Monday, April 14, 2008

How falling spaghettis could lead to more complex nanotechnology self-assembly

Posted: April 14, 2008

(Nanowerk Spotlight) Self-assembly and self-organization are terms (read this discussion about the difference between the two) used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. Self-organizing processes are common throughout nature and involve components from the molecular (e.g. protein folding) to the planetary scale (e.g. weather systems) and even beyond (e.g. galaxies). Self-assembly has become an especially important concept in nanotechnology. As miniaturization reaches the nanoscale, conventional manufacturing technologies fail because it has not been possible (yet) to build machinery that assembles nanoscale components into functional devices (for more on this, read Mind the gap - nanotechnology robotics vision versus lab reality). Until robotic assemblers capable of nanofabrication can be built, self-assembly - together with chemical synthesis - will be the necessary technology to develop for bottom-up fabrication.
The stability of covalent bonds enables the synthesis of almost arbitrary configurations of up to 1000 atoms. Larger molecules, molecular aggregates, and forms of organized matter more extensive than molecules cannot be synthesized bond-by-bond. Self-assembly is one strategy for organizing matter on these larger scales (Source).
The key to using self-assembly as a controlled and directed fabrication process lies in designing the components that are required to self-assemble into desired patterns and functions. Self-assembly reflects information coded – as shape, surface properties, charge, polarizability, magnetic dipole, mass, etc. – in individual components; these characteristics determine the interactions among them.
"It has long been recognized that whereas self-organization near thermodynamic equilibrium tends to attenuate fluctuations, leading to relatively simple geometries, self-organization far from equilibrium can amplify fluctuations into coherent oscillations, leading to much more complex structures" Dr. Ernesto Joselevich tells Nanowerk. While much of the work on molecular self-assembly has focused on equilibrium systems, leading to highly ordered arrays such as crystals, Joselevich points out that this universal principle of 'order through fluctuations' has not yet been widely applied to the self-assembly of complex structures at the nanoscale – although it is the essence of the emergence of order, complexity and life in the universe in spite of the second law of thermodynamics.
Joselevich, a Senior Scientist in the Department of Materials and Interfaces at the Weizmann Institute of Science in Israel, together with PhD students Noam Geblinger and Ariel Ismach, has just published a report in Nature Nanotechnology on an intriguing new type of nanotube structures – serpentines – strikingly more complex than those observed before (Self-organized nanotube serpentines).
"Here we show that combined surface- and flow-directed growth enable the controlled formation of uniquely complex and coherent geometries of single-walled carbon nanotubes, including highly oriented and periodic serpentines and coils" Joselevich explains to Nanowerk. "We propose a mechanism of non-equilibrium self-organization, in which competing dissipative forces of adhesion and aerodynamic drag induce oscillations in the nanotubes as they adsorb on the surface."
carbon nanotube serpentines
SEM image of a nanotube serpentine (Reprinted with permission from Nature Publishing Group)
So far, controlled formation of complex nanotube geometries like rings and loops has been achieved by directed assembly of preformed nanotubes, using templates and microfluidics. In these cases, the alignment was solely determined by the surface of the template, and not affected by external forces such as electric fields or gas flow.
"A few years ago, in experimenting with the growth of carbon nanotubes along atomic steps, we produced arrays of perfectly straight and parallel nanotubes" Joselevich explains the background to his latest paper. "While playing with different substrates, we noticed a few serpentines on some of our samples of single-walled carbon nanotubes (SWCNTs) grown on quartz. This phenomenon could not be explained by our previous mechanism of growth along steps, because it did not make sense why a nanotube growing along a step would suddenly make a U-turn, then after a certain length make another U-turn in the opposite direction, and after precisely the same length make an opposite U-turn, and so on. The serpentine shape was a highly complex organized structure whose formation was a mystery for us.
"One evening, I saw my toddler son playing with spaghetti, and realized that when the spaghetti fell on a bamboo mat, it fell down making wiggles and produced serpentine shapes very similar to those of the nanotubes serpentines that we had observed in our lab experiments."

'Falling-spaghetti mechanism' for the formation of self-organized nanotube serpentines
Joselevich's team hypothesized that the serpentines could form in a two-step mechanism, where the nanotubes first grow standing up from the surface, and at a later stage adsorb on the surface in an oscillatory fashion along the steps. "Once we understood this" he says, "we patterned the catalyst on stripes of amorphous silicon oxide to prevent growth along the steps of the quartz. Then we were able to fabricate thousands of nanotube serpentines and could systematically study their formation and properties."
The serpentines are made of SWCNTs with a very low concentration of defects and appear to have exactly the same electronic properties as regular SWCNTs. Hence they are expected to be either metallic or semiconducting depending on their diameter and chirality. The team's data show that the diameter and chirality remain constant along the entire serpentine, which can be longer than one millimeter.
Joselevich notes that the serpentine shape has very interesting geometric properties. It provides maximum coverage of a certain area by a single line and it packs the maximum contour length of a line on a minimum area. "Hence, you find this shape in many daily useful objects like heating and cooling devices, illumination and irrigation systems, etc. We would like to produce nanodevices that take advantage of the serpentine shape for analogous applications in a miniature size."
The Weizmann Institute team's self-organizing nanotube serpentines is a dramatic example of non-equilibrium self-organization or 'order through fluctuations' at the nanoscale. Joselevich believes that if we learn how to produce complex structures through non-equilibrium self-organization we will be able to produce a lot of new functional nanosystems with potential applications not previously thought possible.
By Michael Berger. Copyright 2008 Nanowerk LLC


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