Showing papers by "Birabar Nanda published in 2019"

Stretchable and dynamically stable promising two-dimensional thermoelectric materials: ScP and ScAs

Abstract: We present two newly designed two-dimensional (2D) thermoelectric materials ScP and ScAs, which are stretchable up to 14% as well as dynamically and thermally stable up to 700 K. From a systematic study using density-functional calculations, ab initio molecular dynamics simulations and phonon studies, we find that these compounds are narrow band gap semiconductors and crystallize in a puckered structure, as is the case for many experimentally realized 2D materials like phosphorene and arsenene. The transport properties of these compounds are estimated using the semi-classical Boltzmann transport approach. The lattice thermal conductivity (kl) in the unstrained system is estimated to be 8.3 and 5 W m−1 K−1 for ScP and ScAs respectively which are less compared to those of pristine phosphorene (24–110 W m−1 K−1) and arsenene (6–30 W m−1 K−1). Furthermore, the kl of these compounds becomes ultra-low (∼0.45 W m−1 K−1), when they are subjected to optimum tensile strain conditions. Highly dispersed bands of ScP and ScAs, due to strong p–d hybridization, give rise to large electrical conductivity (∼108 S m−1) which is two orders higher than that of arsenene and phosphorene. The strain also brings nearly a two- and three-fold increase in the Seebeck coefficient with respect to the unstrained value in these compounds. Overall, the strain tunable large figure of merit (∼0.65–0.9) makes these compounds promising thermoelectric materials.

Large Bulk Photovoltaic Response by Symmetry-Breaking Structural Transformation in Ferroelectric [ Ba ( Zr 0.2 Ti 0.8 ) O 3 ] 0.5 [( Ba 0.7 Ca 0.3 ) Ti O 3 ] 0.5

Abstract: The photovoltaic effect seen in ferroelectric oxides is gaining interest for next-generation solar cells, as these materials do not suffer from the same issues regarding carrier recombination and band gap as do the commonly employed semiconductors. The authors demonstrate a large photovoltaic response in a Pb-free mixed titanate-zirconate system by electrically tuning the material's symmetry-breaking structural transformation, and correlate the effect to the preferred structural symmetry via band-structure calculations. These results give welcome insight for engineering ferroelectric oxides for photovoltaic applications.

Enhanced bulk photovoltaic response in Sn doped BaTiO3 through composition dependent structural transformation

Abstract: Polycrystalline BaTi1-xSnxO3 samples (x = 0.06, 0.07, 0.08, 0.09, 0.10, and 0.11) were synthesized by the solid state technique. The samples exhibit the tetragonal phase at 300 K. In addition, the samples x = 0.06, 0.07, 0.08, and 0.09 also show the orthorhombic phase with enhanced phase fractions upon poling. However, the % orthorhombic phase fractions show an increase up to x = 0.07 and a decrease with an increase in x. The dielectric studies indicate that TC (cubic to tetragonal phase transition) shifts toward lower temperature where the samples x = 0.10 and 0.11 show the tetragonal phase at 300 K. The samples exhibit the maximum remnant polarization and piezoelectric coefficient for x = 0.08. But the bandgap for the x = 0.07 sample shows the value of 2.61 eV before poling and 2.95 eV after poling. A giant photovoltaic (PV) response is seen in the samples with the open-circuit voltage (VOC) as large as 16 V (for x = 0.07). VOC shows a decreasing trend with an increase in the Sn content after x = 0.07, and it did not follow the trend in polarization and the bandgap. The observed results are correlated with the structural symmetry of the compound, and they are validated by the band-structure calculations. The experimental and theoretical studies indicate that the sample with the orthorhombic phase is preferable for the enhanced photovoltaic response in comparison to the tetragonal phase. These studies show a new way to achieve a large photovoltaic response so as to design the system for several device applications such as UV detectors and microactuators.

Shifting of Fermi level and realization of topological insulating phase in the oxyfluoride BaBiO2F

Abstract: The disadvantage of BaBiO$_3$ of not being a topological insulator despite having symmetry protected Dirac state is overcome by shifting the Fermi level (E$_F$) via fluorination. The DFT calculations reveal that the fluorination neither affects the spin-orbit coupling nor the parity of the states, but it acts as a perfect electron donor to shift the E$_F$. We find that 33 % fluorination is sufficient to shift the E$_F$ by $\sim$ 2 eV so that the invariant Dirac state lies on it to make BaBiO$_2$F a topological insulator. The fluorinated cubic compound can be experimentally synthesized as the phonon studies predict dynamical stability above $\sim$ 500 K. Furthermore, the Dirac states are found to be invariant against the low-temperature phase lattice distortion which makes the structure monoclinic. The results carry practical significance as they open up the possibility of converting the family of superconducting oxides, ABiO$_3$ (A = Na, K, Cs, Ba, Sr, Ca), to real topological insulator through appropriate fluorination.

Band engineering via grain boundary defect states for large scale tuning of photoconductivity in Bi1−xCaxFe1−yTiyO3−δ

Abstract: Spark plasma sintered Bi1−xCaxFe1−yTiyO3−δ (BCFTO) (x = y = 0.05 and 0.1) nanoparticle ceramics are studied for photoconductivity properties. As-prepared (AP) BCFTO hosts a large concentration of grain boundary (GB) oxygen vacancies (OV), whereas air annealed (AA) BCFTO have significantly suppressed GB OV. X-ray absorption near edge spectroscopy study confirms that Fe and Ti remain in 3+ and 4+ oxidation states, respectively. Thus, lattice OV created when only Ca2+ is substituted in BiFeO3 are charge compensated in Ca and Ti codoped BiFeO3. This ascertains that BCFTO is devoid of lattice OV. Photoconductivity studies show four orders of more photocurrent arising from GB OV contributions in BCFTO-AP compared to that in BCFTO-AA samples. A large increase in the activation energy for the AA samples (0.4 eV to 1.6 eV) compared to that for the AP samples (0.06 eV to 0.5 eV) is obtained from ln ω vs 1/T Arrhenius plots. This further substantiates the suppression of GB OV resulting in poor photoconductivity. Diffuse band edges observed in Kubelka-Munk plots of BCFTO-AP samples are a consequence of OV defect states occupying the bulk bandgap. In the absence of OV defect states, band edge becomes sharper. Density functional theory (DFT) calculations further support the experimental observations. DFT study shows that the presence of Ca and Ti does not enhance the photocurrent as these codopants do not produce mid-bandgap states. The mid-bandgap defect states are attributed only to the unsaturated bonds and OV at the GB in BCFTO. These studies manifest a critical role of OV residing at the GB in tuning the photoconductivity and, hence, the photoresponse of BCFTO.

Localization Crossover Near Metal-Insulator Transition in Two-Dimension Limit of CaCu3Ru4O12

Abstract: Artificial confinement of electrons by tailoring the layer thickness has turned out to be a powerful tool to harness control over competing phases in complex oxides. We investigate the effect of dimensionality and strain on the electronic band structure and transport properties of d-electron based heavy-fermion metal CaCu3Ru4O12. Transport behavior evolves from metallic to localized regime upon reducing thickness and a metal insulator transition is observed below 3 nm for which sheet resistance crosses $h/e^{2} \approx 25 K{\Omega}$, the quantum resistance in 2D. A strong interplay between inelastic and spin-orbit scattering lengths close to metal insulator transition is observed which results in a weak antilocalization to weak localization crossover in magnetoconducatance upon reducing film thickness. The cause of metal-insulator transition and magnetoconductance crossover are explained using band structure calculation and 2D magnetotransport theory.