Title: Probing the local nature of excitons and plasmons in few-layer MoS2,
Abstract: Excitons and plasmons are the two most fundamental types of collective
electronic excitations occurring in solids. Traditionally, they have been
studied separately using bulk techniques that probe their average energetic
structure over large spatial regions. However, as the dimensions of materials
and devices continue to shrink, it becomes crucial to understand how these
excitations depend on local variations in the crystal- and chemical structure
on the atomic scale. Here we use monochromated low-loss
scanning-transmission-electron-microscopy electron-energy-loss (LL-STEM-EEL)
spectroscopy, providing the best simultaneous energy and spatial resolution
achieved to-date to unravel the full set of electronic excitations in few-layer
MoS2 nanosheets over a wide energy range. Using first-principles many-body
calculations we confirm the excitonic nature of the peaks at ~2eV and ~3eV in
the experimental EEL spectrum and the plasmonic nature of higher energy-loss
peaks. We also rationalise the non-trivial dependence of the EEL spectrum on
beam and sample geometry such as the number of atomic layers and distance to
steps and edges. Moreover, we show that the excitonic features are dominated by
the long wavelength (q=0) components of the probing field, while the plasmonic
features are sensitive to a much broader range of q-vectors, indicating a
qualitative difference in the spatial character of the two types of collective
excitations. Our work provides a template protocol for mapping the local nature
of electronic excitations that open new possibilities for studying
photo-absorption and energy transfer processes on a nanometer scale.