Two-dimensional (2D) MoS₂ as a typical transition metal dichalcogenide (TMD) material has attracted much attention in the last decade due to its intriguing electronic and optical properties. The vast applications of MoS₂ material have led to considerable research attention on the growth of high quality MoS₂ crystals with controlled morphology and size via chemical vapor deposition (CVD).

The thesis begins with an introduction to MoS₂ CVD growth, which focuses on the strategies in MoS₂ morphology and size control. In Chapter 2, we present a custom CVD synthesis system, which employs a unique modification and arrangement of quartz tubes. This system achieves accurate control of MoO₃ and S precursor vapor concentrations, which serves as the first step towards accurate morphology and size control of MoS₂ crystals in CVD growth.

In Chapter 3, we present a unified spatial-temporal model, which we coined the adatom concentration profile (ACP), as a way to describe the growth of MoS₂ crystals, with a full spectrum of shapes from triangles, concave triangles, three-point stars to dendrites. The ACP is a new paradigm to conceptualize the growth of crystals through time, which is expected to be instrumental in guiding the rational shape engineering of MoS₂ crystals. We perform a series of CVD experiments controlling adatom concentration on the substrate and growth temperatures, and present a method for experimentally measuring the ACP in the vicinity of growing islands. We apply a phase-field model of growth that explicitly considers similar variables (adatom concentration, adatom diffusion, and noise effects) and cross-validate the simulations and experiments through the dependence of the ACP and island morphologies as a function of physically controllable variables. Our calculations reproduce the experimental observations with high fidelity.

In Chapter 4, we present a new strategy for regulating the reaction mechanism of CVD grown 2D MoS₂ crystals based on modulating the sulfur (S) and molybdenum (Mo) precursor concentration ratio (S:Mo). By systematically studying crystal density and substrate coverage as a function of the precisely controlled S:Mo, it is found that there is an optimal value of S:Mo (Rop), above which homogeneous reaction becomes dominant. Experimental results show that a relatively low S:Mo favors heterogeneous (as opposed to homogeneous) reaction, resulting in sparse, large-area, and smooth MoS₂ crystals, whereas a relatively high S:Mo favors homogeneous reaction, resulting in dense, small-area and rough-surfaced MoS₂ crystals. In other words, to achieve high quality MoS₂ crystal growth via CVD, it is advantageous to promote heterogeneous reaction by maintaining S:Mo at its optimal value. This new observation is demonstrated by incorporating growth temperature optimization, consistently achieving MoS₂ crystals with lateral dimensions above 500 μm, which is among the largest ever grown.

The thesis ends with a summary in Chapter 5, which also gives a perspective on possible future directions of investigation. Findings from this thesis provide important insights to the rational process design for 2D MoS₂ growth and would also be an instrumental reference towards the growth of other high-quality TMD materials.