Platinum Metals Rev., 1999, 43, (2), 73
Self-Organisation of Nanostructures
A recurrent theme in contemporary research has been the understanding of processes of self-assembly or self-organisation. Perhaps the fascination that this topic exerts is its ubiquitous nature, from the co-ordinated movements of very large flocks of birds to the organisation of living organisms.
The chemist’s (and physicist’s) viewpoint has been to investigate the types of interaction that underpin co-operative phenomena, in attempts to highlight the common denominator responsible for structure recognition. Examples of this abound in the literature, as for instance, the seminal work of Langmuir at the beginning of the century on the ordering of molecules at interfaces resulting from a balance of polar/non-polar intermolecular interactions. In this case, the driving force for molecular recognition and self-organisation can be identified with particular aspects of molecular structure and functionality.
The forces responsible for organisation can be more subtle than those mentioned above, which rely on specific functionalities. One example of this is the ordering and crystallisation processes observed for non-interacting systems composed of particles of different sizes (1). In this case, entropic effects are entirely responsible for phase structure and composition. Importantly, there are now examples that these effects occur not only for systems having dimensions in the micrometre range, but also in the nanoscale dimension (2).
There is at present an intense interest in identifying the forces responsible for self-organisation of nanostructures formed at crystal surfaces. These issues have recently been further explored in a paper by Pohl and colleagues (3). The main question that they address is the nature of the forces responsible for order in a single monolayer of silver on a ruthenium (0001) surface, in particular in relation to the behaviour of vacancy islands formed by exposure to sulfur. The mobility of isolated vacancy islands has been measured by scanning tunnelling microscopy (STM), as previously observed for cobalt layers on copper (111) (4). The analysis of thermal fluctuations of the vacancies and their displacement is a beautiful example of the application of STM to the analysis of nanoscale structures. From the statistical analysis of these displacements, the forces responsible for the stabilisation of the island vacancy lattice can be investigated. These forces result from line tensions, related to the reduction of co-ordination by creating an island, and film strain due to elastic deformation resulting from strain in the film around the edges of islands. The possibility of predicting the structures formed from surface stresses can open up new avenues for designing self-organised surfaces.
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- 3K. Pohl,, M. C. Bartelt,, J. de la Figuera,, N. C. Bartelt,, J. Hrbek and R. Q. Hwang, Nature, 1999, 397, 238
- 4J. de la Figuera,, J. E. Prieto,, C. Ocal and R. Miranda, Solid State Commun ., 1994, 89, 815