title: Tolerance to structural disorder and tunable mechanical behavior in self-assembled superlattices of polymer-grafted nanocrystals.
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.
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA 94720.
authors.forename: X Wendy
authors.identifier: 0000-0001-5477-7040
authors.initials: XW
authors.lastname: Gu
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA 94720.
authors.forename: Xingchen
authors.initials: X
authors.lastname: Ye
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720.
authors.forename: David M
authors.initials: DM
authors.lastname: Koshy
authors.affiliation: Hysitron Inc., Minneapolis, MN 55344.
authors.forename: Shraddha
authors.initials: S
authors.lastname: Vachhani
authors.affiliation: Department of Nuclear Engineering, University of California, Berkeley, CA 94720.
authors.forename: Peter
authors.initials: P
authors.lastname: Hosemann
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA 94720; paul.alivisatos@berkeley.edu.|Department of Materials Science and Engineering, University of California, Berkeley, CA 94720.|Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.|Kavli Energy NanoScience Institute, University of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors.forename: A Paul
authors.initials: AP
authors.lastname: Alivisatos
keywords: buckling; elasticity; nanocomposite; nanoindentation; thin film