Oxygen diffusion through the disordered oxide network during silicon oxidation

Atomic Scale Mechanisms Controlling the Oxidation of Polyethylene: A First Principles Study

Understanding the degradation mechanisms of aliphatic polymers by thermal oxidation and radio-oxidation is very important in order to assess their lifetime in a variety of industrial applications. We focus here on polyethylene as a prototypical aliphatic polymer. Kinetic models describing the time evolution of the concentration of chain defects and radicals species in the material identify a relevant step in the formation and subsequent decomposition of transient hydroperoxides species, finally leading to carbonyl defects, in particular ketones. In this paper, we first summarize the most relevant mechanistic paths proposed in the literature for hydroperoxide formation and decomposition and, second, revisit them using first principles calculations based on Density Functional Theory (DFT). Our results partially confirm commonly accepted reaction energies, but also propose alternative, more favourable, reaction paths. We highlight the influence of the environment-crystalline or not-on the outcome of some of the studied chemical reactions. A remarkable result of our calculations is that hydroxyl radicals play an important role in the decomposition of hydroperoxides. Based on our findings, it should be possible to improve the set of equations and parameters used in current kinetic simulations of polyethylene radio-oxidation.

Strain-driven diffusion process during silicon oxidation investigated by coupling density functional theory and activation relaxation technique

The reaction of oxygen molecules on an oxidized silicon model-substrate is investigated using an efficient potential energy hypersurface exploration that provides a rich picture of the associated energy landscape, energy barriers, and insertion mechanisms. Oxygen molecules are brought in, one by one, onto an oxidized silicon substrate, and accurate pathways for sublayer oxidation are identified through the coupling of density functional theory to the activation relaxation technique nouveau, an open-ended unbiased reaction pathway searching method, allowing full exploration of potential energy surface. We show that strain energy increases with O coverage, driving the kinetics of diffusion at the Si/SiO₂ interface in the interfacial layer and deeper into the bulk: at low coverage, interface reconstruction dominates while at high coverage, oxygen diffusion at the interface or even deeper into the bottom layers is favored. A changing trend in energetics is observed that favors atomic diffusions to occur at high coverage while they appear to be unlikely at low coverage. Upon increasing coverage, strain is accumulated at the interface, allowing the oxygen atom to diffuse as the strain becomes large enough. The observed atomic diffusion at the interface releases the accumulated strain, which is consistent with a layer-by-layer oxidation growth.

Room-temperature metastability of multilayer graphene oxide films

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

Oxygen exchange at the internal surface of amorphous SiO2 studied by photoluminescence of isotopically labeled oxygen molecules

The exchange between lattice and interstitial oxygen species in an oxide was studied by the 16O-18O isotope shift of the a1Deltag(v=0)-->X3Sigmag-(v=1) infrared photoluminescence band of the oxygen molecules (O2) incorporated into the interstitial voids of amorphous SiO2 (a-SiO2) by thermal annealing in 18O2 gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of a-SiO2, is found. The average exchange rate has an activation energy of approximately 2 eV, which is much larger than that for the diffusion of interstitial O2 (approximately 0.8-1.2 eV). The average exchange-free diffusion length of interstitial O2 exceeds approximately 1 microm below 900 degrees C, providing definite evidence that oxygen diffuses as interstitial molecules in a-SiO2.

Vibrational spectra of vitreous SiO2 and vitreous GeO2 from first principles

Using a density-functional approach, we calculate the principal vibrational spectra of vitreous SiO₂ and vitreous GeO₂ and discuss their analogies and differences. For both glasses, we generate model structures consisting of a random network of corner-sharing tetrahedra and differing only by their packing density. The comparison between calculated and measured neutron structure factors supports the validity of our model structures. Our investigation then extends to the vibrational properties, including the inelastic-neutron, infrared, and Raman spectra. For these spectra, good agreement with experiment is also found. Our results support the picture that silica and germania are constituted by a continuous random network of corner-sharing tetrahedra. In particular, the good agreement with experiment for the Raman spectra supports the average intertetrahedral angles of 148° and 135° found in our models of vitreous SiO₂ and vitreous GeO₂, respectively. The concentration of small ring structures in these glasses is also discussed.

New linear-parabolic rate equation for thermal oxidation of silicon

We propose a new oxidation rate equation for silicon supposing only a diffusion of oxidizing species but not including any rate-limiting step by interfacial reaction. It is supposed that diffusivity is suppressed in a strained oxide region near SiO(2)/Si the interface. The expression of a parabolic constant in the new equation is the same as that of the Deal-Grove model, while a linear constant makes a clear distinction with that of the model. The estimated thickness using the new expression is close to 1 nm, which compares well with the thickness of the structural transition layers.

Reaction of the oxygen molecule at the Si(100)-SiO2 interface during silicon oxidation

Using constrained ab initio molecular dynamics, we investigate the reaction of the O2 molecule at the Si(100)-SiO2 interface during Si oxidation. The reaction proceeds sequentially through the incorporation of the O2 molecule in a Si-Si bond and the dissociation of the resulting network O2 species. The oxidation reaction occurs nearly spontaneously and is exothermic, irrespective of the O2 spin state or of the amount of excess negative charge available at the interface. The reaction evolves through the generation of network coordination defects associated with charge transfers. Our investigation suggests that the Si oxidation process is fully governed by diffusion.

Concentration of small ring structures in vitreous silica from a first-principles analysis of the Raman spectrum

Using a first-principles approach, we calculate Raman spectra for a model structure of vitreous silica. We develop a perturbational method for calculating the dielectric tensor in an ultrasoft pseudopotential scheme and obtain Raman coupling tensors by finite differences with respect to atomic displacements. For frequencies below 1000 cm(-1), the parallel-polarized Raman spectrum of vitreous silica is dominated by oxygen bending motions, showing a strong sensitivity to the intermediate range structure. By modeling the Raman coupling, we derive estimates for the concentrations of three- and four-membered rings from the experimental intensities of the Raman defect lines.

Oxidation at the Si/SiO2 interface: influence of the spin degree of freedom

We show, using first-principles spin-polarized total-energy calculations, that depending on the spin configuration of the system, the reaction of an O2 molecule with a Si-Si bond in a suboxidized region might result either in a peroxy linkage defect (for a singlet spin state) or in a perfect Si-O-Si bond plus an interstitial O atom (for a triplet spin state). Even though the singlet has a lower energy than the triplet configuration, we find a rather small probability for triplet to singlet conversion. Therefore, as the O2 in an SiO2 interstitial site has a triplet configuration, this reaction spin dependence may have a strong influence on the high quality of the Si/SiO2 interface.