Singapore University of Technology and Design Singapore Singapore
Two dimensional 2D transition metal dichalcogenides TMDs have received tremendous attention owing to its exceptional physical and chemical properties. Monolayer TMDs are direct bandgap semiconductors exhibiting strong photoluminescence PL, excellent onoff current ratio, edge active relative catalysis, and spin-valleytronics needed for high-performance electronics. However, the yield of monolayer TMDs fabricated via mechanical exfoliation method is typically low, and not suitable for batch production. A chemical exfoliation approach provides better yield of monolayer TMDs, but it is still not a scalable technique with consistent yield. For this work, a unique vertical chemical vapor deposition CVD method was developed to produce batch productions of molybdenum disulfide MoS2 monolayer, a typical TMD material. Raman and PL mappings show the high quality of samples without defects at the edge sites STEM and diffraction data show the high crystal quality. The electrical performance of CVD grown monolayer MoS2 by this technique is comparable to that of mechanically exfoliated samples. Although the common expectation that CVD grown MoS2 should be structurally defective due to thermal stresses during the growth process, the large mobility of samples achieved at 64 cm2V-s RT. Additionally, strong PL intensity is attributed to the p-doping effects of adsorbates at the defect-sites of CVD samples and its PL intensities will decrease significantly when measured in vacuum. First principle calculations were carried out to investigate the charge transfer process between MoS2 and adsorbates and to further clarify the p-doping effect of adsorbates due to the strong electronegativity of adsorbates. Such a p-doping effect from adsorbates reduces the concentration of excess electron in MoS2 and contributes to the radiative process of excitons.