Takeo Minari

Simple and Scalable Gel-Based Separation of Metallic and Semiconducting Carbon Nanotubes

We report a rapid and scalable method for the separation of metallic and semiconducting single-wall carbon nanotubes (SWCNTs); the separation is performed by the selective adsorption of semiconducting SWCNTs on agarose gel. The most effective separation was realized by a simple procedure in which a piece of gel containing SWCNTs and sodium dodecyl sulfate was frozen, thawed, and squeezed. This process affords a solution containing 70% pure metallic SWCNTs and leaves a gel containing 95% pure semiconducting SWCNTs. Field-effect transistors constructed from the separated semiconducting SWCNTs have been demonstrated to function without any electrical breakdown.

Solution‐Processable Organic Single Crystals with Bandlike Transport in Field‐Effect Transistors

www.MaterialsViews.com C O M M Solution-Processable Organic Single Crystals with Bandlike Transport in Field-Effect Transistors U N IC A TI Chuan Liu , Takeo Minari , Xubing Lu , Akichika Kumatani , Kazuo Takimiya , and Kazuhito Tsukagoshi * O N In organic fi eld-effect transistors (OFETs),[1] the best performance of semiconducting oligomers is usually observed in the single-crystalline state rather than in their polycrystalline or amorphous state.[2] As single crystals are free of grain boundaries and molecular disorder, carrier transport is not limited by hopping between localized states but features high mobility (>1 cm2 V−1 s−1) and bandlike transport, i.e. mobility increases with cooling due to lower lattice vibrations or phonon scattering.[2h,3] Based on single-crystal rubrene OFETs, Podzorov et al. observed simultaneously intrinsic bandlike and hopping transport in anisotropic directions, while Hulea et al. reported the mobility transition from metal-like to insulator-like with increasing dielectric constant.[4] Besides for understanding of their fundamental physics, single crystals are also desired for device application. For instance, Briseno et al. demonstrated the patterning of large arrays of high-performance singlecrystal OFETs with microcontact-printed octadecyltriethoxysilane (OTS) fi lms and Tang et al. used predeposited crystals to construct CuPc nanoribbons in various architectures.[5] However, organic single crystals are diffi cult to fabricate and only a few have been applied in OFETs, either by vacuum vapor or, more preferable for device application, by a solution process. The commonly used solution method is based on drying the solution to recrystallize molecules such as bistriisopropylsilylethynyl pentacene (TIPS-PEN), the highest saturated FET mobility ( μ FET ) of which is 1.4 cm 2 V − 1 s − 1 . [ 2 e, 2 h] Yet this method cannot be performed on a polymer dielectric surface unless an orthogonal solvent is used and sometimes it takes a very long time (e.g. several days). [ 2 g] Here, we report use of an advanced method, solvent vapor annealing (SVA), for fabrication of single crystals of dioctylbenzothienobenzothiophene (C8-BTBT, Figure 1 top ) on a polymer surface, which is an air-stable and high-mobility semiconductor. [ 6 ] By further combining our method with a self-organized phase-separation method, [ 7 ]

Ambipolar-transporting coaxial nanotubes with a tailored molecular graphene–fullerene heterojunction

Despite a large steric bulk of C60, a molecular graphene with a covalently linked C60 pendant [hexabenzocoronene (HBC)–C60; 1] self-assembles into a coaxial nanotube whose wall consists of a graphite-like π-stacked HBC array, whereas the nanotube surface is fully covered by a molecular layer of clustering C60. Because of this explicit coaxial configuration, the nanotube exhibits an ambipolar character in the field-effect transistor output [hole mobility (μh) = 9.7 × 10−7 cm2 V−1 s−1; electron mobility (μe) = 1.1 × 10−5 cm2 V−1 s−1] and displays a photovoltaic response upon light illumination. Successful coassembly of 1 and an HBC derivative without C60 (2) allows for tailoring the p/n heterojunction in the nanotube, so that its ambipolar carrier transport property can be optimized for enhancing the open-circuit voltage in the photovoltaic output. As evaluated by an electrodeless method called flash-photolysis time-resolved microwave conductivity technique, the intratubular hole mobility (2.0 cm2 V−1 s−1) of a coassembled nanotube containing 10 mol % of HBC–C60 (1) is as large as the intersheet mobility in graphite. The homotropic nanotube of 2 blended with a soluble C60 derivative [(6,6)-phenyl C61 butyric acid methyl ester] displayed a photovoltaic response with a much different composition dependency, where the largest open-circuit voltage attained was obviously lower than that realized by the coassembly of 1 and 2.

Contact-metal dependent current injection in pentacene thin-film transistors

Contact-metal dependent current injection in top-contact pentacene thin-film transistors is analyzed, and the local mobility in the contact region was found to follow the Meyer-Neldel rule. An exponential trap distribution, rather than the metal/organic hole injection barrier, is proposed to be the dominant factor of the contact resistance in pentacene thin-film transistors. The variable temperature measurements revealed a much narrower trap distribution in the copper contact compared with the corresponding gold contact, and this is the origin of the smaller contact resistance for copper despite a lower work function.

Charge injection process in organic field-effect transistors

The charge injection process in top-contact organic field-effect transistors was energetically observed with displacement of the Fermi level as a result of scanning the gate voltage. Doping of charge-transfer molecules into the metal/organic interface resulted in low interface resistance, which unveiled the bulk transport of the injected charges from the contact metal into the channel. The authors found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection.

Direct evaluation of low-field mobility and access resistance in pentacene field-effect transistors

Organic field-effect transistors (OFETs) suffer from limitations such as low mobility of charge carriers and high access resistance. Direct and accurate evaluation of these quantities becomes crucial for understanding the OFETs properties. We introduce the Y function method (YFM) to pentacene OFETs. This method allows us to evaluate the low-field mobility without the access or contact resistance influence. The low-field mobility is shown to be more appropriate than the currently applied field-effect mobility for the OFETs’ performance evaluation. Its unique advantage is to directly suppress the contact resistance influence in individual transistors, although such contact resistance is a constant as compared to the widely accepted variable one with respect to the gate voltage. After a comparison in detail with the transmission-line method, the YFM proved to be a fast and precise alternative method for the contact resistance evaluation. At the same time, how the contact resistance affects the effective mobi...

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.

Bias stress instability in pentacene thin film transistors: Contact resistance change and channel threshold voltage shift

Bias stress instability in top-contact pentacene thin film transistors was observed to be correlated not only to the channel but also to the metal/organic contact. The drain current decay under bias stress results from the combination of the contact resistance change and the threshold voltage shift in the channel. The contact resistance change is contact-metal dependent, though the corresponding channel threshold voltage shifts are similar. The results suggest that the time-dependent charge trapping into the deep trap states in both the contact and channel regions is responsible for the bias stress effect in organic thin film transistors.

Correlation between grain size and device parameters in pentacene thin film transistors

We develop a general approach to precisely extract the device parameters in top-contact pentacene thin film transistors. The charge trap sites are clarified by analyzing the grain size dependence of the device parameters. The channel mobility and threshold voltage are limited by the charge traps in the channel region, most of which are located not at the grain boundaries but at the organic/insulating-layer interface. The contact resistance decreases by increasing the grain size and is controlled by the charge traps in the contact region, which are suggested to be concentrated at the grain boundaries and at the metal/organic interface.

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.