Nanotribological Properties of Alkanephosphonic Acid Self-Assembled Monolayers on Aluminum Oxide:  Effects of Fluorination and Substrate Crystallinity

Abstract

Two phosphonic acid (PA) self-assembled monolayers (SAMs) are studied on three aluminum oxide surfaces: the C and R crystallographic planes of single crystal alpha-alumina (sapphire) and an amorphous vapor-deposited alumina thin film. SAMs are either fully hydrogenated CH3(CH2)17PO3H2 or semifluorinated CF3(CF2)7(CH2)11PO3H2. Atomic force microscope (AFM) topographic imaging reveals that the deposited films are homogeneous, atomically smooth, and stable for months in the laboratory environment. Static and advancing contact angle measurements agree with previous work on identical or similar films, but receding measurements suggest reduced coverage here. To enable reproducible nanotribology measurements with the AFM, a scanning protocol is developed that leads to a stable configuration of the silicon tip. Adhesion for the semifluorinated films is either comparable to or lower than that for the hydrogenated films, with a dependence on contact history observed. Friction between each film and the tips depends strongly upon the type of molecule, with the fluorinated species exhibiting substantially higher friction. Subtle but reproducible differences in friction are observed for a given SAM depending on the substrate, revealing differences in packing density for the SAMs on the different substrates. Friction is seen to increase linearly with load, a consequence of the tip's penetration into the monolayer.

Use of Eyring Equation to Explore the Frictional Responses of a –CF3 and a –CH3 Terminated Monolayers Self-Assembled on Silicon Substrate

“…This model suggests that higher the adhesion, higher is the friction force. The friction of CF terminated monolayers is found to be greater [2][3][4][5][6][7][8] than that of CH terminated monolayers although the adhesion of the former is [9][10][11] less than or similar to that of the latter. As fluroalkyl and alkane are important tribological molecules, this anomaly has given rise to much debate.…”

Influence of chain ordering on frictional properties of self-assembled monolayers (SAMs) in nano-lubrication

“…Substrates on which silane-based SAMs can form include silicon oxide, aluminum oxide, quartz, glass, mica, zinc selenide, germanium oxide, and gold [1]. Densely packed organophosphonate SAMs result from formation of highly insoluble salts between the dihydrogen phosphonate moiety and tetravalent transition metal ions [59][60][61]. Phosphonate head group can bond to the surface through both monodentate and bidentate coordinations, and the adjacent head groups are linked by inter-molecular hydrogen bonding [62].…”

“…Only when the cross-sectional diameter of the spacer chains of the SAM molecules is no larger than the distance between the anchor groups attached to the substrate can a tightly packed and well ordered monolayer form [6]. Brukman et al observed that octadecylphosphonic acid SAM had lower friction on the C crystallographic plane of single crystal R-alumina (sapphire) than on the R-plane, because the C-plane (with a repeating surface area of 0.196 nm 2 vs. 0.244 nm 2 of the R-plane) could support a denser monolayer [60]. Data from infrared absorption and X-ray photoelectron spectroscopy revealed that SAMs of alkanethiolates with poly(ethylene glycol) terminus on evaporated Au and Ag surfaces had significantly different molecular conformations, which were believed to be controlled by the lattice constants of the substrates and the ordering of the alkyl chains [67].…”

“…Hoque et al also studied the nanotribological properties of alkylphosphonate and perfluoroalkylsilane SAMs on aluminum and copper oxide surfaces and observed that these SAMs on aluminum and copper performed well irrespective of the substrate used [84]. Brukman et al found that octadecylphosphonic acid and semifluorinated CF 3 (CF 2 ) 7 (CH 2 ) 11 PO 3 H 2 SAMs on amorphous alumina exhibited lower friction than on crystalline aluminum oxide due to the difference in their packing densities on these substrates [60].…”

Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution

“…However, the tip can also penetrate the brush, deforming it and dissipating energy in the process. This has been termed "ploughing" [31,36,45], and experiments on self-assembled monolayers have shown that these interactions can be treated as a combination of a load-dependent term (which takes the same form as the macroscopic Amontons' law) [46] and a term reflecting the stress required to retain a sliding contact between the tip and the surface [47][48][49]. This yields where F is the lateral force on the tip, τ is a shear strength, and μ is the friction coefficient.…”