Photocurrent Imaging Using a
Scanning Laser Microscope

 

In this work the direct photocurrent imaging technique using a gaussian scanning laser spot has been used. This allows light-induced currents (induced by thermal or photo effects) to be recorded as pictures where the different colours or shades of grey are used to represent the size of the light-induced current (photocurrent). This technique has mainly been used to study photoelectrochemical reactions (at a solid-liquid interface) although some work looking at solid state physics (at a solid-solid interface) has also been carried out. The work involved therefore is very wide including elements of Physics and Materials Science as well as Chemistry.

 

The Principle Instrumentation Used

The instrument employed is a confocal scanning laser microscope (Bio-Rad MRC 600, containing a 25 mW argon ion laser: 10 mW is at 488 nm, 10 mW at l = 512 nm, and 5 mW is at other similar wavelengths) which had been adapted to record photocurrent images, as well as confocal optical images either separately or simultaneously. The difference being that the photocurrent images show the function at electroactive areas, where as the optical images (which depict the surface reflectivity) just show the topography.

The Photocurrent Imaging Apparatus

 

Subsidiary Techniques Used

In addition, to explore further the element composition of the areas showing interesting features or changes on the photocurrent image an electron probe x-ray microanalyser (Jeol Super Probe JXA-8600, which employs a wavelength dispersive spectrometer) has been used to record microprobe element concentration maps as well as SEM images. The SEM was primarily used to select the region of interest for microprobe analysis. This could easily be compared with the confocal optical image which in turn could be compared with the photocurrent image. As well as confocal optical imaging carried out with the scanning laser microscope, ordinary optical images are also recorded using a regular light microscope (model-Jenalab, manufacturer- Carl Zeiss) attached to a colour video camera (JVC, CCD colour video camera, TK-1085E) and this in turn to a colour video printer (Sony, UP-2200P). This was used as in some cases ordinary light microscopy may be required to see the true different colours of the different areas of the sample as the confocal optical images recorded have a false grey scale or colour scale related to the reflectivity of the sample as opposed to a true colour image.

 

Research Areas

  1. Effect of Light Intensity on Semiconductors
    A study of the effect of laser intensity on; a Si-memory chip, as well as a p-type Si electrode in HCl showed that the features seen on the photocurrent image changed for both systems as the light intensity was increased. This is due to the fact that the penetration depth of the laser into the sample increases with increasing light intensity (Beer-Lambert Law) resulting in features from below the surface of the sample contributing to the photocurrent image. In comparison however the features on the optical image remained the same at each light intensity (as only the surface topography is viewed at each light intensity.

  2. Laser-Induced Thermal Effects
    It was found the image contrast developed as a consequence of the temperature induced transient produced by a rapidly scanned focussed laser spot

  3. The Corrosion of Stainless-Steels
    This technique has also been used to investigate, 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. Microprobe analysis showed that it was crucial for these inclusions to be enriched in Manganese and Sulphur but not Oxygen in a matrix composed generously of the bulk components in stainless steel (Fe, Cr, Ni).

  4. Imaging Extreemly Small Platinum Catalysts
    This technique has also been successfully used to electrochemically characterise extremely small platinum catalyst particles (as used in fuel cells) supported on a glassy carbon surface.

 

References