Oxygen Ionosorption Research Articles

Decoration of Pt/Pd bimetallic nanoparticles on Ru-implanted WS2 nanosheets for acetone sensing studies

Bimetallic decoration is a promising technique for enhancing the gas-sensing characteristics of resistive sensors. In this study, pristine WS2 nanosheets (NSs), Ru-implanted WS2 NSs, separate Pt and Pd nanoparticles (NPs) decorated on Ru-implanted WS2 NSs, and Pt/Pd bimetallic NPs decorated on Ru-implanted WS2 NSs sensors were prepared and utilized for acetone detection in self-heating operations at low applied voltages (1–5 V). The gas sensors were characterized by employing various techniques to explore their phases, morphologies, and chemical compositions. The Ru-implanted WS2 NSs sensor decorated with Pt/Pd bimetallic NPs exhibited good selectivity and fast dynamics toward acetone. Also, the optimized sensor displayed an acceptable response to acetone even in a high-humidity environment. The enhanced acetone sensing of the optimized gas sensor was related to the spillover and catalytic effects of the implanted Ru and Pt/Pd NPs on acetone, enhanced oxygen ionosorption with the formation of defects under implantation, and the formation of Schottky barriers between Pt/Pd and Ru-implanted WS2. We successfully demonstrated the potential of this novel sensing material for room-temperature acetone sensing.

Reversible Entropy-Driven Defect Migration and Insulator-Metal Transition Suppression in VO2 Nanostructures for Phase-Change Electronic Switching.

Oxygen defects are among essential issues and required to be manipulated in correlated electronic oxides with insulator-metal transition (IMT). Besides, surface and interface control are necessary but challenging in field-induced electronic switching towards advanced IMT-triggered transistors and optical modulators. Herein, we demonstrated reversible entropy-driven oxygen defect migrations and reversible IMT suppression in vanadium dioxide (VO2 ) phase-change electronic switching. The initial IMT was suppressed with oxygen defects, which is caused by the entropy change during reversed surface oxygen ionosorption on the VO2 nanostructures. This IMT suppression is reversible and reverts when the adsorbed oxygen extracts electrons from the surface and heals defects again. The reversible IMT suppression observed in the VO2 nanobeam with M2 phase is accompanied by large variations in the IMT temperature. We also achieved irreversible and stable IMT by exploiting an Al2 O3 partition layer prepared by atomic layer deposition (ALD) to disrupt the entropy-driven defect migration. We expected that such reversible modulations would be helpful for understanding the origin of surface-driven IMT in correlated vanadium oxides, and constructing functional phase-change electronic and optical devices.

Layered mesoporous SnO2 for effective ethanol detection at reduced working temperature

Tin oxide is widely studied for ethanol sensing because of its high sensitivity and environmental benignity; yet its operating temperatures generally exceed 200 °C to achieve an acceptable overall sensing performance. In this study, we report a layered mesoporous aggregate of SnO2 nanocrystals 3–5 nm in size, which is synthesized by oxidizing metallic tin particles in a mixed aqueous solution of HNO3 and H2O2 at 80 °C. The aggregate calcinated at 300 °C possesses a high specific surface area of 135 m2 g−1, and 0.182 cm3 g−1 mesopores ca. 2.5 nm in size. At an operating temperature of 180 °C, the SnO2 aggregate exhibits a response of 110 towards 100 ppm ethanol in air. At an even lower working temperature of 140 °C, an ideal overall sensing performance is achieved: a response of 37, a response/recovery time of 26/21 s, together with high selectivity and stability. The layered mesoporous architecture with a high specific surface area contributes to the high response at low operating temperatures. Besides, with increasing operating temperatures from below to beyond 150 °C, it takes much a longer time to achieve a total recovery in resistance. The ethanol sensing mechanism is thus believed to change accordingly from the direct adsorption one to the oxygen ionosorption model.

Au-Decorated 1D SnO2 Nanowire/2D WS2 Nanosheet Composite for CO Gas Sensing at Room Temperature in Self-Heating Mode

We have designed a new ternary structure to enhance the sensing properties of WS2 nanosheet (NS)-based gas sensors at room temperature (RT) in self-heating mode. SnO2 nanowires (NWs, 10–30 wt%) were added to WS2 NSs and then Au nanoparticles (NPs) were deposited on the surface of the resulting composites by UV irradiation. The Au-decorated 10 wt% SnO2–WS2 composition showed the highest gas sensing properties. The presence of SnO2 NWs on the WS2 NSs effectively enhanced the diffusion and adsorption of gas species into deeper parts of the gas sensor. Furthermore, the chemical sensitization of Au (increase in oxygen ionosorption; spillover effect and catalytic effect towards CO) contributed to an enhanced response to CO gas. Gas sensing tests performed in the self-heating mode demonstrated the possibility of realizing a low-voltage, low-power-consumption CO gas sensor based on the Au-decorated 10 wt% SnO2–WS2. The sensor response under 60% relative humidity (RH) conditions was 84% of that under dry conditions, which shows that CO sensing is possible in wet environments at room temperature operation.

Atomistic Descriptions of Gas-Surface Interactions on Tin Dioxide

Historically, in gas sensing literature, the focus on “mechanisms” has been on oxygen species chemisorbed (ionosorbed) from the ambient atmosphere, but what these species actually represent and the location of the adsorption site on the surface of the solid are typically not well described. Recent advances in computational modelling and experimental surface science provide insights on the likely mechanism by which oxygen and other species interact with the surface of SnO2, providing insight into future directions for materials design and optimisation. This article reviews the proposed models of adsorption and reaction of oxygen on SnO2, including a summary of conventional evidence for oxygen ionosorption and recent operando spectroscopy studies of the atomistic interactions on the surface. The analysis is extended to include common target and interfering reducing gases, such as CO and H2, cross-interactions with H2O vapour, and NO2 as an example of an oxidising gas. We emphasise the importance of the surface oxygen vacancies as both the preferred adsorption site of many gases and in the self-doping mechanism of SnO2.

He+ LEIS analysis combined with pulsed jet technique of ethanol sensing by a ZnO surface

Low-energy He+ ion scattering spectroscopy (He+ LEIS) was combined with the pulsed jet technique to analyze the outermost surface of a Zn-ZnO (0001) substrate that is sensing trace ethanol (EtOH, 50 ppmv) contained in the air. The gas sensing response to EtOH by ZnO under static atmospheric pressure was successfully simulated by the periodically pulsed gas jet irradiation onto the surface while keeping the background pressure low enough to operate He+ LEIS. It was found that the gas sensing response to EtOH is due to the surface compositional change analyzed by He+ LEIS. Thus, the gas sensing response to EtOH is caused by adsorption of EtOH-related molecules on the surface, which governs the oxygen ionosorption. It was demonstrated that the combination of He+ LEIS and the pulsed jet technique is a useful approach to analyze the outermost surface of a gas sensor in the realistic working condition.

Hole-supply-rate-controlled methanol-gas-sensing reaction over p-type Co3O4/single-walled carbon nanotube hybrid structures

Co3O4/single-walled carbon nanotube (Co3O4/SWCNT) hybrid structures are fabricated, and their methanol sensing properties and related sensing principles are systematically investigated. A series of Co/SWCNT nanohybrid structures are deposited on silicon dioxide substrates by a co-arc discharge process and converted to Co3O4/SWCNT nanohybrid structures by subsequent methanol treatment and oxidation processes. The morphologies and structures of the nanohybrid composites are investigated by scanning electron microscopy, transmittance electron microscopy, X-ray diffraction, and Raman spectroscopy. The effect of the nanohybrid film thickness and the Co3O4: SWCNT ratio in the films on their physical and gas sensing properties are systematically examined. The best methanol-sensing performance is observed upon measurement at 300 °C for the material fabricated by deposition for 15 min and oxidized at 500 °C. The enhanced gas-sensing performance of the hybrid nanostructures is analyzed to prove that the high carrier supply rate from the transducer determines the reaction rate on the receptor surface (providing the receptor can accommodate the oxygen ionosorption). The hybrid sensor structures also exhibit low detection limit of 50 ppb, good selectivity, repeatability, and long-term stability, demonstrating their potential for practical application to methanol sensing.

Investigations on the Temperature-Dependent Interaction of Water Vapor with Tin Dioxide and Its Implications on Gas Sensing.

This work presents an operando infrared spectroscopic study of the temperature-dependent water adsorption on pristine SnO2 surfaces and discusses the possible implications on the oxygen ionosorption and gas-sensing mechanism. The impact of water on the sensor resistance, CO-sensing performance, and CO conversion was studied, and the obtained phenomenological results provide the basis for discussing the operando spectroscopic investigation findings. The provided information allows identification of three different water adsorption regimes ranging from physisorption and dominant associative adsorption to mainly dissociative water adsorption. In these regions, water has different impacts on the surface composition, sensor resistance, and gas-sensing performance.

Ethanol Gas Sensing by a Zn-Terminated ZnO(0001) Bulk Single-Crystalline Substrate.

Metal oxide semiconductor gas sensors have been widely studied for the selective detection of various gases with trace concentrations. The identification of the reaction scheme governing the gas sensing response is crucial for further development; however, the mechanism of ethanol (EtOH) gas sensing by ZnO is still controversial despite being one of the most intensively studied target gas and sensing material combinations. In this work, for the first time, the detailed mechanism of EtOH sensing by ZnO is studied by using a bulk single-crystalline substrate, which has a well-defined stoichiometry and atomic arrangement, as the sensing material. The sensing response is substantial on the ZnO substrate even with a millimeter-size thickness, and it becomes larger with resistance of the substrate. The large sensing response is described in terms of the adsorption/desorption of the oxygen species on the substrate surface, namely, oxygen ionosorption. The valence state of the ionosorbed oxygen involved in EtOH sensing is identified to be O2– regardless of the temperature. The increase in the sensing response with the temperature is attributed to the enhanced oxidation rate of the EtOH molecule on the surface as analyzed by pulsed-jet temperature-programmed desorption mass spectrometry, which has been newly developed for analyzing surface reactions in simulated working conditions.

Benchmarking β‐Ga2O3 Schottky Diodes by Nanoscale Ballistic Electron Emission Microscopy

AbstractMonoclinic beta‐phase gallium oxide (β‐Ga2O3) is an ultrawide‐bandgap semiconductor, intensively studied as a viable candidate for next‐generation power electronics, optoelectronics, and extreme environment electronics. Schottky contacts to β‐Ga2O3 are of paramount importance to this end; however, they are not yet fundamentally understood. Intrinsic sources of interfacial disorder, including oxygen‐related defects and extrinsic fabrication factors, are thought to greatly determine the properties of such contacts, for example by originating Fermi level pinning and causing patches with different Schottky barrier heights (SBHs). Ballistic electron emission microscopy (BEEM) is used to probe band bending and interfacial inhomogeneity at the nanoscale for prototypical Au/ and Pt/(100)β‐Ga2O3 single crystal Schottky barrier diodes. It is shown that SBH fluctuations amount to 40–60 meV under vacuum, occurring over length scales of tens of nanometers. Furthermore, a remarkable SBH modulation of ≈0.2 eV takes place upon exposure of devices from vacuum to ambient air. Such findings—better obtained by BEEM than by macroscale approaches—point to the existence of an ubiquitous inhomogeneous interfacial layer, controlling band bending and ambient sensitivity via oxygen ionosorption and interface redox chemistry. This study ascribes a key role to interfacial oxygen vacancies, and has practical implications for transport modelling and interface engineering.

Gas Sensing Characteristics of ZnO Thin Films Exposed to Ethanol

There are hundreds of nature and man-made volatile organic compounds (VOC) in atmosphere. Many of them have adverse effects on environments and human health. Ethanol (EtOH) is a frequently encountered VOC in our living environments, and an important interfering gas in breath analyses. Effective monitoring of EtOH is, therefore, beneficial to air pollution control and disease diagnosis. ZnO is an n-type semiconductor, and has been widely studied as a chemoresistive sensor for EtOH detection. In this work, we studied the sensing property of ZnO toward EtOH, and characteristic response features were examined for selectivity improvement. The ZnO sensor was prepared by sputter-depositing a 50 nm-thick ZnO thin film on the Al electrodes, which were thermal evaporated on a thermal oxide substrate and patterned by a stainless steel (SS) hard mask. During the gas sensing test, the ZnO sensor was exposed to a dry air gas mixture of EtOH of various concentrations at different temperatures. The ZnO sensor responses weakly to the EtOH gas mixture below 150oC. However, the sensor has a quick conductance rise upon the exposure to EtOH at 150oC and above, manifesting the nature of n-type oxides according to the oxygen ionosorption model. Figure 1a shows the electrical response of the sensor exposed to 0.15 ppm EtOH at different temperatures. Note that a conductance drops right after the early conductance rise and the characteristic response feature is more apparent at higher temperatures and lower EtOH concentrations. In our previous work, we found that PdO, when exposed to EtOH between 100-250oC, exhibits a similar response feature but in a reverse manner, i.e. a quick conductance drop followed by a rise (see Fig.1b). Since PdO is a p-type oxide, the opposite response behavior suggests that the n-type ZnO sensor has a sensing mechanism likely analogous to the PdO sensor when exposed to EtOH. On the basis of the previous study on the PdO sensor, we ascribe the formation of the characteristic response feature to the combined effects of the reduction of preadsorbed oxygen anions and the ZnO substrate by EtOH and the subsequent adsorption of hydrogen and oxygen adatoms on reduced ZnO nanodomains. Chemical and material characterizations, such as XPS, TDS and DRIFTS have been carried out to examine surface and gaseous species produced in the EtOH sensing reaction In summary, we studied the gas sensing behavior of the sputter-deposited ZnO thin film toward ethanol. A characteristic electrical response feature is observed and its shape greatly depends on the sensing temperature and the EtOH concentration. The response feature is a result of various surface processes, including reduction of oxygen anions and the ZnO substrate, which induces the conductance rise, and adsorption of hydrogen and oxygen adatoms on reduced ZnO nanodomains, which reduces the conductance. The distinct response feature is useful for characteristic feature extraction in pattern recognition algorithms to improve the selectivity of the sensor. Figure 1

High Performance of SnO2-Based Gas Sensor by Introducing Perovskite-Type Oxides

SnO2-bassed gas sensors are widely used for various gases detection such as inflammable gases, toxic gases, bad smell and so on. For materials design of such semiconductor gas sensors, we have reported important three factors i.e. receptor function, transducer function and utility factor. Receptor function in connection with this presentation, concerns the ability of the oxide surface to interact with the target gas. If the sensor is made of a neat oxide, the surface oxygen, especially adsorbed oxygen, on the oxide acts as a receptor. It is well known that the sensors of this type are promoted in sensitivity as the constituent oxides are made smaller in size. This issue was solved completely by developing a theory on the receptor function of small sized oxides [1-3]. The small semiconductors are depleted of electrons in two steps with a progress of oxygen ionosorption (O- and O2-), resulting in the appearance of regional depletion and volume depletion in succession. In the stage of volume depletion, grain size effect is observed. In addition, when the surface is loaded with a foreign receptor like PdO, it acts as a receptor stronger than the adsorbed oxygen. Especially Pd-loaded SnO2 nanoparticles with Pd size of 2.6 nm exhibited significantly high sensitivity to reducing gases [4, 5]. In the above receptor function, there is another method to enhance the sensitivity more. The electric resistance of sensor device is inversely proportional to the surface density of electrons. Therefore the sensor response enhances with increasing oxygen partial pressure. When the special feature of oxygen ionosorption by pulse heating is introduced, the gas sensing properties may be improved by increasing the adsorbed oxygen species on the SnO2 surface. In this presentation, we report the possibility of perovskite-type oxide as a material of oxygen feed. SnO2 nano-particles were synthesized by hydrothermal method using SnCl4·5H2O and NH4HCO3 solution [4]. The obtained SnO2 sol was dried at 120 °C and calcined at 600 °C for 3 h in oxygen atmosphere. The particle size of SnO2 was about 14 nm by TEM image. 0.7 mol.% Pd was loaded on the SnO2 surface by impregnating Pd(NH3)2(NO2)2 solution into aqueous solution of SnO2, followed by ammonia solution treatment and filtration process, then dried at 120 °C and heat-treated at 500 °C in air. The particles size of Pd was about 2.6 nm by CO pulse adsorption method. Perovskite-type oxide, La0.1Sr0.9Co0.4Fe0.6O3- δ (LSCF), was synthesized by AMP method. The powder was grinded for 30 min, followed by further heating at 1050 °C for 5 h. Finally, the LSCF/Pd-SnO2 powders were prepared by directly mixing 15 wt.% LSCF with 0.7%Pd-SnO2. The MEMS device was used for the gas sensor device. The Pt electrodes and heaters were embedded on a silicon-based support membrane (area: 100 μm ×100 μm). To make MEMS type gas sensor, a uniform paste of was firstly prepared by dispersing LSCF/Pd-SnO2 powders into glycerin, and then it was deposited on the membrane by a micro injector. An MEMS sensor coated with Pd-SnO2 nanoparticles was also prepared and tested for comparison. The MEMS sensors were tested in a conventional gas flow apparatus at 250-400 °C by applying appropriate voltage to heaters in pulse mode. The gas sensing performance of MEMS sensors based on Pd-SnO2 and LSCF/Pd-SnO2 was investigated by using 200 ppm target gas such as H2, CO and C7H8. The sensor response (S = Ra/Rg) was defined as the ratio of electric resistance in synthetic air (Ra) and target gas (Rg). The LSCF/Pd-SnO2 MEMS sensor showed fast response, high sensitivity (2 times or more) and stability to reducing gases (H2 and CO) and VOCs (C7H8) at at 250 °C, as compared with Pd-SnO2 MEMS sensor, by pulse heating with extremely low power consumption. It is considered that the excellent gas sensing properties of LSCF/Pd-SnO2 MEMS sensor are attributed to the effective oxgen transport from LSCF to Pd-SnO2 surface by pulse heating. Such pulse-driven MEMS sensor with high performance holds a great promise for practically application. N. Yamazoe, K. Shimanoe, J. Electrochem. Soc., 155(4), J85-J92 (2008).N. Yamazoe, K. Shimanoe, J. Electrochem. Soc., 155(4), J93-J98 (2008).K. Suematsu, M. Yuasa, T. Kida, N. Yamazoe, K. Shimanoe, J. Electrochem. Soc., 159 (4), J136-J141 (2012).N. Ma, K. Suematsu, M. Yuasa, T. Kida, K. Shimanoe, ACS Appl. Mater. interfaces, 7, 5863-5869 (2015).N. Ma, K. Suematsu, M. Yuasa, K. Shimanoe, ACS Appl. Mater. interfaces, 7, 15618-15625 (2015).

Engineering NiO sensitive materials and its ultra-selective detection of benzaldehyde

Ongoing interest in oxide semiconductor as components of gas sensing devices is motivated by environmental monitoring and intelligent control. NiO with different precursor solution were synthesized by aqueous chemical deposition and pyrolysis process. Here the method is quite facile, green and free of surfactant. Their morphology, crystal structure and chemical composition have been systemically characterized by various techniques. Interestingly, the microstructures of NiO can be engineered by different nickel salt (nitrate or chloride). These NiO based gas sensors showed substantially enhanced responses to benzaldehyde target analyte and exhibited fast response-recover feature. The observed gas sensing behavior is explained in terms of oxygen ionosorption mechanism.

Effects of PdO decoration on the sensing behavior of SnO2 toward carbon monoxide

We deposited PdO nanoparticles on SnO2 thin films and studied the CO gas sensing behavior of the PdO-decorated sensors at temperatures below 250°C. The PdO nanoparticles and the SnO2 thin film were both prepared by reactive sputter deposition. The PdO decoration increases the resistivity of the SnO2 thin film as a result of the formation of the P–N junction. The sensing behavior of the sensor at 100°C and below can be understood simply by the oxygen ionosorption model. At temperatures at 150°C and above, the sensor demonstrates a fast response followed by a progressive conductance decay during the CO exposure. The observation is likely related with adsorption kinetics of CO and O2 on Pd nanoislands formed as a result of PdO reduction. PdO reduction and reoxidation occur in the temperature range between 100 and 150°C under the CO sensing condition, and thereby greatly influencing the sensing behavior of the sensor. The sensor signal of the PdO-decorated sensor is about three times that of the bare SnO2 sensor at 150°C and above. The PdO decoration also significantly reduces the recovery time of the SnO2 sensor. The faster recovery is ascribed to the presence of the residual metal Pd phase, which promotes the spill-over effect.

A mechanistic study of hydrogen gas sensing by PdO nanoflake thin films at temperatures below 250 °C.

We prepared PdO nanoflake thin films on the SiO2 substrate by reactive sputter deposition, and studied their sensing response to H2 at temperatures between 25 and 250 °C. In addition to the oxygen ionosorption model, which is used to describe the early H2 sensing response over the temperature range studied, the H2 sensing kinetics of the PdO thin films can be separated into three temperature regimes: temperatures below 100 °C, around 150 °C and above 200 °C. At temperatures below 100 °C, PdO reduction is the dominant reaction affecting the H2 sensing behavior. At temperatures around 150 °C, Pd reoxidation kinetically competes with PdO reduction leading to a complicated sensing characteristic. Active PdO reduction by H2 promotes the continuing growth of Pd nanoislands, facilitating dissociative oxygen adsorption and thus the subsequent Pd reoxidation in the H2-dry air gas mixture. The kinetic competition between the PdO reduction and reoxidation at 150 °C leads to the observation of an inverse of the increase in the sensor conductivity. At temperatures above 200 °C, the PdO sensor exhibits a sensor signal monotonically increasing with the H2 concentration, and the H2 sensing behavior is consistent with the Mars-van-Krevelen redox mechanism.

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO3 Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO3 quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO3, surface covered by unstable W(V) species, slowly oxidized upon standing in room conditions. The WO3 nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics.

PdO Nanoflake Thin Films for CO Gas Sensing at Low Temperatures

We prepared PdO nanoflake thin films by reactive sputter deposition and studied their gas-sensing response to CO at temperatures below 250 °C. The PdO thin film, which has a large open surface area allowing for rapid and extensive CO adsorption, exhibits a complicated CO sensing behavior. At temperatures below 150 °C, the electrical conductance of the thin film drops upon the exposure of the CO/dry air gas mixture. The CO sensing mechanism in this temperature range is described by the oxygen ionosorption model. At 150 °C, oscillatory response occurs when the CO concentration is larger than 2000 ppm. The oscillatory response is due to the reactivity switching between the PdO reduction and reoxidation reactions. In addition, the PdO thin film shows a rapid increase in the conductance before and after the oscillatory response regime. The conductance increase is ascribed to the positive charge accumulation at the interface between the PdO thin film and metal Pd nanoislands, which are formed during the PdO reduction. At temperatures higher than 150 °C, the conductance drops when the PdO thin film is exposed to CO. Besides the ionosorption model, the oxygen vacancy model is used to account for the CO sensing response of the thin film in the high-temperature range. On the basis of the inverse change in the CO sensing sensitivity at 150 °C, a CO sensor of high selectivity, which integrates two PdO sensing elements operating independently at 150 °C and a lower temperature, is proposed.

Modulation of resistance switching in Au/Nb:SrTiO3 Schottky junctions by ambient oxygen

We investigated the room-temperature current-voltage characteristics of Au/Nb:SrTiO3 Schottky junctions under various atmospheres and working pressures. We observed that oxygen partial pressure reversibly modulates junction response, briefly individual specimens behave as high-quality rectifiers in oxygen-rich atmospheres and as bipolar resistive switches in vacuum and inert gases. A two orders of magnitude modulation of resistance switching characterizes samples with the highest content of interfacial oxygen vacancies. We attribute this behavior to oxygen ionosorption and chemical oxidation at the metal-oxide interface. Our results are relevant to oxide devices displaying resistive switching at ambient-exposed interfaces, and might be exploited for gas detection purposes.

Properties of NiO sputtered thin films and modeling of their sensing mechanism under formaldehyde atmospheres

In this work the sensing mechanism of p-type semiconducting NiO thin films under the exposure to formaldehyde is explained. The influence of the sensing layer thickness and annealing treatment on the structural, optical and electrical properties of the samples is studied. The height of the potential barrier is estimated from temperature-stimulated conductance measurements. The potential barrier height is linked to oxygen ionosorption on the semiconductor surface. Furthermore, Fourier transform-IR analysis was carried out in order to determine the chemical reactions that govern the process of gas detection and the temperature range at which they occur. As a result of the study, it is possible to explain how the thickness and annealing treatment affect the sensing mechanism of the samples.

Characterization of the Oxygen Ionosorption Effect on a Single SnO<SUB>2</SUB> Nanowire by Using Conductive Atomic Force Microscopy

We have verified that SnO2 nano-wire has an n-type semiconductor property and it can be a p-type one when it is exposed to O2. We employed conductive AFM system to measure the I-V curve and resistance of single SnO2 nano-wire which had been synthesized on the Au thin film by a thermal process. To analyze a effect of O2 ionosorption into nano-wire, resistance was measured with various O2 concentration and we observed increment and maintenance of resistance which caused by O2 ionosorption. Also, the O2 ionosorption causes a type transfer of semiconductor and this phenomenon was verified by comparing the Schottky property of nano-wire before and after O2 exposure.