Solid-state Chemical Sensors Research Articles

ChemInform Abstract: Perovskite Oxides as Solid State Chemical Sensors

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Changing PL Intensity from GaAs/Al0.3Ga0.7As Heterostructures due to the Chemisorption of SO2: Effects of Heterostructure Geometry

ABSTRACTChemical adsorption of gases onto semiconductor surfaces results in a change in the density of electronic surface states. The corresponding change in the number of surface charges gives rise to a measurable change in the bulk charge distribution within the semiconductor. This phenomenon forms the basis of a novel solid-state chemical sensor device using the intensity of photoluminescence (PL) as a measurement technique. We have observed changes in the PL intensity from MOVPE-grown GaAs/Al0 3Ga0 7As heterostructures due to the chemisorption of SO2 onto “photowashed” GaAs surfaces. The photowashing technique is believed to reduce the density of preexisting surface states by removing As from the GaAs surface oxide. The PL signal-to-noise ratio and PL sensitivity characteristics of these chemical sensors may be dramatically improved by varying semiconductor structure. We have observed a range of PL enhancements upon exposure to 0.6% SO2 in flowing N2 from heterostructure layers with various thicknesses and doping densities. These results are explained by comparison with a FEM numerical model derived to correlate the relative PL intensity from various heterostructures to the surface charge density.

Smart zeolites: new forms of tungsten and molybdenum oxides

In this Account, the authors describe their recent research efforts involving the use of volatile hexacarbonylmolybdenum and -tungstem compounds, as precursors in the synthesis of highly organized assemblies of molecular dimension molybdenum and tungsten oxides, encapsulated within the diamond network of 13-[Angstrom] supercages found in zeolite Y. These assemblies are extremely uniform in terms of their nuclearity and structure, and their exclusive internal confinement inside of and lack of significant perturbation of the zeolite Y host. In these materials, the bulk form of the metal oxide has essentially been reconstituted within the nanoscale void spaces of a crystalline aluminosilicate framework host. The electronic and structural properties of these molecular metal oxide arrays can be easily manipulated as a result of their facile redox interconvertibility, and the further capability of fine tuning their electronic enviornment by choosing which charge-balancing cation is present in the supercage. Bulk forms of WO[sub 3] and MoO[sub 3], which possess octahedral building blocks arranged into open framework, layer, and tunnel structures, are probably most famous for their ability to reversibly accept metal cations and protons into their internal void spaces and electron-charge-transfer equivalents into their conduction bands. This fascinating property enables them to function as intelligentmore » materials in, for example, electrochromic windows, mirrors and displays, rechargeable solid-state batteries, pH-microelectrochemical transistors, and chemical sensors. In addition, their narrow-band-gap semiconductor properties allow them to be usefully exploited in liquid junction solar and electrochemical cells.23 refs., 5 figs., 1 tab.« less

Effect of electrode geometry on gas sensitivity of lead phthalocyanine thin films

The effect of electrode geometry on the gas sensitivity of lead phthalocyanine (PbPc) thin films has been studied. The material PbPc has been chosen for this study because it has a high sensitivity (∼ppm) to oxidizing gases and may be suitable for use in a commercial solid-state chemical sensor. PbPc films of varying thickness (up to 1.5 μm) are vacuum sublimed across ultrathin (0.1 μm) coplanar pairs of gold electrodes lying on a sapphire substrate. The change of the electrical conductance of these devices in atmospheric nitrogen dioxide (NO 2) is measured, over a range of electrode separations for several film thicknesses and temperatures, by a computer-controlled automated system. The temperatures are chosen around 160 °C, at which a peak sensitivity of PbPc to nitrogen dioxide has been observed. The transient and steady-state responses of these devices are analysed in terms of diffusion-rate limited and reaction-rate limited models modified from previous work on thick porous metal oxide semiconducting films. Experimental results show that the response time of the device at temperatures from 130 to 190 °C is insensitive to both the electrode separation and film thickness. Furthermore, the response time, typically 120 s, decreases with increasing nitrogen dioxide concentration (1–9 ppm). These observations are consistent with the reaction-rate limited rather than the diffusion-rate limited model of mass transport in thin PbPc films. The activation energy of the PbPc films is determined from the temperature dependence of the device conductance, and found to be (0.15 ± 0.1) eV; as expected, it is independent of electrode separation and film thickness. The film conductance in air increases with increasing reciprocal electrode separation, but does not follow the linear relationship predicted from the model. Furthermore, the steady-state response (defined as the fractional change in conductance) of the PbPc films to atmospheric nitrogen dioxide shows a gradual but systematic increase with electrode separation, whereas the model predicts a constant value. The deviation of the experimental results from theory suggests the presence of considerable film inhomogeneity. The nature of this deviation is consistent with a film in which both the electrical conductivity and density of adsorption sites are considerably lower near the substrate surface than elsewhere. This observed heterogeneity in the gas sensitivity of thin sublimed PbPc films makes it more difficult to characterize their behaviour and may reduce their commercial viability as chemical sensors.

Odour discrimination with an electronic nose

Smell is probably the least understood and exploited of the principal human senses, yet it is clearly important to both product and process control in many industries, such as foodstuffs, beverages, tobacco and perfumery. Advances in the field of integrated microelectronic devices have led to new instruments, robots, capable of vision and complex touch or taction, but not yet of smell. This paper reviews the research effort that has been carried out at Warwick University over recent years into the development of an electronic instrument that can mimic the human sense of smell. The approach that we have adopted is to construct a microprocessor-controlled system comprising an array of solid-state chemical gas sensors (with overlapping partial sensitivities to odorants) and associated signal processing and pattern recognition. This electronic system is based upon our present knowledge of the biological system. Our earliest electronic nose consisted of an array of only three to twelve tin dioxide thick-film sensors, yet it can discriminate between alcohols, beverages, tobacco blends and coffees. Current efforts are reported towards the fabrication of an integrated microsensor metal oxide array, the development of other electronic devices using polymeric materials, and the implementation of various pattern-recognition techniques, including correlation, principal-component analysis, cluster analysis and artificial neural networks. Finally, the application areas most likely to arouse widespread interest in the next decade are discussed.

Suitable ion-selective sensors for lead and cadmium analysis

Two different chemical sensor assemblies for lead(II) and cadmium(II) ions have been fabricated. The first is based on a traditional ion-selective electrode and the second on a graphite solid-state device. Both chemical sensors have been characterized and tested in standard solutions and also in river water samples. The use of a PVC-PVA-PVAc copolymer as base polymer in the membrane allows us to get a better adhesion on the graphite active surface of the device. The selectivity in the modified membrane is given using derivatives of dioxa diamide or ditioamide. The results obtained by the solid-state chemical sensors are compared with those obtained by ion-selective electrodes. Potentiometric measurements, response time, lifetime, linearity range and selectivity factors have also been reported and discussed.

Ion-implanted chalcogenide glasses as membrane materials for solid-state chemical sensors

Nickel-implanted AgAsS 2 glasses are used as membranes of silver ion sensors. Ni + ion implantation increases the hole conductivity of these glasses by six orders of magnitude and the ionic conductivity by 2.5 orders of magnitude in the vicinity of the inner glass/metal interface and allows highly stable all-solid-state devices to be obtained with fast response, excellent chemical durability and high selectivity. Co/AgAsS 2 and Ni/AgAsS 2 ion-mixed layers produced by proton bombardment do not show reasonable ionic sensitivity and selectivity to Co 2+ and Ni 2+ ions.

Inelastic electron tunneling in aluminum-alumina-gold junctions exposed to thiocyanate ion: sensitive thiocyanate detection

Inelastic electron tunneling spectra are reported for Al-Al 2 O 3 -Au junctions externally doped with KSCN from methanol solution. Spectra of the thiocyanate ion can be observed over a wide range of concentration, 10 -7 -10 -2 M. Because the tunneldiodes are completely fabricated prior to the doping process, and because the doping process is simple and only requires a few minutes, these devices may serve as solid-state chemical sensors. The spectra observed for externally doped junctions are essentially the same as those for junctions doped with KSCN during device fabrication

Development of Tailored Films and Surfaces for Use in Advanced Chemical Sensors

ABSTRACTThe microstructure found at the surfaces and interfaces of active sensing materials can strongly affect the performance of solid state chemical sensors. This paper desribes research efforts aimed at improving oxide semiconductor-based gas sensors which utilize changes in conductance and diode current-voltage measurements as detection schemes. Studies of oxide surface modification and the formation of Pd/SnO2(110) and Pd/TiO2(110) interfaces are discussed as are the electrical changes for these systems induced by H2 or O2 adsorption. The fabrication of heteroepitaxial SnO2 films for use in planar sensing arrays is also reported and the expected advantages of these highly-ordered, uniform films for simplifying the transducing process are described.

A quantitative adhesion test for thin polymer films on silicon substrates

AbstractThe standard 90° peel test was modified in order to apply it to testing adhesion of thin polymer films to surfaces of silicon substrates, such as the silicon nitride passivation layer used in our solid‐state chemical sensors. The goal of this task was to develop a procedure for testing adhesion of both dry and wet films, which would yield repeatable, quantitative results quickly, and with simple sample preparation. It is important to produce valid comparisons of adhesion both over variations of a given polymer composition, and between films made of dissimilar polymer matrices. The method described herein could be applied to adhesion testing of a wide variety of thin polymer layers to solid surfaces.

Interface analysis for solid state electrochemical devices and chemical sensors

Examples are given for recent applications of interface analysis to understand the function of electrochemical devices and chemical sensors based upon solid electrolytes such as stabilized ZrO2, LaF3, and AgI. Gas interaction with particular emphasis on oxygen is studied at various electrodes on these solid electrolytes. The conditions for well-defined surface-spectroscopic measurements on solid ionic conductors are summarized and exemplified. XPS and UPS, in particular, make possible to analyse electrochemical problems and fundamentals in more detail than it is possible with phenomenological methods including the classical electrochemical techniques.

Polyimide-matrix chemical-selective membranes

Preliminary work using polyimide (PI) as the polymer matrix for permselective membranes is reported in this paper. The excellent adhesion of polyimide may be helpful in extending solid-state chemical sensor lifetimes. Electrochemical performance and adhesion to Si 3N 4 of these membranes is compared to that of conventional PVC-based membranes.

Solid-state chemical sensors: Atomistic models and research trends

After presenting a brief survey about thermodynamically and kinetically controlled sensing mechanisms, atomistic models and research trends of solid-state chemical sensors are discussed. Future work will involve new techniques of interface analysis, controlled two-dimensional physical chemistry at interfaces, new materials, new technologies, new microstructured devices and pattern recognition approaches.

Microionic insertion sensors

Abstract A new class of solid state chemical sensor is described having the general configuration: catalytic metalƒinsertion compoundƒ insulator ƒsemiconductor. The insertion layer accepts H atoms produced at the catalyst, and is reduced, leading to an increase in capacitance. The concept is demonstrated for Si/SiO 2 MOS structures incorporating WO 3 , an H-insertion compound, and Pd as the gate layers. The thermodynamic driving force for the reaction of H with WO 3 is sufficient to give rapid response to H 2 at room temperature.

Organic polymer films for solid-state sensor applications

Organic polymers are used as chemically selective layers in all types of solid-state chemical sensors. In most cases the polymer serves only as a convenient matrix which holds the specifically binding molecules such as enzymes, antibodies or ionophores. On the other hand, some organic layers can be designed in such a way that at least one physical parameter of the entire layer is selectively chemically modulated. These may include electrical conductivity, mass, electron work function, dielectric constant, refractive index, etc. Examples of these types of materials and their applications in potentiometric sensors are given.

Solid state chemical sensors

Solid state chemical sensors

A surface chemical view of gas detection

One of the most promising new areas of study, which can provide sensors to meet future requirements for toxic gas instrumentation, is the field of solid-state chemical sensors. Within this category is a method for gas sensing which directly utilizes semiconductor/gas interactions. Devices have been constructed for this application which use organic, elemental, and metal-oxide type materials. Specific results for a cobalt oxide semiconductor material used for the detection of ppm levels of carbon monoxide are reported. The semiconductor surface states and their chemical reactivity are related to changes in the electrical properties of the metal-oxide. The application of principles of surface chemistry to carbon monoxide gas sensing is expressed by this example and a mechanism for MOS toxic gas sensor response is proposed for the system Co 3O 4/CO.