Phase-controlled synthesis for 1T' phase MoS2 and MoSe2 crystals

Abstract

Polymorphism, existence of multiple forms of crystalline substances, has attracted research interest in transition metal dichalcogenides (TMDs) due to its drastic change of properties based on the crystallinity (phase). For example, MoS2 is thermodynamically stable as a trigonal prismatic (2H phase, a semiconducting material) in ambient condition, but it can be metallic when it is octahedral (1T phase). However, it is hard to synthesize the 1T phase of MX2 (M=Mo, W; X=S, Se) because they are metastable in ambient condition. To further study on phase-dependent properties, it is necessary to develop a facile method for phase-controlled synthesis. In this thesis, I describe research findings in two chapters.

First, I report phase-controlled synthesis method for 1T'-MoX2 (X=S, Se) crystals with lateral size up to hundreds of micrometers. The 1T' phase structure was carefully characterized by various techniques including the aberrationcorrected scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) and Raman spectroscopy. By using metastable condition of the 1T' phase in MoS2, the phase transition was achieved by thermal annealing and laser irradiation. In the end, phase-dependent electrochemical property for HER is demonstrated on the basal plane of MoS2.

Second, I used the metallic MoS2 to fabricate the tactile sensor to detect touch and press. The metallic MoS2 nanosheets were prepared by Li intercalation method and integrated into a thin film by the vacuum filtration method. The film exhibited electrically conductive property and response from a touch of fingertips. The results offer the insight for development of a cost-effective tactile sensor.