Vibrational and valence aloof beam EELS: A potential tool for nondestructive characterization of nanoparticle surfaces

In many materials systems, electron beam effects may substantially alter and destroy the structure of interest during observation. This is often true for the surface structures of catalytic nanoparticles where the functionality is associated with thin surface layers which are easily destroyed. The potential application of using aloof beam electron energy-loss spectroscopy as a non-destructive nanoscale surface characterization tool is discussed. Recent developments in monochromators make vibration and valence loss EELS possible in the electron microscope. The delocalization associated with these signals allows spectra to be acquired when the electron beam is position 2 nm or more away from the particle surface. This eliminates knock-on damage and significantly reduces ionization damage. Theoretical and experimental results are employed to explore the potential strengths and weaknesses of monochromated aloof beam EELS for surface analysis. The approach is most favored for surface layers on insulators because the bandgap lowers the background for detection of the vibrational signal and bandgap states. Guided light modes and relativistic effects can complicate the interpretation of the spectra. The effects are suppressed at lower accelerating voltages and particle size especially for low refractive index materials.