Competition as a design concept: polymorphism in self-assembled monolayers of biphenyl-based thiols.

Self-assembled monolayers (SAMs) of two omega-(4'-methylbiphenyl-4-yl)alkanethiols (CH(3)(C(6)H(4))(2)(CH(2))(n)SH, 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 beta-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 beta-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.

Electrochemical characterizations of nickel deposition on aromatic dithiol monolayers on gold electrodes.

The electronic properties of pristine and cross-linked (CL) self-assembled monolayers (SAMs) of [1,1';4',1' '-terphenyl]-4,4' '-dimethanethiol (TPDMT) on Au were studied by electrochemical measurements, including cyclic voltammetry and impedance spectroscopy. In addition, nickel deposition onto the TPDMT and CL-TPDMT substrates was investigated. In all cases, the TPDMT and CL-TPDMT films were found to be insulators, which effectively blocked the ionic permeation of electrolyte, preventing direct access of ions to the Au electrode. At the same time, CL-TPDMT is a better electric insulator than the pristine TPDMT SAM. The top Ni layer in the Ni/CL-TPDMT/Au arrangement was electrically isolated from the Au substrate, and no short circuits occurred. This layer was found to be conductive and relatively stable in the broad potential range in the electrolyte solution.