Low-loss EELS study of oxide-covered aluminum nanospheres

High resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have been used to investigate plasmon losses of aluminum nanospheres. A model based on the dielectric theory allows to attribute the observed features in the loss spectra of particles of different size to either surface or volume losses. Both, surface and volume loss peak, show size dependent characteristics which are reproduced by this model. EEL spectra with negligible surface loss contributions can be used to determine the dielectric function of the observed material by means of the Kramers Kronig relations. For the case of non negligible surface contributions we introduce the notion of dielectric signature. This dielectric signature allows to study the transition where surface effects start to become visible in the spectra. Our analysis suggests that for a system of known composition this approach is a useful tool to determine the importance of surface effects.