International audienceIn this joint experimental and theoretical work, we investigate collective electronic excitations (plasmons) in free-standing, single-layer graphene. The energy- and momentum-dependent electron energy-loss function was measured up to 50eV along two independent in-plane symmetry directions (ΓM and ΓK) over the first Brillouin zone by momentum-resolved electron energy-loss spectroscopy in a transmission electron microscope. We compare our experimental results with corresponding time-dependent density-functional theory calculations. For finite momentum transfers, good agreement with experiments is found if crystal local-field effects are taken into account. In the limit of small and vanishing momentum transfers, we discuss differences between calculations and the experimentally obtained electron energy-loss functions of graphene due to a finite momentum resolution and out-of-plane excitations

PACS 78.67.-n-Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures PACS 78.67.Wj-Optical properties of graphene PACS 73.20.Mf-Collective excitations (including excitons, polarons, plasmons and other charge-density excitations) Abstract-In low-dimensional systems, a detailed understanding of plasmons and their dispersion relation is crucial for applying their optical response in the field of plasmonics. Electron energyloss spectroscopy is a direct probe of these excitations. Here we report on electron energy-loss spectroscopy results on the dispersion of the π plasmons in free-standing graphene monolayers at the momentum range of 0 |q| 0.5Å −1 and parallel to the Γ-M direction of the graphene Brillouin zone. In contrast to the parabolic dispersion in graphite and in good agreement with theoretical predictions of a 2D electron gas of Dirac electrons, linear π plasmon dispersion is observed. As with previous EELS results obtained from single-wall carbon nanotubes, this can be explained by local-field effects in the anisotropic 2D system yielding a significant contribution of the low-energy band structure on the high-energy π plasmon response.

Electronic structures of LiFePO(4) and FePO(4) have been investigated using valence and core electron energy loss spectroscopy (EELS) supported by ab initio calculations. Valence electron energy loss spectra of FePO(4) are characterized by interband transitions found between 0 and 20 eV, which are not observed in LiFePO(4). Spectra are fully analysed using band structure calculations and calculated dielectric functions. In particular, we show that interband transitions observed in FePO(4) spectra originate from the states at the top of the valence band, which have mainly oxygen p character. From core-loss EELS, it is observed that the O-K edge in FePO(4) has a pre-edge peak below the threshold of the main O-K edge. This pre-edge peak is not observed in the O-K spectra of LiFePO(4). The position of the pre-edge peak is determined by a charge transfer process, which shifts the position of the iron 3d bands with respect to the conduction band. The intensity of the pre-edge peak is also determined by the changes in the hybridization of iron 3d and oxygen states as a result of extraction of lithium ions from the LiFePO(4) lattice. We show that the extraction of lithium ions from LiFePO(4) results in large changes in the electronic structure, such that FePO(4) can be considered to be a charge transfer insulator while LiFePO(4) is a typical Mott-Hubbard insulator.

We report Co‐free, Li‐rich Li1+xNi0.5Mn1.5O4 (0