K. Mini Krishna

Host Sensitized White Luminescence from ZnGa2O4 : Dy3 + Phosphor

Dysprosium-doped zinc gallate phosphors with the nominal formulas ZnGa 2(1-x) O 4 :Dy 2x were prepared by the high-temperature conventional solid-state reaction technique, the dopant concentration being varied in the range 0 ≤ x ≤ 0.025. Zn 1-x Ga 2 O 4 :Dy x and Ga 2(1-x) O 3 :Dy 2x samples were also prepared for comparison for a dopant concentration of x = 0.02. Only the intrinsic self-activated emission of the ZnGa 2 O 4 host is observed in the photoluminescence emission spectra of Zn 0.98 Ga 2 O 4 :Dy 0.02, while both the host emission band and characteristic emission lines ( 4 F 9/2 → 6 H J ) of Dy 3+ are observed for ZnGa 2(1-x) O 4 :Dy 2x and Ga 1.98 O 3 :Dy 0.04 phosphors. The luminescent intensity differs in the phosphors due to the different energy-transfer rates from the respective hosts to the luminescent centers. Photoluminescent studies reveal the fact that Dy 3+ ions replace Ga 3+ ions in the host lattice at their octahedral sites. The source of white luminescence in the doped samples is the nonradiative resonant energy transfer via exchange interaction between the host and the activator. The CIE coordinates of the 2.5 atom % Dy 3+ -doped sample (0.32, 0.33) matches well with achromatic white (0.33, 0.33) on the chromaticity diagram.

Hydrothermal Synthesis and Characterization of Undoped and Eu-Doped ZnGa2O4 Nanoparticles

Nanoparticles of ZnGa2O4 and Eu-doped ZnGa2O4 were hydrothermally synthesized varying the process parameters, such as the volume ratio of the cation precursor solutions, temperature and time of growth, and dopant concentration. The nanoparticles were structurally characterized by X-ray diffraction, high-resolution transmission electron microscopy, and selected-area electron diffraction. The studies confirmed the formation of spherically shaped ZnGa2O4 nanoparticles with the standard spinel structure. Photoluminescence (PL) studies show that Eu-doped ZnGa2O4 nanoparticles exhibits a sharp red luminescence due to the intra-4f transitions of Eu3+ ions at an excitation of 397 nm. Luminescence quenching is observed in the nanoparticles at higher Eu concentration. The room-temperature PL measurements of pure ZnGa2O4 nanocrystals monitored at an excitation wavelength of 254 nm gave a peak-shaped spectrum instead of the normally observed bell-shaped spectrum of bulk ZnGa2O4. The bandgap of the ZnGa2O4 nanoparticles is blueshifted compared to the bulk material due to quantum confinement effects. Incorporation of Eu in the nanoparticles was confirmed by inductively coupled plasma atomic emission spectroscopic studies. (c) 2009 The Electrochemical Society. [DOI: 10.1149/1.3070662] All rights reserved.

Effect of Substrate Temperature on the Structural and Luminescent Characteristics of RF-Magnetron-Sputtered ZnGa2O4 : Dy3 + Thin Films

ZnGa 2 O 4 :Dy 3+ phosphor thin films were deposited on quartz substrates by radio frequency (rf) magnetron sputtering and the effect of substrate temperature on its structural and luminescent properties was investigated. Polycrystalline film could be deposited even at room temperature. The crystalline behavior, Zn/Ga ratio, and surface morphology of the films were found to be highly sensitive to substrate temperature. Under UV illumination, the as-deposited films at and above 300°C gave white luminescence even without any postdeposition treatments. The photoluminescent (PL) emission can be attributed to the combined effect of multicolor emissions from the single luminescence center Dy 3+ via host-sensitization. Maximum PL emission intensity was observed for the film deposited at 600°C, and the CIE chromaticity coordinates of the emission were determined to be (x,y) = (0.34, 0.31).

Effect of ZnO Buffer Layer on the Structural and Optical Properties of Zn2GeO4 : Mn2 + Thin Films

Mn 2+ -activated Zn 2 GeO 4 phosphor thin films were deposited on quartz substrates using the radio frequency magnetron sputtering technique. The crystallanity, composition ratio, and luminescent behavior of the films were highly sensitive to the deposition parameters. Zn 2 GeO 4 :Mn 2+ films grown on amorphous substrates were polycrystalline in nature, whereas those deposited on the ZnO buffer layer were highly oriented along the (220) plane under similar deposition conditions. Tauc plot analysis of the Zn 2 GeO 4 :Mn 2+ films grown on ZnO/quartz substrate exhibits strong subband absorption in comparison to the Zn 2 GeO 4 :Mn 2+ film deposited on quartz substrate. The spectral overlap between the subband states and excited levels of Mn 2+ is more favored with the buffer layer presence improving the resonant energy transfer from the host to the activator. The subsequent 4 T 1 - 6 A 1 transition in the Mn 2+ levels results in the green photoluminescent (PL) emission at 540 nm. The maximum PL emission is observed for the film deposited at 650°C. The photoluminescent excitation spectra of Zn 2 GeO 4 :Mn 2+ film on ZnO/quartz substrate showed a blueshift with substrate temperature.