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

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, understanding and controlling the morphologies of TMD monolayers grown by chemical vapor deposition (CVD) is challenging. . Here we report the controlled synthesis and formation mechanism of star-shaped WS₂ monolayers by adding trace concentrations of molybdenum when using a liquid-phase precursor-assisted CVD. 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 used to confirm the presence of polycrystal domains with a 60° lattice misorientations and a mirror twin grain boundaries. 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 addition, we developed a multiscale model which combines density functional theory, the Wulff construction and a phase-field model, demonstrating that the anisotropy of grain boundary (GB) energies due to molybdenum doping can lead to the star-shaped morphologies. Our experimental-theory study provides further insights into controlling the morphology of crystalline TMD monolayers, with implications in 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.