Determination of the orientation of C60 adsorbed on Au(111) and Ag(111)

Highly ordered C60 monolayer self-assembled by using an iodine template on an Au(111) surface in solution

An iodine-modified Au(111) surface, (I/Au(111)), was used as a substrate to prepare a C 60 adlayer by self-organization in a benzene solution. A highly ordered C 60 adlayer was successfully prepared due to the moderate C 60-I/Au(111) interaction. Two lattice structures, (2 square root 3 x 2 square root 3) R30 degrees and p(2 x 2), were imaged for this C 60 adlayer. For the first structure, a featureless ball-like molecular shape was imaged, ascribed to the molecular rotation resulting from a symmetrical location between C 60 and iodine atoms. For the p(2 x 2) structure, the asymmetrical location of C 60 with respect to the iodine atoms freezes the C 60 molecules on the substrate, leading to a clear image of intramolecular structure. The intermediate iodine atoms in the C 60/I/Au(111) adlayer can be desorbed by electrochemically reduction without significantly affecting the ordering of the C 60 adlayer. However, the internal pattern of C 60 disappears in the absence of iodine.

Orientationally ordered C60 on p-sexiphenyl nanostripes on Ag111

Long range orientational ordering within C(60) monolayers is observed on p-sexithiophene (6P) monolayer nanostripes on Ag(111) at 77 K. Low-temperature scanning tunneling microscopy studies reveal that the C(60)-6P intermolecular interaction constrains all C(60) molecules to adsorb on their hexagons atop 6P molecules. The orientation-dependent bond-to-bond Coulomb interaction between charge deficient single bonds and double bonds with excess charge in neighboring C(60) molecules results in the in-plane orientational ordering and contributes to the lowering of the total energy of the orientationally ordered C(60) islands.

Theoretical investigation of electronic structures and properties of C60-gold nanocontacts

A theoretical study was performed to understand the structures and properties of C60-gold nanocontacts. In this investigation, C60 was sandwiched between gold clusters. In the studied clusters, the number of Au atoms varied from 2 to 8 on each side of C60. Molecular geometries of the investigated complexes were optimized at the density functional theory level, employing the B3LYP functional. The 6-31G(d) basis set was used for carbon atoms, while the LANL2DZ effective core potential was used for gold atoms. Geometries of all complexes were optimized under C2h symmetry except for the C60-10Au complex, for which C2 symmetry was assumed. Two different configurations, namely eta(2(5)) and eta(2(6)), for the binding of Au clusters with C60 were considered. It was revealed that complexes corresponding to the latter configuration are more stable than those having the former one. Ground-state geometries of the complexes involving odd numbers of gold atoms on each side of C60 were found to be represented by the triplet configuration. The HOMO-LUMO energy gaps of C60-gold complexes were found to be lower than that of isolated C60. The charge transport properties in the studied system are discussed in terms of molecular orbitals and the Fermi level.

Orientationally ordered (7x7) superstructure of C60 on AU(111)

Long range orientational order within C60 monolayers on Au(111) is observed with low-temperature scanning tunneling microscopy. A unit cell comprised of 49 molecules which adopt 11 different orientations is found. It can be divided in a faulted and an unfaulted half similar to the (7x7) reconstruction of Si(111). A model is proposed which shows how, through a Moiré-like effect, the substrate induces minute changes in the orientation of the C60 molecules. Intermolecular interactions are shown to play a major role in stabilizing the superlattice.

An organic donor/acceptor lateral superlattice at the nanoscale

A precise control of the nanometer-scale morphology in systems containing mixtures of donor/acceptor molecules is a key factor to improve the efficiency of organic photovoltaic devices. Here we report on a scanning tunneling microscopy study of the first stages of growth of 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole, as electron donor, and phenyl-C61-butyric acid methyl ester, as electron acceptor, on a Au(111) substrate under ultrahigh vacuum conditions. Due to differences in bonding strength with the substrate and different interactions with the Au(111) herringbone surface reconstruction, mixed thin films spontaneously segregate into a lateral superlattice of interdigitated nanoscale stripes with a characteristic width of about 10-20 nm, a morphology that has been predicted to optimize the efficiency of organic solar cells.

Bimolecular networks and supramolecular traps on Au(111)

We demonstrate the formation of intermixed phases and self assembled molecular templates on the Au(111) surface. The templates are stabilized by hydrogen bonding between melamine molecules with trigonal symmetry and linear PTCDI (perylene tetra-carboxylic di-imide) molecules. When annealed, these molecules spontaneously form either a chiral intermixed phase or a honeycomb arrangement in which vertexes and edges correspond respectively to melamine and PTCDI molecules. We also observe minority phases with more complex intermolecular junctions. The use of these networks as templates is demonstrated by the controlled capture of fullerenes within the pores of the network to form dimers, hexamers, and heptamers. Our results confirm that bimolecular templates can be realized on a range of substrates.

Metal-molecule interface fluctuations

We have created self-assembled circular chains of C60 laterally bound to a layer of Ag atoms as a model system for characterizing fluctuations at a metal-molecule interface. STM measurements show that the Ag and C60 sides of the interface fluctuate independently, with frequency-dependent amplitudes of magnitude 0.1 nm at approximately 1 Hz for the Ag edge and approximately 0.01 Hz for the C60 ring. The measured frequency spectra of the metal and molecule fluctuation amplitudes will contribute characteristic signatures to transport measurements involving such interfaces.

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.

Ballistic Electron Microscopy of Individual Molecules

We analyzed the transport of ballistic electrons through organic molecules on uniformly flat surfaces of bismuth grown on silicon. For the fullerene C60 and for a planar organic molecule (3,4,9,10-perylene-tetracarboxylic acid dianhydride), the signals revealed characteristic submolecular patterns that indicated where ballistic transport was enhanced or attenuated. The transport was associated to specific electronic molecular states. At electron energies of a few electron volts, this "scanning near-field electron transmission microscopy" method could be applied to various adsorbates or thin layers.

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.

Structural rearrangement of c(70) monolayers induced by octanethiol adsorption

Mixed monolayers of C70 and octanethiol are prepared on Au(111) by a sequential adsorption method. A partial C70 monolayer is first formed and characterized, followed by the vapor deposition of octanethiol. This results in a well-ordered alkanethiol film where single C70 molecules and small molecular clusters are located at domain boundaries and in disordered regions. Substrate step defects have a large influence on the spatial distribution of C70; adjacent to a substrate defect, C70 binds preferentially on the upper terrace and is depleted on the lower terrace. We explain these observations as resulting from the kinetics of alkanethiol monolayer formation, and we present a simple model for the evolution of surface structure in the C70/octanethiol system.

Electronic structure of C60 on Au(887)

We present an analysis of the electronic structure of C60 adsorbed on a vicinal Au(111) surface at different fullerene coverages using photoemission, x-ray absorption, and scanning tunneling microscopy/spectroscopy (STS). STS provides a straightforward determination of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels with respect to the Fermi energy. At C60 coverages of 0.5 and 1 ML a 2.7 eV wide HOMO-LUMO gap is found. The near-edge x-ray absorption fine structure (NEXAFS) spectrum for the 0.5 ML C60 nanomesh structure displays a significant intensity at the low energy side of the LUMO exciton peak, which is explained as due to absorption into HOMO-LUMO gap states localized at individual C60 cluster edges. From 0.5 to 1 ML we observe a rigid shift of the HOMO-LUMO peaks in the STS spectra and an almost complete quenching of the gap states feature in NEXAFS.

Coverage Dependent Supramolecular Structures: C60:ACA Monolayers on Ag(111)

The dependence of supramolecular structure on fractional molecular coverage has been investigated for acridine-9-carboxylic acid (ACA) and the C(60):ACA binary molecular system. The coverage-dependent phase diagram for ACA is first determined from room-temperature STM imaging. At low molecular coverages (theta < 0.4 ML, ML = monolayer), ACA forms a 2-D gas phase. Ordered ACA structures appear with increasing coverage: first a chain structure composed of ACA molecules linked by consecutive O-H...N hydrogen bonds (theta > 0.4 ML), then a dimer structure composed of ACA dimers linked by paired carboxyl-carboxyl hydrogen bonds (theta approximately equal to 1.0 ML). Structures of the C(60):ACA binary system depend on the coverage of predeposited ACA. At intermediate (0.4 ML approximately 0.8 ML) ACA coverages, C(60) deposition results in a hexagonal cooperative structure with the C(60) periodicity nearly 3 times that of the normal C(60) 2-D packing of 1 nm and exists in enantiopure domains. At higher ACA coverages, a C(60) quasi-chain structure is formed in which parallel C(60) chains are spaced by ACA dimer domains. The mechanistic role of the initial ACA phase in the formation of C(60):ACA supramolecular structures is described. Chemically intuitive molecular packing models are presented based on the observed STM images.

C60 thin film growth on graphite: Coexistence of spherical and fractal-dendritic islands

The initial growth stage of C(60) thin film on graphite substrate has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The C(60) layer grows in a quasi-layer-by-layer mode and forms round, monolayer high islands on the graphite surface. The islands are confined by terraces on the graphite surface and the mobility of C(60) fullerenes across steps is low in all layers. The second and all subsequent layers adopt a fractal-dendritic shape, which was confirmed by calculating the fractal dimension (D=1.74 prior to island coalescence) and is in agreement with a diffusion limited aggregation. The profound differences between the growth of C(60) layers on graphite (first layer) and on C(60) surfaces (second and higher layers) are caused by the restriction of the C(60) mobility on the highly corrugated fullerene surfaces. The orientation of the fractal islands follows the hexagonal symmetry of the densely packed (111) surface of the fullerene lattice, which introduces a bias in the direction of molecule movement. The differences in surface topography on the nanoscale determine the mode of film growth in this van der Waals bonded system.

Chiral symmetry breaking in two-dimensional C60-ACA intermixed systems

We have demonstrated a method for fabricating C60 overlayers with controlled spacing and chirality by reactive coadsorption with the aromatic molecule acridine-9-carboxylic acid (ACA). Structural control is achieved by the mismatched symmetries of the coadsorbates, as well as specific intermolecular and adsorbate-substrate interactions. The resulting supramolecular structure has a C60 period nearly three times as large as the normal C60 2D packing of 1 nm and exists in enantiopure domains with robust chirality.

Adlayers of C60-C60 and C60-C70 fullerene dimers formed on au(111) in benzene solutions studied by STM and LEED

Scanning tunneling microscopy (STM) and low-energy electron diffraction were used to reveal the structures of ordered adlayers of [2+2]-type C60-C60 fullerene dimer (C120) and C60-C70 cross-dimer (C130) formed on Au(111) by immersingit in abenzene solution containing C120 or C130 molecules. High-resolution STM images clearly showed the packing arrangements and the electronic structures of C120 and C130 on the Au(111) surface in ultrahigh vacuum. The (2 square root3 x 4square root3)R30 degrees, (2square root3 x 5square root3)R30 degrees, and (7 x 7) structures were found for the C120 adlayer on the Au(111) surface, whereas C130 molecules were closely packed on the surface. Each C60 or C70 monomer cage was discerned in the STM image of a C130 molecule.

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.