Morphology and Size Engineering of Two-Dimensional MoS2 Crystal via Chemical Vapor Deposition


Student thesis: Doctoral Thesis

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Awarding Institution
Award date27 May 2021


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.