NANO LETTERS
“Soft” Metallic Contact to Isolated C60 Molecules
2008 Vol. 8, No. 11 3825-3829
Hendrik Glowatzki,† Benjamin Bro¨ker,† Ralf-Peter Blum,† Oliver T. Hofmann,‡ Antje Vollmer,§ Ralph Rieger,| Klaus Mu¨llen,| Egbert Zojer,‡ Ju¨rgen P. Rabe,† and Norbert Koch*,† Humboldt-UniVersita¨t zu Berlin, Institut fu¨r Physik, Newtonstrasse 15, D-12489 Berlin, Germany, Technische UniVersita¨t Graz, Institut fu¨r Festko¨rperphysik, A-8010 Graz, Austria, Berliner Elektronenspeicherring-Gesellschaft fu¨r Synchrotronstrahlung m.b.H., Albert-Einstein-Strasse 15, D-12489 Berlin, Germany, Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Received July 19, 2008
ABSTRACT C60 adsorbed on a monolayer of hexaazatriphenylene-hexanitrile (HATCN) on Ag(111) is investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy. UPS and quantum-mechanical modeling show that HATCN chemisorbed on Ag(111) displays metallic character. This metallic molecular layer decouples C60 electronically from the Ag substrate and simultaneously acts both as template for the stable adsorption of isolated C60 molecules at room temperature and as “soft” metallic contact for subsequently deposited molecules.
One of the premier challenges in the field of molecular electronics is to realize good electrical contact between a metallic electrode and an individual molecule while retaining intrinsic properties of the organic material. Strong electrodemolecule interactions often lead to significant alterations of the electronic structure of the molecules,1-5 resulting in modified functionality of a device-type structure.6-11 Even for weakly adsorbed (physisorbed) molecules on metals, modifications of the electronic structure have been observed.12-15 To retain the intrinsic (bulk) electronic structure of a molecule, it is thus desired to reduce direct molecule-metal interactions. To achieve this goal, several different approaches have been reported,8,16-18 often involving the insertion of a “buffer” layer between metal and molecules.7,15,19 Such buffer layers can also serve as structural template for subsequently deposited molecules,15,20,21 however, they typically prevent an intimate contact of the active molecule to the metal and introduce an additional barrier for charge transport from/to the electrode because of the wide energy gap of the buffer materials. In addition, stable imaging (e.g., by scanning tunneling microscopy, STM) of individual isolated molecules on surfaces is usually restricted to low temperatures (
