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

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

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