Quantum Espresso Package Research Articles

Modeling field evaporation degradation of metallic surfaces by first principles calculations: A case study for Al, Au, Ag, and Pd

Under the effects of an extreme electric field, the atoms on a metallic surface evaporate by breaking their bonds with the surface. In this work, we present the effects of a high electric field, by the use of computational simulations, for different metallic surface chemistries: Al, Au, Ag, and Pd. To model this bond breaking procedrure (i.e. field evaporation), we use density functional theory through the Quantum-Espresso (QE) simulation package, which incorporates the electric fields by adding a saw-like funcion into the Hamiltonian. This approach, known as dipole correction, was applied to all simulations as is implemented in the QE package. In this work, we calculate the evaporation field (Fe) for all metallic species, which corresponds to the mean field at which atoms can break their bonds from the surface and evaporate. This result is compared with experimantal data from Atom Probe Tomography (APT) and computational data from prior simulations.

Chromium nanostructure formation on the GaAs(1 1 1)–(2 × 2) surface: First principles studies

Structural, electronic and magnetic properties of the chromium (Cr) nanostructure formation on GaAs(1 1 1)–(2 × 2) surfaces are investigated. Studies are performed by first principles total energy calculations within the periodic spin polarized density functional theory plus Hubbard correction (GGA + U) with 1 < U < 5 eV, as developed in the PWscf code of the quantum ESPRESSO package. The chromium adsorption and incorporation into the GaAs(1 1 1)-(2 × 2) atomic structure are investigated on both gallium and arsenic terminated surfaces. Different coverage of Cr is considered from ¼ monolayer (ML) up to a full ML to determine the relaxed configurations and the most favorable structure. High symmetry sites are considered to explore the Cr adsorption and incorporation into the GaAs structure, these are: H3, T4, Top and Bridge. Surface atomic structures contain different number of atoms therefore to determine the most stable atomic geometry the surface formation energies are constructed. On the As terminated surface, under As rich conditions the CrAs bilayer formation is the most favorable structure. On the Ga terminated surface, under Ga rich conditions the Cr trimer formation is the most favorable configuration. Electronic properties of the most stable structures are investigated by calculating the total density of states (DOS) and projected DOS. Results indicate that the most favorable nanostructures exhibit metallic behavior with antiferromagnetic characteristics.

Doping Induced Band-gap widening in Transition-metal doped ZnO Nanocrystals

AbstractPure and transition metal (TM)-doped ZnO nanocrystals were synthesized using a wet-chemical process. Synthesis was carried out in distilled water at 85 °C followed by calcining the as-prepared powders at 280 °C and 600 °C. Co, Mn and Fe doping at 4 and 8 mol % was achieved by adding CoCl2.6H2O, MnCl2 .4H2O and FeCl2 .4H2O respectively during the synthesis. Crystal phase characterization was carried out by X-ray powder diffraction (XRD) which confirmed the formation of ZnO in the wurtzite polymorph. The band gap energy of the nanocrystals was measured by both photoluminescence spectroscopy (using the Near Band Edge Emission) and UV-Vis absorption spectroscopy, using a modified version of the Tauc law. Widening of the band gap energy from 3.23 eV to 3.33 eV with increased doping concentration was observed for all the dopants. Ab-initio simulations of doped and undoped ZnO crystals using density functional theory as implemented in the Quantum Espresso package confirmed the increase in the band gap energies with doping concentration.

Thermoelectric properties of GeTe and Sb2Te3 calculated by density functional theory

The GeTe and Sb2Te3 system materials are compound very narrow band gaps semiconductors in the energy range 0.20-2.80 eV and highly regarded as good performance TE materials. We calculated thermoelelctric properties of GeTe and Sb2Te3 system by using the density functional therory (DFT) and Boltzmann transport therory calculations based on QUANTUM ESPRESSO and BoltzTraP package. The Seebeck coefficient, electrical conductivity, thermal conductivity and power factor dependent on Fermi energy were predicted for future experimental of Ge-Sb-Te materials.

Атомная и электронная структура реконструкций поверхности (111) в кристаллах ZnSe и CdSe

AbstractThe atomic and electron structure of four variants of polar (111)-(2 × 2) surfaces in ZnSe and CdSe terminated by a cation, namely, the ideal, relaxed, reconstructed, and relaxed after reconstruction surfaces, are calculated for the first time from the first principles. The surface is simulated by a film with a thickness of 12 atomic layers and a vacuum gap of ~16 Å in the layered superlattice approximation. Four fictitious hydrogen atoms with a charge of 0.5 electrons each are added for closing dangling Se bonds on the opposite side of the film. Ab initio calculations are performed using the QUANTUM ESPRESSO software based on the density functional theory. It is shown that relaxation results in splitting of atomic layers. We calculate and analyze the band structures and total and layer-wise densities of electron states for four variants of the surface.

Atomic and electron structure of reconstructed (111) surface in ZnSe and CdSe crystals

The atomic and electron structure of four variants of polar (111)-(2 × 2) surfaces in ZnSe and CdSe terminated by a cation, namely, the ideal, relaxed, reconstructed, and relaxed after reconstruction surfaces, are calculated for the first time from the first principles. The surface is simulated by a film with a thickness of 12 atomic layers and a vacuum gap of ~16 A in the layered superlattice approximation. Four fictitious hydrogen atoms with a charge of 0.5 electrons each are added for closing dangling Se bonds on the opposite side of the film. Ab initio calculations are performed using the QUANTUM ESPRESSO software based on the density functional theory. It is shown that relaxation results in splitting of atomic layers. We calculate and analyze the band structures and total and layer-wise densities of electron states for four variants of the surface.

Two-dimensional cadmium selenide electronic and optical properties: first principles studies

Structural, electronic and optical properties of two-dimensional (2D) cadmium selenide (CdSe) structures with $$2\times 2$$ periodicities are investigated. First principles total energy calculations are performed within the periodic density functional theory. Initially, the structural properties are determined using the local density approximation as implemented in the PWscf code of quantum ESPRESSO package. To investigate the electronic properties, the GW method is applied to determine the energy gap within the plasmon pole and the random phase approximations. Optical properties are investigated to determine the dielectric constant and the Bethe–Salpeter theory is used to calculate the exciton binding energies. Zinc blende and wurtzite phases are considered to calculate the bulk energy gaps, which are compared to the experimental values, finding good agreement. The 2D structure exhibits an energy gap larger than that of the bulk, indicating the effects of reduction in dimensionality; these changes can be attributed to the dangling bonds that are present in the 2D layer.

Computational designing ultra-sensitive nano-composite based on boron doped and CuO decorated graphene to adsorb H2S and CO gaseous molecules

In this paper, interactions of carbon monoxide (CO) and hydrogen sulfide (H2S) with doped and decorated graphene were investigated by using density functional theory and quantum-espresso packages. First of all, impurity effects and properties like adsorption mechanism, the more probable position, binding energy, bond length, Lowdin charge analyze and density of state (DOS) have been determined and then the properties of CO and H2S adsorption calculated, and a brief comparison with other studies has been done. All of these lead to tuning the electronic structure of graphene sheet with impurities that show higher affinities with H2S and CO molecules in comparison to pristine graphene. The obtained results from DOS and charge transfer show that electrical conductance of the B doped graphene sheet and CuO Nano particle decorated—Boron doped graphene sheets are significantly changed compared to the pristine graphene sheet by an increase in the electronic states of near the Fermi’s energy states.

First-Principles Study of Structural, Electronic, Mechanical, Thermal, and Phonon Properties of III-Phosphides (BP, AlP, GaP, and InP)

Based on first-principles calculations with generalized gradient approximation as exchange-correlation functional, the structural, electronic, mechanical, thermal, and phonon properties of III-phosphide binary compounds, namely BP, AlP, GaP, and InP, with cubic zincblende structure have been investigated. The calculations were performed in the framework of density functional theory and density functional perturbation theory (DFPT) implemented in the Quantum ESPRESSO package. The results obtained for the structural and electronic properties are in good agreement with available theoretical and experimental results. The results of our electronic calculations indicate semiconducting properties for these binary compounds. Furthermore, the frequency bandgaps and phonon density of states were also investigated. The computed mechanical constants predict that BP, AlP, GaP, and InP are elastically stable. Finally, we determined the heat capacity and entropy for these binary compounds within a quasiharmonic Debye model using DFPT for comparison.

Evolutionary optimization of PAW data-sets for accurate high pressure simulations

We examine the challenge of performing accurate electronic structure calculations at high pressures by comparing the results of all-electron full potential linearized augmented-plane-wave calculations, as implemented in the WIEN2k code, with those of the projector augmented wave (PAW) method, as implemented in Quantum ESPRESSO or Abinit code. In particular, we focus on developing an automated and consistent way of generating transferable PAW data-sets that can closely produce the all electron equation of state defined from zero to arbitrary high pressures. The technique we propose is an evolutionary search procedure that exploits the ATOMPAW code to generate atomic data-sets and the Quantum ESPRESSO software suite for total energy calculations. We demonstrate different aspects of its workability by optimizing PAW basis functions of some elements relatively abundant in planetary interiors. In addition, we introduce a new measure of atomic data-set goodness by considering their performance uniformity over an extended pressure range.

First Principles Calculations of Graphene Doped with Al, P and Si Heteroatoms

In this work, studies of the doping effects on the electronic and structural properties of graphene were performed. Calculations have been done within the periodic density functional theory (DFT) as implemented in PWscf code of the Quantum Espresso Package. Graphene layers have been modeled using the 4x4 periodic supercells. The doping is explored considering phosphorus (P), aluminum (Al) and silicon (Si) heteroatoms. One heteroatom per supercell was considered. Electronic structure results show that the pristine graphene has a linear dispersion at high symmetry K point and zero gap. Band structure of graphene doped with Al atoms exhibit a metal behavior since a valence band crosses the Fermi level. Graphene doped with P also presents a metal behavior but in this case a conduction band crosses the Fermi level. In addition, when the dopant is Si the band structure shows a semiconductor behavior with a 0.3 eV gap. In all cases, the zero gap energy characteristic of graphene was changed by dopant heteroatom. The Dirac lineal dispersion relation is preserved only in the pristine graphene.

First principle study of the conductive type stability in Sn, Li and Li-Ni doped ZnO nanosheet

Abstract In this study, the Sn, Li and Li-Ni doped ZnO nanosheet were studies using density functional theory implemented in Quantum espresso package. The electrical and optical properties of these doping effects on ZnO nanosheet were studied using Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. The dopant ions were substituted on Zn sites in hexagonal ZnO nanosheets. The results showed that, for the n - t ype doping, the Sn-doped ZnO nanosheet is the most stable under O-poor condition compared with the Li doping and Li-Ni co-doping and has donor level at 2.29 eV below conduction band minimum (CBM). On the other hand, for the p -type doping, Li-doped ZnO nanosheet has acceptor level at 0.68 eV above valence band maximum (VBM) and is more energetic favorable than the Li-Ni doped ZnO in O-poor condition. Therefore, this density functional investigation shows that the high stability of ZnO nanosheets can be achieved for both p -type and n -type conductivity depending on the designed growth condition. These results then suggest the possibility to produce both conductive types in ZnO nanosheet for implementation as p-n junction in miniaturized electronics devices.

Computer simulation of edge-terminated carbon nanoribbons

In this paper, we present the results of ab initio simulation of edge-terminated carbon nanoribbons (CNRs). The calculations were performed using the electron density functional theory with the expansion of electron wave functions in plane waves in the Quantum Espresso software package [1]. The effect of various edge termination types on the band structure of graphene nanoribbons is studied. The data obtained showed that hydrogen and fluorine termination has a very weak effect on the structure. Sulfur or bromine termination causes a semiconductor-to-metal transition. The cause of the change in the conductivity type is the appearance of the electron dispersion curve crossing the nanoribbon band gap. At the same time, the dispersion dependences of the ribbon edge-terminated with alternating chlorine and hydrogen atoms do not exhibit such a change, and the curve mentioned above is not observed. The causes of the observed effects are analyzed.

Designing and engineering electronic band gap of graphene nanosheet by P dopants

Chemical doping is promising route to modify and control the electronic properties of graphene Nanosheet (GNS). The electronic properties of the GNS are investigated with and without various concentrations of phosphorus impurities in the various sites by using the first-principles of the density functional theory (DFT) method, which realized in Quantum Espresso packages. My founding results show that the electronic band structure of the GNS is not only depending on the concentrations of phosphorus impurities, but also depending on thegeometrical pattern of phosphorus impurities in the GNS. Furthermore, I found out that there is an electronic band gap with a single phosphorus impurities and it is increased with increasing the concentrations of phosphorus impurities. Also, total energy is affected and decreased with increasing the concentrations of phosphorus impurities, which is led to make all structures are unstable and more reactive. Then, phosphorus impurities are significantly contributing to engineering, control and alter the electronic properties of the GNS, which is very important in various applications.

Electronic properties of MgZnPt2 at extremely high temperatures and pressures

First principles calculations of the structural and electronic properties of MgZnPt2 have been performed using the Quantum ESPRESSO (QE) package based on the density functional theory (DFT). The electronic band structure is computed for MgZnPt2 at (0K, 0GPa), (5500K, 330GPa), and (6000K, 360GPa). The band structures at (0K, 0GPa) and at extremely high temperatures and pressures exhibit a metallic nature. The electronic bands become more and more dispersive with increasing temperature and pressure. In the density of states (DOS) the effect of high temperature and pressure is visible in the broadening of the peaks and the decrease in their height. The charge density shows that more and more electrons become free to participate in conduction as the temperature and pressure increases.

Thymine adsorption on two-dimensional boron nitride structures: first-principles studies.

First-principles total-energy calculations were performed to investigate the structural and electronic properties of thymine (T) adsorption on pristine and Al-doped two-dimensional hexagonal boron nitride (2D-hBN) surfaces. Periodic density functional theory, as developed in the PWscf code of the quantum espresso package, was applied. The pseudopotential theory was used to deal with electron-ion interactions. The generalized gradient approximation was applied to treat the exchange-correlation energies. Van der Waals interactions were incorporated in the calculations. Considering T as an elongated molecule and the interactions through one oxygen atom of the molecule ring, two geometries were explored in pristine and Al-doped systems: in (1) the ring side O interacts with B, and (2) the O at the molecule end interacting with the B. The pristine case yields (4 × 4-a), (5 × 5-b) and (6 × 6-b) as the ground states, , while the doped system shows (4 × 4-a), (5 × 5-a) and (6 × 6-a) as the ground states. Calculations of the adsorption energies indicate chemisorption. Doping enhances the surface reactivity, inducing larger binding energies. The total density of states (DOS) was calculated and interpreted with the aid of the projected DOS. Below the Fermi energy, the DOS graphs indicate that p orbitals make the largest contributions. Above the Fermi level, the DOS is formed mainly by -s and H-s orbitals. The DOS graphs indicate that the structures have non-semiconductor behavior.

Density functional theory study of electronic properties of Bi2Se3 and Bi2Te3

Topological insulators are layered materials via van der Waals interactions with hexagonal unit cell similar to that of graphene. The exciting features of Bi2Se3 and Bi2Te3 topological insulators their zero band gap surface states exhibiting linear dispersion at the Fermi energy. We present here first principles study pertaining to electronics properties of Bi2Se3 and Bi2Te3 compound with and without spin-orbit interaction using density functional theory (DFT). Total density of state (DOS), partial density of state (PDOS) and band structure where determined by Quantum-Espresso simulation package which uses plane wave basis and pseudopotential for the core electrons, while treating exchange-correlation potential with generalized gradient approximation (GGA). From our computations, the obtained results were found to be consistent with the available experimental results.

Density Functional Study of Electronic, Magnetic and Chemical Bonding Properties of Spinel CdCr2O4

This paper is presented a theoretical study of electronic, magnetic and chemical bonding properties of spinel CdCr2O4 with a general formula of AB2X4 (A, B=transition- metal, X=oxides, chalcogenides) using density functional calculation method combined with spin- polarized theory within generalized gradient approximation. Density functional calculation is performed to observe the effects of magnetic ordering on the electronic and chemical bonding properties of spinel CdCr2O4 with both cubic and tetragonal structure from a pyrochlore lattice, using Quantum ESPRESSO package. Consequently, in order to investigate the magnetic properties in paramagnetic, ferromagnetic and antiferromagnetic orderings of spinel CdCr2O4, a first-principles study of the electronic structure as well as chemical bonding properties of spinel CdCr2O4 compound in two different structural form is performed: the cubic structure in order to investigate the desired properties in paramagnetic and ferromagnetic orderings, and tetragonal structure to calculate the same properties in antiferromagnetic ordering

Interaction of CO with an hBN surface doped with Ti and Pt: A First Principles Study

We studied the possible catalytic effect that a transition metal atom could have after being adsorbed in a surface of hexagonal boron nitride (hBN). We considered platinum and titanium, performing ab-initio calculations, including molecular dynamics at 300K, within the Density Functional Theory. We considered an hBN surface either with a vacancy of a Boron atom, or with a vacancy of a Nitrogen atom, and we found that both titanium and platinum are absorbed at the place of the vacancy for both cases considered. Afterwards, we found that this decoration of the hBN surface indeed has a catalytic effect on the adsorption of a carbon monoxide molecule. Possible desorption was explored, at 800 K. To perform the calculations, the Quantum ESPRESSO package code was used. The generalized gradient corrected Perdew-Burke-Ernzerhof (PBE) approximation was used for the exchange and correlation functional.

Ferromagnetism and half metallicity induced by oxygen vacancies in the double perovskite BaSrNiWO6: DFT study

Using the spin polarized density functional theory (DFT) and exploring the Plane-Wave Self-Consistent Field (PWscf) code implemented in Quantum-ESPRESSO package, we investigate the effect of the Oxygen vacancies (VO) and the Oxygen interstitial (Oi) on the double perovskite BaSrNiWO6. This deals with the magnetic ordering and the electronic structure in such a pure sample exhibiting the insulating anti-ferromagnetic (AFM) state. This study shows that the presence of oxygen deficient defects converts the insulating to half metal with ferromagnetic or anti-ferromagnetic states. The magnetic ordering in BaSrNiWO6−δ depends on the position of the Oxygen vacancy in the unit cell. However, it has been shown that the Oxygen interstitial preserves the anti-ferromagnetic propriety. We have computed the formation energies of different positions of the Oxygen vacancy (VO) and the Oxygen interstitial (Oi) in the BaSrNiWO6 compound. We showed that the formation of VO is easier and vice versa for the Oi formation. The obtained results reveal(VO) and the Oxygen interstitial (Oi) that the anti-ferromagnetic can be converted to ferromagnetic in the double perovskite BaSrNiWO6 induced by Oxygen vacancies VO.