Density Functional Perturbation Theory Method Research Articles

Strain-induced modification in thermal properties of monolayer 1T-ZrS2 and ZrS2/ZrSe2 heterojunction.

This paper systematically analyzes the phonon dispersion curves of single-layer ZrS2, ZrSe2, and ZrS2/ZrSe2 heterostructures under different strains. The phonon spectra and thermal parameters of the three structures were obtained based on the density functional perturbation theory method. The upper limits of strain that different monolayers and heterojunctions can withstand were studied. The monolayers ZrSe2 and ZrS2 can withstand up to 8% biaxial tensile strain, and the ZrS2/ZrSe2 heterojunction can withstand up to 6% biaxial tensile strain. In addition, the van der Waals force of the heterojunction may cause phonon tearing in the vertical direction. The application of biaxial tensile strain can adjust the thermal properties of the system to a large extent, which is similar to the strain effect in the pristine case. When the temperature rises, the entropy enthalpy of the three structures also gradually increases, the free energy gradually decreases, and the heat capacity of the system gradually increases until it tends to be stable. Taking single-layer ZrS2 as an example, we analyzed the change curve of thermal properties of single-layer ZrS2 under tensile strain. The results show that with the gradual increase of strain, the crystal's entropy, enthalpy, and free energy change differently. In addition, the heat capacity increases slowly under high temperatures. When all systems reach the limit strain, the phonon spectrum appears to have an imaginary frequency, and the thermal properties decrease significantly. This paper uses the first-principle calculation method based on density functional theory, and the PBE exchange-correlation function based on generalized gradient approximation (GGA) is selected for a specific calculation. The density functional perturbation theory (DFPT) calculates the full kinetic matrix. Because the lattice constants of ZrS2 and ZrSe2 are similar and have similar periodicity, the corresponding unit cells are used for structural optimization and property calculation. The Brillouin zone is integrated using the K points generated by the Monkhorst-pack method. For single-layers ZrS2 and ZrSe2, 8 × 8 × 1K-point grid is selected, and for ZrS2/ZrSe2 heterojunction, 8 × 8 × 2K-point grid is selected. A vacuum layer of 30Å was added in the vertical direction to avoid interlayer interaction. The non-conservative pseudopotential method is used to optimize the structure, and the optimization convergence is set as follows: the cutoff energy is set to 700eV, the convergence threshold of the maximum force between atoms is 0.01eV/Å, the convergence threshold of the maximum energy change is set to 1 × 10-9eV, and the convergence threshold of the maximum displacement is 0.001Å.

Anisotropic ductility and thermoelectricity of van der Waals GeAs.

Wearable thermoelectric applications require materials with both high energy conversion efficiency and excellent flexibility/deformability. Inorganic thermoelectric materials have shown high conversion efficiency, but they are usually brittle and have poor mechanical flexibility, which makes their integration into flexible devices a challenging task. GeAs is a group IV-V binary compound with a van der Waals layered structure, and its thermoelectric response has been reported. Herein, we investigate the mechanical and thermoelectric properties of GeAs crystal by a combination of density functional theory and density functional perturbation theory methods. Our results show that GeAs features a moderately dispersive valence band and multivalley convergence, which give rise to a large Seebeck coefficient and power factor when it is properly p-doped. Remarkably, its electrical transport in the out-of-plane direction even outperforms that in the in-plane direction, while phonon transport is suppressed, leading to a predominant thermoelectric response in the vertical direction. More interestingly, GeAs demonstrates a structural stiffness higher than thermoelectric CuInTe2 and PbTe, and a ductility ratio comparable to a recently discovered plastic semiconductor, InSe. The stress-strain curve simulation reveals that GeAs can withstand deformations up to 20%. These findings showcase GeAs as a ductile thermoelectric material suitable for wearable devices and energy conversion.

Optical and electronic properties of SiTex (x = 1, 2) from first-principles

The optical and electronic properties of α-SiTe, β-SiTe, and RX-SiTe2 are investigated. A detailed analysis of electronic properties is done using standard density functional theory (DFT) and hybrid functional methods. The static dielectric properties are investigated using the density functional perturbation theory method. The optical properties are studied under three different methods: standard DFT, many-body Green's functions, and the Bethe–Salpeter equation. Our calculations show that the SiTe compounds possess extremely high static dielectric constants in their bulk forms [ε0(⊥) = 68.58 and ε0(‖) = 127.29 for α-SiTe, and ε0(⊥) = 76.23 and ε0(‖) = 74.61 for β-SiTe]. The frequency-dependent dielectric functions Im(ε) have very large values (>100) in the optical regime, which are among the highest of layered materials, suggesting them as excellent light absorbents in the corresponding frequencies. α-SiTe exhibits a high degree of optical anisotropy as compared to the other two compounds, consistent with their structural configurations. A strong interlayer excitonic effect is observed in bulk RX-SiTe2. In addition, an analysis of Raman intensity is also performed.

First-principles investigations on the electronic structures, polycrystalline elastic properties, ideal strengths and elastic anisotropy of U3Si2

First-principles calculations based on density functional theory with on-site Coulomb correction were used to examine the electronic structures, polycrystalline mechanical properties, ideal tensile and shear strengths and elastic anisotropy of U3Si2. The present lattice parameters and elastic stiffness constants of single crystal are in good agreement with the experimental data and other theoretical results. Density functional perturbation theory method was used to obtain the dynamical properties of U3Si2, and the well-known Born criteria was investigated to prove the structural stability of the ideal strength calculation model. Also, Voigt–Reuss–Hill model was used to evaluate the polycrystalline properties such as Young’s modulus, the shear modulus, the bulk modulus and so on. The semiempirical formulas for the theoretical estimation of the Debye temperature were adopted. The first-principles computational tensile test (FPCTT) was used to explore the ideal tensile strengths along typical crystal orientations and the ideal shear strengths in several slip systems of P4/mbm U3Si2. Additionally, the elastic anisotropy of P4/mbm U3Si2 was characterized by several anisotropic factors and directional moduli. Within the elastic strain range, the Poisson’s ratios under different single loads were obtained. The least-squares method was used for polynomial fitting of the data obtained by FPCTT method, and the results were satisfactory. These calculated results help to understand the nature of U3Si2 better and provide more useful information to design and develop U3Si2 for a new fuel-cladding solution. The relation curves between stress and strain of U3Si2 were obtained and the ideal tensile and shear strengths of U3Si2 were predicted.

Compressive strain induced dynamical stability of monolayer 1T-MX2 (M = Mo, W; X = S, Se)

The lattice dynamical properties of 1T–MX2 (M = Mo, W; X = S, Se) under different strains were studied by using density functional perturbation theory method. Our results show that all MX2 with 1T phase in our calculations are dynamical instable under zero strain or tensile strain as obvious imaginary frequencies (soft modes) exist. When 3% biaxial compressive strains are applied, the imaginary frequencies remain except that the absolute values of maximum imaginary frequency decrease. With the increase of compressive strain to be 6%, 1T–MoS2, 1T–MoSe2, 1T–WS2 become stable, whereas 1T–WSe2 has small imaginary frequencies. When biaxial compressive strain reaches 9%, all 1T–MX2 are dynamical stable without imaginary frequencies in the phonon dispersion curves. Energy band structures show that all 1T–MX2 are metallic, regardless of zero strain or compressive strain. Therefore, compressive strain could be a practical approach to enhance the stability of 1T–MX2 while maintaining the metallic property.

Structural, dynamical and thermodynamic properties of CdXP2 (X = Si, Ge) from first principles

The structural, elastic, dynamical and thermodynamic properties of CdXP2 (X = Si, Ge) with chalcopyrite semiconductors have been calculated by using the projector augmented wave method within the generalized gradient approximation. The obtained lattice constants and elastic constants of CdXP2 (X = Si, Ge) are compared with the available experimental and other theoretical results, and the agreements between them are quite well. Also, the elastic anisotropy of CdXP2 (X = Si, Ge) have been evaluated. In addition, the phonon dispersion curves and the corresponding phonon density of states of CdXP2 (X = Si, Ge) have been determined by virtue of a linear response approach to density functional perturbation theory method successfully. Finally, the Helmholtz free energy F, internal energy E, entropy S and the constant volume specific heat C V have also been predicted based on quasi-harmonic approximation.

Calculation of Infrared/Raman Spectra and Dielectric Properties of Various Crystalline Poly(lactic acid)s by Density Functional Perturbation Theory (DFPT) Method

We calculated infrared (IR) and Raman spectra of poly(lactic acid) (PLA) polymorphs by employing density functional perturbation theory (DFPT) and a plane wavebasis set. Significant different characteristics are found in the calculated spectra of poly(L-lactic acid) (PLLA) α-form and PLLA/poly(D-lactic acid) (PDLA) stereocomplex (sc) form. Particularly in the carbonyl stretching region, there is only one sharp peak in the sc-form while there are five peaks in the PLLA α-form. A low wavenumber (65 cm(-1)) vibration band of α-PLLA observed in a previous terahertz time-domain spectroscopy study was reproduced in the calculated solid-state PLLA spectra. This band could not be obtained by using DFT (B3LYP/6-31G*) simulation on a single PLA oligomer chain and had been attributed to lattice vibrations in the crystal. The permittivity and polarizability tensors of PLA single crystals were also obtained using the DFPT method and were found to be anisotropic.

Multiphase equation of state and elastic moduli of solid beryllium from first principles

Based on ab initio calculations, we provide a consistent modeling in pressure and temperature of the solid phases of beryllium, including theoretical phase diagram, multiphase equation of state (EOS), and elastic moduli. The quasiharmonic approximation (QHA) allows us to determine the whole theoretical phase diagram: at room temperature, QHA predicts the hexagonal compact $(\ensuremath{\alpha}\text{-hcp})$ phase as the most stable structure up to 400 GPa, where a transition toward the body-centered-cubic $(\ensuremath{\beta}\text{-bcc})$ phase occurs. However, the QHA does not account for the low-pressure-high-temperature bcc phase found experimentally. Combining frozen phonon and density-functional perturbation theory methods, we show that soft phonon modes as reservoirs of entropy may stabilize the low-pressure bcc phase. However the thermodynamic stability of this phase has still to be established. We provide the QHA multiphase EOS in analytic form and an evaluation of the uncertainties on the QHA solid-solid and solid-liquid phase boundaries. Then, we calculate the density and temperature dependence of elastic constants by determining the free energies of strained structures. Consistency between the polycrystalline (i.e., averaged over the elastic constants) and the EOS isothermal bulk moduli is achieved for both phases. Our results are in fair agreement with the most recent experimental data and permit us to raise questions on some experimental moduli.

Room-Temperature Synthesis of Nickel Borides via Decomposition of NaBH4 Promoted by Nickel Bromide

We report the formation of nickel borides, at room temperature and pressure, from the decomposition of NaBH(4) promoted by the addition of nickel bromide at different concentrations in a dispersing organic medium, tetrahydrofuran and pentane. The nickel borides, formed as amorphous powders, were analyzed, and the structure information served as input for modeling a periodic lattice structure with the same composition. Experimentally, the nickel boride phases were predominantly composed of a boron-rich phase with composition NiB(3). Combining FT-IR, X-ray diffraction analyses, and theoretical structure determination, we suggest for it a monoclinic structure, with symmetry group P2(1)/c, lattice parameters a =3.038 Å, b = 8.220 Å, c = 5.212 Å, α = β = 90.00° and γ = 87.57°. The enthalpies of formation of the nickel boride phases, as well as the lattice stability, were calculated using density functional theory and density functional perturbation theory methods.

The calculation of the vibrational frequencies of crystalline compounds and its implementation in the CRYSTAL code.

The problem of numerical accuracy in the calculation of vibrational frequencies of crystalline compounds from the hessian matrix is discussed with reference to alpha-quartz (SiO(2)) as a case study and to the specific implementation in the CRYSTAL code. The Hessian matrix is obtained by numerical differentiation of the analytical gradient of the energy with respect to the atomic positions. The process of calculating vibrational frequencies involves two steps: the determination of the equilibrium geometry, and the calculation of the frequencies themselves. The parameters controlling the truncation of the Coulomb and exchange series in Hartree-Fock, the quality of the grid used for the numerical integration of the Exchange-correlation potential in Density Functional Theory, the SCF convergence criteria, the parameters controlling the convergence of the optimization process as well as those controlling the accuracy of the numerical calculation of the Hessian matrix can influence the obtained vibrational frequencies to some extent. The effect of all these parameters is discussed and documented. It is concluded that with relatively economical computational conditions the uncertainty related to these parameters is smaller than 2-4 cm(-1). In the case of the Local Density Approximation scheme, comparison is possible with recent calculations performed with a Density Functional Perturbation Theory method and a plane-wave basis set.

First Principles Modeling Of High-K Dielectric Materials

ABSTRACTFirst-principles calculations are performed for high-K gate dielectric materials using model bulk and interface systems. Detailed electronic structures and atomic configurations are investigated for transition metal (Hf and Zr) oxide, metal doped silicate bulk system and a model Si-silicate interface system. Pseudo polymorphs of metal oxides are investigated to elucidate the underlying driving mechanisms in microscopic configurations of metal oxides and silicates in amorphous structures. We studied energetics and electronic structure of metal oxide pseudo morph with varying oxygen coordination. Dielectric constants of metal oxide and silicate materials are also investigated using the density functional perturbation theory method implemented in the ABINIT code. Electronic and dielectric properties of silica interface layers between high-κ dielectric and Si substrate are investigated leading to a confirmation that 1 nm is the physical limit of gate oxide thickness. Furthermore silica interface layer is found to have small dielectric constant of 3.4∼3.9.

First Principles Modeling Of High-K Dielectric Materials

ABSTRACTFirst-principles calculations are performed for high-K gate dielectric materials using model bulk and interface systems. Detailed electronic structures and atomic configurations are investigated for transition metal (Hf and Zr) oxide, metal doped silicate bulk system and a model Si-silicate interface system. Pseudo polymorphs of metal oxides are investigated to elucidate the underlying driving mechanisms in microscopic configurations of metal oxides and silicates in amorphous structures. We studied energetics and electronic structure of metal oxide pseudo morph with varying oxygen coordination. Dielectric constants of metal oxide and silicate materials are also investigated using the density functional perturbation theory method implemented in the ABINIT code. Electronic and dielectric properties of silica interface layers between high-κ dielectric and Si substrate are investigated leading to a confirmation that 1 nm is the physical limit of gate oxide thickness. Furthermore silica interface layer is found to have small dielectric constant of 3.4∼3.9.