R. Peat

Photoelectrochemical imaging—part I. Background and theory

Abstract Imaging of spatial variability of electrode processes using signals stimulated by a focused light spot is described. The signals include photovoltage and photocurrent, photoacoustic and photothermal effects, as well as the reflected light intensity. The theory of contrast in the photocurrent image is presented: methods utilizing intensity modulated light are compared with those in which the spot is scanned rapidly without intensity modulation. Effects of variation of intensity modulation frequency and spot scan speed are calculated; blurring, streaking and shadowing effects are explained. For systems which are not photoactive, the feasibility of an imaging procedure based upon the small thermal effect induced by the focused spot on the current for an electrode process is assessed. Methods of imaging utilizing potential variations at constant (usually zero) current are compared with those involving the measurement of current variations at constant potential.

Dissolution and Reaction of Sulfide Inclusions in Stainless Steel Imaged Using Scanning Laser Photoelectrochemical Microscopy

The in situ scanning laser photoelectrochemical microscopy technique has been used to image heterogeneity of the surface chemistry around sulfide inclusions in stainless steel under anodic polarization. The deposition around inclusions of a ring of material deduced to be sulfur is demonstrated. Evidence is presented that this ring of material has its origin in the decomposition of thiosulfate produced by the anodic dissolution of the inclusions. The evidence supports models of the initiation of pitting corrosion which attribute the effect of sulfide inclusions to the acceleration of anodic dissolution kinetics caused by adsorbed sulfur

A Primitive Model for Evaluating the Contrast of Photocurrent Images Obtained at the Semiconductor‐Electrolyte Interface

A model is developed for interpreting the contrast observed in photocurrent images obtained using scanned illumination from a laser source. It combines current theories of carrier generation, surface-state recombination, and electron transfer processes with a primitive site-wise scanning routine to assess the effects of potential, scan rate, beam size, and incident intensity on the form of the image. The results of the model are discussed with reference to scanned photocurrent images obtained with a transparent electrolyte phase contacting n-Si, Bi, and Fe phases

Photoelectrochemical imaging—part II. The passivating oxide film on iron

Abstract Photocurrent imaging of passive iron, both of pure metal and of an impure material in which grain boundary segregation was suspected, showed great heterogeneity with NaOH electrolyte, much less heterogeneity with Na 2 SO 4 electrolyte. Effects of light intensity and electrode potential were spatially variable: the heterogeneity was such as to counsel caution in the interpretation of results based upon measurements averaged over the entire surface. It is speculated that the passive layer formed in Na 2 SO 4 has a duplex morphology, with a uniform overlayer dominating the photoresponse, whereas in NaOH the film comprises a single barrier layer nucleated directly onto the metal.

Photoelectrochemical imaging of sub-monolayer lead deposits on n-GaAs

Abstract A scanning laser microscope has been used to obtain a spatially resolved map of the photocurrent distribution around metal particles at the n-GaAs/electrolyte interface. A minority carrier capture zone that is potential dependent is observed around the metal particles. Mechanisms of image contrast are discussed in terms of particle size, scattering phenomena, screening effects and carrier recombination effects.

Real-time imaging of photoinduced surface reactions using a scanning laser microscope

A novel adaptation of a scanning laser microscope has been developed which produces real-time images representing the non-uniformity of photoelectrochemical reactions at interfaces. The method exploits the photosensitivity of the semiconductor/electrolyte junction to map bulk and surface features such as dislocations, preparation damage, non-uniformity in minority carrier diffusion length, surface-state density, compositional non-uniformity and variations in the thickness of surface layers. Materials that have been examined by the new method include semiconductors, such as silicon and gallium arsenide, and metals covered with thin semiconducting layers, such as corrosion films.

Photocurrent imaging of passive metals

The theory of photoelectrochemical imaging using a scanning laser microscope is presented in outline: the idea of a photocurrent transfer function whose parameters might be spatially variable is introduced and a distinction is made between continuous-scan and step-scan methods of rastering the light spot. In the continuous-scan method interesting blurring, streaking and shadowing effects highlight changes over the surface of different parameters of the photocurrent transfer function. Images of passive copper, bismuth, iron and stainless steel show effects of varying thickness of the passive film and charge carrier recombination rate, effects localised at grain boundaries and around inclusions in the metal and effects showing localised corrosion, inhibition and inhibition breakdown.

Photocurrent distribution across the interfacial region of the n-GaAs/electrolyte junction

A scanning laser microscope has been used to study the photoelectrochemical etching of n-GaAs at the electrolyte interface. Results are presented that show directly, in a qualitative way, the distribution of interfacial charge transfer in the vicinity of microscopic surface defects and the effect of potential and electrolyte composition on this distribution.

Photoelectrochemistry of silicon-delta-doped GaAs structures

Delta doping with silicon has been used to form a contact of metallic conductivity buried 1 μm below the surface of a GaAs MBE-grown structure. The electrochemistry of the high-purity GaAs above the contact could then be investigated. The interfaces in the structure were located by photoelectrochemical etching; the spatial distribution of defects and also some information concerning their origin within the grown layers was obtained by photoelectrochemical imaging.

Scanning Laser Photocurrent Spectroscopy Of Electrochemically Grown Bismuth Sulphide Films

The photoelectrochemical properties of semiconducting bismuth sulphide (Bi2S3) films (in contact with an aqueous electrolyte) grown on bismuth have been investigated. Using an Optically Beam Induced Contrast (OBIC) technique, it has been possible to image these films, and to identify local variations corresponding to grain boundaries and other recombination centres. It is seen that the form of these regions varies with the thickness of the film. Both object scanning and beam scanning have been investigated for generating images and these two methods are compared. Samples have also been investigated by Intensity Modulated Photocurrent Spectroscopy (IMPS). This method involves sinusoidal intensity modulation of the incident laser radiation at different frequencies, and the analysis of the resulting photocurrent response. Our extension of this method has been to map the IMPS response over the specimen surface in order to obtain specific information on local properties.