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Charge transfer induced chemical reaction of tetracyano-p-quinodimethane
adsorbed on graphene
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Published on 28 August 2012 on http://pubs.rsc.org | doi:10.1039/C2RA21756B
Yun Qi,* Ursula Mazur and K. W. Hipps*
Received 9th August 2012, Accepted 24th August 2012
DOI: 10.1039/c2ra21756b
Raman spectroscopy was used to study the interaction of 7,7,8,8-tetracyano-p-quinodimethane
(TCNQ) deposited on graphene films grown by Chemical Vapor Deposition (CVD). Different
thickness layers (1 ML to 7 ML) of TCNQ deposited on single layer graphene were investigated. A
distinct blue shift of the G band of graphene was observed depending on the TCNQ coverage, which
indicated charge transfer from graphene to TCNQ. No charge transfer phenomenon was observed
when TCNQ was adsorbed on graphite, reflecting the intrinsic difference in the electronic structure of
graphene and graphite. The vibrational modes of TCNQ were identified. Moreover, new Raman
bands not associated with TCNQ were discovered on the TCNQ/graphene sample and assigned to
vibrational modes of a,a-dicyano-p-toluoylcyanide (DCTC21). Our observations indicate that charge
transfer occurs between graphene and TCNQ followed by chemical reaction with the atmosphere to
form DCTC21. We estimate the transfer of ¡0.03 electrons per C atom to the TCNQ adlayer. This
study clearly demonstrates the potential of TCNQ to produce p-type graphene. It also demonstrates
that the charged species at the graphene/organic layer may be readily susceptible to chemical reaction
and that the resulting surface species may be more complex than a simple negative (or positive)
ionization of the parent adsorbate. The final charge state of the graphene may depend upon the free
energy change associated with the surface chemical reaction. Any detailed understanding of the
graphene–adsorbate charged interface must include a thorough chemical study of the potential redox
reactions that can occur following deposition of the primary organic species.
1. Introduction
To prepare graphene for electronic devices, it is necessary to
modulate the type and concentration of the carriers in order
to manufacture n-type and p-type graphene. Efforts devoted to
injecting electrons or holes into graphene are demonstrated by
the studies of small gas molecule adsorption on graphene. For
example, NH3, as an n-type dopant,1 can tailor the electronic
band structure of graphene by donating electrons, whereas NO2
adsorbed on graphene behaves as an electron acceptor.2 High
vacuum and low temperature are required for the gas molecules
to achieve stable adsorption and suppress chemical reactivity.
Alternatively, the adsorption of organic molecules that have
good thermal stability and undergo charge transfer to/from
graphene provides a practical approach to engineering the
desired changes in the electronic properties of graphene. In
addition to the stability of adsorption, most organic molecules
adsorb on graphene via p–p stacking. Such noncovalent p–p
molecule/surface interactions are generally non-destructive and
will not perturb the sp2 hybridization of graphene. Thus this
Materials Science and Engineering Program and Department of
Chemistry, Washington State University, Pullman, USA 99164-4630.
E-mail: [email protected]; [email protected]; Fax: +1 509 335 5585;
Tel: +1 509 335 3033
This journal is ß The Royal Society of Chemistry 2012
form of interaction will leave graphene’s superior electronic
properties intact.3 In recent years, numerous organic molecules
have been tested theoretically and experimentally as potential
organic donors or acceptors for graphene, including nitrobenzene,4 metal-phthalocyanine (MPc),5 carboxylic acid derivatives
of PTCDA6 and tetrafluorotetracyanoquinodimethane (F4TCNQ).7,8 Photoemission and Kelvin probe studies of F4TCNQ on graphene have verified charge transfer from graphene
to thin layers of F4-TCNQ, making molecules with cyanogroups promising for use as the electron acceptor in graphene
electronic technology.
The molecule 7,7,8,8-tetracyano-p-quinodimethane (TCNQ)
has attractive properties for electronic and magnetic applications.9
TCNQ has been used as the dopant in carbon nanotubes,
successfully demonstrating its applications in molecular electronics.10 By analogy, new studies of TCNQ doped graphene will
help to develop a comprehensive knowledge of charge transfer
between molecules and this planar carbon allotrope. Recently, a
STM study reported a flat lying TCNQ monolayer adsorbed on
the graphene/Ir surface in ultra high vacuum.11 However, the
authors concluded that no charge transfer between TCNQ and
graphene had occurred. A theoretical calculation, on the other
hand, predicted a transfer of 0.34 e per molecule from graphene to
TCNQ, which was experimentally confirmed by photoemission
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spectroscopy on graphene grown epitaxially on SiC.12 In order to
clarify the discrepancy between experiment and theory, and
especially to make the determination for free standing graphene,
more studies of the interaction between TCNQ and graphene are
justified. Complicating the analysis is the fact that several possible
products of the reduction of TCNQ are known (TCNQ21,
TCNQ22 and TCNQ23) and have been well studied by vibrational
spectroscopy.13 In addition, the oxidation or reduction products
of those molecules are easily formed in ambient conditions.14,15
The species and the chemical state of the surface may be more
complicated than the original TCNQ adsorption. A detailed study
is required for species identification and mechanism analysis.
Raman spectroscopy has been successfully used to study
charge transfer by adsorbates on graphene and enhanced Raman
intensity from organic molecules on graphene was reported.16
Although it is not confirmed that charge transfer is the only
reason for the enhancement of the Raman signal, the Raman
scattering is quite sensitive to any change in the graphene band
structure.17 In this report, we used Raman scattering to discover
the chemical reaction of TCNQ on graphene induced by the
charge transfer. We observed a TCNQ coverage dependent blueshift of the graphene G band suggesting charge transfer from
graphene to TCNQ molecules. New Raman bands which are not
associated with the TCNQ adsorbate and graphene substrate
were observed and were assigned to an oxide decay product of
the TCNQ anion, a,a-dicyano-p-toluoylcyanide (DCTC21). Our
study demonstrates that the charged ions formed on graphene
readily undergo further reactions when exposed to air or other
chemical environments. The eventual chemical state of the
product species on the surface will affect the overall charge state
of the graphene, and may limit the applications of graphene
devices in certain chemical environments.
2. Experimental methods
Graphene films were prepared on polycrystalline Cu foil by
CVD. Ultrahigh purity ethylene (99.95%, Matheson) and 99.99%
purity hydrogen were used during graphene film deposition and
cooling. The detailed procedure of graphene growth and film
transfer is reported in ref. 18. A 0.25 M FeCl3 solution was used
to etch the Cu substrate overnight after growth. Both standard
glass microscope slides and a MgF2 single crystal were used as
graphene supporting substrates. While the results reported here
were observed on both glass and MgF2, we show only the MgF2
spectra. The reason for this is that MgF2 is O22 free and it
cannot contribute to the chemical reaction suggested in the
results and discussion sections. The MgF2 was purchased from
ISP OPTICS and took the form of a disk 199 in diameter and
0.07999 in thickness. MgF2 has a wide transparent range from
0.12 mm to 8.5 mm, and a stable tetragonal structure which resists
mechanical and thermal shock. The MgF2 crystal was cleaned
with a Millipore (MP) water stream and ethanol stream
(separately) before use. After picking up the graphene from the
etching solution, the graphene/MgF2 was rinsed with MP water
several times and soaked in MP water for 4 h to remove any
residual FeCl3 solution. Finally the graphene/MgF2 sample was
dried under N2 and kept in a vacuum chamber. The Raman
spectrum of pristine graphene on MgF2 shows no difference
from that of graphene on a glass slide.
10580 | RSC Adv., 2012, 2, 10579–10584
The TCNQ molecules were purchased from Aldrich Inc. with
98% purity and used as supplied. The deposition was made in a
bell jar deposition system pumped by an LN2 trapped diffusion
pump. The base pressure of the chamber is 5 6 1028 Torr. The
compound was placed in a baffled box sourced from R. D.
Mathis (SM-8) and heated resistively. The deposition was
monitored by a Quartz Crystal Microbalance (QCM) with the
growth rate controlled at 0.05 ML s21. Note that the monolayer
(ML) unit is actually an amount of added TCNQ, rather than an
indication of the amount and nature of the species on the
graphene surface. One ML is defined as the coverage of TCNQ
with a thickness at 3 Å and the density of the crystalline material
(1.42 g cm23 19) assuming a sticking coefficient of 1. This
corresponds to 0.80 nm2 per TCNQ, and is close to what we
expect to be the mean surface density reported for TCNQ on
copper and gold – 0.93 nm2 per TCNQ.20,21
Raman spectra were acquired by using 2 mW of the 514.5 nm
Ar+ laser line as an excitation source. The laser light was focused
onto the sample and the focal spot size was 10 mm. An Olympus
inverted microscope with 650 objective was used to focus the
beam on sample and to collect the scattered radiation. Raman
scattering was analyzed using an Acton spectrometer equipped
with a charge coupled device (CCD) detector. The spectral
resolution is around 5 cm21, and the accuracy is 3 cm21.
Studies of adsorption levels below one monolayer are not
reported because: 1) the signal to noise level achieved in our
experiments do not allow reliable estimates of signal strength to
be made (see error bars in Fig. 2(a)), and 2) time control of the
deposition period is made with a manual shutter. It takes about
1.5 s to open completely and about the same time to close. Thus,
there is a minimum uncertainty in the thickness of at least
0.15 ML. Further, the 0.05 ML s21 deposition rate is an average
over 120 s and we have no confidence that a 10 s exposure would
yield exactly 0.5 monolayers.
3. Results and discussion
There are several ways to prepare graphene films, including
exfoliation from graphite and epitaxial growth on SiC.22,23 The
size of exfoliated graphene is small and not always ideal for
carrying out surface reaction studies. Epitaxial graphene has
large and atomically flat surfaces to work with, but the SiC
substrate exhibits a strong Raman background, and the SiC
peaks obscure both the Raman scattering of graphene and of the
adsorbed molecule.24 An additional problem with graphene/SiC
is that the graphene cannot be physically separated from the
substrate. To make clear and straightforward identification of
the Raman peaks of TCNQ, we fabricated millimeter size
graphene films by CVD on copper, and transferred these films
onto transparent substrates (glass and MgF2) for Raman
measurements.
The spectra in Fig. 1(a) show the Raman shifts of pristine
graphene and TCNQ films adsorbed on graphene with the
TCNQ thickness ranging from 1 ML to 7 ML. Spectra are
normalized such that the 2D band has equal intensity in each
spectrum. These Raman spectra provide a great deal of
information about the graphene substrate and the species
adsorbed on the surface. First, two graphene bands marked as
the G band (near 1585 cm21) and 2D band (near 2700 cm21) can
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Fig. 1 a) Raman spectra of pristine graphene and 1 ML, 3 ML, 5 ML and 7 ML TCNQ deposition on graphene. The bands marked as G and 2D
belong to graphene. The asterisks mark the peak positions of DCTC21. The curves are shifted vertically for comparison. b) Raman spectra of graphite
and 5 ML TCNQ adsorption on graphite. The inset is an expanded view of the graphite G band and the TCNQ CLC peak.
be easily identified. These bands are quite distinct even in the
spectra in which up to 7 ML of added TCNQ is present. Second,
the Raman transitions at 1210 cm21, 1450 cm21, and 2230 cm21
are typical TCNQ Raman shifts,13,25 and their relative intensity
increases with the addition of TCNQ over the coverage range
from 3 ML to 7 ML, as expected. Third, the positions of the
peaks clearly due to TCNQ (vide infra) do not change in position
with increased TCNQ film thickness. Fourth, and most
interesting, the sample with only 1 ML of TCNQ on graphene
(orange curve) has no characteristic of TCNQ vibrations, but
instead has several discernible peaks located at y1640 cm21 and
between 1180 cm21 to 1400 cm21 (marked with asterisks in
Fig. 1). These peaks are characteristic of DCTC21, the oxidation
product of TCNQ.26 The DCTC21 peaks are still visible in the
spectra with heavier TCNQ loading, but their intensities do not
change significantly beyond 3 ML coverage. When the thickness
of TCNQ increases above 10 ML (not shown), the graphene
peaks become very weak and the DCTC21 peaks are barely
observable.
As a two-dimensional film, graphene has the same sp2 orbital
hybridization as graphite. The interest in graphene stems from
the linear energy dispersion near its Fermi level where the energy
levels intersect at K points. Because of this unusual dispersion
curve, the carriers in graphene move as massless fermions with
high velocities. Due to the unique band structure and difference
in work function, the physical properties and chemical behaviour
of graphene differs from graphite in many respects. As illustrated
in Fig. 1(b), the Raman spectrum of 5 ML TCNQ adsorbed on
graphite exhibits graphite bands (G: 1580 cm21, 2D: 2730 cm21)
and TCNQ peaks with no detectable signal from DCTC21; the
same is not true for graphene, Fig. 1(a). Also, there is no
detectable shift in the graphite G band with TCNQ adsorption.
We must conclude, therefore, that on graphite we see primarily a
simple physical adsorption of TCNQ, while on graphene the
same compound experiences a chemical reaction when adsorbed.
It is worth noting that one of the TCNQ Raman bands
(1605 cm21) overlaps the graphene G band, so it is not observed
as a separate peak in Fig. 1(a). However, the graphite G band is
downshifted by y5 cm21 relative to the G peak in graphene,
This journal is ß The Royal Society of Chemistry 2012
making the TCNQ 1605 cm21 peak on graphite clearly visible, as
shown by the inset in Fig. 1(b). Based on the comparison of
TCNQ adsorption on graphene and graphite, we believe there
exists different mechanisms for the initial adsorption of TCNQ
on the two substrates, and that these differences arise from an
electron transfer from the graphene to the TCNQ.
We summarize our experimental Raman shifts in Table 1, and
compare them with previous assignments for TCNQ and the
DCTC anion. The first column in Table 1 contains the peak
positions observed for TCNQ deposited on graphene in this
study. The second column contains the reported Raman bands
of TCNQ bulk powder with strong and very strong intensities in
the range of interest.25 The peaks of 1208 cm21, 1451 cm21,
1598 cm21 and 2221 cm21 are assigned to the C–H bending, CLC
stretching, benzene ring CLC stretching and CMN stretching
modes of TCNQ, respectively. These band positions and
assignments are consistent with our spectral measurements of
TCNQ on graphene and graphite. In the third column, the
strong Raman bands reported for NaDCTC are cited.26 Except
for the peak at 1571 cm21, we observed all the peaks of DCTC21
identified in ref. 26 in our TCNQ/graphene sample. The band at
1571 cm21 has a very low inherent intensity and is extremely
Table 1 Raman peak positions (in cm21) observed for TCNQ/graphene
and related compounds
This study
1183
1210
1290
1353
TCNQa
TCNQ21
c
Assignmenta,b
1184
CHbend
1290
1333
1451
1598
1640
1297
1339
1368
1396
1409
1583
1610
1643
2230
Ref. 25.
b
1280
1450
1605
a
DCTC21
2222
b
2235
CLCstr
CLCring str
CO
2168
2220
CMN
Ref. 26. c Ref. 13.
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weak in the authentic NaDCTC spectrum and therefore too
weak to be seen in the adsorbed DCTC. The fourth column
contains the characteristic bands of TCNQ21. We did not
observe any of those bands, indicating that if there is any
TCNQ21 on the surface it is much lower in concentration than
TCNQ or DCTC. Our observed Raman peak positions agree
well with those reported for the TCNQ and the DCTC anion,
clearly demonstrating the existence of these two molecules on the
graphene surface.
In order to further understand the mechanism of TCNQ
interaction with graphene, the change of the G band with
increasing TCNQ coverage was investigated. A stiffening of the
G peak is observed in Fig. 2, with the position of the G peak
shifting to higher frequency as a function of TCNQ thickness.
The G band of graphene is due to the doubly degenerate zone
centre E2g mode,27 and it reflects the electron–phonon coupling
at the C point. Any injection of electrons or holes will change the
E2g vibrational frequency and shift the G band.28 In Fig. 2(a),
the Raman vibrations near 1600 cm21 in spectra taken from
samples of 3 ML, 5 ML and 7 ML TCNQ loadings on graphene
Fig. 2 a) The G band of graphene as a function of TCNQ thickness.
Spectra taken from 3 ML, 5 ML and 7 ML films were deconvoluted by
Gaussian fit into graphene G bands with moving positions (round head
bars) and TCNQ CLC stretching modes (diamond head bar) with a fixed
position at 1605 cm21. b) Position of G peak as a function of TCNQ
coverage.
10582 | RSC Adv., 2012, 2, 10579–10584
were deconvoluted into two independent peaks using a Gaussian
fit. One of the peaks has a fixed position at 1605 cm21,
corresponding to the TCNQ CLC stretching mode in the benzene
ring, and the other upshifts as the TCNQ loading increases – this
shifting band is the graphene G band. Note that we have used a
fixed position for the TCNQ 1605 cm21 band since none of the
other bands associated with TCNQ shift with coverage. The
dependence of the position of the G band on coverage was
derived from Fig. 2(a) and is exhibited in Fig. 2(b). It is believed
that electron donation by, or hole injection into, graphene makes
the C–C bond length shrink and the electron–phonon interaction
stiffen.29 A similar trend in the G band evolution was observed in
a study of TCNE adsorbed on graphene.30 Thus, the G band
movement demonstrates that in this system graphene is the
electron donor and TCNQ is the acceptor. This charge transfer,
however, was not observed with TCNQ adsorption on graphite
– the G band of graphite showed no movement after the
deposition. We checked the 2D band of graphene before and
after TCNQ deposition, and we also observed an upshift of the
2D band. This movement in the 2D band happens upon
deposition of the first monolayer of TCNQ and it is not
dependant on the coverage beyond that. According to a study of
melamine adsorption on graphene, both G band and 2D bands
upshift due to charge transfer to the adsorbate; but, the 2D peak
may not be appropriate for making a quantitative estimate of the
doping level since the 2D peak is also sensitive to the existence of
any surface stress induced by the adsorption.31 We also note that
substrate stress can produce changes in the G band.32 It is
doubtful that the weak p–p interactions between TCNQ and
graphene can produce stress levels approaching those associated
with films grown on SiC.
Based on our observation and analysis of the G band of
graphene, charge transfer is confirmed in the TCNQ/graphene
system, with charge flowing from the graphene. The presence of
DCTC21 is additional evidence of the charge transfer occurring.
The capture of electrons by the neutral TCNQ molecules will
transform them into anions. However, no spectral bands
characteristic of TCNQ anions are observed. It is known that
the TCNQ anion readily reacts with oxygen33,34 and water35 in
air, and becomes oxidized to form DCTC21. The DCTC anion
has a CLO functional group replacing one of the cyano groups.
The carbonyl group transforms the symmetric D2h TCNQ parent
into an asymmetric DCTC derivative, Fig. 3. The DCTC anion
has been detected as an impurity by XPS during the study of the
TCNQ anion,26 revealing the chemical reactivity of the TCNQ
anion in air. There are several studies on the mechanism by
which TCNQ is oxidized to produce the DCTC21 ion.15,34,36,37
All of them suggest that the reaction has TCNQ anions as the
precursors. We thus postulate that charge transfer from
graphene to TCNQ produces TCNQ anions that further react
with oxygen and results in the formation of DCTC21. The
proposed process of charge transfer and chemical reaction is
illustrated in Fig. 3. If one desires to suppress this process, it
would be necessary to perform all operations under vacuum or
an inert atmosphere and to encapsulate any resulting device prior
to exposure to air.
We interpret the shift in G band with increasing TCNQ load
as depicted in Fig. 2(b), and the rapid change in the 2D band
position with 1 ML coverage, to indicate charge transfer to less
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The half cell voltage of the total process, would be
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ET0 ~
Fig. 3 Proposed process of charge transfer and chemical reaction of the
first TCNQ monolayer on graphene. Carbon is black, nitrogen blue,
oxygen red and hydrogen is gray.
than, or about, one full coverage layer of TCNQ. That is the
reason that we only see the DCTC21 peaks and graphene peaks
in Fig. 1(a) (orange curve) but no TCNQ is observed. The shift in
the G band with coverage greater than 1 ML is probably due to
non-uniform molecular coverage of the graphene surface. One
should ask how many electrons are withdrawn for graphene to
generate a near monolayer of TCNQ21, which subsequently reacts
with air to form DCTC21. Unfortunately, the structure of TCNQ on
free standing graphene is not known. We can, however, use
structures of TCNQ formed on other supports as a guide. On
Cu(111), TCNQ adopts a dense structure having 0.89 nm2 per
TCNQ.20 On Au(111) it adopts a somewhat more open structure
having 0.98 nm2 per TCNQ.21 Using an epitaxial packing motif
similar to that of TCNQ on Cu(111) and placing the molecules as
densely as the CPK models allow, one arrives at a structure with
0.73 nm2 per molecule coverage Thus, there are at least 30 graphene
C atoms per TCNQ molecule in a monolayer. Each C atom in a
single layer of graphene need only donate ¡0.03 electrons. With
increased loading, more of the exposed graphene surface is covered
until electrical equilibrium is established by the negative ions formed,
balancing the holes in the graphene layer. The TCNQ deposited on
top of the negative ions might exchange charge with the negative ion.
However, this is unlikely beyond the second layer since there is a
Franck–Condon barrier to this charge exchange and the TCNQ21
layer may insulate further layers from the graphene. In any case,
there is an electrochemical balance that must be maintained. Once
the system is exposed to air, the TCNQ ions react to form DCTC21.
One can speculate on the possibility of extra (or a reduced) net
charge transfer to graphene resulting from the subsequent chemical
reaction. If, for example, one assumes a reversible overall reaction:
2TCNQz2e{ zO2 u2DCTC{1
(1)
with Gibbs free energy DG0T
This can be written as the sum of two processes:
(5)
where F is the Faraday constant. This shift in electrical potential
(and therefore work function) would occur relative to that of the
direct formation of TCNQ2. This of course is only one of several
possible net reactions, and the adsorption energy has not been
included. Nevertheless, it does demonstrate that the Fermi level
of the final doped graphene will depend upon any chemical
processes involving the reduced species.
An obviously interesting experiment would be to measure the
Raman spectrum and surface potential of as deposited TCNQ on
graphene in UHV and then to continue those measurements
upon exposure to O2 and to H2O. Unfortunately, we do not have
facilities for performing this experiment.
4. Conclusions
We performed Raman scattering measurements on the species
formed when TCNQ is deposited on graphene and on graphite.
The product of air exposure of TCNQ/graphene, DCTC21, was
discovered on the graphene samples, revealing an oxidation–
reduction chemical reaction between graphene and the first few
layers of deposited TCNQ. The same experiment performed with
a graphite surface indicated that only physical adsorption occurs
to a significant extent. The presence of DCTC21 is a clear
indicator of charge transfer from graphene to TCNQ.
Approaching the problem from the graphene side, the variation
in the G band of graphene with TCNQ loading also demonstrated charge transfer from graphene to TCNQ. The mechanism
of chemical reaction producing DCTC21 was discussed. This
study confirmed that TCNQ can be used to p-dope graphene. It
also demonstrates that the graphene to/from adsorbate charge
transfer reactions can lead to highly reactive species that undergo
subsequent reactions when exposed to air or other chemical
environments. Any complete understanding of the graphene
charge state and surface charge transport must include a detailed
knowledge of the actual final surface species present.
Acknowledgements
This material is based upon work supported by the National
Science Foundation under grants CHE-1152951, CHE-1112156,
and CHE-1058435. We thank Dr Bing Zheng for her assistance
with the TOC graphic.
References
TCNQze{ uTCNQ{1
(2)
O2 z2TCNQ{ u2DCTC{1
(3)
DG0T = 2DG0 + DG0O
(4)
with DG0 and
with DG0O .
Then
This journal is ß The Royal Society of Chemistry 2012
0
DGO
{DGT0
~E 0 {
2F
2F
1 X. Wang, X. Li, L. Zhang, Y. Yoon, P. Weber, H. Wang, J. Guo and
H. Dai, Science, 2009, 324, 768–771.
2 S. Zhou, D. Siegel, A. Fedorov and A. Lanzara, Phys. Rev. Lett.,
2008, 101, 086402-1–4.
3 V. Kodali, J. Scrimgeour, S. Kim, J. Hankinson, K. Carroll, W. de
Heer, C. Berger and J. Curtis, Langmuir, 2010, 27, 863–865.
4 S. Saha, R. Chandrakanth, H. Krishnamurthy and U. Waghmare,
Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 155414-1–6.
5 W. Dou, S. Huang, R. Zhang and C. Lee, J. Chem. Phys., 2011, 134,
094705-1–7.
6 H. Huang, S. Chen, X. Gao, W. Chen and A. Wee, ACS Nano, 2009,
3, 3431–3436.
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7 W. Chen, S. Chen, D. Qi, X. Gao and A. Wee, J. Am. Chem. Soc.,
2007, 129, 10418–10422.
8 X. Wang, J. Xu, W. Xie and J. Du, J. Phys. Chem., 2011, 115,
7596–7602.
9 J. Gong and Y. Osada, Appl. Phys. Lett., 1992, 61, 2787–2789.
10 T. Takenobu, T. Takano, M. Shiraishi, Y. Murakami, M. Ata, H.
Kataura, Y. Achiba and Y. Iwasa, Nat. Mater., 2003, 2, 683–688.
11 S. Barja, M. Garnica, J. Hinarejos, A. Vazquez de Parga, N. Martin
and R. Miranda, Chem. Commun., 2010, 46, 8198–8200.
12 J. Sun, Y. Lu, W. Chen, Y. Feng and A. Wee, Phys. Rev. B: Condens.
Matter Mater. Phys., 2010, 81, 155403-1–6.
13 M. Khatkale and J. Devlin, J. Chem. Phys., 1979, 70, 1851–1860.
14 K. Holczer, G. Mihaly, G. Gruner and A. Janossy, J. Phys. C: Solid
State Phys., 1979, 12, 1883–1889.
15 K. Nakatani, T. Sakata and H. Tsubomura, Bull. Chem. Soc. Jpn.,
1975, 48, 2205–2206.
16 X. Ling, L. Xie, Y. Fang, H. Xu, H. Zhang, J. Kong, M. Dresselhaus,
J. Zhang and Z. Liu, Nano Lett., 2010, 10, 553–561.
17 H. Xu, L. Xie, H. Zhang and J. Zhang, ACS Nano, 2011, 5,
5338–5344.
18 Y. Qi, J. Eskelsen, U. Mazur and K. Hipps, Langmuir, 2012, 28,
3489–3493.
19 G. Mondio, F. Neri, G. Curro, S. Patane and G. Compagnini, J.
Mater. Res., 1993, 8, 2627–2633.
20 M. Kamna, T. Graham, J. Love and P. Weiss, Surf. Sci., 1998, 419,
12–23.
21 I. Torrente, K. Franke and J. Pascual, Int. J. Mass Spectrom., 2008,
277, 269–273.
22 K. Novoselov, A. Geim, S. Morozov, D. Jiang, Y. Zhang, S.
Dubonos, I. Grigorieva and A. Firsov, Science, 2004, 306, 666–669.
10584 | RSC Adv., 2012, 2, 10579–10584
23 Y. Qi, S. H. Rhim, G. F. Sun, M. Weinert and L. Li, Phys. Rev. Lett.,
2010, 105, 085502-1–4.
24 C. Coletti, C. Riedl, D. Lee, B. Krauss, L. Patthey, K. Klitzing, J.
Smet and U. Starke, Phys. Rev. B: Condens. Matter Mater. Phys.,
2010, 81, 235401-1–8.
25 A. Pawlukojc, I. Natkaniec, G. Bator, L. Sobczyk and E. Grech,
Chem. Phys. Lett., 2003, 378, 665–672.
26 M. Harris, J. Joagland, U. Mazur and K. Hipps, Vib. Spectrosc.,
1995, 9, 273–277.
27 R. Nemanich and S. Solin, Phys. Rev. B, 1979, 20, 392–401.
28 A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. Saha, U.
Waghmare, K. Novoselov, H. Krishnamurthy, A. Geim, A. Ferrari
and A. Sood, Nat. Nanotechnol., 2008, 3, 210–215.
29 L. Pietronero and S. Strassler, Phys. Rev. Lett., 1981, 47, 593–596.
30 R. Voggu, B. Das, C. Rout and C. Rao, J. Phys.: Condens. Matter,
2008, 20, 472204-1–5.
31 H. Medina, Y. Lin, D. Obergfell and P. Chiu, Adv. Funct. Mater.,
2011, 21, 2687–2692.
32 Z. Ni, Y. Wang, T. Yu and Z. Shen, Nano Res., 2008, 1, 273–291.
33 J. S. Miller, A. H. Reis, E. Gebert, J. Ritsko, W. Salaneck, L.
Kovnat, T. Cape and R. P. Van Duyne, J. Am. Chem. Soc., 1979, 101,
7111–7113.
34 M. Suchanski and R. Van Duyne, J. Am. Chem. Soc., 1976, 98,
250–252.
35 K. Mizoguchi, S. Tsuji, E. Tsuchida and I. Shinohara, Nippon
Kagaku Kaishi, 1977, 8, 395–399.
36 M. Grossel, A. Duke, D. Hibbert, I. Lewis, E. Seddon, P. Horton and
S. Weston, Chem. Mater., 2000, 12, 2319–2323.
37 K. Mizoguchi, S. Tsuji, E. Tsuchida and I. Shinohara, J. Polym. Sci.,
1978, 16, 3260–3274.
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