tuning the doping type and level of graphene with...
TRANSCRIPT
Doping Graphene
Tuning the Doping Type and Level of Graphene with Different Gold Confi gurations
Yaping Wu , Wei Jiang , Yujie Ren , Weiwei Cai , Wi Hyoung Lee , Huifeng Li , Richard D. Piner , Cody W. Pope , Yufeng Hao , Hengxing Ji , Junyong Kang , * and Rodney S. Ruoff *
Au nanoparticles and fi lms are deposited onto clean graphene surfaces to study the doping effect of different Au confi gurations. Micro-Raman spectra show that both the doping type and level of graphene can be tuned by fi ne control of the Au deposition. The morphological structures of Au on graphene are imaged by transmission electron microscopy, which indicate a size-dependent electrical characteristic: isolated Au nanoparticles produce n-type doping of graphene, while continuous Au fi lms produce p-type doping. Accordingly, graphene fi eld-effect transistors are fabricated, with the in situ measurements suggesting the tunable conductivity type and level by contacting with different Au confi gurations. For interpreting the experimental observations, the fi rst-principles approach is used to simulate the interaction within graphene–Au systems. The results suggest that, different doping properties of Au–graphene systems are induced by the chemical interactions between graphene and the different Au confi gurations (isolated nanoparticle and continuous fi lm).
1. Introduction
Graphene with its unique electrical, thermal mechanical,
optical barrier, and chemical properties has aroused wide-
spread interest. [ 1–10 ] The quantum Hall effect, [ 2 ] extremely high
carrier mobility, [ 4 ] and tunable work function make graphene
an attractive material for the fabrication of electronic devices.
Accurate control of the electrical transport properties is con-
sidered critical for the development of graphene devices.
© 2012 Wiley-VCH Verlag Gmb
DOI: 10.1002/smll.201200520
Dr. Y. Wu, W. Jiang, Prof. W. Cai, Prof. J. KangDepartment of PhysicsXiamen UniversityXiamen 361005, People’s Republic of China E-mail: [email protected]
Dr. Y. Wu, Dr. Y. Ren, Dr. W. H. Lee, Dr. H. Li,Dr. R. D. Piner, C. W. Pope, Dr. Y. Hao, Dr. H. Ji, Prof. R. S. RuoffDepartment of Mechanical Engineering andthe Materials Science and Engineering ProgramThe University of Texas at AustinAustin, TX 78712, USAE-mail: [email protected]
small 2012, 8, No. 20, 3129–3136
Surface decoration, using gas molecules or metal adatoms/
nanoparticles, can tune the band structure of graphene for
various electrical applications. [ 11–13 ] For example, several tran-
sition metals with different work functions, such as Ti, Fe, Cu,
Ag, Au, and Pt, [ 14 , 15 ] have been used for graphene band modu-
lation. Both n- and p-type conductivities can be obtained by
selecting metal dopants, and the carrier concentration can be
controlled by the quantity of metal depositied. [ 14 , 16 ] In partic-
ular, band arrangement is not only determined by work func-
tion, but also affected by the interfacial interactions, crucially
for such a thin single-layer graphene. [ 17 ] This means that the
structures of the metal would infl uence the charge transfer
between metal and graphene evidently. Therefore, fi nding
a way to control the interactions and further achieve either
doping type using only one type of metal will be meaning-
fully, especially for gold electrode materials. It would avoid
cross contamination and simplify the process. On the other
hand, the electronic properties and the interactions in the Au–
graphene systems are not fully understood so far, [ 18 , 19 ] e.g.,
the doping effect of Au on graphene is still debated, and both
n- and p-doping of graphene have been reported. [ 14 , 15 , 20–23 ]
Therefore, it is desirable to investigate the doping properties
and interactions between Au and graphene.
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Y. Wu et al.
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Figure 1 . a) The G- and b) 2D-peak positions in Raman spectra as a function of Au deposition time. The black arrow indicates the D ′ band position at 40 s.
2600 2650 2700 27501500 1550 1600 1650 1700
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In this paper, we deposited Au on the surface of mono-
layer graphene at room temperature. Using Raman measure-
ments, the electrical properties of Au-doped graphene were
investigated, while the structures of Au on the graphene
were imaged at a series of deposition stages using transmis-
sion electron microscopy (TEM). In this way, the electrical
properties of the Au–graphene system were revealed to be
dependent on the size and confi guration of Au. Based on the
above results, fi eld-effect transistors (FET) were prepared,
and in situ measurements were performed in high vacuum
to systematically characterize the electrical transport in Au–
graphene systems. The results suggested that, both the doping
type and carrier density of graphene could be tuned by fi nely
controlling the Au deposition. To understand the experi-
mental observations, the fi rst-principles calculations were car-
ried out, and it was found that the charge transfer, and thus,
the doping type were closely associated with the interactions
between graphene and the different Au confi gurations. This
work may lead to possible applications in graphene-based
electronic devices.
2. Results and Discussion
2.1. Raman Scattering Spectra
For investigating the doping effect of different Au con-
fi gurations, monolayer graphene was synthesized and then
transferred onto 285-nm-SiO 2 /Si substrates. The quality
and number of stacking layers were evaluated by Raman
30 www.small-journal.com © 2012 Wiley-VCH Verlag GmbH & Co. KGaA,
spectroscopy. [ 24 ] The D - peak is almost
undetectable, indicative of the absence
of a signifi cant number of defects. The
2D band, centered at 2676 cm − 1 with a
full-width at half-maximum (FWHM) of
34 cm − 1 , shows a symmetric Lorentz shape,
and the intensity ratio of G/2D is about
0.35. These features are typical of high-
quality, monolayer graphene. [ 25 ]
Various amounts of Au were deposited
on the graphene surface to investigate
the size-dependent interaction between
Au and graphene. Raman spectroscopy is
highly sensitive to the electronic structure
of materials, and we used it to examine
the correlation between Au deposition
and doping properties. It is an effective
method for distinguishing the doping type
by comparing the positions of the G- and
2D-peaks between doped and pristine
graphene. [ 26 ] Because the SiO 2 layer under
the graphene fi lm is insulating, its effect on
the electron distribution of graphene is not
considered here. The evolution of Raman
spectrum for graphene with increasing
Au amount is displayed in Figure 1 , with
a comparison to the as-grown graphene.
When increasingly decorated by a small
amount of Au, the G - peak blue-shifts gradually from 1582
to 1587 cm − 1 while the 2D - peak red-shifts from 2676 to
2660 cm − 1 as a function of the increasing deposition time
(0 to ∼40 s), exhibiting typical n-type doping in the low-cov-
erage deposition regime. Reverse shifts from 1587 to 1578 cm − 1
for the G-peak and from 2660 to 2672 cm − 1 for the 2D-peak are
observed after the deposition time further increases, resulting
from a reduction of the n-doped effi ciency. As the deposition
time increases to 70 s, both the G- and 2D-peak show slight
blue-shifts to 1585 and 2682 cm − 1 , respectively, suggesting a
p-type doping. These Raman results indicate the controllable
n-type and p-type doping by depositing differing amounts of
Au. In addition, the defect-induced D ′ band at ∼ 1620 cm − 1
emerges as a shoulder peak of G band at 20 s [ 29 , 30 ] and obvi-
ously increases at 40 s (denoted by the black arrow in Figure
1 a), then slightly decreases at 60 and 70 s. Considering pre-
vious theoretical predictions, [ 17 ] we suggest that the evolve-
ment of the D ′ band in our Raman spectra results from the
defects created by the chemical interaction which breaks
the translational symmetry of the graphene lattice. It is also
responsible for charge-induced renormalization of the elec-
tronic and vibrational energies, leading to the broadening
of the 2D band. Consequently, the formation of a metal–
graphene interface not only results in a redistribution of the
electrons, but also to a metal–graphene chemical interaction.
2.2. TEM Morphology
Bright-fi eld TEM images of graphene/Au, [ 29 ] corresponding
to the deposition times of 10, 20, 30, 40, 60, and 70 s are
Weinheim small 2012, 8, No. 20, 3129–3136
Tuning the Doping Type and Level of Graphene with Different Gold Confi gurations
Figure 2 . a–f) Bright-fi eld TEM images of Au nanoparticles and fi lms on graphene for the deposition times of 10, 20, 30, 40, 60, and 70 s. The inset in (a) is a high-resolution TEM image of a single Au nanoparticle, which shows the lattice structure of the (111) crystal plane of Au.
shown in Figure 2 . With short time depositions (Figure 2 a–c),
the Au atoms assembled into isolated hexagonal nanoparti-
cles, [ 30 ] distributing on the graphene surfaces with relatively
uniform size and shape. The nanoparticles were imaged in
high-resolution TEM (inset of Figure 2 a). The sixfold sym-
metrical lattices have a fringe spacing of about 0.24 nm, thus
identifying the (111) crystal planes of Au, and showing the
high crystallinity of the Au nanoparticles. The diameters of
the nanoparticles are about 20, 27, and 32 nm, with the depo-
sition times of 10, 20, and 30 s, respectively. As the deposition
time is further increased to 40 s, where the Raman spectros-
copy shift reaches a maximum, the nanoparticles are about
45 nm in size and the morphology has evidently changed to
one without a uniform hexagonal shape (Figure 2 d). Fur-
ther deposition coalesces the Au nanoparticles (Figure 2 e),
leading to a large increase in their size. The G and 2D bands
back-shift (deposition time = 60 s) from the critical values
in the Raman spectra, suggesting a gradual reduction in the
level of n-type doping as the Au nanoparticles are coalescing.
Opposite doping occurs when a thin continuous Au fi lm
starts to form (Figure 2 f); the corresponding G and 2D bands
in the Raman spectrum shift to 1585 and 2682 cm − 1 , respec-
tively (Figure 1 ). The Raman results, in conjunction with the
TEM images, demonstrate that the doping type of graphene
is clearly related to the confi guration of Au deposited on its
surface.
2.3. FET Measurements
Raman spectra and TEM images comfi rm that Au nanopar-
ticles and continuous fi lms produce n- and p-type doping
in graphene, respectively. This provides strong motivation
© 2012 Wiley-VCH Verlag Gmbsmall 2012, 8, No. 20, 3129–3136
to fabricate doping-type-tunable graphene FET devices by
controlling the Au deposition on the channel. An optical
image of a graphene FET device is shown in Figure 3 a and
b, and the schematic diagram of the device construction is
shown in Figure 3 d. The uniform color contrast of graphene
indicates uniform thickness. During the in situ FET tests, a
1.0 V direct current was applied between the source and
drain, and a variable gate bias, V g , ranging from − 100 to
+ 100 V was applied on the bottom of the SiO 2 /Si substrate.
Sequentially, the drain–source current ( I ds ) was monitored as
a function of V g through an electrical feedthrough. As-grown
graphene shows p-type behavior in the FET test due to the
pre-existing adsorbates (Figure 3 c). The I ds increases slowly
with decreasing V g . The minimum in the gate-dependent con-
ductivity defi nes the position of the Dirac point, which does
not show up within the measured range. After degassing, the
Dirac point moves to near the zero gate voltage, and the elec-
tron and hole conductions show much better symmetry in our
graphene FET devices. The slope corresponds to the mobility
( μ ) of charge carriers, which are deduced to be about 1400
and 2000 cm 2 V − 1 s − 1 for electrons and holes, respectively,
based on the equation: [ 15 ]
μ = L/(WCgVds)(�Ids/�Vg), (1)
where L and W are the channel length and width, respec-
tively, C g is gate capacitance per unit area, and V ds is drain–
source bias.
Using the same process, three FET devices A, B, and C
were prepared for electrical transport measurements. For
device A, we used a smaller heating current and longer dep-
osition distance to obtain small isolated Au nanoparticles
on the graphene channel (Figure 3 e). Figure 4 a shows the
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Y. Wu et al.
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Figure 3 . a) Optical image of graphene FET device used for electrical transport measurements, with a channel of 5 mm in width and 1 mm in length. b) Optical microscopy image of monolayer graphene taken from the channel region shown in (a). c) I ds as a function of V g taken in high vacuum ( ∼ 1.0 × 10 − 8 mbar). d) Schematic diagram of the FET device constuction. e,f) Schematic diagrams of the FET devices with Au nanopaticles (e) and Au fi lms (f) on the channels.
gate-dependent drain–source current for various deposition
times of Au. The Dirac point shifts toward negative gate volt-
ages from 0 to –6.0 V as the deposition time increases from 0
to 25 s. This is because the Fermi level moves away from the
conical Dirac point for the case of graphene contacted with Au
nanoparticles, resulting in an effective increase of electron den-
sity and hence the n-type doping. For device B, a larger heating
current was used to deposit continuous Au fi lms onto the
graphene channel (Figure 3 f). The evolution of the I ds – V g curve
at a series of deposition times is shown in Figure 4 b. In contrast
www.small-journal.com © 2012 Wiley-VCH Ve
Figure 4 . a,b) The V g dependent I ds at various Au deposition times for devic,d) The deposition-time-dependent hole and electron mobilities for devic
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to the case of Au nanoparticles on device A, the continuous
Au fi lm, acting as an acceptor on device B, produces p-type
doping in the graphene. As a result, the Dirac point shifts from
0 to + 11 V as the deposition time increases from 0 to 25 s. The
evolution of the hole and electron mobilities for devices A and
B are ploted in Figure 4 c and d, respectively. It is clearly seen
that the hole mobility of graphene gradually decreases from
1960 to 1796 cm 2 V − 1 s − 1 after p-doping whereas the electron
mobility reduces from 1118 to 1075 cm 2 V − 1 s − 1 after n-doping.
The slight suppression of carrier mobility results from the addi-
rlag GmbH & Co. KGaA,
ces A (a) and B (b). es A (c) and B (d).
10 20
15 20 25
g (V)
0s
5s
10s
15s
20s
25s
me (s)
tional impurity scattering caused by the Au
deposition. [ 31 ]
Based on the opposite doping types
of devices A and B, device C was gradu-
ally deposited with Au nanoparticles until
a continuous Au fi lm was formed, using
the same condition as that for the sample
in Raman measurements. The amount of
charge transfer was tuned by controlling
the deposition time, and the shift of the
I ds – V g curve was systematically monitored
by the corresponding FET transport meas-
urements after each deposition. The device
was found to be doped stepwise from n- to
p-type, as shown in Figure 5 a; the Dirac
point fi rst shifts to –4, –12, –17, –22, and
–26 V, and then gradually shifts back to
–25, –24, –20, –16, –8 V, passes through 0
V, and then to + 2 and + 6 V. The maximum
of Dirac point shift corresponds to the fre-
quency shift in the Raman spectroscopy
at time, t = 40 s, likewise the signifi cant
change of Au morphology can be observed
in the TEM image at this time mentioned
above. Hole and electron mobilities based
on the deposition time for device C are
Weinheim small 2012, 8, No. 20, 3129–3136
Tuning the Doping Type and Level of Graphene with Different Gold Confi gurations
Figure 5 . a) The V g dependent I ds at various Au deposition times for device C. b) The deposition-time-dependent carrier densities for devices A, B, and C. c,d) The deposition-time-dependent hole (c) and electron mobilitiy (d) for device C.
9.0x10-4
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Device A
Device B
Device C
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ole
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2V
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ploted in Figure 5 c and d, respectively. Although the mobili-
ties of device C are much lower than that of devices A and B,
the evolution of the hole and electron mobilities are similar
with the two devices that, electron mobility decreases after
p-doping while hole mobility decreases after n-doping.
According to the Dirac point shifts, the carrier density
can be estimated from the equation:
n = CgVDirac/e, (2)
where V Dirac is the Dirac point shift relative to that of
pristine graphene, and e is the electron charge. The carrier
densities as a function of deposition time for devices A, B,
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 6 . a) Top and b) side views of the atomic structures of graphene–Au nanoparticle system. c) Top and d) side views of the atomic structures of graphene–Au fi lm. The gray balls represent the carbon atoms in graphene, while the yellow, green, and purple balls represent the fi rst, second, and third layer, respectively, of the Au atoms adsorbed on top of the graphene, with the [111] direction perpendicular to the graphene surface.
small 2012, 8, No. 20, 3129–3136
and C are shown in Figure 5 b. The induced
electron concentration is estimated at
–1 × 10 12 to ∼ 2 × 10 12 cm − 2 . Minus values
indicate a concentration of hole carriers
rather than of electron carriers. Based
on the expression of the change in Fermi
energy: [ 26 ]
�EF(n) = h̄|vF|√
πn, (3)
where the Fermi velocity | v F | is 1.1 ×
10 6 ms − 1 ; the Fermi energy in graphene
can be adjusted between –0.09 and ∼ 0.12
eV, with the sign depending on downshift
or upshift. These electrical transport prop-
erties allow us to tune the Fermi level of
graphene from below to above the Dirac
point; that is, the conductivity type and
carrier density can be selected by control-
ling the Au deposition.
2.4. First-Principles Calculations
To interpret the doping effect of Au nano-
particles/fi lm on graphene, the interac-
tions between graphene and different
confi gurations of Au adsorbates were simulated by the
fi rst-principles calculations. Based on the optimized models
in Figure 6 , the equilibrium separations between Au and
graphene, the binding energies, and the work functions were
calculated, as listed in Table 1 . Compared with the graphene–
Au fi lm, the graphene–Au nanoparticle system has smaller
equilibrium separation and larger binding energy, which also
implies stronger chemical interactions. [ 17 ]
Using the calculated work functions and equilibrium dis-
tances in Table 1 , the band alignments of both contact models
are ploted and shown in Figure 7 a and b, with the vacuum
level and the orginal work function (or Fermi energy) of Au
W Au fi xed as a reference. In the graphene–Au particles contact
system, the difference between W G-Au particle
and W Au defi nes a potential change Δ V of
0.83 eV, which is mainly generated by the
interfacial chemical interaction. [ 17 ] Com-
pared with the orginal work function of
graphene W G , the value W G-Au particle is
lower for about 0.42 eV ( W G-Au particle – W G =
–0.42 eV), leading to an upshift ( Δ E F ) for
the graphene Fermi energy. Hence, the
graphene is n-doped by Au particle. As
graphene contacts with Au fi lms, Δ V is only
0.21 eV deduced from the smaller differ-
ence between W G-Au fi lm and W Au . A 0.20 eV
increase of W G-Au fi rm than W G ( W G-Au fi lm –
W G = 0.20 eV) determines a downshift of
graphene Fermi energy, and deminstrates
that the graphene is p-doped by Au fi lm. It
could be found from the two illustrations
that the chemical interaction induced Δ V
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Y. Wu et al.
31
full papers Table 1. Calculated equilibrium separations, binding energies, and work functions of graphene (G)–Au nanoparticle/fi lm systems.
G Au G–Au nanoparticle
G–Au fi lm
Equilibrium
separation d [Å]
3.10 3.58
Binding energy Δ E [eV] 4.690 5.102 0.036 0.015
Work function W [eV] 4.275 4.893
plays a negative role on the work-function-difference-induced
electron transfer between Au and graphene. As mentioned
above, graphene has stronger chemical interactions with the
Au particles, which creates a much higher Δ V in the graphene–
Au particle model than in the graphene–Au fi lm, and causes
these two systems to have opposite doping types.
The directions of charge transfer is also confi rmed by the
effective potentials of the whole systems. The effective poten-
tials between graphene and Au comprises three terms: [ 32 ]
υeff (r ) = −∫
dr ′ n+(r ′)|r − r ′| +
∫dr ′ n−(r ′)
|r − r ′| + υxc(r ), (4)
where r is the atomic radius, the fi rst two terms represent the
electrostatic potentials caused by the positive background
and by the electronic density, and v xc (r) is the exchange-cor-
relation potential. Figure 7 c and d show the average effective
potentials along the [0001] direction for the graphene–Au
nanoparticle and fi lm confi gurations, respectively. The
34 www.small-journal.com © 2012 Wiley-VCH Ve
Figure 7 . a,b) The band alignment illustrations of graphene–Au nangraphene–Au fi lm (b) systems, using the calculated work functions and eqlisted in Table 1 . The vacuum level and the orginal work function of Au (reference. c,d) Average effective potentials in graphene–Au nanoparticle (cfi lm (d) systems along graphene [0001] direction.
macroscopic averages of each layer were performed using
macroscopic average techniques, which allows a general
description of the effects of interfacial perturbations. [ 33 , 34 ]
In each curve, the left valley represents the potential of Au,
while the right valley represents the potential of graphene.
As shown in Figure 7 c, the effective potential is inclined in
the graphene. Driving from the potential difference, the elec-
trons transfer from the Au nanoparticles to graphene, which
creates a built-in electric fi eld in the same direction. The
donated electrons will raise the Fermi level, inducing n-type
doping in the graphene, which agrees well with the band illus-
tration of Figure 7 a. In contrast, graphene exhibits a relatively
higher effective potential when contacting Au fi lm, as shown
in Figure 7 d. The electrons are inclined to transfer from
graphene to the Au fi lm, resulting in a built-in electric fi eld
directed to the Au from graphene. The Fermi level will down-
shift due to the injected holes, which produces p-type doping
in the graphene, and is consistent with the band structure in
Figure 7 b. These theoretical calculations reveal a close rela-
tionship between the doping type and the interactions within
the graphene–Au systems; qualitatively our experimental
results could be interpreted as graphene being n-doped by
isolated Au nanoparticles and p-doped by a continuous fi lm.
3. Conclusion
Au nanoparticles and fi lms were deposited onto graphene
surfaces at room temperature. Raman spectra indicated that
graphene was n- and p-doped by low and high coverage of
rlag GmbH & Co. KGaA
oparticle (a) and uilibrium distances W Au ) are fi xed as a ) and graphene–Au
Au, respectively. The morphology of Au on
graphene imaged by TEM suggested that
isolated Au nanoparticles yielded n-type
doping while continuous Au fi lms resulted
in p-type doping. Accordingly, graphene
FETs were fabricated, in which both the
doping type and carrier density could
be tuned by fi nely controlling the Au
deposition. The interactions between the
graphene and Au nanoparticle/fi lm were
simulated by fi rst-principles calculations.
The results revealed that the different
doping properties observed in the experi-
ments were derived from the effective
potential difference related to the interfa-
cial interaction within graphene–Au nano-
particle and fi lm systems. This work is of
signifi cant importance for graphene band
modulation and device design.
4. Experimental Section
The fi rst-principles calculations were performed in the framework of density-functional theory (DFT), [ 35 , 36 ] within the generalized gradient approximation (GGA), using the Vienna Ab-initio Simulation Package (VASP). [ 37 ] Calculation models were periodically repeated by an atomic slab consisting of one graphene monolayer
, Weinheim small 2012, 8, No. 20, 3129–3136
Tuning the Doping Type and Level of Graphene with Different Gold Confi gurations
(128 carbon atoms, lattice constant of 2.46 Å), with a Au nanoparticle or one continuous fi lm located on the top, as shown schematically in Figure 6 a and c, respectively. The triangular lattice of Au(111) surface matches the honeycomb lattice of graphene in the unit cells, which was proven to be the most stable confi guration of graphene with Au contact. [ 17 ] An 18 Å thick vacuum layer was applied for separating the slabs to form a surface. The lateral distance between neighboring par-ticles was about 19.8 Å, which was large enough to avoid the overlap of their electronic states in the periodically expanded model. Au 5d6s was treated explicitly as valence. [ 38 , 39 ] Ions and electrons interactions were approximated by ultrasoft pseudopotentials, and the plane-wave cut-off energy was set to 330 eV. A (8 × 8 × 1) Monkhorst–Pack mesh was used to sample the Brillouin zone. A conjugate gradient algorithm was used to optimize the structures by relaxing all the atomic geometries, [ 40 ] with forces on all the atoms converged to within 0.01 eV/Å. Although the part of van der Waals forces between Au atoms and graphene are not involved in DFT calculation, there should be some direct hybridization in our models, espectially between Au 6s orbital and graphene states, because the lowest layer of Au atoms mostly locate on the T site of graphene. [ 17 , 41 ]
Large-area monolayer graphene was grown by chemical vapor deposition on 25- μ m-thick Cu foils (Alfa Aesar, item No. 13382), [ 42 ] and transferred onto 285-nm-SiO 2 /Si substrates ( p + doping, ρ = ∼ 0.002–0.005 Ω cm) using a process similar to that described pre-viously. [ 43 ] We also transferred the as-grown graphene onto 300-mesh TEM copper grids (with lacey carbon supporting fi lms, Ted Pella, Inc.), using the same transfer process. Au deposition was performed in a home-built vacuum system with a base pressure of 1.0 × 10 − 8 mbar. A high-purity Au (99.9999%) source was affi xed to a tungsten fi lament in the upper part of the vacuum chamber, and Au could be evaporated by direct current resistive heating to a temperature around 1000 ° C. By adjusting the heating current and the distance between graphene and the Au source, the deposition rate, and hence the coverage and size of Au nanoparticles could be controlled. The evolution of Raman spectrum for different deposi-tion states was detected by a confocal microprobe Raman system (WITec Alpha300, 532 nm excitation laser, ∼ 6 mW). [ 25 , 27 ] The cor-responding bright-fi eld TEM images were obtained on a JEOL2010F TEM operating at an accelerating voltage of 200 keV.
For fabricating the graphene FET devices, ∼ 500 nm thick Au fi lms were deposited on the graphene surface as source and drain electrodes, and consequently defi ned a transport channel of 5 mm in width and 1 mm in length between the two electrodes. The FET transport measurements were performed in the vacuum chamber where the graphene FET device was attached to a ceramic heater with a temperature tunable up to 150 ° C. Before the deposition and tests, the devices were degassed for 2 h by in situ annealing at 100 ° C to eliminate the pre-existing adsorbates (e.g., H 2 O, O 2 and/or other molecules), and then cooled down to room temperature. Using a similar deposition process, three FET devices A, B, and C were pre-pared with controlled heating currents and deposition distances.
Acknowledgements
We appreciate support from the National “973” Program of China (Grant Nos. 2012CB619301 and 2011CB925600), the National
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Received: March 8, 2012 Revised: June 11, 2012Published online: July 24, 2012
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