Triboelectric Nanogenerators for Preventive Health Monitoring.

With the improvement in life quality, the increased focus on health has expedited the rapid development of portable preventative-health-monitoring devices. As one of the most attractive sensing technologies, triboelectric nanogenerators (TENGs) are playing a more and more important role in wearable electronics, machinery condition monitoring, and Internet of Things (IoT) sensors. TENGs possess many advantages, such as ease of fabrication, cost-effectiveness, flexibility, material-selection variety, and the ability to collect low-frequency motion, offering a novel way to achieve health monitoring for human beings in various aspects. In this short review, we initially present the working modes of TENGs based on their applications in health monitoring. Subsequently, the applications of TENG-based preventive health monitoring are demonstrated for different abnormal conditions of human beings, including fall-down detection, respiration monitoring, fatigue monitoring, and arterial pulse monitoring for cardiovascular disease. Finally, the discussion summarizes the current limitations and future perspectives. This short review encapsulates the latest and most influential works on preventive health monitoring utilizing the triboelectric effect for human beings and provides hints and evidence for future research trends.

A Robust Droplet Triboelectric Nanogenerator with Self-Cleaning Ability Achieved by Femtosecond Laser.

A droplet triboelectric nanogenerator (TENG) has great potential for harvesting the high entropy energy in water. Despite extensive research, it still suffers from low average power density, poor long-term stability, and insufficient flexibility. Here, a porous micronanostructured polytetrafluoroethylene (PTFE) with superhydrophobicity and self-cleaning ability, is generated by femtosecond laser direct processing. The droplet TENG with laser treated PTFE (LT-PTFE) dielectric layer (L-DTENG) can reach a higher output compared with the droplet TENG with a PTFE dielectric layer (P-DTENG). L-DTENG also demonstrated good long-term stability, self-cleaning ability, and flexibility, making it suitable for various applications, including those involving dust and sewage pollution, as well as bending and pressing conditions. Furthermore, a simulation of finite element method (FEM) and an equivalent circuit model are established to understand the working mechanism of L-DTENG. This multifunctional device and theoretical research provide a smart strategy to generate electricity in a complex environment and lay a solid foundation for droplet TENG applications on a large scale.

2D Ruddlesden-Popper Polycrystalline PerovskitePyro-Phototronic Photodetectors.

Two-dimensional (2D) Ruddlesden-Popper (RP) layered halide perovskite has attracted wide attentions due to its unique structure and excellent optoelectronic properties. With inserting organic cations, inorganic octahedrons are forced to extend in a certain direction, resulting in an asymmetric 2D perovskite crystal structure and causing spontaneous polarization. The pyroelectric effect resulted from spontaneous polarization exhibits a broad prospect in the application of optoelectronic devices. Herein, 2D RP polycrystalline perovskite (BA)2 (MA)3 Pb4 I13 film with excellent crystal orientation is fabricated by hot-casting deposition, and a class of 2D hybrid perovskite photodetectors (PDs) with pyro-phototronic effect is proposed, achieving temperature and light detection with greatly improved performance by coupling multiple energies. Because of the pyro-phototronic effect, the current is ≈35 times to that of the photovoltaic effect current under 0 V bias. The responsivity and detectivity are 12.7 mA W-1 and 1.73 × 1011 Jones, and the on/off ratio can reach 3.97 × 103 . Furthermore, the influences of bias voltage, light power density, and frequency on the pyro-phototronic effect of 2D RP polycrystalline perovskite PDs are explored. The coupling of spontaneous polarization and light facilitates photo-induced carrier dissociation and tunes the carrier transport process, making 2D RP perovskites a competitive candidate for next-generation photonic devices.

Optical Force-Induced Nanowire Cut.

One-dimensional nanometer scale-sized materials, such as nanowires, nanotubes, etc., have gradually become new types of structural components, which can be integrated into micro/nano-opto-electromechanical systems. In this paper, optical forces were applied to cut nanowires precisely, which were broken with arbitrary length ratios. The optical force exerted by the optical tweezers proved to be the cause of the fracture of the high-aspect ratio nanowires, and the fracture mechanism of the nanowires was developed. Nanowires of different semiconductor materials were cut with optical tweezers in the experiments. The precise cut with optical tweezers can provide nanowires of appropriate lengths for the construction of nanowire-based structures, which have potential applications for micromachining and microfabrication of micro-electro-mechanical system or semiconductor devices.

Can Vacuum Deposition Apply to Bismuth-Doped γ-CsPbI3 Perovskite? Revealing the Role of Bi3+ in the Formation of Black Phase.

The B-site doped CsPbI3 has been demonstrated to be very promising for photovoltaics owing to its low black phase transition temperature. Though B-site doped black-CsPbI3 perovskites have been successfully achieved by solution-processing, it is unclear whether these systems are available by other methods such as vacuum deposition. In this work, heterovalent doped CsPb1-xBixI3 is targeted. To incorporate Bi3+ into the final film via vacuum deposition, the solid solution precursor Pb1-xBixI2 (0.01 ≤ x ≤ 0.04) is developed. However, these coevaporated films not only are dominated by another hexagonal perovskite phase but also fail to decrease the black phase transition temperature. The role of Bi3+ in the formation of the black phase is further studied by solution methods with different types of precursors. It is demonstrated that the key factor in the low-temperature black phase transition is small grain size, as well as the colloid size within the precursor solution, rather than simple substitution of Pb2+ with Bi3+.

Quantifying and understanding the triboelectric series of inorganic non-metallic materials.

Contact-electrification is a universal effect for all existing materials, but it still lacks a quantitative materials database to systematically understand its scientific mechanisms. Using an established measurement method, this study quantifies the triboelectric charge densities of nearly 30 inorganic nonmetallic materials. From the matrix of their triboelectric charge densities and band structures, it is found that the triboelectric output is strongly related to the work functions of the materials. Our study verifies that contact-electrification is an electronic quantum transition effect under ambient conditions. The basic driving force for contact-electrification is that electrons seek to fill the lowest available states once two materials are forced to reach atomically close distance so that electron transitions are possible through strongly overlapping electron wave functions. We hope that the quantified series could serve as a textbook standard and a fundamental database for scientific research, practical manufacturing, and engineering.

Alternating Current Photovoltaic Effect.

It is well known that the photovoltaic effect produces a direct current (DC) under solar illumination owing to the directional separation of light-excited charge carriers at the p-n junction, with holes flowing to the p-side and electrons flowing to the n-side. Here, it is found that apart from the DC generated by the conventional p-n photovoltaic effect, there is another new type of photovoltaic effect that generates alternating current (AC) in the nonequilibrium states when the illumination light periodically shines at the junction/interface of materials. The peak current of AC at high switching frequency can be much higher than that from DC. The AC cannot be explained by the established mechanisms for conventional photovoltaics; instead, it is suggested to be a result of the relative shift and realignment between the quasi-Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions, which results in electron flow in the external circuit back and forth to balance the potential difference between two electrodes. By virtue of this effect, the device can work as a high-performance broadband photodetector with extremely high sensitivity under zero bias; it can also work as a remote power source providing extra power output in addition to the conventional photovoltaic effect.

All-inorganic perovskite CsPbBr3 microstructures growth via chemical vapor deposition for high-performance photodetectors.

Perovskite cesium lead halide (CsPbBr3) has attracted considerable attention due to its excellent optoelectronic properties and superior stability against moisture, oxygen, light, and heat. In this work, the micro-environment controlled chemical vapor deposition (CVD) method has been adopted to synthesize high-quality single-crystalline CsPbBr3 microstructures, including microwires, microplates and triangular pyramids. Moreover, the structure-activity relationship between the material microstructures and the device properties is illustrated. The results show that photodetectors based on a single horizontal CsPbBr3 microwire exhibit a high responsivity (312.2 A W-1) and a fast response time of 5.8 ms. Photodetectors based on a single CsPbBr3 microplate exhibit a responsivity of 1.74 A W-1 and a response of 10 ms. These results indicate that the CsPbBr3 microwire photodetector is characterized by a higher photodetector performance when compared to the microplate due to its excellent crystallization quality and the Fabry-Pérot cavity effect in the microwire. Furthermore, the flexible CsPbBr3 microwire photodetector was demonstrated on a mica substrate. The results show that the photocurrent can be maintained at 90% after 3000 cycles at a bending radius of 2.5 mm. This work demonstrates the structure-activity photodetector performance, which is essential to develop a full understanding about high-performance optoelectronic devices based on all-inorganic lead halide perovskite materials.

Quantifying the triboelectric series.

Triboelectrification is a well-known phenomenon that commonly occurs in nature and in our lives at any time and any place. Although each and every material exhibits triboelectrification, its quantification has not been standardized. A triboelectric series has been qualitatively ranked with regards to triboelectric polarization. Here, we introduce a universal standard method to quantify the triboelectric series for a wide range of polymers, establishing quantitative triboelectrification as a fundamental materials property. By measuring the tested materials with a liquid metal in an environment under well-defined conditions, the proposed method standardizes the experimental set up for uniformly quantifying the surface triboelectrification of general materials. The normalized triboelectric charge density is derived to reveal the intrinsic character of polymers for gaining or losing electrons. This quantitative triboelectric series may serve as a textbook standard for implementing the application of triboelectrification for energy harvesting and self-powered sensing.

Dramatically Enhanced Broadband Photodetection by Dual Inversion Layers and Fowler-Nordheim Tunneling.

Silicon photonics is now widely accepted as a key technology in a variety of systems. But owing to material limitations, now it is challenging to greatly improve the performance after decades of development. Here, we show a high-performance broadband photodetector with significantly enhanced sensitivity and responsivity operating over a wide wavelength range of light from near-ultraviolet to near-infrared at low power consumption. The specially designed textured top ceiling electrode works effectively as an antireflection layer to greatly improve the absorption of near-infrared light, thereby overcoming the absorption limitation of near-infrared light. Instead of the conventional p-n junction and p-intrinsic-n junction, we introduce a ∼15 nm thick alumina insulator layer between a p-type Si substrate and n-type ZnO nanowire (NW) arrays, which significantly enhances the charge carrier separation and collection efficiency. The photosensing responsivity and sensitivity are found to be nearly 1 order of magnitude higher than that of a reference device of p-Si/n-ZnO NW arrays, significantly higher than the commercial silicon photodiodes as well. The light-induced charge carriers flow across the appropriate thickness of insulator layer via the quantum mechanical Fowler-Nordheim tunneling mechanism. By virtue of the piezo-phototronic effect, the charge density at the interfaces can be tuned to alter the energy bands and the potential barrier distance for tunneling. Additionally, along with the use of incident light of different wavelengths, the influence of the insulator layer on the transport of electrons and holes separately is further investigated. The demonstrated concepts and study would lead to sensitivity improvement, quality enhancement of data transfer, decrease of power consumption, and cost reduction of silicon photonics.

High-performance solar-blind SnO2 nanowire photodetectors assembled using optical tweezers.

One-dimensional semiconducting SnO2 nanowires with wide bandgaps are promising candidates to build many important optoelectronic devices. Because building these devices involves the assembly of nanowires into complex structures, manipulation of the active materials needs to be done with high spatial precision. In this paper, an optical tweezer system, comprising a spatial light-modulator, a microscope, and optical elements, is used to individually trap, transfer, and assemble SnO2 nanowires into two-terminal photodetectors in a liquid environment. After the assembly using optical trapping, the two ends of the SnO2 nanowire photodetectors, which are connected with the electrodes, were further stabilized using a focused laser. During exposure to 275 nm deep-ultraviolet light, the as-assembled photodetectors show a high Iph/Idark ratio of 2.99 × 105, a large responsivity of 4.3 × 104 A W-1, an excellent external quantum efficiency of 1.94 × 105, and a high detectivity of 2.32 × 1013 Jones. The photoresponse-speed of the devices could be improved further using passivation with a polymer. The rise and decay times are about 60 ms and 100 ms, respectively. As a result of this study, we can confirm that non-contact optical trapping can enable the construction of nanowire architectures for optoelectronic, bioelectronic, and other devices.

An Ultra-Low-Friction Triboelectric-Electromagnetic Hybrid Nanogenerator for Rotation Energy Harvesting and Self-Powered Wind Speed Sensor.

Triboelectric nanogenerators (TENGs) are attracting more and more attention since they can convert various mechanical energies into electric energy. However, in traditional TENGs for harvesting rotation energy, most of the contacts between two triboelectric materials are rigid-to-rigid contact with very large friction force, which limits their practical application. Here, we report an ultra-low-friction triboelectric-electromagnetic hybrid nanogenerator (NG). A freestanding mode TENG and a rotating electromagnetic generator (EMG) are integrated together to realize the complementary individual merits. The very soft and elastic contact between the two triboelectric materials in the TENG results into very small friction force. The influences of the type and the dimensions of the dielectric material on the performance of the TENG are studied systematically from theory to experiments. The results indicate that the open-circuit voltage and the transfer charge of the TENG increase with the rotation speed, which is very different from a traditional rotary TENG and is due to the increase of the contact area. The optimized TENG has a maximal load voltage of 65 V and maximal load power per unit mass of 438.9 mW/kg under a speed rotation of 1000 rpm, while the EMG has a maximal load voltage of 7 V and maximal load power density of 181 mW/kg. This demonstration shows that the hybrid NG can power a humidity/temperature sensor by converting wind energy into electric energy when the wind speed is 5.7 m/s. Meanwhile, it can be used as a self-powered wind speed sensor to detect wind speed as low as 3.5 m/s.

High-performance ultraviolet photodetectors based on CdS/CdS:SnS2 superlattice nanowires.

CdS heterostructure nanomaterials are attractive for their potential applications in integrated optoelectronic devices. Herein, the high-quality CdS/CdS:SnS2 superlattice nanowires were synthesized through a micro-environmental controlled co-evaporation technique, which shows periodic emission properties and that their structures are periodic and alternating. For the first time, we demonstrate the fabrication of high-performance ultraviolet photodetectors using unique CdS/CdS:SnS2 superlattice nanowires. The optoelectronic properties of the photodetectors were studied and compared to those devices based on pure CdS nanowires. The as-fabricated photodetectors (under 365 nm) based on CdS/CdS:SnS2 superlattice nanowires showed a high photocurrent to dark current ratio of 10(5), a large photoresponsivity of 2.5 × 10(3) A W(-1), a fast response time of 10 ms and an excellent external quantum efficiency of 8.6 × 10(5) at room temperature, which shows better performance than pure CdS nanowires photodetectors. The results indicate that CdS/CdS:SnS2 superlattice nanowires are very promising potential candidates in nanoscale electronic and optoelectronic devices.

[Study on the Lateral Growth of Ag Nanowires and Its Inhibition].

Ag nanowires (Ag NWs) are synthesized by polyol method under the conditions of different temperature of reaction solution, different addition amount and injection rate of polyvinylpyrrolidone (PVP). The structure and the process of lateral growth of Ag NWs were observed and analyzed by X-ray diffraction (XRD), ultraviolet-visible absorption spectrum (UV-VIS), scanning electron microscopy (SEM) and transmission electron microscope (TEM). It showed that the lateral growth of Ag NWs and longitudinal growth of Ag NWs occurrs at the same time by UV-VIS. And in the later stage of synthesis of Ag NWs, the peak in UV-VIS, which indicated the lateral growth of Ag NWs, red-shifted obviously from 384 nm to 388 nm. This rapid redshift implied that the diameters of Ag NWs increased quickly. In other words, rapid lateral growth of Ag NWs occurred in the later stage of synthesis of Ag NWs. According to the SEM of Ag NWs, in the early stage of the reaction (15~23 min), the diameter of Ag NWs increased by only 20 nm, but in the later stage of reaction (23~30 min), the diameter of Ag NWs increased by nearly 150 nm. The result of SEM observation is consistent with the analysis of UV-VIS. It was also found that the lateral growth of Ag NWs is related not only to the sizes of seeds but also to the thicknesses of the outer Ag layers. Tiny Ag particles with diameters of several nanometers adsorbed onto the side facets of Ag NWs and acted as adsorption points for Ag source. The lateral growth of Ag NWs was caused by the continuous multipoint adsorption of Ag source on the side of Ag NWs. Decreasing the temperature of the reaction solution (from 165 to 155 degree), reducing the injection rate (from 67 to 49 mL·h-1) and the addition amount of PVP in the later stage could inhibit the lateral growth of Ag NWs and increase the aspect ratios of Ag NWs remarkably. The diameters of Ag NWs decreased from 200 nm to 100 nm, but their lengths still maintained above 100 µm.

Preparation and periodic emission of superlattice CdS/CdS:SnS2 microwires.

Semiconductor superlattice micro-/nanowires could greatly increase the versatility and power of modulating electronic (or excitonic, photonic) transport, optical properties. In this communication, we report growth of a semiconductor CdS/CdS:SnS(2) superlattice microwire through a coevaporation technique with microenvironmental control. Such a novel superlattice microwire can modulate the exciton and photons to show multipeak emissions with periods in a wide spectral range, which arise in the 1-d photon crystal and confined exciton emission. This system can be widely used in producing multicolor emissions, low-threshold lasing, study light-matter interaction, slow light engineering, and weak nonlinear optical devices.

The large-scale synthesis of one-dimensional TiO2 nanostructures using palladium as catalyst at low temperature.

The synthesis of TiO(2) nanostructures including nanowires and nanobelts has been demonstrated experimentally by anodization of Ti foil in an electrolyte, and by treatment in a PdCl(2) ethanol solution together with UV light irradiation and annealing at a temperature below 800 degrees C. The TiO(2) nanotube arrays resulting from the anodization were used as source precursor and transformed into nanowires and nanobelts respectively with high efficiency during the subsequent processes. The resulting TiO(2) nanowires and nanobelts, characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman and surface photovoltage (SPV) spectroscopy, are single rutile crystals of high quality. In addition to the synthesis of the nanostructure at low temperature, this method also shows great advantages for the selectable morphology of the final TiO(2) nanostructures via adjustment of the UV light irradiation time and annealing temperature.

Ordered CdS micro/nanostructures on CdSe nanostructures.

Composite structures of aligned and orientation-ordered quasi-one-dimensional CdS micro/nanostructures on CdSe substrates of different shaped nanostructures have been synthesized by using two-step thermal evaporation processes. The CdSe substrate crystalline orientations and local temperatures play their roles in the CdS nanostructure growth step, which is in some contrast with the vapor-liquid-solid (VLS) growth mechanism. Micro-photoluminescence measurements show strong luminescence responses on the six-fold symmetrical CdSe/CdS nanostructure. Controllable growth on various shaped substrates may find applications in obtaining many other aligned orientation-ordered hetero-nano/microstructure materials.