(543c) Self-Assembly of Directionally Interacting Spheres and Rods

There has been considerable recent interest in understanding the self-assembly of mixtures of differently shaped objects. Non-directional entropic and energetic effects are known in the specific case of rods mixed with spheres to exclusively yield layered phases. [1,2] However, here we use computer simulations and theory to demonstrate that the introduction of directional energetic attractions, between pairs of rod ends and between rod ends and isotropically interacting spherical nanoparticles (NPs), can be used to intelligently stabilize specific crystal morphologies through the interplay of energetic and entropic effects. To illustrate, we rationally design rods and spheres to obtain either face-centered cubic (FCC) or hexagonal close-packed (HCP) NP crystals at will. By tuning the relative size of the rods and spheres we control the interaction between non-nearest neighbor NPs in these crystals allowing us to control their relative stability with respect to each other, and with respect to an amorphous phase. Experiments with mixtures of gold spheres isotropically decorated with single stranded DNA (ssDNA) and rods with complementary ssDNA ends, unequivocally verify this behavior. We therefore propose that directionally specific attractions, which reflect only some features of the building block anisotropies, offer an entrée into completely different classes of self-assembly behavior.

References

[1] Adams, Dogic, Keller, and Fraden, Nature 393, 349-352 (1998).
[2] Dogic, Frenkel, and Fraden, Phys. Rev. E. 62, 3925-3933 (2000).