Sequential Nucleation and Growth of Complex Nanostructured Films

Nanostructured films and coatings with controlled surface area, porosity, crystalline orientation, grain sizes, and crystal morphologies are desirable for many applications, including microelectronic devices, chemical and biological sensing and diagnosis, energy conversion and storage (photovoltaic cells, batteries and capacitors, and hydrogen-storage devices), lightemitting displays, catalysis, drug delivery, separation, and optical storage. Meeting the demands of these potential applications, however, will require reliable and economic processes for the production of a large supply of high-quality nanomaterials. Gas-phase reactions have been extensively used to prepare oriented nanostructures including carbon nanotubes, ZnO nanowires, and many other oxide and non-oxide semiconductor materials, but these methods typically require high temperatures (∼ 500–1100 °C) and vacuum conditions, which limit the choice of substrate and the economic viability of high-volume production. These limitations have stimulated research on solution-phase synthesis (sometimes referred to as the soft solution route or chemical bath deposition), which offers the potential for low-cost, industrial-scale manufacturing. Low-temperature (typically < 100 °C), aqueous-phase approaches are particularly attractive because of their low energy requirements, and safe and environmentally benign processing conditions. In aqueous-phase synthesis, oriented nanocrystalline films are deposited on a substrate in aqueous media by heterogeneous nucleation and subsequent growth. The resultant film structure is controlled by a complicated set of coupled processes in both the solution and solid phases. Heterogeneous nuclea-