Controlling nature's spontaneous self-organisation
Physicists from the Netherlands, Spain and the United States have succeeded in controlling the structure of a nanopattern that is formed spontaneously when a single layer of gallium atoms is applied to a silicon base. They also provided the first good theoretical description of this at an atomic level. With this, they have filled a knowledge gap in the modelling of linear structures on a nanoscale. This could, for example, prove helpful in solving problems that occur during the production of ultra-thin layers or wires and connections for microelectronics. The researchers published their findings on 14 September 2007 in Physical Review Letters.
Atomic wires are under the spotlight due to their dimensions: an atom is typically no larger than 0.5 nanometers (a nanometer is a millionth of a millimetre). Dependent on their characteristics they could potentially be used as connecting wires in future ultra-small computer chips on an atomic or molecular scale. With the help of a scanning tunnelling microscope researchers and technologists can themselves place atoms end to end, but it is wiser to let nature produce the atomic wires using the principle referred to as self-organisation.
Physicist Paul Snijders and his colleagues in Delft, Madrid and Tennessee have now discovered how they can control that self-organising process and they have also provided a good theoretical description of this. They worked with gallium atoms, which they precipitated in a single layer onto a silicon base. During this process the gallium atoms spontaneously organise themselves in long rows (wires). Compressive strain arises in these rows that is relieved in the form of an open space in the row, a so-called vacancy. These vacancies are also subject to self-organisation. The vacancies turn out to organise themselves in gently meandering lines of approximately 0.38 nanometres wide which can be almost perfectly straight over a length of up to 50 nanometres. This pattern of vacancies could potentially be used as a kind of template to grow nanowires from another element. However, it needs to be seen whether this can be achieved in practice. The laws of thermodynamics (and specifically entropy) will of course counteract this organisational process. Low-dimensional systems in particular (for example, wires that are so thin that they can be regarded as two- or even one-dimensional) appear to be highly sensitive to entropic effects. The resultant fluctuations cause the vacancy lines to meander instead of being perfectly straight.
Snijders and his colleagues have now discovered that they can control the average distance between the vacancy lines with a precision of within 0.05 nanometres. They can achieve this by varying the temperature of the gallium atoms applied to the surface, through adjustment of their chemical potential. This allows them to shift the balance between the gallium atoms, and therefore the open positions, to greater or smaller distances between the vacancy lines.
Existing mathematical models that describe this process cannot cope with such small dimensions. They predict things that the researchers do not observe, for example that the meandering of the vacancy lines is stronger if the average distance between the lines is greater. The researchers observed that the meandering was constant, even if the distance between the vacancy lines varied. Snijders and his colleagues have now developed a new theoretical model that not only predicts the behaviour of the vacancy lines but also yields outcomes that agree with the observations. Their theory, a combination of calculations with density functional theory and a model based on statistical mechanics, appears to agree closely with experimental observations. This new approach may also prove useful for similar problems in the growth of ultra-thin layers and in nano research.
For further information please contact Dr. Paul Snijders.
Article:
The article "Controlled self-organisation of atom vacancies in monatomic gallium layers" was published in print form on 14 September 2007 in Physical Review Letters; the authors are Paul Snijders1*, Eun Ju Moon2, C. González3, Sven Rogge1, J. Ortega3, F. Flores3 and Hanno Weijtering2,4.
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1* FOM researcher at the Kavli Institute of Nanoscience in Delft when he did this research; current address Oak Ridge National Laboratory, Tennessee
1 Kavli Institute of Nanoscience in Delft
2 University of Tennessee in Knoxville
3 Autonomous University of Madrid
4 University of Tennessee in Knoxville and Oak Ridge National Laboratory, Tennessee