Metal island model

As discussed in previous sections, the mechanisms for the formation of metallic state and charge conduction in conducting polymers have been the subject of intensive study since the occurrence of an insulator to metal transition was reported upon doping. It is proposed that non linear defects such as polarons, solitons and bipolarons have a major role in these systems [36, 95-97]. In earlier synthesised conducting polymers, inhomogeneities often dominated the transport properties, and metallic island models were proposed to... 

Figure 6. Energy band diagram for the metal island model of granular polyaniline film. CB conduction band VB valence band F. P. Region Julfy protonated (metallic) region U P. Region Unprotonated (semiconducting) region.

In the previous generation of conducting polymers, inhomogeneities often dominated the transport properties, and metallic islands" models were constructed to handle such larger scale granularity [65-70]. 

Kohlman et al. [168] and Epstein et al. [65-69] proposed a metallic islands model with separate contributions from crystalline and amorphous phases. They observed e(w) < 0 in the microwave regime and analyzed their data in terms of having two different plasma frequencies for the localized and delocalized electrons, respectively. Their data analysis [65-69,168] yielded A 1-100 /xm and kpX lO , neither of which is consistent with the dc transport properties. Thus, the metallic islands model is unable to provide a quantitative description of the data with acceptable parameters. 

Work by Javadi et al. [195] on a series of poly (anilines) of varying doping/-protonation levels has shown that microwave and DC conductivities follow different temperature relations, respectively T and T, with the microwave conductivity larger than the DC conductivity by several orders of magnitude. These data appear to support a metallic island model for conduction, with a combination of hopping and electronic conduction occurring, depending on dopant level. 

Pt On Graphite-A Model System For The Study Of Physical And Chemical Properties of Small Metal Islands", S. 

There has also been proposed a percolation transition based on the idea of the presence of metallic islands, apart from the soliton doping model (Tomkiewicz et al., 1979, 1981). 

The resistivity of the composite, therefore, is determined by the contribution of metallic (crystalline) and non metallic (amorphous) resistivity. The role of each portion in the determination of DC conductivity is described by temperature functions which identify the contribution of each part in the total electrical transport mechanism. The metallic part is described by the quasi VRH one dimensional exponential model [2,17,18] and an expression describing the amorphous part of the resistivity which is based on the fluctuation-induced tuimeling between metallic islands[3]. 

Metal/semiconductor, 19-2, 19-4—19-10 Metal-containing polythiophenes, 13-33-13-37 Metal-insulator transition (MIT), 16-2 Metallic box model, 15-65-15-66 Metallic islands, 16-2, 16-5, 16-9, 16-17 Metal-oxide-semiconductor FETs, 8-77 Metal-polyaniline composite, 7-26 in-situ metathesis reaction, 7-29 Meta-substituted polyanilines, 7-36-7-38 Microcontact printing, 8-56, 8-58 p, CP, 9-28-9-27 of rr-PATs, 9-28-9-30 Microdisk lasers, 22-56, 22-57-22-61 Micro-fibers, 16-3, 16-5, 16-11-16-12 Micromolding in capillaries (MIMIC), 9-27 Microring laser, 22-21, 22-54-22-57 Microscopic cracks, 9-24 Microtransfer molding (p TM), 9-28-9-27 Microwave electrochromism, 20-49-20-50 Miller—Abrahams theory, 2-4-2-5, 2-19 MM and DD calculations, 1-24 Mobility edge (Ec), 15-8, 15-20, 15-42 Mobility, 2-2-2-3, 2-5, 2-9, 2-17, 2-19, 9-24-9-26, 9-33-9-34... 

This model explains why SEIRA is observed in both s- and p- polarized IRRAS [384] and ATR [391, 405] spectra and in normal-incidence transmission spectra [377] and why the enhancement is not uniformly spread over each metal island but occurs mainly on the lateral faces of the metal islands [378, 384, 385]. The quasi-static interpretation of the SEIRA also defines the material parameters necessary for excitation and observation of SPR (1) The resonance frequency determined from the general Mie condition must be as low as possible and (2) Ime((Ures) must be as small as possible. The maximum enhancement effect should be observed for the absorption bands near the Mie (resonance) frequency of the particle. As mentioned in Section 3.9.1, the resonance frequencies of metal particles lie in the visual or near-IR range. However, they can be shifted into the mid-IR range by (1) increasing the aspect ratio of the ellipsoids, (2) adding the support to an immersion medium, (3) coating the particles by a dielectric shell [24, 406], or (4) varying the optical properties of the support [24, 349, 350, 384]. As emphasized by Metiu [299], the surface enhancement effect is not restricted to metals but can also be observed for such semiconductors as SiC and InSb. 

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