6 research outputs found
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]Âphenacene single-crystal field-effect transistors
(FETs) was modified using 14 electron acceptors with different redox
potentials. The effective hole-injection barrier heights (Ï•<sub>h</sub><sup>eff</sup>s) for [7]Âphenacene single-crystal FETs have
been plotted as a function of the redox potential (<i>E</i><sub>redox</sub>) of the inserted electron acceptors, showing that
the Ï•<sub>h</sub><sup>eff</sup> decreases with increasing <i>E</i><sub>redox</sub>. The highest Ï•<sub>h</sub><sup>eff</sup> occurs without inserted material (electron acceptors), and this
deviates from the otherwise linear relationship between Ï•<sub>h</sub><sup>eff</sup> and <i>E</i><sub>redox</sub>. We
have investigated the temperature dependence of Ï•<sub>h</sub><sup>eff</sup> in an attempt to determine why the Ï•<sub>h</sub><sup>eff</sup> 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 Ï•<sub>h</sub><sup>eff</sup> 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 Ï•<sub>h</sub><sup>eff</sup>. Controlling the interface
between the single crystal and the source/drain electrodes in this
FET produced a very high μ (∼6.9 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>) and low absolute threshold
voltage, i.e., excellent FET characteristics. The topological characterization
of inserted materials on [7]Âphenacene 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 BaTi<sub>2</sub>(Sb<sub>1–<i>y</i></sub>Bi<sub><i>y</i></sub>)<sub>2</sub>O 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 HfO<sub>2</sub> gate dielectric and TCNQ-coated
electrodes shows p-channel characteristics with a smooth hole injection
and a field-effect mobility more than 1 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> 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]Â[hexafluorophosphate],
showing the field-effect mobility of 1.8 × 10<sup>–1</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and low
absolute value, 1.9 V, of threshold voltage
Nanophase Separation in K<sub>1–<i>x</i></sub>Ca<sub><i>x</i></sub>C<sub>8</sub> 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