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We analyze the full relativistic force experienced by swift electrons moving close to planar films for the experimental conditions commonly used in electron energy loss spectroscopy in STEM. In metals the main effects derive from the dispersion of the surface plasmons, which are clearly observed in the EEL spectra. In insulators we explore the role played by the Cherenkov radiation (CR) emitted in the energy gap window. The focus is placed on the transverse force and different factors which may turn this force into repulsive, as reported in recent experimental and theoretical works. ; This project was supported by the ETORTEK project NANOIKER from the Basque Government (BG), project FIS2010-19609-C02-01 from the Spanish Ministry of Science and Innovation and Grant IT-75613 from BG-UPV/EHU. ; Peer Reviewed

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. ; The full retarded electromagnetic force experienced by swift electrons moving parallel to planar boundaries is revisited, for both metallic and dielectric targets, with special emphasis on the consequences in electron microscopy experiments. The focus is placed on the sign of the transverse force experienced by the electron beam as a function of the impact parameter. For point probes, the force is found to be always attractive. The contribution of the induced magnetic field and the causality requirements of the target dielectric response, given by the Kramers-Kronig (K-K) relations, prove to be crucial issues at small impact parameters. For spatially extended probes, repulsive forces are predicted for close trajectories, in agreement with previous works. The force experienced by the target is also explored, with the finding that in insulators, the momentum associated to Cherenkov radiation (CR) is relevant at large impact parameters. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. ; This project was supported by the ETORTEK project NANOIKER from the Basque Government (BG), project FIS2010-19609-C02-01 from the Spanish Ministry of Science and Innovation, and grant IT-75613 from BG-UPV/EHU. ; Peer Reviewed

A general expression for the energy-loss probability in scanning transmission electron microscopy valid for complex microstructures is presented in the framework of classical dielectric theory. Calculations are carried out for small particles half embedded in a planar interface. We used experimental data for the dielectric functions that characterize the media and we take into account the coupling among different multipolar terms. Resonances, present neither in the planar-interface nor in the isolated-sphere energy-loss spectra, are found. The results agree with many experimental results reported in the literature of the last few years. A different behavior between conducting and insulator supports is found. The effect of an oxide coating around the spherical particle is also discussed. ; They thank the Basque Country Government (project PGV9114.1), Gipuzkoako Foru Aldundiaia and Iberdrola for financial help, and Labein for the use of computing facilities. ; Peer reviewed

In the frame of the Self-Energy formalism, we study the interaction between STEM electrons and small particles in the range of the valence electron excitations. We first calculate the energy loss probability for an isolated sphere and study the loss spectrum dependence on the size of the particle and on the relative impact parameter. Then we analyze the loss spectra in more realistic situations: (a) the effect of the coupling between the particle and supporting surface is studied in a simple geometrical model; and (b) we analyze the dependence of the losses on the geometrical shape of the target by considering hemispherical particle. Our results are in a good qualitative (and in simple cases, quantitative too) agreement with several experimental results which show anomalous excitations. We restate the suitability of the dielectric theory to study the surface excitations of these systems. ; They thanks Gipuzkoako Foru Aldundia and the Basque Country Government (Eusko Jaurlaritza) for financial help. ; Peer reviewed

A general expression for the energy loss probability in scanning transmission electron microscopy (STEM) is derived. The method uses a Green's function for the incident and scattered electrons and then folds the specimen into a local response function. Our expression is appropriate to any target geometry and dielectric response. As an application, the energy loss spectrum of a STEM electron moving close to an Al sphere half-embedded in an Al planar surface is calculated. The coupling between different l modes, neglected in earlier theoretical approaches, is taken into account. ; The authors thank the Education Department of the Basque Country Government (project PGV9114.1), Guipuzkoako Foru Aldundia and Iberdrola for help and support and Labein for the use of computing facilities. ; Peer reviewed

The excitations produced by fast electrons impinging perpendicularly on both metallic and semiconductor cylindrical nanowires are investigated within the framework of dielectric theory. The dependence of the electron energy-loss spectra (EELS) on the nanowire radius is studied in detail, and so is the spatial extension of the induced-charge fluctuations associated to the modes that are excited during the loss process. The limits of applicability of dielectric theory to nanowires are discussed. In particular, comparison between the present theory and EELS measurements performed with silicon nanofibers support the use of dielectric theory at the scale of a few nanometers in diameter, and it is shown that this positive result is justified in terms of the longitudinal pattern of the induced surface plasmons. Finally, the effect of nanowire termination on the electron energy-loss probability for electrons passing near the edge is calculated using the boundary charge method, showing that the range of this effect can extend up to tens of nanometers for low-energy m=0 modes. ; Support from the Basque government and Basque Country University is acknowledged. One of the authors E.O. also acknowledges the Spanish Ministerio de Educacion y Cultura for financial support under Contract No. PB98-0780-C02. ; Peer reviewed

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. ; In the framework of dielectric theory, the static non-local self-energy of an electron near an ultra-thin polarizable layer has been calculated and applied to study binding energies of image-potential states near free-standing graphene. The corresponding series of eigenvalues and eigenfunctions have been obtained by numerically solving the one-dimensional Schrödinger equation. The image-potential state wave functions accumulate most of their probability outside the slab. We find that the random phase approximation (RPA) for the non-local dielectric function yields a superior description for the potential inside the slab, but a simple Fermi-Thomas theory can be used to get a reasonable quasi-analytical approximation to the full RPA result that can be computed very economically. Binding energies of the image-potential states follow a pattern close to the Rydberg series for a perfect metal with the addition of intermediate states due to the added symmetry of the potential. The formalism only requires a minimal set of free parameters: the slab width and the electronic density. The theoretical calculations are compared with experimental results for the work function and image-potential states obtained by two-photon photoemission. © 2014 IOP Publishing and Deutsche Physikalische Gesellschaft. ; This work was financed by the Governments of Spain (MAT2011-26534 and FIS2010-19609-C01-01) and the Basque Country (IT-756-13). Computing resources provided by the CTI-CSIC are gratefully acknowledged. ; Peer Reviewed

19 páginas, 12 figuras.-- PACS number(s): 73.20.Mf, 68.37.Ma, 42.50.Wk, 41.75.Fr ; The total momentum transfer from fast electron beams, like those employed in scanning transmission electron microscopy (STEM), to plasmonic nanoparticles is calculated. The momentum transfer is obtained by integrating the electromagnetic forces acting on the particles over time. Numerical results for single and dimer metallic nanoparticles are presented, for sizes ranging between 2 and 80 nm. We analyze the momentum transfer in the case of metallic dimers where the different relevant parameters such as particle size, interparticle distance, and electron beam impact parameter are modified. It is shown that depending on the specific values of the parameters, the total momentum transfer yields a force that can be either attractive or repulsive. The time-average forces calculated for electron beams commonly employed in STEM are on the order of piconewtons, comparable in magnitude to optical forces and are thus capable of producing movement in the nanoparticles. This effect can be exploited in mechanical control of nanoparticle induced motion. ; Financial support from the Department of Industry of the Basque Government through the ETORTEK project inano, from the Spanish Ministerio de Ciencia e Innovación through Project No. FIS207-066711-C02-00, from the Consejo Nacional de Ciencia y tecnología (Mexico) through Project No. 82073, and from the U.S. NSF under Grant No. 0959905 are acknowledged. ; Peer reviewed