جستجوی مقالات مرتبط با کلیدواژه « نظریه تابعی چگالی » در نشریات گروه « مواد و متالورژی »

We report the electronic, structural and optical properties of topological insulator SnBi2Te4 calculated by means of linear augmented plane waves with full potential (FP-LAPW) within the framework of density functional theory (DFT) implemented in Wien2k code. Bulk band gaps of 0.36eV and 0.22eV are predicted by using PBE and LDA approximations, respectively. These gaps indicated that SnBi2Te4 is potentially suitable for room temperature electronics applications. The results are in accordance with other works.

Compared with carbon nanocones, Boron Nitride Nanocones (BNNCs) are characterized by different physical, chemical, and electronic properties. Since BNNCs are in three types of covalent bonds (B-N), B-B, and (N-N), carbon nanocones contain only C-C bonds. Given that the structural properties of nano cones depend on the disclination angle of the cone, the current study aims to discuss the electronic and structural properties of boron nitride nanocones in different sizes (height and disclination angle) using Density Functional Theory (DFT) calculations. To this end, the relevant calculations were done for boron nitride nanocones with the cone heights of 1 to 5 angstroms at the disclination angle of 60°, 3 to 7 angstroms at the disclination angle of 120°, and 4 to 8 angstroms at the disclination angles of 180 and 240°. In this work, the density of states, quantum parameters, analysis of bond order, bond length, polarizability, anisotropic polarization, dipole moment, atomic charges, and frequencies were calculated using the DFT. According to the results, the best structure was identified based on the height and disclination angles to be further used in the next calculations. It is hoped that the results of this project can be useful in expanding the experimental works and effective in the design of nanocones as well in order to absorb gases, drugs, etc.

In this study, the optical and magnetic properties of compounds in hexagonal and orthorhombic phases are investigated. The calculations have been carried out in the framework of the functional density theory with various approximations such as local density approximation, Generalized gradient approximation with the pseudopotential and stationary load method, and using the quantum-espresso computational code. In this study, the optical and magnetic properties are calculated and the results obtained Compare with other available data. The dynamic contribution of the dielectric function indicates that the gap is zero with the strip structure. In calculating the total torque, the uranium atom has the highest share in both combinations. The share of gallium and germanium atoms is negligible.

In this research, we investigate the effects of pressure and enthalpy on the composition of molybdenum disulfide in the framework of the density functional theory and with the Espresso quantum code with flat waves and the exchange potential of the generalized slope approximation of the stationary charge. The lattice constants of the hexagonal structure of molybdenum disulfide compound were calculated and compared with other works and we have seen the agreement of the results. The electronic properties of the compound were plotted and calculated for comparison with pressure application. The band structure of molybdenum disulfide shows the semiconducting nature with a gap of about 2.27 electron volts, which is larger than the results of other researches. The density of states is plotted for the molybdenum disulfide compound and confirms the semiconducting nature of the compound. The gap obtained from the density of states is more consistent with the results of other works and its value is about 1.3 electron volts. It draws the density of partial states and shows the role of d orbital of molybdenum atom and p of sulfur atom in the overall state density. In the following, we apply different pressures, by applying constant positive pressures, the network increases, and first the bandgap decreases, then increases a little in energy, and then decreases again until the structure changes from a semiconducting nature to a metal, and by applying pressures. The lattice constant negativity is reduced and the bandgap is reduced, and in the same application of the initial pressures, it is out of the semiconducting nature and tends towards the metallic property. 

In this research, the structural and phononic properties of the combination of cerium carbide and lanthanum carbide have been investigated using first principle calculations based on density functional theory by Espresso quantum. The results of density of states calculations showed that both cerium carbide and lanthanum carbide compounds have metallic properties. The structural calculations performed using the LDA approximation for both compounds showed almost the same compressibility and hardness. The results of phonon calculations did not show any frequency gap for cerium carbide composition, while similar calculations in the range of 229 cm-1 to 261 cm-1 show a frequency gap for lanthanum carbide composition. The results show that there is no acoustic oscillation in the range of the observed gap and light and heat will not pass through the crystal. 

In this paper we investigated the structural, electronic, optical, dynamic and thermodynamic properties of CuSbTe2 compound by using quantum espresso computational package within density functional theory framework with GGA and mBJ approximations. Density of states crosses the fermi surface that illustrates the metal behavior of this compound. For CuSbTe2 compound the dynamic properties such as phonon dispersion and phonon density of states curves are calculated. Specific heat capacity at constant volume at room temperature reached 48R and at high temperatures is constant which have a good compatibility with experience. The absence of negative phonon frequencies shows the stability of this compound. Thermoelectric properties such as seeback coefficient for CuSbTe2 compound at room temperature (300K) are calculated and its value is 5.5μV/K.

In this paper, the electronic and thermoelectric properties of Sb1-xAxNSr3, A=P, As, and Bi has been investigated. The calculations were done within density functional theory and using pseudo-potential method with plane wave. HSE hybrid functional was used for exchange correlation potential. The electronic results showed that band gap increases by doping P and As atoms and decreases by doping Bi atom. The seebeck coeficient, electrical conductivity and electronic thermal conductivity were calculated in three temperatures of 300, 500 and 700 K. The electrical and electronic thermal conductivity increases with temperature. Seebeck coefficient in the room temperature were obtained higher than that of other temperaturs. The maximum Seebeck coefficient of Sb1-xPxNSr3 was higher than that of others.

In this Paper, the structural, electronic and optical properties of TiO2 compound are investigated and calculated.The calculations were performed using the full potential linear augmented plane wave (PP-PW) method within density functional theory (DFT)and using the Wien2k package.Structural properties such as lattice constants, bulk modulus and compressibility of this combination are calculated and are well compatible with existing experimental values.The presence of a band gap is the reason why it is semiconductor.The study shows that in some points of the energy interval, it has a negative value. Therefore, no waves are emitted in the composition in this energy range.In addition to the optical properties of this compound, due to its importance in the industry, its high refractive index was investigated.

In this research, the electronic and optical properties of the (001) surface of SbNSr3 with SbSr and NSr2 terminations and surface passivation impact on electronic properties were investigated. The calculations were done within density functional theory and using pseudo-potential method. HSE hybrid functional was used for exchange correlation potential. The surface calculations were performed taking into account tetragonal supercell, periodic boundary conditions, sufficient vacuum and minimum force on atoms. Results of the electronic calculations show that despite the semiconducting nature of the SbNSr3 bulk, the electron states were observed near the Fermi level. The active surface bonding was saturated through adsorbing hydrogen atoms on the surface. Due to this, the electron states were removed and opened the band gap in both terminations. The calculated band gap of the SbSr and NSr2 terminations were calculated 0.808 and 1.029 eV, respectively. The optical properties of the SbNSr3 surface were investigated without and with the surface passivation. The results showed that in both terminations, the surface passivation which causes the elimination of surface electron states, leads to an increase in the optical gap. The calculated optical gap in both bare and passivated surfaces for the polarized incident light in the direction perpendicular to the surface was obtained greater than in the direction parallel to the surface. The optical gap of the SbSr termination was calculated to be less than that of the NSr2 termination in agreement with the results of the electronic calculations. It was also observed that the static dielectric constant at both terminations decreases with surface passivation and the static dielectric constant in the z-direction is less than in the x- direction.

In this research, optical properties such as dielectric function; Refractive index; Absorption coefficient
 and electron energy loss spectrum of SnBi2Te4 topological insulator have been calculated by using of Full potential linear augmented plane waves (FP-LAPW) within the framework of density functional theory (DFT) implemented in Wien2k code. PBE and LDA approximations have been used to calculate the exchange-correlation potential. The optical gap and the static refractive index have been obtained about 0.35 eV and 6.2. The absorption coefficient has two main peaks in 2.46eV and 7.36 eV that the second maximum is in the range of UV range and predicts that this material can be used in the field of UV-related devices such as sensors and detectors. The results are consistent with other available data.

Titanium carbide (TiC) and zirconium carbide (ZrC) are among well-known transition-metal carbide ceramics which have received great attention in the past decades. This comes from their excellent properties including high melting temperature, extraordinary strength at high temperatures, chemical and mechanical stability, low density and, good resistance to corrosion and oxidation. All these unique and extraordinary features lead to the widespread applications of these compounds, including in cutting tools, information storage technology, hard and thin coatings as protectors of electronic surfaces protector, and optoelectronic devices. In this paper, the structural and thermodynamic properties of titanium carbide and zirconium carbide as a function of temperature and pressure were investigated, by using the density functional theory method and quasi-harmonic Debye model. The obtained structural results by using different equations of state showed that the structural parameters are in good agreement with the experimental results. The investigation of temperature and pressure effects on the bulk modulus indicated that zirconium carbide and titanium carbide have good strength at high temperatures and pressure, respectively. Also, the Gibbs free energy result showed that zirconium carbide remained stable up to high temperatures and this justifies its high melting temperature. Calculations of thermodynamic properties such as Debye temperature, specific heat capacity at constant volume and pressure, and thermal expansion coefficient also represent the good performance of zirconium carbide and titanium carbide compounds at high temperatures and pressures.

ing the full potential linearized augmented plan wave in framework density functional theory(DFT) with wien2k code. The band structure and energy gap of the bulk structures are calculated with GGA-PBE, GGA+U and GGA+MBJ approximations, and results obtained from the MBJ function are more consistent with the reported experimental results. The optical properties such as real and imaginary parts of dielectric function, absorption coefficient, reflection coefficient, refractive index, extinction coefficient, conductivity and electron energy loss spectrum have been calculated and studied. The results show that the bulk of tetragonal ferrite bismuth is a semiconductor with a optical gap of 2.7 electron volt. Since compound has magnetic properties, all calculations of compound is performed in full spin polarization.

Cathode is the most important part of Li-ion batteries and it determines performance and behavior of these energy storage devices. LiFeSO4F as a cathode material with Tavvorite structure and C2/c space group is investigated by density functional theory (DFT) using Wine2k program package. Calculation were performed by different methods GGA, GGA+U and PBE-Fock-α (a Hybrid Functionals method, HF). Structural calculations were demonstrated structural stability after Lithium ion extraction. Theoretical voltage was evaluated, using total energy of the considered structures. The most agreeable calculated voltage in comparison with the experimental value (3.9 V) belongs to HF method. According to obtained Density of States (DOS) diagrams, lithiated structure is n-type semiconductor and delithiated structure is p-type semiconductor. This material had been suggested as a potentially high rate capable cathode due to its unclose structure and high diffusion coefficient of Li-ions in the structure; however, inversely-biased-diode phenomenon turns it as low rate capable in comparison with oxide cathode materials. For the first time, internist-like and extrinsic-like band gap were calculated and reported for the cathode material.

In this research, the optical properties of tungsten disulfide including dielectric function, the static refractive index, the imaginary part of the dielectric function, optical band gap, energy loss spectrum and its magnetic properties have been studied. Calculations have been done by using Quantum Espresso package which is based on density functional theory and pseudo-potential technique. The static refractive indices of this compound at diffrent x and z directions were calculated 3.66 and 2.55, respectively. The amount of optical band gap, obtained from the imaginary part of dielectric function, was estimated to be 1.45 eV. In addition, bulk plasmon energy, obtained from energy loss spectrum at x and z directions, were obtained to be 17.95 eV and 17.25 eV, respectively.

Cathode material of Li-ion batteries is responsible for performances and behaviours of these high energy storage devices. In this study, olivine, LiFePO4 (LFP), with Pnma space group (orthorhombic) is evaluated by density functional theory (DFT) using Wien2k program. Calculations were performed by different methods, i. e. LSDA, PBE-GGA, LSDA+U, GGA+U, modified Becke-Johnson (mBJ) and PBE-Fock-a (a Hybrid Functionals method, named HF). Assessments showed structural stability after extraction of one Li per formula. The relaxed structure using LSDA (GGA) calculations was underestimated (overestimated) regarding experimental structure data.  According to the calculations, the most shrinkage after Li extraction is occurred in ab plane, which could cause uniform domino-cascade Li extraction in c direction. This phenomenon would lead to constant voltage in charging/discharging. Considering experimental reaction voltage value is 3.2 V, the closest theoretical calculated voltage values belonged to GGA+U, GGA and mBJ methods. According to calculated density of states (DOS) diagrams, lithiated structure (LiFePO4) is N-type semiconductor and delithiated structure (FePO4) is P-type semiconductor. Inversely-biased-diode phenomenon turns this material as low rate capable in comparison by the oxide cathode materials. However, based on the DOS configurations, its rate capability is better than many of polyanion cathodes. Extraction of Li (turning Fe ionic state from II to III) leads to conductivity enhancement. The empty 3d-Fe orbitals are responsible for this phenomenon.

Li2FeSiO4 is one of the most promising cathode materials for application in high energy density Li-ion batteries. In order to evaluate the Li2FeSiO4 theoretically, its four known polymorphs are considered using density functional theory (DFT) framework. First, different DFT methods (LSDA/+U, GGA/+U) were employed for the best known polymorph (Pmn21) in two atomic-radial frame works (RFe=2.0 and RFe=1.75). Then, the results were compared by the experimentally published data in order to determine better atomic-radial framework for each DFT methods, i.e. LSDA(+U) and GGA(+U). The selected optimum methods were used for evaluation of the other polymorphs. Structural properties, structural stability after Li extraction, relative thermodynamic stability, theoretical voltage and electrical properties were calculated for all the polymorphs. In all these investigations polymorphs properties have been compared. Results showed structural stability of all polymorphs after extraction of one Li per formula. The most energetically stable polymorphs were determined to be P21/n and mod-Pmn21, respectively. Using the performed theoretical studies, we proposed a number of mechanisms to understand behaviors of the cathode material.

In this paper, electronic and optical properties of chalchogenid 2( , , ) CuSbX X Se S Te  compounds have been studied in the surface state at (001) direction. Calculations done using pseudo-potential method within the framework of density functional theory (DFT) by using Quantum-Espresso software with the generalized gradient approximation (GGA). Band gap of 2( , ) CuSbX X Se S  in the bulk state is 0.81 and 0.93 eV respectively but in the surface state these compounds are no band gap, which the surface dangling bonds are covered the band gap. Calculations showed that the 2 CuSbTe in the bulk and surface states is metal. The surface energy, work function, density of states and optical properties in the surface state in the (001) direction are calculated with the three times of duplication layers and a 15 angstrom vacuum respectively. The experimental and theoretical results of these compound in the surface state are not available to comparison this calculation with them.

In this paper structural and electronic properties of AgGaX2(X=S, Se) compounds in bulk and its nanolayers in [112] direction were calculated by using Full Potential Linear Augmented Plane Wave (FP-LAPW) based on density functional theory. The studied nanolayer were confined in the orthorhombic supercell and simulated in [112] direction. All calculations were done by applying the periodic boundary condition along nanolayer surfaces, for example x and y Cartesian coordinate and enough vacuum were provided to isolate the system from its neighbors. For investigation of structural and electronic properties of bulk and nano-layers several approximation were used and the results were in good agreement to the experiment and all materials were semiconductors. The effects of dangling bonds and nanolayer thickness on the electronic properties were explored, also cohesive energy were calculated to investigate the structural stability.

In this study, energy and chemical interaction of ZnO and CdS surfaces interfaced with metal-organic framework (MOF), to improve their properties, have been investigated using density functional theory (DFT). Results show that reformation of structures by hybridation with MOF can increase their stability and improve their properties. Comparison of ZnO and CdS structures predict that deposition of MOF on ZnO substrate can be more effective.