Intense terahertz pulse induced exciton generation in carbon nanotubes.

We have investigated the highly nonlinear terahertz (THz) light-matter interaction in single-walled carbon nanotubes (SWNTs). The high-peak THz electric-field (∼0.7 MV/cm) and the low effective mass of carriers result in their ponderomotive energy exceeding the bandgap energy of semiconducting SWNTs. Under such an intense THz pulse irradiation, the interband excitation that results in the generation of excitons occurs, although the THz photon energy (∼4 meV) is much smaller than the gap energy of SWNTs (∼1 eV). The ultrafast dynamics of this exciton generation process is investigated by THz pump and optical probe spectroscopy. The exciton generation mechanism is described by impact excitation process induced by the strong THz E-field. Such intense THz pulse excitation provides a powerful tool to study nonlinear terahertz optics in non-perturbative regime as well as nonlinear transport phenomena in solids with ultrafast temporal resolution.

Optimization and evaluation of networked single-wall carbon nanotubes as a NO(2) gas sensing material.

We have prepared conductometric NO(2) gas sensors based on single-wall carbon nanotube (SWNT) networks. The SWNT properties are modified systematically by varying the annealing temperature between 350 to 550 degrees C under vacuum. Thermal annealing is not only necessary to remove dispersant used for nanotube dispersion but also plays an important role in optimizing the gas sensing abilities. In this paper, the sensing performance is evaluated through three crucial sensing characteristics: sensitivity to NO(2), humidity interfering effect, and sensor stability over repeated use, all examined at room temperature. The sensor annealed at 400 degrees C shows the highest NO(2) sensitivity because of the structural properties, i.e., high specific surface area and the molecular geometry of having all carbon atoms at the tube-surface. The sensor also shows negligible humidity interfering effect and high sensor stability, originating from the hydrophobicity and chemical stability of the material, respectively. In contrast, annealing temperatures higher than 400 degrees C lead to structural defects in SWNTs and thus lower the sensing performance. We experimentally confirm that these SWNT characteristics make SWNTs a suitable gas sensing material from a practical perspective.

Crystal plane dependent growth of aligned single-walled carbon nanotubes on sapphire.

On single-crystal substrates, such as sapphire (alpha-Al 2O 3) and quartz (SiO 2), single-walled carbon nanotubes (SWNTs) align along specific crystallographic axes of the crystal, indicating that the SWNT growth is influenced by the crystal surface. Here, we show that not only the orientation, but also the diameter and chirality of SWNTs are affected by the crystal plane of the sapphire substrate. The aligned SWNTs grown on the A- and R-planes of sapphire have narrower diameter distributions than randomly oriented tubes produced on the C-plane sapphire and amorphous SiO 2. Photoluminescence measurements reveal a striking difference between the aligned SWNTs: near-zigzag tubes are observed on the A-plane and near-armchair tubes on the R-plane. This study shows the route for the diameter and chirality control of SWNTs by surface atomic arrangements of a single-crystal substrate.

[Photocurrent generators derived from non-covalently assembled soft-matter molecular system].

It is rather difficult to design a multilayer photocurrent generator system on the ITO electrode, however, the preparation of thin film with high surface concentration of donor units is indispensable in order to achieve high conversion efficiency. The polymer film of porphyrin bearing pyroles on the electrode was prepared by the potential sweep method. It was indicated that the self-aggregation can be suppressed by encapsulation of the porphyrin unit in the cavity of macro-cyclic host molecule, cyclodextrin. We established the non-equilibrium host-guest system with porphyrins and cyclodextrins for the first time. The photocurrent density and the quantum yield in the porphyrin-cyclodextrin system are remarkably improved. It was demonstrated that the high quantum yield, perhaps 25 times larger, arises from the isolation of the porphyrin unit by cyclodextrin through host-guest interactions.

Photosensitive function of encapsulated dye in carbon nanotubes.

Single-wall carbon nanotubes (SWCNTs) exhibit resonant absorption localized in specific spectral regions. To expand the light spectrum that can be utilized by SWCNTs, we have encapsulated squarylium dye into SWCNTs and clarified its microscopic structure and photosensitizing function. X-ray diffraction and polarization-resolved optical absorption measurements revealed that the encapsulated dye molecules are located at an off center position inside the tubes and aligned to the nanotube axis. Efficient energy transfer from the encapsulated dye to SWCNTs was clearly observed in the photoluminescence spectra. Enhancement of transient absorption saturation in the S1 state of the semiconducting SWCNTs was detected after the photoexcitation of the encapsulated dye, which indicates that ultrafast (<190 fs) energy transfer occurred from the dye to the SWCNTs.

IR-extended photoluminescence mapping of single-wall and double-wall carbon nanotubes.

A simple and efficient technique is described for measuring photoluminescence (PL) maps of carbon nanotubes (NTs) in the extended IR range (1-2.3 mum). It consists of preparing an NT/surfactant/gelatin film and measuring PL spectra using a combination of a tunable Ti-sapphire laser excitation and FTIR detection. This procedure has been applied to a wide range of single- and double-wall NTs unveiling chirality and diameter distributions that have so far been very difficult to measure. The problems associated with deducing these distributions are discussed by comparing absorption and PL mapping data for NT samples prepared under different conditions.

Dispersion and separation of small-diameter single-walled carbon nanotubes.

The dispersion of small-diameter single-walled carbon nanotubes (SWNTs) produced by the CoMoCAT method in tetrahydrofuran (THF) with the use of amine was studied. The absorption, photoluminescence, and Raman spectroscopies showed that the dispersion and centrifugation process leads to an effective separation of metallic SWNTs from semiconducting SWNTs. Since this method is simple and convenient, it is highly applicable to an industrial utilization for widespread applications of SWNTs.

Large-scale separation of metallic and semiconducting single-walled carbon nanotubes.

In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way.