Abstract
The conversion of NIR light into visible light has been studied in Ho3+/Yb3+/Bi3+ co-doped ZnGa2O4 phosphor for the first time. The crystallinity and particles size of the phosphor increase through Bi3+ doping. The absorption characteristics of Ho3+, Yb3+ and Bi3+ ions are identified by the UV–vis-NIR measurements. The Ho3+ doped phosphor produces intense green upconversion (UC) emission under 980 nm excitations. The emission intensity ~ excitation power density plots show contribution of two photons for the UC emissions. The UC intensity of green emission is weak in the Ho3+ doped phosphor, which enhances upto 128 and 228 times through co-doping of Yb3+ and Yb3+/Bi3+ ions, respectively. The relative and absolute temperature sensing sensitivities of Ho3+/Yb3+/5Bi3+ co-doped ZnGa2O4 phosphor are calculated to be 13.6 × 10−4 and 14.3 × 10−4 K−1, respectively. The variation in concentration of Bi3+ ion and power density produces excellent color tunability from green to red via yellow regions. The CCT also varies with concentration of Bi3+ ion and power density from cool to warm light. The color purity of phosphor is achieved to 98.6% through Bi3+ doping. Therefore, the Ho3+/Yb3+/Bi3+:ZnGa2O4 phosphors can be suitable for UC-based color tunable devices, green light emitting diodes and temperature sensing.
Highlights
The conversion of NIR light into visible light has been studied in Ho3+/Yb3+/Bi3+ co-doped ZnGa2O4 phosphor for the first time
The sharp and intense X-ray diffraction (XRD) peaks are observed in both the cases, which show the crystalline nature of phosphor samples
The fluorescence intensity ratio (FIR) values for (5F4) and (5S2) levels of Ho3+ ion follow Boltzmann distribution law and these values have been evaluated by using the following r elation[4,13,40]: FIR = I2 = Be−△E/kT + C I1 (vii) where I1 and I2 stand for the emission intensity of two peaks arising from lower and upper thermally coupled levels (TCLs), (k = 0.695 cm−1 K−1) is Boltzmann’s constant, △E is the energy difference between TCLs (i.e. 5F4 and 5S2 levels) and T refers to absolute temperature, respectively
Summary
The conversion of NIR light into visible light has been studied in Ho3+/Yb3+/Bi3+ co-doped ZnGa2O4 phosphor for the first time. If the band gap of phosphor reduces; the large numbers of the excited ions will be transferred to the higher energy states, which would generate better UC intensity for the ZnGa2O4 materials.
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