Reiko Azumi

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.

Ambipolar organic field-effect transistors based on a low band gap semiconductor with balanced hole and electron mobilities

The authors have demonstrated the thin-film properties and the ambipolar transport of a delocalized singlet biradical hydrocarbon with two phenalenyl radical moieties (Ph2-IDPL). The organic field-effect transistors (OFETs) based on Ph2-IDPL exhibit ambipolar transport with balanced hole and electron mobilities in the order of 10−3cm2∕Vs. The Ph2-IDPL film is an organic semiconductor with a low band gap of 0.8eV and has small injection barriers from gold electrodes to both the highest occupied molecular orbital and the lowest unoccupied molecular orbital. A complementary metal-oxide-semiconductor-like inverter using two identical Ph2-IDPL based ambipolar OFETs shows a sharp inversion of the input voltage with high gain.

Light-induced crawling of crystals on a glass surface

Motion is an essential process for many living organisms and for artificial robots and machines. To date, creating self-propelled motion in nano-to-macroscopic-sized objects has been a challenging issue for scientists. Herein, we report the directional and continuous motion of crystals on a glass surface when irradiated simultaneously with two different wavelengths, using simple azobenzenes as a photoresponsive organic compound. The direction of the motion can be controlled by the position of the light sources, and the crystals can even climb vertical surfaces. The motion is driven by crystallization and melting at the front and rear edges of the crystal, respectively, via photochemical conversion between the crystal and liquid phases induced by the trans–cis isomerization of azobenzenes. This finding could lead to remote-controlled micrometre-sized vehicles and valves on solid substrates.

Highly efficient polarized polymer light-emitting diodes utilizing oriented films of β-phase poly(9,9-dioctylfluorene)

Uniaxially oriented films of β-phase poly(9,9-dioctylfluorene) (PFO) were realized by a friction-transfer technique followed by thermal annealing and vapor treatments. Absorption and photoluminescence (PL) spectra show the characteristics of β-phase: an additional absorption peak at 433nm and redshifted PL peaks compared with those of the usual nematic (N) phase. We fabricated polarized polymer light-emitting diodes utilizing oriented films of β-phase PFO. Highly polarized β-phase emission with an integrated polarization ratio of 51 was observed from the devices. The efficiency of the devices based on β-phase reached 2.0cd∕A, which is two times higher than that based on N-phase.

Simple push coating of polymer thin-film transistors

Organic semiconductors may be processed in solution under ambient conditions; however, liquid manipulation on hydrophobic surfaces is difficult, which may hinder development of devices. Here, a push-coating technique is used to produce large-area semiconducting polymer films over hydrophobic surfaces.

Synthesis and characterization of structurally defined head-to-tail coupled oligo(3-alkylthiophenes)

A very efficient and selective synthesis to regioregular HT-coupled oligo(3-alkylthiophenes) has been developed. A series of benzyl ester-capped oligothiophenes up to the decamer has been synthesized using successive iodination/Suzuki-coupling reaction sequences. For the corresponding hexamer it was shown that saponification and decarboxylation to the HT-coupled oligo(3-alkylthiophene) proceeds in high yields. The dependence of the electronic properties on the chain length of the conjugated oligomers was investigated. The oligomers form very stable and well-ordered adsorbates at the solid–liquid interface and these were imaged by scanning tunneling techniques.

Crystal Melting by Light: X-ray Crystal Structure Analysis of an Azo Crystal Showing Photoinduced Crystal-Melt Transition

Trans-cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π···π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π···π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult.

Doping effect of solution-processed thin-film transistors based on polyfluorene

We report the fabrication of solution-processable organic thin-film transistors (OTFTs) based on poly(9,9′-dioctylfluorene-alt-bithiophene) (F8T2) doped with an electron-acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The 8% doping of the F8T2 film was found to result in an increased hole mobility, up to 0.01 cm2 V−1 s−1, and in an improved on/off ratio, as well as in a low off-current, of the order of 10−11 A. The doped F8T2 device was also found to exhibit a better threshold voltage and reduced hysteresis behavior. These improvements in performance are attributed to the formation of the F8T2-F4TCNQ complex, which results in better hole injection and improved device stability.

The longest oligothiophene ever examined by X-ray structure analysis

The α-conjugated and structurally defined dodecithiophene 1 well crystallizes from solution into single crystals which were investigated by X-ray structure analysis. Molecular conformations and packing of the longest oligothiophene ever structurally characterized were determined. Surprisingly, rather rare syn-conformations at the terminal thiophene rings and partial twisting of alkyl side chains for compact packing were found, whereby the molecules form a layer-like arrangement with partial overlap of their heteroaromatic systems. In comparison, the molecular properties of the oligomer in a 2D self-assembled monolayer adsorbed at the liquid–solid interface were investigated by scanning tunnelling microscopy (STM). In comparison to the 3D bulk material, on graphite, a different molecular arrangement was found. A lamella-type ordering of molecularly resolved oligomers which exhibit an all-anti conformation could be imaged over wide areas. Larger lattice constants were determined which originate from molecule–substrate interactions.

Thermal Behavior of α-Alkylated Oligothiophenes

Abstract The thermal behavior of several α-alkylated quaterthiophenes is investigated. Polarizing microscopy and differential scanning calorimetry reveal that 5,5‴-dipropylquaterthiophene exhibits a nematic liquid crystalline phase while oligothiophenes bearing longer alkyl chains have more ordered high temperature phases.