C60 Surface Research Articles

Effect of target surface on the elastic properties of fast fullerenes

S. Wethekam, M. Busch, ∗ Ch. Linsmeier, and H. Winter Institut fur Physik, Humboldt-Universitat zu Berlin, Newtonstrasse 15, D-12489 Berlin, Germany. Max-Planck-Institut fur Plasmaphysik, EURATOM Association, Boltzmannstr. 2, D-85748 Garching, Germany. C+60 fullerenes with keV energies are scattered at grazing angles of incidence from atomically clean and flat LiF(001), KCl(001), Al(001), Be(0001), Ni(110) surfaces as well as p(2×1) and p(3×1) oxygen superstructures on Ni(110). The elastic properties of C60 are derived from a comparison of experimental data with 3D molecular dynamics simulations for different interaction potentials. In terms of a simple model for the hybridization of C60 with the surface, we find evidence for a close relation between electronic structure of the surface and elasticity of C60.

Negative ions produced in multicharged ions and C60 collisions

We have measured the differential cross-sections in F3+–C60 collisions at low velocity (v=0.18 a.u.). Using coincidence measurements between the ejected electron number and recoil ions, we have estimated the initial charge state of C60 parent ions associated with neutral outgoing projectiles to range from 4+ to 7+. We found that the formation of anions is correlated with the strong excitation of C60 (average excitation energy about 160 eV). The cross-section of the production of the anion has been found to be 25 a.u. This value is in agreement with the scenario of anion formation in two steps. In the first step, F3+ ions are neutralized by multicollision on the C60 surface. In the second step and at the exit part of the collision, the electron attachment process occurs. For the production of a stable anion, the capture of electrons on carbon must occur directly to the ground states of F−, giving many electron vacancies on C60. This process explains the observation of strong electronic and vibronic excitations of C60 leading to electron emission and multifragmentation processes.

Electronic and geometric properties of exohedral potassium-fullerenes

We investigate the structural and electronic properties of KnC60 (n⩽12) clusters in the framework of density-functional theory. The behavior of the potassium atoms over the C60 surface is analyzed. For K12C60, potassium atoms are found to form 4-atom islands on the surface. The charge transfer and the energetics are discussed in relation with available results.

Dependence of microstructures and melt behaviour of polypropylene/fullerene C60 nanocomposites on in situ interfacial reaction

Polypropylene (PP)/fullerene C60 nanocomposites, in which the interfacial reaction between two components was in situ mediated by peroxide, were prepared by a melt mixing method. Structural characterization showed that the interfacial reaction strongly affected the microstructures of PP/C60 nanocomposites at different scales, such as improving the dispersion state of C60 nanoparticles and forming long chain branched structures in which C60 acts as a branching point. This was ascribed to the ability of C60 (as a reactive nanoparticle) for preferentially trapping carbon-centred macroradicals of PP to in situ form long chain branched structures of PP on the surface of C60 during melt mixing. As a result, the shear-responsive sensitivity of the nanocomposite melts in low shear frequency regions was dramatically enhanced compared to parent PP and the nanocomposites without interfacial reaction. The nanocomposites also showed high melt strength. In addition, the transition from the melt state to solid state (crystallization process) occurred at higher temperature after the interfacial reaction between PP and C60 took place.

DFT study on the interaction of Fullerene (C60) with hydroxyl radical (OH)

The interaction of OH radical with C60 has been investigated by means of DFT method in order to elucidate the radical scavenge mechanism of fullerene. The OH radical was examined as an organic radical because the radical has a high reactivity. The DFT calculation showed that the OH radical binds to the carbon atom of C60 and a strong C-O bond is formed. The binding energies were calculated to be 36.4 and 35.2kcal/mol at the B3LYP/6-31G(d) and 6-311G(d,p) levels of theory. The potential energy curve plotted as a function of C-O distance showed that the OH radical approaches to the carbon atom without activation barrier. Also, it was found that structural change from sp2 to sp3-like hybridization occurs easily by the approach of OH. The unpaired electron is distributed widely over the C60 surface in the C60(OH) complex.

ChemInform Abstract: Aza-aziridinofullerene: Interconversion Between Aza-aziridinofullerene and Bisazafulleroid.

An unusual diastereoselective interconversion between 1 and 2 is observed. The strain associated with a tethering moiety on the surface of C60 acts as a driving force for this process.

A molecular dynamics study of the effect of pentacene polymorphs on C60 surface adsorption and diffusional properties and the tendency to form nanowires

Using atomic-scale molecular dynamics and energy minimisation techniques with semi-empirical molecular mechanics 3 potential energy functions, we consider the adsorption of a C60 molecule on pentacene in the known thin film and bulk phases and a series of hypothetical, variably angled, pentacene structures. The thin film phase has a more energetically isotropic surface than the bulk, and exhibits diffusion coefficients that are twice as high, potentially leading to preferable characteristics for ordered film growth. For the variably angled pentacene structures, where the long axis is parallel to the substrate, a relationship was discovered between the angle that the pentacene short axis makes with the surface normal (controlled by the underlying substrate) and the adsorption characteristics of C60. There is a transition of the dominant energy minima from between the pentacene rows at low values of ϕ1 to within the rows at high values of ϕ1, where ϕ1 is the angle the bottom pentacene short axis makes with the surface normal. This shift implies that the likelihood of forming C60 nanowires on pentacene is greater at extreme values of ϕ1 for which there would be a clear preference for C60 to be located between a row or within a row, rather than at intermediate values of ϕ1, where there is no clear preference.

Superstructures and superhydrophobic property in hierarchical organized architectures of fullerenes bearing long alkyl tails

Formation of hierarchically self-organized architectures in organic media and their non-wetting surface features of a series of fullerene-C60 derivatives bearing different numbers of long hydrocarbon chains (1–3) and semiperfluoro-alkyl tails (4) are investigated by means of a variety of techniques, including X-ray diffraction, differential scanning calorimetry, as well as spectroscopic and microscopic methods. All derivatives self-assemble into a bilayer arrangement with their fundamental structural subunit and lamellar distance ranging from 2.88 to 4.85 nm depending on the substituents. The hydrocarbon-C60 derivatives (1–3) provide well-defined three-dimensional microparticles having a nanoflaked outer surface morphology or microparticles composed of many plate-like units in 1,4-dioxane solution, both architectures enhancing the surface water-repellency. The microparticles with a nanoflaked-outer surface obtained from a C60 derivative with semiperfluoro-alkyl chains (4) in a diethoxyethane solution exhibit a surface water-repellency comparable to objects formed from the hydrocarbon-hybrid C60 derivatives. Taking into account the moderate hydrophobic nature of the C60 surface compared to the high hydrophobicity of the hydro- or fluoro-carbons, these results suggest that the C60 moieties are exposed to the outer surface in the supramolecular objects formed in polar solvent conditions and define their non-wetting properties.

Exploring the Surface Reactivity of 3d Metal Endofullerenes: A Density-Functional Theory Study

Changes in the preferential sites of electrophilic, nucleophilic, and radical attacks on the pristine C60 surface with endohedral doping using 3d transition metal atoms were studied via two useful reactivity indices, namely the Fukui functions and the molecular electrostatic potential. Both of these were calculated at the density functional BPW91 level of theory with the DNP basis set. Our results clearly show changes in the preferential reactivity sites on the fullerene surface when it is doped with Mn, Fe, Co, or Ni atoms, whereas there are no significant changes in the preferential reactivity sites on the C60 surface upon endohedral doping with Cu and Zn atoms. Electron affinities (EA), ionization potentials (IP), and HOMO-LUMO gaps (Eg) were also calculated to complete the study of the endofullerene's surface reactivity. These findings provide insight into endofullerene functionalization, an important issue in their application.

Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

A conformationally restricted molecular dyad has been synthesized and subjected to detailed photophysical examination. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C60 residue, and is equipped with hexadecyne units at the boron centre in order to assist solubility. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N-methylpyrrolidine ring at the C60 surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C60 is thermodynamically favourable and could compete with intramolecular energy transfer in the same direction. The driving force for light-induced electron abstraction from Bodipy by the singlet excited state of C60 depends critically on the solvent polarity. Thus, in non-polar solvents, light-induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C60 and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light-induced electron transfer. Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH2Cl2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.

Investigation of C60 Epitaxial Growth Mechanism on GaAs Substrates

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs (111)B, (111)A, and (001) substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by repeating nucleation and step-flow growth mode, as is the case with GaAs and other semiconductor materials. Anomalous oscillations are observed in the initial stage of C60 layer growth on a GaAs (111)B surface with (2×2) structure. These oscillations indicate that the growth of the first C60 layer is completed at the point of approximately 0.5 monolayer coverage by C60 molecules. This phenomenon is explained by a model in which C60 adsorption sites are limited by As trimers adsorbed on the GaAs (111)B surface. A similar relationship between C60 adsorption and surface reconstruction is observed on (001) GaAs substrates, i.e., the first C60 layer on the (2×4) surface is terminated at approximately 0.5 monolayer coverage, while full coverage is needed of the c(4×4) surface. These observed results are strongly supported by the reported results of scanning tunnel microscopy.

Ab-initio molecular dynamical study of a single transition metal atom on fullerene C60: the case of Ta

We report the first-principles Car-Parrinello molecular dynamics study of the behaviour of a single transition metal Ta atom on fullerene C60, at different temperatures, and for both neutral and charged clusters. We seek to characterise the motion of the lone Ta metal atom on the C60 surface, contrasting its behaviour both with that of three Ta atoms, as well as with a single alkali metal atom on the cage surface. Our earlier simulations on C60Ta3 had revealed that the Ta atoms on the surface of the fullerene are affected by a rather high mobility, and that the motion of these atoms is highly correlated due to Ta-atom-Ta-atom attraction. Earlier, experimental studies of a single metal atom (K, Rb) on the surface of a C60 molecule had led to the inference that at room temperature the metal atom skates freely over the surface, the first direct evidence for which was presented by us in earlier first principles molecular dynamical simulations.

Molecular Dynamics Simulation of Fullerene C60 in Ethanol Solution

An ethanol solution containing one or two fullerene C60 molecules was studied via molecular dynamics simulation. We found that the ethanol molecules form several solvation shells around the central fullerene molecule. Radial distribution functions (RDFs) and hydrogen-bond analyses were employed to detect the structure of the ethanol molecules in the solvation shells. The ethanol molecules in the first solvation shell tend to have their nonpolar alkyl groups exposed to the C60 surface while the polar hydroxyl groups point outward to maintain a hydrogen-bond network with a clathrate-like structure. Such orientation of the ethanol molecules in the first solvation shell modulates the orientation of the ethanol molecules in the second solvation shell to have the hydroxyl groups pointing inward. The potential of mean force (PMF) between two C60 molecules in ethanol solution showed that C60 molecules tend to aggregate in the ethanol solution. There is no ethanol molecule in the intersolute area if the distance between the centers of mass of two C60 molecules is shorter than 10.2 Å. The ethanol molecules near the intersolute area tend to have their methyl groups penetrating into the intersolute region if the distance between two C60 molecules is short, although the hydroxyl groups have smaller volume. We analyzed the dynamic properties of the ethanol molecules in different solvation shells and found that the relaxation is much slower than that of water solution of C60 molecules. In addition, the relaxation of the first solvation shell is slower than that in other solvation shells. The lifetime of the hydrogen-bond in the first solvation shell is also longer than that in other solvation shells while the reorientation of the hydrogen-bonded ethanol pair contributes little to break the hydrogen-bonds.

Formation of fullerene clusters in carbon disulfide: Data on small-angle neutron scattering and molecular dynamics

Fullerene solutions in carbon disulfide are studied by small-angle neutron scattering (SANS). In addition to earlier experiments on the given system, the range of measured transmitted impulses is extended and the influence of solution preparation methods on C60 cluster formation in these solutions is studied. It is shown that the formation of large C60 clusters (with a size of about 10 nm) is due to nonequilibrium methods of solution preparation. For nonequilibrium dissolution, there is a 10% excess of the observed fullerene size in the solution over the calculated value. It has been established by simulation of the C60/CS2 interface by molecular dymanics methods that inclusion of how solvent molecules are organized on the C60 surface leads to a decrease in the fullerene size in the solution, observed by using SANS. In this paper, the effect of excess Rg is explained by the presence of small clusters in the solution (approximately 10% of dissolved C60 molecules). It is discovered that there is a time variation in the concentration of the saturated solution. The explanation of this effect using a model of formation and sedimentation of large clusters (with a size of 100 nm or more) is proposed.

Crystal Structures of Copper–Phthalocyanine on C60(111) Surface Grown by Molecular Beam Epitaxy

Crystal orientation and morphologies of copper–phthalocyanine (CuPc) on the C60(111) surface were investigated. CuPc thin films were grown on C60(111) by the molecular beam epitaxy technique. The CuPc molecular column was parallel to the C60(111) surface. CuPc formed two types of structure on C60(111). Structure 1 was shaped similarly to straight fibers and covered the entire surface. Structure 1 lay along three directions equivalent to <112> on C60(111). This indicated the epitaxial growth of CuPc on the C60(111) surfaces. Structure 2 was also fiberlike in shape and dispersed randomly on Structure 1. Both structures were observed on the CuPc/C60(111) surface at growth temperatures of 60–160 °C. The orientation of C60(111) surfaces affected the formation of CuPc crystals even at 60 °C.

How hydrophobic buckminsterfullerene affects surrounding water structure.

The hydrophobic hydration of fullerenes in water is of significant interest as the most common Buckminsterfullerene (C60) is a mesoscale sphere; C60 also has potential in pharmaceutical and nanomaterial applications. We use an all-atom molecular dynamics simulation lasting hundreds of nanoseconds to determine the behavior of a single molecule of C60 in a periodic box of water, and compare this to methane. A C60 molecule does not induce drying at the surface; however, unlike a hard sphere methane, a hard sphere C60 solute does. This is due to a larger number of attractive Lennard-Jones interactions between the carbon atom centers in C60 and the surrounding waters. In these simulations, water is not uniformly arranged but rather adopts a range of orientations in the first hydration shell despite the spherical symmetry of both solutes. There is a clear effect of solute size on the orientation of the first hydration shell waters. There is a large increase in hydrogen-bonding contacts between waters in the C60 first hydration shell. There is also a disruption of hydrogen bonds between waters in the first and second hydration shells. Water molecules in the first hydration shell preferentially create triangular structures that minimize the net water dipole near the surface near both the methane and C60 surface, reducing the total energy of the system. Additionally, in the first and second hydration shells, the water dipoles are ordered to a distance of 8 A from the solute surface. We conclude that, with a diameter of approximately 1 nm, C60 behaves as a large hydrophobic solute.

Tunable Arrays of C60 Molecular Chains

Tunable arrays of C60 linear chains on the α-sexithiophene (6T) bilayer or monolayer surface nanotemplates are formed by careful control of the delicate balance between the intermolecular and the interfacial interactions using different experimental conditions, that is, various C60 and 6T surface coverages and post-annealing temperatures. The figure shows scanning tunneling microscopy images of the arrays.

Solvent-free chemical modification of fullerence C60 by anamino-terminated polyethylene glycol

The solvent-free chemical modification (SFM) of fullerene (C60) by polyethylene glycol with amino terminal (amino-PEG) was studied in order to improve the fullerene dispersability. The effect of the stirring rate and the C60 particle size on the modification yield was investigated in the temperature range 60-120°C for 3-24 h. As a result, 65 % of C60 was modified at 60°C for 24 hr to disperse in ethanol. The effect of C60 particle size on the yield was found to be more significant than that of the stirring rate. Therefore, the SFM is more sensitive to the collision frequency between C60 and amino-PEG compared to the diffusion of C60 in amino-PEG.In order to understand the chemical modification mechanism, simulation was performed based on a shrinking-core model assuming a chemical reaction as a rate limiting step.Using this model it was possible to estimate the number of amino-PEG molecules attached to C60. The results of simulation fit well the gel permeation chromatography (GPC) data collected in the early period of reaction considering that one amino-PEG attached to C60. As for the final period of reaction, the number of attached amino-PEG molecules increases according to the GPC data but the simulation shows opposite tendency.This tendency was explained by decrease in C60 surface activity and by the additional reaction between products in the fluid phase.

X-Ray Photoelectron Spectroscopy and Reflection High EnergyElectron Diffraction of Epitaxial Growth SiC on Si(100) Using C60 and Si

The formation of silicon carbide upon deposition of C60 and Si onSi(100) surface at 850°C is studied via x-ray photoelectronspectroscopy and reflection high energy electron diffraction (RHEED).The C 1s, O 1s and Si 2p core-level spectra and theRHEED patterns indicate the formation of 3C–SiC.

UV laser desorption of nitric oxide from semiconducting C60/Cu(111)

The desorption of NO from a well-characterized, epitaxially grown semiconducting C60 surface is reported. Two different channels are identified in the laser desorption. Both channels yield a comparably high desorption cross section of σ1=7.0×10-17 cm2 and σ2=5.5×10-17 cm2 for the first and second channel, respectively. The laser desorbed NO molecules are detected with rovibrational state selectivity by (1+1) REMPI in the \(A^2\Sigma^+ \to{X}^2 \Pi\,\gamma\)-bands. In the first channel the desorbing molecules are highly excited with an average kinetic energy of 〈Ekin〉=174 meV. The rotational population distribution can be fitted by a rotational temperature of Trot=800 K. A rotational–translational coupling is observed, with velocities ranging from 1000 m/s for low to 1300 m/s for high rotational states. The vibrationally excited population is estimated to be less than 1% of the ground state. The second channel yields less excited molecules and an almost Boltzmann distributed rotational population with a temperature of Trot=280 K. The apparent velocity distribution derived from the pump-probe delay yields molecules much too slow to be explained by even a thermal desorption. This desorption is probably caused by a long-lived electronic excitation in the substrate for which a lifetime of τ≈160 μs is estimated.