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 spectroscopy, providing the best simultaneous energy and spatial resolution achieved to-date to unravel the full set of electronic excitations in few-layer MoS 2 nanosheets over a wide energy range. Using first-principles, many-body calculations we confirm the excitonic nature of the peaks at~2 and~3 eV in the experimental electron-energy-loss spectrum and the plasmonic nature of higher energy-loss peaks. We also rationalise the non-trivial dependence of the electron-energy-loss 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.

Effect of edge plasmons on the optical properties of MoS2 monolayer flakes

“…The existence of such plasmon resonances could strongly influence the optical properties of the flakes and could potentially be of interest for plasmonic photocatalysis where hot electrons generated by the decay of plasmons are used to drive chemical reactions [20,21]. We note in passing that the plasmons in MoS 2 flakes have recently been studied by means of low-loss electron energy loss spectroscopy [22].…”

Probing Polaritons in 2D Materials

“…El‐Demellawi et al investigated the spatial and energy distribution of plasmon polaritons in mono‐ and multilayered Ti 3 C 2 T x flakes with the help of EELS‐TEM (Figure e). In addition, both plasmons and excitons modes in MoS 2 can be excited with the aid of EELS‐TEM …”

Nonlocal dielectric function and nested dark excitons in MoS2

“…In order to test the reliability of the single particle description for thin layers we compare thickness dependent EELS data in the C-exciton region reproduced from Ref. 24 with our model. Figure 3b shows a spectra measured near a thinner part of the sample (red, 5 layer) compared to a thicker part (blue, 10 layer).…”

“…However, the latter requires specialized spectrometers as well. While some EELS studies have been reported [22][23][24][25] the actual available experimental information on the q-dependent exciton landscape is still limited and often derived indirectly from optical experiments.…”