Shape adaptable and highly resilient 3D braided triboelectric nanogenerators as e-textiles for power and sensing.

Combining traditional textiles with triboelectric nanogenerators (TENGs) gives birth to self-powered electronic textiles (e-textiles). However, there are two bottlenecks in their widespread application, low power output and poor sensing capability. Herein, by means of the three-dimensional five-directional braided (3DB) structure, a TENG-based e-textile with the features of high flexibility, shape adaptability, structural integrity, cyclic washability, and superior mechanical stability, is designed for power and sensing. Due to the spatial frame-column structure formed between the outer braided yarn and inner axial yarn, the 3DB-TENG is also endowed with high compression resilience, enhanced power output, improved pressure sensitivity, and vibrational energy harvesting ability, which can power miniature wearable electronics and respond to tiny weight variations. Furthermore, an intelligent shoe and an identity recognition carpet are demonstrated to verify its performance. This study hopes to provide a new design concept for high-performance textile-based TENGs and expand their application scope in human-machine interfacing.

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.

Flexible and durable wood-based triboelectric nanogenerators for self-powered sensing in athletic big data analytics.

In the new era of internet of things, big data collection and analysis based on widely distributed intelligent sensing technology is particularly important. Here, we report a flexible and durable wood-based triboelectric nanogenerator for self-powered sensing in athletic big data analytics. Based on a simple and effective strategy, natural wood can be converted into a high-performance triboelectric material with excellent mechanical properties, such as 7.5-fold enhancement in strength, superior flexibility, wear resistance and processability. The electrical output performance is also enhanced by more than 70% compared with natural wood. A self-powered falling point distribution statistical system and an edge ball judgement system are further developed to provide training guidance and real-time competition assistance for both athletes and referees. This work can not only expand the application area of the self-powered system to smart sport monitoring and assisting, but also promote the development of big data analytics in intelligent sports industry.

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.

Contact-Electrification between Two Identical Materials: Curvature Effect.

It is known that contact-electrification (or triboelectrification) usually occurs between two different materials, which could be explained by several models for different materials systems ( Adv. Mater. 2018, 30, 1706790; Adv. Mater. 2018, 30, 1803968). But contact between two pieces of the chemically same material could also result in electrostatic charges, although the charge density is rather low, which is hard to understand from a physics point of view. In this paper, by preparing a contact-separation mode triboelectric nanogenerator using two pieces of an identical material, the direction of charge transfer during contact-electrification is studied regarding its dependence on curvatures of the sample surfaces. For materials such as polytetrafluoroethylene, fluorinated ethylene propylene, Kapton, polyester, and nylon, the positive curvature surfaces are net negatively charged, while the negative curvature surfaces tend to be net positively charged. Further verification of the above-mentioned trends was obtained under vacuum (∼1 Pa) and higher temperature (≤358 K) conditions. Based on the received data acquired for gentle contacting cases, we propose a curvature-dependent charge transfer model by introducing curvature-induced energy shifts of the surface states. However, this model is subject to be revised if the mutual contact mode turns into a sliding mode or more complicated hard-pressed contact mode, in which a rigorous contact between the two pieces of the same material could result in nanoscale damage/fracture and possible species transfer. Our study provides a primitive step toward understanding the basics of contact-electrification.

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.

Raising the Working Temperature of a Triboelectric Nanogenerator by Quenching Down Electron Thermionic Emission in Contact-Electrification.

As previously demonstrated, contact-electrification (CE) is strongly dependent on temperature, however the highest temperature in which a triboelectric nanogenerator (TENG) can still function is unknown. Here, by designing and preparing a rotating free-standing mode Ti/SiO2 TENG, the relationship between CE and temperature is revealed. It is found that the dominant deterring factor of CE at high temperatures is the electron thermionic emission. Although it is normally difficult for CE to occur at temperatures higher than 583 K, the working temperature of the rotating TENG can be raised to 673 K when thermionic emission is prevented by direct physical contact of the two materials via preannealing. The surface states model is proposed for explaining the experimental phenomenon. Moreover, the developed electron cloud-potential well model accounts for the CE mechanism with temperature effects for all types of materials. The model indicates that besides thermionic emission of electrons, the atomic thermal vibration also influences CE. This study is fundamentally important for understanding triboelectrification, which will impact the design and improve the TENG for practical applications in a high temperature environment.

On the Electron-Transfer Mechanism in the Contact-Electrification Effect.

A long debate on the charge identity and the associated mechanisms occurring in contact-electrification (CE) (or triboelectrification) has persisted for many decades, while a conclusive model has not yet been reached for explaining this phenomenon known for more than 2600 years! Here, a new method is reported to quantitatively investigate real-time charge transfer in CE via triboelectric nanogenerator as a function of temperature, which reveals that electron transfer is the dominant process for CE between two inorganic solids. A study on the surface charge density evolution with time at various high temperatures is consistent with the electron thermionic emission theory for triboelectric pairs composed of Ti-SiO2 and Ti-Al2 O3 . Moreover, it is found that a potential barrier exists at the surface that prevents the charges generated by CE from flowing back to the solid where they are escaping from the surface after the contacting. This pinpoints the main reason why the charges generated in CE are readily retained by the material as electrostatic charges for hours at room temperature. Furthermore, an electron-cloud-potential-well model is proposed based on the electron-emission-dominatedcharge-transfer mechanism, which can be generally applied to explain all types of CE in conventional materials.