Published in

American Chemical Society, Nano Letters, 2(14), p. 1056-1063, 2014

DOI: 10.1021/nl404732m

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Self-Assembly of Octapod-Shaped Colloidal Nanocrystals into a Hexagonal Ballerina Network Embedded in a Thin Polymer Film

This paper is available in a repository.
This paper is available in a repository.

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Abstract

Nanoparticles with unconventional shapes may exhibit different types of assembly architectures that depend critically on the environmental conditions under which they are formed. Here, we demonstrate how the presence of polymer (polymethyl methacrylate, PMMA) molecules in a solution in which CdSe(core)/CdS (pods) octapods are initially dispersed affects the octapod-polymer organization upon solvent evaporation. We show that a fast drop-drying process can induce a remarkable two-dimensional (2D) self-assembly of octapods at the polymer/air interface. In the resulting structure, each octapod is oriented like a "ballerina", i.e., only one pod sticks out of the polymer film and is perpendicular to the polymer-air interface, while the opposite pod (with respect to the octapod's center) is fully immersed in the film and points towards the substrate. In some areas, a hexagonal-like pattern is formed by the "ballerinas", in which the six non-vertical pods, which are all embedded in the film, maintain a pod-pod parallel configuration with respect to neighboring particles. We hypothesize that the mechanism responsible for such a self-assembly is based on a fast adsorption of the octapods from bulk solution to the droplet/air interface during the early stages of solvent evaporation. At this interface, the octapods maintain enough rotational freedom to organize mutually in a pod-pod parallel configuration between neighboring octapods. As the solvent evaporates, the octapods form a ballerina-rich octapod-polymer composite, in which the octapods are in close contact with the substrate. Finally, we found that the resulting octapod-polymer composite is less hydrophilic than the polymer-only film.