Excitonic resonances in thin films of WSe2: From monolayer to bulk material

TL;DR

This study investigates how excitonic resonances in WSe2 thin films evolve from monolayer to bulk, revealing changes in optical properties, Coulomb interactions, and exciton behavior with layer thickness and temperature.

Contribution

It provides detailed optical spectroscopy analysis of WSe2 layers, highlighting the evolution of excitonic features and Coulomb effects across different thicknesses and temperatures.

Findings

Excitonic resonances change with layer thickness due to band structure modifications.

Temperature affects exciton energies and oscillator strength transfer between charged and neutral excitons.

Monolayer WSe2 shows increased photoluminescence yield at higher temperatures.

Abstract

We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated.