Reducing the Molecule-Substrate Coupling in C 60 -Based Nanostructures by Molecular Interactions K. J. Franke, 1, * G. Schulze, 1 N. Henningsen, 1 I. Ferna ´ndez-Torrente, 1 J. I. Pascual, 1 S. Zarwell, 2 K. Ru ¨ck-Braun, 2 M. Cobian, 3 and N. Lorente 3,4 1 Institut fu ¨r Experimentalphysik, Freie Universita ¨t Berlin, Arnimallee 14, 14195 Berlin, Germany 2 Institut fu ¨r Chemie, Technische Universita ¨t Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany 3 Institut de Cie `ncia de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Spain 4 Centre d’Investigacio ´ en Nanocie `ncia i Nanotecnologia (CSIC-ICN), Campus de la UAB, 08193 Bellaterra, Spain (Received 3 August 2007; published 24 January 2008) Codeposition of C 60 and the three-dimensional molecular hydrocarbon 1,3,5,7-tetraphenyladamantane (TPA) on Au(111) leads to the spontaneous formation of molecular nanostructures in which each fullerene is locked into a specific orientation by three surrounding TPA. Scanning tunneling spectroscopy shows that the electronic coupling of C 60 with the surface is significantly reduced in these nanostructures, enhancing the free-molecule properties. As evidenced by density functional theory simulations, the nanostructures are stabilized by 18 local electrostatic forces between C 60 and TPA, resulting in a lifting of the C 60 cage from the surface. DOI: 10.1103/PhysRevLett.100.036807 PACS numbers: 73.22.f, 68.37.Ef, 73.61.Wp, 73.63.b The use of organic molecular thin films on an inorganic surface [1,2] offers the perspective of tuning their elec- tronic functionality by redesigning the basic molecular components. At the interface with a metal, the molecular levels are pinned due to hybridization and charge transfer processes [3], resulting in a substantial broadening of the molecular resonances. The performance of molecular in- terfaces in, for example, electronic devices depends on the facility of charge injection, but also on fundamental pa- rameters regarding carrier mobility and lifetime. Polaronic charge transport and luminescence [4] are strongly im- proved by the localization of electronic states in the mo- lecular layer. It is hence desirable to design strategies that permit us to modify the degree of electronic coupling between molecular entities with states of the metallic support. A recent approach to weaken the electronic interaction between a molecular layer and an inorganic interface is to use dielectric spacers, thin enough to allow charge injec- tion. Ultrathin films of oxides [5 – 9], ionic salts [4,10,11], nitride [12], or alkanethiol layers [13,14] have been used to successfully decrease the electronic overlapping between an atom or a molecule and a metal surface. The results of such spacers are, for example, the sharpening of molecular resonances [13,14], giving rise to strong nonlinearities in charge transport, an increase in their electronic lifetime and coupling with vibrations in the molecule [9], improv- ing photoluminescence efficiency [4,7], or the decrease of exchange interactions of an atom with the metal, hence allowing the control of magnetization at the atomic level [12]. Here, we report on an alternative approach to weaken the electronic coupling between a molecule and a metallic sur- face, based on lateral electrostatic interactions of two or- ganic molecules. These interactions lift one of the species from the surface and at the same time allow the molecule to recover its gas phase shape. We show that C 60 with 1,3,5,7- tetraphenyladamantane (TPA) spontaneously forms nano- structures on Au(111), where one C 60 molecule is sur- rounded by three TPA. These nanostructures are sta- bilized by multiple electrostatic intermolecular bonds be- tween hydrogen atoms and fullerene  states, as resolved by numerical simulations of the bonding induced charge density. Unexpectedly, these bonds are very local and, in- stead of distorting the cage structure, cause the C 60 cage to be reoriented and lifted from the surface, restoring its free- molecule structure and its electronic integrity, as evidenced by scanning tunneling spectroscopy (STS). These results foretell a new approach for tuning molecule-surface cou- pling by the proper functionalization of the interacting molecules, which in this case is proven to be mediated by noncovalent interactions to a conjugated cage. We chose TPA as a hydrocarbon to interact with C 60 [Figs. 1(a) and 1(c)], because its concave carbon skeleton makes this molecule a good candidate to form an inclusion complex with the curved conjugated carbon cage of C 60 . The large difference in gap between TPA and C 60 [20] implies a very small intermolecular hybridization of the frontier orbitals leading to a reduction of electron transport from C 60 into the metal substrate via TPA orbitals. TPA was deposited on the Au(111) surface by sublima- tion under ultrahigh vacuum conditions. The Au(111) sur- face, previously cleaned using standard sputter-annealing methods, was kept at room temperature during the adsorp- tion of about 0.5 monolayers of TPA and subsequent deposition of C 60 . Scanning tunneling microscopy (STM) and STS measurements were then carried out at 4.8 K in custom-made equipment. Figures 1(a) and 1(b) show typical STM images. The most characteristic features are triangular shaped arrange- ments attributed to TPA=C 60 mixed nanostructures. The close-up view in Fig. 1(c) resolves that the smallest nano- PRL 100, 036807 (2008) PHYSICAL REVIEW LETTERS week ending 25 JANUARY 2008 0031-9007= 08=100(3)=036807(4) 036807-1 © 2008 The American Physical Society