Carbon Cage Vibrations of M@C82 and M2@C2 n (M = La,Ce; 2n = 72, 78, 80): The Role of the Metal Atoms

Infrared spectra of La₂@C₈₀ and a series of Ce-based endohedral metallofullerenes (EMFs), including Ce@C₈₂, Ce₂@C₇₂, Ce₂@C₇₈, and Ce₂@C₈₀ are reported. DFT calculations are used for their thorough analysis and assignment. The vibrations of the fullerene cages in all studied EMFs differ from those of their empty, charged cage analogues. Furthermore, EMFs with the same carbon cage but different type of encapsulated species also show significant differences in their cage vibrational patterns. These phenomena are explained by a different coupling of the vibrational modes as well as by the different charge distributions in EMFs and empty, isostructural fullerene anions.     

Ultraviolet Photoelectron Spectroscopy of Two Titanium Metal Atoms Encapsulated Metallofullerenes,Ti2@C80 and Ti2@C84

Abstract

Ultraviolet photoelectron spectra (UPS) of Ti₂@C₈₀ (mixture of two isomers) and two Ti₂@C₈₄ isomers excited by synchrotron radiation light source are presented. The spectra of Ti₂@C₈₀ are complicated compared with those of other metallofullerenes, which is considered with an aid of molecular orbital (MO) calculation. The photoelectron spectra of two Ti₂@C₈₄ isomers are fairly analogous with each other and show less structure than that of Sc₂@C₈₄. This suggests that the symmetry of two isomers differs from D _2d symmetry of Sc₂@C₈₄.     

Using mass-reflectron in the study of fullerenes

A high-resolution mass-reflectron has been successfully used for the mass-spectrometric analysis of metallofullerenes in mixed fullerene extracts. The process of fullerene hydrogenation and the enrichment of fullerenes with the ¹³C isotope were studied by means of thermodesorption with electron-impact ionization. The mass spectra of Y @C₈₂ and Gd@C₈₂ metallofullerenes are presented.     

Dual Electron Acceptor/Electron Donor Character of Endohedral Nitride Clusterfullerenes

Since the discovery of endohedral metallofullerenes, much research effort has focused on their unique electronic and electrochemical features, which vary from those of C₆₀, C₇₀ and make them potential building blocks, especially in the fields of photosynthesis and photovaltaics. Particularly, taking advantage of the rich reductive and oxidative properties of nitride clusterfullerenes, it has been proved that these fascinating compounds could act as either electron acceptor or electron donor. In this mini review, we summarize the dual electron acceptor/electron donor character of endohedral nitride clusterfullerenes and also give our view on the future studies and the open issues in this field.     

Synthesis and Spectroscopic Studies of Endohedral Metallofullerenes Tb@C2N

Abstract

Endohedral metallofullerenes Tb@C_2n were synthesized and extracted with high-yield by K-H carbon-are evaporation and an effective pyridine extraction technique at high-temperature high-pressure. Laser-desorption-ionization time-of-flight (LD-TOF) mass spectrometry, X-ray photoelectron spectroscopy (XPS), solid-state fluorescent emission spectroscopy and gas phase derivation reaction with the self-chemical ionization mass spectrometric ion system of vinyl acetate were employed for studying the electronic structures, fluorescent properties and gas phase reactivities of metallofullerenes Tb@C_2n. The experimental results suggest that endohedral metallofullerenes Tb@C_2n would have the approximate structures of Tb³⁺@C_2n ³⁺ similar to other metallofullerenes, good fluorescent emission properties and active reactivities in gas phase ion-molecular reactions.     

Computations on Metallofullerenes Derivatized during Extraction: La@C80-C6H3Cl2 and La@C82-C6H3Cl2

Recently, an extraction of la metallofullerenes from soot using 1,2,4-trichlorobenzene has been reported for La@C₈₀ and La@C₈₂. In both cases, the cages were derivatized by the solvent (forming La@C₈₀-C₆H₃Cl₂ and La@C₈₂-C₆H₃Cl₂) and the following X-ray analysis disclosed rather unexpected cages: C₈₀(C _2v ;3) and C₈₂(C _3v ;7). In order to explain the challenging observations, a two-step computational treatment is presented. The first step deals with the high-temperature gas-phase formation of the underivatized endohedrals while the second step models the reaction with the solvent. The Gibbs free energies were evaluated for representative temperatures and the computational scheme was able to confirm high relative populations for the observed derivatized cages.     

ISOLATION AND SPECTROSCOPIC STUDY OF A SERIES OF MONOTERBIUM ENDOHEDRAL METALLOFULLERENES

ABSTRACT

A series of monoterbium endohedral metallofullerenes Tb@C₈₀, Tb@C₈₆, and the second isomer of Tb@C₈₂ (II) has been produced and isolated from various hollow fullerenes and endohedral metallofullerenes by an efficient multi-stage chromatographic separation process for the first time. All these newly isolated metallofullerenes are characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry and UV-Vis-NIR absorption spectroscopy. Their possible molecular symmetries are discussed according to their HPLC retention time and absorption spectra.     

Synthesis of Lanthanofullerenes by Laser Ablation Using Pulsed and CW-Nd:YAG Lasers

Abstract

Endohedral lanthanum fullerenes, La@C₆₀, La@C₈₂ and La₂C₈₀, were synthesized by ablation of graphite and La₂O₃ in flowing Ar buffer gas with a 1064 nm beam from a pulsed-Nd: YAG laser in which the graphite was heated by the CW-Nd:YAG laser up to 2500 °C. The La-fullerenes were also prepared by laser ablation of a lanthanum-graphite composite rod. The extractant fullerenes from the carbon soot were analyzed by laser-desorption time of flight mass spectrometry (LD-TOF MS). The relative yield of La@C₈₂, increased with increasing temperature of the rod in the range of 700-2300 °C.     

Quantitative analysis of endohedral metallofullerenes by Rutherford backscattering of protons

Samples containing endohedral metallofullerenes of gadolinium (Gd@C₈₂) and dysprosium (Dy@C₈₂) are investigated by the Rutherford backscattering of protons. It is shown that the concentration of endohedral metallofullerenes can be determined to within a few percent. The measured concentration of endohedral metallofullerenes Gd@C₈₂ and Dy@C₈₂ in samples prepared by selective multistep extraction is ∼60%. Pis’ma Zh. Tekh. Fiz. 25, 24–30 (May 26, 1999)     

Optical and magneto-optical properties of the endohedral complex La@C82 crystal: a Hartree–Fock single-excitation configuration interaction model study

The dielectric constant tensor is used to describe the magneto-optical properties of crystals. Using the Hartree–Fock single-excitation configuration interaction model in conjunction with the local field method, the spin-dependent optical dielectric tensor, refractive index, reflectivity, circular dichroism (CD), and birefringence coefficient (θ) of the simple cubic (sc) phase of the La@C₈₂ crystal are calculated when the La atom is off-center. The calculations show that the spin-dependent optical dielectric tensor of the La@C₈₂ crystal is large. The displacement of the La atom from the center of the cage leads to molecular electric dipole moment which influences the magnetic and optical properties of the La@C₈₂ crystal. The Coulomb interactions and effective magnetic moment cause increasing dielectric tensor, CD, and θ. Our results show that the optical excitation in the region where the energy is lower than 2?eV has the strongest optical absorption bands and refractive index in the La@C₈₂ crystal. Also, our results show that CD and θ of the La@C₈₂ crystal are larger than those of the C₆₀ and C₇₀ crystals. We obtained the E_g of 0.27?eV similar to that of the obtained results of the La@C₈₂ thin film with a semiconducting behavior.     

Effect of Water and Solvent Selection on the SAFA Purification Times for Metallic Nitride Fullerenes

This paper addresses two key parameters in the SAFA purification method for isolating endohedral metallofullerenes. The percent water content in the reactive aminosilica significantly affects the time needed to isolate Sc₃N@I_h -C₈₀ metallic nitride fullerene (MNF). The role of solvent selection in the SAFA process is also discussed. The SAFA purification time needed for isolating Sc₃N@I_h -C₈₀ metallofullerene is influenced by the identity of the electron donating/accepting group on the aromatic solvent.     

Analysis of metallofullerenes using a mass reflectron

A high-resolution mass reflectron has been used to analyze an extract of a fullerene mixture containing the metallofullerene Gd@C₈₂. It is shown that various metallofullerenes can be analyzed by means of thermal desorption followed by electron impact ionization. The C₆₀, C₈₂, and Gd@C₈₂ ion currents are obtained as functions of the evaporation temperature. Doubly charged (Gd@C₈₂)²⁺ metallofullerene ions are identified in the mass spectra. Pis’ma Zh. Tekh. Fiz. 24, 1–5 (March 12, 1998)     

Charge-transfer interaction between C60 fullerene and alkylnaphthalenes

A series of alkylnaphthalenes, namely 1,4-dimethylnaphthalene, 2,6-diethylnaphthalene, 2-ethylnaphthalene and pure naphthalene are not able to form Diels-Alder adducts with C₆₀ fullerene but produce a series of 1:1 charge-transfer complexes (CTC) where the aromatic compounds act as donor and C₆₀ as acceptor. The spectrophotometric analysis of these CTC has permitted to determine the equilibrium constants of the CTC formation at four different temperatures and the relative enthalpies and entropies of formation. The C₆₀-alkylnaphthalenes and C₆₀-naphthalene were identified as weakly bound CTC. Using the Mulliken theory of the CTC also the degree of charge transfer α was determined, confirming the results already suggested by the equilibrium constants, i.e., the weakly interaction between the donor and the acceptor considered.     

Growth characteristics and electrical properties of La2O3 gate oxides grown by thermal and plasma-enhanced atomic layer deposition

We comparatively investigated thermal and plasma-enhanced atomic layer deposition (T-ALD and PE-ALD, respectively) of lanthanium oxide (La₂O₃) films using tris(isopropyl-cyclopentadienyl)lanthanum [La(iPrCp)₃] as a La precursor. H₂O and O₂ plasma were used as reactants for T-ALD and PE-ALD La₂O₃, respectively. Both of the processes exhibited ALD mode growth with good self-saturation behavior and produced pure La₂O₃ films. However, PE-ALD La₂O₃ showed higher growth rate and dielectric constant value than those of T-ALD La₂O₃. In addition, lower leakage current density and interface state density were observed for PE-ALD La₂O₃, compared to those of the T-ALD La₂O₃. These experimental results indicate that the PE-ALD La₂O₃ process using La(iPrCp)₃ precursor can be one of the viable options applicable into future microelectronic industry.     

Spin‐Transparency of the Fullerene Shell of Endometallofullerene La@C82

Abstract

For La@C₈₂ embedded into the polycarbonate film, an intensive ¹H‐electron‐nuclear double resonance (ENDOR) has been revealed. Since La‐EMF does not contain hydrogen atoms, it evidences for the electron spin density on the matrix protons. Furthermore, for the solutions of La‐EMF in hexamethylphosphoramide (HMPA), the paramagnetic shift of the ³¹P NMR signal of bulk solvent molecules has been revealed. These findings testify the partial localization of unpaired electron outside the fullerene cage. The “transparency” of the fullerene shell to the electron spin density may serve the important factor in designing the fullerene‐based materials.     

Aggregation of Endometallofullerene Y@C82 in Polar Solvents

Abstract

Having studied the optical absorption, fluorescence spectra and dynamic light scattering (DLS) of solutions of Y‐EMF in polar solvents, we demonstrated for the first time that the endohedral metallofullerenes (EMF) forms nanoparticles, the mean size of which ranges up to 100 nm.     

Kinetic stability of nitrogen cubane inside the fullerene cage: Molecular dynamics study

Abstract

Using tight-binding molecular dynamics simulations, we study kinetic stability of the nitrogen cubane inside the fullerene carbon cage. The main identified mechanism of N₈ decomposition is the N–N bond breaking with the further rapid “splitting” into the N₂ molecules that can form the strong covalent bonds with the inner surface of the fullerene cage. It is found that the fullerene cage significantly increases the stability of the N₈ cube, but its lifetime at room temperature is insufficient to observe the endohedral complex N₈@C₆₀ without the use of extreme temperature conditions. It can be synthesized and investigated only at cryogenic temperatures.     

Iron promotion of the TiO2 photosensitization process towards the photocatalytic oxidation of azo dyes under solar-simulated light irradiation

The photocatalytic oxidation of the azo dye Orange-II (Or-II) using Fe loaded TiO₂ (Fe–TiO₂) was studied under ultraviolet (UV), visible (vis) and simultaneous UV–vis irradiations using a solar light simulator. Photocatalysts were characterized by means of XRD, SEM-EDX, FTIR and DRS. Fe³⁺ species, identified in XPS analyses, were responsible of the increased absorption of visible light. Moreover, DRS analyses showed a decrease in the bandgap due to Fe³⁺ loading. Photocatalystic tests proved that Fe modification enhanced the TiO₂ photocatalytic activity towards Or-II photodegradation under simultaneous UV–vis irradiation. Even so, the performance of the Fe–TiO₂ samples towards the photodegradation of phenol, under UV irradiation, was lower than TiO₂ suggesting the recombination of the UV photogenerated electron–hole pair. Therefore, results evidence a Fe³⁺ promotion of the electron caption in the photosensitization process of TiO₂ by Or-II acting as a sensitizer. Such process leads to the Or-II photooxidation under UV–vis irradiation by losing energy in electron transferring processes to sensitize TiO₂, and, the formation of reactive oxygen species promoted by the injected electron to the TiO₂ conduction band.     

Photoluminescence spectra of MnIn2S4

MnIn₂S₄ single crystals grown by the directional crystallization method were investigated by using the temperature and excitation power dependencies of photoluminescence (PL) spectra. PL spectra consist of one broad band resulting from donor-acceptor pair recombination. The analysis of the temperature quenching of the PL intensity yields one defect donor level with a thermal ionization energy of about 0.17 eV. The broad band of PL spectra indicates that radiative recombination is related to multiphonon optical processes. The energy of the involved phonon was found to be around 0.025 eV and the energy of the acceptor level is about 0.86 eV.     

Photosensitization of nanoporous TiO2 film with porphyrin-linked fullerene

A porphyrin-linked fullerene derivative, which included COOH groups in its fullerene unit, Por-C₆₀(COOH) was synthesized in order to investigate the possibility of promoting the photosensitization of TiO₂. Por-C₆₀(COOH) molecules produced higher photocurrent per molecule than tetrakis(4-carboxyphenyl)-porphyrin molecules. A dye-sensitized solar cell using Por-C₆₀(COOH), in which a C₆₀ unit was introduced between the porphyrin unit and TiO₂, yielded an improvement in energy conversion for light collection.