Pseudopotential Density Functional Theory Calculations Research Articles

Surface effects in GaN growth

By means of plane wave pseudopotential periodic supercell density functional theory calculations, we investigated the energetics of several GaN(0 0 0 1) surfaces containing atomic and molecular adsorbates. The surface structures were selected in order to get insight into GaN epitaxial growth, in situations where NH 3 molecules are employed as the nitrogen precursor and H 2 molecules as the carrier gas, typical of standard vapor phase growth techniques. Therefore, we considered NH 2 complexes and H atoms as molecular and atomic adsorbates, respectively, assuming they derive from molecular dissociation from the gas flows in the growth chambers. We took into account different adsorption stoichiometries, varying the relative proportion of NH 2 and H adsorbates, and different degrees of clusterization of the ad-atoms and ad-complexes. For each structure, we obtained the relaxed atomic configuration and the corresponding total energy. Hence, we describe here the relative structural stability in terms of a phase diagram that identifies the optimal growth conditions to attain at intermediate stages the most favorable equilibrium surfaces.

Multilayer relaxation of Cu(210) studied by layer-doubling LEED analysis and pseudopotential density functional theory calculations

Multilayer relaxation of Cu(210) surface has been studied by layer-doubling low energy electron diffraction (LEED) analysis and pseudopotential density functional theory (DFT) calculations. An excellent agreement between the calculated and measured $I\ensuremath{-}V$ curves has been achieved as judged by direct inspection and a small Pendry R factor of 0.12. We suggest that the layer-doubling method is a suitable choice for quantitative LEED structural studies on high-index metal surfaces with interlayer spacings down to 0.8 \AA{}. Our pseudopotential DFT calculations have reproduced the relaxation sequence determined by the layer-doubling LEED analysis, i.e., $\ensuremath{-}$ $\ensuremath{-}$ $+$ $\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot},$ with the largest quantitative discrepancy of about 0.04 \AA{}. Comparison is made with LEED and DFT studies on other high-index Cu surfaces. Based on this comparison, a general trend for multilayer relaxations of open metal surfaces is proposed.

Precursor states of atomic hydrogen on the Si(100)-(2×1) surface

Using plane wave pseudopotential density functional theory calculations we have identified for the first time precursor states for hydrogen atom chemisorption on the Si(100)-(2×1) surface. These states exist above clean, partially, and fully monohydride-adsorbed surface dimers. In all three cases the dimer bond is broken in the trapped state. A study of the energetics for atomic desorption, abstraction, chemisorption, and migration was carried out. We find that “hot” hydrogen atoms of energies up to approximately 1.3–1.9 eV can be trapped on the surface. These atoms are highly mobile, and we obtained energetics consistent with experimental data from which precursor-mediated adsorption mechanisms have been inferred. The existence of these states provides an understanding of the non-Langmuirian atomic hydrogen adsorption probability, and also underscores the importance of lattice distortions in the interactions of hydrogen with the silicon surface.

Dissociative Adsorption of H 2 on the H/Si(100) Surface: The Effect of Intradimer π-Bonding Disruption

We have studied dissociative adsorption on the H/Si(100) surface by means of pseudopotential density functional theory calculations. This work is directly motivated by the observations of Biederman et al. [Phys. Rev. Lett., 83, 1810 (1999)]: adsorption is significantly promoted in an interdimer configuration of two adjacent singly occupied dimers.” We find that there is no adsorption barrier on the local site where two quasi-free dangling bonds are available; the adsorption energy is estimated to be 2.6 eV per within the local density approximation. We present maximally localized Wannier functions that clearly illustrate the behavior of dangling orbitals upon the approach of The results suggest that intradimer π-bonding disruption is crucial for the significant promotion of adsorption on Si(100). © 2001 The Electrochemical Society. All rights reserved.

CO and NO adsorption and co-adsorption on the Pd(1 1 1) surface

Adsorption and co-adsorption of nitric oxide (NO) and carbon monoxide (CO) on a Pd(1 1 1) surface were studied using pseudopotential density-functional theory calculations. For individual NO and CO molecules we determined the energetically most stable adsorption sites and harmonic frequencies. The results show that at 0.25 monolayer (ML) coverage both the molecules prefer to reside on a hollow site in agreement with experimental and earlier theoretical results. At half of monolayer coverage pure NO occupies only hollow sites but CO molecules have two energetically degenerate structures namely fcc–hcp and bridge–bridge. The harmonic vibrational frequencies were calculated for the most stable structures and the changes of frequencies from 0.25 ML coverage to 0.5 ML were compared to the corresponding experimental shifts. Our results suggest a hollow site adsorption for NO and a hollow–bridge mixture for CO. CO/NO co-adsorption was studied at 0.5 ML coverage with a molecular ratio 1/1. We investigated 16 co-adsorption geometries and found five almost equally stable structures, where NO is mainly on a hollow site, whereas CO occupies either a hollow or a bridge site. For all the examined structures the interaction between co-adsorbed molecules is repulsive. The strength of the repulsion depends on a inter-molecule distance and sharing of the common Pd atom. We calculated harmonic wave numbers for three structures, and found that different adsorbates do not vibrate in resonance.

External Chemical Reactivity of Fullerenes and Nanotubes

ABSTRACTThe external chemical reactivity of graphene sheet, fullerenes and carbon nanotubes has been investigated. The total reaction energy is analyzed with several contributing terms and formulated as a function of the pyramidal angles of C atoms. We have determined the parameters for the formulae from ab initio simulation of graphene. We have applied them to predict hydrogenation energy of several nanotubes and C60, and demonstrated that the predicted total reaction energies are very close to the results of total energy pseudo-potential density functional theory calculations. This analysis can be used to predict the reaction energy and local bonding configuration of a reactant with diverse fullerenes and nanotubes within 0.1 eV accuracy.

Phonons and static dielectric constant inCaTiO3from first principles

${\mathrm{CaTiO}}_{3}$ has a static dielectric constant that extrapolates to a value greater than 300 at zero temperature. We investigate the origin of this large dielectric response on a microscopic level, using first-principles plane-wave pseudopotential density functional theory calculations. The electronic dielectric tensor and the complete set of zone center phonons and ionic Born effective charges are determined for ${\mathrm{CaTiO}}_{3}$ in its low temperature 20-atom per cell orthorhombic phase via frozen phonon electronic structure, polarization, and force constant calculations. Dispersion theory is then used to obtain the dielectric tensor. The dielectric response is dominated by low frequency $(\ensuremath{\nu}\ensuremath{\approx}90{\mathrm{cm}}^{\ensuremath{-}1})$ polar optical modes in which cation motion opposes oxygen motion. The frequencies of these phonons, and thus the dielectric constant, are predicted to be pressure sensitive.

First-principles study of aluminum and sodium complexes with hydrogen in silicon dioxide

Pseudopotential density-functional theory calculations are reported for the microscopic structure of H and Na complexes with substitutional Al in \ensuremath{\alpha} quartz. Equilibrium sites for the H impurity are discussed and their properties are compared to available experimental data. Present calculations confirm previous beliefs on preferred H sites and on assignments of H-induced infrared absorption bands. The equilibrium configuration for the Al-Na complex is calculated and the electronic properties of both complexes are discussed and compared to that of the Al-hole color center. Impurity-specific electronic states are clearly identified.

Theory of electronic and optical properties of 3C-SiC

We study the electronic and optical properties of cubic (3C) SiC, using a combination of first-principles and tight-binding electronic structure calculations. We employ pseudopotential density functional theory calculations, with appropriate corrections to the energy of conduction bands, to investigate the band structure of this material and obtain band gaps that are in agreement with experimental results. The optical properties are then studied within the framework of the empirical tight-binding model, which is fitted to reproduce the first-principles calculations. This approach allows for a thorough investigation of the dielectric functions, the reflectivity, and the refractive index. Critical points are identified and connected to the appropriate transitions in the band structure. The results are in good agreement with available experimental data. In addition, we investigate spin splitting effects.

Electronic properties of solid ( Al12B) Li in the CsCl structure

Pseudopotential and local density functional theory calculations on the electronic structure and geometry have been carried out for the ( Al 12 B) Li solid where an Al 12 B cluster and a Li atom served as the building blocks of the material. Our results show that ( Al 12 B) Li in the CsCl structure will eventually be an aluminum-like metal when the lattice parameter of the solid and the atomic geometry of the clusters are optimized by the crystal field. The packed clusters of Al 12 B tend to have a cuboctahedral geometry.

First-principles spin-polarized calculations on the reduced and reconstructedTiO2(110) surface

We have performed plane-wave pseudopotential density-functional theory calculations on the stoichiometric and reduced TiO2 ~110! surface, the 231 and 132 reconstructions of the surface formed by the removal of bridging-oxygen atoms, and on the oxygen vacancy in the bulk. The effect of including spin polarization is investigated, and it is found to give a qualitatively different electronic structure compared with a spin-paired description. In the spin-polarized solutions, the excess electrons generated by oxygen reduction occupy localized band-gap states formed from Ti (3d) orbitals, in agreement with experimental findings. In addition, the inclusion of spin polarization substantially lowers the energy of all the systems studied, when compared with spin-paired solutions. However, spin-polarization does not change the relative stability of the two reconstructions, which remain energetically equivalent. @S0163-1829~97!02724-0#