15 research outputs found
Torsional Actuator Powered by Environmental Energy Harvesting from Diurnal Temperature Variation
Inspired
by phase-change technology storing thermal energy in the
form of latent heat, direct conversion of environmental temperature
variation into a useful form of mechanical work is achieved using
the drastic volume change of a phase-change material during solid–liquid
phase transformation. A twisted carbon nanotube yarn in combination
with a phase-change material functions as a backbone of torsional
actuator, powered by the change of environmental temperature, because
nanopores among carbon nanotubes or their bundles are completely filled
with the phase-change material. By the proper selection of infiltrated
material whose melting temperature lies within a diurnal temperature
range, this hybrid yarn can be applied in an autonomous system using
naturally abundant low-grade thermal energy flow
Charge Transport in MoS<sub>2</sub>/WSe<sub>2</sub> van der Waals Heterostructure with Tunable Inversion Layer
Despite numerous
studies on two-dimensional van der Waals heterostructures,
a full understanding of the charge transport and photoinduced current
mechanisms in these structures, in particular, associated with charge
depletion/inversion layers at the interface remains elusive. Here,
we investigate transport properties of a prototype multilayer MoS<sub>2</sub>/WSe<sub>2</sub> heterojunction <i>via</i> a tunable
charge inversion/depletion layer. A charge inversion layer was constructed
at the surface of WSe<sub>2</sub> due to its relatively low doping
concentration compared to that of MoS<sub>2</sub>, which can be tuned
by the back-gate bias. The depletion region was limited within a few
nanometers in the MoS<sub>2</sub> side, while charges are fully depleted
on the whole WSe<sub>2</sub> side, which are determined by Raman spectroscopy
and transport measurements. Charge transport through the heterojunction
was influenced by the presence of the inversion layer and involves
two regimes of tunneling and recombination. Furthermore, photocurrent
measurements clearly revealed recombination and space-charge-limited
behaviors, similar to those of the heterostructures built from organic
semiconductors. This contributes to research of various other types
of heterostructures and can be further applied for electronic and
optoelectronic devices
Negative and Positive Persistent Photoconductance in Graphene
Persistent photoconductance, a prolonged light-induced conducting
behavior that lasts several hundred seconds, has been observed in
semiconductors. Here we report persistent negative photoconductance
and consecutive prominent persistent positive photoconductance in
graphene. Unusually large yields of negative PC (34%) and positive
PC (1652%) and remarkably long negative transient response time (several
hours) were observed. Such high yields were reduced in multilayer
graphene and were quenched under vacuum conditions. Two-dimensional
metallic graphene strongly interacts with environment and/or substrate,
causing this phenomenon, which is markedly different from that in
three-dimensional semiconductors and nanoparticles
Tunable Mobility in Double-Gated MoTe<sub>2</sub> Field-Effect Transistor: Effect of Coulomb Screening and Trap Sites
There
is a general consensus that the carrier mobility in a field-effect
transistor (FET) made of semiconducting transition-metal dichalcogenides
(s-TMDs) is severely degraded by the trapping/detrapping and Coulomb
scattering of carriers by ionic charges in the gate oxides. Using
a double-gated (DG) MoTe<sub>2</sub> FET, we modulated and enhanced
the carrier mobility by adjusting the top- and bottom-gate biases.
The relevant mechanism for mobility tuning in this device was explored
using static DC and low-frequency (LF) noise characterizations. In
the investigations, LF-noise analysis revealed that for a strong back-gate
bias the Coulomb scattering of carriers by ionized traps in the gate
dielectrics is strongly screened by accumulation charges. This significantly
reduces the electrostatic scattering of channel carriers by the interface
trap sites, resulting in increased mobility. The reduction of the
number of effective trap sites also depends on the gate bias, implying
that owing to the gate bias, the carriers are shifted inside the channel.
Thus, the number of active trap sites decreases as the carriers are
repelled from the interface by the gate bias. The gate-controlled
Coulomb-scattering parameter and the trap-site density provide new
handles for improving the carrier mobility in TMDs, in a fundamentally
different way from dielectric screening observed in previous studies
Interfacial Thermal Conductance Observed to be Higher in Semiconducting than Metallic Carbon Nanotubes
Thermal transport at carbon nanotube (CNT) interfaces was investigated by characterizing the interfacial thermal conductance between metallic or semiconducting CNTs and three different surfactants. We thereby resolved a difference between metallic and semiconducting CNTs. CNT portions separated by their electronic type were prepared in aqueous suspensions. After slightly heating the CNTs dispersed in the suspension, we obtained cooling curves by monitoring the transient changes in absorption, and from these cooling curves, we extracted the interfacial thermal conductance by modeling the thermal system. We found that the semiconducting CNTs unexpectedly exhibited a higher conductance of 11.5 MW/m<sup>2</sup>·K than that of metallic CNTs (9 MW/m<sup>2</sup>·K). Meanwhile, the type of surfactants hardly influenced the heat transport at the interface. The surfactant dependence is understood in terms of the coupling between the low-frequency vibrational modes of the CNTs and the surfactants. Explanations for the electronic-type dependency are considered based on the defect density in CNTs and the packing density of surfactants
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
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
Suppression of Interfacial Current Fluctuation in MoTe<sub>2</sub> Transistors with Different Dielectrics
For transition metal
dichalcogenides, the fluctuation of the channel current due to charged
impurities is attributed to a large surface area and a thickness of
a few nanometers. To investigate current variance at the interface
of transistors, we obtain the low-frequency (LF) noise features of
MoTe<sub>2</sub> multilayer field-effect transistors with different
dielectric environments. The LF noise properties are analyzed using
the combined carrier mobility and carrier number fluctuation model
which is additionally parametrized with an interfacial Coulomb-scattering
parameter (α) that varies as a function of the accumulated carrier
density (<i>N</i><sub>acc</sub>) and the location of the
active channel layer of MoTe<sub>2</sub>. Our model shows good agreement
with the current power spectral density (PSD) of MoTe<sub>2</sub> devices
from a low to high current range and indicates that the parameter
α exhibits a stronger dependence on <i>N</i><sub>acc</sub> with an exponent −γ of −1.18 to approximately
−1.64 for MoTe<sub>2</sub> devices, compared with −0.5
for Si devices. The raised Coulomb scattering of the carriers, particularly
for a low-current regime, is considered to be caused by the unique
traits of layered semiconductors such as interlayer coupling and the
charge distribution strongly affected by the device structure under
a gate bias, which completely change the charge screening effect in
MoTe<sub>2</sub> multilayer. Comprehensive static and LF noise analyses
of MoTe<sub>2</sub> devices with our combined model reveal that a
chemical-vapor deposited <i>h</i>-BN monolayer underneath
MoTe<sub>2</sub> channel and the Al<sub>2</sub>O<sub>3</sub> passivation
layer have a dissimilar contribution to the reduction of current fluctuation.
The three-fold enhanced carrier mobility due to the <i>h</i>-BN is from the weakened carrier scattering at the gate dielectric
interface and the additional 30% increase in carrier mobility by Al<sub>2</sub>O<sub>3</sub> passivation is due to the reduced interface
traps
Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS<sub>2</sub>/<i>h</i>‑BN Heterostructure
Layered hexagonal boron nitride (<i>h</i>-BN) thin film
is a dielectric that surpasses carrier mobility by reducing charge
scattering with silicon oxide in diverse electronics formed with graphene
and transition metal dichalcogenides. However, the <i>h</i>-BN effect on electron doping concentration and Schottky barrier
is little known. Here, we report that use of <i>h</i>-BN
thin film as a substrate for monolayer MoS<sub>2</sub> can induce
∼6.5 × 10<sup>11</sup> cm<sup>–2</sup> electron
doping at room temperature which was determined using theoretical
flat band model and interface trap density. The saturated excess electron
concentration of MoS<sub>2</sub> on <i>h</i>-BN was found
to be ∼5 × 10<sup>13</sup> cm<sup>–2</sup> at high
temperature and was significantly reduced at low temperature. Further,
the inserted <i>h</i>-BN enables us to reduce the Coulombic
charge scattering in MoS<sub>2</sub>/<i>h</i>-BN and lower
the effective Schottky barrier height by a factor of 3, which gives
rise to four times enhanced the field-effect carrier mobility and
an emergence of metal–insulator transition at a much lower
charge density of ∼1.0 × 10<sup>12</sup> cm<sup>–2</sup> (<i>T</i> = 25 K). The reduced effective Schottky barrier
height in MoS<sub>2</sub>/<i>h</i>-BN is attributed to the
decreased effective work function of MoS<sub>2</sub> arisen from <i>h</i>-BN induced <i>n</i>-doping and the reduced effective
metal work function due to dipole moments originated from fixed charges
in SiO<sub>2</sub>
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