Interaction of C60 molecules with a (100) Mo surface

The adsorption, initial stages of film growth, and transformation of an adlayer of C₆₀ molecules on a (100) Mo surface upon heating are studied under ultrahigh-vacuum conditions. It is shown that the C₆₀ molecules remain intact in the adsorbed state up to 760 K. Layer-by-layer growth of a fullerite film is observed at room temperature, while tower-shaped crystallites grow up from a loosely packed monolayer with an approximate concentration of C₆₀ molecules equal to (1.3±0.2)×10¹⁴ molecules · cm⁻² at 500–600 K. In the latter case the percentage of the surface occupied by them depends on the temperature and the impinging flux density of fullerene molecules, but after a certain stationary value has been achieved, it scarcely depends on the exposure time. Zh. Tekh. Fiz. 69, 117–122 (November 1999)     

Formation of highly crystalline C<Subscript>60</Subscript> molecular films on a Bi(0001)/Si(111) surface

The results of scanning tunneling microscopy (STM) investigation of controllable growth of C₆₀ adsorption on a Bi(0001)/Si(111) surface are reported. With the use of UHV STM, it has been shown that the most favorable sites for the initial stage of C₆₀ adsorption are the double steps and domain boundaries. At ∼1 monolayer C₆₀ coverage, the modulation pattern caused by the epitaxial relation between C₆₀ and Bi unit cells has been observed. An increase in the C₆₀ coverage up to several monolayers results in the formation of a highly crystalline molecular film. The text was submitted by the authors in English.     

Molecular simulation of C60 adsorption onto a TiO2 rutile (1 1 0) surface

A Monte Carlo molecular simulation study is presented on the adsorption and growth of C₆₀ films on the surface of the (1 1 0) face of rutile. Simulations are performed for a temperature of 600 K using atomistic models both for the fullerene molecules and the TiO₂ surface. It is found in this work that C₆₀ is adsorbed preferably in an ordered arrangement along the surface depressions over the exposed undercoordinated Ti cations. At low densities adsorption occurs preferably at alternate rows, with locations in consecutive rows being occupied appreciably only at higher C₆₀ densities. At low densities, the fullerene molecules tend to aggregate into islands in the surface plane. Additional layers of C₆₀ form only as the density increases, and do so before a monolayer is completed in all consecutive rows. Full monolayer capacity obtained at the highest densities is about 0.9 C₆₀ molecules per nm², but this is only achieved by completing the packing of molecules in interstices at a slightly upper level. The fraction of the molecules that lie closest to the surface only amounts to 0.6 molecules per nm².     

Anomalous character of the interaction of C<Subscript>60</Subscript> molecules with the sulfur-saturated Ta(100) surface

It is shown that, contrary to all previously studied systems, heating to ～800 K in the C₆₀? TaS² monolayer-Ta(100) adsorption system leads to the complete removal of the deposited fullerene molecules. A model is proposed that explains the observed phenomenon by a very weak nonchemisorption interaction between the C₆₀ molecules and the valence-saturated surface of tantalum disulfide that forms layered crystals with van der Waals interaction between layers.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Chemical adsorption of C60 on diamond (100)(2×1) surfaces

Molecular-dynamics simulations (MDSs) and ab initiocalculations are used to investigate the adsorption behavior of C₆₀ molecules on a clean dimer-reconstructed (100)(2×1) diamond surface. C₆₀ molecules have some probability to be adsorbed on the diamond surface at low incident energy (6∼45 eV). Electron-density contours show strong chemical interaction between C₆₀ molecules and the substrate surface. The adsorption property depends strongly on the incident energy and the impacting point. An incident energy of 18 eV may be an appropriate energy to grow a sub-monolayer or monolayer C₆₀ film on a clean C(100)(2×1) surface at room temperature. Received: 5 July 2000 / Accepted: 17 October 2000 / Published online: 28 February 2001     

On the interaction of self-assembled C<Subscript>60</Subscript>F<Subscript>18</Subscript> polar molecules with the Ni(100) surface

The current work is dedicated to investigation of the interaction between self-assembled polar molecules of fullerene fluoride C₆₀F₁₈ with the chemically active surface Ni(100) under radiation and heat treatments. X-ray photoelectron spectroscopy is used in combination with quantum-chemical simulation. For the first time, the transformation of an as-deposited dielectric continuous 2D thin film to a 3D island-type assembly with molecular ordering within the islands is shown to take place. The degree of coverage of the Ni surface by C₆₀F₁₈ islands (0.6–0.7) and their height (~6 nm) are estimated. Quantum-chemical simulation shows that the chemisorption energy of the C₆₀F₁₈ molecule on the Ni surface equals ~6.6 eV and fluorine atoms are located at a distance of 1.9 Å above the Ni surface. The results of the investigation provide an opportunity to create nanoscale ordered structures with local changes in the work function.     

The structure of fullerite C<Subscript>60</Subscript> intercalated with molecular oxygen

A (O₂) _x C₆₀ sample with a high content of oxygen (x ≥ 0.4) and free of technological solvent impurities was obtained by precipitation from solution. For the first time, the results of the determination of the x coefficients using ¹³C NMR and elemental analysis were compared. It was shown by Raman spectroscopy, mass spectrometry, and NMR that the inclusion of oxygen into fullerite was accompanied by a decrease in the frequency of O=O stretching vibrations by no less than 12 cm⁻¹ compared with gaseous O₂. Nevertheless, oxygen exists in the molecular form in (O₂)_0.4C₆₀ and is released in the form of O₂ as the sample is heated to 373 K. The number of oxygen molecules occupying octahedral pores closets to the fullerene molecule takes on all the possible values, from 0 to 6. At room temperature, the (O₂) _x C₆₀ sample lost oxygen much more slowly than similar products prepared by diffusion saturation of pure fullerite with oxygen.     

Thermodynamic properties of C<Subscript>60</Subscript> polyfullerites

This paper discusses the results of calorimetric studies of the 1D C₆₀ (orthorhombic) and 2D C₆₀ (tetragonal and rhombohedral) fullerites, as well as of the graphite-like polyfullerite, which are produced from a starting C₆₀ fullerite subjected to a pressure of 1–8 GPa at temperatures ranging from 300 to 1270 K. The analysis is made primarily of the C _p ⁰ heat capacity measurements performed in adiabatic calorimeters in the 5-to 350-K range.     

Study of the Initial Stage of Fluorinated C<Subscript>60</Subscript>F<Subscript>18</Subscript> Fullerene Adsorption on the Cu(001) Surface

The dynamics of the adsorption and evolution of fluorinated C₆₀F₁₈ fullerene molecules on the Cu(001) surface are studied by real-time ultra-high vacuum scanning tunneling microscopy. Fluorinated fullerene molecules are shown to decompose with time on the Cu(001) surface transforming to C₆₀ molecules. The decay rate depends on the initial molecular coverage. The rapid decay of fluorinated fullerene molecules is observed when the coverage is no higher than 0.2 single layers. As a result, two-dimensional islands consisting of pure C₆₀ molecules are formed on the Cu(001) surface. 2D islands consisting of fluorinated fullerene molecules are formed when the initial molecular coverage is higher than 0.5 single layers. The molecules inside these islands also tend to decompose with time. It is found experimentally that fluorine atoms are removed completely from the initial C₆₀F₁₈ molecules adsorbed on the Cu(001) surface after 250 h when the initial molecular coverage is 0.6 single layers.     

The influence of ultraweak ionizing irradiation on the magnetoplastic effect in single crystals of a C<Subscript>60</Subscript> fullerite

The suppression of plasticity sensitivity of a C₆₀ fullerite to the action of a magnetic field is revealed. It is found that the C₆₀ fullerite undergoes temporary softening due to irradiation with ultralow (<0.1 cGy) doses of β and γ radiation.     

Superparamagnetism of magnetite particles in C<Subscript>60</Subscript> fullerite powder

A new magnetic material, C₆₀ fullerite powder doped by magnetite (Fe₃O₄) nanoparticles, is obtained by heating a mixture of fullerite and iron(III) acetylacetonate. It is shown that the material offers superparamagnetic properties. Surface bonding between the nanoparticles and the fullerite is established.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

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.     

Thermodynamic and dilatometric properties of the dimerized phase of a C<Subscript>60</Subscript> fullerene

This paper reports on the results of complex investigations into the structural, thermodynamic, and dilatometric properties of the C₆₀ dimerized phase prepared under compression of a C₆₀ fullerite at a pressure up to 8 GPa and a temperature of 290 K. It is demonstrated that the dimerized phase has a face-centered cubic structure with a lattice parameter a=14.02±0.05 Å. The dimeric structure of the studied sample is confirmed by x-ray diffraction analysis. According to the dilatometric data, the volume jump observed in the vicinity of the orientational transition for the dimerized phase is estimated to be approximately 30 times less than that for the C₆₀ fullerite. The temperature dependence of the heat capacity of the (C₆₀)₂ crystalline dimer is examined using precision adiabatic vacuum calorimetry under normal pressure in the temperature range from T → 0 K to 340 K. The results obtained are used in the calculations of thermodynamic functions, namely, the heat capacity C _p ⁰ (T), the enthalpy H⁰(T)-H⁰(0), the entropy S⁰(T), and the Gibbs function G⁰(T)-H⁰(0). The fractal dimension D is determined as a function of the heat capacity. The standard entropy of the formation of the (C₆₀)₂ crystalline dimer from a simple compound (graphite) at T=298.15 K and normal pressure is calculated.     

Cleavage of C<Subscript>60</Subscript> fullerite crystals

It is found that, under certain conditions, C₆₀ fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τ_y = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ? 60b³ is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.     

Effect of oxygen and iodine on the optical and magnetic properties of fullerite C<Subscript>60</Subscript>

The effect of oxygen and iodine on the optical and magnetic properties of fullerite C₆₀ is studied by luminescence and EPR spectroscopy within widely varied experimental conditions (temperature of the medium, oxygen or buffer gas pressure, concentration of iodine vapor). It is demonstrated that the efficiency of the singlet oxygen formation when a fullerene sample is irradiated by a neodymium laser at a wavelength of 532 nm and the amplitude of the EPR signal emitted from the unirradiated sample are strongly affected by the concentrations of both oxygen and iodine vapor sorbed by the fullerene sample, as well as by its surface temperature. The spin-spin and spin-lattice relaxation times of paramagnetic centers in fullerite samples studied in the presence of molecular oxygen are determined by the method of microwave radiation absorption saturation.     

SiC bonding in ( C 60 ) n Si m clusters

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C₆₀ molecules. We have studied films obtained by deposition of (C₆₀)_nSi_m clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C₆₀)_nSi_m were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C₆₀)_nSi _n ⁺ , (C₆₀)_nSi _{n - 1} ⁺ and (C₆₀)_nSi _{n - 2} ⁺ species. This observation is in favor of the arrangement of these complexes as polymers where the C₆₀ cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp³ lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C₆₀ molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. Received 15 November 2000     

Epitaxial growth of C<Subscript>60</Subscript> thin films on the Bi(0001)/Si(111) surface

The morphology and atomic structure of C₆₀ fullerene films on the Bi(0001)/Si(111)?7 × 7 surface with different coverages have been studied by scanning tunneling microscopy and spectroscopy and low-energy electron microscopy in high vacuum. It is shown that the most favorable sites for nucleation of C₆₀ islands are double steps and domain boundaries on the surface of epitaxial Bi film.     

Imaging C60 fullerite at 4.5 K by scanning tunneling microscopy

Intramolecular contrast of C₆₀ molecules has been observed by scanning tunneling microscopy at 4.5 K on a C₆₀ fullerite thin film sample. This result provides strong evidence for the freezing of the rotational motion of C₆₀ molecules at low temperature as recently proposed by refinement analysis of neutron diffraction data. Different intramolecular patterns are observed. Interpretation of these patterns is suggested by assigning them to carbon ring structures of the C₆₀ molecules in various orientations.