A. Rivacoba

Plasmonic nanobilliards: controlling nanoparticle movement using forces induced by swift electrons.

Manipulation of nanoscale objects to build useful structures requires a detailed understanding and control of forces that guide nanoscale motion. We report here observation of electromagnetic forces in groups of nanoscale metal particles, derived from the plasmonic response to the passage of a swift electron beam. At moderate impact parameters, the forces are attractive, toward the electron beam, in agreement with simple image charge arguments. For smaller impact parameters, however, the forces are repulsive, driving the nanoparticle away from the passing electron. Particle pairs are most often pulled together by coupled plasmon modes having bonding symmetry. However, placement of the electron beam between a particle pair pushes the two particles apart by exciting antibonding plasmonic modes. We suggest how the repulsive force could be used to create a nanometer-sized trap for moving and orienting molecular-sized objects.

Image potential in scanning transmission electron microscopy

In the framework of the classical dielectric theory, the role of the image potential in electron energy loss spectroscopy (EELS) of fast electrons commonly used in scanning transmission electron microscopy travelling near a surface is studied. Relativistic and dispersive corrections are evaluated to establish the range of validity of this theory. The spatial resolution of the EELS technique is discussed for valence and core electron excitations. The effect of the quantal nature of the probe is also discussed. Finally, several problems involving planar surfaces, small particles, cylinders and truncated targets of interest in nanotechnology are studied.

Energy loss of electrons travelling through cylindrical holes

Abstract Expressions for the energy loss due to interaction with surfaces experienced by fast electrons passing through holes drilled in a coated medium are derived within the framework of the classical dielectric theory. These expressions are evaluated for Al coated with alumina using experimental values of the dielectric constant ϵ(ω) of the different media. Retarded energy loss probabilities are derived and some applications are discussed. Deflection effects due to the image force acting on the particle are also analysed.

Energy loss probability of STEM electrons in cylindrical surfaces

Abstract In the frame of the self-energy formalism, the interaction of STEM electrons with a cylindrical surface is studied. The surface modes of cylindrical interfaces so obtained are proven to fulfil certain sum rules. A general expression for the energy loss probability valid for any beam direction nonparallel to the cylinder axis is presented. In all the cases the so-called begrenzung effect is found.

Energy loss in spheres by penetrating electrons

A classical dielectric theory is developed to provide an expression for the energy loss of fast electrons passing through a metallic sphere at a fixed impact parameter, in the case of spheres coated by a thin oxide layer. The expressions for energy loss are valid for any dielectric function e(ω). A high number of terms of the multipole expansion is required in order to take into account the effects of the oxide layer. Results are compared with experimental measurements of energy loss obtained by STEM techniques.

Deflection of stem electrons by dielectric spheres

Abstract The deflection angle due to the image force for a fast electron (100 keV) travelling very close to the surface of a sphere has been calculated within the frame of classical dielectric theory. The results of the calculation are in disagreement with the existing experimental results.

Relativistic force between fast electrons and planar targets

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.

Cherenkov radiation effects in EELS for nanoporous alumina membranes

Abstract Swift electron energy losses have been investigated in nanoporous alumina, taking into account retardation effects. A simple analytical expression is obtained for the energy loss probability. The electron energy loss spectra observed with the Scanning transmission electron microscope are reasonably well reproduced using a simple model consisting of a cylindrical shell of alumina surrounded by an effective porous medium. The Cherenkov radiation is responsible for energy losses below 8 eV. These losses are very sensitive to the shape of the nanostructures, up to distances much larger than the adiabatic length v / ω .

Image states: idea and theoretical development

Abstract The concept and theoretical development of image states is reviewed. A brief summary of calculations of binding energies and effective masses is presented together with a more detailed discussion of the lifetime of such states. A brief discussion of the dependence of the linewidths with the surface response function is presented.

Plasmon excitations at diffuse interfaces

Energy losses experienced by a fast electron probe moving through a dielectric medium have been studied both numerically and analytically, where the response function varies continuously with position in one transverse direction. The frequent assumption that the loss spectrum should exhibit a peak determined by the plasmon energy in a homogeneous medium with the composition found locally at the probe position can be incorrect. In free electron systems, inhomogeneous effects can cause spectral shape changes as well as peak shifts. Computations for diffuse interfaces between semiconductors with differing band gaps are also reported. Prospects for improved spatial resolution in valence loss spectroscopy at higher momentum transfer are discussed.