Optimal Electron Doping of aC60Monolayer on Cu(111) via Interface Reconstruction

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...

Beating the Stoner criterion using molecular interfaces

“…The potential created by the mismatch of molecular and metal work functions leads to a partial filling of the interface states. [19][20][21] Other molecules with close electron affinity and the potential for 3d z /p coupling could be used to similar effect. In the case of C 60 on metallic substrates such as Cu films, the charge transfer from the metal can be of up to 3e -per molecule and leads to a metallisation of the interface.…”

“…19 However, bulk manganese is paramagnetic and much closer to complying with the Stoner criterion than diamagnetic copper due to the larger exchange interactions and density of states at the Fermi level (DOS(E F )). 26 This may be correlated with the propagation length of the effect, which persists for sample thicknesses five times longer in Mn than in Cu.…”

Large Spatially Resolved Rectification in a Donor–Acceptor Molecular Heterojunction

“…[27][28][29] Annealing allows the C 60 to settle into the surface, displacing Cu atoms beneath each molecule and increasing the hybridization and metallicity to varying degrees. 30,31 Measurements were carried out in a ultrahigh-vacuum (UHV) variable-temperature (VT) STM operating with the sample maintained at 55 K. The Cu(111) crystal was cleaned by sputtering with Ar + ions at 1 keV and simultaneously annealing at 900 K, with a final sputtering cycle at room temperature followed by a brief anneal at 900 K. Pentacene and C 60 were deposited in the same UHV system using an organic molecular-beam evaporator at 510 K and 710 K, respectively. The C 60 film was annealed to ≈570 K before Pn deposition.…”

Resonant core spectroscopies of the charge transfer interactions between C60 and the surfaces of Au(111), Ag(111), Cu(111) and Pt(111)

“…As superspectator and super-Auger processes rely on the transfer of electrons from 75 the surface into the LUMO of the molecule, by tuning the coupling strength of the molecule, in this case C 60 with the metal surface it is in principle possible to observe differences in the intensity of these channels in the RPES. Previous studies have found varying amounts of charge transfer between different metal surfaces and adsorbed C 60 molecules, [8,9,10,11,12,13,14,15] …”