plasma treatment. However, after the plasma was turned off,
one sample was cooled down at a controlled rate of 25 ꢀC/
min, and another one was cooled down naturally (at a rate
approximately 50-100 ꢀC/min). Fig. 2 depicts Raman spectra
of two samples. The sample grown with the cooling rate
25 ꢀC/min presented much stronger and sharper 2D and G
peaks, implying a better crystallinity of the graphene layer
grown with controlled cooling at a lower rate. The observa-
tions revealed that the cooling process played a predominant
role in the growth of graphene, and a lower cooled rate is
preferred for the precipitated carbon to reconstruct to hexag-
onal structure. Moreover, switching off the plasma caused a
steep drop of substrate temperature by 100–150 ꢀC (e.g.,
from 850 to 700 ꢀC for the sample in Fig. 1(b)), which led to
insufficient structure relaxation and hence the formation of
defects as indicated by the appearance of D and D0 peaks.
In this work, the microwave power was varied in a range
from 800 to 1300 W, and only little influence of microwave
power on the quality of graphene sheets was revealed based
on Raman analysis. However, the substrate temperature was
found to play a predominant role in the growth of graphene
sheets. Fig. 3 shows the Raman spectra of graphene sheets
The as-grown graphene sheets were further treated in in
a 5% N2/H2 plasma to incorporate nitrogen into graphene lat-
tice. The graphene sheets were exposed to plasmas for 2 min,
and prolonged treatment was found to lead to etching of the
graphene. Fig. 4(a) shows the Raman spectra of graphene
sheets before and after treatment. The intensities of both G
and 2D peaks were found to decrease dramatically. How-
ever, the relative intensity of D peak with respect to 2D peak
increased, indicating the increase of defect density in the gra-
phene sheet after plasma treatment. XPS (VG ESCALAB
220i-XL equipped with a monochromatic Al Ka (1486.6 eV)
x-ray source) was carried out to study the state of nitrogen in
the graphene sheet. The survey spectrum in the insect of Fig.
4(b) shows clearly carbon, oxygen, and copper (substrate)
signals and a week nitrogen peak at about 398.2 eV. The ox-
ygen signal was believed to be due to the surface adsorption
of oxygen, and the percentage of nitrogen in graphene was
estimated to be about 5%. High–resolution XPS spectra of C
1s and N 1s of nitrogen-doped graphene were depicted in
Figs. 4(b) and 4(c), respectively. The C1s peak at 284.4 eV is
assigned to graphite-like sp2 C, and the small peaks at
285.0 eV and 288.2 eV could be attributed to the N-sp2 C
and N-sp3 C bonds, respectively.29 The deconvolution of N
1s signal reveals three components centered at 398.2 eV,
400.4 eV, and 403.8 eV, as shown in Fig. 4(c), which have
been demonstrated to correspond to pyridine-like N, pyrrole-
like N, and nitrites (NOx), respectively.29,30 The strongest
peak at 398.2 eV implies that most nitrogen atoms incorpo-
rated are in pyridinelike structure (N atom has two carbon
FIG. 3. Raman spectra of graphene films on Cu foils synthesized at sub-
strate temperatures of 400, 500, 600, and 700 ꢀC. The microwave power was
maintained at 1300 W for all samples.
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253107-4
Wang et al.
Appl. Phys. Lett. 100, 253107 (2012)
substrate temperature as compared to the conventional ther-
mal CVD methods. It was revealed that the cooling process
played a predominant role in the growth of graphene, and a
lower cooled rate is preferred for the precipitated carbon to
reconstruct to hexagonal structure. Raman spectroscopy and
Raman mapping were carried to evaluate the quality and uni-
formity of the graphene sheets. The growth temperature was
demonstrated to play a key role in determining the quality of
graphene sheets, and elevated temperatures were shown to
benefit the growth of graphene with reduced defects. In addi-
tion, the plasma treatment of graphene sheets in a nitrogen/
hydrogen gas mixture resulted in doping of graphene sheets
with nitrogen. The XPS analysis revealed that most of the
nitrogen atoms are in the pyridinelike structure in carbon
network.
This work was supported by National Natural Science
Foundation of China (NCFC Grant No. 61176007) and
CityU Applied Research Grant (No. ARG9667045). One of
the authors (C. D. Wang) is grateful to Professor. L. W. Liu,
Suzhou Institute of Nano-tech and Nano-bionics, Chinese
Academy of Science, for helpful suggestions.
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