Localization of electrode reactions by a focused laser beam

Abstract

The localization of electrode reactions in a focused laser beam is compared to other localization techniques such as photolithography or the application of catalysis and inhibition. The geometric resolution of the reactions is given by primary effects of the laser spot and secondary effects of the system (light scattering, thermal and electronic conductivity). A combination of several localization methods is favourable. Tests with natural (Ti) and artificially simulated (Ti/TiN) grain boundaries show a resolution of some µm close to the theoretical resolution, as long as low power densities are used. Some progress has been achieved by full coordination of observation with localized laser radiation. Most of the localized reactions studied are based on photoelectrochemical processes at thin n-type semiconducting films, but localized photochemical copper deposition onto quartz is also presented. Quantitative investigations are carried out with laser-induced Ag-deposition onto anodic TiO₂ layers as an example. Experiments with point arrays as well as velodynamic experiments prove that optically visible spots can be obtained after a minimum consumption of some 10¹⁷ e cm^–2. With increasing power density the reaction becomes delocalized by secondary effects. Depending on the illumination time, i.e. the number of photons absorbed, competition between laser-induced oxide growth and metal deposition was observed on Nb₂O₅ layers. Substrate influences are demonstrated by various examples. Long-range effects are caused by charge-carrier migration. Furthermore, crystallization is induced far away from the laser spot.