The interfacial properties of n-alkylsilane self-assembled monolayers on silicon were investigated by normal force spectroscopy and lateral force measurements and correlated with molecular structure via near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Alkylsilane monolayers (CH3(CH2)n-1SiCl3) with chain lengths of n = 5 and 8 were prepared at room temperature, while n = 12 and 18 were prepared at two temperatures to form both liquid-expanded and liquid-condensed phase films. NEXAFS carbon K-edge spectra were used to compute a dichroic ratio RI, which provides a measure of molecular order. As n decreased from 18 to 5, there was a change in molecular structure from an ordered (RI = 0.41) to a disordered (RI = 0.12) phase, the former being consistent with vertically aligned chains. Normal force spectroscopy data were analyzed with an elastic contact model modified by a first-order elastic perturbation method to extract Youngs modulus, Efilm, and work of adhesion, w, of the film; Efilm decreased from 1.2 GPa to 0.67 GPa and w increased from 48.6 mJ m^-2 to 60.1 mJ m^-2 as n decreased from 18 to 5. Lateral force measurements were used to quantify the reduction in friction in terms of an interfacial shear strength, τ, and a lateral deformation analog, η. It was found that the adsorption of an alkylsilane monolayer reduced the upper bound of τ by two orders of magnitude from roughly 3500 MPa for SiO2 to less than 50 MPa for alkylsilanes with n = 12 and 18. In addition, τ was dependent on contact pressure, with n = 5 and 8 showing the largest pressure dependence due to film disorder and smaller packing density. Conversely, the upper bound of η was pressure invariant, with values of ≈ 3500 MPa for n = 5 and 8 and ≈ 1000 MPa for n = 12 and 18. Lastly, films composed of the liquid-expanded phase were more disordered and exhibit greater works of adhesion and shear strengths than their condensed phase counterparts despite having identical elastic moduli.
Citation: IEEE Journal of Microelectromechanical Systems (Journal of MEMs)
Pub Type: Journals
adhesion, elasticity, friction, self-assembled monolayers, silicon