关键词: atomic-layer MoS₂, hot filament chemical vapor deposition, high-resolution transition electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS)

Abstract: Atomic-layer MoS₂ ultrathin films are synthesized using a hot filament chemical vapor deposition method. A combination of atomic force microscopy (AFM), x-ray diffraction (XRD), high-resolution transition electron microscopy (HRTEM), photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS) characterization methods is applied to investigate the crystal structures, valence states, and compositions of the ultrathin film areas. The nucleation particles show irregular morphology, while for a larger size somewhere, the films are granular and the grains have a triangle shape. The films grow in a preferred orientation (002). The HRTEM images present the graphene-like structure of stacked layers with low density of stacking fault, and the interlayer distance of plane is measured to be about 0.63 nm. It shows a clear quasi-honeycomb-like structure and 6-fold coordination symmetry. Room-temperature PL spectra for the atomic layer MoS₂ under the condition of right and left circular light show that for both cases, the A1 and B1 direct excitonic transitions can be observed. In the meantime, valley polarization resolved PL spectra are obtained. XPS measurements provide high-purity samples aside from some contaminations from the air, and confirm the presence of pure MoS₂. The stoichiometric mole ratio of S/Mo is about 2.0-2.1, suggesting that sulfur is abundant rather than deficient in the atomic layer MoS₂ under our experimental conditions.

Key words: atomic-layer MoS₂, hot filament chemical vapor deposition, high-resolution transition electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS)