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Materials that can depict persistent deep red light under both ultraviolet (UV) and X-ray illumination can be a boon to sustainable economy, particularly for optical imaging, solid state lighting, and anticounterfeiting applications.

In the field of photocatalysis, the suppression of electron–hole recombination through various defects has been an emerging trend to enhance photocatalytic activity.

Defect engineering to tune the light emission properties has been fetching lot of interest among scientific community for designing efficient optoelectronics and light-emitting diodes. Defect evolution in the trivalent ion-doped CeO2 and their dependence on the ionic radii of the trivalent dopant were probed using Rietveld refined powder X-ray diffraction, positron annihilation lifetime and photoluminescence spectroscopy. The photoluminescence of Eu3+, a known structural probe, is monitored with codoping of two other trivalent ions Y3+ and La3+ whose ionic radii are on either side of Eu3+. The studies showed that smaller dopants like Y3+ create oxygen vacancies that are closely associated with Y3+ and codoping does not cause significant enhancement in asymmetry in the emission from Eu3+. At the same time, vacancies created by La3+ are not strongly associated with La3+, and due to their stronger association with Eu3+, the asymmetry in the emission is enhanced. The study shows the usefulness of emission from Eu3+ as probe in understanding the effect of different trivalent dopants using them as codopants to Eu3+. The studies are expected to help in tuning the oxygen vacancy concentration as well as their migration in CeO2 for technological applications such as catalysis, magnetism and conductors. This work highlighted role of defect engineering in tailoring the optical properties of red emitting CeO2:Eu3+ phosphor via aliovalent and contrasting Y3+ and La3+ co-dopant.

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO3 exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO3 in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu3+ ions stabilize at both Ba2+ and Sn4+ sites consistent with the highly negative formation energy of around -6.26 eV. Eu3+ doping leads to an intense 5D0→7F1 orange emission and a feeble 5D0→7F2 red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of EuBa and EuSn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn2+ in the PL of BaSnO3 or Eu2+ in the PL of BaSnO3:Eu3+. Interestingly, when co-doped, Eu3+ stabilizes at Sn4+ sites whereas La3+ stabilizes at Ba2+ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La3+ ions lead to lowering of symmetry around Eu3+ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO3 in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO3 and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO3 is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

Understanding the intricacies and fundamental processes of materials involved in dopant- and defect-based luminescence is of paramount importance for scientists and engineers working towards the design of solid-state lighting, optoelectronics, scintillators, etc. The lack of such fundamental information has restricted the design of new oxide-based materials for light-emitting devices and phosphor-converted materials. Here, we have designed Er2Ti2O7 (ETO), Er2Zr2O7 (EZO), Eu3+-doped ETO (EETO), and Eu3+-doped EZO (EEZO) via high-energy ball milling. Structural analysis using X-ray diffraction (XRD) and Raman spectroscopy suggested the stabilization of the ordered pyrochlore structure for ETO and EETO, whereas EZO and EEZO are stabilized in the defect fluorite structure. The ETO and EZO samples exhibited bright blue emission under ultraviolet irradiation. X-ray absorption near edge structure (XANES) analysis completely rules out contributions from Ti3+ or Zr3+ to the host emission via confirming that they exist as tetravalent ions. Extended X-ray absorption fine structure (EXAFS) analysis confirms the presence of a high density of oxygen vacancies (OVs) near the Ti and Zr sites, respectively, in ETO and EZO. The DFT-calculated charge transition levels qualitatively explain the origin of the blue emission of ETO and EZO with the dominant involvement of ionized oxygen vacancies. Positron annihilation lifetime spectroscopy (PALS) suggested that few changes in the defect density or type occurred upon europium doping in EETO. The defect concentration and type change significantly in EEZO with respect to EZO, which is of significant importance due to the possible agglomeration of vacancies into large-size defect clusters in EEZO. Surprisingly, in both hosts, red/orange narrow emission from europium (585–750 nm, 5D0 → 7FJ) was completely absent. Density of state (DOS) calculations suggested that a possible reason for this is that the Eu f-states are dominantly distributed around the bottom edge of the valence band (VB), far from the Fermi energy, electronic band gap, and top edge of the VB actively participating in the electronic transitions. Similarly, the Eu f-states are distributed around the top edge of the conduction band, far from the electronic band-gap region. We believe this work will be quite helpful for selecting suitable hosts and dopants, band gap engineering, and defect tuning in the pursuit of achieving efficient host-to-dopant energy transfer in Eu3+-doped pyrochlore materials.

Pair matching charge compensation with trivalent and pentavalent dopants in ceria was found to be an attractive strategy in engineering defects with minimal distortions in the lattice and obtaining enhanced catalytic properties. In the present study, charge compensation with a vanadium codopant in trivalent ion doped ceria is studied. Defect evolution in the trivalent ion doped ceria with vanadium codoping has been studied in CeO2:Eu3+, CeO2:La3+,Eu3+ and CeO2:Y3+,Eu3+ systems and the choices of the dopant and co-dopant are triggered by their ionic radius. Eu3+ photoluminescence (PL) is used as a spectroscopic probe to monitor local structural changes around the dopants. Positron lifetime studies showed that oxygen vacancies formed due to trivalent ion doping are weakly associated when larger ions are doped and result in the formation of vacancy aggregates. Positron lifetime studies along with XRD studies show that vanadium codoping effectively removes the vacancies but the distortions are significant when the size mismatch between the pair match used for charge compensation is higher. Photoluminescence demonstrated that the oxygen vacancies associated with Eu are more effectively removed in the case of Y codoped samples. Electron Spin Resonance (ESR) studies suggested that vanadium in excess over the stoichiometric concentration of the trivalent ion can lead to additional defects. These studies are expected to help in tuning the vacancy concentrations as well as controlling the lattice distortions for technological applications such as catalysis, ionic conductivity, etc.

We experimentally show that nanoparticles (NPs) can significantly regulate the network topological defects during a molecularly controlled elastomeric synthesis. Using positron annihilation lifetime spectroscopy, we demonstrate this on well-defined model systems of poly(dimethyl siloxane) elastomers and layered silicate nanoparticles (NPs). The evolutions of topological defects in elastomeric networks prepared from unimodal, bimodal, and NP dispersed bimodal elastomers are sequentially investigated. The extent of NP induced defect regulation is identified by varying the particle concentration from moderately low to an approximate upper limit. The fraction of free volume hole defects present between packed chains in the network generated by molecular control is significantly reduced. The fraction of smaller interstitial cavities near the cross-link sites shows a moderate increase at the lowest NP concentration. However, this fraction decreases at a high NP concentration and is nearly the same as that of bimodal networks that are devoid of NP infusion. Despite the variations in their fractions with NP infusion, the sizes of both these types of defects that remain in the network are minimally affected. The collective topological defects arising from chain induced heterogeneity also show a qualitative reduction upon NP infusion.

Defects evolved during thermal treatment in high Cr content Ni–Cr alloy have been studied using Positron lifetime and coincidence Doppler broadening spectroscopy complemented with microscopic technique. The single-phase gamma-quenched specimen obtained by solutionization at 1200 °C has been thermally aged at 650 °C and 800 °C. Thermal treatment resulted in the phase transformation of the matrix through precipitation of Cr. It is seen that solutionization treatment followed by quenching resulted in the stabilization of thermal vacancies generated at 1200 °C which effectively trap the positrons. The results infer that these vacancies may form vacancy-Cr complexes which act as nucleation site for Cr aggregation. Positron lifetime and momentum distribution of annihilation electrons indicated that in the aged alloys, positrons are trapped in vacancy-like defects (vacancy clusters/vacancy-solute complex) and the interface between Cr precipitate and matrix, the latter becoming more predominant with subsequent ageing. At the highest ageing studied, vacancy-like defects are evolved within the Cr precipitates. The results indicated that positron trapping sites (vacancies and precipitate interface) are not specific to either Ni or Cr atoms which surround the trapping site in nearly equal proportion. Therefore, unambiguous identification of the defect type at each stage of thermal treatment becomes difficult. Considering positron trapping at the surface of precipitates, we have calculated positron specific trapping coefficient to be ~ 1.33 × 10−6 cm3 s−1, which is of the order estimated for large voids and estimated the number density of precipitates.

Designing a bright blue and red phosphor for phosphor-converted white-light-emitting diodes would be extremely beneficial to the lighting industry. Achieving the same through defect and doping engineering in an interesting inorganic layered structure material, Ca2SnO4 (CSO), would fetch a lot of interest among material and optical scientists. There is no report on undoped and doped CSO lattices where extensive effort has been made to understand the origin of host emission, local site occupancy, defect evolution, etc. With the same intention, in this work, we have investigated the photophysical properties of undoped and Eu3+-doped CSO (CSOE). On UV irradiation, CSO displayed beautiful blue emission, and its origin has been explained experimentally using extended X-ray absorption fine structure (EXAFS) and theoretically using density functional theory (DFT) calculations. Both DFT and EXFAS pinpointed the role of oxygen vacancies (OVs) in blue emission of CSO. In fact, by extensive calculations, we also found out that both neutral and charged oxygen vacancies are involved in producing blue emission in CSO. To further tune the luminescence, the Eu3+ ion was doped in CSO, which produces highly pure red emission with an internal quantum efficiency (IQE) of ∼80% and negligible nonradiative transition. Based on EXAFS and lifetime spectroscopy, it was found that the Eu3+ ion is localized in a much higher fraction at the CaO7 site compared to SnO6 octahedra. This was further substantiated using DFT-calculated formation energies. Further, it is expected that aliovalent doping of the trivalent Eu3+ ion at Ca2+/Sn4+ leads to charge-compensating defects, which significantly impact the optical properties of CSOE, and were further investigated using positron annihilation lifetime spectroscopy (PALS). Such complete spectrum of work wherein photophysical properties of functional materials are optimized and studied by deciphering the role of OVs, local site occupancy of the dopant ion, and charge-compensating defects will inculcate a lot of interest within the scientific community in coming forward for research in the area of rare-earth-free phosphor as well as doped phosphor for solving the energy crisis.

Structure, size, and defect manipulations have been found to be highly effective strategies in tuning the properties of optical functional materials. To investigate the same, nanocrystalline and bulk counterparts of Eu3+-doped scheelite BaWxMo1–xO4 solid solutions were synthesized and their optical properties were investigated. Positron annihilation lifetimes were found to be larger in the as-prepared nanocrystalline samples compared to the bulk, suggesting more defects in the nanocrystalline samples. The positron lifetimes increased on Eu3+ doping, suggesting the creation of more defects, with possibilities being W/Mo occupying Ba2+ sites or the co-inclusion of Na+ for charge compensation. The inclusion of Mo was found to inhibit host-to-dopant (Eu3+) energy transfer (HDET) in bulk samples, but very efficient HDET was seen in nanocrystalline samples. A higher degree of asymmetry around the Eu3+ in the nanocrystalline samples compared to the bulk samples was observed, attributed to the large defect density as well as the relaxations around the defects in the former, and this is also manifested as a lower unit cell volume. It was found that doped nanocrystalline scheelites of BaWxMo1–xO4 can be used as red phosphor, while the bulk sample can be used as a color-tunable phosphor in red, orange, yellow, and near white luminescence applications by varying the composition. This work really projects the importance of defect and doping manipulation on the optical properties of undoped and doped, pure and mixed tungstate–molybdate systems.

This work presents a new perspective on alkali metal co-doped rare earth based-phosphors in understanding the distinct role of 3 different alkali metal co-dopants, Li+, Na+ and K+, with the excitation of host and rare earth dopant. The system under investigation is the technologically important Scheelite host and NIR-emitting Nd3+ ion. The Li+ ion was found to improve the crystallinity and reduce the symmetry more efficiently than act as a sensitizer in aiding the host to dopant energy transfer, which results in more emission output from the Nd3+ excitation in comparison to the host. We could also successfully compare the optical output of CaWO4:Nd3+ and SrWO4:Nd3+ Scheelite with and without alkali metal ions in terms of the peak shift and intensity by considering the greater lattice dimensions of the latter when compared to the former. For the larger SrWO4:Nd3+ lattice, K+ and Na+ were found to be better co-dopants in enhancing the NIR emission under the host and Nd3+ excitation, respectively, compared to the smaller Li+ ion. The smaller unit cell dimensions of CaWO4 might facilitate more efficient energy and relaxation processes, making its excited state lifetime shorter in comparison to that of doped SrWO4. For alkali ion co-doping, a higher PL lifetime was observed for Li doping in the case of CaWO4 and Na co-doping in the case of SrWO4 under Nd3+ excitation, which is well in line with the NIR emission spectroscopy. Positron annihilation lifetime spectroscopy suggested the formation of cation vacancies and other associated vacancy clusters in the Nd3+ aliovalent doped CaWO4. Li+ does not act as a charge compensator for the removal of cation vacancies created by Nd3+ substitution in the Ca lattice. On the other hand, Na+ and K+ act as good charge compensators in Nd3+ doping in terms of vacancy removal. This work is highly relevant in understanding the role of alkali co-doping in both the luminescence properties and formation of defects to produce efficient Scheelite-based NIR phosphors.

Materials with optical multifunctionality such as photoluminescence (PL), radioluminescence, and thermoluminescence (TL) are a boon for a sustainable society. BaHfO

Compression-induced (≤1.01 GPa) phase transformation, local deformations, and pore architecture modifications in zeolitic-imidazole framework (ZIF-8) powder have been investigated, ex situ, using X-ray diffraction, extended X-ray absorption fine structure, Fourier transform infrared, and positron annihilation lifetime spectroscopy. Compression-induced reduction in crystallinity leading to partial amorphization of ZIF-8 is observed to be an irreversible phenomenon. The amorphization of ZIF-8 was accompanied by local deformations in imidazolate ring bonding. The study has confirmed the retention of Zn–N tetrahedral arrangement in the amorphous state of the framework through X-ray absorption measurements. The phase transformation and local deformations lead to modifications in powder morphology as well as reduction in internal porosity of the framework. The triplet state of positronium (o-Ps) has been used to investigate the pressure-induced pore architecture modifications. At the lowest applied pressure (0....

Designing new materials for solid state lighting and understanding the various intricacies involved for designing them such as defects, energy transfer and concentration quenching is very important. Such materials will be highly beneficial in optoelectronics, energy and health industry. In this work, an effort has been taken in that direction by exploring room temperature synthesized Dy3+ doped β-Ag2MoO4 using simple co-precipitation method under neutral conditions. Pure β-Ag2MoO4 showed blue – green emission upon shining with UV light. On doping Dy3+an efficient host-dopant energy transfer takes place. The concentration quenching study revealed non-radiative energy transfer in Dy3+ doped β-Ag2MoO4 takes place via Dexter mechanism of exchange interaction. Additionally, on doping Dy3+ ions in the β-Ag2MoO4 a multicolour emission could be observed due to presence of blue, yellow and red bands induced by host sensitized energy transfer. Positron lifetime studies show that the Dy3+ doping creates cation vacancies. Positron lifetimes and asymmetry ratios in PL emission show that Dy3+stabilizes at Ag+ sites. Photoluminescence Lifetime Spectroscopy revealed non-homogenous distribution of Dy3+ ions and its surroundings differ in terms of their vicinity with respect to defects created due to aliovalent doping. Such complete spectrum of work on concentration quenching study, UV excited photoluminescence, defect spectroscopy, local structure of dopant ion and excited state lifetime indicate that the developed phosphor may potentially be used for solid state phosphor for LED application. Future work will be seeing how the material properties change in going to nanodomain and to make it excitable using f-f band (351 nm). The only demerit of this phosphor is the poor absorption in this region.

Open volume defect profiles have been obtained in He, O and (He + O) ion implanted silicon crystal 〈1 0 0〉 using slow positron beam measurements. The p-type Si samples have been implanted with He (30 keV), O (120 keV) and both the ions [He (30 keV) + O(120 keV)] with the dose of 1 × 1015 and 1 × 1017 ions/cm2. The average range of these ions in Si is estimated ∼280 nm. The depth dependent defect profiles have also been studied after isochronal annealing (100–900 °C) of the ion implanted Si crystals. On annealing at temperatures (>500 °C), an increase (decrease) in S-parameter was observed in the He (O) ion implanted samples. The variations observed in the S-E profiles indicate that large size open volume defects are created in He irradiated Si followed by annealing at 700 °C. In the case of O and (He + O) implanted samples, variations in S-E and W-E profiles indicate the formation of oxygen-vacancy clusters, a buried silicon oxide layer or oxygen decorated cavities. The S-E (W-E) profiles have been fitted using a program ‘Variable Energy Positron fit’ to evaluate the characteristic S- and W- parameter values of different regions. It is observed through S-W correlation plots that He ions as well as vacancy defects diffuse out of the silicon crystal at higher annealing temperatures. Oxygen-vacancy cluster defects are identified in O implanted (1 × 1015 ions/cm2) and annealed at 700 and 900 °C. In case of O implanted (1 × 1017 ions/cm2) and co-implanted samples (1 × 1015 ions/cm2 and 1 × 1017 ions/cm2) annealed at 700 and 900 °C, formation of oxygen decorated cavities is proposed.

Poly(ethylene oxide), PEO, based nanocomposites are shown to have higher ionic conductivity compared to pure PEO. In order to investigate the exclusive role of free volume structure of amorphous region of PEO on its electrical conductivity, PEO-silica nanoparticles composite films have been prepared with different silica loadings. The free volume fraction of polymer composites measured using positron annihilation lifetime spectroscopy (PALS) was observed to increase nonlinearly with the increase in loading of silica. The increase in free volume on silica loading has been attributed to PEO-silica interphase as well as newly created amorphous PEO. At higher loading, positron annihilation from regions near silica particles has been confirmed by coincidence Doppler broadening measurements. Thermal analysis of polymer nanocomposites shows a decrease in crystallinity and crystallite melting temperature with the increase in silica loading. The PEO-silica nanocomposite films showed enhanced ac conductivity as well as mechanical properties (higher Young’s modulus and ductility). The observed changes in free volume structure and semicrystalline morphology of PEO matrix are attributed to the physical confinement imposed by presence of silica nanoparticles in PEO matrix. The present study shows that free volume measured using PALS has a direct relation with ac conductivity which is observed to increase drastically after a critical free volume fraction.

Investigation of microenvironment of mesopores in CuO loaded mesoporous materials is vital to predict its catalytic performance. Mesoporous MCM-41 supported CuO catalyst was prepared by a conventional solid state grinding method and characterized by different techniques to understand the microenvironment of mesopores. The samples were characterized using X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Transmission electron microscopy (TEM) and N2 gas adsorption techniques. XRD and TEM measurements have shown the formation of CuO nanocrystallites larger than mesopores size of MCM-41. SAXS and TEM also provide evidence of fine dispersion of CuO on mesopores' inner walls of MCM-41 along with the formation of large size crystallites outside the mesopores. With the loading of CuO, N2 adsorption measurements show a negligible change in pore size along with a continuous reduction in pore volume and surface area of MCM-41. Ortho-positronium (o-Ps) probe was used to investigate the atomic level dispersion of catalyst in MCM-41 pores. A complete absence of positronium formation in the mesopores of CuO loaded samples has been observed which has been unequivocally ascribed to the inhibition of positronium formation on the pores' surfaces. The results indicate that CuO is finely dispersed at atomic level on mesopores' inner walls leading to its layer by layer growth.

Doping of ceria with trivalent lanthanides and pentavalent ions is known to influence its catalytic activity, optical properties and ionic conductivity. In this study, (Nb,Eu)-codoped CeO2 has been synthesized by hydroxide precipitation to study the synergistic effect of (Nb, Eu) codoping on the defect evolution and optical properties of CeO2. Through X-ray diffraction, it was seen that Nb5+ has limited solubility in CeO2 which could be enhanced by codoping with Eu3+. From Rietveld refinement of the X-ray diffraction data, the ceria lattice seems to expand on Eu3+ doping which could be explained taking into account the formation of oxygen vacancies (OVs) in stoichiometric ratios. The observed lower than expected contraction in the Nb5+ doped samples is attributed to lattice expansion due to the creation of oxygen interstitials (OIs). PL studies in terms of the ratio of electric dipole transition (EDT) to magnetic dipole transition (MDT) intensities have shown that Eu3+ lacks inversion symmetry at Ce4+ which invokes the formation of OVs for charge compensation. Interestingly, Nb5+ co-doping restored the inversion symmetry around the Eu3+ site by eliminating the formed OVs. This is achieved by ion pair matching of the dopants. This work has great relevance in understanding the role of defects and co-doping in designing highly efficient catalysts, solid oxide fuel cells and doped phosphors.

Kinetic-energy spectra and angular distributions of the projectile-like fragments were measured in $^{12,13}\mathrm{C}+^{93}\mathrm{Nb}$ reactions at ${E}_{\mathrm{lab}}=65\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ to investigate the reaction mechanisms involving the role of projectile and target structures. It has been observed that one-nucleon pick-up reactions lead to the formation of target-like fragments predominantly in the ground state, whereas nucleon transfer to the target occurs predominantly in the excited states for both $^{12}\mathrm{C}+^{93}\mathrm{Nb}$ and $^{13}\mathrm{C}+^{93}\mathrm{Nb}$ reactions. In the case of $^{93}\mathrm{Nb}(^{12}\mathrm{C}, ^{15}\mathrm{N})^{90}\mathrm{Zr}$ reactions, a significant yield was observed for $^{15}\mathrm{N}$ (``$1p+2n$'' pick-up) indicating the role of the $N=50$ shell in $^{90}\mathrm{Zr}$. Observation of a systematic increase in the forward peaking of the angular distributions of projectile-like fragments with an increasing number of nucleon transfers indicates an increase in the projectile-target overlap with increasing mass transfer. Angular distributions for inelastic scattering as well as transfer channels populating various states of projectile-like fragments and/or target-like fragments were calculated simultaneously by using the coupled reaction channels code fresco and were in reasonable agreement with the experimental data. New (modified) spectroscopic amplitudes have been proposed for some of the unknown (known) overlapping states for calculating transfer cross sections. In the case of $^{13}\mathrm{C}+^{93}\mathrm{Nb}$, the transfer channels corresponding to ``$2n$'' and ``$1p+1n$'' pick-up channels were not observed. This may be due to the higher N/Z ratio of the projectile, highlighting the importance of projectile structure in addition to that of the target.

Design considerations of the intense positron facility being setup at Research reactor at Bhabha Atomic Research Center is presented here. A thin sheet of cadmium will convert the intense thermal neutron flux present near reactor core to a high energy gamma ray flux. These gamma rays will cause electron positron pair production in tungsten mesh assembly which are placed close by. These convertor tungsten mesh assembly are designed to have high surface area and they will act as moderators too thus emitting the slow positrons. GEANT and FLUKA Monte Carlo codes are used to optimize the design parameters of the cadmium sheet and the tungsten mesh assembly. A final intensity close to 2×109 e+/sec is predicted for this facility

Multicolour excitable scheelite type CaWO4:Eu3+ (CEW) and SrWO4:Eu3+ (SEW) nanophosphors were synthesized using high energy ball milling in nano-domain. The synthesized nanophosphors were characterized extensively using Rietveld analysis of X-ray diffraction data, Raman spectroscopy, electron microscopy, energy dispersive spectroscopy and selected area electron diffraction. Differences in optical characteristics of CEW and SEW are probed using time resolved photoluminescence spectroscopy. It was observed that aliovalent doping of Eu3+ in CaWO4 and SrWO4 at divalent Ca/Sr sites leads to formation of cation vacancies as confirmed using positron annihilation lifetime spectroscopy (PALS). PALS also revealed that defect density in SEW is more than CEW which degrades its optical performance by aiding in non-radiative relaxation. The close proximity in ionic radius of Eu3+ and Ca2+ ions leads to better structural homogeneity in case of CEW and less lattice distortion which is reflected in its high emission output and quantum yield. Stark splitting analyses revealed reduction in symmetry around europium ion in SEW as compared to CEW. This is reflected in high asymmetry ratio (EDT/MDT) in case of SEW which directly translates into its high luminescence lifetime. Lifetime spectroscopy also revealed that in both the samples europium ion is localized at two different environments; one closer to defect centre and one is far off. Our study indicates that both these nanophosphors have unique ability to get excited intensely by UV as well as visible light and emit very intense red light which can be explored in phosphor converted white LEDs. Judd–Ofelt analysis indicates better optical performance of CEW as compared to SEW phosphor.

Local structure analysis of dopant ion, understanding host to dopant energy transfer dynamics and defects characterization in a doped material which plays an important role in the designing a highly efficient opto-electronic material. In this connection a new Sm3+ doped Gd2Zr2O7 pyrochlore material was synthesized using gel-combustion technique and was characterized systematically using X-ray diffraction (XRD), time resolved photoluminescence spectroscopy (TRPLS), positron annihilation lifetime spectroscopy (PALS) and density functional theory (DFT) based ab-initio calculation. Based on DFT site selective energetics calculation and luminescence decay measurement, it was observed that the Sm3+ was distributed at both Gd3+ and Zr4+ site with higher Sm3+ fraction at the Gd3+ site. PALS was used to probe the presence of defects in the phosphor. In this work intense orange-red emission is realized through manipulating the energy transfer from host defect emission (oxygen vacancies) to Sm3+ which allows color emission from green in undoped to orange-red in doped samples. Effect of dopant concentration and annealing temperature was probed using TRPLS and PALS. These all information is highly important for researcher looking to achieve pyrochlore based phosphor materials with high quantum yield.

Studies have been carried out to understand the specific role of the alkali charge compensator on the luminescence properties of an alkali ion (Li+, Na+, and K+) codoped SrWO4:Eu phosphor. The oxid...

Uranium and cerium doped yttrium titanate pyrochlores Y2-xAxTi2O7±δ (A = U or Ce) were synthesized by gel combustion method. The Rietveld analysis of these pyrochlores showed that due to the large charge heterogeneity, uranium cannot be incorporated to a large extent at the intended A site of the pyrochlore in air atmosphere causing separation into brannerite phase containing yttrium and yttria stabilized UO2+x. Studies also showed that more than 10 mole% U could be accommodated at A site when reducing conditions were used. Unlike uranium, 15 mole% of Ce could be doped at A site as a single phase in air atmosphere. The observations in the crystal chemistry of these pyrochlores were confirmed by XRD, SEM, Raman and XPS. The gamma irradiation of these pyrochlores showed good radiation stability which was confirmed by XRD and PALS. PALS also confirmed uranium doping is preferably taking place at B site in oxidizing conditions. The thermo-physical properties of these pyrochlores were evaluated by HT-XRD and DSC.

Nanocrystalline (n-SWO) and microcrystalline (b-SWO) samples of SrWO4 were prepared using polyol and solid state diffusion method, respectively and their crystallite sizes were estimated by powder X-ray diffraction employing Williamson-Hall method. Morphostructural characterization using transmission electron microscopy (TEM) revealed the increased thermal agglomeration and size enhancement in b-SWO particles relative to n-SWO. The photoluminescence emission in both the samples was broad in the range of 400–650 nm. The emission consisted mainly of a narrow peak in the blue region and a broader emission in green region. The fraction of emission in the green region was higher for nanocrystalline SrWO4 than its microcrystalline counterpart. The higher intensity of green emission in nanocrystalline sample is attributed to the large concentration of the oxygen related vacancies.

A series of poly(arylene-ethynylene)s (PAEs) containing pentiptycene and tbutylpyrene or pyrene in the main chain were synthesized. Introduction of pyrene moiety in the pentiptycene polymeric backbone made the polymers extremely sensitive and selective towards TNT in aqueous solution as well as vapour phase as compared to the other interfering nitroaromatic compounds (NACs) like DNT and PA. In solid state, one of the polymer P2 showed superior sensitivity towards TNT vapours with 60.4% and 90% fluorescence loss within respective 10 s and 30 s of exposure to TNT vapours, which is much higher than most of the reported NACs sensors. Even towards aqueous NACs, the performance of P2 is the best, with 69.8% fluorescence of P2 film getting quenched within 60 s of immersion in aqueous solution of TNT (50 μM). Fluorescence regain study showed that 94% fluorescence is re-gained after dipping the fluorescence-quenched polymer film in methanol for 20 s. The polymers were found to possess high thermal stability (282–585 °C) and appreciably high fluorescence quantum yield (0.39–0.65). We also report the sub-nanoscopic free volume and its distribution in the polymers, as quantified by positron annihilation life time spectroscopy (PALS).

CeO 2 nanoparticles have been prepared by gel combustion method. The as-prepared nanoparticles were calcined at 500, 550 and 600 °C. The crystallite size of the nanoparticles has been determined using X-ray diffraction. Catalytic activity has been studied by measuring the Tafel slope in oxygen reduction reaction in cyclic voltammetery. The nanoparticles samples showed higher catalytic activity than bulk ceria sample. Surprisingly, smaller size nanoparticles with large surface area showed less catalytic activity than larger size nanoparticles. Positron annihilation, X-ray photo electron spectroscopy and photoluminescence studies indicated the presence of oxygen vacancies as well as larger surface defects. It has been found that surface defect concentration increased with the increase in calcination temperature and the catalytic activity of the nanoparticles is directly correlated to the surface defect concentrations.

The understanding of atomic-scale structure is a prerequisite for establishing the physico–chemical behavior of complex glass systems. To this end, positron annihilation life time spectroscopy (PALS) is an atomic scale probe capable of investigating the subnanoscopic free volume of amorphous materials. In the present work, PALS has been used to quantify the free volume changes as a function of increasing substitution of B2O3 by Al2O3 in strontium borosilicate glasses intended to be used as sealant in solid oxide fuel cells. The free volume parameters; ortho-positronium (o-Ps) life time (τ 3) and intensity (I 3) show composition dependant variations which are correlated to the molar volume and compactness of the glasses through a commutative free volume parameter, $$~\tau _{3}^{3}{I_3}$$ . The effect of change in nanoscopic free volume induced by the substitution of B2O3 by Al2O3 on the glass transition temperature (T g), softening temperature (T s), coefficient of thermal expansion (CTE) and thermal stability of glasses have been studied. A remarkable trend-based variation in these macroscopic properties with change in free volume is observed. We envisage that the findings of this work will provide new insights in establishing subnanoscopic structure and thermo-physical property correlation of complex glass systems containing multiple network former and modifiers.

Phase purity, optical properties and defects in new near white light emitting rare earth free Sr2V2O7:Bi3+phosphors were investigated. Emission spectrum of Sr2V2O7 displayed two peaks at all excitation wavelength approximately around 450 (P1) and 600nm (P2). With increase in the excitation wavelength; emission intensity increases and there is a red and blue shift in Peak P1 and P2, respectively. On bismuth doping, the two Peaks P1 and P2 slowly starts merging and become a single broad peak at 5% doping. Bismuth doping also reduces the emission peak energy. This is attributed to substitution of Bi3+ at Sr2+ site which would distort VO4 tetrahedra, thereby reducing the energy difference between the 3T and 1A levels of VO43− distorted tetrahedral. This dual role of bismuth doping and excitation energy leads to interesting tunability in white emission from cool to warm white. This opens a new arena in phosphor research because of great role of white LEDs (both warm and cool) in outdoor and indoor lighting. Keywords: White light, Pyrovanadate, Photoluminescence, Positron annihilation, Phosphors, Tunable

Nanocrystalline ThO2:Sm3+ was synthesized using wet-chemical route and characterized using X-ray diffraction (XRD), photoacoustic (PA) and photoluminescence (PL) spectroscopy. PA absorptions of Sm3+ doped samples are found to be quite weak as compared to Nd3+, while PL of Sm3+ was intense. As the energy gap between lowest luminescent levels and highest non-luminescent level in samarium ion is around 7000 cm−1; it is highly fluorescing compared to Nd3+ which has close by levels. Through photoacoustic data it was pointed out that large covalent character exists in ThO2:Nd3+ compared to ThO2:Sm3+.

Based on photoluminescence spectroscopy it was inferred that uranium stabilizes as both U(IV) as well as U(VI) in Gd2Zr2O7 which was also corroborated using X-ray photo electron spectroscopy (XPS). Absence of equidistant vibronic structure in emission spectrum of Gd1.98U0.02Zr2O7 confirmed that U(VI) stabilizes in the form of UO66−. Based on luminescence lifetime it was inferred that majority of UO66− stabilizes at both Gd3+/Zr4+ whereas U4+ stabilizes only at Zr4+ sites. The positron lifetime doesn’t change on uranium doping indicating the formation of antisite defect. Infact it is this antisite defect in Gd1.98U0.02Zr2O7 which favours the stabilization of its fluorite phase.

In recent times, high-temperature polymer electrolyte membranes (HTPEMs) have emerged as viable alternatives to the Nafion-based low-temperature-operated polymer electrolyte membrane fuel cells. This is owing to their higher tolerance to fuel impurities, efficient water management, and higher cathode kinetics. However, the most efficacious HTPEMs such as poly(benzimidazole) (PBI) or 2,5-poly(benzimidazole) (ABPBI), which rely on the extent of phosphoric acid (PA) doping level for fuel cell performance, suffer from poor mechanical properties at higher acid doping levels and dopant leaching during continuous operation. To overcome these issues, we report the synthesis of ABPBI membranes and fabrication of ABPBI-zirconium pyrophosphate (α-ZrP)-based nanocomposite membranes by an ex situ methodology using methane sulfonic acid as the solvent. The incorporation of hydrophilic α-ZrP into the membrane resulted in higher dopability of PA (6.5 mol) and proton conductivity (46 mS/cm) of the membranes (10 wt % of α-ZrP) as against the corresponding values of 3.6 mol and 27 mS/cm, respectively, for the pristine membrane. More remarkably, these property improvements could be achieved while simultaneously augmenting the thermomechanical properties and oxidative stability of the membranes. The unit-cell tests showed a marked improvement in the maximum power density for the nanocomposite membrane (335 mW/cm

Optical nanomaterials (ONMs) have attracted significant attention of the global research community owing to their several superior advantages. Some of the ONMs depict negligible photobleaching, bright emission, large electron-hole overlap integral, ease of coating on any surface, large Stoke/Antistoke shifts, higher photostabilites, easy to incorporate inside human body etc. These properties make them technologically highly efficient which helps in minimizing the global issue of energy crisis and improving the standard of living of common man. Several aspects of ONMs such as plasmonic nanomaterials, quantum dots, photonic crystals, and rare earth doped nanoparticles have been discussed. For covering various categories of luminescent nanomaterials (LNMs) based on composition, a brief introduction and importance of metal nanoclusters, carbon nano dots, organic-inorganic nanocomposites, and metal doped nanoparticles have been also included. This review article gives very concise overview of rare earth doped oxide nanoparticles covering several important aspects such as rare earth ion (RE) as dopant, oxide-based host, nanophosphors etc. The importance of LNMs, their advantages as well as drawbacks are also highlighted. Technologically important types of RE doped oxide nanoparticles pertaining to downconversion, upconversion, persistence and radioluminescence are also explained and covered in detail in this review article. Several recent applications involving LNMs in bioimaging, solar cell, thermal sensor, biosensor, anti-counterfeiting etc are explained. In the last section of the review, a comparison between nano and bulk rare earth doped oxide LNMs are discussed in terms of several photophysical properties such as dopant local symmetry, Stark splitting, emission intensity, luminescence lifetime, host to dopant energy transfer, quantum yield etc. Finally, this review article is concluded with summary, outlook, challenges, and future scope on the use of LNMs. Several important aspects of LNMs with focus on RE doped oxide nanoparticles have been covered in this review article which will help general readers in their further research, in this important area, for development of both fundamental sciences and applications involving LNMs.

Epoxy resin nanocomposites containing graphite nanosheets (GNS) and unsaturated polyester resin (UPR) were prepared by incorporating GNS in semi interpenetrating network (semi-IPN) of epoxy resin and unsaturated polyester resin (E-UPR). GNS are prepared by thermal shock treatment of sulphuric acid-graphite intercalation compound (GIC) followed by ultra-sonication. The E-UPR-GNS semi-IPNs were further modified with aromatic amines such as benzidine (Bz) and diphenyl amine (DPA). All the samples were prepared by mixing Triethylenetetramine (TETA) as the curing agent at two different temperatures viz. room temperature (RT) and low temperature (LT; 5 °C). A transition is observed in the conducting properties of the samples: from an electrical insulator to an electrical semiconductor in case of nanocomposites especially on addition of GNS to E-UPR semi-IPNs. A sharp improvement is observed in conductivity after modification with diphenyl amine (DPA) cured at LT. The nanocomposites obtained at LT exhibit homogeneity with minimum anisotropy in conductivity. The morphology, mechanical, thermal and electrical properties were evaluated using various analytical techniques. Positron annihilation spectroscopy was carried out to correlate the measured properties with the free volume and free volume hole size distribution. The results are consistent with the reported studies.