Intermolecular Interactions between a Monomolecular Hydroxyl-Terminated Perfluoropolyether Film and the Sputtered SiNx Surface

Abstract

The intermolecular interactions at the interface between a monomolecular hydroxyl-terminated perfluoropolyether (PFPE) liquid and an amorphous silicon nitride (SiNx) film were investigated using contact angle goniometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results demonstrate that the surface of the sputtered SiNx film is highly oxidized (SiO2) and contain hydroxyl groups (SiOH) that are capable of attractive interactions with the hydroxyl end groups of the PFPE. The attractive interactions lead to a lowering of the polar surface energy with increasing PFPE coverage up to a monolayer. Ab initio quantum chemical computations on model dimers provide the energetic details associated with the PFPE/surface interactions. The primary source of the attractive, adhesive interactions in the PFPE/SiNx system stems from hydrogen bonding between the PFPE and SiNx hydroxyl groups. The binding energy is computed to be approximately −4 kcal/mol. Both the SiNx surface nitrogen and oxygen atoms in the Si3N and Si−O−Si local geometries are weakly basic and hence incapable of providing strong adhesive interaction sites for the PFPE hydroxyl end groups. The results on the kinetics of the adhesive interactions between the hydroxyl-terminated PFPE and the SiNx surface demonstrate a nonclassical time-dependent rate coefficient, k(t) ∝ k o t - h . With increasing temperature, the temporal dependence is observed to change from h = 0.5 (lower temperature) to h = 1.0 (higher temperature), which is interpreted as a two-dimensional melting transition from a solidlike to a liquidlike confined film. Under ambient conditions, adsorbed water competes with PFPE for surface bonding sites on the polar SiNx surface, leading to a significantly reduced level of adhesion for the hydroxyl-terminated PFPE.

Chemically Modified Solid-State Nanopores

“…Our solid-state nanopore device is depicted in Figure (left panel). The SiN membrane surface contains a native oxide layer, which is used here for monolayer self-assembly of organosilanes . Prior to coating, we find that piranha treatment is crucial for removal of contaminants and surface activation.…”

The Adhesion of Monomolecular Hydroxyl-terminated Perfluoropolyether Liquid Films on the Sputtered Silicon Nitride Surface as a Function of End Group Acidity and Mobility

“…The details of the bonding interactions between the PFPE hydroxyl end group and the SiN x surface is finally summarized via dimer interactions using ab initio quantum chemistry. Again for the purposes of this discussion, the sputtered SiN x surface polar group is assumed to be the silanol functionality (SiOH) based upon previous work [9]. The salient features of the Hbonding interactions are summarized in figure 13 and table 3.…”

“…For the film thicknesses utilized in these studies, nominally 25 Å , the stoichiometry of the sputtered SiN x film was determined to be Si 3.0 N 3.0 O 1.6 with significant SiO 2 and surface SiOH groups on the basis of XPS and FTIR analyses, respectively [9]. The functional moiety on the sputtered SiN x surface that is reactive to the PFPE hydroxyl groups is presently considered to derive primarily from the SiOH group based upon the following previous findings: (1) the SiOH functional moiety was detected via FTIR; (2) ESCA showed approximately 30% SiO 2 in sputtered SiN x ; and (3) theoretical studies showed the Si 3 N nitrogen lone pair of electrons was unavailable for chemistry due to Si-N d(p)-p(p) interactions [9]. Throughout the text we will frequently refer to the SiN x film as ''sputtered SiN x '' to distinguish it from stoichiometric Si 3 N 4 .…”

“…In these studies the optimized intermolecular distance between a model PFPE main chain and a model SiN x surface was computed and compared to the d o used to fit the dispersive surface energy data. The model PFPE main chain was represented by CF 3 OCF 3 , and a model SiN x surface site was represented by H 3 SiOH as explained in the experimental section [9]. The results of the computations are presented in figure 3 and table 2.…”

“…However, the tribological properties of these materials is not wellunderstood and appears to vary significantly with the stoichiometry of the film (Si:N:O) and in particular with the extent of film oxidation [4,5]. The tribological robustness of the HDI (head-disk interface) has been linked to the physical state of the adsorbed lubricant thin film and the fundamental understanding of the interactions between perfluoropolyether (PFPE) lubricants and the SiN x surface is only now being quantified [6][7][8][9].…”