Browsing by Author "Y. Dwivedi"

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Analysis of bright red-orange emitting Mn2+:ZnAl2O4 spinel nanophosphor
(Elsevier GmbH, 2018) Pushpa Kumari; Y. Dwivedi; Amresh Bahadur
In the present article, undoped and Mn-doped spinel ZnAl2O4 nanophosphors were synthesized by co-precipitation and hydro-thermal methods. The prepared nanophosphors were characterized using powder X-ray diffraction (XRD), Scanning electron microscopy, UV–Visible-Infrared absorption, photoluminescence etc. XRD pattern reveals the formation of cubical ZnAl2O4 nanocrystals of average crystalline size ∼12 nm. The Mn-doped ZnAl2O4 nanophosphors show bright orange − red emission due to the radiative 4T1 → 6A1 transition (593 nm) on excitation with 342 nm radiation. The emission intensity of the prepared nanophosphors was found to be a function of synthesis method and Mn ion concentrations. Emission intensity is found to reduced appreciably in hydrothermal samples. Photophysics involved for emission and other spectroscopic properties were explained. © 2017 Elsevier GmbH
Color tunable emission from (Gd xY 1-x) 2O 3: Er 3+, Yb 3+ phosphor prepared by combustion method
(2012) C. Joshi; A. Rai; Y. Dwivedi; S.B. Rai
The (Gd xY 1-x) 2O 3: Er 3+, Yb 3+ [x=0.0, 0.1, 0.5, 0.9, 1.0] phosphor samples with 0.5 mol% concentration of Er 3+ and 3.0 mol% of Yb 3+, have been prepared using combustion route. The effect of variation of composition on the morphology, crystallinity and photoluminescence characteristics of the material has been investigated. The samples were post-heated at a temperature 1200°C, for 5 h. We find systematic color tunability from red to green with an increase of Gd 2O 3 content in Y 2O 3 lattice. Further, the post-heated samples show an enhancement of fluorescence intensity for more than fifteen times. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence measurement and Fourier transform infrared (FTIR) techniques in order to find out the structural changes in host lattice. An increase in crystallite size has been marked with increasing Gd content while FTIR analysis explains the reason of variation in the fluorescence of rare earth ions in different host matrices. © 2011 Elsevier B.V. All rights reserved.
Efficient white light generation from 2,3-diphenyl-1,2-dihydro-quinoxaline complex
(2010) Y. Dwivedi; S. Kant; R.N. Rai; S.B. Rai
In this article, we report two organic materials dispersed in transparent poly (methyl methacrylate) matrix for efficient white light simulation under different optical excitations. A newly synthesized complex of benzoin and o-phenyldiamine is observed to be white on illumination with a blue LED. A new concept of white light emitting tube is also demonstrated. A mixture of 2,2"([1,1'-biphenyl]-4,4'diyldi-2,1-ethenediyl)-bis-benzenesulfonic acid disodium salt and complex is optimized to emit white light extended in the violet region on 355 nm laser excitation. The optical quality of the emitted white light is adjudged by the CIE coordinate, correlated color temperature and color rendition index in both the cases. © 2010 Springer-Verlag.
Energy transfer in Er:Eu:Yb co-doped tellurite glasses: Yb as enhancer and quencher
(2009) Y. Dwivedi; Anita Rai; S.B. Rai
Energy transfer has been studied from Er3+ to Eu3+ ions on excitation with NIR photons (796 and 980 nm) with and without Yb3+ ions. It is found that in one case the presence of Yb3+ enhances the fluorescence yield (980 nm excitation) whereas in the other case it quenches (796 nm excitation). Energy transfer from Er3+ ion's levels 4S3/2 and 2H11/2 is verified by decay curve analysis in both the cases. The nature of interaction between the donor (Er) and the acceptor (Eu) ions is found to be dipole-dipole. The energy transfer parameters viz. transfer probability, critical distance etc. have been calculated. © 2009 Elsevier B.V. All rights reserved.
Enhanced luminescence and energy transfer study in Tb:Sm codoped lead fluorotellurite glass
(2014) A. Bahadur; Y. Dwivedi; S.B. Rai
Spectroscopic properties of Sm3+, Tb3+ doped and Tb3+:Sm3+ co-doped lead fluorotellurite glasses have been studied in detail. On the excitation with 355 and 532 nm laser wavelengths, the luminescence properties of the singly and doubly doped glasses have been analyzed. Intensity of characteristic emission bands due to Sm3+ ions is appreciably enhanced in the presence of Tb3+ ions. This is due to the energy transfer from Tb3+ to Sm3+ ions. Different energy transfer parameters have also been calculated, which affirm an efficient energy transfer from Tb3+ to Sm3+ ions. © 2013 Elsevier B.V. All rights reserved.
Enhancement of upconversion intensity in Er3+ doped tellurite glass in presence of Yb3+
(2011) D.K. Mohanty; V.K. Rai; Y. Dwivedi; S.B. Rai
TeO2-PbO glasses doped/codoped with Er3+/Er 3+-Yb3+ ions have been fabricated by melting and quenching method. Efficient frequency upconversion emissions spanning from blue to red regions corresponding to the 2H11/2, 4S 3/2→4I15/2 and 4F 9/2→4I15/2 transitions, respectively, upon excitation with 976 nm diode laser radiation have been observed. The variations observed in the intensity of whole upconversion emission spectra due to the presence of the Yb3+ ions are reported and discussed in detail. © 2011 Springer-Verlag.
Gd 3+ sensitized enhanced green luminescence in Gd:Tb(Sal) 3Phen complex in PVA
(2011) Gagandeep Kaur; Y. Dwivedi; S.B. Rai
Ternary and tertiary complexes of Tb(Sal) 3Phen and Gd:Tb(Sal) 3Phen were synthesized and characterized in PVA polymer. The structural properties of these systems were evaluated on the basis of NMR and FT-IR techniques. The absorption, excitation and emissive properties of the Tb 3+ ion were improved when coordinated with Sal and Phen ligands. Photoluminescence properties of the complexes in solution, crystals and dispersed in PVA film were explored in steady state and in time domain. Selective excitations (487, 355, 310 and 266 nm) of Tb 3+, Sal and Gd 3+ ions reveal an intramolecular energy transfer process. The emission of Gd:Tb(Sal) 3Phen complex in PVA indicates the contribution of Gd 3+ ion to enhance the emission intensity of Tb 3+ ion. On the basis of these investigations, photophysics was widely discussed in terms of energy transfer and encapsulation effect. © 2010 Springer Science+Business Media, LLC.
Green and NIR sensitized luminescent nanophosphor: Preparation, characterization and application
(2012) Gagandeep Kaur; Y. Dwivedi; A. Rai; S.B. Rai
Luminescent nanophosphor of Er, Yb: Gd 2O 3 has been synthesized by well known combustion synthesis. Along with strong UV-Vis upconversion emission, sensor for temperature and magnetic field, nano-heater etc., it can also be used as luminescent ink by dispersing the nanophosphor in aqueous polyvinyl alcohol solution. The stability of the ink has been found to depend strongly on the mixing process of the phosphor powder in the polymer solution. The X-ray diffraction results and TEM images with diffraction pattern show that the particles are in cubic phase of Gd 2O 3 with particles of average size <40 nm. SEM images show flaky structure which makes it suitable for dispersing in PVA and useful for luminescent ink purposes on NIR excitation. The FT-IR and the thermal analysis support the presence of PVA. Upconversion and downconversion luminescence has also been observed with 532 nm excitation and energy transfer mechanism has been explained. The NIR pumping gives strong UC emission bands in red and green regions extending up to extreme UV (240 nm) in this host. © 2012 Elsevier B.V. All rights reserved.
High yield multispectral upconversion emission in Y2Te4O11:Er:Yb nanocrystals on green and NIR excitations
(Royal Society of Chemistry, 2016) Y. Dwivedi; S.B. Rai
Detailed synthesis, structural, thermal and radiative properties of Er and Er:Yb codoped Y2Te4O11 nanocrystals were discussed. Nanocrystals were synthesized by a solid state method and reported to emit bright emissions in the wide range of blue-IR regions on excitation with 266 nm, 532 nm and 808 nm laser radiations via one and two photon absorption processes. We have reported a rarely observed ∼711 nm peak at 266 and 532 nm excitations. The emission intensity increases to a great extent in annealed samples and is explained on the basis of crystal structure. Judd-Ofelt theory has been employed to estimate several radiative parameters and the effect of structural modifications due to annealing. Emission intensity was found to reduce in the presence of Yb3+ ions which is suggested due to backward energy transfer from Er → Yb ions. Power dependence and time resolved spectroscopic techniques were used to explore excitation and emission dynamics. These studies clearly indicate promising and futuristic applications of such material for optical fibers and luminescent devices. © The Royal Society of Chemistry 2016.
Intense white upconversion emission in Pr/Er/Yb codoped tellurite glass
(2008) S.B. Rai; Y. Dwivedi; Anita Ray
Near infrared sensitized white light emitting glass has been developed and demonstrated. The white light comes from a combination of radiations emitted from triple rare earth ions (Er, Pr, and Yb) doped in tellurite glass. White light emission is possible by suitably mixing the red-green-blue (RGB) intensities emitted from these rare earth ions. The 980 nm radiation sensitized Yb3+ ions transfer their excitation energy to the Er3+ and Pr3+ ions to promote them to the upper states. The green and red emissions come mainly from Er3+ ion whereas blue emission from Yb3+ and Pr3+ ions. The RGB intensities could be adjusted by tuning the rare earth ion concentration. The optical quality of white light was adjudged using the Commission Internationale de l' Eclairage coordinates by the rare earth ion concentration and incident power density. © 2008 American Institute of Physics.
Laser induced breakdown spectroscopy diagnosis of rare earth doped optical glasses
(OSA - The Optical Society, 2010) Y. Dwivedi; S.N. Thakur; S.B. Rai
In the present work, rare earth (Nd, Eu, Er, Ho) doped oxyfluoroborate glasses were studied using laser induced breakdown spectroscopy (LIBS) technique. It has been observed that rare earth elements other than the doped one also reveal their presence in the spectrum. In addition the spectral lines of elements constituting the glass matrix have also been observed. Different plasma parameters such as plasma temperature and electron density have been estimated. It is concluded that the LIBS is a potential technique to identify simultaneously the light elements (B, O, F) as well as the heavy elements (Fe, Ba, Ca, Eu, Nd, Ho, Er) present in optical glasses. © 2010 Optical Society of America.
Multicolour emission from surface crystallized Sm:Yb codoped spherulites rooted in borate matrix
(Elsevier B.V., 2018) Y. Dwivedi; S.B. Rai
This article reports on synthesis and spectroscopic characterizations of surface crystallized BaB4O7 nanocrystals doped with Sm and Sm:Yb rooted in borate glass which forms microsized 'spherulites’ while annealing for more than 06 h. Detailed nano and microstructural analysis were done using transmission, scanning electron microscopic and Polaroid optical microscope images; and XRD techniques which revealed the formation of cone - shaped BaB4O7 spherulites. Images under polarized light revealed birefringence property in tiny crystals rooted in glass- ceramic. A bright red dominated orange-red emission was observed under UV (266 nm) laser excitations. Emission intensity was found to be 08 multiple, in ceramics, then the glass counterpart. Formation of tiny crystals reinforces sharp emission peaks (FWHM ~ 1.6 nm) due to Sm3+ ions. We have confirmed formation of Yb2+, Yb3+ and Sm3+ ionic species in the lattice which leads infrared cascade downconversion emissions spanning in the wide range of 380–1000 nm through involvement of intervalence charge transfer from Yb2+ to Yb3+ ions. © 2017 Elsevier B.V.
Observation of avalanche upconversion emission in Pr:Y 2O 3 nanocrystals on excitation with 532 nm radiation
(2012) K. Mishra; Y. Dwivedi; S.B. Rai
In this paper we report the two photon upconversion process in nanosize Pr:Y 2O 3 crystals on excitation with 532 nm photon at room temperature. Upconversion emission intensity shows an abrupt change at the critical pump threshold. On the basis of the temporal evolution of the intermediate state population and power-dependence studies, it is concluded that the significant variation in upconversion emission intensity is due to the photon avalanche process. © Springer-Verlag 2011.
Optical avalanche in Ho:Yb:Gd2O3 nanocrystals
(2011) Y. Dwivedi; A. Bahadur; S.B. Rai
Intense green upconversion emission in Ho:Yb:Gd2O3 spherical nanoparticles is reported through hetrolooping enhanced energy transfer process. Different techniques used for structural characterization divulge the formation of spherical Ho:Yb:Gd2O3 crystals of diameter ∼100 nm. Efficient energy transfer from Yb3+ → Ho3+ ions set spherical nanocrystal to emit in the range of UV-NIR via three NIR photons absorption process. Power dependence and the temporal evolution of upconversion emission intensity suggest the occurrence of a photon avalanche process. The enhancement in emission intensity in nanophosphor sample has been explained and the photophysics involved is correlated with the unique structural properties of the crystallites formed and the time resolved spectroscopy. © 2011 American Institute of Physics.
Optical properties of cerium doped oxyfluoroborate glass
(2013) A. Bahadur; Y. Dwivedi; S.B. Rai
Cerium doped oxyfluoroborate glasses have been prepared and its spectroscopic properties have been discussed. It is found that the absorption edge shifts towards the lower energy side for the higher concentration of cerium dopant. Optical band gap for these glasses have been calculated and it is found that the number of non-bridging oxygen increases with cerium content. The emission spectra of these glasses have been recorded using UV laser radiations (266 and 355 nm) and it is observed that these glasses show bright blue emission. On the basis of excitation and emission spectra we have reported the existence of at least two different emission centers of Ce3+ions. © 2013 Elsevier B.V. All rights reserved.
Optical properties of Eu3+ in oxyfluoroborate glass and its nanocrystalline glass
(Elsevier, 2008) Y. Dwivedi; S.B. Rai
Optical properties of Eu3+ doped oxyfluoroborate glasses containing different concentrations of BaF2 have been studied. The emission is found to be maximum at 20 mol% of BaF2. Transparent oxyfluoroborate glass ceramic with 15-20% volume fraction of fluoride nanocrystals are prepared from the Eu3+ doped glass using heat treatment and the optical properties of Eu3+ ions in such a glass ceramic have been studied and compared with those of the original glasses. Luminescence and absorption measurements reveal higher cross-sections in the glass ceramic. Addition of Bi2O3 during glass formation, with a fixed concentration of BaF2 content, increases the stability of the glass and heat treatment of such a glass leads to increase in its luminescence intensity. © 2008 Elsevier B.V. All rights reserved.
Overtone spectroscopy of chlorine substituted benzene derivatives
(2009) Y. Dwivedi; S.N. Thakur; S.B. Rai
The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400-20,000 cm-1. A large number of bands associated with the fundamental, the overtones and the combination frequencies of C{single bond}H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C{single bond}H stretch vibrations for the six molecules have been determined using local mode model. The C{single bond}H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6-31 + G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C{single bond}H stretching mode and the C{single bond}H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms. © 2008.
Photon avalanche upconversion emission in Ho: Gd2O3 nanophosphor
(2011) Y. Dwivedi; S.B. Rai
This paper report, the structural and spectroscopic properties of Gd 2O3 nano-phosphor activated with Ho3+ ions. XRD, SEM and TEM techniques have been used for the structural characterizations. The phosphor contains spherical nanocrystallites with uniform size of ∼40 nm. Enhanced IR to green and red photon avalanche upconversion is reported under 976 nm laser excitations. Further, annealing shows a significant improvement in emission intensity. Temporal evolution of upconversion emission intensity were measured which suggests the photon-avalanche mechanism is responsible for the upconversion process. The enhancement of emission was explained and the photo-physics involved is correlated with the unique structural properties of the crystallites formed. © 2011 World Scientific Publishing Company.
Preparation and characterization of Tb3 and Tb(sal) 3·nH2O doped PC:PMMA blend
(2011) Y. Dwivedi; A.K. Singh; Rajiv Prakash; S.B. Rai
Tb doped polycarbonate:poly(methyl methacrylate) (TbPC:PMMA) blend was prepared with varying proportions of PC and PMMA. Thermal and spectroscopic properties of the doped polymer have been investigated employing Fourier Transform Infrared (FTIR) absorption and differential scanning calorimetric (DSC) techniques. PC:PMMA blend (with 10 wt% PC and 90 wt% PMMA) shows better miscibility. Optical properties of the dopant Tb3 ions have been investigated using UVvis absorption and fluorescence excited by 355 nm radiation. It is seen that luminescence intensity of Tb3 ion depends on PC:PMMA ratio and on Tb3 ion concentration. Concentration quenching is seen for TbCl3·6H2O concentration larger than 4 wt%. Addition of salicylic acid to the polymer blend increases the luminescence from Tb3 ions. Luminescence decay curve analysis affirms the non-radiative energy transfer from salicylic acid to Tb3 ions, which is identified as the reason behind this enhancement. © 2011 Elsevier B.V. All rights reserved.
Spectral characteristics of intense red luminescence in Pr:Y 2O3 nanophosphor on UV excitation
(2012) Kavita Mishra; Y. Dwivedi; A. Rai; S.B. Rai
Detailed structural and spectroscopic characterizations of Pr:Y 2O3 nanophosphor have been carried out. On ultraviolet light (at 266 nm) excitation, intense red emission along with weak emissions in blue, green and infrared regions have been observed. The phenomena for the observed emission have been thoroughly explained in the present work and it was finally concluded that the direct excitation through low-lying 4f15d band of Pr3+ ions and through the conduction band of Y 2O3 nanophosphor are the main channels for the observed emissions. The calculated quantum efficiency (QE) for the red emission is found to be ~80 % while the CIE coordinates are (0.65, 0.34) quite close to the standard values. These studies clearly indicate that Pr:Y2O 3 nanophosphors are promising and futuristic red luminescent material for the development of various display devices and UV detectors. Time resolved spectroscopy has been further used to investigate luminescence dynamics. © 2012 Springer-Verlag Berlin Heidelberg.