Oxidation of titanium between 25°C and 400°C

The oxidation of Ti between 25°C and 400°C has been studied in an ultra high vacuum system with ellipsometry, Auger spectroscopy and surface potential difference. The surface potential difference is primarily sensitive to the adsorbed layer of oxygen, the Auger spectrometer is sensitive to the oxygen and Ti concentration at the oxide-gas interface and the ellipsometer is sensitive to the film thickness. Consequently, it was possible to follow the various processes separately during oxidation and oxide dissolution. Oxygen adsorbs in a percusor state as a molecule which dissociates and adsorbs in the atomic state. Reaction of atomic oxygen with Ti at the oxide-gas interface causes the stoichiometry to vary with the temperature, oxygen pressure and film thickness. At low temperatures and oxygen pressure the initial film growth follows a logarithmic time law. The predominant transport species is cationic. After initial oxidation, at higher temperatures and oxygen pressure, the predominant transport species is anionic. At constant temperature and oxygen pressure the oxide grows until the rate of growth is equal to the rate of dissolution into the metal. At this point the film thickness remains constant with time. Doping the Ti surface with Au results in a decrease in the oxide dissolution process due to blocking of interstitial diffusion paths by Au atoms.     

Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)

We investigated the mechanism by which a surface oxide layer on Pd(1 1 1) transforms to a PdO(1 0 1) thin film during oxidation with gaseous oxygen atoms in ultrahigh vacuum. Our results provide evidence that the precursor to bulk PdO formation is a distinct oxide phase that forms as small particles, referred to as PdO seeds, after the surface oxide saturates. With increasing oxygen coverage, the PdO seeds grow in size and eventually transform to more stable particles that agglomerate to yield a PdO film. Oxidation effectively ceases when the surface oxide layer is completely replaced by the bulk PdO film, demonstrating that the surface oxide is needed for PdO formation at the conditions studied. Both the kinetics of PdO formation and the final thickness of the PdO thin film depend strongly on the thermal stability of the PdO seeds. Below the decomposition temperature of the seeds (600 K), oxidation follows kinetics similar to Langmuirian adsorption and appears to be limited only by the rate of oxygen adsorption onto the surface oxide. In contrast, PdO formation above 600 K initially exhibits acceleratory kinetics, with the rates starting low but increasing steadily during the initial growth of PdO. We also observe a significant decrease in PdO(1 0 1) film thickness and improved crystallinity when oxidation is conducted below 600 K. We show that the trends observed in the oxidation kinetics and film thickness can be qualitatively explained within the context of a model in which the thermodynamic stability of PdO particles increases with increasing particle size and PdO seeds/particles coexist with a two-dimensional (2D) gas of oxygen atoms adsorbed on the surface oxide layer. This model suggests that the PdO particle-2D gas coexistence relation gives rise to three distinct growth regimes, namely, stable seed nucleation, metastable seed nucleation and oxygen dissolution into the subsurface where the latter is established at 2D gas coverages below the stability limit of a flat PdO surface.     

A model for the initiation of reaction sites during the uranium–hydrogen reaction assuming enhanced hydrogen transport through thin areas of surface oxide

A model for the initiation of hydride sites on uranium metal is described for conditions of constant hydrogen pressure. The model considers variations in hydrogen permeation through the surface oxide film due to intrinsic variations in the oxide thickness. It is proposed that thin areas of surface oxide favour enhanced hydrogen permeation through the oxide and lead to the more rapid initiation of hydride sites. The time and spatial dependence of the hydrogen concentration field in the metal underlying thin areas of oxide is calculated in terms of the local oxide film thickness, the hydrogen diffusion coefficients in the oxide and metal and the hydrogen concentration in the oxide at the gas–oxide interface. The time to precipitate hydride at any location is calculated by assuming that precipitation occurs once the hydrogen concentration in the metal attains the terminal solubility limit of the metal at the prevalent temperature. The model is compatible with the reported temperature and pressure dependence of the hydride induction time. The model can also explain observations such as precipitation of hydride at or beneath the oxide–metal interface and the arrested growth of hydride sites. Finally, an expression is derived for the number of hydride sites initiated on an entire sample surface in any given time by assuming a Gaussian oxide film thickness distribution over the entire sample surface.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

Comparative study of morphology and surface composition of Al–Cr–Fe alloy powders produced by water and gas atomisation technologies

The morphology and surface composition of Al–Cr–Fe alloy powders of 0–63 and 63–100 μm size fractions, produced by gas and water atomisation, have been studied by scanning electron microscopy and Auger electron spectroscopy. While gas atomised particles are of spherical shape, water atomised powders are usually irregular in shape with a complex branched relief. The morphology and composition of surface oxides have been estimated. The surface oxide film is composed of aluminium oxides/hydroxides and contains no Fe and Cr atoms. Two to five water molecules are associated with one Al₂O₃ molecule on the surface of powders. The surface oxide film has a non-uniform thickness, with thick oxide islands separated by thinner oxide film. The parameters of the surface film morphology, such as the island coverage, the oxygen content and the thin film thickness, depend on the atomisation technology used and powder size fraction. Heavily and weakly oxidised powder groups present in all powder fractions are distinguished by Auger spectra analysis. Relationships between heavily and weakly oxidised powder groups are discussed as a function of atomisation technology and size fraction.     

Oxidation of nanocrystalline aluminum by variable charge molecular dynamics

We investigate the oxidation of nanocrystalline aluminum surfaces using molecular dynamics (MD) simulations with the variable charge model that allows charge dynamically transfer among atoms. The interaction potential between atoms is described by the electrostatic plus (Es+) potential model, which is composed of an embedded atom method potential and an electrostatic term. The simulations were performed from 300 to 750 K on polycrystalline samples with a mean grain size of 5 nanometers. We mainly focused on the effect of the temperature parameter on the oxidation kinetic. The results show that, beyond a first linear regime, the kinetics follows a direct logarithmic law (governed by diffusion process) and tends to a limiting value corresponding to a thickness of ∼3 nm. We also characterized at 600 K the effects of an external applied strain on the microstructure and the chemical composition of oxide films formed at the surface. In particular, we obtained a partially crystalline oxide films for all temperatures and we noticed a strong correlation between the degree of crystallinity of the oxide film and the oxidation temperature.     

Oxidation and faceting of polycrystalline tungsten ribbons

We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10^?4 to 5 × 10^?3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and RHEED results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.     

Electric field induced oscillations in the catalytic water production on rhodium: A theoretical analysis

Methods are developed to describe catalytic reactions on multi-facetted surfaces in high electric fields in the conditions of field emission microscopy. These methods are applied to the hydrogen–oxygen–rhodium system for which a mean-field kinetic model is established. This model is shown to reproduce not only the nonequilibrium regimes of bistability and oscillations and their nanopatterns, but also the temperature-programmed desorption spectra of hydrogen and oxygen on rhodium, as well as the equilibrium phase diagram of oxygen on rhodium with the transition to the trilayer surface oxide ORhO. The dependence of the kinetic constants on the surface orientation of the facets is taken into account by expanding them in kubic harmonics suitable for the fcc rhodium crystal. The electric field modifies the gas pressure as well as the activation energies of the different kinetic processes. The tip of the field emission microscope is shown to behave as a nanoreactor.     

FeO(1 1 1) formation by exposure of Fe(1 1 0) to atomic and molecular oxygen

The growth, structure, and morphology of ultrathin iron oxide layers formed on a Fe(1 1 0) single crystal surface are investigated by Auger electron spectroscopy, low energy electron diffraction, and grazing ion scattering. For Fe oxidation by atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude because surface sticking and dissociation are not limiting the growth process. A well-ordered FeO(1 1 1) film with low defect density is only obtained with atomic oxygen. Compared to the bulk, the FeO lattice is laterally compressed by about 5-6% resulting in an in-plane oxygen (Fe) nearest-neighbor distance of 2.87 Å. Independent of the preparation method, long-range structural order is poor if the oxide film thickness is increased to 3-5 layers. This is attributed to the relatively large lattice mismatch between FeO(1 1 1) and Fe(1 1 0).     

Surface structural evolution during the thermal decomposition of a PdO(101) thin film

We used scanning tunneling microscopy (STM) to characterize PdO(101) thin films grown on Pd(111), and the structural changes that occur during isothermal decomposition. We find that the PdO(101) thin films have high-quality surface structures that are characterized by large, crystalline terraces with low concentrations of point defects. Small domains of single-layer oxide are also present on the top layer of relatively thick PdO(101) films grown at 500 K. The thinner PdO(101) films exhibit negligible quantities of such domains, apparently because new domains agglomerate rapidly as the film thickness decreases. We find that the isothermal decomposition rate of a PdO(101) film at 720 K exhibits an autocatalytic regime in which the rate of oxygen desorption increases as the oxide decomposes. Our STM results demonstrate that reduced sites created during oxide decomposition catalyze further PdO decomposition, and reveal strong kinetic anisotropies in the decomposition. The kinetic anisotropies produce one-dimensional reaction fronts that propagate preferentially along the atomic rows of the PdO(101) surface, resulting in the formation of long chains of reduced sites. We also find that reduced sites promote oxygen recombination in neighboring rows of the Pd(101) structure, causing loops and larger aggregates of reduced sites to form. The promotion of decomposition across the atomic rows can qualitatively explain the autocatalytic desorption kinetics. Finally, the STM images provide evidence that underlying PdO(101) layers transfer oxygen to reduced surface domains, thus producing large domains of PdO(101) islands that coexist with reduced domains as well as the larger PdO(101) terraces of the initial surface. Re-oxidation of the surface acts to sustain the autocatalytic decomposition kinetics, and provides a mechanism for oxygen atoms to ultimately evolve from the subsurface of the PdO(101) film.     

Surface-enhanced Raman effect in ultra-thin CuPc films employing periodic silver nanostructures

In order to investigate the interaction of organic molecules with metals by means of Raman spectroscopy, special substrates were designed which combine interference and surface-enhancement mechanisms. Triangular shaped silver nanostructures with an angle bisector and a height of about 80 nm were prepared by nanosphere lithography on silicon substrates with a 100-nm oxide layer. Utilizing these substrates the dependence of the Raman signal intensity on the thickness of copper phthalocyanine (CuPc) was studied in the range from few percentages of a monolayer coverage up to 80 nm using an in situ setup. At an excitation in resonance with the plasmons of the nanostructures (2.6 eV) an increase of the signal was observed during film growth. Contrary to that, excitation at 1.92 eV in resonance with the CuPc absorption band leads to a strongly enhanced Raman signal for submonolayer coverage which hardly changes with the CuPc film thickness in the ultra-low coverage regime.     

The initial oxidation of molybdenum: V. A model for the nucleation of epitaxial oxide and its application to molybdenum

A new model for the phenomenon of epitaxial oxidation nucleation is presented growth characteristics of the oxide suggest that the prenucleation growth is governed by the oxygen adsorption rate of the oxide/gas boundary. As the oxide grows the calculated rate of dissolution of cations at the oxide/metal interface falls until this becomes rate determining. At this point low work function facet sided pits are etched in the substrate with corresponding bulges in the outer oxide film. These bulges are identified as epitaxial nuclei. Based on this model several parameters relating to oxide growth may be calculated. The agreement between theory and experiment is very good. In particular a lower and upper temperature limit for nucleation are found and the pressure dependence is shown to be quite complicated. The basis of the model can be extended to the chemisorption regime. It is shown that above a surface potential of about 1 V cations migrate rapidly through the oxygen layer. The onset of this rapid migration is identified with molecular oxide production and the start of faceting.     

Surface oxidation kinetics of sputtering targets

Films of compounds can be deposited by sputtering a metal or alloy target in an atmosphere containing a suitable reactive gas. Both Al and 90 : 10 In : Sn targets have been sputtered in argon/oxygen mixtures to obtain Al₂O₃ and indium tin oxide films. The experiments were carried out in a planar magnetron sputtering system with both dc and rf excitation. To investigate the kinetics of the reactive sputtering process, the time dependence of the total gas pressure was measured after a change in oxygen flow rate or sputtering power; a capacitance manometer gave accurate and reproducible results. There were simultaneous changes in the rf matching conditions when rf excitation was used. These changes can be attributed to the formation of an oxide on the target surface. The time dependence of the oxygen pressure measured for the Al and In : Sn targets have been used to compare various models of the reactive sputtering process. Fitting of the experimental values to these models yields values of the equilibrium oxide thickness on the target and these have been compared with measured values. For rf sputtering of an Al target at 500 W with flow rates of $\text{3}\text{ml}\text{min}$ and $\text{2.2}\text{ml}\text{min}$ for argon and oxygen respectively, both the calculated and the measured value of the oxide thickness is 100 nm.     

A model for the impregnated cathode emission behaviour under ion bombardment

The electron current emitted by a cathode in the saturation region decrease if the cathode is exposed to ion bombardment. The change in surface coverage with barium (oxide) is used in the model to describe the current behaviour. The steady state current reached (equilibrium current) depends on the gas pressure. The experimental results can be explained in terms of the model and the parameters are determined. The temperature dependence of the barium supply rate for an S-and M-type impregnated cathode are determined.     

Influence of oxygen pressure on the ferroelectric properties of BiFeO3 thin films on LaNiO3/Si substrates via laser ablation

BiFeO₃(BFO) thin films of about 200 nm in thickness have been successfully grown on oxide bottom electrode, LaNiO₃(LNO), via pulsed laser ablation. X-ray diffraction spectrum of the as-deposited BFO film reveals a (100) preferred textured structure. The morphology of the BFO film is found to be strongly dependent on oxygen partial pressure in laser ablation. A saturated hysteresis loop with remanent polarization of 42 μC/cm² and coercive field of 100 kV/cm is obtained at the film deposition at 50 mTorr. The dielectric properties have also been obtained based on the influence of the oxygen pressure.     

Simultaneous and sequential adsorption of deuterium and nitrous oxide by rhenium

The addition of nitrous oxide to a stream of deuterium passing over a rhenium filament reduced the initial sticking probability of the latter gas from 0.24 to 0.09 when the proportion of N₂O exceeded 40%. For the addition of deuterium to nitrous oxide the equivalent figures were 0.45 and 0.30 when deuterium exceeded 30% of the gas phase. These results are attributed to a competition between the two gases for places in the precursor state on the surface. The replacement of adsorbed deuterium from a saturated layer by the oxygen atom of nitrous oxide proceeded initially with a high probability, 0.27, at room temperature and with each oxygen atom replacing one deuterium atom. However, the reaction was incomplete, about 2 × 10¹⁴ atoms cm^?2 of deuterium remaining on the surface. It is suggested that kinetic rather than thermodynamic factors are responsible for the incomplete reaction, possibly as the result of a high activation energy for the migration of deuterium atoms over an oxygenated rhenium surface.     

Transition metal oxides: extra thermodynamic stability as thin films

The oxides of many transition metals wet their own metal surface. The adhesion energy at this interface (E(adh,ox/m)) provides extra stabilization, which lowers the O2 pressure required for oxide stability as a thin film below that required for bulk-oxide stability by the factor exp[(2gamma(g/ox) - E(adh,ox/m))/(tN(ox)RT)], where gamma(g/ox) is the surface energy of the oxide, t is the oxide film thickness, and N(ox) is the oxygen concentration in the bulk oxide (moles O2 per volume). For oxide films only approximately 1 nm thick, this correction can be many orders of magnitude. This may extend to other compounds.     

Photoemission study of the chromium(111) surface interacting with oxygen

The interaction of the Cr(111) surface with O₂ was studied by means of X-ray and UV photoemission and also work function measurements. A strong oxygen adsorption was found even at very low exposures, suggesting a high sticking coefficient. Previous treatments of the clean surface such as argon-ion bombardment or annealing result in significant changes of the surface structure reflected on work function and adsorption kinetics. No work function change was observed in the initial stage of adsorption, ruling out a model of chemisorption on top. In this range the sticking coefficient remains also constant, supporting a model of rapid regeneration of the genuine surface sites and incorporation of oxygen into the lattice. But in contrast with non transition metals like Cs or Sr, oxygen absorbed at room temperature in Cr, remains essentially in the topmost layers of the surface. At room temperature this initial stage of oxygen incorporation is followed by chemisorption on the corrosion film obtained when the uppermost layers are saturated with oxygen. The oxide layer has a stoichiometry close to Cr₂O₃ at saturation, but the detailed electronic structure depends on previous thermal treatments. Exposures at room temperature lead to a thin (about 9 Å), probably amorphous corrosion layer with a maximum work function change Δφ = +0.9 eV. Adsorption followed by heating at 500° C results in a much thicker corrosion film with a limiting work function decrease of Δφ = ?1.2 eV. The XP and UP spectra differ significantly in both cases and suggest a Fermi level shift of nearly 1 eV connected with oxygen adsorption on the Cr₂O₃ surface. The thickness of the corrosion film may be further increased by heating at 500°C in oxygen. The usual XPS spectra of bulk chromium sesquioxide are then clearly observed.     

Photoenhancement mechanism for oxygen chemisorption on GaAs(110) using visible light

Visible light from an argon ion laser (514.5 nm, 3 W/cm²) is seen to increase oxygen chemisorption on cleaved GaAs(110) surfaces up to a final coverage between one and two monolayers. Using photoemission spectroscopy to measure the oxygen coverage after simultaneous exposure of the surface to oxygen and light, we have determined that oxygen uptake for photoenhanced exposures is independent of sample temperature and doping type. In addition, significantly less enhancement is seen for weakly bound oxidizing molecules (N₂O) relative to the effects with molecular oxygen. These results are explained by a photoenhancement mechanism in which energy is released in a surface recombination event, possibly in the form of nonthermal phonons, causing physisorbed gas molecules to dissociate and thereby overcoming a major rate limiting step of the reaction in the dark. This reaction mechanism is supported by calculations of the surface recombination rates and free carrier densities at the surface which show that only the recombination rate is correlated with enhanced oxygen uptake. Other mechanisms and experimental data are also discussed.