Self-Powered Acceleration Sensor for Distance Prediction via Triboelectrification.

Accurately predicting the distance an object will travel to its destination is very important in various sports. Acceleration sensors as a means of real-time monitoring are gaining increasing attention in sports. Due to the low energy output and power density of Triboelectric Nanogenerators (TENGs), recent efforts have focused on developing various acceleration sensors. However, these sensors suffer from significant drawbacks, including large size, high complexity, high power input requirements, and high cost. Here, we described a portable and cost-effective real-time refreshable strategy design comprising a series of individually addressable and controllable units based on TENGs embedded in a flexible substrate. This results in a highly sensitive, low-cost, and self-powered acceleration sensor. Putting, which accounts for nearly half of all strokes played, is obviously an important component of the golf game. The developed acceleration sensor has an accuracy controlled within 5%. The initial velocity and acceleration of the forward movement of a rolling golf ball after it is hit by a putter can be displayed, and the stopping distance is quickly calculated and predicted in about 7 s. This research demonstrates the application of the portable TENG-based acceleration sensor while paving the way for designing portable, cost-effective, scalable, and harmless ubiquitous self-powered acceleration sensors.

Extremely Foldable and Highly Porous Reduced Graphene Oxide Films for Shape-Adaptive Triboelectric Nanogenerators.

Here, a thin and foldable porous reduced graphene oxide (rGO) fabricated by a solvent casting method (SC-rGO) is introduced. The SC-rGO is superior to aluminum as a positive triboelectric material in triboelectric nanogenerators (TENGs), significantly enhancing TENG output performance. The film shows extremely foldable features, where it could be folded by 1/16 size. The electrical properties and device performance of SC-rGO are optimized varying thicknesses from 5 to 30 µm. A 30 µm thick TENG with a non-annealed SC-rGO film (STENG) shows the highest output of about 255 µW cm-2 due to its high carrier concentration, low work function, and high surface area. After annealing, STENG performance is optimized with a 10 µm thick SC-rGO because their work functions decreases, while the corresponding carrier concentrations decrease according to the thickness of the SC-rGO films. The SC-rGO films are highly durable and stable, where their output and conductivity show negligible changes after 100 000 cycles of mechanical deformation. A large SC-rGO with a size of 13 × 3 cm2 is fabricated and is attached inside a person's arm to demonstrate the shape-adaptive characteristics. Consequently, 170 V is obtained and it turns on 19 green light emitting diodes by simply touching the STENG.

Grain Boundary Healing of Organic-Inorganic Halide Perovskites for Moisture Stability.

Although organic-inorganic halide perovskite (OIHP)-based photovoltaics have high photoconversion efficiency (PCE), their poor humidity stability prevents commercialization. To overcome this critical hurdle, focusing on the grain boundary (GB) of OIHPs, which is the main humidity penetration channel, is crucial. Herein, pressure-induced crystallization of OIHP films prepared with controlled mold geometries is demonstrated as a GB-healing technique to obtain high moisture stability. When exposed to 85% RH at 30 °C, OIHP films fabricated by pressure-induced crystallization have enhanced moisture stability due to the enlarged OIHP grain size and low-angle GBs. The crystallographic and optical properties indicate the effect of applying pressure onto OIHP films in terms of moisture stability. The photovoltaic devices with pressure-induced crystallization exhibited dramatically stabilized performance and sustained over 0.95 normalized PCE after 200 h at 40% RH and 30 °C.

Comb-structured triboelectric nanogenerators for multi-directional energy scavenging from human movements.

A triboelectric nanogenerator (TENG) is an emerging energy harvesting technology utilizing multi-directional, wasted mechanical energies stemming from vibrations, winds, waves, body movements, etc. In this study, we report a comb-structured TENG (CTENG) capable of effectively scavenging multi-directional motions from human movements, which include walking, jumping, and running. By attaching CTENG to a person's calf, we obtain a root-mean-square (RMS) power value of 5.28 µW (i.e. 13.12 V and 0.4 µA) for 1 s during mild running action (~5 m/s), which is sufficient for powering 10 light emitting diodes (LEDs). We integrate a CTENG with a simple hand-held pendulum (HHP) system with a natural frequency of 5.5 Hz. The natural frequency and input energy of our HHP system can be easily controlled by changing the holder mass and initial bending displacement, thus producing different output behaviors for the CTENG. Under the optimal HHP-based CTENG system design, the maximum output reaches 116 V at 6.5 µA under 0.1 kg mass and 4 cm bending displacement conditions. The corresponding output energy is 52.7 µJ for an operation time of 10.8 s. Our HHP-CTENG system can sufficiently power 45 LEDs and shows different output performances by varying the driving velocity of a vehicle, thus demonstrating the possibility for a self-powered velocity monitoring system.

Anisotropic drop spreading on superhydrophobic grates during drop impact.

We study the influence of geometric anisotropy of micro-grate structures on the spreading dynamics of water drops after impact. It is found that the maximal spreading diameter along the parallel direction to grates becomes larger than that along the transverse direction beyond a certain Weber number, while the extent of such an asymmetric spreading increases with the structural pitch of grates and Weber number. By employing grates covered with nanostructures, we exclude the possible influences coming from the Cassie-to-Wenzel transition and the circumferential contact angle variation on the spreading diameter. Then, based on a simplified energy balance model incorporating slip length, we propose that slip length selectively enhances the spreading diameter along the parallel direction, being responsible for the asymmetric drop spreading. We believe that our work will help better understand the role of microstructures in controlling the drop dynamics during impact, which has relevance to various engineering applications.

Plasmonic-Photonic Interference Coupling in Submicrometer Amorphous TiO2-Ag Nanoarchitectures.

In this study, we report the crystallinity effects of submicrometer titanium dioxide (TiO2) nanotube (TNT) incorporated with silver (Ag) nanoparticles (NPs) on surface-enhanced Raman scattering (SERS) sensitivity. Furthermore, we demonstrate the SERS behaviors dependent on the plasmonic-photonic interference coupling (P-PIC) in the TNT-AgNP nanoarchitectures. Amorphous TNTs (A-TNTs) are synthesized through a two-step anodization on titanium (Ti) substrate, and crystalline TNTs (C-TNTs) are then prepared by using thermal annealing process at 500 °C in air. After thermally evaporating 20 nm thick Ag on TNTs, we investigate SERS signals according to the crystallinity and P-PIC on our TNT-AgNP nanostructures. (A-TNTs)-AgNP substrates show dramatically enhanced SERS performance as compared to (C-TNTs)-AgNP substrates. We attribute the high enhancement on (A-TNTs)-AgNP substrates with electron confinement at the interface between A-TNTs and AgNPs as due to the high interfacial barrier resistance caused by band edge positions. Moreover, the TNT length variation in (A-TNTs)-AgNP nanostructures results in different constructive or destructive interference patterns, which in turn affects the P-PIC. Finally, we could understand the significant dependency of SERS intensity on P-PIC in (A-TNTs)-AgNP nanostructures. Our results thus might provide a suitable design for a myriad of applications of enhanced EM on plasmonic-integrated devices.

Role of the electric field in selective ion filtration in nanostructures.

Nafion has received great attention as a proton conductor that can block negative ions. Here, we report the effect of a Nafion coating on an anodic aluminium oxide (AAO) nanoporous membrane on its function of ion rejection and filtering depending on the electric field. In our experiments, Nafion, once coated, was used to repel the negative ions (anions) from the coated surface, and then selectively allowed positive ions (cations) to pass through the nanopores in the presence of an electric field. To demonstrate the proof-of-concept validation, we coated Nafion solution onto the surface of AAO membranes with 20 nm nanopores average diameter at different solution concentration levels. Vacuum filtration methods for Nafion coating were vertically applied to the plane of an AAO membrane. An electric field was then applied to the upper surface of the Nafion-coated AAO membrane to investigate if ion rejection and filtering was affected by the presence of the electric field. Both anions and cations could pass through the AAO nanopores without an electric field applied. However, only cations could well pass through the AAO nanopores under an electric field, thus effectively blocking anions from passing through the nanopores. This result shows that ion filtration of electrons has been selectively performed while the system also works as a vital catalyst in reactivating Nafion via electrolysis. A saturated viscosity ratio of Nafion solution for the coating was also determined. We believe that this approach is potentially beneficial for better understanding the fundamentals of selective ion filtration in nanostructures and for promoting the use of nanostructures in potential applications such as ion-based water purification and desalination system at the nanoscale in a massively electrically integrated format.

Effects of mechanical deformation on energy conversion efficiency of piezoelectric nanogenerators.

Piezoelectric nanogenerators (PNGs) are capable of converting energy from various mechanical sources into electric energy and have many attractive features such as continuous operation, replenishment and low cost. However, many researchers still have studied novel material synthesis and interfacial controls to improve the power production from PNGs. In this study, we report the energy conversion efficiency (ECE) of PNGs dependent on mechanical deformations such as bending and twisting. Since the output power of PNGs is caused by the mechanical strain of the piezoelectric material, the power production and their ECE is critically dependent on the types of external mechanical deformations. Thus, we examine the output power from PNGs according to bending and twisting. In order to clearly understand the ECE of PNGs in the presence of those external mechanical deformations, we determine the ECE of PNGs by the ratio of output electrical energy and input mechanical energy, where we suggest that the input energy is based only on the strain energy of the piezoelectric layer. We calculate the strain energy of the piezoelectric layer using numerical simulation of bending and twisting of the PNG. Finally, we demonstrate that the ECE of the PNG caused by twisting is much higher than that caused by bending due to the multiple effects of normal and lateral piezoelectric coefficients. Our results thus provide a design direction for PNG systems as high-performance power generators.

Plasmonic optical interference.

Understanding optical interference is of great importance in fundamental and analytical optical design for next-generation personal, industrial, and military applications. So far, various researches have been performed for optical interference phenomena, but there have been no reports on plasmonic optical interference. Here, we report that optical interference could be effectively coupled with surface plasmons, resulting in enhanced optical absorption. We prepared a three-dimensional (3D) plasmonic nanostructure that consists of a plasmonic layer at the top, a nanoporous dielectric layer at the center, and a mirror layer at the bottom. The plasmonic layer mediates strong plasmonic absorption when the constructive interference pattern is matched with the plasmonic component. By tailoring the thickness of the dielectric layer, the strong plasmonic absorption can facilely be controlled and covers the full visible range. The plasmonic interference in the 3D nanostructure thus creates brilliant structural colors. We develop a design equation to determine the thickness of the dielectric layer in a 3D plasmonic nanostructure that could create the maximum absorption at a given wavelength. It is further demonstrated that the 3D plasmonic nanostructure can be realized on a flexible substrate. Our 3D plasmonic nanostructures will have a huge impact on the fields of optoelectronic systems, biochemical optical sensors, and spectral imaging.

Boosting the Output Performance of the MoS2 Monolayer-Based Piezoelectric Nanogenerator by Artificial Dual Strain Concentration.

Piezoelectric nanogenerators (PENGs) with molybdenum disulfide (MoS2) monolayers have been intensively studied owing to their superior mechanical durability and stability. However, the limited output performance resulting from a small active area and low strain levels continues to pose a significant challenge that should be overcome. Herein, we report a novel strategy for the epoch-making output performance of a PENG with a MoS2 monolayer by adopting the additive strain concentration concept. The simulation study indicates that strain in the MoS2 monolayer can be initially augmented by the wavy structure resulting from the prestretched poly(dimethylsiloxane) (PDMS) and is further increased through flexural deformation (i.e., bending). Based on these studies, we have developed concentrated strain-applied PENGs with MoS2 monolayers. The wavy structures effectively applied strain to the MoS2 monolayer and generated a piezoelectric output voltage and current of around 580 mV and 47.5 nA, respectively. Our innovative approach to enhancing the performance of PENGs with MoS2 monolayers through the artificial dual strain concept has led to groundbreaking results, achieving the highest recorded output voltage and current for PENGs based on two-dimensional (2D) materials, which provides unique opportunities for the 2D-based energy harvesting field and structural insight into how to improve the net strain on 2D materials.

Mechanical characterizations of high-quality quantum dot arrays via transfer printing.

In this study, we investigate the mechanical characteristics of transfer-printed quantum dot (QD) arrays. Transfer printing of a spin-coated QD array by elastomers results in changes of the mechanical properties of the QD films since QDs encapsulated by organic aliphatic ligands can be deformed by physical forces such as pressure. We demonstrate the lateral compaction of a QD film due to the Poisson effect of an elastomeric stamp during transfer printing. Interestingly, the spin-coated QDs on a self-assembled monolayer provide more uniform and denser QD arrays than those on glass. Finally, we demonstrate enhanced device performance of the optoelectronic devices using the printed QD films compared to the spin-coated films on glass.

p-Type polymer-hybridized high-performance piezoelectric nanogenerators.

Enhancing the output power of a nanogenerator is essential in applications as a sustainable power source for wireless sensors and microelectronics. We report here a novel approach that greatly enhances piezoelectric power generation by introducing a p-type polymer layer on a piezoelectric semiconducting thin film. Holes at the film surface greatly reduce the piezoelectric potential screening effect caused by free electrons in a piezoelectric semiconducting material. Furthermore, additional carriers from a conducting polymer and a shift in the Fermi level help in increasing the power output. Poly(3-hexylthiophene) (P3HT) was used as a p-type polymer on piezoelectric semiconducting zinc oxide (ZnO) thin film, and phenyl-C(61)-butyric acid methyl ester (PCBM) was added to P3HT to improve carrier transport. The ZnO/P3HT:PCBM-assembled piezoelectric power generator demonstrated 18-fold enhancement in the output voltage and tripled the current, relative to a power generator with ZnO only at a strain of 0.068%. The overall output power density exceeded 0.88 W/cm(3), and the average power conversion efficiency was up to 18%. This high power generation enabled red, green, and blue light-emitting diodes to turn on after only tens of times bending the generator. This approach offers a breakthrough in realizing a high-performance flexible piezoelectric energy harvester for self-powered electronics.

Highly stretchable hydroxyapatite bionanocomposite for high-performance triboelectric nanogenerators.

Renewable energy has been a focus in recent years. Triboelectric nanogenerators (TENGs) have potential for converting mechanical energy into electricity. However, there are restrictions on the use of biological materials and bionanocomposites, such as the high cost and complexity of the synthesis process, poor stability, and inadequate output performance. To overcome the constraints of TENGs, we have turned to hydroxyapatite, a biological substance with great biocompatibility and high mechanical strength that can be manufactured from waste materials. We successfully developed a negative triboelectric bionanocomposite hydroxyapatite (HA) loaded polydimethylsiloxane (PDMS) to harness energy from biomechanical sources such as wearable devices. A TENG (2 × 2 cm2) with a pushing force of 2 N and different amounts of HA in PDMS can produce highly stable output voltage, current, surface charge density, and power density values of 300 V, 22.4 µA, 90.36 µC m-2, and 27.34 W m-2, which are 6, 9, and 10 times higher than those without HA, respectively. These improvements were attributed to the highest observed surface potential of 1512 mV. After 20 000 cycles of contact-separation, the HA/PDMS-TENG shows exceptionally stable performance. Furthermore, adding HA improves the mechanical properties and the stretchability of the bionanocomposite. The HA/PDMS bionanocomposite exhibits remarkable stretchability of more than 290%. Effectively harvesting energy from body movements, the TENG gadget may be used to charge multiple commercial capacitors, drive up to 100 LEDs, and power a low-power electronic device. Self-powered sensing and wearable devices are made possible by the HA/PDMS-TENG, which allows their large-scale preparation and deployment.

Recent Advances in Functional Fiber-Based Wearable Triboelectric Nanogenerators.

The quality of human life has improved thanks to the rapid development of wearable electronics. Previously, bulk structures were usually selected for the fabrication of high performance electronics, but these are not suitable for wearable electronics due to mobility limitations and comfortability. Fibrous material-based triboelectric nanogenerators (TENGs) can provide power to wearable electronics due to their advantages such as light weight, flexibility, stretchability, wearability, etc. In this work, various fiber materials, multiple fabrication methods, and fundamentals of TENGs are described. Moreover, recent advances in functional fiber-based wearable TENGs are introduced. Furthermore, the challenges to functional fiber-based TENGs are discussed, and possible solutions are suggested. Finally, the use of TENGs in hybrid devices is introduced for a broader introduction of fiber-based energy harvesting technologies.

Emergence of Non-photoresponsive Catalytic Techniques for Environmental Remediation and Energy Generation.

Catalysis plays a crucial role in all the major applications and challenges in the environment, including energy generation and environmental remediation. Although photocatalysts and electrocatalysts are useful in addressing energy and environmental issues, they have some major drawbacks, such as low efficiency and easy charge recombination which limits their applications. Hence, it is imperative to design and explore new catalytic techniques that include non-photoresponsive catalysts. In this review, the detailed possibilities, characteristics and prospects of non-photoresponsive catalysts, such as piezocatalysts, thermocatalysts, pyrocatalysts, and tribocatalysts along with hybrid catalysts are described. The overall mechanism of each catalytic technique and its applications in different fields such as energy generation, environmental remediation, and carbon dioxide reduction are discussed.

Toward Enhanced Humidity Stability of Triboelectric Mechanical Sensors via Atomic Layer Deposition.

Humid conditions can disrupt the triboelectric signal generation and reduce the accuracy of triboelectric mechanical sensors. This study demonstrates a novel design approach using atomic layer deposition (ALD) to enhance the humidity resistance of triboelectric mechanical sensors. Titanium oxide (TiOx) was deposited on polytetrafluoroethylene (PTFE) film as a moisture passivation layer. To determine the effective ALD process cycle, the TiOx layer was deposited with 100 to 2000 process cycles. The triboelectric behavior and surface chemical bonding states were analyzed before and after moisture exposure. The ALD-TiOx-deposited PTFE showed three times greater humidity stability than pristine PTFE film. Based on the characterization of TiOx on PTFE film, the passivation mechanism was proposed, and it was related to the role of the oxygen-deficient sites in the TiOx layer. This study could provide a novel way to design stable triboelectric mechanical sensors in highly humid environments.

Magnetic Force Enhanced Sustainability and Power of Cam-Based Triboelectric Nanogenerator.

Since the first invention of triboelectric nanogenerators (TENGs) in 2012, many mechanical systems have been applied to operate TENGs, but mechanical contact losses such as friction and noise are still big obstacles for improving their output performance and sustainability. Here, we report on a magnet-assembled cam-based TENG (MC-TENG), which has enhanced output power and sustainability by utilizing the non-contact repulsive force between the magnets. We investigate the theoretical and experimental dynamic behaviors of MC-TENGs according to the effects of the contact modes, contact and separation times, and contact forces (i.e., pushing and repulsive forces). We suggest an optimized arrangement of magnets for the highest output performance, in which the charging time of the capacitor was 2.59 times faster than in a mechanical cam-based TENG (C-TENG). Finally, we design and demonstrate a MC-TENG-based windmill system to effectively harvest low-speed wind energy, ~4 m/s, which produces very low torque. Thus, it is expected that our frictionless MC-TENG system will provide a sustainable solution for effectively harvesting a broadband of wasted mechanical energies.

Control of electronic structure of graphene by various dopants and their effects on a nanogenerator.

It is essential to control the electronic structure of graphene in order to apply graphene films for use in electrodes. We have introduced chemical dopants that modulate the electronic properties of few-layer graphene films synthesized by chemical vapor deposition. The work function, sheet carrier density, mobility, and sheet resistance of these films were systematically modulated by the reduction potential values of dopants. We further demonstrated that the power generation of a nanogenerator was strongly influenced by the choice of a graphene electrode with a modified work function. The off-current was well quenched in graphene films with high work functions (Au-doped) due to the formation of high Schottky barrier heights, whereas leakage current was observed in graphene films with low work functions (viologen-doped), due to nearly ohmic contact.

Piezoelectric touch-sensitive flexible hybrid energy harvesting nanoarchitectures.

In this work, we report a flexible hybrid nanoarchitecture that can be utilized as both an energy harvester and a touch sensor on a single platform without any cross-talk problems. Based on the electron transport and piezoelectric properties of a zinc oxide (ZnO) nanostructured thin film, a hybrid cell was designed and the total thickness was below 500 nm on a plastic substrate. Piezoelectric touch signals were demonstrated under independent and simultaneous operations with respect to photo-induced charges. Different levels of piezoelectric output signals from different magnitudes of touching pressures suggest new user-interface functions from our hybrid cell. From a signal controller, the decoupled performance of a hybrid cell as an energy harvester and a touch sensor was confirmed. Our hybrid approach does not require additional assembly processes for such multiplex systems of an energy harvester and a touch sensor since we utilize the coupled material properties of ZnO and output signal processing. Furthermore, the hybrid cell can provide a multi-type energy harvester by both solar and mechanical touching energies.

All-Printed Electronic Skin Based on Deformable and Ionic Mechanotransducer Array.

Mechanoreceptors in human skin possess high sensitivity, wide sensing range, and high sensing resolution for external stimuli. Several attempts have been made to implement electronic skin (e-skin) that can mimic human skin. However, previous attempts are limited by the fundamental resolution problem arising from the use of film-like materials generated through pouring and spinning processes. Here, an all-printed e-skin based on deformable ionic mechanotransducer array (IMA) inspired by the physiological tactile sensing mechanism and the geometric features of mechanoreceptors in human skin is described. First, an ionic mechanotransduction channel is emulated with a piezocapacitive ionic mechanosensory system that engages in ion migration when the polymer matrix is deformed under a mechanical non-equilibrium state. Furthermore, the versatile shapes of the artificial mechanotransducer are tuned by the printing process variables, which results in high sensitivity (2.65 nF kPa-1 ) and high resolution (13.22 cm-2 ) of the device. It is demonstrated that this IMA is fully bio-inspired by the mechanotransduction and papillary structure of the mechanoreceptors. A high-resolution e-skin with a deformable and transparent IMA, which is fabricated by an all-printing methodology, will open up a new market in the field of soft and stretchable sensory platforms.