Role of Long-Range Interactions for the Structure and Energetics of Olympicene Radical Adsorbed on Au(111) and Pt(111) Surfaces

We report on the results of the van der Waals (vdW) inclusive density functional theory (DFT) calculations for the adsorption characteristics of olympicene radical (C₁₉H₁₁) on Au(111) and Pt(111) surfaces. The nature of bonding between the olympicene radical and Au(111) and Pt(111) surfaces is evaluated along with the effects of vdW interactions, and the chemical characteristics of the surfaces, on the adsorption. Our results show a significant increase in the adsorption energies with the inclusion of vdW interactions, with the largest enhancement is obtained with the optimized exchange functionals, while a smaller enhancement is found using the rPW86-vdW2 functional, in agreement with the trends obtained in our earlier observations for other molecular adsorption on similar surfaces. The adsorption of olympicene radical on Au(111) leads to negligible change in the geometric structures of olympicene and the surface layer, while adsorption on Pt(111) reveals significant structural changes. This observation suggests a strong interaction with the Pt(111) surface, while the interaction with the Au(111) surface is rather weak. While the bonding of olympicene on Pt(111) is governed by a combination of the covalent bonding and the vdW forces, on Au(111), the bonding is mostly governed by the vdW forces. Upon adsorption on both surfaces, no interface state is formed, while a charge transfer of ∼0.6 e^– is obtained. We also find the surface work function to be reduced upon adsorption on both surfaces, with the largest change observed on Pt(111) surface, over 1 eV. On the basis of the results obtained using the exchange functionals, we conclude that on Au(111), there is a transition in the nature of bonding from weak physisorption to strong physisorption, while the nature of bonding on Pt(111), is chemisorption, and it is enhanced with the inclusion of vdWs interactions. This is different from the adsorption on Cu(111) surface, for which we observed a transition from weak physisorption to strong chemisorption upon inclusion of vdW forces.