Recently we discovered that under certain conditions new crystal growth (branch) can be induced on specific crystalline planes of the same material. This is a new phenomenon and is in sharp contrast to typical nucleation and growth in which a crystal will simply grow larger in preferred directions depending on the surface energy of the specific crystalline planes. Based on our observation, we developed a sequential nucleation and growth technique offering the power to assemble complex hierarchical crystals step-by-step. However, the key questions of when and how the secondary nucleation takes place have not been answered. Here we systematically study secondary ZnO crystal growth using organic diamine additives with a range of chain lengths and concentration. We found that ZnO branches form for a narrow diamine concentration range with a critical lower and upper critical nucleation concentration limit, which increases by about a factor of 5 for each additional carbon in the diaminoalkane chain. Our results suggest that the narrow window for secondary growth is dictated by the solubility of the ZnO crystals, where the low critical nucleation concentration is determined by slight etching of the surface to produce new nucleation sites, and the upper critical concentration is determined by the supersaturation concentration. Kinetic measurements show that the induction time and growth rate increase with increasing diamine concentration and follow classical nucleation and growth theory. Observations of branch morphological evolution reveal the mechanisms guiding the tunable crystal size and morphology.

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.

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The crystal orientation and piezoelectric properties of solution grown ZnO nanorods on Ag films were measured by quantitative piezoelectric force microscopy (PFM). The polarity of the rods, important for many device applications, was determined to be oriented [0001] from the substrates. This indicates that the prevalence of the [0001] oriented crystals is dominated by the fastest growing direction in solution. The average value of the d33 piezoelectric coefficient was measured to be 4.41pm∕V, with a standard deviation of 1.73pm∕V among the 198 individual rods. For calibration and comparison, PFM measurements were also performed on single crystals of x-cut quartz, z-cut periodically poled and single domain LiNbO3, and z-cut ZnO. Repeated measurements on individual rods establish that the run-to-run variation of a single rod is similar to that of single crystal measurements on quartz and LiNbO3. Hence, the observed rod-to-rod variation is not due to measurement uncertainty. Potential origins of this rod-to-rod difference will be discussed.