Plasmon spectroscopy of graphene and other two-dimensional materials with transmission electron microscopy

Investigations on plasmonic modes of graphene, topological insulators, black phosphorus, boron nitride and two-dimensional carbides by means of transmission electron microscopy are here reviewed. Electron energy loss spectroscopy with atomic resolution allows observing the enhancement of graphene plasmons at substitutional atoms. For topological insulators, the two existing modes in the ultraviolet range have different dispersion relation, thus evidencing band-structure effects on plasmon dispersion. Multiple plasmonic modes in the visible range have been reveled in topological-insulator nanoplatelets. Concerning black phosphorus, the plasmonic spectrum changes with thickness: while the bulk is characterized by a single plasmon mode around 20 eV, other low-energy interband plasmons emerge in few layers. Likewise, an additional mode at 7.5 eV is present in the plasmonic spectrum of few-layer hexagonal boron nitride ((h-BN)). Interband plasmons in graphene, (h-BN), or transition-metal dichalcogenides exhibit frequency dependence on thickness. Such a phenomenon is absent in two-dimensional carbides, for which the bulk plasmon does not change with thickness, while the surface plasmon can be tuned by chemical functionalization and/or thickness.