Grown electrochemically

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

To what extent the films grown electrochemically have decisive advantages over those grown with other techniques is not clear yet. However, one can see that great variety (e.g., ternary and quaternary alloy formations) is possible, and the availability of potentiostatic control and nonaqueous solutions may be helpful. 

A source of doubt in such analyses is whether the depths of the pits grown electrochemically are representative of those expected under natural conditions and therefore appropriate to extrapolate to longer times in predictive models. The data shown for pitting of carbon steel, sketched in Fig. 28, show that they are not. Clearly, growth is accelerated under potentiostatic electrochemical conditions, and the extrapolation of pit depths seriously overestimates the predicted pit depths after long exposure times. This is not surprising, since the use of a... 

Figure 7. Example of AFM images of SiO2 dots grown electrochemically through anodic porous alumina on Si after dissolution of the A12O3 template, (a) 2-D AFM image (scanned area 500 nm x 500 nm), (b) 3-D AFM image (scanned area 0.15 pmx 0.15 pm in (b)). The alumina template was 750 nm thick and it was grown in sulfuric add aqueous solution with the concentration of 6 % in volume under constant voltage bias of 20 V.

Insulating poly(phenol) films can be grown electrochemically by the poten-tiostatic or potentiodynamic mode from aqueous-buffered solutions at neutral pH. The films are usually thin (< 100 nm) because the polymerization process is self-controlled and stops as soon as the electrode surface is completely insulated by the polymer coating, as shown in Fig. 11.8. The unlabeled wave forms in Fig. 11.8 show the decrease in polymerization current to a minimum after three scans in a buffered solution containing the monomer at pH 7.0. Mechanisms for the electrochemical formation of poly(phenol) films have been described by Bartlett and coworkers. The polymer is believed to be a mixture of para-linked and meta-linked units. Figure 11.9 compares the catalytic efficiency of biosensors based on a poly(phenol) film entrapping GOx and that containing GOx and Os-polymer. 

Figure 1.14 Design scheme for a flexible LED structure consisting of vertically oriented single crystalline nanowires grown electrochemically on a polymeric ITO-coated substrate. The top contact consists of p-type polymer (PEDOTiPSS) and an evaporated Au layer. Light Is emitted through the transparent polymer [140]. Reproduced by kind permission from the publisher.

The anode bodies of tantalum, aluminum and most recently niobium capacitors are made of highly porous metals. These bodies are obtained by sintering fine metal powders or by electrochemical etching of thin foils. A thin dielectric layer is then grown electrochemically on the metal surface. Due to the porous structure of the anode bodies, the cathode polymer must be able to penetrate into the pores and coat all internal surfaces in order to utilize the full potential capacitance of the anode. 

Polyaniline forms stable, coherent films when grown electrochemically. Scanning electron microscopy pictures reveal that the film surfaces are fairly smooth [148]. After exposure and handling in air, the films maintain their electrochemical properties and can drive redox reactions such as the oxidation of ferrocene to ferricinium. When the pH of a solution containing a polyaniline electrode is increased above 3, a marked decrease in the electroactivity of the polymer occurs. 

Scanning electron microscopy indicates that thin films of polythiophene (100-200 A), which are grown electrochemically and then peeled off the electrode, have smooth homogeneous surfaces [277]. Defects and contours develop when the film thickness is increased to 0.5-11 /u,m. Powdery deposits were observed at several micrometers. Transmission electron microscopy indicates that undoped polythiophenes exhibit a fibrillar morphology. When undoped, the randomly oriented fibrils are approximately 250 A in diameter when doped to 25%, the fibrils swell to approximately 800 A. The doping process appears to be inhomogeneous as some undoped fibrils remain. 

Polyparaphenylenevinylene films have been grown electrochemically by the reductive coupling of a,a,a, a -tetrabromoxylene [39]. Deposition could be accelerated by the addition of chromium or molybdenum carbonyl complexes. The material produced appeared to be contaminated with both residual chlorine and elec-... 

The transport of water through supported polyaniline membranes grown electrochemically was investigated by Schmidt et al. [71]. A 25-30% increase in water permeation was observed for sulfuric acid-doped polyaniline compared to the undoped polymer. The doped polymer was believed to have a more open structure, accounting for the enhanced water permeation. However, a simpler explanation is the increased hydrophil-icity of doped polyaniline. Analogous increases in methanol transport through both polyaniline and polypyrrole were found [72,73]. 

Fig. 37.5 (A) Low and (B) high magnification scanning electron micrographs of a polypyrrole film that was grown electrochemically onto the surface of a 100 pm wide YBa2Cu307- microbridge supported on a single-crystal MgO(lOO) substrate. (Adapted from Ref. 11.)...

Molten Salts. ZnSe has been grown electrochemically on silicon and germanium substrates by Yamamoto and Yamaguchi (64). Two melts were studied 1) 0.02 zinc chloride, 0.02 selenium chloride, 0.12 potassium chloride and 0.18 lithium chloride (in mole ratio), and 2) 0.003 zinc oxide, 0.003 sodium selenite, 0.4 potassium chloride and 0.5 lithium chloride. In both cases, the potassium and lithium chlorides were the solvent. The melts were operated under dry argon between 430 and 550°C. 

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