Light-Matter Interactions in Two-Dimensional Transition Metal Dichalcogenides: Dominant Excitonic Transitions in Mono- and Few-Layer MoX$_2$ and Band Nesting

Abstract

We report ab initio calculations of the dielectric function of six mono-and bilayer molybdenum dichalcogenides based in a Bethe Salpeter equation+G0W0 (BSE@G0W0)ansatz, focussing on the excitonic transitions dominating the absorption spectrum up to an excitation energy of 3.2 eV. Our calculations suggest that switching chalcogen atoms and the strength of interlayer interactions should affect the detailed composition of the high 'C' peaks in experimental optical spectra of molybdenum dichalcogenides and cause a significant spin-orbit-splitting of the contributing excitonic transitions in monolayer MoSe2 and MoTe2. This can be explained through changes in the electronic dispersion around the Fermi energy along the chalcogen series S→Se→Te that move the van-Hove singularities in the density of states of the two-dimensional materials along the Γ-K line in the Brillouin zone. Further, we confirm the distinct interlayer character of the 'C' peak transition in few-layer MoS2 that was predicted before from experimental data and show that a similar behaviour can be expected for MoSe2 and MoTe2 as well.

Biaxial strain in atomically thin transition metal dichalcogenides

“…The differential reflectance spectrum shown in the bottom panel of Figure 2b displays a peak located at approximately 1.6 eV and various shoulders on a broad increasing background. The peak and the leftmost shoulder have been attributed to the creation of excitons at the K point of the reciprocal space that corresponds to the location of the direct bandgap in TMDCs [11,20,21]. The presence of two features separated in energy by few hundreds meV is due to the spin-orbit splitting of the valence band, which is a common feature of 2D TMDCs and it is schematically depicted in the inset of Fig.…”

Micro-reflectance and transmittance spectroscopy: a versatile and powerful tool to characterize 2D materials

“…Recent hyperspectral, reflectance, ellipsometry and photocurrent spectroscopy experiments, however, present this feature (referred to as the C exciton peak) It originates from singularities in the joint density of states between the first valence and conduction bands near the Γ point of the valence band that leads to multiple optical transitions nearly degenerate in energy. [14,15,[20][21][22][23][24] This is the post-peer reviewed version of the following article: R. Frisenda et al "Micro-reflectance and transmittance spectroscopy: a versatile and powerful tool to characterize 2D materials" Journal of Physics D: Applied Physics (2016) Special issue on 2D materials and electronic devices Micro-reflectance and transmittance are interesting techniques to determine the number of layers of 2D materials. Figure 3a shows, as an example, the differential reflectance spectra acquired for MoS 2 flakes with thickness ranging from 1 to 7 layers.…”

“…The C peak observed in WS 2 and MoSe 2 is believed to have the same origin as the one observed in MoS 2 . [14,15,[20][21][22][23][24]29] In case of WSe 2 several transitions along the KΓ ̅̅̅̅̅ direction contribute to the C exciton. The D exciton transitions stem from the same bands as the B exciton, but occur along the KM ̅̅̅̅̅ direction [17,28].…”

Anti‐Stokes Photoluminescence of Monolayer WS2

“…This is a fundamental difference to the 2D structures studied until now, such as quantum wells of conventional 3D semiconductors (II–VI and III–V compounds), and this gives rise to specific properties. For example, in semiconducting transition metal dichalcogenide (TMDC) monolayers (MLs), the exciton binding energy is on the order of several hundreds of meV, implying the observation of exciton emission at room temperature . Depending on the doping level, the luminescence of charged excitons (trions) is observed at room temperature as well …”

“…For example, in semiconducting transition metal dichalcogenide (TMDC) monolayers (MLs), the exciton binding energy is on the order of several hundreds of meV, implying the observation of exciton emission at room temperature. [12,13] Depending on the doping level, the luminescence of charged excitons (trions) is observed at room temperature as well. [14] Recently, PL up-conversion has been reported in one of the TMDCs, tungsten diselenide WS 2 .…”