Doping and photoelectric properties of C60 films prepared by ionized cluster beam deposition

60 films by means of ionized cluster beam (ICB) deposition. X-ray diffraction (XRD) measurement showed the C₆₀ films to be polycrystalline. The films show negative resistance–temperature coefficients, and their room-temperature resistivity is greater than 10² Ω cm. The films were implanted with 80-keV phosphorus, BBr₃, Ar, and He ions, under doses ranging up to 10¹⁶ cm^-2. The resistivity of the implanted films decreases with increasing doses. n-type electrical conduction was observed for phosphorus-implanted C₆₀ films. The interaction of impinging ions with C₆₀ clusters was found to force the C₆₀ molecules to disintegrate and the films to amorphize. p-type conduction was observed for the C₆₀ films doped with aluminum by simultaneously sputtering aluminum during deposition. C₆₀/Si structures show heterojunction characteristics that can be influenced by light illumination. The photoelectric properties of the films were found to be improved by doping with aluminum. Received: 12 January 1998/Accepted: 24 March 1998     

Branching ratios of successive emission (up to three) of C2n + (n = 1–5) fragments in asymmetrical fission processes of C60 r+ ions (r = 4–6)

Highly charged C₆₀ molecules are produced in collisions between neutral C₆₀ and multiply charged ions within a large range of temperatures. Successive emission of one, two or three light monocharged fragments referred as one-, two- and three-step processes have been observed. The experimental mass branching ratios for the emission of one C_2n⁺ fragment from C₆₀⁶⁺, C₆₀⁵⁺ and C₆₀⁴⁺ ions are compared with the theoretical values using a statistical model. From hotter C₆₀⁶⁺ ions, branching ratios for three-step processes have been measured and the data are in good agreement with an estimation using the branching ratios in one-step process.     

Sm富勒烯的价带光电子能谱

用紫外光电子能谱和同步辐射光电子能谱研究了Sm掺杂C₆₀薄膜的价带电子结构.Sm的价电子大部分转移给C₆₀，化学键以离子性为主.对于任何化学配比都没有观察到费米边，所以Sm富勒烯超导相在室温下为半导体性质.获得了很接近单相Sm_2.75C₆₀的样品在费米能级附近的电子态密度分布.固溶相的光电子发射与Sm_2.75C₆₀有明显区别.Sm_xC_{60关键词： 60}的Sm填隙化合物')" href="#">C₆₀的Sm填隙化合物 价带光电子能谱 电子结构     

Elastic moduli of single-crystal C60

The matrix of elastic constants of the fcc phase of solid C₆₀ has been determined experimentally from measurements of the the velocity of 5 MHz ultrasound in single-crystal samples with different crystallographic orientations. The following values were obtained for the elastic moduli: C ₁₁=14.9±0.9 GPa, C ₁₂=8.8±1.0 GPa, and C ₄₄=6.6±0.18 GPa. The results are compared with theoretical estimates of the elastic moduli and data obtained in previous measurements of the elastic characteristics of solid C₆₀. Fiz. Tverd. Tela (St. Petersburg) 40, 173–175 (January 1998)     

Tunable KrF laser-induced fluorescence of C2 in a sooting flame

2 in a flame, excited by a tunable KrF laser near 248 nm. The first comprises several P and R lines of the (1,0) band of the e ³Π_g-a ³Π_u Fox–Herzberg system, with fluorescence bands extending past 350 nm. The second is the band head region of the (7,1) band of the D ¹Σ_u ⁺←B^′ 1Σ_g ⁺ system, with fluorescence at 232 nm from D to the X ¹Σ_g ⁺ ground state. Neither band has been previously observed in any environment. The flame in these experiments is highly sooting, and the C₂ seen here is likely produced by laser vaporization of the soot with subsequent laser photolysis of a C₂ precursor. In a rich flame, this fluorescence could cause interferences in other studies such as KrF laser Raman scattering. Moreover, signal level calculations suggest native C₂ near 10 ppm could be readily observed using the Fox–Herzberg excitation. Raman measurements of major species (X≥0.01) in the same flame, using the KrF laser, are in good agreement with a model prediction. Received: 2 April 1998/Revised version: 8 June 1998     

Gas-phase aggregation of fullerenes by laser ablation of fullerenes in reactive and nonreactive matrices

60 molecules, whereas little or no coalescence was found in the nonreactive mixture. In the negative ion TOF channel the maximum peak of twice C₆₀ coalescence by laser desorption was always located at masses greater than 120 carbon mass units, which is different from all previous reports. We suggest that the formation mechanism was due to strong interactions between the C₆₀ molecules and silica particles or surface species such as the H or OH group existing in the matrix. Received: 6 May 1996/Revised version: 22 July 1996     

First observation of optical radiation from fullerenes in the gas phase

Optical radiation of C₆₀ fullerenes in the gas phase was observed. The observed emission line λ=258 nm is identified with an electronic transition into the ground state h _g →h _u in C ₆₀. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 683–687 (10 May 1996)     

Spatial characteristics of Kα radiation from weakly relativistic laser plasmas

The spatial dependence of K _α emission generated from laser-produced hot electrons is investigated experimentally and theoretically. In addition, the conversion efficiency of K _α production as a function of laser intensity is measured and compared with modeling results. We use the terawatt Ti:sapphire laser at MPQ and vary the peak intensity from 10¹⁵ to 10¹⁸ W/cm² with a pulse duration of 200 fs. A solid Cu target is placed at various positions in the laser focus, which allows one to vary the intensity but keep the total energy on the target constant. When the target is near best focus, the FWHM of the K _α emission, measured using a knife-edge, is considerably larger than the FWHM of the laser intensity. In measuring the efficiency of K _α production using the fundamental wavelength of the laser, a clear maximum of K _α emission is observed at a position away from best focus, where the peak intensity is down by more than an order of magnitude from the value at best focus. When the second harmonic of the laser is used, the K _α emission is peaked near best focus. The K _α emission from layer targets is used to obtain an estimate of the temperature of the hot electrons. Modeling of K _α production, using a Monte Carlo electron/photon transport code, shows the relationship between incident electron energy and the emitted K _α emission. Efficient K _α generation from the low-intensity wings of the laser pulse contributes to the large spot size of the K _α emission. The lower electron temperatures that are expected for the second harmonic explain the differences in the location of maximum K _α emission for the two wavelengths. We discuss the use of K _α emission in photoionizing inner-shell electrons with the goal of achieving X-ray lasing at short wavelengths. Received: 6 April 1999 / Revised version: 31 May 1999 / Published online: 11 August 1999     

Spectroscopy study of air plasma induced by IR CO2 laser pulses

A spectroscopic study of ambient air plasma, initially at room temperature and pressures ranging from 32 to 101 kPa, produced by high-power transverse excitation atmospheric (TEA) CO₂ laser (λ=9.621 and 10.591 μm; τ _FWHM≈64 ns; power densities ranging from 0.29 to 6.31 GW cm⁻²) has been carried out in an attempt to clarify the processes involved in laser-induced breakdown (LIB) air plasma. The strong emission observed in the plasma region is mainly due to electronic relaxation of excited N, O and ionic fragments N⁺. The medium-weak emission is due to excited species O⁺, N²⁺, O²⁺, C, C⁺, C²⁺, H, Ar and molecular band systems of N ₂⁺(_{2}^{+}( B ²\varSigma _u⁺^{2}\varSigma _{\mathrm{u}}^{+} –X ²\varSigma _g⁺)^{2}\varSigma _{\mathrm{g}}^{+}) , N₂(C³ Π _u–B³ Π _g), N ₂⁺(_{2}^{+}( D² Π _g–A² Π _u) and OH(A² Σ ⁺–X² Π). Excitation temperatures of 23400±700 K and 26600±1400 K were estimated by means of N⁺ and O⁺ ionic lines, respectively. Electron number densities of the order of (0.5–2.4)×10¹⁷ cm⁻³ and (0.6–7.5)×10¹⁷ cm⁻³ were deduced from the Stark broadening of several ionic N⁺ and O⁺ lines, respectively. Estimates of vibrational and rotational temperatures of N ₂⁺_{2}^{+} electronically excited species are reported. The characteristics of the spectral emission intensities from different species have been investigated as functions of the air pressure and laser irradiance. Optical breakdown threshold intensities in air at 10.591 μm have been measured.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

Crystal-field induced mixing of electron states in C60 crystals at high pressure

Optical absorption spectra of thin fullerene (C₆₀) crystals in the range 1.7 to 3.8 eV have been measured at T=300 K and at pressures up to 2.5 GPa. The spectrum shifts toward the red with pressure, and the electron absorption intensity is redistributed among its bands. The intensity of the band associated with the lowest direct electron interband transition monotonically increases with pressure, whereas the intensity of the upper interband feature decreases. Bands related to weak edge absorption in the range between 1.7 and 2.2 eV gradually merge with the band associated with the lowest interband transition, whose intensity rises with pressure. A similar redistribution of intensity among electron transition bands has been observed when comparing the spectrum of an isolated C₆₀ molecule and that of a C₆₀ crystal. The results indicate that the crystal-field induced mixing of electron states is present in solid C₆₀, and they can be discussed in terms of the Craig-McClure model, which was suggested to describe crystal-field induced mixing of electron states in anthracene and naphthalene molecular crystals. Zh. éksp. Teor. Fiz. 113, 313–322 (January 1998)     

Structure and electrical behavior in air of TiO2-doped stabilized tetragonal zirconia ceramics

2 -doped YTZP ([%mol]3 Y₂O₃) compositions sintered in the temperature range of 1300 to 1450 °C, the tetragonal zirconia solid solutions field for the ZrO₂-Y₂O₃-TiO₂ system was established. The solubility of TiO₂ in YTZP was found to be about 12–[%mol]14 at 1450 °C. Structural characterization of the Ti-YTZP tetragonal zirconia solid solutions was carried out using X-ray absorption spectroscopy (EXAFS and XANES) to provide information on the environment of Ti atoms. The electrical behavior in air of the TiO₂-doped tetragonal zirconia solid solutions was studied by impedance spectroscopy in the temperature range of 300 to 800 °C, and it was found that the ionic conductivity decreases with increasing titania content. EXAFS and XANES results show that as the Ti⁴⁺ ions dissolve into the tetragonal zirconia YTZP matrix, a displacement of Ti ions from the center of symmetry takes place, leading to a non-random substitution of Ti⁴⁺ ions on Zr⁴⁺ lattice sites. Ti-O bond distances derived from EXAFS indicate that the Ti ion can be in a square-pyramidal arrangement, i.e., fivefold oxygen-coordinated. As a consequence two kinds of cation–oxygen vacancy associations are created; the high-mobility oxygen-vacancy–eightfold-coordinated cation (Zr⁴⁺) and the low-mobility oxygen-vacancy–fivefold-coordinated cation (Ti⁴⁺). This results in a decrease in the global concentration of moving oxygen vacancies and, therefore, in a decrease of the electrical conductivity. Received: 1 April 1998/Accepted: 28 September 1998     

Comparative Raman study of the Ti complex Cp2Ti(η2-C60) · C6H5CH3 and TixC60 films

Raman spectra from the first Ti fullerene complex Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ are presented. Compared to spectra of pure C₆₀, the spectra of the Ti complex exhibit a number of new peaks due to the symmetry lowering for C₆₀. The A _g(2) mode is downshifted by 12 cm⁻¹ compared to C₆₀, which corresponds to a charge transfer of one electron per Ti-C₆₀ bond. This value (6 cm⁻¹ for one transferred electron) is identical to the downshift of the A _g(2) mode in alkali metal fullerides with ionic bonding. The spectra of Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ were compared to the spectra of evaporated Ti_xC₆₀ films. The A _g(2) mode in Ti₄C₆₀ showed a downshift of about 25 cm⁻¹ compared to pure C₆₀, which corresponds to a charge transfer of one electron per Ti atom; this is similar to the ionic alkali metal fullerides and different from η²-C₆₀-type bonding. From Fizika Tverdogo Tela, Vol. 44, No. 3, 2002, pp. 483–485. Original English Text Copyright ? 2002 by Talyzin, Jansson, Usatov, Burlakov, Shur, Novikov. This article was submitted by the authors in English.     

C60 fullerite with a “stretched” fcc lattice

It is shown that deuteration of C₆₀ fullerite followed by thermal decomposition of the resulting deuteride C₆₀D₂₄ leads to the formation of an fcc lattice with a ₀=14.52 Å in the final product, which according to the IR spectra consists mainly of C₆₀ fullerene molecules. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 239–242 (10 August 1998)     

Phthalocyanine-C60 composites as improved photoreceptor materials?

2 -Pc, in contact with C₆₀ is studied using photoelectron spectroscopy with ultraviolet and X-ray radiation (UPS and XPS, respectively), X-ray absorption near edge spectroscopy (XANES), and optical transmission spectroscopy. A possible improvement of the charge-carrier generation efficiency, which is essential for the performance as photoreceptor material, is thereby found for these materials upon doping with C₆₀. No ground-state charge transfer is detected for the Pcs in contact with C₆₀. The effect of an enhanced photoconductivity is demonstrated for τ-H₂-Pc when it is doped by 5% C₆₀. Received: 16 March 1998/Accepted: 16 March 1998     

The universality of the electron spin relaxation in C60 mono radical anion salts: EPR studies of the model system [P(C6H5)4]2C60X

The C₆₀ radical anion salts [P(C₆H₅)₄]₂C₆₀X (X=Cl, Br, I) are grown via electrocrystallization and used as a model system to study the electron spin and nuclear spin dynamics as well as the molecular dynamics of C₆₀ mono anions in the solid state, which obey universal laws. It is shown that [P(C₆H₅)₄]₂C₆₀X is an exception among the fullerides, since the temperature dependence of the JT distortion, predicted for ionic C₆₀, can be deduced.     

Evidence for a solid phase of dodecahedral C 20

Evidence is presented for the formation of a solid phase based on the smallest fullerene, C₂₀, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10^-4 torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C₂₀, C₂₁ and C₂₂ species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C₂₀ cages as building blocks. Unlike solid C₆₀ and fully protonated C₂₀, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C₂₀ dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell. Received 10 October 2002 / Received in final form 24 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: zafar.iqbal@njit.edu     

Formation of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt and Ni)

Photofragmentation of metal fullerides C₆₀M_x (M=Sm, Pt and Ni) has been studied by excimer laser ablation–TOF mass spectrometry. Metallofullerenes of the type C_nM (n<60) have been observed in both the positive and negative ionic modes, with C₅₉M being the most prominent species. It is supposed that the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage, forming substitutional metallofullerene. The occurrence of the C₅₉M, C₅₈M, C₅₇M clusters in the mass spectra is confirmed by the coincidence of the intensity distribution of the mass peaks with the isotopic abundance pattern calculated from the natural abundance of isotopes of C and M. Odd-numbered high-carbon clusters are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The evolution of the mass spectra of these samples with laser irradiation shots indicates that the transformation process from an externally doped fullerene to the substitutionally doped fullerene involves the loss of metal carbide, MC. The structures of metallofullerenes C_2n+1M and C_2nM with even and odd total numbers of atoms respectively are discussed. Formation mechanisms with the participation of odd-numbered all-carbon fullerene clusters as intermediates are supposed. Received: 18 June 2001 / Accepted: 28 September 2001 / Published online: 2 May 2002     

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.     

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.