High-energy collective electronic excitations in layered transition-metal dichalcogenides

Abstract

We characterize experimentally and theoretically the collective electronic excitations in two prototypical layered transition-metal dichalcogenides, NbSe 2 and Cu 0.2 NbS 2 . The energy-and momentum-dependent dynamical structure factor was measured by inelastic x-ray scattering (IXS) spectroscopy and simulated by time-dependent density-functional theory. We find good agreement between theory and experiment, provided that Nb semicore states are taken into account together with crystal local-field effects. Both materials have very similar spectra, characterized by two main plasmons at 9 and 23 eV, which we show to both have π + σ character on the basis of a detailed analysis of the band structure. Finally, we discuss the role of the layer anisotropy in the dispersion of these plasmons.

Negative plasmon dispersion in 2H-NbS2beyond the charge-density-wave interpretation

“…We note that exchange-correlation effects beyond RPA in this class of materials are not changing qualitatively the results[22,24].…”

Interplay between structure and electronic properties of layered transition-metal dichalcogenides: Comparing the loss function of1Tand2Hpolymorphs

“…In 2H materials, on the other hand, due to the different splitting of the d orbitals, only the metallic states with d z 2 character are able to sustain collective excitations alone. The interband transitions between d states, in fact, strongly mix with the higher-energy transitions involving π and σ bands related to the p z and p xy orbitals of the chalcogen atoms, giving rise to the π + σ plasmon located at 8 eV [22]. This excitation (not shown) is present in 1T systems as well and is located at about 6.6 eV.…”

“…Since in our preliminary tests we found that in the small-energy range considered here the effect of f xc in the adiabatic local-density approximation is negligible (see also Refs. [20] and [21]), in the following we will consider results obtained in the random-phase approximation (RPA) in which f xc = 0 and that has already been proven to be sufficient to obtain a good agreement with available experimental data [18,19,22]. This approximation has been used to evaluate the loss function of single-sheet TMD as well since previous works on graphene and single-wall carbon nanotubes, see e.g.…”

“…We calculate the dynamical charge-density response function of these materials by means of first-principles time-dependent density functional theory (TDDFT). We compare the loss function of 1T polymorphs with that of members of the 2H family [15][16][17][18][19][20][21][22], both in the bulk and in the 2D form [23]. We characterize the different plasmon dispersions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone.…”

Bandstructure modulation of two-dimensional WSe2 by electric field

“…9,10 Layered transition-metal dichalcogenides (TMDs) are an intriguing family of materials that span a broad range of physical properties and have been extensively studied for applications in catalysis, tribology, electronics, photovoltaics, and electrochemistry. [11][12][13][14] Through exfoliation, layered TMDs with strong covalent inplane bonds and weak van der Waals-like coupling between layers, can be made into single-and few-layer flakes. [15][16][17][18][19][20][21] With the relative fabrication easiness compared to one dimensional materials, 2D materials are expected to have a significant impact on next-generation nanoelectronic devices.…”