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 MoS₂/WSe₂ 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 MoS₂/WSe₂ heterojunction <i>via</i> a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe₂ due to its relatively low doping concentration compared to that of MoS₂, which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS₂ side, while charges are fully depleted on the whole WSe₂ 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 MoTe₂ 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) MoTe₂ 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/m²·K than that of metallic CNTs (9 MW/m²·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>_F| > 110 meV, electrons excited by a high-power terahertz pulse <i>E</i>_THz 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>_F| ≤ 110 meV, electron–phonon scattering rate is suppressed owing to the diminishing density of states near the Dirac point. Therefore, <i>E</i>_THz 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>_THz), 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 MoTe₂ 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 MoTe₂ 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>_acc) and the location of the active channel layer of MoTe₂. Our model shows good agreement with the current power spectral density (PSD) of MoTe₂ devices from a low to high current range and indicates that the parameter α exhibits a stronger dependence on <i>N</i>_acc with an exponent −γ of −1.18 to approximately −1.64 for MoTe₂ 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 MoTe₂ multilayer. Comprehensive static and LF noise analyses of MoTe₂ devices with our combined model reveal that a chemical-vapor deposited <i>h</i>-BN monolayer underneath MoTe₂ channel and the Al₂O₃ 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 Al₂O₃ passivation is due to the reduced interface traps

Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS₂/<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 MoS₂ can induce ∼6.5 × 10¹¹ cm^–2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS₂ on <i>h</i>-BN was found to be ∼5 × 10¹³ cm^–2 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 MoS₂/<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 × 10¹² cm^–2 (<i>T</i> = 25 K). The reduced effective Schottky barrier height in MoS₂/<i>h</i>-BN is attributed to the decreased effective work function of MoS₂ 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 SiO₂

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