Size and Composition Dependent Electronic, Structural and Optical Properties of Transition Metal Dichalcogenide Nanoflakes

Nanoparticles and nanostructures of two-dimensional semiconductors are being explored for their potential in photocatalysis, opto-electronics, and energy harvesting applications. Herein, we investigate the size and compo- sition dependence of electronic, structural, and optical properties of triangular transition metal dichalcogenide (MX2, M=Mo, W and X=S, Se, Te) nanoflakes. Structural optimizations reveal that, while all flakes undergo dimerization of X atoms along each edge, in large WS2 flakes the edge S form trimers. All flakes are found to be metallic with dominant contributions to the conducting states from the edges. Our time-dependent density functional theory-based calculations find both surface (2D) and edge (1D) plasmonic exictations at low ener- gies in all small flakes. However, only Se containing flakes are found to support edge plasmons at all sizes. The corresponding plasmon peaks exhibit a red-shift with flake size as expected from quantum confinement effects. Supported by induced charge-density and potential analyses, transition contribution maps as well as trends in generalized plasmonicity indices of the excitations, these findings assume significance given the role of plasmonic nanostructures in the aforementioned applications.