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Large-area and high-quality 2D transition metal telluride

Atomically thin transitional metal ditellurides like WTe2 and MoTe2 have triggered tremendous research interests because of their intrinsic nontrivial band structure. They are also predicted to be 2D topological insulators and type-II Weyl semimetals. However, most of the studies on ditelluride atomic layers so far rely on the low-yield and time-consuming mechanical exfoliation method. Direct synthesis of large-scale monolayer ditellurides has not yet been achieved. Here, using the chemical vapor deposition (CVD) method, we demonstrate controlled synthesis of high-quality and atom-thin tellurides with lateral size over 300 {\mu}m. We found that the as-grown WTe2 maintains two different stacking sequences in the bilayer, where the atomic structure of the stacking boundary is revealed by scanning transmission electron microscope (STEM). The low-temperature transport measurements revealed a novel semimetal-to-insulator transition in WTe2 layers and an enhanced superconductivity in few-layer MoTe2. This work paves the way to the synthesis of atom-thin tellurides and also quantum spin Hall devices.


Introduction
Distinct from most semiconducting transition-metal sulfide and selenide compounds that are only stable in the 2H phase, tellurides exhibit much richer structural variations and electronic properties, from semiconducting in the 2H phase to semi-metallic in the 1T′ phase. Among all telluride compounds, MoTe2 crystal is stable in both 2H and 1T′ phase while WTe2 only form 1T′ phase in nature (Fig. 1a). Therefore, transition metal ditellurides, represented by MoTe2 and WTe2, have attracted tremendous interests recently due to the facile switching between different phases and their unique electronic properties. For instance, large and non-saturating magnetoresistance in WTe2 bulk crystal was reported by Ali et al(1), presumably due to the perfect compensation of the electrons and holes even in an ultrahigh magnetic field. A laser-induced transition between 2H and 1T′ phases in MoTe2 thin film has been demonstrated to show an Ohmic homojunction (2).
Furthermore, high mobility up to 4000 cm 2 /Vs and 10000 cm 2 /Vs were reported in MoTe2 and WTe2, respectively (3,4). MoTe2 and WTe2 atomic layers are also predicted to be promising candidates of type-II Weyl semimetals (5,6), even in their alloys 1T′ (W, Mo)Te2 (7). More importantly, monolayer tellurides like WTe2 and MoTe2 are predicted to be a 2D topological insulator (8). These make transition metal ditellurides an excellent platform for studying fundamental physical phenomena, such as superconductivity (9) and quantum spin Hall effect (8).
They are also promising candidates for various potential applications such as spintronics and highefficiency thermoelectric devices (10,11).
However, most of the novel physical phenomena in transition-metal ditellurides have been demonstrated in mechanically exfoliated layers, e.g., the magnetoresistance in few-layer WTe2 (4) and the superconductivity in bulk 1T′-MoTe2 (12). The exfoliation method is low-yield and timeconsuming and only good for scientific research. Direct synthesis of few-layer and monolayer ditelluride would, therefore, be essential to large-scale applications. Among all the growth techniques, chemical vapor deposition (CVD) has been demonstrated as a facile method in synthesizing monolayer crystals in large scale, including graphene(13), MoS2/WS2 (14,15), (16,17), and their heterojunctions (18)(19)(20), by using different precursors under optimized reaction conditions. Although telluride compounds such as MoTe2 films were successfully synthesized by thermal flux and tellurization of molybdenum films (3,21), controlled synthesis of high-quality telluride atomic layers, even down to monolayer, remains elusive under the existing CVD or physical vapor deposition (PVD) conditions, mainly due to the lower environmental stability and activity of tellurium. For instance, the low chemical reaction activity of W and Mo with Te limits tellurization of W and Mo precursors (powder and oxides), although this method has been widely adopted for the preparation of sulfide and selenide monolayers. More specifically, the electronegativity difference between transitional metal (W or Mo) and Te is very small (~ 0.4 eV or 0.3 eV), indicating a weak bonding between the metals and Te atoms which makes the stoichiometry of ditellurides difficult to be obtained. Furthermore, even though the stoichiometry of WTe2 and MoTe2 is maintained, the as-synthesized product tends to decompose rapidly by emitting Te vapor at high reaction temperature (around 600 o C)(10), instead of evaporating into gas-phase telluride.
Here, we demonstrate a CVD strategy to directly synthesize WTe2 and MoTe2 few-layer and monolayers at large scale. The mixed compounds (weight ratio of the compound metal oxide: metal chlorides: Te is 1:1:1) and Te were used as the source of W (Mo) and Te, respectively. Such configuration of the precursors makes the reaction between Te and the metal sources react easily.
The Te in the mixed compounds can decrease the melting point of the mixed compounds, while the other Te powder in the upstream was used as to keep the Te atmosphere in the whole reaction process. The corresponding chemical reaction is shown in Supplementary Information. Raman spectroscopy was used to characterize the quality of the WTe2 and MoTe2 flakes to confirm their high quality. Two different stacking sequences of WTe2 bilayers were observed by STEM. The asgrown WTe2 atomic layers show non-saturating magnetoresistance which resembles the features of the mechanical exfoliated ones. A novel semimetal-to-insulator transition is observed in the electrical measurements of WTe2 few layers, whereas the few-layer MoTe2 shows enhanced superconducting behavior.

Results
Side view and top view of the crystal structure of 1T′ WTe2 and MoTe2 is shown in Fig. 1a and 1b, respectively. A schematic diagram of the reaction system is shown in Supplementary Fig. S1.  Fig. S2 and S3, respectively. Figure 1c shows a single-crystalline monolayer WTe2 with a length of ~ 350 µm and width of ~ 20 µm. The sharp edges at the two ends of this rectangle suggest the edge of WTe2 in 1T′ phase is terminated by planes that are intersected by 50. Figure 1d shows the optical image of a polycrystalline WTe2 monolayer film with domain size exceeding 100 µm. Figure 1e shows a bilayer WTe2 with a grain boundary at the intersection of two rectangular WTe2 domains Raman spectroscopy was employed to characterize the quality of the WTe2 and MoTe2 atomic layers. Figure 2a shows the Raman spectra of WTe2 films with different thicknesses ranging from monolayer to bulk. For few-layer WTe2 (< 10 L), only four optical vibrational modes, namely 1 10 , 2 3 , 1 7 and 1 9 were identified, compared with that reported in WTe2 crystal and flakes (22,23).
Interestingly, the 1 10 mode was not reported in few-layer WTe2. Furthermore, the intensity of 1 7 peak becomes stronger than other modes as the layer number decreases, similar to the reported result that collected the Raman spectra along b axis of the mechanically exfoliated WTe2 atomic layer (24). These results further confirm the high quality of our as-synthesized WTe2 atomic layers.
The optical image and Raman intensity mapping ( 1 9 , ~ 205 cm -1 ) of WTe2 is shown in Fig. 2b and 2c, respectively. The region for Raman mapping is highlighted in blue dashed square and the mapping size is around 20×20 µm. Raman mapping shows homogeneous intensity across the whole region, which indicates a low defect concentration in the as-synthesized WTe2 monolayers.
We also find that the grain boundary in WTe2 can be easily distinguished from the nearby regions Atom-resolved scanning transmission electron microscope (STEM) was applied to further investigate the atomic structure of the as-synthesized MoTe2 and WTe2 atomic layers. Figure 3a shows a high-resolution Z-contrast STEM image of a monolayer WTe2, revealing the 1T′ phase which composes of quasi-one-dimensional tungsten-tellurium zigzag chains along the a axis of the The as-synthesized MoTe2 atomic layer also maintains the 1T′ phase similar to the WTe2 layer presented above, as shown in Supplementary Fig. S11, where the electron-energy-loss spectra (EELS) are provided for direct comparison between the two materials. In light of the co-existence of different stacking orders in 2D materials, we also found that the CVD-grown 1T′ WTe2 maintains two different stacking sequences in the bilayer that are similar to other 2D materials. Layered CVD-grown ditelluride films provide an excellent platform to study the thicknessdependent electric transport. The electrical properties of the as-synthesized WTe2 and MoTe2 flakes with different thickness were investigated by means of field-and temperature-dependent transport measurements, as shown in Fig. 4. Figure 4a shows the resistivity of WTe2 as a function of temperature with thickness of 4 nm (5 layer), and 2 nm (2 layer). The corresponding optical and AFM images of the WTe2 devices are shown in Supplementary Fig. S14. Above 50 K, the resistance of 4 nm WTe2 decreasing monotonically with decreasing temperature shows a metallic behavior, which is similar to the transport properties of the bulk WTe2, However, an upturn in resistance curve was observed with T further reduced. Such phenomenon may be due to the twodimensional electron-electron interactions at the reduced dimension (32). Interestingly, the bilayer of bilayer WTe2 as a function of the magnetic field is shown in Fig. 4b. These results show that large and non-saturating magnetoresistance is preserved in our CVD-grown WTe2 even down to a bilayer sample, which further demonstrates their high quality. The MR reaches a maximum value of 28% at 2 K. For the thick WTe2 flakes (12 nm), the MR is about 2000% at 25K in a field of 10T, which is shown in Supplementary Fig. S15. These values are close to the order of magnitude of the recent reports(4).
Enhanced superconductivity is also observed in our as-synthesized few-layer MoTe2. The optical image of the MoTe2 device is shown in Supplementary Fig. S16. Figure 4c shows the longitudinal resistance Rxx as a function of temperature T of few-layer MoTe2 device in different perpendicular magnetic fields. Rxx decreases steadily from 300K to 40K, indicating that the sample shows a metallic behavior which is consistent with previous studies on exfoliated samples (3). With T further reduced, the sample gradually becomes superconducting below T=2.5K (the onset of transition) and reaches zero resistance at Tc=0. 5K. Surprisingly, the superconductivity in thinner samples is strongly enhanced compared with Tc=0.1K reported in its bulk counterpart (12). A similar phenomenon was observed in 2H TaS2 (33), and the reason is ascribed to an enhancement of the effective electron-phonon coupling constant in thinner samples. The inset of Fig. 4c  Hc2-Tc phase diagram in Fig. 4d and find a linear relationship between Hc2 and Tc near Tc. This is a characteristic of 2D superconductors and can be explained by the standard linearized Ginzburg-Landau (GL) theory, where (0) is the zero-temperature GL in-plane coherence length and 0 is the magnetic flux quantum. By fitting the experimental data with the above formula, a coherence length of 38 nm was obtained, which is much larger than that in Mo2C (34).

Discussion
In summary, large-scale and atom-thin ditellurides including WTe2 and MoTe2 were synthesized.
Complementary characterizations demonstrated the high quality of as-grown samples. Highresolution STEM imaging resolved that atomic structure of WTe2 and MoTe2 and also observed the domain wall in bi-layer WTe2 where the stacking boundary are revealed between two distinct stacking sequences. Electric transport measurement also revealed the semimetal-to-insulator transition in WTe2 and enhanced superconductivity in MoTe2. Our work will shed light on the synthesis of atom-thin telluride materials and boost the realization of quantum spin Hall devices.  Afterwards, the wafer was immersed in NaOH solution (1M) to etch the SiO2 layer over night.

Synthesis of
After lift-off, the PMMA/WTe2 (MoTe2) film was transferred into DI water for several cycles to wash away the residual contaminants, and then it was fished by a TEM grid (Quantifoil Mo grid).
The transferred specimen was dried naturally in ambient environment, and then dropped into and WTe2 using e-beam lithography. The Ti/Au (5/50 nm) electrodes are deposited using the thermal evaporator, followed by the lift off process. For the measurements of WTe2, the transport measurement is performed in the Quantum Design PPMS system with temperature ranging from 300 K to 2 K, and magnetic field up to 14 T. For the superconductivity measurements of MoTe2, the transport experiment is carried out in a top-loading Helium-3 cryostat in a superconducting magnet. An ac probe current Iac=10 nA at 30.9 Hz is applied from the source to the drain. Then a lock-in amplifier monitors the longitudinal Rxx through two additional electrical contacts.