Determination of charge states of C60 adsorbed on metal surfaces

Thiol-free self-assembled oligoethylene glycols enable robust air-stable molecular electronics

Self-assembled monolayers (SAMs) are widely used to engineer the surface properties of metals. The relatively simple and versatile chemistry of metal-thiolate bonds makes thiolate SAMs the preferred option in a range of applications, yet fragility and a tendency to oxidize in air limit their long-term use. Here, we report the formation of thiol-free self-assembled mono- and bilayers of glycol ethers, which bind to the surface of coinage metals through the spontaneous chemisorption of glycol ether-functionalized fullerenes. As-prepared assemblies are bilayers presenting fullerene cages at both the substrate and ambient interface. Subsequent exposure to functionalized glycol ethers displaces the topmost layer of glycol ether-functionalized fullerenes, and the resulting assemblies expose functional groups to the ambient interface. These layers exhibit the key properties of thiolate SAMs, yet they are stable to ambient conditions for several weeks, as shown by the performance of tunnelling junctions formed from SAMs of alkyl-functionalized glycol ethers. Glycol ether-functionalized spiropyrans incorporated into mixed monolayers lead to reversible, light-driven conductance switching. Self-assemblies of glycol ethers are drop-in replacements for thiolate SAMs that retain all of their useful properties while avoiding the drawbacks of metal-thiolate bonds.

Charging C60 islands with the AFM tip

We show that electrons can be transferred on demand from an AFM tip into single bulk-like C60 islands, which are supported on the insulating NaCl(001) surface. We exemplify this by controlled charge-manipulation experiments conducted in ultrahigh vacuum by noncontact AFM (nc-AFM), electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). KPFM shows a homogeneous contrast at the islands, which is a signature for an equal distribution of the electrons in the T1u band. The charge dissipates during half a day due to an interaction of the charged C60 islands with defects in the near surface region of NaCl. Our results open the perspective in photo-voltaics to study charge attachment, stability and charge exchange with the environment of any C60 bulk-like system.

Vacancy patterning and patterning vacancies: controlled self-assembly of fullerenes on metal surfaces

A density functional theory study accounting for van der Waals interactions reveals the potential of metal surface vacancies as anchor points for the construction of user-defined 2D patterns of adsorbate molecules via a controlled self-assembly process. Vice versa, energetic criteria indicate the formation of regular adsorbate-induced vacancies after adsorbate self-assembly on clean surfaces. These processes are exemplified by adsorbing C₆₀ fullerene on Al(111), Au(111), and Be(0001) surfaces with and without single, triple, and septuple atom pits. An analysis of vacancy-adatom formation energetics precedes the study of the adsorption processes.

Hreels Studies of KxC60 Thin Films

High resolution electron energy loss spectroscopy has been used to investigate the vibrational and electronic excitations of single-phase, crystalline AxC60 films. Substantial changes in intramolecular mode frequencies and intensities with alkali concentration arise from charge transfer effects as well as free carrier screening. For x=4, splitting of the t1u band is observed, resulting in an insulating phase with an estimated gap of 0.3-0.4eV. Screening in the x=3 phase results in enhanced surface sensitivity and deviations from bulk stoichiometry at the surface of metallic films are observed.

Metallization of the C60/Rh(100) interface revealed by valence photoelectron spectroscopy and density functional theory calculations

The electronic structure of single and multiple layers of C(60) molecules deposited on a Rh(100) surface is investigated by means of valence photoemission spectroscopy and density functional theory calculations. The binding of the fullerene monolayer to the metal surface yields the appearance of a new state in the valence band spectrum crossing the Fermi level. Insight into the metallization of the metal/fullerene interface is provided by the calculated electronic structure that allows us to correlate the measured interface state with a strong hybridization between the Rh metal states and the highest and lowest molecular orbitals. This results in a net charge transfer of approximately 0.5e-0.6e from the metal to the p states of the interfacial C atoms. The charge transfer is shown to be very short range, involving only the C atoms bound to the metal. The electronic structure of the second C(60) layer is already insulating and resembles the one measured for C(60) multilayers supported by the same substrate or calculated for fullerenes isolated in vacuum. The discussion of the results in the context of other C(60)/metal systems highlights the distinctive electronic properties of the molecule/metal interface determined by the Rh support.

Tunnelling spectra of individual magnetic endofullerene molecules

The manipulation of single magnetic molecules may enable new strategies for high-density information storage and quantum-state control. However, progress in these areas depends on developing techniques for addressing individual molecules and controlling their spin. Here, we report success in making electrical contact to individual magnetic N@C(60) molecules and measuring spin excitations in their electron tunnelling spectra. We verify that the molecules remain magnetic by observing a transition as a function of magnetic field that changes the spin quantum number and also the existence of non-equilibrium tunnelling originating from low-energy excited states. From the tunnelling spectra, we identify the charge and spin states of the molecule. The measured spectra can be reproduced theoretically by accounting for the exchange interaction between the nitrogen spin and electron(s) on the C(60) cage.

Covalent bonding and hole-electron Coulomb interaction U in C(60) on Be(0001) surfaces

We have investigated the bonding nature and hole-electron Coulomb interaction U in thin C(60) films on Be(0001) surfaces using valence-band and core-level photoemission, inverse photoemission, and near-edge x-ray absorption spectroscopies. The C(60) monolayer had strong covalent bonding with the Be substrate, producing a nearly insulating film, in contrast to a metallic overlayer due to charge transfer observed on many other metallic surfaces. The effect of polarization of surrounding molecules and the image potential decreases the energy gap and U, but the bonding-antibonding contribution increases the gap at the interface. The measured U in thin solid films agrees well with a model calculation using gas-phase values. The deduced hole-electron attraction on the surface is about 0.7 eV larger than the reported hole-hole repulsion determined by Auger spectroscopy. On the basis of the surface-solid difference, the newly estimated value of U for hole-hole correlation places doped C(60) compounds nearer the metallic side of a Mott transition.

Epitaxial supramolecular assembly of fullerenes formed by using a coronene template on a Au(111) surface in solution

Characteristic properties of the coronene layer formed on Au(111) for the epitaxial growth of various fullerenes are described. The electrochemical behavior of the coronene adlayer prepared by immersing a Au(111) substrate into a benzene solution containing coronene was investigated in 0.1 M HClO4. The as-prepared coronene adlayer on Au(111) revealed a well-defined (4 x 4) structure. Structural changes of the array of coronene molecules induced by potential manipulation were clearly observed by in situ scanning tunneling microscopy (STM). Supramolecularly assembled layers of fullerenes such as C60, C70, C60-C60 dumbbell dimer (C120), C60-C70 cross-dimer (C130), and C60 triangle trimer (C180) were formed on the well-defined coronene adlayer on the Au(111) surface by immersing the coronene-adsorbed Au(111) substrate into benzene solutions containing those molecules. The adlayers thus prepared were characterized by comparison with those which were directly attached to the Au(111) surface. The C60 molecules formed a honeycomb array with an internal structure in each C60 cage on the coronene adlayer, whereas C70 molecules were one-dimensionally arranged with the same orientations. The dimers, C120 and C130 molecules, formed an identical structure with c(11 x 4 radical3)rect symmetry. For the C130 cross-dimer molecule, C60 and C70 cages were clearly recognized at the molecular level. It was difficult to identify the adlayer of the C180 molecule directly attached to Au(111); however, individual C180 molecules could be recognized on the coronene-modified Au(111) surface. Thus, the adlayer structures of those fullerenes were strongly influenced by the underlying coronene adlayer, suggesting that the insertion of a coronene adlayer plays an important role in the formation of supramolecular assemblies of fullerenes.

Controlled surface ordering of endohedral fullerenes with a SrTiO(3) template

The ability to select the way in which atoms and molecules self-organize on a surface is important for synthesizing nanometre scale devices. Here we show how endohedral fullerenes (Er(3)N@C(80)) can be assembled into four distinctive arrangements on a strontium titanate surface template. Each template pattern correlates to a particular reconstruction on n-doped SrTiO(3)(001), made in whole or in part by self-assembled arrays of non-stoichiometric oxide nanostructures. Close-packed assemblies of Er(3)N@C(80) molecules are formed, as well as one-dimensional chains and two-dimensional grids. This method of template-assisted molecular ordering provides a new platform for the development of experimental schemes of classical and quantum information processing at the molecular level.

Surprises in the phase diagram of an anderson impurity model for a single C60(n-) molecule

We find by Wilson numerical renormalization group and conformal field theory that a three-orbital Anderson impurity model for a C60(n-) molecule has a very rich phase diagram which includes non-Fermi-liquid stable and unstable fixed points with interesting properties, most notably high sensitivity to doping n. We discuss the implications of our results to the conductance behavior of C60-based single-molecule transistor devices.

Band structure and Fermi surface of electron-doped C60 monolayers

C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali-doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions.

Temperature-dependent fermi gap opening in the c(6x4)-C60/Ag(100) two-dimensional superstructure

High-resolution angle-integrated photoemission of one monolayer of C (60) chemisorbed on Ag(100) shows the reversible opening of a gap at the Fermi level at temperatures 25< or =T< or =300 K. The gap reaches a maximum value of approximately 10 meV at T< or =70 K. This finding is the first evidence of an electronic phase transition in C60 monolayers and has implications on the ongoing debate about surface superconductivity in C60-based bulk materials.

Study of C60/Au(110)-p(6x5) reconstruction from In-plane X-Ray diffraction data

Fullerene molecules absorbed on the highly anisotropic Au(110)-p(1x2) surface induce an ordered p(6x5) superstructure that has been solved by applying the 2D "direct methods" difference sum function to the surface x-ray diffraction data set. We found that the C (60)-gold interface is structurally much more complex than the one previously suggested by scanning tunneling microscopy data [J. K. Gimzewski, S. Modesti, and R. R. Schlittler, Phys. Rev. Lett. 72, 1036 (1994)]. Indeed a large fraction of Au surface atoms are displaced from their original positions producing microscopic pits that may accommodate the fullerene molecules.