Molecular Dynamics Study on the Self-Assembled Monolayer Grown from a Droplet of Alkanethiol

Direct-write lithography, where a nanoscale tip or a stamp serves as a molecular source, is used widely to fabricate self-assembled monolayers (SAMs), nanometers in size. The spatially narrow deposition of molecules creates a droplet, which then spreads to form an ordered SAM. Currently, the dynamics and mechanism for this spreading are largely unknown. Herein, the evolution of a droplet of 1-octadecanethiol into a circular island of SAM, where the sulfur atoms and alkyl chains are densely and orderly packed, was examined by using molecular dynamics simulations. The packing of sulfur atoms preceded the alignment and packing of alkyl chains. The SAM islands resembled the bulk SAM, but it contained defects where the molecules were standing upside down on the surface. We found two mechanisms pertaining to the growth of a SAM island in the direct-write lithography. In the first mechanism, the molecules penetrated into the SAM islands by pushing away the molecules below. In the other mechanism, the molecules diffused, reached the periphery of the SAM islands, and slid down to the surface. The chemisorption of sulfur atoms made the present droplet spread more slowly than a droplet interacting nonspecifically with a surface. A droplet laterally moving across the surface was also simulated to gain insight into the growth of a SAM line. The alkyl chains of the SAM line were directed preferentially toward the line direction.