Self-assembly is scientifically interesting as it occurs inside the most basic form of life – the cell. The cell contains an astonishing range of complex structures such as lipid membranes, folded proteins, structured nucleic acids, protein aggregates, molecular machines, and many others that form by self-assembly. At the same time, self-assembly is technologically important as it provides routes to a range of materials with regular structures: molecular crystals, liquid crystals, and semicrystalline and phase-separated polymers are examples.Self-assembly is a process in which components, either separate or linked, spontaneously form ordered aggregates. Self-assembly can occur with building blocks from molecular to the macroscopic scales, provided that appropriate conditions are met. For customized materials on macroscopic length scales to be made, self-assembly is essential because the number of building blocks that would have to be precisely placed in the absence of a self-directed method are too large and the process is prohibitively time consuming. A Fermi approximation of the time required for building a 1cm cube using 10 nm building blocks with the help of optical tweezers would be in the order of a hundred million days.Self-Assembly is an alternative technique by which materials can be formed. The basic idea behind using self-assembly as a manufacturing technique is to develop building blocks that have sufficient information content inherent in them, such that a desired structure can form spontaneously from a collection of building blocks in a thermal bath.Amongst interesting building blocks that have been used to self-assemble custom materials are colloids. These mixtures of immiscible materials form fine suspensions that have potential photonic applications because the particles that make up the stabilized phase can be tuned to have diameters on the same scale as visible light (500nm) and can form periodic structures. Polystyrene colloidal crystals, consisting of charged monodisperse polystyrene microspheres suspended in water, organize into a face-centered-cubic (fcc) lattices with lattice spacings comparable to the wavelength of light (Clark, N.A. et al, Nature 1979, 281, 57-60). Such colloidal structures may also find applications in nanoscale electronics, miniature diagnostic systems and hierarchically structured catalysts amongst others. However, for the assembly of more complex structures, it has been recognized that anisotropic shape and specific interactions (or “patchiness”) are useful (Glotzer S.C., & Solomon M.J. Nature materials 2007, 6(8), 557-62).
A recent work by Chen et al., Nature (2011), 469 (7330), 381-384, has shown that triblock Janus particles can self-assemble into a Kagome lattice (see Figure 1). A triblock Janus particle has two hydrophobic poles of tunable area and an electrically charged middle band. There are two competing forces that drive self-assembly for suspensions of these particles: the hydrophobic poles agglomerate in aqueous solvents, while the charged middle bands repel. The balance between these forces can be tuned by screening the charged repulsion via the addition of an ionic compound like NaCl, and the particles can stabilize in a kagome lattice. Furthermore, sheets of kagome lattices are reported to stack in parallel layers. An interesting video of this self assembly process can be found at: http://www.nature.com/nature/journal/v469/n7330/extref/nature09713-s3.mov Thus self-assembly seems to offer a viable strategy for generating nanostructures. Recent advances in nanoparticle synthesis have led to the availability of various building blocks including spheres,cubes, rods, and tetrapods at both the nano and micro scales. Specific interactions between these building blocks can also be obtained by modifying chemical composition, changing shape, and by adding functional groups. With such a vast parameter space available to experimentalists, there is much opportunity for simulation to aid in the understanding of how such complicated building blocks assemble.
- N. Khalid Ahmed
PhD Pre-Candidate,
Glotzer Group,
Department of Chemical Engineering,
University of Michigan, Ann Arbor – 48109
nkahm@umich.edu
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