Manjulata Sahu

Intense red emitting monoclinic LaPO4:Eu3+ nanoparticles: host–dopant energy transfer dynamics and photoluminescence properties

LaPO4 nanoparticles were synthesized using complex polymerization method and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Varied concentration of europium ion is doped in the LaPO4 lattice and optical properties and Judd–Ofelt analysis were investigated. It is observed that LaPO4 nanoparticles give violet–blue emission when irradiated with UV light. On doping europium ion the band gap of LaPO4 decreases. Based on DFT calculations it is proposed that energy range over which d and f states of Eu are distributed is coincident with the valence band of LaPO4 so causing an efficient energy transfer from LaPO4 to europium ion. The actual site symmetry for europium ion in lanthanum orthophosphate was also evaluated as D2d based on the Stark splitting pattern although it is C1 for La3+ in LaPO4. The critical energy-transfer distance for the Eu3+ ions was evaluated, based on which the quenching mechanism was verified to be an electric multipolar interaction. It is also observed that the red emission intensity of LaPO4:Eu3+ (2.0 mol%) is almost 85% of a commercial red phosphor, which clearly demonstrates that the as-prepared samples are promising red phosphors under near-UV for use in white-light emitting diodes.

Investigation of free volume characteristics of the interfacial layer in poly(methyl methacrylate)–alumina nanocomposite and its role in thermal behaviour

The free volume characteristics (free volume size, density and size distribution) of the interfacial layer (IL) in PMMA–alumina (0.3 wt%) nanocomposites were investigated using the well-established positron annihilation lifetime spectroscopy (PALS) technique. Differential scanning calorimetry (DSC) measurements were carried out to investigate the glass transition process of the PMMA matrix in these nanocomposites. PALS results show significant variations in the free volume structure of the nanocomposite, contrary to the assumptions made in earlier reports, i.e., free volume in PMMA remains unaffected on alumina loading. The study shows that as a result of strong interfacial interaction between PMMA and alumina, a denser region is created in the vicinity of alumina particles (interfacial layer), having smaller-sized nanoholes with narrower size distribution compared to bulk PMMA. This observation is consistent with the studies wherein an interfacial layer with constrained segmental motion was inferred from spectroscopic and thermal analysis techniques. The glass transition temperature, Tg, determined from DSC is observed to vary with alumina loading qualitatively similar to the variation in the fractional free volume of PMMA. The study indicates that the critical free volume for PMMA to undergo glass transition process depends on the interfacial layer fraction and hence varies as a function of alumina loading.

Solid state speciation of uranium and its local structure in Sr 2 CeO 4 using photoluminescence spectroscopy

An effort was taken to carry our speciation study of uranium ion in technologically important cerate host Sr2CeO4 using time resolved photoluminescence spectroscopy. Such studies are not relevant only to nuclear industry but can give rich insight into fundamentals of 5f electron chemistry in solid state systems. In this work both undoped and varied amount of uranium doped Sr2CeO4 compound is synthesized using complex polymerization method and is characterized systematically using X-ray diffraction (XRD), Raman spectroscopy, impedance spectroscopy and scanning electron microscopy (SEM). Both XRD and Raman spectroscopy confirmed the formation of pure Sr2CeO4 which has tendency to decompose peritectically to SrCeO3 and SrO at higher temperature. Uranium doping is confirmed by XRD. Uranium exhibits a rich chemistry owing to its variable oxidation state from +3 to +6. Each of them exhibits distinct luminescence properties either due to f-f transitions or ligand to metal charge transfer (LMCT). We have taken Sr2CeO4 as a model host lattice to understand the photophysical characteristics of uranium ion in it. Emission spectroscopy revealed the stabilization of uranium as U (VI) in the form of UO66- (octahedral uranate) in Sr2CeO4. Emission kinetics study reflects that uranate ions are not homogeneously distributed in Sr2CeO4 and it has two different environments due to its stabilization at both Sr2+ as well as Ce4+ site. The lifetime population analysis interestingly pinpointed that majority of uranate ion resided at Ce4+ site. The critical energy-transfer distance between the uranate ion was determined based on which the concentration quenching mechanism was attributed to electric multipolar interaction. These studies are very important in designing Sr2CeO4 based optoelectronic material as well exploring it for actinides studies.