Selectively enhanced red upconversion luminescence and phase/size manipulation via Fe³⁺ doping in NaYF₄:Yb,Er nanocrystals

Red upconversion luminescence (UCL) is selectively enhanced by about 7 times via Fe³⁺ codoping into a NaYF₄:Yb,Er nanocrystalline lattice. The maximum red-to-green ratio (R/G) as well as the overall integrated UCL intensity features at an Fe³⁺ content of 20 mol%. The size and phase of nanocrystals are simultaneously manipulated via Fe³⁺ doping with various concentrations by a facile hydrothermal method. Contrary to the literature, the pure hexagonal phase appears when Fe³⁺ concentrations are from 5 to 20 mol%, meanwhile, the size of NaYF₄:Yb,Er nanocrystals reaches its maximum at 10 mol%. The intensified visible UCL especially the dominant red emission is mainly ascribed to the energy transfer (ET) from ²F_7/2, ⁴T_1g > (Yb³⁺-Fe³⁺ dimer) to ⁴F_9/2 (Er³⁺) states as well as the distortion of the crystalline field symmetry upon Fe³⁺ codoping. Dynamic investigation of ⁴S_3/2 and ⁴F_9/2 states under the pulsed laser excitation of 980 nm along with the diffuse reflectance data further supports the proposed mechanism of UC processes. The results show the remarkable promise of Fe³⁺-codoped NaYF₄:Yb,Er nanocrystals as upconverting nanoprobes with high sensitivity and penetrability in deeper tissue for multimodal biomedical imaging.