Semiconducting sensitivity

As a consequence of this surface interaction, charge transfer takes place between the adsorbed species and the semiconducting sensitive material. This charge transfer can take place either with the conduction band or in a localized manner. In the first case, the concentration of the free charge carriers will be influenced. For the understanding of the detection, it is important to deepen our insight in the localized charge transfer case. 

As shown in Figure 22.32 there is a resistance associated with the interface between the semiconducting sensitive layer and the metallic electrode. The importance of this resistance to the overall sensor resistance value depends on the sensing layer and electrode morphology. The possible dependence of the contact resistance on the ambient atmosphere conditions can be described in terms of two contributions (i) the first is dealing only with the electrical contribution of the semiconducting sensitive layer—electrode interface to the overall sensor resistance, (ii) the second describes the possible chemical influence of, for example, the catalytic activity of the contact material in the region close to the contacts. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

See 2-3.1. Typical laboratory conductivity meters have insufficient sensitivity to measure semiconductive and nonconductive liquids, in Appendix B, some tabulated conductivities appear as < suggesting that the instmment used was inappropriate. Some liquids listed as conductive might fall instead into the semiconductive category (e.g., cymene). Eor conductivities less than 100 pS/m especially, highly sensitive picoammeters are required to measure the small currents involved and great care is needed to avoid contamination of both the sample and the test cell. Several ASTM methods are available according to the conductivity range involved [143-146]. 

Semiconducting devices, switches and miniaturised v.h.f. circuits are all particularly sensitive to the slightest reaction on critical surfaces, and in devices calling for the highest levels of reliability even the most inert of the phenolic, epoxide and silicone resins are not considered to be fully acceptablecorrosion of electronic assemblies may often be enhanced by migration of ions to sensitive areas under applied potentials, and by local heating effects associated with current flows. 

Ultrafast photoinduced electron transfer in semiconducting polymers mixed with controlled amounts of acceptors this phenomenon has opened the way to a variety of applications including high-sensitivity plastic photodiodes, and efficient plastic solar cells ... 

Calvet and Guillaud (S3) noted in 1965 that in order to increase the sensitivity of a heat-flow microcalorimeter, thermoelectric elements with a high factor of merit must be used. (The factor of merit / is defined by the relation / = e2/pc, where e is the thermoelectric power of the element, p its electrical resistivity, and c its thermal conductivity.) They remarked that the factor of merit of thermoelements constructed with semiconductors (doped bismuth tellurides usually) is approximately 19 times greater than the factor of merit of chromel-to-constantan thermocouples. They described a Calvet-type microcalorimeter in which 195 semiconducting thermoelements were used instead of the usual thermoelectric pile. 

The Japanese company FIS Inc. has developed a 7-probe with a semiconductive cell made of BaSn03. The functional principle is based on the change in conductivity of the probe. The signal is generated by surface reactions with the local atmosphere and also is sensitive for the intermediate products and the free radicals resulting form combustion [6]. 

G Yu, K Pakpaz, and AJ Heeger, Semiconducting polymer diodes large size, low cost photodetectors with excellent visible-ultraviolet sensitivity, Appl. Phys. Lett., 64 3422-3424, 1994. 

Building a heat flow microcalorimeter is not trivial. Fortunately, a variety of modern commercial instruments are available. Some of these differ significantly from those just described, but the basic principles prevail. The main difference concerns the thermopiles, which are now semiconducting thermocouple plates instead of a series of wire thermocouples. This important modification was introduced by Wadso in 1968 [161], The thermocouple plates have a high thermal conductivity and a low electrical resistance and are sensitive to temperature differences of about 10-6 K. Their high thermal conductivity ensures that the heat transfer occurs fast enough to avoid the need for the Peltier or Joule effects. 

Photoluminescence spectroscopy is used to analyze the electronic properties of semiconducting CNTs [64]. The emission wavelength is particularly sensitive to the tube diameter [65] and chemical defects [66], However, a more dedicated sample preparation is required in order to eliminate van der Waals and charge transfer interactions between bundled CNTs. This can be done via ultrasonication or treatment of the bundles with surfactants that separate individual CNTs and suppress interactions between them [67]. 

Metallic or Semiconducting Single-Walled Carbon Nanotubes Which Gives the Best Performance for a Dye-Sensitized Solar Cell. ChemPhysChem 2012,13 2566-25Z2. 

E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, and Z.L. Wang. Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts . Applied Physics Letters 81 (2002), 1869-1871. 

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