Introduction to Molecular Self-Assembly

Dr. Shashi, Jasty

Material Matters 2006, 1.2, 3.

Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. Self-assembly can occur spontaneously in nature, for example, in cells such as the self-assembly of the lipid bilayer membrane. It usually results in an increase in internal organization of the system. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology.

In self-assembly, the final (desired) structure is ‘encoded’ in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter. Self-assembly is thus referred to as a ‘bottom-up’ manufacturing technique, as compared to lithography being a ‘top-down’ technique.

On a molecular scale, the accurate and controlled application of intermolecular forces can lead to new and previously unachievable nanostructures. This is why molecular self-assembly (MSA) is a highly topical and promising field of research in nanotechnology today. With many complex examples all around us in nature (ourselves included), MSA is a widely observed phenomenon that has yet to be fully understood. Biomolecular assemblies are sophisticated and often hard to isolate, making systematic and progressive analyses of their fundamental science very difficult. What in fact are needed are simpler MSAs, the constituent molecules of which can be readily synthesized by chemists. These molecules would self-assemble into simpler constructs that can be easily assessed with current experimental techniques.

Of the diverse approaches possible for Molecular Self-Assembly, two strategies have received significant research attention – Electrostatic Self-Assembly (or layer- by-layer assembly) and “Self-Assembled Monolayers (SAMs). Electrostatic self-assembly involves the alternate adsorption of anionic and cationic electrolytes onto a suitable substrate. Typically, only one of these is the active layer while the other enables the composite multilayered film to be bound by electrostatic attraction. The latter strategy of Self Assembled Monolayers or SAMs based on constituent molecules, such as thiols and silanes, is the theme for this second issue of Material Matters^TM. For SAMs, synthetic chemistry is used only to construct the basic building blocks (that is, the constituent molecules), and weaker intermolecular bonds such as Van der Waals bonds are involved in arranging and binding the blocks together into a structure. This weak bonding makes solution, and hence reversible, processing of SAMs (and in general, MSAs) possible. Thus, solution processing and manufacturing of SAMs offer the enviable goal of mass production with the possibility of error correction at any stage of assembly. It is well recognized that this method could prove to be the most cost-effective way for the semiconductor electronics industry to produce functional nanodevices such as nanowires, nanotransistors, and nanosensors in large numbers.