research
Structural, electronic & optical properties
Multiple low-energy excitons and optical response of d0 double perovskite Ba2ScTaO6
Large bandgap insulators are considered promising for applications such as photocatalysts, dielectric resonators and interference filters. Based on synchrotron X-ray diffraction, diffuse reflectance measurement and density functional theory, we report the crystal structure, optical response, and electronic properties of the synthesized d0 double perovskite Ba2ScTaO6. In contrast to earlier prediction, the electronic bandgap is found to be large, ∼4.66eV. The optical response is characterized by the presence of multiple exciton modes extending up to the visible range. A detailed investigation of the direct gap excitons based on the Elliot formula is presented. Density functional theory based investigation of the electronic properties within generalized gradient approximation severely underestimates the electronic gap. To reach a quantitative agreement, we consider different available flavors of the modified-Becke–Johnson exchange–correlation potential and discuss their effects on the electronic and optical properties.
Physica B: Condensed Matter 637, 413856 (2022)
Revised crystal structure and electronic properties of Ba(FeNb)O3 ceramics
Ba(Fe1/2Nb1/2)O3 (BFN) ceramics are considered to be a potential candidate for technological applications owing to their high dielectric constant over a wide range of temperature values. However, there exists considerable discrepancy over the structural details. We address this discrepancy through a comparative analysis of the earlier reported structures and combined X-Ray Diffraction (XRD) at room temperature and Neutron Powder Diffraction (NPD) measurements in the range of 5K up to room temperature. Our study reveals a cubic structure with space group Pm3m at all measured temperatures. The local environment of the Fe ions is investigated using X-ray Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) technique. A detailed investigation of the electronic properties of the synthesized BFN ceramics is carried out by combination of theoretical and experimental tools: X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and density functional theory (DFT) within GGA+U. The bandgap is estimated using the diffuse reflectance measurements in the UV-Vis-NIR range and an appropriate value of the electron-electron correlation strength U is estimated based on the UV-Vis-NIR and the XAS spectra.
Indole moiety and biological potency in novel pseudo-peptides
We report the synthesis of three novel pseudo-peptide molecules derived from 2-amino-2-(1H-indole-2-yl) acetamides. The compounds were subjected to spectroscopic characterization (1H, 13C NMR and MS) and their chemical, electronic, and optical properties have been investigated. The tumoricidal properties of the synthesized molecules were evaluated against breast cancer cell lines, MCF-7 and MDA-MB-231. All molecules demonstrated a dose-dependent cytotoxicity against the tested carcinoma cells. To ascertain their potential pharmacological applicability, the prospective reactive centers and molecular sites prone to interaction with water were identified along with possible sensitivity to autoxidation via molecular dynamics (MD) simulation. Further, we have studied the optical response in the presence of different solvents and compared the electronic and optical properties of the pristine molecules. For this, we have calculated molecular electrostatic potential (MEP), average local ionization energy (ALIE) and Fukui functions and mapped their values to the electron density surface. In addition, possible sensitivity of newly synthetized molecules has been also investigated, via DFT calculations of bond dissociation energies for hydrogen abstraction (H-BDE). Furthermore, we highlight the subtle dependence of the properties on the structure and composition of these pseudo-peptides. Our results indicate that these molecules have high pharmaceutical potential and could serve as lead components in new drug formulations.
Effects of octahedral tilting on the electronic structure and optical properties of d0 double perovskites A2ScSbO6 (A=Sr,Ca).
With increasing temperature, Sr2ScSbO6 undergoes three structural phase transitions at approximately 400K, 560K and 650K, leading to the following sequence of phases: P21/n→I2/m→I4/m→Fm3¯m, making it an ideal candidate to study the effects of octahedral tilting while keeping other parameters fixed. To ascertain the isolated effects of octahedral distortions, the electronic and optical properties of the monoclinic P21/n (at room temperature), monoclinic I2/m (at 430K), tetragonal I4/m (at 613K) and the cubic Fm3¯m (at 660K) phases have been studied in terms of the electronic structure, dielectric constant, optical conductivity and electron energy loss spectrum using density functional theory. Ca2ScSbO6, on the other hand, shows only a P21/n phase at room temperature and its properties have been compared with the corresponding Sr compound. UV–Vis spectroscopic studies of the optical properties of the room-temperature phase of these d0 double perovskites have been performed and presence of large direct bandgap for both the compounds have been reported. The electronic bandgaps for the room temperature phases are found to be in good agreement with the corresponding experimental values obtained using the Kubelka-Munk function. Interestingly, in contrast to other Sc-based d0 double perovskites, with increasing octahedral distortions, the effective t2g bandwidth remains unaffected while the states forming the band change due to changes in unit cell orientation, leading to small effects on the electronic and optical properties.
Eigenstate analysis of finite-frequency conductivity in graphene
Eigenstate bases are used to study electrical conductivity in graphene in the presence of short-range diagonal disorder and inter-valley scattering. For the first time, the behavior of graphene in a moderate and weak disorderd regime is presented. For disorder strength, W / t ≥ 5, the density of states is flat. A connection is then established with the work of Abrahams et al. using Microscopic Renormalization Group (MRG) approach. For disorder strength, W / t = 5, results are in good agreement. For low disorder strength, W / t = 2, energy-resolved current matrix elements squared for different locations of the Fermi energy from the band centre is studied. Explicit dependence of the current matrix elements on Fermi energy is shown. It is found that states close to the band centre are more extended and fall off nearly as 1/El 2 as one moves away from the band centre. Further studies on current matrix elements versus disorder strength suggests a cross-over from weakly localized to a very weakly localized system. Using the Kubo-Greenwood formula, conductivity and mobility is calculated. For low disorder strength, conductivity is in a good qualitative agreement with the experiments, even for the on-site disorder. The intensity plots of the eigenstates also reveal clear signatures of puddle formation for very small carrier concentration. We also make comparision with square lattice and find that graphene is more easily localized when subject to disorder.