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

The 1T-phase layered PtX 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 () resolved electron energy loss spectroscopy (-EELS) to study the collective excitations in 1T-PtSe 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 . Interestingly, the absence of long-range screening in the two-dimensional (2D) limit inhibits the formation of long wavelength plasmons. Consequently, in the small- limit, excitations in monolayer PtSe 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-PtSe. 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-PtSe and other 2D materials.