Exchange Bias in van der Waals CrCl3/Fe3GeTe2 Heterostructures.

Exchange bias is a physical phenomenon whereby the spins of a ferromagnet are pinned by those of an antiferromagnet, and this phenomenon has played an undisputed role in magnetic data storage. Over the past few decades, this effect has been observed in a variety of antiferromagnet/ferromagnet systems. New aspects of this phenomenon are being discovered. With the increasing interest in van der Waals (vdW) magnets, we address the question whether the effect can exist in magnetic vdW heterostructures. Here, we report exchange-bias fields of over 50 mT in mechanically exfoliated CrCl3/Fe3GeTe2 heterostructures at 2.5 K, the value of which is highly tunable by the field-cooling process and the heterostructure thickness. We postulate an intuitive picture explaining how the effect arises in this vdW heterostructure, as well as explaining the practical difficulty associated with capturing the effect. This work opens up new routes toward designing spintronic devices made of atomically thin vdW magnets.

High-Energy Gain Upconversion in Monolayer Tungsten Disulfide Photodetectors.

Photodetectors usually operate in the wavelength range with photon energy above the bandgap of channel semiconductors so that incident photons can excite electrons from valence band to conduction band to generate photocurrent. Here, however, we show that monolayer WS2 photodetectors can detect photons with energy even lying 219 meV below the bandgap of WS2 at room temperature. With the increase of excitation wavelength from 620 to 680 nm, photoresponsivity varies from 551 to 59 mA/W. This anomalous phenomenon is ascribed to energy upconversion, which is a combination effect of one-photon excitation and multiphonon absorption through an intermediate state created most likely by sulfur divacancy with oxygen adsorption. These findings will arouse research interests on other upconversion optoelectronic devices, photovoltaic devices, for example, of monolayer transition metal dichalcogenides (TMDCs).

A shadow enabled non-invasive probe for multi-feature intelligent liquid surveillance system.

Liquid detection probes used to identify the features of liquids show great promise in a variety of important applications. However, some challenges, such as sample contamination by direct contact with the liquid, the requirement of additional signal emitters, and complex fabrication, hindered the development of liquid detection probes. Here, we developed a non-invasive shadow probe (SP) for a multi-feature intelligent liquid surveillance system (ILSS). The self-powered SP with the working mechanism of the shadow effect can detect the features of liquids by analyzing the variations of liquid shadows such as the area, wavelength, and brightness. The exact resolution (5 different colors, 6 different concentrations, 6 different levels, 100% accuracy) and fast response time (0.2 ms) are shown by the SP under ambient light conditions (even in 0.003 sun). The ILSS, which integrated the SPs with signal processing circuits and applied the artificial intelligence (AI) technique, successfully detects and synoptically learns about liquids simultaneously. The in-real time ILSS reaches a test accuracy of 99.3% for 10 types of liquids with multiple features. This work showcases a promising solution for non-invasive multi-feature liquid detection, displaying great potential for future applications.

Ultrasensitive Near-Infrared Polarization Photodetectors with Violet Phosphorus/InSe van der Waals Heterostructures.

Near-infrared (NIR) polarization photodetectors with two-dimensional (2D) semiconductors and their van der Waals (vdW) heterostructures have presented great impact for the development of a wide range of technologies, such as in the optoelectronics and communication fields. Nevertheless, the lack of a photogenerated charge carrier at the device's interface leads to a poor charge carrier collection efficiency and a low linear dichroism ratio, hindering the achievement of high-performance optoelectronic devices with multifunctionalities. Herein, we present a type-II violet phosphorus (VP)/InSe vdW heterostructure that is predicted via density functional theory calculation and confirmed by Kelvin probe force microscopy. Benefiting from the type-II band alignment, the VP/InSe vdW heterostructure-based photodetector achieves excellent photodetection performance such as a responsivity (R) of 182.8 A/W, a detectivity (D*) of 7.86 × 1012 Jones, and an external quantum efficiency (EQE) of 11,939% under a 1064 nm photon excitation. Furthermore, the photodetection performance can be enhanced by manipulating the device geometry by inserting a few layers of graphene between the VP and InSe (VP/Gr/InSe). Remarkably, the VP/Gr/InSe vdW heterostructure shows a competitive polarization sensitivity of 2.59 at 1064 nm and can be integrated as an image sensor. This work demonstrates that VP/InSe and VP/Gr/InSe vdW heterostructures will be effective for promising integrated NIR optoelectronics.

Carbon-Coatings Improve Performance of Li-Ion Battery.

The development of lithium-ion batteries largely relies on the cathode and anode materials. In particular, the optimization of cathode materials plays an extremely important role in improving the performance of lithium-ion batteries, such as specific capacity or cycling stability. Carbon coating modifying the surface of cathode materials is regarded as an effective strategy that meets the demand of Lithium-ion battery cathodes. This work mainly reviews the modification mechanism and method of carbon coating, and summarizes the recent progress of carbon coating on some typical cathode materials (LiFePO4, LiMn2O4, LiCoO2, NCA (LiNiCoAlO2) and NCM (LiNiMnCoO2)). In addition, the limitations of the carbon coating on the cathode are also introduced. Suggestions on improving the effectiveness of carbon coating for future study are also presented.

Eco-Friendly Sustainable Synthesis of Graphene Quantum Dots from Biowaste as a Highly Selective Sensor.

Graphene quantum dots (GQDs) have generated a great deal of scientific interest due to their bright fluorescence, good biocompatibility, minimal toxicity and fascinating physicochemical features. However, the ultimate issues regarding the acidic contaminations and high synthesis cost of GQDs remain open challenges for their real-world applications. Herein, we report an eco-friendly, acid-free and sustainable method for the preparation of GQDs using a cost-efficient, and renewable carbon source, 'biomass-waste', which simultaneously solves the risk of contamination from strong acids and high expenditure initiated by expensive precursors. The results demonstrate that GQDs possess a size range of 1-5 nm with an average size of ~3 ± 0.4 nm and a thickness of ~1 nm consisting of 1-3 layers of graphene. As-prepared GQDs demonstrate fascinating size-dependent optical properties and considerable surface grafting. Due to their intriguing optical properties, these GQDs are employed as fluorescence probes to detect ferric ions. A focused and sensitive sensor is developed with a detection limit down to 0.29 µM. This study emphasizes the need for using a reasonably green process and an inexpensive biomass precursor to create high-value GQDs that hold great potential for use in photocatalytic, bioimaging and real-world sensing applications.

All in One, Self-Powered Bionic Artificial Nerve Based on a Triboelectric Nanogenerator.

Sensory and nerve systems play important role in mediating the interactions with the world. The pursuit of neuromorphic computing has inspired innovations in artificial sensory and nervous systems. Here, an all-in-one, tailorable artificial perception, and transmission nerve (APTN) was developed for mimicking the biological sensory and nervous ability to detect and transmit the location information of mechanical stimulation. The APTN shows excellent reliability with a single triboelectric electrode for the detection of multiple pixels, by employing a gradient thickness dielectric layer and a grid surface structure. The sliding mode is used on the APTN to eliminate the amplitude influence of output signal, such as force, interlayer distance. By tailoring the geometry, an L-shaped APTN is demonstrated for the application of single-electrode bionic artificial nerve for 2D detection. In addition, an APTN based prosthetic arm is also fabricated to biomimetically identify and transmit the stimuli location signal to pattern the feedback. With features of low-cost, easy installation, and good flexibility, the APTN renders as a promising artificial sensory and nervous system for artificial intelligence, human-machine interface, and robotics applications.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities.

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.

Observation of Anisotropic Magnetoresistance in Layered Nonmagnetic Semiconducting PdSe2.

Anisotropy in crystals usually has remarkable consequences in two-dimensional (2D) materials, for example, black phosphorus, PdSe2, and SnS, arising from different lattice periodicities along different crystallographic directions. Electrical anisotropy has been successfully demonstrated in 2D materials, but anisotropic magnetoresistance in 2D materials is rarely studied. Herein, we report anisotropic magnetoresistance in layered nonmagnetic semiconducting PdSe2 flakes. Anisotropic magnetoresistance along the two crystalline axes under a perpendicular magnetic field is demonstrated, and the magnetoresistance along the a-axis is apparently different from the magnetoresistance along the b-axis. The magnetoresistance can also be flexibly tuned by applying a gate voltage, leveraging the semiconductor properties of PdSe2. The computed anisotropic electronic density of states and electronic mobility with ab initio density functional calculations support the anisotropic and measured magnetoresistance. Our findings advance the understanding of magnetoresistance in anisotropic transition-metal dichalcogenides and pave the way for potential applications in anisotropic spintronic devices.

Diverse Structures and Magnetic Properties in Nonlayered Monolayer Chromium Selenide.

Thickness-dependent magnetic behavior has previously been observed in chemical vapor deposition-grown chromium selenide. However, the low-dimensional structure in nonlayered chromium selenide, which plays a crucial role in determining the low-dimensional magnetic order, needs further study. Here, we report the structure-dependent magnetic properties in monolayer CrSe2 and Cr2Se3 grown by molecular beam epitaxy. In the monolayer CrSe2, 1T-CrSe2 with a lattice constant of 3.3 Å has a metallic character, coexisting with the 1T″ phase with 2 × 2 surface periodicity. Monolayer CrSe2 can be transformed into Cr2Se3 with a lattice constant of 3.6 Å by annealing at 300 °C. X-ray magnetic circular dichroism (XMCD) measurements combined with DFT calculations reveal that while the MBE-grown monolayer CrSe2 is antiferromagnetic, monolayer Cr2Se3 is ferromagnetic with a Curie temperature of ∼200 K. This work demonstrates the structural diversity in nonlayered chromium selenide and the critical effect of different structures on its electronic and magnetic properties.

Shadow enhanced self-charging power system for wave and solar energy harvesting from the ocean.

Hybrid energy-harvesting systems that capture both wave and solar energy from the oceans using triboelectric nanogenerators and photovoltaic cells are promising renewable energy solutions. However, ubiquitous shadows cast from moving objects in these systems are undesirable as they degrade the performance of the photovoltaic cells. Here we report a shadow-tribo-effect nanogenerator that hybrids tribo-effect and shadow-effect together to overcome this issue. Several fiber-supercapacitors are integrated with the shadow-tribo-effect nanogenerator to form a self-charging power system. To capture and store wave/solar energy from oceans, an energy ball based on the self-charging power system is demonstrated. By harnessing the shadow-effect, i.e. the shadow of the moving object in the energy ball, the charging time shortens to 253.3 s to charge the fiber-supercapacitors to the same voltage (0.3 V) as using pure tribo-effect. This cost-effective method to harvest and store the wave/solar energy from the oceans in this work is expected to inspire next-generation large-scale blue energy harvesting.

Performance Improvement by Ozone Treatment of 2D PdSe2.

Atomic-scale defects in two-dimensional transition metal dichalcogenides (TMDs) often dominate their physical and chemical properties. Introducing defects in a controllable manner can tailor properties of TMDs. For example, chalcogen atom defects in TMDs were reported to trigger phase transition, induce ferromagnetism, and drive superconductivity. However, reported strategies to induce chalcogen atom defects including postgrowth annealing, laser irradiation, or plasma usually require high temperature (such as 500 °C) or cause unwanted structural damage. Here, we demonstrate low-temperature (60 °C) partial surface oxidation in 2D PdSe2 with low disorder and good stability. The combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations provide evidence of atomic-scale partial oxidation with both atomic resolution and chemical sensitivity. We also experimentally demonstrate that this controllable oxygen incorporation effectively tailors the electronic, optoelectronic, and catalytic activity of PdSe2. This work provides a pathway toward fine-tuning the physical and chemical properties of 2D TMDs and their applications in nanoelectronics, optoelectronics, and electrocatalysis.

Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure.

Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS2) monolayer/2D perovskite (C6H5C2H4NH3)2PbI4 (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of -0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS2/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS2 region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 µA/W and 5.7 × 10-5, respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.

High-Performance, Room Temperature, Ultra-Broadband Photodetectors Based on Air-Stable PdSe2.

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high-performance ultra-broadband photodetector based on PdSe2 with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid-infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid-infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W-1 and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe2 , anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Flexible, Cuttable, and Self-Waterproof Bending Strain Sensors Using Microcracked Gold Nanofilms@Paper Substrate.

Rapid advances in functional sensing electronics place tremendous demands on innovation toward creative uses of versatile advanced materials and effective designs of device structures. Here, we first report a feasible and effective fabrication strategy to integrate commercial abrasive papers with microcracked gold (Au) nanofilms to construct cuttable and self-waterproof crack-based resistive bending strain sensors. Via introducing surface microstructures, the sensitivities of the bending strain sensors are greatly enhanced by 27 times than that of the sensors without surface microstructures, putting forward an alternative suggestion for other flexible electronics to improve their performances. Besides, the bending strain sensors also endow rapid response and relaxation time of 20 ms and ultrahigh stability of >18 000 strain loading-unloading cycles in conjunction with flexibility and robustness. In addition, the concepts of cuttability and self-waterproofness (attain and even surpass IPX-7) of the bending strain sensors have been demonstrated. Because of the distinctive sensing properties, flexibility, cuttability, and self-waterproofness, the bending strain sensors are attractive and promising for wearable electronic devices and smart health monitoring system.

Recyclable and Green Triboelectric Nanogenerator.

A recyclable and green triboelectronic nanogenerator (TENG) is developed based on triboelectrification and designed cascade reactions. Once triggered by water, the TENG can fully dissolve and degrade into environmentally benign end products. With features of rapid dissolution, reproductivity, and green electronic, the TENG has potential of serving as clearable energy harvester and nanosensor for health monitoring and motion sensing.

Service Behavior of Multifunctional Triboelectric Nanogenerators.

Triboelectric nanogenerators (TENGs) or TENG-based self-charging systems harvesting energy from ambient environment are promising power solution for electronics. The stable running remains a key consideration in view of potential complex application environment. In this work, a textile-based tailorable multifunctional TENG (T-TENG) is developed. The T-TENG is used as self-powered human body motion sensor, water energy harvester, and formed all textile-based flexible self-charging system by integrating with textile-based supercapacitors. The service behavior and the mechanism of performance retention are also studied when the T-TENG is damaged. As a self-powered human body motion sensor, the T-TENG maintains the stable properties when it is cut. As a water energy harvester, the T-TENG is capable of scavenging mechanical energy from water efficiently even if it is damaged partly. Besides, the charge properties of the self-charging system are systematically investigated when the T-TENG is cut. The investigation on service behavior of T-TENG and TENG-based self-charging system pushes forward the development of highly reliable electronics and is a guide for other nanodevices and nanosystems.

Novel Piezoelectric Paper-Based Flexible Nanogenerators Composed of BaTiO3 Nanoparticles and Bacterial Cellulose.

A piezoelectric paper based on BaTiO3 (BTO) nanoparticles and bacterial cellulose (BC) with excellent output properties for application of nanogenerators (NGs) is reported. A facile and scalable vacuum filtration method is used to fabricate the piezoelectric paper. The BTO/BC piezoelectric paper based NG shows outstanding output performance with open-circuit voltage of 14 V and short-circuit current density of 190 nA cm-2. The maximum power density generated by this unique BTO/BC structure is more than ten times higher than BTO/polydimethylsiloxane structure. In bending conditions, the NG device can generate output voltage of 1.5 V, which is capable of driving a liquid crystal display screen. The improved performance can be ascribed to homogeneous distribution of piezoelectric BTO nanoparticles in the BC matrix as well as the enhanced stress on piezoelectric nanoparticles implemented by the unique percolated networks of BC nanofibers. The flexible BTO/BC piezoelectric paper based NG is lightweight, eco-friendly, and cost-effective, which holds great promises for achieving wearable or implantable energy harvesters and self-powered electronics.

Temperature-dependent electrochemical capacitive performance of the α-Fe2O3 hollow nanoshuttles as supercapacitor electrodes.

The design and optimization of supercapacitors electrodes nanostructures are critically important since the properties of supercapacitors can be dramatically enhanced by tunable ion transport channels. Herein, we demonstrate high-performance supercapacitor electrodes materials based on α-Fe2O3 by rationally designing the electrode microstructure. The large solid-liquid reaction interfaces induced by hollow nanoshuttle-like structures not only provide more active sites for faradic reactions but also facilitate the diffusion of the electrolyte into electrodes. These result in the optimized electrodes with high capacitance of 249 F g(-1) at a discharging current density of 0.5 A g(-1) as well as good cycle stability. In addition, the relationship between charge storage and the operating temperature has been researched. The specific capacitance has no significant change when the working temperature increased from 20 °C to 60 °C (e.g. 203 F g(-1) and 234 F g(-1) at 20 °C and 60 °C, respectively), manifesting the electrodes can work stably in a wide temperature range. These findings here elucidate the α-Fe2O3 hollow nanoshuttles can be applied as a promising supercapacitor electrode material for the efficient energy storage at various potential temperatures.

Flexible and printable paper-based strain sensors for wearable and large-area green electronics.

Paper-based (PB) green electronics is an emerging and potentially game-changing technology due to ease of recycling/disposal, the economics of manufacture and the applicability to flexible electronics. Herein, new-type printable PB strain sensors (PPBSSs) from graphite glue (graphite powder and methylcellulose) have been fabricated. The graphite glue is exposed to thermal annealing to produce surface micro/nano cracks, which are very sensitive to compressive or tensile strain. The devices exhibit a gauge factor of 804.9, response time of 19.6 ms and strain resolution of 0.038%, all performance indicators attaining and even surpassing most of the recently reported strain sensors. Due to the distinctive sensing properties, flexibility and robustness, the PPBSSs are suitable for monitoring of diverse conditions such as structural strain, vibrational motion, human muscular movements and visual control.