Illustration of ALD growth mechanisms and characterizations. (a) Idealized schematic of the mechanisms of ALD process for WS2 growth and in situ Nb doping. The doping concentration could be controlled by adjusting NbS2 cycle numbers. (b) Photographs of 400-cycle WS2 films deposited on 8-inch α-Al2O3/Si wafer, 2-inch sapphire wafer, and pieced GaN substrates. (c) The Raman spectra of annealed WS2 on Si/Al2O3, GaN, and sapphire confirm the successful synthesis of WS2 on each substrate surface.