Near-surface layer radiation color centers in lithium fluoride nanocrystals: Luminescence and composition

Lithium fluoride nanocrystals are irradiated by gamma quanta at 77 K. The radiation color centers formed in a near-surface layer of nanocrystals are studied. Absorption, luminescence and luminescence excitation spectra of the surface defects have been measured. It has been found that the luminescence excitation spectra for aggregated surface centers consist of two or three bands with not very much different intensities. Reactions of the surface centers separately with electrons and with anion vacancies have been investigated. Numbers of anion vacancies and electrons entering into the centers composition have been established and it has been found that FS1, FS1−, FS2, FS2−, FS3+ and FS3 types of the surface centers are formed. The degree of luminescence polarization has been defined and it has been determined that the polarization degree for FS2+ centers changes sign under transition from one excitation band to another. It has been shown that during irradiation at 77 K radiation-induced defects are formed more efficiently on the surface than in the bulk.