DFT study of chain length dependence of alkyl phosphonic acid molecules structure on Al[formula omitted]O[formula omitted] in pure and mixed monolayers

Abstract

The geometrical structure of alkylphosphonic acid molecules on α-Al2O3 (0001) surface with different carbon chain lengths, at different coverages, in pure and mixed models, are investigated using density functional theory (DFT) calculations with optb86b van der Waals (vdW) corrections. Our calculations indicate a clear dependence of the structural characteristics of alkylphosphonic acid molecules on their chain length, and are consistent with previously reported experimental and theoretical results. We show that the orientation of molecules with short alkyl chain is principally forced by the anchor group, while the orientation of molecules with long chain is significantly influenced by the chain flexibility and the intermolecular interaction. This result is well confirmed when mixing short and long alkylphosphonic acid molecules. The mixed model was a useful choice for understanding the ordering of molecules. In the mixed model, we found a clear up-righting of molecules with short alkyl chains and of the bottom part of molecules with long alkyl chains (the part that corresponds to the length of the molecule with short alkyl chain). The up-righting of the small molecule relies to the interaction between the upper tail of long molecule and the short molecule, which in return, forces the bottom tail of long molecule to be more perpendicular to the substrate. However, this interaction causes an increase of the tilt angle of the upper tail of long alkyl chain molecules when the chain length difference is of twelve or eight carbon atoms. Since the adsorption energy depends on the geometrical structure, one can notice that, for more tilted molecules, at low coverage, a higher adsorption energy can be obtained. However, for less tilted molecules, at high coverage, we found a linear dependence of the adsorption energy to the chain length. In the former case, the molecule-substrate interaction is more prominent. However, for the latter case, the molecule-molecule interaction energy increases with increasing chain length. This theoretical study is useful to predict some advanced applications of pure and mixed monolayers in organic devices.