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Jayasimhadri, M.,Jha, Kaushal,Ratnam, B.V.,Woo, Hyun-Joo,Jang, Kiwan,Rao, A.S.,Haranath, D. ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.711 No.-
<P><B>Abstract</B></P> <P>In this work, Tb<SUP>3+</SUP> ions doped lead-germanate-tellurite (LGT) glasses were prepared by conventional melt quenching technique with different dopant concentrations ranging from 0.5 to 3.5 mol %. X-ray diffraction (XRD) and FT-IR analysis were carried out to analyze the structural properties of LGT glass. The excitation spectra revealed a single band centered in the NUV region at 380 nm by monitoring emission at 545 nm. The emission spectra consist of four bands, which are attributed to the <SUP>5</SUP>D<SUB>4</SUB>→<SUP>7</SUP>F<SUB>J</SUB> (J = 3–6) transitions. Among these transitions, the strong emission band was observed at 545 nm corresponding to the <SUP>5</SUP>D<SUB>4</SUB>→<SUP>7</SUP>F<SUB>5</SUB> transition and the optimized doping concentration of Tb<SUP>3+</SUP> ions was 2 mol %. The Huang's theory and I-H model indicate the possibility of energy transfer via electric dipole-dipole interaction between Tb<SUP>3+</SUP> ions. The CIE chromaticity coordinates were (<I>x</I> = 0.282 and <I>y</I> = 0.614) and emits intense yellowish green light. The decay curves measured for <SUP>5</SUP>D<SUB>4</SUB> level for the samples with different doping concentrations and the lifetime for the optimized sample was 548 μs. The results indicate that these glasses have potential applications in solid state lighting and display devices.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Optically transparent Tb<SUP>3+</SUP> doped PbO-GeO<SUB>2</SUB>-TeO<SUB>2</SUB> (LGT) glasses have been prepared. </LI> <LI> Excitation spectrum consists of single band in the NUV region at 380 nm. </LI> <LI> CIE coordinates indicate that Tb<SUP>3+</SUP> doped LGT glass emits yellowish green light. </LI> <LI> Huang's theory applied to reveal the energy transfer process between Tb<SUP>3+</SUP> ions. </LI> <LI> This glass may be potential to use in fabricating GaN based WLEDs. </LI> </UL> </P>
Vikas,Jayasimhadri M.,Haranath D. 한국물리학회 2024 Current Applied Physics Vol.58 No.-
Transparent, Eu3+ doped potassium tungstate tellurite (TKWZBiEu) glass matrices were successfully synthesized via employing the traditional melt quenching method and their thermal, structural and photoluminescent characteristics were thoroughly investigated. To estimate the aggregate weight loss, glass transition temperature (Tg) and thermal stability factor (ΔT) of the prepared host glass matrix, thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) were utilized. The non-crystalline character of the prepared TKWZBiEu glass was studied via XRD profile. Various vibrational functional groups were revealed via employing Fourier transform infrared (FT-IR) spectroscopy. The optical bandgap (Eopt) values for all prepared TKWZBiEu glasses have been evaluated by employing the absorption spectra. Under n-UV and blue excitations, all the prepared TKWZBiEu glasses are demonstrating reddish emission at 614 nm ascribed to the 5D0 → 7F2 transition, in which the intensity is increasing continuously with Eu3+ ion content up to 5.0 mol%. The experimental lifetime (τ) profiles demonstrate the single-exponential nature of prepared TKWZBiEu glasses under n-UV excitation. Furthermore, temperature dependent photoluminescence (TDPL) spectra indicate excellent thermal stability of the TKWZBiEu glass matrix with the highest value of activation energy (ΔE). The prototype organic epoxy resin/ binder-free device has been developed using the 5.0 mol% Eu3+ doped with TKWZBi glass matrix and n-UV LED chip. All the aforementioned findings validate that the optimized TKWZBiEu glass is an auspicious candidate for the red component to fabricate organic epoxy-free w-LEDs.
Ratnam, B.V.,Jayasimhadri, M.,Bhaskar Kumar, G.,Jang, K.,Kim, S.S.,Lee, Y.I.,Lim, J.M.,Shin, D.S.,Song, T.K. Elsevier Sequoia 2013 JOURNAL OF ALLOYS AND COMPOUNDS Vol.564 No.-
<P>An efficient green emitting Tb3+ doped NaCaPO4 (NCP) phosphor was synthesized by using conventional solid-state reaction for solid-state lighting applications. X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), FT-IR, emission and excitation properties were extensively investigated for NCP phosphors. X-ray diffraction analysis confirmed the formation of NaCaPO4 with orthorhombic structure. The excitation spectrum consists of strong 4f-4f transition at around 370 nm, which has higher intensity than the f-d transition. Emission spectra indicated that this phosphor can be efficiently excited by UV light in the range from 250 to 400 nm, and shows strong emission band centered at 547 nm. Analysis of the emission spectra with different Tb3+ concentrations revealed that the optimum dopant concentration for these NCP phosphors is about 5 mol% of Tb3+. Diminishing of D-5(3) level and increasing of D-5(4) level emission intensity with the Tb3+ concentration explained successfully. The emission color was analyzed and confirmed with the help of chromaticity coordinates and color temperature. The excellent luminescent properties of NaCaPO4: Tb3+ phosphor makes it as a potential green phosphor upon near-UV LED excitation. (C) 2013 Elsevier B.V. All rights reserved.</P>
Ratnam, B.V.,Jayasimhadri, M.,Jang, K. Pergamon 2014 Spectrochimica acta. Part A, Molecular and biomole Vol.132 No.-
Rare earth ion activated orthophosphates have a great deal of interest due to their thermal stability for white light emitting diodes. In this regard, thermally stable Sm<SUP>3+</SUP> doped NaCaPO<SUB>4</SUB> (NCP) phosphor was synthesized by conventional solid state reaction technique. The phase and the structure of the as prepared powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), FT-IR, emission and excitation properties were extensively investigated for NCP phosphors. X-ray diffraction analysis confirmed the formation of NaCaPO<SUB>4</SUB> with orthorhombic structure. The excitation spectra indicate that this phosphor can be effectively excited by UV light from 350 to 500nm. All the transitions in the excitation spectrum of Sm<SUP>3+</SUP> start from the ground state <SUP>6</SUP>H<SUB>5/2</SUB> to various excited states. The emission spectra indicated that the emitted radiation was dominated by the emission peak wavelength at 599nm originated from the transition of <SUP>4</SUP>G<SUB>5/2</SUB>→<SUP>6</SUP>H<SUB>7/2</SUB>. The optimum concentration of Sm<SUP>3+</SUP> is determined as 1.0mol% based on the concentration dependent emission spectra. These results suggest that the NaCaPO<SUB>4</SUB>:Sm<SUP>3+</SUP> phosphor is a promising orange emitting phosphor under 404nm excitation with CIE coordinates of x=0.545, y=0.41, which might be used in the development of materials for LED's and other optical devices in the visible region.
Luminescent Properties of Tb<SUP>3+</SUP>- Doped NaCaPO<SUB>4</SUB> Phosphor
B. V. Ratnam,,M. Jayasimhadri,Jaeho Yoon,Kiwan Jang,Ho-Sueb Lee,Soung-Soo Yi,Sung Hoon Kim,Jung Hyun Jeong 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.6
Trivalent-terbium-doped NaCaPO4 powder phosphors were prepared with different processes by using a conventional solid state reaction method. The phase and the structure of the as-prepared samples were characterized by using X-ray diffraction. The excitation and the emission spectra were measured to characterize the luminescent properties of the NaCaPO4:Tb3+ phosphors. The excitation spectrum exhibited one strong band located in the UV region at 274 nm and was assigned to 4f8 → 4f75d1 transition, and some other excitation bands in the longer wavelength region were attributed to the f-f transitions within the Tb3+ (4f8) configuration. The emission spectra were measured upon 370-nm excitation, and the most intense peak was observed in the green region at 547 nm, corresponding to the 5D4 → 7F5 transition. The emitting color of Tb3+- doped NaCaPO4 phosphor is discussed based on the CIE diagram. The lifetime of the 5D4 energy level of Tb3+ ions in the NaCaPO4 phosphor was measured upon 355-nm excitation. From this study, we have successfully optimized the preparation and emission process of the Tb-doped NaCaPO4 green phosphor for use in white- light- emitting diodes. Trivalent-terbium-doped NaCaPO4 powder phosphors were prepared with different processes by using a conventional solid state reaction method. The phase and the structure of the as-prepared samples were characterized by using X-ray diffraction. The excitation and the emission spectra were measured to characterize the luminescent properties of the NaCaPO4:Tb3+ phosphors. The excitation spectrum exhibited one strong band located in the UV region at 274 nm and was assigned to 4f8 → 4f75d1 transition, and some other excitation bands in the longer wavelength region were attributed to the f-f transitions within the Tb3+ (4f8) configuration. The emission spectra were measured upon 370-nm excitation, and the most intense peak was observed in the green region at 547 nm, corresponding to the 5D4 → 7F5 transition. The emitting color of Tb3+- doped NaCaPO4 phosphor is discussed based on the CIE diagram. The lifetime of the 5D4 energy level of Tb3+ ions in the NaCaPO4 phosphor was measured upon 355-nm excitation. From this study, we have successfully optimized the preparation and emission process of the Tb-doped NaCaPO4 green phosphor for use in white- light- emitting diodes.
Bandi, Vengala Rao,Jayasimhadri, M,jeong, Junho,Jang, Kiwan,Lee, Ho Sueb,Yi, Soung-Soo,Jeong, Jung-Hyun Institute of Physics [etc.] 2010 Journal of Physics. D, Applied Physics Vol.43 No.39
<P>A series of red phosphors Ca<SUB>1−<I>x</I></SUB>ZrSi<SUB>2</SUB>O<SUB>7</SUB> : Eu<SUB><I>x</I></SUB> (<I>x</I> = 0.5,1,5,10,12 mol%) were prepared by a solid-state reaction technique at various temperatures and their structural and optical properties were investigated. The x-ray diffraction profiles showed that all peaks could be attributed to the monoclinic phase CaZrSi<SUB>2</SUB>O<SUB>7</SUB> doped with Eu<SUP>3+</SUP>. SEM, FTIR, TG and DTA profiles have also been characterized to explore their structural properties. The luminescence properties of these resulting phosphors have been characterized by photoluminescence spectra. The host matrix itself has shown a strong blue emission which has its maximum intensity at 470 nm. The excitation spectra of CaZrSi<SUB>2</SUB>O<SUB>7</SUB> : Eu<SUP>3+</SUP> revealed two excitation bands at 395 and 464 nm which correspond to the sharp <SUP>7</SUP>F<SUB>0</SUB>–<SUP>5</SUP>L<SUB>6</SUB> and <SUP>7</SUP>F<SUB>0</SUB>–<SUP>5</SUP>D<SUB>2</SUB> transitions of Eu<SUP>3+</SUP> and matches well with the two popular emissions from n-UV/blue GaN-based LEDs. The prominent red emission was obtained at 615 nm by the excitation transitions <SUP>5</SUP>L<SUB>6</SUB>, <SUP>5</SUP>D<SUB>2</SUB> of Eu<SUP>3+</SUP> through the non-radiative energy transfer process from the host to the Eu<SUP>3+</SUP> ion. The effects of charge compensation by monovalent ions on the luminescence behaviour of a red emitting phosphor CaZrSi<SUB>2</SUB>O<SUB>7</SUB> : Eu<SUP>3+</SUP> were investigated. The high colour saturation and the low thermal quenching effect of these phosphors make it a potential red component for white light emitting diodes (w-LEDs).</P>