There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Electrochromic tungsten oxide films: Review of progress 1993–1998

Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol−gel method. Transformations undergone by a colloidal solution of tungstic acid, stabilized by an organic additive such as poly(ethylene glycol) (PEG) 300, as a function of the annealing temperature have been followed by means of a confocal Raman microscope. The shape and size of WO3 nanoparticles, the porosity, and the properties of the films depend critically on preparation parameters, such as the tungstic acid/PEG ratio, the PEG chain length, and the annealing conditions. Well-crystallized WO3 films combine excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm. Particular applications of these nanocrystalline WO3 films include photoelectrochemical and electrochromic devices.

Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications.

In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or...

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

http://cau.ac.kr/~jjang14/BioMEMS/Ivnitski_BSBE_Bacteria_Detection_Bio_Sensors_Review_1999.pdf

Biosensors for detection of pathogenic bacteria

In many fossil fuel burning systems, NOx emissions are minimized by a selective catalytic reduction (SCR) technique where NH3 is injected into the flue gas stream to react with NOx to form environmentally safe gases, such as nitrogen and water vapor. Unfortunately, this process is usually incomplete, resulting in either NOx emissions or excess NH3 (NH3 slip). Therefore, a critical need exits for an in situ sensor array near the stack to provide real-time control of the NH3 injection, and hence, minimize the NOx emissions released into the environment. In the present work, semiconducting metal oxide (SMO) film technology is used to engineer a small, robust, sensitive, and selective sensor array to detect NOx and NH3 emissions. Many thin film tungsten trioxide (WO3) based sensing elements were tested in order to identify two film recipes capable of sensitively and selectively detecting NOx and NH3. The critical parameters inherent in each film recipe are type of substrate material, film thickness, doping, deposition temperature, and operating temperature. The two element sensor array’s response characteristics analyzed include the response and recovery times, rates of reaction, dynamic range, sensitivity, repeatability and selectivity.

A semiconducting metal oxide sensor array for the detection of NOx and NH3

Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.

A lateral field excited liquid acoustic wave sensor

Acoustic waves excited in a piezoelectric medium provide an attractive technology for realizing a family of biosensors that are sensitive, portable, cheap and small. In this paper a wide range of bulk and surface-generated acoustic waves are described and prototype sensing-element geometries are presented. Results obtained using several candidate acoustic wave biosensors are also discussed.

Acoustic wave biosensors

The Debye-Waller factors have been calculated for a number of zinc-blende-type crystals from the eigenfrequencies and eigenvectors of the phonon states as obtained from the modified-rigidion model of lattice dynamics. On the average the contribution from the acoustic-phonon states to the mean-square displacements of the constituent atoms was found to be more significant than that from the optical-phonon states. However, the relative optical-phonon-state contribution was found to be more significant in the lighter atom, and in fact in some cases at low temperatures, it was found to exceed the acoustic-phonon-state contribution. In all the cases studied, the mean-square displacements of the lighter atom were found to be larger. Lindeman's criterion for melting was tested by evaluating the ratio of the sum of the root-mean-square displacements of the constituent atoms to the nearest-neighbor distance. This ratio is approximately constant for the zinc-blende compounds. However, the average ratio for the II-VI compounds was found to be slightly larger than that for the III-V compounds. The calculated results are discussed in relation to available experimental data.

Debye-Waller Factor for Zinc-Blende-Type Crystals

The Brooks-Yu theory is applied to the calculation of the band structure of some III-V semiconductors at finite temperatures. The theory is applied in the context of the empirical pseudopotential method, and utilizes a recent lattice-dynamical calculation of the Debye-Waller factors. The temperature dependence of several band gaps are determined and compared with available experimental data and, in general, good agreement is obtained.

John F. Vetelino

Papers

A semiconducting metal oxide sensor array for the detection of NOx and NH3

A lateral field excited liquid acoustic wave sensor

Acoustic wave biosensors

Debye-Waller Factor for Zinc-Blende-Type Crystals

Temperature Dependence of Energy Gaps of Some III-V Semiconductors