Two-dimensional (2D) van der Waals (vdW) heterostructures herald new opportunities for conducting fundamental studies of new physical/chemical phenomena and developing diverse nanodevice applications. In particular, vdW heterojunction p−n diodes exhibit great potential as highperformance photodetectors, which play a key role in many optoelectronic applications. Here, we report on 2D MoTe 2 /MoS 2 multilayer semivertical vdW heterojunction p−n diodes and their optoelectronic application in self-powered visible−invisible multiband detection and imaging. Our MoTe 2 /MoS 2 p−n diode exhibits an excellent electrical performance with an ideality factor of less than 1.5 and a high rectification (ON/OFF) ratio of more than 10 4 . In addition, the photodiode exhibits broad spectral photodetection capability over the range from violet (405 nm) to near-infrared (1310 nm) wavelengths and a remarkable linear dynamic range of 130 dB within an optical power density range of 10 −5 to 1 W/cm 2 in the photovoltaic mode. Together with these favorable static photoresponses and electrical behaviors, very fast photo-and electrical switching behaviors are clearly observed with negligible changes at modulation frequencies greater than 100 kHz. In particular, inspired by the photoswitching results for periodic red (638 nm) and near-infrared (1310 nm) illumination at 100 kHz, we successfully demonstrate a prototype self-powered visible− invisible multiband image sensor based on the MoTe 2 /MoS 2 p−n photodiode as a pixel. Our findings can pave the way for more advanced developments in optoelectronic systems based on 2D vdW heterostructures.

To effectively improve the energy density and reduce the self-discharging rate of micro-supercapacitors, an advanced strategy is required. In this study, we developed a hydroquinone (HQ)-based polymer-gel electrolyte (HQ-gel) for micro-supercapacitors. The introduced HQ redox mediators (HQ-RMs) in the gel electrolyte composites underwent additional Faradaic redox reactions and synergistically increased the overall energy density of the micro-supercapacitors. Moreover, the HQ-RMs in the gel electrolyte weakened the self-discharging behavior by providing a strong binding attachment of charged ions on the porous graphitized carbon electrodes after the redox reactions. The micro-supercapacitors with HQ gel (HQ-MSCs) showed excellent energy storage performance, including a high energy volumetric capacitance of 255 mF cm−3 at a current of 1 µA, which is 2.7 times higher than the micro-supercapacitors based on bare-gel electrolyte composites without HQ-RMs (b-MSCs). The HQ-MSCs showed comparatively low self-discharging behavior with an open circuit potential drop of 37% compared to the b-MSCs with an open circuit potential drop of 60% after 2000 s. The assembled HQ-MSCs exhibited high mechanical flexibility over the applied external tensile and compressive strains. Additionally, the HQ-MSCs show the adequate circuit compatibility within series and parallel connections and the good cycling performance of capacitance retention of 95% after 3000 cycles.

Due to the lowest formation energies, sulfur vacancies are inevitable in the vapor-phase chemical vapor deposition (CVD) of MoS2, which act as deep donors and induce midgap defect states, making the material intrinsically n-type. The postgrowth oxygen passivation of such defects has been the subject of a large number of recent studies because passivation of defects augments the photoluminescence quantum yield by several orders. In this study, by introducing an SiO2/Si wafer in close proximity to the growth substrate, we were able to supply trace oxygen in situ during the growth while simultaneously enabling chemisorption of oxygen at defect sites on the basal plane of large-area MoS2 monolayers. Low-temperature photoluminescence spectroscopy allowed us to distinguish clearly the nature of oxygen bonding in defective MoS2 grown with and without the trace oxygen. Chemisorption of oxygen enabled elimination of defect-related bound exciton emission at the near band edge transition of MoS2, leading to about 300% enhancement in the photoluminescence. We observed unusual splitting of the first-order A1g Raman mode in monolayer MoS2 films when the sulfur vacancies are not compensated by oxygen. The present study provides new experimental evidence to better distinguish between chemisorption and physisorption of oxygen and may serve as an effective way to tune the optical properties of van der Waals crystals during the large-area CVD process.

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Iron oxyhydroxide (FeOOH) nanostructures of different shapes were successfully synthesized on flexible textile cloth of polyester using a novel and simple technique based on hydrolysis method. The technique used herein is newly designed specifically to improve the efficiency in terms of energy, simplicity and cost involved in large scale synthesis of nanostructured thin films. Additionally, the morphology of nano-sized iron oxyhydroxide could be tuned into different shapes through variation in the type of precursors used for synthesis. The uniformity and adhesion of the depositions were also found to be excellent as examined by qualitative techniques. The as-deposited samples exhibited monoclinic and orthorhombic structures of FeOOH. A significant variation in the shape of as-deposited FeOOH nanostructures with change in precursor was observed through morphological studies, which displayed lance-shaped, rounded clusters and rod-like growth features in different cases. The nanocrystalline FeOOH can be directly applied to attract and trap phosphate from water reservoirs, thus contributing to environmental solutions. The proposed technique can also be utilized to deposit larger areas, which could be suitable for practical applications.