Liquid-vapor Growth of Atomically Thin Metal Tellurides with Controllable Polymorphism

Atomically thin transition metal dichalcogenides (TMDs), together with their polymorphism, provide promising alternatives for next generation electronic devices and a platform to explore exotic quantum phenomena. However, a large-scale synthesis method that can reliably produce high-quality two-dimensional (2D) TMDs with controlled phase is still lacking. Instead, TMDs with high concentration of defects and defect-stabilized metastable crystalline phases are often obtained via conventional chemical vapor deposition. Here we developed a liquid-vapor (LV) technique to exploit liquid precursors to signicantly suppress the equilibrium shift to the decomposition direction and successfully synthesized high-quality TMDs. We highlight the importance of exploiting the synergism of equilibrium and kinetics to facilitate the synthesis reaction (forward) and to impede decomposition (reverse). A high concentration of reactants in the liquid phase also maximizes the kinetic rate of defect repairing. We demonstrated the advantages of LV method by synthesizing diverse high-quality 2D metal tellurides with controllable polymorphs, which would be challenging, if not impossible, to realize by using conventional methods due to weak metal-tellurium bonds, thermal instability and the co-existence of mixed crystalline phases. In particular, we successfully synthesized high-quality monolayer 2H MoTe 2 , which is only possible when Te defect level is substantially suppressed. Our approach provides a new paradigm in high-quality and large-scale materials synthesis and can be readily extended to a variety of quantum materials, potentially accelerating both research and industrial initiatives. nm pulse laser excitation. SEM characterization was performed on Zeiss Merlin High-resolution SEM.

In order to address these challenges, here we present a new liquid-vapor (LV) synthesis mechanism and take full advantage of liquid phase of precursors to synthesize TMDs. Unlike the conventional VSS or other solid precursor-based CVD methods, liquid precursors are introduced as a reservoir, providing su cient amounts of active species that signi cantly facilitate the synthesis reaction of TMDs and suppress the decomposition of the target TMD compounds. Even for TMDs with as-formed defects, the su cient reactants provided in such a LV mode maximize the chance of defects repairing at high temperature. Therefore, the LV method is expected to produce high-quality 2D TMDs, and enable synthesis of TMDs with controlled phases that are unattainable via conventional VSS method.
We experimentally tested this LV method by using metal tellurides as a case study. 2D metal tellurides are highly attractive for topological physics and phase engineering due to their strong spin-orbit coupling and polymorphism [9][10][11][12] . The phase versatility of metal tellurides can greatly facilitate phase engineering such as phase heterostructure 2,13 and phase transition 12 , which are promising for device applications, including high-performance 2D transistors with ohmic contact, topological transistors, recon gurable circuits and memory devices, etc. Despite their great potential, the low electronegativity of tellurium and the weak metal-telluride bonds make this family of 2D TMDs particularly vulnerable to the undesirable chemistry in the VSS mode of CVD processes. Moreover, the tendency of decomposition and oxidation often leads to very defective synthesis, and for many cases, even a complete failure. In addition, 2D metal tellurides exhibit diverse polymorphs with similar formation energies, which makes the precise phase control even more challenging. To our knowledge, CVD grown monolayer 2H MoTe 2 (with phase control) has not been reported before.
In this work, we experimentally demonstrated that the proposed LV method was successful in synthesizing diverse high-quality metal telluride, particularly MoTe 2 , with precise phase and layer-number control. Furthermore, main element tellurides akes (SnTe and GaTe) were also synthesized through the LV approach. Our experimental ndings have unveiled a new LV-facilitated process with on-site nucleation and on-demand reactant supply in the liquid phase, which is fundamentally distinctive from the conventional VSS process. As a general phase-guided method, this LV method can be applied to a wide range of precursors and material systems.
The merits of LV method can be understood from both kinetic and thermodynamic perspectives. The CVD synthesis involves two reaction processes: (1) formation: M (g) +xTe (l) MTe x (s); and (2) decomposition: MTe x (s) MTe (x-y) (s) + yTe (g) (M is metal), as shown in Fig. 1a. We highlight the vital role of decomposition as it is inevitable in conventional CVD growth, and is evidenced by defects and lattice disturbance in CVD-grown 2D metal tellurides ( Supplementary Fig. 1). In order to synthesize high-quality materials, it is crucial to promote the formation process and impede the decomposition process. We use the LV strategy to exploit the synergism of equilibrium and kinetics to achieve this goal. From the kinetics point of view, the reaction rate of formation process sensitively depends on the precursors' concentration. It can be expressed as , where is the rate constant, c(M) and c(Te) denote the concentration of metal-based and tellurium precursors, respectively. Thus, a higher concentration of Te will boost the kinetic growth rate ( ) of 2D metal tellurides. From a thermodynamics point of view, high concentration of Te will impede the decomposition of as-grown tellurides, thus minimize the probabilities of defect creation, and shift the equilibrium to the side of repairing defects according to the Le Châtelier's principle. For the LV growth, liquid reservoirs of Te supply a signi cantly higher concentration of on-site Te precursors than Te vapor in the conventional VSS method, since the Te concentration severely drops along the direction of the gas ow. Since the rate of defects repairing follows a power law against the concentration of Te, high concentration for liquid Te ensures fast repairing rate, which forms stoichiometric 2D tellurides without vacancies. In contrast, for the VSS mode, because of insu cient supply of Te, the growth rate of MTe x should be much slower, and the as-grown MTe x is prone to decompose, thus generating defects (Fig. 1a, right). In short, owing to su cient reactants in liquid, the LV strategy can greatly facilitate growth, and minimize defects through the kinetic and thermodynamic control.
The process of LV growth is illustrated in Fig. 1b. Pre-deposited Te precursor begins to melt and forms liquid droplets when the furnace temperature is ramped up above the Te melting temperature (~450 ℃) (Fig. 1b, ii). Subsequently, as the metal precursor species vaporize, the Te liquid reservoir continuously collects surrounding gaseous precursor species (Fig. 1b, iii). The liquid-vapor-solid triple junction (solid is the substrate) is the major growth front. MTe x continuously precipitates out at the liquid-solid interface ( Fig. 1b, iv and Fig. 1c). As the growth proceeds, droplet vaporizes and the triple phase line moves inward (Fig. 1b, v and Fig. 1d). Consequently, the MTe x ake has inward growth at the triple phase junction, and the growth terminates when the droplet is all consumed (Fig. 1b, vi and Fig. 1e). Simultaneously, the ake grows outward and increase lateral size through attachment of Te atoms from the in-situ saturated Te vapor and metal precursor atoms at the growing edge. Liquid precursor provides nucleation site and boosts growth. Moreover, liquid reservoir prevents defects generation and decomposition by continuously providing su cient Te precursor in-situ during the growth.
Several experimental phenomena in the synthesis of metal tellurides are in good agreement with the proposed LV mechanism. Optical Microscope (OM) images illustrate several key stages during the LV growth of MoTe 2 (Fig. 1, c to e). The droplet-shaped particles were found on MoTe 2 akes when the growth is terminated at an early stage (Fig. 1c). Black regions (labeled as light blue circle in the inset of Fig. 1f) on MoTe 2 akes were sometimes observed after the LV growth. Raman spectrum of the black region indicates that it is a mixture of elemental Te crystal and 2H MoTe 2 , while other regions of the ake only show Raman features of 2H MoTe 2 (Fig. 1f). These black regions could be the residues of droplets.
Furthermore, circular morphology of as-grown MoTe 2 as well as coffee rings suggest the presence of the original Te droplet and many MoTe 2 akes forms from that ( Fig. 1g and Supplementary Fig. 2). The MoTe 2 grown out of the liquid region indicates outward growth also exists, which could result from the reaction between in-situ vaporized Te from liquid reacts and Mo precursors (Fig. 1g, left panel). Moreover, we found that droplet sometimes crawls on the surface and MoTe 2 precipitates out, leaving a trace behind. The MoTe 2 shows the migration trace of the droplet, and the thicker region (whiter region) at the tip would be the location of the droplet residue ( Supplementary Fig. 2). For other metal tellurides such as GaTe system, ower-shaped GaTe was sometimes obtained with a droplet-like center and surrounding "petal" akes ( Fig. 1h). Some traces can be seen, which could be original Te droplet, now shrinking much in size (Fig. 1h). Raman spectra verify that the petals are the precipitated GaTe crystal ( Supplementary  Fig. 3). The GaTe akes precipitate at the liquid-solid interface. The droplet does not completely deplete during the growth, then the liquid in the center condensed during the cooling process.
We start with MoTe 2 as an example to demonstrate the advantages of the LV method. Monolayer 2H MoTe 2 is a promising key material as the on-chip light source for next-generation silicon-based photoelectronics 14 . Moreover, monolayer 2H MoTe 2 provides a great platform for developing phase-change devices because of the reversible phase transition of monolayer 2H to 1T′ MoTe 2 12 . The 1T′ MoTe 2 is the Weyl superconductor with an edge supercurrent 15 , and monolayer 1T′ is a quantum spin Hall insulator 16 . shows hexagonal shape, a typical shape of 2H single crystals (with size ranging from a few to tens of micrometers, Fig. 2a). The step height of the ake is ~1 nm, consistent with the height of exfoliated monolayer 2H MoTe 2 (Fig. 2b).
Raman spectroscopy and polarized second-harmonic generation (SHG) spectroscopy were used to investigate the crystal phase, uniformity and optical anisotropy properties of the 2H MoTe 2 ake. Raman spectrum displays two peaks at 171 cm -1 (A 1g mode) and 236 cm -1 (E 2g 1 mode) (Fig. 2c). The strong intensity of A 1g mode and absence of B 2g 1 mode (290 cm -1 ) con rming the monolayer 2H nature of the ake 23 . The uniformly distributed color in both Raman intensity map of A 1g (Fig. 2d) and E 2g 1 ( Supplementary Fig. 10) modes, together with no B 2g 1 signal over the whole ake region ( Supplementary   Fig. 10), suggest the uniformity of the monolayer ake. The SHG spectrum of the ake (Fig. 2e) displays a strong SHG signal and six-fold SHG pattern, consistent with the SHG feature of monolayer 2H MoTe 2 .

Page 7/17
Furthermore, the SHG mapping (Fig. 2e, inset) shows uniform intensity with hexagonal shape without boundary lines, indicating single crystal nature of the monolayer ake.
Monolayer 2H MoTe 2 lm has also been successfully obtained via LV synthesis. The monolayer 2H lm is uniform and continuous, with only the edge region of the sample showing discontinuity (Fig. 2f).
Atomically resolved annular dark eld scanning transmission electron microscopy (ADF-STEM) reveals the hexagonal atomic lattice of monolayer 2H MoTe 2 (Fig. 2g). X-ray photoelectron spectroscopy (XPS) measurement was conducted to examine the elemental composition, bonding types and phase purity of LV grown monolayer lm. The high-resolution Mo 3d spectrum shows two prominent peaks at 228.3 eV (3d 5/2 ) and 231.5 eV (3d 5/2 ) which are attributed to the Mo-Te bonds (Fig. 2h). The two peaks at 573 eV and 583.4 eV in the Te 3d spectrum can also be assigned to the Mo-Te bonds (Fig. 2h). These features are consistent with that of bulk 2H MoTe 2 crystal and of CVD grown few-layer 2H MoTe 2 17 . The XPS peaks for 1T′ MoTe 2 were not detected in the sample, indicating that our CVD lm is 2H MoTe 2 with high phase purity. Moreover, the atomic ratio between Mo to Te is ~1:2, suggesting the as-grown MoTe 2 is stoichiometric.
The LV grown monolayer 2H MoTe 2 exhibits excellent photoluminescence property. The photoluminescence (PL) spectrum of monolayer lm shows a prominent PL peak centered at ~ 1135 nm (Fig. 2i), consistent with the direct band gap of monolayer 2H MoTe 2 at ~1.09 eV. Repeated measurements on various locations of the same lm yield very similar position and intensity of the peak ( Supplementary Fig. 11). The PL peak of the LV grown sample has a comparable intensity with the exfoliated monolayer sample, and different locations yield PL peaks with similar position and intensity, demonstrating high-quality and homogeneity for the LV grown monolayer 2H MoTe 2 . Owing to the comparable direct bandgap with Si, superior optical property, as well as its 2D nature (without dangling bonds on the surface and can be easily integrate with Si), LV grown monolayer 2H MoTe 2 could be an excellent candidate for integration with Si photonics.
The LV technique can synthesize high-quality 2H MoTe 2 with controlled layer numbers via adjusting the amount of MoCl 5 precursor. The thickness of 2H MoTe 2 is found positively correlated with the amount of MoCl 5 . Thus few-layer MoTe 2 is produced through increasing the amount of MoCl 5 in the ampoule. The step height of the hexagonal shaped ake is about ~2 nm, suggesting it is tri-layer MoTe 2 (Fig. 3a). Angle dependent polarized SHG displays six-folder pattern ( Supplementary Fig. 12a), indicating the noncentrosymmetric nature of the ake. Given the ake is ~2 nm height, the ake should be tri-layer 2H MoTe 2 . SHG mapping of the ake indicates there is no boundaries in the ake, suggesting the ake is single crystal MoTe 2 ( Supplementary Fig. 12b). The Raman spectrum shows three peaks at ~171 cm -1 (A 1g mode), ∼234 cm -1 (E 2g 1 mode), and ∼290 cm -1 (B 2g 1 mode) (Fig. 3b). Since the out-of-plane B 2g 1 mode is Raman inactive for monolayer and bulk 2H MoTe 2 , the ake should be few-layer 2H MoTe 2 . The intensity maps of the A 1g peak and E 2g 1 peak display homogenous color over the ake region, suggesting high homogeneity of the few-layer 2H MoTe 2 ake (Supplementary Fig. S13). Since the ratio of B 2g 1 / E 2g 1 increases as the layer number of 2H MoTe 2 decreases, the uniformly distributed color over the ake in B 2g 1 / E 2g 1 map further indicates the ake has uniform thickness (Fig. 3c). High-resolution transmission electron microscopy (HRTEM) image shows a hexagonal atomic lattice without obvious defects. Moreover, selected-area electron diffraction (SAED) of the ake displays hexagonal symmetry pattern (Fig. 3d). These features are consistent with crystal structure of 2H MoTe 2 , suggesting the high crystalline quality of LV-grown 2H MoTe 2 . Transfer characteristics of a few-layer 2H MoTe 2 transistor displays ambipolar behavior with a current ON/OFF ratio of ~10 3 at room temperature ( Supplementary Fig. 14).
No obvious hysteresis is observed during the gate voltage scanning forward and backward. The near zero charge neutrality point indicates the as-grown MoTe 2 ake is intrinsic without obvious doping, in strong contrast with the heavy p-type doping 2H MoTe 2 grown by previous methods 13,17 . Therefore, LV grown MoTe 2 provides a good materials platform to investigate the intrinsic properties of 2H MoTe 2 .
Phase-selective (2H or 1T′) growth of MoTe 2 can be realized via LV method through tuning temperature or Te concentration. With less amount of Te pre-deposited on substrate and a lower growth temperature, high-quality 1T′ MoTe 2 akes will be obtained. Willow-leave like akes were scattered on mica substrate with certain preferential directions (Fig. 3e). The elongated shape comes from the in-plane anisotropic feature of 1T′ MoTe 2 , with the long direction (preferential growth direction) along the zigzag Mo atomic chain. SAED from 1T′ MoTe 2 shows rectangular pattern, consistent with the lattice structure of 1T′ MoTe 2 ( Supplementary Fig. 15). Raman spectrum of a ake displays several Raman peaks: A g modes at 85 cm -1 , 115 cm -1 , 130 cm -1 , 164 cm -1 , 253 cm -1 and 272 cm -1 , as well as B g modes at 93 cm -1 and 103 cm -1 (Fig. 3f). These features are consistent with the Raman spectrum of monolayer 1T′ MoTe 2 24 .
Interestingly, the intensity of these modes is strongly dependent on the angle of the ake ( Supplementary  Fig. 16). Since Raman intensity is closely related to crystalline orientation because of the anisotropic nature of 1T′ 25 , the angle-dependent Raman intensity suggests that the ake is single crystal. The uniformly distributed color in the Raman intensity map of A g mode at 164 cm -1 (Fig. 3g) further indicates the homogenous and single crystalline feature of the 1T′ MoTe 2 . In contrast, only sparse and small 1T′ MoTe 2 akes are obtained ( Supplementary Fig. 17) via VSS growth.
XPS was utilized to characterize elemental composition and bonding types of the 1T′ MoTe 2 sample. The Mo 3d 5/2 peak at 228.1 eV and Mo 3d 3/2 peak at 231.2 eV can be attributed to Mo-Te bonds. The strong Te 3d 5/2 and Te 3d 3/2 peaks at 572.7 eV and 583.1 eV are also assigned to Mo-Te bonds, consistent with bulk 1T′ samples (Fig. 3h). There is no Mo oxide or Te oxide peaks in the high-resolution XPS spectra, indicating the CVD grown 1T′ MoTe 2 is intrinsic without oxidation. The Te/Mo atomic ratio is estimated to be ~2.01, suggesting the CVD grown 1T′ MoTe 2 is stoichiometric. The source-drain current versus voltage characteristic of the 1T′ MoTe 2 grown from MoCl 5 (Fig. 3i) is linear and symmetric, with a sheet resistance of ~3200 Ω/sq, indicating the metallic nature of the ake.
The generality of the LV approach was demonstrated for the synthesis of diverse 2D metal tellurides, including transition metal (MoTe 2 ), IVA metal (SnTe) and IIIA metal tellurides (GaTe) with multiple crystalline phases (Fig. 4). β-SnTe is a topological crystalline insulator, and few-layer lms show robust ferroelectricity at room temperature 26 . GaTe has extremely high photoresponsivity and good thermoelectric properties, which has promising applications in photonics and thermoelectric devices 27 . Liquid Te droplet works as a preferential reservoir for precursors from the gas phase, and subsequently as seed for the growth of 2D metal tellurides. We choose metal chloride instead of usual metal oxide to address the weak chemical reactivity of Te. The higher reactivity of chloride precursors leads to much faster reaction, synchronously avoid intermediate products and oxidation during growth. Moreover, the total oxygen-free growth condition can further ensure the high quality of as-grown metal telluride. For the synthesis of SnTe, we choose SnCl 2 , H 2 and Te as precursors. After the LV growth, α-SnTe and β-SnTe were successfully obtained. The square shape of the ake (Fig. 4b) corresponds to the cubic crystal structure of β-SnTe. The Raman spectrum of the ake (Fig. 4c) shows two peaks at ~118 and ~139 cm -1 , consistent with the Raman features of nano-sized β-SnTe 28 . Interestingly, the low-temperature phase α-SnTe 29 is also stable at room temperature. The Raman peak at ~128 cm -1 (Fig. 4f) and EDS result ( Supplementary Fig. 18) clearly prove that the trianglar ake is α-SnTe 29,30 . GaTe akes are obtained from GaCl 3, Te and H 2 . Four Raman peaks at ~ 125, 143, 160, 171 cm -1 (Fig. 4i) are the same as that of the GaTe akes exfoliated from bulk crystal 27 , indicating the as-obtained akes are GaTe.
In conclusion, we have developed a universal LV method to synthesize 2D TMDs. The LV method provides an ideal way to continuously supply precursors, facilitates growth, suppresses decomposition, and enables growth of 2D materials/phase that are inaccessible through the conventional VSS growth.
Moreover, LV-grown 2D materials show high quality due to minimum defects. The advantages of LV are demonstrated by synthesis of 2D metal tellurides with high quality, better controlled phase and stoichiometry, which were hard to synthesize previously due to low bonding energy, tendency for decomposition, and complex crystalline phases. In particular, monolayer 2H MoTe 2 lm with superior properties was generated, thus enables diverse applications such as highly e cient near-infrared lightemitting diodes, near-infrared photodetectors, integration with silicon photonics, and phase-change devices. This method is of great signi cance for 2D metal tellurides with both fundamental and technological importance. We anticipate the LV strategy can be readily applied to synthesize many other high-quality quantum materials and heterostructures.

Synthesis of 2D metal tellurides
For the synthesis of MoTe 2 , Te precursor (Sigma-Aldrich, 99.997%) was pre-deposited on substrates by spin-coating Te/chloroform solution (0.1 g Te/6 mL chloroform). The ampoule lled with MoCl 5 precursor (Sigma-Aldrich, 99.99%), substrates and Te powder were loaded into a tube furnace with a chamber of diameter 25 cm. The whole CVD system was purged with Ar and H 2 for 30 min before heating. Then, the substrate and Te powder was heated to the growth temperature in 15 min under a gas ow (Ar 250 sccm, H 2 5 sccm). The ampoule was moved into the heating zone when reaching the growth temperature. The temperature of MoCl 5 during growth is ~200 ℃. After 10 min growth, the system was cooling down to room temperature by taking out the tube, and a mixture gas of Ar 300 sccm and 5 sccm is used to blow away reactants. For the synthesis of SnTe, SnCl 2 (Sigma-Aldrich, 99.99%) was lled in ampoule as the metal precursor. After purging the system with Ar and H 2 for 30 min, the substrate with pre-deposited Te and Te powder were heated to 650 ℃ in 15 min under a gas ow of Ar 100 sccm and H 2 100 sccm. Then SnCl 2 was pushed into furnace and kept at 650 ℃ for 20 min. The system was fast cooling down to room temperature after growth. GaTe was grown from GaCl 3 (Sigma-Aldrich, 99.99%), Te and H 2 . The substrate and Te powder were rst ramped up to 700 ℃ for 15 min (Ar 250 sccm, H 2 5 sccm), then ampoule lled with GaCl 3 was moved into the heating zone. The system was kept at 700 ℃ for 10 min growth under a mixture gas ow of Ar 100 sccm and H 2 100 sccm.

Characterization
Optical images were collected by optical microscopy (Axio Imager, Carl Zeiss). Raman characterization was performed with a Horiba Jobin Yvon HR800 system with a 633 nm excitation laser. The laser spot size was ~1 µm. AFM was conducted on Dimension 3100, Veeco Instruments Inc. in tapping mode. XPS was conducted on a PHI Versaprobe II XPS using Al Ka monochromated X-ray at low pressures of 1.3×10 -8 Torr. XPS spectra were calibrated by shifting the highest peak in C 1s spectrum to 284.5 eV. PL spectra were collected using a home-built confocal microscope setup with a camera and an infrared spectrometer. SHG was collected by a photomultiplier tube using an 800 nm pulse laser excitation. SEM characterization was performed on Zeiss Merlin High-resolution SEM.