Quantification of Fluorine Density in the Outermost Atomic Layer R. D. van de Grampel, †,§ W. Ming,* ,† A. Gildenpfennig, ‡ J. Laven, † H. H. Brongersma, ‡ G. de With, † and R. van der Linde † Laboratory of Coatings Technology, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Calipso BV, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received July 18, 2003 The outermost atomic layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Absolute quantification of fluorine density in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine density of monolayers of a C8F17-thiol on gold was 1.48 × 10 15 F atoms/cm 2 , whereas for PTFE a value of 1.24 × 10 15 F atoms/cm 2 was observed. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a molecular interpretation on the perfluorinated thiol monolayer on gold are given. 1. Introduction 1.1. General. The surface of materials often plays a central role for their applications. Understanding of the physical and chemical properties of surfaces is of para- mount importance for the design of novel materials. Fluorinated polymers belong to a class of materials with exceptional properties. These properties originate from the nature of the carbon-fluorine bond and the orientation of the fluorinated tail. From both a fundamental and an application point of view, it is desirable to gain control over the wetting properties of fluorinated materials. Modern surface analytical methods such as X-ray pho- toelectron spectroscopy (XPS), 1 time-of-flight secondary ion mass spectroscopy (TOF-SIMS), 2 atomic force micros- copy (AFM), 3 scanning force microscopy (SFM), 4 and near- edge X-ray absorption fine structure (NEXAFS) 5 have been used to elucidate many aspects of the chemical composition as well as the structure of fluorinated surfaces. These methods provide useful information on the spatial varia- tion in chemical composition near the surfaces. But none of them can exclusively analyze the composition of the outermost atomic layer. Nevertheless, from the nature of the forces involved, for example, van der Waals interactions, it is expected that the outermost atomic layer of the surface predomi- nantly determines the wettability. Consequently, the wettability of an organic material is not related to the overall chemical composition but rather depends on the chemical nature of the outermost functional groups. Zisman 6 observed that the surface free energy decreases depending on the chemical composition of the material in the sequence -CH 2 ->-CH 3 >-CF 2 ->-CF 3 . There- fore, knowledge of the interfacial behavior (e.g., surface energies) in relation to surface composition will contribute to a better understanding and prediction of adhesion and wetting phenomena. This means that characterization of the chemical composition of the outermost atomic layer is highly relevant. Low-energy ion scattering (LEIS), also referred to as ion scattering spectroscopy (ISS), is a unique technique that exclusively probes the first atomic layer of the surface (even when the sample surface is not smooth). 7 1.2. LEIS. In a LEIS experiment, the sample is bom- barded with low-energy noble gas ions. The energy of the ions that are elastically backscattered provides direct information on the masses of the atoms from which the ions are scattered. 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