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 [1] have been extensively used to prepare oriented nanostructures including carbon nanotubes, [2,3] ZnO nanowires, [4,5] and many other oxide and non-oxide semiconductor materials, [6,7] 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-
335Nanostructured films with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy/chemical conversions. Low-temperature, aqueous chemical routes have been widely investigated for the synthesis of continuous films, and arrays of oriented nanorods and nanotubes. More recently, aqueous-phase routes have been used to produce films composed of more complex crystal structures. In this paper, we discuss recent progress in the synthesis of complex nanostructures through sequential nucleation and growth processes. We first review the use of multistage, seeded-growth methods to synthesize a wide range of nanostructures, including oriented nanowires, nanotubes, and nanoneedles, as well as laminated films, columns, and multilayer heterostructures. We then describe more recent work on the application of sequential nucleation and growth to the systematic assembly of large arrays of hierarchical, complex, oriented, and ordered crystal architectures. The multistage aqueous chemical route is shown to be applicable to several technologically important materials, and therefore may play a key role in advancing complex nanomaterials into applications.-[*] Dr.