Tuning Crystal Phase and Emission Properties of Upconversion Nanocrystals Through Lanthanide Doping

Infrared-to-visible upconversion fluorescent nanocrystals of Yb³⁺/Er³⁺-codoped NaYF₄ and Yb³⁺/Er³⁺/Gd³⁺-tridoped NaYF₄ were synthesized using a modified coprecipitation process. X-ray diffraction and transmission electron diffraction scans of the nanocrystals confirmed that Gd³⁺ doping caused a phase transition to occur in the nanocrystals, changing them from a cubic to a hexagonal phase. Hexagonal phase Yb³⁺/Er³⁺/Gd³⁺-tridoped NaYF₄ nanocrystals displayed much stronger and sharper upconversion luminescence, and larger intensity ratios of red over green emissions relative to their cubic phase counterparts. The influence of the crystal phase on the upconversion emission properties was explored by use of excitation power dependence curves, dynamic fluorescence and Raman spectra. The results suggest that the cubic-to-hexagonal phase transition decreases the crystal field symmetry, and then enhances upconversion luminescence intensity by relaxing forbidden selection rules. The conversion into the hexagonal phase also increases the number of phonon modes, and consequently improves the phonon-assisted energy transfer efficiency from Yb³⁺ to Er³⁺, thus facilitating the output of red emissions.