Controlled synthesis of cubic and hexagonal phase lanthanide doped NaYF4 up-converters

Abstract

Hexagonal phase ytterbium and erbium co-doped sodium yttrium fluoride (NaYF4:Yb,Er) nanocrystals have been demonstrated as the most promising IR-to-visible up-converting materials displaying best luminescence properties among nanophosphors. There is yet an efficient and easy way of synthesizing hexagonal NaYF4:Yb,Er nanocrystals which displays strong up-conversion fluorescence. Co-precipitation using sodium fluoride (NaF) and yttrium, ytterbium and erbium nitrates in presence of disodium ethylenediaminetetraacetic acid (Na2-EDTA) was deployed to synthesize cubic NaYF4:Yb,Er nanocrystals followed by annealing to enable the cubic to hexagonal phase transition at high temperature. Hexagonal with trace amounts of cubic NaYF4:Yb,Er crystals with narrow size distribution are produced. NaF:Ln ratio, annealing temperature and annealing time were found to affect the morphology, size and extend of cubic to hexagonal phase transition. High NaF:Ln ratio, low annealing temperature or short annealing time would result in smaller particles produced. Annealing time is the main parameter to vary the extent of cubic to hexagonal phase transition. Yttrium, ytterbium and erbium chlorides and acetates were used to synthesize NaYF4:Yb,Er nanocrystals using solvothermal method at high temperature of 300°C. Oleic acid and 1-octadecene were used as the surfactant and solvent respectively. Strong agglomerations of particles were obtained with large size distribution. Reaction was far from completion under 1 hr reaction time producing small amount of cubic NaYF4:Yb,Er particles with minimal hexagonal NaYF4:Yb,Er particles. Nano-sized crystals could be potentially used in biology and medicine as bio-detection labels or imaging probes. Larger crystals could be doped in solar cells to convert near infrared light to visible light and subsequently to improve light harvesting efficiency of solar cells.