Takuya Fujimoto

Electric-double-layer field-effect transistors with ionic liquids

Charge carrier control is a key issue in the development of electronic functions of semiconductive materials. Beyond the simple enhancement of conductivity, high charge carrier accumulation can realize various phenomena, such as chemical reaction, phase transition, magnetic ordering, and superconductivity. Electric double layers (EDLs), formed at solid-electrolyte interfaces, induce extremely large electric fields. This results in a high charge carrier accumulation in the solid, much more effectively than solid dielectric materials. In the present review, we describe recent developments in the field-effect transistors (FETs) with gate dielectrics of ionic liquids, which have attracted much attention due to their wide electrochemical windows, low vapor pressures, and high chemical and physical stability. We explain the capacitance effects of ionic liquids, and describe the various combinations of ionic liquids and organic and inorganic semiconductors that are used to achieve such effects as high transistor performance, insulator-metal transitions, superconductivity, and ferromagnetism, in addition to the applications of the ionic-liquid EDL-FETs in logic devices. We discuss the factors controlling the mobility and threshold voltage in these types of FETs, and show the ionic liquid dependence of the transistor performance.

Electrochemical and electrochromic properties of octathio[8]circulene thin films in ionic liquids.

We studied the electrochemistry of the thin films of octathio[8]circulene (1) in an ionic liquid, N,N-diethyl-N-methyl(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI). The compound 1 exhibited simultaneous two-electron oxidation in the oxidation scan and then stepwise reductions to the original neutral state, showing significant electrochromism. This color change was well-interpreted in terms of the n-pi transition that is allowed by the vacancy in the HOMO of 1 after electrochemical oxidation.

Molecular, crystal, and thin-film structures of octathio[8]circulene: release of antiaromatic molecular distortion and lamellar structure of self-assembling thin films.

Carbon–sulfur compounds have been studied extensively as key materials in organic/molecular electronics. A variety of polyand oligothiophenes have been synthesized by organic synthesis and electrochemistry to elucidate their electronic properties. TTF (=1,4,5,8-tetrathiafulvalene) is a very important molecular skeleton as a donor for organic metals and superconductors; [TTF]ACHTUNGTRENNUNG[TCNQ (7,7,8,8-tetracyanoquinodimethane)] was the first organic metal to be developed into organic superconductors in the salts of TMTSF (= tetramethyltetraselenafulvalene), BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene). In these materials, intermolecular S···S contacts play a crucial role in bringing about multidimensional electronic structures that are advantageous to electrical conduction. Cyclic oligothiophenes and thiocirculenes possess promising molecular structures from this perspective; the sulfur atoms are exposed to the outside of the molecular rings. Intermolecular S···S contacts are naturally expected in their crystals. In addition, their molecular structures are of considerable interest because they involve the radialene framework that has been a theoretical and experimental matter of concern with regard to exocyclic double bonds, aromaticity, and p-conjugation. Recently, Nenajdenko et al. synthesized octathio[8]circulene (2) from the parent compound, tetrathiophene (1), and 2 named “sulflower”. They obtained 2 as a red powder and estimated molecular and crystal structures from powder X-ray diffraction data, concluding the presence of a closed packing structure in the solid state with short intermolecular S···S contacts. Although the synthesis of 2 has attracted much attention, there is no single-crystal X-ray analysis; the molecular structure of 2 is open to debating on bond lengths, antiaromaticity. In our previous work, we carried out crystal growths and thin-film fabrications of a planar macrocyclic molecule, tetrakis(thiadiazole)porphyrazine, with strong self-assembling capabilities due to electrostatic S···N contacts and p–p interactions. The investigations revealed a highly ordered molecular arrangement in the thin films, which is nearly the same as that in the bulk crystals. In the present work, we prepared the single crystals and thin films of the sulflower 2. The molecular, crystal, and thin-film structures of 2 are described in detail, on the basis of the results of X-ray singlecrystal analysis, in comparison with those of 1. Single crystals of the parent compound 1 were grown by slow evaporation of the Et2O solution at room temperature. While the X-ray structure of 1 has been reported previously, we obtained a new polymorph belonging to the monoclinic P21/c space group. [17] In this new polymorph, there are two crystallographically independent molecules, whereas their molecular structures are nearly the same. Figure 1 shows top and side views of one of the two molecules. The bond lengths of 1 are shown in Figure S1 (Supporting Information) and Table 1, where the endo bond is the one shared

Double immunostaining with p63 and high‐molecular‐weight cytokeratins distinguishes borderline papillary lesions of the breast

Papillary breast lesions remain a source of diagnostic confusion because the full range of epithelial proliferations may arise within, or secondarily involve, papilloma. The expression of p63 and high‐molecular‐weight cytokeratins (HMWCK) was studied simultaneously in 33 papillary lesions including intraductal papilloma (IP, n = 10), atypical papilloma (AP, n = 8) and intraductal papillary carcinoma (IPC, n = 15) by double immunostaining. The myoepithelial cell nuclei were stained dark brown whereas the cytoplasms of usual ductal hyperplasia (UDH) and myoepithelium were stained purple. The myoepithelial layer was recognized as a dark brown dotted line at the epithelial stromal junction in all IP (10/10), most AP (7/8) and some IPC (7/15), suggesting that the retained myoepithelial layer in the papillary processes does not necessarily guarantee benignity. However, the malignant epithelial cells in AP and IPC were typically recognized as monotonous populations unstained with either chromogen. These monotonous cells contrasted with the proliferating cells of UDH in papilloma, which had intense purple cytoplasm in a mosaic‐like fashion. The present data suggest that the double immunostaining with the two popular antibodies p63 and HMWCK is a useful tool for reproducible classification of papillary breast lesions.

Crystal Structure, Spin Polarization, Solid-State Electrochemistry, and High n-Type Carrier Mobility of a Paramagnetic Semiconductor: Vanadyl Tetrakis(thiadiazole)porphyrazine

We report the synthesis, crystal structure, and magnetic, electrochemical, and carrier-transport properties of vanadyl tetrakis(thiadiazole)porphyrazine (abbreviated as VOTTDPz) with S = ½. X-ray crystal analysis reveals two polymorphs, the α and β forms; the former consists of a 1D regular π stacking, while the latter forms a 2D π network. Molecular orbital calculations suggest a V(4+)(d(1)) ground state and a characteristic spin polarization on the whole molecular skeleton. The temperature dependence of the paramagnetic susceptibility of the α form clearly indicates a ferromagnetic interaction with a positive Weiss constant of θ = 2.4 K, which is well-explained by McConnells type I mechanism. VOTTDPz forms amorphous thin films with a flat and smooth surface, and their cyclic voltammogram curves indicate a one-electron reduction process, which is highly electrochromic, because of a reduction of the porphyrazine π ring. Thin-film field-effect transistors of VOTTDPz with ionic-liquid gate dielectrics exhibit n-type performance, with a high mobility of μ = 2.8 × 10(-2) cm(2) V(-1) s(-1) and an on/off ratio of 10(4), even though the thin films are amorphous.

Dual-gate field-effect transistors of octathio[8]circulene thin-films with ionic liquid and SiO2 gate dielectrics

Dual-gate organic thin-film transistors (OTFTs) of octathio[8]circulene (1) with ionic liquid and SiO2 gate dielectrics lead to good transistor performance with a high on/off ratio (∼105), a low threshold voltage (∼−5 V), a low subthreshold slope (∼150 mV/decade), and low power operation, thus surpassing the performance of the single-gate OTFTs of 1. This operation is a promising method for improving the carrier concentrations in OTFTs and for realizing high-performance OTFTs.

Synthesis, optical properties and charge transport characteristics of a series of novel thiophene-fused phenazine derivatives

A series of tetrathienophenazine derivatives (t-TTP, l-TTP, m-TTP and their alkyl-substituted derivatives) have been synthesized via a simple condensation reaction between diketones and diamines. The redox potentials, UV-vis absorption spectra and fluorescence spectra of these derivatives are significantly affected by the positions of the sulfur atoms and the alkyl groups. The observed electronic properties were well reproduced by theoretical calculations based on density functional theory. X-ray analyses of these derivatives reveal extensive π–π interactions and short S⋯S contacts. The alkyl-substituted derivatives afford highly crystalline thin films by vapor deposition, and show reasonable field effect transistor performance. The conductivity of doped single crystals of these materials was also investigated, and showed an increase by several orders of magnitude upon I2 vapor doping. The π–π stacked structures of these crystals and conductive properties indicate that these thiophene-fused phenazines are useful materials for application in organic electronics.