Self-organization in metal growth

By means of playing around with competing interactions, it is possible to generate indirectly (or by a bottom-up approach) ordered patterns on the nanoscale, patterns that could well be used to grow additional material. But to do so, we must learn about those interactions. Depending on your point of view (or your hype factor) this is just the usual way of doing things, or the next big thing in nanotech. To be fair, self-assembly and self-organization is around us in everyday life. Blacksmiths have hundreds of years of experience modifying the properties of iron and steel to obtain the desired microstructure for a given function. It is only in the late XX century that we have got used to the top-down approach, lithographic techniques in microelectronics that allow us to put every gate, every transistor in the desired location. But this mode of creating a new material (or device) is still the exception.

We focus on surfaces. You can find more information in the Master's short course that you can find here. I would like to point out that the future in many fields, and magnetic recording media is a clear case, lies in the combination of both lithographic techniques with self-assembly. Nowadays Seagate and Hitachi are exploring the use of the self-assembly of diblock copolymer combining the good short range order of self-assembly with the long range control in lithography.

From a basic point of view, a particularly simple -and common- way to promote self-assembly on surfaces are competing interactions of different range. Say for example a short range attractive interaction (due to broken bonds), and some sort of long range interaction. The origin of the long range interaction can be quite different: magnetic dipolar in thin films with perpendicular magnetic anisotropy, electrostatic interaction between polar molecules, or elastic interactions due to the stress difference between surfaces regions (be it by adsorbates or by structure). This produces patterns of the type shown in the figure. A good example of a pattern produced by the latter effect is the one that arises when depositing gold on tungsten at high temperature, where we found that the pattern formation can be understood in a particularly simple model.

Also the generation of misfit dislocations (which can be considered a competition between intralayer interactions vs interlayer interactions) has often been used to produce well ordered patterns, like the nice dislocation pattern we observed (and explained) on Ag(111)/Ru(0001) More information here.

A different way to tackle self-organization is to take advantage not of the thermodynamics of the system (i.e. the equilibrium structure) but kinetic limitations. Maybe tuning the temperature we can take advantage of some activated process on surfaces to self-limit island growth. Such an example is the is the serpentine growth of Pd on Ru, were the growth of Pd self-limits due to some alloying with the substrate at steps edges that limits further growth.