A. Bahadur

Host-Sensitized NIR Quantum Cutting Emission in Nd3+ Doped GdNbO4 Phosphors and Effect of Bi3+ Ion Codoping

Host-sensitized near-infrared quantum cutting (QC) emission has been demonstrated in Nd(3+) doped Gd(1-x)Nd(x)NbO4 phosphors for various x values. Further, the effect of Bi(3+) ion addition as a sensitizer on near-infrared QC is studied in detail. X-ray diffraction confirms a monoclinic structure for pure and Nd(3+) doped phosphors. Pulsed laser excitation at 266 nm of Gd(1-x)Nd(x)NbO4 and Gd(0.99-x)Nd(x)Bi(0.01)NbO4 causes efficient room-temperature energy transfer from the NbO4(3-) to the Nd(3+) ions and the NbO4(3-) and Bi(3+) ions to the Nd(3+) ions, respectively, which emits more than one near-infrared photon for single impinging ultraviolet photon. The emission band of Nd(3+) shows unusual character where the intensity of the (4)F(3/2)-(4)I(9/2) transition at 888 nm is higher than the intensity of the transition (4)F(3/2)-(4)I(11/2) at 1064 nm, due to energy transfer from GdNbO4 host to Nd(3+) ion. Using photoluminescence lifetime studies, the quantum cutting efficiencies are found to be the maximum 166% and 172% for Gd(0.95)Nd(0.05)NbO4 and Gd(0.94)Nd(0.05)Bi(0.01)NbO4, respectively. The present study could establish Nd(3+) ion as an alternative of Yb(3+) ion for near-infrared quantum cutting. This work facilitates the probing of Nd(3+) ions doped phosphor materials for next generation Si-solar cells.

Enhanced white light emission from a Tm3+/Yb3+/Ho3+ co-doped Na4ZnW3O12 nano-crystalline phosphor via Li+ doping

This paper reports white light emission from a Tm3+/Yb3+/Ho3+ co-doped Na4ZnW3O12 nano-crystalline phosphor synthesized through a solution combustion method. The structural measurements reveal the crystalline nature of the synthesized samples. The Tm3+/Yb3+ co-doped Na4ZnW3O12 sample gives intense blue emission due to the 1G4 → 3H6 transition whereas the Ho3+/Yb3+ co-doped Na4ZnW3O12 sample gives intense green and red emissions due to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions, respectively, on excitation with 976 nm. When these three ions viz. Tm3+, Ho3+ and Yb3+; are co-doped in Na4ZnW3O12 the sample gives intense upconverted white light. The as-synthesized sample emits larger emission intensity on annealing at higher temperature. Addition of Li+ in the co-doped phosphor further enhances the emission intensity of white light up to two times and the CIE coordinates of the white light are (0.30, 0.41), which is close to the standard white light (0.33, 0.33). The enhancement in the emission intensity has been discussed due to changes in the local crystal field and reduction in the optical quenching centers. Thus, this nano-crystalline phosphor can be a suitable candidate for white light in various optical applications.

Down shifting and quantum cutting from Eu3+, Yb3+ co-doped Ca12Al14O33 phosphor: a dual mode emitting material

We report the quantum cutting (QC) in a Eu3+, Yb3+ co-doped Ca12Al14O33 phosphor synthesized through combustion method. The X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements reveal the crystalline nature of the phosphor sample. The photoluminescence (PL) spectra of the different samples have been studied using 266 and 394 nm radiation, which give an intense red emission at 612 nm due to 5D0 → 7F2 transition. The addition of Yb3+ in the Eu3+ doped sample reduces the emission intensity of Eu3+ bands in the visible region continuously whereas in the NIR region, the intensity of the Yb3+ band first increases up to 2 mol% and then decreases. This is due to cooperative downconversion energy transfer from Eu3+ to Yb3+ ions and concentration quenching. The decay curve analyses of different samples reveal an efficient energy transfer from Eu3+ to Yb3+ ions. The quantum efficiency (QE) has been calculated for different concentrations of Yb3+ ions and is estimated to be 197%. The possible mechanisms involved in different transitions and energy transfer processes can be understood using a schematic energy level diagram. The phosphor sample is a potential candidate for enhancing the efficiency of c-Si solar cells.

Enhanced green upconversion photoluminescence from Ho3+/Yb3+ co-doped CaZrO3 phosphor via Mg2+ doping

This paper reports enhanced green upconversion photoluminescence from Ho3+/Yb3+ co-doped CaZrO3 phosphor via Mg2+ doping synthesized through a solid state reaction method. The X-ray diffraction measurements confirm a shift in the peak position due to the presence of Mg2+ in the CaZrO3 phosphor. The scanning electron micrographs reveal an increase in the particle size for doping with Mg2+ ions. The Ho3+/Yb3+ co-doped CaZrO3 phosphor gives an intense monochromatic green upconversion emission centered at 543 nm due to 5F4/5S2 → 5I8 transition along with weak UV, blue, red and NIR emissions on excitation at 976 nm. The emission intensity of Ho3+ ions was optimum for 3 mol% Yb3+. The doping of Mg2+ ions slightly changes the band gap of the CaZrO3 phosphor; thereby enhancing the emission intensity significantly. When Mg2+ ions are doped in the Ho3+/Yb3+ co-doped CaZrO3 phosphor the emission intensity of the green band is enhanced by up to 4 times. This enhancement is due to substitution of Ca2+ by the Mg2+ ions, which decreased the lattice parameters and increased the crystallinity. The lifetime of the 5F4/5S2 level increases with the increase in the concentration of Mg2+ ions. Thus, the Ho3+/Yb3+/Mg2+ co-doped CaZrO3 phosphor could be a suitable candidate for intense monochromatic green light and optical devices.

Enhanced upconversion and downshifting emissions from Tb3+, Yb3+ co-doped CaZrO3 phosphor in presence of Li+ ions

This paper reports the enhanced green photoluminescence from Tb3+, Yb3+ co-doped CaZrO3 phosphor in the presence of Li+ ion synthesized through solid state reaction technique. The structural studies show an increase in the particle size and a shrink in crystal lattice due to Li+ co-doping in the phosphor. The phosphor sample emits intense green upconversion emission (UC) due to Tb3+ ions on excitation with 980 nm radiation which is further enhanced ~ 28 times on Li+ co-doping. The lifetime of 5D4 level of Tb3+ ion decreases in the presence of Li+ ions due to increase in asymmetry in crystal field. The downshifting (DS) emission intensity monitored on 378 and 487nm excitations is also enhanced in the presence of Li+ ions. Thus, the Tb3+, Yb3+, Li+ co-doped CaZrO3 phosphor can be a suitable candidate for UC solid state lighting.