Forming Aromatic Hemispheres on Transition‐Metal Surfaces

Abstract

This study details a new method to create materials by integrating polycyclic aromatic hydrocarbons into reactive transition-metal surfaces. We describe the interaction of hexabenzocoronene (1, HBC, Figure 1a) with a clean ruthenium surface. Interest in 1 and its interaction with transition-metal surfaces is driven by its relationship to the end cap of a carbon nanotube, to polynuclear aromatic hydrocarbons, and to reactor carbonization. Even though 1 is a large and multifunctional molecule, its high symmetry (there are only five types of C atom) simplifies the interpretation of the fundamental surface-molecule chemistry. Ruthenium provides a catalytically active metal that is capable of p bonding to polycyclic aromatic hydrocarbons and is known to be active in many processes such as Fischer– Tropsch and olefin metathesis. Our experimental approach utilizes a combination of scanning tunneling microscopy (STM), reflectance absorbance infrared spectroscopy (RAIRS), and temperature-programmed desorption (TPD) to study the surface chemistry of 1. Before depositing 1, we cleaned the (0001) face of a Ru crystal by repeated cycles of Ar sputtering, annealing in oxygen at around 940 8C, and then annealing at 1100 8C. STM images of the crystal prior to chemisorption show large flat terraces (ca. 100 nm wide) in which the individual atoms could be resolved. One of these images is shown in Figure S1A in the Supporting Information. There are a few larger bright features that apparently resulted from underlying damage to the Ru crystal by sputtering and annealing (Figure S1B). Despite these sparse defects, the arrangement of surface atoms is essentially unaffected. These features had no measurable effect on the surface chemistry or spectroscopy described below. Furthermore, the low-energy electron diffraction (LEED) pattern at 59 eV showed a hexagonally close-packed 1! 1 arrangement, as expected for the (0001) surface. No significant amount of surface oxygen atoms was detected by Auger spectroscopy. It is difficult to quantify by Auger electron spectroscopy how much carbon is on the surface, but the amount must be relatively low in view of the high quality of the STM image (Figure S1). Moreover, minor impurities on the surface will not affect the experiments because the surfaces are covered with substantially less than a monolayer of 1. Figure 1 shows the STM measurements of 1 on the Ru single crystal at a base pressure of 3! 10!10 Torr and at room temperature. 17] The features on the surface are hexagonally shaped. We measured the width of these features from the full width at half maximum in the topographic images to counteract tip effects, which tend to inflate the lateral size of molecules on the surface of metals. The diameter of 1 measured in Figure 1d (1.5 0.1 nm) is slightly larger than the diameter of 1 (ca. 1.4 nm) measured from its crystal structure. The important conclusion is that 1 bonds “face-on” to the Ru(0001) surface. We observed no mobility of the molecules on the surface of ruthenium over duration of the experiment (typically several hours). By contrast, we observed high mobility of [*] K. T. Rim, M. Siaj, S. Xiao, M. Myers, L. Liu, M. L. Steigerwald, Prof. G. W. Flynn, Prof. C. Nuckolls Department of Chemistry and The Center for Electron Transport in Molecular Nanostructures Columbia University New York, NY 10027 (USA) Fax: (+1)212-932-1289 (C.N.) or (+1)212-854-8336 (G.W.F.) E-mail: gwf1@columbia.edu cn37@columbia.edu Homepage: nuckolls.chem.columbia.edu V. D. Carpentier, P. H. McBreen Department de Chemie Universite Laval Quebec, QC, G1K 7P4 (Canada) C. Su Department of Molecular Science and Engineering National Taipei University of Technology Taipei 106 (Taiwan) M. S. Hybertsen Center for Functional Nanomaterials Brookhaven National Laboratory Upton, NY 11973-5000 (USA) [**] This work was funded by the Department of Energy under Grant No. DE-FG02-88ER13937 (G.W.F.). Equipment support provided by the National Science Foundation under grant CHE-03-52582 (G.W.F.). We acknowledge financial support from the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number CHE-0117752 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR). We acknowledge support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, US D.O.E. (DE-FG02-01ER15264). C.N. thanks the Alfred P. Sloan Fellowship Program (2004). We thank the MRSEC Program of the National Science Foundation under Award Number DMR-0213574 and by the New York State Office of Science, Technology and Academic Research (NYSTAR) for financial support for MLS and the shared instrument facility. We acknowledge research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. M.S. acknowledges the receipt of an NSERC postdoctoral scholarship. Supporting information for this article (experimental and theoretical methods, STM images, current–voltage characteristics, RAIRS spectrum, and computed structures/frontier orbitals for 3) is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie