Unsolved Problems in Nanotechnology : Chemical Processing by Self-Assembly

The many impressive laboratory demonstrations of controllable self-assembly methods generate considerable hope and interest in self-assembly as a manufacturing method for nano-structured products. The practical realization of this hope will require several achievements in chemical engineering, including the precision manufacturing of precursors, expansion of our ideas of molecular chemistry to super-molecular chemistry from a bonding, kinetics and mechanistic point of view and a focus on self-assembly processes that can be scaled up to interesting levels. This leads to a rich array of research and process development issues for chemical engineers. Chemical engineers are playing very significant roles in developing a new science base in phenomena, structures and potential applications where understanding at nanometer length scales is crucial. Synthesis and demonstration of new catalysts, new multi-component polymers and new biomaterials are all fine examples of areas to which chemical engineers are making major contributions to nanoscale science, in particular but not exclusively, via the self-assembly route that is the subject of this paper. Whether this new science base leads to important technological developments depends even more on chemical engineers. The thesis of this paper is that the opportunities for chemical engineers to advance nanotechnology demand an emphasis on developing suitably effective processing technologies. Treating self-assembly as a serious chemical process is rich with chemical engineering opportunities. Self-assembly is a spontaneous process, guided by information content intrinsic to the assembling units, in which multiple levels of structural organization are built into a product (Kuhn and Ulman, 1995). Owing to the complexity of structures formed and the nominal ease of spontaneous organization, self-assembly is increasingly being examined as a practical chemical processing method. It is akin to crystallization except that the complexity of the structural elements involved is greater in self-assembly and length-scales beyond a single lattice constant emerge in the product. In some cases, self-assembly can result in intricate, interlocking, useful architectures of matter. The resultant structures are being actively explored as new materials, surface treatments, catalysts, membranes, photonic materials and electronic devices, to name a few areas of current engineering exploration. Self-assembly is one of the key mechanisms by which nature builds products, from biological molecules such as proteins to larger structures such as cells and extracellular matrices, the spatial arrangement of atoms is determined, in large part, by information built into the assembling units. Complexity, in the sense of development of emergent properties of an assembly that cannot readily be envisioned from the constituents (Ottino, 2003), can arise spontaneously during self-assembly and often does, especially in biological systems. We are only beginning to develop sufficiently sophisticated synthetic assemblers to mimic biology in this way, as illustrated in Figure 1, from Simon, et al (2001). Figure 1. Schematic diagram showing how complexity of self-assembly may increase as the number of building blocks in a copolymer increases. (adapted from Simon, et al. 2001)