Surface color centers on epitaxial NaCl films

Abstract

The defect formation in epitaxial NaCl films (up to 5.6 nm thick) during low-energy (50--1500 eV) electron bombardment has been studied by a combination of low-energy electron diffraction and energy loss spectroscopy. At temperatures below 200 K two types of color center are generated at the surface, showing distinct electronic transition energies and different kinetics of formation. A rate equation analysis provides evidence for a random distribution of stable, immobile $F$ and $M$ centers on the surface. Their saturation density increases with the film thickness, indicating an indirect generation mechanism involving the whole film, presumably via self-trapping of excitons. The angular distribution of the inelastic electron scattering of excitons reveals their surface polariton character, providing the energy transport from film bulk to surface and within the surface. At room temperature the aggregation of Na clusters is observed. Patterns of high and low surface $F$- and $M$-center densities are potentially invaluable as templates for nanostructure devices on ionic crystal surfaces like NaCl(001), because of the chemical reactivity of surface color centers.