pubs.acs.org/Langmuir © 2010 American Chemical Society
Dynamics of Molecular Adsorption and Rotation on Nonequilibrium Sites Heather L. Tierney, April D. Jewell, Ashleigh E. Baber, Erin V. Iski, and E. Charles H. Sykes* Department of Chemistry, Tufts University, Medford, Massachusetts 02155 Received June 26, 2010. Revised Manuscript Received August 17, 2010 It is generally accepted that important events on surfaces such as diffusion and reactions can be adsorption site dependent. However, due to their short lifetime and low concentration in most systems, adsorbates on nonequilibrium adsorption sites remain largely understudied. Using low-temperature scanning tunneling microscopy, site-dependent adsorption is shown for the molecule butyl methyl sulfide, which is trapped in multiple metastable adsorption sites upon deposition onto a Au(111) surface at 5 K. As this molecule does not have enough energy to diffuse to its preferred adsorption site on the surface, it is possible to study the behavior of individual molecules in a variety of nonequilibrium sites. Here we present atomic-scale data of the same chemical species in three independent, metastable adsorption sites and equilibration to a single equilibrium site as a function of either electrical or thermal excitation. Butyl methyl sulfide exhibits distinctly different physical properties at all four adsorption sites, including rotational dynamics and appearance in scanning tunneling microscopy (STM) images. An energy profile is proposed for the adsorption and equilibration of these species, and a correlation is drawn between rotational barrier and adsorption energy.
Introduction While it is known that reactions can be adsorption site dependent,1,2 it is most often assumed that molecules occupy equilibrium sites when adsorbed on surfaces. This is not always the case; for example, on crowded surfaces, molecules can be forced to adopt less energetically favorable sites.3 A few careful studies have shown reaction dependence on adsorption sites1,2 as well as site-dependent diffusion,2,4 adsorption,5-9 and lateral forces;10-12 however, in most cases, very little is known about mechanisms for diffusion between adsorption sites. It has also been demonstrated that molecules with multiple attachment sites to a surface can exhibit a mismatch with the surface lattice. This forced binding to nonequilibrium adsorption sites can lead to highly directional diffusion properties.13 Since intermediate or precursor states often only exist on the surface in small quantities or for short periods of times, it is very difficult to study their physical properties.14,15 Using lowtemperature scanning tunneling microscopy (STM), it is possible *To whom correspondence should be addressed. (1) Newton, T. A.; Huang, Y. C.; Lepak, L. A.; Hines, M. A. J. Chem. Phys. 1999, 111, 9125–9128. (2) Hwang, I. S.; Lo, R. L.; Tsong, T. T. Surf. Sci. 1998, 399, 173–189. (3) Tysoe, W. T.; Ormerod, R. M.; Lambert, R. M.; Zgrablich, G.; Ramirezcuesta, A. J. Phys. Chem. 1993, 97, 3365–3370. (4) Miwa, J. A.; Weigelt, S.; Gersen, H.; Besenbacher, F.; Rosei, F.; Linderoth, T. R. J. Am. Chem. Soc. 2006, 128, 3164–3165. (5) Maraghechi, P.; Horn, S. A.; Patitsas, S. N. Surf. Sci. 2007, 601, L1–L5. (6) Suzuki, S.; Yamaguchi, Y.; Onishi, H.; Fukui, K.; Sasaki, T.; Iwasawa, Y. Catal. Lett. 1998, 50, 117–123. (7) Stranick, S. J.; Kamna, M. M.; Weiss, P. S. Science 1994, 266, 99–102. (8) Tsukahara, N.; Mukai, K.; Yamashita, Y.; Yoshinobu, J. J. Chem. Phys. 2008, 128. (9) Yoshinobu, J.; Tsukahara, N.; Yasui, F.; Mukai, K.; Yamashita, Y. Phys. Rev. Lett. 2003, 90. (10) Albers, B. J.; Schwendemann, T. C.; Baykara, M. Z.; Pilet, N.; Liebmann, M.; Altman, E. I.; Schwarz, U. D. Nat. Nanotechnol. 2009, 4, 307–310. (11) Schwarz, A.; Allers, W.; Schwarz, U. D.; Wiesendanger, R. Phys. Rev. B 2000, 61, 2837–2845. (12) Ternes, M.; Lutz, C. P.; Hirjibehedin, C. F.; Giessibl, F. J.; Heinrich, A. J. Science 2008, 319, 1066–1069. (13) Kwon, K. Y.; Wong, K. L.; Pawin, G.; Bartels, L.; Stolbov, S.; Rahman, T. S. Phys. Rev. Lett. 2005, 95. (14) Grunze, M.; Kreuze, H. J. Kinetics of Interface Reactions; Springer: Berlin, 1987. (15) Stipe, B. C.; Rezaei, M. A.; Ho, W. J. Chem. Phys. 1997, 107, 6443–6447.
15350 DOI: 10.1021/la102588h
to monitor the properties of these nonequilibrium or precursor species on a molecule-by-molecule basis,16 and if imaging conditions are chosen carefully, metastable species can be imaged without being perturbed.17,18 Previous studies have used step edges and defects to trap molecules;19,20 however, we have chosen molecules that are trapped in multiple metastable adsorption sites on terraces when deposited at low surface temperatures. The molecule under consideration in this paper is butyl methyl sulfide, which is a simple asymmetric thioether (RSR0 ) molecule. This molecule and others like it have been studied as molecular rotors; however, only equilibrated molecules have been considered in previous studies.21-26 As precursor-adsorbed molecules do not have enough energy to make it to their preferred sites (which are most likely
