Abstract
The structures formed by mixtures of dissimilarly shaped nanoscale objects can significantly enhance our ability to produce nanoscale architectures. However, understanding their formation is a complex problem due to the interplay of geometric effects (entropy) and energetic interactions at the nanoscale. Spheres and rods are perhaps the most basic geometrical shapes and serve as convenient models of such dissimilar objects. The ordered phases formed by each of these individual shapes have already been explored, however, when mixed, spheres and rods have demonstrated only limited structural organization to date. Here, we show using experiments and theory that the introduction of directional attractions between rod ends and isotropically interacting spherical nanoparticles (NPs) through DNA base pairing leads to the formation of ordered three-dimensional lattices. The spheres and rods arrange themselves in a complex alternating manner, where the spheres can form either a face-centered cubic (FCC) or hexagonal close-packed (HCP) lattice, or a disordered phase, as observed by in situ X-ray scattering. Increasing NP diameter at fixed rod length yields an initial transition from a disordered phase to the HCP crystal, energetically stabilized by rod-rod attraction across alternating crystal layers, as revealed by theory. In the limit of large NPs, the FCC structure is instead stabilized over the HCP by rod entropy. We, therefore, propose that directionally specific attractions in mixtures of anisotropic and isotropic objects offer insight into unexplored self-assembly behavior of noncomplementary shaped particles.
Highlights
We show here that such a simple modification stabilizes phases not previously observed for mixtures of rods and spheres: close-packed organizations of NPs, either hexagonal close-packed (HCP) or face-centered cubic (FCC) lattices, both accompanied by a complex organization of rods
Our experimental system was constructed from spherical NPs coated with single stranded DNA and rigid DNA bundles capped with ssDNA, the latter serving as rods with customizable end interactions
DNA-based approaches were recently demonstrated as a powerful platform to assemble nanoparticle clusters and lattices.[7, 43,44,45,46]
Summary
Understanding and manipulating the self-assembly of nanoscale objects is important for creating materials that exploit the collective properties of the resulting superstructures, with applications in electronics, optics, and catalysis;[1,2,3] such properties can be dramatically different from those of the building blocks.[4,5,6] Over the past decades, many classes of nanoparticle (NP) superlattices have been assembled using various strategies, e.g., biomolecular recognition, electrostatic forces and entropic effects.[7,8,9,10,11,12,13,14,15,16] Recent studies have shown that incorporation of complex shapes or anisotropic interactions can tremendously enrich the phase behavior of the resulting structures.1721 practical limitations in particle engineering often hinder the translation of those ideas into experimental realization. Even greater organizational complexity might be achieved for mixtures of differently shaped objects.[37] at present, such increased structural diversity is accompanied by phase separation and the formation of disordered states, as has been shown for mixtures of perhaps the simplest shapes: rods and spheres.[38,39,40] Experiments have revealed that rods and spherical particles can assemble into layered structures by maximizing their packing density even in the absence of interparticle interactions While these previous findings were the result of either entropic or non-directional attraction effects, here we reconsider sphere–rod systems by including basic directional attractions, commensurate with a rod geometry, provided by the rod ends. Computational studies, which employed molecular dynamics (MD) simulations with realistic parameters, complemented by a simple mean-field theory, reveal the mechanism of crystal formation and the range of each polymorph’s stability
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