We present an accurate measurement and a quantitative analysis of electron-beam induced displacements of carbon atoms in single-layer graphene. We directly measure the atomic displacement ("knock-on") cross section by counting the lost atoms as a function of the electron beam energy and applied dose. Further, we separate knock-on damage (originating from the collision of the beam electrons with the nucleus of the target atom) from other radiation damage mechanisms (e.g. ionization damage or chemical etching) by the comparison of ordinary ( 12 C) and heavy ( 13 C) graphene. Our analysis shows that a static lattice approximation is not sufficient to describe knock-on damage in this material, while a very good agreement between calculated and experimental cross sections is obtained if lattice vibrations are taken into account.

To increase efficiency of bulk heterojunctions for photovoltaic devices, the functional morphology of active layers has to be understood, requiring visualization and discrimination of materials with very similar characteristics. Here we combine high-resolution spectroscopic imaging using an analytical transmission electron microscope with nonlinear multivariate statistical analysis for classification of multispectral image data. We obtain a visual representation showing homogeneous phases of donor and acceptor, connected by a third composite phase, depending in its extent on the way the heterojunction is fabricated. For the first time we can correlate variations in nanoscale morphology determined by material contrast with measured solar cell efficiency. In particular we visualize a homogeneously blended phase, previously discussed to diminish charge separation in solar cell devices.

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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.

A new UHV spectroscopic X-ray photoelectron emission and low energy electron microscope is presently under construction for the installation at the PM-6 soft X-ray undulator beamline at BESSY II. Using a combination of a sophisticated magnetic beam splitter and an electrostatic tetrode mirror, the spherical and chromatic aberrations of the objective lens are corrected and thus the lateral resolution and sensitivity of the instrument improved. In addition a corrected imaging energy filter (a so-called omega filter) allows high spectral resolution (∆E = 0.1 eV) in the photoemission modes and background suppression in LEEM and small-spot LEED modes. The theoretical prediction for the lateral resolution is 5Å; a realistic goal is about 2 nm. Thus, a variety of electron spectroscopies (XAS, XPS, UPS, XAES) and electron diffraction (LEED, LEEM) or reflection techniques (MEM) will be available with spatial resolution unreached so far.

High-energy collective electronic excitations (plasmons) in freestanding multilayer graphene are studied by momentum-resolved electron energy-loss spectroscopy (EELS). For normal incidence, only the high-energy plasmon band is excited and we measure a blueshift of the π-plasmon dispersion with increasing thickness. The observed transition between two-dimensional and three-dimensional behavior is explained using a layeredelectron-gas (LEG) model. We propose a method to measure all individual plasmon bands by tilting the sample with respect to the electron beam. As a proof of concept, EELS experiments for three-layer graphene are compared with predictions from the LEG model.