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.