Published: September 18, 2011 r2011 American Chemical Society 21334 dx.doi.org/10.1021/jp207568q | J. Phys. Chem. C 2011, 115, 21334–21340 ARTICLE pubs.acs.org/JPCC Impact of the 3d Electronic States of Cobalt and Manganese Phthalocyanines on the Electronic Structure at the Interface to Ag(111) F. Petraki,* ,† H. Peisert, † F. Latteyer, † U. Ayg€ ul, † A. Vollmer, ‡ and T. Chass  e † † University of Tuebingen, IPTC, Auf der Morgenstelle 18, 72076 Tuebingen, Germany ‡ Helmholtz Centre Berlin for Materials and Energy, Electron Storage Ring BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany ’ INTRODUCTION Metal phthalocyanines have been the subject of much interest owing to their unique electronic and optical properties that can be technically exploited. 17 For example, it has been demonstra- ted recently that in particular transition-metal phthalocyanines (TMPcs) possess interesting magnetic properties due to the pre- sence of the partially empty d levels of the central transition- metal atom, 812 and their application to spintronic devices is under investigation. 1316 Therefore, detailed experimental stud- ies of the interfaces between TMPcs and metallic substrates can give valuable insight into the understanding of the processes that take place at device interfaces. In particular, with respect to mag- netic devices, the spin situation of the central metal atom of the TMPc may change distinctly at the interfaces. Because TMPcs are small molecules that can be easily evaporated under con- trolled vacuum conditions, they are well-suited as representative model systems for the investigation of the fundamental proper- ties (electronic, optical, and magnetic) of many other transition- metal-containing materials. In the present work, we focus on interfaces between Ag(111) and CoPc or MnPc. The investigated TMPc’s (TM: Co, Mn) are planar molecules with D 4h symmetry; the central metal atom is surrounded by the Pc ring (Figure 3, inset). Despite a large number of experimental and theoretical studies in the past decades, driven by the excellent perspective for various applications, essential details of the electronic structure and excitations of CoPc and MnPc are still unclear, 13,1722 in particular, the role of the TMPc metal atom in the interface formation. Our studies have been carried out by photoexcited electron spectroscopies such as X-ray photoemission (XPS), X-ray absorption (XAS, NEXAFS), and resonant photoemission spectroscopy (ResPES) with the intention to understand the interference of the 3d states of the central transition-metal atom in the absorption configuration as well as the electronic proper- ties of the interface. XAS provides information about the nature of the empty molecular orbitals of the absorbing atom. It moni- tors transitions from core level to unoccupied states (to the lowest unoccupied molecular orbitals, LUMO). Because of the resonant nature of the electron emission process near the XAS edge, with ResPES, it is possible to shed light on the nature of the spectral features in the valence band. Therefore, this technique is suitable to identify spectral features in the valence band and cor- relate them with specific chemical sites inside the molecule and to unoccupied states. 23 ResPES has been successfully applied also in the case of metal phthalocyanines. 23,24 ’ EXPERIMENTAL METHODS The XAS and the resonant photoemission measurements were carried out at the third generation synchrotron radiation source BESSY II (Berlin) using the Optics-beamline and the end- station SurICat. The photon energies were calibrated comparing the binding energy (BE) of Au 4f 7/2 peak excited by first- and second-order light. XAS were acquired in total-electron yield (TEY) mode, measuring the drain current, with a resolution of ∼100 meV at a photon energy of 400 eV. Detailed description for the experimental process can be found elsewhere. 25 The Ag(111) substrate was cleaned prior to organic film deposition by repea- ted sputtering-annealing treatment, and the cleanliness was chec- ked by XPS. Ultrathin films of CoPc and MnPc, purchased from Aldrich, were thermally evaporated on the substrate in ultrahigh vacuum (base pressure <1  10 8 mbar) from a temperature- controlled evaporation cell. The deposition rate, monitored by a quartz crystal oscillator, was about 1 to 2 Å/min. The thickness of the TMPc films ranged from monolayer to ∼30 Å. The comparison of the film thickness determined using the quartz microbalance Received: August 7, 2011 Revised: September 15, 2011 ABSTRACT: X-ray absorption (XAS) and photoemission (XPS) spectroscopy revealed a strong interaction at the interface between transition-metal Cobalt phthalocyanine (CoPc) and single-crystalline Ag(111) substrate. According the Co L-edge absorption spectra of ultrathin CoPc films, charge transfer between the metal 3d electrons and the underlying metallic substrate occurs. Less intense but comparable effects are also observed at the MnPc/Ag(111) interface for partially filled d levels of Mn. The nature of interacting orbitals is also discussed by means of resonant photoemission spectroscopy (ResPES) as well as in comparison with the CoPc/Au(100) interface studied previously.