Optical Properties and Band Gap of Single- and Few-Layer MoTe2CrystalsClaudia Ruppert, Ozgur Burak Aslan, and Tony F. Heinz*

Departments of Physics and Electrical Engineering, Columbia University, 538 West 120th Street, New York, New York 10027, UnitedStates

*S Supporting Information

ABSTRACT: Single- and few-layer crystals of exfoliated MoTe2 havebeen characterized spectroscopically by photoluminescence, Ramanscattering, and optical absorption measurements. We find that MoTe2in the monolayer limit displays strong photoluminescence. On the basis ofcomplementary optical absorption results, we conclude that monolayerMoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV.This new monolayer material extends the spectral range of atomically thindirect-gap materials from the visible to the near-infrared.

KEYWORDS: Layered materials, transition metal dichalcogenides, 2D materials, MoTe2, electronic and optical properties

Atomically thin two-dimensional crystals have attractedmuch recent attention due to their distinctive proper-

ties.1−3 Monolayers of group-VI transition metal dichalcoge-nides (TMDs) are of particular interest as a family of ultrathinsemiconductors. Crystals of MoS2, MoSe2, and their tungstenanalogs have been shown to transform from indirect to direct-gap semiconductors in the monolayer limit.4−7 Moreover, thesematerials have shown their suitability for access to the valleydegree of freedom through excitation by circularly polarizedlight.8

The availability of a wide range of band gaps is importantboth for fundamental research and applications. For the TMDfamily, the direct gap varies with the choice of chalcogen atom,with single-layer MoS2 and MoSe2 having, for example, gaps of1.854,5 and 1.55 eV,6,9 respectively. The optical gap can betuned continuously between the limits of the stoichiometriccrystals using alloys with different chalcogen concentra-tions.10−13 Modification of the band gap of monolayer TMDsis also possible by applying strain and has yielded shifts around60 meV.14−16 The design of heterostructures of different TMDsallows for further tailoring the bandstructure17,18 and mayreveal fundamentally new effects related to interlayer coupling.Nevertheless, a material with a band gap close to 1 eV has notyet been identified and would be valuable both as a new 2Dsystem and as a building block for more complex structures.Bulk MoTe2 (α-MoTe2) is a semiconductor with an indirect

band gap of about 1.0 eV.19,20 It is composed of hexagonalsheets of Mo atoms that are sandwiched between twohexagonal planes of Te atoms, with trigonal prismaticcoordination (Figure 1a). These Te−Mo−Te units, which werefer to as monolayers, are held together by weak van der Waalsforces and are stacked with 2H symmetry (Figure 1a). Single-and few-layers of MoTe2 are promising candidate systems toexhibit narrow band gaps. They have a characteristic thickness-

dependent Raman spectrum21 and have been predicted toexhibit a phonon-limited room-temperature mobility greaterthan that of MoS2.

22 Transistor devices employing few-layerMoTe2 crystals have recently been demonstrated.23,24 In thisarticle, we present experimental results on the optical propertiesof mechanically exfoliated mono- and few-layer MoTe2. UsingRaman scattering, atomic force microscopy, and opticalcontrast, we identify crystals of 1−5 monolayer thickness.Photoluminescence and optical absorption measurements areapplied to locate the optical gap and to analyze the spectralsignatures related to the direct or indirect character of thetransition. In the monolayer limit, we find that the materialbecomes a direct gap semiconductor with an optical gap of 1.10eV. MoTe2 thus constitutes a new direct-gap monolayermaterial with a significantly lower gap than available heretofore.We prepared our ultrathin layers by mechanical exfoliation of

synthetic semiconducting MoTe2 crystals (supplier: 2Dsemi-conductors). The layers were deposited on either bulk z-cutquartz substrates (SiO2) or on silicon wafers with a 300 nmthick overlayer of thermal oxide (SiO2/Si). We cleaned some ofthe substrates with oxygen plasma prior to exfoliation. Thisallowed us to exfoliate larger samples, as required for thereflection contrast measurements.The Raman spectroscopy measurements were performed in a

commercial micro-Raman setup (Renishaw In-Via) in abackscattering geometry. A 100× objective (NA = 0.85) wasused to collect the scattered photons, which were analyzed in aspectrometer equipped with a grating of 2400 l/mm. The laserpower on the sample was about 5 μW (for 532 nm excitation)and 20 μW (for 633 nm excitation). The measurements of

Received: July 7, 2014Revised: September 29, 2014Published: October 10, 2014

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reflectance contrast were performed on MoTe2 flakes exfoliatedon the transparent SiO2 substrates. The experimental setupconsisted of a quartz tungsten halogen source in combinationwith a microscope using a 50× objective (NA = 0.85). Thereflected light from the sample was dispersed by amonochromator onto a liquid-nitrogen cooled Si or InGaAsCCD array. The peak positions and line widths of thereflectance contrast measurements were extracted by fittingmultiple Lorentzian line shapes to the experimental data.Photoluminescence (PL) measurements were performed in thesame setup with a 100× objective (NA = 0.95) to collect thebackscattered emission. For excitation, we used a HeNe laser ora 647 nm solid-state laser. The excitation power level was keptbelow 10 μW. Reflected and elastically scattered laser radiationwas suppressed by a long-pass filter in front of themonochromator. All experiments were performed at roomtemperature.Before discussing the experimental results, we comment

briefly on the long-term stability of the MoTe2 samples used inthis study. For exfoliated monolayers held under ambientconditions, a decrease in the photoluminescence yield wasconsistently seen within one or two months. The degradationof MoTe2 layers, as judged by the optical contrast and PL yield,was found to depend strongly on the treatment of the substrate.For substrates cleaned by the oxygen plasma prior toexfoliation, the lifetime of monolayers decreased further, evendown to the range of a day. Therefore, the samples were storedin vacuum, and the data presented in this study were obtainedfrom fresh samples. While we cannot exclude the possibilitythat the magnitude of the reflectance contrast spectra wasaffected by sample aging, the energy and line width of the

resonances did not change, even for heavily degradedmonolayers. Encapsulation with h-BN layers improved thestability of the MoTe2 monolayers.We exfoliated micron-sized few-layer flakes of MoTe2 on

both SiO2 and SiO2/Si. An example of MoTe2 layers on a SiO2substrate is shown in Figure 1b. We can identify layerthicknesses varying from the monolayer (1L) through threelayers (3L) in different regions. Measurements with an atomicforce microscope (AFM) were performed on the samples todetermine the layer thickness; a representative height profile fora flake on SiO2/Si is presented in Figure 1(c). The measuredinterlayer separation lies in the range of 0.6−0.65 nm, which isconsistent, within experimental uncertainty, with the bulk layerspacing of 6.982 ± 0.002 Å reported in the literature.25 Thespacing between the monolayer and the substrate determinedby AFM (not shown) varies significantly in different measure-ments and presumably reflects changes in the tip−surfaceinteraction between the substrate and the sample26 and possiblythe presence of adsorbates under the sample.27 To identify thesamples of monolayer thickness, we therefore made use ofRaman spectroscopy. In particular, the inactivity of the RamanB1

2g mode at 289 cm−1 permits unambiguous identification ofmonolayers, as discussed below. After calibration, we could alsosimply use the green channel of the optical images, togetherwith Raman measurements, for rapid and reliable identificationof layer thickness.Figure 2a displays the optical spectra from the visible to the

near-infrared for samples with thicknesses from one to threelayers. The data are presented as reflection contrast spectra,that is, ΔR/R = (RMoTe+S − RS)/RS, where RMoTe+S and RSdenote, respectively, the reflectance of MoTe2 on the substrate

Figure 1. (a) Crystal structure of α-MoTe2. (b) Optical image of few-layer MoTe2 exfoliated on SiO2, showing regions with thicknesses from amonolayer (1L) to three layers (3L). The image contrast has been enhanced to improve the visibility. (c) Representative AFM profile showing therelative heights of a MoTe2 sample exfoliated on SiO2/Si with the corresponding trace indicated by the blue line in the optical image in the inset.

Figure 2. (a) Reflectance contrast ΔR/R spectra for monolayer (1L), bilayer (2L), and trilayer (3L) MoTe2 crystals on SiO2 substrates. (b) Detail ofthe spectrum for the monolayer in the infrared (black curve). The right vertical scale displays the corresponding absorbance for the free-standingmonolayer. We fit the spectrum with multiple Lorentzian peaks (blue curve) with the A and B contributions shown separately (red curve).

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and of the bare substrate. All of the presented spectra weremeasured using samples deposited on a thick, transparent SiO2substrate. For a sample on such a transparent substrate withsufficiently small reflection contrast, the reflection contrastspectrum is proportional to the absorption spectrum of thecorresponding film. In particular, we can obtain the absorbance

of the free-standing layer from the optical contrast as =1/4 (nS

2 − 1)(ΔR/R),28−30 where nS denotes the refractiveindex of the substrate.31

Several peaks are clearly observable in the spectra of Figure2a, which we have labeled according to the bulk assignments ofWilson and Yoffe.32 These spectroscopic features are associatedwith transitions in different parts of the Brillouin zone ofMoTe2. The A, B and A′, B′ pairs have been identified asexcitonic transitions with the A-B splitting arising from spin−orbit interactions.32,33 As for other TMD monolayers, the Aand B peaks are assigned to excitonic peaks associated with thelowest direct optical transition at the K-point.6,34,35 Thesplitting between A, B and A′, B′ has been related to interlayerinteractions.33,36 The additional A′, B′ pair exists in other TMDmaterials, but has recently also been observed in WSe2 atmonolayer thickness,7 where interlayer interactions areobviously absent. For the bulk MoTe2 crystal, the C and Dfeatures have been attributed to regions of parallel bands nearthe Γ point of the Brillouin zone of bulk MoTe2

36 and tosimilar parallel bands in monolayers of other TMDmaterials.37,38

The infrared part of the monolayer spectrum is presented ingreater detail in Figure 2b. In this figure, we also indicate thecorresponding absorbance for a free-standing monolayer,determined according to the relation given above. To analyzeour data further, we fit the resonances in the absorptionspectrum to Lorentzian components. The red line in Figure 2bdisplays the contributions of the A and B features to the fit. Theline shape of the A peak in our experimental data reveals theexcitonic character of the transitions, being well described by asingle Lorenztian feature with no signature of free-carriercontributions to an interband transition.The energy difference between the A and B features for the

other TMD systems is known to arise from the spin−orbit-splitting of the corresponding bands, mostly governed by thevalence-band splitting.6,39 Theory predicts the same behaviorfor MoTe2.

40 The center positions of the two excitonic

transitions are found to be 1.095 ± 0.005 and 1.345 ± 0.005eV. Repeated measurements on other monolayer MoTe2 flakeson transparent substrates yield very similar positions of the Aand B peaks, with deviations of only a few meV. The spectralwidth of the A exciton absorption is about 50 meV (full widthat half-maximum, fwhm), while the B exciton width is severaltimes greater in accordance with the trend for the features inbulk MoTe2 crystals.

32 On the basis of the positions of the Aand B features, we infer a spin−orbit splitting of 250 meV formonolayer MoTe2. This splitting is significantly greater thanthe corresponding value of 150 meV for MoS2.

4

Comparison of the monolayer spectrum with the bilayer andtrilayer spectra shows that most of the peaks shift only slightlyin energy, with the overall reflectance contrast increasingapproximately linearly with thickness. Like other TMDmaterials, the A exciton resonance shifts by about 20 meV tolower energy from monolayer to bilayer, with even smallerredshifts for further increases in layer thickness. The line widthof the A exciton also increases with layer thickness. The Aexciton width of the bilayer is 20 meV larger than that of themonolayer. A more prominent change is seen for the A′/B′peaks, with the A′ feature shifting strongly toward higherenergy with decreasing thickness.In order to gain insight into the nature of the optical

transition of the lowest excitonic state, we measured photo-luminescence (PL) spectra for different layer thicknesses ofMoTe2 crystals. Figure 3a shows the thickness-dependent PLfor a flake exfoliated on a SiO2/Si substrate. The monolayer PLspectrum exhibits a single emission peak, with a maximum at aphoton energy of 1.08 eV and a width of 55 meV (fwhm). Withincreasing layer thickness, the maximum of the PL shifts tosignificantly lower energies. For the bulk, we find a maximum inthe PL intensity around 0.93 eV. The monolayer photo-luminescence is far stronger than that of the bulk. For thesample studied here, the PL is enhanced approximately 3 ordersof magnitude.For the monolayer, the position of the PL peak lies very close

to the A exciton resonance seen in the absorption spectrum.Figure 3b shows a direct comparison of the absorption andemission spectra for the same sample on a SiO2 substrate. Theenergy shift between the two peaks is about 10 meV. Fordifferent monolayers on SiO2/Si substrates, the position of theemission peak varies on the order of ∼10 meV and is mostly

Figure 3. (a) Thickness-dependent photoluminescence (PL) spectra for MoTe2 crystals on SiO2/Si. Results are shown for thicknesses ranging fromone (1L) to five layers (5L), as well as for the bulk crystal. The spectra are shifted vertically for clarity. The bulk data were measured with higherexcitation power, but normalized in the figure assuming linear response. (b) Comparison of PL and reflectance contrast spectra for 1L MoTe2 on aSiO2 substrate. (c) Photon energy of the peak in the PL spectra in (a) and of the A exciton resonance (AX) extracted from reflectance contrastmeasurements for samples of 1−4 layers.

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lower in energy. The best samples show both narrow PLspectra and very slight spectral shifts between the absorptionand emission peaks.For thicker layers, the A exciton resonance in the reflectance

contrast spectra and the PL maxima both shift toward lowerenergies. However, the rate of the red shift of these two featureswith increasing thickness is quite different. Figure 3c comparesthe positions of absorption maxima from Figure 2 with those ofthe emission maxima in Figure 3. Note that the measurementswere performed on different samples, whence the somewhatlarger shift for the monolayer. The A exciton absorption seen inΔR/R measurements displays much smaller shifts and remainsalmost constant for thicker layers, while the PL maximumcontinues to shift to lower energies with increasing thickness.We attribute this shift between features in the absorption (ΔR/R) and emission (PL) spectra to the emergence of an indirecttransition at lower energies, as occurs, for example, in MoS2 andWS2.

4,7,41 Although we do not expect the lower-lying indirecttransition to yield measurable absorption, it will still contributeto the emission spectrum, as is the case for the other TMDmonolayers. We do not observe an additional emission peak inthe PL spectra of thicker layers (2L−5L) near the PL andabsorption maximum of the monolayer. This behavior contraststo reported spectra in other TMD materials in which the directtransition from the A exciton remains visible in the PL spectraof thicker layers.4,7,41 However, the relative strength of the PLpeaks from the A exciton and the indirect transition dependsstrongly on the material and the excitation conditions. The PLfrom the A exciton in few-layer WSe2 is, for example, usuallyrather weak.7 In MoTe2, the width of the features and therelatively small shifts further hinder identification of twoemission peaks. The red shift for the A exciton peak in thereflectance contrast spectra in the 2L sample compared to the1L sample is 21 meV. In comparison, the maximum of the PLpeak seen in Figure 3a shifts by 33 meV. We therefore concludethat the bilayer PL is governed primarily by the emergingindirect transition. For a sample probed at low temperature, wein fact do observe two distinct emission peaks for PL from thebilayer crystal (see Supporting Information for details).The redshift of the PL maximum of ∼100 meV from one to

five layers is relatively small compared to that for MoS2 andWS2 crystals. The observed trend is, however, consistent withtheoretical predictions of a decreased redshift as the chalcogenis changed from S to Se to Te in Mo- and W-based TMDs.42

The result is also compatible with the fact that the redshift ofthe indirect transition is smaller in WSe2 than in WS2.The strength of the PL from monolayer MoTe2 depends

strongly on the choice of substrate. The PL yield is relativelylow for all tested transparent substrates (crystalline SiO2, as wellas crystalline Al2O3), but can be as much as 2 orders ofmagnitude greater for SiO2/Si wafers. Although interferenceeffects are present that enhance the yield for the SiO2/Sisubstrate,43 this can account only for a small part of theobserved increase in PL yield. We note that changes of PL yieldin our samples are typically also accompanied by shifts of about10 meV in the position of the emission maxima. This suggeststhat the difference in PL yield may be linked to charge transfereffects from the substrate, which can shift the ratio of emissionfrom charged and neutral excitons and also alter the totalstrength of the PL emission.44,45

We finally wish to comment briefly on the capability ofRaman measurements for identifying the thickness of differentlayers. Figure 4 shows Raman spectra for the few-layer flake inFigure 1c and for a bulk MoTe2 sample with excitation atwavelengths of 532 and 633 nm. The characteristic phononmodes of MoTe2,

46,47 A1g at 170 cm−1, E12g at 234 cm−1, and

B12g at 289 cm

−1 have recently been observed and characterizedfor mono- and few-layer samples.21 The B1

2g mode, inactive inthe bulk, is allowed for few-layer MoTe2 due to the breaking oftranslational symmetry.48 Because it is also absent for themonolayer, it is observed to have maximum intensity for thebilayer and then to decrease with increasing thickness.The mono- and few-layer Raman response differs strikingly

for excitation at 633 nm compared to that for 532 nmexcitation. In the former case (Figure 4b), the A1g mode isunusually strong. For the monolayer, the A1g mode has by farthe strongest Raman response; for few-layer samples (exceptthe bilayer) this mode is stronger than B1

2g. For 532 nmexcitation, on the other hand, the A1g mode is consistentlyweak. As is common for other few-layer systems, analyzing therelative shift and amplitudes of the E1

2g and B12g Raman modes

aids in the identification of sample thickness but is not alwayssufficiently precise. However, for 633 nm excitation, the Ramanspectra of 1L and 2L samples exhibit such different A1g and B

12g

mode intensities compared with thicker layers that they can bereadily identified. Because the intensities of E1

2g and B12g modes

scale in a similar way with layer thickness for both excitationwavelengths, the peak at 170 cm−1, which we have heretofore

Figure 4. Raman spectra of 1L−5L and bulk MoTe2 for the excitation wavelengths of (a) 532 nm and (b) 633 nm. The spectra are offset for clarity.For simplicity, the Raman modes are labeled according to bulk notation.

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designated as the A1g vibration, in fact may correspond to adifferent type of Raman response. A candidate for the strongresponse at 170 cm−1 is a resonant two-phonon Raman processinvolving acoustic phonons. In other TMDs,49−51 resonantenhancement of such a second-order Raman process has givenrise to a peak near the Raman shift of the A1g optical mode. Adeeper analysis of this phenomenon would require detailedstudies of the Raman response as a function of the laserexcitation wavelength.In summary, we have successfully exfoliated monolayer and

few-layer MoTe2 crystals on various substrates and have appliedabsorption and emission spectroscopy to characterize thesematerials’ optical properties. For the monolayer, the optical gapis located at 1.10 eV and the intense photoluminescence at thepeak of the optical absorption feature provides strong evidencethat monolayer MoTe2 is a direct gap material. The shift of thePL to lower energies with little change in the position of theabsorption feature indicates that thicker layers are indirect-gapsemiconductors, showing a behavior analogous to that exhibitedby MoS2. For the monolayer, we observe the spin−orbitsplitting of the A and B excitons of 250 meV. We presentRaman scattering data showing that in contrast to the case ofexcitation with 532 nm radiation, the spectra for 633 nmexcitation permit unambiguous identification of mono- andbilayer MoTe2 crystals.

■ ASSOCIATED CONTENT*S Supporting InformationLow-temperature photoluminescence spectra of mono- andbilayer MoTe2. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: tony.heinz@columbia.edu.Author ContributionsAll authors contributed to writing of the manuscript and havegiven approval of the final version.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by the National ScienceFoundation through Grant DMR-1124894 for sample prepara-tion and characterization by the Office of Naval Research foranalysis. C.R. acknowledges support from the Alexander vonHumboldt Foundation. We thank Dr. Omer Yaffe for assistancein performing the AFM measurements.

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Nano Letters Letter

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Nano Letters Letter

dx.doi.org/10.1021/nl502557g | Nano Lett. 2014, 14, 6231−62366236