Nobutsugu Minami

A Novel Preparation Method of Organic Microcrystals

Organic microcrystals ranging from several tens nm to µm in size of several chromophores were successfully prepared by simply dispersing ethanol solutions of compounds into stirred water, i.e. by a reprecipitation method. The size of microcrystals was found to depend on concentration of ethanol solutions, dispersing conditions, temperature and so on.

Ultrashort pulse-generation by saturable absorber mirrors based on polymer-embedded carbon nanotubes.

We demonstrate passive mode locking of solid-state lasers by saturable absorbers based on carbon nanotubes (CNT). These novel absorbers are fabricated by spin-coating a polymer doped with CNTs onto commercial dielectric laser-mirrors. We obtain broadband artificial saturable absorber mirrors with ultrafast recovery times without the use of epitaxial growth techniques and the well-established spin-coating process allows the fabrication of devices based on a large variety of substrate materials. First results on passive mode locking of Nd:glass and Er/Yb:glass lasers are discussed. In the case of Er/Yb:glass we report the to our knowledge shortest pulse generated in a self-starting configuration based on Er/Yb:bulk-glass: 68 fs (45 fs Fourier-limit) at 1570 nm wavelength at a pulse-repetition rate of 85 MHz.

Langmuir–Blodgett Films of Single-Wall Carbon Nanotubes: Layer-by-layer Deposition and In-plane Orientation of Tubes

The Langmuir–Blodgett technique has been applied to build optically homogeneous thin films of chemically solubilized single-wall carbon nanotubes (s-SWNTs) which possess good surface spreading properties at the air/water interface. Deposition can be performed in a layer-by-layer fashion up to 100 or more layers either by horizontal lifting or vertical dipping, allowing to readily control the film thickness. Their visible to near-infrared absorption spectra showing the characteristic features of semiconducting and metallic SWNTs prove the intactness of their one-dimensional electronic states during the preparation process. Polarized absorption spectroscopy and atomic force microscope (AFM) observation demonstrate that the tubes are oriented in the direction of the trough barrier (horizontal lifting) or in the dipping direction (vertical dipping). These are attributed to compression-induced or flow-induced orientation, respectively, the latter found to be much stronger than the former. The realization of homogeneous thin films of SWNTs with a controllable thickness and tube orientation should be an important basis for the future development of their scientific understanding and technological applications.

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.

Semiconductor-enriched single wall carbon nanotube networks applied to field effect transistors

Substantial progress on field effect transistors (FETs) consisting of semiconducting single wall carbon nanotubes (s-SWNTs) without detectable traces of metallic nanotubes and impurities is reported. Nearly perfect removal of metallic nanotubes is confirmed by optical absorption, Raman measurements, and electrical measurements. This outstanding result was made possible in particular by ultracentrifugation (150000g) of solutions prepared from SWNT powders using polyfluorene as an extracting agent in toluene. Such s-SWNTs processable solutions were applied to realize FET, embodying randomly or preferentially oriented nanotube networks prepared by spin coating or dielectrophoresis. Devices exhibit stable p-type semiconductor behavior in air with very promising characteristics. The on-off current ratio is 105, the on-current level is around 10μA, and the estimated hole mobility is larger than 2cm2∕Vs.

Cellulose derivatives as excellent dispersants for single-wall carbon nanotubes as demonstrated by absorption and photoluminescence spectroscopy

Sodium carboxymethylcellulose, an etherified derivative of cellulose, has been found to realize stable aqueous dispersion of single-wall carbon nanotubes(SWNTs) that is twenty times more concentrated than when a surfactant is used under the same condition. The dispersion as well as thin films prepared from it exhibits well-resolved near-infrared photoluminescence peaks originating from band-gap transitions in semiconducting SWNTs, a sign of isolated individual tubes. Mechanical stretching of the film strongly aligns the tubes, as demonstrated by considerable dichroism in their absorption spectra. Possessing high optical quality and uniformity, these densely dispersed SWNT films are expected to serve as an important platform for SWNTs’ optical, electrical, and optoelectronic applications, especially because cellulose derivatives are cheap, mass-produced, safe, water-processable, and environmentally benign.

Electrochemical tuning of electronic states in single-wall carbon nanotubes studied by in situ absorption spectroscopy and ac resistance

Electrochemical doping of single-wall carbon nanotube (SWNT) films and concomitant changes in their electronic states were investigated by in situ measurements of optical absorption spectra as well as of ac resistance using a nonaqueous electrolyte solution. A systematic, consistent, and reversible variation of these properties induced by the shift in the electrode potential demonstrated the practicability of fine and continuous tuning of their electronic states. Analysis of the potential dependence of the absorbance at 0.68 eV enabled the estimation of average values of the electron affinity (4.8 eV) and the first ionization potential (5.4 eV) of semiconducting SWNTs.

Near-infrared photoconductive and photovoltaic devices using single-wall carbon nanotubes in conductive polymer films

We have fabricated prototypical Al/single-wall carbon nanotube (SWNT)-polymer/indium tin oxide thin-film devices that exhibit promising photoconductive and photovoltaic responses in a broad spectral range, typically from 300to1600nm. This achievement was made possible by finely dispersed SWNT powders in polymer matrices such as poly-phenylene-vinylene and poly-thiophene. These devices utilize (i) the intrinsic near-infrared light harvesting properties of semiconducting SWNTs, (ii) the electronic transport properties of both semiconducting and metallic SWNTs in combination with those of the polymer matrices, and (iii) probably charge/energy transfer processes between SWNTs and the polymers. By selecting different sources of SWNTs and polymers, we have shown that the optoelectronic properties of these devices are potentially tunable. To support our investigation, several techniques including spectrally resolved photoconductivity, optical absorption, and photoluminescence spectroscopy were utilized.

Fluorescent chromophore functionalized single-wall carbon nanotubes with minimal alteration to their characteristic one-dimensional electronic states{

Soluble, chromophore-functionalized SWNTs (SWNT-NA) have been synthesized via covalent bonding with minimal alteration to the characteristic one-dimensional electronic states of SWNTs. The key step in the molecular design includes incorporating a naphthalimide fluorescent moiety onto SWNTs via the amidation of oxidatively etched SWNTs, where the solubility of the product is brought about by N-octadecyl groups that are attached to naphthalimide. SWNT-NA can be fabricated into in-plane aligned thin films by using the Langmuir–Blodgett (LB) method. The surface pressure–area isotherm is characterized by a steep rise in pressure as well as by a high collapse pressure (up to 40 mN m−1), which indicates that SWNT-NA possesses a good surface spreading property at the air/water interface. The polarized UV-Vis-NIR absorption spectrum of an SWNT-NA LB film (40 layers) by vertical dipping indicates a dichroic ratio of 2.42 at 1340 nm and AFM images show that tubes are preferentially aligned in the dipping direction. Electronic properties of the product were investigated by absorption and luminescence (steady-state and transient) spectroscopy. Fluorescence from the naphthalimide moiety is found to be significantly quenched when bonded to the SWNT backbones.

In situ photoconductivity behavior of C60 thin films: Wavelength, temperature, oxygen effect

Uv-visible absorption, luminescence (excitation, emission), photoconductivity and conductivity measurements have been performed on polycrystalline C60 thin films, in situ or exposed to oxygen. The shape of the emission spectrum is irrespective of the excitation energy, indicating that internal conversion takes place to the lowest localized electronic state. σd and σph in the high temperature regime (T>270K) are thermally activated (T<270K) they are almost temperature independent, for all excitation wavelength above the photoconductivity edge (Egel≈1.7eV or λ≈730nm). Carrier photogeneration and transport processes are invoked in the context of localized electronic state. Oxygen effects on σd and σph are interpreted.