The importance of a solvent in regulating the adhesion forces between surfaces is studied quantitatively with scanning force microscopy. Both samples and tips are coated with alkyl thiolate monolayers of type HS(CH2)10Y and force measurements are conducted as a function of terminal group Y (Y = CH2CH3, CH2OCH3, CO2CH3, CO(NH2), CO2H, and CH2OH) and solvent (water, ethanol, and n-hexadecane). Adhesive forces in water span the greatest range (0.30−12.5 nN), with hydrophobic surfaces adhering most strongly and hydrophilic surfaces most weakly. In ethanol the adhesive forces are substantially smaller and in n-hexadecane they are negligible. In water, these adhesive forces are consistent with the work required to exclude solvent from the tip−sample interface, indicating that solvent exclusion dominates adhesion. Such macroscopic solvent exclusion cannot fully explain the adhesive forces in ethanol. This force data is used to evaluate the tip−sample interfacial energies (γts) of like CH3- and CH2OCH3-terminated surfaces and the surface−vacuum interfacial energies (γsv) of the hydrophilic surfaces. An effective tip radius of ∼30 nm and contact area of ∼10 nm2 (or ∼50 contacting molecules) is estimated from the adhesion between methyl groups in water. Since solvent exclusion regulates adhesion between these model organic surfaces, it provides a source of chemical contrast in force imaging. We explore this chemical contrast with friction force measurements of co-block polyethylene glycol−polyamide polymer surfaces.

Pentacene deposited onto a Ag(111) surface at 300 K is studied using scanning tunneling microscopy at temperatures of 300 and 50 K, providing structural insight into its unusual growth habit. At room temperature, an unexpectedly high pentacene coverage is needed to nucleate ordered pentacene islands, which appear surrounded by a disordered pentacene phase. These room temperature pentacene nuclei are revealed as bilayer structures from their coverage-dependent size evolution and molecularly resolved images of domain boundaries, recorded at 50 K. At this reduced temperature, two different monolayer phases with long-range order and commensurate with the Ag(111) surface lattice further emerge. These two monolayer phases exhibit comparable (0.7 vs 0.8 molecule/nm 2 ) packing densities, but distinct intermolecular registration and alignment with respect to the silver sublattice.

The adsorption and molecular architecture of titanyl phthalocyanine (TiOPc) monolayer films on Ag (111) have been studied with scanning tunneling microscopy. Depending on deposition flux, TiOPc selectively forms three distinct ordered monolayer structures. At lower (<0.1 ML/min) fluxes, molecules assemble into a well-ordered (2 13 × 2 13)R13.9°honeycomb phase comprised of interlocked molecular pairs. This pairing effectively reduces the repulsion between the intrinsic molecular dipole and enhances the attraction between aromatic rings. At intermediate (0.2 ML/min) fluxes, molecules form a metastable ( 21 × 21)R10.9°h exagonal phase of uniformly tilted TiOPc. At higher (0.4 ML/min) fluxes, a misfit dislocation triangular network appears, consisting of uniformly sized TiOPc domains. Molecular models for the three distinct monolayer films are developed. We describe how local electrostatic intermolecular interactions stabilize kinetically accessible structures, driving phase selection.