Tuning of structure and enhancement of upconversion luminescence in NaLuF4:Yb3+,Ho3+ crystals
A series of NaLuF4:Yb3+,Ho3+ nano/micro-crystals with different crystal structures were synthesized via a hydrothermal method using citric acid as a chelating agent. The influences of NaF content, Li+ doping, reaction temperature and reaction time on the crystal structure and shape of the as-synthesized NaLuF4 crystals were systematically investigated. To the best of our knowledge, it is the first time to report Li+ doped α-NaLuF4:Yb3+,Ho3+ nanocrystals and the phase transformation by introducing Li+ in NaLuF4 crystals. As for Li+ doped α-NaLuF4, UC luminescence intensities of green emission (538 nm) and red emission (644 nm) in α-NaLuF4:Yb3+,Ho3+ nanocrystals with 20 mol% Li+ doping are enhanced by 20 and 3.5 times compared to their Li+-free counterpart. As for Li+ doped α/β-mixed NaLuF4, with the increase of Li+ content, the phase transforms from the α/β-mixed phase to hexagonal then to cubic. UC emissions of 538 nm and 644 nm in NaLuF4:Yb3+,Ho3+ crystals doped with 5 mol% Li+ are enhanced by 26.5 and 23 times, respectively. Besides, it is found that with the higher temperature and prolonged time, the morphology of NaLuF4 changes from nanoparticles to microtubes, resulting in the dramatic increase of UC emission intensity. The effects of Li+ doping, reaction temperature and reaction time on the enhancement of UC emission intensity are discussed in detail. This study provides an effective and facile approach to obtain nano/micro-crystals with controllable structures and excellent optical properties.