Supercritical self-assembled monolayer growth

The growth of octadecyltrimethylammonium bromide (C(18)TAB) monolayers on mica was investigated using atomic force microscopy and infrared spectroscopy. A critical temperature was identified below which the monolayer formed via an "islanding" mechanism, that is, nucleation and growth of densely packed two-dimensional (2D) islands within a matrix of a disordered dilute phase. However, above the critical temperature, there was no coexistence of 2D phases during film formation. Instead, the monolayer gradually became better ordered, remaining laterally homogeneous throughout. We show that this corresponds to a critical point in a 2D phase diagram of the monolayer. Additional evidence is provided by the in situ observation of 2D phase separation upon cooling an incomplete monolayer from the one-phase to the two-phase region. The lack of coexisting domains (and domain boundaries) during growth above the critical point provides a possible route for the preparation of essentially defect-free monolayers.