Course Details of the 

PhD Degree in 'Photocurrent Imaging Using a Scanning Laser Microscope'

by Tasneem Fatima Mohiuddin 

at University College London 

(Award date 28th February 1999)

PhD undertaken whilst registered as a full time student from October 1993 - Sept 1998 
[some additional research was also carried out, after the PhD, as a Postdoctoral Research Fellow]. 

 

Main technique photocurrent imaging using a confocal scanning laser microscope (Bio-Rad MRC 600)  Subsidiary techniques confocal optical imaging, light microscope video imaging cyclic voltammetry electron probe x-ray microanalysis / imaging, scanning electron microscopy Areas of Study Physical Chemistry: microscopy, imaging, scanning laser spot technique, photocurrents, laser induced thermal effects, surfaces, solid-solution interfaces, solid-solid interfaces, solid states, photoelectrochemistry, electrochemistry, corrosion, [fuel cell catalysts] stainless-steels, gold, semiconductors, [extremely small Pt catalyst particles on Glassy Carbon]  Abstract This work explored the direct photocurrent imaging technique using a gaussian scanning laser spot. This included 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.  Examples of the deployment of the Photocurrent Imaging Technique: 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)64 - 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. As a detailed example of the application this technique, the processes leading to the initiation of the corrosion of stainless steel, in dilute Cl -, were studied. Changes in the photocurrent image, correlating to the initiation of pitting corrosion, were found to occur at certain inclusions. [Since the completion of the PhD work this technique has also been successfully used to electrochemically characterise extremely small platinum catalyst particles on a glassy carbon surface.]  Supervisor: Prof. David E Williams, of University College London  Internal Examiner: Dr Anthony Kurcernak of Imperial College London External Examiner: Prof. David Schiffrin, from the University of Liverpool