Band structure anion doping

Figure 12 Band structure of anion-doped semiconductor with visible light response from a semiconductor with wide band gap (UV response) (Kudo et al., 2004).

In the oxidized state, condncting pol5nners are doped with coimter anions ( p-doping ) and have a delocalised 7r-electron band structure (55). The fairly large... 

FTIR spectroscopy has been used to monitor the conducting states of a conducting polymer as well as to know if a doping experiment is successful [86, 87], The FTIR and UV-Vis spectra of unsubstituted PANI is similar to that of substituted PANI though with slight band shifts. Doped PANI and its derivatives exist in the emeraldine salt forms which are essentially delocalized polysemiquinone radical cations whose stability is maintained by the presence of dopant anions. The degree of electron delocalization in the polysemiquinone forms of the doped PANI manifests itself in the form of an electronic-like band at ca. 1100 cm 1 associated with polarons [86], The structures of emeraldine base and emeraldine salt form of PANI are presented in Figure 6. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

When the materials become solid solutions in the process of doping, some new luminescence phenomena can occur. In a sohd solution, the dopants enter the crystal lattice without bringing variation to the whole crystal structure and symmetry, whereas the lattice size and composition change. The photoluminescence from each component can present in the solid solution. For instance, the photoluminescence band at 2.5 eV for nano-sized ZnW04 originates from radiative-electron transitions within the WOg anions, and this PL band becomes modulated by the optical adsorption spectra of Ni ions in the ZnxNii xW04 solid solution [65]. 

The loss or acquisition of an electron generates a radical cation or a radical anion, respectively. They are called positive and negative polarons, respectively polaronic states arise within the band-gap. Considering, for the sake of brevity, the case of p-doping, the arising of a polaron requires oxidation and induces distortion in a low number of structural units over which the polaron is delocalized. Further electron extraction requires that different polarons are confined in the same polymer chains, inducing a more marked chain distortion. As a result of distortion and... 

Aside from considerations of the polymer form itself, polyaniline can be doped and derivatized in a variety of ways. First, polyaniline can be polymerized in the presence of a variety of acids, which critically influences the resulting electronic properties [1-15]. The particular acid used and polymerization process employed can affect the degree of crystallinity observed [10-15,17-29]. Multiple dopants and substitutions have been achieved in the hope of increasing both the conductivity and solubility of these materials. The derivatives are simple polyanilines functionalized with complex ions such as aryl-SOj, camphorsulfonates, and perfluoroalkyl (and aryl) sulfonates. Dopants vary from simple anions, to oxyanions, to the more typical iodide ions [10-15,17-29,38-44]. The functionalization and/or doping affects, the band population, and the polyaniline chain conformation in turn influence the resulting electronic and structural properties of the polymer. 

The mechanisms by which these polymers conduct electricity have been a source of controversy ever since conducting polymers were hrst discovered. At first, doping was assumed to remove electrons from the top of the valence band, a form of oxidation, or to add electrons to the bottom of the conduction band, a form of reduction. This model associates charge carriers with free spins, unpaired electrons. This results in theoretical calculations of conduction that are much too small (59). To account for spinless conductivity, the concept of transport via structural defects in the polymer chain was introduced. From a chemical viewpoint, defects of this nature include a radical cation for oxidation effects, or radical anion for the case of reduction. This is referred to as a polaron. Further oxidation or reduction results in the formation of a bipo-laron. This can take place by the reaction of two polarons on the same chain to produce the bipolaron, a reaction calculated to be exothermic see Figure 14.17 (55). In the bulk doped polymer, both intrachain and intrachain electronic transport are important. 

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