Stability Of C60 Research Articles

Fullerene Stability by Geometrical Thermodynamics

AbstractThis work proves that stability of C60 is a geometrical property of the thermodynamics of the system: a significant methodological advance since a detailed treatment of the energetics may be avoidable. This approach may be fruitful, not only for fullerenes but also for general problems of molecular stability and in other applications of conformational chemistry. For the non‐chiral C60, C384, and the weakly‐chiral C28, C76 and C380 (of these, C380 and C384 are classed as “unspirallable”), Schlegel projections are used to show that these fullerenes can all be represented by pairs of spirals counter‐propagating in anti‐parallel (C2) symmetry. For C60, the high symmetry is used to construct an analytical approximation for the spherical double‐spirals, shown mathematically to be Maximum Entropy (MaxEnt) using the formalism of Quantitative Geometrical Thermodynamics (QGT). Therefore C60 is necessarily stable. This MaxEnt stability criterion is general, depending only on the geometry and not the kinematics of the system. The sense and degree of chirality for C76 and C380 is also quantified using a Shannon entropy‐based fragmentation metric.

The dispersion, stability, and resuspension of C60 in environmental water matrices

Environmental waters cover a range of water quality characteristics which could greatly affect the behavior and fate of C60 in the aquatic environment. In this study, the dispersion and stability of C60 in several environmental water matrices during a 70-day extended mixing period were investigated to better understand its environmental behavior and fate in environmental waters. Relatively stable nanoscale aggregates in water (aqu/nC60) could be formed in wastewater influent, while unstable suspensions were obtained in river water, wastewater effluent, seawater, and estuarine water. During the extended mixing under sunlight, oxygen-containing moieties were produced on the surface of the C60 aggregates, independent of the kind of environmental water matrices. Once the mixed system went under quiescent condition, aggregation and sedimentation of aqu/nC60 occurred. However, an extremely short-time disturbance could easily resuspend the C60 aggregates deposited and increase the concentration of aqu/nC60 in the overlying water column. Therefore, the effects of resuspension should be considered when investigating the environmental behavior and fate of C60.

Stability of C60 and N@C60 under thermal and optical exposure

N@C60, a nitrogen atom encapsulated in a fullerene shell, has an electron-nuclear spin system with outstanding coherence properties attractive for quantum computation. The stability of this molecule is known to be limited due to thermal escape of the nitrogen atom from the C60 cage but little is known about the stability towards optical excitation, which is one possible tool for an indirect scheme to manipulate and read out quantum information. Here, we report the results of a systematic study regarding thermal and optical effects on the stability of N@C60. The central result is that stability under intense laser irradiation can be obtained when the sample remains cooled below a certain temperature. Furthermore, the effect of molecules from the atmosphere or matrix (e.g. oxygen, toluene etc.) on N@C60 is discussed with respect to spin read-out experiments and the decomposition of N@C60. As a result, appropriate experimental conditions for optical quantum state read-out of this material are identified.

Study of the fullerene C60 stability in biological substrates using in vitro model system

This paper presents the results of an in vitro study of the unmodified fullerene C60 stability in samples of biological substrates obtained from Wistar rats. The known amounts of fullerene C60 at various concentrations are added to the freshly prepared samples of rat whole blood, homogenates, and liver microsomal fractions; the decrease in its concentration over a day is studied using HPLC. The data testify to the instability of fullerene C60 in biological media in situations reproducing with some degree of approximation the processes of biodegradation of the substance taking place in a living organism (nitrogen-oxygen atmosphere, +37°C, in the dark).

Influence of Electrolytes and Humic Acid on Aggregation Behavior of C60 Nanoparticles in Aquatic System

The aggregation kinetics of C60 nanoparticles have been investigated over a wide range of monovalent and divalent electrolyte concentrations by employing time-resolved dynamic light scattering (DLS). The results showed that the presence of electrolyte made a dramatic decrease in the surface zeta potential and increase in the particle size. The aggregation kinetics of C60 nanoparticles exhibited reaction-limited and diffusion-limited regimes, which was found to be consistent with the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. The critical coagulation concentration (CCC) values of C60 nanoparticles were estimated as 321mM Na+, 295mM K+, 9.6mM Ca2+and 6.7mM Mg2+, which were far higher than the electrolyte concentrations in natural water. The enhanced C60 stability in the presence of humic acid was attributable to steric repulsion. Therefore C60 nanoparticles can be relatively stable in typical aquatic environments.

On the Stability of Fullerene C60 in Aqueous Medium

Fullerene C60 was dissolved in toluene-water mixture under sonochemical activation, and after toluene evaporation a stable aqueous solution remained. The problem of C60 stability in aqueous medium was studied by means of polarization modulation infrared reflection adsorption spectroscopy (PM IRRAS), time-of-flight secondary ion mass spectrometry (TOF SIMS) and atomic force microscopy/scanning tunneling microscopy (AFM/STM) techniques. It was found that C60 clusters are in the aqueous solution stabilized by the presence of thin layer of methylbenzoate produced during sonication.

Stability studies of an electrochemically formed [C60]fullerene-palladium two-component film

The stability of C60 and palladium two-component films, C60/Pd, has been investigated. The effect of different polymerization conditions on the electrochemical stability of the film upon prolonged potential cycling has been studied. Stable voltammetric behavior was observed for polymers formed at potentials less negative than the potential of third C60 reduction step. The incorporation of palladium particles into the structure of C60/Pd polymers increases the polymer stability. The C60/Pd films are doped with supporting electrolyte cations during reduction. The size of these cations is a crucial factor in determining the stability of the film. A strong solvent effect on the potential stability of the film was also observed. The wildest range of stable voltammetric properties was found for acetonitrile and N,N-dimethylformamide. No effect of the temperature on the film stability was observed. The results reported in this work allow for the determination of the optimal conditions for the formation of stable C60/Pd films.

Effect of heating of the electronic subsystem on the thermal stability of the C60 and C20 fullerenes and (C20)2 cluster molecule

The effect of heating of the electronic subsystem on the thermal stability of C60 and C20 fullerenes and a (C20)2 cluster molecule is investigated theoretically. It is demonstrated that the excitation of electrons to upper energy levels in accordance with the Fermi-Dirac distribution function does not lead to a substantial change in the activation energy E a for decay of the C20 fullerene. The stability of the C60 fullerene and the (C20)2 cluster molecule likewise does not change radically. However, the inclusion of corrections associated with the finite sizes of the heat bath leads to the activation energy E a which is in better agreement with the calculated height of the potential barrier preventing the cluster decay.

Equilibrium state of C60, C70, and C72 nanoclusters and local defects of the molecular skeleton

The stability of C60 and C70 fullerenes and C60 and C72 nanotubes devoid of 2–12 atoms of the cluster skeleton was theoretically studied. It was established that Cn molecules with an even number of atoms remain stable, which was confirmed by experimental studies of monomolecular decay of clusters with the number of atoms n≥30. The change in the internuclear distances and in the ionization potential of nanoclusters was determined depending on the number of eliminated atoms. Such defects were shown to decrease the ionization potential of nanoclusters by 0.5–0.8 eV. The electron spectrum was calculated within the Harrison semiempirical tight-binding model in the Goodwin modification. A new parametrization of interatomic matrix elements of the Hamiltonian and atomic terms for carbon nanoclusters was suggested.

Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures

We present 13C NMR spin-lattice relaxation measurements (1/T1) in Na2CsC60 and Rb3C60 from 10 to 700K. The large temperature range of this measurement allow to define unambiguously an increase of 1/T1T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T1T = cst. We attribute this increase to the existence of an additional relaxation channel related to singlet-triplet (ST) excitations of Jahn-Teller distorted C60^{2-} and C60^{4-}. These units are formed within the metal on very short time scales (10^{-14} sec) that do not imply static charge segregation. We show that the amplitude of the ST component depends directly on the density of states, which indicates an interplay between metallic and molecular excitations. Such an interaction is also revealed by the high temperature behavior of Na2CsC60 and CsC60, that we then discuss. A divergence between the behaviors of 1/T1, the NMR shift and the ESR susceptibility is interpreted as the result of a rapid increase of the lifetime of the charge carriers, signaling a tendency to charge localization. In our analysis, the particular stability of C60^{2n-} is then a common feature of all known metallic fullerides and allow to reconcile apparently contradicting properties of these systems.