Fullerene Shell Research Articles

Photoionization of two-shell endohedral atoms

The photoionization of a two-shell endohedral A@CN1@CN2 is considered. Formulas are presented for cross-sections and angular anisotropy parameters, both dipole and non-dipole. The effect of the fullerenes shell upon photoelectron from atom A is taken into account substituting the action of the fullerene by two zero-thickness "bubble potential". The fullerenes shells polarization is included assuming that the radius of the outer shell R2 is much bigger than the inner R1 and both much exceeds the atomic radius r. This permits to express the effect via CN1,N2 polarizabilities, which are connected to their photoionization cross-sections. The interaction between shells CN1 and CN2 is taken into account in the Random Phase Approximation (RPA). The effect of photoelectron scattering by both "bubble potentials" is included in the lowest order and in the RPA frame. As concrete examples, two endohedrals Ar@C60@C240 and Xe@C60@C240 are considered. In the first case we consider 3p and 3s, while in the second 5p, 5s and 4d subshells. A whole variety of new peculiarities are found that deserve experimental verification.

Distortion and preservation of giant resonances in endohedral atoms A@C60

It is demonstrated in this Letter that the effect of the fullerene shell upon atomic Giant resonance decisively depends upon energy of photoelectrons, by which the resonance decay. According to the prediction in [1], the Giant resonance in Xe is strongly modified in the endohedral Xe@C60 being transformed from a single broad and powerful maximum in Xe into four quite narrow but with almost the same total oscillator strength. On the contrary, the 4d Giant resonances in ions Ce3+ (the electronic structure that Ce has, when stuffed into fullerene), in Ce4+, and Eu are considered. In none of them the 4d Giant resonance in endohedrals is affected essentially. This is because the decay of the Giant resonances in these endohedrals proceeds by emission of fast photoelectrons that are almost unaffected by the C60 shell. The results obtained give at least qualitative explanation to the fact that recent observation of 4d Giant resonance in Ce@C82+, where the Giant resonance was observed as a maximum without noticeable structure.

Photoionization of the subvalent subshells of noble gas endohedrals: interference of three resonances

We demonstrate strong interference patterns in the photoionization cross-section of the subvalent subshells of noble gas (NG) endohedral atoms NG@F. This interference is a result of common action of three factors: the effect of neighbouring atomic subshells, reflection of photoelectron waves by the fullerene F shell and resonance modification of the incoming photon beam by the complex effect under the action of the F electrons. We have considered the outer ns-subshells for Ne, Ar, Kr and Xe noble gas atoms. The polarization of the fullerene shell is expressed via the F total photoabsorption cross-section. The photoelectron reflection from the static F potential is taken into account in the frame of the so-called bubble potential which is a spherical δ-type potential. It is assumed that the NG atom is centrally located in the fullerene. It is also assumed in accordance with the available experimental data that the fullerene radius is much bigger than the atomic radius and the thickness of the fullerene shell. These assumptions permit the NG@F photoionization cross-section to be presented as a product of the NG atomic cross-section and two calculated factors that account for polarization of the F electron shell and reflection of photoelectrons by the fullerene static potential.

Photoionization of the outer electrons in noble gas endohedral atoms

We suggest a prominent modification of the outer shell photoionization cross section in noble gas (NG) endohedral atoms NG@C n under the action of the electron shell of fullerene C n . This shell leads to two important effects: a strong enhancement of the cross section due to fullerene shell polarization under the action of the incoming electromagnetic wave and to prominent oscillation of this cross section due to the reflection of a photoelectron from the NG by the fullerene shell. Both factors lead to powerful maxima in the outer shell ionization cross sections of NG@C n , which we call giant endohedral resonances. The oscillator strength reaches a very large value in the atomic scale, 25. We consider atoms of all noble gases except He. The polarization of the fullerene shell is expressed in terms of the total photoabsorption cross section of the fullerene. The photoelectron reflection is taken into account in the framework of the so-called bubble potential, which is a spherical δ-type potential. It is assumed in the derivations that the NG is centrally located in the fullerene. It is also assumed, in accordance with the existing experimental data, that the fullerene radius R C is much larger than the atomic radius r A and the thickness ΔC of the fullerene shell. As was demonstrated recently, these assumptions allow us to represent the NG@C n photoionization cross section as a product of the NG cross section and two well-defined calculated factors.

Giant resonances of endohedral atoms

It is demonstrated for the first time that the effect of a fullerene shell on the photoionization of a “caged” atom in an endohedral can result in the formation of giant endohedral resonances or GER. This is illustrated by the concrete case of the Xe@C60 photoionization cross section that, at 17 eV, exhibits a powerful resonance with total oscillator strengths of about 25. The prominent modification of the 5p6 electron photoionization cross section of Xe@C60 takes place due to the strong fullerene shell polarization under the action of the incoming electromagnetic wave and the oscillation of this cross section due to the reflection of the photoelectron from Xe by the C60. These two factors transform the smoothly decreasing 5p6 cross section of Xe into a rather complex curve with a powerful maximum for Xe@C60, with the oscillator strength of it being equal to 25. We also present the results for the dipole angular anisotropy parameter that is strongly affected by the reflection of the photoelectron waves, but not modified by C60 polarization.

Photoionization of 3d electrons of Xe, Cs and Ba endohedral atoms: comparative analyses

Abstract We demonstrate rather interesting manifestations of co-existence of resonance features in characteristics of the photoionization of 3d-electrons in Xe, Cs and Ba endohedral atoms. It is shown that for all of the considered atoms the reflection by the fullerene shell of photoelectrons produced by the 3d subshell photoionization affects greatly partial photoionization cross-sections of 3d 5/2 and 3d 3/2 levels and respective angular anisotropy parameters, both dipole and non-dipole adding to all of them additional maximums and minimums. The results obtained demonstrate distinctive differences between the three atoms. The calculations are performed treating the 3/2 and 5/2 electrons as electrons of different kinds with their spins “up” and “down”. The effect of the C60 shell is accounted for in the frame of the “orange” skin potential model. It is essential that in the considered photon frequency region the presented resonance features are not affected by the C60polarization.

Electronic structure and conductance of fullerene C20

The electronic structure and electronic conductance of C20 molecule bridged between two Au electrodes are studied by the first principles based on the density functional theory and the nonequilibrium Greens function. The three transmission systems based on C20 molecules are modeled，and the transmission spectra and the molecular orbital distributions are obtained. The reasons why the electronic structure and the electronic transmission are formed are discussed. The results show that the electrons are transported through the C20 bridge on the molecular shell. When one K or one Si atom is inlaid inside C20，the conductivities indicate that the electrons can transfer partly inside the C20 molecule，but most electrons ave transported on the C20 fullerene shell.

Real Time Microscopy, Kinetics, and Mechanism of Giant Fullerene Evaporation

We report in situ high-resolution transmission electron microscopy observing the shrinkage of single-layer giant fullerenes (GF). At temperatures approximately 2000 degrees C, the GF volume reduces by greater than one 100-fold while the fullerene shell remains intact, evolving from a slightly polygonized to a nearly spherical shape with a smaller diameter. The number of carbon atoms in the GF decreases linearly with time until the small subbuckyball cage opens and rapidly disappears. Theoretical modeling indicates that carbon atoms are removed predominantly from the weakest binding energy sites, i.e., the pentagons, leading to the constant evaporation rate. The fullerene cage integrity is attributed to the collective behavior of interacting defects. These results constitute the first experimental evidence for the "shrink-wrapping" and "hot-giant" fullerene formation mechanisms.

Photoionization and vacancy decay of endohedral atoms

We demonstrate the role played by the fullerenes shell in the photoionization and vacancy decay of endohedral atoms A@C 60. It is shown, partly in the frame of a rather simple model that describes the fullerene shell, partly using only the assumption on the smallness of the atom A in comparison to the size of C 60 that it affects photoionization and the vacancy decay of A@C 60 profoundly. Namely, it leads to a number of new resonances in photoionization cross-section and other photoionization characteristics as well as strong modifications of the vacancy decay probabilities and to opening of new decay channels. We will discuss the problem of photon propagation through the C 60 shell and conclude that at any considered frequency ω, 0 ≤ ω ≤ 60 eV the C 60 enhances the incoming radiation. This shows non-metallic dielectric behavior of the 240 collectivized electrons in C 60. We also discuss the effects of C 60 upon the fast electron inelastic scattering. The results obtained are valid qualitatively also for other than C 60 molecules, e.g. for C 70 or C 76.

Nanoparticles of Endohedral Metallofullerenes

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On the possibility of considering the fullerene shell C60 as a conducting sphere

The dynamical and static dipole polarizabilities of the C60 molecule have been calculated on the basis of the experimental data on the cross section of the fullerene photoabsorption. It has been shown that the fullerene shell in the static electric field behaves most likely as a set of separate carbon atoms rather than as a conducting sphere.

Vacancy decay in endohedral atoms

It is demonstrated that the fullerene shell affects dramatically the radiative and Auger vacancy decay of an endohedral atom A@C60. The collectivized electrons of the C60 shell add new possibilities for radiative and non-radiative decays similar to that in ordinary atoms where initial and final state vacancies almost always belong to different subshells. It is shown that the smallness of the atomic shell radii as compared to that of the fullerenes shell provides an opportunity to derive the simple formulas for the probabilities of the electron transitions. It is shown that the radiative and Auger (or Koster-Kronig) widths of a vacancy decay due to electron transition in the atom A in A@C60 acquire an additional factors that can be expressed via the polarizability of the C60 at transition energy. It is demonstrated that due to opening of the non-radiative decay channel for vacancies in subvalent subshells the decay probability increases by five -- six orders of magnitude.

Nanoparticles of endohedral metallofullerenes

This presentation is a mini-review of the recent investigations of our group aimed at understanding the fundamental properties of paramagnetic endohedral metallofullerenes, M@C 2 n . Despite being essentially reactive free radicals, these compounds feature the excellent stability against oxygen on embedding in the polycarbonate films. ENDOR and NMR data testify that partial runoff of the electron spin density outside the fullerene cage takes place. This spin-leakage of the fullerene shell leads to loss of molecular paramagnetism inside nanoparticles of M@C 2 n . Having studied solid powder complexes of La@C 82 and Y@C 82 with HMPA by magic angle spinning (MAS) NMR, we discovered the enormously large NMR shifts of 139La (for La@C 82) and the vast spreading of the shifts of 31P (for La@C 82 and Y@C 82) which may be classified as the Knight shifts caused by the density of conduction electrons at the positions of the NMR observed nuclei in nanoparticles of the M@C 2 n . The clustering of M@C 2 n into nanoparticles and the spin-leakage of the fullerene shell are worth consideration in view of designing novel composite magnetic materials, as well as novel pharmaceuticals for magnetic resonance imaging.

Multiple ionization of the fullerene by a single photon

We consider the single and multiple photoionization processes in a fullerene molecule. The quantum states of C60 are calculated within the Hartree–Fock and the frozen-core approximations. For the description of multiple photoionization probabilities, statistical energy deposition (SED) model was improved and used in addition. The advantage of the SED model is that it is simple enough to be applied to polyatomic systems, where more elaborate calculations are hardly feasible. We account for the electron–electron interaction responsible for the multiple ionization within the random phase approximation with exchange that describes the dynamical polarization of the electronic cloud of the target. As a result, different multi-electron ionization cross-sections of C60 in the photon energy range ∼10–250eV are compared, analyzed, and their orders of magnitude are estimated. We also discuss the diffraction of the photoelectron wave on the fullerene shell which results in oscillations in the photoionization cross-section as a function of the photon energy.

Alkali atoms confined to a sphere and to a fullerene cage

A simple asymptotically correct model potential is considered for the valence electron in an alkali atom. Superposed with the confining infinite barrier potential, this potential yields the energies of various states and the dipole polarizability of the ground state of the confined alkali atom. Superposed with the fullerene shell potential, this potential yields the energies of the alkali atom inside a fullerene cage. Numerical results are presented for Li, Na, K, Rb, and Cs under such confinements. PACS Nos.: 03.65.Ge, 73.21.La, 78.67.Hc

Structure and electrical properties of heat-treated fullerene nanowhiskers as potential energy device materials

Fine carbon tubes “fullerene shell tubes (FSTs)” have been found among the C 60 nanowhiskers heat-treated under vacuum at a temperature higher than 600 °C. FSTs with open ends have been found as well as FSTs with closed spherical ends. It has been also found that the FSTs can be easily prepared by the heat treatment of the C 60 whiskers prepared by using C 60 powders containing ( η 2-C 60)Pt(PPh 3) 2. Since the high temperature-treated fullerene (nano)whiskers exhibit a high specific surface area and a good electrical conductivity, they are expected to be utilized as new electrode materials for fuel cells.

Dynamical screening of a confined atom by a fullerene

In this paper, we elucidate the role of dynamical screening in the photoionization or photoabsorption process of an atom, which is either confined within a fullerene shell or is adsorbed onto its surface. This effect is of a general nature. Thus, it will also appear for impurity atoms or molecules embedded into or attached onto a metallic cluster. We present a formalism which takes dynamical screening into account. A dynamical screening factor is defined, and is shown to lead to an enhancement of the amplitude of the electromagnetic wave within a specific range of frequencies. This range is determined by the characteristics of the fullerene plasmon (or, more generally, by the plasmon of the confining system).

Electrochemical Synthesis and Transformations of Substituted Methano[60]fullerenes

61‐(Dimethoxyphosphoryl)‐61‐(methoxycarbonyl)methano[60]fullerene was produced by using the joint electrochemical reduction of fullerene C60 and (dimethoxyphosphoryl)methoxycarbonyldibromomethane. A few steps of reduction were detected for phosphorylated methano[60]fullerenes. When two electrons per molecule are transferred, the methano fragment is rapidly eliminated (retro‐Bingel reaction). This process involves the step‐by‐step cleavage of two C–C bonds of exo‐carbon with the fullerene shell in combination with the stepwise transfer of other two electrons and a proton to form finally the carbanion of the methano fragment and fullerene dianion.

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

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.

Preservation of endometallofullerene La@C 82 in polycarbonate: EPR, ENDOR, and NMR studies

On embedding in poly(bisphenol A carbonate) film, La@C 82 has featured the exclusive stability against oxygen for more than 1 year's storage of the polymer films on air. The EPR spectral parameters of La@C 82 have occurred to be sensitive to the segmental mobility and the phase transition of the polymer from the glassy to the high-elasticity state. Furthermore, the intense matrix 1H-electron nuclear double resonance (ENDOR) signal discovered in this system testifies the partial transfer of the electron spin density outside the fullerene cage onto the polymer protons. The 139La NMR spectrum of the same suggests that La@C 82 forms nanoparticles, in which the metal atoms strongly interact with the delocalized electrons. The “spin-leakage” of the fullerene shell, along with the tendency of endohedral fullerenes (EMF) to form nanoparticles, is worth consideration in sight of their applications in material sciences.