Two step synthesis of ultrathin transition metal tellurides

Transition metal tellurides (TMTs) are an exciting group of two-dimensional materials with a wide variety of polytypes and properties. Here, we demonstrate a simple and versatile two-step method for producing MoTe2, WTe2, and PtTe2 films via tellurization of thin metals at temperatures between 400 and 700 degrees C. Across this temperature range, monoclinic 1T' phase of MoTe2, orthorhombic T-d phase of WTe2, and hexagonal 2H phase of PtTe2 were formed. Based on x-ray diffraction and Raman analysis, temperatures greater than 600 degrees C were found to produce the best quality MoTe2 and WTe2. In contrast, lower temperatures (400 degrees C) were preferred for PtTe2, which becomes discontinuous and eventually decomposes above 650 degrees C. The presence of H-2 in the tellurization process was critical to facilitate the formation of H2Te, which is known to be more reactive than Te vapor. In the absence of H-2, neither MoTe2 nor WTe2 formed, and although PtTe2 was formed under pure N-2,N- the crystal quality was significantly reduced. Temperature-dependent resistivity (rho) measurements were performed on the best quality TMT films revealing all films to be highly conductive. MoTe2 showed metallic behavior up to 205 K where it underwent a phase transition from the semimetallic T-d to semiconducting 1T' phase. WTe2 exhibited a consistent semiconducting behavior with a small positive increase in rho with decreasing temperature, and PtTe2 showed a metallic dependence from 10 K up to room temperature. Spectroscopic ellipsometry for TMT films provides complex optical constants n and k from ultraviolet to infrared. Published by AIP Publishing.