Aifang Yu

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.

Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators

In this work, by employing halogen elements (fluorine, chlorine, bromine, and iodine) as dopant we demonstrate a unique strategy to enhance the output performance of ZnO-based flexible piezoelectric nanogenerators. For a halogen-doped ZnO nanowire film, dopants and doping concentration dependent lattice strain along the ZnO c-axis are established and confirmed by the EDS, XRD, and HRTEM analysis. Although lattice strain induced charge separation was theoretically proposed, it has not been experimentally investigated for wurtzite structured ZnO nanomaterials. Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to other halogen elements due to the ionic size difference between dopants and oxygen. With its focus on a group of nonmetal element induced lattice strain in ZnO-based nanomaterials, this work paves the way for enhancing the performance of wurtzite-type piezoelectric semiconductor nanomaterials via lattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effective manner.

Nanopillar Arrayed Triboelectric Nanogenerator as a Self-Powered Sensitive Sensor for a Sleep Monitoring System

A flexible and low-cost triboelectric nanogenerator (TENG) based on a patterned aluminum-plastic film and an entrapped cantilever spring leaf is developed as a self-powered sensitive triboelectric sensor for sleep-body movement monitoring. The working mechanism and the impact factors of electric output performance were systematically investigated and elaborated. Due to the patterned nanostructures of the recently designed TENG, both the output voltage and current are greatly enhanced, and thereby the sensitivity of the device is significantly improved. The self-powered and sensitive device has been demonstrated as a smart body motion sensor of sleep monitoring for diagnosis of sleep disorders due to its high sensitivity and excellent stability. This work may promote the application of self-powered TENGs for healthcare and be helpful for the development of real-time mobile healthcare services and smart external portable electronics.

Self-powered acoustic source locator in underwater environment based on organic film triboelectric nanogenerator

Detecting/sensing targets underwater has very important applications in environmental study, civil engineering and national security. In this paper, an organic-film based triboelectric nanogenerator (TENG) has been successfully demonstrated for the first time as a self-powered and high sensitivity acoustic sensor to detect underwater targets at low frequencies around 100 Hz. This innovative, cost-effective, simple-design TENG consists of a thin-film-based Cu electrode and a polytetrafluoroethylene (PTFE) film with nanostructures on its surfaces. On the basis of the coupling effect between triboelectrification and electrostatic induction, the sensor generates electrical output signals in response to incident sound waves. Operating at a resonance frequency of 110 Hz, under an acoustic pressure of 144.2 dBSPL, the maximum open-circuit voltage and short-circuit current of the generator can respectively reach 65 V and 32 μA underwater. The directional dependence pattern has a bi-directional shape with a total response angle of 60°. Its sensitivity is higher than −185 dB in the frequency range from 30 Hz to 200 Hz. The highest sensitivity is −146 dB at resonance frequency. The three-dimensional coordinates of an acoustic source were identified by four TENGs, self-powered active sensors, and the location of the acoustic source was determined with an error about 0.2 m. This study not only expands the application fields of TENGs from the atmosphere to water, but also shows the TENG is a promising acoustic source locator in underwater environments.

A nanogenerator as a self-powered sensor for measuring the vibration spectrum of a drum membrane

A nanogenerator (NG) is a device that converts vibration energy into electricity. Here, a flexible, small size and lightweight NG is successfully demonstrated as an active sensor for detecting the vibration spectrum of a drum membrane without the use of an external power source. The output current/voltage signal of the NG is a direct measure of the strain of the local vibrating drum membrane that contains rich informational content, such as, notably, the vibration frequency, vibration speed and vibration amplitude. In comparison to the laser vibrometer, which is excessively complex and expensive, this kind of small and low cost sensor based on an NG is also capable of detecting the local vibration frequency of a drum membrane accurately. A spatial arrangement of the NGs on the membrane can provide position-dependent vibration information on the surface. The measured frequency spectrum can be understood on the basis of the theoretically calculated vibration modes. This work expands the application of NGs and reveals the potential for developing sound wave detection, environmental/infrastructure monitoring and many more applications.

A flexible p-CuO/n-MoS2 heterojunction photodetector with enhanced photoresponse by the piezo-phototronic effect

Flexible functional devices based on two dimensional (2D) materials are extremely suitable for malleable, portable and sustainable applications, such as health monitoring, electronic skin and optoelectronics. In this work, we developed a flexible photodetector based on a p-CuO/n-MoS2 heterojunction with an enhancement in photocurrent and detection sensitivity. Because of the non-centrosymmetric structure in monolayer MoS2, the piezo-potential induced by applied strain adjusts the band structure at the heterojunction interface and broadens the depletion region based on the piezo-phototronic effect. The border depletion can be discreetly used to improve the photo-generated carrier separation and transport to enhance photoresponse performance. When illuminated by a 532 nm laser, the photocurrent of the heterojunction can be enhanced 27 times under a tensile strain of 0.65% compared to strain free conditions and the detection sensitivity can reach up to 3.27 × 108 Jones. As a result, our research provides a new strategy for novel design and performance optimization of 2D material heterostructures in the application of optoelectronics.

Raman study of 2D anatase TiO2 nanosheets

Herein, we present for the first time a spectroscopic study of two-dimensional (2D) anatase TiO2 nanosheets. Previous publications demonstrated that Raman spectroscopy was mostly employed to characterize the TiO2 nanoparticle size and the phase transition of amorphous-anatase and anatase-rutile. In this study, TiO2 nanosheets were characterized by XRD, AFM and Raman spectroscopy, which demonstrated a shift toward higher frequency and broadening in the full width at half maximum of the characteristic Eg mode by decreasing the thickness of anatase TiO2 with a 2D nanostructure. In contrast to the study of TiO2 nanoparticles, the Raman vibrations can be attributed to phonon confinement in 2D TiO2 nanosheets which can be employed to characterize the thickness of TiO2 nanosheets. In order to effectively identify the thickness of the 2D TiO2 nanostructure, we established a reliable method for the examination by characterizing the shifts of the Eg mode.

Core–Shell-Yarn-Based Triboelectric Nanogenerator Textiles as Power Cloths

Although textile-based triboelectric nanogenerators (TENGs) are highly promising because they scavenge energy from their working environment to sustainably power wearable/mobile electronics, the challenge of simultaneously possessing the qualities of cloth remains. In this work, we propose a strategy for TENG textiles as power cloths in which core-shell yarns with core conductive fibers as the electrode and artificial polymer fibers or natural fibrous materials tightly twined around core conductive fibers are applied as the building blocks. The resulting TENG textiles are comfortable, flexible, and fashionable, and their production processes are compatible with industrial, large-scale textile manufacturing. More importantly, the comfortable TENG textiles demonstrate excellent washability and tailorability and can be fully applied in further garment processing. TENG textiles worn under the arm or foot have also been demonstrated to scavenge various types of energy from human motion, such as patting, walking, and running. All of these merits of proposed TENG textiles for clothing uses suggest their great potentials for viable applications in wearable electronics or smart textiles in the near future.

Self-Powered Random Number Generator Based on Coupled Triboelectric and Electrostatic Induction Effects at the Liquid–Dielectric Interface

Modern cryptography increasingly employs random numbers generated from physical sources in lieu of conventional software-based pseudorandom numbers, primarily owing to the great demand of unpredictable, indecipherable cryptographic keys from true random numbers for information security. Thus, far, the sole demonstration of true random numbers has been generated through thermal noise and/or quantum effects, which suffers from expensive and complex equipment. In this paper, we demonstrate a method for self-powered creation of true random numbers by using triboelectric technology to collect random signals from nature. This random number generator based on coupled triboelectric and electrostatic induction effects at the liquid-dielectric interface includes an elaborately designed triboelectric generator (TENG) with an irregular grating structure, an electronic-optical device, and an optical-electronic device. The random characteristics of raindrops are harvested through TENG and consequently transformed and converted by electronic-optical device and an optical-electronic device with a nonlinear characteristic. The cooperation of the mechanical, electrical, and optical signals ensures that the generator possesses complex nonlinear input-output behavior and contributes to increased randomness. The random number sequences are deduced from final electrical signals received by an optical-electronic device using a familiar algorithm. These obtained random number sequences exhibit good statistical characteristics, unpredictability, and unrepeatability. Our study supplies a simple, practical, and effective method to generate true random numbers, which can be widely used in cryptographic protocols, digital signatures, authentication, identification, and other information security fields.

High-index facets bound ripple-like ZnO nanobelts grown by chemical vapor deposition

Single-crystal ripple-like ZnO nanobelts (NBs) were formed spontaneously in a chemical vapor deposition process. Part of the ZnO NBs exhibits ripple-like structure and “propagates” along the growth direction of the ZnO NBs. The flat side-facets of the ripple-like structures are close to the high-index facets of and the major surfaces of such ripple-like structure belong to the {20} planes.