Fig. 1: Thickness-dependent phase stability of 2M WS₂ on Si/SiO₂ substrate.

a Optical and b atomic force microscope (AFM) images and mappings of E¹_2g Raman mode and photoluminescence (PL) intensities of a piece of exfoliated 2M WS₂ flake with different thicknesses areas at different temperatures. AFM image is measured at room temperature, and the corresponding height profile is shown in Supplementary Fig. 3f. c Optical images, AFM image, and mappings of E¹_2g Raman mode and PL intensities of a piece of exfoliated monolayered (ML) 1T′ WS₂ flake at room temperature in the air and after heated at 450 °C in Ar atmosphere. Mappings measured area is outlined by dotted lines in the optical image. All the scale bars in (a–c) correspond to 2 μm. d Raman spectra of a 2M WS₂ and an intermediate phase and 2H WS₂ that were obtained by heating multilayered 2M WS₂ at 120 °C for 5 min and at 250 °C for 20 min, respectively. e 2M to 2H (or 1T′ to 1H) phase transition temperatures (circle points) and air oxidation temperature (triangle point) as functions of WS₂ layer thickness, measured in the air or Ar atmosphere with temperature elevated by 5 °C and held for 1 min or 15 min in each step. Solid lines are guides to the eye. f The power of laser required to activate 2M to 2H phase transition as a function of WS₂ layer thickness measured in the air. A solid line is a guide to the eye.