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Layered semiconductor molybdenum disulfide nanomembrane based
Schottky-barrier solar cellsMariyappan Shanmugam, Chris A. Durcan and Bin Yu*
Received 21st August 2012, Accepted 27th September 2012
DOI: 10.1039/c2nr32394j
We demonstrate Schottky-barrier solar cells employing a stack of layer-structured semiconductor
molybdenum disulfide (MoS2) nanomembranes, synthesized by the chemical-vapor-deposition method,
as the critical photoactive layer. An MoS2nanomembrane forms a Schottky-barrier with a metal
contact by the layer-transfer process onto an indium tin oxide (ITO) coated glass substrate. Two
vibrational modes in MoS2nanomembranes, E12g(in-plane) and A1g(perpendicular-to-plane), were
verified by Raman spectroscopy. With a simple stacked structure of ITOMoS2Au, the fabricated
solar cell demonstrates a photo-conversion efficiency of 0.7% for110 nm MoS2 and 1.8% for 220 nm
MoS2. The improvement is attributed to a substantial increase in photonic absorption. The MoS2nanomembrane exhibits efficient photo-absorption in the spectral region of 350950 nm, as confirmed
by the external quantum efficiency. A sizable increase in MoS2 thickness results in only minor change in
MottSchottky behavior, indicating that defect density is insensitive to nanomembrane thickness
attributed to the dangling-bond-free layered structure.
Introduction
Schottky-barrier solar cells have the appealing advantage of low-
cost manufacture attributable to their simple and versatile
fabrication process and thin-film-like structure (reduced active
material consumption as compared with pn or pin junctionbulk semiconductor solar cells). Schottky-barrier solar cells rely
mostly on a single semiconductor material (either p-or n-type
doped) in association with a metal, leading to energy barrier
formation and carrier dissociation at the metalsemiconductor
(MS) interface. Various MS material systems have been used
in Schottky-barrier solar cells, including Alamorphous carbon
nitride,1 Au, Al, Mg and CaPbSe quantum dots,2 AlSnS,3 and
AuGaAs.4 Recently, carbon derivatives have attracted attention
due to their promising optical/electrical properties. Schottky
junctions made by carbon nanotubeCdSe,5 grapheneCdS
nanowire,6 graphene, carbon nanotubeSi,7,8 and grapheneSi
nanowire911 have been reported. In general, MS interfacial
behavior and carrier mobility/photon absorption in the semi-conductor material are key factors that impact the overall
performance of Schottky-barrier solar cells. While a group of
semiconductors such as boron doped diamond,12 GaAs,13,14
InP,15 SiGe,16 GaP,17 GaN,18 and SiC19 can be used, the design of
an MS system with appropriate material work function differ-
ence is an essential factor to determine the Schottky-barrier
height and efficiency of carrier separation.
The development of Schottky-barrier solar cells has been
decelerated over the past decade due to several fundamental
material challenges, including interfacial oxide layer formation
and Fermi-level pinning caused by interfacial states (which are
inevitable at MS interfaces).2024
While poor interface properties play a substantial role in theimpeded improvement of energy-conversion efficiency, searching
for a new class of semiconductor materials free of interfacial
states (hence without Fermi level pinning) would be of strategic
significance to the ultimate realization of highly efficient
Schottky-barrier solar cells.
Transition-metal dichalcogenide semiconductors exhibit two
dimensional (2D) layered lattice structures with strong in-plane
covalent bonds and weak interlayer van der Waals interaction.
Molybdenum disulphide (MoS2), a layered semiconductor, has
recently attracted attention in the fields of nanoelectronics,2530
energy storage3133 and photovoltaics.34 Nanosheets of monolayer
or multilayer MoS2can be obtained via mechanical or chemical
exfoliation,35,36similar to theway graphene is produced.Recently,direct assembly of MoS2 nanosheets on SiO2 by the chemical-
vapor-deposition (CVD) method has been demonstrated.37 MoS2exhibits direct energy bandgap (1.85 eV) in monolayer and
indirect bandgap (1.3 eV) in multilayer/bulk form, respec-
tively.38 On electronic properties, it has beenrecentlyreportedthat
MoS2 shows carrier mobility values of 200 cm2 V1 s1 in
monolayer and 517 cm2 V1 s1 in a few layers, suggesting
promising conducting capacity.39,40 Most importantly, the unique
structure of layered semiconductors provides inherent advantages
of forming a smooth surface free of any dangling chemical bonds,
potentially enabling the design of highly efficientSchottky-barrier
College of Nanoscale Science and Engineering, State University of NewYork, Albany, NY-12203, USA. E-mail: [email protected]; Fax: +1 518956-7492; Tel: +1 518 956-7492
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solar cells. Various metal contacts, including Ag, Al, Au, Co and
Fe, have been investigated for the formation of Schottky-barriers
with MoS2 bulk crystals. No Fermi level pinning was observed
even for a high density of material defect (1014 cm3).41
In this paper we demonstrate solar cells employing a layer-
structured semiconductor MoS2 nanomembranemetal (Au)
Schottky barrier. The CVD-assembled MoS2 nanomembrane
also functions as the key photo-active layer. The solar cells are
made by a simple stack structure of ITOMoS2Au. Basicmaterial properties of MoS2, major solar cell device performance
metrics, and the impact of MoS2 nanomembrane thickness are
discussed. The MoS2Au Schottky-barrier interface is charac-
terized by capacitance vs. voltage (CV) measurements.
Experimental section
Large-area (up to several square centimeters) semiconductor
MoS2nanomembranes were synthesized by the CVD approach.
Fig. 1(a) is the schematic of the CVD growth furnace system
showing Ar as a carrier gas and a sulphur (S) source flowing
towards a molybdenum (Mo)-coated SiSiO2 substrate (where
vaporsolid reaction occurs to assemble the MoS2 nano-
membrane). CVD growth of MoS2silicon coated with 100 nm of
SiO2 was used as the substrate for the pre-deposition of Mo. A
thin Mo metal layer was deposited with an approximate thick-
ness of 50 nm by the electron-beam evaporation method. The
CVD growth process started with flowing Ar of 200 sccm in thequartz-tube chamber with a pressure level of 1 Torr. The
furnace was subsequently heated up to 750 C and stabilized over
one hour. Pure S powder, placed upstream in the chamber, was
heated to just above its melting temperature (115 C). The S
vapor flowed over the Mo thin film pre-deposited on the SiO 2Si
substrate for approximately 30 minutes. Vaporsolid reaction
took place at an elevated temperature (750 C), leading to the
growth of an MoS2 nanomembrane driven by increase in
enthalpy. While the MoMo bond is relatively weak (metallic in
Fig. 1 (a) Schematic of theCVD growthsetupshowingthe reaction of sulphur vaporwith Mo filmpre-depositedon SiSiO2 substratefor thegrowth of
an MoS2nanomembrane. (b) Major process steps involved in the synthesis of the MoS2nanomembrane, layer transfer, and subsequent fabrication of a
Schottky-barrier solar cell. The KOH wet etching process was used to detach the MoS 2 nanomembrane from SiO2Si substrate. (c and d) Partially
floating MoS2nanomembrane in KOH solution. (e) Free-floating MoS2nanomembrane in KOH to be transferred onto an ITO-coated glass substrate
for solar cell fabrication.
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nature), the MoS bond is a strong ionic one and therefore more
thermodynamically stable. As the temperature increases, Mo
atoms gain sufficient thermal energy (exceeding the activation
energy) and make covalent bonds with S atoms. After the
completion of CVD growth, the sample was annealed at 1000 C
for one hour to improve the crystalline quality and the unifor-
mity of the resultant nanomembrane. The major steps in the
MoS2 layer transfer sequence are depicted in Fig. 1(b) starting
from the SiSiO2substrate, Mo metal deposition, MoS2growth,and transfer to the target substrate in which the solar cells were
fabricated.
The CVD-assembled MoS2 nanomembrane was transferred
from the growth substrate (SiO2Si) to an indium tin oxide (ITO
serving as transparent electrode) coated device substrate,
involving a wet chemistry etching step of the SiO2 layer in the
growth substrate to yield a freestanding MoS2nanomembrane in
potassium hydroxide (KOH) solution. Fig. 1(c)(e) show the
optical images of the process to separate the MoS2 nano-
membrane from SiO2Si substrate. The MoS2 nanomembrane
was transferred onto the transparent conducting oxide (ITO)
substrate to fabricate the Schottky-barrier solar cell. KOH
solution with a concentration of 15 M was prepared in which theMoS2SiSiO2substrate was immersed to etch away the SiO2. A
slow and smooth KOH etching of SiO2 released the MoS2nanomembrane partially from the Si substrate. Within approx-
imately 10 minutes a completely free-floating MoS2 nano-
membrane in KOH solution was obtained. The thickness of the
MoS2 nanomembrane is determined by that of the Mo film
deposited by electron beam evaporation (it is hence scalable).
The surface morphology and thickness of the as-grown MoS2nanomembrane was characterized by Atomic Force Microscopy
(AFM, dimension 3100). Raman spectroscopy (Horiba Scien-
tific) with a laser of 532 nm was performed on the samples to
confirm the growth of MoS2 via identified vibrational modes.
After transferring the MoS2 nanomembrane onto the ITO-coated glass substrate, deposition of 50 nm of Au contact was
made by electron-beam evaporation to form a Schottky-barrier
(with MoS2). The JV characteristics of the solar cell were
measured under both dark and standard AM 1.5 illumination
conditions using an Agilent B1500A semiconductor parameter
analyzer. An Xe arc lamp was used to simulate the solar light.
The EQE characteristics of the solar cells were measured in the
wavelength range of 350950 nm using a Newport mono-
chromator (equipped with the same Xe arc lamp that was used to
measure the JV characteristics). The CV measurement was
performed on the solar cell using a B1520A-FG Multi Frequency
Capacitance Measurement Unit Module attached to an Agilent
B1500A using a frequency of 1 kHz and AC perturbation of10 mV at room temperature.
Results and discussion
Fig. 2(a) is the micrograph showing the surface morphology of
an MoS2 nanomembrane after being transferred onto the ITO
substrate, as obtained from the 2D mapping measurement of an
Atomic Force Microscope (AFM). The AFM line-scan was
carried out, from point A to point B as shown in Fig. 2(a), to
measure the physical thickness of the MoS2nanomembrane. The
step height between MoS2 and the ITO substrate, as seen in
Fig. 2(b), confirmed the thickness of the MoS2 nanomembrane to
be about 55 nm, an average value from the measured data. We
believe that the as-grown MoS2 nanomembrane on the SiSiO2substrate in our CVD process is highly uniform in thickness, as
reported by Y. Zhan et al.in a similar growth experiment.37 The
variation in AFM line-scan data (approximately 5 nm) could
arise from deformation of the MoS2 nanomembrane as a result
of the layer transfer process. We expect that micro-level wrinkles
might have been formed on the MoS2nanomembrane caused by
various types of mechanical interactions during the wet chem-
istry etching and transferring process.Material characterization using Raman spectroscopy was
conducted on two transferred MoS2 nanomembrane samples
with different physical thicknesses. While the as-assembled MoS2exhibits an average thickness of55 nm, as shown in Fig. 2(b),
stacked MoS2 nanomembranes were prepared via multiple (two
and four, respectively, in our experiment) transfers.
The samples were subsequently used for Raman spectroscopic
investigation. The expected thicknesses of the two samples are
110 nm (two transfers) and 220 nm (four transfers), respec-
tively. Prior research results show that there is a significant
difference in the signature modes in Raman spectra for
Fig. 2 (a) 2D mapping micrograph of AFM surface scan obtained on an
as-assembled MoS2 nanomembrane after transferring onto an ITO
substrate. The line-scan measurement was performed from point A to B
to evaluate the physical thickness of MoS2. (b) Measured step profile ofthe MoS2nanomembrane on ITO substrate.
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monolayer, bilayer and few-layer MoS2.37 In our experiment, a
much thicker stack of MoS2 was employed to enhance the photo-
absorption in the Schottky-barrier solar cells. Fig. 3(a) shows the
Raman spectra of the MoS2 nanomembrane samples with
thicknesses of110 nm and 220 nm, respectively. Two signif-
icant Raman active modes were identified at the wavenumbers of
383 cm1 and 408 cm1 which correspond to E12gand A1gmodes
respectively.
Fig. 3(b) displays schematically both E12g and A1g modes. Inthe E12g mode both Mo and S atoms vibrate along the in-plane
direction (yet opposite to each other), whereas in the A1g mode
the S atoms vibrate in the perpendicular-to-plane direction. The
Raman signature peaks observed in our experiment confirm both
in-plane and perpendicular-to-plane Raman active modes. As we
used relatively thick MoS2 (>100 nm), there are no blue- and red-
shifts for the E12gand A1gmodes, different from that observed in
monolayer/bilayer MoS2 nanomembranes.37 The significant
variation in the peak intensity of Raman spectra, as observed in
the two examined samples, was due to the substantial difference
in physical thickness (100 nm).
Fig. 4(a) shows the schematic cross-sectional view of the MoS2
nanomembraneAu Schottky-barrier solar cell. The MoS2nano-membrane was transferred onto an ITO substrate (serving as the
window layer through which the solar cell is illuminated). Fig. 4(b)
shows the energy band diagram of the fabricated Schottky-barrier
solar cell structure with a stack of ITOMoS2Au.
In general, if the metal work function is higher than that of the
n-type semiconductor, then an ideal Schottky-barrier could be
formed. In our experiment, Au (work function: 5.1 eV) forms abarrier on the MoS2 semiconductor (work function: 4.6 eV).
The ITO used in this study is a heavily n-type doped transparent
conductive oxide with a work function of4.5 eV. It forms an
almost perfect ohmic contact with the n-type doped MoS2nanomembrane. As shown in Fig. 4(b), the Fermi level of ITO is
above the conduction band due to its heavily doped nature.
Although layer-dependent optical properties (e.g., change of
bandgap with number of layers) has been demonstrated in ultra-
thin MoS2(from a monolayer to six layers) as reported by K. F.
Mak et al.,39 the bulk properties dominate in thicker stacks of
MoS2. In our experiment, MoS2 nanomembrane stacks
Fig. 3 (a) Raman spectra obtained on two samples of MoS2 nano-
membranes with thicknesses of110 nm and 220 nm, respectively. The
samples were prepared by multiple transfers/stacking of CVD-grown
MoS2. (b) Schematic showing the two Raman active modes corre-
sponding to the signature peaks observed in MoS2.
Fig. 4 (a) Schematic cross-sectional view of the demonstrated Schottky
barrier solar cell structure showing a stack of MoS2nanomembrane on
an ITO substrate with Au contact. (b) Energy band diagram of the solar
cell with formation of a Schottky-barrier between the MoS2 nano-
membrane and Au metal contact.
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(110 nm and 220 nm) were used for the fabrication of solar
cells. Therefore layer dependency of the optical bandgap
becomes negligible. We consider the MoS2stack with an optical
bandgap of 1.3 eV which effectively absorbs photons in the
wavelength region of 350950 nm. The Schottky-barrier solar
cells are illuminated through the ITO window layer which does
not absorb any photons in the visible light spectral region, as its
bandgap is 3.8 eV and more than 95% optically transparent.
Incident photons generate electronhole pairs in the MoS2nanomembrane and electrons are subsequently excited to the
conduction band (EC) of MoS2. As the work function difference
between Au and MoS2 results in a Schottky-barrier, the photo-
excited electrons move from the EC of MoS2 towards the Fermi
level of ITO (where they can be collected to the load). A larger work
function difference between Au and n-type MoS2would generate a
higher electric field in the depletion region of MoS2. The built-in
electric field facilitates the dissociation of the photo-excited elec-
tronhole pairs, transporting separated carriers towards ITO and
Au contacts. The processes of photo-excitation in MoS2 and elec-
tron transport from MoS2to ITO are demonstrated in Fig. 4(b).
Fig. 5(a) and (b) show the dark and illuminated JVcharac-
teristics of the fabricated Schottky-barrier solar cells with MoS2thicknesses of110 nm and 220 nm, respectively. The output
power as a function of voltage in each solar cell is shown in the
insets of Fig. 5(a) and (b), respectively. For the Schottky-barrier
solar cell with MoS2thickness of 110 nm key metricsJSC(short-
circuit current density), VOC (open-circuit voltage), PMAX(maximum power), FF (fill factor) and h (photo-conversion
efficiency) are 2.52 mA cm2, 590 mV, 1.22 104 W, 0.48 and
0.7%, respectively. For the solar cell with MoS2 thickness of
220 nm, JSC, VOC, PMAX, FFand h are 5.37 mAcm2, 597 mV,
2.99 104 W, 0.55 and 1.8% respectively. It is observed that an
increase of MoS2thickness helps to significantly enhanceJSCof
the solar cell (by 113%) due to more efficient photo-absorption in
the thicker MoS2 stack, and hence creates a noteworthy changein overall device performance. It should be noted that there is no
sizable change in VOC between solar cells with different thick-
nesses of MoS2 stacks (merely 1% change). It is evident that an
increase in thickness of the MoS2 stack would not cause a notable
change in the band structure (hence no change in the Fermi level)
due to the absence of layer-dependent properties.
Fig. 5(c) shows the schematic to compare carrier recombina-
tion in conventional and layered semiconductors. Process 1
represents optical excitation leading to electronhole pair
generation. In conventional semiconductors, the amount of
interface states (recombination centers) in the bandgap is huge
due to unsaturated chemical dangling bonds which capture
electrons from the conduction band (Process 2) and holes fromthe valence band (Process 3), leading to recombination. In
layered semiconductors, the amount of interface states is signif-
icantly less due to the self-saturated surface bonds which do not
actively participate in the recombination process. Thus, we
attribute the enhancement in solar cell performance to the
increase in MoS2thickness which improved photon absorption.
We expect that as a layered material, defect density in MoS2does
not increase drastically with increasing thickness compared to a
conventional semiconductor.
Fig. 6 shows External Quantum Efficiency (EQE) spectra of
the two Schottky-barrier solar cells with different MoS2
thicknesses in the wavelength range of 350950 nm. The
maximum EQE values of 41% and 52% were measured for solar
cells with 110 nm and 220 nm of MoS2stack, respectively, at
wavelength of 600 nm. The increase in MoS2 thickness plays a
vital role in photo-absorption. Our prior work on MoS2-based
Fig. 5 Measured dark and illuminated JV characteristics of the
Schottky-barrier solar cells with a stack of MoS2nanomembrane having
thicknesses of (a) 110 nm and (b) 220 nm. Inset of each figure showsthe power output as a function of voltage for the fabricated solar cell. (c)
Recombination process via electronhole capture by interfacial trap
states present in conventional and layered semiconductors, showing
significantly fewer traps in the latter. This leads to reduced probability of
carrier recombination in a layered semiconductor.
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solar cells34 employing a combination of monolayer/multi-layerstacks of MoS2on TiO2(serving as electron acceptor) has shown
photon absorption of MoS2 in the wavelength region of 350
750 nm. The measured EQE spectra in the extended wavelength
region, as compared with the previously reported values (up to
950 nm),34 suggests that an MoS2 nanomembrane with an
optical bandgap of 1.3 eV could effectively absorb photons in
both visible and near infrared spectral regions due to the absence
of the layer-dependent optical properties of MoS2 as governed by
its physical thickness.
Fig. 7(a) shows the MottSchottky (C2V) characteristics of
the Schottky-barrier solar cells. The C2Vcharacteristics were
used to estimate the Schottky-barrier heights associated with
each of the two fabricated solar cells. Fig. 7(b) shows schemati-cally the energy-level alignment at the MoS2Au interface under
zero, forward and reverse voltage bias conditions. Since the
MoS2 samples are n-type doped, Fermi levels in MoS2 and Au
shift upwards and downwards, respectively, with reference to the
equilibrium Fermi level at zero voltage bias, yielding
the maximum barrier height. In the case of reverse voltage bias,
the situation becomes opposite. We found that both solar cells
(with different MoS2 thickness) resulted in nearly the same
Schottky-barrier height (1.0 eV), as measured from the linear
extrapolation in the reverse-biasing region (from 1 V to 4 V). It
can be understood that a change of the MoS2should not make a
difference in the Schottky-barrier height. While we expect that
MoS2thickness (down to a few layers) may change the Schottky-barrier height, the layer dependence becomes insignificant in the
nanomembranes used in our experiment. The observed linear
behavior of the MottSchottky plots suggests that the MoS2nanomembranes possess a relatively low density of surface
defects.
J. M. Lutheret al.reported a Schottky-barrier solar cell using
PbSe quantum dots.2 The result showed significant variation in
the voltage-dependent C2 for two PbSe samples (thickness
difference > 300 nm).2 In our study, MoS2Au solar cells (with
active layer thickness difference of 100 nm) exhibit no significant
change in the C2 values with reverse bias. This could be
attributed to the layered MoS2 nanomembrane (defect-free or
with negligibly low density of defects) as compared with
conventional semiconductors in which unsaturated danglingbonds make the MS interface highly defective. In layered
semiconductors the crystal lattice planes are weakly bound by
van der Waals interactions and hence the surface exhibits self-
saturated electronic bonds which are no longer active for surface
reaction with the environment. The nature of a self-saturated
surface in layered MoS2 nanomembrane eliminates Fermi level
pinning and interfacial oxide layer formation (both could impede
the transport significantly). The voltage-bias-dependent C2
values are almost identical in both device samples (with different
thicknesses) due to negligible change in the trap states at MoS 2
Au interface where carriers are stored and released, affecting the
junction capacitance. This is a key appealing property of layered
semiconductors for solar cell applications as compared withbulk/thin film semiconductors which possess a particular type of
electronic bonding along and across the crystal planes (leading to
defective bulk and surface upon cleaving).
We project that the performance of the MoS2nanomembrane-
based Schottky-barrier solar cell could be further improved.
Layered semiconductors are typically very sensitive to growth
conditions such as temperature and pressure. Temperature fluc-
tuation across the growth furnace plays a significant role in
influencing the solidvapor reaction between pre-deposited Mo
and S vapor, and eventually the electronic quality of the MoS2nanomembrane. In addition, defects and impurities introduced
Fig. 6 Measured EQE spectra of the Schottky-barrier solar cells with a
stack of MoS2 nanomembranes having thicknesses of 110 nm and
220 nm, respectively, in the wavelength region of 350950 nm. This
confirms an optical bandgap of approximately 1.3 eV for the prepared
MoS2 nanomembranes.
Fig. 7 (a) The measured MottSchottky (C2V) plot of the Schottky-
barrier solar cells with a stack of MoS2 nanomembranes having thick-
nesses of 110 nm and 220 nm, respectively, showing a linear C2
dependency (shown in the circled region) on the applied reverse bias, VR.
(b) Schematic showing MoS2Au interfacial energy level change at zero
(equilibrium), forward, and reverse bias conditions and the effect on
Schottky barrier height.
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during the CVD growth and deformation formed during layer
transfer would all contribute to reduced carrier mobility and
degraded carrier transport properties. We expect that further
optimization of the growth process, use of a cleaner fabrication
environment, and direct assembly of MoS2 nanomembranes on
an ITO substrate would produce less defective materials, leading
to improved solar cell performance.
Conclusions
Schottky-barrier solar cells have been demonstrated using layer-
assembled, CVD-grown MoS2 semiconductor nanomembranes
as the critical photo-active layer. While relatively thick MoS2nanomembranes are used in this experiment, the material is
scalable via a controlled CVD growth process and large-area
solar cell fabrication is potentially feasible. The dependency of
solar cell performance on MoS2 thickness was evaluated. Further
improvement in solar cell performance is expected through
optimizing the growth process, eliminating structural deforma-
tion (induced during fabrication), and minimizing the amount of
surface defects and impurities. The reported results would
promote continued efforts towards developing highly efficientSchottky-barrier solar cells taking advantage of the unique
interfacial properties of layered semiconductor nanostructures.
References
1 Z. B. Zhou, R. Q. Cui, Q. J. Pang, G. M. Hadi, Z. M. Ding andW. Y. Li, Sol. Energy Mater. Sol. Cells, 2002, 70, 487493.
2 J. M. Luther, M. Law, M. C. Beard, Q. Song, M. O. Reese,R. J. Ellingson and A. J. Nozik, Nano Lett., 2008, 8, 34883492.
3 B. Ghosh, M. Das, P. Banerjee and S. Das,Solid State Sci., 2009,11,461466.
4 M. Soylu and F. Yakuphanoglu, Thin Solid Films, 2011, 519, 19501954.
5 L. Zhang, Y. Jia, S. Wang, Z. Li., C. Ji, J. Wei, H. Zhu, K. Wang,
D. Wu, E. Shi, Y. Fang and A. Cao, Nano Lett., 2010,10, 35833589.6 Y. Ye, Y. Dai, L. Dai, Z. Shi, N. Liu, F. Wang, L. Fu, R. Peng,X. Wen, Z. Chen, Z. Liu and G. Qin, ACS Appl. Mater. Interfaces,2010, 2, 34063410.
7 C. C. Chen, M. Aykol, C. C. Chang, A. F. J. Levi and S. B. Cronin,Nano Lett., 2011,11, 18631867.
8 J. Wei, Y. Jia, Q. Shu, Z. Gu, K. Wang, D. Zhuang, G. Zhang,Z. Wang, J. Luo, A. Cao and D. Wu,Nano Lett., 2007,7, 23172321.
9 C. Xie, P. Lv, B. Nie, J. Jie, X. Zhang, Z. Wang, P. Jiang, Z. Hu,L. Luo, Z. Zhu, L. Wang and C. Wu, Appl. Phys. Lett., 2011, 99,133113.
10 G. Fan, H. Zhu, K. Wang, J. Wei, X. Li, Q. Shu, N. Guo and D. Wu,ACS Appl. Mater. Interfaces, 2011,3, 721725.
11 C. Xie, J. Jie, B. Nie, T. Yan, Q. Li, P. Lv, F. Li, M. Wang, C. Wu,L. Wang and L. Luo, Appl. Phys. Lett., 2012, 100, 193103.
12 G. H. Glover, Solid State Electrochem., 1973,16, 973978.13 F. Dubecky and B. Olejnkova,J. Appl. Phys., 1991, 69, 1769.14 C. Ghezzit, E. Gombiaz and R. Moscaf, Semicond. Sci. Technol.,
1991, 6, 3133.15 A. Singh, P. Cova and R. A. Masut, J. Appl. Phys., 1994, 76, 2336.16 F. Lu, D. Gong, J. Wang, Q. Wang, H. Sun and X. Wang, Phys. Rev.
B: Condens. Matter Mater. Phys., 1996, 53, 46234629.17 D. Vanmaekelbergh, A. Koster and F. I. Marn,Adv. Mater., 1997,9,
575578.18 Y. Fukushima, K. Ogisu, M. Kuzuharaand K. Shiojima, Phys. Status
Solidi C, 2009, 6, 856859.19 S. Tongay, T. Schumann and A. F. Hebard, Appl. Phys. Lett., 2009,
95, 222103.20 F. Leonard and J. Tersoff, Phys. Rev. Lett., 2000, 84, 46934696.21 A. Dimoulas, P. Tsipas, A. Sotiropoulos and E. K. Evangelou, Appl.
Phys. Lett., 2006, 89, 252110.22 T. Nishimura, K. Kita and A. Toriumi, Appl. Phys. Lett., 2007, 91,
123123.23 Y. Zhou, W. Han, Y. Wang, F. Xiu, J. Zou, R. K. Kawakami and
K. L. Wang,Appl. Phys. Lett., 2010, 96, 102103.24 S. J.Oh,D. K.Kim and C.R. Kagan, ACS Nano, 2012, 6, 43284334.25 Y. Yoon, K. Ganapathi and S. Salahuddin, Nano Lett., 2011, 11,
37683773.26 Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang,
X. Chen and H. Zhang, ACS Nano, 2012, 6, 7480.
27 H. S. Lee, S. W. Min, Y. G. Chang, M. K. Park, T. Nam, H. Kim,J. H. Kim, S. Ryu and S. Im, Nano Lett., 2012,12, 36953700.
28 D. J. Late, B. Liu, H. S. S. R. Matte, V. P. Dravid and C. N. R. Rao,ACS Nano, 2012, 6, 56355641.
29 Y. Zhang, J. Ye, Y. Matsuhashi and Y. Iwasa, Nano Lett., 2012,12,11361140.
30 S. Ghatak, A. N. Pal and A. Ghosh,ACS Nano, 2011,5, 77077712.31 K. Chang and W. Chen, Chem. Commun., 2011,47, 42524254.32 K. Chang, W. Chen, L. Ma, H. Li, H. Li, F. Huang, Z. Xu, Q. Zhang
and J. Y. Lee, J. Mater. Chem., 2011, 21, 62516257.33 K. Chang and W. Chen, J. Mater. Chem., 2011, 21, 1717517184.34 M. Shanmugam, T. Bansal, C. Durcan and B. Yu, Appl. Phys. Lett.,
2012, 100, 153901.35 K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich,
S. V. Morozov and A. K. Geim, Proc. Natl. Acad. Sci. U. S. A. , 2005,102, 10451.
36 J. N. Coleman, M. Lotya, A. ONeill, S. D. Bergin, P. J. King,U. K.Young, A. Gaucher, S. De, R. J. Smith and I. V. Shvets,et al., Science, 2011, 331, 568571.
37 Y. Zhan, Z. Liu, S. Najmaei, P. M. Ajayan and J. Lou, Small, 2012, 8,966971.
38 H. Liu and P. D. Ye, IEEE Electron Device Lett., 2012, 33, 546548.
39 K.F. Mak, C. Lee, J. Hone, J. Shanand T. F. Heinz, Phys. Rev. Lett.,2010, 105, 136805.
40 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis,Nat. Nanotechnol., 2011,6, 147150.
41 J. R. Lince, D. J. Carre and P. D. Fleischauer, Phys. Rev. B: Condens.Matter Mater. Phys., 1987, 36, 16471656.
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