This paper reports a systematic investigation of the growth and attachment of small gold nanoparticles to the functionalized surfaces of larger silica nanoparticles. Dilution of the gold nanoparticles in mixtures of water and ethanol led to the self-assembly of gold nanoparticles in aggregates of regular size and shape attached to the surfaces of the silica nanoparticles. Functionalization of the surfaces of silica nanoparticles with different terminal groups had a profound influence over the coverage of gold nanoparticles and clusters. While the hydrophilic functional groups NH2 and SH bound the gold nanoparticles, hydrophobic functional groups such as CH3 and PPh2 did not. The coverage of the gold nanoparticles and clusters on the surfaces of the silica nanoparticles was evaluated using transmission electron microscopy.

Molecular-level insight into the frictional properties of fluorinated self-assembled monolayers (SAMs) was achieved by combining two recently developed techniques that operate at the subnanometer scale:  control of the interfacial composition through molecular self-assembly and tribological measurements performed with the atomic force microscope. To explore the origin of frictional forces in fluorinated films, the frictional properties of two classes of alkanethiols adsorbed on single crystal gold were measured and compared. In these studies, films of equivalent chain length, packing density and packing energy, but different termination (methyl vs trifluoromethyl), were characterized and investigated. For these films, in which the only detectable difference was the outermost chemical structure/composition, a factor of 3 increase in the frictional response was observed in going from the hydrogenated to the fluorinated film. These results support the conclusion that chemical structure/composition alone plays an integral role in determining the frictional properties of an interface. We propose that the difference in friction arises predominantly from the difference in size of the methyl and trifluoromethyl groups.

The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.

We report a comparative study of the structure and frictional properties of self-assembled monolayers (SAMs) generated by the adsorption of three homologous 17-carbon alkanethiolssheptadecanethiol, 2,2dipentadecyl-1,3-propanedithiol, and 2-pentadecyl-1,3-propanedithiolsonto the surface of Au(111). The structural properties of these SAMs were characterized by atomic force microscopy, surface infrared spectroscopy, X-ray photoelectron spectroscopy, spectral ellipsometry, and wettability by water and hexadecane. The frictional properties of the SAMs were examined by friction force microscopy. The results demonstrate that the packing density and the related crystalline order of the hydrocarbon chains influence the frictional properties of organic thin films. The origins of the frictional differences measured from these films are discussed in terms of the structure of the films.