First-principles Study on Adsorption of Magnesium Porphyrin on Sodium Chloride Covered Au(111) Surfaces

The adsorption properties of a magnesium porphyrin (MgP) molecule on Au(111) surface covered with up to three layers of sodium chloride (NaCl) were investigated by means of first-principles calculations. The most stable adsorption configuration of MgP on the NaCl/Au(111) heterosurfaces was found to be at the Cl-top site with a 20° angle between the 1 \overline10 lattice direction of NaCl and the Mg–N bond of the molecule. Compared with MgP molecule adsorbed on bare Au(111), the inclusion of NaCl layers can lead to a significant decrease in the adsorption energy of the MgP molecule. The existence of NaCl layers also reduced the charge transfer between the molecule and the surface. For heterosurfaces with two or three monolayers of NaCl, the charge transfer was almost completely suppressed. The obtained partial density of states (PDOS) showed that hybridization between the electronic structures of the adsorbed MgP molecule and the metal surface can be significantly suppressed when NaCl layers were added. For the heterosurface with three layers of NaCl, the PDOS around the Fermi level was almost identical with that of the free molecule, suggesting the electronic structure of the MgP molecule was nicely preserved. Influence of the NaCl layers on the electronic structure of the MgP molecule was mainly found for molecular orbitals (MOs) away from the Fermi level as a result of the large band gap of the NaCl layers.