  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.
