To grow a crystal, of a specific molecule or an extended inorganic solid, the rate limiting step is nucleation, which requires supersaturation for a nuclei to achieve critical size. In a fluid phase, diffusion is fast relative to nucleation, so the average composition is the parameter used to achieve supersaturation. It is possible for a sequence of different crystals may form as the concentration of species in solution evolves with time. In solid state reactions, however, diffusion is much slower than in a fluid and the reacting system is typically heterogeneous. Nucleation typically occurs at the surface of reacting particles. Since all compositions are present at the reacting interfaces, the easiest compound to nucleate forms first and the overall composition does not control what nucleates. We have shown that, by controlling the composition of an amorphous intermediate on a nanoscale, it is possible to control nucleation to obtain new metastable binary and ternary compounds. By introducing more than one diffusion distance we can self-assembly new nanostructured compounds consisting of two or more compounds with different crystal structures that are precisely interleaved on the nanoscale without requiring an epitaxial interface between them. By controlling the nanoarchitecture of a deposited precursor, it is possible to synthesize new metastable compounds with designed nanostructure, including structural isomers. These materials have unprecedented physical properties, including the lowest thermal conductivities ever reported for a fully dense solid, systematic structural changes dependent on nanostructure, and charge density wave transitions. The designed precursors enable diffusion to be followed and quantified over distances of less than a nanometer, providing insights to the mechanism that gives control of the nanoarchitecture of the final product. This ability creates a new "thin film metallurgy" in which nanoarchitecture and composition can be used to tailor physical properties, interfacial structures can be determined for precisely defined constituent thicknesses, and interfacial phenomena and modulation doping can be systematically exploited.

Controlling kinetics to make metastable compounds with targeted structures

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