Synthesis of Vertical MoO₂/MoS₂ Core-Shell Structures on an Amorphous Substrate via Chemical Vapor Deposition

Vertical MoO₂/MoS₂ core-shell structures were synthesized on an amorphous surface (SiO₂) by chemical vapor deposition at a high heating rate using a configuration in which the vapor phase was confined. The confined reaction configuration was achieved by partially covering the MoO₃-containing boat with a substrate, which allowed rapid buildup of the partially reduced MoO_3-x crystals in an early stage (below 680 °C). Rapid temperature ramping to 780 °C enabled spontaneous transition of the reaction environment from sulfur-poor to sulfur-rich, which induced a sequential phase transition from MoO_3-x to intermediate MoO₂ and finally to MoO₂/MoS₂ core-shell structures. The orthorhombic crystal structure of MoO_3-x contributed to the formation of vertical crystals on the amorphous substrate, whereas the nonvolatility of the subsequently formed MoO₂ enabled layer-by-layer sulfurization to form MoS₂ on the oxide surface with minimal resublimation loss of MoO₂. By adjustment of the sulfurization temperature and time, excellent control over the thickness of the MoS₂ shell was achieved through the proposed synthesis method.