G. Seeta Rama Raju

A novel strategy for controllable emissions from Eu3+ or Sm3+ ions co-doped SrY2O4:Tb3+ phosphors

Trivalent rare-earth (RE) ions (Eu(3+), Tb(3+) and Sm(3+)) activated multicolor emitting SrY(2)O(4) phosphors were synthesized by a sol-gel process. The structural and morphological studies were performed by the measurements of X-ray diffraction profiles and scanning electron microscope (SEM) images. The pure phase of SrY(2)O(4) appeared after annealing at 1300 °C and the doping of RE ions did not show any effect on the structural properties. From the SEM images, the closely packed particles were observed due to the roughness of each particle tip. The photoluminescence (PL) analysis of individual RE ions activated SrY(2)O(4) phosphors exhibits excellent emission properties in their respective regions. The Eu(3+) co-activated SrY(2)O(4):Tb(3+) phosphor creates different emissions by controlling the energy transfer from Tb(3+) to Eu(3+) ions. Based on the excitation wavelengths, multiple (green, orange and white) emissions were obtained by Sm(3+) ions co-activated with SrY(2)O(4):Tb(3+) phosphors. The decay measurements were carried out for analyzing the energy transfer efficiency and the possible ways of energy transfer from donor to acceptor. The cathodoluminescence properties of these phosphors show similar behavior as PL properties except the energy transfer process. The obtained results indicated that the energy transfer process was quite opposite to the PL properties. The calculated CIE chromaticity coordinates of RE ions activated SrY(2)O(4) phosphors confirmed the red, green, orange and white emissions.

A facile and efficient strategy for the preparation of stable CaMoO4 spherulites using ammonium molybdate as a molybdenum source and their excitation induced tunable luminescent properties for optical applications

Stable CaMoO4 spherulites were synthesized by a facile hydrothermal method using (NH4)6Mo7O24·4H2O as a Mo source and these spherulites were formed according to the theoretical predictions of the crystal splitting theory. Rietveld refinement and photoluminescence studies confirmed that the CaMoO4 spherulites are defect-free. The CaMoO4 spherulites showed greenish-blue emission and the single emitting component of CaMoO4:Eu3+ spherulites led to a novel excitation induced efficient emission property like organic light emitting diodes. Cathodoluminescent properties of the CaMoO4:Eu3+ exhibited individual emissions from MoO42− clusters and Eu3+ ions. The white color emissions were clearly explained using Gaussian fitting curves. The corresponding CIE chromaticity coordinates provided their emission potentiality in the green, red and white regions for optical and biological applications.

Concentration and penetration depth dependent tunable emissions from Eu3+ co-doped SrY2O4:Dy3+ nanocrystalline phosphor

A series of Dy3+ ion single-doped and Dy3+/Eu3+ ion co-doped white-light emitting SrY2O4 nanocrystalline phosphors were synthesized by a citrate sol–gel method. After the samples were annealed at 1300 °C, the X-ray diffraction patterns confirmed their orthorhombic structure. The particles were closely-packed and their optical properties were monitored by photoluminescence spectroscopy. The Dy3+ ions acted as luminescent centers and substituted Y3+ ions in the SrY2O4 host lattice, where they are located in Cs sites, and the characteristic emission of Dy3+ ions (4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions) with intense yellow emission band at 578 nm occurred. When the Eu3+ ions were co-doped into the SrY2O4:Dy3+ (1 mol%) nanocrystalline phosphor, white-light emission was observed under excitation at 381 nm. The energy transfer between Eu3+ and Dy3+ ions was calculated and the chromaticity coordinates were also presented. The cathodoluminescence (CL) spectra confirmed that the penetration depth is inversely proportional to the total atomic weight and atomic number of the compound. From CL analysis, the emission intensities of Dy3+ ion single-doped and Dy3+/Eu3+ co-doped SrY2O4 nanocrystalline phosphors increased continuously upon increasing both the accelerating voltage from 1 to 5 kV and the filament current from 30 to 47 μA.

Excitation induced efficient luminescent properties of nanocrystalline Tb3+/Sm3+:Ca2Gd8Si6O26 phosphors

The cathodoluminescence and the excitation induced photoluminescence properties have been investigated for the nanocrystalline Tb3+/Sm3+:Ca2Gd8Si6O26 phosphors prepared by a solvothermal reaction method. The XRD patterns confirm their hexagonal structure. The green, orange and white emissions have been obtained by exciting at 275, 378, and 405 nm wavelengths, respectively. The corresponding CIE chromaticity coordinates are found to be in close proximity to the standard points in their respective regions. The cathodoluminescence at low accelerating voltage has also covered the entire visible region, resulting in white emission. These luminescent powders are expected to find potential applications in the development of LEDs and FEDs.

Gd3 + Sensitization Effect on the Luminescence Properties of Tb3 + Activated Calcium Gadolinium Oxyapatite Nanophosphors

The solvothermal synthesis and structural characterization of silicate based oxyapatite Tb 3+ activated Ca 2 Gd 8 Si 6 O 26 (CGS) nanophosphors have been reported. The structure of these phosphors was elucidated by the powder x-ray diffraction (XRD) and further characterized by scanning electron microscopy. The XRD results revealed that the obtained Tb 3+ :CGS shows the characteristic peaks of oxyapatite in a hexagonal lattice structure. The photoluminescence (PL) properties were studied with variations of Tb 3+ concentration and sintering temperature. Under 275 nm excitation, both Tb 3+ ( 5 D 3,4 → 7 F J=6.5.4.3 ) and Gd 3+ ( 6 P 7/2 → 8 S 7/2 ) characteristic emissions associated with 4f-4f transitions have been observed, and when the concentration of Tb 3+ increases above 1 mol % the 5 D 3 emission intensity decreases due to cross relaxation. The Gd 3+ emission intensity decreases with increasing Tb 3+ concentration and the PL intensity of Tb 3+ at 378 nm excitation was much weaker than the obtained intensity with excitation at 275 nm, suggesting that the efficient energy transfer occurred from Gd 3+ to Tb 3+ ions in CGS host lattice. The decay curves of the 5 D 4 level show that the lifetime decreases with increasing crystallite size and concentration of Tb 3+ ions. These luminescent powders are expected to find potential applications such as white light emitting diodes and optical display systems.

Novel rare-earth-free yellow Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 phosphors for dazzling white light-emitting diodes

White light-emitting diode (WLED) products currently available on the market are based on the blue LED combined with yellow phosphor approach. However, these WLEDs are still insufficient for general illumination and flat panel display (FPD) applications because of their low color-rendering index (CRI < 75) and high correlated color temperature (CCT = 6000 K). Although near-ultraviolet (UV) LED chips provide more efficient excitation than blue chips, YAG:Ce3+ phosphors have very weak excitation in the near-UV spectral region. Hence, there is an increasing demand for novel yellow phosphor materials with excitation in the near-UV region. In this work, we report novel self-activated yellow Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 (CZIVT) phosphors that efficiently convert near-UV excitation light into yellow luminescence. The crystal structure and lattice parameters of these CZIVT phosphors are elucidated through Rietveld refinement. Through doping with In3+ and Ta5+ ions, the emission intensity is enhanced in the red region, and the Stokes shift is controlled to obtain good color rendition. When a near-UV LED chip is coated with a combination of CZIVT and commercial blue Ba0.9Eu0.1MgAl10O17 phosphors, a pleasant WLED with a high CRI of 82.51 and a low CCT of 5231 K, which are essential for indoor illumination and FPDs, is achieved.

Eu3+ ion concentration induced 3D luminescence properties of novel red-emitting Ba4La6(SiO4)O:Eu3+ oxyapatite phosphors for versatile applications

A series of Eu3+ ions activated Ba4La6(SiO4)6O (BLSO:Eu3+) phosphors were synthesized by a modified citrate sol–gel process. The structural properties of these phosphors were explored by means of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy. The XRD patterns confirmed their oxyapatite structure with the space group of P63/m after annealing at 1400 °C. The scanning electron microscope image exhibited the irregular morphology of BLSO:Eu3+ particles and the elemental mapping confirmed that the Eu3+ ions were distributed homogeneously on the La3+ ion sites. Photoluminescence (PL) excitation spectra of BLSO:Eu3+ exhibited the charge transfer band (CTB) and intense f–f transitions of Eu3+ ions in the violet and blue wavelength regions. The CTB intensity decreased and the f–f transition of Eu3+ ions increased with increasing the Eu3+ ion concentration due to the presence of defects in the 4f and 6h sites of the BLSO host lattice. This feature facilitates the strong absorption in the near-ultraviolet (NUV) region, which is useful for high color rendering index NUV based white light-emitting diodes for display and lighting applications. The PL spectra displayed intense red emission (5D0 → 7F2) along with considerable orange emission (5D0 → 7F1) with good asymmetry ratios and chromaticity coordinates, and exhibited better emission performance than that of commercial Y2O3:Eu3+ phosphors. The three-dimensional PL spectra revealed their strong emission characteristics under UV, NUV and visible excitation wavelengths. The cathodoluminescence properties were also similar to the PL results, confirming that the BLSO:Eu3+ phosphors emit stable red emission under different excitation sources as compared to the commercial Y2O3:Eu3+ phosphors.

UV-A and UV-B excitation region broadened novel green color-emitting CaGd2ZnO5:Tb3+ nanophosphors

Green color-emitting novel CaGd2ZnO5:Tb3+ (CGZO:Tb3+) nanophosphors were synthesized by a citrate sol–gel method. The structural and morphological properties were elucidated by X-ray diffraction and transmission electron microscope measurements. The photoluminescence properties of orthorhombic-phased CGZO:Tb3+ nanophosphors were studied as a function of Tb3+ ion concentration. The CGZO:Tb3+ nanophosphors revealed the enhanced broadband excitation between ultraviolet (UV)-B and UV-A regions. Under 317 nm excitation, even at dilute Tb3+ ion concentrations, only the emission transitions from 5D4 energy level were exhibited. This unusual behavior is due to the occurrence of nonradiative energy transfer via f–d transition rather than cross-relaxation process. The cathodoluminescence also showed similar behavior at low accelerating voltages. These luminescent powders are expected to find potential applications such as white light-emitting diodes and optical display systems.

La(OH)3:Eu3+ and La2O3:Eu3+ nanorod bundles: growth mechanism and luminescence properties

Oriented attachment assisted self-assembled three-dimensional (3D) flower-like La(OH)3:Eu3+ nanorod bundles were successfully synthesized by a facile wet-chemical method. Hexamethylenetetramine played an important role in the formation of the hexagonal phase of La(OH)3:Eu3+ with respect to the reaction time and its concentration. No other surfactants or capping agents were used. The calcination temperature did not show any influence on the morphological texture, and the La2O3:Eu3+ phase was obtained by a subsequent annealing process. The phase formation and morphological properties were confirmed by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The photoluminescence properties were studied for both the synthesized La(OH)3:Eu3+ and La2O3:Eu3+ samples, and also compared with that of the solid-state reaction based La2O3:Eu3+ phosphor. The 3D flower-like La2O3:Eu3+ nanorod bundles showed an intense red emission due to the hypersensitive 5D0 → 7F2 transition with good asymmetric ratio and chromaticity coordinates. Likewise, a systematic study of the cathodoluminescence (CL) properties was carried out in detail. Furthermore, to estimate the CL potentiality, the La2O3:Eu3+ phosphor was compared with a commercially available Y2O3:Eu3+ red phosphor.

Novel orange and reddish-orange color emitting BaGd2O4:Sm3+ nanophosphors by solvothermal reaction for LED and FED applications.

BaGd2O4 (BG):Sm(3+) nanophosphors were synthesized by a solvothermal reaction method. The powder X-ray diffraction pattern confirmed their orthorhombic structure, and the morphological studies were carried out by taking the scanning and transmission electron microscopy images. The photoluminescence (PL) emission and PL excitation (PLE) spectra were investigated as a function of Sm(3+) ion concentration. The PLE spectra revealed both Gd(3+) and Sm(3+) excitation bands in the shorter and longer wavelength regions, indicating that the efficient energy transfer occurred from the Gd(3+) to Sm(3+) ions in the BG host lattice. The PL spectra exhibited an intense orange emission due to ((4)G5/2→(6)H7/2) transition along with two other moderate intense emission peaks due to the ((4)G5/2→(6)H5/2) and ((4)G5/2→(6)H9/2) transitions. Based on the emission performance related to ((4)G5/2→(6)H7/2) transition, the Sm(3+) ion concentration was optimized to be at 1 mol%. The low-voltage cathodoluminescent (CL) measurements were also performed for BG:1 mol% Sm(3+) nanophosphors as a function of accelerating voltage and filament current. From the CL spectra, the reddish-orange emission was observed. The Commission International De I-Eclairage chromaticity coordinates of BG:Sm(3+) nanophosphors were found to be very close to the chromaticity coordinates of Nichia Corporation developed amber light-emitting diodes.