Total Energy Pseudopotential Calculations Research Articles

First Principle Study to Correlate Location and Activity of Ruthenium Oxide Incorporated in Alkali-Metal Hexatitanates

Ab initio total energy pseudopotential calculations were performed on ruthenium oxide incorporated alkali-metal hexatitanates to study the location and activity of ruthenium oxide. The influence of alkali metals on the activity is also monitored. The photocatalytic activity of alkali-metal hexatitanates in the presence of ruthenium oxide increases in the order Na > K > Rb. To rationalize the role of the hexatitanate structure on its activity, both ruthenium oxides incorporated and unincorporated structures were investigated. One objective is to monitor whether the ruthenium oxide incorporated in the tunnel has photocatalytic activity or not. The present results suggest that the tunnel structure in the Na-hexatitanate framework is well suited for accommodating Ru species and induces strong interaction between the active species and the host oxide. Density of state calculations were performed on both structures (with and without ruthenium oxide) to verify the activity of ruthenium oxide. It is observed that ruthenium oxide residing in the inner tunnels of the hexatitatanate structure plays an active role in photocatalysis. A novel methodology has been formulated to explore other complicated inorganic materials of interest.

F Atom Adsorption on the Fluorinated Si(001) Surface

First-principles pseudopotential total-energy calculations have been performed to investigate the adsorption of a F atom on the F-terminated Si(001)-2×1 surface. We have determined several stable sites for the adsorbed F atom where energy differences and energy barriers between these adsorption sites are relatively small. It is shown that the incoming F atom can be adsorbed on the fully fluorinated Si(001) surface and break the substrate Si–Si bonds, which is contrary to the results of previous empirical potential simulation.

Can atomic force microscopy achieve atomic resolution in contact mode?

Atomic force microscopy operating in the contact mode is studied using total-energy pseudopotential calculations. It is shown that, in the case of a diamond tip and a diamond surface, it is possible for a tip terminated by a single atom to sustain forces in excess of 30 nN. It is also shown that imaging at atomic resolution may be limited by blunting of the tip during lateral scanning.

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.

An ab initio study of the cleavage anisotropy in silicon

Total-energy pseudopotential calculations are used to study the cleavage fracture processes in silicon. It is shown that bonds break continuously and cracks propagate easily on {111} and {110} planes provided crack propagation proceeds in the 〈1̄10〉 direction. In contrast, if the crack is driven in a 〈001〉 direction on a {110} plane the bond breaking process is discontinuous and associated with pronounced relaxations of the surrounding atoms. The discontinuous process is partly a result of some load sharing between the crack tip bond and the neighbouring bond, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the experimentally observed cleavage anisotropy in silicon single crystals.

A Novel Method to Correlate Layer Charge and the Catalytic Activity of 2:1 Dioctahedral Smectite Clays in Terms of Binding the Interlayer Cation Surrounded by Monohydrate

Ab initio total energy pseudopotential calculations were performed on 2:1 dioctahedral smectite to rationalize an understanding of the structure and energetics for the same and to correlate catalytic activity of clay materials with layer charge. We have chosen two clay members from the 2:1 dioctahedral smectite family(1) beidellite and (2) montmorilloniteto monitor the effect of negative charge on the location of interlayer cation. Local softness calculation using Fukui functions generated for clusters derived from the structures of the clay matrixes in the domain of the hard-soft acid-base (HSAB) principle were performed first to locate the active site in clay. Ab initio calculations were followed to determine the most feasible orientation of the single interlayer water molecule in hydrated montmorillonite and beidellite. The role of hydroxyl hydrogen attached with the bridging oxygen connected to octahedral aluminum was also monitored. It is observed that interlayer cation (Na+) has a lesser influence o...

Density functional study of silane adsorption onto Si(100) surface

Total energy pseudopotential calculations based on norm-conserving optimized pseudopotential and density functional theory (DFT) with generalized gradient approximation (GGA) have been used to study the possible pathway and corresponding energy barrier for the initial dissociative adsorption of silane (SiH 4) onto Si(100)-(2×2) surface leading to the formation of SiH 3 and H adsorbed on Si(100)-(2×2) i.e. Si(100)-(2×2)(SiH 3:H). Our calculated results found that the formation of Si(100)-(2×2)(SiH 3:H) after the SiH 4 initial adsorption onto Si(100)-(2×2) surface is energetically favorable. Also the energy barrier, which is calculated to be around 0.72 eV (GGA), occurs due to the elongation of Si–H within SiH 4 accompanying the unbuckling of the buckled Si=Si dimer on Si(100)-(2×2) surface. Finally, the reactive sticking coefficient for SiH 4 on Si(100)-(2×2) is calculated to be around 3×10 −6.

Directional anisotropy in the cleavage fracture of silicon

Total-energy pseudopotential calculations are used to study the cleavage anisotropy in silicon. It is shown that cracks propagate easily on 111 and 110 planes provided crack propagation proceeds in the <1;10> direction. In contrast, if the crack is driven in a <001> direction on a 110 plane the bond breaking process is discontinuous and associated with pronounced relaxations of the surrounding atoms, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the experimentally observed cleavage anisotropy in silicon single crystals.

3×3R30°versus adatom–rest-atom phases on (111) semiconductor surfaces

Total-energy pseudopotential calculations are performed on several competing reconstructions for the Sn-covered and clean Ge(111) surfaces. Our results confirm that the Sn-Ge(111) $\ensuremath{\alpha}$ phase $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{})$ with 1/3 ML Sn coverage, prevails over the adatom--rest-atom $2\ifmmode\times\else\texttimes\fi{}2$ phase (with 1/4 ML Sn coverage) for all allowed values of the Sn chemical potential. This is to be contrasted with the $2\ifmmode\times\else\texttimes\fi{}2$-based reconstructions which are known to prevail on the clean Ge(111) and Si(111) surfaces. Our results suggest that the stabilization of the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ phase for Sn-Ge(111) is caused by the larger size of the adatom, which lowers the energy of the half-filled surface state band, and also increases hybridization with the underlying second-third layer bonding state. The same argument may explain why $\ensuremath{\alpha}$ phases are observed also in Pb-Ge(111), Pb-Si(111), and Sn-Si(111).

The electronic structure and chemical bonding of Ti3GeC2

We have investigated the electronic structure and chemical bonding properties of Ti3GeC2 by performing ab initio pseudopotential total-energy calculations. Our results show that the crystal structure of Ti3GeC2 is a characteristic zigzag chain of Ti-C-Ti-C-Ti-Ge, running parallel to the c-axis. The chemical bonding in Ti3GeC2 is anisotropic and metallic-covalent-ionic in nature with significant contributions from metallic and covalent bonds, which result in the combination of metallic and ceramic properties in Ti3GeC2.

Theoretical investigations of adsorption of fluorine atoms on the Si(001) surface

First-principles pseudopotential total-energy calculations have been performed in order to investigate the initial adsorption of an F atom on the Si(001) surface, the structures of ordered fluorinated surfaces, and their stability against disordering. We have found that the F atom has a high adsorption energy and a high diffusion barrier on the Si(001) surface, and at high F coverage the defected surface containing SiF3 species which is regarded as a precursor for etching is stabilized compared with the ordered surfaces. The thermodynamical instability of the ordered surfaces against local facet formation does not occur at any F coverage. Our results indicate that the disordering of the fluorinated Si(001) surface, which occurs even at low F coverage, may be kinetically induced by the local heating due to the F adsorption.

THE DIFFERENCE BETWEEN THE SURFACE RECONSTRUCTIONS OF AlAs(001) AND GaAs(001)

AlAs(001) is compared with GaAs(001) by ab initio pseudopotential total energy calculation. The results show that either similarities or differences in the stability of the surface reconstructions are obvious. On the Al-rich AlAs surface the reconstruction (2×1)/c (2×2) has an obvious energetic advantage. In this reconstruction the topmost Al atoms abnormally locate below the As atomic layer. Besides, on the As-rich GaAs surface a metastable reconstruction phase of γ(2×4) appears if the surface coverage of As atoms is less than 1, while on the AlAs surface it is fully dimerized (2×1) phase more stable. In addition the thermodynamical analysis also indicates that in the mid-temperature range of MBE growth (e.g. 600°C with As 2 pressure of 2 × 10-6 τ) this (2×1) phase is stable. Further calculations show that 0.2–1 monolayer of Ga atoms deposited on AlAs surface will result in γ(2×4) phase rather than (2×1) phase, while the deposited Ga atoms on the AlAs surface do not go below the topmost As atomic layer. The plausible conclusion is that formation of (2×1) phase on As-rich AlAs surface and the inner location of Al atoms on Al-rich (2×1)/c(2×2) reconstruction surface could be closely related to the obviously different modes of MBE growth on AlAs and GaAs surface, in the mid-temperature range.

Electronic structure of a grain-boundary model inSrTiO3

It is known that grain boundaries (GB's) in strontium titanate $({\mathrm{SrTiO}}_{3})$ play an important and often a controlling role in determining the material's electrical properties. To understand how their electronic structures are related to the GB structures, we have examined two structure models of the \ensuremath{\Sigma}5 GB in ${\mathrm{SrTiO}}_{3}$ obtained by first-principles pseudopotential total energy calculations. The electronic structure of bulk crystal and the relaxed GB models are then studied by using the orthogonalized linear combination of atomic orbitals method. Results are presented for the ground-state structural properties and band structure of bulk ${\mathrm{SrTiO}}_{3},$ the total density of states (DOS), the atom and orbital-resolved partial DOS, effective charges, bond order, charge-density distribution, and near-edge structure of electron energy-loss spectroscopy. It is shown that the GB structures have smaller values of fundamental band gaps, effective charges, and bond orders relative to bulk ${\mathrm{SrTiO}}_{3}.$ There are no GB-induced electronic states within or at the edge of the fundamental band gap. The 100-atom GB model with buckled Sr columns in the GB core is found to be a more likely model. It is also shown that the electron charge distribution across the GB line in ${\mathrm{SrTiO}}_{3}$ is almost balanced.

Ge adatom adsorption, diffusion, and exchange on surfactant-covered Si(111) surfaces

We investigate the effects of various surfactants on the Ge adsorption, diffusion, and exchange on Si(111) surfaces through first-principles pseudopotential total-energy calculations. For surfactant-covered surfaces such as $\mathrm{Si}(111):\mathrm{Ga}\ensuremath{-}1\ifmmode\times\else\texttimes\fi{}1,$ $\mathrm{Si}(111):\mathrm{As}\ensuremath{-}1\ifmmode\times\else\texttimes\fi{}1,$and $\mathrm{Si}(111):\mathrm{Sb}\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$Ge adatoms are generally incorporated into the surfactant layer. On the Ga-covered surface, we find strong interactions between the Ge and Ga atoms, which result in large activation energies for both Ge surface diffusion and exchange with an underlying Ga atom. In the case of As surfactants, the activation energies for adatom diffusion and exchange are much reduced because of the weak couplings between the Ge and As atoms. Similarly, on the Sb-covered surface, the exchange between the adatoms and surfactants takes place easily; however, the surface diffusion is severely suppressed due to a relatively large energy gain by the exchange process.

DX centers in GaAs/Si-δ/AlAs heterostructure

Microscopic mechanisms of impurity spreading in GaAs/Si-δ/AlAs heterostructure have been investigated using an ab initio pseudopotential total energy calculation. Our results showed that silicon atoms can move from the δ-doped plane occupying interstitial positions, favored by the high doped concentration, forming DX centers. The silicon impurity position, out of the δ plane in the AlAs layers, presents an energetically stable configuration, and in the GaAs layers, presents a metastable configuration. As a consequence a silicon doping limit is reached due to the presence of localized deep states inside the band gap, when silicon atoms are in interstitial positions.

Chemical interactions in noncontact AFM on semiconductor surfaces: Si(111), Si(100) and GaAs(110)

Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic force microscopy (AFM) on Si(111), Si(100) and GaAs(110) surfaces. The chemical bonding interaction between a localised dangling bond on the atom at the apex of the tip and the dangling bonds on the adatoms in the surface is shown to dominate the forces and the force gradients and, hence, to provide atomic resolution. The lateral resolution capabilities are tested in both the Si(100) and the GaAs(110) surfaces. In the first case, the two atoms in a dimer can be resolved due to the dimer flip induced by the interaction with the tip during the scan, while in the GaAs(110), we identify the anion sublattice as the one observed in the experimental images.

A density functional study of Ti/MgCl2-supported Ziegler–Natta catalysts

Total energy pseudopotential calculations have been used to examine the adsorption of TiCl4 at both the 110 and 100 surfaces of magnesium chloride. Titanium(IV) chloride is found to bind most strongly on the 100 surface resulting in the formation of a complex with approximately trigonal bipyramidal coordination of titanium, which will dissociate to form TiCl3+ and Cl− with an energy of 127.7 kJ mol−1. Cluster calculations indicate that this site only weakly binds ethene, but does catalyse the formation of C–C bonds with an activation energy consistent with experimental estimates.

Structural transformation in the90°partial dislocation in Si due to Ga impurities

Ga impurities in the 90{degree} Shockley partial dislocation in silicon have been investigated using first-principles total-energy pseudopotential calculations. The results indicate that Ga segregates to the core of the dislocation and destabilizes the asymmetric fourfold coordinated structure, which is known to be the low-energy configuration in pure Si. The segregation energy for Ga in the symmetric core configuration is 0.53 eV/atom. The atomic mechanism for this spontaneous transformation to the symmetric structure is the passivation of quasifive-fold sites in the symmetric core by Ga. {copyright} {ital 1998} {ital The American Physical Society}

Surface-tip interactions in noncontact atomic-force microscopy on reactive surfaces: Si(111)

Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic-force microscopy on Si(111) surfaces. At the distance of closest approach between the tip and the surface, there is an onset of covalent chemical bonding between the dangling bonds of the tip and the surface. Displacement curves and lateral scans on the surface show that this interaction energy and force are comparable to the macroscopic Van der Waals interaction. However, the covalent interaction completely dominates the force gradients probed in the experiments. Hence, this covalent interaction is responsible for the atomic resolution obtained on reactive surfaces and it should play a role in improving the resolution in other systems. Our results provide a clear understanding of a number of issues such as (i) the experimental difficulty in achieving stable operation, (ii) the quality of the images obtained in different experiments and the role of tip preparations and (iii) recently observed discontinuities in the force gradient curves.

DFT–LDA study of NO adsorption on Rh(110) surface

We examine the interaction between NO and the Rh(110) surface using ab initio DFT–LDA pseudo-potential plane-wave total energy calculations. Four different adsorption sites for perpendicular NO are considered. The short-bridge site with linear NO is found to be the optimal adsorption configuration. It is also possible for NO to bond parallel to the surface, and this may be the precursor to NO dissociation.