Fullerene Clusters Research Articles

Ionisation of fullerenes and fullerene clusters using ultrashort laser pulses

We give a brief review of the literature concerning the ultra-short pulse ionisation of fullerenes in the gas phase. Emphasis is placed on the excitation time dependence of different ionisation regimes as manifested by photoelectron spectroscopy. The ionisation rates are modelled for the intermediate situation where the excitation energy is equilibrated between electronic degrees of freedom but not yet coupled to vibrational degrees of freedom. The model is shown to describe many aspects of the experiments. New results are presented on the intra-cluster molecular fusion of fullerene molecules when van der Waals bound clusters of fullerenes are exposed to ultra-short laser pulses. Pump-probe measurements give a decay time constant for the intra-cluster fusion reaction of 520 +/- 55 fs. A comparison with monomer ionisation results suggests that the time window for the fusion reaction is influenced by the coupling of the electronic excitation energy to vibrational degrees of freedom of the molecules in the cluster.

About the formation of C 60 fine particles with reprecipitation method in ethanol/carbon disulfide mixture

We present a short communication to discuss some of the results of two works by Fujitstuka et al. [M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, A. Watanabe, O. Ito, Chem. Lett. (1997) 1211; M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, O. Ito, K. Yase, J. Photochem. Photobiol. A 133 (2000) 45] in which the photochemical and photophysical properties of a dispersion of fullerene fine particles are discussed. One of the simplest way to growth organic fine particles is the reprecipitation method. In this way microcrystals of fullerene molecules can be obtained with a good control on dimension and size distribution of the final aggregates. The existence and the distribution of fullerene clusters in these liquid solutions is usually attained and characterized by dynamic light scattering technique. In this short communication we want to address a common mistake derived from the widely used reprecipitation method. We show that photoncorrelation measurements from [M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, A. Watanabe, O. Ito, Chem. Lett. (1997) 1211; M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, O. Ito, K. Yase, J. Photochem. Photobiol. A 133 (2000) 45] are faulty due to the presence of a microemulsion between the two solvents used. In ethanol/carbon disulfide mixtures, CS 2 droplets dispersed in ethanol mimic in fact the existence of ≈ 300 nm clusters which, actually, do not exist.

Attempts for molecular depth profiling directly on a rat brain tissue section using fullerene and bismuth cluster ion beams

The capabilities of time of flight secondary ion mass spectrometry (TOF-SIMS) have been recently greatly improved with the arrival in this field of polyatomic ion sources. This technique is now able to map at the micron scale intact organic molecules in a range of a thousand Daltons or more, at the surface of tissue samples. Nevertheless, this remains a surface analysis technique, and three-dimensional information on the molecular composition of the sample could not be obtained due to the damage undergone by the organic molecules during their irradiation. The situation changed slightly with the low damage and low penetration depth of the C 60 fullerene ion beams. Recent promising studies have shown the possibility of organic molecular depth profiling using this kind of beams onto model samples. This possibility has been tried out directly onto a rat brain tissue section, which is the most commonly used biological tissue model in TOF-SIMS imaging method developments. The tissue surface has been sputtered with a 10 keV energy fullerene ion beam, and surface analyses were done with a 25 keV Bi 3 + ion beam at regular time intervals. The total depth which was analysed was more than two microns, with total primary ion doses of more than 10 16 ions cm −2. Although not in contradiction with results previously published but with much lower doses, it is found that the molecular damage remains too large, thus making molecular imaging very difficult. In addition, most of the lipids, which are usually the main observable molecules in TOF-SIMS, are concentrated close to the sample surface in the first hundreds of nanometers.

The existence of a plastic phase and a solid–liquid dynamical bistability region in small fullerene cluster(C60)7: molecular dynamics simulation

We have simulated (by the molecular dymanics (MD) method) the dynamics of fullerenes(C60) in an extremely small cluster composed of only as many as sevenC60 molecules. The interaction is taken to be the full 60-site pairwise additive Lennard-Jones(LJ) potential which generates both translational and anisotropic rotational motions ofeach molecule. Our atomically detailed MD simulations discover the plastic phase (notranslations but active reorientations of fullerenes) at low energies (temperatures) of the(C60)7 cluster. We provide the in-depth evidence of the dynamical solid–liquid bistability region inthe investigated cluster. Moreover, we confirm the existence of the liquid phase in(C60)7, the finding of Gallego et al (1999 Phys. Rev. Lett. 83 5258) obtained earlier on the basis ofGirifalco’s model, which assumes single-site only and spherically symmetrical interaction betweenC60 molecules. We have calculated the translational and angular velocity autocorrelationfunctions and estimated the diffusion coefficient of fullerene in the liquid phase.

Formation of fullerene clusters in the system C 60/NMP/water by SANS

Solutions of fullerenes in nitrogen-containing solvents constitute a specific class characterized by the formation of fullerene clusters. In the given work, we report the effect of cluster rearrangement in the system C 60/ N-methylpyrrolidone (NMP) after addition of water (miscible with NMP) as observed by small-angle neutron scattering (SANS). The effect has a critical character and takes place if the water relative content is higher than 40%. Despite a small scattering signal, estimates of the mean scattering length density of the clusters by SANS contrast variation can be done.

Stability and fragmentation of multiply charged clusters of fullerenes

Stability and fragmentation of multiply charged clusters of fullerenes ( C 60 ) n z + are theoretically studied for 2 ⩽ z ⩽ 7. The energy barriers for various fragmentation channels are calculated within the framework of the contact sphere model. For all the cases studied, an emission of a monomer ion C 60 + is found to be energetically the most plausible decay channel for multiply charged clusters. Experimentally measured appearances sizes have been reproduced within a considerable accuracy. Although the liquid drop model essentially explains the appearance sizes, deviations due to geometric shell effects are predicted for highly charged clusters ( z ⩾ 6).

Graphitic onions as reaction cells on the nanoscale

Multishell fullerene clusters (carbon onions) encapsulating iron crystals of a few nanometers in size are subjected to intense electron irradiation at high temperature in an electron microscope. A reaction between graphite and iron, leading to the formation of iron carbide (cementite) in the core of the clusters, is observed in situ. The reaction in these spherical nanoencapsulates is explained by the occurrence of high pressure, mobile carbon interstitial atoms, and the irradiation-induced lowering of the kinetic barrier. Thus, carbon onions can be used as high-pressure cells for inducing a chemical reaction in nanometer-sized particles.

Cluster growth and dissolution of fullerenes in non-polar solvents

The processes of fullerene cluster formation and growth in non-polar solvents are described basing on the numerical solutions of kinetic equations of nucleation theory. The liquid drop model and the limited growth model are analyzed. For supersaturated solutions it is shown, that the model of limited growth is an adequate model and can qualitatively describe the experimental data. The model of fullerene dissolution processes is proposed.

Comment on “Faraday rotation in an electron-positron plasma containing a fraction of ion” [Phys. Plasmas 12, 084507 (2005)

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Synthesis and investigation of iron fullerene clusters

Iron fullerene clusters are prepared by plasma chemical synthesis and investigated using electron magnetic resonance. It is shown that these clusters can be prepared both by plasma chemical synthesis of fullerenes with iron and by mixing of a fullerene solution with a powder of iron nanoparticles coated with carbon shells. A liquid chromatographic technique is proposed for separating iron fullerence clusters.

Molecular dynamics (MD) simulation study of the liquid phase of small fullerene cluster (C 60) 7

We have simulated (MD method) the dynamics of fullerenes (C 60) in extremely small cluster composed of only seven C 60 molecules. The interaction is taken to be the full 60-site pairwise additive Lennard-Jones (LJ) potential, which generates both translational and anisotropic rotational motions of each molecule. Our atomically detailed MD simulations confirm the existence of the liquid phase in (C 60) 7 cluster. For the liquid phase of (C 60) 7, we have calculated the mean square displacement of C 60, the linear velocity autocorrelation functions and the value of the translational diffusion coefficient of fullerene

Aggregation of [70]fullerene in presence of acetonitrile: A chemical kinetic experiment

[70]fullerene solutions in carbon tetrachloride and o-xylene exhibit a noteworthy spectral variation with time when acetonitrile is added. This has been ascribed to self-aggregation of [70]fullerene caused by the repulsion between polar acetonitrile and hydrophobic [70]fullerene, and the aggregation numbers have been determined from a kinetic scheme and also from a scanning electron microscopic study. The numbers thus obtained follow a cuboctahedral stacking pattern proposed recently and also agree with the magic formula n=55+3m (m=1 to 14) proposed by Branz et al. for [60]fullerene clusters [Phys. Rev. B. 66, 094107 (2002)].

Fullerene collisions and clusters of fullerenes

Electron capture processes were studied in glancing collisions between multiply charged C 60 q+ ions and neutral C 60 molecules at low collision energies where nuclear stopping dominates the interaction (typical energy: 10 keV; velocity v = 0.024 a .u . ). In addition, clusters of fullerenes were multiply ionized in collisions with highly charged slow ions and their fragmentation spectra were measured by applying multi-coincidence techniques in a second, separate experiment. Through the first experiment we show that non-fragmenting electron capture collisions do not produce free electrons in C 60 q+ –C 60 collisions ( q = 1–5), i.e., the cross sections for transfer ionization processes are negligible in this charge state regime. This is in contrast to the case of atomic projectile ions where transfer ionization processes, as e.g., C q+ + C 60 → C ( q− r)+ + C 60 r+ → C ( q− r+1)+ + C 60 r+ + e −, are strong for q > 2. These results are rationalized by means of the classical over-the-barrier model for electron transfer between two C 60 molecules, or between an atomic ion and a C 60 molecule, where the molecules are modeled as conducting spheres. Further, the same model may also be used as a basis for understanding the present observations of limitations in the maximum numbers of charges, which might be transferred in non-fragmenting C 60 q+ –C 60 collisions ( q/2 for even q and ( q + 1)/2 for odd q) and in C q+ –C 60 collision (here up to q charges may be transferred). In the same experiment, we have further measured scattering angles, θ, and energy losses, ɛ, for the fullerene projectiles in C 60 q+ –C 60 collisions and we have found low values for both θ and ɛ, which, however, increase with the number of C 2-units lost from the projectile fullerene in electron capture collisions. The critical distances for electron transfer which are deduced from the C 60 q+ –C 60 collision experiment and the Zettergren model are then used to explain the high charge mobility between the individual C 60 molecules in charged (C 60) n van der Waal's clusters of fullerenes, which we observe in the second experiment.

Characterizing the Impact of Preparation Method on Fullerene Cluster Structure and Chemistry

We examined the physical and chemical characteristics of colloidal dispersions of fullerene materials (nC60) produced through several solvent exchange processes and through extended mixing in water only. The nC60 produced via the different methods were unique from each other with respect to size, morphology, charge, and hydrophobicity. The greatest dissimilarities were observed between the nC60 produced by extended mixing in water alone and the nC60 produced by solvent exchange processes. The role of the respective solvents in determining the characteristics of the various nC60 were attributed to differences in the solvent-C60 interactions and the presence of the solvent as a residual in the nC60 structure, indicating the significance of the solvent properties in determining the ultimate characteristics of the colloidal fullerene. Thus, fullerene C60 that may become mobilized through natural processes (agitation in water) may behave in dramatically different ways than those produced through more artificial means. These results highlight the difficulties in generalizing nC60 properties, particularly as they vary in potential toxicity considerations.

Electron correlations in a C20 fullerene cluster

The ground-state properties of C$_{20}$ fullerene clusters are determined in the framework of the Hubbard model by using lattice density-functional theory (LDFT) and scaling approximations to the interaction-energy functional. Results are given for the ground-state energy, kinetic and Coulomb energies, local magnetic moments, and charge-excitation gap, as a function of the Coulomb repulsion $U/t$ and for electron or hole doping $\delta$ close half-band filling ($|\delta| \le 1$). The role of electron correlations is analyzed by comparing the LDFT results with fully unrestricted Hartree-Fock (UHF) calculations which take into account possible noncollinear arrangements of the local spin-polarizations. The consequences of the spin-density-wave symmetry breaking, often found in UHF, and the implications of this study for more complex fullerene structures are discussed.

Enhancement of Light-Energy Conversion Efficiency by Multi-Porphyrin Arrays of Porphyrin-Peptide Oligomers with Fullerene Clusters.

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Numerical study of growth and relaxation of small C 60 nanoclusters

We investigate by means of kinetic Monte Carlo simulations the growth and thermal relaxation of small fullerene nanoclusters modelled by the Pacheco–Prates Ramalho pair potential. The activation barriers for diffusion processes are calculated on the fly by the dimer method. The elementary transitions which are likely to occur around room temperature are figured out. We study island growth on a perfect fullerene cluster and obtain a morphological transition of the island with increasing temperature. At T = 150 K, the islands are small and irregular. Around room temperature, elongated chain islands are obtained while at higher temperature, they are compact with an anti-Mackay stacking. These island morphologies have been shown to influence the character of the growth. Thus, growing fullerene clusters are disordered with rough surface below T = 300 K whereas at T = 450 K the growth occurs facet-by-facet within the growing shell.

Photoinduced Electron‐Transfer Processes in Fullerene‐Based Donor—Acceptor Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Energetic Ion Bombardment of Ag Surfaces by C 60+ and Ga + Projectiles

The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations.

COLLISION INDUCED FRAGMENTATION OF FULLERENE CLUSTERS (C60)n

Clusters of C 60 fullerenes are (multi)-ionised in collisions with O 5+ projectiles at a collision energy of 100 keV. The dominant fragmentation channels are analysed by time-of-flight mass spectrometry for different cluster size distributions. Singly charged [Formula: see text], ions are found to be the dominant fragments, in 25% connected with the loss of one or more C 2-units. This result is explained by the large charge mobility in fullerene clusters. Doubly charged fragments, in form of [Formula: see text] and [Formula: see text], contribute to the observed spectrum with less than 5–10% only and are mainly attributed to the fragmentation of dimers. Singly charged small carbon clusters in the size range n = 7 to 19 are formed with low kinetic energies indicating the importance of thermal dissociation processes. The present experiments confirm earlier conclusions on the charge mobility in fullerene clusters.