Atomic force microscope imaging of molecular aggregation during self-assembled monolayer growth

'Self-assembled' monolayers of amphiphilic surfactant molecules form sponta neously on solid surfaces by exposure to dilute solutions of the adsorbate molecules. Using a combination of atomic force microscopy (AFM), infrared s pectroscopy, and wettability studies, we find that these monolayers form vi a a mechanism that includes nucleation, growth, coalescence, etc, of densel y packed submonolayer islands of the long-chain organic molecules. AFM expe riments permit direct observations of the size and shape of these islands. Molecules that attach to each other covalently and irreversibly (such as al kyltrichlorosilanes) form islands that are fractal with a morphology consis tent with 2D diffusion-limited aggregation. Molecules that interact with ea ch other via softer van der Waals interactions form rounded islands, sugges ting that the island shapes relax via a continuous process of 2D desorption and re-adsorption. In situ AFM measurements allow a quantitative analysis of island nucleation and growth rates as well as determination of the islan d size distribution as a function of coverage. In the growth regime, the nu cleation and growth rates have a power law behavior consistent with a simpl e point island model of 2D cluster growth. The exponents are consistent wit h a critical nucleus of two molecules and the 2D diffusion coefficient corr esponds to a 'hopping time' of about 1 mu s. In the aggregation regime, the island size distributions are shown to scale with a single evolving length scale in accordance with the dynamical scaling approximation. (C) 2000 Els evier Science B.V. All rights reserved.