Robert Freitas (author of Nanomedicine) commented on the new Japanese work:
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.
the same argument I used to show the infeasibility of tiny fingers placing one atom at a time applies also to placing larger, more complex building blocks. Since each incoming “reactive molecule” building block has multiple atoms to control during the reaction, even more fingers will be needed to make sure they do not go astray. Computer-controlled fingers will be too fat and too sticky to permit the requisite control. Fingers just can’t do chemistry with the necessary finesse.
From the abstract of the paper:
The ability to incorporate individual atoms in a surface following predetermined arrangements may bring future atom-based technological enterprises closer to reality. Here, we report the assembling of complex atomic patterns at room temperature by the vertical interchange of atoms between the tip apex of an atomic force microscope and a semiconductor surface. At variance with previous methods, these manipulations were produced by exploring the repulsive part of the short-range chemical interaction between the closest tip-surface atoms. By using first-principles calculations, we clarified the basic mechanisms behind the vertical interchange of atoms, characterizing the key atomistic processes involved and estimating the magnitude of the energy barriers between the relevant atomic configurations that leads to these manipulations.
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.
Noriaki Oyabu, Oscar Custance, Insook Yi, Yasuhiro Sugawara, Seizo Morita, “Mechanical vertical manipulation of selected single atoms by soft nanoindentation using near contact atomic force microscopy,” Phys. Rev. Lett. 90(2 May 2003):176102; http://link.aps.org/abstract/PRL/v90/e176102 (Abstract) http://focus.aps.org/story/v11/st19 (APS story)
ABSTRACT. A near contact atomic force microscope operated at low-temperature is used for vertical manipulation of selected single atoms from the Si(111)–(7×7) surface. The strong repulsive short-range chemical force interaction between the closest atoms of both tip apex and surface during a soft nanoindentation leads to the removal of a selected silicon atom from its equilibrium position at the surface without additional perturbation of the (7×7) unit cell. Deposition of a single atom on a created vacancy at the surface is achieved as well. These manipulation processes are purely mechanical, since neither bias voltage nor voltage pulse is applied between probe and sample. Differences in the mechanical response of the two nonequivalent adatoms of the Si(111)–(7×7) with the load applied is also detected.
NOTE: This landmark paper is the first to report purely mechanical-based covalent bond-making and bond-breaking, i.e., the first experimental demonstration of true mechanosynthesis.
Morita S, Sugimoto Y, Oyabu N, Nishi R, Custance O, Sugawara Y, Abe M, “Atom-selective imaging and mechanical atom manipulation using the non-contact atomic force microscope,” J. Electron Microsc. (Tokyo) 53(2004):163-168.
ABSTRACT. We succeeded in distinguishing between oxygen and silicon atoms on an oxygen-adsorbed Si(111)7 x 7 surface, and also distinguished between silicon and tin atoms on Si(111)7 x 7-Sn intermixed and Si(111) square root(3) x square root(3)-Sn mosaic-phase surfaces using non-contact atomic force microscopy (NC-AFM) at room temperature. Atom species of individual atoms are specified from the number of each atom in NC-AFM images, the tip-sample distance dependence of NC-AFM images and/or the surface distribution of each atom. Further, based on the NC-AFM method but using soft nanoindentation, we achieved two kinds of mechanical vertical manipulation of individual atoms: removal of a selected Si adatom and deposition of a Si atom into a selected Si adatom vacancy on the Si(111)7 x 7 surface at 78 K. Here, we carefully and slowly indented a Si atom on top of a clean Si tip apex onto a predetermined Si adatom to remove the targeted Si adatom and onto a predetermined Si adatom vacancy to deposit a Si atom, i.e. to repair the targeted Si adatom vacancy. By combining the atom-selective imaging method with two kinds of mechanical atom manipulation, i.e. by picking up a selected atom species and by depositing that atom one by one at the assigned site, we hope to construct nanomaterials and nanodevices made from more than two kinds of atom species in the near future.
NOTE: This is another experimental demonstration of true mechanosynthesis, using silicon (Si) adatoms on Si surface.