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

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 interactions 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 a WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in the WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral excitons upon an increase of temperature is demonstrated.

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

“…1(a), 1(b)]. Meanwhile, studies on the sign of the exciton g-factor [27][28][29][30][31] and the dark exciton state 21,[32][33][34] have shown evidence supporting the theory. A direct experimental probe of the spin-polarized band structure and measurement of the magnitude of ∆ !…”

Sub-bandgap Voltage Electroluminescence and Magneto-oscillations in a WSe2 Light-Emitting van der Waals Heterostructure

“…At lower energies, a complex broad band is observed, typical for monolayer WSe 2 samples and which has been attributed to charged and localized (bound) excitons. 13,[32][33][34] This large broad band indicates the presence of a significant amount of impurities in the case of our device. The presence of defects as evidenced by the optical measurements fits into the scenario of a pinned Fermi level for this device.…”

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

“…Today, the 'scotch tape method' is the most used method [12][13][14][15][16] to prepare monolayers (MLs) of WSe 2 from its bulk counterpart. However, WSe 2 layers have been also fabricated using chemical exfoliation [17][18][19], chemical vapor deposition (CVD) [20][21][22][23][24], metal-organic chemical vapor deposition (MOCVD) [25], hydrothermal exfoliation [26], liquid exfoliation [27][28][29], and physical vapor deposition [30,31].…”