Competition as a Design Concept: Polymorphism in Self-Assembled Monolayers of Biphenyl-Based Thiols
journal contributionposted on 25.10.2006, 00:00 by Piotr Cyganik, Manfred Buck, Thomas Strunskus, Andrei Shaporenko, James D. E. T. Wilton-Ely, Michael Zharnikov, Christof Wöll
Self-assembled monolayers (SAMs) of two ω-(4‘-methylbiphenyl-4-yl)alkanethiols (CH3(C6H4)2(CH2)nSH, BPn, n = 4, 6) on Au(111) substrates, prepared from solution at room temperature and subsequently annealed at temperatures up to 493 K under a nitrogen atmosphere, were studied using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BPn SAMs with n = odd, for which only one phase is observed, the even-numbered BPn SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in density and stability. Upon annealing, BP4 and BP6 transform into a β-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diameter. Exchange experiments, monitored by contact angle measurement, reveal that the β-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even- and odd-numbered BPn SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.