Hyun I. Kim

Molecular contributions to the frictional properties of fluorinated self-assembled monolayers

The frictional properties of self-assembled monolayers (SAMs)formed from four different species (n-octyltrichlorosilane,1H,1H,2H,2H-perfluorooctyltrichlorosilane, tridecanethiol, and 13,13,13-trifluorotridecanethiol) were measured on the molecular scale using atomic force microscopy (AFM). On this scale, monolayers containing partially fluorinated alkyl chains exhibited higher frictional properties than monolayers containing analogous fully hydrogenated alkyl chains. Systematic comparison of the frictional properties of these SAMs provided insight into the molecular contributions to the frictional response.

Self-assembled monolayers of CF3-terminated alkanethiols on gold

Abstract The wettability and friction of self-assembled monolayers (SAMs) of terminally fluorinated alkanethiols, CF3(CH2)nSH with n=9–15, and their nonfluorinated analogs, CH3(CH2)nSH with n=9–15, were examined and compared. Surprisingly, the CF3-terminated monolayers were wetted more by water, glycerol and N,N-dimethylformamide than were the CH3-terminated SAMs. The terminally fluorinated films, however, exhibited significantly lower surface energies. The origin of these trends is discussed. The two types of monolayers were also studied by atomic force microscopy (AFM). Compared to the CH3-terminated SAMs, the fluorinated films revealed an approximately 3-fold increase in friction. A new model for the frictional response, which is based on the difference in size between the methyl and trifluoromethyl groups, is proposed.

Characterization of the surface structure of CH3 and CF3 terminated n-alkanethiol monolayers self assembled on Au{111}

Surface elemental and structural characterization of hexadecanethiol and heptadecanethiol (C16 and C17 for short) and 16,16,16-trifluorohexadecanethiol (FC16) self-assembled monolayers (SAMs) on a Au{111} surface have been obtained from time-of-flight scattering and recoiling spectrometry. The clean Au surface was also characterized in order to identify the azimuthal orientation of the SAMs with respect to the substrate. Classical ion trajectory simulations were used to relate the experimental scattering and recoiling data to the surface structure. The scattered and recoiled atoms originate from the outermost five–six atomic layers, and azimuthal anisotropy was observed in the measurements. The results provide a model for the SAMs in which the alkyl chains chemisorb with the S atoms situated above the face-centered-cubic (fcc) threefold sites of the Au{111} surface to form a continuous film with a (√3×√3)R30° structure that fully covers the Au surface. The orientation of the molecular axis azimuth of the ...

Wettability and friction of CF3-TERMINATED monolayer films on gold

Abstract Studies of the wettability and friction of self-assembled monolayers (SAMs) generated from the terminally fluorinated alkanethiols (CF 3 (CH 2 ) n SH, n = 9–15) on gold are reported. These data were compared to those of SAMs generated from the non-fluorinated analogs (CH 3 (CH 2 ) n SH, n = 9–15). The CF 3 -terminated SAMs were less wettable by hexadecane and methylene iodide, but surprisingly more wettable by water and glycerol than were the CH 3 -terminated SAMs. Measurements of friction by atomic force microscopy (AFM) revealed an approximate threefold increase in friction on the CF 3 -terminated films compared to the CH 3 -terminated films. A new model for the frictional response of fluorinated SAMs is proposed.

Generation of atomically flat MgO(100) surfaces: Influence of ambient gas composition during high temperature anneals

Atomic force microscopy, reflection high energy electron diffraction (RHEED), and x-ray photoelectron spectroscopy (XPS) have been used to follow the regeneration of atomically flat magnesium oxide (100) surfaces from samples initially etched in phosphoric acid. This regeneration process has been carried out by annealing samples at high temperatures in a range of gas compositions. The gas compositions investigated include argon, nitrogen, carbon dioxide, high partial pressures of water in nitrogen, and a range of partial pressures of oxygen. While acid etching produces initially rough surfaces, high temperature anneals (1000 °C) are found to significantly reduce the surface roughness independent of gas composition. In contrast, only annealing cycles in the presence of oxygen (O2) produced atomically flat stoichiometric MgO surfaces exhibiting good crystalline character, evidenced by distinct square-shaped terraces with well-defined step edges, a 1×1 RHEED pattern, and a 1:1 ratio between magnesium and oxy...

Comparative studies of perfluorinated lubricants adsorbed on hydrogenated amorphous carbon and amorphous carbon nitride

Temperature-programmed desorption and scanning force microscopy have been used to probe the interaction of a perfluorinated lubricant (Fomblin ZDOL) with hydrogenated amorphous carbon (a:C-Hx) and amorphous carbon nitride (a:C-Nx) substrates, two materials used as hard coatings in disk drive products. Temperature-programmed desorption measurements indicate that the nitride surfaces are more reactive toward this perfluorinated lubricant and, as a result, the thin lubricant film is more tightly bound to this substrate. Frictional force microscopy has been used to measure the coefficient of friction of the lubricated surfaces, 0.18 ± 0.02 for both substrate materials, and finds that frictional properties of these interfaces in the low load regime are influenced more by the presence of the lubricant rather than the adsorbed state of the film. Likewise, similar disjoining pressures were measured for the lubricant adsorbed on the different coating materials and suggest that the ultrathin nature of the adsorbed lubricant film dominates this property rather than adsorption states.