The optical response of monolayer, few-layer and bulk tungsten disulfide

Abstract

We present a comprehensive optical study of thin flakes of tungsten disulfide (WS) with thickness ranging from mono- to octalayer and in the bulk limit. It is shown that the optical band-gap absorption of monolayer WS is governed by competing resonances arising from one neutral and two distinct negatively charged excitons whose contributions to the overall absorption of light vary as a function of temperature and carrier concentration. The photoluminescence response of monolayer WS is found to be largely dominated by disorder/impurity- and/or phonon-assisted recombination processes. The indirect band-gap luminescence in multilayer WS turns out to be a phonon-mediated process whose energy evolution with the number of layers surprisingly follows a simple model of a two-dimensional confinement. The energy position of the direct band-gap response (A and B resonances) is only weakly dependent on the layer thickness, which underlines an approximate compensation of the effect of the reduction of the exciton binding energy by the shrinkage of the apparent band gap. The A-exciton absorption-type spectra in multilayer WS display a non-trivial fine structure which results from the specific hybridization of the electronic states in the vicinity of the K-point of the Brillouin zone. The effects of temperature on the absorption-like and photoluminescence spectra of various WS layers are also quantified.

Probing exciton species in atomically thin WS2–graphene heterostructures

“…Temperature dependences of the exciton and trion peak positions are fitted well with the fitting parameters listed in table 1. The values for E g (0) are comparable to those in previous studies on WS 2 monolayers [31] and indicate that the observed temperature dependence originates from the change in bandgap of WS 2 .…”

“…The trion dissociation energy, i.e. the energy difference between the exciton and trion is related to the Fermi energy by [31,35]:…”

Zeeman spectroscopy of excitons and hybridization of electronic states in few-layer WSe 2 , MoSe 2 and MoTe 2

“…5(c). The X 1s A resonance undergoes a red shift from 1.196 ± 0.001 eV to 1.1307 ± 0.001 eV as the layer thickness is increased from 1L to bulk as has been observed previously also in WSe 2 [25], MoSe 2 [23], WS 2 [3] and MoTe 2 [26,44]. At the same time, the energy difference between the X 1s A and X 2s A resonances decreases from 127 meV to 25 meV as the layer thickness is increased from 2L to bulk (X 2s A is not observed for the 1L flake).…”

“…We calculate the g-factors of a trilayer in the same way as in the bilayer. We introduce the three sets of basis states {|Ψ (1) n }, {|Ψ (2) n }, {|Ψ (3) n }, which belong to the layers z = −l, z = 0 and z = l respectively. In this case the crystal has the mirror symmetry, with the mirror plane z = 0.…”

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe 2 , WSe 2 and WS 2 Monolayers

“…This peculiar evolution originates, as discussed previously in literature [43][44][45][46][47][48], from contribution of two different CX states to the feature observed in absorption-type spectra. The existence of two CX states in 'darkish' materials is a consequence of spin alignment in conduction subbands, which allows an excess electron from either K+ or K− valley to accompany the photo-created electron-hole pair and form a bound CX state.…”