Giant fullerenes formed on C60 films irradiated with electrons field-emitted from scanning tunneling microscope tips

It has been found that spherical large clusters of carbon atoms are formed by irradiation of crystalline C₆₀ films grown on Si(1 1 1)-(7 × 7) surfaces with electrons field-emitted from a scanning tunneling microscope probe tip. The size distribution of the clusters deduced from surface profile measurements suggests that the dominant clusters were not necessarily C_60n (n = 2-4) expected from the simple fusion of C₆₀ molecules. It was proposed that electronic excitations of C₆₀ molecules caused the fragment and coalescence of the molecules to form the giant fullerenes as in the photo-induced similar effects.     

Formation of Sim and SimCn clusters by C60 sputtering of Si

The secondary ion mass spectrum of silicon sputtered by high energy C₆₀⁺ ions in sputter equilibrium is found to be dominated by Si clusters and we report the relative yields of Si_m⁺ (1 ≤ m ≤ 15) and various Si_mC_n⁺ clusters (1 ≤ m ≤ 11 for n = 1; 1 ≤ m ≤ 6 for n = 2; 1 ≤ m ≤ 4 for n = 3). The yields of Si_m⁺ clusters up to Si₇⁺ are significant (between 0.1 and 0.6 of the Si⁺ yield) with even numbered clusters Si₄⁺ and Si₆⁺ having the highest probability of formation. The abundances of cluster ions between Si₈⁺ and Si₁₁⁺ are still significant (>1% relative to Si⁺) but drop by a factor of ∼100 between Si₁₁⁺ and Si₁₃⁺. The probability of formation of clusters Si₁₃⁺-Si₁₅⁺ is approximately constant at ∼5 × 10⁻⁴ relative to Si⁺ and rising a little for Si₁₅⁺, but clusters beyond Si₁₅ are not detected (Si_m≥16⁺/Si⁺ < 1 × 10⁻⁴). The probability of formation of Si_m⁺ and Si_mC_n⁺ clusters depends only very weakly on the C₆₀⁺ primary ion energy between 13.5 keV and 37.5 keV. The behaviour of Si_m⁺ and Si_mC_n⁺ cluster ions was also investigated for impacts onto a fresh Si surface to study the effects that saturation of the surface with C₆₀⁺ in reaching sputter equilibrium may have had on the measured abundances. By comparison, there are very minor amounts of pure Si_m⁺ clusters produced during C₆₀⁺ sputtering of silica (SiO₂) and various silicate minerals. The abundances for clusters heavier than Si₂⁺ are very small compared to the case where Si is the target.The data reported here suggest that Si_m⁺ and Si_mC_n⁺ cluster abundances may be consistent in a qualitative way with theoretical modelling by others which predicts each carbon atom to bind with 3-4 Si atoms in the sample. This experimental data may now be used to improve theoretical modelling.     

Scanning tunneling microscopy of surface-adsorbed fullerenes: C60, C70, and C84

Scanning tunneling microscopy (STM) is used to characterize partial monolayers of C₆₀, C₇₀, and C₈₄ adsorbed on the Au(1 1 1) surface at room temperature and under ambient conditions. A high degree of structural polymorphism is observed for monolayers of each of these fullerenes. For C₆₀, three lattice packings are observed, including a previously unreported 7 × 7 R21.8° structure that is stabilized by adjacent surface step defects. For C₇₀, two lattice packings are observed, and analysis of molecular features in STM images allows molecular binding geometry to be determined. In one of the two observed lattice structures, C₇₀ molecules align their long axis along the surface normal, while in the other, molecules align parallel to the surface and along a gold lattice direction. The parallel geometry is also preferred for isolated and loosely packed molecules on the surface. C₈₄ exhibits a large number of lattice orientations and no long-range order, and likely binds incommensurately on Au(1 1 1). Time series of images of partial C₇₀ monolayers show progressive surface modification as a result of perturbation by the STM tip; this is in contrast to the behavior of C₆₀, where alterations in surface structure at room temperature are thermally driven.     

Energy distributions of atomic and molecular ions sputtered by C60 projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C₆₀⁺ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C₆₀⁺ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C⁺(E^−0.4) > H⁺(E^−1.5) > Ag⁺(E^−3.5). In particular, the distribution of Ag⁺ is not broader than those of Ag₂⁺ and Ag₃⁺ clusters, in sharp contrast with 15 keV Ga⁺ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C₆₀⁺ instead of Ga⁺ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag⁺ and Ag_n⁺ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS⁺ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C₇H₇⁺ and C₁₅H₁₃⁺ and polycyclic fragments, such as C₉H₇⁺ and C₁₄H₁₀⁺, the high energy decay is steep (E⁻⁴ − E⁻⁸). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo₄O₇ has been investigated. It is shown that the oxides CaBaCo₄O_7+δ and CaBaCo_4−xO_7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with T_C~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co³⁺ cations, with respect to the ideal formula CaBaCo₂²⁺Co₂³⁺O_7. This excess Co³⁺ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.     

Stretching the limits of static SIMS with C60

Pristine and Au-covered molecular films have been analyzed by ToF-SIMS (TRIFT™), using 15 keV Ga⁺ (FEI) and 15 keV C₆₀⁺ (Ionoptika) primary ion sources. The use of C₆₀⁺ leads to an enormous yield enhancement for gold clusters, especially when the amount of gold is low (2 nmol/cm²), i.e. a situation of relatively small nanoparticles well separated in space. It also allows us to extend significantly the traditional mass range of static SIMS. Under 15 keV C₆₀⁺ ion bombardment, a series of clusters up to a mass of about 20,000 Da (Au₁₀₀⁻: 19,700 Da) is detected. This large yield increase is attributed to the hydrocarbon matrix (low-atomic mass), because the yield increase observed for thick metallic films (Ag, Au) is much lower. The additional yield enhancement factors provided by the Au metallization procedure for organic ions (MetA-SIMS) have been measured under C₆₀⁺ bombardment. They reach a factor of 2 for the molecular ion and almost an order of magnitude for Irganox fragments such as C₄H₉⁺, C₁₅H₂₃O⁺ and C₁₆H₂₃O⁻.     

Valence fluctuation in Ce2Co3Ge5 and crystal field effect in Pr2Co3Ge5

Polycrystalline samples of ternary rare-earth germanides R₂Co₃Ge₅ (R=La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U₂Co₃Si₅ structure (space group Ibam). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce₂Co₃Ge₅ deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment (μ_eff≈0.95 μ_B) compared to that expected for trivalent Ce ions further supports valence-fluctuating nature of Ce in Ce₂Co₃Ge₅. The observed temperature dependence of magnetic susceptibility is consistent with the ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr₂Co₃Ge₅. An analysis of C_mag data suggests a singlet ground state in Pr₂Co₃Ge₅ separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.     

On the formation and decomposition of C7H8

The kinetics of reactions on the C₇H₈ surface were studied with state-of-the-art ab initio transition state theory (TST) and master equation methodologies. A priori predictions of the capture rate for C₆H₅ + CH₃ and for C₇H₇ + H are obtained from direct variable reaction coordinate TST simulations. These simulations employ small basis set CASPT2 interaction energies coupled with one-dimensional reaction path corrections based on higher level simulations for related reactions. For the C₇H₇ + H reaction, predictions are obtained for both the total rate and for the branching between toluene, o-isotoluene and p-isotoluene. A mapping of the low energy pathways for isomerization from these three C₇H₈ isomers identifies a number of processes with barriers at or below the dissociation threshold. Nevertheless, at combustion temperatures the dissociation rates are predicted to exceed the isomerization rates, and it is reasonable to treat the kinetics of each isomer as a simple single well association/dissociation equilibrium. Master equation simulations yield predictions for the temperature and pressure dependence of each of the recombination and dissociation processes, as well as for the C₇H₇ + H → C₆H₅ + CH₃ bimolecular reaction. These simulations implement collisional energy transfer probabilities based on the work of Luther and co-workers. The theoretical predictions are found to be in satisfactory agreement with the available experimental data for the photodissociation of toluene, the temperature and pressure dependent dissociation of toluene, and the reaction of benzyl radical with H. For the C₆H₅ + CH₃ recombination, the theoretical predictions exceed the experimental measurements of Lin and coworkers by a factor of 2 or more for all temperatures.     

Rotational and hyperfine spectra for a sandwich titanium complex, C5H5TiC7H7

Microwave spectroscopy measurements and density functional theory calculations are reported for the cyclopentadienyl cycloheptatrienyl titanium complex, C₅H₅TiC₇H₇. Rotational transition frequencies for this symmetric-top complex were measured in the 4-13 GHz range using a Flygare-Balle-type pulsed beam spectrometer. The spectroscopic constants obtained for the normal C₅H₅⁴⁸TiC₇H₇ isotopomer are B = 771.78907(38), D_J = 0.0000295(41), and D_JK = 0.001584(73) MHz. The quadrupole hyperfine splittings for C₅H₅⁴⁷TiC₇H₇ were clearly observed and the measured constants are B = 771.79024(32) MHz, D_J = 0.0000395(33), D_JK = 0.001646(24), and eQq_aa = 8.193(40) MHz. Analysis of the experimental and theoretical rotational constants indicates that the η⁷-C₇H₇Ti and η⁵-C₅H₅Ti bond lengths in the gas phase are about 0.02 Å longer than those reported for the solid-state X-ray structure. The calculated Ti-C bond lengths are shorter for the C₇H₇ ligand (r(Ti-C) = 2.21 Å) than for the C₅H₅ ligand (r(Ti-C) = 2.34 Å), and the C₇H₇ H atoms are displaced 0.15 Å out of the C₇ plane, toward the Ti atom.     

Photon-assisted synthesis of C60 polymers by laser irradiation

The harmonics of a free electron laser (FEL) were irradiated in vacuum to surfaces of compressed C₆₀ and a mixture of C₆₀ and I₂. The power and frequency of the fundamental FEL macro-pulse were ca. 0.5 mJ/pulse and 2 Hz, respectively. The irradiation time was 120-180 min. After irradiation of FEL with a typical wavelength of 450 or 345 nm, the Raman peak of Ag(2)-derived vibration mode of C₆₀ shifted to the lower-energy side. The Raman peak shift of the mixture powder sample was greater than that of pure C₆₀. Furthermore, changes of the crystalline structure indicated that various intermolecular combinations occurred by irradiation. These results strongly suggest that three-dimensional polymerization of C₆₀ was promoted by laser irradiation and the effect of photon-assisted hole-doping from iodine atoms to C₆₀ molecules.     

Synthesis, crystal structure and photoconductivity of new molecular complex of C60 with tetrabenzo(1,2-bis[4H-thiopyran-4-ylidene]ethene): Bz4BTPE C60

A new molecular complex of C₆₀ with tetrabenzo(1,2-bis[4H-thiopyran-4-ylidene]ethene), Bz₄BTPE C₆₀ (1) has been obtained. The complex has a layered structure in which closely packed hexagonal layers of C₆₀ alternate with the layers composed of Bz₄BTPE molecules. The complex has a neutral ground state according to UV-vis-NIR spectrum. It has been found that single crystals of 1 show low ‘dark’ conductivity of σ∼10⁻¹⁰ (Ω cm)⁻¹. A 10² increase in photocurrent has been observed upon illuminating the crystal with white light. Photoconductivity of 1 is sensitive to magnetic field with B₀<1 T and increases up to 5% in magnetic field. The photoconductivity spectra of the complex indicate that free charge carriers are generated in the UV-visible range mainly by the Bz₄BTPE excitation (the peaks at 622, 562, 472 and 348 nm) with a possible contribution of charge transfer excitations between neighboring C₆₀ molecules (the peak at 472 nm).     

Hydrogenation of carbon fragments after femtosecond laser ablation of solid C60

We have observed several kinds of hydrocarbon cations after the nanosecond and the femtosecond laser ablation (nsLA and fsLA) of solid C₆₀. The observation indicates that the carbon fragments produced just after laser ablation of the C₆₀ molecule react with the hydrogen atoms and ions coexisting in the ablation plume. In the case of fsLA, clear dependence of the product hydrocarbon species on the ablation laser power has been observed although the dependence is not clearly observed in nsLA. The production of C_nH₅⁺ (n = 8, 10, and 12) is only observed in fsLA suggesting the unique nature of the transient carbon fragments produced by fsLA.     

Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

Intramolecular structures of C60 and C84 molecules on Si(1 1 1)-7 × 7 surfaces by scanning tunneling microscopy

The intramolecular features of carbon 60 and carbon 84 molecules on Si(1 1 1)-7 × 7 surfaces were studied under a UHV-scanning tunneling microscope. Carbon molecules preferentially appear in faulted halves, rather than in unfaulted halves and corner holes; they are embedded in silicon substrates. The orientation and details of the structure of carbon molecules are determined by applying various sample biases to the silicon substrate. As compared with other fullerenes, a bright pentagonal ring with nebulous clusters which represents the cage structure is clearly observed on top of carbon 60 molecules. The bright stripes associated with partitioned curves which depict eight features of asymmetrical C₈₄ molecules are also investigated on Si(1 1 1)-7 × 7 surfaces. The orientations and possible configurations of C₆₀ and C₈₄ are considered in this work. The energy differences for various features of C₆₀ and C₈₄ molecules are estimated and discussed. The corresponding models with respect to each intramolecular feature are proposed and compared with recent theoretical calculation.     

Magnetic phases in lanthanum-strontium manganite-cobaltite La1.25Sr0.75MnCoO6

Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La_1.25Sr_0.75MnCoO₆, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at T_C=217 K and a frequency-dependent transition at T_g<100 K. The high frequency dependence of T_g is indicative of the cluster-glass behavior of La_1.25Sr_0.75MnCoO₆ (7 5 0) at T<T_C within the ferromagnetic state.     

RHEED intensity oscillation of C60 growth on GaAs substrates

Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C₆₀ layer epitaxial growth on GaAs (1 1 1)B and (1 1 1)A substrates. The frequency of the oscillation coincides well with growth rate of C₆₀ layers, suggesting that C₆₀ layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C₆₀ layer growth on GaAs (1 1 1)B substrates with (2 × 2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C₆₀ molecules. This phenomenon is explained by the model that C₆₀ absorption sites are limited due to As-trimers absorbed on (1 1 1)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (1 1 1)A substrates where no As-trimer is absorbed.     

Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl

New triethylammonium salts: [(C₂H₅)₃NH]SbCl₆ (TCA) and [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P2₁/n and P2₁/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C₂H₅)₃NH]SbCl₆ consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.     

Synthesis and characterization of Co7(OH)12(C2H4S2O6)(H2O)2—a single crystal structural study of a ferrimagnetic layered cobalt hydroxide

A novel layered hydrotalcite-like material, Co₇(H₂O)₂(OH)₁₂(C₂H₄S₂O₆), has been prepared hydrothermally and the structure determined using single crystal X-ray diffraction (a=6.2752(19) Å, b=8.361(3) Å, c=9.642(3) Å, α=96.613(5)°, β=98.230(5)°, γ=100.673(5)°, R1=0.0551). The structure consists of brucite-like sheets where 1/6 of the octahedral sites are replaced by two tetrahedrally coordinated Co(II) above and below the plane of the layer. Ethanedisulfonate anions occupy the space between layers and provide charge balance for the positively charged layers. The compound is ferrimagnetic, with a Curie temperature of 33 K, Curie-Weiss θ of −31 K, and a coercive field of 881 Oe at 5 K.     

Solvent-induced optical properties of C60

Different C₆₀ aggregates, i.e. nanoparticles, clusters of nanoparticles and microcrystals in room-temperature solutions, are reported to account for the colors of fluorescence emissions centered at 440, 575 and 700 nm, respectively. And the configurations of C₆₀ aggregation created in solutions are revealed to be closely associated with the characteristic interactions between C₆₀ and solvent molecules. On this basis, aggregation behaviors and thus induced optical properties of C₆₀ have been tentatively controlled through adopting solvent mixtures.