Nongeneric dispersion of excitons in the bulk ofWSe2

Abstract

We combine electron energy-loss spectroscopy (EELS) and density functional theory (DFT) calculations to study the dispersion and effective mass of excitons in the bulk of WSe2. Our EELS data suggest substantial deviations from the generic quadratic momentum dependence along the ΓK-direction. From the DFT-derived Kohn-Sham states we deduce the EELS response without the inclusion of particle-hole attraction to study the possible role of the single-particle band structure on the exciton behavior. Based on this analysis we argue in favor of a strongly momentum dependent particle-hole interaction in WSe2 and other group VI-transition-metal dichalcogenides.

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

“…The next couple of TMDs are WX 2 (X = S and Se) compounds, which are quite intensively studied in recent years 14,19–21,23 , and are known as indirect gap semiconductors in their bulk form, but the value of the indirect gap is not well established for these compounds. The band gap determined from PA measurements for WS 2 and WSe 2 is 1.57 and 1.33 eV, respectively.…”

“…During the analysis of the influence of transition metal atoms exchange on the indirect and direct band gap in TMD it should be noted that the change in the transition metal also leads to a change in the crystallographic structure. HfX 2 crystals can have a 1 T structure with octahedral coordination of the chalcogenides by the metal atom 47 , WX 2 samples are the hexagonal 2 H layered crystals like MoX 2 samples, and ReX 2 samples have a distorted 1 T diamond-chain structure with the triclinic symmetry (3 R polytype) 1,14 . The configuration of the transition metal in this case is 5d 2 6s 2 , 5d 4 6s 2 , and 5d 5 6s 2 for Hf, W, and Re, respectively.…”

“…Van der Waals (vdW) crystals are known for a long time 1–14 but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers 15–25 . Since pioneering reports on optical properties of MoS 2 layers 15,16 , it is well established that the electronic band structure of MoS 2 strongly varies with the number of layers and exhibits indirect-to-direct band gap transition with the size reduction to a single layer.…”

“…Similar studies of optical properties and the electronic band structure have been performed on other van der Waals crystals such as MoSe 2 , MoTe 2 , WS 2 , and WSe 2 22–25 . Some of them were investigated quite recently as bulk materials 13,14 but interest in these materials was low due to no potential applications in semiconductor devices. At this moment the situation is different.…”

Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe2

“…Subsequent to vertical excitation of excitons in the K valleys, excited states swiftly scatter to the Σ valleys [17]. During the subsequent electron relaxation, electrons populate almost exclusively the Σ valleys around the bottom of the conduction band, which is also the lowest energetic state for excitons in bulk WSe 2 [34]. Energy relaxation of the excited electrons therefore occurs through intravalley scattering and intervalley scattering between different Σ valleys (shown as blue arrows).…”