Probing of free and localized excitons and trions in atomically thin WSe₂, WS₂, MoSe₂ and MoS₂ in photoluminescence and reflectivity experiments

Abstract

We report on detailed temperature dependent (T = 7-295 K) optical spectroscopy studies of WSe, WS, MoSe and MoS monolayers exfoliated onto the same SiO/Si substrate. In the high energy region of absorption type (reflectivity contrast-RC) and emission (photo-luminescence-PL) spectra of all the monolayers resonances related to the neutral and charged excitons (X and T) are detected in the entire measured temperature range. The optical amplitudes of excitons and trions strongly depend on the temperature and two dimensional carrier gas (2DCG) concentration. In the low energy PL spectra of WSe and WS we detect a group of lines (L) which dominates the spectra at low temperatures but rapidly quenches with the increase in the temperature. Interestingly, in the same energy range of the RC spectra recorded for WS, we observe an additional line (L ), which behaves in the same way as the L lines in the PL spectra. The optical amplitude of L and T resonances in the RC spectra strongly increases with the growth of the 2DCG concentration. On the base of these observations we identify the L resonance in the RC spectra as arising from the fine structure of the trion. We also propose that the line interpreted previously in PL spectra of WSe and WS as related to the biexciton emission is a superposition of the biexciton, trion and localized exciton emission. We find that with the temperature increase from 7-295 K the total PL intensity decreases moderately in WSe and WS, strongly in MoS and dramatically in MoSe.

Charged excitons in two-dimensional transition metal dichalcogenides: Semiclassical calculation of Berry curvature effects

“…Our results exhibit relatively agreement with those of Ref. [52]. Similar to the findings for exciton binding energy by varying λ s or ε sub , the trion binding energy reacts most sensitively to changes in Ω.…”

“…Accordingly, the exciton binding energy is around 278 meV, using the same λ s , which is somewhat lower than exper-imental values of 340 meV. In order to reproduce the experimental exciton and trion binding energies of about 300 and 42meV at 7K, respectively[52], we use λ s = 28Å for a WS 2 monolayer on SiO 2 substrate. In similar structures but made of WSe 2 monolayer, B 1s = 290 meV for a sample placed on SiO 2 substrate ε sub = 2.1 which is in agreement with reported in Ref [53].…”

Singlet and triplet trions in WS2 monolayer encapsulated in hexagonal boron nitride

“…Following the previous observations [15,16,17], the T S and T T emission peaks are identified as due to, correspondingly, singlet and triplet trions with two different configurations of electrons' spins. The ability to create charged excitons in the structures under study most probably results from unintentional n-type doping of the WS 2 monolayer, which is commonly reported for this material [20,17,18]. Notably, the (optically active) exciton (X A ) in WS 2 monolayer is associated with the top spin-split valence band (VB) subband and the upper spin-split conduction band (CB) subband.…”

Anti‐Stokes Photoluminescence of Monolayer WS2

“…At 30 K, the density of 50 meV phonons is three orders of magnitude smaller than the one of 30 meV phonons, leading to an even lower anti‐Stokes PL in WS 2 than in WSe 2 . Moreover the exciton to trion PL emission intensity ratio is more favorable for the exciton in WSe 2 than in WS 2 facilitating its observation after up‐conversion. The high efficiency of the up‐conversion process is underlined through the observation even at 30 K, where only a low density of phonons of few tens of meV (required for the up‐conversion) are present.…”