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

Pavitra, E.,Seeta Rama Raju, G.,Park, Wook,Yu, Jae Su The Royal Society of Chemistry 2014 NEW JOURNAL OF CHEMISTRY Vol.38 No.1

<P>A series of Dy<SUP>3+</SUP> ion single-doped and Dy<SUP>3+</SUP>/Eu<SUP>3+</SUP> ion co-doped white-light emitting SrY<SUB>2</SUB>O<SUB>4</SUB> 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 Dy<SUP>3+</SUP> ions acted as luminescent centers and substituted Y<SUP>3+</SUP> ions in the SrY<SUB>2</SUB>O<SUB>4</SUB> host lattice, where they are located in C<SUB>s</SUB> sites, and the characteristic emission of Dy<SUP>3+</SUP> ions (<SUP>4</SUP>F<SUB>9/2</SUB>→<SUP>6</SUP>H<SUB>15/2</SUB> and <SUP>4</SUP>F<SUB>9/2</SUB>→<SUP>6</SUP>H<SUB>13/2</SUB> transitions) with intense yellow emission band at 578 nm occurred. When the Eu<SUP>3+</SUP> ions were co-doped into the SrY<SUB>2</SUB>O<SUB>4</SUB>:Dy<SUP>3+</SUP> (1 mol%) nanocrystalline phosphor, white-light emission was observed under excitation at 381 nm. The energy transfer between Eu<SUP>3+</SUP> and Dy<SUP>3+</SUP> 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 Dy<SUP>3+</SUP> ion single-doped and Dy<SUP>3+</SUP>/Eu<SUP>3+</SUP> co-doped SrY<SUB>2</SUB>O<SUB>4</SUB> nanocrystalline phosphors increased continuously upon increasing both the accelerating voltage from 1 to 5 kV and the filament current from 30 to 47 μA.</P> <P>Graphic Abstract</P><P>A white-light emission for indoor illuminations was obtained when a series of Eu<SUP>3+</SUP> ions were co-doped into SrY<SUB>2</SUB>O<SUB>4</SUB>:1Dy<SUP>3+</SUP> nanocrystalline phosphors. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3nj00987d'> </P>

Synthesis and luminescent properties of novel red-emitting CaGd₄O₇: Eu³⁺ nanocrystalline phosphors

Pavitra, E.,Seeta Rama Raju, G.,Park, J.Y.,Ko, Y.H.,Yu, J.S. Elsevier Sequoia 2013 Journal of alloys and compounds Vol.553 No.-

We reported the novel red-emitting phosphors of trivalent europium (Eu<SUP>3+</SUP>) ions activated CaGd<SUB>4</SUB>O<SUB>7</SUB> (CG) prepared by a sol-gel method. After the samples were annealed at 1500<SUP>o</SUP>C, the X-ray diffraction patterns confirmed their monoclinic structure and the transmission electron microscopy image showed the nanocrystallinity of the CG: Eu<SUP>3+</SUP> phosphor. The photoluminescence excitation spectra exhibited the efficient energy transfer from the Gd<SUP>3+</SUP> to Eu<SUP>3+</SUP> by overlapping the f-f transition of Gd<SUP>3+</SUP> excitation band with charge transfer band of Eu<SUP>3+</SUP> ions. The photoluminescence (PL) spectra showed the emission band in the entire visible region at lower Eu<SUP>3+</SUP> concentration and the red emission at higher Eu<SUP>3+</SUP> concentration. All the decay curves were well fitted by single exponential functions. The temperature dependent luminescence measurements were performed for identifying the thermal stability of CG: Eu<SUP>3+</SUP> phosphors at elevated temperatures. The cathodoluminescent (CL) spectra also presented a similar behavior to that observed in PL spectra. Form the PL, CL, and decay analysis the optimum concentration of Eu<SUP>3+</SUP> ions in the CG host lattice was found to be 5mol%. The CIE chromaticity coordinates varied from yellow to red in visible spectral region by increasing the Eu<SUP>3+</SUP> concentration.

A facile large-scale synthesis and luminescence properties of Gd₂O₃:Eu³⁺ nanoflowers

Pavitra, E.,Su Yu, J. North-Holland 2013 Materials letters Vol.90 No.-

Eu<SUP>3+</SUP> ions activated Gd<SUB>2</SUB>O<SUB>3</SUB> flower-like phosphor powders were prepared by a facile synthesis route. The growth mechanism was examined using the scanning electron microscope images as a function of reaction time. The X-ray diffraction pattern confirmed the cubic structure. The excitation spectra showed the efficient energy transfer from the Gd<SUP>3+</SUP> to Eu<SUP>3+</SUP> ions and the emission spectra exhibited the strongest emission band at wavelengths of ~611nm. The Eu<SUP>3+</SUP> concentration was optimized to be 5mol%. The emission intensity and color purity of Gd<SUB>2</SUB>O<SUB>3</SUB>:5mol% Eu<SUP>3+</SUP> were high as compared to the commercial Y<SUB>2</SUB>O<SUB>3</SUB>:Eu<SUP>3+</SUP> phosphor.

Solvothermal synthesis and luminescent properties of Y₂Ti₂O₇:Eu³⁺spheres

Pavitra, E.,Raju, G. Seeta Rama,Yu, Jae Su Wiley (John WileySons) 2013 Physica status solidi. PSS-RRL. Rapid Research Let Vol.7 No.3

Pump power induced tunable upconversion emissions from Er³⁺/Tm³⁺/Yb³⁺ ions tri-doped SrY₂O₄ nanocrystalline phosphors

Pavitra, E.,Seeta Rama Raju, G.,Oh, Ju-hyun,Yu, Jae Su The Royal Society of Chemistry 2014 New journal of chemistry Vol.38 No.8

<P>RE<SUP>3+</SUP> (Er<SUP>3+</SUP>/Tm<SUP>3+</SUP>/Yb<SUP>3+</SUP>) ions tri-doped SrY<SUB>2</SUB>O<SUB>4</SUB> nanocrystalline phosphors were synthesized by a citrate sol–gel method. After annealing at 1300 °C, the structure was identified to be pure orthorhombic and the particles were found to be nearly spherical in shape. The up-conversion emission spectra of the SrY<SUB>2</SUB>O<SUB>4</SUB>:1Er<SUP>3+</SUP> phosphor revealed a bright green emission (<SUP>4</SUP>S<SUB>3/2</SUB>→<SUP>4</SUP>I<SUB>15/2</SUB>) around 551 nm. To enhance the red emission of the Er<SUP>3+</SUP> ions, Yb<SUP>3+</SUP> ions were co-doped with Er<SUP>3+</SUP> ions, leading to a greenish yellow emission under 980 nm excitation. In the case of the 1Er<SUP>3+</SUP>/1Tm<SUP>3+</SUP>/3Yb<SUP>3+</SUP> ions tri-doped phosphor, white-light emission was observed corresponding to the <SUP>1</SUP>G<SUB>4</SUB>→<SUP>3</SUP>H<SUB>6</SUB> (Tm<SUP>3+</SUP>), <SUP>4</SUP>S<SUB>3/2</SUB>→<SUP>4</SUP>I<SUB>15/2</SUB> (Er<SUP>3+</SUP>) and <SUP>4</SUP>F<SUB>9/2</SUB>→<SUP>4</SUP>I<SUB>15/2</SUB> (Er<SUP>3+</SUP>) transitions at 487, 551 and 664 nm, respectively. It was interestingly noticed that, when the exciting pump power was changed from 100 to 900 mW, the blue emission band of the Tm<SUP>3+</SUP> ions was enhanced by the suppression of the red and green emission bands of the Er<SUP>3+</SUP> ions so that the chromaticity coordinates were also moved to the cyan emission region. This feature suggests that the Er<SUP>3+</SUP> ion may also act as a sensitizer for the Tm<SUP>3+</SUP> and Yb<SUP>3+</SUP> ions and a possible energy transfer mechanism was also discussed.</P> <P>Graphic Abstract</P><P>Under near-infrared excitation, efficient green, yellow and white-emissions were achieved from the SrY<SUB>2</SUB>O<SUB>4</SUB> host lattice when doped with Er<SUP>3+</SUP>, Er<SUP>3+</SUP>/Yb<SUP>3+</SUP>, and Er<SUP>3+</SUP>/Tm<SUP>3+</SUP>/Yb<SUP>3+</SUP> ions, respectively. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4nj00163j'> </P>

White light emission from Eu³⁺ co-activated Ca₂Gd₈Si₆O₂₆:Dy³⁺ nanophosphors by solvothermalsynthesis

Pavitra, E.,Seeta Rama Raju, G.,Yu, Jae Su Elsevier 2013 Ceramics international Vol.39 No.6

<P><B>Abstract</B></P> <P>Ca<SUB>2</SUB>Gd<SUB>8</SUB>(SiO<SUB>4</SUB>)<SUB>6</SUB>O<SUB>2</SUB> (CGS) nanophosphors with different concentrations of single-doped Dy<SUP>3+</SUP> ions and co-doped Dy<SUP>3+</SUP>/Eu<SUP>3+</SUP> ions were prepared by a solvothermal synthesis. Very fine particles in the nanometer range could be achieved by this method, as evidenced by transmission electron microscope measurements. The hexagonal phase of the oxyapatite structure was confirmed by X-ray diffraction patterns. The energy transfer between Eu<SUP>3+</SUP> and Dy<SUP>3+</SUP> ions was investigated by photoluminescence excitation and emission properties. These phosphors had absorption bands in the UV and NUV region, which are suitable for the emission wavelength of UV or NUV light-emitting diodes (LEDs). With increasing the Eu<SUP>3+</SUP> ion concentration, the emission peak intensity corresponding to the <SUP>5</SUP>D<SUB>0</SUB>→<SUP>7</SUP>F<SUB>2</SUB> transition increased and the yellow (<SUP>4</SUP>F<SUB>9/2</SUB>→<SUP>6</SUP>H<SUB>13/2</SUB>) emission intensity also increased compared to the blue (<SUP>4</SUP>F<SUB>9/2</SUB>→<SUP>6</SUP>H<SUB>15/2</SUB>) emission intensity due to the increased energy transfer between Dy<SUP>3+</SUP> to Eu<SUP>3+</SUP> ions. Thus, the Eu<SUP>3+</SUP> ions compensated the red emission component of the Dy<SUP>3+</SUP> doped CGS nanophosphors. Such phosphors are expected to have potential applications for NUV based white LEDs.</P>

Tunable emissions from Dy³⁺/Sm³⁺ ions co-activated SrY₂O₄:Er³⁺ nanocrystalline phosphors for LED and FED applications

Pavitra, E.,Seeta Rama Raju, G.,Yu, J.S. Elsevier Sequoia 2014 JOURNAL OF ALLOYS AND COMPOUNDS Vol.592 No.-

Er<SUP>3+</SUP> ions single-doped, Dy<SUP>3+</SUP>/Er<SUP>3+</SUP> ions co-doped, and Er<SUP>3+</SUP>/Dy<SUP>3+</SUP>/Sm<SUP>3+</SUP> ions triple-doped SrY<SUB>2</SUB>O<SUB>4</SUB> nanocrystalline phosphors were synthesized by a citrate sol-gel method. X-ray diffraction patterns confirmed their pure orthorhombic structure after annealed at 1300<SUP>o</SUP>C and the morphology of particles was found to be nearly spherical. The Er<SUP>3+</SUP> ions single-doped phosphor samples showed an intense green emission band around 548nm and the concentration quenching occurred at dilute concentrations (1mol%) because the <SUP>4</SUP>S<SUB>3/2</SUB> energy state of Er<SUP>3+</SUP> ions can be easily quenched by means of cross relaxation to a metastable state. The Dy<SUP>3+</SUP>/Er<SUP>3+</SUP> ions co-doped phosphors exhibited a classic yellowish green emission due to efficient energy transfer from Dy<SUP>3+</SUP> to Er<SUP>3+</SUP> ions under 365nm excitation. By adding a series of Sm<SUP>3+</SUP> ions to the Dy<SUP>3+</SUP>/Er<SUP>3+</SUP> ions co-doped SrY<SUB>2</SUB>O<SUB>4</SUB> phosphors, a pleasant white light emission which is essential for white light-emitting diodes was obtained. Likewise, from the cathodoluminescence measurements, a warm white light emission that is required for field-emission displays was achieved with accelerating voltage of 5kV and filament current of 55μA. A possible energy transfer mechanism was discussed and the energy transfer efficiencies were also calculated.

Evolution of highly efficient rare-earth free Cs(1−x)RbxVO3phosphors as a single emitting component for NUV-based white LEDs

Pavitra, E.,Seeta Rama Raju, G.,Krishna Bharat, L.,Park, Jin Young,Kwak, Cheol Hwan,Chung, Jong Won,Han, Young-Kyu,Huh, Yun Suk The Royal Society of Chemistry 2018 Journal of Materials Chemistry C Vol.6 No.46

<P>Since the commercialization of white light-emitting diodes (WLEDs) in 1996, they are rapidly replacing conventional lighting sources and have become an essential commodity for day-to-day human life. At present, most of the WLEDs existing in the market are rare-earth based, which are very limited, expensive and often not available due to monopolistic supply conditions. Hence, there is a serious demand for novel rare-earth free phosphors to achieve cost-effective and energy-efficient WLEDs with excellent luminous efficacy for general illumination. Herein we report on highly efficient rare-earth free Cs(1−x)RbxVO3phosphors as a single emitting compound for near UV-based WLEDs manufactured by the citrate sol-gel method for the first time. Rietveld refinement is performed for the X-ray diffraction patterns of the CsVO3host and CsVO3:0.25Rb phosphors to reveal their orthorhombic pyroxene structure. The difference in the ionic radii of Rb<SUP>+</SUP>and Cs<SUP>+</SUP>ions led to a more distorted VO4<SUP>3−</SUP>tetrahedron with a broken<I>T</I>dsymmetry, which allows spin forbidden transitions for CsVO3:0.25Rb phosphors, resulting in enhanced internal and external quantum efficiencies of 94.7% and 84.5% along with superior luminescence properties compared to the CsVO3host. When varying the input current from 20 to 200 mA, the fabricated WLED exhibited a good color rendering index of 69.7-81.5 and an extremely high luminous efficacy of 94.8-58.7 lm W<SUP>−1</SUP>, which are among the highest values for the same host lattice. These rare-earth free CsVO3:0.25Rb phosphors with a single emitting center may emerge as a new class of advanced inorganic phosphors for near UV-based WLEDs in the lighting industries.</P>

Novel orange-emitting Ba₂LaNbO₆:Eu³⁺ nanophosphors for NUV-based WLEDs and photocatalytic water purification

Pavitra, E.,Raju, G. Seeta Rama,Ghoreishian, Seyed Majid,Kwak, Cheol Hwan,Park, Jin Young,Han, Young-Kyu,Huh, Yun Suk Elsevier 2019 CERAMICS INTERNATIONAL Vol.45 No.4

<P><B>Abstract</B></P> <P>Energy conservation and environmental safety are the key requirements in the modern world. We report novel orange-emitting double perovskite Ba<SUB>2</SUB>LaNbO<SUB>6</SUB>:Eu<SUP>3+</SUP> (BLN:Eu<SUP>3+</SUP>) nanophosphor fabricated using a citrate sol-gel method for use in general illumination and photocatalysis. After annealing at 800 ℃, the particles exhibited a nanorod-like morphology with monoclinic structure. The photoluminescence emission spectra exhibited an intense <SUP>5</SUP>D<SUB>0</SUB>→<SUP>7</SUP>F<SUB>1</SUB> transition at 594 nm and a moderate <SUP>5</SUP>D<SUB>0</SUB>→<SUP>7</SUP>F<SUB>2</SUB> transition at 615 nm, demonstrating that the Eu<SUP>3+</SUP> ions occupied the La<SUP>3+</SUP> sites with inversion symmetry. The optimal concentration of Eu<SUP>3+</SUP> ions was found to be about 5 mol% for the BLN host lattice. Energy transfer from the NbO<SUB>6</SUB> <SUP>7-</SUP> octahedrons to the Eu<SUP>3+</SUP> ions was clearly witnessed when the BLN:Eu<SUP>3+</SUP> nanophosphors were excited with both the characteristic excitation bands of Eu<SUP>3+</SUP> (<SUP>7</SUP>F<SUB>0</SUB>→<SUP>5</SUP>L<SUB>6</SUB>) and NbO<SUB>6</SUB> <SUP>7-</SUP> octahedrons at 392 and 380 nm, respectively. The thermal quenching temperature of 5 mol% Eu<SUP>3+</SUP> ions doped BLN nanophosphors was found to be 183 ℃, indicating that these nanophosphors are very stable at high temperatures. In addition, the dye removal efficiency of the proposed BLN nanophosphors was verified using Rhodamine B (RhB) dye as a model pollutant under UV irradiation. Compared to a commercial nano-ZnO catalyst, our synthesized BLN nanophosphors showed superior RhB de-colorization efficiency. Therefore, the proposed BLN:Eu<SUP>3+</SUP> nanophosphors are promising multifunctional materials for photocatalysis and general lighting applications.</P>

Synthesis and Luminescent Properties of RE³+ (Eu³+ and Tb³+) Ions Activated CaGd4O7 Novel Phosphors

E.Pavitra,G.Seeta Rama Raju,Yeong Hwan Ko,Jae Su Yu 한국진공학회 2012 한국진공학회 학술발표회초록집 Vol.2012 No.8