Photoluminescence behavior of surface radiation induced color centers in lithium fluoride and influence of nanosized clusters

Influence of pre-irradiation annealing temperature on the efficiency of the subsurface color center formation in γ-irradiated lithium fluoride nanocrystals has been studied. Increase of the annealing temperature reduces the efficiency of formation of these centers. Nanocrystals lose their ability to form subsurface color centers after pre-irradiation annealing at 623 K and higher temperatures. The formed subsurface color centers are partially transformed into the usual centers of the same composition during the post-radiation annealing at certain temperatures. It has been shown that the mechanical fragmentation of lithium fluoride crystals leads to the formation of nanosized clusters in their subsurface layer. Subsurface radiation color centers which are located near the clusters are subjected to the modified crystal field. This circumstance causes strong differences between the luminescent characteristics of subsurface color centers and usual centers of the same composition in the crystal bulk, where there are no clusters. The clusters are completely destroyed after annealing the samples at 623 K and higher temperatures. The obtained results indicate that the presence of nanoclusters is a prerequisite for the formation of subsurface radiation color centers with specific fluorescent properties. The thickness of the subsurface layer, where subsurface color centers can be formed, has been estimated.