Scanning tunneling microscopy of fluorinated fullerene molecules on silicon surfaces

Scanning tunneling microscopy (STM) under ultra-high vacuum conditions is used to study the initial stages of adsorption of C₆₀F₁₈ and C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 x 7 and Si(100)-2 x 1 surfaces. Spatially resolved STM images of individual molecules and ab initio calculations show that the fluorofullerene molecules interact with an Si surface, with the F atoms oriented toward the surface. The large electric dipole moment of the molecules induces strong polarization on the surface, but the charge transfer is weak. The presence of C₆₀F₃₆ isomers with different symmetry—T, C ₃, and C ₁—is revealed in STM images for the first time.     

Initial stage of the adsorption of fluorofullerene molecules on Si surface

Spatially resolved images of an individual C₆₀F₁₈ fluorofullerene molecule on Si(100) − 2 × 1 surface have been obtained using scanning tunneling microscopy. Scanning tunneling microscopy results and ab initio calculations show that the fluorofullerene molecules interact with the Si(100) − 2 × 1 surface with F atoms pointing down towards the surface. The adsorption energy of a C₆₀F₁₈ molecule on Si(100) − 2 × 1 surface is ∼12.1 eV, which is much higher than the adsorption energy of the same molecule on Si(111) − 7 × 7 surface (6.65 eV). C₆₀F₁₈ molecules are located in the troughs in-between the dimer rows occupying the four-dimer site on Si(100) − 2 × 1 surface.     

Scanning tunneling microscopy/spectroscopy study of adsorption of C<Subscript>60</Subscript>F<Subscript>36</Subscript> molecules on the 7 × 7-Si(111) surface

Spatially resolved images of an individual C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 × 7 surface have been obtained by means of scanning tunneling microscopy/spectroscopy (STM/STS). The presence of isomers with different symmetry (T, C ₃, C ₁) has been revealed in STM investigation of initial adsorption stage of C₆₀F₃₆ on silicon surface Si(111)-(7 × 7). The adsorbed fluorofullerene molecule can occupy any adsorption site of silicon surface (corner site, faulted half, unfaulted half) that indicates for strong molecule-substrate interaction. The HOMO-LUMO gap of the adsorbed C₆₀F₃₆ molecules have been estimated from current image tunneling spectroscopy (CITS) and z(V) with engaged feedback measurements. The value of HOMO-LUMO gap observed experimentally was 3 eV. The C₆₀F₃₆ molecules adsorption on Si(111)-(7 × 7) surface was stable and kept equilibrium configuration during several hours.     

Theoretical thermochemistry of the C60F18, C60F36, and C60F48 fluorofullerenes

Relative energies of C₆₀F_N fluorofullerenes are reproduced reasonably well at the B3LYP/6- 311G** level of theory employed in conjunction with isodesmic transfluorination reactions, although overestimation of steric repulsions among non-bonded atoms is evident for species with larger values of N. On the other hand, the MNDO method is found to be less suitable for studies of fluorofullerene thermochemistry. The gas-phase standard enthalpy of formation of the C₆₀F₁₈ species is predicted to lie between ?1500 kJ mol^?1 and ?1400 kJ mol^?1.     

Low-dimensional nanostructures and fullerene films on semiconductor surface

The morphology and atomic structures of C₆₀ fullerene films on a Bi(0001)/Si(111)-7 × 7 surface and adsorption of fluorofullerene C₆₀F _x molecules on a Si(111)-7 × 7 surface have been studied by scanning tunneling microscopy/spectroscopy and low-energy electron microscopy under ultra high-vacuum conditions. It has been shown that initial nucleation of C₆₀ islands on the surface of an epitaxial Bi film occurs on double steps and domain boundaries, while tunnel spectra do not exhibit any significant charge transfer to the lowest unoccupied molecular orbital states. Fluorofullerene molecules allow local (at the nanoscale level) modification of Si surface through local etching.     

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C₆H₆, C₆F₁₂, C₆₀, and C₆₀F₄₈ molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C₆H₆ + proton and C₆₀ + proton systems, starting from a distance of 6 Å from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H⁺ occurs with a probability close to unity and transforms the H⁺ ion into a hydrogen atom and the neutral C₆H₆ and C₆₀ molecules into cation radicals. The mechanism of interaction of low-energy protons with C₆F₁₂ and C₆₀F₄₈ molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C₆F₁₂ and C₆₀F₄₈ molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C₆F₁₂, C₆₀F₄₈), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C₆H₆, C₆₀). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.     

Deformation-induced changes in the structure of fullerites C60/70 during their mechanical activation

Structural changes occurring during the mechanical activation of fullerites C_60/70 have been investigated using X-ray diffraction, IR and UV spectroscopy, and scanning electron microscopy. The complete destruction modes of fullerite have been determined (3.5 h at the intensity of the mill of 4.3 W/g and 28 h at 2.2 W/g). The destruction of the crystal structure of fullerites is accompanied by the destruction of fullerene molecules. The residual solvent, which enters into the composition of C_60/70, is retained during the entire time of mechanical activation. In this case, the low-frequency shift of absorption bands of toluene (729 → 725 cm^?1), which is caused by the deformation of the solvent molecule in the composition of crystal solvates, has been observed. It has been shown that the deformation stability of graphite is substantially lower than in the case of fullerite.     

Buckminsterfullerene, higher fullerenes, and their endohedral and fluorine derivatives

This paper presents an overview of recent works concerned with determination of the electron affinity (EA) and the ionization energy (IE) for higher fullerenes and their endohedral and fluorine derivatives. The numerical values of the electron affinity are analyzed for higher fullerenes up to C₁₀₆ and lanthanum, gadolinium, and scandium endohedral fullerenes, including Sc₃N@C₈₀. Most attention is concentrated on two methods for producing fluorofullerenes, namely, direct fluorination of fullerenes with molecular fluorine in a manganese difluoride matrix and solid-phase reactions between fullerenes and fluorinating agents capable of donating fluorine to fullerene. The structures of three fluorofullerenes (C₆₀F₁₈, C₆₀F₂₀, and C₆₀F₄₈) characterized by a distortion of the carbon cage due to attachment of functional groups are discussed.     

Orientation in Nematic Liquid Crystals Doped with Orange Dyes and Effect of Carbon Nanoparticles

研究了掺杂两种分散染料橙的向列型液晶E7的性质以及碳纳米粒子(单壁碳纳米管或富勒烯C₆₀)的影响. 两种分散染料橙11和13具有较高的溶解度和有序参数,被作为掺杂剂同时使用. 与掺杂单染料相比,同时加入两种染料橙使液晶的有序参数明显提高. 与纯液晶相比,掺杂可引起向列相向各向同性相转变温度的升高.     

A Raman study of polymerised C60

60 are reported. Although prepared according to different routes the Raman spectra of the two polymeric phases of C₆₀ show a quantitative agreement with respect to mode positions and intensity. We conclude from this that both materials have the same structure at least in the short range order, i.e. the same type of bonding and co-ordination between neighbouring C₆₀ molecules. An investigation of the time dependence of the thermal decomposition of high pressure polymerised C₆₀ is also presented. The rate of decomposition of the polymeric phase is found to be multi-exponential at all temperatures investigated. From an Arrhenius-type analysis of the short time data and the long time data, respectively, the activation energy for thermal dissociation of polymeric bonds was found to increase with time. Received: 20 September 1996/Accepted: 11 November 1996     

Formation of doubly charged negative ions under the conditions of the resonant electron capture by fluorofullerenes

Doubly charged negative ions formed when electrons with controlled energies interact with isolated fluorinated fullerene molecules C₆₀F _n (n = 36, 48) have been detected and investigated by resonant electron capture mass spectrometry. The dependence of the intensity of the formation of doubly charged negative ions of fluorofullerenes on the energy of attached electrons has been measured. An original method, which is based on the experimental data and does not require additional calibration quantities, has been developed for estimating the absolute cross section for the formation of doubly charged negative ions. The absolute cross sections for the formation of the most intensely formed ions C₆₀F ₃₆ ^2? and C₆₀F ₄₈ ^2? are estimated to be about 1.1 × 10^?24 and 1.5 × 10^?24 m² at their maximum-yield energies of 2.0 and 1.6 eV, respectively.     

C60晶体中分子取向性的研究

本文用集团模型计算C₆₀晶体的结合能,考察晶体原胞中四个不等价分子之间的相对取向变化时晶体结合能的相应变化。认为低温下C₆₀晶体的有序化结构在能量上是不简并的,存在一个使能量最低的分子相对取向。 关键词：     

Photoinduced quenching of the luminescence of singlet oxygen in fullerene solutions

The photoinduced quenching of the luminescence of singlet oxygen in solutions of C₆₀ and C₇₀ fullerenes in CCl₄ is studied. It is shown that intense pumping of the solutions by ultraviolet and visible radiation leads to formation of relatively long-lived fullerene-oxygen complexes, which are effective quenchers of the ¹Δ_g excited state of singlet oxygen. The behavior of these complexes depending on the experimental conditions (pumping intensity, concentration of fullerenes in the solution, and temperature) is studied. Decomposition of complexes with time is investigated. It is shown that, upon formation and gradual degradation of these complexes, the absorption spectra of the solutions change with time. The C₆₀ fullerene is shown to be more stable to the intense pumping, its concentration being completely restored with the decomposition of the complexes. At the same time, C₇₀ fullerene is partly irreversibly lost and the solution does not reach its initial concentration after decomposition of complexes.     

MC20F20（M=Li，Na，Be和Mg）几何结构和电子性质的密度泛函计算研究

采用密度泛函理论（density functional theory, DFT）中的广义梯度近似（generalized gradient approximation, GGA）对MC₂₀F₂₀（M=Li,Na,Be和Mg）的几何结构和电子性质进行了计算研究.几何结构研发现：随着内掺原子序数的增加,金属原子M对C₂₀F₂₀中的C—C键的影响越来越大,而对C—F键的影响甚微.掺杂能计算表明：MC₂₀F₂₀的掺杂能均为负值,需要在一定的实验条件下才能被合成.内掺碱金属和碱土金属分别产生了两类截然不同的能隙和磁性.其中,内掺碱金属的能隙非常小,且带有1μ_B的净磁矩,表现出磁性;而内掺碱土金属的能隙比C₆₀的能隙还大,净自旋为0,表现出非磁性. 关键词： 富勒烯 几何结构 电子结构 密度泛函     

Optical absorption spectrum and local symmetry of Tm ion in TmAl3(BO3)4 crystal

With the help of group theory analysis of absorption spectra of the transition ³H₆ → ³F₃ of Tm³⁺ ion in TmAl₃(BO₃)₄ crystal, measured at several temperatures from 1.8 till 293 K, it has been shown, that the local symmetry of the Tm³⁺ environment is C₃ and it decreases to C₁ at a low temperature. Effective selection rules and polarizations of lines at high enough temperatures (when the line-width is larger than the splitting in C₃ symmetry) have been obtained.     

Theoretical study on the interaction of heptafluoro-iso-butyronitrile decomposition products with Al (1 1 1)

Seeking environmentally friendly gas-insulated medium has become a research hotspot in recent years. At present, C₃F₇CN (Heptafluoro-iso-butyronitrile) is considered to be a potential SF₆ environment-friendly alternative gas and some achievements have been made in the study of its insulation and decomposition characteristics, but there are few reports on the compatibility between its characteristic decomposition products and materials. The investigation of compatibility between gas-insulated medium and material is an important part of evaluating its comprehensive performance. In this paper, we investigated the interaction between C₂F₅CN, CF₃CN, COF₂ and CF₄ with the aluminium widely used in electrical equipment. It was found that the interaction between C₂F₅CN, CF₃CN and Al (1 1 1) surface is strong. There are obvious charge transfer and electron orbital overlap between the C atom, N atom in CN group and Al (1 1 1). The interaction between COF₂, CF₄ and Al (1 1 1) surface is weak and van der Waal’s forces play the major role. Relevant results reveal the characteristics of C₃F₇CN decomposition products and provide theoretical guidance for evaluating the material compatibility between C₃F₇CN decomposition products and aluminium.     

Capacity fading reason of LiNi0.5Mn1.5O4 with commercial electrolyte

The cycling performances of LiNi_0.5Mn_1.5O₄ (LNMO) were investigated and the reasons of capacity fading were discussed. The results show that LNMO can deliver about 115 mAh?g^?1 at 1C at different temperatures; however, it retains only 61.57 % of its initial capacity after 130th cycles at 60 °C, which is much lower than 94.46 % of LNMO at 25 °C, and the cycling performance at 1C is better than that at 0.5C. The reason of capacity fading of LNMO at 60 °C is mainly due to the lower decomposition voltage of 4.3 V with commercial electrolyte and the larger decomposition current, of which the electrolyte decomposes and interacts with active materials to lead to the larger irreversible capacity loss. While the worse cycling performance at low rate is attributed to the longer interaction time between the electrolyte with the decomposition voltage of 4.5 V and the active materials.     

Nonlinear optical limiting based on energy and charge transfer in fullerene-containing media

The optical limiting efficiency and the possibility of extending the spectral and time ranges of optical limiters based on fullerene-containing media are studied. The optical limiting of pulses of different durations in solutions of C₆₀ and higher fullerenes, the triplet—triplet transfer in multicomponent fullerene-containing solutions, and the electron transfer in the C₆₀-tetramethylbenzidine-perylene system are investigated.

     

The effect of water on the stability of C<Subscript>60</Subscript> fullerene nanowhiskers

The morphology of C₆₀ precipitates synthesized by using isopropyl alcohol (IPA) added with water was investigated in order to know the effect of water on the growth of C₆₀ nanowhiskers (C₆₀NWs) in C₆₀–toluene–IPA solution systems. The stability of C₆₀NWs decreased and granular crystals of C₆₀ were formed in the solutions when IPA added with an excess amount of water was used in the liquid–liquid interfacial precipitation method. The C₆₀NWs were found to be destabilized with time in the solutions added with water. The C₆₀NWs dried in air showed similar Raman profiles irrespective of the use of IPA with and without water addition. The Raman profiles of granular C₆₀ single crystals showed the base lines much flatter than those of C₆₀NWs, indicating that C₆₀NWs possess a disordered crystal structure. By optimizing the growth condition, short C₆₀NWs with aspect ratios ranging from 3 to 10 and an average length of about 1.8 μm were successfully fabricated. The short C₆₀NWs are expected to be applicable for electrodes of organic thick film solar cells.     

The second-order elastic constants of an AgBr-(56.6%) AgCl mixed crystal from 20°C to 400°C

Abstract

Initial measurements of the second-order elastic constants of an AgBr-(56.6%) AgCl mixed crystal have been made from 20—400°C. Over this temperature range, C'₁₁ decreased by 44%, C' by 71%, C₄₄ by 22%, C₁₁ by 53%, C₁₂ by 40%, and B _s by 46%. The decreases in the elastic constants are linear until approximately 280°C, whereupon all but C' begin to decrease more rapidly than linearly with variations from linearity of 4—20% at 400°C and C' remains linear at all temperatures. This anomalous behavior is similar to other silver halide physical properties and may be attributed to the unusually high defect concentration at high temperatures. Similar elastic constant changes are seen in superionic conductors near the superionic transition, indicating that the silver halides may be starting the transition to a superionic state when the halide sublattice melts and the transition is frustrated.