Conducting polymer bandwidth

Band structure calculations have been performed with the valence effective Hamiltonian (VEH) nonempirical pseudopotential technique. The VEH method yields one-electron energies of ab initio double-zeta quality and has been demonstrated to provide accurate estimates of essential electronic properties such as ionization potentials (IP), bandwidths (BW), bandgaps (Eg), and electron affinities (EA) in the context of conducting polymers. All the calculations have been carried out using the VEH parameters previously reported for sulfur, oxygen, and nitrogen atoms and those recently obtained for carbon and hydrogen atoms,... 

In general, conducting polymers can be considered as a special type of semiconducting material. The conductivity of undoped polyconjugated systems is 10 -10 S cm , hence, it can be considered at the semiconductor-insulator boundary. The bandgaps of known polyconjugated systems vary from 0.8 to 4 eV [3]. The bandwidths, parallel and perpendicular to the chain axis, in a typical polyconjugated system like (CH) c are nearly 10 and... 

In polymers with localized electronic states such as polystyrene, which also contains strong energy loss peaks due to tt electronic excitations as seen in Figure 1, positive dispersion is not observed (1 ). These results are shown in Figure 6. As momentum is increased, the strong peak at 7 eV due to tt - tt excitations of the benzene rings in polystyrene shows very little momentum dependence. Since the bandwidth of valence and conduction bands due to overlap of it orbitals on adjacent phenyl rings is <0.1 eV little momentum dependence to the non-vertical... 

Band structure calculations do not support the ID nature of the charged linear polymers. The C-C o bonds effectively interrupt the tt electron network and Pekker et al. [6] suggested that the bandwidth of the chain is mainly determined by overlap between farther lying C atoms. Indeed, the electronic structure calculated for a single chain [72] shows a very narrow conduction electron band. 

It is now possible to design materials possessing permittivities which have low variation with the frequency (e"a(w ) or high (e"ao) ) directly correlated to the intrinsic conductivity of the polymer. To obtain the best performance (large bandwidth) these materials have to be associated in multilayer structures. Moreover, the use of structural laminates allows the integration of two functions (stealth and mechanical). 

A similar exponential temperature dependence, exp( (Oo/kBT), is expected for all quasi-one-dimensional polymers the important parameters are the carrier density (n), the bandwidth (4to), and cOo/kB. For polyacetylene, a 4.1eV/A and coo 0.12 eV. Using these values, we estimate a room temperature value for the intrinsic conductivity of metallic trans-(CH)x which is 2 x 10 S/cm, about four times greater than that of copper. 

It is seen in region I that the amount of absorbed vapour varies linearly with the concentration. The bandwidth, AQ, and the conductance maximum, Gmax, remain unchanged. Such behaviour confirms that the PPy film is rigid. A departure from linearity for G ax and AQ can be seen in region II. The substantial increase of the bandwidth, AQ, and decrease of the conductance, Gmax, correspond to the viscous losses in the polymer. The boundary between region I and II actually corresponds to the onset of plasticization of the EP by the vapour. 

If a periodic polymer chain has a very narrow bandwidth [like the so-called narrow-band (widths of order 10" eV) periodic nucleotide base stacks or base pair stacks in Table 9.11) or a polymer consists of a nonperiodic sequence of different types of units, then the electronic states become localized molecular states and coherent Bloch-type conduction is no longer possible. 

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