| This thesis explores the direct photocurrent imaging technique using a gaussian scanning laser spot. This includes advancements to the technique, its uses and, a study of effects produced by surface heating by the laser. A confocal scanning laser microscope (Bio-Rad MRC 600) which had been adapted to record photocurrent images, as well as confocal optical images was used. The photocurrent image showed the function at electroactive areas, where as the optical image just showed the topography. In addition, to explore further the features seen on the photocurrent image an electron probe microanalyser was used to record microprobe element concentration maps and SEM images. Basics of this technique and the resolution possible were shown by taking photocurrent images at different laser intensities for a Si-memory chip and a p-type Si electrode in HCl. Features on the photocurrent image changed in both cases with increasing light intensity, with which the penetration depth of the laser into the sample increases. The effects of electrode geometry and poisoning on image contrast in photoelectrochemical microscopy were studied using gold microband and disc electrodes in Fe(CN)6 4-solution. It was found the image contrast developed as a consequence of the temperature induced transient produced by a rapidly scanned focussed laser spot, the theory for which is given here. Finally as a detailed example of the application this technique, the processes leading to the initiation of the corrosion of stainless steel in dilute Cr were studied. Changes in the photocurrent image, correlating to the initiation of pitting corrosion, were found to occur at certain inclusions. |
