Information-storage technology relies on systems that can be easily “switched” from one configuration to another by applying an external stimulus. Molecular systems, for example, can be switched by changing their shape or spin state.

The rotaxane molecules used by Cavallini and co-workers are shaped like dumb-bells. Each contains a ring structure that is mechanically locked onto a “thread” by two bulky “stoppers”. A film of these molecules lying side by side resembles an abacus, with the stoppers playing the role of the abacus beads. The molecules are “bistable” in that they can be made to switch by rotating the ring between two states of slightly different energies.

Cavallini and colleagues used the probe of an atomic force microscope to scan the surface of the molecular films using a load force of less than 2 nanonewtons. The researchers then increased the force on the probe and observed that the normally smooth surface of the film becomes perturbed -- the rotaxane beads begin to spread out and form a string of regularly spaced stable pattern of dots that have a uniform width and height.

The team found that they could control the number of dots with the length of the scan and so fabricate any predetermined number of dots –- which will allow them to write information on the dots as strings of bits. They also found that thinner films led to denser and smaller dots.

The scientists say that their approach is better than existing methods of writing with a scanning probe as it allows multiple dots to be encoded at the same time. The team now hopes to speed up this process using parallel writing and to develop an efficient read-out method.