Organizing nanoparticles with different functionalities into multicomponent assemblies is one exciting way to engineer complex materials with novel properties. Conventional understanding of this relies on the approximation that hard spheres assemble. Such theories predict that only a few binary phases (NaCl-, AlB₂- and NaZn₁₃-type) are stable. In general, the formation of binary superlattices is expected if its packing density exceeds the packing density of single-component crystals in ccp (close-packed cubic packing) or hcp (hexagonal packed pacing) structure, which is 0.7405. However, in the collaboration between IBM and the Columbia University MRSEC, we found that nanoparticle superstructures with AB₁₃ stoichiometry can have both dense NaZn₁₃-type and more open polymorphous forms. In both forms the larger particles form a cubic framework. However, 12 small particles form icosahedrons and cuboctahedrons around the thirteenth central small particle in NaZn₁₃-type and its polymorphous form (cub-AB₁₃), respectively. The packing density of the cub-AB₁₃ superlattice (0.7) is too low to provide stability if the assembly of nanoparticles actually followed the rules of hard sphere packing. This observation reveals the shortcomings of structural predictions based on simple space filling models. Both forms of the AB₁₃ superlattice can be formed reproducibly from PbSe ? Pd and Fe₂O₃ ? PbSe colloidal binary mixtures. Such systems demonstrate the potential ordered periodic structures with lower packing density and provide the first example of binary semiconductor ? metal superlattices.

Posted on May 11, 2005.

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(This work is appearing in the Journal of the American Chemical Society)