Structure and optical properties of transition metal dichalcogenides (TMDs) - MX2 (M = Mo, W & X = S, Se) under high pressure and high temperature conditions

Layered structured materials such as transition metal dichalcogenides (TMDs) have gained immense interest in recent times due to their exceptional structural, electrical and optical properties. Recent studies show semiconducting TMDs such as MX2 (M= Mo, W & X = S, Se) could be used as potential shock absorbing material, which has resulted in extensive studies on structural stability of these materials under the influence of high pressure. Understanding the structural stability of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, WS2, and WSe2 under high pressure has been very challenging due to contradicting observations and interpretations reported in the past. Hence, the main objective of this work is to study the crystal structure and optical properties of bulk MX 2 at high hydrostatic pressures up to 51 GPa using a diamond anvil cell with synchrotron radiation in addition to high pressure Raman spectroscopic and high temperature X-ray diffraction (XRD) experiments. Crystal structures of MX2 materials are observed to be stable up to 500 °C with nonlinear thermal coefficients of expansion. Results of high pressure experiments show a pressure induced isostructural hexagonal distortion to a 2Ha-hexagonal P63/mmc phase, in MoS2 around 26 GPa as predicted by theoretical calculations reported earlier. No pressure induced phase transformation is observed in other MX2 (MoSe2, WS2, WSe 2). A semi empirical model based on the energy of interaction of bond electrons is proposed to explain the observed inconsistency between MoS 2 and other TMDs studied. Using this model, it is shown that except MoS2, no other MX2 within the scope of this study undergoes pressure induced phase transition in the pressure range 0 -- 50 GPa. High pressure Raman results show continuous red shifts in dominant vibrational modes with increase in pressure in MX2. Additionally, emergence of a new peak, namely 'd - band' associated with 2Ha structure in MoS2 supports the observation of a isostructural phase transition in high pressure X-ray diffraction experiments. In addition to the studies on bulk MoS2 material, thin film (approximately 100 nm thicknesses) is successfully fabricated via DC magnetron sputtering system and sulfurization technique.