Center for Functional Nanomaterials Seminar

Powerful tools for solving structures rely on elastic scattering and long-range order (periodicity) in materials. No spectroscopic techniques can beat (x-ray, electron and neutron) diffraction in solving crystal structure. However, the diffraction techniques will lose their advantage if materials are amorphous or crystals are extremely small. Alternative approaches have to be considered. Electron energy-loss spectroscopy (EELS), combining with scanning/transmission electron microscopy, has great advantage in probing physical properties and chemistry at nanometer or even atomic scale. It has also been used to deduce structural information, but only succeed in the short range distance, such as coordination. In principle, an EELS spectrum is due to quantum-mechanical transitions that excite a particular inner-shell electron of a specific atom to the unoccupied states, i.e. to the states with an excited electron above the Fermi energy, leaving behind a core hole. According to multiple-scattering theory, the outgoing excited electron can be considered as a quantum electron wave that spread out over the materials. The surrounding atoms can act as scattering centers which scatter the outgoing electron wave back towards the original atom. The amplitudes of the outgoing electron wave and reflected waves add up at the absorbing atom either constructively or destructively, and hence modulate the density of states. Due to the limited lifetime of the excited electron, the excited electron state decays rapidly as a function of time and distance. Therefore the fine structures (peaks and dips) in absorption spectrum are associated with atom correlations over a medium-range distance. However, the interpretation of EELS is usually electronic in nature. Lack of direct relation between the observed EELS and atom structure information could be one among various reasons. In this talk, I will discuss necessity and possibility of establishing direct relations of EELS and structure.