Ernst*, a, M. Böhringera, A. Cossy-Gantnera, C.F. McFaddena, U. Müllera, U. Ellerbeckb
aSwiss Federal Laboratories for Materials Testing and Research (EMPA), Surface Technologies, Ueberlandstrasse 129, CH-8600 Dübendorf, SWITZERLAND bDepartment of Food Toxicology, Veterinary Medical
University of Hannover, Germany
Chiral surfaces are of interest in fields such as stereoselective chemical synthesis, separation of chiral compounds, crystal growth, adsorption of proteins, and optical activity. Studying the adsorption and reaction of chiral molecules on well-characterized chiral surfaces in UHV will provide a framework for understanding the mechanism of stereoselective interaction of molecules with surfaces. The essence of our research is to prepare stable inorganic surfaces which possess densely packed screw dislocations of the same handedness. The average distance between these chiral sites is to be about 1 nm. The chiral surfaces will be fabricated in a two-step process in which an achiral substrate is first modified by the adsorption of one enantiomer of a helical molecule. This will provide a chiral matrix for subsequent physical vapor deposition (PVD) of metals. It is expected that the helical molecules will serve as nucleation sites for the formation of screw dislocations. All steps of the fabrication are performed in ultrahigh vacuum (UHV) and monitored in-situ with scanning tunnelling microscopy (STM).
After successful synthesis and separation of the enantiomers, presently we are studying the interaction of optically pure M- and P-helicene, a polyaromatic phenanthrene derivative (C30H18, see Fig. 1), with well-defined single-crystal surfaces. The next step will be to investigate the initial stages of metal layer growth on a closed packed heIicene monolayer, before thicker and more stable multilayers are grown.
First results of the interaction of heptahelicene with a Ni(111) surface
will be presented. The molecules are deposited onto the single crystal
substrate via molecular beam technique and subsequently characterized with
scanning tunnelling microscopy (STM), temperature programmed desorption
(TPD), x-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy
(NEXAFS), x-ray photo diffraction (XPD), low energy electron diffraction
(LEED), and time-of-flight secondary mass spectrometry (ToF-SIMS). From
these studies a structure model of the closed packed monolayer of helicene
will be presented. In addition, the chiral overlayers were subjected to
experiments investigating the circular dichroic properties in photoelectron
spectroscopy (NEXAFS) and photo emission electron microscopy (PEEM) utilizing
circularly polarized synchrotron radiation.