Different types of phase separation in binary monolayers of long chain alkyltrichlorosilanes on silicon oxide

Abstract

In the present work, we studied the growth and morphology of binary monolayers made with triacontyltrichlorosilane (C30H61SiCl3) as the long chain and hexadecyltrichlorosilane (C16H33SiCl3), octadecyltrichlorosilane (C18H37SiCl3) or eicosyltrichlorosilane (C20H41SiCl3) as the short chain according to the deposition conditions on silicon oxide, using mainly atomic force microscopy, ellipsometry, and contact angle measurements. We distinguished three types of different phase separation. Besides the classical phase separation by islands of long chains in a surrounding phase of shorter molecules, we observed a separation involving dendritic “filaments” of long chains in a shorter chain phase, regardless of the couple of molecules studied. A third sort of phase separation also appears in the case of triacontyltrichlorosilane with octadecyltrichlorosilane or eicosyltrichlorosilane: formation of “holes”, i.e. islands of the shorter molecule in a surrounding phase of long chains. We obtained “hole” and “filament” type phase separation only by working below a critical temperature, which depends on the molecule length. Moreover, the level of ambient relative humidity was shown to have an impact on “hole” versus “filament” phase separation types. We then showed that for phase separation by islands or holes the composition of binary monolayers (RSAM) prepared at low humidity (18% RH) is almost the same as that of the silanisation solution (Rsol). Conversely, for SAMs prepared at high humidity (45% RH) RSAM is always lower than Rsol whatever the type of phase separation by islands or holes. Moreover, concerning phase separation with C30 filaments we observed that RSAM is independent from both the nature of the short molecule and the humidity conditions. Considering a diffusion limited aggregation growth model, we found that at high humidity the monolayer growth is mainly driven by the diffusion coefficient, while at low humidity the deposition rate from solution is the leading parameter of the growth. The efficiency of phase separation is also addressed.