Star-Shaped WS₂ Monolayers with Twin Grain Boundaries Promoted by Molybdenum Atoms

Monolayers of transition-metal dichalcogenides (TMDs) exhibit fascinating properties that make them attractive in optics, electronics^a, spintronics, and valleytronics^b, and it is of vital importance to understand and control their morphology and tune their physical properties^c. However, the origin of their morphology evolution is still highly elusive, which hinders the synthesis of desired morphologies for specific applications. Herein, we report the controlled synthesis and formation mechanism of star-shaped WS₂ monolayers by adding trace concentrations of molybdenum using a liquid-phase precursor-assisted approach. Fluorescence imaging and photoluminescence (PL) mapping of six-arm stars revealed bright lines between adjacent arms. To correlate the morphology and optical properties with the microstructure, second harmonic generation (SHG) microscopy and dark-field transmission electron microscopy (DF-TEM) were implemented to confirm the presence of polycrystal domains with a 60° lattice misorientation and a mirror twin grain boundary. Detailed analysis of the grain boundary and molybdenum atom distribution was assessed using high-resolution, high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The relationship of the growth morphology of WS₂ stars and the molybdenum to tungsten ratio of the precursor was also carefully investigated. In corroboration with the experimental results, we further developed a multiscale model which combines density functional theory, ReaxFF based molecular dynamics simulations, a Wulff construction and a phase-field model, which demonstrated that the anisotropy of grain boundary (GB) energy due to molybdenum doping can lead to the star morphologies of WS₂. Our study provides further insights into controlling the morphology of crystalline TMD monolayers, with implications for the development of field-programmable semiconductor memristor devices.<br/><br/>References<br/>(a) Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N., & Strano, M. S. (2012). <i>Nature nanotechnology</i>, <i>7</i>(11), 699-712.<br/>(b) Tong, W. Y., Gong, S. J., Wan, X., & Duan, C. G. (2016). <i>Nature communications</i>, <i>7</i>(1), 1-7.<br/>(c) Dong, J., Liu, Y., & Ding, F. (2022). <i>NPJ Computational Materials</i>, <i>8</i>(1), 1-11.