MATERIALS SCIENCE: Simply Hexagonal Stacks

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Science  20 Apr 2007:
Vol. 316, Issue 5823, pp. 343b
DOI: 10.1126/science.316.5823.343b

When nanoparticles pack into colloidal crystals, they usually adopt a hexagonally close-packed (hcp) structure in which two layers stack in an ABAB sequence, even though hard-sphere models favor a face-centered cubic (fcc) structure in which three layers stack ABCABC. Talapin et al. found that when they grew superlattices on carbon substrates of more polar, nearly spherical, semiconducting PbS, PbSe, or γ-Fe2O3 nanoparticles that were also somewhat larger (>7 nm in diameter), the layers did not stack in threefold hollow sites but rather directly on top of the underlying nanoparticle in a simple hexagonal (sh) lattice. Transmission electron microscopy images were analyzed to show that the stacking was not simply a projection plane of an fcc lattice. The authors calculated total electrostatic and dispersive energies for different lattices and attribute the stability of the sh superlattices to interactions of nonlocal dipoles between individual nanoparticles and with the underlying substrate, favoring the more open sh packing arrangement. — PDS

Nano Lett. 7, 10.1021/nl070058c (2007).

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