No abstract

We demonstrate the charge state of C 60 on a Cu(111) surface can be made optimal, i.e., forming C 60 3À as required for superconductivity in bulk alkali-doped C 60 , purely through interface reconstruction rather than with foreign dopants. We link the origin of the C 60 3À charge state to a reconstructed interface with ordered (4 Â 4) 7-atom vacancy holes in the surface. In contrast, C 60 adsorbed on unreconstructed Cu(111) receives a much smaller amount of electrons. Our results illustrate a definitive interface effect that affects the electronic properties of molecule-electrode contact. DOI: 10.1103/PhysRevLett.104.036103 PACS numbers: 68.43.Àh, 61.05.jh, 68.35.Ct, 73.20.Àr In bulk fulleride A n C 60 (A ¼ Na, K, etc.) [1], an ''optimal doping'' state favoring superconductivity is known to occur for n ¼ 3, with 3 electrons on each C 60 (C 60 3À ). Since C 60 films on metallic surfaces typically involve substrate-to-C 60 electron transfer that partially populates the C 60 lowest unoccupied molecular orbital (LUMO), it has been of great interest to pursue optimally doped C 60 films. Earlier studies show that the electron transfer amount does not simply depend on the substrate work function [2]. This raises the question of the role of the C 60 =metal interface structure. Although strong C 60 -metal interactions are not expected for, e.g., C 60 on noble metal surfaces, there is increasing evidence of C 60 -induced interface reconstruction for C 60 =Auð110Þ [3], C 60 =Ptð111Þ [4], C 60 =Alð111Þ [5], C 60 =Agð100Þ [6], and even for C 60 =Agð111Þ [7] and C 60 =Cuð111Þ [8], etc. The typical scenario is that C 60 tends to dig a ''vacancy'' in the surface. Calculations, including our own, show this geometry increases the adsorption strength that compensates the energy cost of vacancy creation. No studies, however, have discussed how the electronic structure and hence the charge state of a C 60 film are affected by its interface structure. Here, we discovered that a C 60 monolayer on Cu(111) is optimally electron doped purely by interface reconstruction and without intercalating alkali atoms. We convincingly establish the C 60 3À charge state and trace its origin to a reconstructed interface with ordered (4 Â 4) large 7-atom vacancy holes in the surface. The key link between molecular doping and a reconstructed interface indicates the practical needs of tackling the often neglected difficult interface structure problems which could prove essential in understanding the physics and chemistry of thin film materials.Many inconsistencies between experiment and theory in heteroepitaxial systems, such as the charge state of a C 60 film on a surface, are likely rooted in the application of an incorrect interface model. For C 60 =Cuð111Þ, it has been measured to range from 1-3 electrons per C 60 by photoemission spectroscopy (PES) [9]. Calculations predict a much smaller amount, <0:8e À , for an unreconstructed interface [10]. The electronic band structure measured by a recent PES study is also at odds with theoretical analysi...

Scanning tunneling spectroscopy measurements of Pb islands on Si(111) at high energy resolution reveal a novel pseudogap, or a pseudopeak in special cases, around the Fermi level in addition to the usual quantum well states. These gap or peak features persist to temperatures as high as approximately 80 K and are uniquely related to the quantum well nanostructure of the Pb islands. A systematic analysis indicates that electron-phonon scattering is responsible for the observed electronic structure.