In conjunction with Foresight Update 52
One of the most visually stunning milestones along the path toward molecular manufacturing was the construction of a very small copy of the IBM logo by using a scanning tunneling microscope (STM) to position 35 xenon atoms on a surface of nickel atoms at very low temperatures (see Update 9). But you can’t get very far building structures from xenon atoms, or using only non-covalent associations of atoms. An excellent review of progress toward fabricating molecules using an STM can be found in “STM Control of Chemical Reactions: Single-Molecule Synthesis”, an invited article by S.-W. Hla and K.-H. Rieder published in Ann. Rev. Phys. Chem. 54 (2003) 307-330 and available on the web as a 1.3 MB PDF file at http://www.phy.ohiou.edu/~hla/HLA-annualreview.pdf.
Abstract The fascinating advances in single atom/molecule manipulation with a scanning tunneling microscope (STM) tip allow scientists to fabricate atomic-scale structures or to probe chemical and physical properties of matters at an atomic level. Owing to these advances, it has become possible for the basic chemical reaction steps, such as dissociation, diffusion, adsorption, readsorption, and bond-formation processes, to be performed by using the STM tip. Complete sequences of chemical reactions are able to induce at a single-molecule level. New molecules can be constructed from the basic molecular building blocks on a one-molecule-at-a-time basis by using a variety of STM manipulation schemes in a systematic step-by-step manner. These achievements open up entirely new opportunities in nanochemistry and nanochemical technology. In this review, various STM manipulation techniques useful in the single-molecule reaction process are reviewed, and their impact on the future of nanoscience and technology are discussed.
Hla and Rieder begin with the use of an STM tip to create diffusion of single atoms or molecules across a surface, the process known as lateral manipulation and used by Eigler and Schweizer in 1990 to spell out “IBM” with 35 Xe atoms. By studying how the STM tip interacts with the atom or molecule being manipulated, researchers can distinguish three different manipulation modes: pushing, pulling, and sliding. As an example, an experiment is shown of a diiodobenzene molecule being pulled over copper atoms on a Cu (111) surface at 20 K, and just how it works is explained in detail. Other experiments illustrate effects of tip-target distance and chemical nature, and amount of tunneling current. With lateral manipulation of large molecules at low temperatures, pushing is the main manipulation mode.
The next step beyond pushing atoms and molecules around is chemistry catalyzed by metal atoms on the surface. Breaking covalent bonds can be done in either a field emission regime, in which a high voltage bias on the tip injects high-energy (>3V) tunneling electrons into the molecule to be dissociated, or in a more controlled inelastic tunneling regime (IET). In IET low-energy electrons are injected via a resonance state between the tip or the substrate and the molecule. An example is presented using IET to break the weak C-I bond within iodobenzene without breaking the stronger C-C or C-H bonds. It would of course be even more interesting if you could break a specific strong bond without breaking other strong bonds. Other sections cover using an electric field to transfer atoms or molecules from the surface to the tip or back, and forming bonds between two molecular fragments adsorbed on a surface. In most cases the molecular fragment produced by earlier bond dissociation is now bound to the metal surface, which often results in tilting the fragment at an angle from the surface. Clever manipulation of fragments on the surface can result, for example, in breaking the C-Cu bonds of the fragments on the surface and forming C-C bonds between the fragments, sometimes with the aid of tunneling electrons.