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Owing to magic charge storage behavior, an electret can exhibit an external electrostatic field, which is widely used in numerous domains such as electronics, energy, healthcare, and environment. However, the theory of the charge storage mechanism still needs further development to enhance the performance and stability of the electret. Herein, a stable charge storage model known as the heterocharge-synergy model (HSM) in electrets is proposed and verified, and the electrostatic field superposition effect of electrets is also proved. Based on the HSM and superposition effect, the stable electrostatic field intensity (average of ≈22.49 kV cm

Recently, wearable and breathable healthcare devices for air filtering and real-time vital signs monitoring have become urgently needed since virus and particulate matter (PM) cause serious health issues. Herein, we present a trap-induced dense monocharged hybrid perfluorinated electret nanofibrous membrane (HPFM) for highly efficient ultrafine PM0.3 removal with an efficiency of 99.712% under low pressure drop (38.1 Pa) and high quality factor of 0.154 Pa-1. Furthermore, a recyclable multifunctional healthcare mask is constructed by integrating the HPFM-based nanogenerator, which realizes efficient PM0.3 filtering and wireless real-time human respiration monitoring simultaneously. More importantly, the performance of this mask is still relatively stable even at 100%RH humidity and 92 °C temperature conditions for 48 h, which infers that it can be reused after disinfection. The strategy of fabricating HPFM provides an approach to obtain charge-rich stable electret materials, and the design of multifunctional masks demonstrates their potential application for future personal protection and health monitoring devices.

The combination of grating-based frequency-selective optical feedback mechanisms, such as distributed feedback (DFB) or distributed Bragg reflector (DBR) structures, with quantum dot (QD) gain materials is a main approach towards ultrahigh-performance semiconductor lasers for many key novel applications, as either stand-alone sources or on-chip sources in photonic integrated circuits. However, the fabrication of conventional buried Bragg grating structures on GaAs, GaAs/Si, GaSb, and other material platforms has been met with major material regrowth difficulties. We report a novel and universal approach of introducing laterally coupled dielectric Bragg gratings to semiconductor lasers that allows highly controllable, reliable, and strong coupling between the grating and the optical mode. We implement such a grating structure in a low-loss amorphous silicon material alongside GaAs lasers with InAs/GaAs QD gain layers. The resulting DFB laser arrays emit at pre-designed 0.8 THz local area network wavelength division multiplexing frequency intervals in the 1300 nm band with record performance parameters, including sidemode suppression ratios as high as 52.7 dB, continuous-wave output power of 26.6 mW (room temperature) and 6 mW (at 55°C), and ultralow relative intensity noise (RIN) of < − 165 dB / Hz (2.5–20 GHz). The devices are also capable of isolator-free operating under very high external reflection levels of up to − 12.3 dB while maintaining high spectral purity and ultralow RIN qualities. These results validate the novel laterally coupled dielectric grating as a technologically superior and potentially cost-effective approach for fabricating DFB and DBR lasers free of their semiconductor material constraints, which are thus universally applicable across different material platforms and wavelength bands.

Advances in mobile networks and low-power electronics have driven smart mobile medical devices at a tremendous pace, evoking increased interest in household healthcare, especially for those with cardiovascular or respiratory disease. Thus, flexible battery-free pressure sensors, with great potential for monitoring respiration and heartbeat in a smart way, are urgently demanded. However, traditional flexible battery-free pressure sensors for subtle physiological signal detecting are mostly tightly adhered onto the skin instead of working under the pressure of body weight in a noncontact mode, as the low sensitivity in the high-pressure region can hardly meet the demands. Moreover, a hollow microstructure (HM) with higher deformation than solid microstructures and great potential for improving the pressure sensitivity of self-powered sensors has never been investigated. Here, for the first time, we demonstrated a noncontact heartbeat and respiration monitoring system based on a flexible HM-enhanced self-powered pressure sensor, which possesses the advantages of low cost, a high dynamic-pressure sensitivity of 18.98 V·kPa

Flexible pressure sensors have attracted increasing attention for wide application in wearable electronics and artificial intelligence. To enhance the pressure sensitivity of such sensors, a theoretical study based on modeling and experimental validation is required. Here, we theoretically demonstrated a method for optimizing the sensitivity of flexible piezoelectret-based pressure sensors (FPPS) by investigating the electric-mechanical behavior. A new model was established to estimate the compressibility and analyze the pressure sensitivity of the tunable FPPS. Experimental verification was carried out by fabrication of an FPPS based on a perfluoro (alkoxy alkane) (PFA) electret and tuning of the supporting structures. Based on tuning of the pressure sensitivity of the FPPS, a pressure sensor with a sensitivity of 15 V kPa−1 in the range of 0–2.5 kPa was demonstrated and successfully applied for energy harvesting and self-powered human biological signal detection.

Piezoelectric polymers hold physical properties, such as high mechanical flexibility and smaller acoustical impedance, that piezoelectric ceramics do not have. Nevertheless, owing to the piezoelectricity originates from the crystalline region, it is still a challenge for most semi-crystalline polymers to generate a high piezoelectric response. Here, we develop a class of piezoelectret with sandwiched polymer structure (electret/dielectric/electret, EDE) which is assembled via simple electrostatic interaction. The transparent and mechanical robust EDE piezoelectrets exhibit the piezoelectric constant (d33) up to ~930 pC N-1, and the corresponding physical model is established to investigate major influence factors to the piezoelectric properties, including the relative permittivity and elastic modulus of the electret and dielectric, and the simulated results are in accord with the experimental data. We demonstrate the strain sensors and acoustic transducers based on the ultrathin and flexible EDE piezoelectret, verifying the feasibility of the electrostatic assembled piezoelectrets in the applications of epidermal and implantable electronics. Moreover, various combinations of polymers in the EDE structure can be employed based on our proposed model to explore other unique applications such as transient electronics.

Wearable healthcare devices with monitoring, treating and protecting applications can provide reliable and newfangled interactive service for people. Herein, a bi-functional smart face mask based on electrospun polyetherimide (PEI) electret nonwoven is developed. By virtue of the surplus charges maintained steadily in the PEI nonwoven, the smart face mask possesses bi-functions of removing the particulate matter and generating electricity. Moreover, the unique applications for dynamically monitoring the particulate matter removal ability and human's respiratory rates are demonstrated. Besides offering an unique air filter, this study will promisingly promote the progress of self-powered wearable electronics.

Flexible electret nanogenerators that can convert mechanical energy into electricity have received intensive attention in recent years. However, drawbacks of low outputs and impedance mismatch with traditional electronics limit their extensive applications. In this work, we propose a novel method to enhance outputs by optimizing device structure of a compact multilayer flexible nanogenerator (CMFN). Meanwhile, load resistance corresponding to maximum output power can be reduced to alleviate impedance mismatching. By pressing a 4 × 4 cm2 CMFN with a height of 2 mm, peak output current was reached at ∼3 mA, which was large enough to power a wireless remote system with an effective emitting distance of 30 cm. This study successfully demonstrates a simple and efficient strategy to fabricate flexible electrostatic nanogenerators with outstanding output properties, and promotes progress of self-powered electronics.

Polytetrafluoroethylene (PTFE) is a fascinating electret material widely used for energy harvesting and sensing, and an enhancement in the performance could be expected by reducing its size into nanoscale because of a higher surface charge density attained. Hence, the present study demonstrates the use of nanofibrous PTFE for high-performance self-powered wearable sensors. The nanofibrous PTFE is fabricated by electrospinning with a suspension of PTFE particles in dilute polyethylene oxide (PEO) aqueous solution, followed by a thermal treatment at 350 °C to remove the PEO component from the electrospun PTFE-PEO nanofibers. The obtained PTFE nanofibrous membrane exhibits good air permeability with pressure drop comparable to face masks, excellent mechanical property with tensile strength of 3.8 MPa, and stable surface potential of − 270 V. By simply sandwiching the PTFE nanofibrous membrane into two pieces of conducting carbon clothes, a breathable, flexible, and high-performance nanogenerator (NG) device with a peak power of 56.25 μW is constructed. Remarkably, this NG device can be directly used as a wearable self-powered sensor for detecting body motion and physiological signals. Small elbow joint bending of 30°, the rhythm of respiration, and typical cardiac cycle are clearly recorded by the output waveform of the NG device. This study demonstrates the use of electrospun PTFE nanofibrous membrane for the construction of high-performance self-powered wearable sensors. Electronic supplementary material The online version of this article (10.1186/s11671-019-3091-y) contains supplementary material, which is available to authorized users.

Following the rapid development of lightweight and flexible smart electronic products, providing energy for these electronics has become a hot research topic. The human body produces considerable mechanical and thermal energy during daily activities, which could be used to power most wearable electronics. In this context, fiber-based energy conversion devices (FBECD) are proposed as candidates for effective conversion of human-body energy into electricity for powering wearable electronics. Herein, functional materials, fiber fabrication techniques, and device design strategies for different classes of FBECD based on piezoelectricity, triboelectricity, electrostaticity, and thermoelectricity are comprehensively reviewed. An overview of fiber-based self-powered systems and sensors according to their superior flexibility and cost-effectiveness is also presented. Finally, the challenges and opportunities in the field of fiber-based energy conversion are discussed.

Figure S1. a. SEM images and b. XRD pattern of the PTFE-PEO nanofibrous membrane. Figure S2. a. SEM image and b. pressure drop as a function of gas flow rate of the conducting carbon cloth. Figure S3. a. Output of the NG device with different pressing forces at a frequency of 5â Hz. b. Output of the NG device at different frequencies with a pressing force of 5â N. (PDF 561 kb)

Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm−3 in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications., Two-dimensional atomic crystals are known to be effective electrode materials for energy storage applications. Here, the authors report the preparation of various two-dimensional metal oxides, including those which do not have a layered parent structure, via a salt templating strategy.

Wearable pressure sensors, as essential components of flexible wearable electronics, may play important roles in early diagnosis and mortality prevention for cardiovascular healthcare. However, traditional self-powered pressure sensors can hardly obtain both high sensitivity and wide pressure range, which makes perceiving both cardiac and vascular signals in a wearable self-powered way challenging as these physiological signals not only have imperceptible changes but also span a broad pressure range. Here, we develop a flexible hierarchical elastomer tuned self-powered pressure sensor, which achieves both high-sensitivity and wide pressure range, showing a high sensitivity of 7.989 V kPa−1 and wide pressure range of 0.1–60 kPa simultaneously. Fast response (40 ms) and high signal noise ratio (38 db) with good stability are also achieved. We demonstrate that the self-powered pressure sensor can not only sensitively monitor pulse, artery and heart condition but also monitor the wide range blood pressure, which satisfies the diverse needs of wearable cardiovascular healthcare. As most of the pressure related to human interface is less than 50 kPa, we envision the sensor holds great potential in wearable electronics, mobile healthcare and human-machine interaction.

Flexible pressure sensors possess promising applications in artificial electronic skin, intelligence robot, wearable health monitoring, flexible physiological signal sensing, etc. Herein, we design a flexible pressure sensor with robust stability, high sensitivity, and large linear pressure region on the basis of tetrafluoroethylene-hexafluoropropylene-vinylide (THV)/cyclic olefin copolymer (COC) piezoelectret nanogenerator. According to the theoretical analysis for piezoelectret nanogenerators with imbalanced charge distribution, THV and COC are utilized to promote the electric field inside the piezoelectret for output voltage enhancement. Meanwhile, the compression property of the piezoelectret nanogenerator is facilely tuned. Owing to high inner electric field and optimized compression property, the THV/COC piezoelectret nanogenerator exhibits a high sensitivity of 30 mV/kPa, which is 10 times higher than that of the traditional cellular polypropylene piezoelectret. Simultaneously, the linear pressure region reaches 150 kPa with excellent linearity ( R2 = 0.99963). The device is demonstrated to realize wearable pressure sensing with a wide pressure range from finger typing to fist hammering. This study presents a fabrication strategy for piezoelectret nanogenerators with high sensitivity and large linear pressure region, paving the way for development of wearable and flexible pressure sensing networks.

Intercalation capacitance is emerging as a promising approach to increasing energy density of electrochemical capacitors. However, the number of materials capable of intercalating ions other than small H+ and Li+ is limited. Here we report a K+ preintercalated hydrogenated MoO3 (KyMoO3−x) with enhanced conductivity and widened interlayer gaps. Different cations (Mg2+, Na+, K+ and Li+) were able to intercalate along the (010) facet of KyMoO3−x with high intercalation capacitance (374 F/cm3 at 0.5 A/g in 5 M LiCl). The cation intercalation behavior and the origin of the high capacitance have been systematically studied. Using artificial seawater as electrolyte, high volumetric capacitance (188 F/cm3 at 0.5 A/g) and good rate handling were also achieved. The KyMoO3−x electrode also showed a similar capacitance (196 F/cm3 at 0.5 A/g) and cycling performance (89% of initial capacitance maintained after 10,000 cycles) in natural seawater. When KyMoO3−x was fabricated into a symmetric pseudocapacitor with natural seawater as the electrolyte, the energy density and power density of the electrode reached 0.91 mWh/cm3 and 6.5 W/cm3, respectively. The impressive results indicate KyMoO3−x is a promising high-power handling pseudocapacitor electrode material.

Controlled stacking of different two-dimensional (2D) atomic layers will greatly expand the family of 2D materials and broaden their applications. A novel approach for synthesizing MoS2 /WS2 heterostructures by chemical vapor deposition has been developed. The successful synthesis of pristine MoS2 /WS2 heterostructures is attributed to using core-shell WO3-x /MoO3-x nanowires as a precursor, which naturally ensures the sequential growth of MoS2 and WS2 . The obtained heterostructures exhibited high crystallinity, strong interlayer interaction, and high mobility, suggesting their promising applications in nanoelectronics. The stacking orientations of the two layers were also explored from both experimental and theoretical aspects. It is elucidated that the rational design of precursors can accurately control the growth of high-quality 2D heterostructures. Moreover, this simple approach opens up a new way for creating various novel 2D heterostructures by using a large variety of heteronanomaterials as precursors.

High-speed optical communication systems are built for real-time, massive and remote information exchange. However, any power outage will paralyse the systems and cause a huge loss. Here we constructed a self-powered long-distance optical communication system (SLOCS) utilizing output enhanced parallel connected electret nanogenerators (NGs) as a backup power for the power outage. The output current of the electret NG was promoted from 1.45 μA to 8.14 μA through optimizing the thickness of the electret film. In the SLOCS, a coded message was successfully transmitted for 50 meters by pressing electret NGs. The as-fabricated SLOCS paves the way to a simple and cost-effective strategy for developing a reliable emergency communication system in case of power outage, simultaneously promoting the progress of self-powered electronics.

Wearable active sensors have extensive applications in mobile biosensing and human–machine interaction but require good flexibility, high sensitivity, excellent stability, and self-powered feature. In this work, cellular polypropylene (PP) piezoelectret was chosen as the core material of a sensitivity-enhanced wearable active voiceprint sensor (SWAVS) to realize voiceprint recognition. By virtue of the dipole orientation control method, the air layers in the piezoelectret were efficiently utilized, and the current sensitivity was enhanced (from 1.98 pA/Hz to 5.81 pA/Hz at 115 dB). The SWAVS exhibited the superiorities of high sensitivity, accurate frequency response, and excellent stability. The voiceprint recognition system could make correct reactions to human voices by judging both the password and speaker. This study presented a voiceprint sensor with potential applications in noncontact biometric recognition and safety guarantee systems, promoting the progress of wearable sensor networks.