Evidence of subsurface oxygen vacancy ordering on reduced CeO2(111)

Surface and subsurface oxygen vacancies on the slightly reduced CeO(2)(111) surface have been studied by atomic resolution dynamic force microscopy at 80 K. Both types of defect are clearly identified by the comparison of the observed topographic features with the corresponding structures predicted from recent first-principles calculations. By combining two simultaneously acquired signals (the topography and the energy dissipated from the cantilever oscillation), we are able to unambiguously locate subsurface oxygen vacancies buried at the third surface atomic layer. We report evidence of local ordering of these subsurface defects that suggests the existence of a delicate balance between subtle interactions among adjacent subsurface oxygen vacancy structures.     

Diffusion and coupled fluxes in concentrated alloys under irradiation: a self-consistent mean-field approach

When an alloy is irradiated, atomic transport can occur through the two types of defects which are created: vacancies and interstitials. Recent developments of the self-consistent mean field (SCMF) kinetic theory could treat within the same formalism diffusion due to vacancies and interstitials in a multi-component alloy. It starts from a microscopic model of the atomic transport via vacancies and interstitials and yields the fluxes with a complete Onsager matrix of the phenomenological coefficients. The jump frequencies depend on the local environment through a ‘broken bond model’ such that the large range of frequencies involved in concentrated alloys is produced by a small number of thermodynamic and kinetic parameters. Kinetic correlations are accounted for through a set of time-dependent effective interactions within a non-equilibrium distribution function of the system. The different approximations of the SCMF theory recover most of the previous diffusion models. Recent improvements of the theory were to extend the multi-frequency approach usually restricted to dilute alloys to diffusion in concentrated alloys with jump frequencies depending on local concentrations and to generalize the formalism first developed for the vacancy diffusion mechanism to the more complex diffusion mechanism of the interstitial in the dumbbell configuration. To cite this article: M. Nastar, C. R. Physique 9 (2008).     

Atomic and electronic origins of a type- C defect on Si(001)

We present an extensive set of ab initio calculations for a type- C defect on Si(001). Various models belonging to subsurface defects are studied. A substitutional B in the second surface layer is predicted as a possible atomic origin of this defect. However, H and O coupled with second-layer vacancies and a substitutional C are not responsible for a type- C defect. We also discuss how the electronic structure of a type- C defect contributes to its specific scanning tunneling microscopy images.     

Domain wall conductivity in La-doped BiFeO3

The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO3) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.     

Electronic properties of multi-defected zigzag carbon nanotubes

Electronic properties of multi-defected zigzag single-walled carbon nanotubes are investigated by use of the tight-binding Green's function method. The Stone-Wales defects and the vacancies are considered. We find that the conductance sensitively depends on the realistic defect configurations for the metallic zigzag carbon nanotubes. Interestingly, the electronic transport properties of the nanotubes with three vacancies can be considered as the sum effect of two double-vacancies, while those with Stone-Wal...     

Nonequilibrium Green's function approach to phonon transport in defective carbon nanotubes

We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective carbon nanotubes with increasing temperature.     

Electronic states induced by surface defects on GaSb(110)

Electronic state calculations for point defects on the GaSb(110) surface are presented using a cluster, in order to indicate theoretically the usefulness of the defect model as a mechanism of the Fermi level pinning in Schottky barriers. The results demonstrate that the presence of atomic Ga at surface Sb vacancy sites in addition to surface Ga vacancies gives electronic states localized near the top of the valence band which can be responsible for the pinning observed experimentally.     

The first-principle calculation of structures and defect energies in tetragonal PbTiO3

The first-principle calculation had been adopted to investigate various neutral vacancies and vacancy pairs under seven thermodynamic conditions in bulk PbTiO₃. The electronic structures, atomic relaxations, and formation energies of vacancies were obtained. Depending on the thermodynamic condition, the main and stable defects are different. It was found that V_O is the main defect under the reducing condition, whereas V_Pb becomes dominating under the oxidizing condition. The Pb-O vacancy pair forms more easily than the isolated vacancies under certain thermodynamic condition. Due to the introducing of vacancies, the acceptorlike levels and donorlike levels appear in the cases of the cation and anion vacancies, respectively.     

Calculation of the vibrational spectra of copper crystals with vacancies

The local vibrational spectra of copper crystals containing vacancies are calculated using the pair atomic potential derived in the framework of the resonance pseudopotential theory. The calculations are performed by a recursive method with due regard for the symmetry of the defect region. The frequencies of the vacancy-induced resonance vibrations of different symmetries are determined.     

Diffusion of adatoms and vacancies on otherwise perfect surfaces: A molecular dynamics study

Using computer simulation by the technique of molecular dynamics, we have investigated the influence of the terrace structure on the type and the dynamical aspects of atomic mechanisms for surface diffusion in fcc structure crystals. On the (100) terraces, vacancies are much more mobile than adatoms, while the opposite is true for (111) terraces. On the latter, vacancies migrate through the creation in their vicinity of paired, adatom-vacancy, defects. On the (100) face, the adatom jump length incrases with increasing temperature and reaches a value equal to several times the nearest neighbour distance. Adatoms are also fully delocalized on the (111) face and spend much more time in flight over the surface than by vibrations in the equilibrium sites. Large dynamical correlations are present in the vacancy movement on the (100) face and have been identified as new mechanisms of the defect migration by multiple jumps. On the (110) terrace, despite its anisotropic structure, two-dimensional diffusion takes place by an original atomic exchange mechanism. This mechanism has been identified to be the same as the one proposed by Halicioglu to explain two-dimensional diffusion on (110) Pt terraces, and recently corroborated by the FIM experiments of Wrigley and Ehrlich.     

Defect engineering of ZnO

The defect responsible for the transparent to red color change of nominally undoped ZnO bulk single crystals is investigated. Upon annealing in the presence of metallic Zn as reported by Halliburton et al. and also Ti and Zr a native defect forms with an energy level about 0.7 eV below the conduction band. This change is reversible upon annealing in oxygen. Optical transmission data along with positron depth profiles and annealing studies are combined to identify the defect as oxygen vacancies. Vacancy clustering occurs at about 500 °C if isolated zinc and oxygen vacancies. In the absence of zinc vacancies, clusters form at about 800 °C.     

Thermal migration of iron implanted in aluminium at high doses

Abstract

This paper summarizes recent progress in the understanding of energetic displacement cascades in metals achieved with the molecular-dynamics (MD) simulation technique. Recoil events with primary-knock-on-atom (PKA) energies up to 5 keV were simulated in Cu and Ni. The initial development of displacement cascades was similar in both metals, with replacement collision sequences providing the most efficient mechanism for the separation of interstitials and vacancies. The thermal-spike behavior in these metals, however, is quite different; Cu cascades are characterized by lower defect production and greater atomic disordering than those in Ni. The thermal spike significantly influences various other properties of cascades, such as total defect production and defect clustering.     

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles calculations confirm that the 555-777 defect is energetically much more stable than two separated single vacancies, and the energy of the 555-777 defect is also slightly lower than that of the 5-8-5 double vacancy. In TBMD simulation, it is also found that the four single vacancies reconstruct into two collective 555-777 defects which is the unit for the hexagonal haeckelite structure proposed by Terrones et al. [Phys. Rev. Lett. 84, 1716 (2000)].     

Bias-dependent generation and quenching of defects in pentacene

We describe a defect generation phenomenon that is new to organic semiconductors. A defect in pentacene single crystals can be created by bias-stress and persists at room temperature for an hour in the dark but only seconds with 420 nm illumination. The defect gives rise to a hole trap at Ev+0.38 eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67 eV activation energy with a small (10(8) s(-1)) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.     

Ab initio calculations of transport properties of atomic bridges by the recursion-transfer-matrix method

We present an efficient self-consistent method for approaching quantum transport through atomic-scale structures. Using the recursion-transfer-matrix (RTM) method with a separable form of nonlocal pseudopotentials, scattering waves propagating between metallic electrodes through nano-bridged structures are efficiently calculated on the basis of the density-functional formalism. We performed calculations with this method of the conductance of Al atomic wires with various kinds of single atoms mixed at the contact to one electrode. We found that the transport properties are considerably affected by the bonding nature of the atom at the contact. The conductance is largely determined by the atomic species at the contact and does not change much as the length of the atomic wire increases.     

The influence of atomic displacements on the electrical resistivity due to vacancies in simple metals

The influence of atomic displacements around a vacancy on the electrical resistivity due to vacancies in simple metals has been calculated by the pseudopotential method. The influence of atomic displacements on the electrical resistivity was very large and the resistivity has a minimum at an appropriate relaxation. Therefore it is very important to take into account the effect of the lattice distortions considering the individual atomic displacements around vacancies in the calculation of the electrical resistivity due to vacancies.     

On the theory of void formation during irradiation

A simple theory of the swelling of materials subjected to high energy particle irradiation is developed. Chemical reaction rate equations are used as a basis. Point defects, interstitials and vacancies, are assumed to be produced randomly throughout the solid. They move by random walk through the material until they cease to exist either by recombination with the opposite type of defect or by incorporation into the crystal at sinks such as dislocations, grain boundaries and voids. The rate equations for interstitials and for vacancies, which are coupled via the recombination term, are solved for steady state conditions under irradiation. Defect concentrations, supersaturations, recombination and total sink annihilation rates are obtained in terms of the production rate, sink annihilation probabilities, jump frequencies and thermal equilibrium concentrations of defects. The swelling rate is derived using sink annihilation probabilities at three principally different types of sinks, i.e. voids, sinks which have a bias with regard to the annihilation of interstitials and vacancies (such as dislocations), and sinks with no bias. The defect annihilation probabilities at void, precipitate, dislocation and grain boundary sinks are estimated by using a cellular model and solving the diffusion equation for geometries approximating that of the cells, e.g. a concentric sphere around a void. The relative effects of different types of sinks, i.e. the microstructure, on the swelling rate is discussed. The swelling rate is integrated to give swelling-time or swelling-dose relations, making some simplifying assumptions about the changes in the sink structure as the irradiation proceeds. It is shown that the relation obtained is rather sensitive to the type of assumptions made.     

Effect of oxygen vacancies on adhesion at the Nb/Al2O3 and Ni/ZrO2 interfaces

The atomic and electronic structures of the Nb/Al₂O₃(0001) and Ni/ZrO₂(001) interfaces are calculated using density-functional theory. The formation energy of oxygen vacancies is estimated in bulk materials and in surface layers and interfaces for different uppermost atomic layers of oxide surfaces. The work of separation of metal films from oxide surfaces is determined. The effect of oxygen vacancies on the bonding of transition metals to atoms of a substrate determining adhesion at the metal-oxide interfaces is discussed. It is shown that the Nb(Ni)-O interaction at the interfaces weakens in the presence of surface oxygen vacancies.     

Perturbed γ−γ angular correlations: A spectroscopy for point defects in metals and alloys

Atomic defects which migrate and trap at impurity probe atoms can be labelled by the changes they induce in the hyperfine interactions of the probe nuclei. Many studies have been made using perturbed γ−γ angular correlations (PAC) and the¹¹¹In probe because of the excellent resolution of different sites. Identification of the bound states is the key problem in applying hyperfine interactions methods to point defects studies. In this study three structure-sensitive methods are applied to help identify the atomic structures of various multivacancy complexes in Pt and Au: (1) Quadrupole interaction parameters are compared with results of point-charge calculations of electric-field gradients for 20 structures containing 1–4 vacancies in the fcc lattice. (2) Hyperfine interactions induced by decorating vacancy complexes with hydrogen atoms are measured and interpreted with the assistance of point-charge calculations. (3) Transformations between complexes observed by annihilation of vacancies by mobile self-interstitials are used to test the consistency of the identifications. Using these methods in conjunction with analysis of the trapping behavior which occurs during annealing of damaged samples, structural models are presented for divacancy (2V), 3V and 4V complexes in Pt, and 3V and 4V complexes in Au. The activation temperatures of the 3V defect in Au and Pt are determined to be 162 K and 390 K, respectively, and activation temperatures of defects in Ni, Cu, Pt and Au are compared. For Pt, trapping of H at 1V and 2V complexes is observed to lead to small changes in the quadrupole interactions, consistent with well shielded protonic charges. However, trapping at 3V and 4V complexes leads to very large changes which we attribute to atomic restructuring to the defect complexes. Finally, the application of the same methodology to interpret recent experiments on NiAl, an ordered alloy, is described.