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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>
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>
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.
Sunitha VIJAYAKUMARAN,Ratnam VIJAYAKUMARAN 한국유통과학회 2019 The Journal of Asian Finance, Economics and Busine Vol.6 No.3
The study examines the effects of growth opportunities, debt maturity and liquidity risk on leverage, making use of a large panel of Chinese listed firms. Research on capital structure has broadened its scope from a single capital structure decision (the debt/equity choice) to various attributes of the debt in firms’ capital structure. We use the system Generalized Method of Moments estimator to control for unobserved heterogeneity and the potential endogeneity of regressors. We find a negative relationship between growth opportunities and leverage. Further, we find that while the proportion of short-term debt attenuates the negative effect of growth opportunities on leverage, it negatively affects leverage as predicted by the liquidity risk hypothesis. When we distinguish between state owned firms and private controlled firms, we find evidence that these effects are only relevant to private controlled firms. However, our analysis indicates that the economic implication of liquidity risk effect is much lower for Chinese firms than that observed in the literature for US firms. Our study suggests that these differences can be explained by differences in the institutional environment in which firms operate. This finding related to Diamond’s (1991) liquidity risk hypothesis extends our understanding of the relationship between liquidity risk and the debt maturity choice.
Corporate Governance and Capital Structure Decisions: Evidence from Chinese Listed Companies
Sunitha VIJAYAKUMARAN,Ratnam VIJAYAKUMARAN 한국유통과학회 2019 The Journal of Asian Finance, Economics and Busine Vol.6 No.3
This study examines the impact of corporate governance on capital structure decisions based on a large panel of Chinese listed firms. Using the system Generalized Method of Moments (GMM) estimator to control for unobserved heterogeneity, endogeneity, and persistency in capital structure decisions, we document that the ownership structure plays a significant role in determining leverage ratios. More specially, we find that managerial ownership has a positive and significant impact on firms’ leverage, consistent with the incentive alignment hypothesis. We also find that managerial ownership only affects the leverage decisions of private firms in the post-2005 split share reform period. State ownership negatively influence leverage decisions implying that SOEs may face fewer restrictions in equity issuance and may receive favourable treatments when applying for seasoned equity nancing, thus use less debt. Furthermore, our results show that while foreign ownership negatively influences leverage decisions, legal person shareholding positively influences firms’ leverage decisions only for state controlled firms. We also find that the board structure variables (board size and the proportion of independent directors) do not influence firms’ capital structure decisions. Our findings suggest that recent ownership reforms have been successful in terms of providing incentive to managers through managerial shareholdings to take risky financial choices.