Atom-projected Density Of States Research Articles

From halfmetallicity to dielectric tensor: Atomistic study of phase stability and optical spectrum of Sr2FeMoO6 double perovskite and its potential applications

Structures and the physical properties of the current physical materials and other hypothetical models can be visualized and estimated with considerable success using density functional theory. As promising materials Double Perovskites satisfy the need of fulfillment of energy requirements in current scenario. Probing and investigating such structures theoretically, saves time and curtails the expenditure. This Perspective highlights the optical response functions viz Dielectric Tensor, Polarizability, Reflectivity, Optical conductivity, Absorption Coefficient and Extinction Coefficient of the material in the presence of electromagnetic field of varying energy (0–5eV) and offers an outlook of the potential use of such properties in spintronics and sensors. In our study the effective PBE (Perdew, Burke, Ernzerhof) functional was used to investigate the electronic and optical properties of Sr2FeMoO6 and the thermodynamic stability of the material was estimated using the convex hull approach. The formation energy calculated from total energies of the bulk material (SFMO) and the individual total energies of the constituent elements in their corresponding pure bulk phase indicate the simulated structure to be thermodynamically stable with an energy value of −2.533eV. The study confirms SFMO as an anisotropic dispersive medium, with non-linear optical properties wherein apart from differential rates of absorption along different axis of polarization, nonlinearity and dispersion with respect to optical conductivity, dielectric constant can also be observed. The absorptive bandwidth of the material estimated by density functional approach lies between 2 and 4.5 eV and the absorption function resembles the Gaussian curve. The optical conductivity curve indicates that Sr2FeMoO6 is a direct band gap material with a bandgap of 2.1 eV. The absorption coefficient “μ” peaks at an energy values of 3.2 eV and the material is absorptive for most part of the ultra violet region. The pourbiax analysis of the material has been presented and the stability in presence of varying chemical potentials and pH scales suggest non toxicity and stability associated with the material. The extinction coefficient “K” for the two directions of polarizing field behave analogous to their respective imaginary dielectric functions and the peak value of the extinction coefficient shoots to 0.8. The reflectivity plots indicate the maximum reflectance reaches to about 0.2 (20% reflectance) and then decreases with the increase in the energy of the perturbing field. Spin Polarization and the observed TDOS (total density of sates) APDOS (atom projected density of states) graphics suggest that Sr2FeMoO6 exhibits near-half metallic behaviour and can dramatically enhance device performance by controlling speed and various other properties wherein spin can be manipulated. Consequently, SFMO can be tailored for spintronic applications especially in case of multilayered spin-operated junctions/devices and nonvolatile memory elements, quantum computing and faster data processing speeds.

Diverse structural and electronic properties of carbon-substituted armchair silicene nanoribbons: A first-principles study

The structural and electronic properties of the armchair silicene nanoribbons (ASiNRs) under various C substitutions are investigated using the first-principles calculations. The C substitution-diversified structural and electronic properties are fully identified by the developed first-principles quantities, including the formation energies, phonon calculations, optimal structural parameters, orbital- and atom-projected density of states, and direction-projected spatial charge density distributions. The C substitutions at various adatom concentrations and distributions form the typical C substitution configurations of the single C, ortho-2C, meta-2C, para-2C, and 100% C, whereas the lower buckling height is found at the higher concentrations that become the flat 1D structure at 100% C substitution. The reshaped bandgaps are larger under shorter Si–C bond lengths. The largest opened bandgap of 2.37 eV is found at the highest adatom concentration of the 100% C configuration. The C substitutions generate the quasi σ bonds of the Si-(3s, 3p x , and 3p y ) and C-(2s, 2p x , and 2p y ) orbitals and the quasi π bonds of the Si-3p z and C-2p z orbitals, in which the quasi π orbitals create the long-range quasi π bands that determine the reshaped bandgaps. The quasi σ and π orbitals are hybridized at the middle valence energies that only separate at the deeper valence energies. The weak separation of the σ and π orbitals is responsible for the hybridization mechanism of the mixed sp 2 /sp 3 quasi in the C-substituted systems. The diverse electronic properties of the C-substituted ASiNRs will be very compatible with electronic and optoelectronic applications.

Ab Initio Study of Lattice Dynamics of Dodecaborides ZrB12 and LuB12

We investigate the lattice dynamics of frame-cluster dodecaborides ZrB12 and LuB12 in the framework of the electron density functional theory. Our ab initio calculations are performed using the Quantum Espresso package, with PAW-type pseudopotentials. We compute the phonon properties, such as phonon dispersions and atom-projected densities of states. A comparison is made with a few available ab initio calculations. The results obtained are consistent with inelastic neutron scattering measurements, as well as with experimental Raman spectra. We provide a quantitative confirmation of the experimentally known mixing of the eigenvectors of boron and metal vibrations.

Atomistic Origin of Phase Stability in Oxygen-Functionalized MXene: A Comparative Study

Oxygen-functionalized MXene, M2CO2 (M = group III–V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study on the stability of fully O-functionalized (M2CO2) MXenes. Depending on the position of O atoms, the M2CO2 can exist in two different structural phases. On one side of MXene, the O atom occupies a site which is exactly on the top of the metal atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB′ phase) or on the top of the C atom (CB phase). We find that for M = Sc and Y the CB phase is stable, whereas for M = Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB′. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground sta...

Electronic and optical properties of CuGaS2 nanowires: a study of first principle calculations

In this work we study the electronic and optical properties of fully relaxed CuGaS2 nanowires using the pseudo-potential density functional method. In our calculations we have investigated nanowires with two shapes of hexagonal and triangular with their corresponding diameters in the order of 8 to 15 A in (1–10) growth direction. For CuGaS2 bulk, the geometrical parameters such as anion displacement and equilibrium lattice constant agree well with other theoretical and experimental results. We have shown that for the CuGaS2 nanowires, there is an important contraction of the Cu-S and Ga-S bond lengths in the wires of 2.1% and 1.24% of the bulk value. In addition, in this manuscript the electronic properties such as band structures and atom-projected density of states have been examined. Our results show that while the nanowire diameter increases, the band gap decreases. From partial density of states we found that the greatest valence bands involve atoms which are placed at the surface. The optical constants, the dielectric function, reflectivity, refractive index and absorption of the nanowires have been analyzed. The results show that compared to the CuGaS2 bulk, the corresponding peaks of dielectric functions of CuGaS2 nanowires are blue-shifted. The calculations reveal that the dielectric functions of the nanowires augment while the nanowires’ size increases. It is also found that the peaks related to optical parameters of nanowires are affected by the diameter of the nanowire.

Magneto-electronic and magnetic transport properties of triangular graphene quantum-dot arrays

Graphene (GN), a monolayer two-dimensional (2D) system closely arranged into a benzene ring structure by C atoms, has so far aroused considerable research interest due to its novel electronic, magnetic, mechanical and thermal properties. But 2D GN is a semimetal with zero band gap, and the lowest conduction band touches the highest valence band at Fermi level, leading to the inability to achieve the off effect in the electronic device. Therefore, many researchers are searching the solutions. A simple and feasible method is to convert 2D GN into quasi-one-dimensional (1D) graphene nanoribbons, quantum-dot arrays (QDAs) and zero-dimensional (0D) quantum-dot by tailoring it along a specific single crystallographic direction. The QDAs, due to their structural diversity, have great potential applications in future nano-integrated circuit. In this work, first-principles method based on density functional theory is used to study the magneto-electronic and magnetic transport properties of four 1D quantum-dot arrays (1D QDAs) consisting of triangular graphene nanoflakes with different linking modes. The calculated binding energy suggests that these structures are very stable, and the arrays that are linked by the bottom-side are more stable than that only by the vertex. In particular, it is found that the electronic and magnetic features are not only related to the different magnetic states, but also depend on linking modes. For example, in the non-magnetism state, different QDAs can be a metal or a narrowed band-gap semiconductor. In the ferromagnetic state, different QDAs can be half-metal materials or bipolar magnetic semiconductors with different gaps, and have greatly different magnetic moments from 1.985 to 7.994B/unit cell, reaching a difference almost as large as four times. While in the antiferromagnetic state, all QDAs are semiconductors but with different gaps. These results imply that the linking modes play a crucial role in effectively tuning the electronic and magnetic features for nanostructures. The calculated atom-projected density of states indicates that the highest valence band and the lowest conduction band are determined by the edge C atoms. The half-metallic and bipolar magnetic semiconducting behaviors presented by 1D QDA are extremely important for developing magnetic devices, which is not found in the intrinsic graphene nanoribbons. And, we also investigate the magnetic device properties based on one kind of QDA, and the single or dual spin-filtering effect with the perfect (100%) spin polarization and a rectification ratio of about 104 can be predicted. Particularly, a giant magnetoresistance over 109% is found unambiguously, which is two orders of magnitude higher than the value predicted based on the zigzag graphene nanoribbons and five orders of magnitude higher than previously reported experimental values for the MgO tunnel junction. Our results thus provide strong evidence for the effectiveness of QDAs on the magneto-electronic properties.

From Ba3Ta5O14N to LaBa2Ta5O13N2: Decreasing the optical band gap of a photocatalyst

Yellow LaBa2Ta5O13N2 was successfully synthesized as phase-pure material crystallizing isostructurally to previously reported Ba3Ta5O14N and mixed-valence Ba3TaV4TaIVO15. The electronic structure of LaBa2Ta5O13N2 was studied theoretically with the range-separated hybrid method HSE06. The most stable structure was obtained when lanthanum was placed on 2a and nitrogen on 4h sites confirming Pauling's second rule. By incorporating nitrogen, the measured band gap decreases from ∼3.8eV for the oxide via 2.74eV for Ba3Ta5O14N to 2.63eV for the new oxide nitride, giving rise to an absorption band well in the visible-light region. Calculated fundamental band gaps confirm the experimental trend. The atom-projected density of states has large contributions from N2p orbitals close to the valence band edge. These are responsible for the observed band gap reduction. Photocatalytic hydrogen formation was investigated and compared with that of Ba3Ta5O14N revealing significantly higher activity for LaBa2Ta5O13N2 under UV-light.

Ab initio study on structural, elastic, electronic and optical properties of cuprate based superconductor

AbstractThe structural, elastic, electronic, and optical properties of BaCuO2 are investigated using the plane-wave ultrasoft pseudo-potential technique, which is based on the first-principle density functional theory (DFT) with generalized gradient approximation (GGA). The calculated structural parameters show a good agreement with the experimental and other theoretical results. The optimized lattice parameters, independent elastic constants (C11, C12, C13, C33, C44, and C66), Bulk modulus B, compressibility K, Shear modulus G, and Poisson’s ratio ν, as well as the band structures, total and atom-projected densities of states and finally the optical properties of BaCuO2 has been evaluated and discussed. The band structure and density of states show that these phases have metallic behavior and the major contribution comes from O-2P states for BaCuO2. Further, the first time investigated optical functions reveal that the reflectivity is high in the IR-UV region up to 28.5eV for BaCuO2 respectively, showing...

Structural and electronic properties of CuInS2 nanowire: A study of density functional theory

In this work, the structural and electronic structure of CuInS2 nanowires have been computed via density functional theory by using numerical atomic orbital bases set with local-density approximation. The norm-conserving pseudopotentials of Troullier and Martins are used. In our calculation we used nanowires in two shapes of hexagonal and triangular with their diameters ranging from 8 to 19Å in (1–10) growth direction for both cases of unsaturated and saturated with hydrogen. The geometrical parameters like equilibrium lattice constant and anion displacement are in reasonable agreement with the experimental data and other theoretical results. We show that for the unsaturated nanowires there is a significant contraction of the Cu–S and In–S bond lengths at the edge of the wires of 0.17–3.70%, as for saturated wires at the edge, it is 0.7%. Also electronic properties like band structures and atom-projected density of states have been studied. The band structures confirm that CuInS2 nanowires are semiconductors with a direct band gap and there is hybridization between Cu-d with S-p states in the uppermost valance sub-bands of the CuInS2 nanowires. We show that for saturated nanowires, as the diameter of the nanowire increases, its relative band gap (ΔEg) decreases. But for the unsaturated nanowires, the band gap remains constant with varying diameter and from projected density of states of atoms, the peaks are decreasing from edge to center of nanowires. Also the highest valence bands involve atoms located at the surface. And finally we can see that for all atoms the peaks are increased when the size of nanowires increases.

Electrically dependent bandgaps in graphene on hexagonal boron nitride

We present first-principles calculations on the bandgap of graphene on a layer of hexagonal boron nitride in three different stacking configurations. Relative stability of the configurations is identified and bandgap tunability is demonstrated through the application of an external, perpendicularly applied electric field. We carefully examine the bandgap's sensitivity to both magnitude of the applied field as well as separation between the graphene and hexagonal boron nitride layers. Features of the band structure are examined and configuration-dependent relationships between the field and bandgap are revealed and elucidated through the atom-projected density of states. These findings suggest the potential for opening and modulating a bandgap in graphene as high as several hundred meV.

Effects of strain, d-band filling, and oxidation state on the bulk electronic structure of cubic 3d perovskites

The properties of the d-band structure of the transition metal atom in cubic LaBO(3) and SrBO(3) perovskites (where B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and their dependence on strain, d-band filling, and oxidation state were investigated using density functional theory calculations and atom-projected density of states. The strain dependence of the d-band width is shown to depend systematically on the size of the B atom. We show that the transition metal d-band width and center are linearly correlated with each other in agreement with a rectangular band model. A simple matrix element formalism based on the solid state table can readily predict the strain dependence of the d-band width.

Nonhydrolytic Synthesis and Electronic Structure of Ligand-Capped CeO2−δ and CeOCl Nanocrystals

A novel and versatile nonhydrolytic approach is developed for the synthesis of ligand-passivated CeO2−δ and CeOCl nanocrystals soluble in nonpolar organic solvents based on the condensation of cerium alkoxides with cerium halides. The alkyl group on the metal alkoxides and the specific halide used in the synthesis are observed to considerably influence the composition and size of the obtained nanocrystals. The obtained nanocrystals are <3 nm in diameter and, owing to their surface-capping groups, yield homogeneous and clear solutions in nonpolar organic solvents with no evidence of agglomeration. The electronic structure of the obtained CeO2−δ nanocrystals has been studied using optical absorption spectroscopy and near-edge X-ray absorption fine structure spectroscopy at Ce M- and O K-edges. The latter technique provides detailed insight into the metal valence, geometric structure, and atom-projected density of states in these nanocrystals. Finally, this synthesis method has been expanded to explore the d...