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Riverbeds can have an important impact on large-scale fluxes of biogeochemically active solutes in river corridor systems. The hyporheic zone is an important area of both hydrology and biogeochemistry, particularly in a hydropeaked river system where rapid variations in river stage height, hydraulic gradients, and residence times occur. We measured several biogeochemical and hydrological parameters at three different subsurface depths in the riverbed of the Columbia River in Washington state, a complex hydropeaked river system with significant subsurface heterogeneity. During the study, episodes of significant dissolved oxygen (DO) change were observed. The DO signal changes were the most apparent, compared to more modest changes in other biogeochemical markers. While DO is often associated with biological activity, we ultimately found that the notable DO excursions were associated with hydrologic gradients. Here we describe hydrologic perturbations and biogeochemical responses in terms of hydrobiogeochemical regimes. Two different forms of DO response to hydraulic gradient perturbation were observed, defining different hydrobiogeochemical regimes. The system tips abruptly from one regime to the other, exhibiting threshold behavior that is uncaptured in current estimations of cumulative influences of subsurface processes from reach to earth system scales.Plain language summaryThe water in a river is constantly flowing in and out of the river’s bed. While it is in the bed, microbes process environmentally and ecologically important nutrients and other compounds. The exchange of water between the river and the riverbed is largely controlled by the stage of the river. Many processes, both natural and human, can cause rapid changes in the river stage. These rapid stage changes can cause abrupt changes in the riverbed’s ability to process nutrients and other compounds. In this study, we use dissolved oxygen as a window into riverbed microbial and chemical processing. We see that relatively rapid changes in river stage can cause abrupt changes in the concentrations of chemicals in the riverbed, indicating fundamental changes in how the riverbed processes are occurring. Understanding these abrupt and fundamental changes is important to our overall understanding of the ecological function of river corridor systems.3 key pointsA hydrobiogeochemical regime is defined by the functional form of the relationship of a hydrologic perturbation and biogeochemical marker.Short-term hydrologic perturbations drive tipping points between distinct hydrobiogeochemical regimes.Linear and hysteretic dissolved oxygen dynamics define distinct regimes as observed throughout the hyporheic zone.

Many river corridor systems frequently experience rapid variations in river stage height, hydraulic head gradients, and residence times. The integrated hydrology and biogeochemistry of such systems is challenging to study, particularly in their associated hyporheic zones. Here we present an automated system to facilitate 4-dimensional study of dynamic hyporheic zones. It is based on combining real-timein-situandex-situmeasurements from sensor/sampling locations distributed in 3-dimensions. A novel dissolved oxygen (DO) sensor was integrated into the system during a small scale study. We measured several biogeochemical and hydrologic parameters at three subsurface depths in the riverbed of the Columbia River in Washington State, USA, a dynamic hydropeaked river corridor system. During the study, episodes of significant DO variations (~+/- 4 mg/l) were observed, with minor variation in other parameters (e.g., 3). DO concentrations were related to hydraulic head gradients, showing both hysteretic and non-hysteretic relationships with abrupt (hours) transitions between the two types of relationships. The observed relationships provide a number of hypotheses related to the integrated hydrology and biogeochemistry of dynamic hyporheic zones. We suggest that preliminary high-frequency monitoring is advantageous in guiding the design of long term monitoring campaigns. The study also demonstrated the importance of measuring multiple parameters in parallel, where the DO sensor provided the key signal for identifying/detecting transient phenomena.

The charge of the electron can be determined by simply placing a known number of electrons on one electrode of a capacitor and measuring the voltage, Vs, across the capacitor. If Vs is measured in terms of the Josephson volt and the capacitor is measured in SI units then the fine-structure constant is the quantity determined. Recent developments involving single electron tunneling, SET, have shown bow to count the electrons as well as how to make an electrometer with sufficient sensitivity to measure the charge.

We studied transient chemical speciation during high-temperature solid material pyrolysis and combustion in air. Our objective was to develop a database of chemical burn signatures. The material tested was the thermoplastic PMMA. Material samples were heated in an infrared furnace until they pyrolyzed, ignited, and combusted in air. Time-resolved quantitative measurements of the exhaust species CO2, O2, hydrocarbons, and CO along with exhaust gas temperature were obtained. Two categories of experiments were conducted: (1) pyrolysis tests in which there was no combustion; (2) combustion tests with chemical reaction and heat release. During heating, the sample underwent numerous processes that appear as diagnostic sequences. In the pyrolysis tests, as the furnace temperature was raised, the CO and hydrocarbon (HC) signals underwent transition from one peak to two peaks. In the combustion tests, spontaneous ignition occurred at higher test temperatures as evidenced by an exothermic reaction reported by the thermocouples, leading to three-peak CO and HC profiles. The measured O2/CO2 ratio of 1.3 ± 0.1 agreed with stoichiometric methyl-methacrylate monomer decomposition. Calculations of the power output using two independent methods supported (1) that combustion was experimentally observed in the furnace, and (2) the accuracy of the combustion gas analysis. Copyright © 2012 John Wiley & Sons, Ltd.

The operation of metal-oxide-semiconductor (MOS) devices based on the semiconductor SiC in high temperature environments above 300 °C requires an understanding of the physical processes in these capacitor structures under operating conditions. In this study we have focused on the regime of inversion biasing, where the electrical characteristics of the device are dominated by minority carriers. We report on the direct observation of the high frequency inversion capacitance due to thermal generation of holes in 6H-SiC n-MOS capacitors between 450 and 600 °C by monitoring the 1MHz C-V characteristics of large area, 1000 μm diameter, capacitors in the dark. Our experimental results are consistent with a first order calculation based on the delta depletion approximation.

We have developed a high-temperature gas sensing system for the detection of combustion products under harsh conditions, such as an energy plant. The sensor, based on the wide band gap semiconductor silicon carbide (SiC), is a catalytic gate field-effect device (Pt–SiO 2 –SiC) that can detect hydrogen-containing species in chemically reactive, high-temperature environments. The hydrogen response of the device in an industrially robust module was determined under both laboratory and industrial conditions (1000 sccm of 350 °C gas) from 52 ppm to 50% H 2 , with the sensor held at 620 °C. From our data we find that the hydrogen adsorption kinetics at the catalyst–oxide interface are well fitted by the linearized Langmuir adsorption isotherm. For hydrogen monitoring in a coal gasification application, we investigated the effect of common interferants on the device response to a 20% H 2 gas stream. Within our signal to noise ratio, 40% CO and 5% CH 4 had no measurable effect and a 2000 ppm pulse of H 2 S did not poison the Pt sensing film. We have demonstrated the long-term reliability of our SiC sensor and the robustness of the sensor packaging techniques, as all the data are from a single device, obtained during 5 days of industrial measurements in addition to ∼480 continuous hours of operation under laboratory conditions.

SiC based field-effect devices are attractive for electronic and sensing applications above 250 °C. At these temperatures the reliability of the insulating dielectric in metal-oxidesemiconductor (MOS) structures becomes an important parameter in terms of long-term device performance. We report on the reliability of n-MOS SiC capacitors following thermal stress cycling in the 330 to 630 °C range. As the primary mode of oxide breakdown under these conditions is believed to be due to electron injection from the substrate, the gate leakage current was measured as a function of temperature. The gate dielectric was grown using dry oxidation with a post oxidation NO passivation anneal. For large area, 1 mm diameter, 6H-SiC capacitors we obtain current densities as low as 5nA/cm2 at 630 °C. In addition, gate leakage measurements from arrays of 300 to 1000 2m diameter devices fabricated on different 1cm2 6H-SiC substrates are presented. These are encouraging results for the long-term reliability of SiC field-effect sensors.

Excitation of hexanuclear molybdenum complexes such as Mo6Cl12 and its derivatives in the ultraviolet results in a strongly red-shifted luminescence centered at 750nm. Since oxygen efficiently quenches the luminescence, these thermally stable inorganic complexes are candidate lumophores for real-time, high temperature optical fiber based sensing of oxygen. Sol-gel films containing the acetonitrile complex of Mo6Cl12 were deposited on quartz substrates by dip coating. After drying, the films were heated at 200∘C for 1 h. The luminescence lineshapes of films before and after heating were unchanged, indicating that heating did not adversely affect the cluster photophysics. Compared to solutions of the acetonitrile complex, quenching by oxygen was smaller in the as-prepared films, but heating at 200∘C for 1 h increased the quenching, apparently due to increased oxygen permeability resulting from the loss of water or other small molecules from the matrix. These results confirm the potential of hexanuclear molybdenum complexes such as Mo6Cl12⋅2CH3CN as the lumophores in fiber optic oxygen sensors that can operate up to 200∘C.

Silicon carbide (SiC)-based metal-insulator-semiconductor devices are attractive for gas sensing in automotive exhausts and flue gases. The response of the devices to reducing gases has been assumed to be due to a reduced metal work function at the metal-oxide interface that shifts the flat band capacitance to lower voltages. We have discovered that high temperature (700 K) exposure to hydrogen results not only in the flat-band voltage occurring at a more negative bias than in oxygen, but also in the transition from accumulation (high capacitance) to inversion (low capacitance) occurring over a relatively narrow voltage range. In oxygen, this transition is broadened, indicating the creation of a high density of interface states. We present a model of the hydrogen/oxygen response based on two independent phenomena: a chemically induced shift in the metal-semiconductor work function difference and the passivation/creation of charged states at the SiO/sub 2/-SiC interface that is much slower than the work function shift. We discuss the effect of these results on sensor design and the choice of operating point.

SiC based capacitive devices have the potential to operate in high temperature, chemically corrosive environments provided that the electrical integrity of the gate oxide and metallization can be maintained in these environments. We report on the performance of large area, up to 8 x 10-3 cm2, field-effect capacitive sensors fabricated on both the 4H and 6H polytypes at 600°C. Large area capacitors improve the signal/noise (S/N) ratio which is proportional to the slope of the capacitance-voltage characteristic. At 600 °C we obtain a S/N ~ 20. The device response is independent of polytype, either 4H or 6H-SiC. These results demonstrate the reliability of our field-effect structure, operating as a simple potentiometer at high temperature.

Electric field sensitive scanning probe microscopes (SPMs) can image the electronic properties of a semiconductor through an insulating layer—making the techniques well suited to extract underlying substrate dopant densities. A standard atomic force microscope (AFM) can be used to sense electric field by measuring the electrostatic response of a vibrating cantilever. We have constructed an inexpensive accessory to a commercial AFM to simultaneously acquire images of the spatial variations in substrate doping along with accurate topographic images. Our electric field images have a spatial resolution of 1 μm. In addition, we show explicitly how a simple parallel plate model can be used to extract the magnitude of lateral changes in dopant density from the electric field images. For a substrate doping density of 1×1016 cm−3 we can discern resistivity changes of ∼1 Ω cm.

The most important variable for intensive fish farming is the ability to monitor the oxygen concentration in the water in an autonomous and continuous fashion. We report on a system capable of remote data collection that was deployed in fish pond. The system is capable of tracking the dynamic changes in oxygen concentration encountered in during facility operation. A user friendly computer interface provides output from the sensor located in the fish pond and enables setting alarm levels, which are communicated to the farmer via a commercial cell phone network.

A metal-halide optical indicator for tissue oxygen imaging that circumvents the limitations of organometallic dyes is described. Photobleaching is not observed, advantageous for high speed confocal microscopy. Two photon oxygen imaging is under investigation.

A dissolved oxygen sensor for biological media using the 3O2 quenching of the phosphorescence from MoCl clusters is presented. Real-time measurements for four hours over a physiologically relevant pO2 range show no evidence of photobleaching

The scattering of conduction electrons by neutral impurities in Si has been demonstrated through measurements of spin-dependent transport (SDT) in a two-dimensional electron gas (2DEG). SDT was observed by monitoring the conductivity of a specially fabricated Si accumulation layer transistor operating at 4 K while modulating the electron-spin populations. By utilizing the differences in the singlet and triplet state scattering cross sections, the technique provides the first direct measure of neutral impurity scattering

Scattering by neutral impurities in Si has been demonstrated through measurements of spin dependent transport (SDT) in a two-dimensional electron gas (2DEG). SDT was observed by monitoring the conductivity of a specially fabricated Si accumulation layer transistor operating at 4K while modulating the electron spin populations. By utilizing the differences in the singlet and triplet scattering cross sections, the technique provides the first direct measure of neutral impurity scattering. A signal from ∼108 spins was observed, which demonstrates the enhanced sensitivity of SDT over conventional electron spin resonance methods.

The ability to identify, via remote sensing, the identity of materials prior to self-ignition is important for fire safety. We have developed a system to monitor, in real time, the thermal and chemical properties of specific materials under well controlled heating conditions. Our goal is to develop a library of the pre-combustion “signature” of materials commonly present in buildings. These signatures focus on the gases produced by combustion: CO, CO 2 and total hydrocarbons; as well as oxygen, an essential reactant. An infrared furnace is used to rapidly (few seconds) heat (up to 800 °C) a small sample of combustible material, while output gases are monitored, with 1 s resolution, using a multi-component gas analyzer. Systematic measurements of PMMA, a thermoplastic, are presented. Ultimately these results will be used to provide first responders with remote knowledge of the type of fire prior to entering a site.

Oxygen plays a ubiquitous role in terrestrial biological processes such as respiration and chemical process, i.e. oxidation-reduction reactions. We have developed an optical technique for monitoring oxygen in both gas and liquid phases utilizing nanoscale metal-halide clusters immobilized in a porous matrix. The sensor monitors the luminescence intensity or lifetime from inorganic hexanuclear molybdenum chloride clusters (Mo 6 Cl 12 ) which is efficiently quenched by ground state 3O 2 . Control of the material properties of the matrix at the nanometer scale is requires for fast quantitative output from our reflection mode fiber based sensor. For aqueous applications we can monitor dissolved oxygen in the 0.2–17 mg/L range at temperatures of 10–25 °C. The sensor output was validated using a commercial electro-chemical sensor and the Stern-Volmer fit is linear over the entire measurement range. In the gas phase our device can monitor oxygen with a response time, t 95 , of 30s in the 0.1 – 21 % range.

The goal of this research is to obtain quantitative information on chemical speciation over time during high temperature material thermal decomposition. The long term goal of the research will be to impact structural fire safety by developing a data base of characteristic “burn signatures” for combustible structural materials. In order to establish procedure and to generate data for benchmark materials, the first material tested in these preliminary tests is poly-methyl-methacrylate (PMMA). Material samples are heated in an infrared (IR) heating chamber until they undergo pyrolysis. Time resolved quantitative measurements of the exhaust species CO2 , O2 , HC, and CO were obtained. During heating the PMMA sample undergoes two distinct processes. First, pre-combustion pyrolysis is characterized by the appearance a peak in the THC signal between 600–650 °C. Secondly, at about 900 °C flaming combustion occurs as evidenced by an exothermic reaction reported by the thermocouples. The time sequence of the production of HC, O2 depletion and CO2 production are consistent with combustion in an excess-oxidizer environment.Copyright © 2009 by ASME

We report on a reflection-mode fiber-optic oxygen sensor based on the 3O2 quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0%–21% gaseous oxygen environment have been obtained, a range suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0%–1.0% range. The response time of the fiber probe is theoretically predicted to be 1 s.

An accumulation layer n-channel MOSFET that operates at 4 K has been fabricated. Sharp hysteretic current kinks in the I–V characteristic are observed, where the transistor switches discontinuously between two well defined states. The kinks are modelled in terms of changes in the device potential configuration due to holes generated by avalanche breakdown in the high electric field region near pinch off. Noise measurements which elucidate the physical mechanisms causing the current kinks are presented.

Dissolved oxygen concentration is considered the most important water quality variable in fish culture. Reliable and continuous (24/7) oxygen monitoring of dissolved oxygen (DO) in the 1-11 mg/L range would be of great benefit to the aquaculture industry. We briefly review selected DO sensors from both the Clark and optical sensor categories as well a comparing their differences, both advantages and disadvantages. We introduce our fiber optic technique for continuous monitoring of DO levels in a flowing aqueous stream. An inorganic molybdenum chloride compound was used as the optical indicator in the oxygen sensitive material. The reflection mode fiber sensor is based on the ability of oxygen to quench the red luminescence of the molybdenum chloride indicator. The advantage of our broad-band, optical dissolved oxygen sensor is shown and discussed.

Clean, efficient energy production, such as the gasification of coal (syngas), requires physical and chemical sensors for exhaust gas monitoring as well as real-time control of the combustion process. Following a description of syngas production from coal, we outline the sensing needs for this high temperature, chemically corrosive gas stream. The performance of devices based on the wide-bandgap semiconductor SiC, for monitoring gas composition, flame dynamics and temperature, as well as combustor structural integrity in this extreme environment are reviewed. We have developed a Pt-SiO2-SiC micro-device with a dense Pt catalytic sensing film, for fast (

We present a new apparatus for measuring the response times of a gas sensor with millisecond resolution, while also capturing the slower components of the response such as the steady state value. Laser induced fluorescence (LIF) imaging was used to quantify the exchange rate of the sensor's ambient gas. The millisecond response of high temperature (up to 950 K) field effect SiC sensors for detection of hydrogen containing gases was characterized.

We report on the development of an optical sensor suitable for remote monitoring of oxygen in high temperature environments. The sensor is based on /sup 3/O/sub 2/ quenching of the red emission from hexanuclear molybdenum chloride clusters. A reflection mode fiber optic probe is being developed by immobilizing Mo/sub 6/Cl/sub 12/ in a porous sol-gel matrix at the far end of a high temperature silica fiber. The sensor signal as a function of temperature was determined from in-situ spectroscopic measurements, demonstrating operation up to at least 200/spl deg/C. One of the advantages of the inorganic Mo-cluster is the lack of sensitivity to gases typically present in a coal-fired boiler due to the triplet nature of the transition from the excited to ground state.

A sensor based on the wide bandgap semiconductor, silicon carbide (SiC), has been developed for the detection of combustion products in power plant environments. The sensor is a catalytic gate field effect device that can detect hydrogen containing species in chemically reactive, high temperature environments. For these capacitive sensors we have determined that the optimum sensor operating point in terms of sensor lifetime and response time is at midgap. Detailed measurements of the oxide leakage current as a function of temperature were performed to investigate the high temperature reliability of the devices. In addition, robust metallization and electrical contacting techniques have been developed for device operation at elevated temperatures. To characterize the time response of the sensor responses in the millisecond range, a conceptually new apparatus has been built. Using laser induced fluorescence imaging techniques we have shown that the gas underneath the sensor can be completely exchanged with a time constant under 1 millisecond. Ultrahigh vacuum studies of the surface chemistry of the platinum gate have shown that sensor deactivation by adsorbed sulfur is a possible problem. Investigations on the chemical removal of sulfur by catalytic oxidation or reduction are continuing.

A reflection mode optical sensing system for monitoring oxygen in high temperature gas flows, 25-220degC and 500 sccm, is presented. The sensor is based on 3O2 quenching of the red emission from K2Mo6Cl14, one member of a family of hexanuclear molybdenum chloride clusters. One advantage of using inorganic Mo-clusters as the lumophore is the lack of sensitivity to gases typically present in a coal-fired boiler due to the triplet nature of the transition from the excited to ground state. The sensor is prepared by dip-coating fiber tip in a sol-gel solution containing K2Mo6Cl14 particle clusters. Drying and aging yields a composite film with K2Mo6Cl14 immobilized in a porous silica matrix. The composite film deposited on a planar substrate was characterized via in-situ spectroscopic measurement up to 200degC. The emission intensity is constant following 56 cumulative hours of heating at 200degC, demonstrating long-term stability at high temperature

Silicon carbide based devices are well suited for high temperatures applications such as gas sensors and electronic circuits for control and emissions applications in automobiles and power plants. We have developed a high sensitivity Pt-SiO2-SiC solid-state hydrogen gas sensor. The response time of this metal-oxide-semiconductor field-effect device to hydrogen-containing species at 600degC is in the millisecond regime. In good agreement with standard models, the sensor response to hydrogen concentration is logarithmic over at least four decades of concentration. Based on a detailed understanding of the hydrogen transduction mechanisms, we have determined the optimum sensor biasing conditions for reliable high temperature operation. We report on the measurement accuracy and stability for a sensor that has run for 18 continues days with negligible degradation in performance at 600degC

We show that SiC based metal-insulator-semiconductor (MIS) sensors above 700 K respond to reducing/oxidizing gases by two independent mechanisms: a chemically induced shift in the metal-insulator barrier height and the passivation/creation of charged states at the SiO/sub 2/-SiC interface. The relative magnitude of the two phenomena is determined by the position of the Fermi energy at the insulator-semiconductor interface, i.e. the capacitor bias. The interface state contribution is much slower than the work function shift. For short response times and high sensor-to-sensor reliability, we find that the optimal operating bias of a gas sensor is near midgap. The combination of photoemission measurements and capacitance measurements can discriminate between the two sensor mechanisms.

We report on the development of a fiber based oxygen sensor for power plant applications. The sensor utilizes quenching of the bright red fluorescence from inorganic molybdenum chloride (Mo/sub 6/Cl/sub 12/) clusters by ground state /sup 3/O/sub 2/. We are developing a reflection mode fiber sensor by immobilizing the Mo/sub 6/Cl/sub 12/ clusters in an oxygen permeable sol-gel matrix at the far end of a Au clad high temperature silica fiber (continuous operation to 700 /spl deg/C). In contrast to organic indicators these metal halide compounds are well-suited for oxygen sensing schemes in harsh environments. In this paper we report on the high temperature photophysical parameters of the Mo/sub 6/Cl/sub 12/ clusters obtained via absorption and emission spectroscopy in a controlled gas environment.

Silicon carbide based metal-oxide-semiconductor (MOS) devices are attractive for gas sensing in harsh, high temperature environments. The response of catalytic gate SiC sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. We have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation (high capacitance) to inversion (low capacitance) occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena - a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states at the SiO/sub 2//SiC interface. MIS capacitance sensors typically operate in constant capacitance mode. These results. affect sensor sensitivity since the slope of the C-V curve changes dramatically with gas exposure.

Mo{sub 6}Cl{sub 12}, a cluster compound whose luminescence depends on the ambient concentration of oxygen, is the basis for a real-time oxygen sensor for combustion applications. Previously, the properties of Mo{sub 6}Cl{sub 12} have largely been studied at room temperature; these studies have now been extended to 200 C. Optical microscopy shows that Mo{sub 6}Cl{sub 12} undergoes a steady change in color as it is heated from room temperature to 200 C, changing from canary yellow to crimson and then back to canary yellow. Concurrent thermal gravimetric analyses show a small weight loss for Mo{sub 6}Cl{sub 12} that is consistent with loss of water or HCl from the clusters. These changes are reversible. Absorption and fluorescence emission spectroscopy of Mo{sub 6}Cl{sub 12} heated to 200 C for two hours shows no change in the photophysical parameters compared to the control sample that was not heat cycled.

Silicon carbide based metal/oxide/semiconductor (MOS) devices are well suited for operation in chemically reactive high temperature ambients. The response of catalytic gate SiC MOS sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface, which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. From in-situ C-V spectroscopy, performed in a controlled gaseous environment, we have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation to inversion occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena – a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states (DIT) at the SiO2/SiC interface. Our results are important for both chemical sensing and electronic applications. MOS capacitance gas sensors typically operate in constant capacitance mode. Since the slope of the C-V curve changes dramatically with gas exposure, we discuss how sensor-to-sensor reproducibility and device response time are influenced by the choice of operating point. For electronic applications understanding the environmentally induced changes in DIT is crucial to designing drift-free MOS devices. Our results are applicable to n-type SiC MOS devices in general, independent of the specifics of sample fabrication.

We describe the use of HNQ (2-hydroxy-1,4-naphthoquinone or Lawsone) as a potential sweat sensor material to detect the hydration levels of human beings. We have conducted optical measurements using both artificial and human sweat to validate our approach. We have determined that the dominant compound that affects HNQ absorbance in artificial sweat is sodium. The presence of lactate decreases the reactivity of HNQ while urea promotes more interactions of sodium and potassium ions with HNQ. The interactions between the hydroxyl group of HNQ and the artificial sweat components (salts, lactic acid, and urea) were investigated comprehensively. We have also proposed and developed a portable diode laser absorption sensor system that converts the absorbance at a particular wavelength range (at 455 ± 5 nm, where HNQ has an absorbance peak) into light intensity measurements via a photocell. The absorbance intensity values obtained from our portable sensor system agrees within 10.4% with measurements from a laboratory based ultraviolet-visible spectrometer. Findings of this research will provide significant information for researchers who are focusing on real-time, in-situ hydration level detection.

The implementation of SiC based sensors and electronics for operation in chemically harsh, high temperature environments depends on understanding the SiO2/SiC interface in field effect devices. We have developed a technique to fabricate wedge polished samples (angle ∼ 1×10−4 rad) that provides access to the SiO2/SiC interface via a surface sensitive probe such as xray photoelectron spectroscopy (XPS). Lateral scanning along the wedge is equivalent to depth profiling. Spatially resolved XPS images of the O 1s and Si 2p core levels were obtained of the interfacial region. Samples consist of device-quality thermally grown oxides on 4H-SiC single crystal substrates. The C 1s spectrum suggests the presence of a graphitic layer on the nominally bare SiC surface following thermal oxidation.

We report on a reflection mode fiber optic oxygen sensor based on the 3O2 quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0 - 20% gaseous oxygen environment have been obtained, which is suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0 to 1.0% range. The response time of the fiber probe is less than or equal to 30 s, limited at present by the gas mixing system. The sensor is fabricated by immobilizing the clusters in an oxygen permeable polymer at the end of an UV transmissive optical fiber. For a 1.5 m long fiber pumped at 325 nm, the auto-fluorescence is less than 4% of the reflected luminescence signal from 600 - 850 nm, enabling remote probing of oxygen in harsh environments. The present probe is thermally stable to 200 degrees Celsius, limited by the polymer matrix. Since the clusters are synthesized at 1000 degrees Celsius, immobilization of the clusters in an inert sol-gel matrix is an attractive alternative. This will enable oxygen sensing in gaseous environments at elevated temperatures as well as liquid environments with varying salinity and pH.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The photophysics of metal halide clusters, such as Mo6Cl12, are well suited for oxygen sensing schemes.

We report on an optical oxygen sensor for aqueous media. The phosphorescent signal from the indicator, K2Mo6Cl14, immobilized in a polymer matrix, is quenched by ground state O32. Continuous measurements (Δt=10 s) over 36 h in oxygen atmospheres (0%–21%) were obtained with a signal to noise ratio better than 150. Photobleaching was not observed over ∼13 000 measurements. The senor response at 10, 22, and 37 °C water is governed by bimolecular collisional quenching, as evidenced by a linear fit to the Stern–Volmer equation for dissolved oxygen in the range 0

We report several experiments under ultrahigh vacuum conditions that elucidate the role of oxygen in the functioning of silicon carbide field-effect gas sensors with nonporous platinum gates. The devices studied are shown to be sensitive both to hydrogen and to propene. All of the results are consistent with oxygen acting through its surface reactions with hydrogen. Three specific aspects are highlighted: the need, under some conditions, for oxygen to reset the device to a fully hydrogen-depleted state; competition between hydrogen oxidation and hydrogen diffusion to metal/oxide interface sites, leading to steplike behavior as a function of the oxygen:hydrogen ratio (λ-sensing); and the removal of sulfur contamination by oxygen.

We have investigated the effects of sulfur contamination on a Pt-gate silicon carbide based field-effect gas sensor, under ultrahigh vacuum conditions, at a temperature of 527°C. Exposure to hydrogen sulfide, even in the presence of hydrogen or oxygen at partial pressures of 20–600 times greater than the H2S level, rapidly coated the gate with a monolayer of sulfur. Sulfur contamination reduced the magnitude of the sensor’s response to alternating hydrogen and oxygen pulses by about 70%, as compared to the uncontaminated gate. There was no evidence of irreversible changes in device behavior due to sulfur deposition and removal. The adsorbed sulfur could not be removed by exposure to hydrogen at the pressures accessible. Oxygen was effective at removing the sulfur. The rate of sulfur oxidation was suppressed at high sulfur coverages, but not as strongly as on low-index single-crystal surfaces. These results are discussed in the context of prior experiments on Pt crystals and films.

A monolithic optoelectronic circuit, consisting of a GaAs light‐emitting diode (LED) driven by a Si metal‐oxide‐semiconductor (MOS) transistor, has been fabricated. Light output as a function of applied gate voltage was measured. The LED’s were fabricated in GaAs layers on Ge‐coated Si substrates containing MOS transistors. Normal transistor performance was observed after the GaAs LED fabrication, indicating that GaAs and Si processing technologies appear to be compatible.