Semiconductor colloids: individual nanocrystals, opals and porous silicon

High quality colloidal semiconductor nanocrystals can now be synthesized. Precise physical and mechanistic experiments, and the first steps in controlled self-assembly into opal-like supercrystals, have been carried out. Physical size regimes for the evolution of bulk properties are understood in outline form. Electrical transport in porous Si, an aerogel-like system consisting of partially fused Si nanocrystals, can be understood using electrochemical concepts. Analogies are being drawn between porous silicon and a liquid junction porous TiO2 thin film solar cell.