Abanti Nag

Bio: Abanti Nag is an academic researcher from National Aerospace Laboratories. The author has contributed to research in topics: Seebeck coefficient & Perovskite (structure). The author has an hindex of 13, co-authored 32 publications receiving 995 citations. Previous affiliations of Abanti Nag include Indian Institute of Science & Ludwig Maximilian University of Munich.

The light induced valence change of europium in Sr2SiO4∶ Eu involving transient crystal structure

Abstract: Sr2SiO4 ∶ Eu3+ shows orange-red emission of Eu3+ substitutively present in two different Sr sites. The light-induced spectral changes from orange-red sharp line emission to yellow-white broad band are observed in Sr2SiO4 ∶ Eu at room temperature under irradiation with short UV or X-rays. The spectral changes are attributed to the optically assisted reduction of Eu3+ → Eu2+. The photoreduced Sr2SiO4 ∶ Eu shows emission containing contributions from both Eu2+ and Eu3+ in comparison to chemically reduced samples. This is explained on the basis of preferential reduction of Eu3+ present in Sr(1) sites under irradiation due to unsatisfied EuSr–O–Si bonds. The absence of photoactivity for Ba2SiO4 ∶ Eu3+ (space group = Pnam) as well as Ca2SiO4 ∶ Eu3+ (space group = P21/n) indicates that crystal structure plays an important role in the photoreduction of Sr2SiO4 ∶ Eu3+ because of the prevailing orientational as well as the positional disorder in the latter. Further, the orientationally disordered monoclinic random domains persist within the orthorhombic lattice of Sr2SiO4, resulting in the positionally disordered Sr atoms and orientationally disordered SiO4 tetrahedra. Electron paramagnetic resonance studies confirm the electron trapping by dynamically disordered (SiO4)4− under high energy photon illumination resulting in the formation of radical anion (SiO4)5−. The substitutional studies indicate that the [Eu3+ ← O2−] charge-transfer (CT) state is directly involved in the photoreduction process. The excitation of Sr2SiO4 ∶ Eu3+ produces the [Eu3+ ← O2−] CT state which relaxes and transfers electrons to SiO4 groups due to optically assisted rearrangement of local environment and mediates the electron transfer process to cause photoreduction of Eu3+ to Eu2+. The yellow emission is stable at room temperature and reverts to red on annealing at elevated temperature in Ar atmosphere due to thermally activated detrapping of charge carriers present at the defect centers which, in turn, convert Eu2+ to Eu3+. The thermally activated conversion of Eu2+ → Eu3+ in Sr2SiO4 is optically reversible, thereby resulting in a highly efficient material for application as an optical storage medium.

Oxide thermoelectric materials: A structure-property relationship

Abstract: Recent demand for thermoelectric materials for power harvesting from automobile and industrial waste heat requires oxide materials because of their potential advantages over intermetallic alloys in terms of chemical and thermal stability at high temperatures. Achievement of thermoelectric figure of merit equivalent to unity (ZT ≈ 1) for transition-metal oxides necessitates a second look at the fundamental theory on the basis of the structure–property relationship giving rise to electron correlation accompanied by spin fluctuation. Promising transition-metal oxides based on wide-bandgap semiconductors, perovskite and layered oxides have been studied as potential candidate n- and p-type materials. This paper reviews the correlation between the crystal structure and thermoelectric properties of transition-metal oxides. The crystal-site-dependent electronic configuration and spin degeneracy to control the thermopower and electron–phonon interaction leading to polaron hopping to control electrical conductivity is discussed. Crystal structure tailoring leading to phonon scattering at interfaces and nanograin domains to achieve low thermal conductivity is also highlighted.

Recent Developments in the Field of Inorganic Phosphors

Abstract: Because fossil fuels are becoming scarce and because of the expected climate change, our standard of living can only be maintained by a significant increase in energy efficiency. Large amounts of energy are consumed for lighting and during operation of displays. Thus, the targets are the development of economical light sources like white-light-emitting diodes and display panels with enhanced efficiency. Solar energy is converted into electricity by solar cells, and their efficiency must be improved considerably. A possible contribution might be delivered by phosphors which allow the conversion of thermal radiation into electrical energy. Although the target of energy efficiency is very important, we must not overlook that medical imaging diagnostic methods require efficient and sensitive detectors. For the solution of these central questions, inorganic solid-state materials doped with rare-earth ions are very promising and are therefore in the focus of current research activities.

Mechanism of Phosphorescence Appropriate for the Long-Lasting Phosphors Eu2+-Doped SrAl2O4 with Codopants Dy3+ and B3+

Abstract: The existing mechanisms proposed to explain the phosphorescence of SrAl2O4:Eu2+,Dy3+ and related phosphors were found to be inconsistent with a number of important experimental and theoretical observations. We formulated a new mechanism of phosphorescence on the basis of the facts that the d orbitals of Eu2+ are located near the conduction band bottom of SrAl2O4, that the Eu2+ concentration decreases during UV excitation, and that trace amounts of Eu3+ are always present in these phosphors. In our mechanism, some Eu2+ ions are oxidized to Eu3+ under UV, and the released electrons are trapped at the oxygen vacancy levels located in the vicinity of the photogenerated Eu3+ cations. The phosphorescence arises from the recombination of these trapped electrons around the photogenerated Eu3+ sites with emission at 520 nm. The codopant Dy3+ enhances the phosphorescence by increasing the number and the depth of electron traps, and the codopant B3+ enhances the phosphorescence by increasing the depth of electron tr...

Rational design of carbon nitride photocatalysts by identification of cyanamide defects as catalytically relevant sites

Abstract: The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C3N4) is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches because of a lack of fundamental understanding of its mechanistic operation. Here we employ molecular heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant 'defect'. We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12 and 16 times the hydrogen evolution rate and apparent quantum efficiency (400 nm), respectively, compared with the unmodified melon. Computational modelling and material characterization suggest that this moiety improves coordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the separation of the photogenerated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high-performance heptazine-based photocatalysts.

Persistent Luminescence in Non-Eu2+-Doped Compounds: A Review.

Abstract: During the past few decades, the research on persistent luminescent materials has focused mainly on Eu2+-doped compounds. However, the yearly number of publications on non-Eu2+-based materials has also increased steadily. By now, the number of known persistent phosphors has increased to over 200, of which over 80% are not based on Eu2+, but rather, on intrinsic host defects, transition metals (manganese, chromium, copper, etc.) or trivalent rare earths (cerium, terbium, dysprosium, etc.). In this review, we present an overview of these non-Eu2+-based persistent luminescent materials and their afterglow properties. We also take a closer look at some remaining challenges, such as the excitability with visible light and the possibility of energy transfer between multiple luminescent centers. Finally, we summarize the necessary elements for a complete description of a persistent luminescent material, in order to allow a more objective comparison of these phosphors.