Ellipsometric and neutron diffraction study of pentane physisorbed on graphite

AFM/XPS Analysis of the Growth and Architecture of Oriented Molecular Monolayer by Spin Cast Process and Its Cross-Linking Induced by Hyperthermal Hydrogen

We used atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to comprehensively study the growth and the cross-linking of dotriacontane (C32H66) nanofilms that were deposited on a silicon wafer by the spin-coating process. It was found that the molecular structure of the nanofilms changed with C32H66 concentration at the given spin speed, of which a monolayer of oriented C32H66 molecules, formed at lower deposition concentrations, was composed of a perpendicular orientation state with the molecular long-chain axis perpendicular to the substrate surface and a parallel orientation state, while the perpendicular state was essentially dominant when the nanofilm was formed at higher deposition concentrations. The shortening of the first perpendicular layer in AFM topography could be attributed to the mixing of both parallel and perpendicular lamellas in the first layer. XPS analysis indicated that the average thickness of the layer almost linearly increased with the C32H66 concentration. The monolayer of C32H66 film could be cross-linked by a hyperthermal hydrogen-induced cross-linking (HHIC) at a few eV via kinetic collision to cleave C-H bonds. The water contact angle measurement of extensive HHIC on C32H66 nanofilms disclosed that the static contact angle decreased with the treatment time (or fluence) and saturated after full cross-linking of the film.

Effects of gas or vapor adsorption on adhesion, friction, and wear of solid interfaces

The adsorption of vapor molecules plays an important role in countless fields and is increasingly realized to be critical in tribology, which encompasses adhesion, friction, and wear of surfaces. This feature article reviews experimental methods for quantifying gas and vapor adsorption on flat solid surfaces under equilibrium conditions (ambient pressure and temperature) as well as the effects of these adsorbates on the adhesion, friction, and wear of various materials. Particular attention is given to species that are present in the ambient environment such as water (humidity) and organic vapors. These adsorbed species can have drastic yet varied influences on tribology depending on the surface chemistry of materials. Despite prolonged and ubiquitous observations in a broad range of materials and vapors, a fundamental understanding of the effect of adsorbed gases and vapors on the adhesion, friction, and wear of surfaces has begun only recently through surface-sensitive characterization.

Molecular thin films on solid surfaces: mechanisms of melting

We use molecular dynamics simulations to study the melting of pentane and hexane monolayers adsorbed on the basal plane of graphite. For both of these systems, the temperature-dependent structures and the melting temperatures agree well with experiment. A detailed analysis reveals that a mechanism involving the promotion of molecules to the second layer underlies melting in these systems. In the second-layer promotion mechanism, a small fraction of molecules transition into the second layer around the melting temperature, leaving vacant space in the first layer to facilitate disordering. The second-layer promotion mechanism arises because of the weaker molecule-surface interaction in our study than that in previous studies. The weaker molecule-surface interaction is consistent with experimental temperature-programmed desorption studies.

Transition from van der Waals to H bond dominated interaction in n-propanol physisorbed on graphite

Multilayer sorption isotherms of 1-propanol on graphite have been measured by means of high-resolution ellipsometry within the liquid regime of the adsorbed film for temperatures ranging from 180 to 260 K. In the first three monolayers the molecules are oriented parallel to the substrate and the growth is roughly consistent with the Frenkel-Halsey-Hill (FHH) model that is obeyed in van der Waals systems on strong substrates. The condensation of the fourth and higher layers is delayed with respect to the FHH model. The fourth layer is actually a bilayer. Furthermore, there is indication of a wetting transition. The results are interpreted in terms of hydrogen-bridge bonding within and between the layers.

Accelerated molecular dynamics of temperature-programed desorption

We use accelerated molecular dynamics to simulate temperature-programed desorption (TPD) of n-pentane from the basal plane of graphite in the first atomistic simulations to probe TPD over laboratory time scales. Although the simulated TPD spectra agree with experiment, a detailed analysis reveals underlying kinetic phenomena that contrast the standard experimental interpretation and opens new possibilities for understanding molecular kinetics at solid surfaces.

Thermodynamic investigation of the adsorption of amides on graphite from their liquids and binary mixtures

We report a thermodynamic investigation of the adsorption of saturated and unsaturated (cis- and trans-) alkyl amides onto the surface of graphite from their pure liquids and from binary mixtures. We identify the formation of solid monolayers of the amides at temperatures when the bulk materials are liquid. The extent of this presolidification is much more extensive than other related materials, indicating that these amide layers are significantly more stable. The monolayer stability is found to be greatest for saturated amides. In addition, the stability of unsaturated amides is extremely sensitive to the location of the double bonds in the alkyl chain of the molecules, and trans isomers are found to be more stable than cis. We also address the preferential adsorption and mixing behavior of amide mixtures and amides mixed with other species coadsorbed onto graphite from binary solution. The results indicate that the amide molecules appear to be adsorbed with their principal axis parallel to the graphite surface and that amides are found to be strongly preferentially adsorbed with respect to alkanes. Interestingly the amides appear to mix rather better than might have been expected. There is also evidence of a number of other transitions in the adsorbates.