Oxygen Backgrounds Research Articles

Electrospun ZnO/Pd Nanofibers as Extremely Sensitive Material for Hydrogen Detection in Oxygen Free Gas Phase.

The development of safety sensors is an urgent necessity for the successful use of hydrogen in real conditions, which may differ, in particular, by the oxygen content in the surrounding atmosphere. Palladium-modified zinc oxide shows the high sensitivity when detecting hydrogen in air; however, studies of the sensor properties and the operation mechanism of the ZnO/Pd sensor when reducing gases are detected in an oxygen deficient or inert atmosphere have not been effectuated. In this work, we synthesized the ZnO and ZnO/Pd nanofibers by electrospinning and for the first time determined their sensor properties in the detection of CO, NH3 and H2 in different oxygen backgrounds. The microstructure and composition of nanofibers were characterized by electron microscopy, X-ray diffraction, X-ray fluorescent spectroscopy, and X-ray photoelectron spectroscopy. The interaction with the gas phase was investigated in situ by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100–450 °C towards CO, NH3 and H2 in the N2/O2 gas mixtures containing 0.0005–20% O2. When detecting CO, a decrease in the oxygen concentration from 20 to 0.0005% in the gas phase does not lead to a significant change in the sensor response. At the same time, when detecting NH3 and especially H2, a decrease in oxygen concentration down to 0.0005% results in the dramatic increase in the sensor response of ZnO/Pd nanofibers. This result is discussed in terms of palladium hydride formation, modulation of the potential barrier at the ZnO/Pd interface, as well as changes in the concentration of donor defects and charge carriers in the ZnO matrix. Synthesized electrospun ZnO/Pd nanofibers are extremely promising materials for sensors for detecting hydrogen in an oxygen free atmosphere.

Selective Plasmonic Gas Sensing: H2, NO2, and CO Spectral Discrimination by a Single Au-CeO2Nanocomposite Film

A Au-CeO(2) nanocomposite film has been investigated as a potential sensing element for high-temperature plasmonic sensing of H(2), CO, and NO(2) in an oxygen containing environment. The CeO(2) thin film was deposited by molecular beam epitaxy (MBE), and Au was implanted into the as-grown film at an elevated temperature followed by high temperature annealing to form well-defined Au nanoclusters. The Au-CeO(2) nanocomposite film was characterized by X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). For the gas sensing experiments, separate exposures to varying concentrations of H(2), CO, and NO(2) were performed at a temperature of 500 °C in oxygen backgrounds of 5.0, 10, and ∼21% O(2). Changes in the localized surface plasmon resonance (LSPR) absorption peak were monitored during gas exposures and are believed to be the result of oxidation-reduction processes that fill or create oxygen vacancies in the CeO(2). This process affects the LSPR peak position either by charge exchange with the Au nanoparticles (AuNPs) or by changes in the dielectric constant surrounding the particles. Spectral multivariate analysis was used to gauge the inherent selectivity of the film between the separate analytes. From principal component analysis (PCA), unique and identifiable responses were seen for each of the analytes. Linear discriminant analysis (LDA) was also used and showed separation between analytes as well as trends in gas concentration. Results indicate that the Au-CeO(2) thin film is selective to O(2), H(2), CO, and NO(2) in separate exposures. This, combined with the observed stability over long exposure periods, shows the Au-CeO(2) film has good potential as an optical sensing element for harsh environmental conditions.

Conduction mechanisms in SnO 2 based polycrystalline thick film gas sensors exposed to CO and H 2 in different oxygen backgrounds

The conduction mechanism in polycrystalline SnO 2 thick sensing films was modeled and experimentally investigated by means of simultaneous DC electrical resistance and work function changes measurements under CO and H 2 exposure in different oxygen backgrounds. It was shown that, according to the composition of the ambient atmosphere, the conduction changes from the case in which it is controlled by the surface depletion layers to a situation in which the main contribution comes from free charge transport in the surface accumulation layer. This is significant for the interpretation of work function changes measurements results because the relation between the different measured electrical resistance and surface band bending depends on the conduction model. Furthermore, the CO sensing mechanism dependence on the oxygen amount in the ambient was explained.

The Influence of Pt Doping on the Sensing and Conduction Mechanism of SnO2 Based Thick Film Sensors

The conduction mechanism in polycrystalline Pt doped SnO2 thick films is investigated by means of simultaneous DC and work function changes measurements under exposure to reducing gases (CO and H2) in different oxygen backgrounds. The results are showing that, in contrast to the case of polycrystalline undoped SnO2 thick films - where a continuous switch of the conduction mechanism from a depletion layer controlled model to an accumulation layer one was observed - the situation in case of Pt doping is different: one records an abrupt change in both resistance and work function changes, which may indicate the onset of the reduction of the material. The reduction of the material was further studied by a combination of DC resistance and catalytic conversion measurements.

Influence of oxygen backgrounds on hydrogen sensing with SnO 2 nanomaterials

The influence of different oxygen backgrounds on the sensing of hydrogen with SnO 2 nanomaterials was investigated and a model was proposed. It is based on two hydrogen reaction mechanisms at the surface of tin oxide that can take place simultaneously; the weight of each mechanism depends on the concentration of oxygen in the ambient atmosphere. In the absence of oxygen the adsorbed hydrogen builds surface donors (rooted hydroxyl groups). In the presence of oxygen the reaction between hydrogen and pre-adsorbed oxygen ions dominates. Due to the fact that this behavior is present for very different nanomaterials, the model probably describes a SnO 2 specific feature.

Influence of Material Properties on Hydrogen Sensing for SnO2 Nanomaterials

Two SnO2 materials synthesized from SnCl4 and Sn(CH3COO)2 precursors were investigated concerning their hydrogen sensing properties in different oxygen backgrounds. At very low oxygen concentrations one observes a huge increase in the sensor response but no water formation in the exhaust and very different sensing performance for the different materials. As the oxygen content increases, the sensor response decreases and water production occurs and the sensing performance of the two materials becomes very similar.

Shock front dynamics in the pulsed laser deposition of YBa2Cu3O7−x

The pulsed laser deposition of YBa2Cu3O7−x targets by excimer laser at fluences of 4–10 J cm−2 in low pressure oxygen backgrounds yields emissive plumes with kinetic energies of 50–200 eV, driving the formation of a shock front with Mach numbers of M = 10–50. The propagation of the shock front is independent of atomic species and adequately characterized by the Sedov–Taylor shock model if the dimensionality of the plume is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock expansion is nearly unity at 1 Torr, but decreases rapidly at lower pressures, where the plume expands beyond the laser footprint during ablation. The low oxygen background pressures, 100–1000 mTorr, typically employed for the production of superconducting films is sufficient for the generation of a strong shock front with shock thickness of 5 mm to less than 0.4 mm, but too low to develop three-dimensional flow. Indeed, dimensionality of the expansion ranges from n = 0.8 to 2.4 over the background oxygen pressure range of 25–1000 mTorr. Shock strength is proportional to the Mach number and inversely dependent on pressure, indicating a thickness limited to approximately the collision mean free path.

CO sensing with SnO 2 thick film sensors: role of oxygen and water vapour

The paper investigates the gas response of nanocrystalline SnO 2 based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H 2O) and oxygen (O 2) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO 2. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R 0 and the resistance in the presence of the target gases, R, namely R 0/ R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O 2 and H 2O related surface species and, as a result, different sensing mechanisms can be observed for CO.

Electrode effect on microwave properties of ferroelectric (BaxSr1-x)TiO3 thin films

ABSTRACTMicrowave properties of coplanar waveguide (CPW) transmission lines fabricated on high dielectric materials, such as ferroelectric Ba1−xSrxTiO3 films, are highly sensitive on the dimension and shape of electrodes. A small change in device dimension affects the total electrical length of the CPW, which may mislead the effective dielectric constant of the dielectric layer. Furthermore, extracting dielectric constant of high-k thin films from the measured microwave properties, such as S-parameters, is very difficult. The well known a modified conformal mapping method frequently exhibits an inconsistent dielectric constant for CPW on high-k materials. CPW transmission lines were fabricated on high-k thin films, ferroelectric Ba0.6Sr0.4TiO3, which were deposited by the pulsed laser deposition with partial oxygen backgrounds. A large phase shift angle of 100° at 10 GHz was observed from the CPW (gap = 4 μm, length = 3 mm) with a 40 V of dc bias, which supports that the idea of the tunable microwave device application using ferroelectrics films. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameters by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film calculated by a modified conformal mapping exhibits a gap dependency; dielectric constant (990 ∼ 830) decreases with increasing gap size (4 ∼ 19 μm, respectively). For comparison, dielectric properties have been extracted by extensive EM-simulation using a HFSS™ (Ansoft) with observed dimensions of CPW devices. Total phase, which is closely related with the dielectric constant of the film, is strongly affected by gap size, film thickness, and slanted angle of CPW.

Low Thermal Budget Surface Preparation for Selective Epitaxy. A Study on Process Robustness

The robustness of a low thermal budget surface preparation method for selective silicon epitaxial growth has been investigated. After the HF dip, the stability of hydrogen passivation on Si(100) in deionized water and air has been studied. No significant increase was observed in oxygen and carbon coverage for deionized water rinse times varying from 10 to 1000 s. On wafers exposed to air for up to 10,000 s, carbon coverage on Si(100) stayed at the same level, whereas the oxygen coverage increased steadily. An in situ clean at 800°C for 10 s reduced the interfacial oxygen below the secondary ion mass spectroscopy detection levels on wafers that had been contaminated by exposure to air for up to 1000 s. In situ cleaning was studied in ambients with different partial pressures of intentionally introduced oxygen and nitrogen backgrounds. Oxygen was removed from Si(100) during the in situ clean for nitrogen partial pressures up to . When the oxygen partial pressure is sufficiently high , oxide removal was not complete after in situ cleaning. There was no observable increase in the surface roughness for samples annealed in oxygen partial pressure up to . Hydrogen passivation was removed from the substrates and the surfaces were exposed to vacuum at room temperature for different times. After 10,000 s, the oxygen coverage was less than 2% of a monolayer. The carbon contamination on the surface was instantaneous and no additional carbon accumulation on the surface was observed up to 10,000 s. There was no apparent increase in the defect density for these wait times. © 1999 The Electrochemical Society. All rights reserved.

Growth and Luminescence Properties of III-N:Er Materials Doped During Chemical Beam Epitaxy

AbstractThe incorporation and optical activation of Er in III-N optically-pumped and electroluminescent structures has been studied. For the first time, strong Er3+-related photoluminescence (PL) was measured at 300K for GaN:Er doped during growth on c-plane Al2O3 and Si. Also, room temperature electroluminescence was observed for simple GaN:Er structures on Si. Experiments to evaluate the effects C and 0 on the optical activity of Er indicated that these impurities enhance Er PL and EL in GaN. GaN films doped with Er to a concentration of 3 ×1018 cm−3 with [0] ∼ 1020 cm−3 and [C] ∼1021 cm−3 luminesce at 1.54 pim with an intensity ∼2 orders of magnitude greater than films with oxygen and carbon backgrounds of less than 1019 cm−3. Implantation and activation annealing was also shown to increase Er3+ signal in p(Mg)-i(Er)-n(Si) structures where C and 0 implant profiles were made to coincide with the Er-doped layers. The thermal PL quenching behavior was also markedly different for samples of varying 0 and C content. Er3+ photoluminescence from Gan:Er/Al2O3 samples with high O and C concentrations quenched by only 10% between 15 K and 300 K while the integrated PL signal from samples with lower [O] and [C] quenched ∼85% over the same temperature range.

Er doping of GaN during growth by metalorganic molecular beam epitaxy

1.54 μm photoluminescence has been observed from GaN doped with Er during growth by metalorganic molecular beam epitaxy. Strong Er3+-related photoluminescence (PL) was measured at room temperature for GaN:Er doped during growth on c-plane Al2O3 and Si. Experiments to evaluate the effects C and O on the optical activity of Er indicated that these impurities dramatically enhance Er PL in GaN. GaN films doped with Er to a concentration of 3×1018 cm−3 with [O]∼1020 cm−3 and [C]∼1021 cm−3 luminesce at 1.54 μm with an intensity ∼2 orders of magnitude greater than films with oxygen and carbon backgrounds of less than 1019 cm−3. The thermal PL quenching behavior was also markedly different for samples of varying O and C content. Er3+ luminescence from samples with high O and C concentrations quenched by only 10% between 15 and 300 K while the integrated PL signal from the samples with lower [O] and [C] quenched ∼85% over the same temperature range.

Growth of InN for ohmic contact formation by electron cyclotron resonance metalorganic molecular-beam epitaxy

We have investigated the feasibility of depositing InN from nitrogen plasmas by electron cyclotron resonance metalorganic molecular-beam epitaxy (ECR MOMBE) for use in ohmic contact formation. Resistivity, growth rate, and morphology were evaluated as a function of microwave power, growth temperature, and trimethylindium flux. Fine-grained polycrystalline films with N/In ratios of 1.15 and resistivities as low as 190 μΩ cm were obtained under optimized growth conditions and represent the first demonstration of nitride growth by ECR MOMBE. Carbon and oxygen backgrounds were below the detection level of Auger electron spectroscopy. Ti/Pt/Au contacts deposited on InN yielded contact resistances of 5–6×10−7 Ω cm2 and are thermally stable at normal alloying temperatures (∼380 °C).