INFRARED-SPECTROSCOPY OF 3-DIMENSIONAL SELF-ASSEMBLED MONOLAYERS - N-ALKANETHIOLATE MONOLAYERS ON GOLD CLUSTER COMPOUNDS

Transmission infrared spectroscopy has been used to probe the structur e of alkanethiolate monolayers adsorbed onto nanometer-sized gold clus ters. The alkyl chain lengths vary between propanethiolate and tetraco sanethiolate; specifically the C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24 alkanethiolates have been examined as solid suspensions i n KBr pellets. It has been found that the smaller chain lengths (C3, C 4, and C5) are relatively disordered, with large amounts of gauche def ects present, and thus most resemble the free alkanes in the liquid st ate. The longer length alkanethiolates are predominantly in the all tr ans zigzag conformation. There are detectable amounts of near surface gauche defects, the amount of which decreases with increasing chain le ngth, and a reasonably high percentage of end-gauche defects, the rela tive amount of which increases with increasing chain length. Interanl gauche defects cannot be detected. A model is proposed to explain thes e observations, and the data are compared with that collected for alka nethiolates self-assembled onto the more traditional two-dimensional s ystems.