Projected Density Of States Research Articles

Structure, stability, electronic and thermoelectric properties of strontium chalcogenides

Abstract The present investigation reports a critical insight on the temperature dependent transport properties of strontium chalcogenides (SrX; X = O, S, Se and Te) in their rock salt (rs-SrX) and hexagonal monolayer (h-SrX) phases, for the first time. For an initial check on the stability of these phases (rs-SrX and h-SrX), we have carried out phonon dispersion analysis under PAW-GGA level of calculations. Although, some of these structures are reported to be unstable under lower level (PAW-LDA) of calculations in past, our PAW-GGA computations reveals all the rs-SrX and h-SrX phases to be dynamically stable. The stability of all these structures along with wide band (indirect) gap certainly motivates us to look for their thermoelectric properties, for the first time. The electronic band structures and projected density of states (PDOS) of these materials are computed to understand their electronic properties. Finally, in order to obtain temperature dependent transport properties for the considered series, we have utilized the semi-classical Boltzmann transport equations (BTE). The Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ) and figure of merit (ZT) are calculated using BTE. Couple of monolayers (h-SrS and h-SrTe) and bulk (rock salt) phases (rs-SrO and rs-SrS) are come up with excellent thermoelectric properties through the present investigation.

D-band Center Theory for the Case of Hydrogen Atom Adsorption on Fe(100) and Al(100) Surfaces: A Density Functional Study

The adsorption process occurs in technology that involves the surface. This study aims at investigating the adsorption of hydrogen atom on iron (Fe) and aluminium (Al) surfaces by using d-band center theory based-on density functional theory (DFT) method. DBC theory has proven very useful to understand the formation bond and the tendency of reactivity of transition metal which can learn by electronic structure system. This study obtained electronic structure system which is described by projected density of states (PDOS) using a system that consist of hydrogen atom adsorbed on Fe and Al surfaces. The result of PDOS used to explain the results of adsorption energy. The antibonding states on Fe surface were unfilled, meanwhile on Al surface were filled. Those results was suitable with the adsorption energy calculation results which showed that the bond between hydrogen and Fe surface was stronger than the bond between hydrogen and Al surface.

Tight-binding based modeling to demonstrate metal-insulator transition in VO2 within Mott-Hubbard picture

The mechanism of metal-insulator transition (MIT) in vanadium dioxide (VO2) is still under debate whether it is due to the structural phase transition or electron correlations. A previous study, indicating that MIT appears far below the critical temperature of Tc ≈ 340 K when a small amount of oxygen is taken out from the system, suggested that oxygen screening and electronic correlations play a dominant role in MIT of VO2. To demonstrate how the Mott-Hubbard picture works in this mechanism, we construct a Hamiltonian with a kinetic part constructed within tight-binding scheme and an interaction part treated within the mean-field approximation for the system mimicking the situation of VO2 without and with oxygen deficiencies. Keeping the Hubbard repulsion parameter, U, unchanged, we propose that the MIT is dominantly controlled by the changes of hopping parameters and the on-site energies due to possible orbital reorientations induced by correlation effects. We calculate the projected density of states (PDOS) using Green’s function technique and demonstrate the opening/closing of the insulating gap within the Mott-Hubbard picture. Our results qualitatively reproduce the band renormalization picture inferred from the experimental data suggested by literature. In addition, we find that ferromagnetic electron spin configuration is necessary for the overall MIT scenario to work.

Characterizing Property of States: Effect of Defects on the Coefficient of Thermal Expansion and the Specific Heat Capacity of ZrB2

Thermal storage potential and thermal expansion are characteristic properties for extreme applications. ZrB2 is a candidate for advanced applications in aircraft and fusion reactors. This article presents density functional theory calculations of its states, microstructure and quasi-harmonic levels calculations of thermophysical properties. Band structure highlighted dynamical instability with metallic impurities in ZrB2 structure based on frequency modes. The observed projected density of states (PDOS) appropriate 4d orbital of Zr dominated at low frequency both in perfect crystal in the presence or absence of covalent impurities while B 2s and 2p orbitals dominate higher frequency states. Temperature dependency and anisotropy of coefficient of thermal expansion (CTE) were evaluated with various impurities. Various thermodynamic properties like entropy and free energy were explored for degrees of freedom resulting from internal energy changes in the material. Computed results for heat capacity and CTE were compared to available numerical and experimental data.

A computational-experimental investigation on high ethylene selectivity in ethanol dehydration reaction found on WOx/ZrO2-activated carbon bi-support systems

The high ethylene selectivity exhibited on the zirconia-activated-carbon bi-support catalyst is investigated by experiment and density functional theory–based (DFT) analysis. This bi-support catalyst systems prepared by the physical mixing method for the tungsten catalyst show a significant increase in ethylene selectivity up to 90% compared to the zirconia single support system (~58%) during the ethanol dehydration reaction. Besides, the optimal percent weight ratio of zirconia to activated carbon, which results in the highest ethanol conversion is 50:50. The DFT–based analysis is used to investigate high ethylene selectivity in the bi-support system. It shows that the WO5/zirconia is the most stable model for the zirconia single-support tungsten catalyst represented by the zirconia (101) facet of the tetrahedral phase. The carbon atoms were added to the WO5/zirconia to model the tungsten catalyst on the bi-support system. The Bader charge analysis is carried out to determine the electron transfer in the catalyst. The bonding between ethylene and the WO5 active site on the catalyst is weakened when the system is bi-support, where the added carbon atoms on the catalyst in the ZrO2 region decrease the ethylene adsorption energy. Thus, the desorption and the selectivity of ethylene are promoted. The decrease in adsorption energy can be explained via the analysis of the projected density of states (PDOS) profiles of atom involving the adsorption. It was found that the added carbon in the ZrO2 region induces the electron transfer from the ethylene molecule to the surface, especially to the ZrO2 region. The depletion of the electron around the ethylene molecule weakens the bonds, thus, promote desorption. Hence, the advantages of using the bi-support system in the tungsten catalyst are that the catalyst exhibit (1) high conversion due to the zirconia support and (2) high ethylene selectivity due to the added carbon promoting the desorption of ethylene via the induction of electron from an ethylene molecule to surface.

First principle calculations of structural, electronic, optical and thermoelectric properties of tin (II) oxide

This work aims to study structural, electronic, optical, and thermoelectric properties of Tin(II)Oxide(SnO) by first-principles calculations by using the software VASP within the Perdew-Burke Enzerhof (PBE), HSE03 and HSE06 functionals. Bandstructure, total density of states (TDOS), orbital and site projected density of states (PDOS), dielectric functions of SnO have been calculated. We found that the bandgap of 0.77 eV calculated within HSE06 is in excellent agreement with experimentally established findings. TDOS agrees well with experimentally established x-ray photoelectron spectroscopic (XPS) measurements. By analysis of PDOS contributions of Sn-5s and O-2p at the valence band has been established. Boltzmann semi-classical transport theory is further used to obtain a figure of merit, which has a maximum positive value of 0.14 at around 390 K, and this indicates the possibility of application of SnO as a thermoelectric material.

Explosive vapor detection using novel graphdiyne nanoribbons—a first-principles investigation

We investigated the capability of graphdiyne nanoribbon (GdNR) to detect the existence of explosive vapors like hexogen or cyclonite, hexamethylene triperoxide diamine (HMTD), and 2,4,6-trinitrotoluene (TNT) using ATK-VNL package. In order to determine the sensing response of GdNR towards these explosive vapors, the geometric firmness of the material is first verified with the assistance of cohesive energy. Then, electronic characteristics like the projected density of states (PDOS) spectrum, band structure, and electron density are examined for both isolated and explosive vapor adsorbed GdNR. Further, adsorption attributes like average energy gap variation, enthalpy adsorption, adsorption energy, and Bader charge transfer are explored for explosive vapor adsorbed GdNR. Moreover, there is a need for rapid detection of explosive vapors using solid-state chemical sensors. The scrutinization of these attributes affirms the employment of GdNR as a chief material in a chemical nanosensor to perceive the availability of the mentioned explosive vapors.

Exploring the effect of dopant (Si, P, S, Ge, Se, and Sb) in arsenene: A DFT study

The structural, electronic and magnetic properties of arsenene were investigated using DFT (density functional theory). The charge transfer, large biding energies, and short bond lengths indicate that the doped structures are robust. Si, S, Ge and Se doping induce magnetic state in arsenene. The principal contribution to the magnetic moment is originated in the p-orbital of dopants and adjacent As atoms, as is suggested by the results of the application of PDOS (Projected Density of States). More importantly, the low effective mass of electrons in arsenene doped by Si, P and Sb implies high carrier mobility, which indicates the three types of structures are high efficiency n-type semiconductors.

Theoretical study of the microscopic origin of magnetocrystalline anisotropy in Fe16N2 and its alloys: comparison with the other L10 alloys

We study the magnetocrystalline anisotropy (MCA) energy of Fe16−nXnN2 (), where X = Ti, V, Cr, Mn, Co, Ni, Cu. To understand the microscopic origin and basic mechanism controlling the MCA energy of Fe16−nXnN2, we first examined the behavior of the MCA energy of Fe16N2, focusing on the spin–orbit coupling (SOC), and compared the behavior with other alloy systems (FeCo, FePt and CoPt) with L10 structure. We find that whereas the MCA energy of FeCo is determined by the spin-conserved terms of the SOC energy, the MCA energy of Fe16N2 is determined by mutual competition between spin-conserved and spin-flip terms. We then studied the effect of the transition element X on the phase stability and MCA of Fe16−nXnN2. The MCA energy and cohesive energy are calculated to determine the most stable configuration for each choice of X and n, and compared with those of Fe16N2. For X = V and Cu, both the MCA and phase stability improved noticeably. For X = Co, the MCA energy improves, but Fe16−nConN2 is less stable than Fe16N2. The microscopic mechanism underlying the MCA energy enhancement due to X = V, Cu and Co in Fe16−nXnN2 was studied by examining the data for spin- and site-resolved projected density of states (PDOS), as well as each spin-conserved and spin-flip terms contributing to the SOC energy.

DFT characterization and design of anthracene-based molecules for improving spectra and charge transfer

Four anthracene-based dyes (AN-3, AN-11, AN-12, AN-14) are investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) for dye-sensitized solar cells (DSSCs), involving the stable molecular geometries, the electronic structures, the absorption and fluorescence spectra, and the performance of photoelectric properties. For the simulation of the realistic environment, some important parameters, including energy levels, energy gaps, electron density, projected density of states (PDOS), absorption, vertical dipole moment, first hyperpolarizability, light-harvesting efficiency (LHE), evaluation on electron injection, are calculated for each dye molecule. The relevant electron transfer (ET) and dynamic processes were studied by using the charge different density (CDD) and Newns-Anderson model. The relationship between structure and performance are established. Furthermore, six dyes are designed and examined on the basis of AN-11 to improve optical response and electron injection. It is expected that this study will give theoretical guidance and ideas for finding potential solar cell materials.

Boron trifluoride interaction studies on graphdiyne nanotubes – A first-principles insight

We investigated the adsorption of boron trifluoride (BF3) gas molecules on graphdiyne nanotube (G2YNT) based on first-principles studies. We ensured the stability of G2YNT using cohesive energy and found stable. G2YNT shows semiconductor properties. Thus, it is used as the base material to adsorb BF3 gas molecules. The band structure modification and changes in the electron density is noticed via the projected density of states (PDOS) spectrum. Also, the electron density variation in G2YNT reveals the adsorption of BF3 molecules. The results of the work suggest that G2YNT is a promising material to perceive the existence of toxic BF3 molecules.

CVD grown nitrogen doped graphene is an exceptional visible-light driven photocatalyst for surface catalytic reactions

The photocatalytic potential of large area CVD grown nitrogen doped graphene (NGr) has been explored though the chemical transformation of 4-nitrobenzene thiol into p,p′-dimercaptoazobenzene. Decoration of NGr with Ag nanocubes with rounded edges to form NGr/Ag nanohybrids resulted in a slight increase in the work-function and a decrease in the n-type character of NGr due to ground state transfer of negative charge from NGr to Ag. The Ag nanocubes exhibited a localized surface plasmon resonance (LSPR) at ~425 nm. When the NGr/Ag nanohybrids were illuminated with visible light of wavelength close to the LSPR peak, Kelvin probe force microscopy (KPFM) indicated a dramatic change in surface potential of −225 mV and Raman spectra detected electron accumulation in NGr, which are attributed to a high local field enhancement-mediated hot electron injection into NGr and the formation of long-lived charge separated states. Pristine nitrogen doped graphene and its coupled system with plasmonic Ag nanoparticles showed superior photocatalytic performance compared to bare plasmonic Ag catalyst. While standalone Ag NPs were unable to complete the transformation of 4-NBT into DMAB even at a laser power of 10 mW, NGr/Ag nanohybrids completed this transformation at a laser power of 1 mW, pointing to the high photoreduction strength of NGr/Ag. Density functional theory (DFT) based computational modeling was used to examine the electronic structure of graphene doped with graphitic, pyridinic and pyrrolic nitrogen dopant atoms. DFT results indicated an enhanced chemical reactivity of NGr due to stronger localization of charge at the dopant sites and a pronounced difference in the projected density of states (PDOS) for carbon atoms in proximity to, and distant from, the nitrogen dopant sites.

N-Carbophenes: two-dimensional covalent organic frameworks derived from linear N-phenylenes

N-Carbophene (carbophene) is a novel class of two-dimensional covalent organic frameworks (2DCOF), based on linear N-phenylenes, that have moderate band gaps and low-mobility bands surrounding the Fermi energy; the simplest of which may have been recently synthesized. Using tight-binding density functional theory, the ground state configurations of carbophene single layers, bilayers, and bulk systems were determined. This work finds that carbophenes have formation energies per carbon atom similar to that of graphenylene. The similarity of formation energies between graphenylene and carbophene suggests that when trying to synthesize one, the other may also be synthesized. The formation energies could explain why the first reported synthesis of graphenylene also indicated that they may have synthesized 3-carbophene. Results contained in this work suggests that a carbophene was synthesized instead of graphenylene. The projected density of states (PDOS) demonstrates that the anti-aromatic nature of the cyclobutene units play a direct role in the creation of bands around the Fermi level, making this an exciting material in the theoretical understanding of the nature of aromatic bonds.

Ruthenium decorated boron-doped carbon nanotube for hydrogen storage: A first-principle study

In this paper, the adsorption of molecular hydrogen on Ruthenium (Ru) decorated boron-doped single-walled carbon nanotube (CNT) is theoretically studied, based on the density-functional theory (DFT). The projected density of states (PDOS) and charge density difference are computed, and Mulliken population analysis is conducted to investigate the molecular hydrogen adsorption ability of a boron-doped CNT (BCNT) system and a pure CNT system. As for the pure CNT system, a single Ru atom can absorb up to four H2 molecules with a binding energy of −1.057eV/H2; and for the boron-doped CNT system, the binding energy of H2 molecules increases to −1.151eV/H2 and it displays much higher binding energy of Ru with the increase of 21.19%. This implies that the BCNT system is more stable and has better ability to adsorb hydrogen molecules. The enhanced ability to adsorb H2 molecules implies that Ru decorated BCNT may be a useful and promising nanomaterial for hydrogen energy storage.

Isocyanic acid (HNCO) adsorption on the flat and defective Rh(001) surfaces: Pure DFT and DFT+vdW calculations

Density functional theory (DFT) calculations, including the effect of van der Waals (vdW) forces (vdW-DF), have been performed to study the adsorption of isocyanic (HNCO) on the defective and perfect Rh(001) surfaces. We have considered Rh adatom on Rh(001) surface as a model of defective Rh(001) surface. Our calculations show that on the perfect surface, the HNCO prefers to adsorb on the bridge site, with or without the inclusion of vdW correction. However, the inclusion of vdW forces results in the reduction of adsorption energy relative to values obtained from pure DFT. Regarding the defective surface in particular, the Rh-adatom results in greater stability of the adsorbed HNCO when compared to HNCO adsorbed on a perfect surface. Also, the interaction between the HNCO and the surface has been analyzed in terms of projected density of states (PDOS). It is observed that huge hybridization occurs between the 2p orbital of N (i.e. of HNCO) and Rh 4d band, thus providing the electronic origin of the observed stability.

Exploring the effect of phosphorus doping on the utility of g-C3N4 as an electrode material in Na-ion batteries using DFT method.

The suitability of P-doped g-C3N4 for sodium storage was assessed using density functional theory. The electronic structure of P-doped g-C3N4 was calculated and the results indicate that the presence of the P atom causes the band gap of g-C3N4 to narrow. Na adsorption on a P-g-C3N4 sheet was investigated. Projected density of states (PDOS) analysis revealed that pyridinic nitrogen atoms in g-C3N4 play the main role in Na adsorption. High binding energies were calculated for Na storage on g-C3N4, leading to a high voltage range (1-3V) and a high Na diffusion barrier (2.3eV). Doping the substrate with more P atoms resulted in lower voltages (below 2.2V), easier Na diffusion (with a barrier of 1.2eV), and therefore a material that was better suited than g-C3N4 for use in anodes.

Interaction properties of explosive vapors on γ-graphyne nanosheet – a DFT outlook

We employed ATK-VNL package to explore the electronic and adsorption features of graphyne nanosheet in order to ascertain the possibility to detect explosive vapors like octogen (HMX), pentaerythritol tetranitrate (PETN) and triacetone triperoxide (TATP). Initially, the structural stability of the prime material is ascertained by estimating cohesive energy. The significant electronic features like the projected density of states (PDOS) spectrum, band structure, and electron density are scrutinized for pristine graphyne nanosheets at atomistic levels. Then, upon surface assimilation of target explosive vapors on graphyne nanosheet, the electronic features and surface assimilation attributes like adsorption energy, Bader charge transfer, and average energy gap variation are computed. With the aid of the essential electronic and surface assimilation attributes, it is suggested that graphyne nanosheets can be employed as a prime material in detecting explosive vapors like octogen, pentaerythritol tetranitrate, and triacetone triperoxide.

DFT study on electronic and optical properties of halogen-adsorbed hexagonal boron nitride

In this paper, electronic and optical properties of two-dimensional (2D) hexagonal structure of boron nitride (h-BN), such as its band structures, energy gap, projected density of states (PDOS), dielectric function and adsorption coefficient under adsorption of halogen molecules are studied. A theoretical investigation is presented to find out the most stable atomic site for each halogen molecule to adsorb on a single layer of h-BN. The calculations are performed on a 3 × 3 × 1 supercell of h-BN within the framework of density functional theory (DFT) to have the best consistency with the existing experimental data. The results show that the h-BN is an insulator with a band gap of 4.91 eV. Investigating the most stable site and energy gap’s fluctuation in the h-BN under halogen molecules adsorption shows that F2 and I2 have the same stable position on and parallel to the h-BN, in contrast to Cl2 and Br2 with another same stable site perpendicular to the h-BN layer. It is found out that in all cases halogens decrease the electronic band gap of the h-BN. Real and imaginary parts of dielectric functions for pristine and halogen adsorbed single-layer h-BN are investigated. Optical absorption and reflection curves reveal the optical blindness of the system to the visible range of light, while it fulfills the requirements of ultraviolet (UV) light detection.

DFT STUDY OF METHYL (CH3) AND HYDROXYL (OH) ADSORPTION ON A GOLD (001) SURFACE

We used the DACAPO code with the GGA-PW91 approximation to study the adsorption of methyl (CH3) and hydroxyl (OH) for four- and five-layer gold (Au) (001) slabs. We have determined for each species the best binding site, adsorption energy, the change in the work function, surface energy, surface dipole moment, geometrical parameters and projected density of states (PDOS). We performed spin-unpolarized and spin-polarized DFT calculations for free and adsorbed CH3 and OH species. The most important point is that the spin polarization diminishes the adsorption energies but does not change the geometrical parameters. For the CH3 species, only the top site was found to be stable for different coverages. We found that during the optimization phase, the hollow and bridge sites were found to be unstable. In both cases the CH3 species moves toward the top site. We observe that the adsorption energy decreases when increasing the coverage. However, the OH species was stable in all investigated sites (top, bridge and hollow). We notice that the adsorption energy is dependent on the number of slab layers and the bridge is the best site in adsorption energy. The analysis of the calculated O PDOS of OH radical shows a mixing between the O orbitals and the Au bands.

The differential adsorption mechanism of hexahydrated iron and hydroxyl irons on a pyrite (1 0 0) surface: A DFT study and XPS characterization

In this paper, we have modeled a fractured surface of pyrite (1 0 0). From the viewpoint of bond lengths, we inferred that the chemical adsorption capacity of Fe(III) complexes follows the order [Fe(III)(OH)]2+ > [Fe(III)(OH)3] > [Fe(III)(OH)5]2− > [Fe(III)(OH)2]+ > [Fe(III)(H2O)6]3+ > [Fe(III)(OH)4]− > [Fe(III)(OH)6]3−. The Mulliken population analysis of the quantum chemical model reveals that bonding between the Fe(III) of complexes and the S of the pyrite surface occurs during the adsorption process. The interaction between the S of pyrite (1 0 0) and Fe of [Fe(III)(OH)]2+ most tends to be a covalent bonding, while the interactions between the S of pyrite (1 0 0) and Fe of [Fe(III)(OH)4]−/[Fe(III)(OH)5]2− most tend to be the ionic bonding and are confirmed by projected density of states (PDOS) analysis. The X-ray photoelectron spectroscopy (XPS) study of the S 2p core line reveals distinct contributions of the surface, which is determined to have contributions due to surface S2− monomers, S22− dimers, Sn2−, S0 and surface sulphate. The charge accumulation on the surface S2− monomers is conjectured to be achieved via a transfer of charge from the surface Fe2+ to the surface S2− monomers. The XPS spectra of the Fe 2p, C 1 s, and O 1s also prove that the addition of organic iron compounds (Fe(III)-EDTA and Fe(III)-citrate) and an inorganic iron compound (Fe2(SO4)3) leads to oxidation of Fe2+ and sulfide species of the ruptured pyrite surface, resulting in a higher ratio of Fe3+ to Fe2+ and an increase of surface CO and COOH groups and oxides.