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RISS 인기검색어
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- 저자 : M. M. Ratnam (검색결과 12 건)
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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>
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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.
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Ratnam, B.V.,Sahu, Mukesh K.,Vishwakarma, Amit K.,Jha, Kaushal,Woo, Hyun-Joo,Jang, Kiwan,Jayasimhadri, M. North-Holland 2017 Journal of luminescence Vol.185 No.-
<P><B>Abstract</B></P> <P>Europium activated NaCaPO<SUB>4</SUB> phosphor has been synthesized by various synthesis techniques such as solid-state reaction (SSR), molten salt synthesis (MSS) and sol-gel combustion (SGC) method to optimize the synthesis procedure. The comparative investigations of structural and luminescent properties have been studied to know the best synthesis method. The XRD patterns and Rietveld refinement analysis of the synthesized phosphors confirmed the single phase orthorhombic structure of NaCaPO<SUB>4</SUB>. Excitation spectra indicate the strong absorption in near ultraviolet (n-UV) region and the emission spectra exhibit strong emission band at 595nm corresponds to <SUP>5</SUP>D<SUB>0</SUB>→<SUP>7</SUP>F<SUB>1</SUB> transition under n-UV (λ<SUB>ex</SUB>=392nm) excitation. The SGC route synthesized phosphor exhibit intense emission than that of the SSR and MSS method. Therefore, the effect of dopant (Eu<SUP>3+</SUP>) concentration on the emission intensity and concentration quenching mechanism has been discussed in detail for the Eu<SUP>3+</SUP> doped NaCaPO<SUB>4</SUB> phosphor synthesized by SGC method. The CIE chromaticity coordinates have been calculated for the phosphors synthesized by SSR, MSS and SGC methods to reveal the emitting color and also to know the utility of this phosphor for white LEDs.</P>
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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>
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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.
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Eswaraiah, S.,Kim, Yong Ha,Liu, Huixin,Ratnam, M. Venkat,Lee, Jaewook TERRA SCIENTIFIC PUBLISHING COMPANY 2017 Earth, Planets and Space Vol.69 No.1
<P>We have investigated the coupling between the stratosphere and mesosphere-lower thermosphere (MLT) in the Southern Hemisphere (SH) during 2010 minor sudden stratospheric warmings (SSWs). Three episodic SSWs were noticed in 2010. Mesospheric zonal winds between 82 and 92 km obtained from King Sejong Station (62.22 degrees S, 58.78 degrees W) meteor radar showed the significant difference from usual trend. The zonal wind reversal in the mesosphere is noticed a week before the associated SSW similar to 2002 major SSW. The mesosphere wind reversal is also noticed in 'Specified Dynamics' version of Whole Atmosphere Community Climate Model (SD-WACCM) and Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) simulations. The similar zonal wind weakening/reversal in the lower thermosphere between 100 and 140 km is simulated by GAIA. Further, we observed the mesospheric cooling in consistency with SSWs using Microwave Limb Sounder data. However, the GAIA simulations showed warming between 130 and 140 km after few days of SSW. Thus, the observation and model simulation indicate for the first time that the 2010 minor SSW also affects dynamics of the MLT region over SH in a manner similar to 2002 major SSW.</P>
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