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Proc Natl Acad Sci U S A. 2017 Mar 14;114(11):2836-2841. doi: 10.1073/pnas.1618508114. Epub 2017 Feb 27.

Tolerance to structural disorder and tunable mechanical behavior in self-assembled superlattices of polymer-grafted nanocrystals.

Author information

1
Department of Chemistry, University of California, Berkeley, CA 94720.
2
Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720.
3
Hysitron Inc., Minneapolis, MN 55344.
4
Department of Nuclear Engineering, University of California, Berkeley, CA 94720.
5
Department of Chemistry, University of California, Berkeley, CA 94720; paul.alivisatos@berkeley.edu.
6
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720.
7
Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
8
Kavli Energy NanoScience Institute, University of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

Abstract

Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, even though these nanocrystals are connected only by soft organic ligands (e.g., dodecanethiol or DNA) that are not cross-linked or entangled. Recent developments in the synthesis of polymer-grafted nanocrystals have greatly expanded the library of accessible superlattice architectures, which allows superlattice mechanical behavior to be linked to specific structural features. Here, colloidal self-assembly is used to organize polystyrene-grafted Au nanocrystals at a fluid interface to form ordered solids with sub-10-nm periodic features. Thin-film buckling and nanoindentation are used to evaluate the mechanical behavior of polymer-grafted nanocrystal superlattices while exploring the role of polymer structural conformation, nanocrystal packing, and superlattice dimensions. Superlattices containing 3-20 vol % Au are found to have an elastic modulus of ∼6-19 GPa, and hardness of ∼120-170 MPa. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be critical parameters that govern superlattice elastic and plastic deformation.

KEYWORDS:

buckling; elasticity; nanocomposite; nanoindentation; thin film

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