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Brian W. Goodfellow1, Michael R. Rasch1, Detlef-M. Smilgies2, Brian A. Korgel1

1Dept. of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, TX
2Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY

 

Abstract:
Dense collections of hard sphere particles order into close-packed face-centered cubic (fcc) lattices to maximize free volume entropy. Sterically-stabilized nanocrystals have relatively short-range repulsive interaction potentials and also tend to order into fcc superlattices. However, nanocrystal superlattices with non-close-packed body-centered cubic (bcc) structure are also relatively common. As examples, we have observed bcc superlattices of 1.8 nm dodecanethiol-capped Au nanocrystals, 3.7 nm oleic acid-capped PbS nanocrystals, and 7.9 nm oleic acid-capped PbSe nanocrystals. We argue that bcc superlattices can be favored over fcc when entropic ligand packing frustration overcomes the packing entropy of the spheres. This idea is consistent with our observation of a superlattice thickness-dependent change in structure from hexagonally close-packed monolayers to bcc superlattices in nanocrystal films. We also find that {112} twin planes are common to bcc superlattices.

The organic capping ligands are also central to nanocrystal superlattice phase behavior and structural changes with heating. Small angle X-ray scattering (SAXS) revealed that superlattices of organic ligand-stabilized gold (Au) nanocrystals can undergo a complex series of structural phase transitions at elevated temperature. For example, dodecanethiol-capped Au nanocrystal superlattices can undergo transitions from body-centered cubic (bcc) to hexagonal close-packed (hcp) structure, followed by the formation of simple cubic (sc) AB13 and hexagonal (hex) AB5 binary superlattices before decomposing at high temperature to bicontinuous domains of Au and hydrocarbon. Transmission electron microscopy (TEM) revealed that these transformations result from Au nanocrystal growth during heating, which combined with partial desorption of the ligand shell, forces the observed changes in superlattice symmetry. These observations again suggest that ligand packing entropy plays an important role in determining superlattice structure.

 

 

 

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