X-ray photoelectron spectroscopy analysis of ion¿beam¿induced oxidation of GaAs and AlGaAs

The oxidation of GaAs and AlGaAs targets subjected to O2+ bombardment has been analyzed, using in situ x¿ray photoelectron spectroscopy, as a function of time until steady state is reached. The oxides formed by the O2+ bombardment have been characterized in terms of composition and binding energy. A strong energy and angular dependence for the oxidation of As relative to Ga is found. Low energies as well as near normal angles of incidence favor the oxidation of As. The difference between Ga and As can be explained in terms of the formation enthalpy for the oxide and the excess supply of oxygen. In an AlGaAs target the Al is very quickly completely oxidized irrespective of the experimental conditions. The steady state composition of the altered layers show in all cases a preferential removal of As.

Modifications in the Si valence band after ion-beam-induced oxidation

The changes undergone by the Si surface after oxygen bombardment have special interest for acquiring a good understanding of the Si+-ion emission during secondary ion mass spectrometry (SIMS) analysis. For this reason a detailed investigation on the stoichiometry of the builtup surface oxides has been carried out using in situ x-ray photoemission spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates a strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30°. In this work a special emphasis on the analysis and interpretation of the valence band region was made. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the respective valence band structures also differ. A comparison with experimentally measured and theoretically derived Si valence band and SiO2 valence band suggests that the new valence bands are formed by a combination of these two. This arises from the fact that Si¿Si bonds are present on the Si¿suboxide molecules, and therefore the corresponding 3p-3p Si-like subband, which extends towards the Si Fermi level, forms the top of the respective new valence bands. Small variations in intensity and energy position for this subband have drastic implications on the intensity of the Si+-ion emission during sputtering in SIMS measurements. A model combining chemically enhanced emission and resonant tunneling effects is suggested for the variations observed in ion emission during O+2 bombardment for Si targets.

Gas sensing properties of catalytically modified WO3 with copper and vanadium for NH3 detection

Ammonia gas detection by pure and catalytically modified WO3 based gas sensor was analysed. The sensor response of pure WO3 to NH3 was not only rather low but also presented an abnormal behaviour, probably due to the unselective oxidation of ammonia to NOx. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH3 detection. The introduction of copper and vanadium as catalytic additives improved the response to NH3 and also eliminated the abnormal behaviour. Possible mechanisms of NH3 reaction over these materials are discussed. Sensor responses to other gases like NO2 or CO and the interference of humidity on ammonia detection were also analysed so as to choose the best sensing element.

Barrier for the reaction X20+ + X20+ -> X402+ in alkali-metal clusters related to electron density at the bond midpoint of the supermolecule (X20+)2

Using the extended Thomas-Fermi version of density-functional theory (DFT), calculations are presented for the barrier for the reaction Na20++Na20+¿Na402+. The deviation from the simple Coulomb barrier is shown to be proportional to the electron density at the bond midpoint of the supermolecule (Na20+)2. An extension of conventional quantum-chemical studies of homonuclear diatomic molecular ions is then effected to apply to the supermolecular ions of the alkali metals. This then allows the Na results to be utilized to make semiquantitative predictions of position and height of the maximum of the fusion barrier for other alkali clusters. These predictions are confirmed by means of similar DFT calculations for the K clusters.

Kinematic reduction of reaction-diffusion fronts with multiplicative noise. Derivation of stochastic sharp-interface equations

We study the dynamics of generic reaction-diffusion fronts, including pulses and chemical waves, in the presence of multiplicative noise. We discuss the connection between the reaction-diffusion Langevin-like field equations and the kinematic (eikonal) description in terms of a stochastic moving-boundary or sharp-interface approximation. We find that the effective noise is additive and we relate its strength to the noise parameters in the original field equations, to first order in noise strength, but including a partial resummation to all orders which captures the singular dependence on the microscopic cutoff associated with the spatial correlation of the noise. This dependence is essential for a quantitative and qualitative understanding of fluctuating fronts, affecting both scaling properties and nonuniversal quantities. Our results predict phenomena such as the shift of the transition point between the pushed and pulled regimes of front propagation, in terms of the noise parameters, and the corresponding transition to a non-Kardar-Parisi-Zhang universality class. We assess the quantitative validity of the results in several examples including equilibrium fluctuations and kinetic roughening. We also predict and observe a noise-induced pushed-pulled transition. The analytical predictions are successfully tested against rigorous results and show excellent agreement with numerical simulations of reaction-diffusion field equations with multiplicative noise.

Spatial correlations and cross sections of clusters in the A + B -> 0 reaction

We consider the distribution of cross sections of clusters and the density-density correlation functions for the A+B¿0 reaction. We solve the reaction-diffusion equations numerically for random initial distributions of reactants. When both reactant species have the same diffusion coefficients the distribution of cross sections and the correlation functions scale with the diffusion length and obey superuniversal laws (independent of dimension). For different diffusion coefficients the correlation functions still scale, but the scaling functions depend on the dimension and on the diffusion coefficients. Furthermore, we display explicitly the peculiarities of the cluster-size distribution in one dimension.

Reaction-diffusion fronts under stochastic advection

We study front propagation in stirred media using a simplified modelization of the turbulent flow. Computer simulations reveal the existence of the two limiting propagation modes observed in recent experiments with liquid phase isothermal reactions. These two modes respectively correspond to a wrinkled although sharp propagating interface and to a broadened one. Specific laws relative to the enhancement of the front velocity in each regime are confirmed by our simulations.

Characterization of nano-oxide layers fabricated by ion beam oxidation

In this paper, a remote O2 ion source is used for the formation of nano-oxide layers. The oxidation efficiency was measured in CoFe-oxide films, and a decrease of the oxide layer with the pan angle and the oxidation pressure is observed. For the same oxidation pressure, the oxidation efficiency depends on the O2 content in the Ar-O2 plasma. These results were applied in optimizing the fabrication of Al2O3 barrier for tunnel junctions. This method was also used to fabricate junctions with Fe-oxide layers inserted at the Al2O3-CoFe interface. TEM and magnetization data indicate that after anneal at 385°C, a homogeneous ferromagnetic Fe-oxide layer (Fe3O4?) is formed.

Semiclassical back reaction in the formation of a straight cosmic string

We derive the back reaction on the gravitational field of a straight cosmic string during its formation due to the gravitational coupling of the string to quantum matter fields. A very simple model of string formation is considered. The gravitational field of the string is computed in the linear approximation. The vacuum expectation value of the stress tensor of a massless scalar quantum field coupled to the string gravitational field is computed to one loop order. Finally, the back-reaction effect is obtained by solving perturbatively the semiclassical Einsteins equations.

Use of recovered frying oils in chicken and rabbit feeds: effect on the fatty acid and tocol composition and on the oxidation levels of meat, liver and plasma

The addition of some fat co- and by-products to feeds is usual nowadays; however, the regulations of their use are not always clear and vary between countries. For instance, the use of recycled cooking oils is not allowed in the European Union, but they are used in other countries. However, oils recovered from industrial frying processes could show satisfactory quality for this purpose. Here we studied the effects of including oils recovered from the frying industry in rabbit and chicken feeds (at 30 and 60 g/kg, respectively) on the fatty acid (FA) and tocol (tocopherol + tocotrienol) compositon of meat, liver and plasma, and on their oxidative stability. Three dietary treatments (replicated eight times) were compared: fresh non-used oil (LOX); oil discarded from the frying industry, having a high content of secondary oxidation compounds (HOX); and an intermediate level (MOX) obtained by mixing 50 : 50 of LOX and HOX. The FA composition of oil diets and tissues was assessed by GC, their tocol content by HPLC, the thiobarbituric acid value was used to assess tissue oxidation status, and the ferrous oxidation-xylenol orange method was used to assess the susceptibility of tissues to oxidation. Our results indicate that FA composition of rabbit and chicken meat, liver and plasma was scarcely altered by the addition of recovered frying oils to feed. Differences were encountered in the FA composition between species, which might be attributed mainly to differences in the FA digestion, absorption and metabolism between species, and to some physiological dietary factors (i.e. coprophagy in rabbits that involves fermentation with FA structure modification). The α-tocopherol (αT) content of tissues was reduced in response to the lower αT content in the recovered frying oil. Differences in the content of other tocols were encountered between chickens and rabbits, which might be attributable to the different tocol composition of their feeds, as well as to species differences in the digestion and metabolism of tocols. Tissue oxidation and susceptibility to oxidation were in general low and were not greatly affected by the degree of oxidation of the oil added to the feeds. The relative content of polyunsaturated fatty acids/αT in these types of samples would explain the differences observed between species in the susceptibility of each tissue to oxidation. According to our results, oils recovered from the frying industry could be useful for feed uses.

Spatial correlations and cross sections of clusters in the A + B -> 0 reaction

We consider the distribution of cross sections of clusters and the density-density correlation functions for the A+B¿0 reaction. We solve the reaction-diffusion equations numerically for random initial distributions of reactants. When both reactant species have the same diffusion coefficients the distribution of cross sections and the correlation functions scale with the diffusion length and obey superuniversal laws (independent of dimension). For different diffusion coefficients the correlation functions still scale, but the scaling functions depend on the dimension and on the diffusion coefficients. Furthermore, we display explicitly the peculiarities of the cluster-size distribution in one dimension.

Reaction-diffusion fronts under stochastic advection

We study front propagation in stirred media using a simplified modelization of the turbulent flow. Computer simulations reveal the existence of the two limiting propagation modes observed in recent experiments with liquid phase isothermal reactions. These two modes respectively correspond to a wrinkled although sharp propagating interface and to a broadened one. Specific laws relative to the enhancement of the front velocity in each regime are confirmed by our simulations.