Phase selective CVD growth and photoinduced 1T → 1H phase transition in a WS2 monolayer

Phase engineering in 2D materials offers unique opportunities to control polymorphic phases such as 1H, 1T and 1T′ along with their electronic and optical properties to a great extent. We report the direct chemical vapour deposition (CVD) growth of the metastable 1T phase of a WS2 monolayer and the in situ phase transition characteristics with the aid of Raman, Photoluminescence (PL) and fluorescence microscopy (FM). The kinetics of in situ phase transition from 1T to the 1H phase has been directly observed by fluorescence microscopy imaging. Local phase transition has been triggered by directed white light and a laser beam to investigate the kinetics systematically. The process of 1T → 1H phase transformation follows the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model that is normally observed in classical solids under isothermal conditions. The rate constant of light driven phase transformation was calculated to be 1.6 × 10−8 s−1 in the region which follows the JMAK kinetics perfectly. CVD growth of the 1T phase WS2 monolayer and its in situ direct transformation clearly reveal an important aspect of instability and phase transition kinetics. Controlling the stability of the metastable phases and the associated variation in optical properties across phase transition is a key step in realization of optoelectronic applications in rapidly expanding 2D flatland materials.