Understanding Liquid–Solid-Like Behavior of Tetrahydrofuran Adlayers at Room Temperature between Graphene and Mica: A Born–Oppenheimer Molecular Dynamics Study

Abstract

The phase behavior of adlayers of volatile small organic molecule tetrahydrofuran (THF) at room temperature has been imaged using atomic force microscopy (AFM) through graphene templating (J. Am. Chem. Soc. 2011, 133, 2334−2337). To gain more insight into the dynamical and structural properties of THF adlayers on mica and the effect of graphene templating, the Born−Oppenheimer molecular dynamics simulations (with the BLYP-D functional and a Gaussian plane-wave basis set) are performed. Without the graphene coating, the computed self-diffusion coefficients of THF molecules in the monolayer and bilayer are comparable to that in bulk THF solvent. However, with the graphene coating, the THF monolayer becomes considerably viscous. As the thickness of adlayers increases, the second adlayer of the THF bilayer exhibits even solid-like behavior, consistent with the AFM measurement. Although the motion of THF molecules becomes markedly slower with the graphene coating, the adsorbed THF molecules can still freely tilt and rotate on the mica substrate. Hence, the graphene-coated THF monolayer and bilayer are not strictly as ordered as the monoclinic THF crystal. Nevertheless, with the graphene coating, some THF molecules in the monolayer and bilayer entail certain degrees of crystalline packing as the graphene coating serves as an energy barrier to prevent the volatile THF molecules from evaporation, thereby limiting their motion in both lateral and vertical directions.