Impact of chain length, temperature, and humidity on the growth of long alkyltrichlorosilane self-assembled monolayers

Abstract

In this work, we have studied the growth of self-assembled monolayers (SAMs) on silicon dioxide (SiO₂) made of various long alkyltrichlorosilane chains (16, 18, 20, 24, and 30 carbon atoms in the alkyl chain), at several values of temperature (11 and 20 °C in most cases) and relative humidity (18 and 45% RH). Using atomic force microscopy analysis, thickness measurements by ellipsometry, and contact angle measurements, we have built a model of growth behaviour of SAMs of those molecules according to the deposition conditions and the chain length. Particularly, this work brings not only a better knowledge of the less studied growth of triacontyltrichlorosilane (C₃₀H₆₁SiCl₃) SAMs but also new results on SAMs of tetracosyltrichlorosilane (C₂₄H₄₉SiCl₃) that have not already been studied to our knowledge. We have shown that the SAM growth behaviour of triacontyltrichlorosilane at 20 °C and 45% RH is similar to that obtained at 11 °C and 45% RH for shorter molecules of hexadecyltrichlorosilane (C₁₆H₃₃SiCl₃), octadecyltrichlorosilane (C₁₈H₃₇SiCl₃), eicosyltrichlorosilane (C₂₀H₄₁SiCl₃) and tetracosyltrichlorosilane (C₂₄H₄₉SiCl₃). We have also observed that the monolayers grow faster at 45% than at 18% RH, and surprisingly slower at 20 °C than at 11 °C. Another important result is that the growth time constant decreases with the number of carbon atoms in the alkyl chain except for C₂₄H₄₉SiCl₃ at 11 °C and 18% RH, and for C₃₀H₆₁SiCl₃. To our knowledge, such a chain length dependence of the growth time constant has never been reported. The latter and all the other results are interpreted by adapting a diffusion limited aggregation growth model.