Thin-film sensors modeling

Hoefer, U. Steiner, K. Wagner, E. Contact and sheet resistances of Sn02 thin films from transmission line model measurements. Sensors and Actuators B (1995), p. 59-63. 

The sPS has been exploited as sensitive material for fiber optic chemical sensors based on reflectance measurements and aimed to detection of chloroform and toluene in water and air environments48 50. The refractive index of sPS thin films is estimated to be about 1.578. The effect of the analyte sorption in the crystalline domain was modeled as an increase in the material density, which in turn leads to an increase in the refractive index according to the Lorentz-Lorenz law ... 

Sensor fabrication has been accomplished in various ways. One method [29] combines the phthalocyanine compound with stearic acid to form a bonded substance which is then dissolved in an organic solvent. The mixture is deposited by one of several possible methods onto a substrate and subsequently fired to remove the vehicle. A thin-film model, manufactured by vacuum sublimation over interdigited electrodes on 3 mm x 3 mm alumina substrates [28], exhibited sensitivity to CI2, F2, and BCI3, but not to common reducing species. The films operating continuously did not survive longer than 6 months. 

The performance of the thick-film sensor was similar to that of the thin-film model. Although not verified, the response time may actually be faster however, the greater surface area of the thick film may actually slow response because of... 

Given the nature of the polymer and the conduction pathway, a simple homogeneous model cannot be applied to thin conducting polymer film-electrolyte systems [27,28,31]. For thin films (< lOOnm) with pore sizes estimated to range from 1 to 4 nm, the porous surface-electrolyte interface will dominate the electrical and physical properties of the sensor. Since the oxidation of the porous surface occurs first, the interface properties play a major role in determining device response. To make use of this information for the immunosensor response, the appropriate measurement frequency must be chosen to discriminate between bulk and interface phenomena. To determine the optimum frequency to probe the interface, the admittance spectra of the conducting polymer films in the frequency range of interest are required. 

As shown in Fig. 7.26, when the sensor is exposed to vapor, individual molecules can diffuse into the semiconductor thin film and be adsorbed mostly at the grain boundaries [13], If the adsorbed analytes have large dipole moment, such as H2O ( 2 debye) and DMMP ( 3 debye), the adsorption of those analyte molecules at the grain boundaries close to or at the semiconductor-dielectric interface can locally perturb the electrical profile around the conduction channel, and hence change the trap density in the active layer. We can interpret the trapping effects by a simple electrostatic model discussed briefly in Sect. 7.2. The electric field induced by a dipole with dipole moment of p (magnitude qL in Fig. 7.4) is ... 

Atashbar M. Z., Bazuin B. J., Simpeh M., and Krishnamurthy S., 3-D Finite element simulation model of saw palladium thin film hydrogen sensor, presented at IEEE International Ultrasonics, Ferroelectrics and Frequency control Joint 50th Anniversary Conference, August 24-27, 2004, Montreal, Canada. 

In order for both mass and heat-flow sensors to operate, the thin-film sample must adhere to the top surface of the QCM and be of uniform thickness. The mechanical behaviour of films on the quartz microbalance has been modeled by Kanazawa(12), who examined the amplitude of the shear displacement in the quartz crystal and in the overlying film for several cases. For a 1 volt peak RF applied voltage typical of the Stanford Research Systems oscillator driver, the amplitude of the shear wave of a bare crystal is 132 nm. Mecca [29] has calculated the inertial acceleration at the centre of a similar quartz resonator, and finds that it is roughly 10 g, where g is the gravitational constant. At these extremely high accelerations, powder or polycrystalline samples do not follow the transverse motion of the QCM surface and cannot be used without being physically bound to the surface with a thin adhesive layer. 

The phenomenon of surface-enhanced infrared absorption (SEIRA) spectroscopy involves the intensity enhancement of vibrational bands of adsorbates that usually bond through contain carboxylic acid or thiol groups onto thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule in the same way as reflection-absorption spectra of thin films on smooth metal substrates. When the metal nanoparticles become in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike surface-enhanced Raman scattering, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc. The mechanism of SEIRA is still being discussed but the enhancement and shape of the bands is best modeled by the Bruggeman representation of effective medium theory with plasmonic mechanism pla dng a relatively minor role. At the end of this report, three applications of SEIRA, namely spectroelectrochemical measurements, the fabrication of sensors, and biochemical applications, are discussed. 

U. HOEFER, K. STEINER, E. WAGNER, Contact and sheet resistance of Sn02 thin films from transmission-Une model measurements . Sensors and Actuators, B 26-27, 59-63,1995. 

A. Fischerauer, G. Fischerauer, G. Hagen, et al, Integrated impedance based hydro-carbon gas sensors with Na-zeolite/CrjOj thin-film interfaces From physical modeling to devices, Physica Status Solidi, vol. 208, no. 2, pp. 404-A15, 2011. 

Metallophthalocyanines, MPc, are known for their high thermal stability and very low solubility in any solvents. Transition metal-based phthalocyanines have attracted attention in the areas of fuel cells, gas sensors and biosensors . Most of these applications require the use of phthalocyanines in the form of thin films. Further, it is desirable to have ordered, well-packed, and oriented molecular layers of phthalocyanines on various substrates. The metal phthalocyanines constitute an important class of compounds that provide models for theoretical and experimental studies involving electron-mediated processes one of the main areas of interest is the electrocatalytic reduction of oxygen by metal phthalocyanines. ... 

Mecca VM (2005) From quartz crystal microbalance to fundamental principles of mass measurements. Artal Lett 38 753-767 Bucur RV, Carlsson J-O, Mecea VM (1996) Quartz-crystal mass sensors with glued foil electrodes. Sens Actuators B 37 91-95 Bucur RV, Mecea VM, Carlsson J-O (2003) EQCM with air-gap excitation electrode. Calibration tests with copper and oxygen coatings. Electrochim Acta 48 3431-3438 Mecea V, Bucur RV (1979) The mechanism of the interaction of thin films with resonating quartz crystal substrates the energy transfer model. Thin Solid Films 60 73-84 Mecea V, Bucur RV, Indrea E (1989) On the possibility of thin film structure study with a quartz crystal microbalance. Thin Solid Films 171 367-375... 

Geistlinger H (1994) Accumulation layer model for Ga O thin-film gas sensors based on the Volkenstein theory of catalysis. Sens Actuators B 18-19 125-131... 

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