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We provided an overview of recent advances in 2D-material-based wearable chemical sensors for healthcare applications. We also explored the challenges and opportunities associated with designing and implementing 2D wearable chemical sensors.

In this study, a high-performance triboelectric nanogenerator (TENG) is developed based on cold spray (CS) deposition of composite material layers. Composite layers were fabricated by cold spraying of micron-scale tin (Sn) particles on aluminum (Al) and polytetrafluoroethylene (PTFE) films, which led to improved TENG performance owing to functionalized composite layers as friction layers and electrodes, respectively. As-sprayed tin composite layers not only enhanced the flow of charges by strong adhesion to the target layer but also formed a nano-microstructure on the surface of the layers, thereby increasing the surface area during friction. More importantly, the electricity generation performance was improved more than 6 times as compared to the TENG without CS deposition on it. From parametric studies, the TENG using the cold-sprayed composite layer produced an electrical potential of 1140 V for a simple structure with a 25.4 × 25.4 mm

The graphene boom has triggered a widespread search for novel elemental van der Waals materials thanks to their simplicity for theoretical modeling and easy access for material growth. Group VI element tellurium is an unintentionally p-type doped narrow bandgap semiconductor featuring a one-dimensional chiral atomic structure which holds great promise for next-generation electronic, optoelectronic, and piezoelectric applications. In this paper, we first review recent progress in synthesizing atomically thin Te two-dimensional (2D) films and one-dimensional (1D) nanowires. Its applications in field-effect transistors and potential for building ultra-scaled Complementary metal–oxide–semiconductor (CMOS) circuits are discussed. We will also overview the recent study on its quantum transport in the 2D limit and progress in exploring its topological features and chiral-related physics. We envision that the breakthrough in obtaining high-quality 2D Te films will inspire a revisit of the fundamental properties of this long-forgotten material in the near future.

Efficaciously scavenging waste mechanical energy from the environment is an emerging field in the self-powered and self-governing electronics system which solves battery limitations. it demonstrates enormous potential in various fields such as wireless devices, vesture, and portable electronic devices. Different surface textured PET triboelectric nanogenerators (TENG) were developed by laser pattern method in the previous work, with the line textured TENG device showing improved performance due to a larger surface contact area. Here, polyethylene oxide (PEO) and polyvinyl alcohol (PVA) coated line patterned PETbased TENG was developed for conversion of mechanical energy into useful electric energy. The PEO layer has boosted the TENG output 4 times higher than the PA6-laser patterned PET TENG device (our previous report) and 2-fold higher than a pristine line patterned TENG. It has generated an open-circuit voltage, short circuit current, and instantaneous power density of 131 V, 2.32 μA, and 41.6 μW/cm2, respectively. The as-fabricated device was tested for 10000 cycles for reliability evaluation, which shows no significant performance degradation. In addition, the device was deployed to glowing 10 LEDs with high intensity. Thus, this device is used for harvesting ambient mechanical energy conversion and power micro and nanoelectronic devices.

Spatial manipulation of nanoparticles (NPs) in a controlled manner is critical for the fabrication of 3D hybrid materials with unique functions. However, traditional fabrication methods such as electron‐beam lithography and stereolithography are usually costly and time‐consuming, precluding their production on a large scale. Herein, for the first time the ultrafast laser direct writing is combined with external magnetic field (MF) to massively produce graphene‐coated ultrafine cobalt nanoparticles supported on 3D porous carbon using metal–organic framework crystals as precursors (5 × 5 cm2 with 10 s). The MF‐confined picosecond laser scribing not only reduces the metal ions rapidly but also aligns the NPs in ultrafine and evenly distributed order (from 7.82 ± 2.37 to 3.80 ± 0.84 nm). ≈400% increment of N‐Q species within N compositionis also found as the result of the special MF‐induced laser plasma plume. (). The importance of MF is further exmined by electrochemical water‐splitting tests. Significant overpotential improvements of 90 and 150 mV for oxygen evolution reaction and hydrogen evolution reaction are observed, respectively, owing to the MF‐induced alignment of the NPs and controlled elemental compositions. This work provides a general bottom‐up approach for the synthesis of metamaterials with high outputs yet a simple setup., Magnetically aligned ultrafine cobalt embedded 3D porous carbon metamaterial is synthesized by combining one‐step laser direct writing with external magnetic field. Superfast production speed of 2.5 cm2 s−1 and ultrafine Co nanoparticles (NPs) of 3.80 nm can be obtained massively. Significant improvements of electrocatalytic performances are found owning to magnetic field manipulation which leads to the well‐distributed G@Co NPs.

Nanostructured piezoelectric semiconductors offer unprecedented opportunities for high-performance sensing in numerous catalytic processes of biomedical, pharmaceutical, and agricultural interests, leveraging piezocatalysis that enhances the catalytic efficiency with the strain-induced piezoelectric field. Here, a cost-efficient, high-performance piezo-electrocatalytic sensor for detecting l-ascorbic acid (AA), a critical chemical for many organisms, metabolic processes, and medical treatments, is designed and demonstrated. Zinc oxide (ZnO) nanorods and nanosheets are prepared to characterize and compare their efficacy for the piezo-electrocatalysis of AA. The electrocatalytic efficacy of AA is significantly boosted by the piezoelectric polarization induced in the nanostructured semiconducting ZnO catalysts. The charge transfer between the strained ZnO nanostructures and AA is elucidated to reveal the mechanism for the related piezo-electrocatalytic process. The low-temperature synthesis of high-quality ZnO nanostructures allows low-cost, scalable production, and integration directly into wearable electrocatalytic sensors whose performance can be boosted by otherwise wasted mechanical energy from the working environment, for example, human-generated mechanical signals.

A material with reversible temperature change capability under an external electric field, known as the electrocaloric effect (ECE), has long been considered as a promising solid-state cooling solution. However, electrocaloric (EC) performance of EC materials generally is not sufficiently high for real cooling applications. As a result, exploring EC materials with high performance is of great interest and importance. Here, we report on the ECE of ferroelectric materials with van der Waals layered structure (CuInP2S6 or CIPS in this work in particular). Over 60% polarization charge change is observed within a temperature change of only 10 K at Curie temperature. Large adiabatic temperature change (|{\Delta}T|) of 3.3 K, isothermal entropy change (|{\Delta}S|) of 5.8 J kg-1 K-1 at |{\Delta}E|=142.0 kV cm-1 at 315 K (above and near room temperature) are achieved, with a large EC strength (|{\Delta}T|/|{\Delta}E|) of 29.5 mK cm kV-1. The ECE of CIPS is also investigated theoretically by numerical simulation and a further EC performance projection is provided., Comment: 32 pages, 10 figures

Two-dimensional (2-D) semiconductors have been intensely explored as alternative channel materials for future generation ultra-scaled transistor technology [1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] . However, significant roadblocks (e.g., poor carrier mobilities [9] , [10] , [11] , instability [4,5,10], and vague potential in scaling-up [10,12–15]) exist that prevent the realization of the current state-of-the-art 2-D materials’ potential for energy-efficient electronics. The emergent solution-grown tellurene exhibits attractive attributes, e.g., high room-temperature mobility, large on-state current density, air-stability, and tunable material properties through a low-cost, scalable process, to tackle these challenges [16]. Nevertheless, the fundamental manufacturing science of the hydrothermal processing for tellurene remains elusive. Here, we report on the first systematic, data-driven learning of the process-structure-property relationship in solution-grown tellurene, revealing the process factors’ effects on tellurene’s production yield, dimensions, and transistor-relevant properties, through a holistic approach integrating both the experimental explorations and data analytics. We further demonstrate the application of such fundamental knowledge for developing tellurene transistors with optimized and reliable performance, which can enable the cost-effective realization of high-speed, energy-efficient electronics.

Smart sensing devices with high stretchability and self-powered characteristics are essential in future generation wearable human-integrated applications. Here we report for the first time scalable synthesis and integration of selenium (Se) nanowires into wearable piezoelectric devices, and explore the feasibility of such devices for self-powered sensing applications, e.g., physiological monitoring. The ultrathin device can be conformably worn onto the human body, effectively converting the imperceptible time-variant mechanical vibration from the human body into distinguishable electrical signals, e.g., gesture, vocal movement, and radial artery pulse, through straining the piezoelectric Se nanowires. Our results suggest the potential of solution-synthesized Se nanowire a new class of piezoelectric nanomaterial for self-powered biomedical devices and opens doors to new technologies in energy, electronics, and sensor applications.

Capturing and decoding human motions with textile-based wearables would usher in exciting opportunities for consumer electronics, human–machine interface, and robotics. Transforming textiles into multifunctional materials represents an appealing strategy for implementing human-integrated wearable systems with reconfigurable form factors and designer functions. To this end, significant roadblocks exist primarily due to the challenges in integrating state-of-the-art electronics and power units onto textiles through schemes compatible with the clothing design and processing. Here we present a holistic design and integration of an all-textile triboelectric sensor (ATTS) which is capable of perceiving human motions with a high sensitivity of 1.1 V kPa−1 and a broad pressure-sensitive range from 100 Pa to 400 kPa. The ATTS can be integrated with arbitrary commercial garments. We further demonstrated an ATTS-based smart glove that can capture human gestures for remotely controlling a virtual reality (VR) enabled teleoperated robotic gripper that can dexterously handle delicate objects with a wide range of shapes, sizes, and mechanical properties. Our ATTS technology has the potential for enabling more capable human–machine interfaces and providing dexterity to robots in societally pervasive areas such as teleoperated surgery and advanced manufacturing.

Developing high-capacitance electrode materials with high voltage is always the essential strategy for increasing the energy densities of aqueous supercapacitors. However, it is very difficult to break through the voltage restriction of 2 V due to the narrow electrochemical window of water. Herein, we introduce a double (alkaline–neutral) electrolyte separated with a K+ conductive membrane to assemble a lithium ion capacitor (LIC) with high voltage and high energy density. This electrolyte system adequately exploits the high specific capacitance (256 F g−1) and low potential (−1–0 V vs. Hg/HgO) of nitrogen-rich biomass carbon (NBC) as an anode in alkaline mixed solution, and the large capacitance (440 F g−1) and high redox potential (0–1.2 V vs. SCE) of LiMn2O4 nanorods as a cathode in neutral mixed solution. The as-fabricated NBC//LiMn2O4 LIC realizes a stable working voltage up to 2.3 V and a high energy density of 50 W h kg−1 at 571 W kg−1 as well as excellent rate capability, and is superior to the traditional LIC and many previous carbon-based asymmetric supercapacitors in neutral electrolytes. Moreover, it presents impressive cycling life with 88% capacitance retention after 5000 cycles. These results show that this method has great promise for promoting the electrochemical performance of supercapacitors.

Laser-induced chemical deposition is an economical "grow-in-place" approach to produce functional materials. The lack of precise control over the component density and other properties hinders the development of the method towards an efficient nanomanufacturing technology. In this paper, we provide a mechanism of direct pulsed-laser integration of ZnO nanowire seeding and growth on silicon wafers toward controlled density. Investigation of laser-induced ZnO nucleation directly deposited on a substrate suggested that the coverage percentage of nucleus particles was a function of instantly available area, supplementing the classical nucleation theory for confined area deposition. A processing window was found in which ZnO nanowires only grew from the early deposited nucleated particles as seeds. A study on ZnO nanowire growth showed that the process became transport limited over time, which was important for density-controlled nanowire growth integrated on nucleated seeds. The proposed mechanism provided guidance to integrate nanomaterials using laser-induced chemical deposition with a controlled density and morphology.

The economic production and integration of nanomaterial-based wearable energy storage devices with mechanically-compliable form factors and reliable performance will usher in exciting opportunities in emerging technologies such as consumer electronics, pervasive computing, human–machine interface, robotics, and the Internet of Things. Despite the increased interests and efforts in nanotechnology-enabled flexible energy storage devices, reducing the manufacturing and integration costs while continuously improving the performance at the device and system level remains a major technological challenge. The inkjet printing process has emerged as a potential economic method for nanomanufacturing printed electronics, sensors, and energy devices. Nevertheless, there have been few reports reviewing the scalable nanomanufacturing of inkjet printed wearable energy storage devices. To fill this gap, here we review the recent advances in inkjet printed flexible energy storage technologies. We will provide an in-depth discussion focusing on the materials, manufacturing process integration, and performance issues in designing and implementing the inkjet printing of wearable energy storage devices. We have also compiled a comprehensive list of the reported device technologies with the corresponding processing factors and performance metrics. Finally, we will discuss the challenges and opportunities associated with related topics. The rapid and exciting progress achieved in many emerging and traditional disciplines is expected to lead to more theoretical and experimental advances that would ultimately enable the scalable nanomanufacturing of inkjet printed wearable energy storage devices.

Deformable energy devices capable of efficiently scavenging ubiquitous mechanical signals enable the realization of self-powered wearable electronic systems for emerging human-integrated technologies. Triboelectric nanogenerators (TENGs) utilizing soft polymers with embedded additives and engineered dielectric properties emerge as ideal candidates for such applications. However, the use of solid filler materials in the state-of-the-art TENGs limits the devices' mechanical deformability and long-term durability. The current structural design for TENGs faces the dilemma where the enhanced dielectric constant of the TENG's contact layer leads to an undesirable saturation of the surface charge density. Here, we present a novel scheme to address the above issues, by exploring a liquid-metal-inclusion based TENG (LMI-TENG) where inherently deformable core–shell LMIs are incorporated into wearable high-dielectric-constant polymers. Through a holistic approach integrating theoretical and experimental efforts, we identified the parameter space for designing an LMI-TENG with co-optimized output and mechanical deformability. As a proof of concept, we demonstrated an LMI-TENG based wireless media control system for a self-powered user interface. The device architecture and design scheme presented here provide a promising solution towards the realization of self-powered human-integrated technologies.

When uniform strain is applied to noncentrosymmetric semiconductor crystals, which are piezoelectric, static polarization charges are induced at the surface. If the applied strain is not uniform, these charges can even be created inside the crystal. The applied strain affects electronic transport and also photonic processes, and thus can be used to tune the material properties statically or dynamically. As a result, two new fields have emerged, namely piezotronics and piezo-phototronics. This article reviews the history of the two fields and gives a perspective on their applications. The articles in this issue of MRS Bulletin highlight progress in these two fields, and this article places this progress into perspective.

Tellurium (Te) is a narrow bandgap semiconductor with a unique chiral crystal structure. The topological nature of electrons in the Te conduction band can be studied by realizing n-type doping using atomic layer deposition (ALD) technique on two-dimensional (2D) Te film. In this work, we fabricated and measured the double-gated n-type Te Hall-bar devices, which can operate as two separate or coupled electron layers controlled by the top gate and back gate. Profound Shubnikov-de Haas (SdH) oscillations are observed in both top and bottom electron layers. Landau level hybridization between two layers, compound and charge-transferable bilayer quantum Hall states at filling factor 4, 6, and 8 are analyzed. Our work opens the door for the study of Weyl physics in coupled bilayer systems of 2D materials., Comment: Submitted to Nano Letters

The capability of sensor systems to efficiently scavenge their operational power from stray, weak environmental energies through sustainable pathways could enable viable schemes for self-powered health diagnostics and therapeutics. Triboelectric nanogenerators (TENG) can effectively transform the otherwise wasted environmental, mechanical energy into electrical power. Recent advances in TENGs have resulted in a significant boost in output performance. However, obstacles hindering the development of efficient triboelectric devices based on biocompatible materials continue to prevail. Being one of the most widely used polymers for biomedical applications, polyvinyl alcohol (PVA) presents exciting opportunities for biocompatible, wearable TENGs. Here, the holistic engineering and systematic characterization of the impact of molecular and ionic fillers on PVA blends' triboelectric performance is presented for the first time. Triboelectric devices built with optimized PVA-gelatin composite films exhibit stable and robust triboelectricity outputs. Such wearable devices can detect the imperceptible skin deformation induced by the human pulse and capture the cardiovascular information encoded in the pulse signals with high fidelity. The gained fundamental understanding and demonstrated capabilities enable the rational design and holistic engineering of novel materials for more capable biocompatible triboelectric devices that can continuously monitor vital physiological signals for self-powered health diagnostics and therapeutics.

Atomically thin materials, leveraging their low-dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin-film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain-engineered anisotropic optical and electrical properties in solution-grown, sub-millimeter-size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral-chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain-engineered functionalities.

Dirac and Weyl nodal materials can host low-energy relativistic quasiparticles. Under strong magnetic fields, the topological properties of Dirac/Weyl materials can directly be observed through quantum Hall states. However, most Dirac/Weyl nodes generically exist in semimetals without exploitable band gaps due to their accidental band-crossing origin. Here, we report the first experimental observation of Weyl fermions in a semiconductor. Tellurene, the two-dimensional form of tellurium, possesses a chiral crystal structure which induces unconventional Weyl nodes with a hedgehog-like radial spin texture near the conduction band edge. We synthesize high-quality n-type tellurene by a hydrothermal method with subsequent dielectric doping and detect a topologically non-trivial π Berry phase in quantum Hall sequences. Our work expands the spectrum of Weyl matter into semiconductors and offers a new platform to design novel quantum devices by marrying the advantages of topological materials to versatile semiconductors.

Two-dimensional (2D) semiconductors have been extensively explored as a new class of materials with great potential. In particular, black phosphorus (BP) has been considered to be a strong candidate for applications such as high-performance infrared photodetectors. However, the scalability of BP thin film is still a challenge, and its poor stability in the air has hampered the progress of the commercialization of BP devices. Herein, we report the use of hydrothermal-synthesized and air-stable 2D tellurene nanoflakes for broadband and ultrasensitive photodetection. The tellurene nanoflakes show high hole mobilities up to 458 cm2/V·s at ambient conditions, and the tellurene photodetector presents peak extrinsic responsivity of 383 A/W, 19.2 mA/W, and 18.9 mA/W at 520 nm, 1.55 μm, and 3.39 μm light wavelength, respectively. Because of the photogating effect, high gains up to 1.9 × 103 and 3.15 × 104 are obtained at 520 nm and 3.39 μm wavelength, respectively. At the communication wavelength of 1.55 μm, the tellurene photodetector exhibits an exceptionally high anisotropic behavior, and a large bandwidth of 37 MHz is obtained. The photodetection performance at different wavelength is further supported by the corresponding quantum molecular dynamics (QMD) simulations. Our approach has demonstrated the air-stable tellurene photodetectors that fully cover the short-wave infrared band with ultrafast photoresponse.

Piezoelectricity provides an ideal electromechanical mechanism with emerging applications in wearable devices due to its simplicity and self-powered nature. However, the 3D printing of piezoelectric devices still faces many challenges, including material printability, high energy poling process, and low dimensional accuracy. This study demonstrates, for the first time, a tellurium nanowire-based piezoelectric device fabricated by a hybrid printing method integrating highly complementary aerosol jet printing and extrusion printing in a single printing platform. The aerosol-jet-printed tellurium nanowire demonstrates piezoelectric properties without the need for any poling processing due to the unique properties of the tellurium nanowires. The silver nanowire electrodes printed by aerosol jet printing demonstrate excellent conductivity and stretchability without the need for sintering. An extrusion method is employed to print the silicone films, which serve as the stretchable substrate and the electrical insulation layers between the printed tellurium and silver. The printed wearable piezoelectric devices were attached to a human wrist to detect different hand gestures and to a human neck to detect heartbeat without using an external power source. The fully printed, sintering-free and poling-free, and stretchable piezoelectric device opens enormous opportunities for facile integration with a broad range of printed electronics and wearable devices.

Piezoelectric nanogenerators have attracted intensive interest in harvesting the stray mechanical energy in the environment to power miniaturized electronics and sensors. However, their efficient integration into systems and compatibility with existing technologies for practical applications remains challenging. Here, we report for the first time the systematic, data-driven learning of the process-property-performance relation in ZnO nanowires piezoelectric nanogenerators that are synthesized and integrated through a laser-induced chemical process. An experiment-derived behavioral model was established to reveal the apparent connections between the production parameters and the output performance of the ZnO piezoelectric nanogenerator. We further demonstrated the application of such knowledge for integrating the optimized ZnO nanowires piezoelectric nanogenerator with a photosensor into a self-powered sensor system, exhibiting the potential for future system-level improvements.

Introducing anion vacancies on two-dimensional transition-metal dichalcogenides (TMDs) would significantly improve their catalytic activity. In this work, we proposed a solid-phase reduction (SPR) strategy to simultaneously achieve efficient exfoliation and controlled generation of chalcogen vacancies on TMDs. Consecutive sulfur vacancies were successfully created on the basal plane of the bulk MoS2 and WS2, and their interlamellar distances were distinctly expanded after the SPR treatment (about 16%), which can be conveniently exfoliated by only gentle shaking. The S-vacancy significantly increases the hydrogen-evolution reaction activity of the MoS2 and WS2 nanosheets, with overpotential of -238 and -241 mV at 10 mA cm-2, respectively. We anticipate that our SPR strategy will supply a general platform for the development of TMD-based electrocatalysts for industrial water splitting and hydrogen production in the near future.

Tellurium (Te) is an intrinsically p-type doped narrow bandgap semiconductor with excellent electrical conductivity and low thermal conductivity. Bulk trigonal Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material with high value of thermoelectric figure-of-merit ZT. In view of the recent progress in developing synthesis route of two-dimensional (2D) tellurium thin films as well as the growing trend of exploiting nanostructures as thermoelectric devices, here for the first time we report excellent thermoelectric performance of tellurium nanofilms, with room temperature power factor of 31.7 {\mu}Wcm-1K-2 and ZT value of 0.63. To further enhance the efficiency of harvesting thermoelectric power in nanofilm devices, thermoelectrical current mapping was performed with a laser as a heating source, and we found high work function metals such as palladium can form rare accumulation-type metal-to-semiconductor contacts to 2D Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric 2D Te devices have broad applications as energy harvesting devices or nanoscale Peltier coolers in microsystems., Comment: manuscript+SI, 30 pages

Strain-gated flexible optoelectronics are reported based on monolayer MoS2 . Utilizing the piezoelectric polarization created at the metal-MoS2 interface to modulate the separation/transport of photogenerated carriers, the piezophototronic effect is applied to implement atomic-layer-thick phototransistor. Coupling between piezoelectricity and photogenerated carriers may enable the development of novel optoelectronics.

Due to its extremely high capacity, silicon (Si) is a potentially very useful anode material for lithium (Li) ion batteries. The key aim in the application of Si-based nanostructures to Li ion battery anodes is to improve their cyclic properties. To minimize volume expansion during the insertion and extraction of Li ions, we synthesized unique Si-iron (Fe) based nanomaterials connected by Ge nanostructures using chemical vapor deposition. The Si-Fe based new nanostructures showed a maximum capacity of about 689 mAh/g and stable cyclic properties. These properties are due to their unique nano-composite structures composed by Si and metals.

A light-self-induced pyro-phototronic effect in wurtzite ZnO nanowires is proposed as an effective approach to achieve ultrafast response ultraviolet sensing in p-Si/n-ZnO heterostructures. The relatively long response/recovery time of zinc-oxide-based ultraviolet sensors in air/vacuum has long been an obstacle to developing such detectors for practical applications. The response/recovery time and photoresponsivity are greatly improved by the pyro-phototronic effect.

Sb doped ZnO nanowires grown using the low-temperature hydrothermal method have the longest reported p-type stability of over 18 months. Using this growth system, bulk homojunction films of core-shell ZnO nanowires were synthesized with either n or p-type cores and the oppositely doped shell. Extensive transmission electron microscopy (TEM) characterization showed that the nanowires remain single crystalline, and the previously reported signs of doping remain intact. The electronic properties of these films were measured, and ultraviolet photodetection was observed. This growth technique could serve as the basis for other optoelectronic devices based on ZnO such as light emitting diodes and photovoltaics.

Quantum Hall effect (QHE) is a macroscopic manifestation of quantized states that only occurs in confined two-dimensional electron gas (2DEG) systems. Experimentally, QHE is hosted in high-mobility 2DEG with large external magnetic field at low temperature. Two-dimensional van der Waals materials, such as graphene and black phosphorus, are considered interesting material systems to study quantum transport because they could unveil unique host material properties due to the easy accessibility of monolayer or few-layer thin films at the 2D quantum limit. For the first time, we report direct observation of QHE in a novel low-dimensional material system, tellurene. High-quality 2D tellurene thin films were acquired from recently reported hydrothermal method with high hole mobility of nearly 3000 cm2/(V s) at low temperatures, which allows the observation of well-developed Shubnikov-de Haas (SdH) oscillations and QHE. A four-fold degeneracy of Landau levels in SdH oscillations and QHE was revealed. Quantum oscillations were investigated under different gate biases, tilted magnetic fields, and various temperatures, and the results manifest the inherent information on the electronic structure of Te. Anomalies in both temperature-dependent oscillation amplitudes and transport characteristics were observed that are ascribed to the interplay between the Zeeman effect and spin-orbit coupling, as depicted by the density functional theory calculations.

The economic production of wearable energy storage devices exhibiting mechanically-compliable form factors and reliable performance enable exciting opportunities in emerging technologies of consumer electronics, human-machine interface, and the Internet of Things. Here we report a hybrid scheme for designing and nanomanufacturing ultralight, high-performance supercapacitor electrodes through the hydrothermal growth of two-dimensional MnO2 on three-dimensional printed graphene aerogel (GA). The derived mixed-dimensional hierarchical macroporous composite electrodes exhibit superior specific capacitance and excellent cycling stability after thousands of cycles. We further explored the integration of a contact mode triboelectric nanogenerator and a mixed-dimensional MnO2/GA based supercapacitor into a self-powered system that is capable of converting mechanical signals into electrical power and further storing such scavenged power. The holistic integration of energy harvesting and storage units promises the implementation of self-powered wearable devices with greater intelligence that can scavenge and store environmental energy through sustainable pathways for ubiquitous electronics in societally-pervasive applications.

Due to the non-centrosymmetric crystal structures, wurtzite family semiconducting materials possess piezoelectric properties and exhibit polarizations along certain directions upon straining. Utilizing strain-induced piezoelectric polarization charges to modulate the energy band structures and thus to tune/control the transport processes of charge carriers is referred to as the piezotronic effect. Distinct from the previous studies of c-axis GaN nanowires, here we systematically study the piezotronic-effect-induced modifications of energy band structures and the corresponding influence on electronic transport properties of a-axis GaN nanobelts. The physical mechanism is carefully illustrated and further confirmed by theoretical simulations via finite element analysis. The spatial distributions of local carrier concentration and the energy band diagrams of a-axis GaN under various straining conditions are calculated. This work provides a thorough understanding of strain-gated transport properties of a-axis GaN piezotronic transistors and its future applications in semiconductor devices.

The coupling of piezoelectric and semiconducting properties gives rise to the effect of piezotronics, which regulates charge carrier transport through the modulation of energy barriers at contact interfaces. With piezoelectric semiconductors as the building blocks, extensive progress has been made, covering the fundamental physics level, the individual device level as well as the integrated system level, effectively establishing a new field of study. By manipulating interfacial processes incorporating ionic charges, free electrons/holes, photons and chemicals, novel interdisciplinary effects have been studied and reported. This article aims at reviewing the milestone progress and offering perspectives of this new field of study in applications for multi-functional sensing systems, human-electronics interfacing, MEMS, energy harvesting and so on.

Using polarization charges created at the metal-cadmium sulfide interface under strain to gate/modulate electrical transport and optoelectronic processes of charge carriers, the piezo-phototronic effect is applied to process mechanical and optical stimuli into electronic controlling signals. The cascade nanowire networks are demonstrated for achieving logic gates, binary computations, and gated D latches to store information carried by these stimuli.

Emerging applications in wearable technology, pervasive computing, human-machine interfacing, and implantable biomedical devices demand an appropriate power source that can sustainably operate for extended periods of time with minimal intervention (Wang, Z. L.; et al. Angew. Chem., Int. Ed. 2012, 51, 11700). Self-powered nanosystems, which harvest operating energy from its host (i.e., the human body), may be feasible due to their extremely low power consumption (Tian, B. Z.; et al. Nature 2007, 449, 885. Javey, A.; et al. Nature 2003, 424, 654. Cui, Y.; et al. Science 2001, 291, 851). Here we report materials and designs for wearable-on-skin piezoelectric devices based on ultrathin (2 μm) solution-derived ZnO p-n homojunction films for the first time. The depletion region formed at the p-n homojunction effectively reduces internal screening of strain-induced polarization charges by free carriers in both n-ZnO and Sb-doped p-ZnO, resulting in significantly enhanced piezoelectric output compared to a single layer device. The p-n structure can be further grown on polymeric substrates conformable to a human wrist and used to convert movement of the flexor tendons into distinguishable electrical signals for gesture recognition. The ZnO homojunction piezoelectric devices may have applications in powering nanodevices, bioprobes, and self-powered human-machine interfacing.

A practical self-powered velocity sensor based on the principles of a triboelectric generator for either rectified linear or rotary motion is presented. The effort represents the first successful attempt in integrating a triboelectric generator into a commercial digital circuit for the dual-mode speed sensing. Employing alternating Kapton-copper strips arranged in a spiral configuration wrapped on the inner and outer surfaces of two concentric cylinders, voltage assays for linear and rotary motions can be measured without the need for an external power source. The triboelectric generated output signals when integrated with a digital circuit and a microcontroller unit can be directly processed into remarkably stable, macro-scale output signals for measurements of (0.1−0.6) ms−1±0.5% for linear velocities and (300−700) rpm±0.9% for rotary velocities. We have also discussed the measuring sensitivities and limitations of our device in the paper. We believe our pioneering demonstration of the applied triboelectric technology will have a huge impact in the industrial commercialization of self-powered devices and sensors.

Recent advances achieved in triboelectric nanogenerators (TENG) focus on boosting power generation and conversion efficiency. Nevertheless, obstacles concerning economical and biocompatible utilization of TENGs continue to prevail. Being an abundant natural biopolymer from marine crustacean shells, chitosan enables exciting opportunities for low-cost, biodegradable TENG applications in related fields. Here, the development of biodegradable and flexible TENGs based on chitosan is presented for the first time. The physical and chemical properties of the chitosan nanocomposites are systematically studied and engineered for optimized triboelectric power generation, transforming the otherwise wasted natural materials into functional energy devices. The feasibility of laser processing of constituent materials is further explored for the first time for engineering the TENG performance. The laser treatment of biopolymer films offers a potentially promising scheme for surface engineering in polymer-based TENGs. The chitosan-based TENGs present efficient energy conversion performance and tunable biodegradation rate. Such a new class of TENGs derived from natural biomaterials may pave the way toward the economically viable and ecologically friendly production of flexible TENGs for self-powered nanosystems in biomedical and environmental applications.

Ongoing efforts in triboelectric nanogenerators (TENGs) focus on enhancing power generation, but obstacles concerning the economical and cost-effective production of TENGs continue to prevail. Micro-/nanostructure engineering of polymer surfaces has been dominantly utilized for boosting the contact triboelectrification, with deposited metal electrodes for collecting the scavenged energy. Nevertheless, this state-of-the-art approach is limited by the vague potential for producing 3D hierarchical surface structures with conformable coverage of high-quality metal. Laser-shock imprinting (LSI) is emerging as a potentially scalable approach for directly surface patterning of a wide range of metals with 3D nanoscale structures by design, benefiting from the ultrahigh-strain-rate forming process. Here, a TENG device is demonstrated with LSI-processed biomimetic hierarchically structured metal electrodes for efficient harvesting of water-drop energy in the environment. Mimicking and transferring hierarchical microstructures from natural templates, such as leaves, into these water-TENG devices is effective regarding repelling water drops from the device surface, since surface hydrophobicity from these biomicrostructures maximizes the TENG output. Among various leaves' microstructures, hierarchical microstructures from dried bamboo leaves are preferable regarding maximizing power output, which is attributed to their unique structures, containing both dense nanostructures and microscale features, compared with other types of leaves. Also, the triboelectric output is significantly improved by closely mimicking the hydrophobic nature of the leaves in the LSI-processed metal surface after functionalizing it with low-surface-energy self-assembled-monolayers. The approach opens doors to new manufacturable TENG technologies for economically feasible and ecologically friendly production of functional devices with directly patterned 3D biomimic metallic surfaces in energy, electronics, and sensor applications.

Selenium has attracted intensive attention as a promising material candidate for future optoelectronic applications. However, selenium has a strong tendency to grow into nanowire forms due to its anisotropic atomic structure, which has largely hindered the exploration of its potential applications. In this work, using a physical vapor deposition method, we have demonstrated the synthesis of large-size, high-quality 2D selenium nanosheets, the minimum thickness of which could be as thin as 5 nm. The Se nanosheet exhibits a strong in-plane anisotropic property, which is determined by angle-resolved Raman spectroscopy. Back-gating field-effect transistors based on a Se nanosheet exhibit p-type transport behaviors with on-state current density around 20 mA/mm at Vds = 3 V. Four-terminal field effect devices are also fabricated to evaluate the intrinsic hole mobility of the selenium nanosheet, and the value is determined to be 0.26 cm2 Vs at 300 K. The selenium nanosheet phototransistors show an excellent photoresponsivity of up to 263 A/W, with a rise time of 0.1 s and fall time of 0.12 s. These results suggest that crystal selenium as a 2D form of a 1D van der Waals solid opens up the possibility to explore device applications., Comment: ACS Nano, 2017, 11 (10), pp 10222

Experimental demonstrations of one-dimensional (1D) van der Waals material tellurium (Te) have been presented by Raman spectroscopy under strain and magneto-transport. Raman spectroscopy measurements have been performed under strains along different principle axes. Pronounced strain response along the c-axis is observed due to the strong intrachain covalent bonds, while no strain response is obtained along the a-axis due to the weak interchain van der Waals interaction. Magneto-transport results further verify its anisotropic property, which results in dramatically distinct magneto-resistance behaviors in terms of three different magnetic field directions. Specifically, phase coherence length extracted from weak antilocalization effect, Lϕ ≈ T–0.5, claims its two-dimensional (2D) transport characteristics when an applied magnetic field is perpendicular to the thin film. In contrast, Lϕ ≈ T–0.33 is obtained from universal conductance fluctuations once the magnetic field is along the c-axis of Te, which ind...

The recently introduced triboelectric nanogenerator (TENG) and the traditional electromagnetic induction generator (EMIG) are coherently integrated in one structure for energy harvesting and vibration sensing/isolation. The suspended structure is based on two oppositely oriented magnets that are enclosed by hollow cubes surrounded with coils, which oscillates in response to external disturbance and harvests mechanical energy simultaneously from triboelectrification and electromagnetic induction. It extends the previous definition of hybrid cell to harvest the same type of energy with multiple approaches. Both the sliding-mode TENG and contact-mode TENG can be achieved in the same structure. In order to make the TENG and EMIG work together, transformers are used to match the output impedance between these two power sources with very different characteristics. The maximum output power of 7.7 and 1.9 mW on the same load of 5 kΩ was obtained for the TENG and EMIG, respectively, after impedance matching. Benefiting from the rational design, the output signal from the TENG and the EMIG are in phase. They can be added up directly to get an output voltage of 4.6 V and an output current of 2.2 mA in parallel connection. A power management circuit was connected to the hybrid cell, and a regulated voltage of 3.3 V with constant current was achieved. For the first time, a logic operation was carried out on a half-adder circuit by using the hybrid cell working as both the power source and the input digit signals. We also demonstrated that the hybrid cell can serve as a vibration isolator. Further applications as vibration dampers, triggers, and sensors are all promising.

Summary Due to the polarization of ions in a crystal that has non-central symmetry in piezoelectric-semiconductors such as ZnO, GaN and InN, piezoelectric polarization charges are created at the interface region by applying a strain, which created a potential (piezopotential) in the crystal. Piezotronics is about the devices fabricated using the piezopotential as a “gate” voltage to tune/control charge carrier transport at a contact or junction. The piezo-phototronic effect is to use the piezopotential to control the carrier generation, transport, separation and/or recombination for improving the performance of optoelectronic devices, such as solar cell and LED. Starting from the fundamental physics principles, this article gives an updated review about the fabrication of array of piezotronic devices so that they can be integrated into a system for achieving specific functions as sensors and MEMS. This first demonstration of fabricating array of piezotronic devices and integrated them into a system is a major milestone in nanotechnology, and it represents one of the promising directions of nanoelectronics.