Evolution of transition metal dichalcogenide film properties during chemical vapor deposition: from monolayer islands to nanowalls

Unique properties possessed by transition metal dichalcogenides (TMDs) attract much attention in terms of investigation of their formation and dependence of their characteristics on the production process parameters. Here, we investigate the formation of TMD films during chemical vapor deposition (CVD) in a mixture of thermally activated gaseous H2S and vaporized transition metals. Our observations of changes in morphology, Raman spectra, and photoluminescence (PL) properties in combination with in situ measurements of the electrical conductivity of the deposits formed at various precursor concentrations and CVD durations are evidence of existence of particular stages in the TMD material formation. Gradual transformation of PL spectra from trion to exciton type is detected for different stages of the material formation. The obtained results and proposed methods provide tailoring of TMD film characteristics necessary for particular applications like photodetectors, photocatalysts, and gas sensors.