Heisuke Sakai

Highly polarized luminescence from aligned conjugated polymer electrospun nanofibers

In this contribution we show highly polarized photoluminescence (PL) from aligned polyethyleneoxide: polyphenylenevinylene derivative composite nanofibers. We demonstrate PL polarization ratios (parallel to perpendicular) greater than 13. This ratio is further increased (up to ∼25) by stretching the nanofibers. Stretching also results in an increase in conjugation length, fiber density, and PL lifetime. We argue that the effect of stretching is equivalent to applying a permanent and strong pressure. Our results open up the possibility for new optoelectronic devices and fundamental science studies based on polymer nanofibers.

Synthesis of kinetically stabilized 1,2-dihydrodisilenes.

Kinetically stabilized 1,2-dihydrodisilenes were successfully synthesized and isolated by the introduction of sterically protecting bulky aryl groups. These 1,2-dihydrodisilenes exhibit distinct Si═Si double-bond character in both solution and the solid state. The Si-H bonds in these 1,2-dihydrodisilenes exhibit higher s character than those of typical σ(4),λ(4)-hydrosilanes. Moderate heating of these 1,2-dihydrodisilenes in solution resulted in their isomerization to the corresponding trihydrodisilanes, with an intramolecular hydrogen migration as the rate-determining step.

Tuning of threshold voltage of organic field-effect transistors by space charge polarization

We demonstrate a tunable threshold voltage in an organic field-effect transistor (OFET) using an ion-dispersed gate dielectric. By applying an external electric field (Vex) to the gate dielectrics, the dispersed ions in the gate dielectric are separated by electrophoresis and form space charge polarization. The drain current of the OFET increases more than 1.9 times, and the threshold voltage (Vth) decreases by 22 V (from −35.1 to −13.1 V). The direction and the magnitude of Vth shift are tunable with the applied Vex. The origin of the Vth shift is attributed to the polarization of the gate dielectric.

Organic field effect transistors with dipole-polarized polymer gate dielectrics for control of threshold voltage

The authors demonstrate organic field effect transistors (OFETs) with a dipole-polarized polyurea for the gate dielectrics. In the dielectrics, the internal electric field induces the mobile charge carrier in the semiconductor layer to the semiconductor-dielectric interface. OFETs with dipole-polarized gate dielectrics exhibit lower threshold voltage. With nonpolarized gate dielectrics, the threshold voltage was −11.4V, whereas that decreased to −5.3V with polarized gate dielectrics. In addition to the threshold voltage, polarized gate dielectrics reduced subthreshold swing from 4.1to2.4V/decade at the gate voltage of −20V. These results show that dipole-polarized polyurea gate dielectrics allow us to operate OFETs with lower power consumption.

Fabrication of a submicron-channel organic field-effect transistor using a controllable electrospun single fibre as a shadow mask

We demonstrate a simple and versatile method for the fabrication of a submicron channel for an organic field-effect transistor (OFET) using a single electrospun fibre as a shadow mask. A single electrospun fibre is produced by an alternative switching electrospinning method and is stretched 2.5-fold. The average diameter of the stretched fibres is 302 nm. The stretched fibre is placed on ultrathin dielectric layers of aluminium oxide and a self-assembled monolayer (SAM). During electrode deposition the fibre acts as a very small shadow mask. After removing the fibre, electrodes with very narrow gaps of around 350 nm and with high uniformity are easily obtained. We fabricate an OFET by depositing pentacene as an active layer onto the electrodes. The OFET is operable at low voltages, with a threshold voltage of - 1.1 V and a subthreshold swing of 0.27 V decade(-1), values which are one order of magnitude lower than those obtained with a channel length of 75 µm.

Low-loss waveguiding and detecting structure for surface plasmon polaritons

A simple and low-loss metal/semiconductor surface plasmon polariton (SPP) device consisting of a SPP waveguide and a detector is studied theoretically and experimentally. We demonstrate a simple diffraction structure (a metal grating) where the SPP couples from the waveguide to the detector. The SPP can propagate without large losses at the air/Au interface, and this interface was used for SPP waveguiding. To convert the SPP into an electric signal using internal photoemission, the propagating SPP is coupled into the Au/Si interface by the diffraction structure. The propagation direction of the coupled SPP at the Au/Si interface depends on the slit pitch of the diffraction structure, and the direction can be controlled by adjusting the pitch. The slit pitch is also modeled using a diffraction grating equation, and the results show good agreement with those of simulations using the finite-difference time-domain method. When diffraction structures consisting of a multi-slit structure and a disk array are placed at the end of the waveguide, SPP coupling into the Au/Si interface is also observed. The photocurrents detected at the Au/Si interface are much larger when compared with that detected for the device without the diffraction structure (26 times for the multi-slit structure and 10 times for the disk array). From the polarization angle dependence of the detected photocurrent, we also confirmed that the photocurrent was caused by the SPP propagating at the air/Au interface.

Synthesis of a stable 1,2-bis(ferrocenyl)diphosphene

The synthesis and characterization of a stable 1,2-bis(ferrocenyl)diphosphene, wherein a P=P π-bond connects two ferrocenyl units will be reported. This represents an unprecedented example for a d-π electron system containing a heavier pnictogen π-spacer group. Stabilization of the highly reactive P=P π-bond was achieved by steric protection using two bulky ferrocenyl moieties.

Photoinduced change of dielectric permittivity in molecular doped polymer layer

We demonstrate a large photoinduced change of dielectric permittivity due to the charge separated (CS) state of an electron donor-acceptor linked molecule, 6-[4′-(N,N-diphenylamino)phenyl]-3-ethoxycarbonylcoumarin (DPA–CM), doped in a polymethylmethacryrate (PMMA) matrix. Dielectric permittivity of DPA–CM in PMMA film increases from 4.1 to 7.5 under photoirradiation. Electron spin resonance spectra of the films measured under photoirradiation clearly show the formation of the CS state of DPA–CM in PMMA matrix. The origin of the photoinduced change of the dielectric permittivity is ascribed to the realignment of dipole of the CS state of DPA–CM.

Characterization of an oxide semiconductor prepared by microwave sintering

We fabricated a thin-film transistor (TFT) using amorphous indium gallium zinc oxide (a-IGZO), which was formed through annealing of an IGZO precursor film with a single-mode cavity microwave at 2.45 GHz. The transisitor fabricated with the a-IGZO film prepared by microwave annealing for 15 min showed higher device performance, i.e., a field effect mobility of 5.75 × 10−2 cm2·V−1·s−1, an on/off ratio of 106, and a threshold voltage of 20 V, than that prepared by annealing with a conventional oven for 120 min. The Raman spectra confirm that the device improvement originates from the decrease in the number of –OH groups and removal of organic species for 15 min by microwave annealing. These results suggest that the microwave annealing method has an advantage as the annealing process of solution-processed oxide semiconductors to reduce the process time. It can be applied to the fabrication of TFTs.

Organic Field-Effect-Transistor-Based Memory with Nylon 11 as Gate Dielectric

We present the electrical properties of an organic memory device based on an organic field-effect transistor using a thin film of nylon 11 as a gate dielectric. The transfer characteristics of the memory device showed large hysteresis. A large shift of 37 V in the transfer characteristics was obtained by the application of a writing bias to the gate dielectric. The drain current ratio of the on-state to off-state was ca. 100. The on-state of the memory device showed a clear memory characteristic.