Showing papers by "Birabar Nanda published in 2020"

Density Functional Theory Studies of Si2BN Nanosheets as Anode Materials for Magnesium-Ion Batteries

Abstract: The unique structural characteristics make the 2D materials potential candidates for designing negative electrodes for rechargeable energy storage devices. Here, by employing density functional the...

Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-δ Thin-Film Solar Cells.

Abstract: Designing solid-state perovskite oxide solar cells with large short circuit current (JSC) and open circuit voltage (VOC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large VOC (>50 V). However, they exhibit low JSC (≪μA/cm2) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (VO) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high JSC and VOC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice VO, Ca and Ti cosubstitution is used to bring out only GB-VO. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-δ/Au devices are tested for photovoltaic characteristics. Introducing VO increases the photocurrent by four orders (JSC ∼ 3 mA/cm2). On the contrary, VOC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO3. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-VO. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in JSC. The cosubstitution, following a charge compensation mechanism, decreases the lattice VO concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve VOC (3-8 V) much larger than that of BiFeO3 (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice VO concentration dominates GB-VO concentration. Notwithstanding, both lattice and GB-VO contribute to the increase in JSC; the former weakens ferroelectricity, and as a consequence, undesirably, VOC is lowered well below 0.5 V. Using optimum JSC and VOC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Pressure and inversion symmetry breaking field-driven first-order phase transition and formation of Dirac circle in perovskites

Abstract: Through model Hamiltonian studies and first-principle electronic structure calculations, we have examined the effect of inversion symmetry breaking (ISB) field and hydrostatic pressure on the band topology of halide perovskites by taking ${\mathrm{MAPbI}}_{3}$ as a representative. Our study shows that while hydrostatic pressure induces normal to topological insulator continuous phase transition, the ISB field makes it first order. The pressure smoothly reduces the normal band gap, and without ISB, the system achieves a gapless state before it produces a nontrivial band gap with inverted characters. The ISB field does not stabilize the gapless state, and therefore, the discontinuity in the band gap with pressure gives rise to the first-order transition. Furthermore, in the nontrivial phase, the ISB field forms an invariant surface Dirac circle in the neighborhood of TRIM, which is the first of its kind. The circle is formed due to interpenetration of Dirac cones resembling the band topology of AA-stacked bilayer graphene.

Mechanistic Understanding of NO2 Dissociation on a Rutile TiO2 (110) Surface: An Electronic Structure Study

Abstract: Understanding the mechanism of NO2 interaction on semiconductor surfaces such as TiO2 is a key step in designing the catalytic processes for conversion of NO2 to useful products. In the present wor...

Fluorine intercalated graphene: Formation of a two-dimensional spin lattice through pseudoatomization

Abstract: A suspended layer made up of ferromagnetically ordered spins could be created between two- monolayer or multilayer graphene through intercalation. Stability and electronic structure studies show that, when fluorine molecules are intercalated between two mono/multilayer graphene, their bonds get stretched enough $(\ensuremath{\sim}1.9\text{--}2.0\phantom{\rule{0.16em}{0ex}}\AA{})$ to weaken their molecular singlet eigenstate. Geometrically, these stretched molecules form a pseudoatomized fluorine layer by maintaining a van der Waals separation of $\ensuremath{\sim}2.6\phantom{\rule{0.16em}{0ex}}\AA{}$ from the adjacent carbon layers. As there is a significant charge transfer from the adjacent carbon layers to the fluorine layers, a mixture of triplet and doublet states stabilizes to induce local spin moments at each fluorine site and in turn form a suspended two-dimensional spin lattice. The spins of this lattice align ferromagnetically with nearest-neighbor coupling strength as large as $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. Our finite-temperature ab initio molecular dynamics study reveals that the intercalated system can be stabilized up to a temperature of 100 K with an average magnetic moment of $\ensuremath{\sim}0.6{\ensuremath{\mu}}_{B}/\mathrm{F}$. However, if the graphene layers can be held fixed, the room-temperature stability of such a system is feasible.

Defining the topological influencers and predictive principles to engineer the band structure of halide perovskites

Abstract: Complex quantum coupling phenomena of halide perovskites are examined through ab initio calculations and exact diagonalization of model Hamiltonians to formulate a set of fundamental guiding rules to engineer the band gap through strain. The band-gap tuning in halides is crucial for photovoltaic applications and for establishing nontrivial electronic states. Using ${\mathrm{CsSnI}}_{3}$ as the prototype material, we show that in the cubic phase, the band gap reduces irrespective of the nature of strain. However, for the tetragonal phase, it reduces with tensile strain and increases with compressive strain, while the reverse is the case for the orthorhombic phase. The reduction can give rise to negative band gap in the cubic and tetragonal phases leading to normal to topological insulator phase transition. Also, these halides tend to form a stability plateau in a space spanned by strain and octahedral rotation. In this plateau, with negligible cost to the total energy, the band gap can be varied in a range of 1 eV. Furthermore, we present a descriptor model for the perovskite to simulate their band gap with strain and rotation. Analysis of band topology through model Hamiltonians led to the conceptualization of topological influencers that provide a quantitative measure of the contribution of each chemical bonding towards establishing a normal or topological insulator phase. On the technical aspect, we show that a four orbital based basis set ($\text{Sn}\ensuremath{-}{s,p}$ for ${\mathrm{CsSnI}}_{3}$) is sufficient to construct the model Hamiltonian which can explain the electronic structure of each polymorph of halide perovskites.

Fluorine Intercalated Graphene: Formation of a 2D Spin Lattice through Pseudoatomization

Abstract: A suspended layer made up of ferromagnetically ordered spins could be created between two mono/multilayer graphene through intercalation. Stability and electronic structure studies show that, when fluorine molecules are intercalated between two mono/multilayer graphene, their bonds get stretched enough ($\sim$ 1.9$-$2.0 A) to weaken their molecular singlet eigenstate. Geometrically, these stretched molecules form a pseudoatomized fluorine layer by maintaining a van der Waals separation of $\sim$ 2.6 A from the adjacent carbon layers. As there is a significant charge transfer from the adjacent carbon layers to the fluorine layers, a mixture of triplet and doublet states stabilize to induce local spin-moments at each fluorine sites and in turn form a suspended 2D spin lattice. The spins of this lattice align ferromagnetically with nearest neighbour coupling strength as large as $\sim$ 100 meV. Our finite temperature \textit {ab initio} molecular dynamics study reveals that the intercalated system can be stabilized up to a temperature of 100 K with an average magnetic moment of $\sim$ 0.6 $\mu_{B}$/F. However, if the graphene layers can be held fixed, the room temperature stability of such a system is feasible.

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba2MnTeO6

Abstract: Oxide double perovskites wherein octahedra formed by both 3d elements and sp-based heavy elements give rise to unconventional magnetic ordering and correlated quantum phenomena crucial for futuristic applications. Here, by carrying out experimental and first principles investigations, we present the electronic structure and magnetic phases of Ba2MnTeO6, where Mn^2+ ions with S = 5/2 spins constitute a perfect triangular lattice. The magnetic susceptibility reveals a large Curie- Weiss temperature -152 K suggesting the presence of strong antiferromagnetic interactions between Mn^2+ moments in the spin lattice. A phase transition at 20 K is revealed by magnetic susceptibility and specific heat which is attributed to the presence of a sizeable inter-plane interactions. Below the transition temperature, the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K. Furthermore, muon spin-relaxation reveals the presence of static internal fields in the ordered state and provides strong evidence of short-range spin correlations for T > TN. The DFT+U calculations and spin-dimer analysis infer that Heisenberg interactions govern the inter and intra-layer spin-frustrations in this perovskite. The inter and intra-layer exchange interactions are of comparable strengths (J1 = 4.6 K, J2 = 0.92 J1). However, a weak third nearest-neighbor ferromagnetic inter-layer interaction exists (J3=-0.04 J1) due to double-exchange interaction via the linear path Mn-O-Te-O-Mn. The combined effect of J2 and J3 interactions stabilizes a three dimensional long-range magnetic ordering in this frustrated magnet.