Vibrational structure and organization of various self-assembled alkanethiol monolayers: A HREELS study

High-resolution electron energy loss spectroscopy (HREELS) was used tb inve stigate the molecular structure of self-assembled monolayers formed by adso rption of n-alkanethiols, alpha,omega-alkanedithiols, and alpha-cycloalkyl- omega-alkanethiols on different gold substrates. The HREELS data are report ed as a contribution to a general effort to assess the potential of electro n spectroscopies in the elucidation of structural characteristics of organi c surfaces. Typical fingerprints of the thiol molecules, characteristic of their primary and secondary structure (connectivity, conformation, and orde r), have been found in their HREELS vibrational spectra. We have shown that when the layers are disordered, the spectra are dominated by impact-excite d modes, hampering the identification of the compounds. These data have evi denced many defects in the organization of the monolayers on evaporated gol d substrates. The coherent domains sizes formed by the molecules in the fil ms have been estimated from the angular distribution of the elastic peak as a function of the nature of the alkanethiol chain and of the immersion tim e in thiol solutions.