We report the preparation of nanocomposites of reduced graphene oxide with embedded Fe3O4/Fe nanorings （FeNR@rGO） by chemical hydrothermal growth. We illustrate the use of these nanocomposites as novel electromagnetic wave absorbing materials. The electromagnetic wave absorption properties of the nanocomposites with different compositions were investigated. The preparation procedure and nanocomposite composition were optimized to achieve the best electromagnetic wave absorption properties. Nanocomposites with a GO：cx-Fe203 mass ratio of 1：1 prepared by annealing in HdAr for 3 h exhibited the best properties. This nanocomposite sample （thickness = 4.0 mm） showed a minimum reflectivity of -23.09 dB at 9.16 GHz. The band range was 7.4-11.3 GHz when the reflectivity was less than -10 dB and the spectrum width was up to 3.9 GHz. These figures of merit are typically of the same order of magnitude when compared to the values shown by traditional ferric oxide materials. However, FeNR@rGO can be readily applied as a microwave absorbing material because the production method we propose is highly compatible with mass production standards.
Yi DingLong ZhangQingliang LiaoGuangjie ZhangShuo LiuYue Zhang
A cobaltosic-oxide-nanosheets/reduced-graphene-oxide composite （CoNSs@RGO） was successfully prepared as a light-weight broadband electromagnetic wave absorber. The effects of the sample thickness and amount of composite added to paraffin samples on the absorption properties were thoroughly investigated. Due to the nanosheet-like structure of Co3O4, the surface-to-volume ratio of the wave absorption material was very high, resulting in a large enhancement in the absorption properties. The maximum refection loss of the CoNSs@RGO composite was -45.15 dB for a thickness of 3.6 mm, and the best absorption bandwidth with a reflection loss below -10 dB was 7.14 GHz with a thickness of 2.9 mm. In addition, the peaks of microwave absorption shifted towards the low frequency region with increasing thickness of the absorbing coatings. The mechanism of electromagnetic wave absorption was attributed to impedance matching of CoNSs@RGO as well as the dielectric relaxation and polarization of RGO. Compared to previously reported absorbing materials, CoNSs@RGO showed better performance as a lightweight and highly efficient absorbing material for application in the high frequency band.
Yi DingZheng ZhangBaohe LuoQingliang LiaoShuo LiuYichong LiuYue Zhang
Performance modulation of ZnO optoelectronic devices in the presence of proper piezoelectric polarization charges has been widely reported, whereas relatively less work has been performed about the influence of photoexcitation on piezotronics. In this stud~ we experimentally investigated the performance evolution of ZnO piezotronic strain sensor under various 365 nm UV irradiation densities. The device demonstrated a response ratio of -200 under no illumination and under -0.53% compressive strain, and the response time is approximately 0.3 s. However, tremendous performance degradation was observed with the increase in the illumination densi~, which is attributed to the W-modulated change in the free electron concentration and Schottky barrier height. It was observed that increased carrier density intensifies the screening effect and thus, the modulation ability of piezo-polarization charges weakens. Meanwhile, the deterioration of rectifying behavior at the interface under UV illumination also jeopardizes the device performance.
Sandwich-style memristor devices were synthesized by electrochemical deposition with a ZnO film serving as the active layer between Al-doped ZnO （AZO） and Au electrodes. The carrier concentration of the ZnO films is controlled by adding HNO3 during the growth process. A resulting increase in carrier concentration from 10^17 to 10^19 cm^-3 was observed, along with a corresponding drop in the on--off ratio from 6,437% to 100%. The resistive switching characteristics completely disappeared when the carrier concentration was above 1029 cm-3, making it unsuitable for a memory device. The decreasing switching ratio is attributed to a reduction in the driving force for oxygen vacancy drift. Systematic analysis of the migration of oxygen vacancies is presented, including the concentration gradient and electrical potential gradient. Such oxygen vacancy migration dynamics provide insight into the mechanisms of the oxygen vacancy drift and provide valuable information for industrial production of memristor devices.
Ionic transport in organometal halide perovskites is of vital importance because it dominates anomalous phenomena in perovskite solar cells,from hysteresis to switchable photovoltaic effects.However,excited state ionic transport under illumination has remained elusive,although it is essential for understanding the unusual light-induced effects(light-induced self-poling,photo-induced halide segregation and slow photoconductivity response)in organometal halide perovskites for optoelectronic applications.Here,we quantitatively demonstrate light-enhanced ionic transport in CH3NH3PbI3 over a wide temperature range of 17–295 K,which reveals a reduction in ionic transport activation energy by approximately a factor of five(from 0.82 to 0.15 eV)under illumination.The pure ionic conductance is obtained by separating it from the electronic contribution in cryogenic galvanostatic and voltage-current measurements.On the basis of these findings,we design a novel light-assisted method of catalyzing ionic interdiffusion between CH3NH3I and PbI2 stacking layers in sequential deposition perovskite synthesis.X-ray diffraction patterns indicate a significant reduction of PbI2 residue in the optimized CH3NH3PbI3 thin film produced via lightassisted sequential deposition,and the resulting solar cell efficiency is increased by over 100%(7.5%–15.7%)with little PbI2 residue.This new method enables fine control of the reaction depth in perovskite synthesis and,in turn,supports light-enhanced ionic transport.
In this work, we developed a novel triboelectricity-assisted polymer-free method for the transfer of large-area chemical vapor deposited graphene films. With the assistance of electrostatic forces from friction-generated charges, graphene sheets were successfully transferred from copper foils to flexible polymer substrates. Characterization results confirmed the presence of high quality graphene with less defects and contaminations, compared to graphene transferred by conventional poly（methyl rnethacrylate）-mediated processes. In addition, the graphene samples possessed outstanding electrical transport capabilities and mechanical stability, when studied as electron transfer matrixes in graphene/ZnO hybrid flexible photodetectors. Our results showed a broad application potential for this transfer method in future flexible electronics and optoelectronics.
This paper describes a novel strategy to weaken the piezopotential screening effect by forming Schottky junctions on the ZnO surface through the introduction of Au particles onto the surface. With this approach, the piezoelectric-energyconversion performance was greatly enhanced. The output voltage and current density of the Au@ZnO nanoarray-based piezoelectric nanogenerator reached 2 V and 1 μA/cm^2, respectively, 10 times higher than the output of pristine ZnO nanoarray-based piezoelectric nanogenerators. We attribute this enhancement to dramatic suppression of the screening effect due to the decreased carrier concentration, as determined by scanning Kelvin probe microscope measurements and impedance analysis. The lowered capacitance of the Au@ZnO nanoarraybased piezoelectric nanogenerator also contributes to the improved output. This work provides a novel method to enhance the performance of piezoelectric nanogenerators and possibly extends to piezotronics and piezophototronics.
