The Analytical & Surface Chemistry Program of the Division of Chemistry, in the Directorate of Mathematical & Physical Sciences of the National Science Foundation, will support research by Theodore L. Einstein of the University of Maryland, College Park and Ludwig Bartels of the University of California, Riverside. The goal of this work is to understand how particular organic molecules assemble into large regular honeycomb patterns with open pores having diameters of 5 nm. To explore how general this behavior is, Bartels will perform experiments on different molecules and substrate materials in conjunction with theoretical calculations by Einstein that seek to explain the short-range structure in terms of hydrogen bonds and the large pores in terms of the electronic structure of the substrate. With a command of the interaction mechanisms, Bartels and Einstein will seek to engineer the size of open pores and perhaps also the pattern of the molecular networks. Such surfaces can serve as templates for growing patterned films on surfaces for applications ranging from storage technology to heterogeneous catalysis. Educational benefits include the training of graduate students and postdoctoral associates in state-of-the-art techniques for imaging at the atomic scale and for simulating systems in which comparatively small energies and subtle effects determine the overall geometry. It is also an outstanding opportunity for synergistic and close collaboration between physics-based theory and chemistry-based experiment, offering the involved students highly interdisciplinary training.
The focus of this project concerns the giant honeycomb network formed by a certain type of molecule (nicknamed AQ) that has the shape a short tongue depressor: flat, linear, and-stubby. The network pattern looks much like the grout between hexagonal tiles on an old bathroom floor. The straight segments and the 3-way junctions can be readily explained by the type of attractive forces linking water molecules. However, since there is no tile spacing the stiff grout, it is very surprising that the pattern is so regular, rather than a mixture of large and small hexagons. The resulting array of identical pores has many potential uses, serving as a large set of identical "test tubes" in which the same atoms can be placed. This allows researchers to watch simultaneously and conveniently the various ways in which in which initially identical systems can evolve (rather than watching a single "test tube" over and over again), in analogy to parallel computing in numerical studies. Specifically, small molecules such as carbon monoxide (CO) can stick to the surface inside the pores and undergo chemical reactions when exposed to other gases. Because of the small size and constricted geometry of the pores, the reaction conditions are altered, as explored in detail by our Israeli collaborators. There are biological analogues to this situation. Further, close examination of the geometry shows that when a large number of CO molecules are in the pore, they break into two groups with a distinctive boundary separating them. The fluctuations of this boundary resemble the fluctuations of other boundaries on surfaces, such as borders of islands of atoms or steps between areas having different height. Insight from research on the behavior of these other types of boundaries allows us to assess the ingredients involved in the shape changes of the boundary, which in turn are key to understanding (and ultimately manipulating) how nanostructures move on surfaces. The main science questions are 1) What forces produce the regularity of the hexagonal pattern, and 2) how can we manipulate these forces, e.g. to produce pores of different sizes? To address the first, we initially tried a picture related to the forces that produce checkerboard patterns of atoms on some metal surfaces at low temperatures. The quantum mechanical forces that cause this pattern resemble the waves that spread from the site at which a pebble is dropped into water. The crests and troughs of the ripples, corresponding to unfavorable and favorable separations of a second AQ molecule relative to the first (assumed to be at the center) have a characteristic separation that is particularly large and a decay that is unusually slow in cases where the wave motion is confined to surface (rather than also involving the bulk), as is the case for the metal surface we use. However, there turns out to be a difficulty with this model in that a longer but similar molecule to AQ (i.e., a longer tongue depressor) forms a similar pattern but with somewhat larger pore size. The model we first proposed would predict that the network would form only when pores have the same size as for AQ. We then developed an alternative description in which the pore behaves like a two-dimensional analogy of an atom, which therefore has a characteristic size that is particularly stable. To test this model, it is crucial to know how many electrons from the metal surface are available inside the pore. With collaborators in Sweden, we successfully investigated this issue in detail, as well as ways to adjust the number of electrons in the pore (or the distance between troughs of the wave) by applying an electric field, such as occurs in static electricity, at the surface and thereby make progress on our second science question.
