Growing crystals

one of the basis of the tremendous revolution in electronics has been the enormous advances in materials science. Whether it is the accuracy of photolithography or the ability to fabricate extremely pure materials, microelectronics would not be possible without concomitant growth of material science. An important requirement for the fabrication of many advanced semiconductor and other materials is the control over growth of crystals. Now, scientists at the Bell Laboratories have managed to grow crystals at predetermined sites with almost 100 per cent fidelity. This is an important step in fabricating materials on extremely small scales with applications in a host of technologies including medical science.

Joanna Aizenberg and her colleagues have fabricated a surface with ridges of heights ranging from 3 to 50 micrometer (a millionth of a meter) which are separated from each other by 100 micrometers. This surface is then placed in contact with collections of molecules which have the property of self-assembly. These self-assembling molecules then get bound to the raised ridges on the surface. The whole set-up is then transferred to metal-coated silicon surface and the regions between the ridges is filled up with specific molecules. When the patterned surface is immersed in calcium carbonate solution and exposed to carbon dioxide, calcite crystals grow on certain sites and not on certain other sites. This process can be controlled to an extremely high degree of accuracy.

What is interesting about the experiment is that by varying the nature of the molecules used, the metal coating and the patterned stamp, the number of crystals and the distance between them could be accurately controlled. This is a tremendous breakthrough because now we have precise control over crystal nucleation at distances of the order of sub-micrometer. The dream of material scientists is to achieve this control at the level of a nanometer (a billionth of a meter). This is an important step towards that goal. One of the many potential uses of this technique could be in increasing the life of artificial replacement materials in the body. Calcite crystals invariably form on these materials and if we could encourage the growth of the crystals at sites which are harmless, then the functional life of the materials could be increased.