Alkylperfluorosilane Self-Assembled Monolayers on Aluminum:  A Comparison with Alkylphosphonate Self-Assembled Monolayers

Abstract

The performance of micro- and nanoelectromechanical systems depends on the surface properties of the substrate material, such as chemical composition, roughness, friction, adhesion, and wear. Substrates of aluminum deposited onto Si (Al/Si) have been chemically reacted with perfluorodecyldimethylchlorosilane (PFMS), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and perfluorodecylphosphonic acid (PFDP) and then characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). PFMS/Al self-assembled monolayers (SAMs) were studied by friction force microscopy, a derivative of AFM, to better understand their micro- and nanotribological properties. The adhesion forces for PFMS/Al SAMs were found to be lower when compared to those of bare Al/Si; however, the coefficient of friction for both was comparable. XPS analysis revealed the presence of the corresponding alkyl chain molecules on PFMS/Al, ODP/Al, DP/Al, OP/Al, and PFDP/Al SAMs. The sessile drop static contact angle of pure water demonstrates that all the SAMs are extremely hydrophobic, giving contact angles typically >130° on PFDP/Al, ODP/Al, and PFMS/Al SAMs and >125° on DP/Al and OP/Al SAMs. The surface energy of PFMS/Al SAMs determined by the Zisman plot method is 16.5 ± 2 mJ/m2 (mN/m). The rms surface roughness of ODP/Al, DP/Al, OP/Al, PFMS/Al, and PFDP/Al SAMs, before exposure to warm nitric acid (pH 1.8, 30 min, 60−95 °C), as well as bare Al, is less than 40 nm as determined by AFM. The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid or silane molecule and the oxidized Al/Si surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60−95 °C) show ODP/Al SAMs to be the most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs.

Improving the corrosion performance of hybrid sol–gel matrix by modification with phosphonic acid

“…The Al2p peak after 40 min of sputtering (Figure 10b) was deconvoluted into three components. The curves centered at 72.9 and 74.2 eV have been attributed to Si-O-Al linkages [40] and oxide aluminum [40,41],respectively, whereas the component at 71.2 eV has been attributed to metallic contribution [16,40,42]. Hoque et al [16] identified peaks centered at 71. also contains contributions from the P-O-Al bonds.…”

“…On the other hand, phosphonic acids are an important class of self-assembled molecules with high ability to react with a range of metal oxide surfaces [16], which has been proposed as a new method for corrosion protection [17][18][19]. The formation of phosphonate layer was shown to be a spontaneous process that could be accomplished by simple immersion of the metal into the phosphonic acid solution [17].…”

“…The formation of phosphonate layer was shown to be a spontaneous process that could be accomplished by simple immersion of the metal into the phosphonic acid solution [17]. Thus, metalophosphonate bonds (Me-O-P) are formed through a condensation reaction [16]. This seemingly simple process depends on the metal substrate [20], the chemical nature of the phosphonic acid [18,19], and the pH of the solution [21], thereby requiring a careful assessment.…”

Polymer brushes and self-assembled monolayers: Versatile platforms to control cell adhesion to biomaterials (Review)

“…2͑A͒. Other combinations of adsorbates and substrates include chlorosilanes on silicon oxide, 77,78 aluminum, 79 and titanium; 80 phosphonic acids on aluminum 79 or titanium; 81,82 and catechol derivatives on titanium. 83 Although alkanethiol SAMs on gold are most commonly studied, they suffer from instability of the thiolgold bond leading to facile exchange of the adsorbates.…”

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

“…Alkylphosphonic acids are also an important class of SAM molecules because of their ability to assemble on a number of oxide surfaces, such as tantalum oxide, aluminum oxide, silicon oxide, iron oxide, copper oxide, and mica, and to form robust surface films [78][79][80][81][82][83][84][85][86][87][88][89][90][91][92]. Compared to SAMs of fatty acids, alkanethiols, and alkylsilanes, which have been studied extensively, phosphonic acid-based SAMs have not received much attention [84].…”