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

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.