High-Field Electron Transport and High Saturation Velocity in Multilayer Indium Selenide Transistors.

Creating a high-frequency electron system demands a high saturation velocity (υsat). Herein, we report the high-field transport properties of multilayer van der Waals (vdW) indium selenide (InSe). The InSe is on a hexagonal boron nitride substrate and encapsulated by a thin, noncontinuous In layer, resulting in an impressive electron mobility reaching 2600 cm2/(V s) at room temperature. The high-mobility InSe achieves υsat exceeding 2 × 107 cm/s, which is superior to those of other gapped vdW semiconductors, and exhibits a 50-60% improvement in υsat when cooled to 80 K. The temperature dependence of υsat suggests an optical phonon energy (ℏωop) for InSe in the range of 23-27 meV, previously reported values for InSe. It is also notable that the measured υsat values exceed what is expected according to the optical phonon emission model due to weak electron-phonon scattering. The superior υsat of our InSe, despite its relatively small ℏωop, reveals its potential for high-frequency electronics, including applications to control cryogenic quantum computers in close proximity.

Conductivity-Controlled Polyvinylidene Fluoride Nanofiber Stack for Absorption-Dominant Electromagnetic Interference Shielding Materials.

This work presents a novel method for achieving lightweight electromagnetic interference (EMI) shielding materials with high EMI shielding effectiveness (SE) through absorption-dominant mechanisms, utilizing only organic polymer nanofibers (NFs). Instead of incorporating high-density fillers, the approach involves adjusting the concentrations of iron chloride in the NFs and subsequent vapor phase polymerization (VPP) to control the polymerization density of poly(3,4-ethylenedioxythiophene) (PEDOT) on the surface of polyvinylidene fluoride (PVDF) NFs. This process results in NF layers with varying conductivity, creating a conductivity gradient structure. The conductivity gradient structure of the NF layers significantly enhances absorptivity by reducing impedance mismatches between the shielding material and the surrounding air, as well as between different interlayers. This reduction in impedance mismatches allows for efficient dissipation of absorbed electromagnetic (EM) waves within the highly conductive NF layer. This improved absorptivity is also attributed to the attenuation of EM wave energy through multiple reflections and scattering within the NF pores. Moreover, the gradient structure of the NF layers promotes interfacial polarization, further contributing to the effective absorption of electromagnetic waves. As a result, a high absolute EMI SE (SSEt) of 12,390 dB·cm2 g-1 with low reflectivity (0.32) was achieved without compromising the lightweight and flexible properties.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications.

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

Synergistic Enhancement of Filtering Efficiency and Antibacterial Performance of a Nanofiber Air Filter Decorated with Electropolarized Lithium-Doped ZnO Nanorods.

According to clinical case reports, bacterial co-infection with COVID-19 can significantly increase mortality, with Staphylococcus aureus (S. aureus) being one of the most common pathogens causing complications such as pneumonia. Thus, during the pandemic, research on imparting air filters with antibacterial properties was actively initiated, and several antibacterial agents were investigated. However, air filters with inorganic nanostructures on organic nanofibers (NFs) have not been investigated extensively. This study aimed to demonstrate the efficiency of electropolarized poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) NFs decorated with Li-doped ZnO nanorods (NRs) to improve the filtering ability and antibacterial activity of the ultrathin air filter. The surfactant was loaded onto the ZnO─known for its biocompatibility and low toxicity─nanoparticles (NPs) and transferred to the outer surface of the NFs, where Li-doped ZnO NRs were grown. The Li-doped ZnO NR-decorated NF effectively enhanced the physical filtration efficiency and antibacterial properties. Additionally, by exploiting the ferroelectric properties of Li-doped ZnO NRs and PVDF-TrFE NFs, the filter was electropolarized to increase its Coulombic interaction with PMs and S. aureus. As a result, the filter exhibited a 90% PM1.0 removal efficiency and a 99.5% sterilization rate against S. aureus. The method proposed in this study provides an effective route for simultaneously improving the air filter performance and antibacterial activity.

Enhanced Output Performance of a Flexible Piezoelectric Nanogenerator Realized by Lithium-Doped Zinc Oxide Nanowires Decorated on MXene.

A flexible piezoelectric composite is composed of a polymer matrix and piezoelectric ceramic fillers to achieve good mechanical flexibility and processability. The overall piezoelectric performance of a composite is largely determined by the piezoelectric filler inside. Thus, different dispersion methods and additives that can promote the dispersion of piezoelectric ceramics and optimal composite structures have been actively investigated. However, relatively few attempts have been made to develop a filler that can effectively contribute to the performance enhancement of piezoelectric devices. In the present work, we introduce the fabrication and performance of the composite piezoelectric devices composed of Li-doped ZnO nanowires (Li: ZnO NWs) grown on the surface of MXene (Ti3C2) via the hydrothermal process. Through this approach, a semiconductor-metal hybrid structure is formed, increasing the overall permittivity. Moreover, the Ti3C2 layer can serve as a local ground in the composite so that the ferroelectric phase-transformed Li: ZnO NWs grown on its surface can be more effectively polarized during the poling process. In addition, the NW-covered surface of Ti3C2 prevents the aggregation of metallic Ti3C2 particles, promoting a more uniform electric field distribution during the poling process. As a result, the output performance of the piezoelectric nanogenerator (PENG) fabricated with a Li: ZnO NW/Ti3C2 composite was greatly improved compared to that of the devices fabricated with Li: ZnO NWs without the Ti3C2 platform. Specifically, the Li: ZnO NW/Ti3C2 composite piezoelectric nanogenerator (PENG) demonstrated a twofold higher output power density (∼9 µW/cm2) compared with the values obtained from the PENG devices based on Li: ZnO NWs. The approach introduced in this work can be easily adopted for an effective ferroelectric filler design to improve the output performance of the piezoelectric composite.

Comparative advantages of a type-II superlattice barrier over an AlGaSb barrier for enhanced performance of InAs/GaSb LWIR nBn photodetectors.

The barrier layer in InAs/GaSb LWIR nBn detector is usually composed of AlGaSb alloy, which has a non-negligible valence band offset and is sensitive to chemical solutions. In this work, we investigated a type-II superlattice (T2SL) barrier that is homogeneous with the T2SL absorber layer in order to resolve these drawbacks of the AlGaSb barrier. The lattice mismatch of the T2SL barrier was smaller than that of the AlGaSb barrier. At -70mV and 80 K, the dark current density and the noise equivalent temperature difference of the nBn devices with the T2SL barrier were 4.4×10-6A/cm2 and 33 mK, respectively.

Enhanced Piezoelectric Output Performance of the SnS2/SnS Heterostructure Thin-Film Piezoelectric Nanogenerator Realized by Atomic Layer Deposition.

Recently, the inherent piezoelectric properties of the 2D transition-metal dichalcogenides (TMDs) tin monosulfide (SnS) and tin disulfide (SnS2) have attracted much attention. Thus the piezoelectricity of these materials has been theoretically and experimentally investigated for energy-harvesting devices. However, the piezoelectric output performance of the SnS2- or SnS-based 2D thin film piezoelectric nanogenerator (PENG) is still relatively low, and the fabrication process is not suitable for practical applications. Here we report the formation of the SnS2/SnS heterostructure thin film for the enhanced output performance of a PENG using atomic layer deposition (ALD). The piezoelectric response of the heterostructure thin film was increased by ∼40% compared with that of the SnS2 thin film, attributed to large band offset induced by the heterojunction formation. Consequently, the output voltage and current density of the heterostructure PENG were 60 mV and 11.4 nA/cm2 at 0.6% tensile strain, respectively. In addition, thickness-controllable large-area uniform thin-film deposition via ALD ensures that the reproducible output performance is achieved and that the output density can be lithographically adjusted depending on the applications. Therefore, the SnS2/SnS heterostructure PENG fabricated in this work can be employed to develop a flexible energy-harvesting device or an attachable self-powered sensor for monitoring pulse and human body movement.

Polyvinylidene Fluoride Core-Shell Nanofiber Membranes with Highly Conductive Shells for Electromagnetic Interference Shielding.

As the demand for wireless sensors and equipment is unprecedentedly increasing, the interest in electromagnetic interference (EMI)-shielding materials that can effectively block accompanying electromagnetic interference is also constantly increasing. In particular, flexible and lightweight EMI-shielding materials that exhibit high EMI-shielding effectiveness (SE) have been more actively investigated as they are applicable to various applications. In this work, we reported the fabrication and performance of conducting polymer nanofiber EMI-shielding material, which was realized using electrospun polyvinylidene fluoride (PVDF) core-shell nanofiber membranes with highly conductive shells. Using the chemical polymerization method, core-shell nanofibers with highly conductive shells were employed without compositing with conductive fillers, resulting in shell-conductive lightweight EMI-shielding material without impairing the original properties of the nanofiber. In particular, thanks to the nanofiber structure, the EMI-shielding material exhibits superb flexibility, and the EMI SE was also improved through the enhanced absorption of EM waves and multireflections by the porous nanofiber film structure. Specifically, the developed EMI-shielding material in this work exhibited a SE of ∼40 dB in the X-band, which corresponds to an absolute shielding effectiveness (SSEt) of 16,230 dB·cm2/g at a thickness of 14 µm. Moreover, the high durability and hydrophobicity of the PVDF nanofibers with poly (3,4-ethylenedioxythiophene) (PEDOT)-polymerized shell can also be useful in practical applications.

Enhanced Electrochemical Performance of Micro-Supercapacitors Via Laser-Scribed Cobalt/Reduced Graphene Oxide Hybrids.

The evolution of "smart life," which connects all internet-of-things (IoT) microdevices and microsensors under wireless communication grids, requires microscale energy storage devices with high power and energy density and long-term cyclability to integrate them with sustainable power generators. Instead of Li-ion batteries with a short lifetime, pseudocapacitors with longer or infinite cyclability and high-power density have been considered as efficient energy storage devices for IoT. However, the design and fabrication of microscale pseudocapacitors have difficulties in patterning microscale electrodes when loading active materials at specific points of the electrodes using conventional microfabrication methods. Here, we developed a facile, one-step fabrication method of micro-supercapacitors (MSCs) through the in situ formation of Co metals and the reduced graphene oxides (rGOs) in a one-pot laser scribing process. The prepared Co/rGO MSC thus exhibited four times higher capacitance than the rGO MSC, due to the Faradaic charge capacitance behavior of the Co/rGO composites.

Polybenzimidazole-Benzophenone Composite Nanofiber Window Air Filter with Superb UV Resistance and High Chemical and Thermal Durability.

There is a growing interest in window air filters to protect indoor air quality from ultrafine particulate matter (PM) in outdoor air. The filters for this purpose must achieve high filtering efficiency without compromising the original functions of the window, such as high air permeability and visibility. Several filters meeting these requirements have been developed and demonstrate a high PM2.5 filtering efficiency. However, these filters are installed outside the window or on the window screen guard, thereby requiring high levels of ultraviolet (UV), chemical, and thermal resistance. These requirements have been overlooked so far. In this study, we examine the fabrication and performance of a polybenzimidazole-benzophenone (PBI-BP) composite nanofiber air filter that demonstrates superb UV resistance and chemical and thermal durability. Because of the UV absorbance of the BP in the nanofibers, the filter membrane is robust even under prolonged UV exposure, which is essential for filters for this purpose. The filter membrane is not damaged even after treatment in strong acids or annealing at high temperature up to 400 °C. Thus, the PBI-BP composite filter is suitable for practical application in window air filters and can be adapted to develop filters used under other harsh environments.

Robust Wireless Sensor and Actuator Networks for Networked Control Systems.

The stability guarantee of wireless networked control systems is still challenging due to the complex interaction among the layers and the vulnerability to network faults, such as link and node failures. In this paper, we propose a robust wireless sensor and actuator network (R-WSAN) to maintain the control stability of multiple plants over the spatial-temporal changes of wireless networks. The proposed joint design protocol combines the distributed controller of control systems and the clustering, resource scheduling, and control task sharing scheme of wireless networks over a hierarchical cluster-based network. In particular, R-WSAN decouples the tasks from the inherently unreliable nodes and allows control tasks to share between nodes of wireless networks. Our simulations demonstrate that R-WSAN provides the enhanced resilience to the network faults for sensing and actuation without significantly disrupting the control performance.

Remarkable Output Power Density Enhancement of Triboelectric Nanogenerators via Polarized Ferroelectric Polymers and Bulk MoS2 Composites.

Performance enhancement of triboelectric nanogenerators (TENGs) has been largely limited by the relatively low output current density. Thus, extensive research efforts have been made to increase the output current density. In this respect, this work presents a method to effectively increase output current density of TENGs by adopting polarized ferroelectric polymers and MoS2 composite. Specifically, by compositing bulk MoS2 flakes with both Nylon-11 and PVDF-TrFE, respectively, charge density of each triboelectric charging surface was significantly increased. In addition, proper polarization of both ferroelectric composite layers has also led to an additional increase in the charge density. A combination of them synergistically increases the surface charge density, generating huge output current and the power output density. By optimizing the fabrication process, the output voltage and current density up to ∼145 V and ∼350 µA/cm2 are achieved, respectively. Consequently, the TENG exhibits a recordable output power density of ∼50 mW/cm2, which is one of the highest output power densities reported to date. The method introduced in this work can greatly increase the output current density of TENGs, facilitating the development of high-performance triboelectric energy harvesting devices.

Thermal conductivity enhancement in electrospun poly(vinyl alcohol) and poly(vinyl alcohol)/cellulose nanocrystal composite nanofibers.

The thermal conductivity enhancement of neat poly(vinyl alcohol) and poly(vinyl alcohol) (PVA)/cellulose nanocrystal (CNC) composite was attempted via electrospinning. The suspended microdevice technique was applied to measure the thermal conductivity of electrospun nanofibers (NFs). Neat PVA NFs and PVA/CNC NFs with a diameter of approximately 200 nm showed thermal conductivities of 1.23 and 0.74 W/m-K, respectively, at room temperature, which are higher than that of bulk PVA by factors of 6 and 3.5, respectively. Material characterization by Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis confirmed that the thermal conductivity of the PVA/CNC NFs was enhanced by the reinforcement of their backbone rigidity, while that of the neat PVA NFs was attributed to the increase in their crystallinity that occurred during the electrospinning.

Reusable Polybenzimidazole Nanofiber Membrane Filter for Highly Breathable PM2.5 Dust Proof Mask.

Ultrafine particulate matters (PMs) are an imminent threat to the human respiratory system, as their sizes are comparable to and even smaller than human tissues. To cope with this situation, researchers have developed and commercialized various personal dust proof masks. However, because of the relatively thick filter membrane to guarantee filtering efficiency, a huge pressure drop across the active filter layer is inevitable and breathing through it becomes uncomfortable. In this work, we investigated the performance of electrospun polybenzimidazole (PBI) nanofiber membrane filters that can potentially be used for dust proof masks or other high-performance filters. Thanks to its high dipole moment (6.12) as confirmed by density functional theory (DFT) calculation, the surface potential of the PBI nanofiber air filter, measured by KPFM, was higher than that of other commercially available mask filters. The filter developed in this work provides high PM filtering efficiency of ∼98.5% at much reduced pressure drop (130 Pa) in comparison to those used in commercially available masks (386 Pa) with similar filtering efficiencies. Consequently, an approximately 3-fold higher quality factor (∼0.032), evaluated for PM2.5, in comparison to that of commercial ones (∼0.011) was achieved by using PBI nanofiber. Furthermore, we developed a cleaning method effective for the filter contaminated by both inorganic and organic PMs. Even after several cycles of cleaning, the PBI filter membrane demonstrated negligible damage and retained its original performance because of its mechanical, thermal, and chemical durability.

Transmission Scheduling Schemes of Industrial Wireless Sensors for Heterogeneous Multiple Control Systems.

The transmission scheduling scheme of wireless networks for industrial control systems is a crucial design component since it directly affects the stability of networked control systems. In this paper, we propose a novel transmission scheduling framework to guarantee the stability of heterogeneous multiple control systems over unreliable wireless channels. Based on the explicit control stability conditions, a constrained optimization problem is proposed to maximize the minimum slack of the stability constraint for the heterogeneous control systems. We propose three transmission scheduling schemes, namely centralized stationary random access, distributed random access, and Lyapunov-based scheduling scheme, to solve the constrained optimization problem with a low computation cost. The three proposed transmission scheduling schemes were evaluated on heterogeneous multiple control systems with different link conditions. One interesting finding is that the proposed centralized Lyapunov-based approach provides almost ideal performance in the context of control stability. Furthermore, the distributed random access is still useful for the small number of links since it also reduces the operational overhead without significantly sacrificing the control performance.

An ultraviolet and electric field activated photopolymer-ferroelectric nanoparticle composite for the performance enhancement of triboelectric nanogenerators.

For the development of high performance triboelectric generators (TENGs), it is required to have facile methods to adjust the triboelectric properties of the friction surfaces. In this work, we present the surface charge density modulation of the photopolymer-ferroelectric nanoparticle composite surface by applying ultraviolet (UV) and electric field. By using the photopolymer, the triboelectric surface property was modulated by exposure to UV. In addition, lithographic surface patterning can be easily adopted to enlarge the frictional surface area as well. Furthermore, the use of the PP allows a facile integration of ferroelectric nanoparticles (NPs) in the form of a nanocomposite structure, which can effectively increase the surface charge density by spontaneous dipole coupling of NPs embedded in the PP layer. As a result, approximately 4-fold higher output power has been achieved by applying this approach. The developed TENGs have also demonstrated superior mechanical durability, generating consistent outputs during 104 cyclic frictional contacts. The approach proposed here is a simple and reliable way to enhance the output performance of TENGs.

Interface States in Bilayer Graphene Encapsulated by Hexagonal Boron Nitride.

The threshold voltages at the onset of conduction for electron and hole branches can provide information on band gap values or interface states in a gap. We measured conductivity of bilayer graphene encapsulated by hexagonal boron nitride as a function of back and top gates, where another bilayer graphene is used as a top gate. From the measured conductivity the transport gap values were extracted assuming zero interface trap states, and they are close to the theoretically expected gap values. From a little discrepancy an average density of interface states per energy within a band gap ( Dit) is also estimated. The data clearly show that Dit decreases as a bilayer graphene band gap increases rather than being constant. Despite the decreasing trend of Dit, interestingly the total interface states within a gap increases linearly as a band gap increases. This is because of ∼2 × 1010 cm-2 interface states localized at band edges even without a band gap, and other gap states are equally spread over the gap.

Microneedles integrated with a triboelectric nanogenerator: an electrically active drug delivery system.

In this study, a combined system of microneedles and a triboelectric nanogenerator (TENG) has been developed for drug delivery. A triboelectric device, which converts mechanical energy into alternating current (AC), was chosen to replace the electrophoresis (EP) effect. To directly generate triboelectricity from salmon deoxyribonucleic acid (SDNA)-based microneedles, a triboelectric series of SDNA film and chargeable polymers (polyimide and Teflon) was studied. The electrical output of the two charged polymers was compared to find a material that could be highly charged with SDNA. The electrical output was also compared as a function of the concentration of a drug embedded in the SDNA film, and the results confirmed that drug intercalation affected the carrier diffusion. The mechanical strength of the microneedles was assessed by histological analysis of their penetration into porcine cadaver skin. Furthermore, the output voltage of a system incorporating microneedles and TENG in cadaver skin, and in vitro drug release into gelatin were evaluated to examine potential application as an electrically active drug delivery system. The electrical output voltage of this system was ∼95 V. The mechanism of triboelectric perturbation to the skin has also been discussed. The system developed in this work is a new, facile approach toward effective drug delivery that replaces the existing EP method and expands the application of TENGs.

A soft lithographic approach to fabricate InAs nanowire field-effect transistors.

The epitaxial layer transfer process was previously introduced to integrate high-quality and ultrathin III-V compound semiconductor layers on any substrate. However, this technique has limitation for fabrication of sub-micron nanoribbons due to the diffraction limit of photolithography. In order to overcome this limitation and scale down its width to sub-50 nm, we need either a costly short wavelength lithography system or a non-optical patterning method. In this work, high-quality III-V compound semiconductor nanowires were fabricated and integrated onto a Si/SiO2 substrate by a soft-lithography top-down approach and an epitaxial layer transfer process, using MBE-grown ultrathin InAs as a source wafer. The width of the InAs nanowires was controlled using solvent-assisted nanoscale embossing (SANE), descumming, and etching processes. By optimizing these processes, NWs with a width less than 50 nm were readily obtained. The InAs NWFETs prepared by our method demonstrate peak electron mobility of ~1600 cm2/Vs, indicating negligible material degradation during the SANE process.

Ferroelectric nanoparticle-embedded sponge structure triboelectric generators.

We report high-performance triboelectric nanogenerators (TENGs) employing ferroelectric nanoparticles (NPs) embedded in a sponge structure. The ferroelectric BaTiO3 NPs inside the sponge structure play an important role in increasing surface charge density by polarized spontaneous dipoles, enabling the packaging of TENGs even with a minimal separation gap. Since the friction surfaces are encapsulated in the packaged device structure, it suffers negligible performance degradation even at a high relative humidity of 80%. The TENGs also demonstrated excellent mechanical durability due to the elasticity and flexibility of the sponge structure. Consequently, the TENGs can reliably harvest energy even under harsh conditions. The approach introduced here is a simple, effective, and reliable way to fabricate compact and packaged TENGs for potential applications in wearable energy-harvesting devices.