Momentum-Dependent Oscillator Strength Crossover of Excitons and Plasmons in Two-Dimensional PtSe₂

Abstract

The 1T-phase layered PtX2 chalcogenide has attracted widespread interest due to its thickness dependent metal–semiconductor transition driven by strong interlayer coupling. While the ground state properties of this paradigmatic material system have been widely explored, its fundamental excitation spectrum remains poorly understood. Here we combine first-principles calculations with momentum (q) resolved electron energy loss spectroscopy (q-EELS) to study the collective excitations in 1T-PtSe2 from the monolayer limit to the bulk. At finite momentum transfer, all the spectra are dominated by two distinct interband plasmons that disperse to higher energy with increasing q. Interestingly, the absence of long-range screening in the two-dimensional (2D) limit inhibits the formation of long wavelength plasmons. Consequently, in the small-q limit, excitations in monolayer PtSe2 are exclusively of excitonic nature, and the loss spectrum coincides with the optical spectrum. The qualitatively different momentum dependence of excitons and plasmons enables us to unambiguously disentangle their spectral fingerprints in the excited state spectrum of layered 1T-PtSe2. This will help to discern the charge carrier plasmon and locally map the optical conductivity and trace the layer-dependent semiconductor to metal transition in 1T-PtSe2 and other 2D materials.

High-throughput computational stacking reveals emergent properties in natural van der Waals bilayers

“…The idea of modifying electronic band gaps via layer stacking has been paradigmatic in the field of 2D materials. Well-known examples include the indirect to direct band gap transition in group-VI TMDs 39 and the metal-semiconductor transition in PtSe 2 40 , 41 . In general, a qualitative change in the gap type has dramatic consequences for the material’s properties and possible applications.…”

Momentum and thickness dependent excitonic and plasmonic properties of 2D h-BN and MoS2 restored from supercell calculations

“… 32 Beyond the optical limit, the plasmonic peaks B and C of PtSe 2 were observed to blueshift with sample thickness at finite q in both experimental results and calculations. 33 In conclusion, comprehensive investigations into the intrinsic momentum and thickness dependent behaviours of both excitons and plasmons in 2D materials, which are free of the influence of vacuum spacing, are in high demand at the moment.…”