Scott S. Perry

Preparation and characterization of MgO(100) surfaces

Abstract The preparation of crystalline magnesium oxide surfaces by means of mechanical polishing, acid etching, oxidative annealing and vacuum annealing has been studied on an atomic scale using low energy electron diffraction (LEED) and ultrahigh vacuum (UHV) atomic force microscopy (AFM). Mechanical polishing results in a microscopically smooth and an atomically disordered surface. Although acid etching of the magnesium oxide surface results in LEED patterns indicating a crystalline nature, AFM characterization of these surfaces reveals that surface layers are removed in an anisotropic fashion resulting in a surface roughness on the order of 100 A (which is not probed by the LEED studies). Annealing this same surface in vacuum only modestly reduces the surface roughness while high temperature anneals of an acid etched surface in a 1 atm oxygen environment produces a well ordered surface characterized by 1000 A terraces separated by single and double layer step heights. Further annealing these terraced surfaces to 1000°C under vacuum produced well-ordered and crystallographic surfaces on an atomic scale. Topographic images of the MgO(100) surface obtained under UHV conditions reveal rows of oxygen atoms oriented in a 〈110〉 direction with a spacing of 3.0 A and indicate that this surface preparation results in an unreconstructed, bulk termination of the rock salt structure.

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.

Fundamental measurements of the friction of clean and oxygen-covered VC(100) with ultrahigh vacuum atomic force microscopy: evidence for electronic contributions to interfacial friction

Abstract Atomic force microscopy (AFM) has been employed in an ultrahigh vacuum (UHV) environment to measure the frictional properties of single-crystal vanadium carbide (VC) as a function of reaction with oxygen. The VC(100) surface, prepared by sputtering and annealing in vacuum, was found to be free of oxygen and to exhibit good crystallographic order with atomically flat terraces ∼100–200xa0A in width. The coefficient of friction between a silicon nitride probe tip and the clean VC surface was measured as 0.52±0.04. Chemical modification of this surface was accomplished by exposure to molecular oxygen which produced a saturation coverage of chemisorbed atomic oxygen. The coefficient of friction between the same silicon nitride tip and the “oxidized” VC surface in the wearless regime was measured as 0.32±0.05, representing a ∼40% reduction in friction. This reduction in friction is associated with a reduction in the density of metal d electrons nearest the Fermi level which occurs upon oxygen adsorption. These changes provide evidence for the contribution of an electronic mechanism of energy dissipation at the sliding interface of a solid–solid point contact.

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.

Frictional properties of titanium carbide, titanium nitride, and vanadium carbide: Measurement of a compositional dependence with atomic force microscopy

The frictional properties of TiC(100), TiN(100), and VC(100) surfaces have been investigated on the microscopic scale by atomic force microscopy. In this work, friction measurements were performed under controlled ambient conditions to emulate the use of these materials as hard coatings. A variety of tip materials, silicon nitride, titanium carbide, titanium nitride, and tungsten carbide, were used to investigate the correlation between surface composition and frictional properties of the carbide and nitride substrates. The surface compositions of both clean and air-exposed samples were characterized by x-ray photoelectron spectroscopy to complete the composition/friction correlation. In these studies, the TiC(100) substrate exhibited the lowest frictional response of the substrate samples, regardless of tip composition. The friction measurements on TiN(100) and VC(100) exhibited a strong dependence on counterface composition and were consistently higher than those of TiC. In addition to the compositional...

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.

On the role of electromagnetic field gradients in surface Raman scattering by molecules adsorbed on single crystal metal surfaces

Bands normally forbidden in the vibrational Raman spectra of molecules appear with some intensity when they are adsorbed on solid surfaces. The origin of this intensity can, in principle, be accounted for by two mechanisms. Since the symmetry of the adsorbed molecule is lower than that of the free molecule, relaxed selection rules result. Alternatively, the large gradient in the normal component of the electromagnetic field near a conducting surface can induce scattering through the dipole–quadrupole polarizability. To assess the relative importance of these two mechanisms, the field gradient at the surface of a Ag(111) crystal was varied by using different incident laser frequencies and its effect on the relative intensity of a putative quadrupole mode monitored. The intensity of the mode remained constant over the frequency region in which the field gradient varied. It was concluded that the field gradient mechanism is not important, at least for molecules physically adsorbed on flat surfaces.

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 (1000u2009°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.

Spectroscopic and scanning probe studies of oxygen and water on metal carbide surfaces

Spectroscopic and scanning probe techniques were applied to the study of clean, (100) single-crystal surfaces of titanium carbide (TiC) and vanadium carbide (VC), and their interactions with oxygen and water. We find that oxygen adsorbs dissociatively on both surfaces, but discriminates between them by reacting with TiC to form an oxide while forming a metastable overlayer on VC. Water bonds molecularly and dissociatively at low temperatures and behaves similarly on both materials. These results are interpreted in the context of surface electronic structure.