Use of different sensing materials and deposition techniques for thin-film sensors to increase sensitivity and selectivity

The performances of metal oxide semiconducting materials used as gas-sensing detectors depend strongly on their structural and morphological properties. The average grain size has been proved to play a prominent role and better sensor performances were found in polycrystalline films where the grain size is few tens of nm or smaller. On the other hand, thermal treatments during thin-film deposition and/or sample postprocessing could lead to a grain coalescence, thus decreasing the conductivity of the sensing film. Avoiding such a phenomenon, still keeping optimized processing conditions, will increase the sensor performances, maintaining the resistivity at acceptable values. In this work, new gas-sensing materials and new thin-film deposition procedures have been investigated. Aiming to preserve the sensitivity, to enhance selectivity and to reduce the drift, thin films of WO/sub 3/ and CrTiO/sub 3/ deposited by pulsed-laser ablation (PLA) and of SnO/sub 2/ deposited by rheotaxial growth and thermal oxidation techniques were comparatively characterized. Three issues were mainly addressed: the variation of the conductivity as a function of RH, the sensitivity toward benzene, CO, acetone, and NO/sub 2/, and the selectivity.     

Gas-sensing properties of sprayed films of (CdO)/sub x/(ZnO)/sub 1-x/ mixed oxide

A novel NO/sub 2/ sensor based on (CdO)/sub x/(ZnO)/sub 1-x/ mixed-oxide thin films deposited by the spray pyrolysis technique is developed. The sensor response to 3-ppm NO/sub 2/ is studied in the range 50/spl deg/C-350/spl deg/C for three different film compositions. The device is also tested for other harmful gases, such as CO (300 ppm) and CH/sub 4/ (3000 ppm). The sensor response to these reducing gases is different at different temperatures varying from the response typical for the p-type semiconductor to that typical for the n-type semiconductor. Satisfactory response to NO/sub 2/ and dynamic behavior at 230/spl deg/C, as well as low resistivity, are observed for the mixed-oxide film with 30% Cd. The response to interfering gas is poor at working temperature (230/spl deg/C). On the basis of this study, a possible sensing mechanism is proposed.     

Gas-sensing properties of catalytically modified WO/sub 3/ with copper and vanadium for NH/sub 3/ detection

Ammonia gas detection by pure and catalytically modified WO/sub 3/-based gas sensors was analyzed. Sensor response of pure tungsten oxide to NH/sub 3/ was unsatisfactory, probably due to the unselective oxidation of ammonia into NO/sub x/. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH/sub 3/ detection. The introduction of Cu and V as catalytic additives improved the sensor response to NH/sub 3/. Possible reaction mechanisms of NH/sub 3/ over these materials are discussed. Sensor responses to other gases like NO/sub 2/ or CO and interference of humidity on ammonia detection were also analyzed so as to choose the best sensing element.     

Fabrication, characterisation and application of (8 mol% Y2O3) ZrO2 thin film on Na3Zr2Si2PO12 substrate for sensing CO2 gas

A planar type CO₂ gas sensor employing (8 mol% Y₂O₃) ZrO₂ (YSZ) thin film on Na₃Zr₂Si₂PO₁₂ (Nasicon) substrate with Na₂CO₃ as an auxiliary electrode has been fabricated and tested in laboratory environment between 700–900 K. The YSZ thin film was fabricated on Nasicon and alumina substrate using radio frequency (RF) magnetron sputtering. The film was examined using SEM and X-ray diffraction (XRD) after treating the Nasicon-YSZ bi-layer structure at 1300 K for 2 h. The results indicate that a crack free YSZ film was produced on Nasicon surface that was well bonded to the substrate. The conductivity of sputtered YSZ thin film measured by ac-impedance spectroscopy has been found to be higher than that of YSZ pellet by approximately half an order of magniture. The bi-electrolyte planar sensor displays rapid response (t₉₅ 200 s) to CO₂ compared to the tube type sensor (t₉₅ 700 s) and the measured open circuit voltage of the electrochemical CO₂ sensor has been found to be Nernstian at all temperatures.     

Simultaneous electrochemical detection of nitric oxide and carbon monoxide generated from mouse kidney organ tissues

A planar-type amperometric dual microsensor for simultaneous detection of nitric oxide and carbon monoxide is presented. The sensor consists of a dual platinum microdisk-based working electrode (WE) and a Ag/AgCl counter/reference electrode covered with an expanded poly(tetrafluoroethylene) (Tetra-tex) gas-permeable membrane. The dual WE possesses two different platinized platinum disks (WE1 and WE2, 250 and 25 microm in diameter, respectively). The larger WE1 is further modified with electrochemical deposition of tin. Use of two sensing disks different in their size as well as in their surface modification produces apparently different sensitivity ratios of NO to CO at WE1 and at WE2 (approximately 2 and approximately 10, respectively) that are induced by favorable CO oxidation on the surface of tin versus platinum. Anodic currents independently measured at WE1 and at WE2 are successfully converted to the concentrations of NO and CO in the co-presence of these gases using the differentiated sensitivities at each electrode. The sensor is evaluated in terms of its analytical performance: respectable linear dynamic range (sub nM to microM); low detection limit (approximately 1 nM for NO and <5 nM for CO); selectivity (over nitrite up to approximately 1 mM); and sensitivity (sufficient for analyzing physiological levels of NO and CO). Using the NO/CO dual microsensor, real-time, simultaneous, direct, and quantitative measurements of NO and CO generated from living biological tissue (mouse, c57, kidney) surfaces, for the first time, are reported.     

Iridium oxide as low temperature NO/sub 2/-sensitive material for work function-based gas sensors

This paper presents the results on work function-based NO/sub 2/-sensing properties of iridium-oxide thin films at 130/spl deg/C. Films of 20-nm and 100-nm thickness were deposited on silicon substrates using dc sputtering followed by annealing in oxygen ambient. Sensitivity of these films to different concentrations of NO/sub 2/, H/sub 2/, CO, Cl/sub 2/, and NH/sub 3/ in synthetic air was measured using a Kelvin probe. It was observed that work function of 20-nm-thick iridium-oxide film changed by /spl sim/100 mV on exposure to 5-ppm NO/sub 2/ (German safety limit). Cross sensitivity to other gases (except NH/sub 3/) and interference of humidity was found to be negligibly small. The film was incorporated as a gate electrode in a hybrid suspended gate field effect transistor (HSGFET) structure to examine its suitability in FET-type sensors. The films were characterized using Rutherford backscattering spectroscopy, X-ray diffraction analysis, and scanning electron microscopy to determine their composition, phase, and surface morphology. The results suggest that iridium-oxide film is a promising material for the realization of a FET-based NO/sub 2/ sensor.     

Structural and gas-sensing properties of WO/sub 3/ nanocrystalline powders obtained by a sol-gel method from tungstic acid

WO/sub 3/ nanocrystalline powders were obtained from tungstic acid following a sol-gel process. Evolution of structural properties with annealing temperature was studied by X-ray diffraction and Raman spectroscopy. These structural properties were compared with those of WO/sub 3/ nanopowders obtained by the most common process of pyrolysis of ammonium paratungstate, usually used in gas sensors applications. Sol-gel WO/sub 3/ showed a high sensor response to NO/sub 2/ and low response to CO and CH/sub 4/. The response of these sensor devices was compared with that of WO/sub 3/ obtained from pyrolysis, showing the latter a worse sensor response to NO/sub 2/. Influence of operating temperature, humidity, and film thickness on NO/sub 2/ detection was studied in order to improve the sensing conditions to this gas.     

Ultrahigh-sensitive tin-oxide microsensors for H/sub 2/S detection

Ultrahigh-sensitivity SnO/sub 2/-CuO sensors were fabricated on Si(100) substrates for detection of low concentrations of hydrogen sulfide. The sensing material was spin coated over platinum electrodes with a thickness of 300 nm applying a sol-gel process. The SnO/sub 2/-based sensors doped with copper oxide were prepared by adding various amounts of Cu(NO/sub 3/)/sub 2/.3H/sub 2/O to a sol suspension. Conductivity measurements of the sensors annealed at different temperatures have been carried out in dry air and in the presence of 100 ppb to 10-ppm H/sub 2/S. The nanocrystalline SnO/sub 2/-CuO thin films showed excellent sensing characteristics upon exposure to low concentrations of H/sub 2/S below 1 ppm. The 5% CuO-doped sensor having an average grain size of 20 nm exhibits a high sensitivity of 2.15/spl times/10/sup 6/ (R/sub a//R/sub g/) for 10-ppm H/sub 2/S at a temperature of 85/spl deg/C. By raising the operating temperature to 170/spl deg/C, a high sensitivity of /spl sim/10/sup 5/ is measured and response and recovery times drop to less than 2 min and 15 s, respectively. Selectivity of the sensing material was studied toward various concentrations of CO, CH/sub 4/, H/sub 2/, and ethanol. SEM, XRD, and TEM analyses were used to investigate surface morphology and crystallinity of SnO/sub 2/ films.     

Electrospun La₀.₈Sr₀.₂MnO₃ nanofibers for a high-temperature electrochemical carbon monoxide sensor

Lanthanum strontium manganite (La(0.8)Sr(0.2)MnO(3), LSM) nanofibers have been synthesized by the electrospinning method. The electrospun nanofibers are intact without morphological and structural changes after annealing at 1050?°C. The LSM nanofibers are employed as the sensing electrode of an electrochemical sensor with yttria-stabilized zirconia (YSZ) electrolyte for carbon monoxide detection at high temperatures over 500?°C. The electrospun nanofibers form a porous network electrode, which provides a continuous pathway for charge transport. In addition, the nanofibers possess a higher specific surface area than conventional micron-sized powders. As a result, the nanofiber electrode exhibits a higher electromotive force and better electro-catalytic activity toward CO oxidation. Therefore, the sensor with the nanofiber electrode shows a higher sensitivity, lower limit of detection and faster response to CO than a sensor with a powder electrode.     

CO/sub 2/ laser pulse monitoring instrument based on PVDF pyroelectric array

This paper proposes to extend the use of low-cost, pyroelectric, polyvinylidene fluoride four-quadrant arrays, originally devoted to monitoring the beam point stability of CO/sub 2/ laser beams, to the temporal profiling of the same laser pulse providing an instrument that is not commercially available. The advantage of using a single sensor for both types of measurements can be fully exploited if the pyroelectric sensor bandwidth is made almost flat in the typical frequency range (10 Hz-20 kHz) of the signal spectrum generated by a modulated CO/sub 2/ laser. In this paper, we present the design of an active analog compensation filter aimed to improve the reconstruction accuracy of the laser pulses and its circuit implementation with a quad operational amplifier for an easy integration of the compensation filter on the same board hosting the sensor. Experimental results obtained with modulated CO/sub 2/ laser beams, at pulse repetition rates from 10 to 1000 Hz and variable duty cycle, proved accurate in the laser pulse reconstruction with sensitivity of commercial semiconductor HgCdZnTe sensors.     

Optical and electrical H/sub 2/- and NO/sub 2/-sensing properties of Au/In/sub x/O/sub y/N/sub z/ films

In this paper, we describe the optical and electrical gas-sensing properties of In/sub x/O/sub y/N/sub z/ films with an ultrathin gold promoter overlayer. We have fabricated In/sub x/O/sub y/N/sub z/ films with a nanocrystalline porous structure by RF-sputtering in Ar/N/sub 2/ followed by an annealing process. Gold particles with 20-30-nm diameter have been formed on top of the In/sub x/O/sub y/N/sub z/ films by dc sputtering and an annealing process. We have investigated the optical H/sub 2/and NO/sub 2/-sensing properties (change of absorbance) and also the electrical sensing effect (change of electrical resistance) for these two gases. A combined optical/electrical sensor for H/sub 2//NO/sub 2/ is proposed.     

CO-sensing properties of In/sub 2/O/sub 3/-doped SnO/sub 2/ thick-film sensors: effect of doping concentration and grain size

In/sub 2/O/sub 3/-doped SnO/sub 2/ nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl/sub 4/ 5H/sub 2/O) and indium nitrate. The doping concentration was varied from 7.718/spl times/10/sup -5/ to 3.859/spl times/10/sup -4/ moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300/spl deg/C-900/spl deg/C. Thick-film samples with a thickness of /spl sim/15 /spl mu/m, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220/spl deg/C-240/spl deg/C. A blue shift in the sensing temperature and increase in sensitivity factor (S/sub f/) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of /spl sim/5 is found for the highest doping concentration (3.859/spl times/10/sup -4/ moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, and EDAX techniques. The results are discussed based on elemental analyses and available theories.     

Gas sensing study of hydrothermal reflux synthesized NiO/graphene foam electrode for CO sensing

Nickel oxide nanosheets have been successfully synthesized on the graphene foam (GF) using hydrothermal reflux process for their application as carbon monoxide (CO) gas sensor. X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, energy dispersive spectroscopy, and gas sorption analysis were used to characterize the structure and morphology of the samples. The morphology (SEM), crystal structure (XRD and Raman), and elemental composition (EDS) analysis of NiO/GF composite confirmed the cubic crystal structure of NiO and elemental composition (i.e., Ni, O, and C) of NiO/GF composite. The results reveal that the incorporation of graphene into NiO nanosheets not only improved the surface area of NiO/GF composite, but also enhanced the performance of the composite on CO sensing by improving its conductivity. These results indicate that NiO/GF has potential as electrode material for CO gas sensor.     

Mechanical and electrical characterization of /spl beta/-Ga/sub 2/O/sub 3/ nanostructures for sensing applications

Single crystalline /spl beta/-Ga/sub 2/O/sub 3/ nanowire and nanoribbon materials were synthesized, and electrical and mechanical properties were studied for sensing applications. The structural analysis showed that the Ga/sub 2/O/sub 3/ nanomaterials were stoichiometric and had the same crystal lattice structure as the /spl beta/ phase Ga/sub 2/O/sub 3/ crystal. The mechanical study on individual Ga/sub 2/O/sub 3/ nanowires and nanoribbons showed that they had a bending modulus of around 300 GPa, are flexible (in bending and twisting), and are easy to be cleaved along their crystal lattice. The current-voltage electrical characterization through the thickness of nanoribbon and along the length of nanowire confirmed their semiconducting characteristic. A two-terminal device fabricated with an individual Ga/sub 2/O/sub 3/ nanowire showed good sensing response to ethanol gas at low-operating temperature, which revealed the potential of using such nanostructures for effective sensing applications.     

Comparisons between NO/sub 2/- and O/sub 3/-sensing properties of phthalocyanines and n-InP thin films in controlled and noncontrolled atmospheres

This paper describes two different semiconductor gas sensors devoted to the detection of oxidizing pollutants in the atmosphere. The first sensor consists of thin films of phthalocyanines as sensing layers (CuPc, ZnF/sub 16/Pc, and LuPc/sub 2/) evaporated onto alumina substrate fitted with interdigitated electrodes. The second sensor is realized with a mineral monocrystalline semiconductor: n-doped epitaxial layer grown on a semi-insulating substrate of indium phosphide. Each sensor has been submitted to low-controlled concentrations of ozone and nitrogen dioxide, and their detection characteristics, such as response time, stability, and sensitivity, are described. Comparison of these two sensors shows their complementary sensing characteristics, and NO/sub 2/ and O/sub 3/ act in the same way. Measurements under noncontrolled atmosphere (urban air) have been realized and have demonstrated the potentialities of these structures to be used as oxidizing pollutant detectors. Proposed methods to improve the detection of oxidizing species in urban air are discussed.     

A novel NO/sub 2/ gas sensor based on Bis[phthalocyaninato] samarium complex/silicon hybrid charge-flow transistor

A new kind of sandwich-like bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] samarium complex Sm[Pc/sup */]/sub 2/(Pc/sup */=Pc(OC/sub 8/H/sub 17/)/sub 8/) is used as film-forming material. Pure Sm[Pc/sup */]/sub 2/ and mixture of Sm[Pc/sup */]/sub 2/ and octadecanol(OA) deposited from both pure water and 10/sup -4/M Cd/sup 2+/ subphases are investigated. It is found that a mixture of 1:3 Sm[Pc/sup */]/sub 2/:OA forms an excellent material for the fabrication of the gas-sensing Langmuir-Blodgett (LB) film by studying the film-forming characteristics. A new gas sensor has been fabricated by incorporating the multilayer LB film into the gate electrode of a metal-oxide-semiconductor field effect transistor, forming an array of charge-flow transistor. On the application of a gate voltage (V/sub GS/), greater than the threshold voltage (V/sub TH/), a delay was observed in the response of the drain current. This is due to the time taken for the resistive gas-sensing film to charge up to V/sub GS/. This delay characteristic was found to depend on the concentration of NO/sub 2/. Results are presented showing that the device can detect reversibly the concentration of NO/sub 2/ gas down to 5 ppm at room temperature.     

Oxygen sensor based on the fluorescence quenching of a ruthenium complex immobilized in a biocompatible Poly(Ethylene glycol) hydrogel

An optically based system has been developed for use as an oxygen sensor for a cell culture bioreactor. Electrochemical sensors based on the Clark oxygen electrode are typically used with cell-culture bioreactors. These sensors, however, are subject to long-term drift, due in part to biofouling, and require penetrating the bioreactor with the probe in order to perform a measurement. We report an implantable sensor that, when used with an external fiber-optic probe, takes advantage of the oxygen stimulated fluorescence quenching of dichloro(tris-1,10-phenanthroline) ruthenium (II) hydrate. This fluorophore was immobilized in a photopolymerized hydrogel made from poly(ethylene glycol) diacrylate (PEG-DA), a polymer known to minimize protein and cell adhesion. A low-average molecular weight PEG-DA (MW = 575) was employed to hinder the fluorophore from leaching. The PEG-DA precursor solution contained 40% H/sub 2/O such that, upon polymerization, the gel was already in the hydrated state. Sensor hydrogels stored in H/sub 2/O for several months retained their physical shape and sensitivity to oxygen. The sensor showed a high degree of reproducibility across a range of oxygen concentrations that are typical for cell culture experiments (0-9.1 ppm O/sub 2/), and a linear model produced a strong correlation (R /sup 2/= 0.995) compared with a commercial electrochemical probe. No drift or hysteresis was identified in the sensor across cycles of varying oxygen concentrations in this range.     

Study of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ solid solution for low-temperature resistive oxygen gas sensors

A noble type of oxygen-sensitive and electrical-conductive material, ZrO/sub 2/-based with /spl alpha/-Fe/sub 2/O/sub 3/ thick-film gas sensor, was investigated for low operating temperature. Amorphous-like solid solutions of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ powders were derived using the high-energy ball milling technique, and their physical and microstructural properties were characterized from DTA, XRD, TEM, and XPS. The oxygen gas-sensing properties of the screen-printed thick-film gas sensors fabricated from such mechanically-alloyed materials were characterized systematically. Very good sensing properties were obtained with a relative resistance value of 82 in 20% oxygen, and at a low operating temperature of 320/spl deg/C. AC impedance spectra and thermally stimulated current were characterized to investigate the conduction properties of the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, in air and nitrogen (carrier gas), respectively. It was found that the Arrhenius plots of /spl sigma/T versus 1000/T have two distinct gradients corresponding to two activation energies in the high and low temperature regions. The transition temperature occurs at about 320/spl deg/C that corresponds to an optimal operating temperature of the gas sensor. It is believed that the high oxygen vacancy concentration present in the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, and the dissociation of the associated oxygen vacancy defect complexes at 320/spl deg/C are the critical factors for the high relative resistance to oxygen gas at low operating temperature.     

Using polypyrrole as the contrast pH detector to fabricate a whole solid-state pH sensing device

In this paper, we use the extended gate field effect transistor (EGFET) and the coated wire electrode (CWE) to design a differential pH-sensing device. The SnO/sub 2//ITO glass structure is the EGFET used as the pH sensor because of its excellent pH sensitivity of about 57.10 mV/pH. The contrast pH sensor is the polypyrrole/SnO/sub 2//ITO glass structure CWE, which has the lower pH sensitivity of about 27.81 mV/pH, and we use the third SnO/sub 2//ITO glass structure as the reference electrode to serve the base potential of the electrolyte solution. The pH sensitivity of this differential pH-sensing device is about 30.14 mV/pH and it is linear. Hence, this device is a good pH sensor. By using this technology, the differential pH-sensing device has a lot of advantages, such as simple fabrication, solid-state electrodes, easy packaging, low cost, etc.     

Zr/ZrO2 sensors for in situ measurement of pH in high-temperature and -pressure aqueous solutions

The aim of this study is to develop new pH sensors that can be used to test and monitor hydrogen ion activity in hydrothermal conditions. A Zr/ZrO2 oxidation electrode is fabricated for in situ pH measurement of high-temperature aqueous solutions. This sensor responds rapidly and precisely to pH over a wide range of temperature and pressure. The Zr/ZrO2 electrode was made by oxidizing zirconium metal wire with Na2CO3 melt, which produced a thin film of ZrO2 on its surface. Thus, an oxidation-reduction electrode was produced. The Zr/ZrO2 electrode has a good electrochemical stability over a wide range of pH in high-temperature aqueous solutions when used with a Ag/AgCl reference electrode. Measurements of the Zr/ZrO2 sensor potential against a Ag/AgCl reference electrode is shown to vary linearly with pH between temperatures 20 and 200 degrees C. The slope of the potential versus pH at high temperature is slightly below the theoretical value indicated by the Nernst equation; such deviation is attributed to the fact that the sensor is not strictly at equilibrium with the solution to be tested in a short period of time. The Zr/ZrO2 sensor can be calibrated over the conditions that exist in the natural deep-seawater. Our studies showed that the Zr/ZrO2 electrode is a suitable pH sensor for the hydrothermal systems at midocean ridge or other geothermal systems with the high-temperature environment. Yttria-stabilized zirconia sensors have also been used to investigate the pH of hydrothermal fluids in hot springs vents at midocean ridge. These sensors, however, are not sensitive below 200 degrees C. Zr/ZrO2 sensors have wider temperature range and can be severed as good alternative sensors for measuring the pH of hydrothermal fluids.