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

Abstract

‘Self-assembled’ monolayers of amphiphilic surfactant molecules form spontaneously on solid surfaces by exposure to dilute solutions of the adsorbate molecules. Using a combination of atomic force microscopy (AFM), infrared spectroscopy, and wettability studies, we find that these monolayers form via a mechanism that includes nucleation, growth, coalescence, etc. of densely packed submonolayer islands of the long-chain organic molecules. AFM experiments permit direct observations of the size and shape of these islands. Molecules that attach to each other covalently and irreversibly (such as alkyltrichlorosilanes) form islands that are fractal with a morphology consistent with 2D diffusion-limited aggregation. Molecules that interact with each other via softer van der Waals interactions form rounded islands, suggesting 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 island size distribution as a function of coverage. In the growth regime, the nucleation and growth rates have a power law behavior consistent with a simple point island model of 2D cluster growth. The exponents are consistent with a critical nucleus of two molecules and the 2D diffusion coefficient corresponds to a ‘hopping time’ of about 1 μs. 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.