Nanografting: modeling and simulation

We present a simple phenomenological model of the nanografting process with an emphasis on the formation of binary self-assembled monolayers. This model includes dynamical processes that are involved in natural growth experiments, including molecular deposition, surface diffusion, and the phase transition from physisorption to chemisorption, and we show that it predicts domain formation in ungrafted deposition that matches experiment. The one-order-of-magnitude faster kinetics that is found in the nanografting experiments compared to natural self-assembly (or unconstrained self-assembly) is described with a key assumption that the deposition rate is greatly enhanced in the small region confined between the back side of the AFM tip and the edge of the previously deposited self-assembled monolayer. Monte Carlo simulations based on this model reproduce experimental observations concerning the variation of SAM heterogeneity with AFM tip speed. Our simulations demonstrate that the faster the AFM tip displaces adsorbed molecules in a monolayer, the more heterogeneous are the monolayers formed behind the tip, as this allows space and time for the formation of phase-segregated domains.