Effect Of Self-assembled Monolayers Research Articles

Odd−Even Effects in Self-Assembled Monolayers of ω-(Biphenyl-4-yl)alkanethiols: A First-Principles Study

Conjugated molecules with a saturated alkyl linker between a thiol docking group and the pi-conjugated core have been shown to form self-assembled monolayers (SAMs) with a high degree of long-range order and uniformity. Additionally, pronounced odd-even effects have been observed in a number of properties characterizing these SAMs. We focus on omega-(biphenyl-4-yl)alkanethiols with n = 0-6 -(CH2)n- units deposited on Au(111) and investigate the microscopic origin of these odd-even effects in terms of the local sulfur-gold bonding geometry by employing first-principles calculations. An additional structural parameter, the torsion angle between the two phenyl rings in the biphenyl moiety, is identified and its relation to the experimentally observed odd-even effects is discussed. More importantly, we address relevant quantities for the application of these SAMs in molecular electronic devices, in particular, the modification of the work function of the underlying metal substrate and the energetic alignment of the molecular orbitals in the SAM with the Fermi level. While no clear trend emerges for the former, we find pronounced odd-even effects for the latter. Furthermore, the insertion of a single methylene unit between the biphenyl core and the thiol appears to largely decouple the valence electronic systems of the pi-conjugated segment and the gold substrate. Our results thus provide a solid theoretical basis for the interface energetics in this important class of systems.

The effect of self-assembled monolayers on polarization-dependent two-photon photoemission and on the angular distribution of the photoelectrons

The effect of a self-assembled organized organic monolayer on the two-photon photoemission from semiconductor substrates was investigated. It has been found that the monolayer affects the relative yield of photoelectrons emitted by p-polarized versus s-polarized light. In addition, the monolayer affects the angular distribution of the ejected electrons. The effect on the photoelectron yield is attributed to the monolayer "smoothing" the electronic potential on the surface by eliminating surface states and dangling bonds. The effect on the angular distribution is attributed to a post-ejection interaction between the photoelectrons and the adsorbed molecules.

Impact of self‐assembly composition on the alternate interfacial electron transfer for electrostatically immobilized cytochrome c

We report on the effects of self-assembled monolayer (SAM) dilution and thickness on the electron transfer (ET) event for cytochrome c (CytC) electrostatically immobilized on carboxyl terminated groups. We observed biphasic kinetic behavior for a logarithmic dependence of the rate constant on the SAM carbon number (ET distance) within the series of mixed SAMs of C(5)COOH/C(2)OH, C(10)COOH/C(6)OH, and C(15)COOH/C(11)OH that is in overall similar to that found earlier for the undiluted SAM assemblies. However, in the case of C(15)COOH/C(11)OH and C(10)COOH/C(6)OH mixed SAMs a notable increase of the ET standard rate constant was observed, in comparison with the corresponding unicomponent (omega-COOH) SAMs. In the case of the C(5)COOH/C(2)OH composite SAM a decrease of the rate constant versus the unicomponent analogue was observed. The value of the reorganization free energy deduced through the Marcus-like data analysis did not change throughout the series; this fact along with the other observations indicates uncomplicated rate-determining unimolecular ET in all cases. Our results are consistent with a model that considers a changeover between the alternate, tunneling and adiabatic intrinsic ET mechanisms. The physical mechanism behind the observed fine kinetic effects in terms of the protein-rigidifying omega-COOH/CytC interactions arising in the case of mixed SAMs are also discussed.

The Effect of Self-Assembled Monolayers on Interfacial Fracture

This paper describes a series of experiments and analyses that were used to examine crack growth near sapphire/epoxy interfaces. Adhesion of the epoxy to the sapphire was enhanced by coating the sapphire with mixtures of two silane coupling agents that form self-assembled monolayers. A new biaxial loading device was used to conduct a series of mixed-mode fracture experiments. Crack opening interferometry, atomic force microscopy, and angle-resolved X-ray photoelectron spectroscopy allowed cohesive zone sizes, fracture surface topographies, and loci of fracture to be established. The experiments were complemented by finite element analyses that accounted for the rate- and pressure-dependent yielding of the epoxy. The analyses also made use of traction-separation laws to represent the various interphases that were produced by the mixed monolayers. The intrinsic toughness (defined as the area underneath the traction-separation curve) of the bare sapphire interfaces was independent of mode-mix and lower than values from previous experiments with glass/epoxy and quartz/epoxy specimens. The increase in overall toughness with mode-mix was completely accounted for by viscoplastic dissipation in the epoxy outside the cohesive zone. The minimum toughness of the coated sapphire interfaces was about five times higher than the mode-mix independent intrinsic toughness of the uncoated specimens. The increase in overall toughness with mode-mix was almost completely accounted for by increases in the intrinsic toughness as the traction-separation law varied with mode-mix. As a result, viscoplastic dissipation outside the cohesive zone was minimal. Atomic force fractography and X-ray photoelectron spectroscopy indicated that the crack growth mechanisms and the loci of fracture in the coated and uncoated specimens were quite different.

Electronic memory effects in self-assembled monolayer systems

We report stable and reproducible switching and memory effects in Self-Assembled Monolayers (SAMs). We demonstrate realization of negative differential resistance (NDR) and charge storage in electronic devices that utilize single redox-center-contained SAM as the active component; and compare the effects of various redox centers to switching and storage behavior. The devices exhibit electronically programmable and erasable memory bits with bit retention times greater than 15 min at room temperature.

Electronic transport of molecular systems

We report stable and reproducible switching and memory effects in self-assembled monolayers (SAMs). We demonstrate realization of negative differential resistance (NDR) and charge storage in electronic devices that utilize single redox-center-contained SAM as the active component; and compare the effects of various redox centers to switching and storage behavior. The devices exhibit electronically programmable and erasable memory bits with bit retention times greater than 15 min at room temperature.