CaMoO_4:RE~(3+),Yb~(3+),M~+ phosphor with controlled morphology and color tunable upconversion

摘要

CaMoO_4:RE~(3+),Yb~(3+) (RE = Er, Ho, Tm) phosphors were successfully synthesized by a facile hydrothermal method. XRD patterns confirmed tetragonal structure under different RE~(3+) and M~+ ions doping conditions. Particles shapes and sizes were confirmed by SEM and TEM analyses. Particles shape and size were well tuned by control of solution pH; spherical balls consisting of nano-grains at low pH of ～2, rice grain shapes at moderate pH of ～6, and thin flakes at higher pH of ～12, were observed. Fine tunability of upconversion (UC) emission color was achieved by doping multiple RE~(3+) ions within a single CaMoO_4 host. Blue, green and orange upconverted emission were observed by doping Tm~(3+), Er~(3+) and Ho~(3+) in the CaMoO_4, respectively. Further, the emission colors were well tuned by the combination of Tm, Er and Ho ions and their concentrations. CaMoO_4:Tm~(3+),Ho~(3+),Yb~(3+) exhibited perfect white emission with well tunability from cool white to warm white colors. Substitution of part of Ca ions by M~+ (M = Li, Na, K, Rb) ions affected the crystal field symmetry around RE~(3+) ions and hence changed the transition probabilities between their f-f transition levels, consequently intensified the UC intensities. The blue (Tm~(3+)), green (Er~(3+)), and orange (Ho~(3+)) upconversion intensities of CaMoO_4:RE~(3+),Yb~(3+), 0.10K~+ phosphors increased by 60, 50 and 40 folds compared to the unsubstituted analogues, respectively. The K substituted CaMoO_4: RE~(3+),Yb~(3+),K~+ phosphors exhibited intense UC emissions visible by naked eye even pumped by less than 1 mW laser power and can have potential application in displays and variety of other applications.