Probing the interaction at the C60-SiC nanomesh interface

Synchrotron-based high-resolution photoemission spectroscopy (PES) and in situ scanning tunneling microscopy (STM) are used to investigate the interaction at the C₆₀-SiC nanomesh interface during the sequential deposition and subsequent desorption of C₆₀ molecules. A weak charge transfer occurs at the C₆₀-nanomesh interface, involving electrons transferring from nanomesh to C₆₀ overlayer. The interface interaction originated from the weak charge transfer at the C₆₀-nanomesh interface is stronger than C₆₀ intermolecular interaction (e.g., van-der-Waals force), facilitating the layer-by-layer growth for the first two layers of C₆₀ on SiC nanomesh. The highly corrugated nanomesh surface results in an anisotropic diffusion and high diffusion barrier of C₆₀ on top, and thereby leads to the formation of irregularly shaped C₆₀ islands under submonolayer condition. In contrast, C₆₀ diffusion on HOPG and Ag(1 1 1) surfaces is rather isotropic, resulting in the formation of hexagonally shaped C₆₀ islands with smooth domain boundaries. STM results show the partial desorption of C₆₀ molecules from the SiC nanomesh surface after annealing the 1 ML C₆₀ sample (complete wetting layer of C₆₀ on SiC nanomesh) at around 150 °C for 20 min. Thorough desorption of C₆₀ molecules and full recovery of the clean SiC nanomesh are observed after annealing at around 200 °C for 20 min. In situ PES and STM experiments clearly demonstrate that C₆₀ adsorption and desorption processes do not affect the underlying SiC nanomesh structure, revealing its thermal stability and chemical inertness to C₆₀ molecules.     

Epitaxial C60 thin films on Bi(0 0 0 1)

We have used the Bi(0 0 0 1)/Si(1 1 1) template to grow highly ordered C₆₀ epitaxial thin films and analyzed them using scanning tunneling microscopy and low-energy electron microscopy. The in situ low-energy electron microscope investigations show that the initial nucleation of the C₆₀ islands on the surface takes place at surface defects, such as domain boundaries and multiple steps. The in-plane lattice parameters of this C₆₀ film turns out to be the same as that of the bulk fcc(1 1 1) C₆₀. The line-on-line epitaxial structure is realized in spite of a weak interaction between the C₆₀ molecules and Bi(0 0 0 1) surface, while scanning tunneling spectroscopy indicates that there is a negligible charge transfer between the molecules and the surface.     

Intramolecular structures of C60 and C84 molecules on Si(1 1 1)-7 × 7 surfaces by scanning tunneling microscopy

The intramolecular features of carbon 60 and carbon 84 molecules on Si(1 1 1)-7 × 7 surfaces were studied under a UHV-scanning tunneling microscope. Carbon molecules preferentially appear in faulted halves, rather than in unfaulted halves and corner holes; they are embedded in silicon substrates. The orientation and details of the structure of carbon molecules are determined by applying various sample biases to the silicon substrate. As compared with other fullerenes, a bright pentagonal ring with nebulous clusters which represents the cage structure is clearly observed on top of carbon 60 molecules. The bright stripes associated with partitioned curves which depict eight features of asymmetrical C₈₄ molecules are also investigated on Si(1 1 1)-7 × 7 surfaces. The orientations and possible configurations of C₆₀ and C₈₄ are considered in this work. The energy differences for various features of C₆₀ and C₈₄ molecules are estimated and discussed. The corresponding models with respect to each intramolecular feature are proposed and compared with recent theoretical calculation.     

Theoretical investigation of the reflectivity of fullerene-(C60, C70)/AlN multilayers in UV region

Theoretical calculations via a transfer matrix method (TMM) were performed to investigate the possibility of fullerene/AlN multilayer films acting as one-dimensional (1D) photonic band gap (PBG) crystals. The response within and out of the periodic plane of (C₆₀, C₇₀)/AlN multilayers was studied. (C₆₀, C₇₀)/AlN multilayer films presented incomplete PBG behavior in UV region. C₆₀/AlN multilayers with two pairs of 49 nm-C₆₀ and 21 nm-AlN layers exhibited a high reflectivity of 90.4% at a wavelength of about 200 nm. As a consequence, this photonic crystal may be important for achieving materials with an incomplete band gap in the UV region.     

Empty-state scanning tunneling microscopy image of C2H2 on Si(0 0 1)--new evidence for paired end-bridge di-σ configuration

New evidence of the paired end-bridge configuration in the room-temperature adsorption geometries of C₂H₂ molecules on Si(0 0 1) is presented by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. The distinct four-leaved feature occupying two adjacent Si dimer sites in the experimental empty-state STM images are well reproduced by simulations of the paired end-bridge adsorption configuration. Calculated energetics suggests that the Si(0 0 1) surface is covered by paired end-bridge structures at the saturation coverage of 1 ML, in agreement with the existing experiments.     

Molecular simulation of C60 adsorption onto a TiO2 rutile (1 1 0) surface

A Monte Carlo molecular simulation study is presented on the adsorption and growth of C₆₀ films on the surface of the (1 1 0) face of rutile. Simulations are performed for a temperature of 600 K using atomistic models both for the fullerene molecules and the TiO₂ surface. It is found in this work that C₆₀ is adsorbed preferably in an ordered arrangement along the surface depressions over the exposed undercoordinated Ti cations. At low densities adsorption occurs preferably at alternate rows, with locations in consecutive rows being occupied appreciably only at higher C₆₀ densities. At low densities, the fullerene molecules tend to aggregate into islands in the surface plane. Additional layers of C₆₀ form only as the density increases, and do so before a monolayer is completed in all consecutive rows. Full monolayer capacity obtained at the highest densities is about 0.9 C₆₀ molecules per nm², but this is only achieved by completing the packing of molecules in interstices at a slightly upper level. The fraction of the molecules that lie closest to the surface only amounts to 0.6 molecules per nm².     

RHEED intensity oscillation of C60 growth on GaAs substrates

Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C₆₀ layer epitaxial growth on GaAs (1 1 1)B and (1 1 1)A substrates. The frequency of the oscillation coincides well with growth rate of C₆₀ layers, suggesting that C₆₀ layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C₆₀ layer growth on GaAs (1 1 1)B substrates with (2 × 2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C₆₀ molecules. This phenomenon is explained by the model that C₆₀ absorption sites are limited due to As-trimers absorbed on (1 1 1)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (1 1 1)A substrates where no As-trimer is absorbed.     

Sputtering of amorphous ice induced by C60 and Au3 clusters

Molecular dynamics simulations were performed to study the behavior of cluster SIMS. Two predominant cluster ion beam sources, C₆₀ and Au₃, were chosen for comparison. An amorphous water ice substrate was bombarded with incident energy of 5 keV. The C₆₀ cluster was observed to shatter upon impact creating a crater of damage approximately 8 nm deep. Although Au₃ was also found to both break apart and form a damage crater, it continued along its initial trajectory causing damage roughly 10 nm deep into the sample and becoming completely imbedded. It is suggested that this difference in behavior is due to the large mass of Au relative to the substrate water molecule.     

The structure of ordered Au films on TiOx

The growth of Au on an ultra-thin, ordered Mo(1 1 2)-(8 × 2)-TiO_x, was investigated using scanning tunneling microscopy (STM), low energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Wetting of the TiO_x surface by Au was observed with STM and LEISS, and the ordering of the Au films was atomically resolved with STM. TPD showed that Au binds more strongly to the reduced TiO_x film than to bulk TiO₂, but more weakly than to the Mo substrate. The Au-TiO_x binding energy is greater than Au-Au in bulk Au. The oxidation state of Ti in the TiO_x film was deduced by XPS and from the Ti-O phonon shifts relative to bulk TiO₂. The TiO_x/Mo(1 1 2) film structure and those for the (1 × 1)- and (1 × 3)-Au/TiO_x/Mo(1 1 2) surfaces are discussed.     

STM investigations of Au(1 1 1) electrodes coated with vitamin B12 derivatives

Vitamin B₁₂ derivatives immobilized at flame-annealed Au(1 1 1) electrode surfaces have been investigated in close correlation with their structural properties and spatial arrangement at the electrode substrate by scanning tunneling microscopy (STM) in air and in aqueous 0.1 M NaClO₄ solution. The investigated compounds were symmetrical (B₁₂C₁₀S-SC₁₀B₁₂) and nonsymmetrical (B₁₂C₁₀S-SC₁₀) dialkyl disulfide derivatives of vitamin B₁₂, attached to the electrode surfaces by the S-Au bond. The ex situ and in situ STM experiments show the formation of a surface layer, whose packing density and structure is presumably controlled by the spatial arrangement of the large cobyrinate head groups. In presence of the symmetrical B₁₂ compound, a disordered surface layer is observed. Voltammetric investigations show that, in 0.1 M NaClO₄, this layer becomes unstable at potentials approximately ? −1000 mV vs. MSE and is almost completely removed at more negative potentials. The STM imaging properties of the nonsymmetrical B₁₂ surface layer show a significant dependence on the tunneling distance. In particular, at small tunneling distances, a highly regular hexagonal surface pattern is observed that suggests strongly the presence of an ordered surface assembly. Modeling of the B₁₂ head group has been performed to provide information for a structure-related interpretation of the high-resolution STM images. The investigations are first STM results obtained at such B₁₂ modified electrodes.     

Investigation of orientation and packing of H8Si8O12 arrays on graphite by scanning tunneling microscopy

The layer structures of H₈Si₈O₁₂ molecules on highly oriented pyrolitic graphite have been investigated by scanning tunneling microscopy. Two kinds of ordered assemblies of H₈Si₈O₁₂ monolayers are observed, with a unit cell of 6.5 Å × 6.5 Å and 7.2 Å × 9.4 Å, respectively. On the basis of the shapes and sizes of the H₈Si₈O₁₂ STM images and the heights of the H₈Si₈O₁₂ monolayers, H₈Si₈O₁₂ can be adsorbed with one face of its cage structure in contact with the substrate surface or with a tilted orientation.     

Scanning tunneling microscopy study on a BC3 covered NbB2(0 0 0 1) surface

We have investigated a BC₃ covered NbB₂(0 0 0 1) surface using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and low energy electron diffraction (LEED). The STM images reveal characteristic features of a Moiré pattern reflecting an incommensurate relation of the BC₃ sheet with the substrate: bright protrusions with the periodicity of the substrate lattice are modulated in intensity with the periodicity of the BC₃ lattice. As a result, the surface exhibits nm-scale patchy regions with either the √3 × √3 or the 1 × 1 structure of the substrate. The two-dimensional Fourier transformation pattern of the STM image is consistent with the LEED pattern proving the epitaxial and incommensurate relationship between BC₃ surface sheet and substrate. No feature of a predicted superconducting gap was found in STS spectra measured at 5 K.     

Stretching the limits of static SIMS with C60

Pristine and Au-covered molecular films have been analyzed by ToF-SIMS (TRIFT™), using 15 keV Ga⁺ (FEI) and 15 keV C₆₀⁺ (Ionoptika) primary ion sources. The use of C₆₀⁺ leads to an enormous yield enhancement for gold clusters, especially when the amount of gold is low (2 nmol/cm²), i.e. a situation of relatively small nanoparticles well separated in space. It also allows us to extend significantly the traditional mass range of static SIMS. Under 15 keV C₆₀⁺ ion bombardment, a series of clusters up to a mass of about 20,000 Da (Au₁₀₀⁻: 19,700 Da) is detected. This large yield increase is attributed to the hydrocarbon matrix (low-atomic mass), because the yield increase observed for thick metallic films (Ag, Au) is much lower. The additional yield enhancement factors provided by the Au metallization procedure for organic ions (MetA-SIMS) have been measured under C₆₀⁺ bombardment. They reach a factor of 2 for the molecular ion and almost an order of magnitude for Irganox fragments such as C₄H₉⁺, C₁₅H₂₃O⁺ and C₁₆H₂₃O⁻.     

Giant fullerenes formed on C60 films irradiated with electrons field-emitted from scanning tunneling microscope tips

It has been found that spherical large clusters of carbon atoms are formed by irradiation of crystalline C₆₀ films grown on Si(1 1 1)-(7 × 7) surfaces with electrons field-emitted from a scanning tunneling microscope probe tip. The size distribution of the clusters deduced from surface profile measurements suggests that the dominant clusters were not necessarily C_60n (n = 2-4) expected from the simple fusion of C₆₀ molecules. It was proposed that electronic excitations of C₆₀ molecules caused the fragment and coalescence of the molecules to form the giant fullerenes as in the photo-induced similar effects.     

Elastic theory of surface deformation in C60 adsorption

In molecular physisorption, beyond attractive and repulsive forces, a third contribution arises from the elastic deformation of the surface. In this paper the dimple appearing on a gold surface upon physisorption of a C₆₀ molecule is studied and its effect on the adsorption energy is evaluated.The depth and shape of the dimple are studied on the basis of the theory of elasticity. The equilibrium equation is solved in the presence of the body forces originating from a suitable Au-C₆₀ interaction potential and the solution for the deformation field is obtained by means of a two-step procedure. In the first step a curl-free deformation field is obtained, which takes the effects of the body forces into account, but generates mismatch of the boundary conditions. In the second step the solution of the equilibrium problem in the absence of body force is generated, which compensates the previous boundary condition violation. Thanks to linearity, the final solution is obtained as the sum of the results of the two steps. An (approximate) iterative scheme is applied for taking the surface deformation into account in imposing that no forces act on the Au surface.When the distance z₀ of the centre of the C₆₀ cage from the surface is rather large, the dimple is represented by a bulging surface, but, as the C₆₀ approaches the Au surface, the surface recedes and a well formed dimple appears. The corresponding depth at the mechanical equilibrium position (z₀ = 4.8 Å) turns out to be 36.3 pm.     

Gold cluster formation on C60 surfaces observed with scanning tunneling microscopy: Au-cluster beads and self-organized structures

The growth mechanism of Au-clusters on fullerene layers has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The fullerene layers, which serve as substrates, are formed on a graphite surface and exhibit the typical combination of round and fractal shapes, and small sections of the original graphite substrate are exposed. The immobile Au-clusters are concentrated on the C₆₀ terminated surface section, and the original fullerene island structures are preserved. A preferential nucleation of Au-clusters is observed at the C₆₀-graphite edges while the C₆₀-C₆₀ edges remains undecorated. These Au-clusters are placed directly on the edge and shared by the graphite and fullerene layer. They form bead-like structures, which densely populate this edge, while the first layer C₆₀ islands are clearly depleted of Au-clusters. A roughness analysis of the fullerene surface indicates the presence of Au atoms (or very small clusters), which are embedded in the fullerene surface, and likely situated in the troughs in between the large molecules. These Au atoms are highly mobile and cannot be individually resolved at room temperature. The analysis of the spatial and size distributions of Au-clusters provides the basis for the development of a qualitative model, which describes the relevant surface processes in the Au-fullerene system. The simultaneous deposition of Au and fullerene on graphite leads to the formation of highly organized structures, in which Au-clusters are embedded in a ring of fullerene molecules with a constant width of about 4 nm. The mechanism for the formation of these structures is highly speculative at present and further experiments will be pursued in the near future. A comprehensive analysis of the Au-C₆₀ system is presented, which contributes to the advancement in our understanding of the metal-fullerene interaction and furthers the development of composite materials of interest in the synthesis of solar cells and metal contacts to organic materials.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Mid-infrared polarization spectroscopy of C2H2: Non-intrusive spatial-resolved measurements of polyatomic hydrocarbon molecules for combustion diagnostics

Polarization spectroscopy in the mid-infrared (IRPS) has been applied to the detection of acetylene molecules making use of the asymmetric C-H stretching vibration at around 3 μm. The infrared laser pulses were produced through difference frequency generation in a LiNbO₃ crystal pumped by a Nd:YAG and dye laser system. By directly probing the ro-vibrational transitions with IRPS, sensitive detection of molecules with otherwise inaccessible electronic states was realized with high temporal and spatial resolution by using a pulsed laser and a cross-beam geometry. Detection sensitivities of 2 × 10¹³ molecules/cm³ (10 ppm in 70 mbar gas mixture) of C₂H₂ were achieved using the P(1 1) line of the (0 1 0(1 1)⁰)-(0 0 0 0⁰ 0⁰) band. The dependence of the IRPS signal on the pump laser fluence, acetylene mole fraction, and buffer gas pressure of Ar, N₂, H₂, and CO₂ has been studied experimentally. The investigation demonstrates the quantitative nature of IRPS for sensitive detection of polyatomic IR active molecules. In order to fully demonstrate the technique for combustion applications, nascent acetylene molecules were measured in a low pressure methane/oxygen flame.     

Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

Permeability of C60 films deposited on polycarbonatesyloxane to N2, O2, CH4, and He gases

In this study, we report on the gas permeability of non-polymerized and polymerized fullerene films (thickness about 0.5 μm) grown on an organic polymer substrate, polycarbonatesyloxane (PCS), using a high vacuum deposition method. The photopolymerized C₆₀ films were prepared by a simultaneous thin film deposition and UV-vis irradiation method which was reported previously [V.A. Karachevtsev, P.V. Mateichenko, N.Y. Nedbailo, A.V. Peschanskii, A.M. Plokhotnichenko, O.M. Vovk, E.N. Zubarev, A.M. Rao, Carbon 42 (2004) 2091]. Raman spectroscopy revealed that ∼90% of the C₆₀ molecules are covalently linked to neighboring C₆₀ molecules in the photopolymerized film after 20 h of film deposition/irradiation. Permeability of the resulting membranes consisting of polymer PCS base and fullerene films to the N₂, O₂, CH₄, and He gases has been investigated. Our experiments revealed that the gas permeability properties are dependent on the age of the membrane. In particular, the aged membrane exhibited an enhanced permeability for O₂ and He gases in comparison to N₂ and CH₄, respectively.