Realization of a structural phase transition in few-layer WS₂

In recent years, two-dimensional (2D) tungsten disulfide (WS₂) with different crystal structures has attracted significant attention due to its interesting physical properties. For instance, 2H-phase WS₂ is a semiconductor with layer thickness-dependent band structures, while the metallic 2M-phase WS₂ is a topological superconductor candidate. However, no progress has been made in realizing structural phase transitions between 2H and 2M phases of WS₂ thus far. In this work, we report the structural phase transition from 2M to 2H in WS₂ atomic layers by controlling the sample temperature using either a laser or thermal heating. We first characterized the treated 2M WS₂ thin flakes using Raman spectroscopy and observed the characteristic spectrum of 2H WS₂. Furthermore, we confirmed such a phase transition through the photoluminescence (PL) measurements. No PL signal has been obtained in the pristine 2M WS₂ thin layer, whereas a clear PL peak ~ 1.9 eV can be detected after the thermal treatment, consistent with the semiconducting nature of 2H WS₂. Our results may open a way of locally engineering the phase and physical properties of 2D transition metal dichalcogenides for functional device applications.