No abstract

The adsorption structures of alkanethiol self-assembled monolayers (SAMs) on Au were studied for various alkyl chain lengths [CH 3 (CH 2 ) n-1 SH, where n ) 2, 4, 6, 8, 10, 11, 12, 14, 15, 16, and 18] in the frequency range from 150 to 3500 cm -1 by means of high-resolution electron-energy-loss spectroscopy (HREELS). In the spectra, we observed a clear odd-even effect depending on the alkyl carbon number, which confirms that the signals were obtained from well-ordered domains of alkanethiol SAMs. Thus, the multiple Au-S stretching modes observed for several alkanethiol SAMs correspond to multiple adsorption sites in the ordered alkanethiol SAMs on Au(111). The variety of the adsorption sites depending on the alkyl chain length implies that the adsorption sites in alkanethiol SAMs are not simple as generally believed, but instead are complicated.

The structural changes as a function of the immersion time of predeposited hexanethiol self-assembled monolayers (SAMs) chemisorbed on an Au(111) surface in diethyl ether at room temperature have been investigated by scanning tunneling microscopy (STM). High-resolution STM images show molecular-scale morphological changes of the monolayers resulting from the desorption of molecules from the surface and provide a significant implication for the stability of the monolayers in pure organic solvent. Defects in the ordered monolayer were mainly initiated at domain boundaries and around depressions, followed by molecular defects in closely packed domains. After the immersion of predeposited SAMs in pure diethyl ether for 10 days, the structure of the monolayer was completely changed to a disordered phase. Meanwhile, the recrystallization of the resulting disordered phase at 4 °C for 6 months gave rise to a well-ordered 6 × √3 structure having alternating missing rows in the nearest-neighbor direction, reflecting that the disordered phase originates from low surface coverage. This is the first STM result to show that the low-density molecular structure appeared by the recrystallization of the disordered phase.

Time-dependent phase transitions of closely packed alkanethiol self-assembled monolayers (SAMs) on Au(111) were investigated by scanning tunneling microscopy (STM). We report a new phase, the 6 × √3 superlattice, that exhibits deviations from the hexagonal packing arrangement formed through long-term rearrangement of the usual c(4 × 2) superlattice of alkanethiol SAMs. Our STM images show clearly systematic phase transitions with molecular-scale images from the c(4 × 2) phase to the 6 × √3 phase. This result implies that the monolayers in the c(4 × 2) phase are still in the nonequilibrium state, although it has been regarded to be in the equilibrium state in a number of previous studies. The striking implication in this study is to provide new features for closely packed alkanethiol SAMs for the first time.