First-principles Molecular Dynamics Method Research Articles

First-principles study of the electronic properties of transition metal nitride surfaces

Transition metal nitride (TMN) surfaces are investigated by using the first-principles molecular dynamics method. Electronic and structural properties of five systems (TiN, ZrN, NbN, HfN and TaN(001)-1×1) are calculated. The optimized surface structures and electronic properties (charge densities, electronic band structures, work function, etc.) are obtained. All calculated electronic states of surfaces are metallic. By the full structural optimization of the surface, the nitrogen and transition metal atoms on the top layer move outward and inward, respectively. This trend of atomic displacements on the outermost layer is similar to our previous results of transition metal carbide (TMC) surfaces. In most surfaces, the values of the work function for the TMN surfaces are lower than those for the TMC surfaces. The values of the work function for relaxed ZrN, NbN, HfN and TaN surfaces are lower than those for unrelaxed (ideal) surfaces. This lowering of the work function is different from our previous results of TMC surfaces.

Electronic Structures of MgB2under Uniaxial and Hydrostatic Compression

Electronic and lattice properties of MgB{$_2$} under uniaxial and hydrostatic compression are calculated. Lattice properties are optimized automatically by using the first-principles molecular dynamics (FPMD) method. Features of the electronic band structures under uniaxial and hydrostatic compression are quite different each other.

Ground-state structure ofβ−C3N4by first-principles calculations

The ground-state structure of $\ensuremath{\beta}\ensuremath{-}{\mathrm{C}}_{3}{\mathrm{N}}_{4}$ has been examined systematically by the first-principles molecular-dynamics (FPMD) method combined with a variable unit-cell shape algorithm based on the Parrinello-Rahman method. For each configuration with the possible symmetry of ${P6}_{3}/m,$ ${P6}_{3},$ and $P3,$ we optimized both atomic coordinates and unit cell shapes at the same time. We observed that the results of each symmetry converged to almost the same atomic structure. We conclude that the ground-state structure of $\ensuremath{\beta}\ensuremath{-}{\mathrm{C}}_{3}{\mathrm{N}}_{4}$ has ${P6}_{3}/m$ symmetry.

Geometric and electronic structures ofSiO2/Si(001)interfaces

Interface structures of ${\mathrm{SiO}}_{2}/\mathrm{Si}(001)$ are studied by using the first-principles molecular-dynamics method. Three crystalline phases of the ${\mathrm{SiO}}_{2},$ cristobalite, quartz, and tridymite, are stacked on the Si(001) substrate and are fully relaxed. When the ${\mathrm{SiO}}_{2}$ layer is very thin (\ensuremath{\sim}7 \AA{}), the lowest-energy structure is the tridymite, followed by the quartz phase. As the ${\mathrm{SiO}}_{2}$ layer becomes thicker (\ensuremath{\sim}15 \AA{}), the quartz phase has lower energy than the tridymite phase. The cristobalite phase on Si(001) is unstable due to large lattice mismatch, and transforms into a different crystal structure. No defects appear at the interface after the successive bond breaking and rebonding, but the energy of the resulting structure is the highest irrespective of the thickness. Calculations of the local density of states show that the band-gap change occurs on the ${\mathrm{SiO}}_{2}$ side, resulting in an effective decrease of the oxide thickness by 2--5 \AA{}.

TiC(100) surface relaxation studied with low-energy electron diffraction intensity analysis

The atomic structure of TiC(100) was determined precisely by low-energy electron diffraction intensity analysis. The surface atomic structure is relaxed; the topmost C atoms are displaced outward and Ti atoms inward with respect to the truncated bulk atomic positions. The results agree qualitatively with earlier theoretical work [D. L. Price, J. M. Wills, and B. R. Cooper, Phys. Rev. Lett. 77, 3375 (1996)] and quantitatively with the latest theoretical results based on the first-principles molecular dynamics method [K. Kobayashi, Jpn. J. Appl. Phys., Part 1 39, 4311 (2000)].

K-0101 窒化ケイ素の機械的特性の第一原理分子動力学計算による評価(J01-1 第一原理強度解析)(J01 第一原理計算・分子動力学法による材料強度評価)

The ideal tensile strength of beta silicon nitride were determined by first-principle molecular dynamics method based on density functional theory with local density approximation. From evaluated stress-strain curves, we found that the tensile strength along c-axis 75GPa is slightly larger than that along a-axis 72GPa. The calculated elastic constants from the stress-strain curves, C_11 and C_33, are in good agreement with the previous theoretical and experimental results.

First-Principles Study of the Surface Electronic Structures of Transition Metal Carbides

Surfaces of transition metal carbides (TiC, ZrC, NbC, HfC and TaC(001)-1×1) are investigated using the first-principles molecular dynamics (FPMD) method. By the full structural optimization of the surface, the carbon and transition metal atoms on the top layer move outward and inward, respectively. All the calculated electronic states of surfaces are metallic. A non-linear core correction is considered for pseudopotentials of transition metals and plays an important role in the structural optimization.

Electronic Structure of High-Pressure Alumina Polymorphs

Electronic properties are investigated for three alumina polymorphs (corundum, Rh2O3 (II) and Pbnm perovskite), which are predicted as the stable structures under different pressure range, by means of the first-principles molecular dynamics method within local density functional framework. The similarity in electronic properties of the polymorphs of alumina is observed. The effect of possible phase transitions on ruby (Cr+3-doped Al2O3) fluorescences is discussed.

Quantum distribution of protons in solid molecular hydrogen at megabar pressures

Solid hydrogen, a simple system consisting only of protons and electrons, exhibits a variety of structural phase transitions at high pressures. Experimental studies based on static compression up to about 230 GPa revealed three relevant phases of solid molecular hydrogen: phase I (high-temperature, low-pressure phase), phase II (low-temperature phase) and phase III (high-pressure phase). Spectroscopic data suggest that symmetry breaking, possibly related to orientational ordering, accompanies the transition into phases II and III. The boundaries dividing the three phases exhibit a strong isotope effect, indicating that the quantum-mechanical properties of hydrogen nuclei are important. Here we report the quantum distributions of protons in the three phases of solid hydrogen, obtained by a first-principles path-integral molecular dynamics method. We show that quantum fluctuations of protons effectively hinder molecular rotation--that is, a quantum localization occurs. The obtained crystal structures have entirely different symmetries from those predicted by the conventional simulations which treat protons classically.

Structural properties of Ga clusters on Si(111)

Using the first-principles molecular-dynamics method in the energy-minimization application, we calculate the detailed geometry of Ga decamers on the Si(111) surface observed by scanning tunneling microscopy (STM). The calculated relative heights of Ga adatoms agree reasonably well with STM data. Our total-energy results support the Ga-Si honeycomblike triangular decamer model proposed by Lai and Wang [Phys. Rev. B 60, 1764 (1999)] to explain the Ga clusters observed in their STM measurements.

Ab initio studies of Ge addimers on the Si(100) surface

The atomic and electronic structures of Ge dimers adsorbed on top of a Si(100) flat surface has been investigated by first-principles molecular dynamics method. Four high-symmetry configurations have been considered and thoroughly relaxed. The most stable configuration for Ge dimers is found to be on the trough, in the middle of, and parallel to the substrate Si dimers. These results are consistent with recent experimental studies of the system using the scanning tunneling microscopy. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 708–712, 2000

Ab Initio Study of SiC/Metal Polar Interfaces: Relation Between Interface Structure and Schottky-Barrier Height

ABSTRACTAb initio calculations of the polar interfaces between thin films of titanium and cubic silicon-carbide (SiC) have been performed by using the first-principles molecular dynamics method. Stable configurations, adhesive energies and Schottky-barrier height (SBH) for the Si-terminated and the C-terminated interfaces are obtained. The C-terminated interface has covalent C-Ti bonds, while the Si-terminated interface has shown metallic nature. Adhesive energy/SBH of the C-terminated interface is larger/smaller than that of the Si-terminated one, respectively. In order to examine a conventional SBH model, work functions of SiC slab with Si and with C surface and Ti slab have been calculated and SBHs have been estimated from the difference of work functions. In estimated SBHs between the interfaces, the relationship depend on the crystal orientation as (111) and (001).

First-Principles Calculations of the Tensile Strength and Fracture of a Grain Boundary in SiC

The tensile strength and fracture of the non-polar interface of the {122}Σ=9 coincidence boundary in cubic SiC have been examined by using the techniques of the first-principles molecular-dynamics method based on the density-functional theory. This interface is very strong because of the reconstruction of interfacial bonds. The tensile strength is about 80 % of that of bulk crystal. The Young's modulus and the fracture toughness are also comparable to the bulk values. The back Si-C bond of the interfacial C-C bond is broken first because of the large strength of the C-C bond. Critical bond stretchings for the Si-C bond breaking are observed. Changes in the electronic structure are analysed.

Structure and electronic property of [formula omitted] interface

Stable structures and electronic states of Si(100) SiO 2 interface have been investigated with the first-principles molecular dynamics method. Quartz, tridymite, and pseudo β-cristobalite are employed as the initial structures of SiO 2 at the interface to find the stable ones by structural optimization. It is found that the optimized tridymite-type SiO 2 on Si is most stable for a thin (about 7 Å) SiO 2 layer. For a thicker (about 15 Å) layer, however, this structure becomes less stable than the others, and the optimized quartz-type SiO 2 structure becomes most stable. Variation of the band gap perpendicular to the interface has also been investigated. In the SiO 2 region from the structural interface to a point about 1 Å away from it, the band gap remains as narrow as that of silicon. The dramatic change of the band gap takes place in the SiO 2 region from about 1 to 4 Å away from the interface.

Theoretical Study of Na Clusters by First Principle Molecular Dynamics Calculations

Based on molecular dynamics simulations with first-principle forces, we have studied the Na clusters. Comparing with available high-precision experimental data (e.g., geometric structure of Na3), we then can assess validity of various first-principle theoretical methods. The first-principle molecular dynamics method is a powerful tool to study properties of clusters. Therefore, we can elucidate evolution of growth of Na clusters.

Macrocycle-Macrocycle Interactions within One-dimensional Cu Phthalocyanine Chains

Interactions between planar Cu phthalocyanine molecules, CuPc , arranged in vertical stacking sequence are studied by first-principles density functional (DF) and semiempirical molecular dynamics (MD) methods. Dependence of electronic structure upon interplanar spacing and staggering angle is analyzed in order to interpret observed electrical conductivity and magnetic coupling behavior of partially oxidized Cu ( pc ) I . MD predictions of equilibrium structure of dimers and tetramers are in excellent agreement with experiment on the linear chain species. DF predictions of interaction energy of dimers are compared with MD and experiment.

Structures and Electronic States of Si/SiO2 Interface.

Stable structures and electronic states of Si(100)/SiO2 interface are investigated using the first-principles molecular dynamics method. Quartz, tridymite, and pseudo beta-cristobalite are employed as the initial structures of the SiO2 at the interface to find the stable ones by the structural optimization. It is found that the optimized tridymite-type SiO2 structure on Si is the most stable for thin (about 7A) SiO2 layer. For the thicker (about 15A) layer, however, this structure becomes less stable and the optimized quartz-type SiO2 structure is the most stable. The band gap variation along the direction perpendicular to the interface is also investigated for the optimized structures. In the SiO2 region within 1A from the structural interface, the band gap remains as narrow as that of silicon. The drastic change of the band gap takes place in the SiO2 region between 1 and 4A.

First-principles study on electronic and elastic properties of BN, AlN, and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at Γ point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1–D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones.

Ab Initio Study of Grain Boundaries in SiC and Si

We have obtained the atomic and electronic structure of the {122}Σ=9 grain boundaries in Si and SiC using the first-principles molecular-dynamics method based on the density-functional theory. The results are compared with our previous tight-binding calculations. The agreement between the present and tight-binding results is rather good about the energy and configurations, although serious disagreement exists about the electronic structure of the SiC boundary. The results are also compared with our recent high-resolution electron microscopy observation in SiC, which shows the real existence of the calculated configuration.

Ab Initio Tensile Tests of Grain Boundaries in SiC

Ab initio tensile tests, namely, first-principles calculations of the tensile strength and fracture have been applied to a coincidence boundary in SiC for the first time, using the first-principles molecular-dynamics method beased on the density-functional theory. The non-polar interface of the {122}Σ=9 boundary has been dealt with. The stable configuration contains reconstructed five-membered and seven-membered rings and interfacial C-C and Si-Si wrong bonds. This interface is very strong because of the reconstruction, although the C-C and Si-Si bonds have large effects. The back bond of the C-C bond is broken first because of the short bond length and large strength of the C-C bond. Then the interfacial Si-C bonds are broken, and the Si-Si bond is broken. The ab initio tensile test is a powerful tool to investigate the intrinsic strength and fracture of interfaces through the behavior of electrons.