from Semiconductor, Topological Insulator, to Weyl Semimetal

Polymorphic Layered MoTe

from Semiconductor, Topological Insulator, to Weyl Semimetal

Raman Sankar,*

G. Narsinga Rao,

I. Panneer Muthuselvam,

Christopher Butler,

Nitesh Kumar,

G. Senthil Murugan,

Chandra Shekhar,

Tay-Rong Chang,

Cheng-Yen Wen,

Chun-Wei Chen,

Wei-Li Lee,

M.-T. Lin,

Horng-Tay Jeng,

Claudia Felser,

and F. C. Chou*

†Institute of Physics, Academia Sinica, Taipei 10617, Taiwan

‡Center for Condensed Matter Sciences,^§Department of Physics, and^⊥Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

#Max Planck Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße, 01187 Dresden, Germany

∥Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan

¶National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

▽Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan

*^S Supporting Information

ABSTRACT: Large size (∼2 cm) single crystals of layered MoTe₂in both 2H- and 1T′-types were synthsized using TeBr4

as the source of Br₂ transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity Cp(T) measurement results reveal afirst order structural phase transition near ∼240 K for 1T′-MoTe2, which has been identified to be the orthorhombic Td-phase of MoTe₂ as a candidate of Weyl

semimetal. The STM study revealed different local defect geometries found on the surface of 2H- and Td-types of MoTe6units in trigonal prismatic and distorted octahedral coordination, respectively.

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The two-dimensional (2D) layered transition metal dichalco- genides (TMDs), MX₂(M = Ta, Mo, or W and X = Se, S, or Te), have attracted significant attention due to their rich physical properties including superconductivity, large thermo- electric effect, and anomalous magnetoresistance.¹⁻⁵These 2D materials have shown promising potential for next-generation devices as an alternative to graphene as a 2D monolayer material.^6,7In search of materials having both a band gap like a typical semiconductor and a high charge carrier mobility like graphene, the recently demonstrated MoTe₂-based high mobility device opened a new area for future applications.⁸

The recent theoretical prediction and following discovery of WTe₂as a topological type-II Weyl semimetal⁹with extremely large magnetoresistance⁵ have triggered extensive attention to uncover the origin of these intriguing emerging physical properties identified in TMDs, which arise from rich variations of intralayer packing of MX₆octahedral or trigonal prismatic units and interlayer stacking via van der Waals (vdW) interlayer interactions.^10,11The two most common structures of TMDs are the 2H-type (space group P6₃/mmc with MX₆ in trigonal

prismatic coordination) and the 1T-type (space group P3̅m1 with edge-shared MX₆ in octahedral coordination).¹⁰ In addition to the common 2H-type structure, MoTe₂ has also been found in a variation of 1T-type (α), namely the 1T′-type (β), which is composed of buckled layers of edge-sharing MoTe₆octahedra in monoclinic symmetry of space group P2₁/ m.¹²⁻¹⁴ Although the name 1T′-type has been discussed theoretically in parallel to those of 1T- and 1H-types by assuming there is one layer per unit cell,¹⁵the experimentally identified two layers per unit cell following the space group P2₁/m operation suggests that the correct categorization for the recently discussed 1T′-type should be named 2T′-type following the convention of TMDs.¹⁶So as not to be confused with those discussed in recent publications, we will continue to use the term 1T′ in this paper.

Recent theoretical investigations showed that the separation of Weyl points as well as the length of a Femi arc can be

Received: October 12, 2016 Revised: December 19, 2016 Published: December 20, 2016

pubs.acs.org/cm

adiabatically tuned as a function of Mo-doped WTe₂,¹⁷and the orthorhombic Td phase of pristine MoTe₂also possesses type- II Weyl nodes,¹⁸ which have much larger Weyl node pair separations than those found in WTe₂.^9,18Hence, Td-MoTe₂is expected to provide more convincing experimental evidence via angle-resolved photoemission spectroscopy compared to that of WTe₂. On the basis of the transport studies of single crystal β(1T′)-MoTe2 in the literature, the hysteretic behavior of resistivity near ∼247 K corresponds to the structural phase transition from the 1T′-type to the low temperature orthorhombic Td-phase,^14,19 although a recent investigation focusing on the few-layered 1T′-MoTe2did not confirm the Td phase existence.²⁰

1T′-MoTe2and Td-MoTe₂phases differ in their β angles, i.e., the former is monoclinic withβ = 93.9° slightly deviated from a right angle, but the latter is orthorhombic.¹⁹Surprisingly, this small distortion makes a crucial difference in the symmetry- induced electronic property change. Although the monoclinic 1T′ phase has both time-reversal and inversion symmetry, the Td phase MoTe₂ of missing inversion symmetry becomes a promising candidate for the investigation of Weyl fermions.¹⁸

It is clear that the growth of temperature-sensitive polymorphic single crystals of MoTe₂ is a challenge. Here, with the aim of growing large and high quality MoTe₂single crystals of controlled structure types, we have demonstrate the successful growth of polymorphic single crystals using a novel chemical vapor transport (CVT) method that involves a mixture of Mo and TeBr₄ as the precursor, where TeBr₄ decomposed at 420 °C becomes the natural source of Br2 as the transport agent in the CVT growth. By controlling the temperature profile of the source, large crystals (∼2 cm) of MoTe₂ have been grown successfully in less than a week.

Details of the CVT growth method as well as a comprehensive characterization of the as-grown single crystals are presented, including X-ray diffraction (XRD), Raman spectroscopy, scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and transport measurements.

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Synthesis and Crystal Growth. The chemical vapor transport method was employed using TeBr₄as Br₂ source, which acts as the transport agent and allows an effective and faster vapor transport to produce the necessary supersaturation of the expectedfinal product. A three-zone muffle furnace was fabricated for this purpose, having typical temperature profiles for the growth of single crystals of 2H- MoTe2 and 1T′-MoTe2, as shown in Figure S1. A stoichiometric amount of Mo/Te = 1:2 (6N purity for Te and 5N for Mo) was sealed into an evacuated quartz ampule and heated for 2 days at 750 °C.

Approximately 10 g of the prereacted MoTe₂ powder was placed together with a variable amount of TeBr₄(purity 4N) (120 mg) at one end of the silica ampule (length of 40 cm with inner diameter of 1.8 cm and outer diameter of 2.0 cm). Bromine concentration in the range of 2.6−3.7 mg was yielded sufficiently from ∼5 mg of TeBr4per cm³, which provided high transport rates of nearly 150 mg per day. All preparation steps before the quartz tube evacuatedflame sealing were carried out in an argon-filled glovebox of oxygen, and the water level was kept below∼1 ppm. The loaded ampule was evacuated and flame sealed before loading into the tube furnace for CVT growth. For the growth of 1T′-MoTe2single crystals, the end of the ampule containing the prereacted material was held at 1050°C, and the growth end was maintained at a temperature near 950°C with a temperature gradient near 2.5°C/cm for a week. Shiny 1T′-MoTe2single crystals of sizes up to 17× 10 × 2 mm³were obtained, as shown in the inset ofFigure 1(a). For the growth of 2H-MoTe₂ single crystals, the prereacted material was held at 800°C, and the crystals were grown at the end

maintained at a temperature of 750°C with a temperature gradient near 1.25°C/cm for approximately a week. Shiny 2H-MoTe2 single crystals of sizes up to 21× 16 × 2 mm³were obtained, as shown in the inset ofFigure 1(b).

The crystal structure and phase purity of the samples were checked by synchrotron X-ray powder diffraction (SXRD) using a wavelength ofλ = 0.619927 Å (BL01C2, NSRRC, Taiwan). Raman spectroscopy was performed in the backscattering configuration using 632 nm excitation lasers. Thefield-cooled (FC) and zero-field-cooled (ZFC) magnetization curves were measured in a commercial vibrating sample magnetometer (VSM, Quantum Design, USA) in the presence of an applied magneticfield of 10 kOe. The heat capacity (Cp) and transport measurements were carried out using the physical properties measurement system (PPMS, Quantum Design, USA).

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The room-temperature SXRD patterns of the pulverized as- grown MoTe₂ single crystal are shown in Figure S2. All diffraction peaks for single crystals grown at the low temperature (750 °C) profile can be indexed with space group P6₃/mmc (No. 194) of hexagonal 2H-type,¹²whereas the crystals grown at high temperature (950°C) can be indexed with space group P2₁/m (No. 11) of monoclinic 1T′-type at room temperature.¹³In addition, the low temperature (T = 200 K) diffraction peaks are indexed with space group Pmn21(No.

31) of orthorhombic Td-MoTe₂. The refined lattice parameters are listed inTable 1and found to be consistent with the values reported in the literature.12,13,21−25The diffraction patterns for the as-grown single crystals with planes of preferred orientation of (00l) reflections only are shown inFigure 1.

2H-MoTe₂ crystallizes in a layer-type structure with Mo atoms surrounded by six Te atoms in a trigonal prismatic coordination similar to that of 2H-MoS₂.¹²2H-MoTe₂has been characterized by a stacking sequence of XyXYxY··· (X,Y: Te atom; y,x: Mo atom) with all monatomic planes in 2D hexagonal close packing,²⁶ as shown in Figure 2(a).

Alternatively, the 2H-type is easier to describe using the face- sharing hexagonal close packing of trigonal prismatic MoTe₆ units in each layer, and these layers are held together by the weak van der Waals force that gives rise to the quasi-two- dimensional character, as shown inFigure 2(d). On the other hand, 1T′-MoTe2could be viewed as a distortion of the typical 1T-type with layers composed of edge-sharing MoTe₆- Figure 1.X-ray diffraction patterns of (a) 2H- and (b) 1T′-MoTe2

single crystals taken from the platelike pieces of preferred (00l) orientation. The as-grown single crystal samples are shown in the insets.

octahedra, which is demonstrated by the monoclinic distortion of β ≈ 93.9° from the original hexagonal lattice, as shown in Figure 2(b). The distortion from 1T to 1T′ not only breaks the in-plane hexagonal symmetry with the added mirror symmetry, the Te monolayers above and below the Mo monolayer also rearrange in a zigzag deviation along the z-direction relative to that of the flat Mo monolayer. The low temperature orthorhombic Td-MoTe₂ phase differs from the 1T′-MoTe2

phase by a small deviation from the right angle forβ, i.e., the monoclinic angle of 1T′ has a slightly larger (β = 93.55°) than that ofβ = 90° for Td with orthorhombic symmetry, as shown in Figure 2(c). It should also be noted that the structure symmetry of the 1T′-WTe2reported in the literature is identical to the Td-MoTe₂phase instead of the 1T′-MoTe2.²⁷

The electronic structure of 2H-MoTe₂, 1T′-MoTe2, and Td- MoTe₂were calculated using a 30× 30 × 6 and 8 × 16 × 4 Table 1. Crystal Structures and Lattice Parameters of MoTe₂Single Crystals

crystal type space group a (Å) b (Å) c (Å) angle

2H-MoTe₂ P6₃/mmc 3.4474(2) 3.4474(2) 9.3528(3) β = 90°; γ = 120°

1T′-MoTe2 P2₁/m 3.4692(4) 6.3348(4) 13.8835(2) β ≈ 93.9°

Td-MoTe₂ Pmn2₁ 3.4783(4) 6.3563(2) 13.8935(2) β = 90°

Figure 2.Crystal structures of (a) 2H-MoTe₂, (b) 1T′-MoTe2, and (c) Td-MoTe₂; the MoTe₆unit has a trigonal prismatic coordination for 2H and a distorted octahedral coordination for 1T′ and Td. MoO6coordination and projection views for 2H-MoTe₂along the [001] and Td-MoTe₂along the [100] directions are shown in (d) and (e), respectively.

Figure 3.Band structures and density of states of (a) 2H-MoTe2, (b) 1T′-MoTe2, and (c) Td-MoTe2. Blue and green lines indicate the Mo d-orbital and Te p-orbital, respectively. The correspondingfirst Brillouin zones (BZ) of all samples are shown below. The insets (b1) and (b2) of 1T′-MoTe2

are enhanced images around EF, which show a well-defined continuous energy gap throughout the whole BZ. The insets (b1) and (b2) of Td-MoTe2

show the enhanced band structure around EF, which shows a pair of Weyl points alongΓ-X and is fully gapped along the Γ-S direction.

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Monkhorst−Pack k-mesh over the Brillouin zone (BZ), respectively, as shown in Figure 3. The first-principles calculations were based on the generalized gradient approx- imation (GGA)²⁸using the full-potential projected augmented wave method²⁹as implemented in the VASP package.³⁰ The spin−orbit coupling (SOC) was included self-consistently. An indirect band gap of ∼0.81 eV is obtained for 2H-MoTe2

between the valence band maximum atΓ point and conduction band minimum at the center ofΓ-K, which is consistent with the transport measurement results. All of the bands are found to be spin degenerate due to the spatial inversion symmetry.

The Mo-4d states dominate the energy band from E_F to approximately−1.0 eV, whereas the Te-5p states are of lower energy. It is noteworthy that the valence band energy splitting at K point is primarily due to the SOC instead of the interlayer interaction (Figure 3(a)), which is similar to that found in 2H- MoS₂and 2H-WS₂.³¹

The electronic structure of 1T′-MoTe2 (Figure 3(b)) is dramatically different from that of 2H-MoTe2. The finite density of states (DOS) at E_F indicates a metallic phase, and this result is consistent with a metallic behavior of resistivity as shown inFigure 8. The orbital characters around E_Fare mainly derived from Mo-4d and Te-5p orbitals. Mo electron-like and Te hole-like band edges cross each other near E_F, which results in a band inversion and being gapped due to SOC. Hence, even though 1T′-MoTe2 has a metallic ground state, the valence band remains separated from the conduction band by a continuous energy gap throughout the entire BZ in the presence of SOC (below, Figure 3(b)). The band inversion feature and well-defined energy gap implies that 1T′-MoTe2

might be characterized as a Z₂ topological insulator (below, Figure 3(b)).¹⁵

The third structure of MoTe₂ is orthorhombic Td phase.

Contrary to monoclinic 1T′-MoTe2phase that possesses spatial inversion symmetry, the crystal inversion symmetry has been broken in Td-MoTe₂, which results from slight lattice structure distortion.Figure 3(c) shows the band structure of Td-MoTe₂ with SOC. The electronic structure exhibits significant band splitting due to the breaking of spatial inversion symmetry except for the time-reversal invariant points. Similar to the 1T′ phase, Td-MoTe₂shows a metallic ground state, whereas the orbital characters around E_F are mainly contributed from Mo- 4d and Te-5p orbitals. Zooming in around E_F along Γ-X (below, Figure 3(c)) shows that two singly generated bands cross each other and form two Weyl nodes with opposite chirality, which is consistent with a recent theoretical prediction.¹⁸

The scanning transmission electron microscopy (STEM) images of 2H-MoTe₂and 1T′-MoTe2were obtained in 200 kV JEOL 2100F equipped with a probe corrector for the spherical aberration. The TEM specimens were prepared by mechanical exfoliation and ion beam thinning (Gatan Precision Ion Polishing System II, PIPS II). Figure 4(a) is the high-angle annular dark-field (HAADF) STEM image of 2H-MoTe2along the [0001] zone axis. The intensity of the HAADF-STEM image is sensitively proportional to the atomic number Z,^1.7so that the positions of Mo atoms appear brighter in the figure.

From the selected-area diffraction pattern (inset of Figure 4(a)), the (11̅00) lattice planes are labeled, and the plane spacing is found to be near 3.0 Å, which is consistent with the 2H-MoTe₂ hexagonal lattice structure of lattice constant a = 3.52 Å. The HAADF-STEM image of the 1T′-MoTe2phase is shown inFigure 4(b) along the [001] zone axis. The (100) and

(010) lattice plane spacings, as labeled in the figure, are consistent with the lattice constants of a = 6.33 Å and b = 3.48 Å for the 1T′-MoTe2phase.

The scanning tunneling microscopy (STM) measurements were performed for both 2H-MoTe₂ and Td-MoTe₂ at 4.5 K and 1T′-MoTe2at room temperature using an Omicron LT- STM and an electrochemically etched tungsten tip. dI/dV (V) curves were acquired using the lock-in technique with a bias modulation of 10 mV. Crystals for STM measurements were prepared by in vacuo cleavage at room temperature in a preparation chamber at a base pressure lower than 1× 10⁻¹⁰ mbar before transfer to an STM chamber with a base pressure lower than 5× 10⁻¹¹mbar.Figure 5shows atomically resolved topography images taken on (a,b) 2H-MoTe₂, (c, d) Td- MoTe₂, and (f, g) 1T′-MoTe2 crystals and the relationship between the observed atomic corrugations of Te surface layer and the corresponding Te surface lattice modeling. The topography for the 2H structure exhibits a clear hexagonal surface lattice, whereas the Td and 1T′ structures have rectangular surface lattices that are indistinguishable using STM topography imaging, which can only probe the uppermost atomic layer and cannot discern the difference in angle β between the two structures. Tunneling spectroscopy curves acquired at 4.5 K as shown inFigure 5(e) suggest a metallic behavior for the Td-MoTe₂surface, which is consistent with the band structure calculation of apparent n-type, on the other hand, the 2H-MoTe₂surface shows a band gap of around 1.13 eV. These results are consistent with the calculation shown above and with previous predictions.⁹ Interestingly, a Figure 4.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images for (a) 2H-MoTe₂and (b) 1T′- MoTe₂. The selected-area diffraction pattern for (a) is along the [0001] zone axis and the [001] zone axis for (b). The plane spacing and the lattice planes are labeled.

comparison of enhanced STM topography of Td-MoTe₂taken at 4.5 and 77 K, as shown in Figure 5(h), reveals clear protrusions seen at the unit cell edges at 4.5 K (marked by black arrows), which are absent at 77 K. This difference is consistent with a similar one recently observed in WTe₂, which has been attributed to the onset at low temperature of Umklapp interference of quasiparticles induced by Rashba-type split- ting.³²Therefore, the topography at 77 K is expected to better represent the true surface lattice.

Step edges were observed in the as-cleaved MoTe₂surfaces, as shown inFigure 6, which is helpful to probe the interlayer stacking by exploring the relative orientations of two adjacent MoTe₂trilayers in stack, as illustrated inFigure 6(b and d). It has been nearly impossible to distinguish the reversed orientation of MoTe₆ units in either trigonal prismatic or octahedral coordination via the surface Te alone, mostly because the Te monolayer on the surface is always arranged in hexagonal close packing. However, we might distinguish the reversed orientation by taking advantage of the different local geometries for the Te vacancy defect sitting on surfaces of reversed orientation. Two types of characteristic defects can be identified in 2H-MoTe2and Td-MoTe₂at 4.5 K (Figure 6), as indicated by the dark arrowhead-like defects of symmetry m for Td-MoTe₂and the bright three-lobed defects of symmetry 3m for 2H-MoTe₂. In particular, the orientations for the two types of defects are seen reversed from one region to the next region that is one trilayer below, which is consistent with the 2H (P6₃/ mmc) and Td (Pmn2₁) symmetries of required reverse orientation between neighboring layers. Although 2H-MoTe₂ is able to maintain its hexagonal symmetry per layer, Td-MoTe₂ has lost its hexagonal symmetry with the additional in-plane mirror symmetry breaking (Figure 2), which is consistently reflected on the observed defect local symmetry of reversed orientations between neighboring layers exposed to the surface,

Figure 5.STM and STS measurement results on vacuum-cleaved 2H-, Td-, and 1T′-MoTe2acquired with V_bias= 0.6 V and I = 0.15 nA. Atomically resolved STM on 2H- MoTe₂, acquired at 4.5 K, is shown in (a) along with the corresponding FFT (inset). An enhanced image displaying the correspondence between the STM image and the Te surface lattice is shown in (b). The corresponding images for Td-MoTe₂obtained with V_bias=

−0.1 V and I = 0.2 nA at 4.5 K are shown in (c) and (d), respectively. dI/dV (V) acquired at 4.5 K for 2H- and Td-MoTe2are shown in (e). An STM topography image acquired for 1T′-MoTe2(same sample as in (c) and (d) at 294 K) is shown in (f) with its FFT shown in the inset. An enhanced image showing the correspondence to the 1T′ MoTe2lattice is shown in (g). A side-by-side comparison of atomic scale topography of Td-MoTe₂ taken at 4.5 and 77 K is shown in (h) with the features possibly caused by Umklapp interference at 4.5 K marked by black arrows.

Figure 6.Stacking of trilayers in 2H- and Td-MoTe₂ at 4.5 K. (a) Large-scale STM topography map (V_bias= 1 V, I = 0.3 nA), including a single trilayer step-edge and enhanced images (insets, V_bias= 1 V, I = 0.3 nA) taken on its left- and right-hand sides, showing a reversal of orientation of the characteristic dark arrowhead-shaped defects located in the two adjacent trilayers. (b) Topographic line profile taken across the step edge (along the dashed line in the large scale map), showing a height comparable with half the c-axis lattice parameter. The corresponding STM topography images (V_bias= −1 V, I = 0.2 nA) for both the large-scale and enhanced images) and line profile for the 1T′ structure (c and d), also revealing a reversal of orientation for the bright three-lobed defects (bound by black triangles) in the two adjacent trilayers.

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i.e., the former has the three-lobe shape of 3m symmetry and the latter has the arrowhead-like shape of m symmetry only.

The 2H and 1T′ phases of MoTe2of various thicknesses have been characterized with Raman spectroscopy at room temper- ature, as shown in Figure 7. The 2H-MoTe₂ crystal shows

Raman peaks between 100 and 300 cm⁻¹ (Figure 7(a)), including the prominent peak of the in-plane E¹_2g mode at

∼234 cm⁻¹, the out-of plane A_1gmode at∼173 cm⁻¹, and the out-of-plane bulk inactive B¹_2g phonon mode (∼290 cm⁻¹).

The intensity of the Raman modes shows significant thickness dependence from the bulk sample to the nanothickness near

∼10 layers. The presence of B2g1mode in the 10-layers sample indicates that the synthesized sample is atomically thin. Besides the observed first-order Raman peaks, several second-order resonant peaks with relatively low intensities at∼144, 183, and 204 cm⁻¹ are also observed in 2H-MoTe₂. The observed characteristic peaks of 1T′-MoTe2 are A_g (108.2 cm⁻¹), A_g (129.2 cm⁻¹), A_g(164.8 cm⁻¹), and A_g(261.5 cm⁻¹), as shown inFigure 7(b). Furthermore, a peak related to the Raman mode (B_g) at 180 cm⁻¹was observed at room temperature,³³whereas this mode disappeared at low temperature, as shown inFigure S3, which could be related to the expected 1T′ to Td phase transition near ∼240 K. The observed phonon modes match with those reported in the literature,³³⁻⁴⁰and the phase purities for both phases are confirmed in addition to the XRD structural analysis (Figure 1). The most prominent peak for 2H-MoTe₂is 234 cm⁻¹corresponding to the E_2gphonon, in contrast to 166 cm⁻¹corresponding to the A_gphonon for 1T′-MoTe2, which is consistent with the expected easier excitation for Te atoms in the distorted trilayer along the c-direction for the latter.³⁵

The electrical resistivitiesρ(T) as a function of temperature for both 1T′- and 2H-MoTe2 are shown in Figure 8. Figure 8(a) shows the resistivity of 2H-MoTe₂ as a function of temperature; a slight decrease upon cooling from 300 to 160 K is observed and then increases at lower temperature, which suggests the existence of a thermally excitable narrow gap of Figure 7.Room-temperature Raman spectra for (a) 2H-MoTe2and

(b) 1T′-MoTe2with various layer thicknesses.

Figure 8.(a) Resistivity of 2H-MoTe₂as a function of temperature. Inset shows the Arrhenius behavior below 50 K of afitted gap size of ∼35.3 meV. (b) Hall resistivity of 2H -MoTe₂as a function of magneticfield at different temperatures (50 K data are displayed in the lower left corner in larger scale). The temperature dependence of Hall mobility is shown in the inset. (c) Resistivity of 1T′-MoTe2as a function of temperature. Inset presents thefitting result according to the Fermi liquid theory (red solid line). (d) Hall resistivity of 1T′-MoTe2as a function of magneticfield at different temperatures with temperature dependence of Hall mobility displayed in the inset.

35.3 meVfitted with an Arrhenius law below 50 K (inset of Figure 8(a)). Further, we alsofit the data to the variable range hopping mechanism ρ = ρ0exp(T₀/T)^1/3 for two dimensions and found the hopping parameter (T₀^1/3) to be equal to 80 K^1/3. This gives rise to a differential activation energy of 31 meV at 50 K. Activation energy obtained from both mechanisms is on the same order. Figure 8(b) shows the Hall resistivity ρxy at several temperatures for the 2H-MoTe₂ single crystal. ρxy data shows a good linearity relative to the magneticfield up to 9 T above 50 K, indicating the dominance of electron carriers. Electronic mobility decreases with rising temperature (inset ofFigure 8(b)) of∼400 cm²V⁻¹s⁻¹at 50 K. Because the band gap size of the ideal 2H-MoTe₂has been estimated to be near∼1.0 eV (Figure 3), it is possible that the identified narrow gap from the transport study is the gap between the impurity and the conduction bands of n-type doping introduced by Te vacancies.²¹The magneticfield of 9 T does not affect the resistivity behavior with apparently no magnetoresistance.

Theρ(T) curve of 1T′-MoTe2indicates a metallic behavior of∼4.4 × 10⁻⁴Ω cm at room temperature as shown inFigure 8(c). A hysteretic anomaly in resistivity is observed between 240 and 265 K upon the warming and cooling cycles, which has been assigned corresponding to a first-order structural phase transition from the 1T′ phase to a low-temperature orthorhombic Td phase.¹⁴ This 1T′-to-Td phase transition can also be identified in the magnetic susceptibility and specific heat measurements, as shown in Figures S4 and S5. The magnetic susceptibilities of single crystal 1T′-MoTe2 as a function of temperature were measured in an applied magnetic field of 10 kOe for field applied along the ab-direction, and a hysteretic χ(T) step type anomaly near ∼254 K is found. In addition, the heat capacity C_p(T) shows an anomaly of onset near ∼250 K. Fitting Cp with the electronic and phonon contributions in C_p/T =γ + βT², whereγ is the normal-state electronic contribution andβ is the lattice contribution to the specific heat, the fitted results yield γ = 3.22 mJ mol⁻¹K⁻²and β = 0.849 mJ mol⁻¹ K⁻⁴ for Td-MoTe₂. Considering the saturation value (74.93 J mol⁻¹K⁻¹at 300 K) as the Dulong− Petit classical limit, the derived Debye temperature (β = N(12/

5)π⁴RΘD‑3(R = 8.314 J mol⁻¹K⁻¹) is found to be 190 K, which is of the same order and consistent with that found in orthorhombic WTe₂.⁴¹

The ρ(T) behavior does not show any noticeable change under a 9 T magneticfield for both 1T′-MoTe2and Td-MoTe₂ (Figure 8(c)), which is surprising compared to WTe₂ (orthorhombic Td structure) being identified as a large magnetoresistance material due to perfect electron and hole compensation.⁵Such a compensation of electron and holes are absent in Td-MoTe₂.²⁰The low temperature part (T < 50 K) of theρ(T) data is well fitted by ρ(T) = ρ0+ AT²following the Fermi liquid model, as shown by the solid red line in the inset ofFigure 8(c). The obtainedfitting parameters are the residual resistivityρ0= 5.97 × 10⁻⁵Ω cm and coefficient A = 1.42 × 10⁻⁸Ω cm K⁻², which indicates the Fermi liquid-like behavior for Td-MoTe₂single crystal below 50 K.⁴²In semimetals like 1T′MoTe2, the temperature-dependent resistivity can be expressed as ρ = a + bT + cT², where the first term is from electron-defect scattering, the second term is electron−electron scattering, and the third term is electron−phonon scattering. At low temperature, because of negligible phonon contribution, the resistivity varies almost as T², commonly known as the Fermi liquid behavior. At higher temperature, although the T²

term is present, because of large phonon contribution (large value of coefficient b), the resistivity varies almost linearly. The ρxyas a function of magneticfield at different temperatures for 1T′-MoTe2(at T = 300 K) and Td-MoTe₂(below 260 K) is shown in Figure 8(d). Below 260 K, ρxy shows a nonlinear behavior with magneticfield for the Td-MoTe2 single crystal.

Electrons remain the majority charge carriers in the whole temperature range for both 1T′-MoTe2and Td-MoTe₂single crystals. Td-MoTe₂has an electron mobility of 133 cm²V⁻¹s⁻¹ at 2 K and decreases with increasing temperature, whereas electron mobility of 1T′-MoTe2is very small at the level of∼3 cm²V⁻¹s⁻¹at 300 K (inset ofFigure 8(d)). The decrease in the carrier mobility of both 2H and 1T′ forms of MoTe2

decreases with temperature. This is very common in metals and semiconductors. This is mostly due to the increased phonon density at higher temperatures, which results in the smaller τ (average interval between two scattering events). As mobility is directly related to τ (μ = qτ/m*, where q is the electronic charge and m* is the effective mass), mobility also decreases with temperature. The effect of the phonon can easily be seen in the resistivity in metals. In 1T′-MoTe2, due to the negligible electron−phonon scattering contribution at low temperature, the rate of decrease in resistivity decreases and almost saturates.

Interestingly, the mobility also saturates at low temperature and decreases thereafter upon increasing temperature. These transport properties are consistent with those estimated from tunneling spectroscopy of STM shown inFigure 5.

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Large and high-quality single crystals of layered MoTe₂in both 2H- and 1T′-types have been grown using a CVT method, and the low temperature Td phase can also be generated from the as-grown 1T′-MoTe2. The three phases of distinctly different layer structures have been confirmed rigorously with XRD, Raman spectroscopy, STEM, and STM studies. The physical properties have also been compared through electrical resistivity, magnetic susceptibility, and specific heat measure- ments. Scanning tunneling spectroscopy confirms the reversed stacking relationship between the adjacent MoTe₂trilayers via the local symmetry of the point defects. The metallic behavior of Td-MoTe₂is found to be consistent with the predicted Weyl semimetal phase, and the n-type semiconducting behavior for 2H-MoTe₂ has also been confirmed via resistivity and STM tunneling spectroscopy.

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*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemma- ter.6b04363.

Crystal growth profiles, refined room-temperature SXRD patterns, Raman spectra for 1T′-MoTe2 single crystals with various temperatures, magnetic susceptibilities, and heat capacity data (PDF)

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*E-mail: [email protected], [email protected].

tw.*E-mail:[email protected].

ORCID

Raman Sankar: 0000-0003-4702-2517 Chemistry of Materials

Notes

The authors declare no competingfinancial interest.

■

R.S. and F.C.C. acknowledge the support provided by the Academia Sinica research program on Nanoscience and Nanotechnology under project number NM004. F.C.C.

acknowledges support from the Ministry of Science and Technology in Taiwan under project number MOST-102- 2119-M-002-004. We thank the Nanoscience and Technology thematic research program of Academia Sinica, Taiwan.

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TutiŠ, E. From Mott state to superconductivity in 1T-TaS₂. Nat.

Mater. 2008, 7, 960−965.

(2) Ang, R.; Wang, Z. C.; Chen, C. L.; Tang, J.; Liu, N.; Liu, Y.; Lu, W. J.; Sun, Y. P.; Mori, T.; Ikuhara, Y. Atomistic origin of an ordered superstructure induced superconductivity in layered chalcogenides.

Nat. Commun. 2015, 6, 6091−6096.

(3) Mak, K.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. A New Direct- Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805−136808.

(4) Chen, K. X.; Wang, X. M.; Mo, D. C.; Lyu, S. S. Thermoelectric Properties of Transition Metal Dichalcogenides: From Monolayers to Nanotubes. J. Phys. Chem. C 2015, 119, 26706−26711.

(5) Ali, M. N.; Xiong, J.; Flynn, S.; Tao, J.; Gibson, Q. D.; Schoop, L.

M.; Liang, T.; Haldolaarachchige, N.; Hirschberger, M.; Ong, N. P.;

Cava, R. J. Large, non-saturating magnetoresistance in WTe2. Nature 2014, 514, 205−208.

(6) Thoutam, L. R.; Wang, Y. L.; Xiao, Z. L.; Das, S.; Luican-Mayer, A.; Divan, R.; Crabtree, G. W.; Kwok, W. K. Temperature-Dependent Three-Dimensional Anisotropy of the Magnetoresistance in WTe₂. Phys. Rev. Lett. 2015, 115, 046602−046606.

(7) Castro Neto, C.; Novoselov, K. Two-Dimensional Crystals:

Beyond Graphene. Mater. Express 2011, 1, 10−17.

(8) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.

Single-layer MoS₂transistors. Nat. Nanotechnol. 2011, 6, 147−150.

(9) Soluyanov, A. A.; Gresch, D.; Wang, Z.; Wu, Q. S.; Troyer, M.;

Dai, Xi; Bernevig, B. A. Type-II Weyl semimetals. Nature 2015, 527, 495−498.

(10) Podberezskaya, N. V.; Magarill, S. A.; Pervukhina, N. V.;

Borisov, S. V. Crystal Chemistry of Dichalcogenides MX₂. J. Struct.

Chem. 2001, 42, 654−681.

(11) Ataca, C.; Şahin, H.; Ciraci, S. Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure. J. Phys. Chem. C 2012, 116, 8983−8999.

(12) Brown, B. E. The crystal structures of WTe2 and high- temperature MoTe₂. Acta Crystallogr. 1966, 20, 268−274.

(13) Puotinen, D.; Newnham, R. E. The crystal structure of MoTe₂. Acta Crystallogr. 1961, 14, 691−692.

(14) Hughes, H. P.; Friend, R. H. Electrical resistivity anomaly inβ- MoTe₂ (metallic behavior). J. Phys. C: Solid State Phys. 1978, 11, L103−L105.

(15) Qian, X.; Liu, J.; Fu, L.; Li, J. Quantum spin Hall effect in two- dimensional transition metal dichalcogenides. Science 2014, 346, 1344−1347.

(16) Levy, F. A. Intercalated Layered Materials; D. Reidel Publishing:

Dordrecht, Holland, 1979.

(17) Chang, T-Ro; Xu, S.-Y.; Chang, G.; Lee, C.-C.; Huang, S.-M.;

Wang, B. K.; Bian, G.; Zheng, H.; Sanchez, D. S.; Belopolski, I.;

Alidoust, N.; Neupane, M.; Bansil, A.; Jeng, H.-T.; Lin, H.; Zahid Hasan, M. Prediction of an arc-tunable Weyl Fermion metallic state in Mo_xW_1-xTe₂. Nat. Commun. 2016, 7, 10639−9.

(18) Sun, Y.; Wu, S.-C.; Ali, M. N.; Felser, C.; Yan, B. Prediction of Weyl semimetal in orthorhombic MoTe2. Phys. Rev. B: Condens.

Matter Mater. Phys. 2015, 92, 161107-1−161107-7.

(19) Clarke, R.; Marseglia, E.; Hughes, H. P. A low-temperature structural phase transition inβ-MoTe2. Philos. Mag. B 1978, 38, 121−

126.

(20) Keum, D.; Cho, S.; Kim, V.; Choe, D.-H.; Sung, H.-J.; Kan, M.;

Kang, H.; Hwang, J.-Y.; Kim, S. W.; Yang, H.; Chang, K. J.; Lee, Y. H.

Bandgap opening in few-layered monoclinic MoTe2. Nat. Phys. 2015, 11, 482−486.

(21) Vellinga, M. B.; de Jonge, R.; Haas, C. Semiconductor to metal transition in MoTe2. J. Solid State Chem. 1970, 2, 299−302.

(22) Wieting, T. J. Electrical conductivity of thin single crystals of the IVB-VIB dichalcogenides. J. Phys. Chem. Solids 1970, 31, 2148−2151.

(23) Al-Hilli, A. A.; Evans, B. L. The preparation and properties of transition metal dichalcogenide single crystals. J. Cryst. Growth 1972, 15, 93−101.

(24) Bromley, R. A.; Murray, R. B.; Yoffe, A. D. The band structures of some transition metal dichalcogenides: III. Group VIA: trigonal prism materials. J. Phys. C: Solid State Phys. 1972, 5, 759−778.

(25) Davey, B.; Evans, H. L. The optical properties of MoTe₂and WSe₂, Phys. Status. Solidi (a). Phys. Status. Solidi (a) 1972, 13, 483−

491.

(26) Wilson, J. A.; Yoffe, A. D. The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties. Adv. Phys. 1969, 18, 193−335.

(27) Dawson, W. G.; Bullett, D. W. Electronic structure and crystallography of MoTe₂and WTe₂. J. Phys. C: Solid State Phys. 1987, 20, 6159−6174.

(28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.

(29) Blochl, P. E. Projector augmented-wave method. Phys. Rev. B:

Condens. Matter Mater. Phys. 1994, 50, 17953−17979. Kresse, G.;

Joubert, J. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys.

1999, 59, 1758−1775.

(30) Kresse, G.; Hafner, J. Ab initio molecular dynamics for open- shell transition metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 13115−13118. Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane- wave basis set. Comput. Mater. Sci. 1996, 6 (6), 15−50.

(31) Latzke, D. W.; Zhang, W.; Suslu, A.; Chang, T.-R.; Lin, H.; Jeng, H.-T.; Tongay, S.; Wu, J.; Bansil, A.; Lanzara, A. Electronic structure, spin-orbit coupling, and interlayer interaction in bulk MoS₂and WS₂. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 235202-1− 235202-6.

(32) Li, Q.; Yan, J.; Yang, B.; Zang, Y.; Zhang, J.; He, K.; Wu, M.;

Zhao, Y.; Mandrus, D.; Wang, J.; Xue, Q.; Chi, L.; Singh, D. J.; Pan, M.

Interference evidence for Rashba-type spin-split on semimetallic WTe2

surface; arXiv:1602.08256v1 [cond-mat.mtrl-sci], 2016.

(33) Park, J. C.; Yun, S. J.; Kim, H.; Park, J. H.; Chae, S. H.; An, S. J.;

Kim, J. G.; Kim, S. M.; Kim, K. K.; Lee, Y. H. Phase-Engineered Synthesis of Centimeter-Scale 1T′- and 2H-Molybdenum Ditelluride Thin Films. ACS Nano 2015, 9, 6548−6554.

(34) Lin, Y.-F.; Xu, Y.; Wang, S.-T.; Li, S.-L.; Yamamoto, M.;

Aparecido-Ferreira, A.; Li, W.; Sun, H.; Nakaharai, S.; Jian, W.-B.;

Ueno, K.; Tsukagoshi, K. Ambipolar MoTe₂ Transistors and Their Applications in Logic Circuits. Adv. Mater. 2014, 26, 3263−3269.

(35) Cho, S.; Kim, S.; Kim, J. H.; Zhao, J.; Seok, J.; Keum, D.; Baik, J.; Choe, D.-H.; Chang, K. J.; Suenaga, K.; Kim, S. W.; Lee, Y. H.;

Yang, H. Phase patterning for ohmic homojunction contact in MoTe₂. Science 2015, 349, 625−628.

(36) Joshi, J.; Stone, I.; Beams, R.; Krylyuk, S.; Kalish, I.; Davydov, A.; Vora, P. Phonon Anharmonicity in Bulk Td-MoTe2. Appl. Phys.

Lett. 2016, 109, 031903−031907.

(37) Chen, S. Y.; Goldstein, T.; Venkataraman, D.;

Ramasubramaniam, A.; Yan, J. Activation of New Raman Modes by Inversion Symmetry Breaking in Type II Weyl Semimetal Candidate T′-MoTe2. Nano Lett. 2016, 16, 5852−5860.

(38) Beams, R.; Cançado, L. G.; Krylyuk, S.; Kalish, I.; Kalanyan, B.;

Singh, A. K.; Choudhary, K.; Bruma, A.; Vora, P. M.; Tavazza, F.;

Davydov, A. V.; Stranick, S. J. Characterization of Few-Layer 1T′

MoTe₂ by Polarization-Resolved Second Harmonic Generation and Raman Scattering. ACS Nano 2016, 10, 9626−9636.

(39) Zhang, K.; Bao, C.; Gu, Q.; Ren, X.; Zhang, H.; Deng, K.; Wu, Y.; Li, Y.; Feng, J.; Zhou, S. Raman signatures of inversion symmetry breaking and structural phase transition in type-II Weyl semimetal MoTe₂; arXiv:1606.05071, 2016.https://arxiv.org/abs/1606.05071.

(40) Naylor, C. H.; Parkin, W. M.; Ping, J.; Gao, Z.; Zhou, Y. R.;

Kim, Y.; Streller, F.; Carpick, R. W.; Rappe, A. M.; Drndic, M.;

Kikkawa, J. M.; Johnson, A. T. C. Monolayer Single-Crystal 1T′- MoTe₂ Grown by Chemical Vapor Deposition Exhibits Weak Antilocalization Effect. Nano Lett. 2016, 16, 4297−4304.

(41) Callanan, J. E.; Hope, G. A.; Weir, R. D.; Westrum, E. F., Jr. The preparation and lowtemperature heat capacity at temperatures from 6 to 326 K. J. Chem. Thermodyn. 1992, 24, 627−638.

(42) Zandt, T.; Dwelk, H.; Janowitz, C.; Manzke, R. Quadratic temperature dependence up to 50 K of the resistivity of metallic MoTe₂. J. Alloys Compd. 2007, 442, 216−218.

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