Highly Sensitive and Fast Responding Flexible Force Sensors Using ZnO/ZnMgO Coaxial Nanotubes on Graphene Layers for Breath Sensing

The authors report the fabrication of highly sensitive, rapidly responding flexible force sensors using ZnO/ZnMgO coaxial nanotubes grown on graphene layers and their applications in sleep apnea monitoring. Flexible force sensors are fabricated by forming Schottky contacts to the nanotube array, followed by the mechanical release of the entire structure from the host substrate. The electrical characteristics of ZnO and ZnO/ZnMgO nanotube-based sensors are thoroughly investigated and compared. Importantly, in force sensor applications, the ZnO/ZnMgO coaxial structure results in significantly higher sensitivity and a faster response time when compared to the bare ZnO nanotube. The origin of the improved performance is thoroughly discussed. Furthermore, wireless breath sensing is demonstrated using the ZnO/ZnMgO pressure sensors with custom electronics, demonstrating the feasibility of the sensor technology for health monitoring and the potential diagnosis of sleep apnea.

Piezotronic Transistors Based on GaN Wafer for Highly Sensitive Pressure Sensing with High Linearity and High Stability

Piezotronic effect utilizing strain-induced piezoelectric polarization to achieve interfacial engineering in semiconductor nanodevices exhibits great advantages in applications such as human-machine interfacing, micro/nanoelectromechanical systems, and next-generation sensors and transducers. However, it is a big challenge but highly desired to develop a highly sensitive piezotronic device based on piezoelectric semiconductor wafers and thus to push piezotronics toward wafer-scale applications. Here, we develop a bicrystal barrier-based piezotronic transistor for highly sensitive pressure sensing by p-GaN single-crystal wafers. Its pressure sensitivity can be as high as 19.83 meV/MPa, which is more than 15 times higher than previous bulk-material-based piezotronic transistors and reaches the level of nanomaterial-based piezotronic transistors. Moreover, it can respond to a very small strain of 3.3 × 10-6 to 1.1 × 10-5 with high gauge factors of 1.45 × 105 to 1.38 × 106, which is a very high value among various strain sensors. Additionally, it also exhibits high stability (current stability of 97.32 ± 2.05% and barrier height change stability of 95.85 ± 3.43%) and high linearity (R2 ∼ 0.997 ± 0.002) in pressure sensing. This work proves the possibility of designing a bicrystal barrier as the interface to obtain a strong piezotronic effect and highly sensitive piezotronic devices based on wafers, which contributes to their applications.

A Novel Hollow Graphene/Polydimethylsiloxane Composite for Pressure Sensors with High Sensitivity and Superhydrophobicity

Flexible pressure sensors have attracted great interest as they play an important role in various fields such as health monitoring and human-machine interactions. The design of the pressure sensors still faces challenges in achieving a high sensitivity for a wide sensing range, and the interference of water restricts the applications of the sensors. Herein, we developed a graphene-polydimethylsiloxane film combining a hierarchical surface with nanowrinkles on it and a hollow structure. The microstructure design of the composite can be facilely controlled to improve the sensing and hydrophobic performance by tailoring the microsphere building units. Attributed to the irregular surface and hollow structure of the sensing layer, the optimized sensor exhibits a superior sensitivity of 1085 kPa-1 in a 50 kPa linear range. For practical applications, the nanowrinkles on the surface of the microspheres and the polymer coating endow the composite with waterproof properties. Inspired by the dual receptors of the skin, two designed microstructured films can simply integrate into one with double-sided microstructures. The sensing performance and the water-repellence property allow the sensor to detect physiological signals under both ambient and underwater conditions. Furthermore, underwater stimuli detection and communication are demonstrated. This method of fabricating a flexible sensor shows great potential in wearable and robotic fields.

Electric pulse-tuned piezotronic effect for interface engineering

Investigating interface engineering by piezoelectric, flexoelectric and ferroelectric polarizations in semiconductor devices is important for their applications in electronics, optoelectronics, catalysis and many more. The interface engineering by polarizations strongly depends on the property of interface barrier. However, the fixed value and uncontrollability of interface barrier once it is constructed limit the performance and application scenarios of interface engineering by polarizations. Here, we report a strategy of tuning piezotronic effect (interface barrier and transport controlled by piezoelectric polarization) reversibly and accurately by electric pulse. Our results show that for Ag/HfO2/n-ZnO piezotronic tunneling junction, the interface barrier height can be reversibly tuned as high as 168.11 meV by electric pulse, and the strain (0-1.34‰) modulated current range by piezotronic effect can be switched from 0-18 nA to 44-72 nA. Moreover, piezotronic modification on interface barrier tuned by electric pulse can be up to 148.81 meV under a strain of 1.34‰, which can totally switch the piezotronic performance of the electronics. This study provides opportunities to achieve reversible control of piezotronics, and extend them to a wider range of scenarios and be better suitable for micro/nano-electromechanical systems.

Low-dimensional nanostructures for monolithic 3D-integrated flexible and stretchable electronics

Flexible/stretchable electronics, which are characterized by their ultrathin design, lightweight structure, and excellent mechanical robustness and conformability, have garnered significant attention due to their unprecedented potential in healthcare, advanced robotics, and human-machine interface technologies. An increasing number of low-dimensional nanostructures with exceptional mechanical, electronic, and/or optical properties are being developed for flexible/stretchable electronics to fulfill the functional and application requirements of information sensing, processing, and interactive loops. Compared to the traditional single-layer format, which has a restricted design space, a monolithic three-dimensional (M3D) integrated device architecture offers greater flexibility and stretchability for electronic devices, achieving a high-level of integration to accommodate the state-of-the-art design targets, such as skin-comfort, miniaturization, and multi-functionality. Low-dimensional nanostructures possess small size, unique characteristics, flexible/elastic adaptability, and effective vertical stacking capability, boosting the advancement of M3D-integrated flexible/stretchable systems. In this review, we provide a summary of the typical low-dimensional nanostructures found in semiconductor, interconnect, and substrate materials, and discuss the design rules of flexible/stretchable devices for intelligent sensing and data processing. Furthermore, artificial sensory systems in 3D integration have been reviewed, highlighting the advancements in flexible/stretchable electronics that are deployed with high-density, energy-efficiency, and multi-functionalities. Finally, we discuss the technical challenges and advanced methodologies involved in the design and optimization of low-dimensional nanostructures, to achieve monolithic 3D-integrated flexible/stretchable multi-sensory systems.

Robust and sensitive conductive nanocomposite hydrogel with bridge cross-linking-dominated hierarchical structural design

Conductive hydrogels have a remarkable potential for applications in soft electronics and robotics, owing to their noteworthy attributes, including electrical conductivity, stretchability, biocompatibility, etc. However, the limited strength and toughness of these hydrogels have traditionally impeded their practical implementation. Inspired by the hierarchical architecture of high-performance biological composites found in nature, we successfully fabricate a robust and sensitive conductive nanocomposite hydrogel through self-assembly-induced bridge cross-linking of MgB2 nanosheets and polyvinyl alcohol hydrogels. By combining the hierarchical lamellar microstructure with robust molecular B─O─C covalent bonds, the resulting conductive hydrogel exhibits an exceptional strength and toughness. Moreover, the hydrogel demonstrates exceptional sensitivity (response/relaxation time, 20 milliseconds; detection lower limit, ~1 Pascal) under external deformation. Such characteristics enable the conductive hydrogel to exhibit superior performance in soft sensing applications. This study introduces a high-performance conductive hydrogel and opens up exciting possibilities for the development of soft electronics.

Applications of flexible electronics related to cardiocerebral vascular system

Ensuring accessible and high-quality healthcare worldwide requires field-deployable and affordable clinical diagnostic tools with high performance. In recent years, flexible electronics with wearable and implantable capabilities have garnered significant attention from researchers, which functioned as vital clinical diagnostic-assisted tools by real-time signal transmission from interested targets in vivo. As the most crucial and complex system of human body, cardiocerebral vascular system together with heart-brain network attracts researchers inputting profuse and indefatigable efforts on proper flexible electronics design and materials selection, trying to overcome the impassable gulf between vivid organisms and rigid inorganic units. This article reviews recent breakthroughs in flexible electronics specifically applied to cardiocerebral vascular system and heart-brain network. Relevant sensor types and working principles, electronics materials selection and treatment methods are expounded. Applications of flexible electronics related to these interested organs and systems are specially highlighted. Through precedent great working studies, we conclude their merits and point out some limitations in this emerging field, thus will help to pave the way for revolutionary flexible electronics and diagnosis assisted tools development.

Printing conformal and flexible copper networks for multimodal pressure and flow sensing

Flexible multimodal sensors with ultrasensitive detection capabilities are an indispensable component of wearable electronics and are highly sought-after involving a wide range of signal monitoring such as artificial skin and soft robotics. Here we report a flexible and wireless multimodal sensor using low-temperature additive manufacturing of copper nanoplates on elastic polyurethane substrates for temperature, pressure, and flow monitoring. The positive temperature coefficient and piezoresistive performance of the copper nanoplate network translates to a reliable temperature, steady-state and dynamic pressure/flow sensing for detecting pressures as small as 0.64 Pa with a response time of 130 ms, as well as velocity detection ranging from 2.5-6.8 m s-1. Additionally, by incorporating a printed antenna, it enables a self-powered, battery-free system, offering a wireless readout of printed multimodal sensors with superior real-time sensing performance in conjunction with wearable flexibility.

A high-performance dual-functional organic upconversion device with detectivity approaching 1013 Jones and photon-to-photon efficiency over 20

Organic upconversion devices (UCDs) are a cutting-edge technology and hot topic because of their advantages of low cost and convenience in the important applications of near-infrared (NIR) detection and imaging. However, to realize utilization of triplet excitons (T1), previous UCDs have the drawback of heavily relying on toxic and costly heavy-metal-doped emitters. More importantly, due to poor performance of the detecting unit and/or emitting unit, improving their detectivity (D*) and photon-to-photon conversion efficiency (ηp-p) is still a challenge for real applications. Here, we report a high-performance dual-functional purely organic UCD that has an outstanding D* approaching 1013 Jones and a high ηp-p of 20.1% in the NIR region, which are some of the highest values among those reported for UCDs. The high performance is credited to the excellent D* of the detecting unit, exceeding 1014 Jones, and is also attributed to efficient T1 utilization via a dual reverse intersystem crossing channel and high optical out coupling achieved via a high horizontal dipole ratio in the emitting unit. The high D* and ηp-p enable the UCD to detect 850 nm light at as little as 0.29 μW cm-2 and with a high display contrast of over 70 000 : 1, significantly improving the potential of practical applications of UCDs in NIR detection and imaging. Furthermore, a fast rise time and fall time of 8.9 and 14.8 μs are also achieved. Benefiting from the high performance, consequent applications of low-power pulse-state monitoring and fine-structure bio-imaging are successfully realized with high quality results by using our organic UCDs. These results demonstrate that our design not only eliminates dependence of UCDs on heavy-metal emitters, but also takes their performance and applications to a high level.

A Selective-Response Hypersensitive Bio-Inspired Strain Sensor Enabled by Hysteresis Effect and Parallel Through-Slits Structures

Flexible strain sensors are promising in sensing minuscule mechanical signals, and thereby widely used in various advanced fields. However, the effective integration of hypersensitivity and highly selective response into one flexible strain sensor remains a huge challenge. Herein, inspired by the hysteresis strategy of the scorpion slit receptor, a bio-inspired flexible strain sensor (BFSS) with parallel through-slit arrays is designed and fabricated. Specifically, BFSS consists of conductive monolayer graphene and viscoelastic styrene-isoprene-styrene block copolymer. Under the synergistic effect of the bio-inspired slit structures and flexible viscoelastic materials, BFSS can achieve both hypersensitivity and highly selective frequency response. Remarkably, the BFSS exhibits a high gage factor of 657.36, and a precise identification of vibration frequencies at a resolution of 0.2 Hz through undergoing different morphological changes to high-frequency vibration and low-frequency vibration. Moreover, the BFSS possesses a wide frequency detection range (103 Hz) and stable durability (1000 cycles). It can sense and recognize vibration signals with different characteristics, including the frequency, amplitude, and waveform. This work, which turns the hysteresis effect into a "treasure," can provide new design ideas for sensors for potential applications including human-computer interaction and health monitoring of mechanical equipment.

Knittle Pressure Sensor Based on Graphene/Polyvinylidene Fluoride Nanocomposite Coated on Polyester Fabric

In this paper, a knittle pressure sensor was designed and fabricated by coating graphene/Polyvinylidene Fluoride nanocomposite on the knitted polyester substrate. The coating was carried out by a dip-coating method in a nanocomposite solution. The microstructure, surface properties and electrical properties of coated layers were investigated. The sensors were tested under the application of different pressures, and the corresponding sensor signals were analyzed in terms of resistance change. It was observed that the change in resistance was 55% kPa-1 with a sensitivity limit of 0.25 kPa. The sensor model was created and simulated using COMSOL Multiphysics software, and the model data were favorably compared with the experimental results. This investigation suggests that graphene-based nanocomposites can be used in knittle pressure sensor applications.

Self-Powered Sensing in Wearable Electronics─A Paradigm Shift Technology

With the advancements in materials science and micro/nanoengineering, the field of wearable electronics has experienced a rapid growth and significantly impacted and transformed various aspects of daily human life. These devices enable individuals to conveniently access health assessments without visiting hospitals and provide continuous, detailed monitoring to create comprehensive health data sets for physicians to analyze and diagnose. Nonetheless, several challenges continue to hinder the practical application of wearable electronics, such as skin compliance, biocompatibility, stability, and power supply. In this review, we address the power supply issue and examine recent innovative self-powered technologies for wearable electronics. Specifically, we explore self-powered sensors and self-powered systems, the two primary strategies employed in this field. The former emphasizes the integration of nanogenerator devices as sensing units, thereby reducing overall system power consumption, while the latter focuses on utilizing nanogenerator devices as power sources to drive the entire sensing system. Finally, we present the future challenges and perspectives for self-powered wearable electronics.

Dual-modal piezotronic transistor for highly sensitive vertical force sensing and lateral strain sensing

Mechanical sensors are mainly divided into two types (vertical force sensing and lateral strain sensing). Up to now, one sensor with two working modes is still a challenge. Here, we demonstrate a structural design concept combing a piezoelectric nano/microwire with a flexible polymer with protrusions that enables a dual-modal piezotronic transistor (DPT) with two working modes for highly sensitive vertical force sensing and lateral strain sensing. For vertical force sensing, DPT exhibits a force sensitivity up to 221.5 N-1 and a minimum identifiable force down to 21 mN, corresponding to a pressure sensitivity of 1.759 eV/MPa. For lateral strain sensing, DPT can respond to a large compression strain (~5.8%) with an on/off ratio up to 386.57 and a gauge factor up to 8988.6. It is a universal design that can integrate vertical force sensing and lateral strain sensing into only one nanodevice, providing a feasible strategy for multimodal devices.

Design, Fabrication, and Characterization of Inkjet-Printed Organic Piezoresistive Tactile Sensor on Flexible Substrate

In this paper, we propose a novel tactile sensor with a "fingerprint" design, named due to its spiral shape and dimensions of 3.80 mm × 3.80 mm. The sensor is duplicated in a four-by-four array containing 16 tactile sensors to form a "SkinCell" pad of approximately 45 mm by 29 mm. The SkinCell was fabricated using a custom-built microfabrication platform called the NeXus which contains additive deposition tools and several robotic systems. We used the NeXus' six-degrees-of-freedom robotic platform with two different inkjet printers to deposit a conductive silver ink sensor electrode as well as the organic piezoresistive polymer PEDOT:PSS-Poly (3,4-ethylene dioxythiophene)-poly(styrene sulfonate) of our tactile sensor. Printing deposition profiles of 100-micron- and 250-micron-thick layers were measured using microscopy. The resulting structure was sintered in an oven and laminated. The lamination consisted of two different sensor sheets placed back-to-back to create a half-Wheatstone-bridge configuration, doubling the sensitivity and accomplishing temperature compensation. The resulting sensor array was then sandwiched between two layers of silicone elastomer that had protrusions and inner cavities to concentrate stresses and strains and increase the detection resolution. Furthermore, the tactile sensor was characterized under static and dynamic force loading. Over 180,000 cycles of indentation were conducted to establish its durability and repeatability. The results demonstrate that the SkinCell has an average spatial resolution of 0.827 mm, an average sensitivity of 0.328 mΩ/Ω/N, expressed as the change in resistance per force in Newtons, an average sensitivity of 1.795 µV/N at a loading pressure of 2.365 PSI, and a dynamic response time constant of 63 ms which make it suitable for both large area skins and fingertip human-robot interaction applications.

Low-Profile, Large-Range Compressive Strain Sensing Using Micromanufactured CNT Micropillar Arrays

Tactile sensors, or sensors that collect measurements through touch, have versatile applications in a wide range of fields including robotic gripping, intelligent manufacturing, and biomedical technology. Hoping to match the ability of human hands to sense physical changes in objects through touch, engineers have experimented with a variety of materials from soft polymers to hard ceramics, but so far, all have fallen short. A grand challenge for developers of "human-like" bionic tactile sensors is to be able to sense a wide range of strains while maintaining the low profile necessary for compact integration. Here, we developed a low-profile tactile sensor (∼300 μm in height) based on patterned, vertically aligned carbon nanotubes (PVACNT) that can repetitively sense compressive strains of up to 75%. Upon compression, reversible changes occur in the points of contact between CNTs, producing measurable changes in electrical admittance. By patterning VACNT pillars with different aspect ratios and pitch sizes, we engineered the range and resolution of strain sensing, suggesting that CNT-based tactile sensors can be integrated according to device specifications.

Flexible Microstructured Capacitive Pressure Sensors Using Laser Engraving and Graphitization from Natural Wood

In recent years, laser engraving has received widespread attention as a convenient, efficient, and programmable method which has enabled high-quality porous graphene to be obtained from various precursors. Laser engraving is often used to fabricate the dielectric layer with a microstructure for capacitive pressure sensors; however, the usual choice of electrodes remains poorly flexible metal electrodes, which greatly limit the overall flexibility of the sensors. In this work, we propose a flexible capacitive pressure sensor made entirely of thermoplastic polyurethane (TPU) and laser-induced graphene (LIG) derived from wood. The capacitive pressure sensor consisted of a flexible LIG/TPU electrode (LTE), an LIG/TPU electrode with a microhole array, and a dielectric layer of TPU with microcone array molded from a laser-engraved hole array on wood, which provided high sensitivity (0.11 kPa^-1), an ultrawide pressure detection range (20 Pa to 1.4 MPa), a fast response (~300 ms), and good stability (>4000 cycles, at 0-35 kPa). We believe that our research makes a significant contribution to the literature, because the easy availability of the materials derived from wood and the overall consistent flexibility meet the requirements of flexible electronic devices.

Recent progress in flexible micro-pressure sensors for wearable health monitoring

In recent years, flexible micro-pressure sensors have been used widely in wearable health monitoring applications due to their excellent flexibility, stretchability, non-invasiveness, comfort wearing and real-time detection. According to the working mechanism of the flexible micro-pressure sensor, it can be classified as piezoresistive, piezoelectric, capacitive and triboelectric types. Herein, an overview of flexible micro-pressure sensors for wearable health monitoring is presented. The physiological signaling and body motions contain a lot of health status information. Thus, this review focuses on the applications of flexible micro-pressure sensors in these fields. Additionally, the contents of sensing mechanism, sensing materials and performance of flexible micro-pressure sensors are introduced in detail. Finally, we predict the future research directions of the flexible micro-pressure sensors, and discuss the challenges in practical applications.

Recent Advances in Polymer Composites for Flexible Pressure Sensors

Pressure sensors show significant potential applications in health monitoring, bio-sensing, electronic skin, and tactile perception. Consequently, tremendous research interest has been devoted to the development of high-performance pressure sensors. In this paper, recent progress on the polymer composite-based flexible pressure sensor is reviewed. The parameters of pressure sensors, including sensitivity, linear response range, detection limit, response speed, and reliability, are first introduced. Secondly, representative types of pressure sensors and relevant working principles are introduced and discussed. After that, the applications in human physiology monitoring, health monitoring, artificial skin, and self-powered smart system are listed and discussed in detail. Finally, the remaining challenges and outlook of polymer composite-based flexible sensors are summarized at the end of this review paper. This work should have some impact on the development of high-performance flexible pressure sensors.

Emerging 2D Metal Oxides: From Synthesis to Device Integration

2D metal oxides have aroused increasing attention in the field of electronics and optoelectronics due to their intriguing physical properties. In this review, an overview of recent advances on synthesis of 2D metal oxides and their electronic applications is presented. First, the tunable physical properties of 2D metal oxides that relate to the structure (various oxidation-state forms, polymorphism, etc.), crystallinity and defects (anisotropy, point defects, and grain boundary), and thickness (quantum confinement effect, interfacial effect, etc.) are discussed. Then, advanced synthesis methods for 2D metal oxides besides mechanical exfoliation are introduced and classified into solution process, vapor-phase deposition, and native oxidation on a metal source. Later, the various roles of 2D metal oxides in widespread applications, i.e., transistors, inverters, photodetectors, piezotronics, memristors, and potential applications (solar cell, spintronics, and superconducting devices) are discussed. Finally, an outlook of existing challenges and future opportunities in 2D metal oxides is proposed.

A Flexible and Highly Sensitive Pressure Sense Electrode Based on Cotton Pulp for Wearable Electronics

Flexible pressure sensors with high sensitivity have great potential applications in wearable electronics. However, it is still a great challenge to prepare sense electrodes with high flexibility, high sensitivity, and high electrochemical performance. Here, we propose a novel and simple method for carbonizing cotton fibers as excellent electrically conductive materials. Moreover, carbonized cotton fiber (CCF) and polydimethylsiloxane (PDMS) were assembled into a flexible sense electrode. The CCF/PDMS electrode shows a high sensitivity of 10.8 kPa^-1, a wide response frequency from 0.2-2.0 Hz, and durability over 900 cycles. The combined CCF/PDMS sensors can monitor human movement and pulse vibration, showing the enormous potential for use in wearable device technology. Additionally, the CCF/PDMS can be used as electrodes with a specific capacitance of 332.5 mF cm^-2 at a current density of 5 mA cm^-2, thanks to their high electrical conductivity and hydrophilicity, demonstrating the promising prospect of flexible supercapacitors.

Vertical strain engineering of Van der Waals heterostructures

Van der Waals materials and their interfaces play critical roles in defining electrical contacts for nanoelectronics and developing vehicles for mechanoelectrical energy conversion. In this work, we propose a vertical strain engineering approach by enforcing pressure across the heterostructures. First-principles calculations show that the in-plane band structures of 2D materials such as graphene, h-BN, and MoS₂as well as the electronic coupling at their contacts can be significantly modified. For the graphene/h-BN contact, a band gap in graphene is opened, while at the graphene/MoS₂interface, the band gap of MoS₂and the Schottky barrier height at contact diminish. Changes and transitions in the nature of contacts are attributed to localized orbital coupling and analyzed through the redistribution of charge densities, the crystal orbital Hamilton population, and electron localization, which yield consistent measures. These findings offer key insights into the understanding of interfacial interaction between 2D materials as well as the efficiency of electronic transport and energy conversion processes.

2D Zinc Oxide - Synthesis, Methodologies, Reaction Mechanism, and Applications

Zinc oxide (ZnO) is a thermally stable n-type semiconducting material. ZnO 2D nanosheets have mainly gained substantial attention due to their unique properties, such as direct bandgap and strong excitonic binding energy at room temperature. These are widely utilized in piezotronics, energy storage, photodetectors, light-emitting diodes, solar cells, gas sensors, and photocatalysis. Notably, the chemical properties and performances of ZnO nanosheets largely depend on the nano-structuring that can be regulated and controlled through modulating synthetic strategies. Two synthetic approaches, top-down and bottom-up, are mainly employed for preparing ZnO 2D nanomaterials. However, owing to better results in producing defect-free nanostructures, homogenous chemical composition, etc., the bottom-up approach is extensively used compared to the top-down method for preparing ZnO 2D nanosheets. This review presents a comprehensive study on designing and developing 2D ZnO nanomaterials, followed by accenting its potential applications. To begin with, various synthetic strategies and attributes of ZnO 2D nanosheets are discussed, followed by focusing on methodologies and reaction mechanisms. Then, their deliberation toward batteries, supercapacitors, electronics/optoelectronics, photocatalysis, sensing, and piezoelectronic platforms are further discussed. Finally, the challenges and future opportunities are featured based on its current development.

Silicon flexoelectronic transistors

It is extraordinarily challenging to implement adaptive and seamless interactions between mechanical triggering and current silicon technology for tunable electronics, human-machine interfaces, and micro/nanoelectromechanical systems. Here, we report Si flexoelectronic transistors (SFTs) that can innovatively convert applied mechanical actuations into electrical control signals and achieve directly electromechanical function. Using the strain gradient-induced flexoelectric polarization field in Si as a "gate," the metal-semiconductor interfacial Schottky barriers' heights and the channel width of SFT can be substantially modulated, resulting in tunable electronic transports with specific characteristics. Such SFTs and corresponding perception system can not only create a high strain sensitivity but also identify where the mechanical force is applied. These findings provide an in-depth understanding about the mechanism of interface gating and channel width gating in flexoelectronics and develop highly sensitive silicon-based strain sensors, which has great potential to construct the next-generation silicon electromechanical nanodevices and nanosystems.

Progress in Microtopography Optimization of Polymers-Based Pressure/Strain Sensors

Due to the wide application of wearable electronic devices in daily life, research into flexible electronics has become very attractive. Recently, various polymer-based sensors have emerged with great sensing performance and excellent extensibility. It is well known that different structural designs each confer their own unique, great impacts on the properties of materials. For polymer-based pressure/strain sensors, different structural designs determine different response-sensing mechanisms, thus showing their unique advantages and characteristics. This paper mainly focuses on polymer-based pressure-sensing materials applied in different microstructures and reviews their respective advantages. At the same time, polymer-based pressure sensors with different microstructures, including with respect to their working mechanisms, key parameters, and relevant operating ranges, are discussed in detail. According to the summary of its performance and mechanisms, different morphologies of microstructures can be designed for a sensor according to its performance characteristics and application scenario requirements, and the optimal structure can be adjusted by weighing and comparing sensor performances for the future. Finally, a conclusion and future perspectives are described.

Carbon-Based Flexible Devices for Comprehensive Health Monitoring

Traditional public health systems suffer from incomprehensive, delayed, and inefficient medical services. Convenient and comprehensive health monitoring has been highly sought after recently. Flexible and wearable devices are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Using carbon materials with overall superiorities can facilitate the development of wearable and flexible devices with various functions and excellent performance, which can comprehensively and real-time monitor human health status and prevent diseases. Herein, the latest advances in the rational design and controlled fabrication of carbon materials for applications in health-related flexible and wearable electronics are reviewed. The fabrication strategies, working mechanism, performance, and applications in health monitoring of carbon-based flexible devices, including electromechanical sensors, temperature/humidity sensors, chemical sensors, and flexible conductive wires/electrodes, are reviewed. Furthermore, integrating multiple carbon-based devices into multifunctional wearable systems is discussed. Finally, the existing challenges and future opportunities in this field are also proposed.

A Flexible Pressure Sensor Based on Silicon Nanomembrane

With advances in new materials and technologies, there has been increasing research focused on flexible sensors. However, in most flexible pressure sensors made using new materials, it is challenging to achieve high detection sensitivity across a wide pressure range. Although traditional silicon-based sensors have good performance, they are not formable and, because of their rigidity and brittleness, they are not suitable for fitting with soft human skin, which limits their application in wearable devices to collect various signals. Silicon nanomembranes are ultra-thin, flexible materials with excellent piezoresistive properties, and they can be applied in various fields, such as in soft robots and flexible devices. In this study, we developed a flexible pressure sensor based on the use of silicon nanomembranes (with a thickness of only 340 nm) as piezoresistive units, which were transferred onto a flexible polydimethylsiloxane (PDMS) substrate. The flexible pressure sensor operated normally in the range of 0-200 kPa, and the sensitivity of the sensor reached 0.0185 kPa^-1 in the low-pressure range of 0-5 kPa. In the high-pressure range of 5-200 kPa, the sensitivity of the sensor was maintained at 0.0023 kPa^-1. The proposed sensor exhibited a fast response and excellent long-term stability and could recognize human movements, such as the bending of fingers and wrist joints, while maintaining a stable output. Thus, the developed flexible pressure sensor has promising applications in body monitoring and wearable devices.

Fundamentals and Applications of ZnO-Nanowire-Based Piezotronics and Piezo-Phototronics

The piezotronic effect is a coupling effect of semiconductor and piezoelectric properties. The piezoelectric potential is used to adjust the p-n junction barrier width and Schottky barrier height to control carrier transportation. At present, it has been applied in the fields of sensors, human-machine interaction, and active flexible electronic devices. The piezo-phototronic effect is a three-field coupling effect of semiconductor, photoexcitation, and piezoelectric properties. The piezoelectric potential generated by the applied strain in the piezoelectric semiconductor controls the generation, transport, separation, and recombination of carriers at the metal-semiconductor contact or p-n junction interface, thereby improving optoelectronic devices performance, such as photodetectors, solar cells, and light-emitting diodes (LED). Since then, the piezotronics and piezo-phototronic effects have attracted vast research interest due to their ability to remarkably enhance the performance of electronic and optoelectronic devices. Meanwhile, ZnO has become an ideal material for studying the piezotronic and piezo-phototronic effects due to its simple preparation process and better biocompatibility. In this review, first, the preparation methods and structural characteristics of ZnO nanowires (NWs) with different doping types were summarized. Then, the theoretical basis of the piezotronic effect and its application in the fields of sensors, biochemistry, energy harvesting, and logic operations (based on piezoelectric transistors) were reviewed. Next, the piezo-phototronic effect in the performance of photodetectors, solar cells, and LEDs was also summarized and analyzed. In addition, modulation of the piezotronic and piezo-phototronic effects was compared and summarized for different materials, structural designs, performance characteristics, and working mechanisms' analysis. This comprehensive review provides fundamental theoretical and applied guidance for future research directions in piezotronics and piezo-phototronics for optoelectronic devices and energy harvesting.

Application of Piezoelectric Material and Devices in Bone Regeneration

Bone injuries are common in clinical practice. Given the clear disadvantages of autologous bone grafting, more efficient and safer bone grafts need to be developed. Bone is a multidirectional and anisotropic piezoelectric material that exhibits an electrical microenvironment; therefore, electrical signals play a very important role in the process of bone repair, which can effectively promote osteoblast differentiation, migration, and bone regeneration. Piezoelectric materials can generate electricity under mechanical stress without requiring an external power supply; therefore, using it as a bone implant capable of harnessing the body's kinetic energy to generate the electrical signals needed for bone growth is very promising for bone regeneration. At the same time, devices composed of piezoelectric material using electromechanical conversion technology can effectively monitor the structural health of bone, which facilitates the adjustment of the treatment plan at any time. In this paper, the mechanism and classification of piezoelectric materials and their applications in the cell, tissue, sensing, and repair indicator monitoring aspects in the process of bone regeneration are systematically reviewed.

Polymer-like Inorganic Double Helical van der Waals Semiconductor

In high-performance flexible and stretchable electronic devices, conventional inorganic semiconductors made of rigid and brittle materials typically need to be configured into geometrically deformable formats and integrated with elastomeric substrates, which leads to challenges in scaling down device dimensions and complexities in device fabrication and integration. Here we report the extraordinary mechanical properties of the newly discovered inorganic double helical semiconductor tin indium phosphate. This spiral-shape double helical crystal shows the lowest Young's modulus (13.6 GPa) among all known stable inorganic materials. The large elastic (>27%) and plastic (>60%) bending strains are also observed and attributed to the easy slippage between neighboring double helices that are coupled through van der Waals interactions, leading to the high flexibility and deformability among known semiconducting materials. The results advance the fundamental understanding of the unique polymer-like mechanical properties and lay the foundation for their potential applications in flexible electronics and nanomechanics disciplines.

Mixed Dimensional ZnO/WSe2 Piezo-gated Transistor with Active Millinewton Force Sensing

This work demonstrates a mixed-dimensional piezoelectric-gated transistor in the microscale that could be used as a millinewton force sensor. The force-sensing transistor consists of 1D piezoelectric zinc oxide (ZnO) nanorods (NRs) as the gate control and multilayer tungsten diselenide (WSe₂) as the transistor channel. The applied mechanical force on piezoelectric NRs can induce a drain-source current change (ΔI_ds) on the WSe₂ channel. The different doping types of the WSe₂ channel have been found to lead to different directions of ΔI_ds. The pressure from the calibration weight of 5 g has been observed to result in an ∼30% I_ds change for ZnO NRs on the p-type doped WSe₂ device and an ∼-10% I_ds change for the device with an n-type doped WSe₂. The outcome of this work would be useful for applications in future human-machine interfaces and smart biomedical tools.

A warm hug from a robot: A dual-mode e-skin with programming compliance

Recent achievements in the field of electronic skin (e-skin) have provided promising technology for service robots. However, the development of a bionic perception system that exhibits superior performance in terms of safety and interaction quality remains a challenge. Here, we demonstrate a biomimetic soft e-skin that is composed of an array of capacitors and air pouches. It is a single platform that shows dual-mode sensing capabilities of tactile sensing and proximity perception. We optimized the shape and area of the electrode via simulation of the approach of a robot to an object. Moreover, the compliance and temperature of the e-skin can be actively adjusted by tuning the pressure and heat of the air inside the pouches. The e-skin provided dual-mode sensing feedback and soft touch for humanoid service robots, for example, when a robot hugged a man, which illustrated the potential of this e-skin for applications in human-robot interactions.

Roles of Low-Dimensional Nanomaterials in Pursuing Human-Machine-Thing Natural Interaction

A wide variety of low-dimensional nanomaterials with excellent properties can meet almost all the requirements of functional materials for information sensing, processing, and feedback devices. Low-dimensional nanomaterials are becoming the star of hope on the road to pursuing human-machine-thing natural interactions, benefiting from the breakthroughs in precise preparation, performance regulation, structural design, and device construction in recent years. This review summarizes several types of low-dimensional nanomaterials commonly used in human-machine-thing natural interactions and outlines the differences in properties and application areas of different materials. According to the sequence of information flow in the human-machine-thing interaction process, the representative research progress of low-dimensional nanomaterials-based information sensing, processing, and feedback devices is reviewed and the key roles played by low-dimensional nanomaterials are discussed. Finally, the development trends and existing challenges of low-dimensional nanomaterials in the field of human-machine-thing natural interaction technology are discussed.

Nanoscale mapping of surface strain in tapered nanorods using confocal photoluminescence spectroscopy

The strain occurs spontaneously at the heterogeneous interfaces of virtually all crystalline materials. Consequently, the analysis across multiple interfaces requires a complementary characterization scheme with a resolution that fits the deformation scale. By implementing two-photon confocal laser scanning nanoscopy with an axial resolution of 10 nm, we extract the surface strain from the photoluminescence (PL) spectra, epitomized by a 2-fold enhancement at the tapered tips in comparison to the substrate of ZnO nanorods. We firstly traced the well-established contribution from quantum confinement (QC) to PL shift in three geometrically classified regions: (I) a strongly tapered region where the diameter increases from 3 to 20 nm; (II) a weakly tapered region with a gradually increasing diameter from 20 to 58 nm; (III) round cylindrical region interfacing the sapphire substrate. The measured PL shift influenced by the deformation is significantly stronger than the attained QC effect. Particularly, surface strain at the strongly tapered region turned out to drastically increase the PL shift which matches well with the analysis based on the surface to volume ratio incorporating mechanical parameters such as the compliance tensor component, strain dislocation constant, and surface stress. The surface strain increased at a lower temperature, further disclosing its inherent dependence on the thermal expansion coefficients in clear contrast to the temperature-invariant characteristics of QC.

Finger-inspired rigid-soft hybrid tactile sensor with superior sensitivity at high frequency

Among kinds of flexible tactile sensors, piezoelectric tactile sensor has the advantage of fast response for dynamic force detection. However, it suffers from low sensitivity at high-frequency dynamic stimuli. Here, inspired by finger structure—rigid skeleton embedded in muscle, we report a piezoelectric tactile sensor using a rigid-soft hybrid force-transmission-layer in combination with a soft bottom substrate, which not only greatly enhances the force transmission, but also triggers a significantly magnified effect in d₃₁ working mode of the piezoelectric sensory layer, instead of conventional d₃₃ mode. Experiments show that this sensor exhibits a super-high sensitivity of 346.5 pC N⁻¹ (@ 30 Hz), wide bandwidth of 5–600 Hz and a linear force detection range of 0.009–4.3 N, which is ~17 times the theoretical sensitivity of d₃₃ mode. Furthermore, the sensor is able to detect multiple force directions with high reliability, and shows great potential in robotic dynamic tactile sensing.

Designing efficient tactile sensors under high-frequency dynamic stimuli remains a challenge. Here, the authors demonstrate piezoelectric tactile sensor with sensitivity of 346.5 pCN−1, wide bandwidth of 5–600 Hz and a linear force detection range of 0.009–4.3 N using a rigid-soft hybrid force-transmission-layer in combination with a soft bottom substrate.

Morphological Engineering of Sensing Materials for Flexible Pressure Sensors and Artificial Intelligence Applications

Various morphological structures in pressure sensors with the resulting advanced sensing properties are reviewed comprehensively. Relevant manufacturing techniques and intelligent applications of pressure sensors are summarized in a complete and interesting way. Future challenges and perspectives of flexible pressure sensors are critically discussed.

Red carbon dot directed biocrystalline alignment for piezoelectric energy harvesting

Herein, using chitin-derived chitosan, we first demonstrate the luminous carbon dot-directed large-scale biocrystalline piezo-phase alignment. This further significantly facilitates the piezo-energy harvesting of Earth-abundant natural biopolymers. A very small, yet moderate, number of red-emission carbon quantum dots (R-CQDs) allow a highly preferential macroscopic alignment of chitosan based, electrospun hybrid nanofibers and a highly preferential microscopic alignment of internal chitosan piezo-phase crystalline lamellae. Meanwhile, R-CQD hybridized bionanofibers maintain the long-wavelength photoluminescence excitation/emission of encapsulated, monodisperse R-CQDs. The piezoelectric voltage output and piezoelectric current output of hybrid bionanofibers reach up to 125 V cm^-3 and 1.5 μA cm^-3, respectively. They are more than 5 and 6 times higher than those of the state-of-the-art pristine ones, respectively. Moreover, the proof-of-concept red-emission bionanofibrous piezoelectric nanogenerator shows a highly durable, highly stable, and highly reproducible piezoresponse in over 10 000 continuous load cycles. As a reliable renewable energy source, it demonstrates the fast charging of external capacitors and the direct operation of commercial electronics. In particular, as a self-powered wearable tactile healthcare sensor, it attains ultrahigh mechanosensitivity in sensing a broad range of human biophysiological pressures and strains.

Understanding the Enhancement Mechanism of ZnO Nanorod-based Piezoelectric Devices through Surface Engineering

ZnO is a typical piezoelectric semiconductor, and enhancing the piezoelectric output of ZnO-based devices is essential for their efficient applications. Surface engineering is an effective strategy to improve the piezoelectric output of ZnO-based devices, but its unclear regulation mechanism leads to a lack of reasonable guidance for device design. In this work, the regulation effect of the barrier layer in ZnO-based piezoelectric devices is systematically investigated from the carrier perspective through surface engineering, resulting in a significant improvement (nearly 10-fold) in the output performance of piezoelectric devices. The regulation mechanism of the ZnO-Cu₂O p-n heterojunction devices on piezoelectric output is revealed in terms of built-in electric field, depletion layer width, and junction capacitance. These findings facilitate further insight into the enhancement mechanism of the piezoelectric output of ZnO-based devices and provide reasonable ideas for efficient device design.

Self-Powered Tactile Sensor for Gesture Recognition Using Deep Learning Algorithms

A multifunctional wearable tactile sensor assisted by deep learning algorithms is developed, which can realize the functions of gesture recognition and interaction. This tactile sensor is the fusion of a triboelectric nanogenerator and piezoelectric nanogenerator to construct a hybrid self-powered sensor with a higher power density and sensibility. The power generation performance is characterized with an open-circuit voltage V_OC of 200 V, a short-circuit current I_SC of 8 μA, and a power density of 0.35 mW cm^-2 under a matching load. It also has an excellent sensibility, including a response time of 5 ms, a signal-to-noise ratio of 22.5 dB, and a pressure resolution of 1% (1-10 kPa). The sensor is successfully integrated on a glove to collect the electrical signal output generated by the gesture. Using deep learning algorithms, the functions of gesture recognition and control can be realized in real time. The combination of tactile sensor and deep learning algorithms provides ideas and guidance for its applications in the field of artificial intelligence, such as human-computer interaction, signal monitoring, and smart sensing.

Atomic Layer Assembly Based on Sacrificial Templates for 3D Nanofabrication

Three-dimensional (3D) nanostructures have attracted widespread attention in physics, chemistry, engineering sciences, and biology devices due to excellent functionalities which planar nanostructures cannot achieve. However, the fabrication of 3D nanostructures is still challenging at present. Reliable fabrication, improved controllability, and multifunction integration are desired for further applications in commercial devices. In this review, a powerful fabrication method to realize 3D nanostructures is introduced and reviewed thoroughly, which is based on atomic layer deposition assisted 3D assembly through various sacrificial templates. The aim of this review is to provide a comprehensive overview of 3D nanofabrication based on atomic layer assembly (ALA) in multifarious sacrificial templates for 3D nanostructures and to present recent advancements, with the ultimate aim to further unlock more potential of this method for nanodevice applications.

Intelligent Nanomaterials for Wearable and Stretchable Strain Sensor Applications: The Science behind Diverse Mechanisms, Fabrication Methods, and Real-Time Healthcare

It has become a scientific obligation to unveil the underlying mechanisms and the fabrication methods behind wearable/stretchable strain sensors based on intelligent nanomaterials in order to explore their possible potential in the field of biomedical and healthcare applications. This report is based on an extensive literature survey of fabrication of stretchable strain sensors (SSS) based on nanomaterials in the fields of healthcare, sports, and entertainment. Although the evolution of wearable strain sensors (WSS) is rapidly progressing, it is still at a prototype phase and various challenges need to be addressed in the future in special regard to their fabrication protocols. The biocalamity of COVID-19 has brought a drastic change in humans’ lifestyles and has negatively affected nations in all capacities. Social distancing has become a mandatory rule to practice in common places where humans interact with each other as a basic need. As social distancing cannot be ruled out as a measure to stop the spread of COVID-19 virus, wearable sensors could play a significant role in technologically impacting people’s consciousness. This review article meticulously describes the role of wearable and strain sensors in achieving such objectives.

Optimizing Piezoelectric Nanocomposites by High-Throughput Phase-Field Simulation and Machine Learning

Piezoelectric nanocomposites with oxide fillers in a polymer matrix combine the merit of high piezoelectric response of the oxides and flexibility as well as biocompatibility of the polymers. Understanding the role of the choice of materials and the filler-matrix architecture is critical to achieving desired functionality of a composite towards applications in flexible electronics and energy harvest devices. Herein, a high-throughput phase-field simulation is conducted to systematically reveal the influence of morphology and spatial orientation of an oxide filler on the piezoelectric, mechanical, and dielectric properties of the piezoelectric nanocomposites. It is discovered that with a constant filler volume fraction, a composite composed of vertical pillars exhibits superior piezoelectric response and electromechanical coupling coefficient as compared to the other geometric configurations. An analytical regression is established from a linear regression-based machine learning model, which can be employed to predict the performance of nanocomposites filled with oxides with a given set of piezoelectric coefficient, dielectric permittivity, and stiffness. This work not only sheds light on the fundamental mechanism of piezoelectric nanocomposites, but also offers a promising material design strategy for developing high-performance polymer/inorganic oxide composite-based wearable electronics.

Research on the High Sensitivity Detection Method of Carbon Nanotube/Polydimethylsiloxane Composites Structure

In this paper, a carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composite force-sensitive structure with good flexibility is proposed and fabricated, and the measurement of scanning electron microscopy (SEM) and Raman are carried out. The equivalent circuit of force-sensitive test of structure is performed and analyzed under direct current (DC) and alternating current (AC) conditions. Under AC conditions, experimental results further show that the sensitivity and sensitivity factors of force-sensitive structures are 0.15 KPa−1 and 2.17 in the pressure range of 600–1000 KPa compressive stress and 20–50% tensile stress, respectively. These results are increased by 36.4% and 38.2% compared to the results of compressive stress (0.11 KPa−1) and tensile stress (1.57) under DC conditions, respectively. It shows that the carbon nanotube/PDMS composite has higher test accuracy under AC conditions.

Electrooculography and Tactile Perception Collaborative Interface for 3D Human-Machine Interaction

The human-machine interface (HMI) previously relied on a single perception interface that cannot realize three-dimensional (3D) interaction and convenient and accurate interaction in multiple scenes. Here, we propose a collaborative interface including electrooculography (EOG) and tactile perception for fast and accurate 3D human-machine interaction. The EOG signals are mainly used for fast, convenient, and contactless 2D (XY-axis) interaction, and the tactile sensing interface is mainly utilized for complex 2D movement control and Z-axis control in the 3D interaction. The honeycomb graphene electrodes for the EOG signal acquisition and tactile sensing array are prepared by a laser-induced process. Two pairs of ultrathin and breathable honeycomb graphene electrodes are attached around the eyes for monitoring nine different eye movements. A machine learning algorithm is designed to train and classify the nine different eye movements with an average prediction accuracy of 92.6%. Furthermore, an ultrathin (90 μm), stretchable (∼1000%), and flexible tactile sensing interface assembled by a pair of 4 × 4 planar electrode arrays is attached to the arm for 2D movement control and Z-axis interaction, which can realize single-point, multipoint and sliding touch functions. Consequently, the tactile sensing interface can achieve eight directions control and even more complex movement trajectory control. Meanwhile, the flexible and ultrathin tactile sensor exhibits an ultrahigh sensitivity of 1.428 kPa^-1 in the pressure range 0-300 Pa with long-term response stability and repeatability. Therefore, the collaboration between EOG and the tactile perception interface will play an important role in rapid and accurate 3D human-machine interaction.

Piezoelectric nanogenerators for personalized healthcare

The development of flexible piezoelectric nanogenerators has experienced rapid progress in the past decade and is serving as the technological foundation of future state-of-the-art personalized healthcare. Due to their highly efficient mechanical-to-electrical energy conversion, easy implementation, and self-powering nature, these devices permit a plethora of innovative healthcare applications in the space of active sensing, electrical stimulation therapy, as well as passive human biomechanical energy harvesting to third party power on-body devices. This article gives a comprehensive review of the piezoelectric nanogenerators for personalized healthcare. After a brief introduction to the fundamental physical science of the piezoelectric effect, material engineering strategies, device structural designs, and human-body centered energy harvesting, sensing, and therapeutics applications are also systematically discussed. In addition, the challenges and opportunities of utilizing piezoelectric nanogenerators for self-powered bioelectronics and personalized healthcare are outlined in detail.

Artificial fast-adapting mechanoreceptor based on carbon nanotube percolating network

Most biological sensors preferentially encode changes in a stimulus rather than the steady components. However, intrinsically phasic artificial mechanoreceptors have not yet been described. We constructed a phasic mechanoreceptor by encapsulating carbon nanotube film in a viscoelastic matrix supported by a rigid substrate. When stimulated by a spherical indenter the sensor response resembled the response of fast-adapting mammalian mechanoreceptors. We modelled these sensors from the properties of percolating conductive networks combined with nonlinear contact mechanics and discussed the implications of this finding.

Revolution in Flexible Wearable Electronics for Temperature and Pressure Monitoring—A Review

In the last few decades, technology innovation has had a huge influence on our lives and well-being. Various factors of observing our physiological characteristics are taken into account. Wearable sensing tools are one of the most imperative sectors that are now trending and are expected to grow significantly in the coming days. Externally utilized tools connected to any human to assess physiological characteristics of interest are known as wearable sensors. Wearable sensors range in size from tiny to large tools that are physically affixed to the user and operate on wired or wireless terms. With increasing technological capabilities and a greater grasp of current research procedures, the usage of wearable sensors has a brighter future. In this review paper, the recent developments of two important types of wearable electronics apparatuses have been discussed for temperature and pressure sensing (Psensing) applications. Temperature sensing (Tsensing) is one of the most important physiological factors for determining human body temperature, with a focus on patients with long-term chronic conditions, normally healthy, unconscious, and injured patients receiving surgical treatment, as well as the health of medical personnel. Flexile Psensing devices are classified into three categories established on their transduction mechanisms: piezoresistive, capacitive, and piezoelectric. Many efforts have been made to enhance the characteristics of the flexible Psensing devices established on these mechanisms.

Highly sensitive strain sensors based on piezotronic tunneling junction

Piezotronics with capacity of constructing adaptive and seamless interactions between electronics/machines and human/ambient are of value in Internet of Things, artificial intelligence and biomedical engineering. Here, we report a kind of highly sensitive strain sensor based on piezotronic tunneling junction (Ag/HfO₂/n-ZnO), which utilizes the strain-induced piezoelectric potential to control the tunneling barrier height and width in parallel, and hence to synergistically modulate the electrical transport process. The piezotronic tunneling strain sensor has a high on/off ratio of 478.4 and high gauge factor of 4.8 × 10⁵ at the strain of 0.10%, which is more than 17.8 times larger than that of a conventional Schottky-barrier based strain sensor in control group as well as some existing ZnO nanowire or nanobelt based sensors. This work provides in-depth understanding for the basic mechanism of piezotronic modulation on tunneling junction, and realizes the highly sensitive strain sensor of piezotronic tunneling junction on device scale, which has great potential in advanced micro/nano-electromechanical devices and systems.

Strain-induced piezoelectric polarization can be used to modulate the interface electrical transport. Here, the authors achieved a piezotronic tunneling strain sensor at device scale with optimized performance based on the structure of Ag/HfO2/n-ZnO.