High O2 Pressure Research Articles

Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study

We present a study of the surface stoichiometry and contamination of La0.7Sr0.3MnO3 thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hν=150 to 350eV) and traditional Mg-Kα XPS (hν=1253.6eV) whilst annealing incrementally to ≈510°C in low pressures of O2. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O2 pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270°C, it is reduced to ≈0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films.

Optimization of annealing processes on superconductivity of FeSr2YCu2O6+δ

Abstract FeSr2YCu2O6+δ (Fe1212) shows superconductivity only after proper annealing in N2 flow, subsequently, annealing in O2 flow and under high O2 pressure. In order to optimize each of the annealing temperature for superconductivity, Fe1212 samples were annealed at various temperature in each annealing process. The optimized annealing condition is annealing in N2 flow at 790 °C, annealing in O2 flow and under high O2 pressure at 270 °C. The resistance began to decrease at 64 K and dropped to zero at 40 K. Superconductivity of Fe1212 was affected by temperature of N2 annealing process rather than that of O2 annealing. The N2 annealing affects the offset superconducting transition temperature rather than the onset superconducting transition temperature. This implies the inter-grain superconductivity in ceramic Fe1212 is affected by N2 annealing temperature.

Communication: Time-domain measurement of high-pressure N2 and O2 self-broadened linewidths using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N(2) and O(2) from 1-20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N(2) and O(2) from ~0.06 to 2.2 cm(-1) and the line separation of O(2) triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures.

Laser deposition of iron in oxygen atmosphere

Abstract Iron oxide films were produced by pulsed laser deposition (PLD) of 57Fe metal in an oxygen atmosphere and their compositions were studied by Mössbauer spectroscopy. The effects of gas-phase reactions were investigated by varying the pressure of O2 gas or an O2/Ar gas mixture. When PLD was performed in a high-pressure O2 atmosphere, the main product in the film was trivalent iron oxide particles. When the O2 pressure was reduced, hematite Fe2O3 became dominant in the film, while wüstite FeO was produced at very low O2 pressures. PLD in an O2/Ar gas mixture produced films of trivalent iron oxide particles and hematite solid, but wüstite was not produced. Increasing the substrate temperature during deposition induced annealing of the films, reducing the lattice defect density. X-ray diffraction patterns were obtained to confirm the assignments, and the surface morphologies of the films were investigated by scanning electron microscopy.

Generation of Reactive Oxygen Species on Au/TiO2 after Treatment with Hydrogen: Testing the Link to Ethanol Low‐Temperature Oxidation

The gold catalyzed aerobic oxidation of alcohols is currently of great interest for the following reasons: 1) biomass-derived alcohols are a promising, renewable organic feedstock; 2) they use cheap, green oxidants, such as molecular oxygen (air) ; and 3) gold catalysts show extraordinarily high activities under mild conditions. The mechanistic aspects of gold catalyzed oxidation reactions of CO, H2, and other small molecules have been extensively studied, but a generally accepted oxidation mechanism for alcohols under similar conditions has not yet been formulated (however, see references [7–10]). Notably, the efficient low-temperature (50–120 8C) gold catalyzed oxidation of alcohols is known to proceed only in the liquid phase during the course of a long-term batch reaction with high oxygen pressure or intensive O2 flow. [1–4] On the other hand, we recently found that metallic gold nanoparticles supported on TiO2 gave rise to “double peak” catalytic activity in the gasphase oxidation of ethanol to acetaldehyde. The temperature, at which the first peak of activity occurred, 120 8C, was unusually low for the gas-phase reaction of primary alcohols. In contrast, gold supported on Al2O3 and SiO2 showed more usual behaviors, which were analogous to the second peak activity of Au/TiO2 at temperatures above 200 8C. [11] To account for these results, we proposed that specific active oxygen species form on the Au/TiO2 surface under mild reaction conditions and suggested hydrogen as a probable cofactor in their generation. Indeed, H2 can be produced concurrently as a result of ethanol anaerobic dehydrogenation and can thus participate in catalytic activity. In the present work, our efforts were focused on the role of hydrogen in the catalytic activity of gold supported on TiO2, Al2O3, and SiO2 matrixes to provide insights into the different profiles of ethanol oxidation. For this purpose, the O2 isotope exchange technique was applied in order to estimate the relative activity of the surface oxygen species. Oxygen isotope exchange (OIE) is a very sensitive direct method for evaluating the reactivity of surface oxygen atoms, which is a subject of primary interest in heterogeneous oxidation catalysis. Usually, as in the case of metal oxides, OIE is observed at 220–700 8C, although some metal oxides induce OIE at low temperatures after calcination and cooling in a vacuum. A highly reactive oxygen species, so called aoxygen, is generated on the Fe-ZSM5 zeolite surface after treatment with N2O. These species readily oxidize organics (benzene, methane, etc.) as well as perform OIE at room temperature. A special but relevant case, although photoinduced, is the OIE over TiO2, which also occurs at room temperature due to the involvement of reactive oxygen species, probably O anion radicals. Notably, a correlation was found between the TiO2 activity for the photoinduced OIE and the oxidation of isobutane, methanol, and ethanol over TiO2 upon ultraviolet (UV) irradiation. Generally, in a heterophase system comprising molecular dioxygen (O2) in the gas phase and oxygen on the surface of a solid (O), two isotope exchange reactions take place: 1) the hetero-exchange reaction [Equation (1)] , which is conveniently monitored by a fraction of the O isotope in the gas phase (a), and 2) the homo-exchange reaction [Equation (2)] , which is monitored by the parameter y = *C34 C34 (the difference between the equilibrium (*C34) and the current (C34) values for the fraction of asymmetric isotopic OO molecules). Obviously, the low-temperature activity of a catalyst in an oxygen homoexchange can indicate the involvement of very active surface oxygen species by analogy with the cases of a-oxygen and illuminated TiO2. [17]

Enhanced Superconductivity of the Pb-Based 1212 Cuprates in the (Pb0.75S0.25)(Sr2−x Ba x )(Y0.4Ca0.6)Cu2O z System by High-Pressure O2 Annealing

We have already reported on substitution effects of Ba for Sr in the sulfur contained Pb-based 1212 compound with the composition of (Pb0.75S0.25)(Sr2−x Ba x ) (Y0.4Ca0.6)Cu2O z . It was found that each sample with x=0.3–0.6 showed a resistivity drop originating from superconductivity, and the highest onset temperature of the drop was about 32 K for a sample with x=0.5, but the volume fraction was very small. More recently, we have investigated effect of high-pressure O2 annealing on superconductivity of the Ba-substituted samples with x=0.4–0.6. As a result, the sample with x=0.5 is found to show an onset of the resistivity drop at the highest temperature of about 41.7 K and it shows an onset of the diamagnetic signal at about 42.0 K. These transition temperatures are higher by ∼10 K than those of the previously reported sample with x=0.5. Moreover, the superconducting volume fraction is increasing more by the high-pressure O2 annealing. These results indicate that the high-pressure O2 annealing effectively promotes superconductivity of the present (Pb0.75S0.25)(Sr2−x Ba x )(Y0.4Ca0.6)Cu2O z system.

Performance of a Sulfide-Oxidizing, Sulfur-Producing Membrane Biofilm Reactor Treating Sulfide-Containing Bioreactor Effluent

Sulfide-containing waste streams are generated in mining, petrochemical plants, tanneries, viscose rayon manufacture, and the gasification of coal. Colorless sulfur bacteria can oxidize sulfide to elemental sulfur (S°), which can be recovered, when oxygen is their electron acceptor. This study evaluated sulfide oxidation and S° recovery in an oxygen-based membrane biofilm reactor (MBfR) treating the effluent from a sulfidogenic anaerobic baffled reactor. Sulfide oxidation efficiency (37-99%) and S° recovery (64-89% of oxidized sulfide) could be controlled by manipulating the sulfide loading, oxygen pressure to the fibers, and hydraulic retention time (HRT). For example, too-low oxygen pressure decreased S° recovery due to decreased sulfide oxidation, but too-high oxygen pressure lowered S° recovery due to its oxidation to sulfate. Most importantly, high sulfide oxidation (>98%) and conversion to S° (>75%) could be achieved together when the sulfide loading was less than 1.7 mol/m²·d and the O₂ pressure was sufficient to give an O₂ flux of at least 1.5 mol/m²·d. However, higher sulfide loading could be compensated by a higher O₂ pressure, and the best performance occurred when the sulfide loading was high (2 molS/m²·d), the O₂ pressure was high (∼1 atm), and the HRT was short (1.9 h). Membrane fouling caused a low O₂ flux, which led to low sulfide-oxidation efficiency, but fouling could be reversed by mild acid washing.

The respiratory down‐regulation debate

The respiratory down‐regulation debate

The effect of He plasma treatment on properties of organosilicate glass low-k films

The effect of low-pressure He plasma on properties of nanoporous organosilicate glasses low-k films with 24% and 33% open porosity is studied. The influence of ions, VUV radiation, and metastable atoms are extracted separately using a special experimental system designed for this purpose. The low-k films treated in He plasma were exposed to O or H atoms in the downstream of high-pressure O2 or H2 rf discharge. The changes in chemical composition and structure occurring in low-k films were measured before and after all treatments. The loss probabilities of oxygen and hydrogen atoms on the low-k film surface were measured for both treated and pristine films. It is shown that the film pretreatment in He plasma leads to the noticeable densification of the top surface layer up to complete sealing all the films studied. The sealing layer prevents O atoms from deep penetration to the film bulk and carbon extraction. The sealing mechanism related to the joint impact of low-energy ions and VUV photons with metastable atoms in He plasma is discussed in detail.

Effect of Ba Substitution for Sr in the Pb-based 1212 Cuprate Containing Sulfur of (Pb0.75S0.25)Sr2(Y,Ca)Cu2O z

We already reported on the synthesis of new Pb-based 1212 layered cuprates containing sulfur in the (Pb0.75S0.25)Sr2(Y,Ca)Cu2Oz system. There none of the samples showed superconductivity until after annealing under high O2 pressure. More recently, we have discovered that the samples could be substituted by Ba for Sr up to x=0.6 in the composition of (Pb0.75S0.25)(Sr2−xBax)(Y0.4Ca0.6) Cu2Oz. Then if the Ba-substitution is performed for Sr, some of the almost-single phase samples are found to show resistivity drops and magnetic anomalies without annealing under high O2 pressure. Among the samples, a sample with the nominal composition of (Pb0.75S0.25)(Sr1.5Ba0.5)(Y0.4Ca0.6) Cu2Oz is found to show the highest onset temperatures of a resistivity drop at about 35 K and a magnetic anomaly at about 31.5 K. These are higher than the highest values for the superconducting Ba-free sample of (Pb0.75S0.25)Sr2 (Y0.4Ca0.6)Cu2Oz which was annealed under high O2 pressure.

New Pb-based 1212 Cuprate Superconductors Containing Sulfur, (Pb0.75S0.25)Sr2 (Y1−x Ca x )Cu2O z

New Pb-based 1212 layered cuprates containing sulfur have been synthesized in the (Pb0.75S0.25) Sr2 (Y1−xCax) Cu2Oz system. X-ray diffraction analysis shows that the almost single-phase samples are obtained within a region of 0.0≤x≤0.5. The crystal structure of each sample has a tetragonal symmetry with the typical lattice constants a=0.3837 nm and c=1.186 nm. As Ca content x is increasing, the semiconductor-like behavior is suppressed. But after only annealing under ambient O2 pressure, none of the samples show any trace of superconductivity. On the other hand, when the samples are annealed under high O2 pressure of about 13.6 MPa, they show resistivity dropping phenomenon in a region of 0.5≤x≤0.7. Among them, the lowest resistivity sample with x=0.6 has an onset temperature of the resistivity dropping at about 22.5 K. Moreover, this sample shows a diamagnetic signal at about 21.5 K. These phenomena are attributed to superconductivity.

The effect of oxygen pressure on the structure, morphology and photoluminescence intensity of pulsed laser deposited Gd2O2S:Tb3+ thin film phosphor

Luminescent Gd2O2S:Tb3+ phosphor thin films were grown on Si (100) substrates, using the pulsed laser deposition technique. The films were grown in 100 to 300 mTorr oxygen gas (O2) atmospheres when the substrate temperature was kept constant at 400 or 600°C. The effect of the O2 ambient on the structure and morphological properties of the films were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM), respectively. Spherical nanoparticles deposited on the Si (100) substrates were shown to crystallize in the hexagonal structure of Gd2O2S. The photoluminescence (PL) spectra of all the films were characterized by a stable green emission peak with a maximum at 545 nm. Improved PL intensity was observed from the films deposited at higher oxygen pressures and higher substrate temperatures. Particles sizes of the nanoparticles deposited under the different conditions varied between 19 and 36 nm for the different samples. Smaller and more densely packet particles were produces at the higher O2 pressures and the higher temperature.

Hyperbaric oxygen-stimulated proliferation and growth of osteoblasts may be mediated through the FGF-2/MEK/ERK 1/2/NF-κB and PKC/JNK pathways

We investigated whether the hyperbaric oxygen (O2) could promote the proliferation of growth-arrested osteoblasts in vitro and the mechanisms involved in this process. Osteoblasts were exposed to different combinations of saturation and pressure of O2 and evaluated at 3 and 7 days. Control cells were cultured under ambient O2 and normal pressure [1 atmosphere (ATA)]; high-pressure group cells were treated with high pressure (2.5 ATA) twice daily; high-O2 group cells were treated with a high concentration O2 (50% O2) twice daily; and high pressure plus high-O2 group cells were treated with high pressure (2.5 ATA) and a high concentration O2 (50% O2) twice daily. Hyperbaric O2 significantly promoted osteoblast proliferation and cell cycle progression after 3 days of treatment. Hyperbaric O2 treatment stimulated significantly increased mRNA expression of fibroblast growth factor (FGF)-2 as well as protein expression levels of Akt, p70S6K, phosphorylated ERK, nuclear factor (NF)-κB, protein kinase C (PKC)α, and phosphorylated c-Jun N-terminal kinase (JNK). Our findings indicate that high pressure and high O2 saturation stimulates growth-arrested osteoblasts to proliferate. These findings suggest that the proliferative effects of hyperbaric O2 on osteoblasts may contribute to the recruitment of osteoblasts at the fracture site. The FGF-2/MEK/ERK 1/2/Akt/p70S6K/NF-κB and PKC/JNK pathways may be involved in mediating this process.

(Invited) Oxidation, Diffusion and Desorption in a Ge/GeO2 System

The effects of oxidation, diffusion, and desorption in a Ge/GeO2 system on the Ge/GeO2 stack formation process for Ge-CMOS are discussed. The GeO desorption is a key to understanding and to establishing the Ge gate stack formation process. High-pressure O2 oxidation has dramatically improved Ge/GeO2 MIS capacitors. This aspect of gate stack formation in Ge is quite different from that in Si.

Particle-in-cell/Monte Carlo collisions treatment of an Ar/O2 magnetron discharge used for the reactive sputter deposition of TiOx films

The physical processes in an Ar/O2 magnetron discharge used for the reactive sputter deposition of TiOx thin films were simulated with a 2d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) model. The plasma species taken into account are electrons, Ar+ ions, fast Arf atoms, metastable Arm* atoms, Ti+ ions, Ti atoms, O+ ions, O2+ ions, O− ions and O atoms. This model accounts for plasma–target interactions, such as secondary electron emission and target sputtering, and the effects of target poisoning. Furthermore, the deposition process is described by an analytical surface model. The influence of the O2/Ar gas ratio on the plasma potential and on the species densities and fluxes is investigated. Among others, it is shown that a higher O2 pressure causes the region of positive plasma potential and the O− density to be more spread, and the latter to decrease. On the other hand, the deposition rates of Ti and O are not much affected by the O2/Ar proportion. Indeed, the predicted stoichiometry of the deposited TiOx film approaches x=2 for nearly all the investigated O2/Ar proportions.

Control of Properties of GeO2 Films and Ge/GeO2 Interfaces by the Suppression of GeO Volatilization

Control of both GeO2 film quality and GeO2/Ge interface properties were successfully demonstrated, based on the understanding of GeO kinetic behaviors in GeO2/Ge stacks. The GeO volatilization was driven by GeO generation at GeO2/Ge interface, but the volatilization rate was limited by GeO diffusion process in GeO2 film. It was found that the gap states in GeO2 films induced by GeO volatilization were detectable as a sub-gap photo-absorption by spectroscopic ellipsometry. We have successfully demonstrated two approaches to control the GeO volatilization: the cap layer introduction and the annealing in high-pressure O2 ambient. The characteristics of Ge MOS capacitors were significantly improved by both of those two approaches. In addition, a Ge MOSFET fabricated with GeO2, thermally grown in high-pressure O2 ambient, showed a relatively high electron mobility.

磁性超伝導体ＦｅＳｒ２ＬｎＣｕ２Ｏ６＋ｙの超伝導と磁性

The magnetic superconductor FeSr2YCu2O6+y exhibits superconductivity below 59 K, only when it is properly annealed in a N2 flow and subsequently annealed in an O2 flow and under high O2 pressure. FeSr2LnCu2O6+y (Ln = Nd, Eu, Gd and Tb), where Ln is substituted for Y, was also studied. While FeSr2GdCu2O6+y and FeSr2EuCu2O6+y show superconductivity below 37 K and 31 K, respectively, FeSr2NdCu2O6+y and FeSr2TbCu2O6+y do not show superconductivity. A peak was observed at 20 K in the temperature dependence of susceptibility under a magnetic field of 1 T for FeSr2YCu2O7.65, and this implies the magnetic order of the Fe ion. To study the magnetism of the Fe ion in FeSr2YCu2O7.65, non-superconducting FeSr2Y(Cu0.95Zn0.05)2O7.65 was studied. The magnetism of the Fe ion is mainly antiferromagnetic.

Impact of High Pressure O2 Annealing on Amorphous LaLuO3/Ge MIS Capacitors

Amorphous LaLuO3/Ge MIS capacitors were fabricated and dielectric properties were investigated. LaLuO3 has a high dielectric constant (k~23) and an amorphous structure after annealed at 600oC. Its energy band-gap is reasonably large (5.5~5.8 eV). The C-V characteristics annealed in a small amount of O2 ambient were quite good and showed a high saturated capacitance thanks to a negligible interface layer formation. Furthermore, it has been demonstrated that the hysteresis in C-V curves was reduced by high pressure O2 annealing. Although an optimization of the trade-off between EOT increase and hysteresis reduction will be the next challenge for LaLuO3 on Ge, the high pressure O2 annealing will make a strong combination with amorphous LaLuO3.

Study of Kinetic Behaviors of GeO in GeO2/Ge Stacks

The kinetic behaviors of GeO in GeO2/Ge stacks, which would be crucially important to control the qualities of both GeO2 film and GeO2/Ge interface, were investigated. It was clarified that the GeO volatilization was driven by GeO generation at GeO2/Ge interface, but the volatilization rate was limited by GeO diffusion process in GeO2 film. The volatilization from (111) surface was slower than that from (100), which would come from the difference of reactivity between those surfaces. It was found that the gap states in GeO2 films induced by GeO volatilization were detectable as sub-bandgap photo-absorption. The extinction coefficients of GeO2 films determined by spectroscopic ellipsometry indicated that the gap states were generated by annealing in both N2 and O2 under normal pressure. However, the annealing in high-pressure O2 effectively annihilates those gap states.

Real-time scanning tunneling microscopy observations of the oxidation of a Ti∕Pt(111)-(2×2) surface alloy using O2 and NO2

The authors have used scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) to study the nascent oxidation of an ordered Ti∕Pt(111)-(2×2) surface alloy exposed to oxygen (O2) or nitrogen dioxide (NO2) under ultrahigh vacuum conditions. The Ti∕Pt(111)-(2×2) surface alloy was formed by depositing an ultrathin Ti film on Pt(111) and annealing to 1050K. This produces an alloy film in which the surface layer is pure Pt and the second layer contains Ti atoms in a (2×2) structure, which causes the pattern observed by STM and LEED. Real-time imaging of the surface at 300K was carried out by continuously scanning with the STM while either O2 or NO2 was introduced into the chamber. O2 exposures did not cause any gross structural changes; however oxygen was detected on the surface afterward using AES. Annealing this surface to 950K resulted in the formation of an ordered TiOx overlayer as characterized by both LEED and STM. In contrast, NO2 exposures caused definite changes in the surface morphology at 300K, and the root-mean-square roughness increased from 3.5to7.1Å after a large NO2 exposure. No ordered structures were produced by this treatment, but annealing the surface to 950K formed an ordered pattern in LEED and corresponding clear, well-resolved structures in STM images. We account for these observations on the disruption or reconstruction of the Ti∕Pt(111)-(2×2) surface alloy by arguments recalling that Ti oxidation is an activated process. The energetic barrier to TiOx formation cannot be surmounted at room temperature at low oxygen coverages, and annealing the surface was necessary to initiate this reaction. However, the higher oxygen coverages obtained using the more reactive oxidant NO2 lowered the chemical potential in the system sufficiently to overcome the activation barrier to extract Ti from the alloy at room temperature and form a disordered TiOx film. These results illustrate the importance of the surface oxygen coverage in nucleating the room temperature oxidation of the Pt–Ti surface alloys and further show the ability of NO2 in ultrahigh vacuum studies for probing the chemistry that will occur at higher O2 pressure.