Edge, size, and shape effects on WS2, WSe2, and WTe2 nanoflake stability: design principles from an ab initio investigation

Abstract

An improved atomistic understanding of the W-based two-dimensional transition-metal dichalcogenides (2D TMDs) is crucial for technological applications of 2D materials, since the presence of tungsten endows these materials with distinctive properties. However, our atomistic knowledge on the evolution of the structural, electronic, and energetic properties and on the nanoflake stability of such materials is not properly addressed hitherto. Thus, we present a density functional theory (DFT) study of stoichiometric (WQ₂)_n nanoflakes, with Q = S, Se, Te, and n = 1,…,16, 36, 66, and 105. We obtained the configurations with n = 1,…,16 through the tree growth algorithm whereas the nanoflakes with n = 36, 66, and 105 were generated from fragments of 2D TMDs with an abundant diversity of shapes and edge configurations. We found that all the most stable nanoflakes present the same Q-terminated edge configuration. Furthermore, in isomers with n = 1,…,16 sizes, nanoflakes with triangular shapes and their derivatives, such as the rhombus geometry, define magic numbers, whereas for n > 16, triangular shapes were also found for the most stable structures, because they preserve the edge configuration. A strong modulation of the Hirshfeld charges, depending on chalcogen species and core or edge position, is also observed. The modulation of the Hirshfeld charge due to the nature of the W metal atoms makes the energetic 1D → 1T′ transition of (WQ₂)_n differ in nanoflake size in relation to (MoQ₂)_n nanoflakes. Our analysis shows the interplay between edge configuration, coordination environment, and shape that determines the stability of nanoflakes, and allows us to describe design principles for stable 1T′ stoichiometric nanoflakes of various sizes.