Shortly before I launched Metamodern, Science published a remarkable paper by Sugimoto et al. describing atom-by-atom manipulation of a monatomic layer of tin (Sn) on silicon (Si). The animation to the right shows the steps in constructing a pattern of Si atoms that spells ‘Si’. Each frame is an atomic force microscope image made with the same tip used to construct the pattern, part of a commercially available silicon cantilever.
The process works by interchanging atoms between the tip and the surface. Some tips deposit Si, others deposit Sn, some alternate. Once a suitable tip is found, the process is reproducible. The authors have simulated the process using density functional methods and computing resources from the Barcelona Supercomputing Center, home of the beautiful MareNostrum machine (I couldn’t resist the temptation to include a picture).
The ‘Si’ structure took 1.5 hours to make. Most of this time was spent imaging, and some of this time was spent waiting for a new Si atom to appear on the tip. I say “appear”, because the tips recharge spontaneously, from material already on the tip, not by picking up atoms from the surface.
- Mechanosynthesis isn’t restricted to processes like those found in ribosomes and nonribosomal peptide synthases: Physics allows mechanosynthetic processes quite different from those based on positional control of conventional reactive monomers.
- Fabrication processes are are often discovered, not designed. I doubt that the researchers expected to see this behavior, and the spontaneous recharging of tips is (to me) very surprising.
- For a wide range of applications, quantum chemistry works. Simulations showed how atom interchange can work, and simulations of the same kind could be used to discover and design other processes — which could seldom be implemented without a way to make atomically precise tips that meet the design specification.
- Techniques for atomically precise fabrication are accumulating. The first contributions came from chemical synthesis, more came from biotechnology, and yet others from materials science. Direct physical manipulation methods are becoming part of the toolkit.
Techniques for direct physical manipulation have thus far remained a laboratory curiosity, but they may gain practical value, and even now they illustrate direct positional control of the sort that will be characteristic of advanced mechanosynthetic methods.