Showing papers by "Rodney S. Ruoff published in 1993"
TL;DR: In this article, the room temperature solubility of pure C[sub 60] has been determined in 47 solvents, and the solubilities cover a wide range, from 0.01 mg/mL in methanol to 50 mg/m in 1-chloronaphthalene.
Abstract: The room temperature solubility of pure C[sub 60] has been determined in 47 solvents. The solubilities cover a wide range, from 0.01 mg/mL in methanol to 50 mg/mL in 1-chloronaphthalene. The solubilities in CS[sub 2], toluene, and hexane, three of the commonly employed solvents, are 7.9, 2.8, and 0.04 mg/mL, respectively. An examination of the solubilities of C[sub 60] as a function of the solvent properties such as index of refraction, dielectric constant, molecular size, Hildebrand solubility parameter, and H-bonding strength reaffirms the century-old principle like dissolves like. No single solvent parameter can uniformly predict the solubility of C[sub 60], but a composite picture of solvents with high solubility for C[sub 60] emerges: large index of refraction, dielectric constant around 4, large molecular volume, Hildebrand solubility parameter equal to 10 cal[sup 1/2] cm[sup [minus]3/2], and tendency to act as a moderate strength nucleophile. 18 refs., 4 figs., 1 tab.
1,003 citations
TL;DR: Observation of crystals of pure encapsulated α-LaC2 that were exposed to air for several days before analysis indicates that the LaC2 is protected from degradation by the carbon polyhedral shells of the nanoparticles, a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.
Abstract: Single-domain microcrystals of LaC2 encapsulated within nanoscale polyhedral carbon particles have been synthesized in a carbon arc. Typical particle sizes are on the order of 20 to 40 nanometers. The stoichiometry and phase of the La-containing crystals have been assigned from characteristic lattice spacings observed by high-resolution transmission electron microscopy and energy dispersive spectroscopy (EDS). EDS spectra show that La and C are the only elements present. Characteristic interatomic distances of 3.39 and 2.78 angstroms identify the compound inside the nanoparticle cavities as α-LaC2, the phase of LaC2 that is stable at room temperature. Bulk α-LaC2 is metallic and hydrolytic. Observation of crystals of pure encapsulated α-LaC2 that were exposed to air for several days before analysis indicates that the LaC2 is protected from degradation bythe carbon polyhedral shells of the nanoparticles. A high percentage of the carbon nanoparticles have encapsulated LaC2 single crystals. These carbon-coated metal crystals form a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.
609 citations
TL;DR: In this paper, the van der Waals forces between adjacent carbon nanotubes can deform them substantially, destroying this cylindrical symmetry, and the authors predict that these effects should be observable at least for tubes as small as 20 A in diameter.
Abstract: THE discovery of carbon nanotubes1,2 has stimulated many theoretical studies of their physical properties3–12, and their bulk synthesis13 should soon make possible experimental measurements of these properties14. All studies so far have assumed that the nanotubes have perfect cylindrical symmetry. Here we show that van der Waals forces between adjacent nanotubes can deform them substantially, destroying this cylindrical symmetry. We present transmission electron microscopy images of two adjacent aligned tubes, about 100 A in diameter, which show flattening of the tubes along the contact region. Calculations on two-layer nested nanotubes indicate that these deformations can be explained on the basis of van der Waals interactions. We predict that these effects should be observable at least for tubes as small as 20 A in diameter, and suggest that they may have a significant influence on the tubes' physical properties.
447 citations
TL;DR: In this paper, the authors investigated the temperature-dependent solubility of C60 in hexane, toluene and CS and concluded that dissolution is endothermic below room temperature and exothermic above.
Abstract: SOLUBILITIES of solids in liquids exhibit a temperature dependence that is correlated with the energetics of dissolution. For organic solutes in organic solvents the common experience is that solubility increases on warming (that is, dissolution is normally endothermic), forming the basis for purification by crystallization by slow cooling of hot solutions1. Interactions between inorganic compounds and water are often more energetic, and may lead to complicated temperature-dependent solubilities, often associated with the formation of hydrated solid phases2. We have investigated the temperature-dependent solubility of C60 in hexane, toluene and CS2. We observe a solubility maximum near room temperature (around 280 K) for all three solvents. Although the solubility of C60 in these three solvents differs by several orders of magnitude, the temperature dependence of the relative solubilities is much the same in each case. We conclude that dissolution is endothermic below room temperature and exothermic above. We interpret this change as being due to a phase change in solid C60 presumably the phase change observed previously in the absence of a solvent3–5, modified by solvent wetting. A solubility maximum (or minimum) for organic compounds in non-electrolytes is highly unusual, and may be unprecedented. The effect may have consequences for solvent-extraction techniques of fullerene purification.
173 citations
TL;DR: In this article, the mass spectra of the metallofullerenes found in arc-produced primary soots have been studied for a large variety of alkaline earth and lanthanide elements.
Abstract: Laser desorption (LD) and thermal desorption (TD) mass spectra of the metallofullerenes found in arc-produced primary soots have been studied for a large variety of alkaline earth and lanthanide elements. The metallofullerene ratios found in the LD spectra indicate that two distinct groups are observed: Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Ho, Er, and Lu (group A) and Ca, Sr, Sm, Eu, and Yb (group B). The TD spectra of most of these same soots also separate into two groups that contain the same elements as groups A and B. Group A metallofullerenes show strong signals in both LD and TD spectra. Group B metallofullerenes are distinguished by their presence in the LD spectra but absence in the TD spectra. From the general ionic behavior of the elements of these groups, and recent studies of the endohedral oxidation states, we propose that the oxidation states are +3 for group A and +2 for group B. C[sub 70] metallofullerenes are anomalous in that they are absent in TD spectra for all group A and B elements, even at T = 750[degrees]C, but present in LD spectra. 31 refs., 4 figs., 2 tabs.
111 citations
90 citations
TL;DR: In this paper, the potential energy function of a spherical fullerene, C N, near a graphite surface was predicted based on summing all the interactions between the C atoms in the fullerenes and those on graphite sheet.
Abstract: We develop a model that predicts the potential energy function W(R) of a spherical fullerene, C N , near a graphite surface. R is the distance between the fullerene center of mass and the surface. The model is based on summing all the interactions between the C atoms in the fullerene and those on a graphite sheet. The atom-atom potential employed is a Lennard-Jones 12-6 potential with parameters chosen from previous treatments of graphite and of C 60 (s)
77 citations
TL;DR: In this paper, the formation energy of M@C60 endohedral complexes was analyzed using Born-Haber cycle and the stability of these complexes was calculated using a Born cycle that was originally applied to determine the formation enthalpy of solid C60 intercalation compounds.
58 citations
TL;DR: In this article, the authors performed thermal conductivity measurements on C60 and C70 crystals grown by sublimation and found that C60 is nearly temperature independent down to 260K and shows a sharp orientational phase transition at 260K, indicated by a 25% jump in k.
Abstract: We have performed thermal conductivity measurements on C60 and C70 crystals grown by sublimation. For single crystal C60, the thermal conductivity k is ∼0.4 W/m K at room temperature and is nearly temperature independent down to 260K. We observed a sharp orientational phase transition at 260K, indicated by a 25% jump in k. Below 90K, k is time dependent, which manifests itself as a shoulder-like structure at ∼85K. The temperature and time dependence of k below 260K can be described by a simple model which accounts for the thermally activated hopping of C60 molecules between two nearly degenerate orientations, separated by an energy barrier of ∼240 meV. It is found that solvents have a strong influence on the physical properties of C70 crystals. For solvent-free C70 crystal, k is about constant above 300K. There is a broad first-order phase transition in k at 300K with a 25% jump. We associate this transition with the aligning of the fivefold axes of the C70 molecules along the c-axis of the hexagonal lattice. Upon further cooling, k increases and is time independent.
52 citations
IBM1
TL;DR: In this paper, both electron diffraction and high-resolution microscopy have been used to assess the degree of crystallinity, the orientational ordering and the nature of defects present in these face-centered-cubic films.
Abstract: Thin films (25-2500 A) of C60 molecules have been deposited on both (001) NaCl and mica substrates at varying temperatures by resistive evaporation. Both electron diffraction and high resolution microscopy have been used to assess the degree of crystallinity, the orientational ordering and the nature of the defects present in these face-centered-cubic films. For NaCl, optimum conditions yielded polycrystalline films with a tendency towards a 〈110〉 orientation, while for mica, extended single crystal films have been fabricated which exhibit a 〈111〉 direction normal to the film surface.
48 citations
TL;DR: Fullerenes were extracted from graphite soot produced by the Huffman-Kratschmer process by using a number of solvents, including refluxing in xylenes, 1,3,5-trimethylbenzene, α,α,α-trichlorotoluene, and 1-methylnaphthalene as discussed by the authors.
Abstract: Fullerenes were extracted from graphite soot produced by the Huffman-Kratschmer process by using a number of solvents, including refluxing in xylenes, 1,3,5-trimethylbenzene, 1,2,4-trichlorobenzene, α,α,α-trichlorotoluene, and 1-methylnaphthalene, and by using high-pressure, high-temperature o-xylene and supercritical carbon tetrachloride. The extract was dried and analyzed using laser desorption/Fourier transform mass spectrometry in order to measure fullerene molecular weight distributions.
TL;DR: The coupling, bond-cleavage, and dehydrogenation reactions clearly demonstrate the ability of fullerenes to shuttle H atoms in alkylaromatics and thereby cause both bondcleavage and coupling reactions as discussed by the authors.
Abstract: The coupling, bond-cleavage, and dehydrogenation reactions clearly demonstrate the ability of fullerenes to shuttle H atoms in alkylaromatics and thereby cause both bond-cleavage and coupling reactions. Together, these reactions suggest the possible use of fullerenes in hydrocracking and re-forming operations
TL;DR: The results indicate that alkali and alkaline-earth intercalated fullerite compounds are most stable.
Abstract: We construct a thermodynamic Born-Haber cycle to predict formation energies of donor- and acceptor-based C 60 fullerite intercalation compounds with different stoichiometries. Energies associated with the individual steps in the cycle contain important related information such as the lattice constant, bulk modulus, and phonon spectra of the compounds. Our results indicate that alkali and alkaline-earth intercalated fullerite compounds are most stable
TL;DR: In this article, the stability of MxC60 solids and M@C60 molecules using Born-Haber cycle was analyzed and the stability trends across the periodic table was elucidated.
TL;DR: In this paper, the stabilities of lanthanide and actinide intercalated fullerites are calculated with a Born-Hess thermodynamic cycle, and the equilibrium structure and stoichiometry, as well as intercalant valency, are predicted.
TL;DR: In this article, a linear regression fit of the enthalpies of vaporization of polycyclic aromatic hydrocarbons as a function of 1 χ v was used to predict enthalpy of sublimation of each C 2 n solid.
TL;DR: In this paper, the mass spectra of the metallofullerenes found in arc-produced primary soots have been studied for a large variety of alkaline earth and lanthanide elements.
Abstract: Laser desorption (LD) and thermal desorption (TD) mass spectra of the metallofullerenes found in arc-produced primary soots have been studied for a large variety of alkaline earth and lanthanide elements. The metallofullerene ratios found in the LD spectra indicate that two distinct groups are observed: Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Ho, Er, and Lu (group A) and Ca, Sr, Sm, Eu, and Yb (group B). The TD spectra of most of these same soots also separate into two groups that contain the same elements as groups A and B. Group A metallofullerenes show strong signals in both LD and TD spectra. Group B metallofullerenes are distinguished by their presence in the LD spectra but absence in the TD spectra. From the general ionic behavior of the elements of these groups, and recent studies of the endohedral oxidation states, we propose that the oxidation states are +3 for group A and +2 for group B. C[sub 70] metallofullerenes are anomalous in that they are absent in TD spectra for all group A and B elements, even at T = 750[degrees]C, but present in LD spectra. 31 refs., 4 figs., 2 tabs.