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Interfacial Mechanical Properties of n-Alkylsilane Monolayers on Silicon Substrates



Brian G. Bush, Frank W. DelRio, Cherno Jaye, Daniel A. Fischer, Robert F. Cook


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 Young’s 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.
IEEE Journal of Microelectromechanical Systems (Journal of MEMs)


adhesion, elasticity, friction, self-assembled monolayers, silicon


Bush, B. , DelRio, F. , Jaye, C. , Fischer, D. and Cook, R. (2013), Interfacial Mechanical Properties of n-Alkylsilane Monolayers on Silicon Substrates, IEEE Journal of Microelectromechanical Systems (Journal of MEMs), [online], (Accessed October 16, 2021)
Created February 1, 2013, Updated February 19, 2017