A green emitting Gd(0.95-x)HoxYb0.05NbO4 phosphor (x = 0.001, 0.005, 0.01, 0.02, 0.03 and 0.035) has been synthesized by the solid-state reaction method. The phosphor samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and upconversion (UC)/down-shifting (DS) photoluminescence measurements. The XRD analysis confirms the formation of pure phase GdNbO4. The FTIR and Raman spectra reveal the modes of vibration in GdNbO4. It also confirms that this host has a lower phonon frequency (806 cm(-1)) in comparison to the other well-known compounds of this family. The photoluminescence excitation (PLE) spectrum of GdNbO4 shows two broad bands at 270 and 303 nm corresponding to the NbO4(3-) group and Gd(3+) ion, respectively. On 270 nm excitation it shows a weak emission band with maximum at 442 nm. The intensity of this emission band strengthens on excitation with 303 nm. The PL measurements have shown the energy transfer from host to Ho(3+) ions. In addition, of Bi(3+) ions, the intensity of the PL band corresponding to the NbO4(3-) group increases, which facilitates a better energy transfer from host to the Ho(3+) ions. On 980 nm diode laser excitation, the phosphor shows strong green and rather weak red UC emission peaks. The influence of an external magnetic field on the UC emission has also been studied. It is found that the UC emission of Ho(3+) ions decreases in the presence of a magnetic field. It also shows the existence of optical bistability because of the presence of hysteresis behavior. Although this host has a low phonon frequency and shows paramagnetic behavior, it is not well explored yet. Our studies reveal that this host could have significant scientific and technological importance.
Schematic representation of energy bands/defect states, energy transfer and emission in Na+, Eu3+:CaGa2O4 usable for solar blind UV converter application.
Intense green upconversion emission in Ho:Yb:Gd2O3 spherical nanoparticles is reported through hetrolooping enhanced energy transfer process. Different techniques used for structural characterization divulge the formation of spherical Ho:Yb:Gd2O3 crystals of diameter ∼100 nm. Efficient energy transfer from Yb3+ → Ho3+ ions set spherical nanocrystal to emit in the range of UV-NIR via three NIR photons absorption process. Power dependence and the temporal evolution of upconversion emission intensity suggest the occurrence of a photon avalanche process. The enhancement in emission intensity in nanophosphor sample has been explained and the photophysics involved is correlated with the unique structural properties of the crystallites formed and the time resolved spectroscopy.
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