Effect Of Transition Metal Codoping And Core-shell Coating On Upconversion Luminescence Of Nanocrystals

Red upconversion luminescence（UCL） is selectively enhanced via transition metal codoping into NaYF₄:Yb,Er nanocrystalline lattice. About 7 times enhancement of upconversion intensity was achieved by copping Fe³⁺.The maximum red-to-green ratio（R/G） as well as the overall integrated UCL intensity features at Fe³⁺ content of 20 mol%. Contrary to the literature, the size and phase of nanocrystals are simultaneously manipulated via transition metal doping with various concentrations by a facile hydrothermal method.The intensified visible UCL especially the dominant red emission is mainly ascribed to the energy transfer（ET） from |2F7/2, 4T1 g >(Yb3+–Fe³⁺ dimer) to 4F9/2（Er3+） states as well as the distortion of the crystalline field symmetry upon transition metal codoping. Dynamic investigation of 4S3/2 and 4F9/2 states under the pulsed laser excitation of 980 nm along with the diffuse reflectance data further supports the proposed mechanism of UC processes. Further experiments are performed to enhance the upconversion luminescence intensity by means of the silica shell coating. At present, there are few reports about the effect of the reaction time and the layer thickness. The strongest fluorescence intensity was obtained after 4 hours of reaction under the optimal reaction concentration.The water solubility of nanocrystals is greatly improved via silica coating. The results show the remarkable promise of transition metal-codoped NaYF₄:Yb,Er nanocrystals as upconverting nanoprobes with high sensitivity and penetrability in deeper tissue for multimodal biomedical imaging.