Probing the local nature of excitons and plasmons in few-layer MoS2

Excitons and plasmons are the two most fundamental types of collectiveelectronic excitations occurring in solids. Traditionally, they have beenstudied separately using bulk techniques that probe their average energeticstructure over large spatial regions. However, as the dimensions of materialsand devices continue to shrink, it becomes crucial to understand how theseexcitations depend on local variations in the crystal- and chemical structureon the atomic scale. Here we use monochromated low-lossscanning-transmission-electron-microscopy electron-energy-loss (LL-STEM-EEL)spectroscopy, providing the best simultaneous energy and spatial resolutionachieved to-date to unravel the full set of electronic excitations in few-layerMoS2 nanosheets over a wide energy range. Using first-principles many-bodycalculations we confirm the excitonic nature of the peaks at ~2eV and ~3eV inthe experimental EEL spectrum and the plasmonic nature of higher energy-losspeaks. We also rationalise the non-trivial dependence of the EEL spectrum onbeam and sample geometry such as the number of atomic layers and distance tosteps and edges. Moreover, we show that the excitonic features are dominated bythe long wavelength (q=0) components of the probing field, while the plasmonicfeatures are sensitive to a much broader range of q-vectors, indicating aqualitative difference in the spatial character of the two types of collectiveexcitations. Our work provides a template protocol for mapping the local natureof electronic excitations that open new possibilities for studyingphoto-absorption and energy transfer processes on a nanometer scale.