Raman of 2D-MoS2: Disentangling the Metallic Phase Conundrum

Abstract

2D Molybdenum disulfide (2D-MoS2) is thermodynamically stable and hence easily synthesized in its semiconducting 2H phase. In contrast, the metallic-phase 1T-MoS2 is a highly sensitive, metastable, and complex phase that is not naturally occurring. This heightened sensitivity and instability have resulted in a widespread misrepresentation in academic literature. Based on the current understanding, the majority of Raman investigations incorrectly identify the spectrum of MoO3 as 1T-MoS2 due to the samples oxidation when exposed to intense laser irradiation in the presence of air, as commonly performed. Therefore, this work focuses on conducting a comprehensive Raman analysis of 1T-MoS2 by investigating the impact of oxidizing atmosphere, laser power, irradiation time, and temperature changes on the sample. Controlling the Raman conditions has allowed to introduce a novel spectrum that signifies a new phase triggered by laser heating. This has led to further confusion in the identification of the metallic MoS2 polymorphs. This study seeks to expose the current challenges in characterizing the metallic phase of MoS2, especially when using Raman spectroscopy, which, till now, is considered the most reliable technique for this purpose. These results provide guidance to the next experimental Raman studies and possible practical applications for materials engineering.