Molecular Dynamics of Self-Assembled Monolayer Formation in Soft Nanolithgoraphy

Molecular dynamics simulation is performed to study the growth mechanism of self-assembled monolayer in the AFM tip-assisted soft nanolithography such as in dip-pen nanolithography. We investigate how the droplet created around the tip spreads out to become a monolayer on the substrate. The previous diffusion model assumes that molecules diffuse on top of molecules already adsorbed on the substrate. In contrast, our molecular simulation shows that a molecule on top pushes out a molecule below it and the molecule just pushed out in turn pushes out a molecule next to it. The monolayer grows through such a serial pushing. The present large scale (40 nm diameter) simulation reveals new features. For a relatively weak adsorbatesubstrate binding, the monolayer has irregular branches. As the adsorbate-substrate binding strengthens, the monolayer becomes compact, and reflects the rotational symmetry of substrate. A substrate with a hexagonal symmetry results in a hexagonal monolayer. An extremely strong moleculesubstrate binding removes such an effect of the substrate anisotropy, giving rise to a circular monolayer. The monolayer periphery shows an initial diffusional growth in its time dependence followed by a slow expansion. The rates of self-assembled monolayer growth exhibit a turnover behavior with increase in the attractive force between the adsorbate and substrate.