Superparamagnetic properties of C60Co3 complexes

Preparation of fullerites containing cobalt and analyses of reactions based on semiempirical quantum calculations are described. The magnetic properties of thermally treated C₆₀Co₃ samples: Curie constant (C≈3500 emu K/mol Oe) temperature and field dependencies of magnetization and nonequilibrium effects of magnetization are interpreted in terms of superparamagnetic blocking model of the compound.     

Magnetism of C60-based molecular complexes: High-field-magnetization and magnetooptical study

Magnetization study of the C₆₀ · TMTSF · 2CS₂ molecular complex in magnetic fields up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)₂C₆₀ at T=1.8 K for the frequency range 60–90 GHz in magnetic fields up to 32 T provide experimental evidence that paramagnetic centers with reduced g-factor values g<1 control the magnetic properties of these solids. A model is suggested in which the renormalization of the g factor is due to the dynamic Jahn-Teller effect involving negative C₆₀ ions that appear as defects in the crystalline structure with a weak charge transfer.     

C60 molecular bearings

An ultralubricated system is reported which confines a C60 monolayer between graphite plates. C60 molecules act as molecular bearings, assisted by the nanogears of six-membered carbon rings between C60 molecules and graphite, in which the mean dynamical frictional forces are zero up to a high load of 100 nanonewtons. A stick-slip rolling model with a step rotation of a C60 molecule is proposed. This ultralubricated system, very promising for the realization of nano- and micromachines, is expected to open a new field of molecular bearings.     

内陷在C60中的氢原子的性质

在短程球形势阱的模型下,运用线性变分法并采用B-样条作为展开基函数计算了内陷于C60几何中心的氢原子能谱和波函数,并计算了势阱深度对能谱的影响,详细讨论了内陷氢原子表现出的一系列特殊性质,从而对低维半导体材料性能的研究提供了有效的数据;同时这一工作也表明,用线性变分法结合B-样条函数在处理这类问题时是非常有效的。     

Dielectric properties of ultrathin films of molecular crystals

Dielectric properties of thin films (TF) of molecular crystals, including the effect of size and the boundary surfaces were analysed using Green's function method. Polarisability of molecules in various film layers and dielectric susceptibility of TF were calculated. A comparison with crystal bulk has shown that dielectric properties of TF are strongly influenced both by the sample dimensions and by the boundary conditions.The frequency dependence of the dielectric susceptibility has also been derived. One obtains the monotonous variation for the frequencies above and below exciton band. However, for the frequencies within the exciton band there appears to be complicated, non-monotonous dependence. The relationship between the dielectric susceptibility and the film width for the same frequency shows a complex, oscillatory behaviour. Furthermore, the amplitude of these oscillations increases with increasing film width, demanding the introduction of a damping factor. Finally, the thickness dependence of dielectric susceptibility was analysed in the Cole-Cole plot.     

Magnetic properties of C60 polymers

Magnetic properties of various C60 polymers are calculated using tight-binding molecular-dynamics and ab initio methods. Our results suggest a mechanism involving an interplay between structural defects and sp(3) hybridization to be responsible for the origin of this magnetism. The onset of magnetism is found to occur much more readily for the Rh-C60 polymeric phase with defects than for any of the other polymers, in agreement with the recent experiment. Our estimate of the magnetic moment is also in very good agreement with the value observed in experiment.     

C60、C240、C60@C240富勒烯分子压缩特性的分子动力学研究

采用Tersoff-Brenner势的分子动力学方法,研究了双石墨层作用下C60、C240及C60@C240富勒烯分子的压缩力学特性.根据计算结果,讨论了三种分子压缩过程中几何构形、能量、压缩载荷等的变化及其差异.研究表明,压缩过程中,仅C240分子出现了“塌陷“现象,塌陷时,该分子的能量及外载一度下降;相同压缩应变下,C240的体积压缩率以及C60@C240的能量吸收率最大,而C60的体积压缩率及能量吸收率均最小; C60@C240分子的最大承载能力及C240的最大承受变形能力最大,而C60分子的最大承载和最大承受变形能力均最小;在C60@C240分子的压缩中,当应变小于20%时,内笼C60的体积及其能量变化很小;C60与C240之间的范德华尔能在整个压缩C60@C240分子的能量变化中仅仅占有非常小的份额.     

C60 molecular configurations leading to ferromagnetic exchange interactions in TDAE*C60

The charge-transfer salt tetrabis(dimethylamino)ethylene-fullerene (C₆₀), or TDAE*C₆₀, is a rare exception among pure organic crystalline systems, because it shows a transition to a ferromagnetic (FM) state with fully saturated s=1/2 molecular spins at a respectable T _c=16 K. In spite of extensive experimental and theoretical work over the past ten years, the origin of the ferromagnetism in TDAE*C₆₀ has remained a mystery. To resolve this problem, we performed a comparative structural study of two different magnetic forms of TDAE*C₆₀ crystals, one of which is magnetic and the other is nonmagnetic at low temperatures, and fully correlated their structural properties (particularly, the intermolecular orientations) with their magnetic properties. We identified the relative orientation of C₆₀ molecules along the c axis as the primary variable that controls the ferromagnetic order parameter and showed that both FM and low-temperature spin-glass-like ordering are possible in this material, depending on the orientational state of the C₆₀ molecules. Thus, we resolved the apparent contradictions intrinsic to different macroscopic measurements and opened a path to a microscopic understanding of p-electron FM exchange interactions.     

Effect of C60 molecular rotation on nanotribology

The effect of C60 molecular rotation on the nanotribological properties of C60 single crystal surfaces has been studied by atomic/frictional force microscopy. The orientational order-disorder phase transition, in which the high temperature C60 free rotation is reduced to a low temperature hindered rotation, is shown to give rise to an abrupt change in friction and adhesion. This change in frictional force is quantitatively consistent with the observed change in adhesion. The similar slopes of the friction versus load curves in both phases indicate that the friction coefficient in the two phases remains about the same. Hence the C60 rotation does not provide an additional energy dissipation channel in the friction process.     

Structural properties of vapor-grown C60 crystals

Vapor grown crystals of C₆₀ with thin flat triangular, rhombic or trapezoidal shapes of size to 1×2×0.001 mm as well as prismastic crystals typically 0.5×0.4×0.2 mm have been grown using a high temperature vapor transport method. Room temperature X-ray precession photography shows these crystals exhibit diffraction patterns consistent with those for either (a) single or (b) twinned crystals of the previously reported face-centered cubic structure or (c) a more complex cell of hexagonal symmetry with a=10.010(2) Å and c=49.064(11) Å. This latter from actually is a multiple twin containing both the face-centered cubic and the close-packed hexagonal structure types. The sharp diffraction maxima for either the single or the twinned crystals demonstrate that they consist of large coherent domains and are essentially free of planar defects parallel to the twin planes.     

Dielectric and optical properties of C60 material studied by ellipsometry and quantitative IR and UV/VIS spectroscopy

Optical properties in the spectral range from 0.06 to 5.5 eV of fullerite films on different substrates, C₆₀ powder, and dissolved fullerene material are investigated by spectroscopic ellipsometry and optical transmission and reflection measurements. Depolarization effects are taken into account during determination of the dielectric function of fullerite films by ellipsometry. The optical gap for C₆₀ films is found to be 1.63 eV. Three optical absorption bands are observed at 2.69, 3.53, and 4.49 eV.The dielectric function in the infrared shows the four characteristic infrared vibrational modes. The interference pattern seen in UV/VIS reflection measurements are used for high-precision thickness determination of the films. The Clausius-Mossotti formula is successfully applied to reproduce the experimental optical data measured in C₆₀/dichlormethane solutions. Deviations between theory and experiments provide interesting information about the intermolecular interaction of the C₆₀ molecules. A tentative interpretation of the measured absorption bands is presented.