With support from the Chemical Measurement and Imaging Program in the Division of Chemistry, Professors Schwarz and Altman at Yale University are developing a new approach for the visualization and characterization of single molecule surface chemistry. This new approach offers dramatic advances over existing pathways to explore surface chemistry over large ensembles of molecules and reactions; instead each step of a reaction is individually induced by the tip of a scanning probe microscope, the specific and unique reaction pathway is chosen at will, and energy barriers between potential minima on that pathway are quantified. During each step, the interactions responsible to drive the reaction are characterized with unprecedented precision, which has the potential to reveal the influence of nearby surface defects or other molecules, thereby opening a whole new avenue to the study of surface chemistry and catalysis. The technique and the novel results it can generate are being illustrated using hydrogenation, dehydrogenation, and carbon-carbon bond formation in aromatic compounds as examples. These reactions are chosen because of their extraordinary technological importance for the world's chemical and petrochemical industry. The ability to "see chemistry in action" combined with controlling and quantifying every detail of it is expected to facilitate outreach to the general public.
More specifically, the new approach builds on recent advances in scanning probe microscopy, which have made it possible to not only image molecules on surfaces, but also to 1) map the entire surface potential around the molecule, thereby uncovering sites of enhanced local reactivity; 2) translate molecules, atoms, and clusters while measuring the diffusion barrier between sites, thereby detecting these barriers as a function of the chemical environment; 3) split molecules (dissociation) using energy provided by the tip; and 4) form molecules and molecular bonds through voltage pulses induced by the tip after the reactants have been arranged properly on the surface. This research, for the first time, combines these elements to achieve a complete quantitative picture of all of the elementary steps involved in surface reactions. The new approach is centered on measuring the force needed to push molecules and/or atoms together so that they can react; by integrating along the path, the energy barriers and depths of the potential minima can be recovered. To achieve the necessary stability of the molecules on the surface, all investigations take place at low temperatures using a home-built combined scanning tunneling/atomic force microscope. First, benzene, iodobenzene, and hydrogen are deposited on the (111) surface of platinum. Subsequently, individual H and I atoms are produced by applying voltage pulses and the resulting radicals and atoms are manipulated to determine possible manipulation paths and diffusion barriers between potential minima while the potential energy landscapes are being mapped. Finally, biphenyl are produced by linking two benzyl radicals or benzene by hydrogenating benzyl, either spontaneously or through the application of bias voltage pulses once the species are brought close together. As a result, complete energetic information is obtained on a single-molecule level.
