6 research outputs found
Transient Carrier Cooling Enhanced by Grain Boundaries in Graphene Monolayer
Using
a high terahertz
(THz) electric field (<i>E</i><sub>THz</sub>), the carrier scattering in graphene was studied with
an electric field of up to 282 kV/cm. When the grain size of graphene
monolayers varies from small (5 μm) and medium (70 μm)
to large grains (500 μm), the dominant carrier scattering source
in large- and small-grained graphene differs at high THz field, i.e.,
there is optical phonon scattering for large grains and defect scattering
for small grains. Although the electron–optical phonon coupling
strength is the same for all grain sizes in our study, the enhanced
optical phonon scattering in the high THz field from the large-grained
graphene is caused by a higher optical phonon temperature, originating
from the slow relaxation of accelerated electrons. Unlike the large-grained
graphene, lower electron and optical phonon temperatures are found
in the small-grained graphene monolayer, resulting from the effective
carrier cooling through the defects, called supercollisions. Our results
indicate that the carrier mobility in the high-crystalline graphene
is easily vulnerable to scattering by the optical phonons. Thus, controlling
the population of defect sites, as a means for carrier cooling, can
enhance the carrier mobility at high electric fields in graphene electronics
by suppressing the heating of optical phonons
Unsaturated Drift Velocity of Monolayer Graphene
We
observe that carriers in graphene can be accelerated to the
Fermi velocity without heating the lattice. At large Fermi energy
|<i>E</i><sub>F</sub>| > 110 meV, electrons excited by
a
high-power terahertz pulse <i>E</i><sub>THz</sub> relax
by emitting optical phonons, resulting in heating of the graphene
lattice and optical-phonon generation. This is owing to enhanced electron–phonon
scattering at large Fermi energy, at which the large phase space is
available for hot electrons. The emitted optical phonons cause carrier
scattering, reducing the drift velocity or carrier mobility. However,
for |<i>E</i><sub>F</sub>| ≤ 110 meV, electron–phonon
scattering rate is suppressed owing to the diminishing density of
states near the Dirac point. Therefore, <i>E</i><sub>THz</sub> continues to accelerate carriers without them losing energy to optical
phonons, allowing the carriers to travel at the Fermi velocity. The
exotic carrier dynamics does not result from the massless nature,
but the electron–optical-phonon scattering rate depends on
Fermi level in the graphene. Our observations provide insight into
the application of graphene for high-speed electronics without degrading
carrier mobility
Coaxial Fiber Supercapacitor Using All-Carbon Material Electrodes
We report a coaxial fiber supercapacitor, which consists of carbon microfiber bundles coated with multiwalled carbon nanotubes as a core electrode and carbon nanofiber paper as an outer electrode. The ratio of electrode volumes was determined by a half-cell test of each electrode. The capacitance reached 6.3 mF cm<sup>–1</sup> (86.8 mF cm<sup>–2</sup>) at a core electrode diameter of 230 μm and the measured energy density was 0.7 μWh cm<sup>–1</sup> (9.8 μWh cm<sup>–2</sup>) at a power density of 13.7 μW cm<sup>–1</sup> (189.4 μW cm<sup>–2</sup>), which were much higher than the previous reports. The change in the cyclic voltammetry characteristics was negligible at 180° bending, with excellent cycling performance. The high capacitance, high energy density, and power density of the coaxial fiber supercapacitor are attributed to not only high effective surface area due to its coaxial structure and bundle of the core electrode, but also all-carbon materials electrodes which have high conductivity. Our coaxial fiber supercapacitor can promote the development of textile electronics in near future
Electrical Transport Properties of Polymorphic MoS<sub>2</sub>
The
engineering of polymorphs in two-dimensional layered materials
has recently attracted significant interest. Although the semiconducting
(2H) and metallic (1T) phases are known to be stable in thin-film
MoTe<sub>2</sub>, semiconducting 2H-MoS<sub>2</sub> is locally converted
into metallic 1T-MoS<sub>2</sub> through chemical lithiation. In this
paper, we describe the observation of the 2H, 1T, and 1T′ phases
coexisting in Li-treated MoS<sub>2</sub>, which result in unusual
transport phenomena. Although multiphase MoS<sub>2</sub> shows no
transistor-gating response, the channel resistance decreases in proportion
to the temperature, similar to the behavior of a typical semiconductor.
Transmission electron microscopy images clearly show that the 1T and
1T′ phases are randomly distributed and intervened with 2H-MoS<sub>2</sub>, which is referred to as the 1T and 1T′ puddling phenomenon.
The resistance curve fits well with 2D-variable range-hopping transport
behavior, where electrons hop over 1T domains that are bounded by
semiconducting 2H phases. However, near 30 K, electrons hop over charge
puddles. The large temperature coefficient of resistance (TCR) of
multiphase MoS<sub>2</sub>, −2.0 × 10<sup>–2</sup> K<sup>–1</sup> at 300 K, allows for efficient IR detection
at room temperature by means of the photothermal effect
Junction-Structure-Dependent Schottky Barrier Inhomogeneity and Device Ideality of Monolayer MoS<sub>2</sub> Field-Effect Transistors
Although
monolayer transition metal dichalcogenides (TMDs) exhibit superior
optical and electrical characteristics, their use in digital switching
devices is limited by incomplete understanding of the metal contact.
Comparative studies of Au top and edge contacts with monolayer MoS<sub>2</sub> reveal a temperature-dependent ideality factor and Schottky
barrier height (SBH). The latter originates from inhomogeneities in
MoS<sub>2</sub> caused by defects, charge puddles, and grain boundaries,
which cause local variation in the work function at Au–MoS<sub>2</sub> junctions and thus different activation temperatures for
thermionic emission. However, the effect of inhomogeneities due to
impurities on the SBH varies with the junction structure. The weak
Au–MoS<sub>2</sub> interaction in the top contact, which yields
a higher SBH and ideality factor, is more affected by inhomogeneities
than the strong interaction in the edge contact. Observed differences
in the SBH and ideality factor in different junction structures clarify
how the SBH and inhomogeneities can be controlled in devices containing
TMD materials
Unraveled Face-Dependent Effects of Multilayered Graphene Embedded in Transparent Organic Light-Emitting Diodes
With increasing demand
for transparent conducting electrodes, graphene has attracted considerable
attention, owing to its high electrical conductivity, high transmittance,
low reflectance, flexibility, and tunable work function. Two faces
of single-layer graphene are indistinguishable in its nature, and
this idea has not been doubted even in multilayered graphene (MLG)
because it is difficult to separately characterize the front (first-born)
and the rear face (last-born) of MLG by using conventional analysis
tools, such as Raman and ultraviolet spectroscopy, scanning probe
microscopy, and sheet resistance. In this paper, we report the striking
difference of the emission pattern and performance of transparent
organic light-emitting diodes (OLEDs) depending on the adopted face
of MLG and show the resolved chemical and physical states of both
faces by using depth-selected absorption spectroscopy. Our results
strongly support that the interface property between two different
materials rules over the bulk property in the driving performance
of OLEDs