Influence of molecular structure on phase transitions:: A study of self-assembled monolayers of 2-(aryl)-ethane thiols

Self-assembled monolayers (SAMs) prepared on Au(111) substrates from solutions of omega-(4'-methylbiphenyl-4-yl)ethane thiol (CH3(C6H4)(2)(CH2)(n)SH, n = 2, BP2), at room temperature and subsequently annealed at temperatures of up to 423 K were studied using scanning tunneling microscopy, low-energy electron diffraction, high-resolution X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy. Upon annealing a phase transition occurs from the low-temperature (5 root 3 x 3) structure common to all SAMs prepared from the series of BPn homologues with n = even studied so far, to a new structure which is markedly different from the high-temperature phases of the higher BPn homologues. Although its basic structure can be approximated by a (2 root 3 x 2) unit cell, the regular occurrence of line defects running exclusively along the 11 (2) over bar direction is the most characteristic feature of this new phase. Irrespective of these defects the phase transition dramatically improves the stability of the BP2 monolayer as demonstrated by exchange experiments. In contrast to BP2, SAMs made from the closely related 2-phenylethane thiol (C6H5(CH2)(2)SH, P2) do not show any phase transition. The differences between BP2, its higher homologues, and P2 highlight the subtleties of the interplay of different factors determining the structure of a SAM.