Fundamental aspects of activated nanocrystal luminescence and possible applications

Presented results of complex study of relaxation processes and interionic interaction in Y₂SiO₅:Pr³⁺ and Lu₂SiO₅:Ce³⁺ nanocrystals clearly show two fundamental aspects: the phonon quantum confinement gives rise to the new fluorescence dynamics of doped ions; the developed surface of nanocrystals stimulates the irregular distribution of doped ions within the nanocrystal volume and could be the reason of new atomic arrangement of nanocrystal. Fluorescence spectrum of isolated Y₂SiO₅:Pr³⁺nanocrystal demonstrates the intense fluorescence from the high crystal field components of split ¹D₂ manifold of Pr³⁺ as the result of a suppression of phonon-assisted relaxation under the phonon quantum confinement. The direct comparison of the data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. This effect is caused by uphill diffusion of doped ions and preferred Pr segregation at the nanocrystal surface layer that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺. Lu₂SiO₅:Ce³⁺ nanocrystals which average size is 5 nm do not demonstrate the effect of energy storage as the result of atomic packing changing that does not permit the existence of electronic traps.