Self-Assembly and Nanostructures: Fabricating Without a Top-Down Tool
23 Apr, 2007 01:27 pm
Controlling growth of materials opens the way to fabricate regular highly perfect structures without any top-down tool making it far less cost intensive and widening the application range. Nature is a master in self-assembly of proteins. Complex macromolecules can be formed from proteins which have a specific shape and functional groups defining their interactions. All the main components of biological cells are built from protein structures.
Examples: Soap molecules consist of head and tail groups. The tails tend to line up forming oriented molecular layers which can form spheres, cylinders and bilayers. The figure below shows examples from our laboratory: nano-needles of histidine molecules are formed through self- assembly and islands of gold nanoparticles are self-assemble into self-similar shapes or dendrites and oriented square shaped silver platelets are formed spontaneously when reducing the cooling rate of evaporated films.
Figure: A) Histidine nano-needles with diameters in the 50-100nm range (3), B) self-similar islands of gold nanoparticles (4), C) oriented square silver platelets 80x80x6nm (5).
Intrinsic dynamics: The assembly of atoms, molecules and nanoparticles can be influenced by taking advantage of the atomic interaction and Brownian motion. The Brownian motion is absolutely fundamental in the nanometer range and implies that there is a dynamics at the nanometer range which has drastic consequences on the formation and the stability of nanostructures. Defects are constantly created and repaired. Earlier more radical views on nanotechnology focus on building nanotstructures atom by atom. Considering the intrinsic dynamics of matter at the nanometer scale, we see that building of nanostructures is more subtle and cannot be accomplished without taking into account the different physical context at the nanometer scale as compared to the macroscopic scale. While self-assembly does not necessarily lead to spontaneous formation of electrical circuits it is used in a number of situations such as in sensors or in a first step on predefined electrodes. The field of self-assembly of complex nanostructures is a very promising field of scientific activity. Looking at nature we can see the huge application range for self-assembled nanostructures. But there are some doubts how perfect self-assembled nanostructres can be made. To build a highly complex circuit one needs high perfection. The complexity of molecules, their shape and functional groups or even the purity of the molecules or particles are factors which limit the perfection of larger nanostructures. Liquid crystals which are used as a light switch in displays show, however that molecular assemblies once observed in a laboratory decades ago can be improved and developed into reliable commercial products.
1 ‘Soft Machines: nanotechnology and life’ by Richard A L Jones, Oxford University Press (2004)
2 ‘Understanding nanotechnology’ S Fritz, Scientific American (2002)
3 V Sonois et al, to appear in Chem. Phys. Lett. (2007)
4 WS Bacsa, C Amien, B Chaudret, unpublished
5 D Berner, L Zuppiroli, M Caumont, WS Bacsa, NSTI Nanotechnology Proceedings (2006)