Masataka Kano

Improvement of subthreshold current transport by contact interface modification in p-type organic field-effect transistors

The charge injection efficiency of organic field-effect transistors (OFETs) is found to be a critical factor determining the subthreshold characteristics of these devices. OFETs fabricated using a wide band gap organic semiconductor and gold source/drain contacts display large threshold voltage and poor subthreshold characteristics. Insertion of a metal-oxide charge injection layer at the contact/semiconductor interface lower the injection barrier height, resulting in marked improvements in threshold voltage and subthreshold slope and strong suppression of the short-channel effect. The improved subthreshold characteristics are attributed to enhanced charge injection and the consequent promotion of charge accumulation.

Surface selective deposition of molecular semiconductors for solution-based integration of organic field-effect transistors

A bottom-up fabrication technique for the preparation of self-organizedorganic field-effect transistors(OFETs) on flexible plastic substrates is presented. Solution-based self-organization of OFETs is achieved by patterning the insulator surface with solution-wettable and unwettable regions. The proposed method satisfies several important requirements of printable electronics, including reduction in energy consumption, minimization of facilities, and the on-demand use of molecular materials.Self-organizedOFETs display an average mobility of 0.53 cm 2 / ( V s ) , on/off ratio of 10 9 , and subthreshold slope of 0.18 V/dec, with near-zero and narrowly distributed threshold voltage. An inverter circuit prepared using these devices is demonstrated with high signal gain.

Controlled Self‐Assembly of Organic Semiconductors for Solution‐Based Fabrication of Organic Field‐Effect Transistors

The solubility and low processing temperatures of organic semiconductors enable fabrication of electronic devices using relatively simple printing technologies, and hold promise for realizing flexible plastic devices by environment-friendly production methods at low cost. In particular, by effectively using the self-assembling ability of molecules, production methods for organic semiconductor devices are expected to become more efficient in terms of energy and material consumption. We have developed two solution-based methods for self-organized formation of organic semiconductor crystals, including area selective nucleation of crystalline semiconductor films and direct formation of organic single crystals. These bottom-up methods of device fabrication, wherein the intrinsic functionalities of molecules are utilized for spontaneous assembly, may become a core technology for future plastic electronics.

Selective organization of solution-processed organic field-effect transistors

Semiconductor channels of organic field-effect transistors have been directly self-organized from a solution phase. The alkyl-modified surface was locally patterned by using a phenyl self-assembled monolayer (SAM) for the channels. Drop-cast small organic molecules were selectively crystallized on the phenyl SAM region. The self-organized process allows the simultaneous formation of polycrystalline transistor arrays from the patterned channels. The phenyl SAM under the channel is critical for the improvement of device stability. Further optimization of the deposition process realized direct growth of a single crystal channel from solution between prefabricated electrodes, and the single-crystal transistors exhibited excellent performance.

All-Solution-Processed Selective Assembly of Flexible Organic Field-Effect Transistor Arrays

We have developed an all-solution fabrication process for plastic electronic devices, in which all components of devices spontaneously assemble with the desired geometry. The entire process can be carried out under ambient atmosphere. By forming surface molecular templates, complete active matrix arrays of organic field-effect transistors, including gate, source and drain electrodes, and organic semiconductor layers, have been patterned from solution with excellent operational characteristics and uniformity on a plastic substrate. This bottom-up method of device fabrication, in which the functional properties of molecules are utilized for their spontaneous assembly, may become a core technology for plastic electronics.

Control of device parameters by active layer thickness in organic field-effect transistors

We investigate the effect of the parasitic access resistance on device parameters, particularly the threshold voltage (VT) and the contact resistance (RC), of staggered organic field-effect transistors by varying the active layer thickness (ds). At low gate voltages, RC decreases as ds increases due to the free carrier density increasing in the semiconductor film. At high gate voltages, RC increases as ds increases due to increasing access resistance. These factors degrade the device parameters with increasing ds. The contribution of the change in ds on the VT shift is assessed by subtracting the contact effect from the apparent VT.

Response Time-Shortened Zinc Oxide Scintillator for Accurate Single-Shot Synchronization of Extreme Ultraviolet Free-Electron Laser and Short-Pulse Laser

We report an over one-order-of magnitude improvement in the response time of conventional hydrothermal method-grown zinc oxide (ZnO) scintillator by introducing additional quenching channels via intentional indium ion doping. A 3-ps fluorescence decay time constant is achieved, therefore making it the fastest scintillator operating below 100 nm to date. Using this indium-doped ZnO, relative jitter between extreme ultraviolet free electron laser (EUV-FEL) probe and optical pump pulses is evaluated to be less than 3 ps. Moreover, pulses from these sources can be synchronized with 3-ps accuracy through in-situ observation of relative time difference in single-shot base.

Fabrication of In-Doped ZnO Scintillator Mounted on a Vacuum Flange

High quality In-doped ZnO single crystal was grown using the hydrothermal method. The growth rate for both <;0001>; and <;000-1>; directions is about 80 μm/day. The X-ray rocking curve (XRC) full width at half maximum (FWHM) of the (0002) reflection is 24.2 arcsec. In order to make it possible to use this material as a scintillator for in situ imaging of soft X-ray laser with high spatial resolution, we prepared an In-doped ZnO wafer (9.0 mm x 9.0 mm x 0.75 mm) and mounted it on a vacuum flange. The decay time constant is evaluated to be 120 ps, therefore making it an ideal scintillator for in situ imaging device of soft X-ray laser with high spatial resolution. Furthermore, In-doped ZnO scintillator with such short decay time constant is suitable for high accuracy alignment and synchronization of ultrafast, short wavelength laser sources for pump-and-probe experiments.

Metal valence structures in 1-D mixed-metal complexes [Ni1-xPdxX(chxn)2]X2 studied by solid NMR

13C magic-angle-spinning NMR spectra in solid were measured in mixed-metal 1-D complexes [Ni 1-x Pd x X(chxn) 2 ]X 2 (X: Cl, Br; 0.0≤x ≤1.0) where the antiferromagnetically coupled paramagnetic -X-Ni 3+ -X-Ni 3+ - chains are formed at x=0.0, while the mixed-valence -X-Pd 2+ -X-Pd 4+ - state is made at x=1.0. Upon increasing x from 0.0, 13 C spectra in the C α position in the chxn ligand was broadened and shifted from original Pd 2+ and Pd 4+ positions indicating the fluctuation of Pd valency in mixed states. A marked spectrum broadening observed in C β is also attributable to the effect from the partial mixing of paramagnetic Pd 3+ sites in consistent with reported ESR results.

Indium-Doped ZnO Scintillator With 3-Ps Response Time for Accurate Synchronization of Optical and X-Ray Free Electron Laser Pulses

Accurate synchronization of pulses from an extreme ultraviolet free electron laser (EUV-FEL) and optical pulses from a Ti:sapphire laser is demonstrated. By using an indium (In)-doped zinc oxide (ZnO) crystal, pulses from these sources can be synchronized with ~ 3 -ps accuracy, an improvement of over three orders of magnitude compared to the widely-used cerium-doped yttrium aluminum garnet (Ce:YAG) scintillator. This dramatic improvement in response time will be valuable for pump-probe experiments using FEL and other ultrafast sources, and is expected to create new possibilities for time-resolved short wavelength spectroscopy.