Doping experiments

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

A nonelectronic method of measuring impurity concentrations is that of absorption spectroscopy. From Eq. (36a) it is seen that ani = avnini0, where a i is the absorption constant due to electronic transitions from level i to the conduction band. The total impurity concentration Nt can be related to ni0 by a knowledge of EF. The photon-capture cross section independently measuring Nt in some sample. This process has been carried out for Cr impurity (Martin, 1979) as well as (EL2) (Martin, 1981) in GaAs. The same considerations hold for photoconductivity measurements, except that t also needs to be known, as seen from Eq. (35). 

The identification of an impurity, defect, or impurity-defect complex by some particular technique must nearly always be accomplished in conjunction with doping experiments. Thus, the well-known, sharp, zero-phonon photoluminescence lines at 0.84 eV in GaAs are almost certainly associated with Cr, as established by Cr-doping experiments (Koschei et al., 1976). However, some care must be taken here. For example, a dominant electron trap (EL2) in -doped GaAs is probably not associated with O, according to recent experiments (Huber et al., 1979). Thus, the doping must be accompanied by a positive identification of the relevant impurity concentration, say by SSMS, or SIMS. These general considerations apply to all the techniques discussed below. 

It was found that enhanced chirality is achieved by doping CLCs with ester molecules instead of BSMs. The same doping experiment on CLCs was... 

Electrochemical (cyclovoltametric) investigations of the ladder-type poly-(para-phenylene) species 71 support the results of the chemical oxidation (doping) experiments both in solution and in the solid state (film). A reversible oxidation takes place and it is well-separated into two waves especially in the solid-state experiment. These are assigned to the formation of radical cationic (79) and dicationic species (80), respectively. The halfwave potential (E1/2) for the first oxidation wave lies between 0.75 V (solution experiment) and 0.95 V (solid state - film) - versus a standard calomel electrode SCE) [106]. Consequently, one has to search for an alternative synthetic process to generate the ladder-type poly(phenylenemethide)s 77 or polymers containing extended segments of the fully unsaturated structure desired. The oxidation of polymeric carbanions appeared suitable, but it proved necessary to work under conditions which completely exclude water and air. 

Fig. 3. Structural features of oxygen incorporation into the LCO lattice, (a) Phase diagram of LCO determined partially in electrochemical doping experiments [251] tetragonal phase (I), oxygen-poor (II) and oxygen-rich (III) orthorhombic phases, (b) Lattice parameters vs. 6 dependence measured in situ in the course of electrochemical doping [267]. (c) Schematic representation of LCO structure [266] O, = incorporated excess oxygen species.

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. 

A complication in doping experiments with silica is that the state of the impurity (A13+) may depend on the presence of other impurities. In quartz and fused silica, aluminum can be put in a substitutional position only if a charge-compensating cation such as Na+ is also present (86). If this is also true for silica gel, then the concentration of color that can be produced in a given sample will depend on the concentrations of both aluminum and some monovalent cation. 

NMR spectroscopy will provide a simpler spectral pattern, when compared with NMR spectroscopy, because a given N resonance line may correspond to a given structure. Therefore, the structure of doped and dedoped N-labeled polypyrrole films can be successfully studied by high resolution solid-state NMR [14, 15]. Doped and dedoped samples were prepared by electrochemical polymerization [16] using 20-30% N-labeled pyrrole. To obtain a dedoped sample, the electrodes were inverted after the doping experiment and the same voltage applied. 

Let us now consider the picture that emerges from the decomposition and doping experiments about the roles of donor and acceptor stacks in the two phase transitions and in the intermediate and low temperature phases of TTF-TCNQ. 

Electrochemical studies of several polymetallaynes have also been reported. Reduction of organic spacer groups can be achieved and the oxidation of substituents (e.g., ferrocene) can be reversible. However, electrochemical oxidation of the platinum centers in polymetallaynes appears to occur in two irreversible redox steps (presumably involving Pt VPt and Pt VPt couples). This observation suggests that the chemical doping experiments that were used to increase the electrical conductivities of the polymers described above are not simple processes. 

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