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Low-energy electron diffraction and reflection-absorption infrared spectroscopy were used to study the monolayers formed by the adsorption of n-alkane thiols [HS(CH2)mCH3] on both (111) and (100) single-crystal gold substrates. Samples were prepared by dosing either from solution (m=15, 17, 18, and 21) or in ultrahigh vacuum (m=0–9). On Au(111), ordered surface structures are obtained which can be indexed as (n√3×√3)R30°, where n varies from 1 to 6. On Au(100), the adsorption of short chain thiols leads to the formation of a c(2×2) overlayer while the longer chain homologs show additional diffraction spot splittings. It is also found that chain length influences both the character of the diffraction seen and perturbs the reactive sticking probability of molecules dosed in UHV. Infrared studies reveal that the polymethylene chains of the monolayers formed on Au(100) are comprised of nearly all-trans conformations and are less canted than the comparable structures formed on Au(111). A simple model is proposed to account for the differences seen on the two surfaces and a mechanism for the adsorption and assembly of organic thiols into ordered monolayer phases on gold is presented.

The influences of temperature on the infrared spectra of a docosylthiol (C22) monolayer on a gold substrate are reported. The data suggest that this adsorbate exhibits a complex phase state at 300 K in which gauche conformations concentrated at the chain termina figure prominently. The detailed temperature dependence of the spectral band intensities is found to be very similar in many regards to that of bulk hydrocarbon crystals. At low temperature, band splittings are observed which suggest the lowest energy structure for this adsorbate is one containing two chains per unit cell. A simple lattice argument, consistent with steric models and low-energy electron diffraction data, is presented to explain these observations.