Characterisation of alkyl-functionalised Si(1 1 1) using reflectometry and AC impedance spectroscopy

The past few years have seen a dramatic increase in the study of organic thin-film systems that are based on silicon-carbon covalent bonds for bio-passivation or bio-sensing applications. This approach to functionalizing Si wafers is in contrast to gold-thiol or siloxane chemistries and has been shown to lead to densely packed alkyl monolayers. In this study, a series of alkyl monolayers CH₃(CH₂)_nCH=CH₂; n = 7, 9, 11, 13, 15] were directly covalent-linked to Si(1 1 1) wafers. The structures of these monolayers were studied using X-ray reflectometry (XRR) and AC impedance spectroscopy. Both techniques are sensitive to the variation in thickness with each addition of a CH₂ unit and thus provide a useful means for monitoring molecular-scale events. The combination of these techniques is able to probe not only the thickness, but also the interfacial roughness and capacitance of the layer at the immobilized surface with atomic resolution. Fundamental physical properties of these films such as chain canting angles were also determined.