Exploring CVD Fabrication of Transition Metal Dichalcogenides for Nanoscrolling

Abstract

The continuous demand for smaller, faster, and more efficient electronic devices has driven research into two-dimensional (2D) materials that can overcome the physical limitations of Si brought on by quantum effects. Among these, transition metal dichalcogenides (TMDs) stand out due to their inherent bandgaps and highly tunable properties. In particular, the one-dimensional (1D) nanostructure formed by rolling up sheets of 2D materials, known as a nanoscroll, has immense potential for emergent optoelectronic properties brought on by its uniquely non-uniform strain field. Stacked and scrolled TMD heterostructures offer a promising route toward realizing novel optoelectronic phenomena driven by broken centrosymmetry and interlayer coupling. However, it is not well reported how the morphology of the initial 2D TMD sheet affects the final scrolled structure, which is a significant barrier to the deterministic control of nanoscrolls. This thesis explores the synthesis and morphological control of molybdenum-based TMDs, specifically MoS₂ and MoSe₂, using chemical vapor deposition (CVD), with an emphasis on understanding how 2D flake morphology governs the formation and properties of 1D nanoscrolls. Through systematic modification of CVD parameters, it was found that synchronizing temperature ramps between precursor zones greatly improved MoS₂ flake uniformity, yielding smaller, triangular monolayers with consistent morphology. Subsequent scrolling experiments demonstrated that flake shape and substrate adhesion critically influence scrolling yield and integrity, establishing a clear relationship between 2D precursor structure and final scroll geometry. In parallel, attempts to extend hydrogen-free CVD growth to MoSe₂ revealed significant challenges associated with selenium’s low reactivity, resulting instead in dominant Mo oxidation processes. MoO_X phase evolution was investigated via these results, with a detailed structural study being carried out on novelly synthesized 2D α-MoO_X nanobelts. Altogether, the findings advance the understanding of how CVD growth parameters dictate morphology and transformation pathways in molybdenum-based 2D materials, highlighting both the opportunities and challenges of fabricating non-hydrogen TMD heterostructures and strain-engineered nanoscrolls for future optoelectronic applications.