YAMAMOTO ET AL. VOL. 8 NO. 4 3895–3903 2014 3895 March 21, 2014
C 2014 American Chemical Society
Strong Enhancement of Raman
Scattering from a Bulk-Inactive
Vibrational Mode in Few-Layer MoTe
2
Mahito Yamamoto,†,#,* Sheng Tsung Wang,‡,#Meiyan Ni,†,§,#Yen-Fu Lin,†,^Song-Lin Li,†Shinya Aikawa,† Wen-Bin Jian,‡Keiji Ueno,)Katsunori Wakabayashi,†and Kazuhito Tsukagoshi†,*
†International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan,‡Department
of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan,§School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei 230009, China,^Department of Physics, National Chung Hsing University, Taichung, 40227, Taiwan, and )Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan.#M. Yamamoto, S. T. Wang, and M. Ni contributed equally to this work.
A
tomic layers of group VI transition metal dichalcogenides (MX2, whereM = Mo and W and X = S, Se, and Te) have attracted much attention for a wide variety of applications ranging from spin-and valley-tronics to catalysts for hydrogen evolution reaction.15Of particular interest are their electronic and optoelectronic appli-cations due to their large band gaps, together with the indirect-to-direct transitions in single-layers.610Field effect transistors with high on/off current ratios have been demon-strated using atomically thin MX2.1114
Single-layers of MoS2and WSe2have been
used in optoelectronic devices including photodetectors and photoemitters.1520Thanks to their large mechanical strength,21 few-layers of MoS2 and WS2 have been used
to fabricateflexible electronic devices.2225 Additionally, atomically thin MX2films have
potential for thermoelectric applications, owing to their low thermal conductivities.26
A crucial step toward the application of atomically thin MX2 is to understand its
phonon properties. Phonons couple to elec-trons and limit carrier mobility at room
temperature,27,28 along with Coulomb
impurities.29,30Heat propagates in a crystal via predominantly acoustic phonons; thus, pho-nons determine its thermal conductivity.31,32 Additionally, soft mode phonons determine the mechanical strength of the crystal.33 Previously, Raman spectroscopy has been used to investigate the phonon properties of atomically thin MX2, including electron
phonon coupling34and the effects of heat-ing3537 and strain38,39 on the phonons. Moreover, Raman spectroscopy has shown that the lattice dynamics in MoS2, MoSe2,
WS2, and WSe2depend sensitively on their
thicknesses4043and, hence, can be used to identify the number of layers at the atomic scale.
Here, we present, for thefirst time, Raman spectroscopy of atomically thin layers of R-MoTe2. Bulk MoTe2is an indirect band gap
semiconductor with a bandgap of 1.0 eV, but MoTe2 is expected to exhibit a direct
band gap of 1.1 eV in its single-layer,4446 similar to MoS2, MoSe2, WS2, and WSe2.
Moreover, bulk MoTe2 has been observed
to undergo a transition from a diamagnetic
* Address correspondence to [email protected], [email protected]. Received for review February 7, 2014 and accepted March 21, 2014. Published online
10.1021/nn5007607
ABSTRACT Two-dimensional layered crystals could show phonon properties that are markedly distinct from those of their bulk counterparts, because of the loss of periodicities along thec-axis directions. Here we investigate the phonon properties of bulk and atomically thinR-MoTe2using Raman spectroscopy. The Raman spectrum of
R-MoTe2shows a prominent peak of the in-plane E 1
2gmode, with its frequency upshifting
with decreasing thickness down to the atomic scale, similar to other dichalcogenides.
Furthermore, wefind large enhancement of the Raman scattering from the out-of-plane B12gmode in the atomically thin layers. The B12gmode is Raman
inactive in the bulk, but is observed to become active in the few-layerfilms. The intensity ratio of the B12gto E12gpeaks evolves significantly with
decreasing thickness, in contrast with other dichalcogenides. Our observations point to strong effects of dimensionality on the phonon properties of MoTe2.
KEYWORDS: transition metal dichalcogenides . Raman spectroscopy . density functional theory . molybdenum ditelluride . MoS2.
MoSe2. WSe2
semiconductingR-phase (trigonal prismatic) to a para-magnetic β-phase (distorted octahedral) at high temperatures,47,48offering unique potential for appli-cations. However, though a few Raman spectroscopy studies of bulk MoTe2 have been reported,4951the
lattice dynamics in atomically thin MoTe2has yet to be
investigated.
The Raman spectrum of MoTe2shows a prominent
peak of the in-plane E12gmode at∼235 cm1, with a
small out-of-plane A1gpeak at∼174 cm1. The E12g
mode upshifts, while the A1gmode downshifts with
decreasing thickness. Additionally, we find a strong peak at∼291 cm1in the atomically thin crystals. This peak is not observed in the bulk crystals, but the intensity is enhanced with decreasing thickness, down to bilayers. However, this peak is absent in single-layer MoTe2. We assign, using density functional theory
(DFT) and group theory analysis, the peak as a bulk-Raman inactive mode of B12g. The activation of the B12g
mode in atomically thin MoTe2is due to translation
symmetry breaking along the c-axis direction. These findings suggest strong effects of symmetry breaking on the phonon properties of atomically thin MoTe2. RESULTS AND DISCUSSION
Bulk crystals of MoTe2were prepared through
chem-ical vapor transport,52 and were determined to have
a 2Hb-structure (R-phase) by using X-ray photoelectron
spectroscopy and X-ray diffraction.14 Atomically thin MoTe2 films were mechanically exfoliated from the
bulk crystals onto silicon substrates with oxide layers on top. Figure 1a is a typical optical image of atomically thin MoTe2deposited on a 90 nm-thick SiO2substrate.
We determine the thicknesses of the MoTe2 films
optically and using atomic force microscopy (AFM) in the tapping mode and Raman spectroscopy (see Figure S2 in Supporting Information for the identifica-tion of the number of layers from the Raman peak
intensity ratios).53Figure 1b is an AFM image of the area inside the white dashed lines indicated in Figure 1a. The profile along the blue line in Figure 1b shows a single layer spacing of∼0.7 nm and a double layer spacing of ∼1.4 nm (Figure 1c). Hence, the thickness of theflake in the scanned area is identified to be a single-layer- to six-layers-thick (Figure 1a,b). The larger height of single-layer MoTe2shown in Figure 1c
is due to either trapped contaminations at the MoTe2SiO2 interface, an artifact caused by the
tapping mode AFM, or a combination of both. We compare the optical contrasts between MoTe2of
various thicknesses and the SiO2substrate to establish
a reference for the identification of the number of layers. Figure 1d shows the optical contrast differences between the MoTe2and 90 nm-thick SiO2surfaces in
gray scale for the red, green, and blue channels, as functions of the number of layers (the contrast di ffer-ence is normalized with the contrast of the SiO2
sur-face; see Methods and Section S3 in Supporting Information for details).54The contrast difference var-ies clearly with thickness up to 10 layers for each color channel and can be used to identify the thicknesses of the thin MoTe2 films on SiO2, similar to other
two-dimensional dichalcogenides (see Figures S3 and S4 in Supporting Information for the optical and the corre-sponding gray scale images of MoTe2 with various
thicknesses and the optical contrast differences be-tween the MoTe2and 285 nm-thick SiO2surfaces).5456
TheR-MoTe2crystal has a 2HbMX2structure. 47,52
The 2HbMX2crystal consists of layers of a trigonal
prismatic XMX structure, as represented in Figure 2a. In the 2Hbstructure, the XMX layers are stacked in an
AbABaB sequence, where the upper and lower cases represent X and M atoms. Therefore, bulk 2HbMX2
belongs to the D46hsymmetry group with M and X
atoms in the D3hand C3vpoint groups, respectively. The
irreducible representations of the phonons in bulk MX2
Figure 1. (a) Typical optical image of single- to multilayer MoTe2on 90 nm-thick SiO2. The number of layers (NL with N = 16)
is indicated. (b) An AFM image of the area surrounded by white dashed lines in (a). (c) A profile of the MoTe2flake along the
blue line indicated in (b), showing a single-layer thickness of∼0.7 nm between the layers. (d) Optical contrast differences between the MoTe2and SiO2surfaces for the red, green, and blue channels of the optical images, as functions of thickness.
The contrast difference is normalized with the optical contrast of SiO2for each channel.
YAMAMOTO ET AL. VOL. 8 NO. 4 3895–3903 2014 3897
at the Brillion zone center (theΓ point) are
Γbulk ¼ A1gþ 2A2uþ 2B2gþ E1gþ 2E1uþ E2uþ B1uþ 2E2g
(1) where E1g, E12g, E22g, and A1gare Raman-active and E11u,
E21u, A12u, and A22uare infrared active. The other modes
are optically inactive. (Figure 2b; see also Figure S1 in Supporting Information for atomic displacements of all theΓ point phonon modes in bulk 2HbMX2). Because
of the loss of translation symmetry along the c-axis direction, single- and few-layers of 2HbMX2 belong
to different space groups, depending on the parity of the number of layers. Atomically thin crystals with an odd number of layers belong to the D13hsymmetry
group without inversion symmetry, while crystals with an even number of layers belong to the D33dsymmetry
group with inversion symmetry.57,58 The irreducible representations of theΓ point phonons of the N-layer MX2are thus Γodd ¼ 3N 1 2 (A 0 1þ E00)þ 3Nþ 1 2 (A 00 2þ E0) (2a)
for an odd number of N and Γeven ¼
3N
2 (A1gþ A2uþ Egþ Eu) (2b) for an even number of N, respectively.57,58Group theory predicts that the phonon modes in the single- and few-layer crystals exhibit different optical activities from those of the corresponding modes in the bulk. Among the phonon modes in single- and few-layer MX2, A10, E00,
A1g, and Egare Raman active, A200, A2u, and Euare infrared
active, and E0 is both Raman and infrared active. In Figure 2b, wefind some phonon modes have equivalent atomic displacements but have different optical activities
for single- and bilayer and bulk MX2. For example, the
bulk-inactive E2u mode becomes infrared active for
bilayers (Eu) and Raman active for a single-layer (E00).
Below, we denote the phonon modes of atomically thin crystals with the irreducible representations of the corre-sponding modes in the bulk, according to the literature. Raman spectroscopy of atomically thin 2HbMoS2,
MoSe2, WS2, and WSe2has shown peaks of the in-plane
E12g and out-of-plane A1g modes.4043 Furthermore,
peaks of an interlayer shear mode of E2
2g have been
observed at very low frequencies in few-layer MoS2and
WSe2.57,5961 The E1gmode is forbidden in the
back-scattering configuration in the bulk, but has been de-tected in few-layers of WSe2.58
We performed Raman spectroscopy of MoTe2, using
a solid-state laser with an excitation wavelength of 532 nm and a grating with 1800 grooves per milli-meter, unless otherwise noted. Figure 3a shows the Raman spectra of single- tofive- and 30-layer MoTe2.
The MoTe2films show prominent peaks of the E12g
mode at∼235 cm1and relatively weak peaks of the A1gmode at∼174 cm1, as previously observed in the
bulk crystals.4951The frequencies of these modes in MoTe2are smaller than those observed in MoS2and
MoSe2because of the larger weight of Te (see Section
S4 in Supporting Information for the peak positions of the E12gand A1gmodes in MoS2, MoSe2, and MoTe2).8,40
The peak near 235 cm1splits into two lines in 30-layer MoTe2. While one of the peaks is the E12gmode, another
peak may be an infrared active mode of E21u(the two
modes are a conjugate pair with almost the same frequencies with a small shift induced by the interlayer interactions; see Figure 2b). The Raman activation of the E21umode has been reported in bulk MoS2under the
resonance condition,62,63but the cause of the activation
Figure 2. (a) Crystal structure of 2HbMX2in a repeat unit (two layers). The metal (M) and chalcogen (X) atoms are
represented in yellow and green, respectively. (b) Phonon modes of single-layer (1L), bilayer (2L), and bulk 2HbMX2at theΓ
point in the unit cell. Bilayer and bulk MX2have phonon modes with equivalent atomic displacements, but different
irreducible representations and optical activities.“R”, “IR”, and “Ina” indicate the Raman active, infrared active, and optically inactive modes. The black dashed lines connecting the X atoms indicate interlayer interactions.
in MoTe2is unclear. We observe small peaks near 138 and
185 cm1. These peaks may be the second-order Raman modes, as observed in other dichalcogenides.4143
Figure 3b shows the peak positions of the E12gand
A1gmodes in MoTe2as functions of thickness. Wefind
the E12gmode upshifts by∼1.5 cm1, while the A1g
mode downshifts by ∼2 cm1, with decreasing the number of layers from 30-layers to single-layer, as observed in other dichalcogenides.4043The softening of the A1gmode in atomically thin layers is caused by
the smaller effects of interlayer interactions that induce restoring forces to MoTe2molecules,40while the
stif-fening of the E1
2gmode may be due to effects of the
boundary surface layers that lead to more effective forces to the MoTe2 molecules with decreasing
thickness.64We observe consistently an abrupt decrease in the A1gfrequency at trilayer thickness. The A1gpeak in
trilayer MoTe2may be its“in-phase” vibrational mode,
where the Te atoms in all three layers vibrate in phase (in the“out-of-phase” mode, the Te atoms in the middle layer vibrate 180 out-of-phase with respect to the outside layers). The“in-phase” and “out-of-phase” A1g
modes in trilayer MoTe2are expected to have lower and
higher frequencies, respectively, than the A1gmode of
single-layer MoTe2, because of interlayer interactions.65
However, we observe no clear peak of the“out-of-phase” A1g mode in trilayer MoTe2, likely because of the
low spectral resolution of our Raman measurements. Further work using higher resolution Raman spectros-copy is needed to determine the cause of a decrease in the A1gfrequency at trilayer thickness.
Figure 3c shows the frequency differences ωbulk
ω1Lfor MoS2, MoSe2, and MoTe2, whereωbulkandω1L
are the frequencies of the E12gand A1gmodes of the
bulk and single-layer crystals, respectively (see Section S4 in Supporting Information for the thickness-depen-dence of the E12gand A1gmode frequencies in MoS2
and MoSe2). For the A1gmode, MoS2shows largerωbulk
ω1Lthan MoSe2 and MoTe2, while MoSe2and MoTe2
show the small difference in ωbulk ω1L. We alsofind no
clear dependence ofωbulk ω1Lon compounds for the
E12gmode. These observations imply that the frequency
difference between the bulk and single-layer crystals has a complex variation for the compounds, depending on the interlayer interactions, the molecular weights, and the surface effects.64
In addition to the E12gpeak, we observe consistently
a strong peak at∼291 cm1in atomically thin MoTe2
(Figure 3a), which has been previously unassigned in Raman spectroscopy of bulk MoTe2.4951This peak is
also observed using a 633 nm excitation wavelength (see Figure S8 in Supporting Information for the Raman spectrum). Figure 4a plots the intensity ratio of the peak at∼291 cm1to the E1
2gpeak as a function of
thickness. The relative intensity is enhanced signi fi-cantly with decreasing thickness and becomes the strongest in bilayer MoTe2. However, the peak is
invariably absent in single-layer MoTe2, as shown in
Figure 3a. The peak position has no clear thickness dependence (inset of Figure 4a). To investigate the spatial variation of the peak intensity, we perform Raman intensity mapping of atomically thin MoTe2.
Figure 4c is a Raman intensity map of the E12gmode
(∼ 235 cm1) of the single- to few-layerflake shown in Figure 4b. The Raman map shows a homogeneous intensity distribution over the surfaces of each layer, except at the edges, indicating that the crystal quality is spatially uniform. Raman intensity mapping of the peak at∼291 cm1on the same flake in Figure 4d shows no detectable intensity in the single-layer re-gions, but strong intensities are observed in the bilayer region. The intensity is reduced on tri- and four-layer surfaces. Similar to the E12gpeak mapping, the peak
intensity shows small spatial variations for each layer thickness.
These observations suggest that the phonon mode at∼291 cm1 is Raman inactive in the bulk, but is intrinsically Raman-active in few-layer MoTe2, rather
than activated externally, e.g., by defects or oxidation of MoTe2.62To identify the phonon mode of the peak at
291 cm1, we calculate all of the phonon modes of R-MoTe2at theΓ point for single- to trilayer and bulk
Figure 3. (a) Raman spectra of single- tofive-layer and 30-layer MoTe2. The excitation wavelength is 532 nm. The Raman
intensity of 30-layer MoTe2is magnified by 10 times. The peak at ∼291 cm1in few-layer MoTe2is identified as the B12gmode
in the text. (b) Peak positions of E1
2g(red circles) and A1g(black squares) for MoTe2as functions of the number of layers. (c)
Frequency differences between the bulk and single-layer crystals, ωbulk ω1L, for MoS2, MoSe2, and MoTe2. The red circles
represent the E12gmode, and the black squares represent the A1gmode.
YAMAMOTO ET AL. VOL. 8 NO. 4 3895–3903 2014 3899
crystals, by employing DFT. The DFT calculations were performed using the Vienna ab Simulation Package (VASP) within the local density approximation (LDA; see Methods for details).66,67Table 1 shows the
calcu-lated frequencies of theΓ-point phonon modes for single- to trilayer and bulk crystals of R-MoTe2with
their Raman/infrared activities, determined by group theory (see Section S6 in Supporting Information for all the phonon frequencies calculated with DFT). The calculated frequencies of the E12gand A1gmodes in
bulk MoTe2and the corresponding modes in
single-and few-layer MoTe2crystals are in reasonable
agree-ment with the observed peak positions for each layer thickness, thus demonstrating the validity of the DFT calculations. In Table 1, wefind no Raman-active
modes near 291 cm1in single-layer and bulk MoTe2,
which is in consistent with the observations. The bi-and trilayer MoTe2 crystals both have Raman-active
modes of A1gand A10near the observed peak positions
of 291 cm1. Additionally, the Raman active A1gmode
is present near 291 cm1in four-layer MoTe2(see Table
S2 in Supporting Information for the calculated fre-quencies of the phonon modes of four-layer MoTe2).
Thus, we conclusively assign the observed peaks in few-layer MoTe2 at ∼291 cm1 as the out-of-plane
vibrational modes of A1gfor an even number of layers
and A10for an odd number of layers.
The A1gand A10modes in few-layer crystals
corre-spond to the B12gmode in the bulk, which is optically
inactive in 2HbMX2 (see Figure 2 and Table 1).
Figure 4. (a) Intensity ratio of the peak at∼291 cm1(which is identified to be the B12gmode in the text) to the E 1
2gpeak of
MoTe2, as a function of the number of layers. The inset is a plot of the peak position of the B12gmode as a function of
the number of layers. (b) An optical image of single- and few-layer MoTe2on 90 nm-thick SiO2. The number of layers (NL with
N = 1 to 4) is indicated. (b,c) Raman intensity maps of the MoTe2films shown in (b) at frequencies of (c) ∼ 235 cm1and
(d)∼ 291 cm1. The background intensities are subtracted. The scale bars are 5μm.
TABLE 1.Calculated Frequencies (in cm1) of theΓ Point Phonon Modes of Single-Layer (1L), Bilayer (2L), Trilayer (3L), and Bulk Crystals ofr-MoTe2a
in-plane out-of-plane
1L E00(R) 120.2 E0(RþIR) 242.3 A10(R) 178.1 A200(IR) 300.5
2L Eu(IR) 119.8 Eg(R) 121.0 Eu(IR) 241.0 Eg(R) 241.0 A2u(IR) 177.9 A1g(R) 180.2 A2u(IR) 297.7 A1g(R) 298.7
3L E00(R) 121.4 E0(RþIR) 241.0 A10(R) 179.4 A10(R) 298.5
E0(RþIR) 120.5 E00(R) 241.0 A200(IR) 178.3 A200(IR) 298.2
E00(R) 119.7 E0(RþIR) 239.9 A10(R) 177.0 A200(IR) 295.9
bulk E2u(Ina) 119.3 E1g(R) 121.6 E21u(IR) 239.8 E12g(R) 239.9 B1u(Ina) 176.3 A1g(R) 179.9 A22u(IR) 290.4 B12g(Ina) 296.9 aOnly high frequency phonons are shown.“R”, “IR”, and “Ina” indicate Raman- and infrared-active modes and optically inactive modes. The Raman/infrared activities of the
phonon modes are determined from group theory.
A recent Raman spectroscopy study, along with DFT calculations and group theory analysis has reported that the B12gmode of WSe2becomes Raman active at
the two-dimensional limit, because of translation sym-metry breaking.58 Furthermore, few-layer MoSe2 has
been observed to show a weak Raman peak possibly from the B12g mode.65 Symmetry-breaking-induced
Raman activation of a vibrational mode has also been observed in different groups of layered materials such as Bi2Te3and Bi2Se3.6870The peak intensities of the
symmetry-breaking-activated modes are reduced with increasing the number of layers because the crystals become more bulk-like with thickness. Indeed, we find the intensity ratio of the B1
2g peak to the E 1
2g
peak in MoTe2 decreases with increasing thickness
(Figure 4a). Accordingly, we determine crystal symme-try breaking along the c-axis direction as the cause of the Raman activation of the B12gmode in few-layer
MoTe2. However, the B 1
2g peak in atomically thin
MoTe2is more strongly enhanced than in MoSe2and
WSe2. Figure 5a,b are Raman spectra of single- and
bilayer WSe2and MoSe2flakes that are mechanically
cleaved from the bulk crystals onto 285 nm-thick SiO2.
The WSe2and MoSe2bilayers show peaks of the B12g
mode at∼310 and ∼355 cm1, which are absent in their single-layers. The intensities of the B12gpeaks in
WSe2and MoSe2are extremely weak, compared with
those of their most prominent peaks of the E12gor A1g
modes (the E12gand A1gmodes are nearly degenerate
in WSe2and, thus, they are indistinguishable
experi-mentally in our Raman measurements). In Figure 5c, we show the intensity ratio of the B12g peak to each
prominent peak of bilayer WSe2, MoSe2, and MoTe2.
The relative intensities of the B12gpeaks of WSe2and
MoSe2are nearly 10 times smaller than that of MoTe2.
The strong enhancement of the B12gpeak intensity in
atomically thin MoTe2is likely due to the large
polariz-ability of the Te atom.50Additionally, the B1
2gpeak is
persistently observed in MoTe2with thicknesses
rang-ing from two- to 13-layers, as shown in Figure 4a, but is diminished rapidly within several-layers in WSe2and
MoSe2.58,65 These results suggest strong effects of
dimensionality on the Raman activity of the B1
2gmode
in MoTe2.
In addition to translation symmetry breaking, the optical activity of a vibrational mode in an atomically thin layered material might change due to coupling to an underlying substrate. Raman spectroscopy of atom-ically thin 2HbTaSe2supported on SiO2shows a peak
of the Raman E1gmode that is forbidden in the
back-scattering geometry, but this peak is observed to be absent in free-standing TaSe2. The observations imply
that interactions between TaSe2and SiO2lead to the
Raman-activation of the E1gmode.71Lastly, we
inves-tigate the substrate effects on the Raman spectrum of atomically thin MoTe2. The atomically thin MoTe2films
were exfoliated onto SiO2substrates that were
pre-patterned with arrays of pits with diameters of 25 μm. The inset in Figure 6 is an optical image of trilayer MoTe2 deposited on a SiO2substrate with pits with
diameters of 2μm. Figure 6 shows the Raman spectra of trilayer MoTe2, supported on SiO2and suspended
over a pit. Both the supported- and suspended-MoTe2
films show the B1
2gpeaks with small differences in the
Figure 5. (a,b) Raman spectra of single-layer (black lines) and bilayers (red lines) of (a) WSe2and (b) MoSe2. The E12gpeak
intensities of the bilayer crystals are normalized with those of the single-layer crystals. (c) The intensity ratio of the B12gpeak
to each prominent peak (either E1
2gor A1g) of bilayer WSe2, MoSe2, and MoTe2.
Figure 6. Raman spectra of trilayer (3L) MoTe2supported
on SiO2(black line) and suspended over a hole (red line). The
Raman spectrum of the suspended MoTe2flake is
normal-ized with the E12gpeak intensity of the supported MoTe2
flake. The inset is an optical image of 3L MoTe2on 300
nm-thick SiO2with periodic arrays of holes with diameters of 2
μm. The edge of the MoTe2flake is highlighted by the black
dashed lines.
YAMAMOTO ET AL. VOL. 8 NO. 4 3895–3903 2014 3901
intensities, suggesting no obvious effect from the substrate on its Raman activation. Furthermore, we ob-serve a small peak of the E1gmode at∼120 cm1in both
the supported- and suspended-MoTe2 samples (see
Figure S9 in Supporting Information for the expanded Raman spectra of MoTe2 near ∼120 cm1). Thus, in
contrast with TaSe2, the activation of the E1gmode in
trilayer MoTe2is due to crystal symmetry breaking rather
than substrate effects, as previously observed in WSe2.58
Wefind the suspended MoTe2films show lower Raman
frequencies than the supported-MoTe2films. The redshift
in the Raman peaks in the suspendedfilms is due to the larger thermal effects induced by the incident laser, because of the lack of heat dissipation paths.3537
CONCLUSIONS
In summary, we have exfoliated atomically thin crystals ofR-MoTe2and investigated their
thickness-dependent phonon properties with Raman spectros-copy. Similar to other dichalcogenides, the Raman E12g
peak of MoTe2upshifts, while the A1gpeak downshifts,
with decreasing thickness down to the atomic scale. However, we have observed a strong peak in atom-ically thin MoTe2, which has been unassigned in bulk
MoTe2. The peak intensity is enhanced largely with
decreasing thickness, but the peak vanishes at single-layer thickness. We assign, by using group theory and DFT calculations within LDA, the observed peak as the bulk-Raman inactive B12gmode. The activation of the
B12gpeak at atomically thin thickness is due to
transla-tion symmetry breaking along the c-axis directransla-tion, rather than substrate effects. The relative peak inten-sity of the B12gmode in atomically thin MoTe2is much
stronger than those observed in WSe2and MoSe2. These
observations suggest strong effects of symmetry break-ing on the phonon properties of atomically thin MoTe2.
In contrast to other dichalcogenides, very little is known about the electronic and optical properties of MoTe2in
its atomically thin form. Our results could provide insight into the phonon properties of atomically thin MoTe2and
also a strategy for identifying the number of layers of MoTe2at the atomic scale for further studies.
METHODS
Experimental Details. Bulk crystals of R-MoTe2, 2HbMoSe2,
and 2HbWSe2 were synthesized through chemical vapor
transport.52For MoS
2, commercially available crystals were used
(Furuuchi Chemical Corporation). Atomically thin crystals of MoTe2, MoS2, MoSe2, and WSe2were exfoliated mechanically
from the bulk crystals onto SiO2with a thickness of either 90 or
285 nm, using adhesive tape. To obtain the suspended MoTe2
samples, atomically thin MoTe2 films were deposited onto
300 nm-thick SiO2 substrates that were prepatterned with
periodic arrays of pits with diameters ranging from 2 to 5μm. The patterns were fabricated using photolithography and dry etching processes. The thicknesses of the atomically thin crystals were identified optically and with Raman spectroscopy and atomic force microscopy.40,43,53For the optical contrast analysis, gray scale
images were extracted from the bare optical images for the red, green, and blue components by using ImageJ.54Raman
spectros-copy was performed in the backscattering configuration using 532 and 633 nm excitation lasers, a 100 objective, and a grating with 1800 grooves/mm (Tokyo Instruments, Inc.). The laser power was kept below 0.1 mW to avoid any damage to the samples. Atomic force microscopy was performed at room temperature in the tapping mode with silicon cantilevers (Seiko Instruments Inc.).
Calculation Details. Density functional theory calculations were performed using the Vienna ab Simulation Package.66,67
The core and valence electrons in the atoms were described with the projector-augmented wave method. The exchange-correlation functional was treated with the local density approx-imation, which gives a good description of the geometrical and phonon properties of layered materials. The plane-wave basis set cutoff was 500 eV for MoTe2. Monkhorst-Pack k-point meshes of 9
9 1 and 9 9 5 were used for the thin films and the bulk, respectively. To avoid interactions between the periodic images of the thin films in the stacking direction, a vacuum thickness of 15 Å was used. All of the structures were completely optimized with convergence thresholds of 106eV for the energies and 103eV/Å for the forces. The phonon frequencies and eigenvectors were calculated using density functional perturbation theory.
Conflict of Interest: The authors declare no competing financial interest.
Acknowledgment. We thank Yasushi Morihira and Hiroyuki Watabe from Tokyo Instruments, Inc., for their technical support
in the Raman measurement. This research was supported by a Grant-in-Aid (Kakenhi No. 25107004) from the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), initiated by the Council for Science and Technology Policy (CSTP) of Japan, and Experiment-Theory Fusion trial project by MANA.
Supporting Information Available: The Γ-point phonon modes of bulk 2HbMX2, identification of the number of layers
of MoTe2from the Raman intensity ratios, optical contrast
differ-ences between atomically thin MoTe2 and SiO2surfaces, peak
positions and intensities of the E12gand A1gmodes and their
thickness-dependences for MoS2, MoSe2, and MoTe2, the 633
nm-excited Raman spectrum of atomically thin MoTe2, the calculated
phonon frequencies of MoTe2, and Raman spectra of
supported-and suspended-MoTe2. This material is available free of charge via
the Internet at http://pubs.acs.org.
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