Existence Of Oxygen Vacancies Research Articles

Room-temperature ferromagnetic properties in Mn-doped rutile [formula omitted] thin films

Room-temperature ferromagnetic properties of Mn-doped reduced titanium dioxide ( Ti 1 - x Mn x O 2 - δ ) thin films with rutile structure synthesized by a sol–gel method have been detected for a limited range of Mn composition ( x ) . The Ti 1 - x Mn x O 2 - δ films were found to be p-type semiconducting with hole concentration near 10 19 cm - 3 . The observed ferromagnetism in the Ti 1 - x Mn x O 2 - δ films is not related to the hole concentration but related to x. The room-temperature ferromagnetism is attributable to magnetic polaron formed by trapped electron in oxygen vacancy and magnetic ions around it. Thus, the existence of oxygen vacancies is necessary for the room-temperature ferromagnetism.

Effect of strontium substitution on the activity of La2−x Sr x NiO4 (x = 0.0–1.2) in NO decomposition

The aim of this work is to study the effect of Sr substitution on the redox properties and catalytic activity of La 2- x Sr x NiO 4 ( x = 0.0－1.2) for NO decomposition. Results suggest that the x = 0.6 sample shows the highest activity. The characterization (TPD, TPR, etc.) of samples indicates that the x = 0.6 sample possesses suitable abilities in both oxidation and reduction, which facilitates the proceeding of oxygen desorption and NO adsorption. At temperature below 700℃, the oxygen desorption is difficult, and is the rate-determining step of NO decomposition. With the increase of reaction temperature ( T >700℃), the oxygen desorption is favorable and, the active adsorption of NO on the active site (NO + V o + Ni 2+ NO - -Ni 3+ ) turns out to be the rate-determining step. The existence of oxygen vacancy is the prerequisite condition for NO decomposition, but its quantity does not relate much to the activity.

Cerium oxide based nanometric powders: synthesis and characterization

Nanometric powders of solid solutions of cerium oxide were obtained by a modified glycine nitrate procedure. Solid solutions of the host compound CeO2 with one or more dopants in the lattice were synthesized. Rare earth cations (Re=Yb, Gd and Sm) were added to ceria in total concentration of x= 0.2 that was kept constant. The criterion in doping was to keep the value of lattice parameter of ceria unchanged. The lattice parameters were calculated by using the model that takes into account the existence of oxygen vacancies in the structure.

SYNTHESIS OF In2O3 NANOWIRES ENHANCED BY ANODIC ALUMINA MEMBRANE

Large-scale single crystalline In 2 O 3 nanowires were successfully synthesized on anodic alumina membranes by a simple thermal evaporation method at 570°C. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy studies revealed the formation of single crystalline In 2 O 3 nanowires with diameters of 50–100 nm and lengths of up to a few hundreds of micrometers. Cathodeluminescence study revealed existence of oxygen vacancies evidenced by a strong and broad emission at 470 nm with a shoulder at 400 nm. The growth mechanism of the nanostructures is also discussed.

Structural and surface approach to the enhanced photocatalytic activity of sulfated TiO2 photocatalyst

TiO2 materials prepared by sol–gel method and then impregnated with sulfuric acid and further calcined at different temperatures show high photon efficiencies for the photocatalytic degradation of phenol under UV-illumination. Best photocatalyst was obtained after calcination around 700°C, giving specific activities (i.e. per m2) significantly higher than those exhibited by similarly prepared non-sulfated TiO2 or by pure Degussa P25. Structural analysis of these new materials by XRD, TG–DTG and Raman spectroscopy shows that once calcined at 700°C the material was a well-crystallized, high surface area and sulfate-free 90% anatase. Surface characterization in this work by XPS, LEIS and 1H MAS-NMR confirms a complete loss of the sulfate and OH-groups, and a low XPS O/Ti-atomic ratio with the O(1s) peak shifted to higher binding energies (1.7 versus 2±0.1 and 530.4eV versus 529.8eV, respectively, against the reference materials). This indicates the existence of oxygen vacancies, which give a broad band at 400–600nm in the reflectance spectra. However, LEIS spectra show an O/Ti composition at the topmost exposed atomic surface layer similar to that of TiO2 reference materials. Adsorption microcalorimetry of pyridine gives a profile of acid sites quite similar to those found for reference anatase samples, what discards the presence of superacid sites as the origin of its enhanced UV-photoefficiency. A mechanism is proposed, on the basis of earlier results in the literature for acidic TiO2 surfaces, to explain the nature of these materials. We also try to correlate the contribution of the oxygen vacancies within the anatase sub-surface layers to the high photon UV-efficiency of the system and, likely, to an enhanced vis-photoactivity of these materials.

Effect of oxygen deficiencies and the substitution of fluorine on the electronic states in YBa 2Cu 3O 7 − δ superconductors

We present a systematic theoretical study of fluorine-substituted YBa 2Cu 3O 7 crystal based on accurate first-principles all-electron total-energy calculations, using the full-potential linearized augmented plane wave (FP-LAPW) approach in the framework of density functional theory (DFT), and the generalized gradient approximation (GGA) for the exchange-correlation term. The electronic structure, energy levels and the density of states (DOS) at equilibrium volume with different oxygen deficiencies distribution or F substitution are studied. The existence of oxygen vacancies and the substitution of oxygen by one, two and up to three fluorine atoms on several selective sites change the electronic band structure in CuO 2 plane between two Ba planes. Thus, doping with charge carriers in the above materials is simulated and the resulting changes in the electronic distribution are compared to experimental and/or ab initio results.

Effects of the oxygen vacancy on NO adsorption at the TiO2 surface

The desorption spectrum of NO adsorbed on the surface of TiO2 powder was determined by temperature programmed desorption. The molecular orbital theory was u tilized to investigate the crystal cluster model of NO adsorbed on TiO2 and th e change of band gap in the adsorption process. The results indicate that there exist two peak temperatures, 450 and 980K, as N2 desorption followed by the NO adsorption on TiO2 The intensity of N2 desorption decre ases if the TiO2 powder is pretreated in an O2 atmosphere. The nitrogen atoms of the adsorbed NO gas recombine into N2 for desorption while oxygen atoms occupy th e oxygen vacancies in the TiO2 surface. The calculation analysis from the mole cular orb ital theory supports the deduction that the existence of oxygen vacancy on TiO2(110) surface offers a suitable condition of crystal surface for NO ad sorption.

Influence of Oxygen Vacancies and Strain on Electronic Reliability of SiO2-x Films

AbstractWe performed a quantum chemical molecular dynamics analysis for SiO2-x structure under strain to make clear the effect of the strain and intrinsic defects on both electronic and structural characteristics of SiO2-x. The SiO2-x showed a large change of the structure during the simulation. This is mainly because that the Si-O bonds near an oxygen vacancy were broken and a free silicon monoxide molecule was generated in the SiO2-x structure. The magnitude of the band gap of the SiO2-x decreased drastically due to the formation of the free monoxide. In addition, the band gap decreased further under large tensile strain of about 10%. We can conclude therefore, that both the existence of oxygen vacancies and tensile strain in SiO2-x films deteriorate the electronic reliability of the oxide film seriously.

1815 高誘電率薄膜材料のバンドギャップに及ぼす結晶欠陥と格子ひずみの影響(J06-3 マイクロ・ナノ構造解析法と応用,J06 計算マイクロ・ナノメカニクス)

In order to make clear the effect of the strain and intrinsic defects on both electronic and structural characteristic of HfO_2, we performed a quantum chemical molecular dynamics analysis for HfO_<2-x> structures under strain. The band gap value of HfO_2 decreased drastically due to the oxygen vacancy. In addition, the band gap decreased further by the absolute value of strain. We can conclude, therefore, that both the existence of oxygen vacancies and strain in HfO_2 films deteriorate the electronic reliability of the oxide film seriously.

Photoluminescence properties of SnO2 nanowhiskers grown by thermal evaporation

SnO2 nanowhiskers were synthesized by thermal oxidation with and without a gold film as a catalyst. The SEM images reveal wire-like and rod-shaped nanowhiskers about several hundred micrometers in length and 100 nm in diameter. The three observed Raman peaks at 474, 632, and 774 cm−1 indicate the typical rutile phase which is in agreement with the X-ray diffraction results. The photoluminescence properties were measured at room temperature. The peaks at 342 nm corresponding to the excitation transitions from the conduction band to the valence band of the SnO2 nanowhiskers were not observed. However, a strong emission band at ∼600 nm was detected indicating the existence of oxygen vacancies in both samples. A new emission band at ∼398 nm was also observed in the sample with the gold film and it could be attributed to the near band-edge emission.

ENHANCED PHOTOLUMINESCENCE OF Eu(III)-ANCHORED POROUS ANODIC ALUMINA FILM

We report here the enhanced luminescence of Eu(III)-anchored porous anodic alumina prepared by self-assembling Eu(III) acetylacetonate, and investigate the luminescence mechanisms. Porous anodic alumina can emit visible light due to a lot of oxygen vacancies formed in the anodic oxidation. The existence of oxygen vacancies resulted in e−-h+ pairs when excited. Eu(II) exists stably by forming Eu2+-hole complexes. The enhanced luminescence of Eu(III)-anchored porous anodic alumina is attributed to the complex luminescence of e−-h+ through luminescence center Eu2+.

Perovskite-type BaTiO3 ceramics containing particulate SiC

BaTiO3-based composites with nanosized SiC particulates were successfully fabricated by a hot-pressing technique in an argon atmosphere. Crystal structure and phase transformation behaviour were investigated by X-ray diffraction analysis, linear thermal expansion analysis and internal friction measurement. It was confirmed that the added SiC particulates were uniformly distributed within the matrix BaTiO3 grains, with some larger particulates located at the BaTiO3 grain boundaries. In addition, there were no reaction phases between BaTiO3 matrix and SiC particulates. The crystal structure gradually changed from tetragonal to cubic phase with respect to the SiC content. The Curie temperature, Tc, was lowered as the SiC content increased. Moreover, the transformations in the low-temperature range almost disappeared above 1 vol% SiC. The diffused phase transformation phenomenon was observed as the SiC content increased up to 3 vol%. The results were associated with the grain-size reduction, the existence of oxygen vacancies and the residual stresses associated with the thermal expansion mismatch between matrix and SiC particulate. The influence on the domain structure development of SiC particulates dispersed within the matrix grains was also discussed.

Influence of the fluoride ion on the piezoelectric properties of a pzt ceramics

In order to decrease dielectric losses of PZT type piezoelectric ceramics, it is necessary to introduce oxygen vacancies in ABO3 perovskite lattice by heterovalent cationic substitutions (for instance Fe, Mn, Ni in place of B ions). Hence, it is possible, at room temperature, to reduce the dielectric loss by an order of magnitude of corresponding stoichiometric materials. This phenomenon is accompanied by an increase of electric conductivity which appears especially at temperatures higher than hundred degrees. This conductivity is very depending on the fabrication process and the result is an important lack of reliability during the industrial manufacture. This is certainly due to a mobility of charges which is the result of the existence of oxygen vacancies. The partial introduction of fluorine into the lattice can greatly reduce the losses and increase the polarization stability under high stress levels. This study concerns the mechanism of the fluorine introduction and its consequences on electric and ...

Synthesis, Crystal Structure, and Electrical Properties of the Pyrochlores Pb 2- xLn xRu 2O 7- y( Ln = Nd, Gd)

The lead-lanthanoid pyrochlores Pb 2- x Ln x Ru 2O 7- y ( Ln = Nd, Gd) have been synthesized and characterized by X-ray diffraction, neutron diffraction, and electrical resistivity measurements. The solid solutions synthesized from metal nitrates and ruthenium oxide showed two monophasic regions in both neodymium and gadolinium systems: an oxygen-vacancy-ordered phase with F4̄3m symmetry for the lead-rich region, 0 ≤ x ≤ 0.2, and a vacancy-disordered phase with Fd3̄ m symmetry for the lanthanoid-rich region, 1.0 ≤ x ≤ 2.0. Electrical resistivity measurements showed a metallic behavior for the lead-rich region of the solid solutions (0 ≤ x ≤ 0.2). In the lanthanoid-rich region, a change from metallic to semiconducting behavior was observed at 1.0 < x < 1.2 and 1.2 < x < 1.5 in Pb 2- x Nd x Ru 2O 7- y and Pb 2- x Gd x RU 2O 7- y , respectively. From x = 1.0 to 2.0, the lattice parameters increase in Pb 2- x Nd x Ru 2O 7- y and decrease in Pb 2- x Gd x Ru 2O 7- y . However, Rietveld analysis of Pb 2- x Ln x Ru 2O 7- y at room temperature indicates similar structural changes in both systems; the Ru-O(1) bond length decreases and the bend in RuO 6 zigzag chains increases with increasing x. The analysis of neutron diffraction data for PbNdRu 2O 7- y indicates the existence of oxygen vacancies, y = 0.188(8), with no evidence for long-range ordering. The relationship between the structural changes and the electrical properties is discussed.

Role of Point Defects in Dielectric Breakdown of SiO2 Formed by Plasma-Enhanced Chemical Vapor Deposition of Tetraethoxysilane

The intrinsic breakdown strength of thin SiO2 films formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane was measured using a self-healing technique by applying short-duration voltage pulses. Point defects in the films and other film qualities were examined by instrumental analyses and by optical absorption and luminescence using synchrotron radiation. The intrinsic breakdown strength decreases when the deposition temperature becomes higher than 400° C, in spite of the fact that the optical gap and the density increase and the impurity content decreases. Based on the fact that the film deposited at high temperatures shows absorption at 7.6-eV and luminescence at 4.4-eV, the existence of oxygen vacancies ( ≡Si-Si≡) is suggested. The intrinsic breakdown strength is increased by decreasing oxygen vacancies through oxygen treatment. From these results, it is considered that oxygen vacancies play a crucial role in lowering the breakdown strength. Three possible mechanisms are discussed.

Links between oxide, interface, and border traps in high-temperature annealed Si/SiO2 systems

Evidence is provided to show that enhanced hole-, interface-, and border-trap generation in irradiated high-temperature annealed Si/SiO2/Si systems are all related either directly, or indirectly, to the presence of oxygen vacancies. We find that the calculated oxygen vacancy due to high-temperature anneals from 800 to 950 °C in metal-oxide-semiconductor capacitors closely matches the radiation-induced oxide-trapped charge. This strongly suggests that oxygen vacancies (or vacancy-related complexes) are the dominant hole trapping sites in this particular case. Along with the increase in radiation-induced oxide-trap charge, we observe a concomitant increase in the interface- and border-trap densities. This suggests that in devices that receive high-temperature anneals, all these phenomena are linked to the existence of oxygen vacancies either directly, or indirectly, perhaps via hole trapping at these vacancies.

Degradation of Multilayer Ceramic Capacitors with Nickel Electrodes

The mechanism of degradation, or IR drop, of BaTiO3‐based Ni‐electrode chip capacitors has been studied, with special attention to the microstructure. Degradation occurred mainly at the center of the capacitors. Longer‐life capacitors were obtained under conditions of low oxygen partial pressure, 10−12 atm, during sintering, and had larger grain sizes and smaller quantities of grain boundary phase. The existence of oxygen vacancies in the grain boundary phase was confirmed by cathode luminescence measurement. Double layers along the surfaces of the Ni electrodes were also related to the life of the capacitors.

Structure images ofa- andb-axis projections in YBa2Cu3O7-δsuperconductors observed by HREM at 200 kV

Abstract Studies of oxygen vacancies in high-Tc YBa2Cu3O7-δ superconductors have been attempted by high-resolution electron microscopy (HREM). HREM images taken near Scherzer defocus show a good correspondence with the projection of the structure proposed from neutron and X-ray diffraction results. Y, Ba and Cu atoms are seen as dark spots with different contrast depending on the atomic scattering factors. The spots arising from the positions of oxygen vacancies located at (0,0,1/2) in Y planes and (½, 0, 0) in Cu(1) planes are brighter than those corresponding to occupied oxygen positions. The simulated images confirm that the brightest spots which appear at unoccupied atomic oxygen positions (½ 0,0) in Cu(1) planes appear not only because of the existence of the oxygen vacancies themselves, but also due to the shift of the Ba atoms away from the Cu(1) planes; on the other hand the brighter spots located at (0,0,½) in Y planes are due merely to the existence of oxygen vacancies.

Identification of oxygen vacancies in YBaCuO superconductors by high-resolution electron microscopy

High-resolution electron microscopy studies as well as computer simulation studies on oxygen vacancies in Y-Ba-Cu-O superconductors show that the existence of oxygen vacancies at {0, 1/2, 0} sites leads to the distortions of structural images. For certain imaging conditions, the spots corresponding to the column of oxygen vacancies can be distorted heavily, so that the indirect image of oxygen vacancies in Y-Ba-Cu-O superconductors may be acquired. The results reveal that even in local area the oxygen vacancies may be distributed regularly.

Effect of oxygen deficiencies and the substitution of fluorine on the electronic states in YBa 2Cu 3O 7∓δ superconductors

The electronic structure for copper and its environmental ions in the high Tc superconductors pure and F doped YBa 2Cu 3O 7∓δ with different oxygen deficiencies distribution or F substitution are studied in the framework of local density theory. The one electronic HFS equation for various embedded cluster models are solved self-consistently to obtain the wave functions, energy levels (energy band) and the density of states (DOS). The existence of oxygen vacancies and the substitution of fluorine change the electronic band structure in CuO plane between two Ba planes. The influence on the Tc of these YBa 2Cu 3O 7 superconductors are discussed.