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

TL;DR

This study investigates how the electronic structure of atomically thin WS2 and WSe2 sheets evolves with decreasing layer thickness, revealing an indirect-to-direct bandgap transition similar to MoS2, with implications for optoelectronic applications.

Contribution

It provides experimental evidence of the indirect to direct bandgap transition in WS2 and WSe2 monolayers and compares their optical properties to MoS2, highlighting the effects of Se p-orbitals and interlayer coupling.

Findings

Monolayer WS2 and WSe2 exhibit strong photoluminescence due to direct bandgap.

Few-layer WS2 and WSe2 show both indirect and direct gap emissions, indicating high-quality synthetic crystals.

Thickness-dependent optical features reveal the influence of Se p-orbitals and interlayer interactions.

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.