Size-dependent upconversion luminescence in CaF2:Yb3+,Tm3+ nanocrystals

CaF2:YB,Tm nano- and micro-crystals with different sizes were synthesized by a simple hydrothermal method. All the nanocrystals present intense ultraviolet (UV). visible, and near infrared (NIR) upconversion (UC) luminescence. The relationship between the UC luminescence intensities and the sample sizes from 50 nm to 1200 nm was investigated. The UV UC luminescence intensity of the sample with the particle size around 600 nm was the highest among all the samples, but the NIR UC luminescence intensity of the sample was the lowest. Our experimental results indicated that there was a size between 50 nm and microscale at which the UV emission intensity was the highest for CaF2 matrix. The size-dependent UC luminescence was proposed and analyzed based on experimental data. (C) 2012 Elsevier B.V. All rights reserved.