Cation doping band structure

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

One-dimensional stacking of the molecules may provide a framework for possible band structure formation. In this case, repeat units should preferably have an overall planar geometry. This concept can be extended to two-dimensional (2D) structural arrangements where interactions between the molecules are found along two directions as in cation-radical salts derived from TTF-like molecules such as BEDT-TTF (BEDT-TTF = C10H8S8, bis(ethylenedithio)-tetrathiafulvalene) (13), or even to three-dimensional (3D) systems such as the doped A Ceo fullerides (47). 

Figure 11 Band structure of cation-doped photocatalyst with visible light response from a semiconductor with wide band gap (UV response) (Kudo, 2003).

We have begun a study of the stabilization of seml-conductlng and metallic oxides with other metal cations that will font covalent metal-O-Cu bonds and a two level electronic band structure. These materials will be essentially semiconductors where the conductivity arises from doping to produce mixed-valence compounds. Ue chose to begin our study with cations that adopt tetrahedral coordination and focus on how to create structures that Incorporate distorted octahedral, square pyramidal and square planar coordination of copper compatible with still other electropositive (Ionic) cations. The mixed valency Introduced by doping can then be accommodated on the copper metal and adjacent oxygen atom sites by an accompanying bond polarization around the cation with tetrahedral coordination. 

Electronic band structure. Figure 3.7 shows the electronic levels or bands proposed by Bredas and co-workers for PPy, on the basis of semi-empirical theoretical calculations, as it is progressively doped from its neutral undoped state to a maximum of ca. 35% doping. In its neutral state, PPy is predicted to have a very large (valence —x onduction) band gap of 3.2 eV. When one electron is removed to form a polaron (radical cation), two polaronic levels appear in the band gap, as shown in Figure 3.7(b). The lower of these polaron levels is half filled for such partly doped PPys (with one positive charge for every 4 to 6 monomer units), as confirmed by an ESR... 

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. 

---