August 05, 2014
C 2014 American Chemical Society
Monolayer MoSe
2
Grown by Chemical
Vapor Deposition for Fast Photodetection
Yung-Huang Chang,†,OWenjing Zhang,‡,§,OYihan Zhu,^Yu Han,^Jiang Pu,)Jan-Kai Chang,zWei-Ting Hsu,# Jing-Kai Huang,†Chang-Lung Hsu,†Ming-Hui Chiu,^Taishi Takenobu,),4Henan Li,1Chih-I Wu,z
Wen-Hao Chang,#Andrew Thye Shen Wee,‡and Lain-Jong Li†,^,*
†Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 11529, Taiwan,‡Department of Physics, National University of Singapore, 117542 Singapore, §
SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education, Guangdong Province, Shenzhen University, Shenzhen 518060, China,^Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia, )Department of Advanced Science and Engineering, Waseda University, Tokyo 169-8555, Japan,zInstitute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan,#Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan,4Department of Applied Physics, Kagami Memorial Laboratory for Material Science and Technology, Waseda University, Tokyo 169-8555, Japan, and1School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore.OThese authors contributed equally.
T
ransition metal dichalcogenides (TMDs) with the formula MX2, where M is atransition metal (Mo, W, and so on) and X is a chalcogen (S, Se, or Te), have attracted much attention due to their layer structure and semiconducting properties.16 These two-dimensional (2D) materials are in the form of XMX, where a plane of metal atoms is sandwiched between two planes of chalcogen atoms by covalent interaction, and different layers are held together by van der Waals interactions.7,8 These layer materials exhibit many distinctive charac-teristics such as outstanding flexibility,9 moderate carrier mobility,10,11 and layer-dependent electronic and optical prop-erties.1220 Thus, the TMD materials can serve as transparent and flexible field-effecttransistors(FETs),2124photodetectors,25
photovoltaic cells,26,27light-emitting diodes,28,29 and catalysts.3032In particular, TMD mate-rials have been reported to absorb up to 510% of incident sunlight within a thick-ness less than 1 nm, which is about 1 order of magnitude higher absorption than GaAs and Si.26 The phototransistors based on monolayer MoS2show outstanding
photo-responsivity even up to a few thousand
A/W.25,33 Recently, a graphene/MoS
2
hy-brid phototransistor was demonstrated to be able to provide a photogain of more than 108.34However, the long response time of 430 s for monolayer MoS2-based
phototransistors, caused by persistent photocarriers generated from trapped defects or charged impurity states,25,33,34 limits their application for fast photon detection.
* Address correspondence to [email protected]. Received for review June 17, 2014 and accepted August 5, 2014. Published online
10.1021/nn503287m
ABSTRACT Monolayer molybdenum disulfide (MoS2) has
be-come a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects sig-nificantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum
diselenide (MoSe2) monolayers have been successfully synthesized
by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers
reveals that the MoSe2monolayer shows a much weaker bound
exciton peak; hence, the phototransistor based on MoSe2presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2
monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2and MoS2and is
useful for guiding future designs in 2D material-based optoelectronic devices.
KEYWORDS: transition metal dichalcogenides . photoresponse . MoSe2. MoS2. two-dimensional materials
CHANG ET AL. VOL. 8 NO. 8 8582–8590 2014 8583 Recently, monolayer MoSe2 has started to gain
attention because it has many interesting electronic and optical properties similar to those of monolayer MoS2, such as a direct band gap, strong
photolumines-cence (PL), and a large exciton binding energy.3541It is known that sulfur defects in the MoS2monolayer
greatly affect the electronic transport and optical properties. Therefore, it would be meaningful to care-fully compare the properties of monolayer MoS2and
MoSe2. One important question is whether the MoSe2
monolayer grown by chemical vapor deposition (CVD) exhibits better optoelectronic characteristics. In this study, we synthesized monolayer MoSe2on sapphire
by the gas phase reaction between MoO3 and Se
powders in a hot-wall tube furnace system, using a CVD method which has been reported elsewhere.4,6 X-ray photoemission spectroscopy (XPS) and transmis-sion electron microscopy (TEM) measurements con-firm that the MoSe2 film is highly crystalline. By
comparing the electrical devices fabricated with these two monolyers, we demonstrate that both MoS2and
MoSe2 monolayers exhibit a comparable mobility
value for the electron transport. It is also noteworthy that electrical and ultraviolet photoemission spectros-copy (UPS) measurements demonstrate that MoS2is
heavily n-doped but MoSe2 is less n-doped.
Low-temperature photoluminescence study reveals that the MoSe2 monolayer has a much weaker bound
exciton peak, indirectly suggesting that the MoSe2
monolayer possesses fewer defects or impurities. Most interestingly, the CVD MoSe2 monolayer exhibits a
much faster response time (<25 ms) than MoS2,
mak-ing it superior for fast photodetection applications.
RESULTS AND DISCUSSION
Synthesis of the MoSe2 Monolayer. The experimental
setup for growing monolayer MoSe2using MoO3and
Se powder precursors in a hot-wall CVD system is schematically illustrated in Figure 1a. Hydrogen was introduced as a reducing agent during the growth process.5 The morphology of the MoSe2 grown on
sapphire substrates varies with the distance of the sub-strate from the MoO3source, as shown in Figures 1b,c.
The optical micrograph (OM) in Figure 1b shows that
Figure 1. (a) Schematic illustration for the growth of MoSe2layers on sapphire substrates by the selenization of MoO3
powders in a CVD furnace. (b) OM. (c) AFM image of the monolayer MoSe2flakes and monolayer film grown at 800 C, where
the difference is the location of the substrates in the furnace. (d) AFM image and (e) OM of a monolayer MoSe2film grown at
800C on a sapphire substrate.
the sparsely distributed triangular crystals (lateral size∼5 μm) are found in most areas of the substrate located furthest from the MoO3 source. When the
substrate is close to the MoO3source, the nucleation
density becomes much higher such that these small domains easily merge to form a continuous film, as shown in the atomic force microscopy (AFM) image in Figure 1c. Moreover, the cross-sectional height profile in Figure 1d shows that the thickness of the MoSe2
film is∼0.7 nm, corresponding to a monolayer and con-sistent with published monolayer thickness.6,37Figure 1e shows the OM of the MoSe2monolayer. In addition to the
MoSe2monolayer, we occasionally observe the growth of
second-layer MoSe2on top of some monolayer flakes, as
shown in Supporting Information Figure S1. The occasion-ally found MoSe2second layers are normally in the areas
with high substrate roughness or with some particles, likely due to the rough surfaces or particles that are able to assist the nucleation of the second layer.
Structural Characterization of MoSe2. The layer
depen-dence of Raman features has been reported for TMDs
such as MoS2, MoSe2, and WSe2.7,16,38,40In Figure 2a,
the synthesized monolayer MoSe2exhibits two
char-acteristic peaks located at 241.2 and 286.7 cm1, asso-ciated with the out-of-plane A1gmode and in-plane
E2g1 mode, respectively.6,35 Furthermore, the Raman
peak at∼353 cm1, which has been demonstrated to relate to the interlayer interaction, is not observed in our monolayer MoSe2.38This suggests that the CVD
synthesized MoSe2is indeed one monolayer. Figure 2b
displays the PL spectrum for monolayer MoSe2. Only a
strong peak located at 793 nm is observed, attributed to the direct band gap emission from A excitons.6,35,37 It is noted that the indirect gap emission is absent in the monolayer, and the strong A exciton emission from the monolayer is in good agreement with a recent report.38,41In addition, the XPS spectra for the CVD synthesized monolayer MoSe2in Figure 2c,d confirm
the stoichiometry of MoSe2. The peaks at 228.8 and 232
eV are attributed to the doublet Mo 3d5/2and Mo 3d3/2
binding energies, respectively, for Mo4þ.42,43The peaks corresponding to the Se 3d5/2and Se 3d3/2orbitals of
Figure 2. (a) Raman spectrum for the monolayer MoSe2, obtained in a confocal Raman spectrometer excited by a 473 nm
laser. (b) Photoluminescence spectra for the CVD monolayer MoSe2, obtained in a microscopic PL system (excitation
wavelength 532 nm). (c,d) XPS spectra of the monolayer MoSe2film, where the (c) Mo 3d and (d) Se 3d binding energies are
identified. (e) High-resolution TEM image of monolayer MoSe2with an inset showing its Fourier transform pattern. (f)
Enlarged TEM image, where the Mo and Se atoms are identified. The inset is a simulated HRTEM image of monolayer MoSe2.
CHANG ET AL. VOL. 8 NO. 8 8582–8590 2014 8585 divalent selenide ions (Se2) are observed at 54.4 and
55.2 eV.42,43Aberration-corrected high-resolution TEM (HRTEM) images demonstrate a high crystallinity of monolayer MoSe2, in which the Se and Mo atomic
columns can be directly identified (Figure 2e,f). The remarkable contrast difference between Se and Mo is in good agreement with the simulated HRTEM image for monolayer MoSe2(Figure 2f). The lattice constant of
the 2D hexagonal lattice is measured directly from the image and the corresponding Fourier transform and determined to be a = 0.323 nm, consistent with that of bulk MoSe2.
Electric Double-Layer Transistors. The electrical charac-teristics of the electric double-layer transistors (EDLTs) were measured directly for the monolayer MoSe2films
on sapphire substrates. The detailed description of EDLT fabrication was reported in previous studies3,5
and is illustrated in Figure S2. The output character-istics for the MoSe2devices are also shown in Figure S2.
Figure 3a shows the OM top view image of the MoSe2
EDLT, where the channel width and length are 90 and 1000 μm, respectively. Figure 3c displays the p- and n-channel drain current as a function of the reference voltage VRfor monolayer MoSe2EDLT at the applied
drain voltage VD=0.1 and 0.1 V, respectively. Note
that VRis the measured voltage between the
electro-lytes and MoSe2, that is, the voltage for the electric
double layer on MoSe2surfaces. Since the gate voltage
applied on the top Pt metal is partially consumed by the electric double layer on the gate electrode, VRis
used instead for the gate dependence measurements. Although some reports have claimed that MoSe2 is
an n-type semiconductor,44 we clearly observe an ambipolar transport behavior instead since the elec-tric double layer exhibits a higher gating efficiency. Figure 3c shows that the threshold voltage for the n-channel is 0.64 V in the forward scan, smaller than the 1.46 V value for the p-channel, indicating that the MoSe2is an n-type preferred ambipolar
semiconduc-tor. However, a unipolar n-type electrical characteristic is presented for MoS2using the same EDLT technology
as shown in Figure 3b. Furthermore, we did not observe any hole transport current in the same nega-tive gate voltage range (1.6 to 0 V) for MoS2. The
field-effect mobility was calculated using the standard for-mula in the linear region,μ = (L/WCiVD) (ΔID/ΔVR),
whereμ is the field-effect mobility, W is the channel width, VD is the drain voltage, Ci is the measured
specific capacitance of the ion gel, L is the channel length, and IDis the drain current. The highest carrier
mobility obtained for the MoSe2 monolayer is
23 cm2/Vs for electron transport and 15 cm2/Vs for hole transport. For the MoS2monolayer, the electron
mobility is around 17 cm2/Vs. As shown in the insets of
Figure 3. (a) Optical micrograph (top view) for the MoSe2EDLT device, where the top and bottom photos were taken before
and after the ion gel/top-gate deposition. (b,c) Typical transfer curves of the (b) MoS2and (c) MoSe2EDLT devices, where the
inset in each graph shows the transfer curve plotted in a log scale.
Figure 3bd, the current ONOFF ratio is as high as 104105for both MoSe
2and MoS2. The comparative
EDLT measurement results for both MoSe2and MoS2
are listed in Table 1. In brief, MoSe2exhibits a similar
electron mobility value and ONOFF current ratio compared with MoS2, which also makes it a promising
candidate for FET applications.
Ultraviolet Photoemission Spectroscopy. UPS is used to explore the energy level alignment with respect to the Fermi energy (EF). The MoS2and MoSe2monolayers are
separately transferred onto Si substrates coated with 60 nm thermally evaporated Au. The Au layer serves as a reference for EF, assigned to 0 eV.45,46As shown in
Figure 4a, the valence band (EV) for MoS2and MoSe2on
Au/Si substrates is, respectively, located at 1.75 and 1.20 eV below EFby linearly extrapolating the leading
edge of the spectrum to the baseline. In addition, the work function (Φ) can be calculated using Φ = hν
Eonset, where hν is the incident photon energy (40.8 eV)
and Eonsetis the onset level related to the secondary
electrons, as shown in Figure 4b.47Hence, theΦ for MoS2and MoSe2on Au/Si substrates is 4.20 and 4.27
eV, respectively. Note that the work function value obtained for monolayer MoS2is consistent with several
other reports.4851In addition, the optical band gaps of the CVD monolayer MoS2and MoSe2are determined
to be∼1.83 and ∼1.51 eV, respectively, from the ab-sorption spectra (Figure S3). Thus, the energy band diagrams of CVD monolayer MoS2and MoSe2relative
to the Fermi level EFof gold films are illustrated in
Figure 4c,d. The energy separationΔE between the conduction band of MoS2and the EFof Au is∼0.08 eV,
indicating that CVD monolayer MoS2 is heavily
n-doped, consistent with unipolar n-type electrical transport behavior. On the other hand, the energy separationΔE between the conduction band of MoSe2
and the EFof Au is∼0.31 eV, indicating that the CVD
monolayer MoSe2is slightly n-doped, consistent with
the n-type preferred ambipolar electrical transport behavior.
Optical Properties. To examine the gate-dependent photoresponse characteristics, the MoS2 and the
MoSe2films are transferred onto 300 nm SiO2/Si
sub-strates, and the phototransistors are patterned using standard photolithography. Figure S4 shows the opti-cal image of the interdigitated electrodes used, where the contact metal layers, 10 nm Ti and 80 nm Au, are deposited by thermal evaporation. The 532 and
TABLE 1. Summarization of the MoS2and MoSe2from
EDLT Measurements
mobility (cm2/Vs)
hole electron ON/OFF type
MoS2 ∼17 104105 n-type
MoSe2 ∼15 ∼23 10
4
105
ambipolara
aThe n-type preferred.
Figure 4. Ultraviolet photoemission spectroscopy and energy band diagrams. (a) UPS spectra, near the Fermi level energy and valence band maximum, of the monolayer MoS2and MoSe2film transferred onto the 60 nm Au-coated Si substrates. (b)
Onset level (Eonset) of the UPS spectra, where the work function (Φ) can be calculated by Φ = hν Eonset;hν is the incident
photon energy of 40.8 eV. (c,d) Energy band diagrams of the monolayer (c) MoS2and (d) MoSe2film, where Au metal is used as
a reference. Note that the band gap energies used in diagram are optical gaps.
CHANG ET AL. VOL. 8 NO. 8 8582–8590 2014 8587 650 nm CW lasers are used as the light sources to study
the MoS2and MoSe2phototransistors. Figure 5a
pre-sents the typical transport curves of the two photo-transistors in the dark and under illumination. In the dark, the n-channel subthreshold voltage Vth of the
MoSe2 and MoS2is ∼7 V and ∼46 V, respectively,
corroborating that the MoS2 on SiO2 is much more
heavily n-doped. The mobilities of MoSe2and MoS2
transistors are∼0.012 cm2/Vs at Vg= 60 V (VgVth=
53 V) and∼0.021 cm2/Vs at Vg= 17 V (VgVth= 53 V),
respectively. Compared with the devices on sapphire, the mobilities of the devices on SiO2are about 3 orders
of magnitude lower, which may be caused by several factors including the possible degradation by transfer processes, the scattering effect from charge impurities added during the transfer process, and damage caused by patterning. Under illumination at a power density of ∼0.31 and 0.59 W/cm2, the OFF state currents of both devices increase by∼3 orders of magnitude. The photogain (G), given by G = hνIph/(ηeP0),33is an index
to quantify the conversion efficiency of incident photons to photogenerated carriers, where Iphis the
net photocurrent, P0is the absorbed laser power, h is
Planck's constant, e is the charge of an electron, andν is the frequency of incident light. Assumingη = 100%, the photogain of the MoS2phototransistors is∼0.2,
much higher than∼5 104of the MoSe2
photo-transistor in the OFF state. The MoSe2band diagrams in
Figure 4c,d indicate that the electron Schottky barrier of MoSe2is higher,52resulting in the smaller photogain
of MoSe2on SiO2. This also accounts for the lower OFF
state dark current for MoSe2(∼1012A vs∼1010A for
MoS2).
Figure 5c,d shows the time-resolved photocurrent for monolayer MoSe2 and MoS2 films, respectively,
under ON/OFF light illumination at Vds= 1 V in ambient
environment. A fast rise and decay response time shorter than∼25 ms for MoSe2is observed. However,
the photocurrent of the MoS2phototransistors takes
more than 30 s to saturate and decay. The persistent photocurrent is normally attributed to defect or charge impurity states inside the band gap.53 Figure 5e,f shows the PL spectra of the monolayer MoSe2 and
MoS2at room and low temperatures. We observe only
Figure 5. Optical properites of CVD MoSe2and MoS2monolayers. (a)IdVgtransfer characteristics of the photodetectors
based on CVD MoSe2and MoS2monolayers in dark and under illumination atVds= 1 V. (b) Power density dependence of the
photogain atVds= 1 V for the photodetectors at the OFF state, whereVgis 0 V for the MoSe2andVgis48 V for the MoS2. (c,d)
Time-resolved photocurrents of the photodectors based on MoSe2atP = 0.59 W/cm 2
and MoS2atP = 0.31 W/cm 2
. (e,f) PL spectra of monolayer MoSe2and MoS2, respectively, at room temperature (RT) and 12 K.
one peak at room temperature,∼1.55 eV for the MoSe2
and∼1.88 eV for the MoS2, corresponding to the direct
band-to-band A excitonic transition. The peak intensity for monolayer MoSe2at room temperature is stronger
than that of monolayer MoS2, and the full width at
half-maximum of∼42 meV for monolayer MoSe2is smaller
than the∼51 meV value of monolayer MoS2. When the
temperature decreases to 12 K, an additional sub-band-gap emission at the low energy side between 1.4 and 1.9 eV appears for the monolayer CVD MoS2,
which could be attributed to the defect or charge impurity states.15,53Due to the additional binding to defects or charge impurities, the energy of the bound exciton peak is lower than that of the free exciton peak.15 The large width of the bound exciton peak
indicates the presence of different kinds of defects or charge impurity sites since different binding energies are needed. In contrast, no sub-band-gap emission is observed at the low-energy side for the CVD MoSe2
monolayer, implying better crystalline quality and few-er defects or charge impurity states, thus resulting in higher band-to-band free exciton recombination rates. Some reports have pointed out that the persistent photoconductance arises from the defects or charge impurity states inside the band gap.33,54,55Hence, the extra photogenerated carriers in the MoS2arising from
the defects or charge impurity states could be respon-sible for its slow light response time. Figure S5 shows the HRTEM image for the CVD MoS2monolayers with
abundantly disordered atom arrangement highlighted by dashed circles, which is in clear contrast to the HRTEM for CVD MoSe2in Figure 2e,f.
Moreover, the CVD MoS2shows significantly lower
stability in ambient environment, where its PL intensity normally decays in 2 or 3 days. We notice that the shelf-lifetime can be increased to more than 1 week if the sample is stored in a drybox, indicating that the moisture may react or catalyze the formation of defects in MoS2. By contrast, PL measurements suggest that
the CVD MoSe2 monolayer exhibits a much longer
shelf-lifetime, typically more than 4 weeks, in the same ambient storage condition. These observations are also in line with a relatively larger amount of defects in MoS2revealed by TEM.
CONCLUSIONS
In conclusion, we have synthesized crystalline monolayer MoSe2 by the gas phase selenization of
MoO3in a hot-wall CVD chamber. From the analyses of
EDLT and UPS measurements, the MoSe2 exhibits a
slight n-type preferred ambipolar behavior, while the MoS2shows heavily n-doped electrical characteristics.
In addition, the defect-less crystalline structure for the MoSe2is identified through the low-temperature PL,
whereas relatively abundant defects occur in MoS2.
This study shows that CVD synthesized monolayer MoSe2has great potential inflexible transparent
op-toelectronic applications.
METHODS
Growth of MoSe2Monolayers. The MoO3powders (0.0250.3 g)
were placed in a ceramic boat located in the heating zone center of the furnace. The Se powders were placed in a separate quartz boat at the upper stream side maintained at 270C during the reaction. The sapphire substrates for growing MoSe2
were placed at the downstream side near MO3powders, where
the Se and MoO3vapors were brought to the targeting sapphire
substrates by an Ar/H2flowing gas (Ar = 4070 sccm, H2= 10
sccm, chamber pressure = 25350 Torr). The center heating zone was heated to 700900 C at a ramp rate 15 C/min. After the growth temperature was reached, the heating zone was kept for 15 min and the furnace was then naturally cooled to room temperature.
Fabrication of EDLT Devices. For the source and drain electro-des, Au contacts with Ni adhesion layers (70 nm/2 nm) were thermally deposited onto the surface of the MoSe2films. The ion
gels, a mixture of a triblock copolymer, poly(styrene-block-methyl methacrylate-block-styrene) (PSPMMA-PS; MPS = 4.3 kg/mol, MPMMA = 12.5 kg/mol, Mw= 21.1 kg/mol), and
an ionic liquid, 1-ethyl-3-methylimidazoliumbis(trifluoromethyl-sulfonyl)imide ([EMIM][TFSI]) in an ethyl propionate solution, were used as the top-gate dielectrics. Note that the weight ratio of the polymer, ionic liquid, and solvent was maintained at 0.7:9.3:20. This solution was drop-casted onto and covered the surfaces of MoSe2
film and the source and drain electrodes. The transistor channel was then covered with a thin Pt foil (thickness of 0.03 mm) to form the top-gate electrode. Finally, a thin gold wire was inserted into the gel films, between the channel and top-gate metal, as the reference electrode. All electrical characterizations were per-formed using a semiconductor parameter analyzer (Agilent E5270) in a shield probe station inside a N2-filled glovebox.
Characterizations. Photoluminescence spectra were excited by a green light laser with 532 nm wavelength and a 0.9 NA objective (spot size: 0.7μm). Raman spectra were collected in a NT-MDT confocal Raman microscopic system (laser wave-length = 473 nm and laser spot size∼0.5 μm). The Si peak at 520 cm1was used as reference for wavenumber calibration. The AFM images were performed in a Veeco Dimension-Icon system. Chemical configurations were determined by X-ray photoelectron spectroscopy (Phi V5000). XPS measurements were performed with an Mg KR X-ray source on the samples. The energy calibrations were made against the C 1s peak to eliminate the charging of the sample during analysis. The valence band UPS were performed using He I (21.2 eV) and He II (40.8 eV) photon lines as excitation sources, and the photoelectrons were analyzed with a hemispherical analyzer with an overall resolution of 0.05 eV. The absorbance spectra were obtained using a JASCO-V-670 UVvis spectrophot-ometer. The MoSe2films were transferred onto a copper grid
for TEM observation. HRTEM imaging was performed on an aberration-corrected and monochromated G2 cubed Titan
60-300 electron microscope under 60 kV. The electrical mea-surements were performed using a Keithley semiconductor parameter analyzer, model 4200-SCS. All the measurements were achieved at room temperature in ambient air. Continuous wavelength 532 and 650 nm lasers were used to measure the photoresponse of the devices, and the spot size was∼1 mm.
Conflict of Interest: The authors declare no competing financial interest.
Acknowledgment. This research was supported by Acade-mia Sinica, National Science Council Taiwan (102-2119-M-001-005) and AFOSR-BRI USA. T.T. was partially supported by the
CHANG ET AL. VOL. 8 NO. 8 8582–8590 2014 8589 Funding Program for the Next Generation of World-Leading
Researchers and Grants-in-Aid from MEXT (26107533“Science of Atomic Layers” and 25000003 “Specially Promoted Research”). Supporting Information Available: Raman spectra are in-cluded. This material is available free of charge via the Internet at http://pubs.acs.org.
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