C60 Crystallites Research Articles

Well-ordered molecular heterojunction of epitaxial C60 on single-crystal dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

A well-ordered molecular arrangement is a necessary condition for “band transport” in molecular semiconductor materials, and thus it is important for donor–acceptor molecular junctions for applications in advanced organic optoelectronic devices. In this study, the heteroepitaxial growth of an acceptor material C60 on a single-crystal (001) surface of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), a representative high-mobility donor material, is demonstrated. Surface X-ray diffraction analysis indicated spontaneous alignment of the nearest-neighbor molecular direction of the C60 crystallites uniquely to the a -axis of the DNTT.

Bimodal exciton-plasmon light sources controlled by local charge carrier injection.

Electrical charges can generate photon emission in nanoscale quantum systems by two independent mechanisms. First, radiative recombination of pairs of oppositely charged carriers generates sharp excitonic lines. Second, coupling between currents and collective charge oscillations results in broad plasmonic bands. Both luminescence modes can be simultaneously generated upon charge carrier injection into thin C60 crystallites placed in the plasmonic nanocavity of a scanning tunneling microscope (STM). Using the sharp tip of the STM as a subnanometer-precise local electrode, we show that the two types of electroluminescence are induced by two separate charge transport channels. Holes injected into the valence band promote exciton generation, whereas electrons extracted from the conduction band cause plasmonic luminescence. The different dynamics of the two mechanisms permit controlling their relative contribution in the combined bimodal emission. Exciton recombination prevails for low charge injection rates, whereas plasmon decay outshines for high tunneling currents. The continuous transition between both regimes is described by a rate model characterizing emission dynamics on the nanoscale. Our work provides the basis for developing blended exciton-plasmon light sources with advanced functionalities.

Structural-phase state of chromium-fullerite-chromium films subjected to heat treatment in vacuum

The patterns of change in the structure and the phase composition of chromium-fullerite-chromium films subjected to heat treatment in vacuum at different temperatures (470, 570, and 620 K) are studied via scanning electron and atomic force microscopies, X-ray diffraction, reflection-electron diffraction, and X-ray spectral microanalysis. It is found that sample annealing at 470 and 570 K leads to recrystallization of the fullerite and the emergence and growth of a new CrxC60 phase, accompanied by volumetric changes that generate internal mechanical stresses. During the process of stress relaxation, the film starts to crack, and lamellar crystallites of the fullerite phase start to grow. Film destruction and the growth of C60 crystallites on the surface are not observed in chromium-fullerite-chromium films annealed at 620 K.

Deposition of fullerene C60 on the surface of MCM-41 via the one-step wet impregnation method: Active catalysts for the singlet oxygen mediated photooxidation of alkenes

A series of C60 catalysts supported on MCM-41 was prepared for the singlet oxygen-mediated oxidation of alkenes to the corresponding hydroperoxides (ene or Schenck reaction). The one-step wet impregnation method, followed by heating at 180°C, was used for dispersing effectively 6, 9 and 12wt% C60 onto the MCM-41 surface. N2 adsorption–desorption measurements, FT-IR, XRD, DRS and TGA were used to characterize the prepared photocatalysts. The photocatalytic activity of the C60/MCM-41 series was determined by using the photooxygenation of 2-methyl-2-heptene as a probe reaction. The results of the present study were compared with those obtained in our previous studies where the same photocatalysts had been prepared by using the successive wetness impregnation method.It was found that very effective C60/MCM-41 photocatalysts can be synthesized by applying the one-step wet impregnation method. All three photocatalysts exhibited similar activity in terms of substrate conversion, whereas their intrinsic activity which is expressed by the TON or TOF values, decreased with increasing C60 content. The replacement of the successive incipient wetness impregnation by the simpler wet impregnation method provokes an increase in the photocatalytic activity of the samples with C60 content equal to 9 and 12wt% but it does not affect the photocatalytic activity of the sample with C60 content equal to 6wt%, which exhibits the maximum intrinsic activity. The influence of the deposition method and the C60 loading on the photocatalytic performance of the C60/MCM-41 catalysts was rationalized on the basis of the different size of the supported C60 crystallites.

Ionic liquid-mediated epitaxy of high-quality C60 crystallites in a vacuum

Ionic liquid-mediated vacuum deposition was demonstrated as a novel growth technique for high-quality C60 crystallites. In this process the ionic liquid worked as a solvent, i.e. the gas phase C60 precursors were continuously fed into the ionic liquid on a substrate in a vacuum chamber, from which the nucleation and the subsequent growth of C60 crystallites proceeded. The impact of varying the substrate on the growth behaviour was intensively examined in terms of not only the lattice-mismatch between C60 and the substrates, but also the wetting behavior of the ionic liquid on the substrates. This led to a significant improvement of the C60 crystallinity and epitaxial growth was successfully found on certain substrates. In particular, molecularly smooth and thick C60 hexagonal-shaped crystallites epitaxially grew on a MoS2 substrate, the average size of which reached as large as 10 μm.

Effect of pressure on the formation of superelastic hard particles in a metal-fullerene system and the tribological properties of composite materials reinforced with such particles

Raman spectroscopy, X-ray diffraction, and microhardness and modulus of elasticity measurements are used to study the influence of compacting pressure (5, 8 GPa) on the structure and properties of the phases prepared from fullerene soot extract (mixture of C60 and C70 crystallites) in a mixture with a cobalt powder. Carbon particles synthesized during high-temperature treatment at a pressure of 5 or 8 GPa and reinforcing composite samples have a universal hardness H u (hardness measured from the total (elastic and plastic) strain under loading) of 12 or 25 GPa, respectively. After heating of samples to 900°C, the values of H u of the particles decrease to 9–11 GPa at elastic recovery of the phase more than 85%. The dry friction coefficients of iron- and cobalt-based composite materials in contact with tool steel are 0.08 and 0.04, respectively.

Intermolecular Interactions and Dynamics in Polymer/C60 Blends

Solid-state NMR and X-ray scattering are used to study intermolecular interactions in miscible blends of C60 with polystyrene, poly(9-vinylcarbazole), and phase-separate blends with poly(ethylene oxide). Miscible C60 blends prepared by solution precipitation with polystyrene and poly(9-vinylcarbazole) are purple in color, show intermolecular C60−polymer cross-polarization, and do not show the scattering peaks from C60 crystallites. The C60 dynamics measured using the chemical shift anisotropy filter pulse sequence shows that C60 rotates rapidly in the blend and averages the anisotropic line shape, while blending with C60 has a minor effect on the host polymer dynamics. These results demonstrate that C60 interacts weakly with polymers in miscible blends.

Small-angle X-ray scattering study of the structure of powder fullerene C60 and fullerene soot

Powder samples of fullerene C60 and fullerene soot have been studied by the small-angle X-ray scattering method. The radii of gyration of scattering elements have been determined by constructing small-angle diffraction patterns in Guinier coordinates. The data obtained agree well with the results of wide-angle X-ray scattering study, the available data on the structure of the powder fullerene C60 prepared by the Huffman-Kratschmer technique, and the structure of the C60 molecules. Conglomerates of two C60 molecules, along with crystallites ∼20 nm in size that are distributed in an amorphous matrix, are present in fullerene powders. Fullerene soot contains C60 crystallites 20–25 nm in size and graphite crystallites ∼2–3 nm in size that are distributed in an amorphous matrix.

Oriented growth of oxygen-free C60 crystallites on silicon substrates

This is the first report of preparation of C60 fullerite films on silicon substrates coated by a layer of natural oxide. The crystallites are about 1 µm in size. The films were found to have an enhanced stability to atmospheric oxygen. The films were obtained by a modified method of discrete evaporation in a quasi-closed volume. The principal features of the method are the quasi-equilibrium condensation conditions, a high substrate temperature (up to 300 °C), and alternation of deposition with recrystallization in a single technological cycle. The method is characterized by high film condensation rates (up to 2000 A/min) and an economical expenditure of the starting material. A study has been made of the surface structure and morphology, and of the depth profile of the film optical constants and elemental composition.

Raman studies of nonlinear phenomena in fullerene crystallites

The resonant Raman scattering of C60 crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm−1 is observed. No feature at 1459 cm−1 is observable. With increasing intensities, the 1468 cm−1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the π-electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed.

Direct observation of nucleation, cluster growth, and structure of solvent-free crystalline C60 and C70

C60 and its functionalized derivatives have attracted wide interest due to a great potential for various applications. The C60 molecule has icosahedral symmetry, and nearly spherical shape. Crystalline C60 is a van der Waals bonded molecular crystal with a structure characteristic of cubic close-packed pseudospheres. Its crystal lattice is face-centered-cubic with Fm(-3)m symmetry at ambient temperature and becomes a simple-cubic structure below -18°C with a symmetry of Pa(-3) through a first order transition, C70 is the next-larger fullerene with an elongated “rugby-ball” shape. Its crystal lattice is an fee structure at ambient temperature with many properties similar to those of C60. In this paper we describe our electron microscopy studies of nucleation, cluster growth and structure of pure C60 and C70 crystallites formed on various substrates.

Nucleation, Cluster Growth, and Structure of Crystalline C60 and C70

ABSTRACTThe nucleation and cluster growth of C60 and C70 crystallites on various substrates at ambient temperature have been investigated using electron microscopy. It was found that the initial nucleation is closely associated with surface defects, and the fullerenes are much more strongly bonded to each other than to the substrate. Sublimed C60 or C70 crystallites nucleate at the step edge in the liquid state and are aligned with the step walls and terraces through the process of coalescence. Reflection Electron Microscopy (REM) studies have shown an abnormal profile of C60 grown crystals as a result of the interaction of C60 molecules with the surface strain field during crystal growth. Transmission electron diffraction patterns reveal a twin structure with (110) habit plane for the low temperature ordered phase.