By Wolfgang Bacsa

, Nanostructured Materials, NanoOptics

Filed under : Scitizen >> Technology >> NanoScience >> Self-Assembly and Nanostructures: Fabricating Without a Top-Down Tool

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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.

What is self-assembly based on? Self-assembly needs a balance between two apparent opposing phenomena such as Brownian motion and attractive interactions between atoms and molecules. Atoms are in constant motion and collide with each other. As the particle size gets smaller the number of collisions with neighboring atoms or molecules is reduced; there is a lack of momentum balance and the particle is moved in one direction. Random collisions of atoms with the particle have the effect that particles are finally moved in arbitrary fashion and we speak of Brownian motion. Brownian motion depends on kinetic energy of the atoms or molecules and is proportional to temperature. Brownian motion tends to bring atoms or molecules apart while surface forces tend to bring them together. Or differently put Brownian motion makes atoms explore the configuration space and to find a position where the interaction is strongest. The right balance between these two apparently opposing phenomena leads to self-assembly and happens in the nanometer range. It is fascinating to see how we can generate order from disorder. While things tend to end up in disorder where entropy is increasing, local order can be generated by exporting heat energy to the surrounding medium (1). The system tends to reduce energy by maximizing the interaction between atoms creating ordered structures at low temperature and the system tends to favor disorder or to increase entropy with higher temperature. At low temperature the dynamics is slow and at high temperature the dynamics is fast. The scientists talk in this context about minimizing the free energy or maximizing interaction and entropy. The free energy has a term describing the energy of interaction and entropy. The entropy term is directly proportional to temperature. The Brownian motion makes the self assembled structures dynamic, defects appear and disappear. A damaged structure can form again; it can repair itself. Self assembly is observed between atoms, molecules and nanoparticles. The effect of Brownian motion on larger particles is reduced and disappears with increasing size. Self-assembled structure can be used as templates and the structure is reproduced in a second medium which then can become rigid. Zeolites or biological skeletons are formed this way. Self-assembled structures can be hierarchical; the spheres or cylinders can form arrays, sheets or three dimensional crystals. Molecular interactions can be multiple they can be based on hydrogen bonding, hydrophobic forces, van der Waals or electrostatic forces and pi-pi interaction. Self-assembly happens in solution phase and on surfaces or in a combination of both.

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.

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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.

References:

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)

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