Controlling molecular alignment in an organic monolayer with a sacrificial liquid crystal template

A method is presented for preparing organic films with controlled orientational order based on a sacrificial monolayer template. The procedure involves a two-step process wherein an oriented organic template formed in the first step is replaced by a second compound, deposited in the second step. The approach enables orientational control in monolayer films over molecular to macroscopic length scales, and provides a useful model for investigating fundamental aspects of competitive molecular adsorption and molecular ordering and disordering at surfaces. Films of the liquid crystal 4′-octyl-4-cyanobiphenyl oriented by magnetic fields served as templates for the subsequent deposition of the straight-chain alkane n-tetracontane (C40, C40H82), which displaced the template through a process of competitive adsorption. When displacement was performed at low temperatures, some order was communicated from the template to C40. The degree of order depended on the temperature at which the displacement process occurred, ranging from s=0.59±0.05 to 0.16 ± 0.03, where s is the two-dimensional orientational order parameter. Based on measurements of molecular alignment statistics, scanning tunneling microscopy observations of the detailed structures of the films, and the effects of temperature, a mechanism for template-mediated ordering is proposed.