Two-Step Growth of Uniform Monolayer MoS Nanosheets by Metal-Organic Chemical Vapor Deposition.

To achieve large area growth of transition metal dichalcogenides of uniform monolayer thickness, we demonstrate metal-organic chemical vapor deposition (MOCVD) growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure (AP). Following sulfurization, the MOCVD-grown continuous MoS film transforms into compact triangular crystals of uniform monolayer thickness as confirmed from the sharp distinct photoluminescence peak at 1.8 eV. Raman and X-ray photoelectron spectroscopies confirm that the structural disorders and chalcogen vacancies inherent to the as-grown MOCVD film are substantially healed and carbon/oxygen contaminations are heavily suppressed. The as-grown MOCVD film has a Mo/S ratio of 1:1.6 and an average defect length of ∼1.56 nm, which improve to 1:1.97 and ∼21 nm, respectively, upon sulfurization. The effect of temperature and duration of the sulfurization process on the morphology and stoichiometry of the grown film is investigated in detail. Compared to the APCVD growth, this two-step growth process shows more homogenous distribution of the triangular monolayer MoS domains across the entire substrate, while demonstrating comparable electrical performance.