Near-Infrared-to-Blue Upconversion in Colloidal BaYF5:Tm3+, Yb3+ Nanocrystals

Tetragonal barium yttrium fluoride (BaYF5) nanocrystals doped with 0.5 mol % Tm3+ and 15 mol % Yb3+ (BaYF5:Tm3+, Yb3+) were synthesized using the thermal decomposition method yielding rectangular-shaped nanocrystals (15 nm x 5 nm) that can (up)convert near-infrared light to higher energies such as blue, via a process known as upconversion. The upconversion spectrum of the BaYF5:Tm3+, Yb3+ nanocrystals, following excitation with 980 tun, revealed that the upconverted blue emission from the (1)G(4) -> H-3(6) transition was more intense than the infrared H-3(4) -> H-3(6) emission at high excitation densities (90 W/cm(2)) contrary to what is normally observed for Tm3+/Yb3+ codoped nanomaterials. On the other hand, the infrared emission dominates at lower excitation densities (15 W/cm(2)) demonstrating a lack of excited Yb3+ ions to carry on the upconversion beyond the H-3(4) excited state to the (1)G(4) excited state. A saturation of the upconversion process was observed in the power dependence studies at excitation densities above 57 W/cm(2), causing a deviation in the expected number of photons required for the upconversion process. The upconversion properties of BaYF5:Tm3+, Yb3+ nanocrystals are elucidated and discussed.