Formation mechanisms and packing structures of alkoxyl and alkyl monolayers on Si(111): Theoretical studies with quantum chemistry and molecular simulation models

Quantum chemistry and molecular mechanics calculations are performed to study formation mechanisms and packing structures of octadecanal and octadecene monolayers on Si(111). The radical chain mechanism is investigated by density functional theory with cluster models. Transition states of key steps involving abstracting a neighboring hydrogen atom from the surface are confirmed with the six-membered ring structures. Energy barriers for abstractions of the surface H to form new reactive surface Si radicals in the substitution of Si(111) via Si-O and Si-C linkages are 18.05 and 14.97 kcal/mol, respectively. Based on the radical chain mechanism, we investigate the linear and zigzag packing structures of alkoxyl chains on Si(111) with substitution percentages of 50%, 66.7%, and 75% using a series of two-dimensional repeating cells. By comparison of packing energies of octadecanoyl chains at different substitution percentages, 66.7% is predicted to be an optimal substitution percentage, which agrees with experimental observations. At this surface substitution, packing structures of the monolayer such as tilt angles and film thickness are well correlated with experimental data. The difference in packing structures between monolayers on Si(111) via Si-O and Si-C linkages is rationalized by their different van der Waals radii of surface linkage groups and tilt angles of chains.