Synthesis of methoxy-substituted picenes: substitution position effect on their electronic and single-crystal structures.

A series of picenes having methoxy groups was synthesized through Pd-catalyzed Suzuki-Miyaura couplings or Wittig reaction/intramolecular cyclization sequences, and their physicochemical properties and single-crystal structures were evaluated. The substitution position effects between the outer 1,12-, 2,11-, and 4,9-position and the inner 3,10-position are quite different; the former showed the same electronic structure as that of picene, but the latter results in a HOMO geometry different from those of picene and other methoxy picenes. In addition, crystal structures of four types of methoxy-substituted picenes 4a-c,e strongly depend on their substitution position and number of methoxy groups, which dramatically changes the structures from the fully anisotropic 1D π-stacked structure to a unique 3D herringbone structure due to steric hindrance of methoxy groups. The calculations of transfer integrals based on their single-crystal structures reveal that the methoxy picenes have intermolecular overlaps less effective than that of the parent nonsubstituted picene. These results are attributed not only to the packing structure but also to electronic structures such as the HOMO distribution. The preliminary OFET of the representative 4c,e showed hole mobilities significantly lower than that of picene due to their less effective intermolecular overlaps, as predicted by the calculated transfer integrals.

Fabrication of high performance/highly functional field-effect transistor devices based on [6]phenacene thin films.

Field-effect transistors (FETs) based on [6]phenacene thin films were fabricated with SiO2 and parylene gate dielectrics. These FET devices exhibit field-effect mobility in the saturation regime as high as 7.4 cm(2) V(-1) s(-1), which is one of the highest reported values for organic thin-film FETs. The two- and four-probe mobilities in the linear regime display nearly similar values, suggesting negligible contact resistance at 300 K. FET characteristics were investigated using two-probe and four-probe measurement modes at 50-300 K. The two-probe mobility of the saturation regime can be explained by the multiple shallow trap and release model, while the intrinsic mobility obtained by the four-probe measurement in the linear regime is better explained by the phenomenon of transport with charge carrier scattering at low temperatures. The FET device fabricated with a parylene gate dielectric on polyethylene terephthalate possesses both transparency and flexibility, implying feasibility of practical application of [6]phenacene FETs in flexible/transparent electronics. N-channel FET characteristics were also achieved in the [6]phenacene thin-film FETs using metals that possess a small work function for use as source/drain electrodes.

Semiconductor-metal transition in Bi2Se3 caused by impurity doping.

Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor-metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the EF is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the EF at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the EF or the crossover between the bulk and the top surface transport.

Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules.

The magnitude of the field-effect mobility µ of organic thin-film and single-crystal field-effect transistors (FETs) has been overestimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the µ values using the effective field-effect mobility, µeff, a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of µeff depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of µeff for each FET. The FET performance based on the values of µeff obtained in this study is discussed in detail. In particular, the µeff values of single-crystal FETs are almost consistent with the µ values that were reported previously, but the µeff values of thin-film FETs were much lower than those previously reported for µ, owing to a high absolute threshold voltage, |V th|. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the µ values of single-crystal FETs with phenacene molecules, is well reproduced from the µeff values. The FET performance is discussed based on the newly evaluated µeff values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

Fabrication of ring oscillators using organic molecules of phenacene and perylenedicarboximide.

Organic field-effect transistors (FETs) can be applied to radio-frequency identification tags (RFIDs) and active-matrix flat-panel displays. For RFID application, a cardinal functional block is a ring oscillator using an odd number of inverters to convert DC voltage to AC. Herein, we report the properties of two ring oscillators, one formed with [6]phenacene for a p-channel FET and N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) for an n-channel FET, and one formed with 3,10-ditetradecylpicene ((C14H29)2-picene) for a p-channel FET and PTCDI-C8 for an n-channel FET. The former ring oscillator provided a maximum oscillation frequency, f osc of 26 Hz, and the latter a maximum f osc of 21 Hz. The drain-drain voltage, V DD, applied to these ring oscillators was 100 V. This may be the first step towards a future practical ring oscillator using phenacene molecules. The values of field-effect mobility, µ in the p-channel [6]phenacene FET and n-channel PTCDI-C8 FET, which form the building blocks in the ring oscillator with an f osc value of 26 Hz, are 1.19 and 1.50 × 10-1 cm2 V-1 s-1, respectively, while the values in the p-channel (C14H29)2-picene FET and n-channel PTCDI-C8 FET, which form the ring oscillator with an f osc of 21 Hz, are 1.85 and 1.54 × 10-1 cm2 V-1 s-1, respectively. The µ values in the p-channel FETs are higher by one order of magnitude than those of the n-channel FET, which must be addressed to increase the value of f osc. Finally, we fabricated a ring oscillator with ZrO2 instead of parylene for the gate dielectric, which provided the low-voltage operation of the ring oscillator, in which [6]phenacene and PTCDI-C8 thin-film FETs were employed. The value of f osc obtained in the ring oscillator was 24 Hz. In this ring oscillator, the V DD value applied was limited to 20 V. The durability of the ring oscillators was also investigated, and the bias stress effect on the f osc and the amplitude of the output voltage, V out are discussed. This successful operation of ring oscillators represents an important step towards the realization of future practical integrated logic gate circuits using phenacene molecules.

Photochemical synthesis and device application of acene-phenacene hybrid molecules, dibenzo[n]phenacenes (n = 5-7).

Dibenzo[n]phenecenes (DBnPs, n = 5-7) were conveniently synthesised through Mallory photocyclization as the key step. Effective mobilities of single-crystal field-effect transistors of DBnPs were evaluated to demonstrate that C2h-symmetrical DB6P shows higher performance than C2v-symmetrical DB5P and DB7P.

Facile synthesis of picenes incorporating imide moieties at both edges of the molecule and their application to n-channel field-effect transistors.

Picene derivatives incorporating imide moieties along the long-axis direction of the picene core (C n -PicDIs) were conveniently synthesized through a four-step synthesis. Photochemical cyclization of dinaphthylethenes was used as the key step for constructing the picene skeleton. Field-effect transistor (FET) devices of C n -PicDIs were fabricated by using ZrO2 as a gate substrate and their FET characteristics were investigated. The FET devices showed normally-off n-channel operation; the averaged electron mobility (µ) was evaluated to be 2(1) × 10-4, 1.0(6) × 10-1 and 1.4(3) × 10-2 cm2 V-1 s-1 for C4-PicDI, C8-PicDI and C12-PicDI, respectively. The maximum µ value as high as 2.0 × 10-1 cm2 V-1 s-1 was observed for C8-PicDI. The electronic spectra of C n -PicDIs in solution showed the same profiles irrespective of the alkyl chain lengths. In contrast, in thin films, the UV absorption and photoelectron yield spectroscopy (PYS) indicated that the lowest unoccupied molecular orbital (LUMO) level of C n -PicDIs gradually lowered upon the elongation of the alkyl chains, suggesting that the alkyl chains modify intermolecular interactions between the C n -PicDI molecules in thin films. The present results provide a new strategy for constructing a high performance n-channel organic semiconductor material by utilizing the electronic features of phenacenes.

Synthesis of the extended phenacene molecules, [10]phenacene and [11]phenacene, and their performance in a field-effect transistor.

The [10]phenacene and [11]phenacene molecules have been synthesized using a simple repetition of Wittig reactions followed by photocyclization. Sufficient amounts of [10]phenacene and [11]phenacene were obtained, and thin-film FETs using these molecules have been fabricated with SiO2 and ionic liquid gate dielectrics. These FETs operated in p-channel. The averaged measurements of field-effect mobility, <µ>, were 3.1(7) × 10-2 and 1.11(4) × 10-1 cm2 V-1 s-1, respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO2 gate dielectrics. Furthermore, [10]phenacene and [11]phenacene thin-film electric-double-layer (EDL) FETs with ionic liquid showed low-voltage p-channel FET properties, with <µ> values of 3(1) and 1(1) cm2 V-1 s-1, respectively. This study also discusses the future utility of the extremely extended π-network molecules [10]phenacene and [11]phenacene as the active layer of FET devices, based on the experimental results obtained.

Transistor Properties of 2,7-Dialkyl-Substituted Phenanthro[2,1-b:7,8-b']dithiophene.

A new phenacene-type molecule with five fused aromatic rings was synthesized: 2,7-didodecylphenanthro[2,1-b:7,8-b']dithiophene ((C12H25)2-i-PDT), with two terminal thiophene rings. Field-effect transistors (FETs) using thin films of this molecule were fabricated using various gate dielectrics, showing p-channel normally-off FET properties with field-effect mobilities (µ) greater than 1 cm2 V-1 s-1. The highest µ value in the thin-film FETs fabricated in this study was 5.4 cm2 V-1 s-1, when a 150 nm-thick ZrO2 gate dielectric was used. This implies that (C12H25)2-i-PDT is very suitable for use in a transistor. Its good FET performance is fully discussed, based on electronic/topological properties and theoretical calculations.

Recent progress on carbon-based superconductors.

This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.

Synthesis and transistor application of the extremely extended phenacene molecule, [9]phenacene.

Many chemists have attempted syntheses of extended π-electron network molecules because of the widespread interest in the chemistry, physics and materials science of such molecules and their potential applications. In particular, extended phenacene molecules, consisting of coplanar fused benzene rings in a repeating W-shaped pattern have attracted much attention because field-effect transistors (FETs) using phenacene molecules show promisingly high performance. Until now, the most extended phenacene molecule available for transistors was [8]phenacene, with eight benzene rings, which showed very high FET performance. Here, we report the synthesis of a more extended phenacene molecule, [9]phenacene, with nine benzene rings. Our synthesis produced enough [9]phenacene to allow the characterization of its crystal and electronic structures, as well as the fabrication of FETs using thin-film and single-crystal [9]phenacene. The latter showed a field-effect mobility as high as 18 cm(2) V(-1) s(-1), which is the highest mobility realized so far in organic single-crystal FETs.

Transition-metal-catalyzed facile access to 3,11-dialkylfulminenes for transistor applications.

Novel [6]phenacenes (fulminenes) with two long alkyl chains at the axis positions were synthesized. This short synthesis comprises the following three steps: (1) ruthenium-catalyzed direct C-H bond arylation; (2) conversion of directing groups by Wittig reaction; and (3) bismuth- or gold-catalyzed cyclization of vinyl ether. Organic field-effect transistor devices fabricated with a thin film of 3,11-di(tetradecyl)fulminene exhibited typical p-channel normally-off properties.

Transistor application of alkyl-substituted picene.

Field-effect transistors (FETs) were fabricated with a thin film of 3,10-ditetradecylpicene, picene-(C14H29)2, formed using either a thermal deposition or a deposition from solution (solution process). All FETs showed p-channel normally-off characteristics. The field-effect mobility, µ, in a picene-(C14H29)2 thin-film FET with PbZr0.52Ti0.48O3 (PZT) gate dielectric reached ~21 cm2 V(-1) s(-1), which is the highest µ value recorded for organic thin-film FETs; the average µ value (<µ>) evaluated from twelve FET devices was 14(4) cm2 V(-1) s(-1). The <µ> values for picene-(C14H29)2 thin-film FETs with other gate dielectrics such as SiO2, Ta2O5, ZrO2 and HfO2 were greater than 5 cm2 V(-1) s(-1), and the lowest absolute threshold voltage, |Vth|, (5.2 V) was recorded with a PZT gate dielectric; the average |Vth| for PZT gate dielectric is 7(1) V. The solution-processed picene-(C14H29)2 FET was also fabricated with an SiO2 gate dielectric, yielding µ=3.4×10(-2) cm2 V(-1) s(-1). These results verify the effectiveness of picene-(C14H29)2 for electronics applications.

An extended phenacene-type molecule, [8]phenacene: synthesis and transistor application.

A new phenacene-type molecule, [8]phenacene, which is an extended zigzag chain of coplanar fused benzene rings, has been synthesised for use in an organic field-effect transistor (FET). The molecule consists of a phenacene core of eight benzene rings, which has a lengthy π-conjugated system. The structure was verified by elemental analysis, solid-state NMR, X-ray diffraction (XRD) pattern, absorption spectrum and photoelectron yield spectroscopy (PYS). This type of molecule is quite interesting, not only as pure chemistry but also for its potential electronics applications. Here we report the physical properties of [8]phenacene and its FET application. An [8]phenacene thin-film FET fabricated with an SiO2 gate dielectric showed clear p-channel characteristics. The highest µ achieved in an [8]phenacene thin-film FET with an SiO2 gate dielectric is 1.74 cm(2) V(-1) s(-1), demonstrating excellent FET characteristics; the average µ was evaluated as 1.2(3) cm(2) V(-1) s(-1). The µ value in the [8]phenacene electric-double-layer FET reached 16.4 cm(2) V(-1) s(-1), which is the highest reported in EDL FETs based on phenacene-type molecules; the average µ was evaluated as 8(5) cm(2) V(-1) s(-1). The µ values recorded in this study show that [8]phenacene is a promising molecule for transistor applications.