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

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

Controllable Fabrication of Large-Area Wrinkled Graphene on a Solution Surface

It is unavoidable to form wrinkles, which are folds or creases in a material, in graphene, whenever the graphene is prepared by micromechanical exfoliation from graphite or chemical vapor deposition (CVD). However, the controllable formation and structures of graphene with nanoscale wrinkles remains a big challenge. Here, we report a liquid-phase shrink method to controllably fabricate large-area wrinkled graphene (WG). The CVD-prepared graphene self-shrinks into a WG on an ethanol solution surface. By modifying the concentration of the ethanol solution, we can easily and efficiently obtain WG with a uniform distribution of wrinkles with different heights. The WG shows high stretchability and can withstand more than 100% tensile strain and up to 720° twist. Furthermore, electromechanical response sensors based on double-layer stacking of WG show ultrahigh sensitivity. This simple, effective, and environmentally friendly liquid-phase shrink method will pave a way for the controllable formation of WG, which is an ideal candidate for application in highly stretchable and highly sensitive electronic devices

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 <i>n</i>-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

Resistance Switching and Failure Behavior of the MoO_<i>x</i>/Mo₂C Heterostructure

With the rapid demand for high-performance and power-efficient memristive and synaptic systems, more 2D heterostructures with improved resistance switching (RS) properties are still urgently in need for next-generation devices. Here, we report the RS behaviors of vertical MoOx/Mo2C heterostructures fabricated by controllable thermal oxidation and uncover the failure behavior for the first time. It is found that the MoOx/Mo2C heterostructure exhibits bipolar RS with a low set/reset voltage of +0.5/–0.3 V, an ultralow power consumption of 5 × 10–8 W, and an on/off ratio of 102, which is ascribed to the transport of the internal oxygen ions of MoOx. Furthermore, the failure behavior of RS behaviors of the MoOx/Mo2C heterostructure under a higher work voltage is revealed. It indicates that the amorphization of the pristine crystalline MoOx layer could block the movement of the internal oxygen ions in the vertical direction. The excellent RS performance induced by the synergy of MoOx and Mo2C and the demonstration of the failure behavior enable the potential applications of the 2D heterostructure in related memory devices and biological neural networks

Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer MoS₂ Film with Spatial Homogeneity and the Visualization of Grain Boundaries

MoS₂ monolayer attracts considerable attention due to its semiconducting nature with a direct bandgap which can be tuned by various approaches. Yet a controllable and low-cost method to produce large-scale, high-quality, and uniform MoS₂ monolayer continuous film, which is of crucial importance for practical applications and optical measurements, remains a great challenge. Most previously reported MoS₂ monolayer films had limited crystalline sizes, and the high density of grain boundaries inside the films greatly affected the electrical properties. Herein, we demonstrate that highly crystalline MoS₂ monolayer film with spatial size up to centimeters can be obtained via a facile chemical vapor deposition method with solid-phase precursors. This growth strategy contains selected precursor and controlled diffusion rate, giving rise to the high quality of the film. The well-defined grain boundaries inside the continuous film, which are invisible under an optical microscope, can be clearly detected in photoluminescence mapping and atomic force microscope phase images, with a low density of ∼0.04 μm^–1. Transmission electron microscopy combined with selected area electron diffraction measurements further confirm the high structural homogeneity of the MoS₂ monolayer film with large crystalline sizes. Electrical measurements show uniform and promising performance of the transistors made from the MoS₂ monolayer film. The carrier mobility remains high at large channel lengths. This work opens a new pathway toward electronic and optical applications, and fundamental growth mechanism as well, of the MoS₂ monolayer

Ultrastretchable and Stable Strain Sensors Based on Antifreezing and Self-Healing Ionic Organohydrogels for Human Motion Monitoring

Ionic hydrogels, a class of intrinsically stretchable and conductive materials, are widely used in soft electronics. However, the easy freezing and drying of water-based hydrogels significantly limit their long-term stability. Here, a facile solvent-replacement strategy is developed to fabricate ethylene glycol (Eg)/glycerol (Gl)-water binary antifreezing and antidrying organohydrogels for ultrastretchable and sensitive strain sensing within a wide temperature range. Because of the ready formation of strong hydrogen bonds between Eg/Gl and water molecules, the organohydrogels gain exceptional freezing and drying tolerance with retained deformability, conductivity, and self-healing ability even stay at extreme temperature for a long time. Thus, the fabricated strain sensor displays a gauge factor of 6, which is much higher than previously reported values for hydrogel-based strain sensors. Furthermore, the strain sensor exhibits a relatively wide strain range (0.5–950%) even at −18 °C. Various human motions with different strain levels are monitored by the strain sensor with good stability and repeatability from −18 to 25 °C. The organohydrogels maintained the strain sensing capability when exposed to ambient air for nine months. This work provides new insight into the fabrication of stable, ultrastretchable, and ultrasensitive strain sensors using chemically modified organohydrogel for emerging wearable electronics

Nonsolid TiO_<i>x</i> 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

Ultraviolet Random Laser Based on a Single GaN Microwire

Random lasing (RL) from self-constructed localized cavities based on micropits scatters in a single GaN microwire (MW) was investigated. The spectra and spatial resolution of RL exhibits that the lasing modes originated from different regions in the MW. Temperature-dependent lasing measurement of GaN RL shows an excellent characteristic temperature of about 52 K. In addition, the dependence of spatial localized cavities’ dimension on the pumping intensity profile and temperature was studied by fast Fourier transform spectroscopy. For GaN RL, the optical feedback was supported by localized paths through the scattering effect of micropits in the MW. The scattering feedback mechanism for RL can avoid the enormous difficulty in fabricating artificial cavity structures for GaN. Hence, the results in this paper represent a low-cost technique to realize GaN-based ultraviolet laser diodes without the fabrication difficulty of cavity facets