Effect of van der Waals interactions in the DFT description of self-assembled monolayers of thiols on gold

The structure and energetic properties of self-assembled monolayers (SAMs) of alkanethiol derivatives (simple alkanethiols, mercaptoalkanoic acids and aminoalkanethiols with different chain length) adsorbed on the metallic Au(111) surface are investigated through periodic DFT calculations. To sort out the effect of van der Waals (vdW) interactions on the DFT calculations, the results of the standard GGA-PBE functional are compared with those obtained with approaches including the vdW interactions such as those incorporating the Grimme's (GGA-PBE-D2) and the Tkatchenko-Scheffler's (GGA-PBE-TS) schemes, as well as with the optB86b-vdW density functional. The most significant difference between the two sets of results appears for the adsorption energies per thiol molecules: The standard functional predicts energy values 30-40 % lower than those obtained when the van der Waals interactions are taken into account. This is certainly due to a better description of the lateral interactions between the chains of the thiols when including the van der Waals effects. Differences are also found between the adsorption energies predicted by density functionals taking into account the vdW corrections, with values increasing in the order GGA-PBE-D2 < GGA-PBE-TS < optB86b-vdW. Furthermore, the functionals considering dispersion interactions favor much more tilted orientations of the SAMs over the surface with respect to those found using the standard GGA functional (the SAMs' tilt angles increase from 17 degrees-24 degrees to 37 degrees-46 degrees), being the former in closer agreement with available experimental data. In contrast, the SAMs' precession angle and monolayer thickness are less affected by the type of DFT exchange-correlation functional employed. In the case of low surface coverage, the chains of the thiols adopt more tilted configurations and tend to lay side-down onto the surface.