Sensing compact layer

In a certain sense, interfaces between two liquids are simpler than those between a metal and a liquid, since they do not involve solid-state properties. However, for a long time the structure of liquid-liquid interfaces has remained a matter of controversy. Essentially, there are two different views one holds that the interface is sharp and contains a compact layer of solvent molecules into which the ions cannot penetrate. The other view posits the... 

Figure 22.32 illustrates the way in which the metal-semiconductor junction, built at electrode-sensitive layer interfaces, influences the overall conduction process. For compact layers they appear as a contact resistance (Rc) in series with the resistance of the metal oxide layer (see Figure 22.13). For partly depleted layers Rc could be dominant, and the reactions taking place at the three-phase boundary, electrode-metal oxide-atmosphere, control the sensing properties.

The solvent also acts as a dielectric medium, which determines the field diji/dx and the energy of Interaction between charges. Now, the dielectric constant e depends on the inherent properties of the molecules (mainly their permanent dipole moment and polarizability) and on the structure of the solvent as a whole. Water is unique in this sense. It is highly associated in the liquid phase and so has a dielectric constant of 78 (at 25 C), which is much higher than that expected from the properties of the individual molecules. When it is adsorbed on the surface of an electrode, inside the compact double layer, the structure of bulk water is destroyed and the molecules are essentially immobilized... 

The development of a well-ordered metal surface during electrodeposition is of considerable theoretical and practical importance since it affects the electrocatalytic processes that occur later on it. In this sense the porosity, the surface roughness and the compactness of the deposit define the formation of the oxide layers, especially, in the case of the non-noble metals. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

The classical, state-of-the-art preparation technology for SMOX-based gas sensors - thick, porous sensing layers - is not the best choice for p-type materials. In their case, the direct readout of the changes in the surface band bending would be more efficient in the case of a resistive readout, thin, compact films with electrodes deposited on the top would be more appropriate. 

The equation above builds on a shear model that assumes that all material in the distance range O-dgcM oscillates with the crystal and thus contributes fully to the tme sensed mass, whereas the material located further away from the surface does not oscillate with the crystal and does not contribute to the sensed mass. This is clearly a simplification, as are the optical models used for evaluating the ellipsometric thickness. It has been shown that the QCM thickness and the ellipsometric thickness are similar for relatively compact and homogeneous layers [28]. We do not expect this to be the case for more diffuse polymer layers since the ellipsometric thickness is directly influenced by the segment density profile [29], whereas the QCM thickness is influenced by the amount of water that oscillates with the crystal, and tlus quantity is at present an unknown function of the segment density profile. 

The double-layer capacitance is composed of several contributions. In a geometrical sense the double layer in "supported" systems is represented by the compact "Helmholtz" or "Stem" layer. The electrostatically attracted solvated species reside in the "outer Helmholtz plane" (OHP), and specifically adsorbed species reside closer to the electrode in the "inner Helmholtz plane" (IHP). The double-layer structure is completed by a "diffuse" layer, composed of electrostatically attracted species at some distance from the electrode surface. The fuU thickness of the double layer can be defined as the external boundary of the diffuse layer separating it from the bulk solution, where the measured potential becomes equal to that of the bulk solution and no local potential gradient driven by the difference between the electrode potential ( )j and the solution potential can be determined (Figure 5-4). 

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