The Doping Process

Electrochemical doping is carried out by the electrochemical synthesis of the polymer in a solvent containing the dopant. This allows an easy control of the doping rate and of the amount of dopant. After doping, the material still contains solvent molecules [181]. 

Chemical doping is carried out by exposure of the clean film material to the doping gases. The doping rate and the amount of dopant cannot easily be controlled. Also, a suspension of the solid polymeric material in a solvent can be doped by dissolved reagents. A new method is solid-state doping under UHV conditions [181,182]. 

The mechanism and kinetics of electrochemical reactions at electrodes covered with PT studied using the voltammetric behavior of PT films depend on the nature and concentration of mobile charge carriers within the polymer and therefore on the potential range [193,194]. The dependence of the processes during the electrochemical reduction and oxidation on the medium in contact with the PT film can be revealed by in situ Raman spectra [195]. The doped and neutral states of PMT and poly(3-thienylacetic acid) and the anion and cation dopants can be clearly identified by an improved specular reflectance IR spectroscopy. This method allows the use of the same electrode as that used in 

The electrochemical doping process of a PT film with CIO4 proceeds as follows [206]  

At higher overpotentials the oxidation process is more intensive and faster. When PT is oxidized at a much higher overpotential than the oxidation potential, both overoxidation and polymer degradation (loss of electrical conductivity) coexist with the reversible oxidation [206,217]. The problem of the degradation of electronic properties by overoxidation resulting from nucleophilic attack of water at the dication (bipolaron) charge carriers [218] can be avoided by the use of extremely dry non-nucleophilic electrolyte solutions [219,220]. PMT films made inactive by overoxidation in the presence of CP can be reactivated both electrochemically and chemically to produce a partially chlorinated conducting polymer [220], 

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These conjugated polymers can be chemically and electrochemically reduced and reoxidized in a reversible manner. In all cases the charges on the polymer backbone must be compensated by ions from the reaction medium which are then incorporated into the polymer lattice. The rate of the doping process is dependent on the mobiHty of these charge compensating ions into and out of the polymer matrix. 

Bellosta von Colbe, J.M., B. Bogdanovic, M. Felderhoff, A. Pommerin, and F. Schuth, Recording of hydrogen evolution—a way for controlling the doping process of sodium alanate by ball milling, /. Alloys Compd., 370, 104-109, 2004. 

The doping process may be controlled by changing the integrated flux of neutrons, making possible the approximate prediction of the resistivity of thermistors as a function of temperature. 

Application to the two-layer resist system. Photobleachable resist systems that have a strong absorption before exposure and that bleach completely upon UV exposure alleviate the light reflection from the substrate. A photobleachable resist system formed by means of the doping process liras reported in our previous paper (9). This resist system consists of two layers in which a diazonium salt is distributed in both the top and bottom layers. When exposed to i-line, the diazonium salt in... 

The exposure curve of the two-layer resist based on the doping process is shown in Figure 8. The two-layer resist system has a high contrast and high resolution capability. Submicron line-and-space patterns are obtained using this two-layer resist system (Figure 9). 

A second major event in the saga of polymer conductors was the discovery that the doping processes of polyacetylene could be promoted and driven electrochemically in a reversible fashion by polarising the polymer film electrode in a suitable electrochemical cell (MacDiarmid and Maxfield, 1987). Typically, a three-electrode cell, containing the (CH) film as the working electrode, a suitable electrolyte (e.g. a non-aqueous solution of lithium perchlorate in propylene carbonate, here abbreviated to LiC104-PC) and suitable counter (e.g. lithium metal) and reference (e.g. again Li) electrodes, can be used. 

The doping process (here of the p-type) takes place with formation of the charged polycation accompanied by diffusion of the electrolyte counterion X ... 

In an attempt to illustrate in a simple way the general concept of the doping process in polymers, let us consider the p-doping (oxidation) process of polypyrrole. In the undoped state, polypyrrole is a poor electronic conductor with an energy gap of 3.2 eV between the conduction band (CB) and the valence band (VB) ... 

Also the case of polyaniline is somewhat different from that of heterocyclic polymers. It has been proposed (MacDiarmid and Maxfield, 1987) that the doping process does not induce changes in the number of electrons associated with the polymer chain but that the high conductivity of the emeraldine salt polymers is related to a highly symmetrical 7r-delocalized structure. 

Methods for monitoring the doping processes of conducting polymers... 

The proposed mechanism of the doping processes in conducting polymers implies oxidation (p-doping) or reduction (n-doping) of the polymer with... 

The evolution of the band structure - and thus of the doping process -may be conveniently monitored by detecting in situ the optical absorption during the electrochemical process, by placing the cell directly into the spectrophotometer (Danieli et al, 1985). 

As repeatedly stressed, the doping processes imply the diffusion of electrolyte counterions to compensate for the electric charge assumed by the polymeric chain and thus polymers are expected to experience changes of mass upon doping. Consequently, by monitoring these changes it is possible to control the nature and the extent of the doping processes. 

In order to evaluate conducting polymers as possible electrode materials for novel devices, it is essential to investigate the kinetics of the doping processes. 

Fig. 9.10 shows a typical CV of a (CH), film in a LiClO -propylene carbonate electrolyte. The voltammogram presents well-defined peaks both in the anodic (doping) and in the following cathodic (undoping) scans this confirms that the doping process of polyacetylene, as suggested by (9.10), can indeed be driven electrochemically and in a reversible way. 

Furthermore, the fact that the scan exhibits two peaks suggests the presence in the basic polyacetylene structure of at least two (if not more) different structural sites for the doping process, as effectively confirmed by independent structural studies (Shacklette et al, 1985). Finally, the... 

As schematically illustrated in Fig. 9.11, one may assume that the doping process of (9.10) proceeds via the following main steps ... 

Since the kinetics of the doping processes is expected to depend upon the nature of the counterion, particularly its size (which may influence the mobility throughout the polymer host), it is possible to control the diffusion kinetics by selecting the nature of the supporting electrolyte employed in the electrodeposition process. 

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