Soliton doping

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). 

Fukutome H. and Sasai, M. (1983) "Theory of Electronic Structures and Lattice Distortions in Polyacetylene and Itinerant Peierls Systems. II" Prog. Theor. Phys. 69, 1-19. StafstrGm, S. and Chao, K.A. (1984) "Polaron-Bipolaron-Soliton Doping in Polyacetylene" Phys Rev. B 30, 2098-2103. 

Some photoelectron spectroscopy studies of /ram-polyacctylcne in the pristine [28, 46], p-lype doped [47-52J and n-lype doped [52J forms, can be found in the literature. Although no new well defined structure is detected upon doping, which would correspond to doping induced soliton levels, the density of states close to... 

The most interesting aspect of solitons is their charge-spin relation [ 11. Chaiged solitons are spinless, while neutral solitons have spin 1/2. This property provided a natural explanation for the fact that in doped /rrm.v-polyacctylenc the electrical... 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Although the conductivity of polyacetylene is generally discussed in terms of solitons, the question of the precise nature of the major charge-carriers continues to be a subject of debate, with conflicting evidence from different experiments. Spectro-electrochemical studies provide evidence that the charge in doped polyacetylene is stored in soliton-like species (although this is not the only possible interpretation [142, 143]), with absorptions in the optical spectra corresponding to transitions to states located at mid-gap [24,89, 119]. The intensity of the interband transitions... 

FIGURE 57. Coupled soliton modes in n-doped polyacetylene. Reprinted with permission from Reference 66. Copyright (1990) American Chemical Society... 

The degeneracy of the ground state of polyacetylene influences its charge distribution. In fact, upon doping the charges, which in other polymers, such as the heterocyclics, would pair to form bipolarons, are here readily separated to form two positively charged solitons ... 

At high doping levels, when the coulombic attractions to the counterions are screened, the bipolarons or the solitons may become highly... 

Solitons are considered to be important defect states in these conjugated polymers (see Fig. 6.48). It has however been shown that correlation energy is the more important factor in giving rise to the energy gap in (CH) (Soos Ramasesha, 1983). Other polymers related to polyacetylene are polythiophene, polypyrrole, poly-phenylenesulphide, and polyparaphenylene (Section 3.3). Extensive measurements on doped polyacetylenes have been reported in the last five years and these materials, unlike other conducting polymers such as (SN), seem to have good technological potential. 

Figure 6.48 (a) Effect of doping on the electrical conductivity (solid line) and thermopower (broken line) of polyacetylene. (Following Etemad et al, 1982.) (b) solitons in trans-polyacetylene (i) neutral, (ii) positive and (iii) negative solitons. Arrow marks the boundary between the two symmetric configurations. A, acceptor D, donor. (Following Subramanyam Naik, 1985.)... 

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