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.

Ultrafast and Low-Power 2D Bi2O2Se Memristors for Neuromorphic Computing Applications.

Memristors that emulate synaptic plasticity are building blocks for opening a new era of energy-efficient neuromorphic computing architecture, which will overcome the limitation of the von Neumann bottleneck. Layered two-dimensional (2D) Bi2O2Se, as an emerging material for next-generation electronics, is of great significance in improving the efficiency and performance of memristive devices. Herein, high-quality Bi2O2Se nanosheets are grown by configuring mica substrates face-down on the Bi2O2Se powder. Then, bipolar Bi2O2Se memristors are fabricated with excellent performance including ultrafast switching speed (<5 ns) and low-power consumption (<3.02 pJ). Moreover, synaptic plasticity, such as long-term potentiation/depression (LTP/LTD), paired-pulse facilitation (PPF), and spike-timing-dependent plasticity (STDP), are demonstrated in the Bi2O2Se memristor. Furthermore, MNIST recognition with simulated artificial neural networks (ANN) based on conductance modification could reach a high accuracy of 91%. Notably, the 2D Bi2O2Se enables the memristor to possess ultrafast and low-power attributes, showing great potential in neuromorphic computing applications.

Flexible GaN-based microscale light-emitting diodes with a batch transfer by wet etching.

Flexible inorganic GaN-based microscale light-emitting diodes (µLEDs) show potential applications in wearable electronics, biomedical engineering, and human-machine interfaces. However, developing cost-effective products remains a challenge for flexible GaN-based µLEDs. Here, a facile and stable method is proposed to fabricate flexible GaN-based µLEDs from silicon substrates in an array-scale manner by wet etching. Circular and square µLED arrays with a size and pitch of 500 µm were fabricated and then transferred to a flexible acrylic/copper substrate. The as-fabricated flexible µLEDs can maintain their structure intact while exhibiting a significant increase in external quantum efficiency. This Letter promotes the application of simple and low-cost flexible µLED devices, especially for virtual displays, wearables, and curvilinear displays.

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.

Effect of backside dry etching on the device performance of AlGaN/GaN HEMTs.

AlGaN/GaN heterojunction-based high-electron-mobility transistors (HEMTs) have significant advantages of high carrier concentration, high electron mobility, and large breakdown voltage, and show promising potential as power devices. Being widely used in semiconductor manufacturing, dry etching process is capable of fabricating microstructures and thinning substrate from backside, which is good for developing flexible devices. Here, we investigate the effect of backside dry etching of Si substrate on the physical and electrical properties of AlGaN/GaN HEMTs. The physical properties were characterized by scanning electron microscope, Raman spectra, and x-ray diffraction (XRD). After the dry etching process, the peak red-shift of GaNE2mode indicates an increase of tensile stress, and the XRD rocking curve of GaN film shows to a certain extent decreased dislocation density. Furthermore, the maximum saturation current density and maximum transconductance of the HEMTs are improved by 21.1% and 25.5%, respectively. The approach of backside dry etching for thinning Si substrate would contribute to the optimization of GaN heterojunction-based devices, and also provide inspirations for the development of flexible and robust power devices.

Piezotronic Synapse Based on a Single GaN Microwire for Artificial Sensory Systems.

Tactile information is efficiently captured and processed through a complex sensory system combined with mechanoreceptors, neurons, and synapses in human skin. Synapses are essential for tactile signal transmission between pre/post-neurons. However, developing an electronic device that integrates the functions of tactile information sensation and transmission remains a challenge. Here, we present a piezotronic synapse based on a single GaN microwire that can simultaneously achieve the capabilities of strain sensing and synaptic functions. The piezotronic effect in the wurtzite GaN is introduced to strengthen synaptic weight updates (e.g., 330% enhancement at a compressive stress of -0.36%) with pulse trains. A high gauge factor for strain sensing (ranging from 0 to -0.81%) of about 736 is also obtained. Remarkably, the piezotronic synapse enables the neuromorphic hardware achievement of the perception and processing of tactile information in a single micro/nanowire system, demonstrating an advance in biorealistic artificial intelligence systems.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications.

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

Hybrid Aerogel Triboelectric Nanogenerator Based on the Synergistic Effect of Solid-Solid/Gas-Solid Triboelectricity and Piezoelectric Polarization.

Aerogels, due to their unique interconnected 3D networks, and large number of air-filled pores, extend the structural characteristics and physicochemical properties of the nanoscale to the macro level. However, aerogels made from a single component can hardly meet the needs of multifunctional energy harvesting/supply situations. Here, a BaTiO3-based hybrid aerogel (BTO HA) with 3D network structure was prepared. When the BTO HA is used as the electrode of triboelectric nanogenerator (BTO HA-TENG), high electrical output performances were obtained, which is due to the synergistic effect of solid-solid contact electrifications between the two electrification layers, the gas-solid contact electrifications between the inner surface of BTO HA and the air filled in the aerogel pores, and the piezoelectricity of the doped BaTiO3 nanoparticles. The BTO HA-TENG exhibited excellent fatigue resistance and structural stability after 12,000 cycles of alternatively contact/separation tests, and it can not only provide stable power supply for commercial capacitors, drive small mobile electronic devices but also can be used as a self-powered sensor to monitor human motion signals. Compared with traditional TENGs depending on surface charge transfer, the BTO HA-TENG exhibited its unique advantage that it can generate and transfer triboelectric charges by 3D volume, which boost TENGs' electrical output performances.

Ultralight, Elastic, Hybrid Aerogel for Flexible/Wearable Piezoresistive Sensor and Solid-Solid/Gas-Solid Coupled Triboelectric Nanogenerator.

Aerogels have been attracting wide attentions in flexible/wearable electronics because of their light weight, excellent flexibility, and electrical conductivity. However, multifunctional aerogel-based flexible/wearable electronics for human physiological/motion monitoring, and energy harvest/supply for mobile electronics, have been seldom reported yet. In this study, a kind of hybrid aerogel (GO/CNT HA) based on graphene oxide (GO) and carboxylated multiwalled carbon nanotubes (CMWCNTs) is prepared which can not only used as piezoresistive sensors for human motion and physiological signal detections, but also as high performance triboelectric nanogenerator (TENG) coupled with both solid-solid and gas-solid contact electrifications (CE). The repeatedly loading-unloading tests with 20 000 cycles exhibit its high and ultrastable piezoresistive sensor performances. Moreover, when the obtained aerogel is used as the electrode of a TENG, high electric output performance is produced due to the synergistic effect of solid-solid, and gas-solid interface CEs (3D electrification: solid-solid interface CE between the two solid electrification layers; gas-solid interface CE between the inner surface of GO/CNT HA and the air filled in the aerogel pores). This kind of aerogel promises good applications for human physiological/motion monitoring and energy harvest/supply in flexible/wearable electronics such as piezoresistive sensors and flexible TENG.

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.

Excitation-dependent perovskite/polymer films for ultraviolet visualization.

Ultraviolet (UV) visualization has extensive applications in military and civil fields such as security monitoring, space communication, and wearable equipment for health monitoring in the internet of things (IoT). Due to their remarkable optoelectronic features, perovskite materials are regarded as promising candidates for UV light detecting and imaging. Herein, we report for the first time the excitation-dependent perovskite/polymer films with dynamically tunable fluorescence ranging from green to magenta by changing the UV excitation from 260 to 380 nm. And they still render dynamic multi-color UV light imaging with different polymer matrixes, halogen ratios, and cations of perovskite materials. The mechanism of its fluorescence change is related to the chloride vacancies in perovskite materials. A patterned multi-color ultraviolet visualization pad is also demonstrated for visible conversion of the UV region. This technique may provide a universal strategy for information securities, UV visualizations, and dynamic multi-color displays in the IoT.

High-Uniformity Threshold Switching HfO2-Based Selectors with Patterned Ag Nanodots.

High-performance selector devices are essential for emerging nonvolatile memories to implement high-density memory storage and large-scale neuromorphic computing. Device uniformity is one of the key challenges which limit the practical applications of threshold switching selectors. Here, high-uniformity threshold switching HfO2-based selectors are fabricated by using e-beam lithography to pattern controllable Ag nanodots (NDs) with high order and uniform size in the cross-point region. The selectors exhibit excellent bidirectional threshold switching performance, including low leakage current (<1 pA), high on/off ratio (>108), high endurance (>108 cycles), and fast switching speed (≈75 ns). The patterned Ag NDs in the selector help control the number of Ag atoms diffusing into HfO2 and confine the positions to form reproducible filaments. According to the statistical analysis, the Ag NDs selectors show much smaller cycle-to-cycle and device-to-device variations (C V < 10%) compared to control samples with nonpatterned Ag thin film. Furthermore, when integrating the Ag NDs selector with resistive switching memory in one-selector-one-resistor (1S1R) structure, the reduced selector variation helps significantly reduce the bit error rate in 1S1R crossbar array. The high-uniformity Ag NDs selectors offer great potential in the fabrication of large-scale 1S1R crossbar arrays for future memory and neuromorphic computing applications.

Atomic threshold-switching enabled MoS2 transistors towards ultralow-power electronics.

Power dissipation is a fundamental issue for future chip-based electronics. As promising channel materials, two-dimensional semiconductors show excellent capabilities of scaling dimensions and reducing off-state currents. However, field-effect transistors based on two-dimensional materials are still confronted with the fundamental thermionic limitation of the subthreshold swing of 60 mV decade-1 at room temperature. Here, we present an atomic threshold-switching field-effect transistor constructed by integrating a metal filamentary threshold switch with a two-dimensional MoS2 channel, and obtain abrupt steepness in the turn-on characteristics and 4.5 mV decade-1 subthreshold swing (over five decades). This is achieved by using the negative differential resistance effect from the threshold switch to induce an internal voltage amplification across the MoS2 channel. Notably, in such devices, the simultaneous achievement of efficient electrostatics, very small sub-thermionic subthreshold swings, and ultralow leakage currents, would be highly desirable for next-generation energy-efficient integrated circuits and ultralow-power applications.

Strain-controlled power devices as inspired by human reflex.

Bioinspired electronics are rapidly promoting advances in artificial intelligence. Emerging AI applications, e.g., autopilot and robotics, increasingly spur the development of power devices with new forms. Here, we present a strain-controlled power device that can directly modulate the output power responses to external strain at a rapid speed, as inspired by human reflex. By using the cantilever-structured AlGaN/AlN/GaN-based high electron mobility transistor, the device can control significant output power modulation (2.30-2.72 × 103 W cm-2) with weak mechanical stimuli (0-16 mN) at a gate bias of 1 V. We further demonstrate the acceleration-feedback-controlled power application, and prove that the output power can be effectively adjusted at real-time in response to acceleration changes, i.e., ▵P of 72.78-132.89 W cm-2 at an acceleration of 1-5 G at a supply voltage of 15 V. Looking forward, the device will have great significance in a wide range of AI applications, including autopilot, robotics, and human-machine interfaces.

Piezotronic Effect Modulated Flexible AlGaN/GaN High-Electron-Mobility Transistors.

Flexible electronic technology has attracted great attention due to its wide range of potential applications in the fields of healthcare, robotics, and artificial intelligence, etc. In this work, we have successfully fabricated flexible AlGaN/GaN high-electron-mobility transistors (HEMTs) arrays through a low-damage and wafer-scale substrate transfer technology from a rigid Si substrate. The flexible AlGaN/GaN HEMTs have excellent electrical performances with the Id,max achieving 290 mA/mm at Vgs = +2 V and the gm,max reaching to 40 mS/mm. The piezotronic effect provides a different freedom to optimize device performances, and flexible HEMTs can endure the larger mechanical distortions. Based on the piezotronic effect, we applied an external stress to significantly modulate the electrical performances of flexible HEMTs. The piezotronic effect modulated flexible AlGaN/GaN HEMTs exhibit great potential in human-machine interface, intelligent microinductor systems, and active sensors, etc, and introduce an opportunity to sensing or feedback external mechanical stimuli and so on.

Low-Voltage Oscillatory Neurons for Memristor-Based Neuromorphic Systems.

Neuromorphic systems consisting of artificial neurons and synapses can process complex information with high efficiency to overcome the bottleneck of von Neumann architecture. Artificial neurons are essentially required to possess functions such as leaky integrate-and-fire and output spike. However, previous reported artificial neurons typically have high operation voltage and large leakage current, leading to significant power consumption, which is contrary to the energy-efficient biological model. Here, an oscillatory neuron based on Ag filamentary threshold switching memristor (TS) that has a low operation voltage (<0.6 V) with ultralow power consumption (<1.8 µW) is presented. It can trigger neuronal functions, including leaky integrate-and-fire and threshold-driven spiking output, with high endurance (>108 cycles). Being connected to an external resistor or a resistive switching memristor (RS) as synaptic weight, the TS clearly demonstrates self-oscillation behavior once the input pulse voltage exceeds the threshold voltage. Meanwhile, the oscillation frequency is proportional to the input pulse voltage and the conductance of RS synapse, which can be used to integrate the weighted sum current. As an energy-efficient memristor-based spiking neural network, this combination of TS oscillatory neuron with RS synapse is further evaluated for image recognition achieving an accuracy of 79.2 ± 2.4% for CIFAR-10 subset.

Piezo-phototronic Effect Enhanced Efficient Flexible Perovskite Solar Cells.

Tremendous work has been made recently to improve the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs); the best reported value is now over 23%. However, further improving the PCEs of PSCs is challenged by material properties, device stability, and packaging technologies. Here, we report a new approach to increase the PCEs of flexible PSCs via introducing the piezo-phototronic effect in the PSCs by growing an array of ZnO nanowires on flexible plastic substrates, which act as the electron-transport layer for PSCs. From the piezo-phototronic effect, the absolute PCE was improved from 9.3 to 12.8% for flexible perovskite solar cells under a static mechanical strain of 1.88%, with a ∼40% enhancement but no change in the components of materials and device structure. A corresponding working model was proposed to elucidate the strategy to boost the performance of the PSCs. These findings present a general approach to improve PCEs of flexible PSCs without changing their fundamental materials.

Shape-Adaptive, Self-Healable Triboelectric Nanogenerator with Enhanced Performances by Soft Solid-Solid Contact Electrification.

The viable application of soft electronics/robotics relies on the development of power devices which are desired to be flexible, deformable, or even self-healable. We report here a shape-adaptive, self-healable triboelectric nanogenerator (SS-TENG) for harvesting biomechanical energies. The use of a viscoelastic polymer, normally known as Silly Putty, as the electrification material and as the matrix of a carbon-nanotube-filled composite (CNT-putty) electrode endows the SS-TENG the capability of adapting to arbitrary irregular surfaces and instantaneous healing from mechanical damage (almost completely recovered in 3 min without extra stimuli). Furthermore, the output performances of the SS-TENG have also been significantly improved because (i) the ideal soft contact is achieved at the solid-solid interfaces for more effective contact electrification and (ii) the introduced cation dopants make the putty even more tribo-negative than polytetrafluoroethylene. The SS-TENG can be adhered to any curvy surface, tailored, and reshaped into arbitrary configurations and utilized as a power supply for small electronics, suggesting promising applications in soft electronics/robotics in the future.

A Threshold Switching Selector Based on Highly Ordered Ag Nanodots for X-Point Memory Applications.

Leakage interference between memory cells is the primary obstacle for enlarging X-point memory arrays. Metal-filament threshold switches, possessing excellent selectivity and low leakage current, are developed in series with memory cells to reduce sneak path current and lower power consumption. However, these selectors typically have limited on-state currents (≤10 µA), which are insufficient for memory RESET operations. Here, a strategy is proposed to achieve sufficiently large RESET current (≈2.3 mA) by introducing highly ordered Ag nanodots to the threshold switch. Compared to the Ag thin film case, Ag nanodots as active electrode could avoid excessive Ag atoms migration into solid electrolyte during operations, which causes stable conductive filament growth. Furthermore, Ag nanodots with rapid thermal processing contribute to forming multiple weak Ag filaments at a lower voltage and then spontaneous rupture as the applied voltage reduced, according to quantized conductance and simulation analysis. Impressively, the Ag nanodots based threshold switch, which is bidirectional and truly electroforming-free, demonstrates extremely high selectivity >109, ultralow leakage current <1 pA, very steep slope of 0.65 mV dec-1, and good thermal stability up to 200 °C, and further represents significant suppression of leakage currents and excellent performances for SET/RESET operations in the one-selector-one-resistor configuration.