Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance Cg. However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance CEDL between the ionic liquid and graphene involves the series connection of Cg and the quantum capacitance Cq, which is proportional to the density of states. We investigated the variables that determine CEDL at the molecular level by varying the number of graphene layers n and thereby optimising Cq. The CEDL value is governed by Cq at n < 4, and by Cg at n > 4. This transition with n indicates a composite nature for CEDL. Our finding clarifies a universal principle that determines capacitance on a microscopic scale, and provides nanotechnological perspectives on charge accumulation and energy storage using an ultimately thin capacitor.

A method for inducing nonuniform strain in graphene films is developed. Pillars made of a dielectric material (electron beam resist) are placed between graphene and the substrate, and graphene sections between pillars are attached to the substrate. The strength and spatial pattern of the strain can be controlled by the size and separation of the pillars. Application of strain is confirmed by Raman spectroscopy as well as from scanning electron microscopy (SEM) images. From SEM images, the maximum stretch of the graphene film reaches about 20%. This technique can be applied to the formation of band gaps in graphene.Comment: Appl. Phys. Express, in pres

Picene is a phenacene-type aromatic hydrocarbon molecule with five benzene rings. We have fabricated picene single crystal (SC) field-effect transistors (FETs) with solid gate and ionic liquid gate dielectrics. Although the picene SC FET showed a large hole-injection barrier without any modification of interface between source/drain electrodes and picene SC, such a large hole-injection barrier could be effectively reduced by modifying the interface with tetracyanoquinodimethane (TCNQ). Picene SC FET with an HfO2 gate dielectric and TCNQ-coated electrodes shows p-channel characteristics with a smooth hole injection and a field-effect mobility more than 1 cm2 V–1 s–1 in two-terminal measurement. Picene SC FET could be operated even in bottom-contact structure by modifying the interface with octanethiol. Furthermore, picene SC FET operated with ionic liquid gate dielectric, [1-butyl-3-methylimidazolium][hexafluorophosphate], showing the field-effect mobility of 1.8 × 10–1 cm2 V–1 s–1 and low absolute value, 1.9 V, of threshold voltage.

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 cm2 V−1 s−1, which is the highest mobility realized so far in organic single-crystal FETs.

We report electronic structure and physical properties of metal-doped picene as well as selective synthesis of the phase that exhibits 18 K superconducting transition. First, Raman scattering is used to characterize the number of electrons 2 transferred from the dopants to picene molecules, where a softening of Raman scattering peaks enables us to determine the number of transferred electrons.From this we have identified that three electrons are transferred to each picene molecule in the superconducting doped picene solids. Second, we report pressure dependence of T c in 7 K and 18 K phases of K 3 picene. The 7 K phase shows a negative pressure-dependence, while the 18 K phase exhibits a positive pressure-dependence which cannot be understood with a simple phonon mechanism of BCS superconductivity. Third, we report a new synthesis method for superconducting K 3 picene by a solution process with monomethylamine, CH 3 NH 2 . This method enables us to prepare selectively the K 3 picene sample exhibiting 18 K superconducting transition. The method for preparing K 3 picene with T c = 18 K found here may facilitate clarification of the mechanism of superconductivity.Corresponding author: Takashi Kambe, kambe@cc.okayama-u.ac.jp & Yoshihiro Kubozono, kubozono@cc.okayama-u.ac.jp 3 I. IntroductionRecently a new class of organic superconductors has been discovered in aromatic systems. They are solids of hydrocarbons that include picene, coronene, phenanthrene and 1,2:8,9-dibenzopentacene, 1-6 doped with metal atoms. Namely, the superconductivity was first discovered in potassium-doped picene, K 3 picene, which showed two different superconducting transition temperatures, one with T c = 7 K and the other as high as 18 K. 1 This has been followed by other studies, and the highest T c among these hydrocarbon superconductors to date attains 33 K observed in K 3.45 dibenzopentacene, 6 whose T c is much higher than the highest T c (14.2 K at 8.2 GPa 7 in β'-(BEDT-TTF) 2 ICl 2 ) in charge-transfer organic superconductors. Thus the hydrocarbon superconductors are very attractive from viewpoints of development of new high-T c superconductors as well as fundamental physics of superconductivity.Theoretical calculations for picene superconductors were also achieved, which suggests that the electron-phonon coupling is strong, 8,9 the conduction band comprises four bands arising from two LUMO orbitals, 10 and that strong hybridization between the dopants and molecules invalidates a rigid-band picture. 10The departure from the rigid-band picture was experimentally evidenced by photoemission spectroscopy. 11 This photoemission study clearly showed a metallic ground state for potassium-doped picene films. Our recent resistivity data also indicate a metallic behavior for the K 3 picene phase. 12 Further, a Pauli paramagnetic susceptibility was observed for a K 3 picene bulk sample. 1 These results support a metallic ground state for K 3 picene.The T c for the solid K 3 picene was found to be either 7 or 18 K, 1,2 while the T c of K 3 phenant...

No abstract

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.

The characteristics of field‐effect transistors (FETs) fabricated from thin films and single crystals of phenacene molecules are fully reported in this review together with the electronic and crystal structures of phenacenes. Phenacene molecules possess a low HOMO level and a wide band gap. The highest mobility observed in the phenacene thin‐film FETs is 7.4 cm2 V–1 s–1 for [6]phenacene, and in single‐crystal FETs the highest value is 6.3 cm2 V–1 s–1 for [7]phenacene. The phenacene thin‐film FETs show O2‐sensing properties unlike their single‐crystal FETs. The bias‐stress effect is fully investigated for phenacene single‐crystal FETs. Furthermore, the low‐voltage operation of phenacene single‐crystal FETs with electric‐double‐layer (EDL) capacitors is reported. The temperature dependence of phenacene single‐crystal FETs is reported to clarify the transport mechanism, which is suggestive of band‐like transport.