Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.inorgchem.3c02254

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DOI： 10.1021/acs.jpcc.3c06294

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.inorgchem.3c02405

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Chemical Society of Japan  

DOI： 10.1246/bcsj.20230155

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.108.035146

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DOI： 10.1021/acs.chemmater.3c00391

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DOI： 10.1021/acs.inorgchem.3c00744

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.elspec.2023.147295

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Abstract

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 E_F 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 E_F 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 E_F or the crossover between the bulk and the top surface transport.

DOI： 10.1038/s41598-023-27701-5

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Other Link： https://www.nature.com/articles/s41598-023-27701-5

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DOI： 10.1021/acs.inorgchem.2c03365

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.jpcc.2c06505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.jpcc.2c04879

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3-Decyl-10-phenyl[5]phenacene (PhC10-PIC) molecule shows good FET characteristics in case of using ZrO₂ gate dielectric, owing to strong fastener effect and large π-overlap.

DOI： 10.1039/d2tc03383f

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DOI： 10.1021/acs.jpcc.2c01972

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1557/s43578-022-00536-y

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Other Link： https://link.springer.com/article/10.1557/s43578-022-00536-y/fulltext.html

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The structural and superconducting properties of alkali-Bi-based compounds, KBi2 and RbBi2, were investigated over a wide pressure range for the first time. The samples of KBi2 and RbBi2 were prepared using a liquid ammonia (NH3) technique, and demonstrated superconductivity with superconducting transition temperatures, T-c, of 3.50 and 4.21 K at ambient pressure, respectively. The onset superconducting transition temperature, T-c(onset), of KBi2 decreased slightly; however, it suddenly jumped at 2 GPa and increased gradually with pressure, indicating the presence of two superconducting phases in the low-pressure range. The pressure-dependent X-ray diffraction patterns indicate that the KBi2 sample decomposed into KBi and Bi at pressures higher than 2.5 GPa. Moreover, a discontinuous change in T-c(onset) was observed for KBi2 at 9 GPa, which reflects the decomposition of KBi2 into KBi and Bi. By contrast, the value of T-c(onset) of RbBi2 was almost constant over a pressure range of 0-8 GPa. Thus, the superconducting properties and stability of alkali-Bi-based compounds against pressure were comprehensively explored in this study. In addition, the superconducting Cooper pair symmetry was investigated from the magnetic field dependence of T-c of KBi2 at 0.790 and 2.32 GPa, and of RbBi2 at 1.17 GPa, indicating the exact deviation from the simple s-wave paring model, which may be due to the complex electronic structure of Bi. The results elucidated the exotic superconducting properties of KBi2 and RbBi2 based on the pressure and magnetic field dependence of T-c and verified the chemical stability of KBi2 under pressure.

DOI： 10.1039/d2cp00679k

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The magnitude of the field-effect mobility mu of organic thin-film and single-crystal field-effect transistors (FETs) has been over-estimated 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 mu values using the effective field-effect mobility, mu(eff), 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 mu(eff) 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 mu(eff) for each FET. The FET performance based on the values of mu(eff) obtained in this study is discussed in detail. In particular, the mu(eff) values of single-crystal FETs are almost consistent with the mu values that were reported previously, but the mu(eff) values of thin-film FETs were much lower than those previously reported for mu, owing to a high absolute threshold voltage, vertical bar V-th vertical bar. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the mu values of single-crystal FETs with phenacene molecules, is well reproduced from the mu(eff) values. The FET performance is discussed based on the newly evaluated mu(eff) values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

DOI： 10.1021/acsomega.1c06932

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DOI： 10.1088/1361-648x/ac244b

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DOI： 10.1021/acs.jpcc.1c06207

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The topological insulator PdBi2 exhibits two different crystal phases at ambient pressure, i.e., 'alpha-PdBi2' and ' -PdBi2'. The pressure dependence of crystal structure and superconductivity of alpha-PdBi2 has been fully elucidated thus far. However, the physical properties of beta-PdBi2 crystals under pressure have not been sufficiently investigated. In this study, we fully investigate the crystal structure and superconductivity of beta-PdBi2 under pressure based on synchrotron x-ray diffraction (XRD) patterns. The temperature dependence of beta-PdBi2 indicates its superconductivity with a superconducting transition temperature (T-c) as high as 4.10 K, and its crystal structure is tetragonal [space group of I4/mmm (no. 139)]. The XRD patterns at 0-22.0 GPa indicate no structural phase transitions, and the unit cell volume shrinks monotonically with pressure, unlike the behavior of alpha-PdBi2. Furthermore, alpha-PdBi2 transformed to beta-PdBi2 under pressure. This suggests that beta-PdBi2 is stable under pressure. The superconductivity is clearly observed at 0-11.8 GPa, and the value of T-c is almost constant at similar to 4.4 K. The temperature dependence of the upper critical field at ambient pressure and 10.7 GPa indicates that the superconductivity is not attributed to a simple s-wave dirty limit but an s-wave clean or p-wave polar model. This is the first systematic study of superconductivity of topological insulator beta-PdBi2 under pressure.

DOI： 10.1088/1361-648x/abd99c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>Dibenzo[<italic>n</italic>]phenacenes (<bold>DBnP</bold>, <italic>n</italic> = 5–7) were applied to the active layers of p-channel FETs and their effective mobilities were evaluated to demonstrate that <italic>C</italic>_2h-symmetrical <bold>DB6P</bold> showed superior performance to the <italic>C</italic>_2v-symmetrical homologues.</p>

DOI： 10.1039/d1cc01294k

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsami.0c20140

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We investigated the pressure dependence of electric transport in a superconducting sample, Ba0.77Na0.23Ti2Sb2O, to complete the phase diagram of superconducting transition temperature (T-c) against pressure (p). This superconducting sample exhibits a T-c value of 5.8 K at ambient pressure. Here, the superconductivity of the recently reported sample was investigated over a wide pressure range. The T-c value monotonously decreased with pressure below 8 GPa. Interestingly, the T-c value rapidly increased above 8 GPa and slowly declined with pressure above 11 GPa. Thus, a new superconducting phase was discovered above similar to 9 GPa. The crystal structure of Ba0.77Na0.23Ti2Sb2O was also elucidated at 0-22.0 GPa with synchrotron X-ray powder diffraction. Consequently, an evident relation between the crystal structure and the superconductivity was revealed, namely, a clear structural phase transition was observed at 8-11 GPa, where the T-c value rapidly increased against pressure. This study provides detailed information on the superconductivity of Ba0.77Na0.23Ti2Sb2O under pressure, which will lead to a comprehensive understanding of pressure-driven superconductivity.

DOI： 10.1021/acs.inorgchem.0c02836

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The pressure dependence of BaBi₃ superconductivity is investigated, demonstrating only the presence of the α phase, and <italic>p</italic>-wave superconductivity.

DOI： 10.1039/d1cp00042j

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<p>A ring oscillator consisting of p-channel and n-channel organic FETs.</p>

DOI： 10.1039/d1ra00511a

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The application of an electric field can be a new technique to control the electronic properties of two-dimensional materials. In a conventional double-gate structure, an ionic liquid is used in the top gate to produce a large electric field successfully. However, the material in the bottom gate has been restricted to solid dielectrics, which limits the applicable electric field. In this study, we combine the direct electron transfer and electrostatic gating to produce a large electric field. Bilayer graphene (BLG), which exhibits a band gap under a perpendicular electric field, was placed on electron-donating self-assembled monolayers with an amino end group (NH2-SAMs). As the NH2-SAMs were prepared on SiO2/Si substrates, the electron density of the BLG was increased further by applying a positive bottom gate voltage through the SiO2 dielectric. Additionally, a negative top gate voltage was applied with an ionic gel to produce an upward electric field. The transport gap in the BLG was evaluated based on the temperature dependence of the minimum conductivity at the charge neutrality point of the BLG. Comparing the results obtained from the BLG without the NH2-SAMs, we found that the NH2-SAMs donated electrons whose sheet density was 3.9 x 10(12)/cm(2), or in other words, they produced an additional electric displacement field of 0.72 V/nm without a significant reduction of the mobility. Furthermore, we successfully evaluated the actual carrier density controlled by the top gate voltage based on a simple capacitor model. The combination of the two methods to accumulate carriers offers a unique opportunity to explore a novel electronic property by enhancing the controllability of the electric field and the carrier density.

DOI： 10.1021/acs.jpcc.0c07537

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.102.155118

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.155118/fulltext

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<p>A possible Dirac semimetal, BaTi₂Bi₂O, indicates a sign of topologically nontrivial nature in superconductivity at high pressure.</p>

DOI： 10.1039/d0cp04771f

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DOI： 10.1088/1361-648x/abaad2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>Picenediimide derivatives serve as the active layer of <italic>n</italic>-channel thin-film field-effect transistors displaying a maximum charge carrier mobility as high as 2.0 × 10⁻¹ cm² V⁻¹ s⁻¹.</p>

DOI： 10.1039/d0ra06629j

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<p>A superconducting KBi₂ sample was successfully prepared using a liquid ammonia (NH₃) technique.</p>

DOI： 10.1039/d0ra04541a

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DOI： 10.1088/1361-648x/ab9911

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>Ditetradecyl-substituted [7]phenacene was prepared and applied in thin-film FET devices, displaying higher mobility compared to parent [7]phenacene.</p>

DOI： 10.1039/d0tc00272k

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/2053-1591/ab7c85

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© 2019 IOP Publishing Ltd. Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, T c, as high as 6.6 K at ambient pressure, in order to complete the T c-pressure (p ) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T c value decreased with an increase in pressure, but the onset superconducting transition temperature, (R-T plot), increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R-T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

DOI： 10.1088/1361-648X/ab4605

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© 2019, The Author(s). 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 SiO 2 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 cm 2 V −1 s −1 , respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO 2 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) cm 2 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.

DOI： 10.1038/s41598-019-39899-4

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© 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft. We prepared two superconducting phases of (NH3) y Li x FeSe0.5Te0.5, which show superconducting transition temperatures (T c's) as high as 20.2 and 29.5 K at ambient pressure, here called the 'low-T c phase' and 'high-T c phase'. The temperature dependence of electrical resistance (R) was measured for the low-T c phase of (NH3) y Li x FeSe0.5Te0.5 over a pressure (p) range of 0-14 GPa, and for the high-T c phase of (NH3) y Li x FeSe0.5Te0.5 over 0-19 GPa, yielding double-dome superconducting T c-p phase diagrams, i.e. two superconducting phases (SC-I and SC-II) were found for both the low-T c and high-T c phases under pressure. For the low-T c phase, the maximum T c was 20.2 K at 0 GPa for SC-I, and 19.9 K at 8.98 GPa for SC-II. For the high-T c phase, the maximum T c was 33.0 K at 1.00 GPa for SC-I, and 24.0 K at 11.5-13.2 GPa for SC-II. These results imply that the maximum T c value of the high pressure phase (SC-II) does not exceed the maximum value of the SC-I, unlike what was shown in the T c-p phase diagrams of (NH3) y Li x FeSe and (NH3) y Cs x FeSe investigated previously. Nevertheless, the double-dome T c-p phase diagram was found in metal-doped FeSe0.5Te0.5, indicating that this feature is universal in metal-doped FeSe1-zTe z . Moreover, no structural phase transitions were observed for either the low-T c or high-T c phases of (NH3) y Li x FeSe0.5Te0.5 over the wide pressure range of 0-15.3 GPa, and the T c-lattice constant (c) plots for both phases were recorded to determine the critical point separating SC-I and SC-II.

DOI： 10.1088/1367-2630/ab5034

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DOI： 10.7567/1347-4065/ab4ef5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.orgel.2019.06.021

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© 2019 American Physical Society. We systematically investigated the pressure dependence of electrical transport and the crystal structure of topological insulator, Bi2-xSbxTe3-ySey, which showed no superconductivity down to 2.0 K at ambient pressure. The Bi2-xSbxTe3-ySey crystal showed two structural phase transitions under pressure, from rhombohedral structure (space group No. 166, R3m, termed phase I) to monoclinic structure (space group No. 12, C2/m, termed phase II), and from phase II to another monoclinic structure (space group No. 12, C2/m, termed phase III). Superconductivity appeared when applying pressure; actually the superconductivity of all Bi2-xSbxTe3-ySey samples emerged in phase I. The superconducting transition temperature, Tc, increased against pressure in a pressure range of 0-15 GPa for all Bi2-xSbxTe3-ySey samples, and the maximum Tc was 5.45 K, recorded at 13.5 GPa in Bi2-xSbxTe3-ySey at x=0 and y=1.0. The magnetic field (H) dependence of the R-T plot for Bi2-xSbxTe3-ySey was measured to characterize the superconducting pairing mechanism of pressure-induced superconducting phase.

DOI： 10.1103/PhysRevB.100.094525

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DOI： 10.1088/1367-2630/ab4159

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The pressure dependence of the crystal and electronic structures of (NH3)(y)Cs0.4FeSe, which has two pressure-induced superconducting domes of the SC1 and SC2 phases, was investigated by x-ray diffraction and emission spectroscopy at low temperatures. We found a pressure-induced change in the crystal structure from tetragonal (T) to collapsed tetragonal (cT) and peculiar pressure dependence of the magnetic moment between the SC1 and SC2 phases at a low temperature. The electronic structure also markedly changed between the two phases. The results suggest that the increase in the electron-electron correlation accompanying the increase in the magnetic moment may relate the high T-c in the SC2 phase. Theoretical calculations using the multi-orbital Hubbard model revealed that the spin susceptibility decreases in the SC1 phase and increases in the SC2 phase with the T -> cT phase transition.

DOI： 10.7566/JPSJ.88.074704

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<p>The transport properties of 3,10-ditetradecylpicene ((C₁₄H₂₉)₂-picene) and [6]phenacene thin-film field-effect transistors (FETs) on Si and plastic substrates are reported, in which SiO₂ and parylene are used as gate dielectrics, respectively.</p>

DOI： 10.1039/c8tc05824e

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DOI： 10.1038/s41598-019-41906-7

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DOI： 10.1088/2053-1591/aae9d2

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Publishing type：Part of collection (book)   Publisher：Springer Singapore  

DOI： 10.1007/978-981-13-3417-7_8

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Language：English   Publishing type：Part of collection (book)   Publisher：World Scientific Publishing Co.  

In this chapter, the principles behind the mechanism of operation of organic transistors are described, as they are both important in their own right and also a representative example of organic electronics. Accordingly, the recent progress in organic transistors will be explored in depth. The term “organic transistor” generally refers to the field-effect transistor (FET), which uses thin films of organic materials. The study of these “organic FETs” began in the 1980s.1 Initially, the available fieldeffect mobility (µ), which is one determinant of the performance of an FET, was ca. 10-5 cm2 V-1 s-1, a value that is insufficient for practical applications. In the late 1990s, it was found that pentacene, in which five benzene rings are connected in a linear structure (Fig. 8.1.1), and C60, in which 60 carbon atoms form a spherical structure, were good candidates for the active layer of an FET device. The µ value became ~1 cm2 V-1 s-1 in FETs using these molecules.2, 3 This µ value raised the expectation that organic FETs might in the future become practical devices. On the other hand, since the year 2000, some work on FETs has demonstrated that the physical properties of materials can be controlled by carrier accumulation, such as the field-induced carrier doping of C60 for inducing superconductivity. These studies were not the case, but the interest in field-induced carrier doping to control the physical properties of materials has attracted much attention from researchers in solid-state physics. Consequently, some control of the physical properties of inorganic materials has been achieved by field-induced carrier accumulation using a device with the structure of an FET.4, 5 Research on organic FETs has been brought to a new stage by the participation of many researchers from other fields, where FET performance was pursued not only to find practical applications, but also to investigate fundamental physics. In 2003, a device with excellent FET properties that used a single crystal was fabricated using rubrene (Fig. 8.1.1).6 This showed that transistor operation in an organic FET could be achieved using not only thin films but also single crystals. An FET using a single crystal is superior to a thin-film FET for studying the intrinsic nature of organic materials because many interfering factors such as structural defects, grain boundaries, and impurities can be removed from the active layer. Generally, FET performance is also higher in a single-crystal FET than in a thin-film FET. Clearly, these studies have led to remarkable advances in the research on organic FETs. Furthermore, Hall-effect measurements were achieved in single crystals of organic molecules, 7, 8 and angle-resolved photoemission spectroscopy (ARPES) was performed on some organic single crystals.9, 10 Based on these results, it was suggested that electron transport in organic singlecrystal FETs occurs by band transport rather than hopping transport. Since then, the idea that electron transport in an organic thin-film FET is explained by simple hopping transport has been modified due to analyses based on the multiple shallow trap-and-release (MTR) model for the temperature-dependence of transport properties in organic thinfilm FETs exhibiting a high µ value.11-13 The MTR model assumes that the carriers in a band are captured by trap states and thermally excited up to the conduction (or valence) band, i.e. this concept is substantially based on band transport. Furthermore, the reduction of µ value caused by thermal scattering of phonons was determined from the temperature-dependence of µ in organic single-crystal FETs.14, 15 This constituted the direct observation of band transport in organic single-crystal FETs. These studies have enabled the full discussion of the mechanism of organic FET operation. An FET using pentacene (Fig. 8.1.1) as the active layer shows typical pchannel operation with Au source/drain electrodes, but shows n-channel operation with Ca source/drain electrodes.16 So an organic FET can operate both as p-channel and n-channel depending on the electrode metal, a flexibility termed “ambipolar”. Thus, an organic FET can operate in an ambipolar way by changing the metal of the source/drain electrodes (or changing the work function of the electrodes), unlike the operation of Si MOSFETs, which can operate in either p-channel or n-channel through the formation of an inversion layer in which minority carriers in the bulk become the majority in the channel region. Also, neither organic thin films nor crystals are doped with any impurities, unlike the Si crystal used in Si MOSFETs, which can be doped with an acceptor (p-type Si) or with a donor (n-type Si) material. In Section 8.1.1, we will fully examine organic FETs based on the background described above.

DOI： 10.1142/9789813273672_0010

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Surface structure relaxation of organic semiconductors affects the properties of organic devices, although such relaxation has not been well explored. Only two examples have been experimentally reported; tetracene shows a large surface relaxation, while rubrene exhibits no relaxation. Therefore, a systematic investigation of the surface relaxation is conducted on [ n]phenacenes ( n = 5, 7, and 9). Electron density analyses are performed based on the synchrotron surface X-ray scattering with the aid of first-principles calculations. The results show little surface relaxation in [ n]phenacenes.

DOI： 10.1021/jacs.8b08811

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DOI： 10.1021/acs.jpcc.8b02978

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We prepared two superconducting phases, which are called "low-Tc phase" and "high-Tc phase" of (NH3)yNaxFeSe showing Tc's of 35 and 44 K, respectively, at ambient pressure, and studied the superconducting behavior and structure of each phase under pressure. The Tc of the 35 K at ambient pressure rapidly decreases with increasing pressure up to 10 GPa, and it remains unchanged up to 22 GPa. Finally, superconductivity was not observed down to 1.4 K at 29 GPa, i.e., Tc&lt
1.4K. The Tc of the 44 K phase also shows a monotonic decrease up to 15 GPa and it weakly decreases up to 25 GPa. These behaviors suggest no pressure-driven high-Tc phase (called "SC-II") between 0 and 25 GPa for the low-Tc and high-Tc phases of (NH3)yNaxFeSe, differing from the behavior of (NH3)yCsxFeSe, which has a pressure-driven high-Tc phase (SC-II) in addition to the superconducting phase (SC-I) observed at ambient and low pressures. The Tc-c phase diagram for both low-Tc and high-Tc phases shows that the Tc can be linearly scaled with c (or FeSe plane spacing), where c is a lattice constant. The reason why a pressure-driven high-Tc phase (SC-II) was found for neither low-Tc nor high-Tc phases of (NH3)yNaxFeSe is fully discussed, suggesting a critical c value as the key to forming the pressure-driven high-Tc phase (SC-II). Finally, the precise Tc-c phase diagram is depicted using the data obtained thus far from FeSe codoped with a metal and NH3 or amine, indicating two distinct Tc-c lines below c=17.5Å.

DOI： 10.1103/PhysRevB.97.094505

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We investigated the pressure dependence of electric transport and crystal structure of Ag-doped Bi2Se3. In the sample prepared by Ag doping of Bi2Se3, the Bi atom was partially replaced by Ag, i.e., Ag0.05Bi1.95Se3. X-ray diffraction patterns of Ag0.05Bi1.95Se3 measured at 0-30 GPa showed three different structural phases, with rhombohedral, monoclinic, and tetragonal structures forming in turn as pressure increased, and structural phase transitions at 8.8 and 24 GPa. Ag0.05Bi1.95Se3 showed no superconductivity down to 2.0 K at 0 GPa, but under pressure, superconductivity suddenly appeared at 11 GPa. The magnetic field (H) dependence of the superconducting transition temperature Tc was measured at 11 and 20.5 GPa, in order to investigate whether the pressure-induced superconducting phase is explained by either p-wave polar model or s-wave model.

DOI： 10.1103/PhysRevB.97.104503

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We report the crystal structure and physical properties of Sr2-xLaxIrO4 synthesized by mechanical alloying. The magnetic transition temperature TN and electrical resistivity decreased with increasing La doping, consistent with previous studies involving single-crystalline samples. We also identified the relationship between TN and tetragonal distortion (c/a) in this system. This result suggests that the magnetism of the Sr214 system is strongly correlated with its crystal structure. Zero-field muon spin rotation/relaxation studies revealed that short-range antiferromagnetic ordering is realized in Sr1.9La0.1IrO4
also, the spin-glass state is stabilized in the low-temperature region. The Ir moment estimated from the longitudinal field μSR results is 0.045μB, ten times smaller than that of Sr2IrO4 (∼0.4μB), indicating that electrons are introduced into the Ir atoms.

DOI： 10.1103/PhysRevB.97.064425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have studied the local structure of layered Na-x(NH3)(y)MoSe2 system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe2 largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature (Theta(E)) of Mo-Mo distance hardly changes (similar to 264 K) indicating that bond strength of this distance remains unaffected by intercalation. On the other hand, Mo-Se distance suffers a softening, revealed by the decrease of (Theta(E)) from similar to 364 K to similar to 350 K. The results indicate that Na+ ion transported by NH3 molecules may enter between the two MoSe-layers resulting reduced Se-Se coupling. Therefore, increased hybridization between Se 4p and Mo 4d orbitals due to inter-layer disorder is the likely reason of metallicity in intercalated MoSe2 and superconductivity at low temperature.

DOI： 10.1016/j.jpcs.2017.07.011

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

A band gap is opened in bilayer graphene (BLG) by applying an electric field perpendicular to the layer, which offers versatility and controllability in graphene-based electronics. The presence of the band gap has been confirmed using double-gated BLG devices in which positive and negative gate voltages are applied to each side of BLG. An alternative method to induce the electric field is electron and hole doping of each side of BLG using electron-transfer adsorbates. However, the generation of the band gap by carrier doping is still under investigation. Here, we determined whether the electron/hole doping can produce the electric field required to open the band gap by measuring the temperature dependence of conductivity for BLG placed between electron-donor self-assembled monolayers (SAMs) and electron-acceptor molecules. We found that some devices exhibited a band gap and others did not. The potentially irregular and variable structure of SAMs may affect the configuration of the electric field, yielding variable electronic properties. This study demonstrates the essential differences between gating and doping.

DOI： 10.1038/s41598-017-11822-9

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We have produced a superconducting binary-elements intercalated graphite, CaxSr1-xCy, with the intercalation of Sr and Ca in highly-oriented pyrolytic graphite; the superconducting transition temperature, T(c1)was similar to 3 K. The superconducting CaxSr1-xCy sample was fabricated with the nominal x value of 0.8, i.e., Ca0.8Sr0.2Cy. Energy dispersive X-ray (EDX) spectroscopy provided the stoichiometry of Ca0.5(2)Sr0.5(2)C-y for this sample, and the X-ray powder diffraction (XRD) pattern showed that Ca0.5(2)Sr0.5(2)Cy took the SrC6-type hexagonal-structure rather than CaC6-type rhombohedral-structure. Consequently, the chemical formula of CaxSr1-xCy sample could be expressed as 'Ca0.5(2)Sr0.5(2)C6'. The XRD pattern of Ca0.5(2)Sr0.5(2)C6 was measured at 0-31 GPa, showing that the lattice shrank monotonically with increasing pressure up to 8.6 GPa, with the structural phase transition occurring above 8.6 GPa. The pressure dependence of Tc was determined from the DC magnetic susceptibility and resistance up to 15 GPa, which exhibited a positive pressure dependence of T-c up to 8.3 GPa, as in YbC6, SrC6, KC8, CaC6 and Ca0.6K0.4C8. The further application of pressure caused the rapid decrease of Tc. In this study, the fabrication and superconducting properties of new binary-elements intercalated graphite, CaxSr1-xCy, are fully investigated, and suitable combinations of elements are suggested for binaryelements intercalated graphite.

DOI： 10.1038/s41598-017-07763-y

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have studied new superconductors prepared by metal doping of two-dimensional (2D) layered materials, FeSe and FeSe0.5Te0.5, using ethylenediamine (EDA). The superconducting transition temperatures (T(c)s) of metal-doped FeSe and metal-doped FeSe0.5Te0.5, i.e., (EDA)(y)MxFeSe and (EDA)(y)MxFeSe0.5Te0.5 (M: Li, Na, and K), were 31-45 K and 19-25 K, respectively. The stoichiometry of each sample was clarified by energy dispersive x-ray (EDX) spectroscopy, and the x-ray powder diffraction pattern indicated a large expansion of lattice constant c, indicating the cointercalation of metal atoms and EDA. The pressure dependence of superconductivity in (EDA)(y)NaxFeSe0.5Te0.5 has been investigated at a pressure of 0-0.8GPa, showing negative pressure dependence in the same manner as (NH3)(y)NaxFeSe0.5Te0.5. The T-c-c phase diagrams of MxFeSe and MxFeSe0.5Te0.5 were drawn afresh from the T-c and c of (EDA)(y)MxFeSe and (EDA)(y)MxFeSe0.5Te0.5, showing that the T-c increases with increasing c but that extreme expansion of c reverses the T-c trend.

DOI： 10.1103/PhysRevB.96.014502

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The synthesis of a hexacyclic fused polyheteroaromatic system, namely carbazolo[2,1-a] carbazole, via a simple two-step route is reported. Additionally, the characterization of its electronic structure is described, corresponding to a very stable molecule that is also transparent to visible light. Solid state packing is determined by X-ray diffraction analysis which, in combination with the computational calculation of the charge transfer parameters, reveals an adequate molecular arrangement to achieve a low anisotropic environment for charge transport. This p-conjugated system is used as an organic semiconductor for the fabrication of thin film and single crystal organic field-effect transistors (OFETs). Furthermore, the evaluation of different gate dielectrics results in transistors with very good hole mobilities and low operating voltages. In agreement with these results, carbazolo[2,1-a] carbazole becomes one of the best organic semiconductors belonging to the family of carbazole-based azaphenacenes.

DOI： 10.1039/c7tc02139a

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Field-effect transistors (FETs) were fabricated using exfoliated single crystals of Mo(Se1-x Te-x)(2) with an x range of 0 to 1, and the transistor properties fully investigated at 295 K in four-terminal measurement mode. The chemical composition and crystal structure of exfoliated single crystals were identified by energy-dispersive x-ray spectroscopy (EDX), single-crystal x-ray diffraction, and Raman scattering, suggesting the 2H - structure in all Mo(Se1-x Te-x)(2). The lattice constants of a and c increase monotonically with increasing x, indicating the substitution of Se by Te. When x &lt; 0.4 in a FET with a thin single crystal of Mo(Se1-x Te-x)(2), n-channel FET properties were observed, changing to p-channelor ambipolar operation for x &gt; 0.4. In contrast, the polarity of a thick single-crystal Mo(Se1-x Te-x)(2) FET did not change despite an increase in x. The change of polarity in a thin single-crystal FET was well explained by the variation of electronic structure. The absence of such change in the thick single-crystal FET can be reasonably interpreted based on the large bulk conduction due to naturally accumulated electrons. The mu value in the thin single-crystal FET showed a parabolic variation, with a minimum mu at around x = 0.4, which probably originates from the disorder of the single crystal caused by the partial replacement of Se by Te, i.e., a disorder that may be due to ionic size difference of Se and Te.

DOI： 10.1103/PhysRevB.95.245310

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Electrostatic carrier-doping is attracting serious attention as a meaningful technique for producing interesting electronic states in two-dimensional (2D) layered materials. Ionic-liquid gating can provide the critical carrier density required to induce the metal-insulator transition and superconductivity. However, the physical properties of only a few materials have been controlled by the electrostatic carrier-doping during the past decade. Here, we report an observation of superconductivity in a 2D layered material, LaOBiS2, achieved by the electrostatic electron-doping. The electron doping of LaOBiS2 induced metallic conductivity in the normally insulating LaOBiS2, ultimately led to superconductivity. The superconducting transition temperature, T-c, was 3.6 K, higher than the 2.7K seen in LaO1-xFxBiS2 with an electron-doped BiS2 layer. A rapid drop in resistance (R) was observed at low temperature, which disappeared with the application of high magnetic fields, implying a superconducting state. This study reveals that electron-doping is an important technique for inducing superconductivity in 2D layered BiS2 materials. Published by AIP Publishing.

DOI： 10.1063/1.4972400

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

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 (mu) greater than 1cm(2) V-1 s(-1). The highest mu value in the thin-film FETs fabricated in this study was 5.4 cm(2) 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.

DOI： 10.1038/srep38535

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From the C 1s and K 2p photoelectron holograms, we directly reconstructed atomic images of the cleaved surface of a bimetal-intercalated graphite superconductor, (Ca, K) C-8, which differed substantially from the expected bulk crystal structure based on x-ray diffraction (XRD) measurements. Graphene atomic images were collected in the in-plane cross sections of the layers 3.3 angstrom and 5.7 angstrom above the photoelectron emitter C atom and the stacking structures were determined as AB-and AA-type, respectively. The intercalant metal atom layer was found between two AA-stacked graphenes. The K atomic image revealing 2 x 2 periodicity, occupying every second centre site of C hexagonal columns, was reconstructed, and the Ca 2p peak intensity in the photoelectron spectra of (Ca, K) C-8 from the cleaved surface was less than a few hundredths of the K 2p peak intensity. These observations indicated that cleavage preferentially occurs at the KC8 layers containing no Ca atoms.

DOI： 10.1038/srep36258

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Na-intercalated FeSe0.5Te0.5 was prepared using the liquid NH3 technique, and a superconducting phase exhibiting a superconducting transition temperature (T-c) as high as 27 K was discovered. This can be called the high-T-c phase since a 21 K superconducting phase was previously obtained in (NH3)(y)NaxFeSe0.5Te0.5. The chemical composition of the high-T-c phase was determined to be (NH3)(0.61(4))Na-0.63(5) Fe0.85Se0.55(3) Te-0.44(2). The x-ray diffraction patterns of both phases show that a larger lattice constant c (i.e., FeSe0.5Te0.5 plane spacing) produces a higher T-c. This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher T-c. The high-T-c phase converted to the low-T-c phase within several days, indicating that it is a metastable phase. The temperature dependence of resistance for both phases was recorded at different magnetic fields, and the critical fields were determined for both phases. Finally, the T-c versus c phase diagram was prepared for the metal-doped FeSe0.5Te0.5, which is similar to that of metal-doped FeSe, although the T-c is lower.

DOI： 10.1103/PhysRevB.94.174505

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Potassium-doped picene (K(3.0)picene) with a superconducting transition temperature (TC) as high as 14 K at ambient pressure has been prepared using an annealing technique. The shielding fraction of this sample was 5.4% at 0 GPa. The TC showed a positive pressure-dependence and reached 19 K at 1.13 GPa. The shielding fraction also reached 18.5%. To investigate the chemical composition and the state of the picene skeleton in the superconducting sample, we used energy-dispersive x-ray (EDX) spectroscopy, MALDI-time-of-flight (MALDI-TOF) mass spectroscopy and x-ray diffraction (XRD). Both EDX and MALDI-TOF indicated no contamination with materials other than K-doped picene or K-doped picene fragments, and supported the preservation of the picene skeleton. However, it was also found that a magnetic K-doped picene sample consisted mainly of picene fragments or K-doped picene fragments. Thus, removal of the component contributing the magnetic quality to a superconducting sample should enhance the volume fraction.

DOI： 10.1088/0953-8984/28/44/444001

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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.

DOI： 10.1088/0953-8984/28/33/334001

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We report syntheses of new superconducting metal-doped MoSe2 materials (MxMoSe2). The superconducting MxMoSe2 samples were prepared using a liquid NH3 technique, and can be represented as '(NH3)(y)MxMoSe2'. The T(c)s of these materials were approximately 5.0 K, independent of x and the specific metal atom. X-ray diffraction patterns of (NH3)(y)NaxMoSe2 were recorded using polycrystalline powders. An increase in lattice constant c showed that the Na atom was intercalated between MoSe2 layers. The x-independence of c was observed in (NH3)(y)NaxMoSe2, indicating the formation of a stoichiometric compound in the entire x range, which is consistent with the x-independence of T-c. A metallic edge of the Fermi level was observed in the photoemission spectrum at 30 K, demonstrating its metallic character in the normal state. Doping of MoSe2 with Li and K also yielded superconductivity. Thus, MoSe2 is a promising material for designing new superconductors, as are other transition metal dichalcogenides.

DOI： 10.1038/srep29292

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Metal intercalation to graphite produces various types of superconductors. The highest superconducting transition temperature T-c (onset temperature, T-c(onset), of 11.5 K) was found in Ca intercalated graphite, denoted CaC6.T-c(onset) increased up to 15.1 K at 7.5 GPa, implying a positive pressure dependence. However, no new metal-intercalated graphite superconductors with T-c(onset) higher than 11.5 K at ambient pressure have so far been reported. To search for new graphite superconductors, we successfully synthesized binary-element-intercalated graphite, CaxK1-xCy. Their structure resembles that of KC8. T-c increased continuously with increasing x. Furthermore, the pressure dependence of T-c in Ca0.6K0.4C8 was investigated over a wide pressure range from 0-43 GPa. T-c (= 9.6 K at 0 GPa) increased to 11.6 K at 3.3 GPa, and decreased to 2.0 K at 41 GPa. This behavior is similar to that of CaC6, albeit with a lower maximum T-c. Xray diffraction patterns were measured under high pressures of 0-24 GPa, and suggest a structural transition at 15 GPa. Evidence is given for superconducting graphite involving binary metal intercalation. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.carbon.2016.01.071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The superconducting transition temperature T-c of ammoniated metal-doped FeSe (NH3)(y)MxFeSe (M: metal atom) has been scaled with the FeSe plane spacing, and it has been suggested that the FeSe plane spacing depends on the location of metal atoms in (NH3)(y)MxFeSe crystals. Although the crystal structure of (NH3)(y)LixFeSe exhibiting a high T-c (similar to 44 K) was determined from neutron diffraction, the structure of (NH3)(y)MxFeSe exhibiting a low T-c (similar to 32 K) has not been determined thus far. Here, we determined the crystal structure of (NH3)(y)Cs0.4FeSe (T-c = 33 K) through the Rietveld refinement of the x-ray diffraction (XRD) pattern measured with synchrotron radiation at 30 K. The XRD pattern was analyzed based on two different models, on-center and off-center, under a space group of 14/mmm. In the on-center structure, the Cs occupies the 2a site and the N of NH3 may occupy either the 4c or 2b site, or both. In the off-center structure, the Cs may occupy either the 4c or 2b site, or both, while the N occupies the 2a site. Only an on-center structure model in which the Cs occupies the 2a and the N of NH3 occupies the 4c site provided reasonable results in the Rietveld analysis. Consequently, we concluded that (NH3)(y)Cs0.4FeSe can be assigned to the on-center structure, which produces a smaller FeSe plane spacing leading to the lower T-c.

DOI： 10.1103/PhysRevB.93.104508

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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.

DOI： 10.1038/srep21008

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Language：English   Publishing type：Part of collection (book)   Publisher：Pan Stanford Publishing Pte. Ltd.  

DOI： 10.1201/b19365

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Doping with the optimum concentration of carriers (electrons or holes) can modify the physical properties of materials. Therefore, improved ways to achieve carrier doping have been pursued extensively for more than 50 years. Metal-intercalation is one of the most important techniques for electron doping of organic / inorganic solids, and has produced superconductors from insulators and metallic solids. The most successful examples are metal-intercalated graphite and C-60 superconductors. Metal intercalation has been performed using solid-reaction and liquid solvent techniques. However, precise control of the quantity of intercalants in the target solids can be difficult to achieve using these methods, as that quantity depends largely on the initial conditions. Here we report an electrochemical method for metal-intercalation, and demonstrate the preparation of superconductors using organic and inorganic materials (C-60 and FeSe). The metal atoms are effectively intercalated into the spaces in C-60 and FeSe solids by supplying an electric current between electrodes in a solvent that includes electrolytes. The recorded superconducting transition temperatures, T-c's, were the same as those of metal-intercalated C-60 and FeSe prepared using solid-reaction or liquid solvent techniques. This technique may open a new avenue in the search for organic / inorganic superconductors.

DOI： 10.1038/srep18931

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Language：English   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2150

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DOI： 10.11316/jpsgaiyo.71.2.0_2177

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DOI： 10.11316/jpsgaiyo.71.2.0_1691

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DOI： 10.11316/jpsgaiyo.71.2.0_2174

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DOI： 10.11316/jpsgaiyo.71.1.0_2377

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DOI： 10.1002/aelm.201500085

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We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH3)(y)NaxFeSe. To clarify the origin of the multiple superconducting phases, the variation of T-c was fully investigated as a function of x in (NH3)(y)NaxFeSe. The 32 K superconducting phase is mainly produced in the low-x region below 0.4, while only a single phase is observed at x = 1.1, with T-c = 45 K, showing that the T-c depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH3)(y)NaxFeSe does not change as x increases up to 1.1, i.e., the space group of I4/mmm. The lattice constants, a and c, of the low-T-c phase (T-c = 32.5 K) are 3.9120(9) and 14.145(8) angstrom, respectively, while a = 3.8266(7) angstrom and c = 17.565(9) angstrom for the high-T-c phase (similar to 46 K). The c increases in the high T-c phase, implying that the T-c is directly related to c. In (NH3)(y)LixFeSe material, the T-c varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of T-c is different from that of (NH3)(y)NaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.

DOI： 10.1038/srep12774

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

'Aromatic hydrocarbon' implies an organic molecule that satisfies the (4n + 2) pi-electron rule and consists of benzene rings. Doping solid aromatic hydrocarbons with metals provides the superconductivity. The first discovery of such superconductivity was made for K-doped picene (K(x)picene, five benzene rings). Its superconducting transition temperatures (T-c's) were 7 and 18 K. Recently, we found a new superconducting K(x)picene phase with a T-c as high as 14 K, so we now know that K(x)picene possesses multiple superconducting phases. Besides K(x)picene, we discovered new superconductors such as Rb(x)picene and Caxpicene. A most serious problem is that the shielding fraction is &lt;= 615% for K(x)picene and Rb(x)picene, and it is often similar to 1% for other superconductors. Such low shielding fractions have made it difficult to determine the crystal structures of superconducting phases. Nevertheless, many research groups have expended a great deal of effort to make high quality hydrocarbon superconductors in the five years since the discovery of hydrocarbon superconductivity. At the present stage, superconductivity is observed in certain metaldoped aromatic hydrocarbons (picene, phenanthrene and dibenzopentacene), but the shielding fraction remains stubbornly low. The highest priority research area is to prepare aromatic superconductors with a high superconducting volume-fraction. Despite these difficulties, aromatic superconductivity is still a core research target and presents interesting and potentially breakthrough challenges, such as the positive pressure dependence of T-c that is clearly observed in some phases of aromatic hydrocarbon superconductors, suggesting behavior not explained by the standard BCS picture of superconductivity. In this article, we describe the present status of this research field, and discuss its future prospects. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2015.02.015

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Electron transfer between graphene and an adsorbed molecule is systematically studied using a variety of electron acceptor/donor molecules. Doping efficiency is determined by the difference between the Fermi energy of graphene and the lowest unoccupied molecular orbital or highest unoccupied molecular orbital level of the molecule. The maximum carrier density accumulated by molecular adsorption is limited by the small density of states in graphene.

DOI： 10.1002/aelm.201500073

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The pressure dependence of the superconducting transition temperature (T-c) and unit cell metrics of tetragonal (NH3)(y)Cs0.4FeSe were investigated in high pressures up to 41 GPa. The T-c decreases with increasing pressure up to 13 GPa, which can be clearly correlated with the pressure dependence of c (or FeSe layer spacing). The T-c vs. c plot is compared with those of various (NH3)(y)MxFeSe (M: metal atoms) materials exhibiting different T-c and c, showing that the T-c is universally related to c. This behaviour means that a decrease in two-dimensionality lowers the T-c. No superconductivity was observed down to 4.3 K in (NH3)(y)Cs0.4FeSe at 11 and 13 GPa. Surprisingly, superconductivity re-appeared rapidly above 13 GPa, with the T-c reaching 49 K at 21 GPa. The appearance of a new superconducting phase is not accompanied by a structural transition, as evidenced by pressure-dependent XRD. Furthermore, T-c slowly decreased with increasing pressure above 21 GPa, and at 41 GPa superconductivity disappeared entirely at temperatures above 4.9 K. The observation of a double-dome superconducting phase may provide a hint for pursuing the superconducting coupling-mechanism of ammoniated/non-ammoniated metal-doped FeSe.

DOI： 10.1038/srep09477

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry  

DOI： 10.1039/c5tc90044a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

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.

DOI： 10.1021/ol503723j

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_1718

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_1719

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Field-effect transistors (FETs) with single crystals of a new phenacene-type molecule, [8]phenacene, were fabricated and characterized. This new molecule consists of a phenacene core of eight benzene rings, with an extended p-conjugated system, which was recently synthesized for use in an FET by our group. The FET characteristics of an [8]phenacene single-crystal FET with SiO2 gate dielectrics show typical p-channel properties with an average field-effect mobility, &lt;mu &gt;, as high as 3(2) cm(2) V-1 s(-1) in two-terminal measurement mode, which is a relatively high value for a p-channel single-crystal FET. The hmi was determined to be 6(2) cm(2) V-1 s(-1) in four-terminal measurement mode. Low-voltage operation was achieved with PbZr0.52Ti0.48O3 (PZT) as the gate dielectric, and an electric-double-layer (EDL) capacitor. The &lt;mu &gt; and average values of absolute threshold voltage, &lt;vertical bar V-th vertical bar &gt;, were 1.6(4) cm(2) V-1 s(-1) and 5(1) V, respectively, for PZT, and 4(2) x 10(-1) cm(2) V-1 s(-1) and 2.38(4) V, respectively, for the EDL capacitor; these values were evaluated in two-terminal measurement mode. The inverter circuit was fabricated using [8]phenacene and N, N'-1H, 1H-perfluorobutyldicyanoperylene-carboxydi-imide single-crystal FETs. This is the first logic gate circuit using phenacene molecules. Furthermore, the relationship between mu and the number of benzene rings was clarified based on this study and the previous studies on phenacene single-crystal FETs.

DOI： 10.1039/c5tc00960j

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Language：English   Publisher：ROYAL SOC CHEMISTRY  

DOI： 10.1039/c5tc90044a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Field-effect transistors (FETs) have been fabricated with thin films of a series of 2,9-dialkylated phenanthro [1,2-b:8,7-b'] dithiophene derivatives (C-n-PDTs). The FET characteristics of C-n-PDT thin-film FETs with an SiO2 gate dielectric as well as high-k gate dielectrics were recorded, and the dependence of the field-effect mobility, mu, on the number (n) of carbon atoms in the alkyl chains was investigated, showing that the 2,9-didodecylphenanthro[1,2-b:8,7-b'] dithiophene (C-12-PDT) thin-film FET displays superior properties, with mu s as high as 1.8 cm(2) V-1 s(-1) for the SiO2 gate dielectric and 2.2 cm(2) V-1 s(-1) for the HfO2 gate dielectric. The average mu values, &lt;mu &gt;, reach 1.1(5) and 1.8(6) cm(2) V-1 s(-1), respectively, for the SiO2 and ZrO2 gate dielectrics. Low-voltage operation, showing an absolute average threshold voltage &lt;|V-th|&gt; of similar to 11 V, was implemented, together with the above high &lt;mu &gt; of similar to 2 cm(2) V-1 s(-1). Also, a flexible FET was fabricated with a parylene gate dielectric. The results of this study show the potential of the C-12-PDT molecule for application in a high-performance transistor.

DOI： 10.1039/c4tc02413c

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_1246

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Language：English   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_1894

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_2751

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_2236

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Language：English   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_1994

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Language：English   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_1995

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We fabricated picene thin-film field-effect transistors (FETs) with an ionic liquid gel and ionic liquid sheet as the gate electrolyte, and then used electron spin resonance (ESR) to investigate the carrier injection process in the organic electric double layer (EDL) FET. The ESR spectra strongly depended on the morphology of gate electrolytes. Three types of carrier injection processes in the EDL-FET were observed by examining the applied-bias time, organic-layer thickness, and gate-voltage dependencies of the electric-field-induced ESR spectrum: (1) interface injection due to electrostatic EDL formation, (2) bulk injection due to penetration of ions (electrochemical bulk doping), and (3) electrochemical reaction. These findings are significant for designing novel materials using the EDL-FET technique because three different carrier injection processes may lead to different physical properties, even in the same organic material. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2014.08.043

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

In this study, we obtained the first cation radical solid of a highly symmetric (D-6h) polyaromatic hydrocarbon, coronene, by electrooxidation. The (coronene)(3)Mo6Cl14 salt, which is formed with an O-h-symmetric molybdenum cluster unit Mo6Cl142-, has an isotropic cubic structure with Pm (3) over barm symmetry. The presence of two orientations for the coronene molecules related by an in-plane 90 degrees rotation (merohedral disorder) allows for fourfold symmetry along the &lt; 100 &gt; direction. The disorder has dynamic features because H-2 NMR spectroscopic studies revealed that the coronene molecules undergo an in-plane flipping motion. The observation of two motional sites with significantly different rotational rates (300 Hz and 5 MHz at 103 K) in an approximate 2: 1 ratio appears to be consistent with the splitting of a Raman-active A(1g) mode, confirming a random charge-disproportionated state instead of a uniform partially-charged state. The slower-and faster-rotating species are assigned to charge-rich and charge-poor coronenes, respectively, with respect to C-H center dot center dot center dot Cl hydrogen bonds with neighboring Mo6Cl142- cluster units. The electrical conductivity of the salt is rather high but is well-described by a three-dimensional (3D) variable-range hopping mechanism, which is possibly associated with the random charge disproportionation. These results provide a significant step forward in developing an isotropic 3D pi-conducting system composed of planar pi-conjugated molecules.

DOI： 10.1002/ejic.201400119

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Language：English   Publisher：WILEY-V C H VERLAG GMBH  

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 cm(2)V(-1)s(-1) for [6]phenacene, and in single-crystal FETs the highest value is 6.3 cm(2)V(-1)s(-1) for [7]phenacene. The phenacene thin-film FETs show O-2-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.

DOI： 10.1002/ejic.201402168

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CHEMICAL SOC JAPAN  

The continuous flow reaction technique has been applied to the photocyclization of 1,2-diarylethenes, the so-called Mallory reaction, to afford phenacenes in high chemical yields and efficiencies (114-288 mg h(-1)). The present technique will allow us to produce several grams of phenacenes at a time.

DOI： 10.1246/cl.140182

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

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 pi-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 m 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 mu was evaluated as 1.2(3) cm(2) V-1 s(-1). The mu 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 m was evaluated as 8(5) cm(2) V-1 s(-1). The mu values recorded in this study show that [8] phenacene is a promising molecule for transistor applications.

DOI： 10.1038/srep05330

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

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 pi-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.

DOI： 10.1021/jo500543h

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

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, m, in a picene-(C14H29)(2) thin-film FET with PbZr0.52Ti0.48O3 (PZT) gate dielectric reached similar to 21 cm(2) V-1 s(-1), which is the highest mu value recorded for organic thin-film FETs; the averagem mu value (&lt;mu &gt;) evaluated from twelve FET devices was 14(4) cm(2)V(-1) s(-1). The, m. values for picene-(C14H29)(2) thin- film FETs with other gate dielectrics such as SiO2, Ta2O5, ZrO2 and HfO2 were greater than 5 cm(2) V-1 s(-1), and the lowest absolute threshold voltage, vertical bar V-th vertical bar, (5.2 V) was recorded with a PZT gate dielectric; the average jVthj for PZT gate dielectric is 7(1) V. The solution-processed picene-(C14H29)(2) FET was also fabricated with an SiO2 gate dielectric, yielding mu = 3.4 x 10(-2) cm(2) V-1 s(-1). These results verify the effectiveness of picene-(C14H29)(2) for electronics applications.

DOI： 10.1038/srep05048

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DOI： 10.1103/PhysRevB.89.144509

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The interface between the single crystal and the Au source/drain electrodes in [7]phenacene single-crystal field-effect transistors (FETs) was modified using 14 electron acceptors with different redox potentials. The effective hole-injection barrier heights (phi(eff)(h)s) for [7]phenacene single-crystal FETs have been plotted as a function of the redox potential (E-redox) of the inserted electron acceptors, showing that the phi(eff)(h) decreases with increasing E-redox. The highest phi(eff)(h) occurs without inserted material (electron acceptors), and this deviates from the otherwise linear relationship between phi(eff)(h) and E-redox. We have investigated the temperature dependence of phi(eff)(h) in an attempt to determine why the phi(eff)(h) value without inserted material is so high, which suggests that no additional barrier, such as a tunneling barrier, is formed in the device. We conclude that the pure Schottky barrier in this FET is lowered very significantly by the insertion of an electron acceptor. The gate-voltage dependence of phi(eff)(h) suggests a slight reduction of Schottky barrier height owing to hole accumulation. Furthermore, the clear correlation between threshold voltage and redox potential suggests a relationship between threshold voltage and phi(eff)(h). Controlling the interface between the single crystal and the source/drain electrodes in this FET produced a very high mu (similar to 6.9 cm(2) V-1 s(-1)) and low absolute threshold voltage, i.e., excellent FET characteristics. The topological characterization of inserted materials on [7]phenacene single crystals are achieved using atomic force microscope (AFM) and X-ray diffraction (XRD). The results show that the single crystals are not completely covered with the inserted materials and the inhomogeneous modification of inserted materials for single crystals effectively leads to the drastic change of hole-injection barrier between source/drain electrodes and single-crystal active layer.

DOI： 10.1021/jp4107469

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We study the electronic properties in few-layer graphenes (FLGs) classified by even/odd layer number n. FLGs with even n have only parabolic energy dispersions, whereas FLGs with odd n have a linear dispersion besides parabolic ones. This difference leads to a distinct density of states in FLGs, experimentally confirmed by the gate-voltage dependence of the electric double-layer capacitance. Thus, FLGs with odd n are unique materials that have relativistic carriers originating in linear energy dispersion. © 2013 American Chemical Society.

DOI： 10.1021/nl402404z

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Alkali-metal-intercalated FeSe materials, (NH3) yM0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)yCs 0.4FeSe shows a superconducting transition temperature (T c) as high as 31.2 K, which is higher by 3.8 K than the Tc of nonammoniated Cs0.4FeSe. The Tcs of (NH 3)yK0.4FeSe and (NH3) yRb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The Tc of (NH 3)yCs0.4FeSe shows a negative pressure dependence. A clear correlation between Tc and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.88.094521

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The magnetic ground state of the fcc phase of the Mott insulator Cs 3C60 was studied using a low-temperature electron spin resonance technique, and antiferromagnetic resonance (AFMR) below 1.57 K was directly observed at ambient pressure. The AFMR modes for the fcc phase of Cs3C60 were investigated using a conventional two-sublattice model with uniaxial anisotropy, and the spin-flop field was determined to be 4.7 kOe at 1.57 K. The static magnetic exchange interactions and anisotropy field for fcc-Cs3C60 were also estimated. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0953-8984/25/36/366001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Growth and structure of picene thin films on SiO2 surface were studied from ultrathin film to thick film regimes (nominal film thickness upto 100 nm) by using X-ray diffraction and atomic force microscopy. We found that from initial film growth picene form crystalline grains with a certain multilayer height, in which their crystalline ab-plane orients parallel to the substrate surface. With increasing the film thickness the number of the grains increases with keeping the grain height until the substrate surface is fully covered. These results indicate that picene films exhibit island growth mode on SiO2 surface.

DOI： 10.1080/15421406.2013.807718

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Single crystal field-effect transistors (FETs) using [6] phenacene and [7] phenacene show p-channel FET characteristics. Field-effect mobilities, mu s, as high as 5.6 x 10 (1) cm(2) V (1) s (1) in a [6] phenacene single crystal FET with an SiO2 gate dielectric and 2.3 cm(2) V (1) s (1) in a [7] phenacene single crystal FET were recorded. In these FETs, 7,7,8,8-tetracyanoquinodimethane (TCNQ) was inserted between the Au source/drain electrodes and the single crystal to reduce hole-injection barrier heights. The mu reached 3.2 cm(2) V (1) s (1) in the [7] phenacene single crystal FET with a Ta2O5 gate dielectric, and a low absolute threshold voltage vertical bar V-TH vertical bar (6.3 V) was observed. Insertion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ) in the interface produced very a high mu value (4.7-6.7 cm(2) V (1) s (1)) in the [7] phenacene single crystal FET, indicating that F(4)TCNQ was better for interface modification than TCNQ. A single crystal electric double-layer FET provided mu as high as 3.8 x 10 (1) cm(2) V (1) s (1) and vertical bar V-TH vertical bar as low as 2.3 V. These results indicate that [6] phenacene and [7] phenacene are promising materials for future practical FET devices, and in addition we suggest that such devices might also provide a research tool to investigate a material's potential as a superconductor and a possible new way to produce the superconducting state. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2013.03.035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance C-g. However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance C-EDL between the ionic liquid and graphene involves the series connection of C-g and the quantum capacitance C-q, which is proportional to the density of states. We investigated the variables that determine C-EDL at the molecular level by varying the number of graphene layers n and thereby optimising C-q. The C-EDL value is governed by C-q at n, 4, and by C-g at n &gt; 4. This transition with n indicates a composite nature for C-EDL. 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.

DOI： 10.1038/srep01595

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Field-effect transistor (FET) devices using thin crystals of FeSe1-xTex (x = 0.9 and 1.0) have been fabricated with an electric-double-layer (EDL) capacitor. Despite the presence of substantial quantities of electron and hole carriers in the bulk due to the semimetallic electronic structure of FeSe1-xTex, we have observed p-channel depletion-type FET characteristics, in contrast to the n-channel normally on FET characteristics of a Bi2Se3 EDL FET. In FeSe1-xTex, the mobile carriers, holes, are depleted in the channel region by accumulating electrons, resulting in a decrease in conductivity. This result is consistent with the experimentally observed positive Hall coefficient at room temperature. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4795626]

DOI： 10.1063/1.4795626

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Graphene has two kinds of edges which have different electronic properties. A singular electronic state emerges at zigzag edges, while it disappears at armchair edges. We study the edge-dependent transport properties in few-layer graphene by applying a side gate voltage to the edge with an ionic liquid. The devices indicating a conductance peak at the charge neutrality point have zigzag edges, confirmed by micro-Raman spectroscopy mapping. The hopping transport between zigzag edges increases the conductance.

DOI： 10.1021/nl3044844

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DOI： 10.1103/PhysRevB.87.060505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Information on the interfacial electronic structure in organic photovoltaics (OPVs) is essential for fully understanding features of device operation such as the photocurrent generation and relative energy band offsets at the donor/acceptor interface, which directly affect the open circuit voltage (V-oc). Kelvin probe (KP) measurements fully reveal the energy level alignment in a prototype OPV with a copper phthalocyanine (CuPc)/fullerene (C-60) planar heterojunction. Energy level pinning at the CuPc/C-60 junction fixes the energy band offsets of C-60. A downward energy shift of about 0.9 eV appears at the C-60/bathocuproine junction, which may act as a hole-blocking barrier. A combination of KP and current density-voltage measurements indicates that photocurrent generation depends strongly on the magnitude of the upward energy shift at the CuPc/C-60 junction. The dependence of V-oc on the substrate work function is also discussed in terms of the energy level alignment at indium tin oxide/CuPc/C-60 junctions. (c) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2012.09.025

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

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.

DOI： 10.1039/c3cp53598c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

A new picene-type molecule, phenanthro[1,2-b : 8,7-b'] dithiophene, has been synthesized for use in organic field-effect transistors (OFETs). The molecule consists of a phenanthrene core with two thiophene rings fused on the ends. This molecule can be recognized as a picene analogue. The electronic structure of the molecule was determined by its optical absorption spectrum together with a theoretical calculation based on density functional theory (DFT). The topological and electronic structures of thin films produced by direct thermal evaporation of the compounds and by deposition from a solution were characterized by optical imaging, X-ray diffraction, and atomic force microscopy. FET devices were fabricated with these thin films, and showed field-effect mobility as high as 10(-1) cm(2) V-1 s(-1).

DOI： 10.1039/c3ra44050h

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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 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- 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 can not be understood with a simple phonon mechanism of BCS superconductivity. Third, we report a synthesis method for superconducting K(3)picene by a solution process with monomethylamine CH3NH2. 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.

DOI： 10.1103/PhysRevB.86.214507

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Here we report an investigation of the growth of picene by supersonic molecular beam deposition on thermal silicon oxide and on a self-assembled monolayer of hexamethyldisiloxane (HMDS). In both cases film morphology shows a structure with very sharp island edges and well-separated islands which size and height depend on the deposition conditions. Picene films growth on bare silicon covered with hydrophobic HDMS shows islands characterized by large regular crystallites of several micrometers; on the other hand, films growth on silicon oxide shows smaller and thicker islands. We analyzed the details of the growth model and describe it as a balancing mechanism involving the weak interaction between molecules and surface and the strong picene-picene interaction that leads to a different Schwoebel-Ehrlich barrier in the first layer with respect to the successive one Finally, we study. the charge transport properties of these films by fabricating field-effect transistors devices in both top and bottom contact configuration. We notice that substrate influences the electrical properties of the device and we obtained a maximum mobility value of 1.2 cm(2) V-1 s(-1) measured on top contact devices in air.

DOI： 10.1021/jp304561s

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Structure and morphology of picene films under vacuum and O-2 atmosphere were studied by in situ synchrotron X-ray scattering. We observed that picene films exhibit a highly oriented and ordered structure, which is similar to the one reported for picene single crystals. Furthermore, we found that the film structure determined under vacuum remains nearly unchanged under O-2 atmosphere. The results provide new insights into a high hole mobility and O-2 gas sensing mechanism previously reported for picene thin film-based organic field-effect transistors. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2012.07.006

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Transistor characteristics and O-2 gas sensing properties are investigated for picene thin film field-effect transistors (FETs) with ZrO2, Ta2O5, HfO2 and BaxSr1-xTiO3 (x = 0.4). The low-voltage operation is achieved using the above oxides with high gate dielectric constant, contrary to SiO2 gate dielectric. The O-2 gas sensing is achieved at 11 s intervals without any bias stress by an application of pulse drain and gate voltages (V-D and V-G), in contrast to previous result in which the O-2 gas sensing was performed at least at 1 h step because of bias stress effect. The actual O-2 sensing-speed in the picene thin film FET was similar to 10 s for 3.8 Torr O-2, and the O-2 sensing limit was concluded to be 0.15-0.38 Torr. Furthermore, it has been found that the O-2 sensing properties are observed only under irradiation of light with wavelength below 400 nm. From the result, we have presented two scenarios for O-2 sensing mechanism in picene thin film FET. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.snb.2012.05.030

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Transistor characteristics are studied for field-effect transistors (FETs) with thin films of [6]phenacene, which has six benzene rings and W-shape structure. The molecular alignment preferable for FET transport is found to be formed in [6]phenacene thin films. The transistor shows clear p-channel FET characteristics with field-effect mobility mu as high as 3.7 cm(2) V-1 s(-1). The similar O-2 sensing properties to picene FET are observed in [6]phenacene thin film FET. The bias stress properties are observed in [6]phenacene thin film FET. The pulse-voltage application suppresses the bias-stress effect and it enables a continuous O-2 sensing in [6]phenacene FET. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747201]

DOI： 10.1063/1.4747201

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We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with a laser for photoconduction. A hole effective mass of 2.24m(0) and the hole mobility mu(h) &gt;= 9.0 cm(2)/Vs (298 K) were deduced in the Gamma-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.

DOI： 10.1103/PhysRevLett.108.226401

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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 cm(2) 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 X 10(-1) cm(2) V-1 s(-1) and low absolute value, 1.9 V, of threshold voltage.

DOI： 10.1021/jp300052p

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Thin film transistors (TFTs) with ionic liquid gate dielectrics, [1-ethyl-3-methylimidazolium][bis(trifluoromethanesulfonyl) imide] (emim[TFSI]) and [1-butyl-3-methylimidazolium][hexafluorophosphate] (bmim[PF6]), are fabricated with thin films of one dimensional (1D) hydrocarbon, [7]phenacene. P-channel characteristics are observed for [7] phenacene TFTs with both ionic liquids by use of platinum electrode. The field-effect mobility mu for [7]phenacene TFT with bmim[PF6] was recorded to be 0.28 cm(2) V-1 s(-1). The value of absolute threshold voltage, vertical bar V-TH vertical bar, was less than 2.5 V, showing low-voltage operation. The accumulation of hole in the [7]phenacene TFTs with ionic liquids was confirmed from the voltage or time dependence of capacitance in metal-insulator-semiconductor structure, which shows that these TFTs operate electrochemically and the carriers are accumulated in the whole of [7]phenacene thin films. (C) 2011 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.orgel.2011.08.016

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Superconductivity was recently discovered in solid potassium-intercalated picene (K(3)22ph), in which the picene molecule becomes trianionic (22ph(3-)). In this Letter, we conduct a theory-based study of the superconductivity of 22ph(3-) within the framework of BCS theory. We estimate the density of states N(epsilon(F)) on the Fermi level to be 2.2 states per (eV molecule spin) by using the theoretical intramolecular electron-phonon coupling l(x) and the experimental superconducting transition temperature T-c of 18 K. The theoretical value is consistent with the 1.2 states per (eV molecule spin) determined experimentally for K(3)22ph with T-c = 18 K, indicating the validity of our theoretical treatment and the electron-phonon mechanism for superconductivity. The predicted l(x), 0.206 eV, for 22ph(3-) is larger than any value reported for organic superconductors, so picene may have the largest l(x) among the superconductors reported so far.

DOI： 10.1103/PhysRevLett.107.077001

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The electronic properties of Eu2.75C60 are studied using magnetic susceptibility and electron spin resonance (ESR) from 2 to 300 K. Both the magnetic susceptibility and the ESR parameters clearly show an anomaly around the valence transition temperature, T-V = 70 K. The magnetic susceptibility shows weak temperature dependence above T-V, while it changes drastically to Curie-Weiss behavior below T-V. The low-temperature susceptibility can be reproduced by assuming the moment of free Eu2+ ions. This result reveals that Eu2.75C60 changes from the intermediate valence state to the divalent state below T-V. Although ESR signals above T-V should be attributed to conduction electrons, the ESR intensity below T-V follows the Curie-Weiss law with a distinct increase in the g-factor. This should be associated with a strong localization of pi electrons. We also found that, below similar to 17 K, the isothermal magnetization exhibits a weak hysteresis and thermoremanent magnetization appears. These results suggest that valence-ordered Eu2.75C60 undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Neel temperature, T-N = 17 K.

DOI： 10.1103/PhysRevB.83.245103

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A facile formation of picene was achieved by photosensitization of 1,2-di(1-naphthyl)ethane using 9-fluorenone as a sensitizer. This sensitized photoreaction is the first photochemical cyclization of ethylene-bridged naphthalene moieties to afford the picene skeleton. 5,8-Dibromopicene, prepared by this procedure using 1,2-di[1-(4-bromonaphthyl)]ethane as the substrate, was readily converted to novel functionalized picenes by conventional substitution and cross-coupling reactions.

DOI： 10.1021/ol200874q

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Field-effect transistors (FETs) with solid gate dielectrics are fabricated with thin films of the one-dimensional (1D) extended hydrocarbon [7]phenacene, which contains seven benzene rings. p-channel FET characteristics are observed for these FETs, with a mobility of 0.75 cm(2) V(-1) s(-1) at 100 Torr of O(2). The O(2) gas-sensing effect is examined for the [7]phenacene FET and for the 1D hydrocarbon picene FET. These FETs&apos; trap density and contact resistance are investigated with the multiple shallow trap and release model and the transfer line method. Unlike picene FETs, [7]phenacene FETs have few charge traps and are therefore air-stable. (C) 2011 American Institute of Physics. [doi:10.1063/1.3540648]

DOI： 10.1063/1.3540648

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The electronic structure of metallofullerene Ce@C-82 is probed by resonant x-ray emission spectroscopy at the Ce L-3 absorption edge. We observed a satellite structure in x-ray absorption and resonant emission spectra for Ce@C-82, which, we show, corresponds to the charge transfer induced by the core-hole potential in the final state, similarly to Pr@C-82. This charge-transfer satellite may be a common feature in metallofullerenes. The temperature dependence of the electronic structure is also investigated.

DOI： 10.1143/JPSJ.80.014702

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

New carbon-based superconductors are synthesized by intercalating metal atoms into the solid-phase hydrocarbons picene and coronene. The highest reported superconducting transition temperature, T-c, of a hydrocarbon superconductor is 18 K for K(3)picene. The physics and chemistry of the hydrocarbon superconductors are extensively described for Axpicene (A: alkali and alkali earth-metal atoms) for x = 0-5. The theoretical picture of their electronic structure is also reviewed. Future prospects for hydrocarbon superconductors are discussed from the viewpoint of combining electronics with condensed-matter physics: modification of the physical properties of hydrocarbon solids is explored by building them into a field-effect transistor. The features of other carbon-based superconductors are compared to clarify the nature of hydrocarbon superconductors.

DOI： 10.1039/c1cp20961b

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We use photoemission spectroscopy to study electronic structures of pristine and K-doped solid picene. The valence band spectrum of pristine picene consists of three main features with no state at the Fermi level (E(F)) while that of K-doped picene has three structures similar to those of pristine picene with new states near E(F), consistent with the semiconductor-metal transition. The K-induced change cannot be explained with a simple rigid-band model of pristine picene but can be interpreted by molecular-orbital calculations considering electron-intramolecular-vibration interaction. Excellent agreement of the K-doped spectrum with the calculations points to importance of electron-intramolecular-vibration interaction in K-doped picene.

DOI： 10.1103/PhysRevB.82.195114

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O-2 gas sensing behaviors are studied in picene thin film field-effect transistors (FETs) with hydrophobic polymer (Cytop(TM) or polystyrene) coated SiO2 gate dielectrics. Picene thin film FETs show a rapid reduction of hysteresis in transfer curves recorded in forward and reverse measurement modes, compared to a picene FET with hexamethydisilazane-coated SiO2. The picene FETs show very sensitive O-2 gas sensing effects down to similar to 10 ppm. A quantitative analysis is presented of the gate voltage (V-G) dependent mobility induced by O-2 exposure, i.e., the suppression of drain current at high V-G. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2010.06.003

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Self-assembled monolayers (SAMs) are efficient tuners of metal work functions, and have been widely used to modify source/drain electrodes of organic field-effect transistors (OFETs) to achieve effective charge injection from the electrodes into organic semi-conducting layers. The device characteristics of OFETs with Au electrodes modified with 1-alkanethiol SAMs were investigated and displayed an anomalous hysteresis opposite to that generally observed. Measurements of OFETs with different SAM molecular species or different average surface densities indicate that an increase in drain currents by tunneling-barrier narrowing accompanied by a structural order-disorder switching of the SAMs causes such hysteretic behavior. Furthermore, time evolution measurements of OFET drain currents indicate that an electric field perpendicular to the surface of the SAM-modified electrodes, i.e., a drain voltage, induces the switching. The anomalous hysteresis unveiled in this study is indicative of an electrically-stimulated switching of SAMs, which has many implications with respect to the development of future molecular switches. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2010.02.017

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Flexible picene thin film field-effect transistors (FETs) have been fabricated with parylene gate dielectric on polyethylene terephthalate substrates. The picene thin film FETs show p-channel output/transfer characteristics and the field-effect mobility mu reaches similar to 1 cm(2) V(-1) s(-1) in vacuum. The FET shows a clear O(2) gas sensing effect and negligible hysteresis in the transfer curves, indicating a possible application of the transistor as O(2) selective gas sensor. Furthermore, it has been found that the parylene gate dielectric can eliminate a reduction in on-state drain current caused by continuous bias-voltage application which is observed if a SiO(2) gate dielectric is used.

DOI： 10.1063/1.3360223

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds(1), boron-doped diamond(2) and-most prominently-iron arsenides such as LaO(1-x)F(x)FeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (T(c)) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (K(x)picene) shows T(c) values of 7K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (&lt;2 K), similar to the behaviour of Ca-intercalated graphite(1). The Tc of 18K is comparable to that of K-intercalated C(60) (ref. 4). This discovery of superconductivity in K(x)picene shows that organic hydrocarbons are promising candidates for improved T(c) values.

DOI： 10.1038/nature08859

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Low voltage operation of picene thin film field-effect transistor (FET) has been realized with 40 nm thick SiO(2) gate dielectrics coated by two polymers, Cytop (TM) and polystyrene. The picene FETs operated in low absolute gate voltage vertical bar V(G)vertical bar below 15 V for Cytop (TM) coated SiO(2) and 30 V for polystyrene coated SiO(2) gate dielectrics, and they showed a significant O(2) gas sensing effect down to similar to 10 ppm. Photoemission spectrum clarified that O(2) molecules penetrate into the thin films at O(2)/picene mole ratio of 1: 1. X-ray diffraction pattern of picene thin films showed highly oriented growth on the polymer-coated SiO(2).

DOI： 10.1063/1.3257373

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Resonant x-ray emission spectroscopy (RXES) was performed on the metallofullerene Pr@ C-82 at the Pr L-3 absorption edge. We verify not only nearly three-electron charge transfers from the metal to the cage but also back-electron transfer observed as a charge transfer satellite. The results are compared to theoretical calculations with a single-impurity Anderson model. Theory shows that the electronic structure of endohedral atom in the cage is atomiclike. The satellite structure originates from the charge transfer, i.e., dynamical screening effect, induced by the core-hole potential in the final state rather than from the valence fluctuation of the rare-earth metal in the ground state. We also performed the RXES measurement of Pr2O3 for comparison.

DOI： 10.1103/PhysRevB.80.205403

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C-60 and picene thin film field-effect transistors (FETs) in bottom contact structure have been fabricated with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PED-OT:PSS) electrodes for a realization of mechanical flexible organic FETs. The C-60 thin film FETs showed n-channel enhancement-type characteristics with the field-effect mobility mu value of 0.41 cm(2\) V-1 s(-1), while the picene thin film FET showed p-channel enhancement-type characteristics with the p of 0.61 cm(2) V-1 s(-1). The p values recorded for C-60 and picene thin film FETs are comparable to those for C-60 and picene thin film FETs with Au electrodes. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2009.01.006

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A C-70 close-packed surface was formed by a heating of the Si surface, which is covered with C-70 molecules. The close-packed surface is assigned to high-temperature hexagonal close-packed phase. The stability of C-70 close-packed surface and formation of nanometer scale structures are studied by the application of local electric field to the close-packed surface. An application of local electric field from scanning tunneling microscope tip to the C-70 close-packed surface caused molecular scale evaporation. The application of local electric field near strain in the surface produced a very large void by an evaporation of more than 20 of C-70 molecules.

DOI： 10.1063/1.3075959

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Transport characteristics and trap states are investigated in picene thin film field-effect transistor under O(2) atmosphere on the basis of multiple shallow trap and release (MTR) model. The channel transport is dominated by MTR below 300 K. It has been clarified on the basis of MTR model that the O(2)-exposure induces a drastic reduction in shallow trap density to increase both the field-effect mobility mu and on-off ratio. We also found that the O(2)-exposure never caused an increase in hole carrier density. Actually, a very high mu value of 3.2 cm(2) V(-1) s(-1) is realized under 500 Torr of O(2).

DOI： 10.1063/1.3076124

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

We have investigated the electronic structures of single-wall carbon nanotubes (SWCNTs) encapsulating C-70 fullerene and metallofullerenes M@C-82 (M = La, Gd, Dy), C-70 peapods (C-70@SWCNT) and metaltofullerene peapods (M@C-82@SWCNT), using photoemission spectroscopy. The photoemission spectra of C-70 and M@C-82 peas are obtained by subtracting the spectrum of pristine SWCNTs from those of the corresponding peapods. The electronic structure of the C-70 peas is similar to that of the C-70 film. For the M@C-82 peapods, on the other hand, the electronic structures of the M@C-82 peas are different. For the La@C-82 peapods, the singly occupied molecular orbital (SOMO) for the La@C-82 peas is located nearer to the Fermi level when compared with that in the La@C-82 film. For the DY@C-82 peapods, the effective Dy valence is estimated to be +2.9. This result demonstrates that effective charges move back from the C82 cage to Dy following encapsulation of DY@C-82 peas in the SWCNTs because of the strong hybridization interaction between Dy 4f and C 2p electrons, taking into account the Dy valence of +3 in the Dy@C-82 film. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.200879571

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A field-effect transistor (FET) with thin films of picene has been fabricated on SiO(2) gate dielectric. The FET showed p-channel enhancement-type FET characteristics with the field-effect mobility, mu, of 1.1 cm(2) V(-1) s(-1) and the on-off ratio of &gt;10(5). This excellent device performance was realized under atmospheric conditions. The p increased with an increase in temperature, and the FET performance was improved by exposure to air or O(2) for a long time. This result implies that this device is an air (O(2))-assisted FET. The FET characteristics are discussed on the basis of structural topography and the energy diagram of picene thin films.

DOI： 10.1021/ja803291a

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C(60) field-effect transistors (FETs) have been fabricated with parylene gate dielectric on Si/SiO(2), on polyethylene terephthalate, and commercially available transparent sheet substrates. The best performance of the C(60) FET device is achieved with parylene as gate dielectric: field-effect mobility of 0.41 cm(2) V(-1) s(-1) and on-off ratio of similar to 10(7). The excellent FET characteristics are recorded without any annealing, and the devices were kept in He atmosphere after an exposure to air. This result suggests the parylene gate dielectric to be highly H(2)O repellent. The mechanical flexibility and air-exposure effect were studied for the C(60) FET with parylene gate dielectric. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2959819

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The photoion yield spectra of an endohedral metallofullerene Pr@C(82) were measured in the photon energy range of 100-150eV by using time-of-flight mass spectrometry. Parent ions Pr@C(82)(+), Pr@C(82)(2+) and Pr@C(82)(3+) were observed in t he mass spectra. The photoion yield spectra of Pr@C(82)(2+) showed a broad peak at 120-140eV that was assigned to the 4d-4f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C(82) were evaluated from the photoion yield spectra to be 37 +/- 12 Mb at 110eV (off-resonance) and 52 +/- 13 Mb at 130eV (on-resonance). These cross sections of Pr@C(82) were compared with the results of Ce@C(82), the only metallofullerene whose photoionization properties have ever been studied near the 4d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C(82) and Ce@C(82), but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms. (c) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jqsrt.2007.11.005

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Transport properties of C(60) field-effect transistors (FETs) have been investigated in the temperature range between 160 and 300 K. Activation energy was estimated from temperature dependence of resistance at the linear region and of current at the saturation region for various channel lengths. Variation of activation energy values is attributed to carrier injection barrier at contact between source electrode and C(60) channel, and barriers to carrier hopping between trap states in the channel of C(60). (c) 2008 American Institue of Physics.

DOI： 10.1063/1.2917469

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C(60) field-effect transistor (FET) has been fabricated with a single Schottky barrier formed by an insertion of 1-dodecanethiol at the interface between the active layer and the gate dielectric. The suppression of drain current is observed at low drain-source voltage, showing a formation of the carrier injection barrier. Furthermore, a clear difference between forward and reverse drain currents is observed in the FET in a high temperature region, showing that this FET device is close to an ideal single Schottky diode. The quantitative analysis for carrier injection barrier has been achieved with thermionic emission model for a single Schottky barrier. (c) 2008 American Institue of Physics.

DOI： 10.1063/1.2919799

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The dissolution of metal clusters in metal island films by the simultaneous application of electric field and temperature is reported. The consequent fading of surface plasmon resonance greatly modifies the optical properties of the samples. The dissolution process is verified in island films of different metals, obtained under different conditions and covered by different dielectric materials, as well as on multilayer dielectric stacks showing interferential properties. The tailoring possibilities of the optical behavior of metal island films combined with the inexpensive technical requirements of this approach open up the possibility to produce low-cost photonic heterostructures. (c) 2008 American Institute of Physics.

DOI： 10.1063/1.2908886

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Field-effect transistor (FET) device has been fabricated with Langmuir-Blodgett films of C(60) dendrimer. The device showed n-channel normally off characteristics with the field-effect mobility of 2.7x10(-3) cm(2) V(-1) s(-1) at 300 K, whose value is twice as high as that (1.4x10(-3) cm(2) V(-1) s(-1)) for the FET with spin-coated films of C(60) dendrimer. This originates from the formation of ordered pi-conduction network of C(60) moieties. From the temperature dependence of field-effect mobility, a structural phase transition has been observed at around 300 K. Furthermore, the density of states for impurity levels was estimated in the Langmuir-Blodgett films. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2824818

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The C-76 field-effect transistor (FET) showed n-channel normally-off like behavior with n-channel field-effect mobility, P., of 3.9 x 10(-4) CM2 V-1 s(-1), and the highest on-off ratio, 125, among higher fullerenes FETs. The carrier transport in the C-76 FET followed a thermally-activated hopping transport model. The normally-off like properties Of C76 FET could be reasonably explained in terms of the electronic structure of thin films determined by photoemission spectroscopy. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2007.10.012

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Field-effect transistor with thin films of pentacene has been fabricated with Au electrodes modified by 1-hexadecanethiol (C16H33SH), and the hole-injection barriers have been determined from the temperature dependence of output properties on the basis of the thermionic emission model for double Schottky barriers. The large tunneling barriers are formed by the insulating C16H33SH at the interfaces between the Au electrodes and pentacene thin films. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2789699

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Partial cross sections for the photoion formation from C-60 and C-70 were determined from the yields of singly, doubly, and triply charged ions which were measured by mass spectrometry combined with tunable synchrotron radiation at h nu = 25-120 eV. The dependence of the detection efficiencies on the mass-to-charge ratio was evaluated by using the formula proposed by Twerenbold et al. Corrections of the detection efficiency were found to be critical for obtaining accurate partial cross sections for photoionization of fullerenes. Revisions were made of the partial cross-section curves for single and double photoionization of C-60 and C-70. The curve for triple photoionization of C-70 was newly proposed. The ratios between the cross sections for double and single photoionization increase with h nu and reach saturated values of 0.78 at 85 eV for C-60 and similar to 1.3 at 100 eV for C-70. In contrast, the ratios at 120 eV between the cross sections for triple and single photoionization of C-60 and C-70 amount to 0.14 and similar to 0.38, respectively. The formation mechanism of multiply charged fullerene ions was discussed in terms of valence-electron excitation to antibonding unoccupied orbitals and/or spherical standing waves inside the cavity of a fullerene. This excitation could be followed by Spectator Auger processes and transmission of the excess electronic energy among numerous vibrational degrees of freedom.

DOI： 10.1021/jp0726034

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Field-effect transistors (FETs) with thin films of C-60 have been fabricated with Au electrodes modified by a series of 1-alkanethiols. All C-60 FETs show n-channels of normal FET properties. It has been found that the output properties for the FETs with the Au electrodes modified by 1-alkanethiols with long alkyl chains are largely affected by the carrier-injection barrier (i.e., the current vs drain/source voltage plots exhibited concave-up nonlinearity at low voltage regions). The output properties are substantially dominated by an additional tunneling barrier of 1-alkanethiols inserted into the junction of the Au electrode and C-60 thin films, and the parameters associated with the junction barrier height and tunneling efficiency were determined from the output properties based on the thermionic emission model for double Schottky barriers.

DOI： 10.1021/jp0708751

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C-60 field-effect transistors (FETs) have been fabricated with source/drain electrodes of three different materials, indium tin oxide (ITO), Au, and Pt. High field-effect mobility mu(FE) of FETs with ITO electrodes, 1.6x10(-1) cm(2)/V s, shows that ITO is a potential material for the electrodes of organic electronics. Although the highest Schottky barrier and the lowest mu(FE) were expected, mu(FE) of FET with Pt electrodes (1.4x10(-1) cm(2)/V s) is higher than that of FET with Au electrodes (9.6x10(-2) cm(2)/V s). The result suggests that modification of local electronic structure at the interface between electrodes and C-60 affects device performance. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2709523

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Carrier (electron or hole) injection from a scanning tunneling microscope tip causes various surface modifications on the molecular scale. We report that injection into C-60 close-packed layers forms a ring-shaped distribution of C-60 polymers. This can be explained on the basis of the radial propagation and energy dissipation of carriers. Subsequent electron or hole injections enlarge the ring, showing that both carriers can induce both polymerization and depolymerization. Furthermore, we demonstrate visualization of carrier scattering by injecting carriers into C-60 layers with grain boundaries.

DOI： 10.1103/PhysRevLett.97.196101

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The authors fabricated random-network single-walled carbon nanotube field-effect transistors (FETs) with high-kappa gate insulators. The gate insulator in the FETs was changed from SiO2 to Ba0.4Sr0.6Ti0.96O3 (BST, epsilon(s)=100), which was fabricated by a sol-gel method. The gate-switching voltage of a FET with a BST insulator is about one-tenth of that of a FET with a SiO2 insulator, and the values of the transconductance and the on/off ratio are 0.18 mu S and 10(5), respectively. In addition, hysteresis in the operation of the FETs was dramatically decreased, probably because of improvements in the surface condition of the insulator. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2388150

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Polymerization and depolymerization of C-60 molecules are induced by charge carrier injection from a scanning tunneling microscope tip into C-60 close-packed layers. These reactions are hard to be achieved at single molecular precision because of a spatial spread of injected carriers. We report that the carrier-spreading effect produces a ring-shaped distribution of C-60 polymers around the carrier injection point. Formation of the polymer ring of C-60 is explained by energy dissipation of spreading carriers. Therefore, the polymer ring can be recognized as a direct reflection of carrier propagation. Subsequent electron or hole injections enlarge the ring. This result shows that both electrons and holes can induce both polymerization and depolymerization of C-60 molecules.

DOI： 10.1002/pssb.200669154

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Field-effect transistor (FET) devices have been fabricated with thin films of fullerodendrons (I), (II) and (III) by using solution process. The n-channel normally-off FET properties have been observed in the fullerodendron (I) and (II) FET devices with SiO2/Si substrate. The values of field-effect mobility, mu, of the fullerodendron (I) and (II) FETs were determined to be 4.5 x 10(-4) and 1.4 x 10(-3) cm(2) V-1 s(-1), respectively, at 300 K. On the other hand, the n-channel normally-on FET properties have been observed in the fullerodendron (II) FET devices with three types of polymer gate insulators on poly(ehylene terephthalate) substrates.

DOI： 10.1002/pssb.200669178

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Field-effect transistor (FET) device with thin films of C-60 has been fabricated with Eu electrodes exhibiting small work function. The C-60 FET device shows n-channel FET properties with high field-effect mobility, 0.50 cm(2) V-1 s(-1). Furthermore, nonvanishing drain current, i.e., normally on, is observed in this FET device. This originates from small energy barrier for electron from Eu source electrode to lowest unoccupied molecular orbital of C-60. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2337990

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Field-effect transistor (FET) devices with thin films of perylene have been fabricated with various metal electrodes exhibiting work function phi from 2.5 to 5.1 eV. All perylene FET devices show p-channel FET properties. The p-channel field-effect mobility mu(p) and the on-off ratio in the perylene FET increase with an increase in phi of the metal electrodes. The n-channel conduction is also observed for the FET devices with Eu and Sr electrodes exhibiting small phi. These results can be reasonably explained on the basis of energy barrier for hole or electron. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2266596

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n-channel field-effect transistor (FET) devices have been fabricated with thin films of fullerodendron on SiO2/Si, polyimide/Au/poly(ethylene terephthalate), and polyvinyl alcohol/Au/poly(ethylene terephthalate) substrates by using solution processes. The value of field-effect mobility mu of the fullerodendron FET reaches 1.7 x 10(-3) cm(2) V-1 s(-1) at 300 K. The mobility gap and optical gap have been estimated to be 0.15 and 1.4 eV, respectively. The channel conduction in the FET device follows thermally activated hopping-transport mechanism below 300 K. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2198098

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Field-effect transistor (FET) devices have been fabricated with thin films of perylene on SiO2 and polyimide gate insulators, and p-channel FET properties have been found in both FET devices. The perylene FET devices with SiO2 and polyimide gate insulators exhibited field-effect mobility, mu, values of 7.0x10(-3) and 3.7x10(-4) cm(2) V-1 s(-1), respectively, at 300 K under vacuum of 10(-6) Torr. These FET devices were found to operate under atmospheric condition after exposure to air. The mu value increased with increasing temperature from 160 to 280 K, showing a hopping carrier transport. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2182024

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We report a new technique for inscribing characters, pictures and patterns on a nanometer scale by pin-point shooting at single C-60 molecules. An exact and sharp electron/hole injection precisely removes a single target C-60 molecule in the close-packed C-60 layer. This leaves a small dark void on the scanning tunneling microscope image. Furthermore, a pin-point shooting at a C-60 molecule adjacent to a void can cause the molecule to slide and fill up the void. Such a movement of a single C-60 molecule provides a new way for precisely recording characters, pictures and patterns on a nanometer scale. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2005.12.052

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The ion yield curves for C-70-2n(z+) (n = 1-8, z = 2 and 3) produced by photoionization of C-70 were measured in the photon energy (hv) range of 25 - 150 eV. The appearance hv values were higher by ca. 34 eV than the thermochemical thresholds for dissociative ionization of C-70 leading to C-70-2n(z+). Evaluation was made on the upper limits of the internal energies of the primary C-70(z+) above which C-70-2n+2(z+), fragments cannot escape from further dissociating into C-70-2n(z+)+C-2. These critical internal energies agreed well with appearance internal energies of C-70(z+) theoretically obtained corresponding to the threshold for the formation of C-70-2n(z+). The photofragmentation of the parent C-70(z+) ions is considered to be governed by the mechanism of internal conversion of their electronically excited states, statistical redistribution of the excess energy among a number of vibrational modes, and sequential ejection of the C-2 units.

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A flexible C-60 field-effect transistor (FET) device has been fabricated with a polyimide gate insulator on the poly(ethylene terephthalate) substrate, and n-channel normally off FET properties are observed in this FET device. The field-effect mobility, mu, is estimated to be similar to 10(-2) cm(2) V-1 s(-1) at 300 K. Furthermore, the C-60 FET has been fabricated with a high-dielectric Ba0.4Sr0.6Ti0.96O3 (BST) gate insulator, showing n-channel properties; the mu value is estimated to be similar to 10(-4) cm(2) V-1 s(-1) at 300 K. The FET device operates at very low gate voltage, V-G, and low drain-source voltage, V-DS. Thus these C-60 FET devices possess flexibility and low-voltage operation characteristic of polyimide and BST gate insulators, respectively. (C) 2005 American Institute of Physics.

DOI： 10.1063/1.2081134

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Ambipolar field-effect transistor (FET) devices were fabricated with a heterostructure of C-60 and pentacene, and their p- and n-channel field-effect mobilities were studied as a function of thickness of pentacene thin-films. The observed dependences of the mu values were interpreted in terms of the morphology of the thin films and the band structure of C-60/pentacene heterostructure. A complementary metal-oxide-semiconductor (CMOS) circuit was fabricated by integration of two ambipolar FETs, aiming at realization of a new CMOS inverter circuit composed of FETs with the same device structure. The gain of four, the threshold voltage of 85 V, and the complex output characteristics were explained on the basis of the properties of the component FET devices. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2005.07.096

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A fullerene field-effect transistor (FET) device has been fabricated with thin films of C-88, and n-channel normally on depletion-type FET properties have been found in this FET device. The C-88 FET exhibited a high mobility, mu, of 2.5x10(-3) cm(2) V-1 s(-1) at 300 K, in fullerene FETs. The carrier transport showed a thermally activated hopping transport. The n-channel normally on FET properties and the hopping transport reflect the small mobility gap and low carrier concentration in the channel region of C-88 thin films. (c) 2005 American Institute of Physics.

DOI： 10.1063/1.1994957

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A new material of higher fullerene, RbxC84, was synthesized by intercalating Rb metal into C-84 crystals. The RbxC(84) crystals showed a simple cubic (sc) structure with lattice constant, a, of 16.82 (2) angstrom at 6.5 K, and 16.87 (2) angstrom at 295 K. The Rietveld refinements were achieved with the space group, Pa (3) over bar, based on a model that the C-2 axis of D2d-C84 aligned along [111]. The sample composition was determined to be Rb-8.8(7) C-84. The ESR spectrum at 303 K was composed of a broad peak with peak-to-peak linewidth Delta H-pp of 220 G, and a narrow peak with Delta H-pp of 24 G. Temperature dependence of the broad peak clearly showed a metallic behavior. The metallic behavior was discussed based on a theoretical calculation. This finding of new metallic phase in a higher fullerene is the first step for a development of new types of fullerene materials with novel physical properties such as superconductivity.

DOI： 10.1103/PhysRevB.71.224118

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A field-effect transistor (FET) device was fabricated with thin films of C-2v isomer of Pr@C-82. This device apparently showed n-channel normally on type FET properties, where non-zero current was observed at the gate-source voltage, V-GS, of 0 V. Normally off FET properties were observed by subtraction of the non-zero current from the drain current. Thus the normally on properties are ascribed to the high bulk current caused by the small energy gap of approximate to 0.3 eV. The field-effect mobility for this FET was 1.5 x 10(-4) cm(2) V-1 s(-1) at 320 K, being comparable to those of other endohedral metallofullerene FET devices. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2005.05.019

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The yield curves for C-60-2n(+) (n = 1-3) produced by photoionization of C-60 are measured in the hv range of 25-150 eV. The appearance energies increase with increasing n. Evaluation is made on the upper limits of the internal energies of the primary C-60(+) above which C-60-2n+2(+) fragments (n &gt;= 1) cannot escape from further dissociating into C-60-2n(+) + C-2. These limits agree well with the theoretical internal energies of C-60(+) corresponding to the threshold for the formation of C-60-2n(+), on the assumption that the binding energies of C-60-2n+2(+) are equal to those proposed by Foltin et al. [M. Foltin, M. Lezius, P. Scheier, T.D. Mark, J. Chem. Phys. 98 (1993) 9624]. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.084

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The absolute photoabsorption cross section curve of C-60 has been determined by means of mass spectrometry with the photon source of monochromatized synchrotron radiation of hv = 24.5-150 eV. Description has been made on a high-temperature source of gaseous fullerenes and an efficient time-of-flight mass spectrometer. The cross section was estimated by assuming an approximate expression of the number density of C-60 in the ionization region. The resultant values were 762, 24 1, and 195 Mb at hv = 24.5, 90, and 110 eV, respectively, with about 10% errors. The cross section curve was then normalized at hv = 25 eV to the absolute photoabsorption cross section reported by Jaensch and Kamke [R. Jaensch, W. Kamke, Mol. Mater. 13 (2000) 143], the most reliable data so far available in the valence excitation region of C-60. Accordingly, the present cross section data were altered to 407, 144, and 114 Mb at hv = 25, 90, and 110 eV, respectively. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.110

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We report an ambipolar operation in field-effect transistors of C-60 and metallofullerene Dy @ C-82 by modification of semiconductor/metal electrode interface with perfluoroalkylsilane (FAS) molecules. Kelvin probe experiments revealed that the work function of the gold surface modified with FAS molecules increased by 0.55 eV as compared to the untreated gold. Hole injection into fullerenes is qualitatively understood in terms of this work-function change induced by the FAS molecules. The present results indicate that the charge injection from electrodes to organic semiconductors can be controlled simply by modification of semiconductor/metal interface without changing materials themselves. (c) 2005 American Institute of Physics.

DOI： 10.1063/1.1903109

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The photoion yield curves for Dy@C-82(z+) (z= 1 and 2) from Dy@C-82 are measured by using synchrotron radiation in the photon energy range from 24.5 to 39.5 eV. Correction has been made to compensate the effect of transient change of the density of Dy@C82 in the interaction region, with the help of the yield curve of C-60(2+) produced from C-60 remaining as a trace impurity in the sample. The yield of Dy@C-82(+) exhibits a gradually descending curve with a flat region at 30-33 eV, similarly to the yield curve Of C-60(+) from C-60. The total photoabsorption cross section of Dy@C-82 was evaluated to be (1.2 &PLUSMN; 0.4) x 10(-20) m(2) at the photon energy of 39.5 ev. &COPY; 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.ijms.2005.02.001

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The yield curves for photoions from Ce@C-82 are measured by using synchrotron radiation in the photon energy range from 90 to 160 eV. Parent Ce@C-82(z+) and fragment ions C-60(z+) and C-70(z+) are observed in a mass spectrum (z= 1 and 2). The yield curves for doubly charged ionic species exhibit broad resonance in the photon energy region of from 120 to 140 eV which is ascribed to the 4d --&gt; 4f giant dipole resonance of the encapsulated Ce atom. The total photoabsorption cross section of Ce@C-82 was determined from partial photoionization cross sections for formation of the parent and fragment ions to be 5.3(-1.1)(+1.8) and 19.6(-3.9)(+6.5) Mb at photon energies of 110 and 130 eV, respectively. (C) 2005 American Institute of Physics.

DOI： 10.1063/1.1846674

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Oxidation of the [Au(6)Ni(32)(CO)(44)](6-) cluster anion in acetonitrile solutions by air and by Me(3)NO has been monitored using Ni K- and Au L(III)-edge XAFS spectroscopy. EXAFS data for the pristine cluster correspond reasonably well to its known crystal structure and point to a formation of bimetallic Ni-Au nanoparticles in the oxidation products. Air oxidation of the acetonitrile solutions without additional oxidants/promoters goes very slowly whereas addition of Me(3)NO results in immediate precipitation. The nanoparticles formed upon the oxidation are suggested to have a core of gold atoms and an outer shell of nickel oxide.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The ion yield curves for C-60-2n(z+) (n = 1-5, z = 1-3) produced by photoionization of C-60 are measured in the photon energy (hnu) range of 25 - 150 eV. The appearance hnu values are higher by 30 - 33 eV than the thermochemical thresholds for dissociative ionization of C-60 leading to C-60-2n(z+). Evaluation is made on the upper limits of the internal energies of the primary C-60(z+) above which C-60-2n+2(z+) fragments (n greater than or equal to 1) cannot escape from further dissociating into C-60-2n(z+) +C-2. These upper limits agree well with the theoretical internal energies of C-60(z+) corresponding to the threshold for the formation of C-60-2n(z+). The photofragmentation of C-60(z+) is considered to be governed by the mechanism of internal conversion of the electronically excited states of C-60(z+), statistical redistribution of the excess energy among a number of vibrational modes, and sequential ejection of the C-2 units.

DOI： 10.1039/b416503a

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Structures and electronic states in the ground state of endohedral fullerence are investigated by XAFS analysis and XANES spectrum. The distances between the Eu ion and the neighboring C atoms show clearly that the Eu ion is inside the C-60 cage. The endohedral element, Eu, in C-60 is located away from the central position of C-60 cage by 1.4 angstrom. We show the detailed location of the Eu ion in the C-60 cage by XAFS and the first principles calculation. The perturbation theory indicates that the distortion of C-60 cage and the shift of the Eu ion are attributed to electron-nuclear interaction, pseudo-Jahn-Teller type interaction. XANES spectrum of Eu L-III-edge shows that the valence of the Eu ion is close to +2. The electron con. guration is thus f(7), showing the total symmetry (A(1g)) of I-h group. The details of the structures and the electronic states will be discussed with both the experimental and theoretical results.

DOI： 10.1238/Physica.Topical.115a00507

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Scanning tunneling microscopy images for two isomers of Ce@C-82 were observed on Si(111)-(7x7) at 295 K. The Ce@C-82 molecules in the first layer were bound to the Si surfaces, and the motions were frozen even at 295 K. The multilayer of the Ce@C-82 isomer I (Ce@C-82-I) produced a close-packed structure in the surface layer by annealing the Si substrate at 473 K. The distance between the nearest-neighboring molecules was 1.15(4) nm whose value was consistent with that, 1.12 nm, estimated from x-ray diffraction of the Ce@C-82-I crystals. This implies that the close-packed structure is dominated by van der Waals forces, as in crystals of Ce@C-82-I. The internal structure of Ce@C-82-I was observed in the first layer due to a freeze of molecular motion caused by strong interactions between the molecule and the Si adatoms in the surface. Scanning tunneling spectroscopy revealed that the energy gaps for Ce@C-82-I and -II in the first layer opened to gap energies, E-g of 0.7 and 1.0 eV, respectively. This fact suggests that these molecules are semiconductors with smaller value of E-g than those for C-60 and C-70.

DOI： 10.1103/PhysRevB.70.235421

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Ambipolar field-effect transistor (FET) device was fabricated with heterostructure of thin films of C-60 and pentacene. Three types of device structures in the C-60/pentacene heterostructure FET device were studied in order to realize the best ambipolar properties. In the middle-contact type FET device of C-60 and pentacene, the mobility mu in p-channel operation was estimated to be 6.8x10(-2) cm(2) V-1 s(-1), while the mu in n-channel operation was 1.3x10(-3) cm(2) V-1 s(-1). This ambipolar FET device is available for a practical building-block to form CMOS integrated circuits with low-power consumption, good-noise margins, and ease of design. (C) 2004 American Institute of Physics.

DOI： 10.1063/1.1818336

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Electronic properties of the major and minor isomers of Pr@C-82, I (C-2nu,) and 11 (C,), are studied by Raman scattering, resistivity measurement, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The valences of the Pr atom in both isomers are determined to be +3 based on the Raman shift in the Pr-C-82 stretching mode. The transport properties showed that both isomers are normal semiconductors with a small energy gap (E-g). STM of isomer I shows internal structures dependent on bias voltage V-s, and STS shows that this isomer is a semiconductor with E-g = 0.7 eV. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2004.07.072

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X-ray diffractions and electronic transports for the Ce@C-82 isomers I and II, which refer to major and minor isomers, respectively, are studied in a wide temperature region to clarify the structural and electronic properties characteristic of individual isomers. The X-ray diffraction patterns observed at 295 K can be indexed based on simple cubic (sc) structures with lattice constants, a's, of 15.78(1) Angstrom for isomer I and 15.74(4) Angstrom for isomer II. Rietveld analyses are achieved for these X-ray diffraction patterns with a space group of Pa(3) over bar. Temperature dependence of a for isomer I shows a drastic change around 170 K, which implies existence of a structural phase transition. The structural phase transition above 300 K cannot be detected for Ce@C82 isomer I in contrast with La@C-82 isomer I in which the phase transition at 400 K was detected by differential scanning calorimetry and dielectric constant measurements. The temperature dependence of a for isomer II indicates no structural phase transition from 100 to 300 K. The pressure dependence of a for isomer I exhibits a monotonic decrease with an increase in pressure. This result implies no pressure- induced structural phase transition for isomer I. The temperature dependence of resistivities for thin films of these isomers is studied by a four-probe method, and it shows naffow-gap semiconductor-like behaviors. The energy gaps of isomers I and II are 0.33 and 0.55 eV, respectively. The difference in the structural and electronic properties among the isomers of metallofullerenes will attract much interest in chemistry and materials science.

DOI： 10.1021/jp049787w

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Fullerene field-effect transistors (FETs) were fabricated with thin films of C-84, which showed n-channel normally-on depletion-type FET characteristics. The C-84 FET device exhibited the highest mobility, mu, of 2.1x10(-3) cm(2) V-1 s(-1) among normally-on fullerene FETs. The carrier transport of this FET device can be interpreted as thermally activated hopping transport. Carrier type (n-channel) and transport mechanism (hopping) reflect the electronic properties of the C-84 molecule. (C) 2004 American Institute of Physics.

DOI： 10.1063/1.1695193

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Location of the Dy ion in the minor isomer of Dy@C-82 was determined by Dy L-III-edge EXAFS. The Dy ion is found to lie near the fused bond between the two hexagon rings. This finding is consistent with that predicted by a quantum-chemical calculation [Chem. Phys. Lett. 282 (1998) 325]. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2004.02.065

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Photoion yields from gaseous fullerenes, C-60 and C-70, for production of singly and doubly charged ions are measured by mass spectrometry combined with tunable synchrotron radiation at hnu=25-150 eV. Since the signal of triply or highly charged ions is very weak, the total photoionization yield curve can be estimated from the sum of the yields of the singly and doubly charged ions. A distinct feature appears in the resultant curve of C-60 which is absent in the calculated total photoabsorption cross section previously reported. This difference is attributed to C-60(2+) ions chiefly produced by spectator Auger ionization of the shape resonance states followed by tunneling of the trapped electron or by cascade Auger ionization. Ratios between the yields of doubly and singly charged ions for C-60 and C-70 are larger than unity at hnu&gt;50 eV. These ratios are quite different from those reported in the experiments using electron impact ionization. (C) 2004 American Institute of Physics.

DOI： 10.1063/1.1651478

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Structure of isomer-separated crystalline C-82 has been studied by powder x-ray diffraction with synchrotron radiation. The Rietveld refinement was achieved by assuming a simple cubic lattice (space group Pa (3) over bar). The C-2(a) symmetry for the C-82 molecule was supported in this analysis. Transport properties of thin film of C-82 have been studied by resistivity measurement. The thin film showed a narrow-gap semiconductorlike behavior with gap energy of 0.43 eV. The field effect transistor (FET) of a C-82 thin film showed an n-channel normally-on depletion-type behavior, and the mobility was 1.9x10(-3) cm(2) V-1 s(-1) whose value was one of the largest mu among normally-on FETs with fullerenes. A hopping transport was found as channel conduction for the C-82 FET above 150 K.

DOI： 10.1103/PhysRevB.69.165412

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The molecular structure of La-2@C-80 Was investigated theoretically and experimentally. The most stable configuration was found to be D-3d symmetry. La ions can move between ten equivalent D-3d configurations via D-2h configuration. The resulting dodecahedral trajectory of La ions agrees well with an X-ray powder diffraction experiment.

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N-channel field effect transistors (FETs) were fabricated with thin films of C-60 and Dy@C-82. A typical enhancement-type FET property was observed in C-60 FET above 220 K. The mobility Of C-60 FET increased with increasing temperature. This fact suggests hopping transport as the conduction mechanism, with the activation energy of 0.29 eV. The Dy@C-82 FET was found to be a normally-on type FET, which has a property different from that for C-60 and C-70 FETs. A complementary metal oxide semiconductor (CMOS) logic gate circuit was first fabricated with C-60 and pentacene thin-film FETs. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2003.07.025

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A photoionization spectrometer has been developed for measuring the ion yields for fullerenes in the photon energy range of 23-200 eV. Gaseous fullerenes were supplied from a high-temperature oven, ionized by irradiation of monochromatized synchrotron radiation, and detected after analysis with a time-of-flight mass spectrometer. The fluxes of the synchrotron radiation and fullerene beams were monitored concurrently with the acquisition of the ion signal counts in order to obtain reliable photoionization efficiency curves. The performance of the apparatus was examined by measuring the efficiency curve of C-60(+) produced from C-60. The spectrum demonstrated better statistics than the previous results in the same photon energy region. Three distinct features were newly observed in the higher-energy side of the prominent resonance at similar to20 eV. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1590748

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Photoion yield spectra Of C-60 in the gas phase were measured from 23 to 180 eV by synchrotron radiation. Two peaks at 26 and 34 eV and a flat area ranging 40-50 eV are newly observed in the high-energy side of the giant resonance at similar to20 eV. These features are assigned to the shape resonance on photoionization. of the valence electrons Of C-60; the ionized electron is temporarily trapped inside a centrifugal barrier. Above similar to50 eV the yield curve shows a steady decrease with increasing photon energy like the photoabsorption cross section of atomic carbon. Thus, the spectrum is interpreted as essentially determined by photoionization of the 2s orbitals of carbon atoms. (C) 2003 Elsevier Science B.V. Aft rights reserved.

DOI： 10.1016/S0009-2614(03)00613-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The structure of the alkaline-earth fulleride Ca3.01C60 is refined using maximum entropy data analysis of synchrotron powder diffraction data. Despite the size and complexity of the structural problem, the fulleride anion orientations and the details of multiple occupancy of the octahedral interstitial sites in the fcc anion array are determined. The power of the maximum entropy technique in solving underdetermined problems in powder crystallography is thus demonstrated.

DOI： 10.1021/cm020171y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

High temperature-treated compounds of mixed powders of cerium metal and C-60 solid show fairly strong diamagnetic signal in the zero-field-cooling process and distinct ferromagnetic behavior in the field-cooling path below 13.5K and 15K, respectively. In the resistivity measurement of a compacted and long-term heated pellet of the powder a distinct drop of the resistivity with decreasing temperature is recognized around 20K, which is followed by a steep rise of the resistivity below 14K. These findings indicate an existing competition of superconductivity and ferromagnetism in this compound.

DOI： 10.1016/S0379-6779(02)00825-1

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Rare-earth-metal-doped fullerides with nominal composition of (formula presented) (formula presented) Eu) adopt a pseudomonoclinic structure in which (formula presented) dimers glued with rare-earth ions are involved. High-pressure powder x-ray diffraction experiments revealed that these compounds undergo a reversible first-order structural phase transition at 1.5 GPa, associated with 2.7%–2.9% reduction of the unit cell volume. Structural analyses showed that the rare-earth ions, which are located close to the edge of tetrahedral sites at ambient pressure, move back to the center of the tetrahedral sites. Simultaneously, (formula presented) molecules are realigned so that the fivefold (long) axes are perpendicular to the (formula presented) or (formula presented) plane at high pressure. The derived charge density map indicates that the transition is regarded as a structural change from dimers to three-dimensional polymers of fullerenes. These features are ascribed to the unique bonding nature in rare-earth (formula presented) compounds. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevB.67.094101

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DOI： 10.1021/ja0288690

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The crystal structure of (formula presented) isomer I at 298 K has been determined by Rietveld refinement for x-ray powder diffraction with synchrotron radiation. Isomer I shows a simple cubic structure (sc: (formula presented) with a lattice constant a of 15.78(1) Å. The (formula presented) axis of a (formula presented) cage aligns along the [111] direction of this crystal lattice. The (formula presented) cage is orientationally disordered to satisfy a (formula presented) symmetry along [111], which is requested in this space group. The large thermal parameter for the Dy atom estimated from the x-ray diffraction probably reflects a large disorder caused by a floating motion of the Dy atom inside the (formula presented) cage as well as a ratchet-type motion of the (formula presented) molecule. The electronic transport of thin film of (formula presented) shows a semiconducting behavior. The energy gap (formula presented) is estimated to be 0.2 eV. Further, the variation of valence from (formula presented) to (formula presented) is found by metal doping into the (formula presented) crystals. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevB.67.115410

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELECTROCHEMICAL SOCIETY INC  

Structures and physical properties of metallofullerenes are studied by Xray diffraction, XANES, Raman, electric transport and scanning tunneling microscope (STM) measurements. Furthermore, the field effect transistors (FET's) are fabricated with thin films of fullerenes, and their characteristics are studied. The molecular orientation of metallofullerene in the crystals has been found to be closely associated with the structural phase transition from the X-ray diffraction study of Ce endohedral C-82 (Ce@C-82). The C-60 FET shows an n-channel enhancement-type property, while the Dy@C-82 FET shows an n-channel depletion-type property. The STM image of Dy@C-82 molecules on a well-defined Si(111) - (7 x 7) surface is observed at 300 K, showing a random adsorption of molecules even after annealing at 200 degreesC.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELECTROCHEMICAL SOCIETY INC  

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) of Dy@C(82) and Dy@C(60) adsorbed on Si(111)-(7x7) surface have been measured at room temperature. The Dy@C(82) molecules in the first layer are randomly adsorbed on Si(111)-(7x7) surface, and the motion of the molecules freezes even at room temperature. No second layer islands are observed before the completion of the first layer. The band gap for Dy@C(82) molecule in the first layer is estimated to be 0.1 eV by STS. The STM image reveals that Dy@C(60) molecule is nearly spherical.

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AC conductivity measurements with an impedance analyzer were carried out for a hydrogen-doped coordination polymer, N,N&#039;-bis-(2-hydroxyethyl) dithiooxamidatocopper(II), in order to estimate the protonic conductivity (σp). The log σp was linearly increased from 2.6 χ 10-9 to 2.2 χ 10-6 S cm-1 with relative humidity (RH) from 45 to 100 % at 300 K. A slight hysteresis of protonic conductivity was observed upon increasing and decreasing RH, which implies that H3O+ is generated by a reaction between water molecule and acid-base polymer near RH - 100%.

DOI： 10.1080/713738672

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The intermediate phases Cs3+alphaC60 (alpha=0.0-1.0), have been prepared, and their structure and physical properties are studied by x-ray powder diffraction, Raman, ESR, electric conductivity, and ac susceptibility measurements under ambient and high pressures. The x-ray powder diffraction pattern of Cs3+alphaC60 (alpha=0.0-1.0) can be indexed as a mixture of the body-centered-orthorhombic (bco) and cubic (A15) phases. The A15 phase diminishes above 30 kbar. The broad ESR peak due to the conduction electron (c-ESR) is observed only for the phases around alpha=0.0 in Cs3+alphaC60. The resistivity of the Cs3+alphaC60 (alphanot equal0) sample follows the granular metal theory and/or Sheng model even in the sample exhibiting a broad ESR peak. No superconducting transition is observed up to 10.6 kbar in Cs3+alphaC60 (alphanot equal0). These results present that bco phase of Cs3+alphaC60 (alpha=0) is a final candidate for a pressure-induced superconductor.

DOI： 10.1103/PhysRevB.65.235425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The bonding nature between rare earth metals and fullerene molecules has been investigated. The electron density distribution for nominal Sm3C70, calculated by a maximum entropy method (MEM) based on the Rietveld analysis of synchrotron X-ray diffraction pattern, unambiguously demonstrated a covalent Sm-C bond, which is almost as strong as the interatomic bonding of crystal Si. Furthermore, the Sm bridges two C-70 molecules, producing a C-70-Sm-C-70 dimer structure. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(02)00297-X

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Crystal structures of (CH3)(3)CNH3BF4 were analyzed at 296, 270, 240, 180, and 40 K by single crystal X-ray diffraction in order to investigate the mechanism of the phase transition occurring at 219 K. All the structures analyzed are monoclinic, belonging to the same space group of P2(1)/c (#14) with Z = 8. The asymmetric unit of the crystals is composed of two (CH3)(3)CNH3+ ions and two BF4- ions. One of the two anions in the asymmetric unit is disordered in four orientations in the temperatures of 180-296 K, and in two orientations at 40 K. For the other anion and one of the two cations, disordering in two orientations was also detected at 40 K. A continuous change in the orientations of the two anions was observed from room temperature to 40 K. The phase transition is mainly concerned with the change in the number of disordered orientations of one of the two anions, as well as the change in the orientations of the two anions. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-2860(01)00899-7

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Crystals of Dy@C82 isomer I are studied by x-ray powder diffraction with synchrotron radiation in wide temperature and pressure regions. The isomer I of Dy@C82 shows a simple cubic structure with lattice constant a of 15.85(3) Å at 298 K, while the isomer II shows a face-centered cubic structure with a of 15.75(4) Å. The structural phase transition of the second order is indicated for the isomer I at 300-310 K by the temperature dependence of x-ray diffraction and differential scanning calorimetry. Further, the pressure dependence of the lattice constant is studied for the isomer I up to 60 kbar, which can be fitted by a Murnaghan equation of state.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Crystals of Dy@C-82 isomer I are studied by x-ray powder diffraction with synchrotron radiation in wide temperature and pressure regions. The isomer I of Dy@C-82 shows a simple cubic structure with lattice constant a of 15.85(3) Angstrom at 298 K, while the isomer It shows a face-centered cubic structure with a of 15.75(4) Angstrom. The structural phase transition of the second order is indicated for the isomer I at 300-310 K by the temperature dependence of x-ray diffraction and differential scanning calorimetry. Further. the pressure dependence of the lattice constant is studied for the isomer I up to 60 kbar, which can be fitted by a Murnaghan equation of state.

DOI： 10.1103/PhysRevB.65.073405

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Solid-phase reaction of cerium and C-60 powders under 600 degreesC may yield unusual magnetic materials which involve fairly strong dia- and ferro-magnetism at almost similar low temperature range, below 13.5 K. The temperature dependence of the electrical resistivity of this material may indicate that the diamagnetism is due to the superconductivity and there are possible interplays between the superconductivity and the ferromagnetism in this material. (C) 2002 Published by Elsevier Science Ltd.

DOI： 10.1016/S0038-1098(02)00291-0

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DOI： 10.1107/S0108767302097854

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We have studied crystal structure, magnetism, and electric transport properties of a europium fulleride (formula presented) and its Sr-substituted compounds, (formula presented) They have a bcc structure, which is an isostructure of other (formula presented) (formula presented) represents an alkali atom or an alkaline-earth atom). Magnetic measurements revealed that magnetic moment is ascribed to the divalent europium atom with (formula presented) spin, and a ferromagnetic transition was observed at (formula presented) In (formula presented) we also confirm the ferromagnetic transition by heat-capacity measurement. The striking feature in (formula presented) is very large negative magnetoresistance at low temperature
the resistivity ratio (formula presented) reaches almost (formula presented) at 1 K in (formula presented) Such large magnetoresistance is the manifestation of a strong (formula presented) interaction between conduction carriers on (formula presented) and (formula presented)(formula presented) electrons of Eu. © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.134431

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

AC conductivity measurements with an impedance analyzer were carried out for a hydrogen-doped coordination polymer, N,N'-bis-(2-hydroxyethyl)dithiooxamidatocopper(II), in order to estimate the protonic conductivity (sigma(p)). The log sigma(p) was linearly increased from 2.6x10(-9) to 2.2x10(-6) S cm(-1) with relative humidity (RH) from 45 to 100% at 300 K. A slight hysteresis of protonic conductivity was observed upon increasing and decreasing RE, which implies that H3O+ is generated by a reaction between water molecule and acid-base polymer near RH similar to 100%.

DOI： 10.1080/713738672

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Language：English   Publishing type：Research paper (scientific journal)  

The intermediate phases (formula presented) (formula presented) have been prepared, and their structure and physical properties are studied by x-ray powder diffraction, Raman, ESR, electric conductivity, and ac susceptibility measurements under ambient and high pressures. The x-ray powder diffraction pattern of (formula presented) (formula presented) can be indexed as a mixture of the body-centered-orthorhombic (bco) and cubic (formula presented) phases. The (formula presented) phase diminishes above 30 kbar. The broad ESR peak due to the conduction electron (formula presented) is observed only for the phases around (formula presented) in (formula presented) The resistivity of the (formula presented) (formula presented) sample follows the granular metal theory and/or Sheng model even in the sample exhibiting a broad ESR peak. No superconducting transition is observed up to 10.6 kbar in (formula presented) (formula presented) These results present that bco phase of (formula presented) (formula presented) is a final candidate for a pressure-induced superconductor. © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.235425

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Language：English   Publishing type：Research paper (scientific journal)  

Crystals of (formula presented) isomer I are studied by x-ray powder diffraction with synchrotron radiation in wide temperature and pressure regions. The isomer I of (formula presented) shows a simple cubic structure with lattice constant a of 15.85(3) Å at 298 K, while the isomer II shows a face-centered cubic structure with a of 15.75(4) Å. The structural phase transition of the second order is indicated for the isomer I at 300-310 K by the temperature dependence of x-ray diffraction and differential scanning calorimetry. Further, the pressure dependence of the lattice constant is studied for the isomer I up to 60 kbar, which can be fitted by a Murnaghan equation of state. © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.073405

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In view of the lack of information on the structure of M@C60 (M: alkaline earth and lanthanide metal) because of its instability and difficulties in purifying it, we have carried out and report here some structural investigations of Dy@C60. We obtained a pure sample of Dy@C60 by high-performance liquid chromatography with aniline as an eluent, and studied it by Dy LIII-edge x-ray-absorption fine structure and Raman scattering. Our results show conclusively that the structure is endohedral (with the metal inside the C60 cage) and that electron transfer takes place between Dy and C60. The Dy in Dy@C60 is located at an off-center position 1.25-1.30 Å from the center of the C60 cage. The valence of the Dy is shown to be +3 on the basis of a Dy LIII-edge x-ray absorption near edge structure study. The Ag(2) Raman peak also shows that three electrons have been transferred from the Dy atom to the C60 cage. The UV-VIS-IR spectrum suggests that the highest-occupied-lowest-unoccupied-molecular-orbital gap is small.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Two isomers of Dy@C(82) were separated by high performance liquid chromatography (HPLC). and their UV-VIS absorption spectra were measured to characterize these isomers. The crystalline powder of Dy@C(82) was obtained by removing the solvent (toluene) at 250 degreesC under vacuum. The X-ray diffraction pattern can be indexed with fcc crystal lattice, as that in La@C(82). The lattice constant a at 298 K, 15.86(1) Angstrom, is dose to that of La@C(82), 15.78 Angstrom. The distances between Dy and the first and second nearest C atoms are determined to be 2.52(2) and 2.86(2) Angstrom, respectively. on the basis of Dy L(III)-edge EXAFS. The XANES shows that the valence of the Dy atom in Dy@C(82) is +3. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(01)00234-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Cs K-edge XAFS of Cs(3)C(60) which is a pressure-induced superconductor were measured at 21 and 34 kbar by using a diamond anvil cell (DAC) in order to obtain the structural information under high pressure, and to clarify the origin of the pressure-induced superconductivity. The distances and the mean square displacements between the Cs and C atoms are consistent with those determined by X-ray powder diffraction. Consequently, the high-pressure XAFS can give the reliable structural-information on a fullerene superconductor under high pressure. We also show the procedure of the analysis of high-pressure XAFS with DAC in detail.

DOI： 10.1107/S0909049501000218

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

Structure of metal endohedral fullerenes is studied by XAFS and XANES. The Sc-Sc distance of 2.23(1) Angstrom determined from Sc K-edge XAFS supports the formation of a triangular Sc-3 cluster in Sc-3@C-82 as is found by MEM analysis for the X-ray diffraction. Gd L-III-edge XAFS of Gd@C-82 shows that the first and the second neigboring Gd-C distances are 2.51(2) and 2.85(4) Angstrom, respectively. The La-La distance of La-2@C-80 has been determined to be 3.90(1) Angstrom at 40 K. This value does not change when increasing temperature [3.90(2) Angstrom at 240 K]. The position and the valenece of the Eu atom in Eu@C-60 are also discussed based on Eu L-III-edge XAFS and XANES.

DOI： 10.1107/S0909049500017155

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The structure of La-2@C-80 is studied by La K-edge XAFS from 40 to 295 K. The distances between the La atom and the first nearest C atoms have been determined to be 2.42(1) Angstrom at 40 K and 2.44(2) Angstrom at 295 K, and those between the La atom and the second nearest C atoms to be 2.97(2) Angstrom at 40 K and 2.98(3) Angstrom at 295 K. The La-La distance has been determined to be 3.90(1) Angstrom at 40 K and 3.88(2) Angstrom at 295 K. The temperature dependence of the mean-square displacement of La-C is also studied to get an insight into the dynamical behavior of two La atoms in the C-80 cage. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(01)00048-3

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Crystal structure and magnetic properties of EuxSr6-xC60 were studied. Ferromagnetic transition was observed at 10-14 K, and magnetic moment is ascribed to Eu3+. The divalent state of Eu is also confirmed by Eu L-III-edge XANES experiments. The fact that ferromagnetic transition is observed even at a dilute case (x = 1) suggests the magnetic interaction is caused through C-60 molecules, that is, pi-f interaction, rather than direct exchange interaction between Eu atoms.

DOI： 10.1063/1.1420116

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Temperature dependence of resistivity, p, of a two-dimensional polymer phase of Na4C60 has been studied from 190 to 300 K. The p was 6.9 x 10(3) Ohm cm at 300 K, which is higher by six orders than those of metallic fullerides. The temperature dependence of p shows a semiconducting behavior in this temperature region. This result is different from the previous result derived from ESR which suggests a metallic property above 100 K. The gap energy, E-g, was estimated to be 0,8 eV. This value is close to those of other A(4)C(60) (A: K, Rb and Cs).

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Physical properties of bco Phase of Cs3+alphaC60 (alpha = 0.0 - 1.0) and Al5 phase of Cs3C60 are studied by X-ray diffraction, ESR, AC susceptibility, resistivity and Raman. The ESR of Cs3.00(6)C60 showed a broad peak of similar to380 G due to conduction electron, while no broad peak was observed in the ESR of bco Cs3+alphaC60 (alpha not equal 0.0) and Al5 phase, This shows that only beo phase of Cs3C60 is metallic. The AC susceptibility of bco phase of Cs3.2(3)C60 and Cs3.5(1)C60 showed no superconducting transition above 1.3 K even under high pressure, and the resistivity was 0.52 Ohm cm for both samples.

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In view of the lack of information on the structure of (formula presented) (M: alkaline earth and lanthanide metal) because of its instability and difficulties in purifying it, we have carried out and report here some structural investigations of (formula presented) We obtained a pure sample of (formula presented) by high-performance liquid chromatography with aniline as an eluent, and studied it by Dy (formula presented)-edge x-ray-absorption fine structure and Raman scattering. Our results show conclusively that the structure is endohedral (with the metal inside the (formula presented) cage) and that electron transfer takes place between Dy and (formula presented) The Dy in (formula presented) is located at an off-center position 1.25–1.30 Å from the center of the (formula presented) cage. The valence of the Dy is shown to be +3 on the basis of a Dy (formula presented)-edge x-ray absorption near edge structure study. The (formula presented) Raman peak also shows that three electrons have been transferred from the Dy atom to the (formula presented) cage. The UV-VIS-IR spectrum suggests that the highest–occupied–lowest-unoccupied-molecular-orbital gap is small. © 2001 The American Physical Society.

DOI： 10.1103/PhysRevB.64.113403

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The local lattice instability around the imperfection doped in the host matrix is invetigated with ultra-violet absorption spectra and x-ray excitation spectra. The local environment of a cuprous ion in NaCl shrinks, and in NaBr cuprous ion is lacated away from the regular lattice site. This phenomenon is discussed with the pseudo Jahn-Teller effect.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Raman scattering is studied for a pressure-induced superconductor Cs3C60 in a pressure region from 1 bar to 62 kbar. The center Frequency omega(0) for H-g(1) and H-g(2) Raman peaks increase by applying pressure, but the Increase shows a saturation in the high-pressure region. On the other hand, the omega(0) for A(g)(1) and A(g)(2) modes increase monotonically in all pressure regions. The electron-phonon coupling constant for Cs3C60 shows a rapid decrease up to 30 kbar and an increase above 30 kbar. This result may be associated with a transformation From a multiphase (body-centered orthorhombic and A 15 phases) to a single phase around 20 kbar. X-ray powder diffraction pattern at 11 K under a pressure of 40 kbar shows that a superconducting phase for Cs3C60 is body-centered orthorhomhic.

DOI： 10.1103/PhysRevB.62.5366

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Eu endohedral C-60, Eu@C-60, has been extracted with aniline from soot prepared by are-heating of a graphite/Eu2O3 composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@C-60(+). The W-VIS absorption spectrum of this sample has a red-shift in the onset (&gt; 900 nm) in comparison with those for C-60 and C-70, as expected from electron transfer from the Eu atom to the C-60 cage. The valence state of the Eu atom in Eu@C-60 has been determined to be +2 by Eu L-m-edge XANES. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(99)01309-3

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Crystal structure and magnetic properties of EuxSr6-xC60 (x = 1, 3,6) were studied. They all have a bce structure which is an isostructure to other M6C60 (M is an alkali or alkaline earth metal). The lattice constants follow the Vegard's law, suggesting a formation of solid solution at x = 1 and 3. Magnetic measurements reveals all Eu atoms are in the divalent state with a magnetic moment of 7 mu (B) (S = 7/2), which is consistent with XANES experiments. Ferromagnetic correlation develops at low temperature in all three samples and seems to be caused the pi -f interaction through C-60 rather than the direct exchange interaction between Eu atoms.

DOI： 10.1063/1.1342461

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

X-ray diffractions of two-dimensional Na4C60 are studied at various temperatures above 11 K in a pressure region up to 67 kbar. All X-ray diffraction patterns measured in this study can be analyzed by using a body-centered monoclinic (bcm) structure of space group I2/m. New Bragg reflections which cannot be assigned to I2/m are not observed in all X-ray diffraction patterns. The beta at ii K and 67 kbar is the smallest (93.66(8)degrees) among those of Na4C60 lattice of bcm. In this crystal, the center-to-center distance, 9.57(2) Angstrom, between the C-60 molecules associated with van der Waals contacts approaches to that, 9.20(2) Angstrom, between the C-60 molecules in a polymer chain, suggesting the realization of three-dimensional polymeric structure in Na4C60 at a higher pressure than 67 kbar.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

Conversion He+ ion yield (HIY) XAFS measurement, in which He+ ion is collected using the conversion electron yield (CEY) cell under atmospheric He gas environment, has been attempted at Eu K-absorption edge (48.5keV) for europium dioxymonocyanamide. The shape of Eu K-XAFS spectrum measured by the HIY method was similar to that measured by the conventional transmission method, while the signal-to-background ratio decreased with an increase in absorption edge energy, owing to low Auger electron emission probability. The evidences suggest that the HIY method is still applicable to the XAFS measurement for materials which could not be pulverized at such higher energy region.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

The Rb K-edge XAFS spectra for the stable phase of RbC60 which is a quasi one-dimensional polymer were measured in the temperature range from 14.6 to 210 K in order to clarify the origin of the metal-insulator transition around 50 K. The distances and mean-square relative displacements between the Rb atom and the neighboring C atoms determined by XAFS exhibited no change around 50 K, implying that the metal-insulator transition originates from the SDW instability.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

An x-ray absorption fine-structure (XAFS) spectroscopy beamline, BL01B1, was installed at a bending magnet source at SPring-8 and has been open to users since October 1997. It was designed for XAFS experiments covering a wide energy range. Position tables and automatical control programs were established to adjust the x-ray optics and achieve the designed performance of the beamline under each experimental condition. This has enabled conventional XAFS measurements to be made with a good data quality from 4.5 to 110 keV.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

XAFS spectra near K absorption edges of Pt (78.4 keV) and Pb (88.0 keV) were measured in transmission mode with Si (511) planes of an adjustable inclined double-crystal monochromator at bending-magnet beamline BL01B1 at SPring-8. The energy resolution was estimated as good as 7 eV at Pt K-edge, benefitted from the high brilliant x-rays from the SPring-8 storage ring. Blunt edge jumps and reduction of MAPS amplitude were observed owing to the finite lifetime of the core hole as predicted theoretically. Local structure parameters have been successfully evaluated from EXAFS signal above the Kedge.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

Eu Lm-edge XAFS spectrum for EuC60 was measured at room temperature in order to clarify the position of the Eu atom. The distances between the Eu atom and the neighboring C atoms determined by XAFS showed clearly that the Eu atom was in the inside of C-60 cage, and located on the off-center by 1.4 Angstrom. A XANES spectrum showed that the Eu atom took the mixed valence states of +2 and +3.

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DOI： 10.1103/PhysRevB.59.15062

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Structure and physical properties of a pressure induced superconductor Cs3C60 were studied by X-ray powder diffraction and ESR methods. All X-ray diffraction patterns below 300 K under ambient pressure (I bar) showed that Cs3C60 took a body-centered orthorhombic (bco) structure as main phase and an A15 structure as minor phase. The ESR at 1 bar showed that Cs3C60 was metallic from 2 to 290 K. The pressure dependence of X-ray powder diffraction showed a disappearence of A15 phase above 20 kbar. The compressibility for the bco phase of Cs3C60, 3.1 x 10(-3) kbar(-1), was similar to those of Rb3C60 and K3C60.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

The crystal structure of ammonium hydrogen succinate has been determined at 80 and 20 K in order to clarify the structural change at the second-order phase transition at 170 K. The geometries of all the hydrogen bonds in the structure were analyzed by combining new geometric data with data obtained previously at 297, 190 and 150 K. The O ... O distances of two O-H ... O hydrogen bonds involving the hydrogen succinate ions increase as the temperature decreases below T-c and the N ... O distance of a weak bifurcated hydrogen bond involving the ammonium ion decreases below T-c. It is noted that the phase transition occurs so that the bifurcated hydrogen bond is retained below T-c. There was no evidence of a descent in space group from P (1) over bar to P1 below T-c and the formation of a superstructure below T-c was not detected.

DOI： 10.1107/S0108768198005448

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Temperature dependence of Raman scattering in a pressure-induced superconductor Cs3C60 was studied under ambient pressure between 2 and 290 K. The contributions of the intramolecular vibration modes to electron-phonon coupling were evaluated by comparing the linewidths of the modes in Cs3C60 with those in pristine C-60. The total electron-phonon coupling constant lambda was approximate to 0.18, approximately one-third of the values for Rb3C60 and K3C60. Temperature-dependent Raman scattering in the superconductor Rb3C60 and a non-superconductor Rb6C60 was also measured for comparison. (C) 1998 Elsevier Science B.V. All rights reserved.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The structure and electronic properties of Cs3C60 under ambient pressure were studied by X-ray diffraction (XRD) and ESR. The XRD showed that Cs3C60 takes a body-centered orthorhombic structure as the main phase. No clear structural change was observed down to 9 K. The temperature dependence of the ESR peak suggested that Cs3C60 exhibits a metallic behavior but no metal-insulator transition down to 1.9 K, contrary to a previous prediction. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(98)00598-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Na-60 was prepared and identified by Raman spectroscopy. Differential scanning calorimetry showed the existence of two stable phases above and below 280 K. The high-temperature phase was supercooled down to 205 K and further transformed to a glass state below 205 K. X-ray diffraction at 295 K could be indexed by a face-centered cubic lattice. The Na-23-NMR spectra below 252 K exhibited the second-order nuclear quadrupole effect accompanied by a descent in symmetry at the site occupied by the Na+ ion. An ESR study has shown that the fee NaC60 becomes nonmagnetic below 180 K. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(98)00394-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

XAFS (X-ray absorption fine-structure) spectra were measured near K-absorption edges of Ce (40.5 keV), Dy (53.8 keV), Ta (67.4 keV and Pt (78.4 keV). The blunt K-edge jump due to the finite lifetime of the core hole was observed. This makes it difficult to extract EXAFS (extended X-ray absorption fine-structure) functions at low k values. Local structure parameters can be evaluated from the EXAFS spectra above K-absorption edges in the high-energy region as well as from those above L-III-edges. It was found that the finite-lifetime effect of the core hole is effectively taken into the photoelectron mean-free-path term, as predicted theoretically.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The temperature dependence of atomic displacement in superconducting K2RbC60 and non-superconducting Rb6C60 has been studied on the basis of X-ray powder diffraction. Rietveld refinement of X-ray powder diffraction data for K2RbC60 at 10.2 K revealed that the occupancy of Rb+ ions at an octahedral site is not 100% but 71%. It has been found that atomic displacements for C atom and metal ions in K2RbC60 show an increase around its superconducting critical temperature. Such an increase has not been observed in Rb6C60. (C) 1998 Elsevier Science Ltd.

DOI： 10.1016/S0038-1098(97)10195-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The crystal structure of chloranil above and below the phase transition temperature, T-c (= 90 K), has been studied by single crystal X-ray diffraction at 20, 40, 60, 70, 80, 85, 90, 95, 100, 110, 130, and 150 K. Below T-c the difference, Delta phi, between the angle of an a axis and an O...O vector in a chloranil molecule [projected to (010) plane] at any temperature and the angle at T-c increases with a power law relation with a decrease in temperature. It shows that the Delta phi is an order parameter. It has been found that the mean-square-displacements of O and Cl atoms increase around T-c. The intermolecular nearest O...C distance decreases with a decrease in temperature above T-c. However, it splits into two types below T-c, and their mean value does not decrease with a decrease in temperature. We have concluded that two types of molecular rotations of clockwise and counterclockwise for neighboring molecules induce this phase transition to avoid too small O...C distance below 90 K. (C) 1997 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(97)80006-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Rb K-edge XAFS of Rb6C60 and Ba K-edge XAFS of Ba6C60 and K3Ba3C60 have been studied, all of which take a bcc structure. The radial structure functions obtained by Fourier transform of XAFS spectra for these compounds showed two pronounced envelopes which can be attributed to the scattering between metal ion and the neighboring C atoms and that between metal ions. The mean-square-displacements for the metal-C in these compounds were larger than that for the t-site Rb-C in Rb3C60 because of the distribution in metal-C distances. From the temperature dependent XAFS, it has been found that the mean-square-displacement for Rb-C in a non-superconductor Rb6C60 shows no anomaly around superconductingtransition temperature, contrary to a superconductor Rb3C60. (C) 1997 Elsevier Science B.V.

DOI： 10.1016/S0921-4534(97)01576-1

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DOI： 10.1039/a700706j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDITIONS PHYSIQUE  

The x-ray excitation spectra around Cu K-edge in sodium chloride crystals containing a small amount of Cu+ ions are measured at 11K to 200K. We found Cu+ ion displacements and growths to micro-clusters (micro-crystals) of CuCl2 in sodium chloride crystal by x-ray irradiation. The trace of the formation is presented by observing the XAFS spectra and XEOL (X-ray Excitation Optical Luminescence) intensities at various temperatures. Ar low temperature below about 50 K, the micro-clusters or the micro-crystals of CuCl2 hardly create. The higher temperature moves the ions faster and further, and makes more active to form the CuCl2 micro-crystals in bulk. The process of moving of the Cu+ ion is discussed.

DOI： 10.1051/jp4/1997106

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDITIONS PHYSIQUE  

Rb and Cs K-edge EXAFS spectra have been measured in the temperature range from 10 to 290 K, in order to investigate the temperature dependence of local structure around Rb+ and Cs+ ions in various alkali doped fullerenes. It has been found that the alkali metal ions in Rb3C60, KRbC60 and Na2CsC60 occupy the center of the tetrahedral and octahedral sites of C-60 crystals. The metal ion in the octahedral site exhibits a large fluctuation in comparison with that in the tetrahedral site. The increase in sigma(2) of the t-site Rb-C for Rb3C60 has been observed around T-c, while the increase in sigma(2) has not clearly been observed for the o-site Rb-C in K2RbC60. The local structure around Rb+ ion in non-superconducting alkali doped fullerene Rb6C60 has also been studied to compare with that in superconducting fullerenes.

DOI： 10.1051/jp4:19972186

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AKADEMIE VERLAG GMBH  

Local structures around Pb(II) and Sn(II) in CH(3)NH(3)PbX(3) (X=Cl, Br, I) and CH(3)NH(3)SnX(3) (X=Br, I) were studied by Pb L(III)-edge and Sn K-edge EXAFS in the temperature range of 10 to 293 K. The M-X distances (M=Pb, Sn; X=Cl, Br, I), the coordination numbers, and the Debye-Waller factors were determined in three, four, three, five, and two solid phases of CH3NH3PbCl3, CH3NH3PbBr3, CH3NH3PbI3, CH3NH3SnBr3, and CH3NH3SnI3, respectively. Five kinds of deformed octahedra consisting of halogens were observed.

DOI： 10.1002/1521-396X(199702)159:2<277::AID-PSSA277>3.0.CO;2-C

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CHEMICAL SOC JAPAN  

The Dy endohedral C-60 (Dy@C-60) was extracted with aniline under ultrasonic irradiation from the soot prepared by are-heating of Dy2O3/graphite composite rod under He pressure of 80 Torr. The purification of Dy@C-60 was performed by using high performance liquid chromatography (HPLC) technique. Furthermore, the Dy endohedral C-82 (Dy@C-82) was effectively extracted from the soot with N,N-dimethylformamide (DMF).

DOI： 10.1246/cl.1997.1019

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELECTROCHEMICAL SOCIETY INC  

Metal Endohedral C-60 (M@C-60 M: Ca, Y, Cel Pr, Nd, Gd) were extracted with aniline from the soots synthesized by are-heating of metal oxide/graphite rods. The extracted solutions, which contain C-60, C-70 and metal endohedral higher fullerenes other than M@C-60, were purified by high performance liquid chromatography (HPLC) technique with aniline as eluent. The laser desorption time-of-flight (LD-TOF) mass spectra for purified solutions exhibited pronounced peaks for M@C-60, indicating the enrichment of M@C-60 in the extracted solutions.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CHEMICAL SOC JAPAN  

The orange and green forms of the Schiff base-oxovanadium(IV) complex, [VO{sal-(RR)-stien}]. MeOH (1, green), [VO{sal-(RR)-stien}]. 2CHCl(3) (2, green), and [VO{sal-(RR)-stien}]. CH3CN (3, orange) (H-2{sal-(RR)-stien}=N,N'-disalicylidene-(RR)-1,2-diphenyl- 1,2-ethanediamine), have been prepared, and their crystal structures were determined by an X-ray method. The green forms, 1 and 2, contain mononuclear square pyramidal molecules of the complex, while the orange form 3 consists of individual complexes stacked to give an infinite linear ... V=0 ... V=0 ... chain. 1, which crystallizes as monoclinic in space group P2(1) with Z=4, has the cell parameters a=16.684(2), b=16.923(4), c=9.249(3) Angstrom, beta=105.76(2)degrees, and V=2513(1) Angstrom(3). 2, which crystallizes as triclinic in space group P1 with Z=2, has the cell parameters a=13.187(3), b=13.469(3), c=10.450(3) Angstrom, alpha=103.72(2), beta=103.20(3), gamma=63.03(2)degrees, and V=1591.2(7) Angstrom(3). 3 crystallizes as triclinic in space group P1 with Z=2. The cell parameters are a=13.300(3), b=13.667(4), c=7.551(1) Angstrom, alpha=90.44(2), beta=94.51(2), gamma=97.36(2)degrees, and V=1357(1)Angstrom(3). The final R indices are R=0.059 and R(w)=0.062 for 1, R=0.098 and R(w)=0.090 for 2, and R=0.053 and R(w)=0.042 for 3. The structural characteristics of these complexes are compared and discussed. 3 shows piezochromism (mechanochromism); its color changes from orange to green upon grinding. 1 and 2 turn orange when heated at 120 degrees C for a few minutes. The extent of conversion either by grinding or heating was studied by EXAFS (X-ray absorption fine structure) and IR spectroscopy. The orange crystals turn green when exposed to chloroform vapor, and the green crystals turn orange upon exposure to acetonitrile vapor.

DOI： 10.1246/bcsj.69.3207

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

A powder X-ray diffraction study has been carried out in the temperature range from 8.9 to 289.4 K in order to clarify the temperature dependence of the lattice constant and the lattice vibrations of Rb3C60 crystals. A large increase in vibrational displacements of C atoms in Rb3C60 has been found around 50 K. The phenomenon is of interest in relation to the superconducting transition. Copyright (C) 1996 Published by Elsevier Science Ltd

DOI： 10.1016/0038-1098(96)00399-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Rb K-edge EXAFS has been studied for K2RbC60 at 21 and 39 K and for KRb2C60 at 10 and 27 K. It is shown that the Rb+ ions in K2RbC60 OCCUPY the center of the octahedral site, contrary to a previous report based on Rb K-edge EXAFS at room temperature that the Rb+ ions are off-centered. It is also found that the Rb+ ions in KRb2C60 occupy the octahedral and two tetrahedral sites.

DOI： 10.1016/0009-2614(96)00324-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

For crystals of ammonium hydrogen succinate it is known that the space group is &lt;P(1)over bar&gt; with Z = 2 at 293 K and the second-order phase transition occurs around 170 K. X-ray crystal structure analyses above and below 170 K have been carried out in order to study the change in mode of short hydrogen bonds between the hydrogen succinate ions. The space group was determined to be &lt;P(1)over bar&gt; at 150 and 190 K by structure analysis. No ordering of the H-atom positions in the short hydrogen bonds occurs by the phase transition. The hydrogen bonds show a decrease in the O ... O distances with a decrease in temperature from 290 to 190 K, but no significant change in the geometries between 190 and 150 K. Disorder of the NH4+ ion is not observed at 297, 190 and 150 K. Significant change through the phase transition is found only in the geometry of one of the N-H ... O hydrogen bonds between ammonium and hydrogen succinate ions.

DOI： 10.1107/S0108768195012432

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CHEMICAL SOC JAPAN  

Endohedral metallofullerene Ce@C-60 has been enriched in the extracted solution of the soot prepared by are-heating of CeO/graphite composite rods. The enrichment of Ce@C-60 in aniline solution containing the hollow and the other endohedral fullerenes have been performed by using high performance liquid chromatography (HPLC) technique. The laser desorption time-of-flight(LD-TOF) mass spectrum shows clearly the enrichment of Ce@C-60.

DOI： 10.1246/cl.1996.1061

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