Solvothermal Route to Colloidal Upconverting NaYF4: Yb3+/Er3+ and Yb3+/Tm3+ Nanocrystals

Monodispersed nanocrystals of NaYF4: Yb3+, Er3+/Tm3+ that exhibited a high luminescence intensity were chemically synthesized by a non-aqueous solvothermal approach. These nanocrystals were easily dispersed in non-polar solvent and subsequently formed a transparent colloidal solution. Under 980 nm NIR excitation, green and blue emissions were observed from colloidal solutions of the NaYF4: Yb3+, Er3+/Tm3+, respectively. The effects of different solvents used and solvent concentration on the shape, size and size distribution of the nanocrystals have been investigated. It has been revealed that the doping ions diffused into NaYF4 lattice, and they formed a solid solution. The fluorescence photos, spectra, luminescence mechanism, and the formation mechanism of the nanocrystals have also been demonstrated. Emission spectrums of the nanocrystals prepared by this method show much more complicated emission peaks than by other methods.