C60 Monolayer Research Articles

Dependence of the band structure of C60 monolayers on molecular orientations and doping observed by angle resolved photoemission

Abstract We present angle resolved photoemission studies of C60 monolayers deposited on Ag surfaces. The electronic structure of these monolayers is derived from the partial filling of the narrow, 6-fold degenerated, C60 conduction band. By comparing the band structure in two monolayers deposited, respectively, on Ag(111) and Ag(100), we show that the molecular degree of freedom, in this case the relative orientations between C60 molecules, is essential to describe the band structure. We further show that the evolution of the band as a function of doping does not follow a rigid band-filling picture. Phase separation is observed between a metallic and an insulating phase, which might be a result of strong correlations.

Gold cluster formation on a fullerene surface

The growth of Au clusters on a fullerene thin film was investigated by in situ photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as molecular templates and serve to create a cluster superstructure. To gain insight into the fullerene-Au interaction two types of experiments were performed: (i) the deposition of Au on a fullerene surface, and (ii) the deposition of fullerenes on a Au surface. In both experiments an island growth mode is observed. The deposition of submonolayer amounts of C60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C60 monolayer. In addition a constant band bending in the fullerene film is detected, but the UPS valence-band spectra show that there is no charge transfer from the Au to the C60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C60 surface, the analysis of the Au core level does not reveal a specific bonding or nucleation site for Au atoms and clusters. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene molecules. The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to 150 atoms for the deposition of a nominal coverage of 1.5 nm. These data are derived from an analysis of the d-band splitting and the Au 4f core-level shift due to delayed photohole relaxation. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temperatures of 650 degrees C. For temperatures up to 450 degrees C a continuous growth of the clusters is detected, which is accompanied by a slight drop in Au concentration in the range of XPS for annealing temperatures higher than 350 degrees C. This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. The film shows a very good thermal stability and even after annealing at 650 degrees C there is still a fullerene film detectable in the photoelectron spectroscopy spectra.

Charge transfer and formation of conducting C60 monolayers at C60/noble-metal interfaces

The resistance of a conducting C60 monolayer formed on a polycrystalline Ag film was found to be 0.7±0.1kΩ by in situ resistance measurements. By another series of in situ resistance measurements, the surface scattering cross sections, whose magnitude represents the relative amount of transferred charge, were evaluated as 100Å2 for C60/Au, and 150Å2 for C60/Cu and C60/Ag systems. However, comparison with previous results obtained for monolayers formed on Au and Cu films showed that the resistances of conducting C60 monolayers do not show a simple dependence on the transferred charge. Atomic force microscopy measurements revealed that the grain size of the underlying noble metals also plays an important role.

Rocking-motion-induced charging ofC60onh−BN∕Ni(111)

One monolayer of C60 on one monolayer of hexagonal boron nitride on nickel is investigated by photoemission. Between 150 and 250 K the work function decreases and the binding energy of the highest occupied molecular orbital (HOMO) increases by approx. 100 meV. In parallel, the occupancy of the, in the cold state almost empty, lowest unoccupied molecular orbital (LUMO) changes by 0.4 electrons. This charge redistribution is triggered by onset of molecular rocking motion, i.e. by orientation dependent tunneling between the LUMO of C60 and the substrate. The magnitude of the charge transfer is large and cannot be explained within a single particle picture. It is proposed to involve electron-phonon coupling where C60- polaron formation leads to electron self-trapping.

Superlubricity of C60 Intercalated Graphite Films

The frictional behavior of the C60 intercalated graphite films with a large size of 2.3 × 2.3 mm2 is reported. The C60 intercalated graphite films consist of alternating close-packed C60 monolayers and graphite layers (graphenes), and thus infinite sliding planes are formed between each C60 monolayer and graphene. The intercalation of C60 molecules into graphite films results in superlubricity where both static and dynamic frictional forces are observed to be zero. [DOI: 10.1380/ejssnt.2005.21]

Preparation of Nano-Structured C60 Thin Films

Sapphire (0001) substrates were annealed at 1000°C for 12 h in air to obtain flat surfaces with a step-terrace structure. Au films were deposited on the substrates using the molecular beam epitaxy (MBE) method, at various substrate temperatures. The surface morphology of the films was observed by means of atomic force microscopy (AFM). A quasi one-dimensional alignment of Au particles was partially prepared along the steps at the substrate temperature of 620°C. On the other hand, an atomically flat Au thin film was obtained at the low substrate temperature of 130°C. A C60 monolayer was obtained by re-evaporating the C60 molecules deposited on the Au films, which was confirmed by X-ray photoelectron spectroscopy (XPS).

Thermal Diffusion of C60 Molecules and Clusters on Au(111)

The structure and dynamics of partial monolayers of C60 on the Au(111) surface were observed by scanning tunneling microscopy (STM) under ambient conditions. At submonolayer coverages, C60 molecules group into islands, and while the molecules in the interior of islands are quite stable, the C60s at island edges are less so, and their motions can be observed in real time using STM. The motion of surface-adsorbed C60 is predominantly the result of thermal diffusion; the extent of perturbation by the STM tip is determined to be minimal, using a quantitative analysis of a time series of STM images. Further analysis shows that motion of single C60 molecules is relatively uncommon, and that most diffusion of C60 on the surface occurs through the correlated or cooperative motion of molecular clusters ranging in size from 2 to 8 C60 molecules. Cluster diffusion is explained by a proposed “anchoring” mechanism, in which the stability of each C60 varies according to its position and orientation relative to the surface and to neighboring molecules.

Surface composition of K3C60 multilayers and C[formula omitted] monolayers investigated by high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectroscopy (HREELS) in reflection geometry was used to probe the surface of K3C60 multilayer films as well as C603− monolayers adsorbed on Ag(1 1 1) and Cu(1 1 1). The −3 charge state of the adsorbed fullerenes was obtained by distilling K-saturated C60 monolayers. We found that roughly 2 electrons are donated by the K atoms on Ag(1 1 1) and slightly less than 1.5 electron in the case of Cu(1 1 1), the charge difference being provided by the metallic substrates. The surface of the K3C60 multilayer films has a charge state of about −1.5 and is thus underdoped.

The understanding of electronic excitations ofC60 on graphite

Electron energy loss spectroscopy has been used to probe the coverage-dependent electronic excitationsfrom a C60 covered graphite surface. In the energy region corresponding to the optical band-gap of the bulkC60 solid, the dipole-scattering contribution to the loss features is simulated bya double dielectric layer model. We find that a good agreement betweenthe simulated spectra and the experimental results can be achieved for aC60 multilayer.However, an additional energy loss channel is required in order to describe the electronic excitationsfor a C60 monolayer. The possible significances of this finding are discussed.

HREELS study of the intercalation of K atoms in C60 monolayers

Extra- and intramolecular vibrations of K-doped C60 monolayers adsorbed on Ag(1 1 1) and Cu(1 1 1) have been studied by high-resolution electron energy loss spectroscopy (HREELS) in reflection geometry. Intramolecular modes are used to assess the electronic occupancy of C60, which is increased by intercalating K atoms. Regardless of the charge donated by the substrate, extramolecular modes have characteristic frequencies for K1C60, K2C60, and K4C60 monolayers. When the K concentration exceeds four, the K-doped monolayers exhibit different structures on Ag(1 1 1) and Cu(1 1 1).

Adsorption and Self‐Assembly of C70 Molecules at the Au(111)/n‐Tetradecane Interface: A Scanning Tunneling Microscopy Study

An STM study of C70 (sub‐)monolayers deposited at an n‐tetradecane/Au(111) interface (see Figure) is reported. The adsorption of C70 and C60 are compared in the light of their geometric and electronic differences. While C60 is a strained spherical cage, C70 has a reduced symmetry and an ellipsoidal shape. The structural differences between the C60 and C70 (sub‐)monolayers are discussed in terms of commensurability between the adsorbate and the Au (111) lattices.

Molecular resolution imaging ofC60 on Au(111) by non-contact atomic force microscopy

Non-contact atomic force microscopy (NC-AFM) was used to study thin films ofC60 on Au(111). After observing the Au(111) reconstruction, 2–3 monolayers ofC60 were deposited onto the Au surface. The close-packedC60 surface was imaged by NC-AFM with molecular resolution. Enhanced corrugation and a stretchingof the C60 lattice were observed at step edges. Based on a calculation of the force required todisplace an edge molecule, it is proposed that the edge effects are a result oftip-induced displacements of edge molecules. While imaging small clusters ofC60, some molecules were removed, leading to structural rearrangements of the clusters.

A Study of Nano-scale Mechanics-Basic Principle for Nano-fabrication and -machines-

We discuss recent studies of nano-size mechanics from the stand points of nano-fabrication and nano-machine. First, a nano-mechanical simulator developed by us is explained. Theoretical simulations of noncontact atomic force microscopy images by taking thermal fluctuation into consideration are in very good agreement with experimental results. Next, a new nano-size super-lubricated system that is not subject to friction, “C60 molecular bearing,” is described. This sandwiched structure which confines a C60 monolayer between graphite plates, exhibits superlubrication with zero dynamical frictional forces. Observed results and a theoretical concept of, “step rotation model” to interpret the mechanism of super lubrication, are explained by using a picture of “single molecular bearing.” This research is the first proposal as an artificial nano-machine using fullerene, which can be called the smallest bearing in the world.

Resistance measurements of conducting C60 monolayers formed on Au and Cu films

The resistances of conducting C60 monolayers formed on Au and Cu films were found to be 0.9±0.2 and 2.4±0.4 kΩ, respectively, by in situ resistance measurements. Although the amount of charge transferred to each C60 molecule from the Cu film was greater than that from the Au film, the conducting C60 monolayer formed on the Cu film had higher resistance than that formed on the Au film. This result is consistent with resistance data for alkali fullerides.

Organic Light-Emitting Diodes with a Nanostructured Fullerene Layer at the Interface between Alq3 and TPD Layers

Organic light-emitting diodes (OLEDs) with a nanostructured fullerene (C60) layer at and near the interface between the electron- and hole-transport layers were fabricated using a vacuum evaporation method. The OLEDs had a fundamental structure of indium-tin-oxide (ITO) anode/copper phtalocyanine (CuPc)/N,N'-dephenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. The electroluminescent (EL) properties have been investigated for the OLEDs with a nanostructured C60 layer at the TPD/Alq3 interface and at the different positions in the Alq3 layer near the TPD/Alq3 interface. The dependences of the EL properties on the thickness of the inserted C60 layer and on the position from the interface were examined. The OLEDs with a C60 monolayer inserted at the TPD/Alq3 interface showed the highest efficiency.

Lateral confinement of image electron wave function by an interfacial dipole lattice

Image-potential states on Cu(111) surfaces covered by thin films of C60 fullerene have been characterized by angle-resolved two-photon photoemission spectroscopy. Metal-to-molecule electron transfer within the first layer creates a 4×4 superlattice of surface dipoles. We show that such a surface dipole lattice provides lateral confinement of image-electron wave functions. Measurements of parallel dispersion indicate that the n=1 image state is localized in the presence of one monolayer of C60 but becomes delocalized by the addition of a second layer. Quantum mechanical calculations explain this in terms of the screening of the dipole potential, thus, restoring the free-electron behavior parallel to the surface. These results show that a surface dipole lattice can effectively control the interfacial electronic structure.

C60Molecular Bearings

An ultralubricated system is reported which confines a C60 monolayer between graphite plates. C60 molecules act as molecular bearings, assisted by the nanogears of six-membered carbon rings between C60 molecules and graphite, in which the mean dynamical frictional forces are zero up to a high load of 100 nanonewtons. A stick-slip rolling model with a step rotation of a C60 molecule is proposed. This ultralubricated system, very promising for the realization of nano- and micromachines, is expected to open a new field of molecular bearings.

単層Ｃ６０薄膜の形成と摩擦特性

A C60 monolayer film can grow on a graphite (Highly Oriented Pyrolitic Graphite: HOPG) substrate at a substrate temperature of 150°C. The frictional behavior of a C60 monolayer film between graphite substrates is studied using a homebuilt surface force apparatus. The mean frictional force from the C60 monolayers is estimated to be approximately 2mN, which is one-fifth that of C60 multilayer films. This result indicates that C60 molecules of the C60 monolayer sandwiched between graphite substrates roll via the nanogears of six-membered carbon rings between C60 molecules and the upper and lower graphite substrates. The C60 monolayer film maintains durability with a low frictional force of 2 mN for more than one hundred scans under a load of 10mN. This behavior does not change in the scan velocity range of 10 to 20 μm/s. Hence a novel frictional mechanism is realized in the graphite substrates sandwiching a C60 monolayer film system. It should be emphasize that the graphite substrates sandwiching a C60 monolayer film system is very promising for the realization of nanomachines and micromachines.

Synthesis of C60 self-assembled monolayer on Au ultrathin films

We have studied the possibility of room temperature superconductivity andnanoscaled conducting systems by using self-assembled monolayers of theC60 derivatives C60–O–C8SH (C60-SAMs). In this study the C60-SAMswere synthesized on Au ultrathin films. The films were prepared on MgOsingle-crystal substrates whose surfaces had clear step–terrace structures. Themorphologies of the C60-SAM/Au films synthesized were observed byatomic force microscopy and the resistivities of the films were studied.

Frictional behavior of C60 monolayer films on graphite (HOPG)

ABSTRACTThe frictional behavior of a C60 monolayer film between graphite substrates is studied using a homebuilt surface force apparatus. The mean frictional force from the C60 monolayers is estimated to be approximately 2mN, which is one-fifth that of C60 thin films. The C60 monolayer films exhibit a low frictional force of 2mN up to one hundred scans under a normal stress of 8MPa, which indicates that the C60 monolayer film is highly promising for use as a lubricant. Key words, C60 monolayer film, Lubrication, Mechanical properties, Micromachines