Wave functions for two- and three-electron atoms and isoelectronic ions

We have developed simple wave functions for two- and three-electron atoms and ions, which have the correct structure when one of the electrons is far away, or when two of the particles are close to each other. These essentially parameter-free wave functions allow us to deduce fairly accurate values for the energies, , for multipolar polarizabilities of two-electron atoms and ions, and for the coefficients of the asymptotic density. Received: 5 August 1998 / Received in final form: 27 November 1998     

A simple model potential description of the alkaline earth isoelectronic sequences

We have developed a simple model potential with a hard core and the correct large-r Coulombic behaviour, to describe the interaction of an electron with a closed shell. One has an exact, analytic ground state wave function for this potential. This potential is used to develop two-electron perturbed and unperturbed wave functions, with the correct asymptotic behaviour and cusp conditions. These wave functions allow us to obtain accurate values for the two-electron energies, polarisabilities, hyperpolarisabilities, and dispersion coefficients of alkaline earth sequences. Many of these results are the only ones available in the literature. Received 29 July 1999 and Received in final form 16 November 1999     

Spectral patterns and ultrafast dynamics in planar acetylene

A diabatic correlation diagram procedure is used to classify energy and intensity patterns in a planar model for coupled stretches and bends of acetylene using an effective spectroscopic fitting Hamiltonian. Analysis with polyad phase spheres accounts for the observed patterns in terms of classical phase space structure and bifurcations of the normal modes. Received 29 August 2000 and Received in final form 14 September 2000     

A density functional theoretic study of novel silicon-carbon fullerene-like nanostructures: Si40C20, Si60C20, Si36C24, and Si60C24

Fullerene-like silicon nanostructures with twenty and twenty-four carbon atoms on the surface of the Si₆₀ cage by substitution, as well as inside the cage at various orientations have been studied within the generalized gradient approximation to density functional theory. Full geometry optimizations have been performed without any symmetry constraints using the Gaussian 03 suite of programs and the LANL2DZ basis set. Thus, for the silicon atom, the Hay-Wadt pseudopotential with the associated basis set is used for the core electrons and the valence electrons, respectively. For the carbon atom, the Dunning/Huzinaga double zeta basis set is employed. Electronic and geometric properties of these nanostructures are presented and discussed in detail. Optimized silicon-carbon fullerene like nanostructures are found to have increased stability compared to the bare Si₆₀ cage and the stability depends on the number and the orientation of carbon atoms, as well as on the nature of silicon-carbon and carbon-carbon bonding.     

The continuum gauge field-theory model for low-energy electronic states of icosahedral fullerenes

The low-energy electronic structure of icosahedral fullerenes is studied within the field-theory model. In the field model, the pentagonal rings in the fullerene are simulated by two kinds of gauge fields. The first one, non-abelian field, follows from so-called K spin rotation invariance for the spinor field while the second one describes the elastic flow due to pentagonal apical disclinations. For fullerene molecule, these fluxes are taken into account by introducing an effective field due to magnetic monopole placed at the center of a sphere. Additionally, the spherical geometry of the fullerene is incorporated via the spin connection term. The exact analytical solution of the problem (both for the eigenfunctions and the energy spectrum) is found.     

Combined experimental and theoretical DFT study of molecular nanowires negative differential resistance and interaction with gold clusters

Electric-field-assisted assembly has been used to place rod-shaped metal nanowires containing 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecules onto lithographically defined metal pads. These junctions exhibited negative differential resistance. The quantum chemical approach was used to compare the properties of Au-bonded 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecule and a molecule that does not exhibit the negative differential resistance, Au-bonded 4-[[4-(phenylethynyl) phenyl] ethynyl] benzenthiol. The influence of the static electric field and charge variation were modelled for both systems.     

Structure and non-collinear magnetism of iron linear chains

Atomic structures and non-collinear magnetic moments are calculated by the first principle molecular dynamics for Fe₅ and Fe₆ linear chains with several fixed and free chain-lengths. The dimerization appears in the optimized atomic structures of all the chains. For the Fe₅, the magnetic arrangement is parallel for a large chain-length and changes to non-collinear with decreasing the chain-length. For the Fe₆, the magnetic arrangement is antiparallel in a unit of dimer for a small chain-length and changes to non-collinear with increasing the chain-length. These magnetic behaviors are simulated by a simple J₁-J₂ Heisenberg model.     

Electron transport in double-walled carbon nanotubes

The transport properties of finite length double-walled carbon nanotubes subject to the influences of a transverse electric field and a magnetic field with varying polar angles are investigated theoretically. The electrical conductance, thermal conductance and Peltier coefficient dependences on the external fields and symmetric configuration are studied in linear response regime. Prominent peak structures of the electrical conductance are predicted when varying the electric field strength. The features of the conductance peaks are found to be strongly dependent on the external fields and the intertube interactions. The heights of the electrical and thermal conductance peaks display the quantized behavior, while those of the Peltier coefficient do not. The conductance peaks are found to be broadened by the finite temperature.     

Structural dependence of exciton in carbon nanotubes

The binding energies and sizes of excitons, and energy splitting of the bright-dark excitons in single-walled carbon nanotubes have been calculated using the nonorthogonal tight-binding model, supplemented by the long-range Coulomb interaction. It is found that the binding energies and the sizes of excitons not only depend on tube's diameter d, but also its chirality. However, the splitting of the bright-dark excitons mostly depends on 1/d². Our obtained results show that the curvature effect is very important for the exciton excitations in the SWNTs, especially in the smaller diameter ones.     

Direct observation of the non-supported metal nanoparticle electron density of states by X-ray photoelectron spectroscopy

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.     

Electronic instabilities in 3D arrays of small-diameter (3, 3) carbon nanotubes

We investigate the electronic instabilities of the small-diameter (3, 3) carbon nanotubes by studying the low-energy perturbations of the normal Luttinger liquid regime. The bosonization approach is adopted to deal exactly with the interactions in the forward-scattering channels, while renormalization group methods are used to analyze the low-energy instabilities. In this respect, we take into account the competition between the effective e–e interaction mediated by phonons and the Coulomb interaction in backscattering and Umklapp channels. Moreover, we apply our analysis to relevant experimental conditions where the nanotubes are assembled into large three-dimensional arrays, which leads to an efficient screening of the Coulomb potential at small momentum-transfer. We find that the destabilization of the normal metallic behavior takes place through the onset of critical behavior in some of the two charge stiffnesses that characterize the Luttinger liquid state. From a physical point of view, this results in either a divergent compressibility or a vanishing renormalized velocity for current excitations at the point of the transition. We observe anyhow that this kind of critical behavior occurs without the development of any appreciable sign of superconducting correlations.     

Structure and magnetism in carbon nanotubes including magnetic wire

We have studied the electronic structure of the carbon nanotubes which include Fe atomic wire with using the density functional theory. As the stable geometries, we obtained the straight and zigzag wires, which have ferromagnetic and antiferromagnetic alignments, respectively. The antiferromagnets consists of the two ferromagnetic dimers which couple in antiparallel alignment. We presents the band dispersions and the density of states for the magnetic nanotubes. The electronic structure at the Fermi level consists of the Fe 3d and C 2pπ states, which shows a strong hybridization between them.     

New organic FET-like photoactive device, experiments and DFT modeling

We present the possible construction of an organic FET-like photoactive device in which source-drain current through a phthalocyanine ( H₂Pc film is affected by a photo-induced dipolar field in a photoactive “gate” electrode. The influence of the dipolar electric field on charge transfer between H₂Pc molecules is modeled by DFT quantum-chemical calculations on H₂Pc dimers and tetramers.     

Predicting experimental signatures for the oxidation of magnesia supported palladium clusters by density functional theory

We showed in a recent density functional study that small palladium cluster on a MgO surface with F-centers can be oxidized to epitaxial Pd_xO_y nano-oxides below room temperature [1]. Here, we employ density functional theory in order to explore different methods for an experimental verification of the Pd_xO_y formation. The electronic density of states (DOS) of bare, O₂-decorated and of oxidized palladium cluster was calculated. For many cluster sizes a clear difference in the DOS could be observed allowing for a detection of the oxidation with surface sensitive spectroscopic methods. In addition, adsorption sites and stretch frequencies of a single CO molecule on bare and oxidized Pd₄ clusters were calculated. While CO prefers hollow sites on Pd₄, top adsorption sites are found for Pd₄O₂. Markedly different CO stretch frequencies indicate a possible discrimination of bare clusters and oxides by Fourier transform infrared spectroscopy.     

Novel metal-encapsulated caged clusters of silicon and germanium

We report the recent findings of metal (M) encapsulated clusters of silicon from computer experiments based on ab initio total energy calculations and a cage shrinkage and atom removal approach. Our results show that using a guest atom, it is possible to wrap silicon in fullerenelike (f) structures, as sp² bonding is not favorable to produce empty cages unlike for carbon. Transition M atoms have a strong bonding with the silicon cage that are responsible for the compact structures. The size and structure of the cage change from 14 to 20 Si atoms depending upon the size and valence of the M atom. Fewer Si atoms lead to relatively open structures. We find cubic, f, Frank-Kasper (FK) polyheral type, decahedral, icosahedral and hexagonal structures for M@Si_n with n = 12-16 and several different M atoms. The magic behavior of 15 and 16 atom Si cages is in agreement with experiments. The FK polyhedral cluster, M@Si₁₆ has an exceptionally large density functional gap of about 2.35 eV calculated within the generalized gradient approximation. It is likely to give rise to visible luminescence in these clusters. The cluster-cluster interaction is weak that makes such clusters attractive for cluster assembled materials. Further studies to stabilize Si₂₀ cage with M = Zr, Ba, Sr, and Pb show that in all cases there is a distortion of the f cage. Similar studies on M encapsulated germanium clusters show FK polyhedral and decahedral isomers to be more favorable. Also perfect icosahedral M@Ge₁₂ and M@Sn₁₂ clusters have been obtained with large gaps by doping with divalent M atoms. Recent results of the H interaction with these clusters, hydrogenated silicon fullerenes as well as assemblies of clusters such as nanowires and nanotubes are briefly presented.     

Electronic properties of double-layer carbon nanotubes

The electronic spectra for double-wall zigzag and armchair nanotubes are found. The influence of nanotube curvatures on the electronic spectra is also calculated. Our finding that the outer shell is hole doped by the inner shell is in the difference between Fermi levels of individual shells which originate from the different hybridization of π orbital. The shift and rotation of the inner nanotube with respect to the outer nanotube are investigated. We found stable semimetal characteristics of the armchair DWNTs in regard of the shift and rotation of the inner nanotube. We predict the shift of k_F towards the bigger wave vectors with decreasing of the radius of the armchair nanotube.     

Water-silica interaction in clusters

The interaction of water with SiO ₂ is an important problem in geophysics, materials physics, and environmental science. In this paper, we present recent results on studies of H ₂ O-silica clusters from first-principles Born-Oppenheimer molecular dynamics calculations. Bond strength and chemical stability are investigated as a function of cluster size and chemical composition. Both physisorption and chemisorption of water molecules on the clusters are discussed via analysis of energetics. Calculations of clusters are compared with the results from extended surfaces. The validity of clusters as models of surfaces is discussed.     

A comparative study on the interaction between tin(II) porphyrin and selected group 8B metals for potential application as reduction catalysts

We study the interaction between tin(II) porphyrin (SnPor) with platinum and non-precious Group 8B metals (iron, cobalt and nickel) by density functional theory and discuss the electronic properties of the resulting products. We also model the interaction of the resulting compounds with water where applicable. Our studies indicate that, SnPor-Ni possesses electronic properties similar to SnPor-Pt, suggesting that it may possess similar photocatalytic properties for reduction reactions, such as converting water to hydrogen gas.     

Electronic properties of Ge–Si nanoparticles

Using a parameterized density-functional tight-binding method we have calculated the electronic and structural properties of Ge–Si nanoparticles. Starting with a spherical part of a zinc-blende/diamond crystal (with the center of the sphere at the mid-point of a nearest-neighbour bond) we have constructed initial structures that subsequently were allowed to relax. Structures consisting solely of Ge atoms or solely of Si atoms were studied, together with core-shell structures for which one semiconductor forms a shell on the core of the other semiconductor. Moreover, homogeneous, ordered SiGe structures as well as structures with a semisphere of one semiconductor and a semisphere of the other were also considered. In analysing the results special emphasis is put on identifying particularly stable structures, on explaining the occurrence of those, on the spatial distribution of the frontier orbitals, and on the variation of the total energy with structure and composition.