Theoretical and experimental comparison of SnPc, PbPc, and CoPc adsorption on Ag(111)

Abstract

A combination of normal-incidence x-ray standing-wave (NIXSW) spectroscopy, x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density-functional theory (DFT) has been used to investigate the interaction of a number of phthalocyanine molecules (specifically, SnPc, PbPc, and CoPc) with the Ag(111) surface. The metal-surface distances predicted by the DFT calculations for SnPc/Ag(111) $(2.48\text{ }\text{\AA{}})$ and CoPc/Ag(111) $(2.88\text{ }\text{\AA{}})$ are in good agreement with our NIXSW experimental results for these systems ($2.31\ifmmode\pm\else\textpm\fi{}0.09$ and $2.90\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{\AA{}}$, respectively). Good agreement is also found between calculated partial density-of-states plots and STM images of CoPc on Ag(111). Although the DFT and $\text{Pb}\text{ }4f$ NIXSW results for the Pb-Ag(111) distance are similarly in apparently good agreement, the $\text{Pb}\text{ }4f$ core-level data suggest that a chemical reaction between PbPc and Ag(111) occurs due to the annealing procedure used in our experiments and that the similarity of the DFT and $\text{Pb}\text{ }4f$ NIXSW values for the Pb-Ag(111) distance is likely to be fortuitous. We interpret the $\text{Pb}\text{ }4f$ XPS data as indicating that the Pb atom can detach from the PbPc molecule when it is adsorbed in the ``Pb-down'' position, leading to the formation of a Pb-Ag alloy and the concomitant reduction in Pb from a ${\text{Pb}}^{2+}$ state (in bulklike films of PbPc) to ${\text{Pb}}^{0}$. In contrast to SnPc, neither PbPc nor CoPc forms a well-ordered monolayer on Ag(111) via the deposition and annealing procedures we have used. Our DFT calculations show that each of the phthalocyanine molecules donate charge to the silver surface, and that back donation from Ag to the metal atom (Co, Sn, or Pb) is only significant for CoPc.