TEM-EELS of Low-D Materials Combining High Energy and Momentum Resolution

A major mission of condensed-matter physics is to understand material properties via the knowledge of the energy vs. momentum (q) dispersion and lifetime of fundamental excitations. Recent developments of EELS in TEM with a combined high energy & q-resolution is a perfect tool to determine them. This opens the so-far unexplored possibility to investigate dispersion and lifetime of phonons, plasmons & excitons in nanomaterials including molecules, 1D & 2D materials and heterostructures with few nm of lateral resolution on samples as thin as an atomic monolayer. In this presentation I give an overview on our recent progress in analysing fundamental excitations such as phonons, excitons, and plasmons in 2D materials such as graphene, h-BN and transition metal dichalcegonides (TMDC) using EELS with complementary high energy and momentum resolution in comparison to previous results. I will show how we can understand the full phonon dispersion of an apolar material like graphene [1] and use the ultrahigh momentum resolution to make the link to surface phonon polaritons close to the optical limits in h-BN. For graphene we also show new results on the plasmon dispersion including the gap opening close to the optical limit unravelling the Dirac cone in the excitation spectrum [2] concomitant to the direct observation of a vanishing EELS cross section approaching the optical limit [3]. For monolayer TMDC using ultra high q resolution we determined the exciton dispersion and deciphered the intense postgap absorptions and disentangling plasmon from excitons from their different momentum dependence [4-6].


References
¹ R. Senga, K. Suenaga, P. Barone, S. Morishita, F. Mauri, T. Pichler, Nature 573 (2019) 247
² A. Guandalini, R. Senga, Y.C. Lin, K. Suenaga, A. Ferretti, D. Varsano, A. Recchia, P. Barone, F. Mauri, T. Pichler, C. Kramberger, Nanoletters 23, 11835 (2023)
³ A. Guandalini, R. Senga, Y.C. Lin, K. Suenaga, P. Barone, F. Mauri, T. Pichler, C. Kramberger, https://arxiv.org/abs/2406.
⁴J. Hong, R. Senga, T. Pichler, K. Suenaga, Phys. Rev. Lett. 124 (2020) 087401.
⁵ J. Hong, M. Koshino, R. Senga, T. Pichler, H. Xu, K. Suenaga, ACSNano 15 (2021) 7783.
⁶ J. Hong, M.K. Svendsen, M. Koshino, T. Pichler, H. Xu, K. Suenaga, K.S Thygesen, ACSNano 16, 12328 (2022).

Acknowledgement
We thank the MORE-TEM consortium for support and the EU for funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program grant agreements No 951215 (MORE-TEM).