Solid C60 Research Articles

C n films (n=50, 52, 54, 56, and 58) on graphite: Cage size dependent electronic properties

Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50< or =n<60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.

Model for compression of fullerenes and carbon nanotubes

The new materials of carbon, solid C60 and carbon nanotubes (CNT) [1,4] are important for various technological applications. One of the reasons for their technological applications could be based upon the fact that they have a lot of empty space in their molecules as well as in their unit cells when bulk quantities of these are taken in the form of either C60 solid or nanoropes. Their compressibility studies have revealed extremely interesting results showing tremendous reversible compression which has aroused curiosity and interest. CNT have been found to be compressible reversibly up to about 65% of their original volume when subjected to pressures of about 20 kbar. This enormous volume compression in such materials under moderate pressures is conceived to be of great significance for understanding and interpreting the mechanism based on molecular level understanding related to shock compression in these materials. We undertake a model study accounting for unit cell deformations by applying pressures up to 3 Gpa and develop a pressure compression relation. We use the anharmonicity of the intermolecular potential energy to calculate Gruneisen parameters and the temperature increase due to an adiabatic pressure application. This work uses some components calculated from a rigid molecule model, whereas others are taken from experimental pressure compression data.

NMR evidence for the charge transfer from interstitial NO or O2 to molecule C60 in solid C60

Solid C60 was stored in NO and O2 under high pressure, and the molecule NO and O2 were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Their 13C NMR MAS spectra composed of a primary resonance at 143.7 ppm, accompanied by several minor peaks shifted downfield respectively. The spectra at various temperatures from 210 to 300 K have been measured, and the chemical shifts of the minor peaks referenced to the primary resonance versus inverse temperature show straight lines. But, the lines do not go through origin, and their intercepts are equal to 0.13 ppm or its multiple. Due to a non-zero intercept at high-temperature limit, we proposed that there is a charge transfer from NO and from O2 to C60. The quantity of the charge transferred has been calculated, that is about 0.065 qe- (qe- for the elementary charge on an electron).

Polarized near-edge x-ray-absorption fine structure spectroscopy of C60-functionalized 11-amino-1-undecane thiol self-assembled monolayer: Molecular orientation and Evidence for C60 aggregation

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of pi* resonance associated with C60 pi network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57 degrees with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid C60 and /or 1-ML C60/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2 orbitals. The latter implied hybridization between N Pz of -NH2 group of thiolate SAM and pi levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C60 from Ih to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to pi* graphitic LUMO, signified the formation of aggregated clusters, (C60)n of C60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n approximately 5.

Photoinduced Electron Transfer in Solid C60 Donor/Acceptor Complexes Studied by Light-Induced Electron-Spin Resonance

ABSTRACT Light-induced ESR (LESR) spectroscopy was used to study photoinduced electron transfer (PIET) in the crystals of the C60 donor/acceptor complexes. High-field LESR signals attributed to (g = 1.9984−1.9992 and ΔH = 3−6 G at 100 K) were observed in fullerene complexes with Leuco Crystal Violet (LCV·C60·C6H5Cl, 1), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA·C60, 2), and bianthrone ((BA)2C60, 3), whereas a low-field LESR signal in 2 was attributed to TMPDA•+. Microwave power studies reveal two independent spins in 2 and show high relaxation rates for photoinduced spins on . The LESR signals consist of a major long-lived “persistent” component and a minor “prompt” one, which disappears immediately after light is off. The maximal intensity of the LESR signals was observed for the excitation of 1–3 below 2 eV. This range of photon energy corresponds to direct intermolecular charge transfer from donor to C60 molecules. PIET is temperature activated with Ea = 23, 43 and 49 meV for 1, 2, and 3, respectively. Photoinduced spin susceptibility attains the maximum at 30 K in 1 and decreases with temperature down to 4 K indicating a possible antiferromagnetic interaction of spins, while in 2 it increases with the temperature decrease down to 4 K.

Radiative Recombination of Frenkel Excitons in Solid c60 Under Radiation Damages

The behavior of the emission of the excitons localized at different X-centers at the radiation damages by electron beams with an energy of 1.8 MeV and argon ions in the plasma of glow discharge is studied. The nature of X-centers, which contribute to the emission of excitons near energies of 1.5 and 1.6 eV, is discussed. It is shown that the essential reconstruction of the energy spectrum of excitation is observed with increase in the radiation dose, as a result of the radiation damages of C60 molecules.

On the Effect of the Rotational State of C60 Molecules on Electronic Transport in the C60 Fullerite

A phenomenological scenario is suggested in an attempt to explain controversies in the published data concerning the effect of orientational order on electronic transport in solid C60. The main factor of crucial importance is the dependence of the charge hop probability on the mutual orientation of the molecules. Magnitude and the sign of the conductivity response to the changes in the rotational state of C60 across the orientational phase transition are the results of the interplay between the changes in dynamic and static disorder that accompany the transition and may vary in the samples with different structure perfection.

Feynman–Hibbs correction in classical molecular dynamics simulations of solid C60 under a progressively raised pressure and temperature

Feynman–Hibbs corrections in molecular dynamics computer calculations of solid C60 are implemented within the classical effective pair potential approach. The potential model chosen for the site–site interactions is of the Buckingham exp-6 type (Rigby 1986 The Forces Between Molecules (New York: Oxford University Press)), fitted to the Cheng and Klein or the Kitaïgorodsky potential. Structural and thermodynamic properties of the C60 crystal lattice are investigated over a wide range of pressures and temperatures, and data obtained are compared to experimental results and those obtained from a classical approach (Baameur et al 2000 Phys. Status Solidi 220 821). It will be demonstrated that ‘quantum corrections’ applied in a simple quasi-classical simulations have a great influence upon most calculated properties and increase their accuracy when compared to classical approaches. The observed transition pressure with the distorted lattice and cell contraction compares favourably well with experimental data and equation of state calculations. All measured properties are reported together with the phonon density of states at different pressures.

Probing highly deep levels in solid C70 with isothermal capacitance transience-time spectroscopy

A technique, referred to as the isothermal capacitance transience-time spectroscopy, has been developed to probe the deep levels at insulator/semiconductor interfaces or in semiconductor materials with fairly wide band gaps held at moderately elevated temperatures. The highly deep levels in the solid C70/p-GaAs heterojunction were studied with this technique. Two discrete hole traps, H1 and H2, were found to exist in the solid C70 layer at the positions of Ev+0.856 eV and Ev+1.037 eV, respectively.

Solid C60 doped with C70: Order-to-disorder transition studied with dielectric and 13C nuclear magnetic resonance spectroscopies

C 60 crystals containing C70 at various dopant levels from 0% to 10% have been grown via a method of “solvent dilution.” Their crystallographic structures as revealed by x-ray diffraction exhibit an extra hcp lattice, and this stacking fault character increases with C70 concentration. The same trend applies to the shift of transition temperature in dielectric spectra, and in 13C nuclear magnetic resonance spectra, an additional resonance appeared besides the primary resonance in doped samples. We explain such phenomena as follows. The additional resonance arises from a diamagnetic ring-current effect in C70. C60 and C70 molecules in an hcp phase undergo a cylindrical motion around their threefold axis, which is parallel to the crystallographic sixfold axis, instead of “spherical” motion in fcc C60 crystal. The lower molecular rotations correspond to lower free energy of the high-temperature phase. The free energy barrier between the high- and the low-temperature phases thus decreases, accounting for the suppression of orientational order–disorder transition temperature.

Solid C 60 layer growth on AlN (0001) surface for C 60 FET structure by MBE

One of the most important problems is how to form an excellent interface between the insulator and the C60 layers in the C60 field-effect doping technique. In this paper, we propose a new structure of the C60 field-effect transistor which consists of an epitaxial AlN layer as the insulator and a C60 epitaxial layer grown on an AlN (0001) surface. We have mainly investigated solid C60 growth on the AlN (0001) surface by the MBE technique in the present work. C60 grains with face centred cubic structure have been obtained on a smooth AlN surface. Four types of the C60 grains were obtained, that is, the 0° orientation and its twin grains and the 30° orientation and its twin grains. We have found that the 0° orientation grains only can be obtained by controlling the growth temperature. The results strongly indicate that an excellent interface between the AlN insulator and the C60 layer can be formed for device applications. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamics of diatomic molecules confined in a chemical trap II. Nuclear magnetic resonance experiments on CO in solid C60

A detailed nuclear magnetic resonance (NMR) study of CO isolated in a matrix of C60 is reported. CO in C60 shows both quantum and classical behaviour on the NMR timescale. Below 10 K orientational quantum tunnelling for part of the CO molecules is inferred from an analysis of the line shapes. Above 30 K, however, the system can be treated classically due to the dense rovibronic level structure at energies above 30 cm−1.

Dynamics of diatomic molecules confined in a chemical trap I. Nuclear magnetic resonance experiments on hydrogen in solid C60

A detailed nuclear magnetic resonance analysis of the low-temperature dynamics of hydrogen (H2, D2, and HD) trapped in a solid matrix of C60 is presented. Experimental evidence for hindered rotational and localized three-dimensional translational motion for hydrogen within the ‘box’ formed by the C60 molecules is given.

Orientational correlations in the high-temperature phase of solid C60

The diffuse scattering in the high-temperature phase of solid C60 forms a very complicated pattern. It is analyzed from microscopic interactions within a mean-field theory. Different local orientational configurations of the C60 molecules are found.

Surface-Enhanced Raman Scattering of C60 and C70 in Gold Hydrosols

Employing pyridine molecules as media, hydrophobic C60 and C70 molecules were transported into gold hydrosols and adsorbed on the surface of gold nanoparticles, thus resulting in high-quality surface-enhanced raman scattering (SERS) spectra with enhancement factors of 8 × 104 and 6 × 105, respectively. Comparison of SERS spectra with normal Raman spectra of solid C60 and C70 indicates that these molecules absorb on gold nanoparticles by one of their pentagonal faces.

Intermolecular potential and the equation of state of solid C60

First-principles total-energy calculations are performed for the low-temperature ordered phase of solid C60 in the wide range of lattice parameter. The intermolecular potential obtained from these calculations is successfully modeled in terms of the interaction between carbon atoms on different C60 molecules and the multipole Coulombic interaction between molecules. This model enables us to calculate structural and thermodynamic properties of both the orientationally ordered and disordered phases in a consistent manner. We find that the equation of state at finite temperature obtained for this potential is not in good agreement with experiments and the intermolecular potential is much shallower than that expected from the experimental heat of sublimation. These discrepancies are interpreted as arising from the limited capability of the density-functional calculations to appropriately incorporate the effect of electron correlations at large separation, which is responsible for the long-range behavior of the van der Waals interaction between molecules. In order to circumvent this difficulty we take an empirical approach to estimate the additional van der Waals interaction, which is not taken into account in the current density-functional calculations.

Strong superconductivity with local Jahn-Teller phonons in C60 solids.

We analyze fulleride superconductivity at experimental doping levels, treating the electron-electron and electron-phonon interactions on an equal footing, and demonstrate that the Jahn-Teller phonons create a local (intramolecular) pairing which is surprisingly resistant to the Coulomb repulsion, despite the weakness of retardation in these low-bandwidth systems. The requirement for coherence throughout the solid then yields a very strong doping dependence to T(c), one consistent with experiment and much stronger than expected from standard Eliashberg theory.

Effect of interstitial Ar, Ne, He, and O2 on the glass and phase transitions in solid C60

C 60 pellets charged with He, Ne, Ar, or O2 under high pressure, have been studied with complex impedance and high-resolution C13 nuclear magnetic resonance spectroscopes. Ar and O2 were found to depress both the face-centered-cubic–simple cubic and the glass transitions. Storage at room temperature hardly affected these temperatures shifts in the first case, but the shifts decreased exponentially in the O2 case. The effective barriers for the glass transition are determined as 218±15 and 182±20 meV in Ar0.49C60 and (O2)0.5C60, respectively. Neither He nor Ne had a discernible influence on the temperatures and the barrier energy. These observations may be explained as follows. (1) The intercalation of Ar causes the C60 lattice to expand, and the resulting “free volume” increment suppresses Tg. (2) The reduction in effective barriers (by ∼70 meV) weakens the restorative force of the cage libration mode, which decreases Tc. (3) For (O2)0.5C60, an anharmonic coupling between the cage libration and the tumbling of O2 diatomic molecules further modified the potential surface and reduced the effective barriers, therefore inducing a larger temperature shift. (4) Its anharmonic tumbling motion makes O2 more susceptible to the “paddle wheel” action of rotating C60 so that it diffuses readily at room temperature.

Evidence for a solid phase of dodecahedral C 20

Evidence is presented for the formation of a solid phase based on the smallest fullerene, C20, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10-4 torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C20, C21 and C22 species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C20 cages as building blocks. Unlike solid C60 and fully protonated C20, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C20 dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell.

Structural and electrical properties of an electron-beam-irradiated C60 film

The structural and electrical properties of an electron-beam (EB)-irradiated C60 film have been examined at room temperature, using in situ infrared (IR) spectroscopy and ex situ four-probe measurements. IR results show that the irradiated film is neither graphite nor carbon nanotube-like but a peanut-shaped C60 polymer. Current–voltage curve shows that the polymer exhibits a metallic property with a drastically reduced resistivity of 7 Ω cm in comparison with 108–1014 Ω cm for solid C60. This indicates the possibility of applying C60 molecules in EB nanofabrication processes and large potential for developing carbon-based nanodevices.