Fullerene Clusters Research Articles

FORMATION OF FULLERENES AND CARBON CLUSTERS BY LASER IRRADIATION OF POLYMERIC FULLERENE OXIDE (FULLERENE OZOPOLYMER)

ABSTRACT Polymeric fullerene oxide (PFO) prepared by prolonged ozonation of C60 fullerene has been laser irradiated and the resulting products formed have been studied by ion cyclotron resonance mass spectrometry. It has been found that PFO produces a complete set of carbon clusters from C60 up to C164. The mechanism of formation of this set of fullerenic clusters implies necessarily a laser-induced carbonization step of the PFO substrate. Once the PFO target has been changed into the opportune carbon nanostructure by the laser radiation, the sequence of fullerene carbon clusters has been produced.

Formation of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt and Ni)

Photofragmentation of metal fullerides C60Mx (M=Sm, Pt and Ni) has been studied by excimer laser ablation–TOF mass spectrometry. Metallofullerenes of the type CnM (n<60) have been observed in both the positive and negative ionic modes, with C59M being the most prominent species. It is supposed that the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage, forming substitutional metallofullerene. The occurrence of the C59M, C58M, C57M clusters in the mass spectra is confirmed by the coincidence of the intensity distribution of the mass peaks with the isotopic abundance pattern calculated from the natural abundance of isotopes of C and M. Odd-numbered high-carbon clusters are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The evolution of the mass spectra of these samples with laser irradiation shots indicates that the transformation process from an externally doped fullerene to the substitutionally doped fullerene involves the loss of metal carbide, MC. The structures of metallofullerenes C2n+1M and C2nM with even and odd total numbers of atoms respectively are discussed. Formation mechanisms with the participation of odd-numbered all-carbon fullerene clusters as intermediates are supposed.

Structural phase transitions in the process of an A(60) cluster formation.

The explanation of the existence of the two critical parameters corresponding to the formation of an A(60) fullerene cluster in the lattice gas model is given. The presented treatment suggests the possibility of the existence of the two phases of the same symmetry type -- high-density phase and low-density phase. These theoretical results can be applied to the carbon fullerene or nanotube research.

Effect of intracavity light-erosion plasma on the output parameters of a tunable laser

The possibility of using intracavity laser spectroscopy for studying the formation of carbon and metallocarbon fullerene molecular clusters synthesised in light-erosion heterogeneous plasma is analysed. The condensation of the emission spectrum of a tunable laser was used as an indicator of formation of a fullerene mass in the plasma.

Fluctuation of the interaction-induced polarizability anisotropy in fullerene cluster—MD study

We have calculated (MD simulation) the interaction-induced polarizability anisotropy correlation function for fullerene cluster (C 60) 13. The corresponding frequency spectrum (Fourier transform of polarizability anisotropy correlation function) lies in the microvawe region.

On the mechanism of fullerene formation in a carbon plasma

A conception about the mechanism of fullerene formation in carbon plasma is developed based on the influence of electron concentration on the stability of carbon clusters. Thermodynamic estimates for the efficiency of formation of spheroidal, flat clusters and linear carbon chains depending on the charge of the reacting particles are described. Strong influence of cluster charge upon the geometry and stability of flat clusters is shown. The charge variation of flat clusters can promote both their folding to curved structures and their dissociation. It is concluded that the fluctuations of electron concentration in a carbon plasma can result in the accumulation of fullerene clusters and the dissociation of flat clusters.

Excitation spectra of free fullerene clusters

In this work we study the single electron emission from fullerene clusters upon the impact of low-energy electrons. The calculations of the quantum ground states of the fullerene are performed within the Hartree–Fock and the jellium shell model. The interaction between the incoming projectile and knocked out electrons is described on the basis of random phase approximation with exchange, which leads to the concept of screening of the inter-electronic interaction. We compare the results of the calculations with available experimental data for the ionization of C 60 by electron impact and show that neglect of polarization leads to results at variance with experimental finding.

Small-angle neutron-scattering study of anomalous C60 clusterization in toluene

This paper reports on a small-angle neutron-scattering study (in the momentum transfer range q=0.005–0.08 A−1) of the structure of C60 fullerene clusters in a saturated toluene solution at 20°C. Large-scale fractal structures formed by small clusters with radius RC∼30 A are shown to persist in the solution after prolonged storage (for one year) at 20°C. The results are discussed in terms of the well-known fullerene aggregation models.

Correlation effects in the (e, 2e) process on C60

The effect of electronic correlations on the electron-impact induced ionization spectrum of free fullerene clusters is investigated. The unperturbed valence states of the clusters are described within the Hartree-Fock scheme based on a jellium shell model. To account for the many-body response of the cluster to an external perturbation, we utilize the random-phase approximation with exchange in order to describe the interaction of an incoming projectile with the metallic cluster, which leads to nonlocal screening of the interelectronic interaction. Within this model, we evaluate the ionization transition amplitudes numerically and compare the results with available experimental data for a C60 cluster. We point out that the neglect of the collective response of the cluster leads to results that are at variance with experimental findings.

Electron density as the main parameter influencing the formation of fullerenes in a carbon plasma

Thermodynamic estimates are presented for the formation of spheroidal and flat carbon clusters from reactant species of different charges. Charge is shown to strongly influence the geometry and stability of flat clusters. Changes in the charge of flat clusters can promote both their folding to spheroidal structures and their dissociation. It is concluded that the fluctuations of electron concentration in carbon plasmas can result in the accumulation of fullerene clusters and the dissociation of flat clusters.

Unusual electrocatalytic behavior of ferrocene bound fullerene cluster films

Fullerene clusters containing a redox couple (ferrocene) have been assembled as nanostructured films on conducting glass and carbon electrodes using an electrophoretic approach. These films show irreversibility in the reduction waves when the electrochemical scan is limited to cathodic cycles (potential range of 0 to −0.8 V versus SCE). However, the reversibility in the cyclic voltammograms can be restored by extending the scans to the anodic region (+0.75 to −0.8 V versus SCE). The ferrocene couple reoxidizes the stabilized C60 anion–tetrabutylammonium complex in the cluster assembly during the oxidation cycle. We present here a simple approach of incorporating a redox active species for improving the electrochemical activity of fullerene films. The electrochemical measurements that illustrate the electrocatalytic property of C60 cluster films are described.

Production of multiply charged fullerene and carbon cluster beams by a 14.5 GHz ECR ion source

We report our first results on the study of normal and endohedral fullerene plasmas and beams produced by a 14.5 GHz electron cyclotron resonance (ECR) ion source (ECRIS). A mixed plasma of mainly N, N2, C, C2, and C60 was produced in the plasma chamber. Formation of caged nitrogens in the fullerene balls requires a careful, unusual tuning of the 14.5 GHz ECRIS. We deposited macroscopic quantities of the solid solution of N@C60 in C60 within the ECRIS. The N@C60/C60 ratio measured off-line using electron-spin-resonance spectroscopy may exceed typical values using conventional, non-ECR techniques. High intensities of C60Q+ ions (Q=1–5) were recorded in the extracted beam spectra. The highest measured C60+ beam current was 410 nA at 700 V extraction voltage. The method may be applied to produce carbon cluster beams from fullerenes, as well. We produced CnQ+ ions (n=2–15, Q=1,2) with intensities of 10–100 nA.

Formation of various types of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt, Ni, La, Y, and Rh)

A photofragmentation study of metal fullerides C60Mx (M=Sm, Pt, Ni, La, Y, and Rh) by excimer laser ablation-TOF mass spectrometry shows that many kinds of metallofullerenes have been observed in both the positive and negative ionic modes. For C60Mx (M=Sm, Pt, and Ni), the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage forming substitutional metallofullerene. While in the case of metal fullerides C60Mx (M=La, Y), evidence of the encapsulation of La and Y atoms in fullerene cages forming endohedral fullerenes has been observed. Different from the above two cases, the laser ablation of rhodium fulleride C60Rhx(x≈1) produces two sequences of rhodium-containing fullerene clusters C2nRh and C2n+1Rh(2n=50–62). Odd-numbered all-carbon clusters in the fullerene regime are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The structures of metallofullerenes C2n+1M and C2nM with an even and odd number of total atoms, respectively, are discussed. Formation mechanisms with the participation of odd-numbered all-carbon fullerene clusters as intermediates are supposed.

Nanostructured Fullerene Films

Spherical clusters of C60 aggregates, formed in a toluene/acetonitrile mixture, are assembled electrophoretically as nanostructured thin films on conducting glass. AFM (see Figure) shows that the C60 clusters (100–300 nm diameter) are packed as 3D arrays, giving the film a highly nanoporous morphology. With rising deposition voltage, the packing density of the fullerene clusters within the film increases.

Theoretical study of the structural and electronic properties of two-dimensionally polymerized fullerene clusters with 2, 3, 4, and 7C60molecules

The structural and electronic properties of clusters of up to 7 crosslinked ${\mathrm{C}}_{60}$ molecules are calculated at the semiempirical AM1 and PM3 levels within the linear scaling divide and conquer scheme (DCM) of Yang. An uniaxial contraction of $\ensuremath{\sim}1.2%$ along the $\stackrel{\ensuremath{\rightarrow}}{c}$ direction of the monomer ${\mathrm{C}}_{60}$ due to the polymerization is found through our geometry optimization procedure with no symmetry restrictions. The clustering formation of four ${\mathrm{C}}_{60}$ through the suggested [2+2] cycloaddition mechanism is a very favorable event, which is consistent with other theoretical results. HOMO-LUMO gap energies decrease as the cluster size increases, predicting the possible onset of conductor behavior as size increases. Our results are in agreement with the two-dimensional ${\mathrm{C}}_{60}$ polymer model proposed by Oszlanyi and Forro. The usefulness of the DCM to study these materials is shown.

Model of C60 fullerene, (H2O)60 water, and other similar clusters.

An analytical calculation of the partition function for the lattice gas model of a finite fullerene type cluster and of an infinite nanotube type cluster is presented. The method of calculation is based on the Vdovichenko random walk approach. There appear to be two values of critical temperature, and the heat capacity in their vicinities is proportional to (T-T(c))(-1).

Molecular Polarizability of Sc and C (Fullerene and Graphite) Clusters

A method (POLAR) for the calculation of the molecular polarizability <α> is presented. It uses the interacting induced dipoles polarization model. As an example, the method is applied to Scn and Cn (fullerene and one-shell graphite) model clusters. On varying the number of atoms, the clusters show numbers indicative of particularly polarizable structures. The <α> are compared with reference calculations (PAPID). In general, the Scn calculated (POLAR) and Cn computed (POLAR and PAPID) are less polarizable than what is inferred from the bulk. However, the Scn calculated (PAPID) are more polarizable than what is inferred. Moreover, previous theoretical work yielded the same trend for Sin, Gen and GanAsm small clusters. The high polarizability of the Scn clusters (PAPID) is attributed to arise from dangling bonds at the surface of the cluster.

Fullerene growth and the role of nonclassical isomers

We report results of a survey of the energetics and stability of classical and 1-heptagon nonclassical fullerene clusters in the range of sizes ${\mathrm{C}}_{30}$ to ${\mathrm{C}}_{70},$ using both total-energy tight-binding and first-principles density functional theory methods. We conclude from this survey that 1-heptagon isomers of high stability exist at every nuclearity, and that these nonclassical isomers play an important part in the growth mechanism of fullerenes.

Growth and Formation of Fullerene Clusters

Theoretical studies on the stability and electronic structure of small carbon clusters assuming chain, ring, bowl, and fullerene structures have been carried out using a linear combination of atomic orbitals molecular orbital approach within a density functional formalism. Our studies on clusters containing between 12 and 60 atoms indicate three regimes for the growth and formation of carbon clusters. In clusters containing less than 20 atoms, the most stable geometry is the ring arrangements. Between 20 and 28 atoms, clusters with very different geometry have comparable energies. For clusters with larger than 30 atoms, the fullerene structures are the most stable structures. An analysis of the electronic structure shows a distinct correlation between the geometry and the nature of electronic states.

Inelastic Interaction of an Electron with a C60 Cluster

The method of intersecting beams of C60 fullerene clusters and of electrons is used to investigate the production of C 60 + and C 60 − ions and the radiation in the wavelength range from 300 to 800 nm for the electron energy E e ranging from zero to 100 eV. The absolute values of the ionization and electron-attachment cross sections [σ+(E e ) and σ−(E e ), respectively] are determined. A maximum of σ+(E e ) of 0.53 nm2is observed at E e = 52 eV. For a C60 cluster excited by electron impact, “delayed” (∼150 μs) ionization initiated by collision with the surface and ionization due to thermionic emission (its characteristic time at E e = 60 eV is 6 μs) is observed. It is found that, for E e 7.5 eV, C 60 − ions proved to be unstable to electron autodetachment. In the region of intersection of C60 and electron beams, radiation of a quasicontinuous spectrum described by a modified Planck formula for the thermal emission of spherical particles of diameter d ≪ λ is recorded. For E e > 47 eV, the brightness temperature proved to be 3150 ± 50 K. It is found that this radiation is emitted predominantly by hot C 60 +* ions produced as a result of thermionic emission from C 60 * . The rate of radiation loss of energy by a C 60 +* ion and the cross section for the formation of radiating C 60 +* ions are found to be, respectively, 5.5 × 105 eV/s at T = 3150 K and 2 × 10−2 nm2 at E e = 60 eV.