ABSTRACT
Highly ordered alkanethiol self-assembled monolayers (SAMs) on gold substrates are suitable models of boundary layer lubricants and may be used in actual nanoscale device applications. Here, such monolayers were studied by spectroscopic methods as a function of tribological wear (rubbing) using a pin-on-disk microtribometer. The coefficient of friction (COF) (ratio of the frictional force to the load) was measured with the tribometer, and reflectance infrared spectra and X-ray photoelectron spectra were obtained as the monolayer film failed and the COF changed. The results show that it is possible to correlate disorder in the monolayer film with tribological failure of the film, and that continued rubbing produces a chemical change in the monolayer film. Disorder in the monolayer is distinct from the influence of wear in the underlying gold substrate. Aged SAMs, having sulfonate rather than thiol headgroups and initially less well ordered, behave differently to the well-ordered freshly prepared SAMs. Interestingly, they show a lower COF over many more cycles of exposure to the rubbing pin. The impact of the mechanism of film failure in boundary layer lubrication is discussed.
ABSTRACT
Thin oxide films (from one to tens of monolayers) of SiO2, MgO, NiO, Al2O3, FexOy, and TiO2 supported on refractory metal substrates have been prepared by depositing the oxide metal precursor in a background of oxygen (ca 1 x 10(-5) Torr). The thinness of these oxide samples facilitates investigation by an array of surface techniques, many of which are precluded when applied to the corresponding bulk oxide. Layered and mixed binary oxides have been prepared by sequential synthesis of dissimilar oxide layers or co-deposition of two different oxides. Recent work has shown that the underlying oxide substrate can markedly influence the electronic and chemical properties of the overlayer oxide. The structural, electronic, and chemical properties of these ultrathin oxide films have been probed using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS), ion-scattering spectroscopy (ISS), high-resolution electron energy loss spectroscopy (HREELS), infrared reflectance absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS).
