Chemical Vapor Growth of Monolayer MoS2 with In-Plane Preferred Orientations

Abstract

The isolation of graphene from bulk graphite has marked the start of a new era: two-dimensional (2D) materials. With remarkable properties distinct from their bulk counterparts, 2D materials have become a focus of research in the past decade, such as hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDCs). TMDCs with a generalized formula of MX2, where M is one transition metal and X is one chalcogen, exhibit a wide variety of unique properties from semiconductor (WS2, WSe2, etc) to metal and superconductor (NbS2, TaSe2, etc), to be promising candidates for electronics, optoelectronics and other applications. Owing to the development of chemical vapor deposition (CVD), the large single-domain flake and continuous polycrystalline film of monolayer MX2 can be synthesized successfully. However, the orientation of monolayer TMDCs is usually random on the substrates due to the misoriented nucleation, which is disadvantageous to their mechanical, optical and electrical properties of atomically thin TMDCs because the grain boundaries and defects can be formed when the neighboring flakes encounter and merge together. Therefore, control over the nucleation and growth orientation to minimize or even eliminate the grain boundaries is quite essential for the growth of 2D materials. In this thesis, it reports that the monolayer single-domain flakes and continuous films of molybdenum disulfide (MoS2) could be synthesized on both SiO2/Si and c-plane sapphire substrates by using a CVD method. Effects of different growth temperature, growth time, gas flow rates of mixed Ar/H2 and growth region were studied in detail to obtain the large monolayer MoS2. In addition, after preparation of the c-plane sapphire with a terrace-structured surface by annealing, the aligned growth of monolayer MoS2 with in-plane preferred orientations was achieved. The morphology and thickness of as-grown samples were characterized by optical microscope, atomic force microscope (AFM) and Raman spectroscopy. Photoluminescence (PL) measurements showed a higher-intensity but narrower-width peak of aligned monolayer MoS2 flakes compared to that of random-distributed monolayer MoS2 flakes, indicating the aligned monolayer MoS2 has higher optical qualities and less doping level. The homogeneously high-quality as-synthesized monolayer MoS2 can be utilized in various future applications.