4 ' - dithiodipyridine on Au ( 111 ) : a Combined STM , STS , and DFT Study

We studied the adsorption of 4,4'-dithiodipyridine (PySSPy) on Au(111) under ultra-high vacuum conditions and at low-temperature both experimentally by means of scanning tunneling microscopy and spectroscopy (STM and STS), and theoretically by density functional theory (DFT). We find PySSPy molecules – characterized by their elongated appearance in STM – assembled in islands which reside exclusively in fcc regions of the herringbone reconstructed Au (HB) terraces. A triangular structure motif dominates the local arrangement of the PySSPy. DFT calculations reveal a virtually planar adsorption geometry of the PySSPy with the S-S bond of the molecule almost parallel to the Au-Au bonds of the substrate underneath. Though van der Waals forces dominate the stability of the adsorbed PySSPy, there is also a covalent contribution to the PySSPy/Au interaction. As a consequence, the PySSPy structures are relatively unstable. As found by STS, the highest occupied molecular orbital (HOMO) is located at around -0.7 eV below the Fermi energy (Ef) in good agreement with the HOMO provided by DFT. The position of the lowest unoccupied molecular orbital (LUMO) is about 3.2 eV above Ef leaving the HOMO-LUMO gap practically unchanged compared to a free molecule. The molecules exhibit a strong propensity to dissociate into monomers forming pyridine thiolates (PyS) at step edges or elbow sites of the HB. At room temperature, all PySSPy on the substrate dissociate resulting in linear chains of PyS characterized by a drop of the LUMO by about -0.9 eV.