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M13 bacteriophage (phage) nano- and microfibers were fabricated using electrospinning. Using liquid crystalline suspension of the phage, we successfully fabricated nano- and microscale pure phage fibers. Through a near field electrospinning process, we fabricated the desired phage fiber pattern with tunable direction and spacing. In addition, we demonstrated that the resulting phage fibers could be utilized as an electrostatic-stimulus responsive actuator. The near field electrospinning would be a very useful tool to design phage-based chemical sensors, tissue regenerative materials, energy generators, metallic and semiconductor nanowires in the future.

Despite their high power density, aqueous-based electric double-layer capacitors (EDLCs) possess relatively low energy density due to the limitation of potential window (~1.0 V) and low specific capacitance of active materials. To increase the energy density without sacrificing the power density, it is highly desired to achieve a simultaneous improvement of both specific capacitance and potential window of EDLCs. However, this remains a major challenge that is not been solved up to date. This work demonstrates that by inserting a polarized-polyvinylidene fluoride (PVDF) ferroelectric layer underneath the graphene, a simultaneous enhancement of both areal capacitance and potential window is achieved, in which the areal capacitance increases from 5.5 to 7.5 μF/cm2 (i.e., 36%) (or 55 F/cm3 to 75 F/cm3 in terms of volumetric capacitance), and the potential window expands from 1.0 V to 1.5 V. This results in a threefold increase in the areal energy density of the capacitor. The enhancement in capacitance can be explained by the Gouy–Chapman–Stern model. The widening of potential window is due to the shift of the Fermi level of graphene caused by the doping effect of the polarized-PVDF layer.

With the development of technology, electronic devices are becoming more miniaturized and multifunctional. With the development of small electronic devices, they are changing from the conventional accessory type, which is portable, to the patchable type, which can be attached to a person's apparel or body, and the eatable/implantable type, which can be directly implanted into the human body. In this regard, it is necessary to address various technical issues, such as high-capacity/high-efficiency small-sized battery technology, component miniaturization, low power technology, flexible technology, and smart sensing technology. In addition, there is a demand for self-powered wireless systems in particular devices. A piezoelectric/triboelectric nanogenerator (PENG/TENG) can generate electric energy from small amounts of mechanical energy such as from blood flow and heartbeats in the human body as well as human movement, so it is expected that it will enable the development of self-powered wireless systems. Due to their unique properties, such as flexibility, transparency, mechanical stability, and nontoxicity, 2D materials are optimal materials for the development of implantable and patchable self-powered nanodevices in the human body. In this Review, the studies related to patchable and implantable devices for the human body using PENGs/TENGs based on 2D materials are discussed.

Controlling the shape, geometry, density, and orientation of nanomaterials is critical to fabricate functional devices. However, there is limited control over the morphological and directional characteristics of presynthesized nanomaterials, which makes them unsuitable for developing devices for practical applications. Here, we address this challenge by demonstrating vertically aligned and polarized piezoelectric nanostructures from presynthesized biological piezoelectric nanofibers, M13 phage, with control over the orientation, polarization direction, microstructure morphology, and density using genetic engineering and template-assisted self-assembly process. The resulting vertically ordered structures exhibit strong unidirectional polarization with three times higher piezoelectric constant values than that of in-plane aligned structures, supported by second harmonic generation and piezoelectric force microscopy measurements. The resulting vertically self-assembled phage-based piezoelectric energy harvester (PEH) produces up to 2.8 V of potential, 120 nA of current, and 236 nW of power upon 17 N of force. In addition, five phage-based PEH integrated devices produce an output voltage of 12 V and an output current of 300 nA, simply by pressing with a finger. The resulting device can operate light-emitting diode backlights on a liquid crystal display. Our approach will be useful for assembling many other presynthesized nanomaterials into high-performance devices for various applications.

Next-generation memory and energy harvesting devices require a higher output performance for charging lectric devices. Generally, it is very limited to control charge transfer through triboelectricity in triboelectric materials. Here, using ferroelectric polarization, it is found that both the amount and direction of charge transfer in triboelectric materials can be controlled. The ferroelectric-dependent triboelectricity in a ferroelectric co-polymer film is explored using atomic force microscopy (AFM). Ferroelectric surfaces are rubbed with the AFM tip after poling with positive and negative bias voltages to achieve a triboelectric effect. The surface potential of the positively (negatively) poled area becomes smaller (larger) after rubbing the film surface with the AFM tip. Furthermore, the power output from the triboelectric nanogenerator is dependent on the ferroelectric polarization state. The results indicate that the amount and direction of the charge transfer in triboelectricity can be controlled by the ferroelectric polarization state.

Piezoelectric materials are excellent generators of clean energy, as they can harvest the ubiquitous vibrational and mechanical forces. We developed large-scale unidirectionally polarized, aligned diphenylalanine (FF) nanotubes and fabricated peptide-based piezoelectric energy harvesters. We first used the meniscus-driven self-assembly process to fabricate horizontally aligned FF nanotubes. The FF nanotubes exhibit piezoelectric properties as well as unidirectional polarization. In addition, the asymmetric shapes of the self-assembled FF nanotubes enable them to effectively translate external axial forces into shear deformation to generate electrical energy. The fabricated peptide-based piezoelectric energy harvesters can generate voltage, current, and power of up to 2.8 V, 37.4 nA, and 8.2 nW, respectively, with 42 N of force, and can power multiple liquid-crystal display panels. These peptide-based energy-harvesting materials will provide a compatible energy source for biomedical applications in the future.

Currently, integration of energy harvesting and storage devices is considered to be one of the most important energy-related technologies due to the possibility of replacing batteries or at least extending the lifetime of a battery. This review aims to describe current progress in the various types of energy harvesters, hybrid energy harvesters, including multi-type energy harvesters with coupling of multiple energy sources, and hybridization of energy harvesters and energy storage devices for self-powered electronics. We summarize research on recent energy harvesters based on the piezoelectric, triboelectric, pyroelectric, thermoelectric, and photovoltaic effects. We also cover hybrid cell technologies to simultaneously generate electricity using multiple types of environmental energy, such as mechanical, thermal, and solar energy. Energy harvesters based on the coupling of multiple energy sources exhibit enhancement of power generation performance with synergetic effects. Finally, integration of energy harvesters and energy storage devices is introduced. In particular, self-charging power cells provide an innovative approach to the direct conversion of mechanical energy into electrochemical energy to decrease energy conversion loss.

Transparent and flexible (TF) microheaters are required in wearable devices, labs-on-chip, and micro-reactors. Nevertheless, conventional microheaters are rigid or opaque or both. Moreover, the resistances of conventional metallic microheaters are too low to be effectively powered by wearable energy harvesters. Here, we demonstrate the first TF microheaters by taking advantage of chemical vapor deposition (CVD)-grown graphene heating tracks and of a hexagonal boron nitride (h-BN) sheet for passivation; the h-BN sheet increases the maximum temperature by ~80%. Our TF microheaters show excellent temperature uniformity and can reach temperatures above 200 °C in just 4 s, with power consumption as low as 39 mW. Additionally, since the CVD-graphene sheet resistance is orders of magnitude higher than that of typical metallic heaters, our devices can be effectively powered by wearable energy harvesters. As a proof-of-concept, we demonstrate the first self-powered, wearable microheater which achieves a temperature increase of 8 °C when operated by a sound driven textile-based triboelectric nanogenerator. This is a key milestone towards next generation microheaters with applications in portable/wearable personal electronics, wireless health, and remote and mobile environmental sensors.

Here micropatterned poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) films-based piezoelectric nanogenerators (PNGs) with high power-generating performance for highly sensitive self-powered pressure sensors are demonstrated. The microstructured P(VDF-TrFE)-based PNGs reveal nearly five times larger power output compared to a flat film-based PNG. The micropatterning of P(VDF-TrFE) polymer makes itself ultrasensitive in response to mechanical deformation. The application is demonstrated successfully as self-powered pressure sensors in which mechanical energy comes from water droplet and wind. The mechanism of the high performance is intensively discussed and illustrated in terms of strain developed in the flat and micropatterned P(VDF-TrFE) films. The impact derived from the patterning on the output performance is studied in term of effective pressure using COMSOL multiphysics software.

The search for harvesting both the mechanical and solar energies from a single hybrid system is of significant value and represents a new trend in energy harvesting technologies. This single hybrid system can utilize both the energy sources easily available from nature and most importantly it is clean and sustainable. It is a novel technique involving completely different physical principles utilized for scavenging different types of energies. This report presents studies of a hybrid power generator made a direct-current piezoelectric nanogenerator based on ZnO nanosheets and a bulk heterojunction organic solar cell based on an inverted structure. The device shows much larger electric power output compared to its two individual power output components, which facilitates more effective multi-type energies harvesting and clarifies a mechanism for realizing multi-functional energy devices.

Piezoelectric semiconductor materials have emerged as the most attractive material for nanogenerator (NG)-based prototype applications, such as piezotronics, piezophotonics and energy harvesting, due to the coupling of piezoelectric and semiconducting dual properties. Understanding the mechanisms for high power generation, charge transport behavior, energy band modulations, and role of depletion width in piezoelectric semiconducting p–n junction, through piezoelectric charges developed by external mechanical strains, are essential for various NGs. Here, we demonstrate enhancement of the output power of one-dimensional zinc oxide (ZnO) nanowires (NWs)-based NG using a p-type semiconductor polymer, by controlling their energy band at depletion width in the piezoelectric semiconducting p–n junction interface and native defects presented in as-grown ZnO NWs. The piezoelectric output performance from the P3HT-coated ZnO NWs-based NG was several times higher than that from the pristine ZnO NWs-based NG, under application of the same vertical compressive strain. Holes from the p-type P3HT polymer significantly reduced the piezoelectric potential screening effect caused by free electrons in ZnO. Theoretical investigations using COMSOL multiphysics software were also carried out, in order to understand the improvement in the performance of surface passivated ZnO NWs-based NG, in terms of free carriers concentration and holes diffusion, due to the formation of p–n junction at the interface of ZnO and P3HT, and depletion width change.

Here, we report the synthesis of lead-free single-crystalline two-dimensional (2D) vanadium(V)-doped ZnO nanosheets (NSs) and their application for high-performance flexible direct current (DC) power piezoelectric nanogenerators (NGs). The vertically aligned ZnO nanorods (NRs) converted to NS networks by V doping. Piezoresponse force microscopy studies reveal that vertical V-doped ZnO NS exhibit typical ferroelectricity with clear phase loops, butterfly, and well-defined hysteresis loops with a piezoelectric charge coefficient of up to 4 pm/V, even in 2D nanostructures. From pristine ZnO NR-based NGs, alternating current (AC)-type output current was observed, while from V-doped ZnO NS-based NGs, a DC-type output current density of up to 1.0 μAcm(-2) was surprisingly obtained under the same vertical compressive force. The growth mechanism, ferroelectric behavior, charge inverted phenomena, and high piezoelectric output performance observed from the V-doped ZnO NS are discussed in terms of the formation of an ionic layer of [V(OH)4(-)], permanent electric dipole, and the doping-induced resistive behavior of ZnO NS.

Here we report a new type of stretchable transparent piezoelectric nanogenerator (NG) using an organic piezoelectric material consisting of poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] sandwiched with mobility-modified chemical vapor deposition-grown graphene electrodes by ferroelectric polarization into P(VDF-TrFE). This new type of NG has a very high sensitivity and mechanical durability with fully flexible, rollable, stretchable, foldable, and twistable properties. We also investigated the mobility-modified graphene electrodes with ferroelectric P(VDF-TrFE) remnant polarization, and a mechanism is proposed for switching the mobility of the carriers by the ferroelectric remnant polarization. Upon exposure to the same input sound pressure, the measured output performance of the stretchable NG with a thin polydimethylsiloxane stretchable rubber template is up to 30 times that of a normal NG with a plastic substrate. Upon exposure to an air flow at the same speed, the measured output voltage from the stretchable NG is about 8 times larger than that of the normal NG.

In this work, a sulfur (S) vacancy passivated monolayer MoS2 piezoelectric nanogenerator (PNG) is demonstrated, and its properties before and after S treatment are compared to investigate the effect of passivating S vacancy. The S vacancies are effectively passivated by using the S treatment process on the pristine MoS2 surface. The S vacancy site has a tendency to covalently bond with S functional groups; therefore, by capturing free electrons, a S atom will form a chemisorbed bond with the S vacancy site of MoS2 . S treatment reduces the charge-carrier density of the monolayer MoS2 surface, thus the screening effect of piezoelectric polarization charges by free carrier is significantly prevented. As a result, the output peak current and voltage of the S-treated monolayer MoS2 nanosheet PNG are increased by more than 3 times (100 pA) and 2 times (22 mV), respectively. Further, the S treatment increases the maximum power by almost 10 times. The results suggest that S treatment can reduce free-charge carrier by sulfur S passivation and efficiently prevent the screening effect. Thus, the piezoelectric output peaks of current, voltage, and maximum power are dramatically increased, as compared with the pristine MoS2 .

Here we report a fully flexible, foldable nanopatterned wearable triboelectric nanogenerator (WTNG) with high power-generating performance and mechanical robustness. Both a silver (Ag)-coated textile and polydimethylsiloxane (PDMS) nanopatterns based on ZnO nanorod arrays on a Ag-coated textile template were used as active triboelectric materials. A high output voltage and current of about 120 V and 65 μA, respectively, were observed from a nanopatterned PDMS-based WTNG, while an output voltage and current of 30 V and 20 μA were obtained by the non-nanopatterned flat PDMS-based WTNG under the same compressive force of 10 kgf. Furthermore, very high voltage and current outputs with an average value of 170 V and 120 μA, respectively, were obtained from a four-layer-stacked WTNG under the same compressive force. Notably it was found there are no significant differences in the output voltages measured from the multilayer-stacked WTNG over 12 000 cycles, confirming the excellent mechanical durability of WTNGs. Finally, we successfully demonstrated the self-powered operation of light-emitting diodes, a liquid crystal display, and a keyless vehicle entry system only with the output power of our WTNG without any help of external power sources.

The past decade has been especially creative for nanogenerators as energy harvesting devices utilizing both piezoelectric and triboelectric properties. Most recently, self-charging power units using both nanogenerators and energy storage systems have begun to be investigated for portable and wearable electronics to be used in our daily lives. This review focuses on these hybrid devices with self-charging combined with energy harvesting storage systems based on the most recent reports. In this research update, we will describe the materials, device structures, integration, applications, and research progress up to the present on hybrid devices.

Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal-boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS2 exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal-stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe2 bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe2 bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)-grown WSe2 monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD-grown WSe2 bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe2 bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe2 for which the device performance becomes degraded above a strain of 0.63%.

Poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)), as a ferroelectric polymer, offers great promise for energy harvesting for flexible and wearable applications. Here, this paper shows that the choice of solvent used to dissolve the polymer significantly influences its properties in terms of energy harvesting. Indeed, the P(VDF-TrFE) prepared using a high dipole moment solvent has higher piezoelectric and pyroelectric coefficients and triboelectric property. Such improvements are the result of higher crystallinity and better dipole alignment of the polymer prepared using a higher dipole moment solvent. Finite element method simulations confirm that the higher dipole moment results in higher piezoelectric, pyroelectric, and triboelectric potential distributions. Furthermore, P(VDF-TrFE)-based piezoelectric, pyroelectric, and triboelectric nanogenerators (NGs) experimentally validate that the higher dipole moment solvent significantly enhances the power output performance of the NGs; the improvement is about 24% and 82% in output voltage and current, respectively, for piezoelectric NG; about 40% and 35% in output voltage and current, respectively, for pyroelectric NG; and about 65% and 75% in output voltage and current for triboelectric NG. In brief, the approach of using a high dipole moment solvent is very promising for high output P(VDF-TrFE)-based wearable NGs.

Direct current (DC) piezoelectric power generator is promising for the miniaturization of a power package and self-powering of nanorobots and body-implanted devices. Hence, we report the first use of two-dimensional (2D) zinc oxide (ZnO) nanostructure and an anionic nanoclay layer to generate piezoelectric DC output power. The device, made from 2D nanosheets and an anionic nanoclay layer heterojunction, has potential to be the smallest size power package, and could be used to charge wireless nano/micro scale systems without the use of rectifier circuits to convert alternating current into DC to store the generated power. The combined effect of buckling behaviour of the ZnO nanosheets, a self-formed anionic nanoclay layer, and coupled semiconducting and piezoelectric properties of ZnO nanosheets contributes to efficient DC power generation. The networked ZnO nanosheets proved to be structurally stable under huge external mechanical loads.

Mechanical energy is a potential energy source for self-powered electronic devices. Due to their unique semiconducting and piezoelectric properties, wurtzite-structured nanomaterials have been considered as potential candidates for piezoelectric nanogenerators that convert mechanical energy into electricity. In the present work, we report on the growth of Ga-doped ZnO (GZO) nanorods and investigate the performance of nanogenerators fabricated from undoped ZnO (UZO) nanorods, low Ga-doped ZnO (LGZO) nanorods, and high Ga-doped ZnO (HGZO) nanorods. A nanogenerator integrated with LGZO nanorods exhibited a current density of 1.2 microA/cm2, an enhancement over the 0.4 microA/cm2 and 0.7 microA/cm2 current densities of nanogenerators integrated with UZO and HGZO nanorods, respectively.

It is essential to stabilize and enhance the power output performance of piezoelectric nanogenerators (NGs) in order to use them as a sustainable power source for wireless sensors, microelectronics, and optoelectronic devices. Here, we report a facile route to stabilize and maximize the output performance of a zinc oxide (ZnO) nanorod (NR)-based NG in the presence of ultraviolet (UV) light by reducing the piezoelectric potential screening effect caused by the free electrons in the ZnO NRs via surface passivation. We experimentally and theoretically investigated the piezoelectric potential drop in the ZnO NR-based NG due to the piezoelectric potential screening effect. A simple thermal annealing treatment was applied to the pristine ZnO NRs in air atmosphere to stabilize the output performance of the NG even in the presence of UV light. The piezoelectric output voltage from the surface-passivated NG was approximately 25 times higher than that from the NG with no passivation under UV light illumination.