Electrode poly films

Deposition of Pb has also been carried out at modified gold electrodes. The films of 3-mercaptopropane sulfonate on poly-crystaUine gold electrodes were prepared from an acidic solution. At such electrodes,... 

So far, we have ignored the primary reason poly films are used in integrated circuits. Heavily-doped poly is used as a gate electrode, and the electrical conductivity of this material is of prime importance. Therefore, we have to inquire into the feasibility of doping poly as it is being deposited by CVD. 

In electrochemical polymerization a solution of monomer is oxidized or reduced at the electrode surface to generate reactive radical species which couple together and produce an adherent polymer film at the electrode. The films can be electronically conducting, as in the case of pyrroles, anilines, thiophenes, etc redox conductors in which conduction occurs by self-exchange between discrete redox sites attached to the polymer, as in metal poly(pyridine) complexes or insulating, as in the case of phenols, 1, 2-diaminobenzene, etc. 

In this report Compton et al describe the synthesis and use of poly-(vinylfluoranthene) and its use in mediating the reduction of isoflurane. The polymer was deposited on a 2platinum electrode. This film exhibits reversible electrochemistry in acetonitrile and catalytic behaviour when isoflurane is present. Unfortunately the films were unstable and could not be used for long term monitoring of the gas. 

In protecting silver from tarnishing, an electrode-posited film of poly(amino triazole) has been tested. The protection by poly(amino triazole) is not reliable for all nuances of silver. In contrast, a film formed with hexadecane thiol shows satisfactory properties [80]. 

The Conditions During the Preparation of the Polymer Film. Electro-polymerized PT films have a more compact morphology in contrast to chemically synthesized PT films [146]. Poly[l,2-bis(3-alkyl-2-thienyl)ethylene] prepared chemically is a bulk powder, in contrast to electrochemically prepared polymers which form homogeneous films (see also Sect. 5.3) [147]. The surface of elec-tropolymerized PTT films is also influenced by the current density. PTT films prepared at a current density of 0.4 mA cm (7.5 min) have typically rough surfaces. PTT films prepared at a current density of 0.05 mA cm (60 min), with the same quantity of electricity, have a compact homogeneous surface [146,148]. These characteristics are independent of the material of the electrodes. PTT films electrochemically prepared at room temperature have a more homogeneous and more compact and smooth surface than at — 5 °C, independently of the current density, with the same quantity of electricity [148]. 

Fig. 20.38 Illustration of the electric potential distribution (< ) across the interface between a metal electrode (Me) coated with a neutral polymer (poly) film and an electrolyte solution containing ions that are partitioned into the polymer. El, E2, E3 are three applied electrode potentials at which the polymer is neither oxidized nor reduced. The electrical double layers (1) and (2) correspond to two different bulk concentrations of ions in the polymer, d.p. is the distribution potential.

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

A number of approaches are available to improve the morphology and homogeneity of electrochemically deposited conducting polymer films. Priming of the electrode surface with a monolayer of adsorbed or covalently bonded monomer leads to more compact deposits of polyaniline,87,88 poly thiophene,80 and polypyrrole.89,90 Electrode rotation has been shown to inhibit the deposition of powdery overlayers during poly(3-methylthiophene) deposition.81... 

In the dual-electrode techniques, the potential of each electrode is controlled with a bipotentiostat so that a small constant potential difference is maintained across the polymer film as its potential is slowly scanned, relative to a reference electrode. Figure 10 shows the results of this type of experiment for poly(3-methylthiophene) in SO20).37... 

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

Cyclic voltammetric studies involving polymers, 558 and the nature of charge carriers, 561 and the nucleation loop, 557 of poly (3-methylthiophene), 564 and parallel-band electrodes, 570 Cyclic voltammograms as a function of scan rate, 559 involving polymerization, 559 with polyanaline, 566 of polypyrrole film, 581... 

A great variety of suitable polymers is accessible by polymerization of vinylic monomers, or by reaction of alcohols or amines with functionalized polymers such as chloromethylat polystyrene or methacryloylchloride. The functionality in the polymer may also a ligand which can bind transition metal complexes. Examples are poly-4-vinylpyridine and triphenylphosphine modified polymers. In all cases of reactively functionalized polymers, the loading with redox active species may also occur after film formation on the electrode surface but it was recognized that such a procedure may lead to inhomogeneous distribution of redox centers in the film... 

Metal-free poly-4-vinyl-4 -methyl-2,2 -bipyridine films on electrodes have been prepared by the electroreductive polymerization of a Rh complex and subsequent leaching of the metal by a strong complexand. The films can incorporate a variety of transition metals... 

Tetra(o-aminophenyl)porphyrin, H-Co-Nl TPP, can for the purpose of electrochemical polymerization be simplistically viewed as four aniline molecules with a common porphyrin substituent, and one expects that their oxidation should form a "poly(aniline)" matrix with embedded porphyrin sites. The pattern of cyclic voltammetric oxidative ECP (1) of this functionalized metal complex is shown in Fig. 2A. The growing current-potential envelope represents accumulation of a polymer film that is electroactive and conducts electrons at the potentials needed to continuously oxidize fresh monomer that diffuses in from the bulk solution. If the film were not fully electroactive at this potential, since the film is a dense membrane barrier that prevents monomer from reaching the electrode, film growth would soon cease and the electrode would become passified. This was the case for the phenolically substituted porphyrin in Fig. 1. 

Fig. 2. Curve A Eleotropolymerization of ImH H2(o-NH2)TPP in 0.1M Et NClO /CH CN by sweeping potential at 200mV/s on Pt electrode. Numbers represent scan number. Curve B Cyclic voltammogram of an electropolymerized film of poly-[H2(o-NH2)TPP] on a Pt electrode, in 0.1M Et NClO /CH CN at 200 mV/s. Integration of the charge under the wave shows that coverage is 3.5X10 9 mol/cm of the porphyrin sites. Curve C Rotated disk electrode voltammetry of the Os(lII,Il) reaction for 0.2 mM...

Figure 4 shows the application (6) of potentials to the Pt and Au electrodes of the sandwich (vs. a reference electrode elsewhere in the contacting electrolyte solution) so that they span the E° of the poly-[Co(II/I)TPP] couple (Fig. 4B). There is a consequent redistribution of the concentrations of the sites in the two oxidation states to achieve the steady state linear gradients shown in the inset. Figure 4C represents surface profilometry of a different film sample in order to determine the film thickness from that the actual porphyrin site concentration (0.85M). The flow of self exchange-supported current is experimentally parameterized by applying Fick s first law to the concentration-distance diagram in Fig. 4B ...

Fig. 4. Voltammograms in 0.1M Eti+NClOi+ZCH CNj rT = 1.2X1 O 8 mol/cm. Curve As Cyclic voltammetry of Pt/poly-Co -NI TPP at 20 mv/s = 200pA/cm. Curve B Four-electrode voltammetry of Pt/poly-Co( -NHi(,)TPP/Au sandwich electrode with E u = 0.0 Vj Ep scanned negatively at 5 mV/s = 400pA/cm. Curve Ci Surface profilometry of a poly-Co(o-NH2)TPP film on Sn02/glassj Tt = 7.6X10 9 mol/cm. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

Figure 3 shows the cyclic vo1tammogram of an aceton i tr i 1 e solution containing 9-fluorenone on a bare Pt electrode and on a Polv-1 coated Pt electrode using II as the electrolyte. The only significant difference between the vo1tammograms is the presence of the poly-I reduction and oxidation waves for the coated electrode. We have previously determined by other means that poly-I films should not be permeable to molecules as large as 9-... 

Figure 1. Cyclic voltammogram on a tin oxide electrode modified with a thin film of poly-1. A sweep rate of 50mV/s was emp 1 oved in CH3CN containing 0.1 M TBAPFS. E vs. Ag+(0.1 M AgN03 in DMSO)/Ag.

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