Strain-mediated rapid growth of vertically oriented 2D MoSe₂: Insights into the growth mechanism

Abstract

Horizontally aligned transition metal dichalcogenides (TMDCs) are well-suited for charge transport applications, while vertically oriented TMDCs are advantageous for high surface area applications such as catalysis, water splitting, and energy storage. However, the mechanism governing these structural and morphological transition remains unclear. This study provides a comprehensive growth time profile and demonstrates that parameters such as strain, distribution of grain boundaries, randomness and interlayer distance plays the critical role in driving the desirable morphological evolution. Here, we investigate the growth dynamics of MoSe₂ thin films synthesized via chemical vapor deposition (CVD), with a focus on understanding and optimizing the horizontal-to-vertical transition as a function of growth time. Scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM) were used to trace the stacking and structural order, while Raman spectroscopy and photoluminescence spectroscopy (PL) were used to investigate the variation of stacking and optical order. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical environment of the films, and field-effect transistors (FET) measurements were used to assess electrical properties such as mobility and surface carrier density. To support the experimental findings, a computational multilayer stacking framework was developed to project desirable randomness in a controlled manner through the variation of interlayer distance and simulating extent of strain mediation through wide range of unstrained, compressive, tensile and even highly diffusive strain states. This model helps to establish the relationship between the interlayer distortion and randomness with the optical asymmetry, providing insights into strain-mediated widely distributed direct to indirect optical transitions. This can further serve as an optical marker especially for these highly randomized directional vertical oriented flakes. Overall, this study presents a fundamental understanding of strain-induced morphological transitions in MoSe₂ thin films and offers a framework for tracing and tuning the optical and electronic properties in anisotropic 2D materials.