Solid C60 Research Articles

Seebeck coefficient and thermal conductivity in doped C60

Pressed bulk samples of C60 doped with P, Co, Al, and Bi have been investigated for their thermoelectric properties. These samples show extremely low thermal conductivity, typically in the range of 0.1–0.3W∕Km at room temperature. The Seebeck coefficients of Co, Al, and Bi doped C60 solids are in the tens of μV∕K; however, for P doped C60 samples, a very large Seebeck coefficient in the order of 103μV∕K was observed. The value of the Seebeck coefficient seems to depend sensitively on the P concentration and changes sign upon annealing at 100°C. Ab initio density functional theory calculations show that the calculated electronic structures and the activation energies strongly depend on the dopants in C60 solids. The high Seebeck coefficient in studied P doped C60 is due to the system’s unique dopant and concentration.

Polymerization of solid C60 under C 60 + cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C 60 + cluster ions is reported. The Raman spectra of the irradiated C60 films reveal a new peak rising at 1458 cm−1 with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C60 induced by the cluster ion impacts. The aligned C60 polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C60 polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C60 polymeric phase indirectly confirms the dominant role of shock waves in the detected C60 phase transformation.

Infrared spectroscopy on the fullerene C70 under pressure

AbstractWe present pressure‐dependent infrared transmittance measurements on fullerene C70 at room temperature in the pressure range up to 4 GPa. The vibrational modes of C70 exhibit a nearly linear pressure dependence with a sluggish anomaly at around 0.8 GPa. This anomaly could be attributed to the change in the orientational ordering of the C70 molecules. The sluggish character of the transition could be either due to the continuous change of the orientational order or the coexistence of two phases in the solid C70. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanical properties of solid C60 studied with density functional tight binding method augmented by an empirical dispersionterm

The bulk modulus of solid fcc C60 was calculated with a density functional tight binding method, augmented by an empiricalvan der Waals force. The predicted modulus of 9.1 GPa is in good agreementwith the experimental measurements. We found that the geometric structures ofC60 molecules in the solid fcc phase are considerably changed under strong compression, due tovariations in the type of hybridization of carbon atoms. We also observed a reduction ofthe HOMO–LUMO gap under compression which is attributed to the overlap ofπ orbitals fromneighboring C60 molecules. Finally, we showed that the dispersion energy correction in the adoptedscheme plays an essential role for the quantitative description of the weakly boundC60 solid.

The effect of ammonia intercalation on the superconducting properties of the alkali-metal doped fulleride

A theoretical investigation of the electronic properties of the relatively high-Tc alkali-metal doped superconducting fullerides when they are further exposed to ammonia molecules as dopants is presented. Our numerical method relies on a group theoretical technique for the formation of molecular orbitals (MO's) for the solid C60 using 60 radial atomic orbitals (AO's) derived in a previous work. We also take into consideration the fact that the Fermi energies of the fullerides compare favourably with their phonon energies in the choice of a Tc formula. The effect of the ammonia molecules, as intercalants, on the density of states at the Fermi level (N(εf)) and consequently superconductivity are then discussed JONAMP Vol. 11 2007: pp. 455-460

Temporal pulsewidth and the wavelength dependences of the product ions obtained by laser ablation of solid C60

By ablating solid C60 with a laser pulse, we observe various processes such as the prompt- and the delayed-ionization of C60, the fragmentation into molecular ions and the formation of cluster ions. We found these processes show distinct dependences on the temporal pulse width, the power and the wavelength of the ablation laser. From the observations, we could confirm efficient coupling of laser energy to C60 through the molecular absorption even with a laser pulse width less than the electron-phonon coupling time of the C60 molecule.

Orientation ordering and the transition to the orientational glass state in fullerite C60

We propose a model for describing the low-temperature transition to the orientational glass state in solid molecular C60 in the framework of a theory similar to spin glass theory. We find a replica symmetric solution and also break the replica symmetry. The obtained results agree with experimental data on the partial retention of the orientational long-range order in the glass phase and on the presence of a broad maximum on the curve corresponding to the temperature dependence of the orientational part of the heat capacity.

Hydrogen absorption and desorption kinetics in fullerite C60 single crystals. Low-temperature micromechanical and structural characteristics of the interstitial solid solution C60(H2)x

The microhardness HV and lattice parameter a of C60 single crystals are measured at room temperature as functions of the hydrogen saturation time t for several values of the saturation temperature (250, 300, and 350°C) at a fixed hydrogen pressure p=30atm. According to the measurements of HV and a, the kinetics of hydrogen absorption is described by a simple exponential law with a single, temperature-dependent characteristic time. In highly saturated samples the microhardness is 4 times greater than for the initial C60 crystal, while the lattice parameter is 0.2% larger. The temperature dependence of the microhardness HV and lattice parameter a of C60(H2)x crystals is investigated in the temperature interval 77–300K. The introduction of hydrogen lowers the temperature of the fcc–sc phase transition, and the transition becomes strongly broadened in temperature. The dependence of the microhardness of the saturated sample on the hold time in air at room temperature is described by the sum of two exponentials with different characteristic times. Kinetics of this kind is presumably due to two processes: desorption of hydrogen from the sample, which causes a decrease of the microhardness, and a simultaneous penetration of gaseous impurities into the sample from the surrounding air, which is accompanied by hardening. The influence of the H2 molecules on the characteristic of the intermolecular interaction in fullerite C60 is discussed and the intercalation-induced processes of dislocation slip and microfracture.

Direct Ionization Desorption of Fullerene by Intense Femtosecond Laser Fields

We investigated the direct ionization and desorption of solid C60 by the irradiation of femtosecond nonresonant (neutral and cationic states) pulses at 1.4 µm. We concluded that ion desorption is induced by Coulomb explosion. Initially formed highly charged molecular ions were annihilated with neighboring neutral or lower-charged ions, which finally resulted in singly charged C60. The electronically and vibrationally excited hot ions were formed by the release of excess energy after the charge redistribution, electron-hole recombination, and electron impact processes leading to delayed (thermal) ionization. Singly charged ions were finally emitted due to the high density of the surface charge.

Wavelength-dependent fragmentation and clustering observed after femtosecond laser ablation of solid C60

We report here the resonance effect in femtosecond laser ablation of solid C60 by investigating wavelength and fluence dependence of product ion species. When the ablation laser wavelength is far from the molecular absorption band of C60, we observe both C60-2n+ fragment ions and C60+2n+ cluster ions as well as C60+ parent ion. Delayed ionization of C60 is not significant. When the ablation laser wavelength is near resonant with the molecular absorption, we observe C60+ and some amount of C60-2n+ fragment ions depending on the laser fluence. Delayed ionization of C60 is significant in this case, which indicates high internal energy of C60 molecule. From the observations, we confirm the strong coupling of femtosecond laser energy with C60 molecule when the molecular absorption is high at the ablation laser wavelength.

Ozone Reaction with Carbon Nanostructures 2: The Reaction of Ozone with Milled Graphite and Different Carbon Black Grades

Synthetic graphite particles 0.2 microns in diameter react with O3 at room temperature with evolution of CO2. Graphite is less reactive with ozone than carbon blacks having different surface area ranging from 120 to 8 m2/g, but graphite reactivity is comparable to that shown by powdered solid C60 and C70 fullerenes. The kinetic rate constant derived from the ozone consumption k appears in relation with the rate of CO2 evolution suggesting a very simple reaction stoichiometry at the early stages. The reaction between ozone and graphite or carbon blacks essentially involves two stages: the gasification of the surface to CO2 and its functionalization with oxygenated chemical groups, mainly as COOH but also other oxygenated chemical moieties. The pseudofirst order kinetics appears adequate to describe the heterogeneous reaction between ozone and the selected carbon materials. The discussion of the results is focused on the role played by fullerene-like carbon nanostructures, present in graphene sheets to explain the observed gasification rates and surface functionalization.

The Role of Carbon Nanostructures in the Ozonization of Different Carbon Black Grades, Together with Graphite and Rubber Crumb in an IR Gas Cell

Four carbon black grades (namely N234, N375, N660, and N991) having surface area of 120, 90, 36, and 8 m2/g have been studied together with a micronized synthetic graphite and a rubber crumb in their reaction with ozone. The study was conducted at room temperature in heterogeneous and static conditions inside an infrared gas cell. Using gas‐phase infrared spectroscopy it has been followed the evolution of CO2 formed by the O3 attack directed toward the carbon surface and the consumption of ozone admitted into the cell using the ozone absorption band at 1056 cm−1. The pseudofirst kinetic rate constant k derived from the ozone consumption in presence of carbon materials was determined for all the carbon materials studied. The reaction between ozone and carbon blacks essentially involves two stages: the gasification of the surface to CO2 and its functionalization with oxygenated chemical groups, mainly under the form of COOH but also many other groups. The pseudofirst order kinetics appears adequate to describe the heterogeneous reaction between ozone and the selected carbon materials. Graphite reacts with O3 in the conditions selected in this work with evolution of CO2. Graphite is less reactive with ozone than carbon blacks but its reactivity is comparable to that shown by powdered solid C60 and C70 fullerenes. Rubber crumb can be considered a carbon material in fact it reacts with ozone releasing CO2 at least at the early stages of exposure. This fact is really surprising and a mechanism involving free carbon black on the crumb surface has been advocated to justify this result. The discussion of the results has been focused on the role played by carbon nanostructures, in particular by the fullerene‐like structures present in the graphene sheets to explain the phenomena of gasification and surface functionalization.

Ionization and fragmentation of solid C60 by femtosecond laser ablation

Ionization and fragmentation of solid C(60) dispersed on a silicon plate are investigated by femtosecond laser ablation. Bimodal mass distribution with large fragment ions C(60-2n) (+) (0< or =n< or =11) and small fragment ions C(n) (+) (13< or =n< or =28), formation of dimer ion (C(60))(2) (+), and delayed ionization of C(60) have been observed as reported in gas phase experiments with nanosecond laser excitation. Metastable dissociation of small fragment ions C(n) (+) has been observed for the first time, which suggests different structures of fragment ions compared with those of well-studied carbon cluster ions. From these observations, strong coupling of laser energy to electronic degrees of freedom of solid C(60) has been revealed for femtosecond laser ablation as compared with excitation in the gas phase.

Evaluation of the adsorption free energy of light guest molecules in nanoporous host structures

We present a simple method for estimation of the adsorption free energy of lightweight featureless molecules in nanoporous structures. The partition function of the system is computed quantum mechanically within the ideal gas approximation, and leads directly to the free adsorption energy. Storage capacities can be estimated by combining the ideal gas results with the real gas equation of state. Two possible implementations of the approach are discussed. The assumptions and potential sources of errors in the calculations are analyzed. The performance of the method is illustrated for graphitic slit pores and solid C60.

Molecular dynamics calculations of solid C60 under high pressure

Recently, we have proposed the new intermolecular interaction model of C60. Using this new model, we have analyzed the molecular rotational motions of C60 in solid state with the constant-temperature and constant-pressure molecular dynamics method at the temperature regions 180–340 K under the pressure regions 100–500 MPa. By analyzing the density of states of the molecular rotational motions, we found that the orientational phase transitions which involve the slightly volume change of the C60 crystal, are induced at 260 ∼ 280 K under 100 MPa, 280 ∼ 300 K under 200 MPa, 300 ∼ 320 K under 300 MPa and 320 ∼ 340 K under 400 MPa. The temperature/pressure variations of the order parameter also shows that C60 molecules rotate rapidly even below the orientational phase transition temperature.

Synthesis of Superhard and Ultrahard Materials by 3D-polymerization of C60, C70 Fullerenes Under High Pressure (15 GPa) and Temperatures up to 1820 K

Solid fullerenes C60 and C70 have been treated at high pressure of 15 GPa and high temperatures of 520 - 1820 K for a time of exposure of 60 s and a quenching rate of 300 K s−1 using a toroid-type apparatus. X-ray diffraction and Raman spectra confirm the realization of 3D-polymerized phases in these solids at 15 GPa. The pressure/temperature maps of synthesis of metastable carbon phases on the basis of C60 and C70 have thus been extended to 15 GPa. The longitudinal and shear sound wave velocities were measured by acoustic microscopy techniques. A maximum sound wave velocity of (21±1)×105 m s−1 was observed in the sample synthesized from C60 at T = 1170 K. The elastic constants were calculated using experimental data. The acoustic microscopy images of experimental samples have been investigated.

Single and Collective Electron Excitations in the Solid C60F18

The electronic structure of the solid polycrystalline fluorinated fullerene C60F18 has been investigated by reflection electron‐energy‐loss spectroscopy for the first time. The elementary excitation spectrum of the solid C60F18 was compared with the respective spectrum of the solid C60, and an explanation of their significant similarity in the range of π→π* transitions was suggested.

Cryogenic apparatus for diffuse reflection infrared spectroscopy with high-pressure capabilities

A cryogenic apparatus for obtaining diffuse reflection infrared spectra at liquid helium temperature is described. The system combines a commercially available reflection accessory and cold finger cryostat with a custom designed vacuum housing and high-pressure sample cell. This enables spectra to be obtained from room temperature to 10K and at gas loading pressures as high as 100bars. Initial experiments on H2 in solid C60 show the power of the system to obtain spectra that are significantly enhanced relative to those at room temperature.

ON THE USE OF THE WHOLE EIGENVALUE SPECTRUM TO OBTAIN SINGLE MOLECULE BAND STRUCTURES AND SOLID BAND GAPS FOR MOLECULAR ELECTRONICS STUDIES

A Density Functional microscopic model of C60, consisting of seven molecular orbitals (3 occupied and 4 empty), was employed to get the best values for the half-width and the scanning distance of the mathematical function used to obtain the theoretical DOS spectrum. It is found that the convolution of the whole eigenvalue spectrum with a Gaussian function should be made with values of 0.1 eV for both, the broadening parameter and the scanning distance. The theoretical calculation of the VB-CB band gap for the solid from results belonging to isolated C60 is in a relatively good agreement with most results. We were able to simulate the experimental band gap starting from the theoretical DOS spectrum and using some data. Regarding the HOMO-LUMO gap, the isolated molecule results are in very good agreement with results for solid C60. Finally it is suggested that experimentalists select their results based on an a priori knowledge of theoretical results because they intend to calculate some properties needing them

Antibacterial Activity of Fullerene Water Suspensions: Effects of Preparation Method and Particle Size

Fullerene research in biological systems has been hindered by the compound's relative insolubility in water. However, C60 molecules can be made to aggregate, forming stable fullerene water suspensions (FWS) whose properties differ from those of bulk solid C60. There are many different protocols for making FWS. This paper explores four of these methods and establishes the antibacterial activity of each resulting suspension, including a suspension made without intermediary solvents. The aggregates in each polydisperse suspension were separated by size using differential centrifugation and tested for antibacterial activity using Bacillus subtilis as a test organism. All suspensions exhibited relatively strong antibacterial activity. Fractions containing smaller aggregates had greater antibacterial activity, although the increase in toxicity was disproportionately higher than the associated increase in putative surface area. This suggests the need for improved understanding of the behavior of FWS towards organisms and in the environment to determine how C60 can be safely used and disposed.