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Bioinspired yarn/fiber structured hydro-actuators have recently attracted significant attention. However, most water-driven mechanical actuators are unsatisfactory because of the slow recovery process and low full-time power density. A rapidly recoverable high-power hydro-actuator is reported by designing biomimetic carbon nanotube (CNT) yarns. The hydrophilic CNT (HCNT) coiled yarn was prepared by storing pre-twist into CNT sheets and subsequent electrochemical oxidation (ECO) treatment. The resulting yarn demonstrated structural stability even when one end was cut off without the possible loss of pre-stored twists. The HCNT coiled yarn actuators provided maximal contractile work of 863 J/kg at 11.8 MPa stress when driven by water. Moreover, the recovery time of electrically heated yarns at a direct current voltage of 5 V was 95% shorter than that of neat yarns without electric heating. Finally, the electrothermally recoverable hydro-actuators showed a high actuation frequency (0.17 Hz) and full-time power density (143.8 W/kg).
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Nanotubos de Carbono , Biomimética , Eletricidade , Contração Muscular , Nanotubos de Carbono/química , ÁguaRESUMO
Clean interface and low contact resistance are crucial requirements in two-dimensional (2D) materials to preserve their intrinsic carrier mobility. However, atomically thin 2D materials are sensitive to undesired Coulomb scatterers such as surface/interface adsorbates, metal-to-semiconductor Schottky barrier (SB), and ionic charges in the gate oxides, which often limits the understanding of the charge scattering mechanism in 2D electronic systems. Here, we present the effects of hafnium dioxide (HfO2) high-κ passivation and SB height on the low-frequency (LF) noise characteristics of multilayer molybdenum ditelluride (MoTe2) transistors. The passivated HfO2 passivation layer significantly suppresses the surface reaction and enhances dielectric screening effect, resulting in an excess electron n-doping, zero hysteresis, and substantial improvement in carrier mobility. After the high-κ HfO2 passivation, the obtained LF noise data appropriately demonstrates the transition of the Coulomb scattering mechanism from the SB contact to the channel, revealing the significant SB noise contribution to the 1/f noise. The substantial excess LF noise in the subthreshold regime is mainly attributed to the excess metal-to-MoTe2 SB noise and is fully eliminated at the high drain bias regime. This study provides a clear insight into the origin of electronic signal perturbation in 2D electronic systems.
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Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to promote gate-voltage scaling using a high-k dielectric material. The atomically flat surface of thin STO layer epitaxially grown on Nb-doped STO single-crystal substrate offers good adhesion between the high-k film and graphene, resulting in nonhysteretic conductance as a function of gate voltage at all temperatures down to 2 K. The two-terminal conductance quantization under magnetic fields corresponding to quantum Hall states survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. Our experimental data prove that the operating voltage reduction can be successfully realized due to the underlying high-k STO thin film, without any noticeable degradation of graphene device performance.
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Layered hexagonal boron nitride (h-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 h-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of h-BN thin film as a substrate for monolayer MoS2 can induce â¼6.5 × 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 h-BN was found to be â¼5 × 1013 cm-2 at high temperature and was significantly reduced at low temperature. Further, the inserted h-BN enables us to reduce the Coulombic charge scattering in MoS2/h-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 â¼1.0 × 1012 cm-2 (T = 25 K). The reduced effective Schottky barrier height in MoS2/h-BN is attributed to the decreased effective work function of MoS2 arisen from h-BN induced n-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO2.
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The combination of quantum Hall conductance and charge-trap memory operation was qualitatively examined using a graphene field-effect transistor. The characteristics of two terminal quantum Hall conductance appeared clearly on the background of a huge conductance hysteresis during a gate-voltage sweep for a device using monolayer graphene as a channel,hexagonal boron-nitride flakes as a tunneling dielectric and defective silicon oxide as the charge storage node. Even though there was a giant shift of the charge neutrality point, the deviation of quantized resistance value at the state of filling factor 2 was less than 1.6% from half of the von Klitzing constant. At high Landau level indices, the behaviors of quantum conductance oscillation between the increasing and the decreasing electron densities were identical in spite ofa huge memory window exceeding 100 V. Our results indicate that the two physical phenomena, two-terminal quantum Hall conductance and charge-trap memory operation, can be integrated into one device without affecting each other.
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We report electrochemically powered, all-solid-state torsional and tensile artificial yarn muscles using a spinnable carbon nanotube (CNT) sheet that provides attractive performance. Large torsional muscle stroke (53°/mm) with minor hysteresis loop was obtained for a low applied voltage (5 V) without the use of a relatively complex three-electrode electromechanical setup, liquid electrolyte, or packaging. Useful tensile muscle strokes were obtained (1.3% at 2.5 V and 0.52% at 1 V) when lifting loads that are â¼25 times heavier than can be lifted by the same diameter human skeletal muscle. Also, the tensile actuator maintained its contraction following charging and subsequent disconnection from the power supply because of its own supercapacitor property at the same time. Possible eventual applications for the individual tensile and torsional muscles are in micromechanical devices, such as for controlling valves and stirring liquids in microfluidic circuits, and in medical catheters.
Assuntos
Músculo Esquelético/química , Nanotubos de Carbono/química , Catéteres , Humanos , Músculo Esquelético/ultraestrutura , Nanotubos de Carbono/ultraestrutura , Resistência à TraçãoRESUMO
Negative capacitance gives rise to subthreshold swing (SS) below the fundamental limit by efficient modulation of surface potential in transistors. While negative-capacitance transition is reported in polycrystalline Pb(Zr0.2Ti0.8)O3 (PZT) and HfZrO2 (HZO) thin-films in few microseconds timescale, low SS is not persistent over a wide range of drain current when used instead of conventional dielectrics. In this work, the clear nano-second negative transition states in 2D single-crystal CuInP2S6 (CIPS) flakes have been demonstrated by an alternative fast-transient measurement technique. Further, integrating this ultrafast NC transition with the localized density of states of Dirac contacts and controlled charge transfer in the CIPS/channel (MoS2/graphene) a state-of-the-art device architecture, negative capacitance Dirac source drain field effect transistor (FET) is introduced. This yields an ultralow SS of 4.8 mV dec-1 with an average sub-10 SS across five decades with on-off ratio exceeding 107, by simultaneous improvement of transport and body factors in monolayer MoS2-based FET, outperforming all previous reports. This approach could pave the way to achieve ultralow-SS FETs for future high-speed and low-power electronics.
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Hydrogen (H2) gas has recently become a crucial energy source and an imperative energy vector, emerging as a powerful next-generation solution for fuel cells and biomedical, transportation, and household applications. With increasing interest in H2, safety concerns regarding personal injuries from its flammability and explosion at high concentrations (>4%) have inspired the development of wearable pre-emptive gas monitoring platforms that can operate on curved and jointed parts of the human body. In this study, a yarn-type hydrogen gas sensing platform (HGSP) was developed by biscrolling of palladium oxide nanoparticles (PdO NPs) and spinnable carbon nanotube (CNT) buckypapers. Because of the high loading of H2-active PdO NPs (up to 97.7 wt %), when exposed to a flammable H2 concentration (4 vol %), the biscrolled HGSP yarn exhibits a short response time of 2 s, with a high sensitivity of 1198% (defined as ΔG/G0 × 100%). Interestingly, during the reduction of PdO to Pd by H2 gas, the HGSP yarn experienced a decrease in diameter and corresponding volume contraction. These excellent sensing performances suggest that the fabricated HGSP yarn could be applied to a wearable gas monitoring platform for real-time detection of H2 gas leakage even over the bends of joints.
Assuntos
Nanopartículas , Nanotubos de Carbono , Dispositivos Eletrônicos Vestíveis , Humanos , Hidrogênio , GasesRESUMO
Coil-structured carbon nanotube (CNT) yarns have recently attracted considerable attention. However, structural instability due to heavy twist insertion, and inherent hydrophobicity restrict its wider application. We report a twist-stable and hydrophilic coiled CNT yarn produced by the facile electrochemical oxidation (ECO) method. The ECO-treated coiled CNT yarn is prepared by applying low potentiostatic voltages (3.0-4.5 V vs Ag/AgCl) between the coiled CNT yarn and a counter electrode immersed in an electrolyte for 10-30 s. Notably, a large volume expansion of the coiled CNT yarns prepared by electrochemical charge injection produces morphological changes, such as surface microbuckling and large reductions in the yarn bias angle and diameter, resulting in the twist-stability of the dried ECO-treated coiled CNT yarns with increased yarn density. The resulting yarns are well functionalized with oxygen-containing groups; they exhibit extrinsic hydrophilicity and significantly improved capacitance (approximately 17-fold). We quantitatively explain the origin of the capacitance improvement using theoretical simulations and experimental observations. Stretchable supercapacitors fabricated with the ECO-treated coiled CNT yarns show high capacitance (12.48 mF/cm and 172.93 mF/cm2, respectively) and great stretchability (80%). Moreover, the ECO-treated coiled CNT yarns are strong enough to be woven into a mask as wearable supercapacitors.
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Predicting and preventing disasters in difficult-to-access environments, such as oceans, requires self-powered monitoring devices. Since the need to periodically charge and replace batteries is an economic and environmental concern, energy harvesting from external stimuli to supply electricity to batteries is increasingly being considered. Especially, in aqueous environments including electrolytes, coiled carbon nanotube (CNT) yarn harvesters have been reported as an emerging approach for converting mechanical energy into electrical energy driven by large and reversible capacitance changes under stretching and releasing. To realize enhanced harvesting performance, experimental and computational approaches to optimize structural homogeneity and electrochemical accessible area in CNT yarns to maximize intrinsic electrochemical capacitance (IEC) and stretch-induced changes are presented here. Enhanced IEC further enables to decrease matching impedance for more energy efficient circuits with harvesters. In an ocean-like environment with a frequency from 0.1 to 1 Hz, the proposed harvester demonstrates the highest volumetric power (1.6-10.45 mW cm-3 ) of all mechanical harvesters reported in the literature to the knowledge of the authors. Additionally, a high electrical peak power of 540 W kg-1 and energy conversion efficiency of 2.15% are obtained from torsional and tensile mechanical energy.
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Charge carrier scattering at grain boundaries (GBs) in a chemical vapor deposition (CVD) graphene reduces the carrier mobility and degrades the performance of the graphene device, which is expected to affect the quantum Hall effect (QHE). This study investigated the influence of individual GBs on the QH state at different stitching angles of the GB in a monolayer CVD graphene. The measured voltage probes of the equipotential line in the QH state showed that the longitudinal resistance (Rxx) was affected by the scattering of the GB only in the low carrier concentration region, and the standard QHE of a monolayer graphene was observed regardless of the stitching angle of the GB. In addition, a controlled device with an added metal bar placed in the middle of the Hall bar configuration was introduced. Despite the fact that the equipotential lines in the controlled device were broken by the additional metal bar, only the Rxx was affected by nonzero resistance, whereas the Hall resistance (Rxy) revealed the well-quantized plateaus in the QH state. Thus, our study clarifies the effect of individual GBs on the QH states of graphenes.
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HfO2-based ferroelectric thin films deposited via atomic layer deposition have been extensively studied as promising candidates for next-generation ferroelectric devices. The conversion of an amorphous Hf1-xZrxO2 film to the ferroelectric phase (non-centrosymmetric orthorhombic phase) has been achieved through annealing using a post-thermal process. However, in this study, we present the first report of ferroelectricity of hafnium-zirconium-oxide (HZO) thin films deposited via atomic layer deposition using cyclopentadienyl-based precursors without additional post-thermal processing. By increasing the deposition temperature using a cyclopentadienyl-based cocktail precursor, the conditions of the as-deposited HZO thin film to crystallize well with an orthorhombic phase were secured, and excellent ferroelectric properties with a large remanent polarization (2Pr â¼ 47.6 µC cm-2) were implemented without crystallization annealing. The as-deposited HZO thin film possessed very stable ferroelectric properties without a wake-up effect or significant fatigue up to 106 cycles. Futhermore, we demonstrated the applicability to devices using negative capacitance and non-volatile memory characteristics. This result suggests that a new strategy can be applied to ferroelectric devices where subsequent processing temperature constraints are required, such as back-end-of-line processes and ferroelectric-based flexible device applications.
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We report unusual absolute negative resistance phenomena in twisted superconducting yarns consisting of niobium-nitride (NbN) nanowires formed on a template of aligned carbon nanotube (CNT) sheets. In the vicinity of the superconducting critical temperature and critical current, the electrical resistance with a standard four-probe configuration exhibits negative values for many wire-shaped twisted yarns. This anomalous behavior at the superconducting transition stage is analyzed using a simplified circuit model, where the charge conduction is determined by the combination between the intra- and internanofiber transports inside the yarn. The superconducting transition of intrafibrillar transport along CNT-templated NbN nanowires was distinguished from that of an interfibrillar one, where the latter exhibits the ensemble property of superconducting weak links among adjacent NbN nanowires. Furthermore, the topological similarity between the sheet of an aligned array of nanowires and the yarn of twisted nanofibrils enables the occurrence of this anomaly. This study indicates that the quantitative network-based approach is effective for the analysis of anomalous charge conduction through nanowire-based anisotropic materials.
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Dielectrics have long been considered as unsuitable for pure electrical switches; under weak electric fields, they show extremely low conductivity, whereas under strong fields, they suffer from irreversible damage. Here, we show that flexoelectricity enables damage-free exposure of dielectrics to strong electric fields, leading to reversible switching between electrical states-insulating and conducting. Applying strain gradients with an atomic force microscope tip polarizes an ultrathin film of an archetypal dielectric SrTiO3 via flexoelectricity, which in turn generates non-destructive, strong electrostatic fields. When the applied strain gradient exceeds a certain value, SrTiO3 suddenly becomes highly conductive, yielding at least around a 108-fold decrease in room-temperature resistivity. We explain this phenomenon, which we call the colossal flexoresistance, based on the abrupt increase in the tunneling conductance of ultrathin SrTiO3 under strain gradients. Our work extends the scope of electrical control in solids, and inspires further exploration of dielectric responses to strong electromechanical fields.
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The advanced Hall magnetic sensor using an ion-gated graphene field-effect transistor demonstrates a high current-normalized sensitivity larger than 3000 V/AT and low operation voltages smaller than 0.5 V. From commercially available graphene-on-SiO2 wafers, large-area arrays of ion-gated graphene Hall element (ig-GHE) samples are prepared through complementary metal-oxide-semiconductor-compatible fabrication processes except the final addition of ionic liquid electrolyte covering the exposed graphene channel and the separate gate-electrode area. The enhanced carrier tunability by ionic gating enables this ig-GHE device to be extremely sensitive to magnetic fields in low-voltage-operation regimes. Further electrical characterization indicates that the operation window is limited by the nonuniform carrier concentration over the channel under high bias conditions. The drain-current-normalized magnetic resolution of the device measured using the low-frequency noise technique is comparable to the previously reported values despite its significant low power consumption.
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The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO3 , graphene/LaAlO3 /SrTiO3 , graphene/ferroelectric oxide, MoS2 /SrTiO3 , and FeSe/SrTiO3 heterostructures include voltage scaling in field-effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next-generation optoelectronic devices.
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We directly synthesize MoWS2 nanosheets on conductive carbon nanotube yarn (MoWS2/CNTY) and carbon fiber cloth (MoWS2/CC) through a fast and low-temperature thermolysis method to obtain flexible catalyst electrodes that show high performance in the hydrogen evolution reaction. Small Tafel slopes of 41.8 and 46.7 mV dec-1 are achieved for MoWS2/CC and MoWS2/CNTY, respectively, by optimizing the density of the exposed active edge sites of vertically aligned MoWS2 on CNTY and CC. Furthermore, the catalyst electrodes demonstrate good electrocatalytic stability over 36 h. The proposed technique for fabricating high-performance, binder-free, and flexible catalyst electrodes is more accessible and faster than conventional methods.
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Highly deformable and electrically conductive fibres with multiple functionalities may be useful for diverse applications. Here we report on a supercoil structure (i.e. coiling of a coil) of fibres fabricated by inserting a giant twist into spandex-core fibres wrapped in a carbon nanotube sheath. The resulting supercoiled fibres show a highly ordered and compact structure along the fibre direction, which can sustain up to 1,500% elastic deformation. The supercoiled fibre exhibits an increase in resistance of 4.2% for stretching of 1,000% when overcoated by a passivation layer. Moreover, by incorporating pseudocapacitive-active materials, we demonstrate the existence of superelastic supercapacitors with high linear and areal capacitance values of 21.7 mF cm-1 and 92.1 mF cm-2, respectively, that can be reversibly stretched by 1,000% without significant capacitance loss. The supercoiled fibre can also function as an electrothermal artificial muscle, contracting 4.2% (percentage of loaded fibre length) when 0.45 V mm-1 is applied.
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The transport behaviors of MoS2 field-effect transistors (FETs) with various channel thicknesses are studied. In a 12 nm thick MoS2 FET, a typical switching behavior is observed with an Ion/Ioff ratio of 106. However, in 70 nm thick MoS2 FETs, the gating effect weakens with a large off-current, resulting from the screening of the gate field by the carriers formed through the ionization of S vacancies at 300 K. Hence, when the latter is dual-gated, two independent conductions develop with different threshold voltage (VTH) and field-effect mobility (µFE) values. When the temperature is lowered for the latter, both the ionization of S vacancies and the gate-field screening reduce, which revives the strong Ion/Ioff ratio and merges the two separate channels into one. Thus, only one each of VTH and µFE are seen from the thick MoS2 FET when the temperature is less than 80 K. The change of the number of conduction channels is attributed to the ionization of S vacancies, which leads to a temperature-dependent intra- and interlayer conductance and the attenuation of the electrostatic gate field. The defect-related transport behavior of thick MoS2 enables us to propose a new device structure that can be further developed to a vertical inverter inside a single MoS2 flake.
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A simple but powerful device platform for electrothermal local annealing (ELA) via graphite Joule heating on the surface of transition-metal dichalcogenide, is suggested here to sustainably restore intrinsic electrical properties of atomically thin layered materials. Such two-dimensional materials are easily deteriorated by undesirable surface/interface adsorbates and are screened by a high metal-to-semiconductor contact resistance. The proposed ELA allows one to expect a better electrical performance such as an excess electron doping, an enhanced carrier mobility, and a reduced surface traps in a monolayer molybdenum disulfide (MoS2)/graphite heterostructure. The thermal distribution of local heating measured by an infrared thermal microscope and estimated by a finite element calculation shows that the annealing temperature reaches up to >400 K at ambient condition and the high efficiency of site-specific annealing is demonstrated unlike the case of conventional global thermal annealing. This ELA platform can be further promoted as a practical gas sensor application. From an O2 cycling test and a low-frequency noise spectroscopy, the graphite on top of the MoS2 continuously recovers its initial condition from surface adsorbates. This ELA technique significantly improves the stability and reliability of its gas sensing capability, which can be expanded in various nanoscale device applications.