Kinetics of Formation of Long-Chain n-Alkanethiolate Monolayers on Polycrystalline Gold

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Abstract

The effect of immersion time on the ordering properties of self-assembled docosanethiolate (C22) monolayers has been studied by reflection/absorption infrared spectroscopy (RAIRS). Starting with a chemically cleaned polycrystalline gold surface, we have found that an immersion time of 45 s in micromolar thiol solutions is sufficient to produce ordered “monolayer” films. These films have RAIR spectral features (peak position, line width, and integrated intensity) similar to those obtained with much longer immersion times. Decreasing the immersion time below 45 s gradually increases the disorder. The temperature dependence of the CH2 antisymmetric stretching (d-) peak position of thiol films obtained with an immersion time of 120 s is identical to ones obtained with very long immersion time (0.5 h to several days). In contrast, films obtained with a 5 s immersion time disorder at a lower temperature. Apart from immersion time and temperature, a large solvent effect has been observed based on real-time RAIRS measurements of the continuing film ordering during aging in vacuum. Finally, a new assignment of the shoulder appearing around 2931 cm-1 is proposed. We present arguments suggesting that the generally accepted Fermi resonance assignment of this shoulder should be modified to include a contribution from another vibrational mode.

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