Momentum-dependent oscillator strength crossover of excitons and plasmons in two-dimensional PtSe2

TL;DR

This study combines first principles calculations and q-EELS to explore the excitation spectrum of 1T-PtSe2, revealing a crossover from excitonic to plasmonic behavior with momentum and thickness, highlighting fundamental differences in 2D materials.

Contribution

It provides the first detailed momentum-resolved analysis of excitons and plasmons in 1T-PtSe2, elucidating their distinct dispersion behaviors in 2D and bulk forms.

Findings

Monolayer PtSe2 exhibits excitonic excitations at small q.

Two interband plasmons dominate at finite momentum transfer.

Absence of long-range screening inhibits long-wavelength plasmons in 2D.

Abstract

The 1T-phase layered PtX2 chalcogenides 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. Our work unravels the excited state spectrum of layered 1T-PtSe2 and establishes the qualitatively different momentum dependence of excitons and plasmons in 2D materials.