C60 Clusters Research Articles

Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20

Fullerenes are graphitic cage structures incorporating exactly twelve pentagons. The smallest possible fullerene is thus C20, which consists solely of pentagons. But the extreme curvature and reactivity of this structure have led to doubts about its existence and stability. Although theoretical calculations have identified, besides this cage, a bowl and a monocyclic ring isomer as low-energy members of the C20 cluster family, only ring isomers of C20 have been observed so far. Here we show that the cage-structured fullerene C20 can be produced from its perhydrogenated form (dodecahedrane C20H20) by replacing the hydrogen atoms with relatively weakly bound bromine atoms, followed by gas-phase debromination. For comparison we have also produced the bowl isomer of C20 using the same procedure. We characterize the generated C20 clusters using mass-selective anion photoelectron spectroscopy; the observed electron affinities and vibrational structures of these two C20 isomers differ significantly from each other, as well as from those of the known monocyclic isomer. We expect that these unique C20 species will serve as a benchmark test for further theoretical studies.

Nanostructure of laser pyrolysis carbon blacks: observation of multiwall fullerenes

A study of the nanostructure of both as-produced and thermally treated laser pyrolysis carbon blacks which comprise unusual types of nanoparticles and fullerenes is reported. It has been shown that those blacks are characterized by two-level nanostructure, i.e. they consist of amorphous nanoparticles 30–40 nm in size and C60 clusters. Heat treatment at 3000°C causes restructurization with appearance of fullerene-like multishell nanoparticles of ∼20 nm size, higher fullerenes and multishell (cage-inside-cage) fullerenes. Transmission electron microscopy observation showed the occurrence of three different cage-inside-cage clusters: two-shell 1.4 nm size (C60@C240), two-shell 2.0 nm size (C240@C560) and three-shell 2.0 nm size (C80@C240@C560).

Coulomb explosion patterns of fast C60 clusters in solids

The molecular dynamics method is used to simulate Coulomb explosion of fast C60 ion clusters in an Al target, taking into account dynamical screening of interionic interactions by the electron gas of the target. It is found that the wake forces in the medium are strong enough, depending on cluster speed, to stabilize the whole cluster against Coulomb explosion, or to compress the trailing part of the cluster, for prolonged times of penetration through the target. This is encouraging news for such cluster-ion beams applications where massive energy depositions in small volumes of targets are desired.

Molecular-dynamics simulations of energetic C60 impacts on (2×1)-(100) silicon

Single impacts of energetic C60 clusters on (2×1)-(100) silicon substrates are studied by molecular-dynamics simulations. The role of impact energies and internal cluster energy are investigated in detail. Six different energy regimes can be identified at the end of the ballistic phase: At thermal energies below 20 eV the fullerene cages undergo elastic deformation, while impinging on the surface, and are mostly chemisorpted on top of the (2×1)-dimer rows. Between 20 and 100 eV the cage structure is preserved after the collision, but the cluster comes to rest within a few monolayers of the silicon surface. At energies of 100–500 eV the cluster partially decomposes and small coherent carbon caps are embedded in the surface. At higher energies up to 1.5 keV complete decomposition of the fullerene cluster occurs and an amorphous zone is formed in the subsurface area. At energies greater than approximately 1.5 keV craters form and above 6 keV sputtering becomes significant. In all cases the substrate temperature is of minor influence on the final result, but the projectile temperature is important for impacts at lower energies (<1.5 keV). For high energy impacts the ballistics resemble that of single atom impacts. Nearly 1:1 stoichiometry is obtained for impact energies around 1 keV. These results reveal an interesting possibility for controlled implantation of C in Si at high local concentrations, which might allow the formation of silicon carbide.

Electron emission devices formed by energetic cluster impacts on TiO2 rutile

Nanotips at the surface of refractory materials were produced by bombarding the surfaces with energetic clusters. The goal is to realize new electron emission devices with low applied voltages. The nanotips were produced on single crystal surfaces of TiO2 rutile by 20–40 MeV C60 cluster beams and were investigated by atomic force microscopy. Protrusions of nanometer sizes (10–25 nm) were observed at the surface. These nanotips result from the damage induced by the high level of electronic energy losses (up to 80 keV/nm), leading to the local amorphization of the oxide that surrounded the particle trajectories. First experiments on electron emission from such bombarded surfaces covered with a 5 nm platinum film were performed. Electron emission was observed above a low threshold field with excellent stability even in vacuum to 10−7 Torr. The electron energy spectrum was also measured and has very specific characteristics.

Electrodeposition of C60Cluster Aggregates on Nanostructured SnO2Films for Enhanced Photocurrent Generation

Clusters of C60 formed in acetonitrile (ACN)/toluene solutions are deposited as thin films on nanostructured SnO2 electrodes under the influence of an electric field. These films exhibit remarkable photoelectrochemical activity under visible light excitation. The photocurrent observed with these novel electrodes (∼0.14 mA/cm2 obtained with visible light excitation, λ > 425 nm, 130 mW/cm2) is 2−3 orders of magnitude larger than previously reported values for a fullerene system. Direct electron transfer between the excited C60 clusters deposited on SnO2 nanocrystallites and the redox couple (I3-/I-) in solution is responsible for the enhanced photocurrent generation.

Fullerene genesis by ion beams III. On the absence of latent tracks in GeV ion irradiated graphite

Irradiation of natural graphite with ∼ 1 GeV lead and uranium ions initiates a phase change to fullerene (C60) along the particle trajectories. High resolution transmission electron microscopy (Dunlop et al. [1]) reveals no tracks in the bulk in either kinematic or diffraction contrast. Small amoeboid-like surface patches are formed however, random in distribution and proportional in number, but not equivalent to the total ion fluence. The evidence suggests that molecules of C60 and other clusters are ejected from the entry and exit surfaces by electronic sputtering and are then deposited. A mechanism referred to as particle activated prompt anneal (or PAPA) is proposed to explain the absence of latent tracks in the bulk. A reverse cooling and annealing wave permits recovery by epitaxy of the disorder and phase changes induced by the primary heat wave. Radiation enhanced recovery processes may also play a role. PAPA can additionally explain the absence of induced fission fragment tracks in graphite, diamond, silicon and germanium.

COULOMB EXPLOSIONS FOR SWIFT Ｃ60 ION-CLUSTERS PENETRATING IN SOLIDSⅡ—THE MOLECULAR DYNAMICS SIMULATIONS

We study the interaction of the swift Ｃ60 ion-clusters with solids. The dynamic interacting forces acting on the individual ions in the cluster are derived based on the linear-response dielectric theory with the PLA dielectric function, and a set of equations describing the motion of the individual ions is obtained. It has been shown that by solving the equations of the motion, the wake effects of the dynamic interactions result in the strong asymmetric structures of the high-velocity C60 clusters, i.e., the leading ions in the cluster explode very fast, while the trailing ions in the cluster almost remain in stable states.

Annealing Temperature Dependence of the Size of C60 Clusters in C60-Doped Silicon Oxide Films

C60-doped silicon oxide thin films were prepared by spin-coating a viscous solution formed upon soaking at 40°C an acidic toluene/ethanol solution of C60, phenyltriethoxysilane, and tetraethoxysilane with a C60–to–Si molar ratio of 2.5 × 10−3. The films were submitted to annealing at 300–500°C in Ar to investigate variation in the size of C60 clusters embedded in the films by photoluminescence spectroscopy. The film before annealing was found to contain the clusters consisting of ca. 60 C60 molecules, suggesting that C60 is present well-dispersed in the film. The molecules in the film aggregated to increase the size with increasing annealing temperature, indicating that the molecules diffuse easily in the film upon heating and therefore the size of the clusters is controllable with the annealing temperature.

Charge-Transfer Complexation of C60 with Diphenyltetrathiafulvalene (DPTTF) Capped Gold Clusters

4,4′-Diphenyltetrathiafulvalene (DPTTF) capped gold nanoparticles of 3- to 5-nm diameter have been prepared and their thermal stability has been investigated by variable temperature FT–IR spectroscopy. The DPTTF molecules at the cluster surface undergo charge-transfer complexation with C60 to form Au/DPTTF.C60. Direct evidence of complexation is observed in the x-ray photoelectron and infrared spectra of the clusters. The orientational ordering transition of C60 in Au/DPTTF.C60 clusters occur at a higher temperature than pure C60 indicating increased resistance to orientational disorder due to charge transfer. Variable-temperature infrared spectroscopic studies yield complementary information. Both FT–IR and mass spectra show that some of the sites on the gold surface are occupied by the phase-transfer reagent used in the cluster preparation.

Copper-oxygen substructures in carbon allotropes (graphite and fullerites)

A comparative study of crystalline graphite and copper-doped fullerite membranes is reported. It is assumed that C60 clusters form complexes with oxygen and copper similar to those known to exist in graphite. Above room temperature, these complexes, first, change the symmetry of the fullerite lattice and, second, are responsible for the nonmonotonic temperature dependence of the electrical resistance.

Transferability of the Tight-binding Total Energy Parametrization Method for Carbon: Application to Electronic Structural Calculations of Solid C60

We have studied the transferability of the tight-binding total energy parametrization (TBTE) method for carbon, which has been presented by the Naval Research Laboratory group through electronic structural calculations of solid C60. The TBTE method fails in describing the weak van der Waals interactions among C60 clusters. It has, however, excellent transferability for evaluating electronic properties;thus it is expected to be a new tool for studying diverse carbon systems.

Formation and Optical Absorption of Clusters of the p-tert-butylcalix[8]arene:C60 Complex

Depending on the initial concentrations of fullerene (C60) and p-tert-butyl calix[8]arene (C[8]A) in toluene, two regimes of their interaction were distinguished. The changes in optical absorption of C60:C[8]A mixed solutions observed over a period of time suggest that the following consecutive processes take place upon interaction between C60 and C[8]A in toluene: (1) molecular inclusion, and (2) growth and sedimentation of the insoluble C60/C[8]A clusters. At increased concentrations of the components, process (2) is suppressed and stable C60/C[8]A colloids can be formed. Spectral features of these colloids in different solvents clearly indicate the difference between self-aggregated C60 and small clusters of the complex. A specific influence of the C[8]A cage on the entrapped C60 molecules is also confirmed by IR spectroscopy of the complex in the solid state.

Non-Linear Transports in Evaporated C60 Cluster Films

We studied nonlinear transport in evaporated C60 films by means of the I-V characteristics at room temperature. SET-like transport behavior was observed in the results of studies of the C60 cluster films at room temperature. It is especially thought to be related to a certain metal insulator transition in a low-dimensional system because of the large difference between the bulk and surface transports of the small clusters of C60.

Raman spectra and calculated vibrational frequencies of size-selected C16, C18, and C20 clusters

The surface plasmon polariton-enhanced Raman spectra of size-selected C16, C18, and C20 clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C20 are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm−1 region for C16, C18, and C20 suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C20. The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions.

Adhesion enhancement of thin gold films on sapphire by electronic processes using atomic and fullerene ions

Sapphire (α-Al2O3) substrates were irradiated with either swift heavy ions (208Pb and 238U) or C60 clusters at various fluences. After thin gold films deposition these samples were submitted to thermal treatments in a temperature ranging from 300°C to 800°C. Scanning electron microscopy observations show that the pre-damage enhances strongly the gold adhesion on sapphire surfaces. The thermal stability of the total wetting is increased of ≈200°C. From the different modifications induced by these pre-irradiations, the initial disorder rate seems to be the main parameter responsible by the observed effect.

Characterisation by various microscopic techniques of the damage created by MeV C60 ions in amorphous Ni3B

In the work presented here, we study the damage resulting from the high rates of energy deposition in electronic processes during the slowing down of energetic projectiles in amorphous Ni3B. More precisely, we study the structural modifications of the targets irradiated at 300 K in a wide range of fluences (109–5×1011 cm−2) with 30 MeV C60 ions. The experimental results mainly concern the surface features and were obtained by Transmission Electron Microscopy (TEM) using in particular phase contrast and topographical contrast imaging. In the case of C60 ions, TEM reveals that surface damage consists of approximatively 20 nm diameter hollows, which are surrounded by protuding rims. This seems to indicate that the anisotropic growth process, which is specific of amorphous structures, is induced by each individual C60 cluster ion. Previous results obtained using GeV heavy monoatomic projectiles showed that the plastic deformation of the targets started only above an incubation fluence, i.e. after a significant spatial overlap of the damaged regions.

Doping and photoelectric properties of C 60 films prepared by ionized cluster beam deposition

films by means of ionized cluster beam (ICB) deposition. X-ray diffraction (XRD) measurement showed the C60 films to be polycrystalline. The films show negative resistance–temperature coefficients, and their room-temperature resistivity is greater than 102 Ω cm. The films were implanted with 80-keV phosphorus, BBr3, Ar, and He ions, under doses ranging up to 1016 cm-2. The resistivity of the implanted films decreases with increasing doses. n-type electrical conduction was observed for phosphorus-implanted C60 films. The interaction of impinging ions with C60 clusters was found to force the C60 molecules to disintegrate and the films to amorphize. p-type conduction was observed for the C60 films doped with aluminum by simultaneously sputtering aluminum during deposition. C60/Si structures show heterojunction characteristics that can be influenced by light illumination. The photoelectric properties of the films were found to be improved by doping with aluminum.

The concept of effective electronic stopping power for modelling the damage cross-section in refractory oxides irradiated by GeV ions or MeV clusters

Single crystals of Al2O3 and LiNbO3 were bombarded at room temperature with either GeV ions (up to 238U) or MeV carbon clusters (up to C60). Depending on the irradiation conditions, the electronic stopping power S e ranged from 3 to 76keV nm−1. Rutherford Backscattering Spectrometry in Channeling geometry (RBS-C) evidenced lattice disorder at the sample surface which was ascribed to the high density of electronic energy deposited by the incident projectile. The damage cross-sections A d varied from 3 to 350 nm2, the highest values being obtained, at a given S e, in LiNbO3 irradiated with C60 clusters. For each target, A d was found to depend on both S e and the projectile velocity. In this paper, we propose a modelling of A d based on the new concept of effective electronic stopping power S*e. In our approach, the target is characterized by two parameters: a diffusion length L and a critical density of energy W c. The calculation of S*e takes into account L, W c and the radial distribution of energy deposited by the projectile. Provided a correct choice of L and W c, this model predicts a linear law between A d and S*e, and consequently unifies the results obtained from both ion and cluster irradiations. This was confirmed by fitting the A d measurements in Al2O3 and LiNbO3. According to our calculations, the different damage sensitivities between these two materials are mainly ascribable to the diffusion length, which is higher in Al2O3 (L = 5.2 nm) than in LiNbO3 (L = 1.5 nm).

Indium-containing heterofullerenes and their η5-π-complexes

The molecular and electronic structure of hypothetical metallofullerenes In5C55 (1a) and In10C60 (2a) were simulated by the MNDO/PM3 method. Formally, heterofullerene1a is obtained from the C60 cluster by replacement of the carbon atoms at α-positions relative to one of the pentagons by In atoms, and cluster2a is obtained from the C70 cluster by replacement of the carbon atoms framing the polar pentagons of this fullerene by In atoms. Along with clusters1a and2a, their η5-π-complexes ln(η5-1a (1b) and ln2(2η5-2a) (2b) with one (1b) and two (2b) exohedral In atoms coordinated to the pentagons (pent *) isolated by In atoms were also studied. The energies of the In—pent * bonds in1b and2b are approximately equal to 104 kcal mol−1.