Sol-gel derived barium orthovanadate phosphors for white light-emitting diodes

Krishna Bharat, L.,Nagaraju, Goli,Khaja Hussain, Sk.,Yu, Jae Su Elsevier 2018 Dyes and pigments Vol.150 No.-

<P><B>Abstract</B></P> <P>The self-luminescent Ba<SUB>3</SUB>(VO<SUB>4</SUB>)<SUB>2</SUB> materials were synthesized by a sol-gel method. The structural and morphological properties of the prepared materials were analyzed by X-ray diffraction (XRD) pattern and scanning electron microscope images. The XRD pattern shows that the sample crystallized in the rhombohedral phase with a space group of R 3 ¯ m (166). The XRD pattern is consistent with the Joint Committee on Powder Diffraction Standards card values without any other impurity peaks. The self-luminescent properties of the samples were investigated by measuring the photoluminescence (PL) excitation and emission spectra by using a spectrofluorometer. The PL spectra revealed that the materials can give visible bluish-green emission under ultraviolet irradiation. The materials were further employed in making near-ultraviolet excitable white light-emitting diodes. The Ba<SUB>3</SUB>(VO<SUB>4</SUB>)<SUB>2</SUB> phosphor along with a red emitting phosphor was mixed in silicone epoxy and applied to the near-ultraviolet light-emitting diode to obtain a bright white-light emission with a color rendering index of 80 and a correlated color temperature of 4075 K.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The Ba<SUB>3</SUB>(VO<SUB>4</SUB>)<SUB>2</SUB> rare-earth free phosphors were prepared by a sol-gel method. </LI> <LI> Under NUV excitation (365 nm), the phosphors reveal a bluish-green visible emission. </LI> <LI> The WLEDs made with the Ba<SUB>3</SUB>(VO<SUB>4</SUB>)<SUB>2</SUB> and a red-phosphor give good CRI and CCT values. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Effect of transition metal ion (Nb⁵⁺) doping on the luminescence properties of self-activated Ca₂AgZn₂V₃O₁₂ phosphors

Bharat, L. Krishna,Krishna, Kurugundla Gopi,Yu, Jae Su Elsevier 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.699 No.-

<P><B>Abstract</B></P> <P>Ca<SUB>2</SUB>AgZn<SUB>2</SUB>V<SUB>3</SUB>O<SUB>12</SUB> self-activated vanadate phosphors were synthesized via a citrate assisted sol-gel method. The phase formation of the garnet structure was verified by X-ray diffraction (XRD) and Rietveld refinement analyses. From XRD patters, it is clear that the prepared samples were crystallized in cubic phase with a space group Ia 3 ¯ d (230). The photoluminescence excitation spectrum showed a broad band in the UV region and a broad emission spectrum was observed in the wavelength range of 400–750 nm. The doping of transition metal ion (Nb<SUP>5+</SUP>) in the vanadium site changed the luminescence properties. Doping of Nb<SUP>5+</SUP> ions increased the VO bond length from 1.7016 to 1.7148 Å which causes the increase in luminescence intensity and the red shift of emission spectrum. The increase in the luminescence intensity may also be attributed to the emission from Nb<SUP>5+</SUP> ions. The thermal stability of the materials was measured and the activation energy was found to be 0.254 eV. The luminescence decay time was also measured and the values were found to be 158 and 154 μs for undoped and doped samples, respectively. The prepared phosphors exhibited a greenish yellow emission under UV excitation, indicating that they can be potential candidates for the application in UV-based WLEDs.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The self-activated Ca<SUB>2</SUB>AgZn<SUB>2</SUB>V<SUB>3</SUB>O<SUB>12</SUB> vanadate phosphor was prepared via a sol-gel method. </LI> <LI> The prepared phosphors exhibited a greenish yellow emission under UV excitation. </LI> <LI> The doping of Nb<SUP>5+</SUP> ions in the vanadium site changes the luminescence properties. </LI> <LI> The red shift in the emission spectrum is due to the increase in VO bond length. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Synthesis and luminescent properties of trivalent rare-earth (Eu³⁺, Tb³⁺) ions doped nanocrystalline AgLa(PO₃)₄ polyphosphates

Krishna Bharat, L.,Jeon, Y.I.,Yu, J.S. Elsevier Sequoia 2014 JOURNAL OF ALLOYS AND COMPOUNDS Vol.614 No.-

The AgLa(PO<SUB>3</SUB>)<SUB>4</SUB> phosphors activated with trivalent rare-earth (Eu<SUP>3+</SUP>, Tb<SUP>3+</SUP>) ions were prepared by a sol-gel synthesis method. The crystal structure of the compound was studied by X-ray diffraction patterns and found to be crystallized in the monoclinic system with a space group P2<SUB>1</SUB>/c, indicating the calculated lattice parameters of a=10.08A, b=13.12A, and c=7.314A. The Fourier-transform infrared spectrum, photoluminescence excitation/emission spectra, and decay curves were examined to study the optical properties. The analysis of the Eu<SUP>3+</SUP> ions related emission spectrum revealed the presence of highly symmetric sites for the activator ions. The Tb<SUP>3+</SUP> ions related emission spectrum exhibited a <SUP>5</SUP>D<SUB>3</SUB> emission due to the prolonged calcination at high temperatures, which reduces the residual hydroxyl ions. The optical properties show that this host material is suitable for phosphor materials and laser crystals.

Stannic Oxide Nanoplate-Assembled Hierarchical Microstructures: Synthesis and Photocatalytic Properties

Bharat, L. Krishna,Reddy, L. Sivananda,Yu, Jae Su American Scientific Publishers 2017 Science Of Advanced Materials Vol.9 No.7

Preparation of Eu³⁺ ions activated Ca₂La₈(SiO₄)₆O₂ oxyapatite nanophosphors through two-step surfactant-free method and their optical and electrical properties

Bharat, L Krishna,Dugasani, Sreekantha Reddy,Seeta Rama Raju, G,Yu, Jae Su IOP 2017 Nanotechnology Vol.28 No.37

<P>Eu<SUP>3+</SUP> ions activated Ca<SUB>2</SUB>La<SUB>8</SUB>(SiO<SUB>4</SUB>)<SUB>6</SUB>O<SUB>2</SUB> (CLSO):Eu<SUP>3+</SUP> nanophosphor samples were synthesized by a mixed solvothermal and hydrothermal method. The samples were carefully studied using various characterization techniques. The XRD patterns of CLSO:Eu<SUP>3+</SUP> and CLSO confirmed that the samples were crystallized in hexagonal phase with a space group of P6<SUB>3</SUB>/m (176). The morphology of the nanoparticles was studied by varying the reaction parameters such as growth, temperature and time. The photoluminescence (PL) excitation and PL emission spectra exhibited the typical Eu<SUP>3+</SUP> bands in the wavelength range of 200–550 nm and 400–750 nm, respectively. The intensity of the <img ALIGN='MIDDLE' ALT='${}^{5}{\rm{D}}_{0}\to {}^{7}{\rm{F}}_{2}$' SRC='http://ej.iop.org/images/0957-4484/28/37/375601/nanoaa7dadieqn1.gif'/> electric dipole (ED) transition peak was strong in the PL emission spectrum which imparts the red color when observed under ultraviolet light. The ED transition peak intensity increased when the sample was calcined at an elevated temperature of 700 °C, indicating improved asymmetry ratio and good chromaticity coordinates. The electrical properties of the prepared materials were studied by spin-coating the powder dispersed solutions on the silica substrate. The output current values were also measured for the CLSO nanoparticles prepared under different growth conditions. These results showed the advantages of CLSO nanoparticles for their application in optics and feasibility in nanoelectronic and energy harvesting devices.</P>

Hexadentate ligand-assisted wet-chemical approach to rare-earth free self-luminescent cocoon-shaped barium orthovanadate nanoparticles for latent fingerprint visualization

Bharat, L. Krishna,Nagaraju, Goli,Yu, Jae Su Elsevier 2018 Sensors and actuators. B Chemical Vol.271 No.-

<P><B>Abstract</B></P> <P>Designing nanomaterials with controlled size and shape for desired functions and devices is becoming a prerequisite for current researchers. Nanomaterials also have a significant impact on forensic sciences for the visualization of latent fingerprint. In this context, we report the synthesis of nanostructured barium orthovanadate (Ba<SUB>3</SUB>V<SUB>2</SUB>O<SUB>8</SUB>) rare-earth free materials via a novel wet-chemical method. The growth of cocoon-shaped nanoparticles is explained by the variation of ethylene glycol and ethylenediaminetetraacetic acid concentrations. In addition, the luminescence properties of the self-luminescent materials are of high quality under ultraviolet light illumination and are useful for the detection of latent fingerprints. The latent fingerprints and their Galton details were clearly observed on different hydrophilic and hydrophobic substrates. The preceding results suggest that the cocoon-shaped Ba<SUB>3</SUB>V<SUB>2</SUB>O<SUB>8</SUB> rare-earth free nanomaterials can serve as a significant candidate for the detection and individualization of latent fingerprints.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Ba<SUB>3</SUB>V<SUB>2</SUB>O<SUB>8</SUB> cocoon-shaped nanoparticles were prepared in a double-solvent system. </LI> <LI> Herein, EG and EDTA play an important role in obtaining cocoon-shaped nanoparticles. </LI> <LI> Calcinating the sample increases the crystallinity and optical properties. </LI> <LI> The nanoparticles can assist the latent fingerprint detection and individualization. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Long-wave UVA radiation excited warm white-light emitting NaGdTiO₄: Tm³⁺/Dy³⁺/Eu³⁺ ions tri-doped phosphors: Synthesis, energy transfer and color tunable properties

Bharat, L. Krishna,Du, Peng,Yu, Jae Su Elsevier 2016 JOURNAL OF ALLOYS AND COMPOUNDS Vol.666 No.-

<P><B>Abstract</B></P> <P>NaGdTiO<SUB>4</SUB> (NGT) phosphors doped with different activator ions (Tm<SUP>3+</SUP>, Dy<SUP>3+</SUP>, and Eu<SUP>3+</SUP>) were synthesized by a conventional solid-state reaction method in an ambient atmosphere. These phosphors were characterized by scanning electron microscope images, transmission electron microscope images, X-ray diffraction patterns, Fourier transform infrared spectra, and photoluminescence spectra. All the samples were crystallized in an orthorhombic phase with a space group of Pbcm (57). In Tm<SUP>3+</SUP>/Dy<SUP>3+</SUP> ions co-doped samples, white-light emission was observed under near-ultraviolet (NUV) excitation. In addition, the energy transfer between Tm<SUP>3+</SUP> and Dy<SUP>3+</SUP> ions was proved to be a resonant type via an electric dipole–dipole mechanism and the critical distance of energy transfer was calculated to be 19.91 Å. Furthermore, Tm<SUP>3+</SUP>/Dy<SUP>3+</SUP>/Eu<SUP>3+</SUP> ions tri-doped NGT phosphors demonstrated warm white-light emission by appropriately tuning the activator content, based on the principle of energy transfer. These NUV wavelength excitable phosphors exhibit great potential as a single-phase full-color emitting phosphor for white light-emitting diode applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The pebble shaped NaGdTiO<SUB>4</SUB> particles were prepared by solid-state reaction method. </LI> <LI> Tm<SUP>3+</SUP> and Dy<SUP>3+</SUP> single doping gives respective blue and cool white light emission. </LI> <LI> The Tm<SUP>3+</SUP>/Dy<SUP>3+</SUP> ions co-doped samples give CIE values near to standard white light. </LI> <LI> Addition of Eu<SUP>3+</SUP> ions shifts the CIE values towards warm white light region. </LI> <LI> This single phase white light emitting phosphors have lower CCT values (<5000 K). </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Thermal stability and luminescence characteristics of rare-earth ions doped Y<SUB>6</SUB>WO<SUB>12</SUB> phosphors for solid-state lighting and display applications

Krishna Bharat LANKAMSETTY,Jae Su YU 한국진공학회 2016 한국진공학회 학술발표회초록집 Vol.2016 No.8

Near-ultraviolet excitation-based bluish-green emitting K₂ZnSiO₄: Eu²⁺ nanophosphors for white light-emitting applications

Krishna Bharat, L.,Hussain, Sk. Khaja,Yu, Jae Su Applied Science Publishers 2017 Dyes and pigments Vol.145 No.-

<P><B>Abstract</B></P> <P>The K<SUB>2</SUB>ZnSiO<SUB>4</SUB>: Eu<SUP>2+</SUP> nanophosphor materials were synthesized via a sol-gel method. The morphology of the synthesized materials was examined by field-emission scanning electron microscope images. The structural properties were analyzed from X-ray diffraction (XRD) pattern and Fourier transform infrared spectrum. The K<SUB>2</SUB>ZnSiO<SUB>4</SUB> host lattice crystallizes in a cubic phase, and the XRD pattern matches well with the standard JCPDS card values and does not show any impurity peaks. The luminescence properties were studied using the photoluminescence (PL) excitation and PL emission spectra. The broadband excitation in the near-ultraviolet (NUV) region and the broadband emission in the visible region make this phosphor material suitable for the application of NUV-based white light-emitting diodes. The thermal stability of the material was tested by calculating the activation energy value which was found to be 0.296 eV. The synthesized K<SUB>2</SUB>ZnSiO<SUB>4</SUB>: Eu<SUP>2+</SUP> nanophosphor was then coated onto the NUV light-emitting diode along with a red-emitting phosphor to get visible white-light emission with a color rendering index value of 81 and a correlated color temperature value of 5730 K.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The K<SUB>2</SUB>ZnSiO<SUB>4</SUB>: Eu<SUP>2+</SUP> nanophosphors were prepared by a sol-gel method. </LI> <LI> The NUV excitation (375 nm) gives broadband bluish-green emission. </LI> <LI> The samples have good thermal stability with an activation energy value of 0.296 eV. </LI> <LI> The LED made with K<SUB>2</SUB>ZnSiO<SUB>4</SUB>: Eu<SUP>2+</SUP> and red-emitting phosphors gives cool white-light emission. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Energy transfer and luminescence properties of Ce³⁺/Dy³⁺ co-doped Sr₃SiO₅ phosphors for WLEDs

Krishna, Kurugundla Gopi,Bharat, L. Krishna,Yu, Jae Su Elsevier 2017 CERAMICS INTERNATIONAL Vol.43 No.2

<P><B>Abstract</B></P> <P>The Sr<SUB>3</SUB>SiO<SUB>5</SUB>: Ce<SUP>3+</SUP>/Dy<SUP>3+</SUP> phosphors were prepared via a citrate assisted sol-gel method. The prepared samples were characterized by X-ray diffraction pattern, scanning electron microscopy, Fourier transform infrared spectroscopy and fluorescence spectroscopy. When excited with near-ultraviolet (NUV) (360nm) irradiation, the photoluminescence emission spectra showed a broad band blue emission of Ce<SUP>3+</SUP> ions combined with the sharp emission peaks of Dy<SUP>3+</SUP> ions. Energy transfer occurred between the Ce<SUP>3+</SUP> and Dy<SUP>3+</SUP> ions and it increased with the increase of Dy<SUP>3+</SUP> ion concentration. The luminescence properties of the prepared phosphor materials could be adjusted by changing the concentration ratio of the Ce<SUP>3+</SUP> and Dy<SUP>3+</SUP> ions. The prepared phosphor materials also exhibited good thermal stability.</P>