Chemisorption of single fluorine atoms on the surface of zigzag single-walled carbon nanotubes: A model calculation

We report a model calculation of the chemisorption energies ΔE_ads of single fluorine atoms on the outer surface of zigzag single-walled carbon nanotubes (Z-SWCNTs) (p,0) with p ranging from 11 to 21. A simplified model based on an effective-mass theory is adopted to describe the electronic structure of the nanotubes. Chemisorption is treated within the Anderson–Newns approach, which takes account of Coulomb interaction between adsorbate electrons. Considering adsorption of an adatom directly on top of a surface carbon atom, we find that in the case of a fluorine atom bonded to the sidewall of the nanotubes, the absolute values of ΔE_ads are in the range for Z-SWCNTs with typical diameters of , larger |ΔE_ads| values being associated with semiconducting tubes. For the latter ones, |ΔE_ads| decreases rather significantly as the radius R of the tubes increases, tending towards the “infinite” radius graphene case, whereas for metallic tubes |ΔE_ads| slightly increases with increasing R. The localized acceptor states induced by a fluorine atom in the band gap of the semiconducting tubes are found to be responsible for such difference in the behaviour of ΔE_ads for the two above-mentioned types of tubes. The results obtained shed light on the possible mechanism of the atomic fluorine adsorption-induced hole-doping of the semiconducting tubes, which might significantly affect the transport properties of these tubes.     

Polarization anisotropy in the optical properties of silicon ellipsoids

A new real space quantum mechanical approach with local field effects included is applied to the calculation of the optical properties of silicon nanocrystals. Silicon ellipsoids are studied and the role of surface polarization is discussed in details. In particular, surface polarization is shown to be responsible for a strong optical anisotropy in silicon ellipsoids, much more pronounced with respect to the case in which only quantum confinement effects are considered. The static dielectric constant and the absorption spectra are calculated, showing that the perpendicular and parallel components have a very different dependence on the ellipsoid aspect ratio. Then, a comparison with the classical dielectric model is performed, showing that the model only works for large and regular structures, but it fails for thin elongated ellipsoids.     

Diffusion properties of Cu(0 0 1)-c(2 × 2)-Pd surface alloys

Structural and diffusion properties of a Cu(0 0 1)-c(2 × 2)-Pd surface and sub-surface ordered alloys are studied by using interaction potentials obtained from the embedded-atom method. The calculated diffusion energies are in agreement with observed kinetics of the surface alloy formation and confirm stability of the underlayer alloy. Activation energy of planar diffusion of palladium at the initial stage of the alloy formation as well as the activation energy of the overlayer-underlayer diffusion of the Pd atoms are in good agreement with those obtained by the scanning tunneling microscopy and low energy electron diffraction measurements, respectively.     

Reflection characterization of multilayer surface films

The reflection of linearly polarized light from a multilayer system of ultrathin dielectric surface films is investigated both analytically in the long-wavelength limit and numerically by the standard way of calculating the reflection characteristics for the layered medium. The second-order approximate formulas for reflection coefficients and characteristic reflection angles are derived and their accuracy is estimated. It is shown that approximate expressions obtained for reflection parameters of multilayer system in the long-wavelength limit are of immediate interest to the solution of the inverse problem for ultrathin layered surface structures. Innovative possibilities for optical diagnostics are generated by means of polarizing and principal angles. For determining the parameters of multiple surface layers an appropriate method is found by combining differential reflectance with ellipsometry.     

Computational study of the surface properties of aluminum nanoparticles

We calculate the surface energy, surface stress, and lattice contraction of Al nanoparticles using ab initio density functional and empirical computational techniques. Ab initio calculations are carried out using the siesta pseudopotential method combined with the generalized gradient approximation. Empirical calculations are conducted using the embedded atom method. The ab initio density functional approach predicts the surface energies of Al nanoclusters to be in the range of 0.9-2.0 J/m². These values are consistent with the surface energy of bulk aluminum and are close to the surface energies of silver nanoparticles calculated in our previous study. In contrast to our previous results for Ag nanoparticles, we found a significant discrepancy between the theoretical values of surface energy and stress for Al nanoclusters. This result could be explained by a greater degree of surface reconstruction in Al clusters than in Ag clusters.     

Electronic theory of the chemisorption effect on surface segregation in transition-metal alloys

The tight-binding Ising model generalized for a system consisting of a transition-metal alloy with chemisorbed adatoms is used to calculate the effect of hydrogen chemisorption on surface segregation in a Cu---Ni alloy. The simple model of chemisorption and of the band structure of the alloy is assumed. The charge-neutrality condition is incorporated into the model.     

First principles study of the electronic and geometric structure of Cu(5 3 2)

Using density functional theory with the generalized gradient approximation and ultra-soft pseudo-potentials, we have calculated structural relaxations of the Cu(5 3 2) surface which contains steps and kinks. We find the relaxation pattern to oscillate dramatically for atoms in the first 10 layers before decaying rapidly in the bulk. The most striking feature is an outward expansion of the relative interlayer separation d₁₂ of 25%. We also find serious discrepancies with relaxation pattern and relaxation amplitudes calculated using embedded atom method potentials that may reflect the limitation of these potentials to accurately describe systems with complex geometries. Full potential calculations reveal a dispersionless surface state along a high symmetry direction in the surface Brillouin zone. Valence charge density along several planes show excess of charge around kink atom and the nature of bonding with other (5 3 2) atoms.     

Evaluation of different models for the dissociation of silane on the Si(1 1 1)7 × 7 surface using the extended Brenner empirical potential

Several models have been proposed in the literature for the initial stages of the dissociative chemisorption of silane (SiH₄) on the Si(1 1 1)7 × 7 surface. In this paper, geometry optimisation calculations using the extended Brenner empirical potential have been performed to determine which of these models yields the minimum energy structure. The lowest energy configurations are found to correspond to the dissociation of silane into SiH₂ and two hydrogen atoms. The minimum energy structure involves the adsorption of the two hydrogen atoms onto the dangling bonds of an adjacent adatom and rest atom, and the insertion of the remaining SiH₂ fragment into one of the adatom backbonds. These results are discussed in the light of the existing experimental data.     

Finding the reconstructions of semiconductor surfaces via a genetic algorithm

In this article we show that the reconstructions of semiconductor surfaces can be determined using a genetic procedure. Coupled with highly optimized interatomic potentials, the present approach represents an efficient tool for finding and sorting good structural candidates for further electronic structure calculations and comparison with scanning tunneling microscope (STM) images. We illustrate the method for the case of Si(1 0 5), and build a database of structures that includes the previously found low-energy models, as well as a number of novel configurations.     

LCAO study of the Cu(1 1 0)-p(2 × 1)O surface

We have performed semi-empirical LCAO calculations of the electronic structure of the Cu(1 1 0)-p(2 × 1)O surface. This has been done accounting for the Cu-Cu interactions by means of a recently proposed set of parameters, which give very good results for the bulk as well as for the surfaces of lowest Miller indices. Furthermore, the O-O interactions, which have been neglected in the preceding similar studies, have been taken into account. The resulting surface bands are in very good agreement with the overall set of the available experimental data. Several issues concerning the physical properties of this surface are addressed in the present paper: the changes induced on the clean surface bands by the adsorption and the reconstruction; the arrangement of the Cu and O atoms in the added rows; the position of the p_y antibonding band of the oxygen. In particular, we have found that the latter has an energy of −0.2 eV at the point. This result confirms an experimental indication in the same direction previously reported by Courths et al. [R. Courths, S. Hüfner, P. Kemkes, G. Wiesen, Surf. Sci. 376 (1997) 43].     

Study of scanning tunneling microscopy images and probable relaxations of the SrTiO3(100) surface by electronic structure calculations

Partial electron density plots were calculated for a model SrTiO₃(100) surface with √5 × √5 ordered oxygen vacancy to examine why the bright spots of the scanning tunneling microscopy (STM) images of SrTiO₃(100) observed in ultrahigh vacuum (UHV) correspond to the oxygen vacancy sites. Possible dependence of the image on the polarity and magnitude of the bias voltage was also discussed on the basis of partial electron density plot calculations. Our study strongly suggests that the UHV STM imaging involves the lowest-lying d-block level of every two Ti³⁺ centers adjacent to an oxygen vacancy, the tip-sample distance involved in the UHV STM experiments is substantially larger than that involved in typical ambient-condition STM imaging, and the Ti⁴⁺ and Ti³⁺ sites of SrTiO₃(100) are reconstructed.     

An energetic criterion to compare the evolution of thermally-excited surface disturbances and nanoindentation-induced defects on thin polymer films

A criterion is developed to predict the resulting evolution process of the following surface defects on thin (17 nm) polystyrene (PS) films on silicon (Si): (i) nanoindentation-induced indents which grow after being heated above the glass transition temperature of PS, T_g, leading to dewetting; (ii) nanoindentation-induced indents which level at temperatures above the T_g, resulting in a flat polymer surface and (iii) indents which are formed and grow spontaneously by thermal treatment above the T_g (thermal film break up). The criterion is based on the concept of the excess surface energy, ΔF_γ, which was introduced in previous reports for cases (i) and (ii). Here, a similar energetic term is used which corresponds only to the effect of the depressions, ΔF_γ(D). The effect of the rims which surround the depressions in cases (i) and (ii) is not taken into account. Measurements of ΔF_γ(D), performed by atomic force microscopy, prior to any treatment above the T_g suggest that growing depressions (cases i and iii) correspond to ΔF_γ(D) > 1.5 × 10⁻¹⁶ J while for healing depressions (case ii) ΔF_γ(D) < 1.8 × 10⁻¹⁶ J. A critical region of ΔF_γ(D) exists from 1.5 × 10⁻¹⁶ J to 1.8 × 10⁻¹⁶ J. Depressions which correspond to this, rather short, region can either grow or heal.     

Stable unidimensional arrays of coherent strained islands

We investigate the equilibrium properties of arrays of coherent strained islands in heteroepitaxial thin films of bidimensional materials. The model we use takes into account only three essential ingredients: surface energies, elastic energies of the film and of the substrate and interaction energies between islands via the substrate. Using numerical simulations for a simple Lennard-Jones solid, we can assess the validity of the analytical expressions used to describe each of these contributions. A simple analytical expression is obtained for the total energy of the system. Minimizing this energy, we show that arrays of coherent islands can exist as stable configurations. Even in this simple approach, the quantitative results turn out to be very sensitive to some details of the surface energy.     

Longitudinal wave propagation in a piezoelectric nanoplate considering surface effects and nonlocal elasticity theory

The propagation characteristics of the longitudinal wave in a piezoelectric nanoplate were investigated in this study. The nonlocal elasticity theory was used and the surface effects were taken into account. In addition, the group velocity and phase velocity were derived and investigated, respectively. The dispersion relation was analyzed with different scale coefficients, wavenumbers, and voltages. The results showed that the dispersion degree can be strengthened by increasing the wavenumber and scale coefficient.     

Simple geometrical explanation of Gurtin-Murdoch model of surface elasticity with clarification of its related versions

It is showed that all equations of the linearized Gurtin-Murdoch model of surface elasticity can be derived, in a straightforward way, from a simple second-order expression for the ratio of deformed surface area to initial surface area. This elementary derivation offers a simple explanation for all unique features of the model and its simplified/modified versions, and helps to clarify some misunderstandings of the model already occurring in the literature. Finally, it is demonstrated that, because the Gurti...     

Reflection characterization of anisotropic ultrathin dielectric films on absorbing isotropic substrates

The reflection of linearly polarized light from an ultrathin anisotropic dielectric film on isotropic absorbing substrate is investigated analytically in the long-wavelength limit. All analytical results are correlated with the numerical solution of the anisotropic reflection problem on the basis of rigorous electromagnetic theory. Simple analytical approach developed in this work not only gives a physical insight into the reflection problem but also provides a way of estimating the necessary experimental accuracy for optical diagnostics by reflection characteristics. It is shown that obtained expressions are of immediate interest for determining the parameters of anisotropic surface layers. Innovative possibilities for optical diagnostics of anisotropic properties of ultrathin dielectric layers upon absorbing materials are discussed.     

Modeling of a surface plasmon polariton interferometer

We model the operation of a micro-optical interferometer for surface plasmon polaritons (SPPs) that comprises an SPP beam-splitter formed by equivalent scatterers lined up and equally spaced. The numerical calculations are carried out by using a relatively simple vectorial dipolar model for multiple SPP scattering [Phys. Rev. B 67 (2003) 165405]. The SPP beam-splitter is simulated elucidating the influence of system parameters, such as the angle of SPP beam incidence, scattering particle size, and inter-particle distance, on the splitting efficiency and phase difference between the transmitted and reflected beams. It is found that the splitting efficiency is very sensitive to the size of scatterers and angle of incidence. Comparing our simulations with experimental data available in the literature, we conclude that this approach can be used, with certain limitations, for modelling of SPP components assembled of individual scatterers, e.g., beam-splitters and interferometers, and suggest further improvements of the model used.     

Surface structure and electron density dependence of scattered Ne ion fractions from the Si(1 0 0)-(2×1) surface

The magnitudes and azimuthal anisotropies of 4 keV Ne⁺ scattered ion fractions from the Si(1 0 0)-(2×1) two-domain surface have been measured by means of time-of-flight scattering and recoiling spectrometry. The absolute values of these ion fractions as well as their dependence on surface structure and electron density have been determined. By investigating the trajectories of the scattered Ne⁺, a clear correlation is demonstrated between these experimentally observed surviving ion fractions of Ne⁺ and the fraction of ions that scatters from the topmost layer of the surface. This is interpreted in terms of a model in which the neutralization probability of Ne⁺ is proportional to the local substrate electronic charge density.     

Photoelectron diffraction at low energies: the applicability of single scattering calculations

The energy dependence of the 4f core level photoemission intensity from Ta(100) has been modeled at low electron kinetic energies using the single scattering cluster (SSC) approximation. The results show a strikingly similar shape to that of a full multiple scattering calculation, particular at electron kinetic energies less than 75 eV. A comparison with the experimentally measured surface/bulk core level intensity oscillations indicates that the first interlayer spacing of the Ta(100) surface is contracted by 11 ± 3%, consistent with previous determinations. These results suggest that SSC calculations can model low energy photoelectron diffraction data from surface shifted core levels with sufficient accuracy to enable a quantitative structural determination of clean, unreconstructed surfaces.