Material characterizations of ALD grown WS2 films without doping. (a) The XPS fine spectra of W 4f and S 2p for as-deposited and annealed 400-cycle WS2 film. Both WS2 and WS2+x peaks were observed, with the W/S ratio of 1 : 2.7. Only WS2 was observed in fine spectra of annealed WS2, indicating the necessity of annealing, and the W/S ratio was reduced to 1 : 2.1. (b) The Raman spectra of annealed WS2 films with varying thickness. The Raman peak separation increased with increasing film thickness. (c) The plane-view and cross-sectional TEM result of 400-cycle annealed WS2 film and the plane-view TEM result and statistical analysis of film grains by selected area electron diffraction (SAED) patterns. A layered structure was clearly observed in cross-sectional TEM. The average grain size of 259 WS2 grains was 55 nm, with the maximum size up to 160 nm.