Ryoma Hayakawa

Optically and Electrically Driven Organic Thin Film Transistors with Diarylethene Photochromic Channel Layers

We achieved drain-current switching of diarylethene-channel field-effect transistors with light- and electric-field effects. The drain current was reversibly changed by alternating ultraviolet and visible light irradiation. Stress is placed on the fact that the on/off ratio realized by light irradiation was 1 × 10(2) (1 × 10(4)%) and this value is much larger than those in other photochromism-based transistors. These results indicate that the drain current was effectively controlled by light irradiation. Furthermore, the on and off states modulated by light were maintained without light irradiation even after 1 week, exhibiting that our transistor works as an optical memory. We clarified that the light-driven modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization. These findings have the potential to attain high-performance optoelectrical organic devices including optical sensors, optical memory, and photoswitching transistors.

Recent progress in photoactive organic field-effect transistors

Abstract Recent progress in photoactive organic field-effect transistors (OFETs) is reviewed. Photoactive OFETs are divided into light-emitting (LE) and light-receiving (LR) OFETs. In the first part, LE-OFETs are reviewed from the viewpoint of the evolution of device structures. Device performances have improved in the last decade with the evolution of device structures from single-layer unipolar to multi-layer ambipolar transistors. In the second part, various kinds of LR-OFETs are featured. These are categorized according to their functionalities: phototransistors, non-volatile optical memories, and photochromism-based transistors. For both, various device configurations are introduced: thin-film based transistors for practical applications, single-crystalline transistors to investigate fundamental physics, nanowires, multi-layers, and vertical transistors based on new concepts.

Optical switching of carrier transport in polymeric transistors with photochromic spiropyran molecules

We demonstrate the optical switching of transistor properties by adopting the photoisomerization of spiropyran (SP) molecules dispersed in a poly(triarylamine) (PTAA) channel layer. The drain current (IDS) was reduced by ultra-violet (UV) light irradiation, while visible (VIS) light irradiation restored the IDS to its original value. The UV and VIS light irradiation brought about the photoisomerization reactions of SP, i.e., an ionic polarized open-ring isomer was induced by UV and a less polarized closed-ring isomer was induced by VIS. To clarify the mechanism of the photoisomerization-induced optical switching, we evaluated the variations in transistor properties, namely IDS, carrier mobility and threshold voltage, when subjected to UV and VIS light irradiation. We found that the ionic polarized open-ring SP isomer worked as a carrier scattering site that effectively suppressed carrier transport in the polymer channel.

Self-Assembled Molecular Nanowires of 6,13-Bis(methylthio)pentacene : Growth, Electrical Properties, and Applications

We demonstrate a comprehensive study of self-assembled molecular nanowire, including molecular design, one-dimensional crystal growth, resistivity measurement of individual wire, and application to a field-effect transistor. Appropriate molecular design and control of interfacial interactions lead to single crystalline wire growth with an extensive pi-stacking motif. Resistivity measurements of an individual molecular wire indicate that these structural features are advantageous for electrical transport. Finally, field-effect transistors with single- and double-wire channels were fabricated to give some indication of the potential application of the molecular wires.

Continuous hydrothermal synthesis of nickel oxide nanoplates and their use as nanoinks for p-type channel material in a bottom-gate field-effect transistor

Nickel oxide nanoplates were continuously synthesized by hydrothermal reaction using a flow-type reactor. The products had a thickness of approximately 10 nm and a lateral size of 100-500 nm. The nanoplates were purified and drop-cast on a bottom-gate substrate and used as the channel material in a field-effect transistor after annealing at 300 degrees C. The I(d)-V(d) profile showed that the NiO nanoplates worked as the p-type semiconductor. This result suggests that various electronic devices can be prepared using metal oxide nanomaterials, which exhibit various properties including magnetism, ferroelectronics and catalysis as well as stability and safety in air and water.

Unique device operations by combining optical-memory effect and electrical-gate modulation in a photochromism-based dual-gate transistor.

We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers, photochromic spiropyran (SP)-doped poly(triarylamine) (PTAA) and pristine PTAA, as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect.

Shot Noise of 1,4-Benzenedithiol Single-Molecule Junctions

We report measurements of the shot noise on single-molecule Au-1,4-benzenedithiol-Au junctions, fabricated with the mechanically controllable break junction (MCBJ) technique at 4.2 K in a wide range of conductance values from 10(-2) to 0.24 conductance quanta. We introduce a simple measurement scheme using a current amplifier and a spectrum analyzer and that does not imply special requirements regarding the electrical leads. The experimental findings provide evidence that the current is carried by a single conduction channel throughout the whole conductance range. This observation suggests that the number of channels is limited by the Au-thiol bonds and that contributions due to direct tunneling from the Au to the π-system of the aromatic ring are negligible also for high conductance. The results are supported by quantum transport calculations using density functional theory.

Growth and electrical properties of N,N′-bis(n-pentyl)terrylene- 3,4:11,12-tetracarboximide thin films

A n-type semiconductor molecule N,N′-bis(n-pentyl)terrylene-3,4:11,12-tetracarboximide (TTCDI-5C) was synthesized. Theoretical calculations predict several advantages in electrical properties, including large adiabatic electron affinity and small reorganization energy. The molecule was deposited on SiO2 surfaces and the structure of the resultant thin film was studied. Grain size and thin film crystallinity improve as the temperature increases. Top-contact organic field effect transistors (OFETs) using TTCDI-5C as the semiconductor layer were fabricated using SiO2 as the gate dielectric. Values of charge carrier mobility up to 7.24×10−2cm2V−1s−1 and current on/off ratios higher than 104 were obtained, demonstrating the potential of TTCD-5C for use in OFETs.

Optically Controllable Dual-Gate Organic Transistor Produced via Phase Separation between Polymer Semiconductor and Photochromic Spiropyran Molecules

We produced an optically controllable dual-gate organic field-effect transistor by a simple one-step spin-coating of a mixed solution of photochromic spiropyran (SP) and poly(3-hexylthiophene) (P3HT). Postannealing enhanced polymer chain ordering of P3HT to induce phase separation into an SP-rich lower layer and an SP-free upper layer. These layers worked independently as transistor channels with distinct optical responsivity. The top channel was optically inactive, but the bottom channel was optically active, because of the photoisomerization of SP. These results demonstrate the potential of our technique to produce a multifunctional photoactive organic transistor by a simple process.

Interface structure and the chemical states of Pt film on polar-ZnO single crystal

The interface structures and the chemical states between the Pt layer and polar-ZnO single crystal were investigated. Two-dimensional x-ray diffraction images revealed that Zn-polar ZnO had less mosaicity than the O-polar one. Angle-resolved hard x-ray photoelectron spectroscopy was used to investigate the chemical states of the interface. At the interface of the Pt/Zn-polar ZnO, zinc diffusion into the Pt layer, forming PtZn and ZnPtO bonds, was found. For the O-polar ZnO, in contrast, PtO was predominantly formed at the interface. Zinc diffusion prevented oxidization of the Pt layer and affected the mosaicity of the interface.