Optical Second Harmonic Generation of Thiols Adsorbed on Gold
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OPTICAL SECOND HARMONIC GENERATION OF THIOLS ADSORBED ON GOLD 3 1 2 2 M. Buck 1 , F. Eisert , J. Fischer , M. Grunze , F. Triger
1)Angewandte Physikalische Chemie der Universitit Heidelberg, Im Neuenheimer Feld 253, W-6900 Heidelberg, Fed. Rep. Germany 2)Physikalisches Institut der Universitdt Heidelberg, Philosophenweg 12, W-6900 Heidelberg, Fed. Rep. Germany 3)Fachbereich Physik, Universitdt Kassel, Heinrich-Plett-Str. 40, W-3500 Kassel, Fed. Rep. Germany
Abstract The adsorption kinetics of self-assembling n-alkyl thiol films on polycrystalline gold substrates was studied in situ by optical second harmonic generation (SHG). The relation between the SHG signal change and the thiol coverage was established by comparison with ex situ X-ray photoelectron spectroscopy measurements. We find that the temporal dependence of the adsorption is well described by Langmuir adsorption kinetics. The source of the SHG signal is found to be linearly related to the nonlinear susceptibility of the adsorbate-substrate interaction. Experiments with chain lengths ranging from 1 to 22 showed, that the saturation coverage for thiols longer than hexanethiol is equal.
Introduction Long chain alkane thiols are known to self-assemble to well-ordered films up to the coverage of one monolayer. The binding to the surface is mediated by the sulphur atom and the hydrocarbon chains are parallel oriented at an angle of approximately 350 with respect to the surface normal [1,2]. Recently SHG has emerged as a powerful technique to investigate interfaces [3-5]. Since thiols form well defined films they are well suited as model systems to study fundamental aspects of signal generation in second harmonic and sum-frequency experiments [6-8]. Furthermore, thiol films are promising systems for applications in microelectronics and may serve as passivation layers to prevent corrosion [9]. Exchange of the methyl end group by other functional groups make thiols also candidates as versatile adhesion links [10]. Thiols also appear as byproducts in catalytic reactions if the catalyst is contaminated with sulphur [11,12]. The present paper summarizes the results of recent experiments carried out by optical second harmonic generation (SHG) to monitor the adsorption of thiols of Mat. Res. Soc. Symp. Proc. Vol. 237. @1992 Materials Research Society
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different chain lengths on polycrystalline gold surfaces. In contrast to the well established surface analytical tools SHG combines advantages like intrinsic sensitivity to interface properties, high time resolution and the possibility of in situ diagnostics under realistic experimental conditions. Also, the method is nondestructive.
Experimental Details of the setup of the SHG experiment have been described elsewhere [6]. The p-polarized fundamental of a pulsed Nd-YAG laser with 7 ns pulse width was incident on the gold surface at an angle of 450. In all experiments a fluence of 45 mJ/cm 2 was used at a pulse repetition rate of 10 Hz. No influence of the laser radiation on the adsorption kinetics was detectabl
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