Interfacial Mechanical Properties of $n$ -Alkylsilane Monolayers on Silicon Substrates

The interfacial properties of n-alkylsilane monolayers on silicon were investigated by normal force spectroscopy, lateral force measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Monolayers of (CH₃( CH₂)n - 1SiCl₃) with chain lengths n = 5, 8, 12, and 18 were prepared and NEXAFS spectra were used to compute the dichroic ratio, R_I. As n decreased from 18 to 5, the film structures change from ordered (R_I = 0.41) to disordered (R_I = 0.12) states. Normal force spectroscopy data were analyzed with a modified elastic adhesive contact model to extract Young´s modulus, E_film, and the work of adhesion, w, of the film; E_film decreased from 1.2 to 0.67 GPa, and w increased from 48.6 to 60.1 mJ·m^-2 as n decreased from 18 to 5. Lateral force measurements quantified the reduction in friction via an interfacial shear strength, τ , and a lateral deformation analog, η. Monolayer adsorption reduced τ from 3500 MPa for SiO₂ to less than 50 MPa for n = 12 and 18 alkylsilanes and was dependent on contact pressure. Conversely, η was pressure invariant, with values of ≈3500 MPa for n = 5 and 8 and ≈1000 MPa for n = 12 and 18.