Abstract

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 1a 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 variety of new problematics in nanophysics.

© 2009 Optical Society of America