Hiroyuki Niwa

Missing small-bandgap metallofullerenes: their isolation and electronic properties.

In contrast to the extensively studied empty C60 and C70 fullerenes, the extraction of the corresponding endohedral metallofullerenes M@C60 and M@C70 (e.g., M = Sc, Y, La, Ce, or Gd) (Figure 1a) has been a long-standing challenge ever since the experimental discovery in 1985 and the first macroscopic production in 1991 of metallofullerenes. It has been well-known that M@C60 and M@C70 are insoluble in common fullerene solvents such as toluene and carbon disulfide, although the yields of M@C60 and M@C70 in raw soot are fairly high according to mass spectrometric analysis. Besides M@C60 and M@C70, there are a large number of other insoluble, the so-called small-bandgap (or small HOMO– LUMO gap) metallofullerenes such as M@C72 and M@C74. These metallofullerenes are highly reactive because of their open-shell electronic configurations or small bandgaps, and they tend to form insoluble polymerized solids in raw soot. The highly reactive small-bandgap metallofullerenes can be stabilized either by electrochemical reduction or chemical functionalization. Diener and Alford reported that insoluble polymerized Gd metallofullerenes can be reduced into soluble closed-shell anions by an electrochemical method. Exohedral derivatization is another way to stabilize smallbandgap fullerenes and metallofullerenes. For example, C74 can be extracted through exohedral fluorination or trifluoromethylation. Similarly, insoluble metallofullerenes La@C2n (2n = 72, 74, and 80) become soluble in organic solvents after functionalization with dichlorophenyl groups. However, the most interesting La@C60 and La@C70 fullerenes are still unavailable. Arc discharge is the most commonly used technique for the production of metallofullerenes, which is typically performed with a metal/graphite composite rod in a helium atmosphere. 7] It is convenient to introduce additional gaseous or solid reagents into the arc-discharge chamber to produce new types of fullerenes or metallofullerenes. For instance, trifluoromethyl derivatives of C60 were produced using polytetrafluoroethene (PTFE) as a source for functional groups during arc discharge. Here we demonstrate an arcdischarge method for producing derivatives of small-bandgap metallofullerenes using PTFE. As shown schematically in Figure 1b, PTFE is placed near the area where arc discharge occurs. Because of the high-temperature of the arc zone, PTFE is evaporated together with the metal/graphite rod during arc discharge. As a consequence, a series of trifluoromethyl derivatives of insoluble metallofullerenes M@C2n(CF3)m (e.g., 2n = 60, 70, 72, or 74) are formed effectively. Surprisingly, these derivatives, including those of M@C60 and M@C70, are totally soluble and stable in such organic solvents as toluene and carbon disulfide, which is important for further purification and characterization. This arc-discharge method can be applied for various metals. Similar results have been obtained in the present laboratory for metals such as yttrium, gadolinium, or dysproFigure 1. a) Molecular structural models for missing metallofullerenes M@C60 and M@C70. b) The arc-discharge apparatus. c) Production, extraction, and purification process for Y metallofullerenes functionalized with trifluoromethyl groups.

Large fullerenes in mass spectra

Fullerenes have been studied for nearly three decades and enormous advances have been made. Mass spectrometry is commonly used for investigations on the distribution of fullerenes formed from evaporated graphite targets, and soot produced from such targets. We report distributions of fullerenes formed by graphite evaporation by use of a pulsed supersonic cluster source and compare them to certain distributions synthesised by other techniques, such as arc discharge and combustion methods. We highlight the fact that physical processes can occur during the mass spectral analysis of fullerenes under certain conditions that may skew the observed distribution of cage sizes present in a sample. In some cases, an analysis of fullerene-containing soot can greatly exaggerate the relative abundance of large fullerenes compared to C60 and medium-sized fullerenes, depending on the particular experimental setup.

Measurements of Electroluminescence Intensity Distribution in the Direction of Gate Width of n^+ Self-Aligned Gate GaAs Metal-Semiconductor Field-Effect Transistors

We studied electroluminescence (EL) intensity distributions in the direction of gate width of n+ self-aligned gate GaAs metal-semiconductor field-effect transistors (MESFETs). Nonuniform EL was observed along the gate-width direction, suggesting the nonuniform high-field formation in the direction of the gate. It has also been found that the EL at the source side shows gentler distribution than that at the drain side. These features suggest that the nonuniform hole distribution generated by impact ionization at the drain side becomes gradual during the process of their flow into the source and gate electrodes.

Electroluminescence Measurement of n+ Self-Aligned Gate GaAs MESFETs

We studied the electroluminescence (EL) for n+ self-aligned gate GaAs metal-semiconductor field-effect transistors (MESFETs) at room temperature. It has been found that the spatial distribution of the EL intensity is dependent on the luminescence energy. The EL peak with bandgap energy is observed at a region between the source and the gate metals, while the EL with an energy higher than the band gap energy is observed on the drain-side edge of the gate. By studying the correlation between the integrated EL intensity and the drain/gate current, it is concluded that the electron-hole recombination is a dominant luminescence mechanism for both the high and low energy regions for the present n+ self-aligned gate GaAs MESFETs.

Crystalline functionalized endohedral C 60 metallofullerides

Endohedral metallofullerenes have been extensively studied since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, most of these studies have focused on metallofullerenes larger than C60 such as (metal)@C82, and there are no reported purified C60-based monomeric metallofullerenes, except for [Li@C60]+(SbCl6)− salt. Pure (metal)@C60 compounds have not been obtained because of their extremely high chemical reactivity. One route to their stabilization is through chemical functionalization. Here we report the isolation, structural determination and electromagnetic properties of functionalized crystalline C60-based metallofullerenes Gd@C60(CF3)5 and La@C60(CF3)5. Synchrotron X-ray single-crystal diffraction reveals that La and Gd atoms are indeed encapsulated in the Ih-C60 fullerene. The HOMO-LUMO gaps of Gd@C60 and La@C60 are significantly widened by an order of magnitude with addition of CF3 groups. Magnetic measurements show the presence of a weak antiferromagnetic coupling in Gd@C60(CF3)3 crystals at low temperatures.While endohedral metallofullerenes have demonstrated advantageous electronic and magnetic properties, the isolation of monomeric M@C60 remains highly challenging. Here, the authors prepare trifluoromethylated Gd@C60 and La@C60, where the functionalization of C60 allows for the stabilization of these otherwise reactive species.

Determining addition pathways and stable isomers for CF3 functionalization of endohedral Gd@C60

Using density functional theory approaches, we follow the sequential addition of CF3 functional groups to the surface of the metallic endofullerene species Gd@C60. The presence of gadolinium in the interior of the cage strongly influences the addition sequence. The calculations are able to successfully identify end points in the addition sequence at Gd@C60(CF3)n, n = 3 and two isomers at n = 5, in predictive agreement with experiment. Inverting the algorithm to determine the most labile groups also identifies the correct positively charged Gd@C60(CF3)4+ isomer, as confirmed by experimental mass spectra. The importance of surface mobility, notably at later stage addition, is discussed.

CF2-bridged C60 dimers and their optical transitions

Fullerene dyads bridged with perfluorinated linking groups have been synthesized through a modified arc-discharge procedure. The addition of Teflon inside an arc-discharge reactor leads to the formation of dyads, consisting of two C60 fullerenes bridged by CF2 groups. The incorporation of bridging groups containing electronegative atoms lead to different energy levels and to new features in the photoluminescence spectrum. A suppression of the singlet oxygen photosensitization indicated that the radiative decay from singlet-to-singlet state is favoured against the intersystem crossing singlet-to-triplet transition.