Self-assembly (SA) of long-chain alkanethiols on copper was studied. Two factors were found to have
substantial influence on the SA process: (i) the chemical reactivity of Cu toward substances present in the
adsorption solution, particularly the solvent, and (ii) surface pretreatment, which influences the amount of
oxide and the surface morphology. Both factors are less important in the case of SA onto gold because of
its chemical inertness. Monolayers of octadecanethiol (C18SH) were adsorbed from different solvents (ethanol,
toluene, and bicyclohexyl) at various thiol concentrations onto Cu surfaces pretreated in several different
ways. The monolayers were characterized by contact-angle measurements, grazing-incidence Fourier transform
infrared spectroscopy, and scanning force microscopy. Ethanol, the most common solvent for alkanethiol
SA, is found to have a negative effect on monolayer SA apparently because of its chemical reactivity toward
copper. With toluene as a solvent, better oriented and more crystalline monolayers are obtained provided
that a higher thiol concentration is used to compensate for the higher solubility of thiols in toluene. Treatment
of the Cu surface prior to SA is shown to significantly improve the SA by reducing the amount of surface
oxide and the surface corrugation. The effect of the solvent is more critical than surface oxidation; hence,
high-quality monolayers are formed in the presence of thin oxide layers on Cu surfaces. Superior C18SH
monolayers, in terms of organization and crystallinity, are obtained by SA from toluene onto Cu surfaces
sputtered-annealed in high vacuum, even when the Cu surface is subjected to short exposure to air before
SA.
A new kind of multilayers based on metal-ion coordination was constructed on gold surfaces, where
molecular layers are successively added using a highly controlled step-by-step procedure. A bifunctional ligand
is used as the base layer, bearing a cyclic disulfide group to attach to the gold surface and a bishydroxamate
group capable of ion binding. An 8-coordinating metal ion such as Zr4+ or Ce4+ is then coordinated to the
bishydroxamate site, followed by exposure to a second ligand possessing four hydroxamate groups. The
tetrahydroxamate molecule ligates to the metal ion (bound to the base layer) using two of its four hydroxamate
groups and is free to bind a second metal ion at its other end. A sequence of adsorption steps using metal ions
and tetrahydroxamate ligands was carried out, resulting in an ordered metal−organic multilayer. Multilayer
structures comprising up to 10 tetrahydroxamate/metal ion layers were constructed, with full characterization
at each step of multilayer formation using ellipsometry, contact angle measurements, X-ray photoelectron
spectroscopy, and Fourier transform infrared spectroscopy. The multilayer morphology and mechanical properties
were studied by scanning force microscopy. It is shown that different base ligands induce dramatic differences
in the morphology and stiffness of the final multilayer. The possibility to construct segmented multilayers
containing Zr4+ and Ce4+ ions at defined locations is presented.
We are grateful to Ada Yonath and her group of the Max-Planck-Insitut fur strukturelle Molekularbiologie, Hamburg for use of laboratory facilities and to the staff of HASYLAB for assistance. We thank Meir Lahav, Isabelle Weissbuch, and Ivan Kuzmenko for discussions.
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