Optical properties of silver thin films, derived from REELS

Complete text of publication follows. The differential surface excitation probabilities (DSEPs) for medium-energy electrons traveling in Ag are extracted from reflection electron energy loss spectroscopy (REELS) spectra by using the elimination-retrieved algorithm of Werner. Based on a surface Kramers- Kronig dispersion relationship, the electronic structure and optical properties of Ag thin film are determined. The results show that the optical property curves between surface thin layer and bulk are similar in shape,whereas they deviate quantitatively for energies below 40 eV, which may be caused by the different electronic structures at surface and in bulk. The present data would be an appropriate approximation to the optical properties of thin film, whose thickness is characterized by the inelastic mean free path of the electrons in a REELS experiment. Figure 1 shows the comparison on the surface energy loss functions (SELFs), Im-1/(1 + ε), between the present result obtained from REELS spectra of Ag thin film and that derived from the optical data for the surface of bulk specimen. It can be seen that the peak intensity of surface plasmon for thin film is lower than for the surface of bulk specimen but the peak width is broader. Furthermore, the obtained surface plasmon energy in a thin film (3.6 eV) is slightly lower than that at the surface of bulk specimen (3.65-3.7 eV). Ritchie had pointed out in 1957 that for a quite thin film the electrostatic fields originating from both two interfaces overlap and results in the varied surface plasmon energy branches with thickness. With the REELS spectra measured for low-energy electrons, the complex dielectric functions as well as optical constants of the surface layer can be determined if the surface and bulk excitations could be successfully separated from each other in a REELS spectrum. Since the reflection energy loss process takes place in a space region of the order of the inelastic mean free path, it is possible, in principle, to apply this procedure to probe the electronic structure and physical properties of individual nanostructures with REELS. Acknowledgements. This work was supported by the National Natural Science Foundation of China (grant no. 10874160), National Basic Research Program of China (2007CB925004), 111 Project, Chinese Education Ministry and Chinese Academy of Sciences, the grant Bolyai from the Hungarian Academy of Sciences and TeT grant no. 554 CHN-28/2003.