Molecular self-assembly is a strategy for nanofabrication that involves designing molecules and supramolecular entities so that shape-complementarity causes them to aggregate into desired structures. Self-assembly has a number of advantages as a strategy: First, it carries out many of the most difficult steps in nanofabrication--those involving atomic-level modification of structure--using the very highly developed techniques of synthetic chemistry. Second, it draws from the enormous wealth of examples in biology for inspiration: self-assembly is one of the most important strategies used in biology for the development of complex, functional structures. Third, it can incorporate biological structures directly as components in the final systems. Fourth, because it requires that the target structures be the thermdynamically most stable ones open to the system, it tends to produce structures that are relatively defect-free and self-healing.
Self-assembly also poses a number of substantial intellectual challenges. The brief summary of these challenges is that we do not yet know how to do it, and cannot even mimic those processes known to occur in biological systems at other than quite elementary levels. Although there are countless examples of self-assembly all around us--from molecular crystals to mammals--the basic rules that govern these assemblies are not understood in useful detail, and self-assembling processes cannot, in general, be designed and carried out "to order". Many of the ideas that are crucial to the development of this area--"molecular shape", the interplay between enthalpy and entropy, the nature of non-covalent forces that connect the particles in self-assembled molecular aggregates--are simply not yet under the control of investigators.
In addition, there are issues of function in self-assembled aggregates that need solution. The most promising avenues for self-assembly are presently those based on organic compounds, and organic compounds, as a group (although with exceptions), are electrical insulators; thus, many ideas for information processing and electrical/mechanical transduction will require either fundamental redesign in going from the macroscopic systems presently used to self-assembled systems, or the development of new types of organic molecules that show appropriate properties.
This talk will outline some of these issues, and illustrate one of the approaches to self-assembled structures that has been particularly successful: that is, self-assembly on surfaces. There are now a range of different molecular systems that self-assemble--that is, form ordered, monomolecular structures--by the coordination of molecules to surfaces. These systems--self-assembled monolayers (SAMs)--are reasonably well understood, and increasingly useful technologically. The crucial dimension in SAMs is the thickness perpendicular to the plane of the monolayer: this dimension, and the componsition along this axis, can be controlled very simply at the scale of 0.1 nm by controlling the structures of the molecules making up the monolayer. SAMs also provide tailorable functions: for example, by changing the structures of the organic molecules in straightforward ways, interfacial free energies can be controlled. Complementary techniques such as microcontact printing (uCP) allow the in-plane dimensions of structures in SAMs to be controlled easily at the scale of 500 nm, and with difficulty at smaller dimensions.
SAMs represent one type of structure that is using molecular self-assembly to build structure and function on the nanometer scale. They are not a general solution to the problem of building functional nanostructures, but the lessons learned from them will be valuable in building more versatile systems. They also are among the first of the self-assembled systems to move into technology, and there are lessons that can be learned from them about technology transfer and nanotechnology.