CaMoO4:RE3+,Yb3+,M+ phosphor with controlled morphology and color tunable upconversion

Abstract

CaMoO4:RE3+,Yb3+ (RE=Er, Ho, Tm) phosphors were successfully synthesized by a facile hydrothermal method. XRD patterns confirmed tetragonal structure under different RE3+ 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 RE3+ ions within a single CaMoO4 host. Blue, green and orange upconverted emission were observed by doping Tm3+, Er3+ and Ho3+ in the CaMoO4, respectively. Further, the emission colors were well tuned by the combination of Tm, Er and Ho ions and their concentrations. CaMoO4:Tm3+,Ho3+,Yb3+ 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 RE3+ ions and hence changed the transition probabilities between their f–f transition levels, consequently intensified the UC intensities. The blue (Tm3+), green (Er3+), and orange (Ho3+) upconversion intensities of CaMoO4:RE3+,Yb3+,0.10K+ phosphors increased by 60, 50 and 40 folds compared to the unsubstituted analogues, respectively. The K substituted CaMoO4:RE3+,Yb3+,K+ phosphors exhibited intense UC emissions visible by naked eye even pumped by less than 1mW laser power and can have potential application in displays and variety of other applications.