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 |2F7/2, 4T1g > (Yb3+–Fe3+ dimer) to 4F9/2 (Er3+) states as well as the distortion of the crystalline field symmetry upon Fe3+ 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. 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.