Increased Lateral Density in Alkanethiolate Films on Gold by Mercury Adsorption

Abstract

We present a method for increasing the lateral chain density in self-assembled monolayers (SAMs) of alkanethiols on polycrystalline gold. This method relies on exposure of the alkanethiolate monolayers to mercury vapor and subsequent reimmersion into the thiol solution. Mercury adsorption on the gold surface induces a structural rearrangement in the alkanethiolate monolayers, as indicated by changes in dichroism in the Fourier transform infrared (FTIR) spectra, in line shape in the sum frequency generation (SFG) spectra, and in the macroscopic wetting behavior of the monolayers. X-ray photoelectron spectroscopy (XPS) data show that saturation of the thiolate samples with mercury occurs after 20−30 min of exposure to air saturated with mercury vapor. For 100 nm evaporated gold films a mercury bulk concentration of 14−16 atom % was determined by energy-dispersive X-ray analysis (EDX). Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) data indicate that after mercury adsorption the monolayers consist of gold thiolate and mercury thiolate molecules. From the FTIR and XPS data we conclude that the mercury-exposed SAMs exhibit an inhomogeneous structure with differently tilted domains. As determined from the IR experiments, the average tilt angle of the alkyl chains in hexadecanethiolate monolayers decreases by ∼16° by mercury adsorption and by an additional ∼3° after reimmersion into the thiol solution. The corresponding changes obtained from near edge X-ray absorption fine structure (NEXAFS) spectra are ∼9° and ∼3°, respectively.