Robert A. Freitas Jr. brings to our attention a major step on the road to advanced nanotech, published a couple weeks ago in Science (abstract). He writes:

This paper reports purely mechanical-based covalent bond-making and bond-breaking (true mechanosynthesis) involving atom by atom substitution of silicon (Si) atoms for tin (Sn) atoms in an Sn monolayer surface on a Si(111) surface; also demonstrates atomically precise exchange of lead (Pb) and indium (In) on Si(111) surface. This is the first report of a complex pattern being drawn on a 2D surface, literally atom by atom, purely via mechanical forces.

Working on a single atomic layer of tin atoms grown on a single-crystal silicon surface, the Japanese-European collaboration maneuvered an atomic force microscope (AFM) tip precisely (plus or minus 0.01 nm) over a single silicon atom defect in the tin surface, and were able to reversibly exchange a tin atom on the apex of the tip and the silicon atom on the surface. These experiments were done at room temperature and, unlike earlier demonstrations in which a scanning tunneling microscope (STM) tip was used to interchange atoms weakly bond to a metallic surface through use of an electrical bias, this demonstration used mechanical force to interchange strongly bound atoms.

To characterize what was happening between the atoms involved, the researchers did a first principles quantum mechanics simulation of the tip-surface interactions. The simulations show that the key step happens when the outermost atom of the tip and the target atom on the surface have an equal number of bonds with the surrounding atoms so that they lose the property of being part of the tip or the surface.

The method used here of vertical interchange of atoms between tip and surface was found to be about ten times faster than previous lateral manipulations of atoms with the AFM. Using vertical manipulation as an atomic pen, the authors wrote the chemical symbol for silicon (Si) with 12 silicon atoms on the tin surface. In supplementary material, the authors report doing similar manipulations with lead and indium atoms on a silicon surface. They propose that:

This manipulation technique may pave the way toward selective semiconductor doping, practical implementation of quantum computing, or atomic-based spintronics. The possibility of combining sophisticated vertical and lateral atom manipulations with the capability of AFM for single-atom chemical identification may bring closer the advent of future atomic-level applications, even at room temperature.