Self-assembled monolayers in the context of epitaxial film growth

Self-assembled monolayers of amphiphilic surfactant molecules form spontane ously on solid surfaces by exposure to dilute solutions of the adsorbate mo lecules. We report observations of the growth and desorption/dissolution pr ocesses for monolayers of octadecylphosphonic acid (OPA, CH3(CH2)(17)PO(OH) (2)) on mica. We find that these monolayers form via a mechanism that inclu des nucleation, growth, coalescence, etc, of densely-packed submonolayer is lands of the long-chain organic molecules. In situ AFM measurements allow a quantitative analysis of island nucleation and growth rates as well as det ermination of the island size distribution as a function of coverage. In th e growth regime, the nucleation and growth rates have a power law behavior consistent with a simple point island model of 2D cluster growth. In the ag gregation regime, the island size distributions are shown to scale with a s ingle evolving length scale in accordance with the dynamical scaling approx imation. The desorption/dissolution process proceeds by nucleation of holes in the monolayer that grow and percolate across the sample, leaving isolat ed islands that gradually decrease in size. The relative rates of hole grow th and hole nucleation suggest that removing a molecule from the monolayer/ hole boundary is about 5 x 10(4) times more likely than removing a molecule from within a continuous region of monolayer. The coverage kinetics during dissolution can be quantitatively described by a model that incorporates d esorption from; "hole" regions and diffusive solution-phase transport throu gh a stagnant layer of finite thickness that can be altered by solvent flow or stirring. If the monolayer is brought into contact with a small enough volume of stagnant solvent, the surface coverage eventually stabilizes due to the buildup of adsorbate molecules in solution. Under these conditions o f steady-state surface coverage, the local dynamical processes of island sh rinkage, growth, and nucleation continue, eventually leading to a distincti ve (decaying exponential) island size distribution characteristic of the sy stem. (C) 2001 Elsevier Science B.V. All rights reserved.