Clustering Of Oxygen Atoms Research Articles

Enhanced Osteogenic Differentiation of Stem Cells on Phase-Engineered Graphene Oxide.

Graphene oxide (GO) has attracted significant interest as a template material for multiple applications due to its two-dimensional nature and established functionalization chemistries. However, for applications toward stem cell culture and differentiation, GO is often reduced to form reduced graphene oxide, resulting in a loss of oxygen content. Here, we induce a phase transformation in GO and demonstrate its benefits for enhanced stem cell culture and differentiation while conserving the oxygen content. The transformation results in the clustering of oxygen atoms on the GO surface, which greatly improves its ability toward substance adherence and results in enhanced differentiation of human mesenchymal stem cells toward the osteogenic lineage. Moreover, the conjugating ability of modified GO strengthened, which was examined by auxiliary osteogenic growth peptide conjugation. Overall, our work demonstrates GO's potential for stem cell applications while maintaining its oxygen content, which could enable further functionalization and fabrication of novel nano-biointerfaces.

A defect model for UO2+x based on electrical conductivity and deviation from stoichiometry measurements

Electrical conductivity of UO2+x shows a strong dependence upon oxygen partial pressure and temperature which may be interpreted in terms of prevailing point defects. A simulation of this property along with deviation from stoichiometry is carried out based on a model that takes into account the presence of impurities, oxygen interstitials, oxygen vacancies, holes, electrons and clusters of oxygen atoms. The equilibrium constants for each defect reaction are determined to reproduce the experimental data. An estimate of defect concentrations and their dependence upon oxygen partial pressure can then be determined. The simulations carried out for 8 different temperatures (973–1673 K) over a wide range of oxygen partial pressures are discussed and resulting defect equilibrium constants are plotted in an Arrhenius diagram. This provides an estimate of defect formation energies which may further be compared to other experimental data or ab-initio and empirical potential calculations.

Density functional theory study of the initial oxidation of the Pt(111) surface

We used density functional theory calculations to examine the initial stages of oxidation of the Pt(111) surface. Consistent with prior studies, our calculations predict that oxygen atoms adsorb on fcc sites and form $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $p(2\ifmmode\times\else\texttimes\fi{}1)$ structures at coverages of 0.25 and 0.50 ML, respectively. In addition to various surface configurations of oxygen on fcc sites, we examined subsurface oxygen and clustering of oxygen atoms on the surface. We find that subsurface oxygen is not the precursor to the oxidation of the Pt(111) surface. Instead, we predict a strong preference for the formation and growth of one-dimensional Pt oxide chains within the $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure. In particular, at coverages above 0.50 ML, additional oxygen atoms prefer to aggregate between the close-packed oxygen rows formed by the $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure and induce large buckling $(\ensuremath{\sim}1.8\text{ }\text{\AA{}})$ and modification of the charge of the surface Pt atoms. The result is an oxide compound with threefold and fourfold Pt-O coordination that grows as a one-dimensional chain running parallel to the oxygen rows of the $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure. Furthermore, half of the oxygen atoms in the Pt oxide chains reside near hcp sites, contrary to some reports that oxygen atoms reside only on the fcc sites on Pt(111). Our results agree well with a recent scanning tunneling microscopy study and suggest a precursor mechanism to the oxidation of metal surfaces involving Pt oxide chain formation and growth on terraces at moderate oxygen coverages. Our results should have important implications to current models of NO and CO oxidation on Pt(111) and potentially on studies of the initial oxidation of other transition-metal and bimetallic surfaces.

Effects of surface steps on oxygen migration and clustering on Si( [formula omitted]) surfaces studied by EHA

The effects of surface steps on mobility and clustering of oxygen atoms has been investigated quantum mechanically using the Hartree-Fock theory at semi-empirical level according to the Extended Debye Hückel Approximation. Clusters of large size (approximately 200 atoms) represent steps of a silicon surface vicinal to (1 0 0). A map of the energy experienced by the deposited atom is constructed by moving the atom on the terraces and down the step. Clustering is studied by constructing a similar map for two oxygen impurities located at a variable distance. The results indicate that, due to the interaction with the underlying silicon dimers, a barrier in the range 0.4 eV exists at the step edge. The effect, however, depends both on the path by which the diffusing atom approaches the step edge and on the step structure. Clustering is favored provided that the participants are at a distance below a critical value. These results are in qualitative agreement with experimental observations on nucleation of oxide films.

Optical investigations of oxygen ordering and persistent photo-doping in tetragonal YBCO

We present optical measurements of the tetragonal phase of YBCO. The measurements give strong evidence that the doping for low oxygen content is driven by the clustering of oxygen atoms in the chain plane, giving rise to metallic islands with local orthorhombic structure. By increasing the oxygen content, a percolation transition to macroscopic metallic behaviour takes place at an oxygen content between x=6.1 and x=6.2. The state oxygen order in the chains can be changed by either annealing the sample or by a long-time illumination with visible laser light. From the optical investigation of this persistent photo-conductivity effect, it can be concluded that the additional conductivity due to illumination is most likely due to oxygen order and not to a charge transfer and trapping of electrons at some defects.

Study of Oxygen Diffusion and Clustering in Silicon Using an Empirical Interatomic Potential

AbstractThe diffusion path and diffusivity of oxygen in crystalline silicon are computed using an empirical interatomic potential which was recently developed [1] for modelling the interactions between oxygen and silicon atoms. The diffusion path is determined by constrained energy minimization, and the diffusivity is computed using jump rate theory. The calculated diffusivity D=0.025 exp(-2.43eV/kBT) cm2/sec is in excellent agreement with experimental data. The same interatomic potential also is used to study the formation of small clusters of oxygen atoms in silicon. The structures of these clusters are found by NPT molecular dynamics simulations, and their free energies are calculated by thermodynamic integration. These free energies are used to predict the temperature dependence of the equilibrium partitioning of oxygen atoms into clusters of different sizes. The calculations show that, for given total oxygen concentration, most oxygen atoms are in clusters at temperature below 1300K, and that the average cluster size increases with decreasing temperature. These results are in qualitative agreement with the effects of thermal annealing on oxygen precipitation in silicon crystals.

Clustering of oxygen atoms in silicon at 450 degrees C: A new approach to thermal donor formation.

Clustering of oxygen atoms in silicon at 450 \ifmmode^\circ\else\textdegree\fi{}C has been correlated with a group of infrared vibrational absorption bands observable at room temperature. The bands are related to the formation of thermal donors and show a good correlation with results from different experimental techniques. It is suggested that three categories of thermal donors are developing corresponding to absorption bands at about 975, 988, 1000, 1006, and 1012 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Monoenergetic Positron Beam Studies of Oxygen in Single Crystal Silicon - Stress Induced Clustering of Oxygen Atoms in Silicon

ABSTRACTA monoenergetic positron beam has been used to investigate the state of interstitial oxygen in Czochralski (CZ)-grown Si with either thermally grown S1O2 (100 nm thick) or silicon oxide (p-SiOx) deposited by plasma enhanced chemical vaper deposition technique on the surface. Both the growth of thermal SiO2 and the deposition of SiOx film resulted in a reduction of the doppler-broadening line shape parameter (S-parameter) for the positron annihilation in the bulk silicon region. Annealing at 450δC, the removal of oxide overlayer or long-term aging at room temperature caused the S-parameter to return to its intrinsic value. It was thought that tensile stress in silicon, induced by the thermal oxidation or the deposition of SiOx films which had compressive internal stress themselves, enhanced the rearrangement of oxygen atoms and caused the formation of oxygen clusters in silicon crystal. Oxygen interstitial clusters can trap positrons leading to the lower S-parameter value for annihilation in the bulk silicon region, because of large overlap with core electrons. The above results suggest that oxygen atoms can absorb lattice strain by clustering and thus prevent the generation of dislocations against external stress in the Si lattice. This results yield an additional explanation of the high mechanical strength of CZ Si crystal.

Clustering of oxygen atoms around carbon in silicon

Transmission electron microscope studies of carbon-doped Czochralski silicon, when combined with previous infrared data on the same specimens, reveal a double peak in the carbon-sited oxygen-cluster size distribution after 64 h at 750 °C. The first peak, which represents most of the carbon and oxygen in the specimen, is comprised of clusters with an average of two oxygens per carbon atom. These clusters can survive 64 h at 1000 °C although they are not created by such an anneal, suggesting that carbon atoms have difficulty trapping a first oxygen atom at 1000 °C. The second peak in the distribution near 104 oxygen atoms in size is populated with regular {111}-octahedral precipitates having large dilatational strain fields. The two peaks in the size distribution, and their dependences on heat treatment, indicate roles for both seeding (creation of metastable clusters below critical size) and nucleation (achievement of energetic stability) in the formation of carbon-sited precipitates. The observations confirm a trend toward octahedral precipitate morphologies in carbon-doped specimens. However, the trend may result not from site differences but from effects of carbon or point defects on strain energy during precritical cluster growth. Finally, differences between secondary defects associated with precipitation in low- and high-carbon specimens suggest that substitutional carbon atoms at 1000 °C act as sites for silicon self-interstitial condensation near precipitates.

High temperature (600–800°C) thermally activated deformation behaviour of α-Zircaloy-4-Oxygen alloys

The high temperature thermally activated deformation behaviour of Zircaloy-4-oxygen alloys containing oxygen up to 1 wt.% has been investigated. Décrémentai unloading technique (Dip technique) was employed to determine the internal stress ( τ μ ) and thus the effective stress ( τ ∗). Strain rate change tests were done to determine strain rate sensitivity ( m) and activation volume ( V∗). Activation energy ( ΔG μ ) was calculated from the above data. Oxygen was found to produce an increase in y.s. (0.2%), τ ∗ and ΔG μ 0 and a decrease in m and V∗ while τ μ was not affected significantly. These experimental results support the view that the predominant rate controlling mechanism for deformation in this temperature regime is the thermally activated breaking of attractive junctions for these alloys. Oxygen atoms, pairs or clusters of oxygen atoms, are ruled out as barriers, instead the strengthening is attributed to an increase in the core width of the dislocations due to the presence of oxygen. This in turn results in a more stable attractive junction, which is reflected in the increasing values of ΔG μ 0 with the increase in oxygen. Finally it is suggested that the atypical temperature dependence of τ μ / μ and ΔG μ is a direct consequence of dynamic recovery.

Role of oxygen atoms in the phonon conductivity of Si containing 5 ×1017oxygen atoms percm3

The reduction in the phonon conductivity of silicon crystals containing 5 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ oxygen atoms per ${\mathrm{cm}}^{3}$ near the maximum is interpreted on the basis of the phonon scattering by clusters of oxygen atoms, each atom forming an isosceles triangle with two neighboring silicon atoms at its lattice site. Klemens's theory for the scattering of phonons by clusters of substitutional impurities is assumed for interstitial impurities and is used to explain the results. The scattering of phonons by clusters is negligible in the case of samples containing less than 5 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ oxygen atoms.