Evolution of Electronic Structure in Atomically Thin Sheets of WS₂ and WSe₂

TLDR

Geometrical confinement in exfoliated layered materials causes energy dispersion to evolve with decreasing thickness, and MoS₂ has been shown to transition from an indirect to a direct bandgap at monolayer thickness. The study reports differential reflectance and photoluminescence spectra of mono‑ to few‑layer WS₂ and WSe₂ to demonstrate that their band structures undergo an indirect‑to‑direct transition at monolayer thickness. The authors measured differential reflectance and photoluminescence across mono‑ to few‑layer WS₂ and WSe₂ samples to probe band‑structure evolution. The spectra reveal a strong PL quantum‑yield enhancement in monolayer WS₂ and WSe₂ due to exciton recombination at the direct band edge, while few‑layer samples show both strong indirect‑gap emission and distinct direct‑gap hot‑electron emission, indicating high‑quality synthetic crystals and a pronounced influence of Se p‑orbitals and interlayer coupling on the d‑electron band structure.

Abstract

Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion with decreasing layer thickness. Molybdenum disulphide (MoS2) was recently found to exhibit indirect to direct gap transition when the thickness is reduced to a single monolayer. This leads to remarkable enhancement in the photoluminescence efficiency, which opens up new opportunities for the optoelectronic applications of the material. Here we report differential reflectance and photoluminescence (PL) spectra of mono- to few-layer WS2 and WSe2 that indicate that the band structure of these materials undergoes similar indirect to direct transition when thinned to a single monolayer. Strong enhancement in PL quantum yield is observed for monoayer WS2 and WSe2 due to exciton recombination at the direct band edge. In contrast to natural MoS2 crystals extensively used in recent studies, few-layer WS2 and WSe2 show comparatively strong indirect gap emission along with distinct direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe2.