Sadanori Kuroshima

EXCITED-STATE PROPERTIES OF C60

Abstract The excited singlet-state and triplet-state properties of C 60 in toluene have been measured by picosecond and nanosecond laser-flash photolysis. S 1 has absorption maxima at 513, 759 and 885 nm with differential molar extinction coefficients (Δϵ S ) of 4500, 3700 and 6300 M −1 cm −1 , respectively. T 1 has absorption maxima at 457, 509 and 747 nm with Φ T Δϵ T =3400, 3000 and 15000 M −1 cm −1 , respectively. The lifetime of S 1 is 1.2 × 10 −9 s and that of T 1 is ⩾ 2.8 × 10 −4 s. T 1 also undergoes triplet—triplet annihilation with a rate constant ⩾ 4.8 × 10 9 M −1 s −1 .

Crystal growth of C60 thin films on layered substrates

The process of the C60 thin film crystal growth on layered materials such as MoS2 and mica is studied in detail with a combination of atomic force microscopy (AFM) and reflection high‐energy electron diffractometry, and is compared to that on alkali‐halide (NaCl) substrates. AFM shows that a single crystal containing triangle‐shaped grains of 1–2 μm size with a (111) surface terrace is grown on the MoS2 substrate over a large area through a layer‐by‐layer process, while the grains on mica are 200–400 nm in size and distribute with some disorder in the orientation. The epitaxial characteristics of the C60 crystal growth on these layered substrates are discussed in comparison with that on alkali‐halides.

Picosecond and nanosecond studies of the excited state properties of C70

Abstract The excited singlet and triplet state properties of C 70 in toluene have been measured by picosecond and nanosecond laser flash photolysis. Between 450 and 950 nm, S 1 has a maximum at 675 nm with a differential molar absorption coefficient of 3800 M −1 cm −1 while T 1 has a maximum at 579 nm where the product of the triplet quantum yield and the differential molar absorption coefficient is 1800 M −1 cm −1 . S 1 decays with a lifetime of 670 ps while the T 1 lifetime (> 200 μs) is shortened by diffusion limited triplet—triplet annihilation (≈ 8 × 10 9 M −1 cm −1 ).

Crystal growth and structure of fullerene thin films

Abstract The growth and structure of C60 and C70 thin film crystals on alkali halide and layered material substrates are described. It is demonstrated that fairly large grains can be grown with the specific orientation to the substrate lattice, especially on layered substrates. The defects in the films fabricated on alkali halides are studied by transmission electron microscopy in detail. The growth mechanism of C60 and C70 thin films on these substrates is discussed on a basis of reflection high-energy electron diffraction and atomic force microscopy observation and compared with that on other substrates. The unusual growth characteristics of C60 and C70 are discussed and compared with those encountered in thin films of conventional van der Waals crystals.

Structures of C60 thin films fabricated on alkali halide substrates by organic MBE

Abstract C 60 thin film crystals have been fabricated on the (001) surface of alkali halide substrates, KCl, KBr, and NaCl, and their structures have been studied. TEM observations show that the hexagonal close packing (hcp) with lattice parameters a = 10.0 A and c = 16.3 A and the face-centered cubic (fcc) with a = 14.2 A coexist in the C 60 thin film crystals. The ratio of hcp to fcc is very dependent on the kind of substrate and on the temperature during crystal growth. The coexistence of two phases is considered to be related to the difficulties in achieving alkali-doped M 3 C 60 thin films.

SUPERCONDUCTIVITY IN SILICON BASED BARIUM-INCLUSION CLATHRATES

Abstract Clathrates of type Ba 8 TM x Si 46− x (with TM = Au, Ag, Cu) have been investigated. We report on superconductivity in Ba-inclusion silicon based clathrates when doped with transition metals. It is clarified that the transition metals are necessary in order to form the clathrates as major phases, but they destroy the superconductivity. On lowering the amount of TM, an increase in superconducting fraction has been observed, with a maximum at x ≈1. After purification a superconducting fraction of up to 50% was obtained. The critical temperatures range from 5 to 6 K depending on both, x and the type of TM.

Highly efficient field emission from carbon nanotube-nanohorn hybrids prepared by chemical vapor deposition.

Electrically conductive carbon nanotubes (CNTs) with high aspect ratios emit electrons at low electric fields, thus applications to large-area field emission (FE) devices with CNT cathodes are attractive to save energy consumption. However, the poor dispersion and easy bundling properties of CNTs in solvents have hindered this progress. We have solved these problems by growing single-walled CNTs (SWNTs) on single-walled carbon nanohorn (SWNH) aggregates that have spherical forms with ca. 100-nm diameters. In the obtained SWNT-SWNH hybrids (NTNHs), the SWNTs diameters were 1-1.7 nm and the bundle diameters became almost uniform, that is, less than 10 nm, since the SWNTs were separated by SWNH aggregates. We also confirmed that a large-area FE device with NTNH cathodes made by screen printing was highly and homogeneously bright, suggesting the success of the hybrid strategy.

ESR studies of K-doped C60

Abstract Precise electron spin resonance (ESR) spectroscopic studies have been done for K x C 60 . Three ESR peaks corresponding to the three important crystal phases of K x C 60 (face-centered cubic K 3 C 60 , body-centered tetragonal K 4 C 60 , and body-centered cubic K 6 C 60 ) are reported. From the temperature dependencies of the ESR intensity for these three phases, it is confirmed that the superconducting K 3 C 60 phase is metallic, and that both of the non-superconducting K 4 C 60 and K 6 C 60 phases are semiconducting or insulating. The electronic properties of K x C 60 are discussed in comparison with those of Rb x C 60 and K x C 70 .

Site selective behavior of alkali metals in binary doped C60

Abstract The site selective behavior of binary doped C60 superconductors with compositions of A2Cs1C60 and A1Cs2C60 (A=Na, K, Rb) was observed by X-ray diffraction. All the A2Cs1C60s indicate the ordering between octahedral and tetrahedral sites, while the situations for the A1Cs2C60 compositions are different depending on the kind of metal A. It is shown that the relationship between the ionic radius of metal A and the size of tetrahedral site plays an important role in site selectivity.

Spiral growth of C60

We have observed the spiral growth of C60 grown on cleaved graphite using atomic force microscopy. Triangular terraces in the spirals consist of a monolayer of C60 having a 10 A step height which agrees well with (111) lattice constant of fcc‐C60. One of the characteristic features of the C60 spiral is the presence of a line defect running from the spiral core and crossing the spiral terraces. As a result, the original helicoid plane transforms into a stack of terraces. We think the C60 spiral growth is terminated by the introduction of a line defect at the early stages of the film growth, resulting from the weak van der Waals forces.