Comparison of ultraviolet upconversion luminescence between thin film and bulk of calcium fluoride co-doped with Yb³⁺/Tm³⁺ ions

A calcium fluoride thin film co-doped with Yb³⁺/Tm³⁺ was deposited on Si substrates by the spincoating method using trifluoroacetic acid as a fluorine source. The scanning electron microscopy (SEM) image of the sample showed that a thin film with some cracks was formed. After annealing, the cracks disappeared, and the film became thinner. The roughness of the thin film was about 150 nm, which was confirmed by atomic force microscopy (AFM). Moreover, the upconversion luminescence properties of the thin film after annealing were studied. Under infrared light excitation, strong ultraviolet (UV) upconversion luminescence of the thin film was observed. To compare the upconversion properties, Yb³⁺/Tm³⁺ co-doped calcium fluoride bulk material that has the same components with the thin film was also synthesized. Compared with the bulk material, the UV upconversion emission of thin film was more intense. The mechanism of UV upconversion was determined by analyzing the upconversion spectra. The decrease in the size of the CaF ₂ Yb³⁺/Tm3+ particles to the nano-scale in the thin film contributed to the enhancement of the UV UC luminescence.