Direct Synthesis of Large‐Scale WTe₂ Thin Films with Low Thermal Conductivity

Abstract

1 of 9) 1605928applications. [8][9][10][11][12][13][14] A spin quantum Hall state is also predicted in the distorted octahedral phase (1T′) of MX 2 in the monolayer limit, further extending applications of TMDs into spintronics and lowdissipation electronics. [13] As a part of the TMDs family, WTe 2 has recently attracted great interest due to its giant, nonsaturating magnetoresistance (MR) observed in bulk crystals, [15] and its predicted Weyl state. [16] Pressure-induced superconductivity and large spin-orbit coupling are also observed. [17,18] In addition, the lattice thermal conductivity of WTe 2 is predicted to be smaller than that of WSe 2 due to the heavier atom mass and the lower in-plane crystal symmetry. [19] Studies on WTe 2 have so far been carried out using bulk crystals or mechanically exfoliated flakes. Although mechanical exfoliation can produce high-quality flakes down to a monolayer, scaling it to obtain large-area thin films for practical applications is challenging. Thus, direct synthesis of WTe 2 thin films is desirable for potential electronic and thermal propertyrelated applications, but has yet to be realized due to the low bonding energy of W-Te. Synthesizing WTe 2 directly into largescale thin films is challenging due to its very small standard Gibbs free energy of reaction (−26.2 kJ mol −1 ) compared to that of WSe 2 (−135.0 kJ mol −1 ). [20,21] Additionally, the low melting point of the forming Te-W binary eutectic and high melting point of W (3422 °C) restrict the reaction efficiency between W and Te. Only recently, direct synthesis of MoTe 2 thin films, another interesting TMD [22] with a lower standard Gibbs free energy of reaction (−64.3 kJ mol −1 ) than WTe 2 , has been demonstrated via chemical vapor deposition synthesis (all values of standard Gibbs free energy of reaction are taken at 1100 K). [21,23,24] So far, no direct synthesis of large-area, highly crystalline WTe 2 thin films has been reported.Here, we demonstrate a large-area, facile synthesis of WTe 2 and MoTe 2 thin films by reacting sputtered metal films with H 2 Te, an intermediate vapor phase formed from Te vapor and H 2 carrier gas, through atmospheric pressure chemical vapor reaction. The synthesized films are polycrystalline whose grain size increases with increasing metal film thickness. Based on time-domain thermoreflectance (TDTR), [25,26] the in-plane thermal conductivity of our polycrystalline WTe 2 thin film is less than 2 W m −1 K −1 , at least 7.5 times smaller than that of single-crystalline exfoliated flakes (15 ± 3 W m −1 K −1 ) at room temperature. Through-plane thermal conductivity of our WTe 2 thin films was measured to be 0.8 W m −1 K −1 at room temperature, which is lower than that of the recently reported Large-scale, polycrystalline WTe 2 thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H 2 Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single-crystallin...

In‐Plane Anisotropic Thermal Conductivity of Few‐Layered Transition Metal Dichalcogenide Td‐WTe2

“…Previous experiments have shown that the in-plane thermal conductivity is 15 ± 3 W m −1 K −1 for Td-WTe 2 flakes with thicknesses greater than 500 nm, [18] verifying our calculations. For bulk Td-WTe 2 , the calculated thermal conductivities along the a-, b-, and c-axis orientations are 14.93, 14.56, and 4.83 W m −1 K −1 , respectively.…”

“…To understand the role of these thermally active phonon modes, we calculate the in-plane cumulative thermal conductivity as a function of the phonon www.advmat.de www.advancedsciencenews.com frequency. [18] This isotropy can be attributed to comparable group velocities along the a-and b -axis orientations ( Figure S9, Supporting Information). Figure 4b presents the thin film lattice thermal conductivity of Td-WTe 2 computed for a 14 × 14 × 14 wavevector grid.…”

“…[14] Semimetal Td-WTe 2 is theoretically predicted to have a low thermal conductivity because of its heavy atomic mass and low Debye temperature, which makes it a prominent candidate for thermoelectric materials. [18,19] Although the distinct crystal structure of Td-WTe 2 implies notable anisotropic properties, experimental demonstrations of such in-plane anisotropic thermal conductivity are still lacking, even in layered 2D TMDs. The recent first-principles calculations predicted that anisotropy exists in the in-plane thermal conductivity of bulk Td-WTe 2 along different lattice directions, with a maximum value along the [010] direction (a-axis) whereas the minimum value is in the [100] direction (b-axis).…”

“…Accordingly, the theoretical curve of δT m /δP, which is related to the value of thermal conductivity, can be obtained, as shown in Figure 3b. [18] Furthermore, we also extracted the thermal conductivities from other suspended samples of 12.6, 21.1, and 27.0 nm in thickness ( Figure S8, Supporting Information), as summarized in Figure 3c. Thus, we can extract the thermal conductivity of suspended Td-WTe 2 by Adv.…”

Emerging in‐plane anisotropic two‐dimensional materials

“…The highly anisotropic thermal conductivity has also been observed in low‐symmetry TMDs such as T d WTe 2 , and ReS 2. Zhou et al reported the anisotropic thermal conductivity of WTe 2 flakes, where the thermal conductivity along the in‐plane is 15 times higher than along through‐plane.…”