Synthesis and Photochemistry of Quantum‐Size Semiconductor Particles in Solution and in Modified Layers

We describe the preparation of highly monodisperse, quantum‐size CdS particles. In these samples a band splitting into discrete exciton‐like states is observed. A mechanism for the excitonic fluorescence is presented involving shallow traps as a reservoir for electrons which finally recombine with free positive holes across the bandgap. Quantum‐size CdS and PbS particles prepared in situ on highly porous, polycrystalline TiO2 electrodes work as highly efficient sensitizers. Photocurrent yields of more than 70% are reported. In the case of PbS, it is shown that an efficient electron transfer is possible only with particles in the size of the quantization regime.