High-resolution dynamic pressure sensor array based on piezo-phototronic effect tuned photoluminescence imaging

Color-Shifting Iontronic Skin for On-Site, Nonpixelated Pressure Mapping Visualization

Mimicking the function of human skin is highly desired for electronic skins (e-skins) to perceive the tactile stimuli by both their intensity and spatial location. The common strategy using pixelated pressure sensor arrays and display panels greatly increases the device complexity and compromises the portability of e-skins. Herein, we tackled this challenge by developing a user-interactive iontronic skin that simultaneously achieves electrical pressure sensing and on-site, nonpixelated pressure mapping visualization. By merging the electrochromic and iontronic pressure sensing units into an integrated multilayer device, the interlayer charge transfer is regulated by applied pressure, which induces both color shifting and a capacitance change. The iontronic skin could visualize the trajectory of dynamic forces and reveal both the intensity and spatial information on various human activities. The integration of dual-mode pressure responsivity, together with the scalable fabrication and explicit signal output, makes the iontronic skin highly promising in biosignal monitoring and human-machine interaction.

Soft Sensors and Actuators for Wearable Human-Machine Interfaces

Haptic human-machine interfaces (HHMIs) combine tactile sensation and haptic feedback to allow humans to interact closely with machines and robots, providing immersive experiences and convenient lifestyles. Significant progress has been made in developing wearable sensors that accurately detect physical and electrophysiological stimuli with improved softness, functionality, reliability, and selectivity. In addition, soft actuating systems have been developed to provide high-quality haptic feedback by precisely controlling force, displacement, frequency, and spatial resolution. In this Review, we discuss the latest technological advances of soft sensors and actuators for the demonstration of wearable HHMIs. We particularly focus on highlighting material and structural approaches that enable desired sensing and feedback properties necessary for effective wearable HHMIs. Furthermore, promising practical applications of current HHMI technology in various areas such as the metaverse, robotics, and user-interactive devices are discussed in detail. Finally, this Review further concludes by discussing the outlook for next-generation HHMI technology.

A novel SERS sensor array based on AuNRs and AuNSs inverse-etching for the discrimination of five antioxidants

Antioxidants play an important role in life health and food safety. Herein, an inverse-etching platform based on gold nanorods (AuNRs) and gold nanostars (AuNSs) for high-throughput discrimination of antioxidants was constructed. Under the action of hydrogen peroxide (H2O2) and horseradish peroxidase (HRP), 3,3',5,5'-tetramethylbenzidine (TMB) would be oxidized to TMB+ or TMB2+. HRP reacts with H2O2 to release oxygen free radicals, which then react with TMB. Au nanomaterials can react with TMB2+, at the same time, Au was oxidized into Au (I), leading to the etching of the shape. Antioxidants, with good reduction ability, would prevent the further oxidation of TMB+ to TMB2+. So the presence of antioxidants will prevent further oxidation while avoiding the etching of Au in the catalytic oxidation process, thereby achieved inverse etching. Distinctive surface enhanced Raman scattering (SERS) fingerprint of five antioxidants were obtained based on the differential ability to scavenge free radicals. Five antioxidants, including ascorbic acid (AA), melatonin (Mel), glutathione (GSH), tea polyphenols (TPP), and uric acid (UA) were successfully distinguished by using linear discriminant analysis (LDA), heat map analysis and hierarchical cluster analysis (HCA). The study exhibits an effective inverse-etching based SERS sensor array for the response of antioxidants, which has great reference value in the field of human disease and food detection.

Vertically stacked quantum well diodes for multifunctional applications

Dual-functioning multiple quantum well (MQW) diodes can simultaneously transmit and receive information through visible light. Here, we report vertically stacked red, green, and blue (RGB) MQW diodes for light detection and display applications. Both blue and green MQW diodes are monolithically integrated with distributed Bragg reflector (DBR) filters to realize the separation of light. The versatile RGB MQW transmitter/receiver system not only creates full-color display but also effectively separates RGB light into various colors. These results open feasible routes to generate multifunctional device for the development of full-color display and light receiver.

Miniature GaN optoelectronic temperature sensor

The combination of plastic optical fiber (POF) with monolithically integrated transmitter and receiver is becoming increasingly attractive for the development of miniature optoelectronic sensing systems. Here, we propose a temperature sensing system by integrating a GaN optoelectronic chip with a POF and aluminum (Al) reflector. Owing to the overlap between electroluminescence and responsivity spectra of multiple quantum well (MQW) diodes, both the transmitter and the receiver having identical MQW structures are monolithically integrated on a tiny GaN chip by using the same fabrication process flow. Environmental temperature change leads to thermal deformation in the Al reflector, which reflects the transmitted light back with a light pulse. The reflected light is coupled into the guided POF again and sensed by the on-chip receiver. Finally, the temperature information is read out as electrical signals. When the ambient temperature changes from 20.1°C to 100°C, the optically induced electrical signal decreases from -3.04 µA to -3.13 µA. The results suggest that the monolithically integrated GaN device offers a promising option for optoelectronic temperature sensing systems.

Multilevel Simultaneous Lighting-Imaging System

In a III-nitride multiple quantum well (MQW) diode biased with a forward voltage, electrons recombine with holes inside the MQW region to emit light; meanwhile, the MQW diode utilizes the photoelectric effect to sense light when higher-energy photons hit the device to displace electrons in the diode. Both the injected electrons and the liberated electrons are gathered inside the diode, thereby giving rise to a simultaneous emission-detection phenomenon. The 4 × 4 MQW diodes could translate optical signals into electrical ones for image construction in the wavelength range from 320 to 440 nm. This technology will change the role of MQW diode-based displays since it can simultaneously transmit and receive optical signals, which is of crucial importance to the accelerating trend of multifunctional, intelligent displays using MQW diode technology.

Full-duplex visible light communication system using a single channel

Multiple quantum well (MQW) III-nitride diodes can emit light and detect light at the same time. In particular, given the overlapping region between the emission spectrum and the detection spectrum, the III-nitride diode can absorb photons of shorter wavelengths generated from another III-nitride diode with the same MQW structure. In this study, a wireless visible light communication system was established using two pairs of identical III-nitride diodes with different wavelengths. In this system, two green light diode chips were used to transmit and receive green light signals on both sides. We have integrated two blue light chips with optical filtering in the middle of the optical link to carry out blue light communication, with one end transmitting and one end receiving. Simultaneously, green light was allowed to pass through two blue light chips for optical communication. Combined with a distributed Bragg reflection (DBR) coating, we proposed using four chips in one optical path to carry out optical communication between chips with the same wavelength and used the coating principle to gate the optical wavelength to filter the clutter of green light chips on both sides to make the channel purer and the symbols easier to demodulate. Based on this multifunctional equipment, advanced single-optical path, III-nitride, full-duplex optical communication links can be developed for the deployment of the Internet of Things.

Self-Powered Electrodeposition System for Sub-10-nm Silver Nanoparticles with High-Efficiency Antibacterial Activity

Recently, silver nanoparticles (AgNPs) have been widely applied in sterilization due to their excellent antibacterial properties. However, AgNPs require rigorous storage conditions because their antibacterial performances are significantly affected by environmental conditions. Instant fabrication provides a remedy for this drawback. In this study, we propose a self-powered electrodeposition system to synthesize sub-10-nm AgNPs, consisting of a triboelectric nanogenerator (TENG) as the self-powered source, a capacitor for storing electrical energy from the TENG, and an electrochemical component for electrodeposition. The self-powered system with larger capacitance and discharging voltage tends to deliver smaller AgNPs due to the nucleation mechanism dominated by current density. Furthermore, antibacterial tests reveal that compared to direct current (DC) electrodeposition, the TENG-based electrodeposition can synthesize finer-sized AgNPs (<10 nm) with overwhelming antibacterial effect against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) (with 100% efficiency at 2 h). This work provides a new strategy for the self-powered, instant, and controllable electrodeposition of nanoparticles.

Coexistence of light emission and detection in a III-nitride quantum well diode

The demand for on-chip multifunctional optoelectronic systems is increasing in today's Internet of Things era. III-nitride quantum well diodes (QWDs) can transmit and receive information through visible light and can be used as both light-emitting diodes (LEDs) and photodetectors (PDs). Spectral emission-detection overlap gives the III-nitride QWD an intriguing capability to detect and modulate light emitted by itself. In this paper, the coexistence of light emission and detection in a III-nitride QWD is experimentally demonstrated, and a wireless video communication system through light is established. When approximately biasing and illuminating at the same time, the III-nitride QWD can achieve light emission and detection simultaneously. This work provides a foundation for the development of multifunctional III-nitride QWDs and the realization of device-to-device data communication.

High Performance, Stable, and Flexible Piezoelectric Nanogenerator Based on GaN:Mg Nanowires Directly Grown on Tungsten Foil

Rapid development of micro-electromechanical systems increases the need for flexible and durable piezoelectric nanogenerators (f-PNG) with high output power density. In this study, a high-performance, flexible, and highly stable f-PNG is prepared by directly growing the Mg-doped semi-insulating GaN nanowires (NWs) on a 30-µm-thick tungsten foil using vapor-liquid-solid growth mechanism. The direct growth of NWs on metal foil extends the overall lifetime of the f-PNG. The semi-insulating GaN NWs significantly enhance the piezoelectric performance of the f-PNG by reducing free electron density. Additionally, the direct integration of NWs on the tungsten foil improves the conductivity, resulting in current enhancement (2.5 mA) with an output power density of 13 mW cm^-2 . The piezoelectric performance of the f-PNG is investigated under several bending angles, actuation frequencies, continuous vibrations, and airflow velocities. The maximum output voltage exhibited by the f-PNG is 20 V at a bending angle of 155°. The f-PNG is connected to the backside of an index finger to monitor finger bending behavior by changing the current density. Depending on its flexibility and sensitivity, the f-PNG can be used as a health-monitoring sensor to be mounted on joints (fingers, hands, elbows, and knees) to monitor their repeated bending and relaxation.

Enhanced Photoluminescence of Flexible InGaN/GaN Multiple Quantum Wells on Fabric by Piezo-Phototronic Effect

Fabric-based wearable electronics are showing advantages in emerging applications in wearable devices, Internet of everything, and artificial intelligence. Compared to the one with organic materials, devices based on inorganic semiconductors (e.g., GaN) commonly show advantages of superior characteristics and high stability. Upon the transfer of GaN-based heterogeneous films from their rigid substrates onto flexible/fabric substrates, changes in strain would influence the device performance. Here, we demonstrate the transfer of InGaN/GaN multiple quantum well (MQW) films onto flexible/fabric substrates with an effective lift-off technique. The physical properties of the InGaN/GaN MQWs film are characterized by atomic force microscopy and high-resolution X-ray diffraction, indicating that the transferred film does not suffer from huge damage. Excellent flexible properties are observed in the film transferred on fabric, and the photoluminescence (PL) intensity is enhanced by the piezo-phototronic effect, which shows an increase of about 10% by applying an external strain with increasing the film curvature to 6.25 mm^-1. Moreover, energy band diagrams of the GaN/InGaN/GaN heterojunction at different strains are illustrated to clarify the internal modulation mechanism by the piezo-phototronic effect. This work would facilitate the guidance of constructing high-performance devices on fabrics and also push forward the rapid development of flexible and wearable electronics.

Recent Advances in Flexible Tactile Sensors for Intelligent Systems

Tactile sensors are an important medium for artificial intelligence systems to perceive their external environment. With the rapid development of smart robots, wearable devices, and human-computer interaction interfaces, flexible tactile sensing has attracted extensive attention. An overview of the recent development in high-performance tactile sensors used for smart systems is introduced. The main transduction mechanisms of flexible tactile sensors including piezoresistive, capacitive, piezoelectric, and triboelectric sensors are discussed in detail. The development status of flexible tactile sensors with high resolution, high sensitive, self-powered, and visual capabilities are focused on. Then, for intelligent systems, the wide application prospects of flexible tactile sensors in the fields of wearable electronics, intelligent robots, human-computer interaction interfaces, and implantable electronics are systematically discussed. Finally, the future prospects of flexible tactile sensors for intelligent systems are proposed.

Enhanced Heat Dissipation in Gallium Nitride-Based Light-Emitting Diodes by Piezo-phototronic Effect

As a new generation of light sources, GaN-based light-emitting diodes (LEDs) have wide applications in lighting and display. Heat dissipation in LEDs is a fundamental issue that leads to a decrease in light output, a shortened lifespan, and the risk of catastrophic failure. Here, the temperature spatial distribution of the LEDs is revealed by using high-resolution infrared thermography, and the piezo-phototronic effect is proved to restrain efficaciously the temperature increment for the first time. We observe the temperature field and current density distribution of the LED array under external strain compensation. Specifically, the temperature rise caused by the self-heating effect is reduced by 47.62% under 0.1% external strain, which is attributed to the enhanced competitiveness of radiative recombination against nonradiative recombination due to the piezo-phototronic effect. This work not only deepens the understanding of the piezo-phototronic effect in LEDs but also provides a novel, easy-to-implement, and economical method to effectively enhance thermal management.

Dynamic real-time imaging of living cell traction force by piezo-phototronic light nano-antenna array

Dynamic mapping of the cell-generated force of cardiomyocytes will help provide an intrinsic understanding of the heart. However, a real-time, dynamic, and high-resolution mapping of the force distribution across a single living cell remains a challenge. Here, we established a force mapping method based on a "light nano-antenna" array with the use of piezo-phototronic effect. A spatial resolution of 800 nm and a temporal resolution of 333 ms have been demonstrated for force mapping. The dynamic mapping of cell force of live cardiomyocytes was directly derived by locating the antennas' positions and quantifying the light intensities of the piezo-phototronic light nano-antenna array. This study presents a rapid and ultrahigh-resolution methodology for the fundamental study of cardiomyocyte behavior at the cell or subcellular level. It can provide valuable information about disease detection, drug screening, and tissue engineering for heart-related studies.

High-Performance Piezo-Phototronic Devices Based on Intersubband Transition of Wurtzite Quantum Well

III-nitride semiconductors play much more important roles in the areas of modern photoelectric applications, whereas strong polarization in their heterostructures is always a challenge to restrict the efficiency and performance of photoelectric devices. In this study, piezo-phototronic effect on near-infrared intersubband absorption is explored based on polar GaN/AlN quantum wells. The results show that externally applied pressure leads to the redshift of absorption wavelength by reducing polarization field of the quantum well. The sensitivity to estimate pressure-dependent intersubband absorption wavelength is almost two orders of magnitude higher than interband photoelectric devices. Additionally, such sensitivity is further enhanced by 2.6 times at 20 GPa as a result of piezo-phototronic effect. This study paves avenue for designing high-performance near-infrared piezo-phototronic devices based on intersubband transition.

Biomimetic Tactile Sensors Based on Nanomaterials

Tactile sensor technology has been researched extensively in response to the increasing demand for robotic and wearable healthcare systems. Studies on tactile sensory systems have primarily focused on achieving two goals: (1) developing technologies with high sensing abilities that mimic the biological functions and characteristics of the sensory systems of human skin and (2) satisfying the requirements of wearable device applications by fabricating mechanically flexible devices with low-power data analysis and processing abilities. In this Perspective, we present recent advances in artificial tactile sensory systems, which are based on biomimetic technologies that exhibit functional features of biological systems including mechanoreceptors and human skin sensory neurons for human-machine interfaces. We also discuss the opportunities, current challenges, potential solutions, and future investigative directions pertaining to this field.

Ultraflexible and transparent electroluminescent skin for real-time and super-resolution imaging of pressure distribution

The ability to image pressure distribution over complex three-dimensional surfaces would significantly augment the potential applications of electronic skin. However, existing methods show poor spatial and temporal fidelity due to their limited pixel density, low sensitivity, or low conformability. Here, we report an ultraflexible and transparent electroluminescent skin that autonomously displays super-resolution images of pressure distribution in real time. The device comprises a transparent pressure-sensing film with a solution-processable cellulose/nanowire nanohybrid network featuring ultrahigh sensor sensitivity (>5000 kPa-1) and a fast response time (<1 ms), and a quantum dot-based electroluminescent film. The two ultrathin films conform to each contact object and transduce spatial pressure into conductivity distribution in a continuous domain, resulting in super-resolution (>1000 dpi) pressure imaging without the need for pixel structures. Our approach provides a new framework for visualizing accurate stimulus distribution with potential applications in skin prosthesis, robotics, and advanced human-machine interfaces.

Piezoelectric polymer nanofibers for pressure sensors and their applications in human activity monitoring

Miniaturized, wearable and self-powered sensors are crucial for applications in artificial intelligence, robotics, healthcare, and communication devices. In particular, piezoelectric polymer-based sensing systems have the advantages of light weight, large piezoelectricity and mechanical flexibility, offering great opportunities in flexible and stretchable electronic devices. Herein, free-standing large-size nanofiber (NF) membranes have been fabricated by an electrospinning technique. Our results show that the as-synthesized P(VDF–TrFE) NFs are pure β-phase and exhibit excellent mechanical and thermal properties. Besides having high sensitivity and operational stability, the fibrous sensor can generate remarkable electrical signals from applied pressure, with an output voltage of 18.1 V, output current of 0.177 μA, and power density of 22.9 μW cm⁻². Moreover, such sensors also produce significant electrical performance of up to a few volts under human mechanical stress, thereby allowing for the monitoring of biomechanical movement of the human foot, elbow, and finger. Our study sheds light onto the use of piezoelectric polymers for flexible self-powered sensing electronics and wearable devices.

Miniaturized, wearable and self-powered sensors are crucial for applications in artificial intelligence, robotics, healthcare, and communication devices.

Electronic Skin for Closed-Loop Systems

Electronic skin (e-skin) uses advanced electronics and sensor arrays to manufacture human-skin-like robotics skin. The creation of e-skin has been made possible due to various physics effects/mechanisms, innovative materials, structural designs, and advanced fabrication techniques. In this Perspective, we describe the current advances in and emerging uses of e-skin for closed-loop systems, with a view toward applications in smart robotics, Internet of Things, and human-machine interfaces.

Piezotronics and Piezo-phototronics of Third Generation Semiconductor Nanowires

With the fast development of nanoscience and nanotechnology in the last 30 years, semiconductor nanowires have been widely investigated in the areas of both electronics and optoelectronics. Among them, representatives of third generation semiconductors, such as ZnO and GaN, have relatively large spontaneous polarization along their longitudinal direction of the nanowires due to the asymmetric structure in their c-axis direction. Two-way or multiway couplings of piezoelectric, photoexcitation, and semiconductor properties have generated new research areas, such as piezotronics and piezo-phototronics. In this review, an in-depth discussion of the mechanisms and applications of nanowire-based piezotronics and piezo-phototronics is presented. Research on piezotronics and piezo-phototronics has drawn much attention since the effective manipulation of carrier transport, photoelectric properties, etc. through the application of simple mechanical stimuli and, conversely, since the design of new strain sensors based on the strain-induced change in semiconductor properties.

Gel-Based Artificial Photonic Skin to Sense a Gentle Touch by Reflection

This work demonstrates that engineering a three-dimensional photonic crystal (3DPC) structure in a highly flexible gel is a potential method to achieve flexible tactile artificial photonic skin (p-skin) for future visible-light communication (VLC). We investigated the photonic output modes of 3DPC-coated gel-based pressure sensors and explored their ability to sense low pressures (<10 kPa) through reflection. Such sensors with high sensitivity, fast response, and adjustable detection range can be fabricated in arrays of dots covering large, complex/uneven surfaces and are promising in the development of stimuli-responsive soft materials for future artificial intelligence, health monitoring, and photonic communication systems.

Enhanced luminescence from InGaN/GaN nano-disk in a wire array caused by surface potential modulation during wet treatment

Here we have demonstrated the profound impact of surface potential on the luminescence of an array of InGaN/GaN nano-disk in a wire heterostructure. The change in surface potential is brought about by a combination of dry and successive wet-processing treatments. The photoluminescence (PL) properties are determined as a function of size and height of this array of nano-disks. The observed characteristics are coherently explained by considering a change in quantum confinement induced by the change in surface potential, quantum-confined Stark effect, exciton binding energy and strain relaxation for varying surface potential. The change in hole bound state energy due to parabolic potential well near the side-wall is found to be the dominating factor. The PL peak position, full width at half-maximum, strain relaxation and integrated PL intensity are studied as a function of incident power and temperature. The devices demonstrate higher integrated PL intensity and slope efficiency.

A large-scale, ultrahigh-resolution nanoemitter ordered array with PL brightness enhanced by PEALD-grown AlN coating

III-nitride solid-state microdisplays have significant advantages, including high brightness and high resolution, for the development of advanced displays, high-definition projectors, head-mounted displays, large-capacity optical communication systems, and so forth. Herein, a high-brightness InGaN/GaN multiple-quantum-well (MQW) nanoemitter array with an ultrahigh resolution of 31 750 dpi was achieved by combining a top-down fabrication with surface passivation of plasma-enhanced atomic layer deposition (PEALD)-grown AlN coating. With regard to the nanometer-level top-down etching, the surface damage or defects on the newly-formed sidewall play a significant role in the photoluminescence (PL) quality. Note that these arrays can be effectively passivated by the PEALD-grown AlN coating with an over 345% PL enhancement. In addition, a sharp band bending at the interface of the luminescent InGaN QW and the AlN coating layer can electrically drift away the photogenerated electrons from the surface traps; this leads to enhancement of the bulk PL radiative recombination with a fast PL decay rate. Therefore, we have demonstrated a feasible way for realizing an advanced nanoemitter array with both high brightness and ultrahigh resolution for future smart displays, high-resolution imaging, big-data optical information systems and so on.

Eu2+/Eu3+-Based Smart Duplicate Responsive Stimuli and Time-gated Nanohybrid for Optical Recording and Encryption

With the rapid development of information science, it is urgent that memory devices possessing high security, density, and desirable storage ability should be developed. In this work, a smart duplicate response of stimuli has been developed and a time-gate nanohybrid based on variable valence Eu²⁺/Eu³⁺ coencapsulated has been fabricated and acts as active material in the multilevel and multidimensional memory devices. The luminescence lifetime of Eu³⁺ in this nanohybrid gave a stimuli response due to which the energy level of the coordinated ligand could be modulated. Furthermore, by a simple sintering procedure, Eu³⁺ was partially in situ reduced to Eu²⁺ with a short lifetime in the system. And the in situ reduction ensured both Eu³⁺ and Eu²⁺ ions' uniform distribution in the nanohybrid and simultaneous response upon light excitation of variable valence Eu ions. Interestingly, Eu³⁺ revealed a prolonged lifetime because of the presence of an energy-transfer effect of Eu²⁺ → Eu³⁺. Such a nanohybrid had abundant luminescent properties, including the short lifetime of Eu²⁺, the energy transfer from the Eu²⁺ to Eu³⁺ ions, and the stimuli response of the Eu³⁺ lifetimes when exposed to acidic or basic vapor, thus giving birth to interesting recording and encryption performance in spatial-temporal dimensions. We believe that this research will point out a new direction for the future development of multilevel and multidimensional optical recording and encryption materials.

Strain-Controlled Recombination in InGaN/GaN Multiple Quantum Wells on Silicon Substrates

This paper reports the photoluminescence (PL) properties of InGaN/GaN multiple quantum well (MQW) light-emitting diodes grown on silicon substrates which were designed with different tensile stress controlling architecture like periodic Si δ-doping to the n-type GaN layer or inserting InGaN/AlGaN layer for investigating the strain-controlled recombination mechanism in the system. PL results turned out that tensile stress released samples had better PL performances as their external quantum efficiencies increased to 17%, 7 times larger than the one of regular sample. Detail analysis confirmed they had smaller nonradiative recombination rates ((2.5~2.8)×10^-2 s^-1 compared to (3.6~4.7)× 10^-2 s^-1), which was associated with the better crystalline quality and absence of dislocations or cracks. Furthermore, their radiative recombination rates were found more stable and were much higher ((5.7~5.8) ×10^-3 s^-1 compared to [9~7] ×10^-4 s^-1) at room temperature. This was ascribed to the suppression of shallow localized states on MQW interfaces, leaving the deep radiative localization centers inside InGaN layers dominating the radiative recombination.

A wet-chemistry-based hydrogel sensing platform for 2D imaging of pressure, chemicals and temperature

Human skin can perceive pressure, chemical compounds and temperature with a high spatial resolution via specific receptors. Inspired by human skin, we present a wet-chemistry based hydrogel sensing platform for 2D imaging sensitive to specific external stimuli, e.g., pressure, chemicals and temperature. This platform is composed of a hydrogel pyramidal array on a single electrode. Each pyramid serves as a spatially separated reservoir for chemical reactions, enabling independent pixels for sensing without individual electrodes. Depending on the electrochemiluminescence (ECL) electrolyte for specific stimuli, our platform possesses 2D imaging capabilities with high sensitivity for pH and temperature in addition to pressure via the deformation of the viscoelastic hydrogel. This work represents an important step toward the application of sensitive chemical reactions for various external stimuli to biocompatible electronic skins based on hydrogels without addressing circuit for each pixel.

Electric-field driven photoluminescence probe of photoelectric conversion in InGaN-based photovoltaics

The spatial distribution of electric field in photovoltaic multiple quantum wells (MQWs) is extremely important to dictate the mutual competition of photoelectric conversion and optical transition. Here, electric-field-driven photoluminescence (PL) in both steady-state and transient-state has been utilized to directly investigate the internal photoelectric conversion processes in InGaN-based MQW photovoltaic cell. As applying the reversed external electric field, the compensation of the quantum confined stark effect (QCSE) in InGaN QW is beneficial to help the photoabsorbed minor carriers drift out from the localized states, whereas extremely weakening the PL radiative recombination. A directly driven force by the reversed external electric field decreases the transit time of photocarriers drifting in InGaN QW. And hence, the overall dynamic PL decay including both the slow and fast processes gradually speeds up from 19.2 ns at the open-circuit condition to 3.9 ns at a negative bias of -3 V. In particular, the slow PL decay lifetime declines more quickly than that of the fast one. It is the delocalization of photocarriers by electric-field drift that helps to further enhance the high-efficiency photoelectric conversion except for the tunneling transport in InGaN-based MQW photovoltaics. Therefore, it can be concluded that the electric-field PL probe may provide a direct method for evaluating the photoelectric conversion in multilayer quantum structures and related multijunction photovoltaic cells.

Superhydrophobic WS2-Nanosheet-Wrapped Sponges for Underwater Detection of Tiny Vibration

Underwater vibration detection is of great importance in personal safety, environmental protection, and military defense. Sealing layers are required in many underwater sensor architectures, leading to limited working-life and reduced sensitivity. Here, a flexible, superhydrophobic, and conductive tungsten disulfide (WS2) nanosheets-wrapped sponge (SCWS) is reported for the high-sensitivity detection of tiny vibration from the water surfaces and from the grounds. When the SCWS is immersed in water, a continuous layer of bubbles forms on its surfaces, providing the sensor with two special abilities. One is sealing-free feature due to the intrinsic water-repellent property of SCWS. The other is functioning as a vibration-sensitive medium to convert mechanical energy into electric signals through susceptible physical deformation of bubbles. Therefore, the SCWS can be used to precisely detect tiny vibration of water waves, and even sense those caused by human footsteps, demonstrating wide applications of this amphibious (water/ground) vibration sensor. Results of this study can initiate the exploration of superhydrophobic materials with elastic and conductive properties for underwater flexible electronic applications.

Piezo-Phototronic Matrix via a Nanowire Array

Piezoelectric semiconductors, such as ZnO and GaN, demonstrate multiproperty coupling effects toward various aspects of mechanical, electrical, and optical excitation. In particular, the three-way coupling among semiconducting, photoexcitation, and piezoelectric characteristics in wurtzite-structured semiconductors is established as a new field, which was first coined as piezo-phototronics by Wang in 2010. The piezo-phototronic effect can controllably modulate the charge-carrier generation, separation, transport, and/or recombination in optical-electronic processes by modifying the band structure at the metal-semiconductor or semiconductor-semiconductor heterojunction/interface. Here, the progress made in using the piezo-phototronic effect for enhancing photodetectors, pressure sensors, light-emitting diodes, and solar cells is reviewed. In comparison with previous works on a single piezoelectric semiconducting nanowire, piezo-phototronic nanodevices built using nanowire arrays provide a promising platform for fabricating integrated optoelectronics with the realization of high-spatial-resolution imaging and fast responsivity.

Piezotronic effect tuned AlGaN/GaN high electron mobility transistor

The piezotronic effect utilizes strain-induced piezoelectric polarization charges to tune the carrier transportation across the interface/junction. We fabricated a high-performance AlGaN/GaN high electron mobility transistor (HEMT), and the transport property was proven to be enhanced by applying an external stress for the first time. The enhanced source-drain current was also observed at any gate voltage and the maximum enhancement of the saturation current was up to 21% with 15 N applied stress (0.18 GPa at center) at -1 V gate voltage. The physical mechanism of HEMT with/without external compressive stress conditions was carefully illustrated and further confirmed by a self-consistent solution of the Schrödinger-Poisson equations. This study proves the cause-and-effect relationship between the piezoelectric polarization effect and 2D electron gas formation, which provides a tunable solution to enhance the device performance. The strain tuned HEMT has potential applications in human-machine interface and the security control of the power system.

Enhanced Solar Cell Conversion Efficiency of InGaN/GaN Multiple Quantum Wells by Piezo-Phototronic Effect

The piezo-phototronic effect is the tuning of piezoelectric polarization charges at the interface to largely enhance the efficiency of optoelectronic processes related to carrier separation or recombination. Here, we demonstrated the enhanced short-circuit current density and the conversion efficiency of InGaN/GaN multiple quantum well solar cells with an external stress applied on the device. The external-stress-induced piezoelectric charges generated at the interfaces of InGaN and GaN compensate the piezoelectric charges induced by lattice mismatch stress in the InGaN wells. The energy band realignment is calculated with a self-consistent numerical model to clarify the enhancement mechanism of optical-generated carriers. This research not only theoretically and experimentally proves the piezo-phototronic effect modulated the quantum photovoltaic device but also provides a great promise to maximize the use of solar energy in the current energy revolution.

Transparent, Flexible Piezoelectric Nanogenerator Based on GaN Membrane Using Electrochemical Lift-Off

A transparent and flexible piezoelectric nanogenerator (TF PNG) is demonstrated based on a GaN membrane fabricated by electrochemical lift-off. Under shear stress on the TF PNG by finger force (∼182 mN), the GaN membrane effectively undergoes normal stress and generates piezoelectric polarization along the c-axis, resulting in the generation of piezoelectric output from the TF PNG. Although the GaN layer is 315 times thinner than the flexible polyethylene terephthalate (PET) substrate, the low Young's modulus of PET allows the GaN membranes to absorb ∼41% of the applied strain energy, which leads to their large lattice deformation under extremely low applied stress. Maximum output voltage and current values of 4.2 V and 150 nA are obtained, and the time decay of the output voltage is discussed.

Piezo-phototronic effect enhanced UV photodetector based on CuI/ZnO double-shell grown on flexible copper microwire

In this work, we present a facile, low-cost, and effective approach to fabricate the UV photodetector with a CuI/ZnO double-shell nanostructure which was grown on common copper microwire. The enhanced performances of Cu/CuI/ZnO core/double-shell microwire photodetector resulted from the formation of heterojunction. Benefiting from the piezo-phototronic effect, the presentation of piezocharges can lower the barrier height and facilitate the charge transport across heterojunction. The photosensing abilities of the Cu/CuI/ZnO core/double-shell microwire detector are investigated under different UV light densities and strain conditions. We demonstrate the I-V characteristic of the as-prepared core/double-shell device; it is quite sensitive to applied strain, which indicates that the piezo-phototronic effect plays an essential role in facilitating charge carrier transport across the CuI/ZnO heterojunction, then the performance of the device is further boosted under external strain.

Progress in Piezo-Phototronic-Effect-Enhanced Light-Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non-central symmetric crystal structures. The three-way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo-phototronics. This effect can efficiently manipulate the emission intensity of light-emitting diodes (LEDs) by utilizing the piezo-polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo-phototronic-effect-enhanced LEDs is reviewed; following their development from single-nanowire pressure-sensitive devices to high-resolution array matrices for pressure-distribution mapping applications. The piezo-phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.

Flexible Self-Powered GaN Ultraviolet Photoswitch with Piezo-Phototronic Effect Enhanced On/Off Ratio

Flexible self-powered sensing is urgently needed for wearable, portable, sustainable, maintenance-free and long-term applications. Here, we developed a flexible and self-powered GaN membrane-based ultraviolet (UV) photoswitch with high on/off ratio and excellent sensitivity. Even without any power supply, the driving force of UV photogenerated carriers can be well boosted by the combination of both built-in electric field and piezoelectric polarization field. The asymmetric metal-semiconductor-metal structure has been elaborately utilized to enhance the carrier separation and transport for highly sensitive UV photoresponse. Its UV on/off ratio and detection sensitivity reach to 4.67 × 10(5) and 1.78 × 10(12) cm·Hz(0.5) W(1-), respectively. Due to its excellent mechanical flexibility, the piezoelectric polarization field in GaN membrane can be easily tuned/controlled based on piezo-phototronic effect. Under 1% strain, a stronger and broader depletion region can be obtained to further enhance UV on/off ratio up to 154%. As a result, our research can not only provide a deep understanding of local electric field effects on self-powered optoelectronic detection, but also promote the development of self-powered flexible optoelectronic devices and integrated systems.

Piezo-Phototronic Effect Controlled Dual-Channel Visible light Communication (PVLC) Using InGaN/GaN Multiquantum Well Nanopillars

Visible light communication (VLC) simultaneously provides illumination and communication via light emitting diodes (LEDs). Keeping a low bit error rate is essential to communication quality, and holding a stable brightness level is pivotal for illumination function. For the first time, a piezo-phototronic effect controlled visible light communication (PVLC) system based on InGaN/GaN multiquantum wells nanopillars is demonstrated, in which the information is coded by mechanical straining. This approach of force coding is also instrumental to avoid LED blinks, which has less impact on illumination and is much safer to eyes than electrical on/off VLC. The two-channel transmission mode of the system here shows great superiority in error self-validation and error self-elimination in comparison to VLC. This two-channel PVLC system provides a suitable way to carry out noncontact, reliable communication under complex circumstances.

Piezotronic-Enhanced Photoelectrochemical Reactions in Ni(OH)2-Decorated ZnO Photoanodes

Controlling the interface electronic band structure in heterostructures is essential for developing highly efficient photoelectrochemical (PEC) photoanodes. Here, we presented an enhanced oxygen evolution reaction (OER) by introducing the piezotronics concept, i.e., piezoelectric polarization (Ppz)-induced band engineering. In a Ni(OH)2-decorated ZnO photoanode system, appreciably improved photocurrent density of sulfite (SO3(2-)) and hydroxyl (OH(-)) oxidation reactions were obtained by physically deflecting the photoanode. Both theoretical and experimental results suggested that the performance enhancement was a result of the piezoelectric Ppz-endowed enlargement of the built-in electric field at the ZnO/Ni(OH)2 interface, which could drive an additional amount of photoexcited charges from ZnO toward the interface for OER. This strategy demonstrates a new route for improving the performance of inexpensive catalysts-based solar-to-fuel production.