Doped systems

The range of potential sensitizers for photopolymerization of diacetylenes widened considerably when Wegner and coworkers discovered sensitized photopolymerization in LB-multilayer systems doped with certain dyes. It is known for long that dye molecules carrying long aliphatic substituents can easily be incorporated into LB-multilayer assemblies without destroying the sample architecture Therefore, the topotactic requirements for the polymerization reaction to proceed are retained. 

Sensitization by charge transfer requires that the LUMO of the diacetylene monomer be below the excited singlet level of the dye. The latter is at Is — Eg where I, and Eji are ionization potential and energy of the first singlet state of the dye, respectively. On the basis of polarographic data I, — E j —2.7. .. —3.3 eV is estimated for cyanine dyes The lowest unoccupied orbital of a previously neutral diacetylene 

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Significant variations in the properties of polypyrrole [30604-81-0] ate controlled by the electrolyte used in the polymerization. Monoanionic, multianionic, and polyelectrolyte dopants have been studied extensively (61—67). Properties can also be controlled by polymerization of substituted pyrrole monomers, with substitution being at either the 3 position (5) (68—71) or on the nitrogen (6) (72—75). An interesting approach has been to substitute the monomer with a group terminated by an ion, which can then act as the dopant in the oxidized form of the polymer forming a so-called self-doped system such as the one shown in (7) (76—80). 

The systems discussed in this chapter give some examples using different theoretical models for the interpretation of, primarily, UPS valence band data, both for pristine and doped systems as well as for the initial stages of interface formation between metals and conjugated systems. Among the various methods used in the examples are the following semiempirical Hartree-Fock methods such as the Modified Neglect of Diatomic Overlap (MNDO) [31, 32) and Austin Model 1 (AMI) [33] the non-empirical Valence Effective Hamiltonian (VEH) pseudopotential method [3, 34J and ab initio Hartree-Fock techniques. 

Figure 5-8. UPS vuIlmkc bund spectra of duped Im/i.t-polyacclylenc for K-dopcd, neutral, and CIO4-doped systems, from lop lo bolloin (adapted from Ref. 52 ).

The four Eqs. [(8.3)—(8.6)] are simplified using chemical and physical intuition and appropriate approximations to the electroneutrality Eqs. (8.7) and (8.10). Brouwer diagrams similar to those given in the previous chapter can then be constructed. However, by far the simplest way to describe these equilibria is by way of polynomials. This is because the polynomial appropriate for the doped system is simply the polynomial equation for the undoped system, together with one extra term, to account for the donors or acceptors present. For example, following the procedure described in Section 7.9, and using the electroneutrality equation for donors, Eq. (8.9), the polynomial appropriate to donor doping is ... 

Rahmandoust et al. [212] used finite element models of SWCNTs to calculate the mechanical properties of these doped systems, and obtained results that were in agreement with existing literature values calculated using more complex methods. Subse-... 

CNTs can be easily doped by noncovalent means via molecular adsorption, an aspect that has been considerably exploited to develop ultrasensitive field effect transistor sensors [88-91]. However, substitutional doping with B and N to confer p and n character to the CNTs has also been carried out [92]. Such doped systems can be more susceptible to react with donors or acceptors molecules (depending on the doping) allowing the chemically reactivity to increase. 

DOS in doped systems including broadening due, to coulombic centers... 

Finally we recall that in the Sr-doped system the maximum Tc is achieved for a Sr content of 0.15, which corresponds to a formal valence per Cu of 2.15 (i.e., 0.15 holes per Cu). In the Ce-doped system the maximum in Tc (24K) is achieved for a Ce content of 0.15, which corresponds to a formal valence per Cu of 1.85 (i.e., 0.15 electrons per Cu). Thus, a maximum Tc occurs in the n-type materials at the same electron concentration as the hole concentration in the p-type materials which produces a maximum Tc and indicates a doping symmetry. The symmetry between the n-type and p-type materials is further reflected in Figure 4 where we have shown that... 

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