Combined STM and FTIR characterization of terphenylalkanethiol monolayers on Au(111):: Effect of alkyl chain length and deposition temperature

Self-assembled monolayers (SAMs) of 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)(2)(CH2)(n)-SH (TPn, n = 1-6) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRRAS). When the SAMs were prepared at room temperature (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both molecular orientation and packing density. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2 root 3 x root 3)R30 degrees molecular arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5 root 3 x 3) rectangular unit cell was found. The packing density for the even-numbered TPn SAMs is 25% lower than that for the odd-numbered TPn SAMs. When the SAMs were prepared at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing density. In the case of TP2, instead of the (2 root 3 x root 3)R30 degrees structure formed at room temperature, a c(5 root 3 x 3) structure was observed. For TP6 SAMs, the room-temperature c(5 root 3 x 3) structure was replaced by a (6 root 3 x 2 root 3)R30 degrees structure.