Mapping electron excitations in the visible-UV range using sub-nm resolved STEM-EELS spectrum imaging

Summary form only given. For decades, the electron energy loss spectroscopy (EELS) in a transmission electron microscope has been used to explore electronic and electromagnetic excitations of solids. In particular, the low-loss energy domain (from few eV to 50 eV) has been exploited for studying dielectric properties of materials. However, so far, only excitations in the UV range and above were investigated due to severe limitations in the detection of lower energy spectral features hidden by the strong contribution of the transmitted beam to the measured spectrum. Recently, significant improvements occurred either instrumentally for significantly reducing the zero-loss tail intensity in the experimentally measured spectrum (instrumental development of monochromators) or for retrieving by a posteriori data processing the spectral information hidden by this zero-loss tail (optimization of deconvolution techniques). In parallel, the EELS spectrum-imaging mode in a scanning transmission electron microscope (STEM) allows to record the variation of the EELS signal at a sub-nanometer scale (typically, in our STEMVG HB501 the nominal probe diameter is 0.5 nm and the accuracy in position is 0.2 nm). Therefore, by combining the spectrum-imaging approach with the above mentioned a posteriori deconvolution techniques it is now possible to probe with unprecedented spatial resolution spectral features that were so far only measurable with optical techniques. As an example, fig. 1 displays the spatial variation of the visible spectral range plasmon modes along a line scan of 64 points joining an apex of the triangular particle of figure 1 a to the opposite side of the triangle. With the support of well adapted models to simulate the optical response of nano-objects and the associated local low-loss EEL spectra (models both based on a classical dielectric continuum description or discrete dipole approximation), these new possibilities open the route to the exploration of a large v- ariety of new problematics in nanophysics. Some examples will be reviewed: - The optical response of individual silver nanoparticles [1]. More specifically, from the mapping of the different plasmon excitations in the visible range, the interplay between local effects (local electromagnetic field enhancement) and long range effects (symmetry of the excitation modes) will be discussed. -The optical response of new types of gold nanoparticles, namely nanodecahedra and nanostars. -The optical response of Split Ring Resonators. The analogies and differences between EELS and optical measurements will be also stressed, emphasizing that the quantity measured in both EELS and optical near field microscopy, namely the Local Electromagnetic Density of States, is the same.