Dynamic modulation of a surface-enhanced Raman scattering signal by a varying magnetic field.

Surface-enhanced Raman scattering (SERS) signals are fundamental for spectroscopy applications. However, existing substrates cannot perform a dynamically enhanced modulation of SERS signals. Herein, we developed a magnetically photonic chain-loading system (MPCLS) substrate by loading magnetically photonic nanochains of Fe3O4@SiO2 magnetic nanoparticles (MNPs) with Au nanoparticles (NPs). We achieved a dynamically enhanced modulation by applying an external stepwise magnetic field to the randomly dispersed magnetic photonic nanochains that gradually align in the analyte solution. The closely aligned nanochains create a higher number of hot spots by new neighboring Au NPs. Each chain represents a single SERS enhancement unit with both a surface plasmon resonance (SPR) effect and photonic property. The magnetic responsivity of MPCLS enables a rapid signal enhancement and tuning of the SERS enhancement factor.

Engineering Support and Distribution of Palladium and Tin on MXene with Modulation of the d-Band Center for CO-resilient Methanol Oxidation.

The efficiency of direct methanol fuel cell (DMFC) is largely determined by the activity and durability of methanol oxidation reaction (MOR) catalysts. Herein, we present a CO-resilient MOR catalyst of palladium-tin nano-alloy anchored on Se-doped MXene (PdSn0.5 /Se-Ti3 C2 ) via a progressive one-step electrochemical deposition strategy. MOR mass activity resulting from Pd/Se-Ti3 C2 catalyst (1046.2 mA mg-1 ) is over 2-fold larger than that of Pd/Ti3 C2 , suggesting that the introduction of Se atoms on MXene might accelerate the reaction kinetics. PdSn0.5 /Se-Ti3 C2 with Se-doping progress of MXene and the cooperated Pd-Sn sites has a superior MOR mass activity (4762.8 mA mg-1 ), outperforming many other reported Pd-based catalysts. Both experimental results and theoretical calculation reveal that boosted electron interaction of metal crystals with Se-doped MXene and optimized distribution of Pd-Sn sites can modulate the d band center, reduce adsorption energies of CO* at Pd site and enhance OH* generation at Sn site, resulting in highly efficient removal of CO intermediates by reaction with neighboring OH species on adjacent Sn sites.

Enhanced random laser by metal surface-plasmon channel waveguide.

Compared with conventional lasers, the random laser is realized through strong multiple scatterings in disordered gain system. In this paper, random lasing (RL) in one-dimensional metal surface plasmon (SP) waveguide with gold-plated self-formed silicon pyramids was investigated comprehensively. Consequently, the emission intensity of RL was enhanced dramatically and the RL threshold was reduced significantly through Au-coated Si spiky tips. Meanwhile, one-dimensional metal SP channel waveguides confined the emitting light in a certain direction with a small divergence angle. Using FDTD simulations, it was found that the enhancement effect for RL is likely attributed to the localized surface plasmon (LSP) field. In addition, the LSP field nearby the spiky tips can enhance field-molecule interaction, which was benefit for lasing in small scale. The results in this letter supplied a feasible method to realize the application of RL in subwavelength optical elements.

Highly Sensitive, Flexible MEMS Based Pressure Sensor with Photoresist Insulation Layer.

Pressure sensing is a crucial function for flexible and wearable electronics, such as artificial skin and health monitoring. Recent progress in material and device structure of pressure sensors has brought breakthroughs in flexibility, self-healing, and sensitivity. However, the fabrication process of many pressure sensors is too complicated and difficult to integrate with traditional silicon-based Micro-Electro-Mechanical System(MEMS). Here, this study demonstrates a scalable and integratable contact resistance-based pressure sensor based on a carbon nanotube conductive network and a photoresist insulation layer. The pressure sensors have high sensitivity (95.5 kPa-1 ), low sensing threshold (16 Pa), fast response speed (<16 ms), and zero power consumption when without loading pressure. The sensitivity, sensing threshold, and dynamic range are all tunable by conveniently modifying the hole diameter and thickness of insulation layer.

Tailoring carbon nanotube density for modulating electro-to-heat conversion in phase change composites.

We report a carbon nanotube array-encapsulated phase change composite in which the nanotube distribution (or areal density) could be tailored by uniaxial compression. The n-eicosane (C20) was infiltrated into the porous array to make a highly conductive nanocomposite while maintaining the nanotube dispersion and connection among the matrix with controlled nanotube areal density determined by the compressive strains along the lateral direction. The resulting electrically conductive composites can store heat at driven voltages as low as 1 V at fast speed with high electro-to-heat conversion efficiencies. Increasing the nanotube density is shown to significantly improve the polymer crystallinity and reduce the voltage for inducing the phase change process. Our results indicate that well-organized nanostructures such as the nanotube array are promising candidates to build high-performance phase change composites with simplified manufacturing process and modulated structure and properties.

High Sensitivity and Wide Linear Range Flexible Piezoresistive Pressure Sensor with Microspheres as Spacers for Pronunciation Recognition.

Flexible piezoresistive pressure sensors have received great popularity in flexible electronics due to their simple structure and promising applications in health monitoring and artificial intelligence. However, the contradiction between sensitivity and detection range limits the application of the sensors in the medium-pressure regime. Here, a flexible piezoresistive pressure sensor is fabricated by combining a hierarchical spinous microstructure sensitive layer and a periodic microsphere array spacer. The sensor achieves high sensitivity (39.1 kPa-1) and outstanding linearity (0.99, R2 coefficient) in a medium-pressure regime, as well as a wide range of detection (100 Pa-160.0 kPa), high detection precision (<0.63‰ full scale), and excellent durability (>5000 cycles). The mechanism of the microsphere array spacer in improving sensitivity and detection range was revealed through finite element analysis. Furthermore, the sensors have been utilized to detect muscle and joint movements, spatial pressure distributions, and throat movements during pronouncing words. By means of a full-connect artificial neural network for machine learning, the sensor's output of different pronounced words can be precisely distinguished and classified with an overall accuracy of 96.0%. Overall, the high-performance flexible pressure sensor based on a microsphere array spacer has great potential in health monitoring, human-machine interface, and artificial intelligence of medium-pressure regime.

Flexible, Transparent and Conductive Metal Mesh Films with Ultra-High FoM for Stretchable Heating and Electromagnetic Interference Shielding.

Despite the growing demand for transparent conductive films in smart and wearable electronics for electromagnetic interference (EMI) shielding, achieving a flexible EMI shielding film, while maintaining a high transmittance remains a significant challenge. Herein, a flexible, transparent, and conductive copper (Cu) metal mesh film for EMI shielding is fabricated by self-forming crackle template method and electroplating technique. The Cu mesh film shows an ultra-low sheet resistance (0.18 Ω â–¡-1), high transmittance (85.8%@550 nm), and ultra-high figure of merit (> 13,000). It also has satisfactory stretchability and mechanical stability, with a resistance increases of only 1.3% after 1,000 bending cycles. As a stretchable heater (ε > 30%), the saturation temperature of the film can reach over 110 °C within 60 s at 1.00 V applied voltage. Moreover, the metal mesh film exhibits outstanding average EMI shielding effectiveness of 40.4 dB in the X-band at the thickness of 2.5 µm. As a demonstration, it is used as a transparent window for shielding the wireless communication electromagnetic waves. Therefore, the flexible and transparent conductive Cu mesh film proposed in this work provides a promising candidate for the next-generation EMI shielding applications.

Stretchable, Transparent, and Ultra-Broadband Terahertz Shielding Thin Films Based on Wrinkled MXene Architectures.

With the increasing demand for terahertz (THz) technology in security inspection, medical imaging, and flexible electronics, there is a significant need for stretchable and transparent THz electromagnetic interference (EMI) shielding materials. Existing EMI shielding materials, like opaque metals and carbon-based films, face challenges in achieving both high transparency and high shielding efficiency (SE). Here, a wrinkled structure strategy was proposed to construct ultra-thin, stretchable, and transparent terahertz shielding MXene films, which possesses both isotropous wrinkles (height about 50 nm) and periodic wrinkles (height about 500 nm). Compared to flat film, the wrinkled MXene film (8 nm) demonstrates a remarkable 36.5% increase in SE within the THz band. The wrinkled MXene film exhibits an EMI SE of 21.1 dB at the thickness of 100 nm, and an average EMI SE/t of 700 dB µm-1 over the 0.1-10 THz. Theoretical calculations suggest that the wrinkled structure enhances the film's conductivity and surface plasmon resonances, resulting in an improved THz wave absorption. Additionally, the wrinkled structure enhances the MXene films' stretchability and stability. After bending and stretching (at 30% strain) cycles, the average THz transmittance of the wrinkled film is only 0.5% and 2.4%, respectively. The outstanding performances of the wrinkled MXene film make it a promising THz electromagnetic shielding materials for future smart windows and wearable electronics.

Engineering Symmetry-Breaking Centers and d-Orbital Modulation in Triatomic Catalysts for Zinc-Air Batteries.

Unraveling the configuration-activity relationship and synergistic enhancement mechanism (such as real active center, electron spin-state, and d-orbital energy level) for triatomic catalysts, as well as their intrinsically bifunctional oxygen electrocatalysis, is a great challenge. Here we present a triatomic catalyst (TAC) with a trinuclear active structure that displays extraordinary oxygen electrocatalysis for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), greatly outperforming the counterpart of single-atom and diatomic catalysts. The aqueous Zn-air battery (ZAB) equipped with a TAC-based cathode exhibits extraordinary rechargeable stability and ultrarobust cycling performance (1970 h/3940 cycles at 2 mA cm-2, 125 h/250 cycles at 10 mA cm-2 with negligible voltage decay), and the quasi-solid-state ZAB displays outstanding rechargeability and low-temperature adaptability (300 h/1800 cycles at 2 mA cm-2 at -60 °C), outperforming other state-of-the-art ZABs. The experimental and theoretical analyses reveal the symmetry-breaking CoN4 configuration under incorporation of neighboring metal atoms (Fe and Cu), which leads to d-orbital modulation, a low-shift d band center, weakened binding strength to the oxygen intermediates, and decreased energy barrier for bifunctional oxygen electrocatalysis. This rational tricoordination design as well as an in-depth mechanism analysis indicate that hetero-TACs can be promisingly applied in various electrocatalysis applications.

Controllable synthesis of spongy carbon nanotube blocks with tunable macro- and microstructures.

Macroscopic carbon nanotubes (CNTs) with uniform structures are in great demand for use in composites and environmental materials. Here we demonstrate the controlled synthesis of spongy CNT blocks with isotropic properties and flexible, freestanding structures. The formation mechanism of the isotropic CNT sponges is discussed, based on its open-ended structure and initial formation in the vapor phase. The microstructure of the CNT sponges can be tuned by changing the flow rate of the carrier gas, resulting in CNT sponges with diameters ranging from 30.2 to 47.8 nm and wall thicknesses from 7 to 16 nm. The bulk density (5-25 mg cm(-3)), mechanical strength of the CNT sponges, and filling rate of ferromagnetic catalyst in the CNT sponges can also be modulated by controlling the supply rate of the carbon source, suggesting potential applications in mechanical energy absorption and environmental materials.

Flexible Solid-State Metal-Air Batteries: The Booming of Portable Energy Supplies.

The rapid development of portable and wearable electronics has given rise to new challenges and provoked research in flexible, lightweight, and affordable energy storage devices. Flexible solid-state metal-air batteries (FSSMABs) are considered promising candidates, owing to their large energy density, mechanical flexibility, and durability. However, the practical applications of FSSMABs require further improvement to meet the demands of long-term stability, high power density, and large operating voltage. This Review presents a detailed discussion of innovative electrocatalysts for the air cathode, followed by a sequential overview of high-performance solid-state electrolytes and metal anodes, and a summary of the current challenges and future perspectives of FSSMABs to promote practical application and large-scale commercialization in the near future.

Ultra-sensitive, Multi-directional Flexible Strain Sensors Based on an MXene Film with Periodic Wrinkles.

The fast-growing motion capturing/monitoring technique has raised a great demand for flexible strain sensors. For capturing complex motions (e.g., facial motion), both the strain amplitude and direction should be accurately detected. Although some reported sensors based on anisotropic conductive networks are proved to show accurate localization of strain directions, it is still a great challenge to achieve both high sensitivity and a high sensing range in these designs. Here, a self-assembled Ti3C2Tx MXene film with parallel and periodic wrinkles is fabricated on a stretchable poly(dimethylsiloxane) substrate for constructing multi-directional strain sensors. During stretching, relative slip and crack will occur between the stacked MXene nanosheets, which contribute to high structural sensitivity in the MXene film. Meanwhile, the wrinkled structure contributes to high stretchability. As a result, the sensor based on the film with one-dimensional periodic wrinkles shows a large sensing range (>50%) and a gauge factor of 45. Furthermore, the sensor can accurately detect both the strain amplitude and direction by using the MXene film with two-dimensional wrinkles. It shows distinguishable electrical responses when detecting different-amplitude human/robot motions such as joint bending and walking. Additionally, the directions in complex human motions (e.g., facial motion) can also be well-tracked. This work provides an effective strategy to detect multi-directional motions.

Engineering Smart Composite Hydrogels for Wearable Disease Monitoring.

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

Flexible, Transparent, and Wafer-Scale Artificial Synapse Array Based on TiOx /Ti3 C2 Tx Film for Neuromorphic Computing.

A high-density neuromorphic computing memristor array based on 2D materials paves the way for next-generation information-processing components and in-memory computing systems. However, the traditional 2D-materials-based memristor devices suffer from poor flexibility and opacity, which hinders the application of memristors in flexible electronics. Here, a flexible artificial synapse array based on TiOx /Ti3 C2 Tx film is fabricated by a convenient and energy-efficient solution-processing technique, which realizes high transmittance (≈90%) and oxidation resistance (>30 days). The TiOx /Ti3 C2 Tx memristor shows low device-to-device variability, long memory retention and endurance, a high ON/OFF ratio, and fundamental synaptic behavior. Furthermore, satisfactory flexibility (R = 1.0 mm) and mechanical endurance (104 bending cycles) of the TiOx /Ti3 C2 Tx memristor are achieved, which is superior to other film memristors prepared by chemical vapor deposition. In addition, high-precision (>96.44%) MNIST handwritten digits recognition classification simulation indicates that the TiOx /Ti3 C2 Tx artificial synapse array holds promise for future neuromorphic computing applications, and provides excellent high-density neuron circuits for new flexible intelligent electronic equipment.

Strong and reversible modulation of carbon nanotube-silicon heterojunction solar cells by an interfacial oxide layer.

Deposition of nanostructures such as carbon nanotubes on Si wafers to make heterojunction structures is a promising route toward high efficiency solar cells with reduced cost. Here, we show a significant enhancement in the cell characteristics and power conversion efficiency by growing a silicon oxide layer at the interface between the nanotube film and Si substrate. The cell efficiency increases steadily from 0.5% without interfacial oxide to 8.8% with an optimal oxide thickness of about 1 nm. This systematic study reveals that formation of an oxide layer switches charge transport from thermionic emission to a mixture of thermionic emission and tunneling and improves overall diode properties, which are critical factors for tailoring the cell behavior. By controlled formation and removal of interfacial oxide, we demonstrate oscillation of the cell parameters between two extreme states, where the cell efficiency can be reversibly altered by a factor of 500. Our results suggest that the oxide layer plays an important role in Si-based photovoltaics, and it might be utilized to tune the cell performance in various nanostructure-Si heterojunction structures.

Anchoring Oxidized MXene Nanosheets on Porous Carbon Nanotube Sponge for Enhancing Ion Transport and Pseudocapacitive Performance.

Two-dimensional (2D) MXene nanosheets are attractive for electrochemical energy storage applications due to their superior surface-controlled charge storage capacity. However, the slow ion transport in the closely packed electrode limits their electrochemical performances. Meanwhile, the restricted surface-controlled pseudocapacitance of MXene nanosheets requires to be enhanced. Herein, a well-controlled electrophoretic deposition strategy is developed to disperse Ti3C2Tx nanosheets into a freestanding, porous carbon nanotube (CNT) sponge. The constructed Ti3C2Tx@CNT hybrid sponge can provide high-speed ion-transport pathways for the charge-discharge process. Furthermore, by tuning the deposition potential, the inserted MXene nanosheets can be partially oxidized, boosting the pseudocapacitance performance. A large gravimetric capacitance of 468 F g-1 at 10 mV s-1 and a retention of 79.8% at 100 mV s-1 can be achieved in the Ti3C2Tx@CNT electrode. Meanwhile, the highest areal capacitance of 661 mF cm-2 at 1 mA cm-2 was obtained in the sample with high-loading Ti3C2Tx. For the assembled symmetric supercapacitor, 92.8% of the capacitance is retained after 10 000 cycles of the charge-discharge process at 10 mA cm-2. Thus, this study develops a promising electrophoretic deposition strategy for dispersing 2D MXene nanosheets and boosting their pseudocapacitive performance, resulting in a high-capacitive electrochemical energy storage electrode.

Metal-Organic Framework-Derived Core-Shell Nanospheres Anchored on Fe-Filled Carbon Nanotube Sponge for Strong Wideband Microwave Absorption.

Metal-organic frameworks (MOFs) are booming as a promising precursor for constructing lightweight, high-efficiency microwave absorbing (MA) material. However, it is still a challenge to rationally design three-dimensional (3D), porous MOF-derived MA materials with a stable structure and strong and wideband MA performance. Herein, a 3D hybrid nanostructure (CNT/FeCoNi@C) comprising MOF-derived magnetic nanospheres and Fe-filled carbon nanotube (CNT) sponge has been controllably fabricated to enhance the absorption ability and broaden the effective absorption bandwidth (EAB). The magnetic nanospheres are uniformly anchored on the CNT skeleton, forming hybrid network structures, which enhance interface polarization, electron transportation, and impedance matching. The minimum reflection loss (RL) and EAB of the as-prepared CNT/FeCoNi@C sponges reach -51.7 dB and 6.0 GHz, respectively, outperforming most reported MOF-based wave absorbers. This work provides not only a novel design of MOF-derived 3D nanostructures but also an effective guide for the optimization of electromagnetic properties and absorbing performance in MA material.

Nonsolid TiOx Nanoparticles/PVDF Nanocomposite for Improved Energy Storage Performance.

Nanofiller/polymer nanocomposites are promising dielectrics for energy harvesting to be applied in wearable and flexible electronics. The structural design of the nanofillers plays a vital role to improve the energy storage performance of the related nanocomposites. Here, we fabricate a flexible device based on nonsolid titanium oxide (TiOx) nanoparticles/poly(vinylidene fluoride) (PVDF) to achieve enhanced energy storage performance at low loading. The room-temperature oxidation method is used to oxidize two-dimensional MXene (Ti3C2Tx) flakes to form partially hollow TiOx nanoparticles. Taking advantage of this structure, the flexible TiOx nanoparticles/PVDF nanocomposite with an ultralow loading content of 1 wt % nanofillers shows high energy storage performance, including a dielectric constant of ≈22 at 1 kHz, a breakdown strength of ≈480 MV m-1, and an energy storage density of 7.43 J cm-3. The finite element simulation further reveals that the optimization of the energy storage performance is ascribed to the lower electric potential among the partially hollow TiOx nanoparticles, which enhances the breakdown strength of the nanocomposites. This work opens a new avenue to structurally design and fabricate low-loading polymer-based nanocomposites for energy storage applications in next-generation flexible electronics.

Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring.

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors. However, traditional humidity sensors suffer from a trade-off between deformability, sensitivity, and transparency, and thus the development of high-performance, stretchable, and low-cost humidity sensors is urgently needed as wearable electronics. Here, ultrasensitive, highly deformable, and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels. Concomitantly, a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area, which can be applied to different polymer networks and facilitate the development of flexible integrated electronics. In addition, sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network, exhibiting excellent water adsorption capacity. Through the synergistic optimization of structure and composition, the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH, which is unprecedented. Moreover, the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity. As a proof of concept, we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission, enabling real-time monitoring of human health status. This work provides a general strategy to construct high-performance, stretchable, and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Ultrasensitive, Stretchable, and Fast-Response Temperature Sensors Based on Hydrogel Films for Wearable Applications.

Conductive hydrogels can be used in wearable electronics integrated with skin, but the bulk structure of existing hydrogel-based temperature sensors limits the wearing comfort, response/recovery speeds, and sensitivity. Here, stretchable and transparent temperature sensors based on a novel thin-film sandwich structure (TFSS) are designed, which display unprecedented thermal sensitivity (24.54%/°C), fast response time (0.19 s) and recovery time (0.08 s), a broad detection range (from -28 to 95.3 °C), high resolution (0.8 °C), and high stability. The thin hydrogel layer (12.15 µm) is encapsulated by two thin elastomer layers, which prevent the water evaporation and enhance the heat transfer, leading to the boosted stability and accelerated response/recovery speeds. The nondrying and antifreezing capabilities are further promoted by the hydratable lithium bromide (LiBr) incorporated in the hydrogel, enabling it to avoid dehydration in an extremely arid environment and freeze below subzero temperatures (freezing point below -120 °C). A comparative study reveals that the thermal sensitivity displayed by the TFSS sensor in capacitance mode is several times higher than that in conventional conductance/resistance mode above room temperature. Importantly, a new mechanism based on a horizontal plate capacitance model is proposed to understand the high sensitivity by considering the permittivity and geometry variations of TFSS. The thin TFSS, stretchability and transparency enable the sensor to be conformally and comfortably attached to human skin for real-time and reliable monitoring of various human motions, physical states, skin temperature, etc., without affecting the appearance.