The formation of dimerized molecules of C60 and their solids

The formation of dimerized molecules of C₆₀ was studied using the Brenner potential. Several structures are obtained which have been classified into three categories, viz., dumbbells, and fused and coalesced dimers, similar to those obtained earlier, using the Tersoff potential. However, there are differences in the binding energies obtained using these two potentials. From these formations, we chose four forms of the composite dimer molecule as cyclo dumbbell, peanut, capped armchair (5,5) and zigzag nanotubes (10,0) to form crystalline solids. Calculations have been performed by placing them in various crystal structures, i.e. monoclinic, hexagonal and cubic close packed. To obtain stable dimerized crystal structures, the dimer molecules are considered to be rigid, interacting via atom-atom interaction of 6-exponential form. The monoclinic phase has been found to be energetically most stable for each of the dimers. Various structural, thermodynamic and phonon related properties of the stable dimer phases were investigated.     

Stable molecular orientations of a C60 dimer in a photoinduced dimer row

We present a theoretical study on stable orientations of photoinduced C₆₀ dimers in a C₆₀ monolayer film, on the basis of first-principles calculations within the framework of the density functional theory. Using Tersoff and Hamann’s tunnel current theory, current images of scanning tunneling microscopy (STM) for a C₆₀ dimer in several different orientations are reproduced from the spatial distribution of the wave functions of an isolated C₆₀ dimer. Comparing the resultant current images with experimentally obtained STM images, in consequence, two stable orientations of C₆₀ dimers are determined: one is that a C-C double bond shared by two adjacent hexagons faces to vacuum, and the other is that a C-C single bond shared by a hexagon and an adjacent pentagon faces to vacuum. These orientations are supported by empirical total-energy calculations.     

New solid solvates of C60 and C70 fullerenes: The relationship between structures and lattice energies

The goal of this study was to establish the relationship between the X-ray and the thermodynamic data on fullerene’s solvated crystals. X-ray diffraction study of 12 solid solvates of C₆₀ and C₇₀ with different aromatic solvents have been performed. It has been demonstrated that the solid solvates under consideration were typical van-der-Waals complexes with the negative excess volumes, packing coefficients from 0.72 to 0.78, stable due to the formation of the fullerene to solvent bonds, reasonably described by six to 12 Lennard-Jones potentials. The atom-atom potential method has been used to describe both the crystal structures and the thermodynamics of the solid solvates. The minimization of the lattice energy with respect to the cell and rigid body parameters (T=0 K) has led to the crystal structures very close to the experimental ones. The minimum energies found have reasonably reproduced the calorimetric lattice energies of the solvates. The theory has also demonstrated its ability to account for the trends in thermodynamic stability of solid solvates, e.g., has predicted correctly the low stability of the hypothetical monoclinic solvate C₆₀·2C₆H₁₄.     

Effect of heat-treatment and hydrostatic loading upon the poro-elastic properties of a mortar

This contribution aims at identifying experimentally the poro-elastic properties of a cement-based material under different levels of confining pressure, and after a heat-treatment up to 400 °C. The model material used is a normalized mortar, with a (W/C) ratio of 0.5. After a given heating/cooling cycle, drained bulk modulus K_b, solid matrix bulk modulus K_s and Biot's coefficient b are measured at different confining pressure levels (with a maximum of 25 MPa).Results show that under drained conditions, mortar stress-strain relationship evolves with increasing heat-treatment temperature from linear elastic with brittle failure (up to 105 °C heat treatment) to plastic and ductile (from 200 °C and above). Plastification testifies of material degradation under gradual confining pressure. At the microstructure scale, this is attributed to thermal damage after heat treatment above 105 °C, which consists mainly in various micro-cracking. This leads to easier failure of solid skeleton bridges (or trabecules), and to pore network collapse. Concomitantly to this, at given confining pressure P_c, secant drained bulk modulus K_b decreases monotonously, for heat-treatment temperatures above 105 °C. On the opposite, at given heat-treatment temperature above 105 °C, secant drained bulk modulus K_b increases when confining pressure is increased. This testifies of a solid matrix rigidification in the elastic domain, and it is attributed to increased skeleton compactness linked with pore network collapse. This is directly attributable to heat treatment followed by confinement.At given confining pressure P_c, matrix bulk modulus K_s and Biot's coefficient b increase with heat-treatment temperature above 105 °C. The increase in b means that mortar becomes less and less cohesive and more and more of a granular nature. Moreover, Biot's coefficient b and solid matrix bulk modulus K_s are independent of confining pressure P_c for intact mortar, whereas they decrease for heat-treated mortars when P_c increases. From literature analysis alone, it was quite unexpected that after heat treatment, K_s should vary under confinement. This is interpreted as the closure, under confining pressure, of micro-void connections and of micro-cracks created by heat-treatment. Therefore, increasing confinement induces more and more occluded pores in the solid matrix, whereby K_s diminishes.     

New superhard carbon allotropes based on C20 fullerene

Three new carbon allotropes have been uncovered by compressing hexagonal C₂₀ solid based on density functional tight binding level simulations. They are identified to be energetically more favorable and stable than C₂₀ molecule, with the phase III structure being the energetically most favorable. The volume compression calculations suggest that the phase III has a very high anti-compressibility with a bulk modulus of 427 GPa. All of the predicted structures are semiconductors with varied bandgaps and the phase III has a direct band gap of 5.1 eV. Our results indicate that solid C₂₀ can be converted to a transparent superhard material under cold compression.     

First-principles study of structural,mechanical, and thermodynamic properties of cubic Y2O3 under high pressure

The structural, mechanical, and thermodynamic properties of cubic Y₂O₃ crystals at different hydrostatic pressures and temperatures are systematically investigated based on density functional theory within the generalized gradient approximation. The calculated ground state properties, such as equilibrium lattice parameter a₀, the bulk modulus B₀, and its pressure derivative B₀′ are in favorable agreement with the experimental and available theoretical values. The pressure dependence of a/a₀ and V/V₀ are also investigated. Furthermore, the elastic constants C_ij, bulk modulus B, shear modulus G, Young's modulus E, the ductile or brittle (B/G), Vickers hardness H_v, isotropic wave velocities and sound velocities are calculated in detail in a pressure range from 0 to 14 GPa. It was found that the Debye temperature decreases monotonically with an increase in pressure, the calculated elastic anisotropic factors indicate that Y₂O₃ has low anisotropy at zero pressure, and that its elastic anisotropy increases as the pressure increases. Finally, the thermodynamic properties of Y₂O₃, such as the dependence of the heat capacities C_V and C_P, the thermal expansion coefficient α, the isothermal bulk modulus, and the Grüneisen parameter γ on temperature and pressure, are discussed from 0 to 2000 K and from 0 to 14 GPa, respectively, applying the non-empirical Debye model in the quasi-harmonic approximation.     

Fluorination of pressure-polymerized C60 phases

The reactivity of O-, T- and R-phases of the high pressure-high temperature (HPHT) polymerized C₆₀ towards gaseous fluorine in the temperature range of 50-250 °C was investigated. The reaction products were characterized by FTIR, powder X-ray diffraction, SEM, EDX, and VTP-EIMS to determine the bulk stoichiometries, bonding patterns, phase compositions, crystalline structures and thermal decomposition behavior of the fluorinated polymers. At 1 h isothermal treatment duration, fluorinated products with various bulk stoichiometries were obtained from different polymer phases with the R-phase showing the highest fluorine uptake. At 250 °C, all C₆₀ polymers showed partial decomposition to unfluorinated C₆₀ monomer under fluorine atmosphere. At 200 °C, the fluorination of R-phase yielded a pure fluoropolymer most likely having a {C₆₀F_x}_n (x = 36-44) composition. The same fluoropolymer was presumably obtained from O- and T-phases in lower yields. The linear chain structure was suggested for this new fluorocarbon polymer in agreement with the molecular mechanics modeling calculations.     

Electronic structure of C60 semiconductors under controlled doping with B,N, and Co atoms

We present our recent studies of ab initio density functional theory (DFT) calculations of the electronic structures of several selected n- and p-type doped C₆₀ semiconductors. A super-cell approach was used. We performed a series of ab initio density functional computations to systematically study the changes of the electronic structure of C₆₀ semiconductors doped with boron, nitrogen and cobalt atoms. We found that boron and cobalt doped, face-centered cubic (FCC) C₆₀ solids have the electronic structures of n-type semiconductors. Nitrogen doped FCC C₆₀ solid has an electronic structure similar to those of a p-type semiconductor, with shallow impurity energy levels near the top of the valence bands of the host material.     

Formation of palladium fullerides and their thermal decomposition into palladium nanoparticles

Formation of palladium fullerides from various Pd:C₆₀ compositions was studied by XRD, Raman spectroscopy and TEM. Raman spectra of Pd_xC₆₀ samples show close similarity to spectra of C₆₀ polymers obtained under high pressure/high temperature conditions and suggest the formation of chain-like and two-dimensional polymeric structures in Pd_xC₆₀ of various compositions. Thermal decomposition of Pd_xC₆₀ results in the formation of fine 5-10 nm size nanoparticles of palladium which can be used for catalytic applications.     

Phase transitions in single crystal tubes formed from C60 molecules under high pressure

Pure single crystal tubes formed from C₆₀ molecules, with a face-centered cubic (fcc) structure were fabricated by a liquid–liquid interfacial precipitation method using C₆₀ powder. A bulk transition from fcc to a simple cubic structure and a surface transition from (1 × 1) to (2 × 2) have been observed around 246 K (bulk transition temperature T_B) and 214 K (surface transition temperature T_S), respectively, during the measurement of the temperature dependence of electrical resistance. The initiation of the two transitions under pressure was investigated using a piston cylinder high pressure apparatus and it was found that both T_B and T_S increase with increasing pressure. And the C₆₀ molecules at the surface of the tube exhibit the same behavior of that in the bulk at a pressure of about 2.1 GPa.     

Mapping distributions of mechanical properties and formation ability on the ternary B―C―N phase diagram

Based on first-principles calculations, we present the distributions of mechanical properties and formation ability of amorphous B_xC_yN_z solids on the ternary B-C-N phase diagram. Along the C-BN isoelectronic line, the formation energy shows symmetric distributions; the Young's modulus and ratio of bulk modulus and shear modulus (B/G) show zonal distributions. Amazingly, for some peculiar compositions (B: 13-17 at.%; C: 48-52 at.%; N: 33-35 at.%), B-C-N solids exhibit certain ductile characteristic that is comparable to metals. On the phase area (B: 15-30 at.%; C: 50-60 at.%; N: 20-30 at.%), B-C-N solids possess both excellent hardness and good formation ability. These theoretical results provide valuable guidance for intentionally synthesizing B_xC_yN_z materials with desirable mechanical properties.     

Molecular memory with atomically smooth graphene contacts

We report the use of bilayer graphene as an atomically smooth contact for nanoscale devices. A two-terminal bucky-ball (C₆₀) based molecular memory is fabricated with bilayer graphene as a contact on the polycrystalline nickel electrode. Graphene provides an atomically smooth covering over an otherwise rough metal surface. The use of graphene additionally prohibits the electromigration of nickel into the C₆₀ layer. The devices exhibit a low-resistance state in the first sweep cycle and irreversibly switch to a high-resistance state at 0.8 to 1.2 V bias. In the subsequent cycles, the devices retain the high-resistance state, thus making it write-once read-many memory.     

Pressure dependent mechanical properties of calcium carbides

The mechanical properties of newly synthesized Ca₂C₃ and Ca₂C under pressure have been studied by using the first-principles calculations with generalized gradient approximation. The equilibrium geometry, elastic stiffness constants, various moduli, and Pugh's ratio of the C2/m phase of Ca₂C₃ and the C2/m and Pnma phases of CaC₂ are systematically studied. The elastic stiffness constants of C2/m-Ca₂C₃ under 0–30GPa, C2/m-Ca₂C under 0–7.5 GPa, and Pnma-Ca₂C under 7.5–30 GPa satisfy the Born?Huang mechanical criteria. The three phases of calcium carbides exhibit ductile characteristics. The surface constructions of bulk and Young's moduli illustrate the mechanical anisotropy of Ca₂C₃ and Ca₂C. Our results are consistent with previously obtained experimental and theoretical data and have significant implications for the application of calcium carbides.     

Micromechanical properties of glassy and rubbery polymer brush layers as probed by atomic force microscopy

Carboxylic acid terminated polystyrene and polybutylacrylate were grafted from melt onto a silicon substrate modified with the epoxysilane monolayer. The tethered layers fabricated from polymers of different molecular weights are smooth, uniform, mechanically stable, and cover homogeneously the modified silicon surface. Micromechanical properties of the dry glassy and rubbery brush layers were measured with atomic force microscope. We observed that for the PS layers with the thickness higher than 7 nm, the average value of the elastic moduli reached 1.1 GPa, which is close, but still lower than the expected for bulk polymer. The elastic modulus of PS polymer brush layers dramatically depends upon molecular weight and follows the inverse law with segment molecular weight, M_c of 18,000 known for bulk PS. This result indicates that the process of the formation of the physical network within polymer melt of chains tethered to a solid substrate is similar to that occurring in unconstrained polymer melt. Under these conditions, three PS brush layers studied in this work represent different cases of chains without stable entanglements for M<M_c as well as chains with stable entanglements for brushes with M∼M_c. This transition shows itself in significant reduction of the compliance reflected in twofold increase in elastic modulus. Our estimation predicts that modest lowering of ‘limiting’ elastic modulus of 1.4 GPa can be expected for thicker polymer brushes.     

Two-dimensional condensation of pyrimidine oligonucleotides during their self-assemblies at mercury based surfaces

For the first time it is shown that homopyrimidine oligodeoxynucleotides (ODNs) adsorbed at mercury or amalgam electrode surface can condensate upon applying negative potentials (around −1.35 V vs. Ag/AgCl/3M KCl). This 2D condensation resulted in formation of capacitance pits on the C-E curves resembling those observed earlier with monomeric nucleic acid bases, nucleosides and nucleotides. Differences in behavior of the condensed layers of dT₃₀ and dC₃₀ ODNs, reflecting different physico-chemical and electrochemical properties of thymine and cytosine, were observed. Formation of the ODN condensed film involved reorientation of the oligonucleotide molecules firmly adsorbed at the electrode and took place even in the absence of any ODN in the bulk of solution. Homopurine ODNs did not form these two-dimensional (2D) condensed monolayers under the same conditions. A preliminary thermodynamic analysis of the condensed ODN layers is presented.     

Solvated structure of C60 nanowhiskers

This work characterizes the structure of C₆₀ nanowhiskers prepared by the liquid-liquid interfacial precipitation method in the C₆₀-saturated m-xylene and isopropyl alcohol system. Transmission electron microscopy and X-ray diffraction measurement show that the C₆₀ nanowhiskers had a hexagonal structure with cell dimensions a = 2.407 nm and c = 1.018 nm which is different from pristine C₆₀. The structure of the C₆₀ nanowhiskers in solution is different from that of the solvated structure reported for the C₆₀ nanotubes but similar to that reported for the C₆₀ bulk crystal solvated with m-xylene. X-ray diffraction analysis also showed a shift to fcc structure after solvent evaporation. The C₆₀ nanowhiskers prepared using toluene as solvent also showed a similar solvated structure, and a more rapid structural change into fcc upon drying was again observed.     

Sound velocities and elastic properties of PbTiO3 and PbZrO3 under pressure: First principles study

The elastic constants and sound velocities as a function of pressure for perovskite materials PbTiO₃ (PTO) and PbZrO₃ (PZO) were investigated by first principles calculations. Under ambient pressure, the calculated structural parameters were calculated and found to be in good agreement with known values. To study properties under pressure, PTO and PZO were calculated at several reduced volumes, each of which corresponds to the system under pressure. The C₁₁, C₁₂ and C₄₄ elastic constants are all found to increase with pressure for the pressure range studied. Because the sound velocities are directly derived from the elastic constants, the relationships between the sound velocities and pressure also follow similar trends. The longitudinal modes are all larger than those of the transverse modes.     

Deuterofullerenes

We describe the results of investigations of the products obtained in the results of the interaction of solid C₆₀ and D₂ (P=2 MPa) at different temperature. It was shown that at first with increasing temperature the amount of deuterium absorbed by fullerite increases, then at above 723 K it begins to decrease. This decreasing content of absorbed deuterium is accompanied by deuteromethane evolution that has been found experimentally at 773 K. This points to the destruction of the closed carbon cage at least in some of the C₆₀ or C₆₀D_x molecules.     

On the electrochemical behaviour in aprotic media of dihydropyridines and tetrahydrobipyridines derived from NAD+ model compounds

The electrochemical behaviour of 3-cyano-(or 3-carbamoyl-) 1-methyl-(or benzyl) 1,4-dihydropyridines (4a, b) and the corresponding 1, 1′, 4, 4′ tetrahydro-4,4′-bipyridines (5a, b) or 1, 1′, 6, 4′-tetrahydro-bipyridines (6a, b) in DMF was investigated using ac and dc voltammetrhy, cyclic voltammetry, coulometry, uv and ir spectrometry.At a vitrous carbon electrode all the compounds examined can be oxidized and reduced in a single step. The oxidation process involves loss of two electrons per molecule of reactant to give the corresponding pyridinium salt. The reduction process (which occurs at very negative potentials) involves saturation of the C₅C₆ (dihydropyridines 4a, b) and probably the C₅C₆, C_5′C_6′ (dimers 5a, b) or the C₄C₅, C_5′C_6′ (dimers 6a, b) double bonds, two and four electron per molecule, respectively, being used up.     

Thermal dimerization of fatty ester hypdroperoxides

Summary When autoxidized fatty esters and purified fatty hydroperoxides were decomposed in the absence of oxygen at 210°C., the principal reaction was dimerization of the fatty acid chains with elimination of the hydroperoxide groups. Dimers isolated by molecular distillation (60 to 90% of the polymer) have approximately 1 mole hydroxyl, 0.5 mole carbonyl, and two double bonds per mole of dimer. Diene conjugation in the dimers from polyunsaturated fat hydroperoxides varied from 10 to 23%. The infrared spectra of the dimers were similar to those of the original fatty esters except for one striking band at 2.9 μ, which is attributed to the secondary hydroxyl group. Thecis-trans diene in the polyunsaturated hydroperoxides was isomerized to thetrans-trans configuration on dimerization. The methyl oleate hydroperoxide dimer showed only absorption for isolatedtrans double bond. The dimer was not split either by catalytic hydrogenation or by hydrogen iodide, indicating a carbon-carbon bond between the monomer units. On oxidation with permanganate and periodate, the dimeric acids behaved like a monounsaturated mixture containing double bonds in the C₆, C₇, C₈, C₉, and C₁₀ positions in the oleate dimer and in the C₈, C₉, and C₁₀ positions in the safflower ester dimer. Although the dimers showed no peroxidic oxygen iodometrically with potassium iodide, a reduction occurred with hydriodic acid that may indicate the presence of intramolecular peroxide groups and/or allylic alcohol or carbonyl groups. Bromination with N-bromosuc-cinimide and dehydrobromination with N,N-dimethyl aniline produced no aromatization. Subsequent oxidation of the dehydrobrominated dimer yielded 2.6% residue, which was not aromatic. This evidence indicates that the dimer does not have a six-membered cyclic structure. Dimerization of the hydroperoxides is suggested as occurring through alkyl or alkoxy hydroperoxide radicals to give carbon-carbon linked fatty acid dimers and some higher polymeric units. Presented at 33rd fall meeting, American Oil Chemists’ Society, Los Angeles, Calif., Sept. 28–30, 1959. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.