Parity Effects in Few-Layer Graphene

We study the electronic properties in few-layer graphenes (FLGs) classified by even/odd layer number <i>n</i>. FLGs with even <i>n</i> have only parabolic energy dispersions, whereas FLGs with odd <i>n</i> 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 <i>n</i> are unique materials that have relativistic carriers originating in linear energy dispersion

Edge-Dependent Transport Properties in Graphene

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

Systematic Control of Hole-Injection Barrier Height with Electron Acceptors in [7]phenacene Single-Crystal Field-Effect Transistors

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 (ϕ_h^effs) for [7]­phenacene single-crystal FETs have been plotted as a function of the redox potential (<i>E</i>_redox) of the inserted electron acceptors, showing that the ϕ_h^eff decreases with increasing <i>E</i>_redox. The highest ϕ_h^eff occurs without inserted material (electron acceptors), and this deviates from the otherwise linear relationship between ϕ_h^eff and <i>E</i>_redox. We have investigated the temperature dependence of ϕ_h^eff in an attempt to determine why the ϕ_h^eff 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 ϕ_h^eff 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 ϕ_h^eff. Controlling the interface between the single crystal and the source/drain electrodes in this FET produced a very high μ (∼6.9 cm² 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

Synthesis of Methoxy-Substituted Picenes: Substitution Position Effect on Their Electronic and Single-Crystal Structures

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

Synthesis of Methoxy-Substituted Picenes: Substitution Position Effect on Their Electronic and Single-Crystal Structures

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

Superconducting Behavior of BaTi₂(Sb_1–<i>y</i>Bi_<i>y</i>)₂O under Pressure

The crystal structure and superconducting properties of a new type of titanium–pnictide superconductor, BaTi2(Sb1–yBiy)2O (y = 0.2, 0.5, and 0.8), are comprehensively investigated over a wide pressure range to elucidate the effect of substituting Bi for Sb on the superconducting behavior. The behavior of superconducting properties under pressure changes drastically with y, as expected from the double-dome Tc–y phase diagram obtained at ambient pressure. In this study, three BaTi2(Sb1–yBiy)2O samples (y = 0.2, 0.5, and 0.8) are considered, which correspond to the first superconducting dome, nonsuperconducting part, and second superconducting dome, respectively, in the Tc–y phase diagram. The crystal of BaTi2(Sb1–yBiy)2O with y = 0.2 shows a clear collapse transition, i.e., a drastic shrinkage of the lattice constant c at ca. 5 GPa. Strictly speaking, the collapsed crystal phase coexists with the noncollapsed phase above 5 GPa. On the other hand, BaTi2(Sb1–yBiy)2O with y = 0.8 shows a continuous change in the crystal lattice with pressure, i.e., no collapse transitions. The pressure dependence of Tc for BaTi2(Sb1–yBiy)2O with y = 0.2 shows a drastic increase in Tc at approximately 5 GPa, where the collapse transition occurs, indicating a clear pressure-induced superconducting phase transition related to the collapse transition. The value of Tc for BaTi2(Sb1–yBiy)2O with y = 0.8 increases slightly up to ∼2 GPa and is almost constant at 2–13 GPa. It is found that the superconducting behavior under pressure can be unambiguously classified by y based on the double-dome Tc–y phase diagram, indicative of distinguishable superconducting features at different y values. In this study, we comprehensively discuss the superconducting properties of the exotic material, BaTi2(Sb1–yBiy)2O, with a double-dome Tc–y phase diagram

Synthesis of Methoxy-Substituted Picenes: Substitution Position Effect on Their Electronic and Single-Crystal Structures

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

Characteristics of Single Crystal Field-Effect Transistors with a New Type of Aromatic Hydrocarbon, Picene

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

Nanophase Separation in K_1–<i>x</i>Ca_<i>x</i>C₈ Revealed by X‑ray Fluorescence Holography and Extended X‑ray Absorption Fine Structure

Even at low Ca concentrations, the binary-element intercalated graphite K1–xCaxC8 exhibits high superconducting transition temperatures Tc, closer to that of CaC6 (11.5 K) than KC8 (0.169 K). To investigate the behaviors of K and Ca within the graphite matrix, their local structures have been investigated by using X-ray fluorescence holography and extended X-ray absorption fine structure (EXAFS) spectroscopy. The atomic images reconstructed from the K–Kα and Ca–Kβ holograms showed that K0.7Ca0.3C8 did not take the solid-solution type random distribution of Ca and K atoms; instead, a nanoscale phase separation of CaC6 and KC8 was observed, which was also supported by the EXAFS results. While the lattice constant of K0.7Ca0.3C8 was close to that of KC8, we found a nanoscale Ca layer dispersed within the sample. The Ca nanolayer was offset from the center between the C sublayers. The superconducting behavior found in K1–xCaxC8 was discussed with two scenarios of percolation of CaC6 and deformation of graphene based on such a specific inhomogeneous binary element monatomic layers. This study represents an important step for understanding the superconducting properties in a nanoscale phase-separated system