ALBERT

Description: Dual-mode luminescence (downshifting-DS and upconversion-UC) properties of Pr 3+ /Yb 3+ co-doped Y 1−x Gd x NbO 4 (x = 0.0, 0.5, and 1.0) phosphors synthesized by solid state reaction technique have been explored with and without Gd 3+ ion. The structural characterizations (XRD, SEM, and FTIR) confirm the pure phase of YNbO 4 phosphor. Further, with the Gd 3+ ion co-doping, the YNbO 4 phosphors having a random shape and the large particle size are found to be transformed into nearly spherical shape particles with the reduced particle size. The optical band gaps (E g ) of Y 1−x Gd x NbO 4 (x = 0.00, 0.25, 0.50, and 1.00) calculated from UV-Vis-NIR measurements are ∼3.69, 4.00, 4.38, and 4.44 eV, respectively. Moreover, YNbO 4 phosphor is a promising blue emitting material, whereas Y 1−x−y−z Pr y Yb z Gd x NbO 4 phosphor gives intense green, blue, and red emissions via dual-mode optical processes. The broad blue emission arises due to (NbO 4 ) 3− group of the host with λ ex  = 264 nm, whereas Pr 3+ doped YNbO 4 phosphor gives dominant red and blue emissions along with comparatively weak green emission on excitation with λ ex  = 300 nm and 491 nm. The concentration dependent variation in emission intensity at 491 nm ( 3 P 0 → 3 H 4 transition) and 612 nm ( 1 D 2 → 3 H 4 transition); at 612 nm ( 1 D 2 → 3 H 4 transition) and 658 nm ( 3 P 0 → 3 F 2 transition) of Pr 3+ ion in YNbO 4 phosphor with λ ex  = 300 nm and 491 nm excitations, respectively, has been thoroughly explored and explained by the cross-relaxation process through different channels. The sensitization effect of Bi 3+ ion co-doping on DS properties of the phosphor has also been studied. The observed DS results have been optimized by varying the concentration of Pr 3+ and Bi 3+ ions, and the results are explained by the well-known simple band structure model. The study of Gd 3+ co-doping reveals noticeable differences in DS characteristics of Y 1−x Pr x NbO 4 phosphors: the overall decrement and increment (except for 612 nm emission) in intensity of DS emission on excitation with λ ex  = 264 nm and 491 nm, respectively. These observations have been thoroughly explained, and the 1 D 2 → 3 H 4 transition (612 nm) of Pr 3+ ion is found to be strongly dependent on surrounding environment of the host matrix. The UC properties of Y 0.95−x Pr x Yb 0.05 NbO 4 phosphors have been explored using Near Infra-Red (NIR) excitation. The material gives intense green and relatively weak blue and red UC emissions with λ ex  = 980 nm. Interestingly, the UC emission intensity is further enhanced in the case of Y 0.949−x Pr 0.001 Yb 0.05 Gd x NbO 4 phosphors. In addition, the less explored laser induced heating effect with the pump power as well as the irradiation time on the UC emission has been explored in Y 0.949−x Pr 0.001 Yb 0.05 Gd x NbO 4 (x = 0, 0.5, and 0.949) phosphor samples, and subsequently, this feature has been found to be superior for Gd 0.949 Pr 0.001 Yb 0.05 NbO 4 phosphor. The comparative study between the two hosts, viz., YNbO 4 and GdNbO 4 shows that GdNbO 4 is better than YNbO 4 for UC emission behavior; however, a reverse is observed as for as DS behavior is concerned only for the particular excitation wavelength (λ ex  = 264 nm).