Microscopic imaging of the efficiency of electron injection from excited sensitizer dye into nanocrystalline ZnO film

We constructed a transient absorption microscope as a new tool for studying heterogeneous photochemical systems. We have used the instrument to study the process of electron injection from an excited dye into a nanocrystalline ZnO film. From microscopic images of ground-state absorption and fluorescence, we found that the distribution of dye molecules on the surface was not homogeneous and that aggregates between dye molecules and Zn2+ ions were formed on the surface. However, the distribution of oxidized dye molecules generated by electron injection was homogeneous, according to the microscopic image of transient absorption. This indicates that dye molecules that are active for electron injection are attached directly to the ZnO surface and that aggregates that are inactive for electron injection are formed on this layer. (C) 2004 Elsevier B.V. All rights reserved.