Interfacial behavior of alkyltrichlorosilane monolayers on silicon: Control of flat-band potential and surface state distribution using chain length variation

The passivating behavior of octadecyltrichlorosilane (C₁₈), dodecyltrichlorosilane (C₁₂) and octyltrichlorosilane (C₈) self-assembled monolayers (SAMs) on Si(1 0 0), has been quantitatively compared using cyclic voltammetry and impedance analysis in the presence and absence of an external redox probes like ferrocene. In all these cases, Fourier transform infra red (FTIR) spectroscopy and contact angle measurements give clear evidence for the presence of a closely packed, oriented and hydrophobic monolayer. The electron transfer behavior of ferrocene is found to be drastically affected by the presence of monolayer and the reasons for such significant variation are analyzed in terms of the change in resistance, dielectric thickness and coverage of the monolayer. Double layer capacitance is found to decrease systematically with increasing the chain length of the monolayer suggesting a smooth variation in the “plane of closest approach” with the thickness of the monolayer, despite the presence of a space charge layer on Si electrode. Comparison of the electrochemical properties of the SAM-derivatized Si electrodes with that of a bare Si electrode using impedance analysis exhibits a four order of magnitude decrease in the apparent rate constant of ferrocene oxidation due to the barrier provided by various monolayers (C₈, C₁₂, C₁₈). A peak in the capacitance-potential curve presumably, due to surface states, is suppressed with an increase in the chain length of the monolayer. In addition, a positive shift in flat-band potential (E_fb) with the monolayer chain length, suggests the covalent coupling of the silane monolayer.