Symposium ES21—Nanogenerators and Piezotronics

Piezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. For wurtzite structures such as ZnO, GaN, InN and ZnS, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. The effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is named piezotronics, with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic units. This effect was also extended to 2D materials such as MoS₂. Piezo-phototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential. The objective of this talk is to introduce the fundamentals of piezotronics and piezo-phototronics and to give an updated progress about their applications in energy science (LED, solar) and sensors (photon detector and human-CMOS interfacing).

.Z. Wu, X.N. Wen, Z.L. Wang “Pixel-addressable matrix of vertical-nanowire piezotronic transistors for active/adaptive tactile imaging”, Science, 340 (2013) 952-957.
.F. Pan, L. Dong, G. Zhu, S. Niu, R.M. Yu, Q. Yang, Y. Liu, Z.L. Wang* “Micrometer-resolution electroluminescence parallel-imaging of pressure distribution using piezoelectric nanowire-LED array”, Nature Photonics, 7 (2013) 752-758.
Z.L. Wang “Piezopotential Gated Nanowire Devices: Piezotronics and Piezo-phototronics”, Nano Today, 5 (2010) 540-552.
[4] Q. Yang, W.H. Wang, S. Xu and Z.L. Wang^* “Enhancing light emission of ZnO microwire-based diodes by piezo-phototronic effect”, Nano Letters, 11 (2011) 4012–4017.
[5] W.Z. Wu, L. Wang, Y.L. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X. Zhang, Y. Hao, T.F. Heinz, J. Hone, and Z.L. Wang “Piezoelectricity of single-atomic-layer MoS₂ for energy conversion and piezotronics", Nature, 514 (2014) 470-474.
[6] W.Z. Wu and Z.L. Wang “Piezotronics and piezo-phototronics for smart adaptive electronics and optoelectronics”, Nature Review Materials, 1 (2016) 16031 doi:10.1038/natrevmats.2016.31.

Energy harvesting systems based on triboelectric nanomaterials are in great demand, as they can provide routes for the development of self-powered devices which are highly flexible, stretchable, mechanically durable, and can be used in a wide range of applications. Our recent research interest mainly focuses on the fabrication of high-performance triboelectric nanogenerators (TENGs) based on various kinds of nanomaterials. Flexible TENGs exhibit good performances and are easy to integrate which make it the perfect candidate for many applications, and therefore crucial to develop. In this presentation, I firstly introduce the fundamentals and possible device applications of TENGs, including their basic operation modes. Then the different improvement parameters will be discussed. As main topics, I will present a couple of recent achievements regarding highly stretchable transparent flexible TENGs, textile-based wearable TENGs, highly robust and efficient TENGs with multifunctional materials, etc. The recent research and design efforts for enhancing power generation performance of TENGs to realize self powering of portable and wearable sensors and electronics will also be discussed in this talk. Finally I am going to introduce a graphene tribotronic touch sensor which is based on coplanar coupling of a single electrode mode TENG (S-TENG) and a graphene FET. When any object (e.g. human finger) comes into contact with friction layer of the S-TENG, the charges are produced due to well-known triboelectric effect. The triboelectric charges act as a gate bias to the graphene FET and modulates its current transport. The tribotronic sensors have displayed a sensitivity of ~2% kPa^-1, a limit of detection <1 kPa, and a response time of ~30 ms. Furthermore, the devices can effectively detect touch stimuli from both bare and gloved fingers which can be a limitation with capacitive touch screens.

Structural phase-change materials are of great importance for applications in information storage devices. Thermally driven structural phase transitions are employed in phase-change memory to achieve lower programming voltages and potentially lower energy consumption than mainstream nonvolatile memory technologies. However, the waste heat generated by such thermal mechanisms is often not optimized and could present a limiting factor to widespread use. The potential for electrostatically driven structural phase transitions has recently been predicted and subsequently reported in some two-dimensional materials, providing an athermal mechanism to dynamically control properties of these materials in a nonvolatile fashion while achieving potentially lower energy consumption. In this work, we employ DFT-based calculations to make theoretical comparisons of the energy required to drive electrostatically-induced and thermally-induced phase transitions. Determining theoretical limits in monolayer MoTe₂ and thin films of Ge₂Sb₂Te₅, we find that the energy consumption per unit volume of the electrostatically driven phase transition in monolayer MoTe2 at room temperature is 9% of the adiabatic lower limit of the thermally driven phase transition in Ge₂Sb₂Te₅. Furthermore, experimentally reported phase change energy consumption of Ge₂Sb₂Te₅ is 100–10,000 times larger than the adiabatic lower limit due to waste heat flow out of the material, leaving the possibility for energy consumption in monolayer MoTe₂-based devices to be orders of magnitude smaller than Ge₂Sb₂Te₅-based devices.

For many years, researchers have sought metal oxide catalysts that efficiently split water in sunlight to produce hydrogen fuel. On the surfaces of oxide semiconductors with polar domains, electrons are attracted to positively terminated domains where they promote reduction reactions and holes are attracted to negatively charged domains where they promote oxidation. The separation of charge carriers reduces charge carrier recombination and the back reaction of the reduced and oxidized products. Charged domains can arise at the surfaces because of piezoelectricity, flexoelectricity, or differences in the chemical termination of the surface. Here, we report results showing that it is possible to optimize the overall photochemical reactivity of SrTiO₃ and BaTiO₃ by controlling relative areas of the polar surface regions and the surface charge through solution pH. For SrTiO₃, the fraction of the surface terminated by positive or negative charges is controlled through thermochemical treatments. For BaTiO₃, domain orientations are altered by controlling the cooling rate. The solution pH can then be used to 'tune' the surface potential to a point where the photoanodic and photocathodic reactions occur at maximum and equal rates, which is the point of optimized reactivity.

Electrical impedance spectroscopy (EIS) has been recognized as a powerful diagnostic tool for its extraordinary sensitivity. It has been widely used in evaluating electrical properties of materials and their interfaces with surface-modified techniques. Typically, EIS test applies a harmonic voltage perturbation to the target, then measures the response current, which can yield invaluable information of material electrical properties and structures as so-called Impedance spectroscopy. In practical, impedance spectroscopy measurement on a multiphasic / multifunctional electro-active composite material is conducted through probing two electrodes at specified point or surface, while the specification of probing sites is generally arbitrary. The following work deals with the impedance tomography based characterization of ZnO/ZnO naowire-Epoxy-BaTiO3 thin films with a variation in ZnO volume fraction from 1-10%. Impedance tomography data will also be used for generating dependent and independent data sets towards data analytics based optimization of surface properties and development of fabrication-processing-property relationships. The surface micro-structure and topography will analyzed using scanning electron microscopy and profilometry. The piezoelectric and dielectric properties of these composites will also be characterized.

The creations of photons in response to mechanical stimulus in a crystal that has noncentral symmetry are the great fundamental physics responsible for numbers of important technologies. The underlying mechanism and complete theory for a precise explanation of the mechanical-photonic energy conversion phenomena is vital important. We take commercial piezoelectric LiNbO₃ matrics as the example to interpret the detail mechanisms of energy conversions for the photon-generation through a native point defects study. It was found the Frenkel and Schottky type complex pairs as well as the antisite pair defects acting as energy harvesting and migration centers, which are very easy to form and active. It does to be the extra deep electron or hole traps levels near the valence or conduction band edge, respectively. That is the substantial energy reduction via a spontaneous equilibrium transformation from the complementarily charged individuals into agglomerated complexes. Such energy gain for both two processes turns to be independent to the variations of synthesis chemical potentials. In addition, the complex defects actually form independent to the variations of the chemical potentials. This leads to a coupling and exchange effect by them to continuously collect and transport host charges along the path via localized states to the deep recombination levels. The initiating energy barrier is small which ambient thermal stimulation or quantum tunneling can accomplish. The native sensitizers such as V_Nb2O5, V_LiNbO3, Nb_Li are also the energy conversion centers to non-radiative resonant energy transfer onto the activator center at the O_i to transfer the energy into photon emissions. A generalized energy conversion mechanism has been unraveled in this work. This gives a solid theoretical reference for developing the mechanical-photonic energy conversion materials.

As the third-generation semiconductors, III-Nitrides exhibits great potentials in the solid-state lighting, display, power device, photovoltaics and so on. The piezoelectric property is the great difference between III-Nitrides and previous semiconductors (Silicon, Germanium, etc.). Prof. Wang pointed it out that the piezo-potential can be used as a gate to tune/control the carrier generation, transport, separation and/or recombination via external strain, and thus tuning the device performances [1].

This report focuses on piezo-phototronic effects in III-nitrides. Firstly, in the framework of the quantum perturbation theory and constitutive equations, we proposed a self-consistent model to study the piezo-phototronic effects in quantum structure [2,3]. This model matches well with the optical excitation in InGaN/GaN quantum well under the various external stress field. Furthermore, we studied the carrier dynamic process in piezo-phototronic effects with the transit piezo-phototronic model and the time-resolved photoluminescence for the first time. The piezoelectric field was partly “canceled”, which increased the overlap of wavefunctions to decrease the carrier decay time. Thus, the maximum speed of a single chip was increased from 54 MHz up to 117 MHz in a blue LED chip under 0.14% compressive strain. Finally, the piezo-phototronic effect was used to effectively improve the conversion efficiency of InGaN/GaN quantum well and compensated the thermal degradation in high power InGaN/GaN micro-strip LED arrays [4,5]. These researches deepen our understanding on carrier’s excitation and transportation under external strain filed, and exhibits important applications in communication, lighting and energy collections.

Reference
1.X. Wang, J. Song, J. Liu, Z. L. Wang, Science 2007, 316 ,102
2.Xin Huang, Chunhua Du, Yongli Zhou, Chunyan Jiang, Xiong Pu, Wei Liu, Weiguo Hu*, Hong Chen*, and Zhong Lin Wang*, ACS Nano 2016 10 (5), 5145-5152
3.Xin Huang, Chunyan Jiang, Chunhua Du, Liang Jing, Mengmeng Liu, Weiguo Hu*, and Zhong Lin Wang*, ACS Nano 10 (12), 11420-11427, 2016
4.Chunyan Jiang, Liang Jing, Xin Huang, Mengmeng Liu, Chunhua Du, Ting Liu, Xiong Pu, Weiguo Hu* and Zhong Lin Wang*, ACS Nano 11 (9), 9405–9412, (2017)
5.Chunhua Du, Liang Jing, Chunyan Jiang, Ting Liu, Xiong Pu, Jiangman Sun, Dabing Li* and Weiguo Hu*, Materials Horizons, DOI 10.1039/C7MH00876G

Dielectric materials, commonly referred to as electrical insulators, have received much attention owing to their strong electron bonding, good support of electric fields, and low energy loss. In particular, polarization and depolarization that occur in dielectrics as a result of an external electric field has been investigated as a means to efficiently charge and discharge electricity, which are very useful when applied to capacitors. Dielectrics have also been widely used as charge accepting materials in triboelectric nanogenerator (TENG). So far, various dielectric materials such as polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA), polyimide (PI), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) have been used without any modifications and the electrical signals were not enough. Very recently, the modification of properties such as compressibility surface potential, and hydrophobicity in a few TENGs has been reported, however, most of them have focused on partial reports on the effects of the output performance. Here, we present rational design of effective polymer dielectrics for high-output triboelectric nanogenerator based on PDMS, PI, and PVDF. The stretchable materials as positive charged materials were also presented. This will make the TENG generate higher output power, compared with the PTFE-based TENG.

Piezotronics is a promising new class of semiconductor-devices that offer important implications for the realization of flexible nano-electronic, nano-sensor, and microelectromechanical (MEMS) applications. However, reproducibility in both synthesis and sensitivity has been a materials challenge for achieving wide-scale adaptation of these applications. We present a novel method for the synthesis and study of the piezoelectric effect of single-crystalline zinc oxide (ZnO) nanosheets (NS). Here we show reliable synthesis of free-standing two-dimensional (2D) nanometer thick ZnO achieved using Ionic Layer Epitaxy (ILE). In this method, a highly-packed surfactant monolayer guides crystal evolution underneath the ionized headgroups at the air-water interface. Using dynamic contact electrostatic force microscopy (DC-EFM) we measure the piezo response coefficient (d₃₃) of our as-synthesized NSs in the nanometer scale from 1 to 3 nm. The d₃₃ was found in the range of 10-60 pm/V. The thickness-related piezoelectric responses were studied as well. Comparing to reported piezo response values of bulk ZnO, the piezo response of our ZnO NSs offer significant implications for next-generation nanomaterials as building blocks for piezotronics.

Harvesting energy from our environment or biomechanical motion requires the development of mechanical energy harvesting devices that are compliant to curvilinear surfaces or wearable on human body. Triboelectric nanogenerators (TENGs) generate electricity by triboelectrification and electrostatic induction from ambient energy that are ubiquitous and constantly available. Wearable TENGs based on textiles are attractive options as it can be incorporated onto garments and versatile for high portability. We developed two kinds of textile-TENGs, one for harvesting energy from water flow and another for harvesting biomechanical energy from human motion. A wearable all-fabric-based TENGs with self-cleaning and antifouling properties can be realized with the use of hydrophobic cellulose oleoyl ester nanoparticles (HCOENPs), a low-cost and nontoxic coating material to achieve superhydrophobic coating. An all-fabric-based dual-mode device harvests both the electrostatic energy and mechanical energy of water, achieving the maximum instantaneous output power density of 0.30 W m^-2. To generate energy from subtle biomechanical motion, an all-textile nanogenerator utilizing HCOENPs encapsulated black phosphorus was presented with considerably high output (~250–880 V, ~0.48–1.1 μA cm⁻²) with a small force (~5 N) and low frequency (~4 Hz), which can power small electronic gadgets. On the other hand, ionic based materials can be utilized to generate signal from touch and decode environmental changes. Based on the ionic conductive material, a stretchable, transparent and self-healable nanogenerator has been developed. To realize an extremely stretchable and self-healable device, we designed a thermoplastic elastomer that is 3D printable and it can serve as the triboelectric active layer and simultaneously as the matrix for current collector. This optimized elasmomer leads to enhanced interface compatibility that harnesses the outstanding output performance.

The reliable production of two-dimensional (2D) crystals are essential for exploring new science and implementing novel technologies in the 2D limit. However, ongoing efforts are limited by the vague potential in scaling-up, restrictions in growth substrates and conditions, small sizes and/or instability of synthesized materials. Here we report the fabrication of large-area, high-quality 2D tellurene by a substrate-free solution process. The crystals exhibit process-tunable thicknesses from a monolayer to tens of nanometers, and lateral sizes up to 100 um. The chiral structure of tellurene gives rise to strong in-plane anisotropic properties and giant thickness-dependent shifts in Raman vibrational modes, unseen in 2D layered materials. The tellurene transistors can exhibit air-stable performance at room temperature for over two months, with on/off ratios on the order of 10⁶ and field-effect mobility ~700 cm²/Vs. More strikingly, with scaled channel length and integration with high-k dielectrics, the record-high on-state current density of 1.06 A/mm is demonstrated, surpassing all state-of-the-art 2D-based transistors. Our versatile solution process allows access to a broad range of characterization and application of tellurene as a new 2D semiconductor for high-performance, energy-efficient electronics in future ubiquitous applications.
1. Wang, Y. X., Qiu, G., Wang, R. X., Huang, S. Y., Wang, Q. X., Liu, Y. Y., Du, Y. C., Goddard, W. A., Kim, M. J., Xu, X. F., Ye, P. D., Wu, W. Z., “Field-effect transistors made from solution-grown two-dimensional tellurene”, Nature Electronics, 2018, 1, 228-236.
2. Wu, W. Z., Qiu, G., Wang, Y. X., Wang, R. X., Ye, P. D., “Tellurene: its physical properties, scalable nanomanufacturing, and device applications”, Chem. Soc. Rev., 2018, 47, 7203-7212.
3. Qiu, G., Wang, Y. X., Nie, Y. F., Zheng, Y. P., Cho, K., Wu, W. Z., Ye, P. D., “Quantum transport and band structure evolution high magnetic field in few-layer tellurene”, Nano Letters, 2018, 18, 5760-5767.
4. Jin S. Y., Wang, Y. X., Motlag, M., Gao, S. J., Xu, J., Nian, Q., Wu, W. Z., Cheng, G. J., “Large area direct laser shock imprinting of 3D biomimic hierarchical metal surface for triboelectric nanogenerator”, Adv. Mater., 2018, 30 (11), 1705840.
5. Gao, S. J., Wang, Y. X., Wang, R. X., Wu, W. Z., “Piezotronic effect in 1D van der Waals solid of elemental tellurium nanobelt for smart adaptive electronics", Semicond. Sci. Technol., 2017, 32, 104004.
6. Du, Y., Qiu, G., Wang, Y. X., Si, M., Xu, X., Wu, W. Z., Ye, P. D., “One-dimensional van der Waals material tellurium: Raman spectroscopy under strain and magneto-transport”, Nano Lett., 2017, 17 (6), 3965-3973.

Implantable medical devices have made a major impact in improving healthcare. In recent years, these devices have increasingly become active rather than passive – for example, cardiac pacemakers are those of the active type. A key challenge for these active systems is their need for an internal electrical power source. A downside of present active systems is the limitation of their internal batteries, which are rigid, bulky and must be changed frequently, thus requiring recurring surgical procedures, with associated complication risks and additional healthcare costs. One compelling solution would be to employ energy harvesting as a means of recharging or completely replacing batteries for active implantable medical devices. Energy harvesting — through chemical reactions, heat extraction, blood flow, and natural mechanical movements of organs — could help address energy depletion issues in implantable devices. However, most energy harvesting units being considered today are like conventional batteries, in that they also rely on rigid electronics and subcomponents, and therefore, are incapable of providing intimate mechanical contact with soft tissue.

We have developed a new class of biocompatible piezoelectric mechanical energy harvesters (PZT MEHs) that are soft and flexible, with extremely low bending stiffness, allowing them to conform to and laminate on the heart as well as on other soft tissues. These devices are the first-of-its-kind flexible generators that convert mechanical energy from internal organ movements into electric energy to power medical devices. We designed and fabricated the entire device, and performed in vitro and in vivo tests, and analyzed the data to show that it is a feasible new invention. The co-integrated collection of such energy-harvesting elements with rectifiers and microbatteries provides an entire flexible system, capable of viable integration with the beating heart via medical sutures and operation with efficiencies of ∼2%.

This conformal mechanical energy harvesters can yield significant amounts of electrical power from motions of internal organs, up to and exceeding levels relevant for practical use in implants. Under the rhythmic contraction of the heart muscle, the device bends and relaxes, enabling to supply enough trickle charge — a steady stream of charging current at low rate — to satisfy the needs of a pacemaker. Thus, this technology could extend the battery life of implanted medical devices or even eliminate the need of battery replacement that altogether would spare patients from repeated operations and the risk of surgical complications. Another potential application of this technology is in powering health monitors/sensors worn epidermally on skin.

Wearable healthcare devices and point-of-care diagnostics are essential tools to provide affordable healthcare globally and to deliver personalised medicine. These devices are able to remotely monitor patient health in real time, thus improving diseases management and reducing medical cost. Furthermore, the physiological data collected can be used by a machine learning software to provide personalised treatments.

One of the biggest challenges that wearable technology faces is power supply[1]. In this context, triboelectric generators are promising as they rely on motion-generated surface charge transfer between materials with different electron affinities to convert mechanical energy, for example from body movements, into useful electricity. Fibre-based triboelectric generators are therefore particularly promising for wearable applications as they can harvest the kinetic energy from body motion and be seamlessly integrated into “smart” textiles at the same time. Current fibre-based triboelectric generators are limited by low power output. Two of the most effective methods to improve this are increasing surface area and altering the surface chemistry of the material. Higher surface area allows for higher charge transfer density, while tailored surface charge properties improve the charge transfer mechanisms.

Electrospinning can be used to effectively tailor the surface chemistry of fibres by alternating voltage polarities in a single-step manufacturing process[2]. In our work, we have enhanced the triboelectric performance of poly(methyl methacrylate) and polyvinylidene fluoride by engineering both its surface area (due to the fibre geometry) as well as the surface chemistry (by controlling the polarity of applied voltage during electrospinning). Scanning electron microscopy and electrical measurements were used to study the effect of surface area enhancement. X-ray photoelectron spectroscopy and Kelvin probe force microscopy have demonstrated that the improvement in triboelectric output is correlated to molecular reorientation on the fibre surface caused by the applied voltage during electrospinning. The suitability of surface-modified electrospun fibres as high-performance materials for fibre-based triboelectric generators is demonstrated in this work, including applications in smart textiles.

[1] A. Tricoli, N. Nasiri, S. De, Wearable and Miniaturized Sensor Technologies for Personalized and Preventive Medicine, Adv. Funct. Mater. 27 (2017) 1–19. doi:10.1002/adfm.201605271.
[2] U. Stachewicz, C.A. Stone, C.R. Willis, A.H. Barber, Charge assisted tailoring of chemical functionality at electrospun nanofiber surfaces, J. Mater. Chem. 22 (2012) 22935–22941. doi:10.1039/c2jm33807f.

A sensing system is the core in human machine interaction (HMI), which connects the human being and machine with understanding the human instruction or senses. In this presentation, I will introduce the sensors based on triboelectric nanogenerator (TENG) technology, including an eye motion triggered self-powered mechnosensational communication system (1), a self-powered auditory sensor for social robotics and hearing aids (2), a quantization rotation sensing and gesture control of a robot joint (3), and a self-powered 2D barcode recognition system for personal identification. This work expresses notable advantages of using TENG technology to build a new generation of sensing systems for meeting the challenges in HMI.
References
[1] Sci. Adv. 2017; 3: e1700694.
[2] Sci. Robot. 2018, 3: eaat2516.
[3] Nano Energy, doi.org/10.1016/j.nanoen.2018.10.044
[4] Nano Energy 43 (2018) 253–258

The reliable production of atomically-thin crystals is essential for exploring new science and implementing novel technologies in the 2D limit. In this talk, I will discuss our recent discovery of a new 2-D material, tellurene, synthesized by a substrate-free solution process. The tellurene crystals exhibit process-tunable thicknesses from a monolayer to tens of nanometers, and lateral sizes ~ 100 um. Our prototypical tellurene transistor device, which is air-stable, shows an excellent all-around figure of merits compared to existing 2D materials. We further carry out the first experimental exploration of piezotronic effect in tellurene and systematically investigate the piezotronic transport properties. The fundamental understanding of piezotronic coupling in tellurene is expected to provide insights for the development of 2-D material piezotronics, leading to the realization of “smarter” electronics for a multitude of emerging technologies, e.g., wearable electronics, soft robotics, medical prosthetics, and human-machine interface.

1. Wu, W. Z., Qiu, G., Wang, Y. X., Wang, R. X., Ye, P. D., “Tellurene: its physical properties, scalable nanomanufacturing, and device applications”, Chem. Soc. Rev., 2018, 47, 7203-7212.
2. Wang, Y. X., Qiu, G., Wang, R. X., Huang, S. Y., Wang, Q. X., Liu, Y. Y., Du, Y. C., Goddard, W. A., Kim, M. J., Xu, X. F., Ye, P. D., Wu, W. Z., “Field-effect transistors made from solution-grown two-dimensional tellurene”, Nature Electronics, 2018, 1, 228-236.

The recent exploration of two dimensional (2D) planes as piezoelectric structures has accelerated. Mechanical displacements, such as vibration, bending and stretching, are ubiquitously present in the ambient environment and 2D structures may facilitate their sensing and the harvesting of their kinetic energy to power miniaturised devices based on such piezoelectric materials. Peculiar qualities by 2D materials, including their lateral strength and high crystallinity along the planes, large surface area to mass ratios and compatibility with surface fabrication processes, provide the concept of 2D piezoelectric with great prospect for future industries.
Field of 2D piezoelectric materials can benefit from the emergence of crystals featuring high piezoelectric coefficients such as gallium phosphate (GaPO₄). This 2D material is an archetypal piezoelectric with wide-ranging industrial applications. However, it does not naturally crystallise in a stratified structure and hence cannot be exfoliated using any of the conventional methods. Here we present a low temperature liquid metal 2D printing and synthesis strategy. Interfacial oxide layer of liquid gallium is exfoliated and surface-printed, followed by the vapour phase reaction between the gallium oxide sheet and phosphoric acid. The method gives access to large-area, uniform 2D GaPO₄ nanosheets of unit cell thickness (~1.1 nm), while featuring wafer scale lateral dimensions. The unit cell thick nanosheet presents a large effective out-of-plane piezoelectric coefficient of nearly 8 pm/V. The developed liquid metal based process is also suitable for the synthesis of free-standing 2D GaPO₄ nanosheets over micron sized cavities. The presented low temperature process is compatible with a variety of electronic device fabrication procedures, providing a facile route for the development of 2D piezoelectric devices for sensing and energy harvesting.

References:
1- Zavabeti, A. et al. A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science 358, 332, (2017).
2- Carey, B. J. et al. Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals. Nat. Commun. 8, 14482, (2017).
3- Daeneke, T. et al. Liquid metals: fundamentals and applications in chemistry. Chem. Soc. Rev., 47, 4073, (2018).
4- Syet, N. et al. Printing two-dimensional gallium phosphate out of liquid metal. Nat. Commun. 3618, (2018).

The properties of systems and, in particular, of transducers are greatly affected by their dimensions due to both classic scaling laws and, at nanoscale, quantum effects. For this reason, nanoscale piezoelectric transducers can help to overcome the limits of conventional piezoelectrics.
In particular, nanoscale materials can withstand larger deformations (which translate into higher piezopotentials, more robust and flexible devices and extended dynamic ranges), have higher piezoelectric coefficients, offer superior mechanical force-to-displacement sensitivities and operate at higher speed. Moreover, several materials which are not piezoelectric in their bulk form due to the opposite orientations of adjacent atomic layers, are piezoelectric as single layer. Besides, the working mechanisms of some devices are only possible at nanoscale; for instance, it has been suggested [1] that floating ghost gates can be created by tunneling triboelectrification [2] for tuning piezotronic devices with the extraordinary spatial resolution of AFM tips. Additionally, at nanoscale there are many more degrees of freedom for designing piezoelectric transducers [3] and, in comparison with conventional piezoelectrics, there can be more choices for the types of mechanical input, the positions of the contacts, the dimensionalities, and the shapes [3,4]. Downscaling also uniquely offers the opportunity to fabricate piezoelectric transducers which can be used in vivo for a variety of tasks [5,6].
Piezoelectric nanotransducers may offer outstanding advantages in comparison with conventional piezoelectrics, but practical applications can be complicated by many difficulties, with special reference to synthesis, characterization, modeling and device implementation. We will discuss some of the most important open challenges and also some general strategies to address them, namely, with reference to piezoelectric quasi-1D nanostructures (e.g. ZnO nanowires), rational design of solution-growth synthesis procedures [7], real time growth monitoring [8], dynamic synthesis procedures [8] and 3D geometrical characterizations from conventional SEM images [9].

References
[1] R. Hinchet, U. Khan, C. Falconi, S.W. Kim, Piezoelectric properties in two-dimensional materials: Simulations and experiments, Mater. Today. 21 (2018) 611–630. doi:10.1016/j.mattod.2018.01.031.
[2] S. Kim, T.Y. Kim, K.H. Lee, T.H. Kim, F.A. Cimini, S.K. Kim, R. Hinchet, S.W. Kim, C. Falconi, Rewritable ghost floating gates by tunnelling triboelectrification for two-dimensional electronics, Nat. Commun. 8 (2017) 1–7. doi:10.1038/ncomms15891.
[3] C. Falconi, G. Mantini, A. D’Amico, Z.L. Wang, Studying piezoelectric nanowires and nanowalls for energy harvesting, Sensors Actuators, B Chem. 139 (2009) 511–519. doi:10.1016/j.snb.2009.02.071.
[4] R. Araneo, G. Lovat, P. Burghignoli, C. Falconi, Piezo-semiconductive quasi-1D nanodevices with or without anti-symmetry, Adv. Mater. 24 (2012) 4719–4724. doi:10.1002/adma.201104588.
[5] C. Falconi, A. D’Amico, Z.L. Wang, Wireless nanosensors and nanoactuators for in-vivo biomedical applications, in: Eurosensors, Göteborg, 2006.
[6] C. Falconi, A. D’Amico, Z.L. Wang, Wireless Joule nanoheaters, Sensors Actuators, B Chem. 127 (2007) 54–62. doi:10.1016/j.snb.2007.07.002.
[7] G. Arrabito, V. Errico, Z. Zhang, W. Han, C. Falconi, Nanotransducers on printed circuit boards by rational design of high-density, long, thin and untapered ZnO nanowires, Nano Energy. 46 (2018) 54–62. doi:10.1016/j.nanoen.2018.01.029.
[8] A. Orsini, C. Falconi, Real-time monitoring of the solution growth of ZnO nanorods arrays by quartz microbalances and in-situ temperature sensors, Sci. Rep. 4 (2014) 1–7. doi:10.1038/srep06285.
[9] A. Mencattini, A. Orsini, C. Falconi, 3D Reconstruction of Quasi-1D Single-Crystal Nanostructures, Adv. Mater. 27 (2015) 6271–6276. doi:10.1002/adma.201503522.

TiO₂ is a well-known photocatalyst used for water splitting, pollutant removal, antibacterial application, etc. How to increase the efficiency by overcoming their limitation including fast recombination of photoexcited carriers, limited light utilization are still great challenge.
In this talk, I will introduce our recent research progresses in piezo-phototronic effect enhanced photodynamic bacteria killing wound healing, and photoelectrocatalysis. For increase of the photodynamic induced bacteria killing based on TiO₂, an internal piezoelectric field was built by inserting a ferroelectric semiconductor barium titanate (BaTiO₃) nanolayer between TiO₂ and plasmonic gold nanoparticles (Au NPs). Plasmonic Au NPs broadened the light absorbance to visible region and generated hot electrons. BaTiO₃ with piezo-phototronic effect created a favorable band bending for facilitating electron transportation, thus enhancing the separation of electron-hole pairs and photoinduced bacteria killing. With the enhancement of photoelectrochemical performance, it was used as a skin antibacterial coating for photodynamic bacterial killing and promotion of skin wound healing in vitro and in vivo. We also used the piezoelectric effect enhanced full-spectrum Photoelectrochemical photoelectrocatalysis with multilayered coaxial titanium dioxide/barium titanate/silver oxide (TiO₂/BTO/Ag₂O) nanorod array as the photoanode. The vertically grown nanorods ensured the good electron conductivity, which enabled the fast transport of the photogenerated electrons for photoelectrocatalysis. The polar charge-created field induced by the polarized BaTiO3 nanolayer can effectively promote the separation and suppress the recombination of the photocarriers generated by TiO₂ and Ag₂O. The solarcatalytic performance of the heterostructure was evaluated via the PEC performance under full solar spectrum from UV, visible, to NIR light. The ternay hetero-nanorod array exhibited highly improved PEC activity and high stability. These works open new avenue for developing heterojunction for improving photocatalysis with piezoelectric effect.

References
1. Xin Yu, Zhenhuan Zhao, Jian Zhang, Weibo Guo,Jichuan Qiu, Deshuai Li, Xiaoning Mou, Linlin Li*, Aixue Li*, Hong Liu*. Small. 2016, 12, 2759.
2. Xin Yu, Zhenhuan Zhao, Jian Zhang, Weibo Guo, Linlin Li*, Hong Liu*, Zhong Lin Wang*. CrystEngComm 2017, 19, 129.
3. Jichuan Qiu, Kun Zhao, Linlin Li, Xin Yu, Weibo Guo, Shu Wang, Xiaodi Zhang, Caofeng Pan*, Zhong Lin Wang*, HongLiu*. Nano Res. 2017, 10, 776.
4. Xin Yu, Shu Wang, Xiaodi Zhang, Anhui Qi, Xiran Qiao, Zhirong Liu, Mengqi Wu, Linlin Li*, Zhong Lin Wang*. Nano Energy, 2018, 46, 29

Triboeletric nanogenerator（TENG）has been considered to be a more effective technology to harvest various types of mechanic vibration energies such as wind energy, water energy in the blue energy and so on. Considering the vast energy from the blue oceans, harvesting of the water energy has attracted huge attention. There are two major types of “mechanical” water energy, water wave energy in random direction and water flow kinetic energy. Here, we successfully developed multifunctional TENGs that can be used to harvest the blue energy. Moreover, surface charge density of tribo-layer is the most key-point parameter for developing high performance TENG. Most of the previous works were focus on the surface structural/chemical modification, nevertheless the internal space of the tribo-layer and its mechanism exploration were less investigated. Herein, internal space charge zones are built through imbedding ravines and gullies criss-cross gold layers in near-surface of tribo-layer, which leads to the high output performance of TENG. At the same time, the tribo-laryer of TENGs have been fabricated via dispersing the lead-free piezoelectric materials to increase the output performances, the fundamental mechanism of the TENG has been discussed from the perspectives of dielectric modulation.

Piezoelectricity is defined as the interconversion between mechanical and electrical energy induced by charge redistribution and separation when mechanical or electrical stimuli are applied to materials that do not have inversion symmetry. Although various conventional inorganic piezoelectric materials exhibit strong piezoelectric property, these materials often require substances that are harmful to the environment / human body or harmful to the environment during the fabrication process. Many natural biomaterials (eg, amino acid, peptide, protein, and virus) that can be synthesized in an environmentally friendly manner have been found to have piezoelectric properties and it is comparable to conventional piezoelectric materials. Recently, self-assembled diphenylalanine (FF) nanotubes have been reported to have strong piezoelectric properties comparable to conventional piezoelectric materials. However, the difficulty of fabricating unidirectionally polarized FF nanotubes that can convert mechanical forces into electrical energy has been a major impediment to the realization of high power bio-piezoelectric devices. Here, we develop a novel fabrication process to synthesize large-scale aligned and uni-polarized FF nanotubes. We use a meniscus-driven self-assembly process which is environmental friend process. By controlling the thermodynamic and kinetic parameters of fabrication process, we synthesized FF nanostructures ranging from vesicles to aligned peptide nanotubes. The resulting horizontally aligned FF nanotubes have unidirectional polarization. Furthermore, this synthesis process allows us to easily scale-up to form large-area coatings that we use to develop piezoelectric energy generator.

With the advancement in technology and the urgent need for clean energy sources, triboelectric nanogenerator also known in short as TENG starts gaining lots of attention in recent years. TENG works on the principle of contact electrification, in which when two different materials come into contact charges are generated on the surface of the materials.[1–3] The amount of charges generated depend on various parameters, like surface roughness, work function, polarizability and hydrophobicity of the materials. [4–6]
In the present work, we have modified surface roughness, hydrophobicity and polarizability of polyvinylidene fluoride (PVDF) by incorporating hydrothermally synthesized ZnO nanorods into PVDF. The changes in the above properties of PVDF lead to the enhanced triboelectric effect of PVDF. The films were prepared using a simple drop casting method, and no surface treatment or complex film synthesis techniques were not employed. TENGs fabricated by coupling ZnO-PVDF film with polytetrafluoroethylene (PTFE) showed a much enhanced triboelectric output of voltage of ~119 V and short circuit current of ~1.6 µA. The instantaneous output power is 65.6% more as compared to PVDF/PTFE based TENG. The enhancement in the performance of the TENG has been correlated with the changes in the properties of PVDF. We believe that this work will provide a simple yet effective way of enhancing the triboelectric effect of materials mainly ferroelectric polymers.

References
1. L. Lin, S. Wang, S. Niu, C. Liu, Y. Xie, and Z. L. Wang, ACS Appl. Mater. Interfaces 6, 3031 (2014).
2. Q. Zhang, Q. Liang, Q. Liao, F. Yi, X. Zheng, M. Ma, F. Gao, and Y. Zhang, Adv. Mater. 29, (2017).
3. J. Qian, X. Wu, D.-S. Kima, and D.-W. Lee, Sensors Actuators A Phys. 263, 600 (2017).
4. P. Bai, G. Zhu, Z. H. Lin, Q. Jing, J. Chen, G. Zhang, J. Ma, and Z. L. Wang, ACS Nano 7, 3713 (2013).
5. S. G. Lee, J.-W. Ha, E.-H. Sohn, I. J. Park, and S.-B. Lee, Appl. Surf. Sci. 390, 339 (2016).
6. H. H. Singh and N. Khare, Nano Energy 51, 216 (2018).

The concepts such as Internet of Things—with an aim to equip the regular, everyday objects with intelligence and interconnect them into a network of smart devices—are thought to revolutionize the way of living. The widespread of these devices would require cost- and waste- efficient processing, which cannot be realized by a traditional photolithography-based processing. Inkjet printing—an additive patterning technique—is a powerful alternative to the traditional technology, and enables a direct deposition of micro-sized structures with digitally-defined geometry. Yet, due to strongly interdisciplinary nature of inkjet printing, it is still facing major problems when it comes to the realization of fully-printed devices.
To approach this challenge, we have systematically studied the inkjet printing process and developed a method for a reliable fabrication of all-oxide ferroelectric thin-film capacitors. The capacitors consist of lanthanum nickelate (LNO) electrodes and lead zirconium titanate (PZT) ferroelectric layer. To prepare the inks we first synthesized sol-gel precursor solutions and adjusted their physical properties by the addition of ethanolamine and ethylene glycol. The solvent composition was selected on the basis of our previous work to suppress the coffee stain formation of dried structures [A. Matavz et al., Appl. Phys. Lett. 113, 012904, 2018]. One of the most demanding issues in multilayer inkjet printing is the control over the wetting behavior throughout the fabrication process. To achieve this, we employed a few-nanometer-thick polymeric layer, which redefines the surface properties after each deposition step and ensures the repeatability during the fabrication of LNO/PZT/LNO stacks. The polymeric layer decomposes during annealing and does not influence the performance of capacitors.
The developed method proved to be extremely versatile: we printed LNO/PZT/LNO capacitors on silicon wafers, alumina substrates and nickel foils. The capacitors with 400 nm thick PZT layer show excellent electrical properties: the capacitance density of 30 nF/mm² (corresponding to dielectric permittivity of 1350), typical ferroelectric hysteresis and piezoelectric coefficient of 80 pm/V. The printing method offers also the possibility of multilayer capacitor (MLC) processing. We successfully realized MLCs composed of three PZT layers on silicon wafers, which exhibit electromechanical properties superior to the single PZT layer capacitors. The results demonstrate the large potential of inkjet printing for the fabrication of high-quality ferroelectric devices and imply its role in a next-generation production of sensors, energy harvesters, and microelectromechanical systems.

The development of the transparent and flexible power sources has attracted much interest for their potential in a broad range of applications from artificial skins to touch screen. Among them, a triboelectric nanogenerator (TENG), which is one of energy-harvesting devices, has been extensively investigated owing to several advantages such as cost-effectiveness, structural simplicity, wide material selection, and high conversion efficiency. Despite these attractive advantages, it remains a challenge to achieve a high output performance while preserving both the transparency and the flexibility because of the opaque and less flexible metal.
Here, we report a highly efficient, transparent and flexible TENG based on a transparent conducting polymer (TCP) and metallic nanowires. Metallic nanowire-embedded TCP layer, which played a role as a contact layer as well as an electrode, showed high tribonegativity as well as high transparency and flexibility. The output performance of the TENGs was optimized considering the triboelectric property of a counter contact layer and the physico-chemical properties of a TCP film. In our TCP-based TENG, the maximum power density was shown to be about 1.5 mW/cm² for the external resistance of about 2 MΩ. This value is a quite large compared with the previous transparent and flexible TENGs. As a realistic application example of the TCP-based TENG, we demonstrated a user-interactive system for instantaneous touch visualization based on a transparent and flexible TENG and a voltage-driven display device. The electro-optic devices were operated by only one or a few gentle touch to our TCP-based TENGs. Our work will provide a viable framework to devise flexible and transparent power sources and sensors.

Acknowledgement
This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2018R1A6A3A01011608).

Functional tactile sensing device is mandatory for next-generation robotics and human-machine interfaces since the emulation of touching requires large-scale pressure sensor arrays with high-spatial resolution, high-sensitivity, and fast-response^[1]. Some tactile sensors fabricated with organic transistors or micro-structured rubber layer pressure sensor arrays have been reported^[2], whose working mechanism is based on changes in capacitance or resistance. Our group have demonstrated pressure sensor array base on piezotronic and piezo-phototronic effects. A spatial resolution of 120 μm was achieved for piezotronic pressure sensor array and an ultra-high resolution of 2.7 μm was derived from piezo-phototronic pressure sensor array using ZnO nanowire (NW)/p-GaN LEDs array^[3]. These devices provide stable, fast response, as well as parallel-reading detections of spatial pressure distributions. However, the lacking of flexibility with a rigid sapphire substrate prevents the NW-LEDs array device from applications as smart skin; and the pressure measuring range of the device is in a relatively high pressure region. Therefore, a flexible pressure mapping system with moderate spatial-resolution become necessary and may find numerous potential applications in human-machine interfaces.
Organic/inorganic hybridized LEDs are drawing tremendous attentions because of their high flexibility. Among them, n-ZnO and p-poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) hybridized inorganic/organic LED has aroused great technological and scientific interests due to the excellent optoelectronic properties of ZnO and the high flexibility, low-cost, easy fabrication, potential for large area deposition and commercial availability of PEDOT:PSS. Here, we demonstrate a flexible LED array composed of PEDOT:PSS and patterned ZnO NWs for mapping spatial pressure distributions. By utilizing strain-induced negative piezoelectric polarization charges presented at local interface of the pn junction, piezo-phototronic effect has been applied to modify the band structure through reducing the barrier height for hole injections from PEDOT:PSS side, and thus facilitate the recombination between electrons and holes in ZnO side for enhancement of light emitting intensity. Therefore, the pressure distribution is obtained by parallel-reading the illumination intensities of LED pressure sensor array. The spatial resolution is achieved as high as 7 μm by fabricating ZnO nanowires on flexible substrate. By controlling the growth conditions of the ZnO nanowire array, a wide range of pressure measurements from 40 MPa to 100 MPa are derived under different ZnO morphologies. These devices may find prospective applications as electronic skins by taking advantage of their high spatial-resolution, flexibility and wide pressure mapping range.

Reference
[1] a) S. C. B. Mannsfeld, B. C. K. Tee, R. M. Stoltenberg, C. V. H. H. Chen, S. Barman, B. V. O. Muir, A. N. Sokolov, C. Reese, Z. Bao, Nature Materials 2010, 9, 859; b) J. J. Boland, Nature Materials 2010, 9, 790; c) K. Takei, T. Takahashi, J. C. Ho, H. Ko, A. G. Gillies, P. W. Leu, R. S. Fearing, A. Javey, Nature Materials 2010, 9, 821.
[2] a) B. C. K. Tee, A. Chortos, R. R. Dunn, G. Schwartz, E. Eason, Z. A. Bao, Advanced Functional Materials 2014, 24, 5427; b) Y. Wang, L. Wang, T. T. Yang, X. Li, X. B. Zang, M. Zhu, K. L. Wang, D. H. Wu, H. W. Zhu, Advanced Functional Materials 2014, 24, 4666; c) C. Y. Hou, H. Z. Wang, Q. H. Zhang, Y. G. Li, M. F. Zhu, Advanced Materials 2014, 26, 5018; d) S. Jung, J. H. Kim, J. Kim, S. Choi, J. Lee, I. Park, T. Hyeon, D. H. Kim, Advanced Materials 2014, 26, 4825; e) Q. Sun, D. H. Kim, S. S. Park, N. Y. Lee, Y. Zhang, J. H. Lee, K. Cho, J. H. Cho, Advanced Materials 2014, 26, 4735.
[3] a) W. Z. Wu, X. N. Wen, Z. L. Wang, Science 2013, 340, 952; b) C. F. Pan, L. Dong, G. Zhu, S. M. Niu, R. M. Yu, Q. Yang, Y. Liu, Z. L. Wang, Nat. Photonics 2013, 7, 752;

Triboelectrification is a well-known effect that occurs anywhere and anytime in nature and in our life. Although this effect has been known for over 2600 years and each and every material exhibits triboelectrification, as a key material’s genome, its quantification has not been standardized and quantified in material science. The only available tool is a triboelectric series that gives a qualitative ranking about the triboelectric polarization of some common materials. Here, for the first time, we introduce a universal standard method to quantify the triboelectric series for a wide range of polymers, establishing a fundamental materials property of quantitative triboelectrification. By measuring the tested materials with a liquid metal in an environment under well-defined and fixed conditions, the proposed method standardizes the experimental set up for uniformly quantifying the surface triboelectrification of general materials. The normalized triboelectric charge density (TECD) was defined and derived to reveal the intrinsic character of polymers for gaining or losing electrons. A table is given regarding the TECD of over 50 materials. This first quantitative triboelectric series will be a textbook standard for implementing the application of triboelectrification for energy harvesting and self-powered sensing.

Energy harvesting that can scavenge various kinds of mechanical energy from several different sources have attracted great attention. In particular, a flexible energy-harvesting device consisting of piezoelectric materials has been studied as a power source for flexible and wearable electronic devices because the piezoelectric materials can effectively convert to electrical energy from mechanical energy. Recently, many researchers have attempted to achieve highly efficient energy-harvesting devices using several fabrication methods. However, there are still the challenge problems with flexible piezoelectric nanogenerators. Here we report the flexible piezoelectric nanogenerators (PNGs) based on composite thin films comprising amine-functionalized lead zirconate titanate (PZT) nanoparticles (PZT-NH₂ NPs) and a thermoplastic triblock copolymer. Without additional dispersants, the uniform distribution of PZT-NH₂ NPs in the polymer composite improves the piezoelectric power generation compared to that of a PNG device using pristine PZT NPs. This unique composite behavior allows the PZT-NH₂ NP-based flexible PNG to exhibit a high output voltage of 65 V and current of 1.6 μA without time-dependent degradation. This alternating energy from the PNG can be used to charge a capacitor and operate light-emitting diodes through a full bridge rectifier. Furthermore, the proposed PNG is demonstrated as a promising energy harvester for potential applications in self-powered systems.

Triboelectric nanogenerators(TENGs) have gained much attention thanks to their relatively high output power generation by scavenging vibrating resources that exist in our living environment. To date, a number of approaches, such as surface patterning, dielectric permittivity modulation, chemical functionalization, have been introduced to enhance output performance of the TENGs. However, the TENG’s output performance has still been limited due to relatively low output current density on the triboelectric frictional surfaces. Therefore, simple and effective methods are necessary to boost the output current density of the TENG, which further extend the performance limit of the TENGs.

In this report, we adopted the composite structure consisting of MoS₂ flakes and ferroelectric Nylon 11 polymer to improve output current density. Here, ferroelectric Nylon 11 provides a knob to adjust surface triboelectric charge density by surface polarization. In addition, MoS₂ provides the trap states to further increase charge density in the composite structure. Using these two knobs, the output charge density was greatly improved in comparison to the TENG without the MoS₂ particles. Specifically, 5-fold higher current density was obtained by using the polarized MoS₂-Nylon 11 composite layer compared with the device fabricated only with non-polarized Nylon 11. Consequently, the maximum output power density of ~40 mW/cm² was achieved in our work. The approach introduced here provides simple strategy to effectively extend the output performance limit of TENGs.

Acknowledgement: This subject is supported by Korea Ministry of Environment(MOE) as Advanced Technology Program for Environmental Industry Program.

Novelty: This paper reports a novel multi-function self-powered pressure sensor with hierarchical wrinkle structure which is thin and flexible. Due to the sensitive hierarchical wrinkle structure, the sensor shows the sensitivity of 2.0 kPa^-1 and the excellent response performance of 0.15 ms (risetime) and 0.7 ms (releasetime). In addition, the surface charges deriving from the contact of PDMS and ITO on the wrinkle structure enable the sensor to be a generator and differentiate various mechanical stimuli.
Background: With the development of electronic skins and wearable devices, flexible sensors based on capacitive effect due to its stability, are now concerned about by more and more people. But most of them require multi-step process and complicated fabrication because of complexly designed dielectric layer. PDMS wrinkle structure fabricated by C₄F₈ plasma treating has been applied to triboelectric nanogenerator (TENG) and have the advantage of simple fabrication. However, the sensor with monolayer wrinkle structure shows low sensitivity. Here, we introduce hierarchical PDMS wrinkle structure into the capacitive effect pressure sensor to improve the performance of the sensor and meanwhile integrate the feature of energy harvesting.
Fabrication Process: The vacuum degassed mixture of PDMS base and cross-linker is spin coated on the indium tin oxide (ITO) coated polyethylene terephthalate (PET) film. Then heat the thin PDMS/ITO/PET film at 100 degree celsius for 45 seconds. Next implement a C₄F₈ plasma treatment for 1000 seconds, after which heat the film till the curing of the thin PDMS layer. Finally, cover the wrinkle-structrued film with another ITO/PET film. The thickness and area of the sensor are about 260 μm and 6.25 cm², respectively.
Working principle: As a capacitive effect sensor, the ITO layers work as two conductive electrodes and the dual-scale wrinkle structure, the thin PDMS layer and the air work as dielectric layer. While as a generator, the PDMS and ITO work as charges generation surface and the air between the wrinkle gap enables PDMS layer and ITO layer to contact and separate. The hierarchical wrinkle structure manages differentiating normal pressure and bending.
Experimental Results: Since extending C4F8 plasma treating time to 1000s can produce hierarchical wrinkle structure on the PDMS surface, the sensitivity in the relatively low pressure regime (0-500 Pa) can get an obvious promotion from 0.5 kPa^-1 (50s plasma treating), 1.0 kPa^-1 (500s plasma treating) to 2.0 kPa^-1 (1000s plasma treating). Moreover, in the relatively high pressure regime (more than 1500 Pa), the sensitivity of the sensor gets a slight promotion from 0.1 kPa^-1 (50s & 500s plasma treating time) to 0.2 kPa^-1 (1000s plasma treating time). Moreover, the response performance is excellent with the release-time is 0.7 ms and the rise-time is even only 0.15 ms. When pressure is applied, the small wrinkle firstly works and then sense the pressure quickly and sensitively by forming deformation, thus the sensitivity of the pressure sensor in the relatively low pressure regime can be highly enhanced and the response time is extremely short. When the pressure is applied extremely high, the big wrinkle begins to work and shows a sensitivity of 0.2kPa^-1 once bigger than that of a PDMS plat.
As a TENG, the output voltage of the sensor under bending stimuli is about 300 V, while that of the sensor under pressure stimuli is only about 75 V. The output current of the former is about 3μA, while the latter is about 17 μA. The wave pattern under pressure stimuli is more sharp than that under bending stimuli. The reason for the feature of mechanical stimuli differentiation of the device can be attributed to the hierarchical wrinkle structure. The two forces can make hierarchical wrinkles contact with the PET/ITO with different deformation which leads to different contact area thus results in output voltage and current.

Helicenes are a unique group of helically conformed aromatic compounds which have been shown to possess potentially useful properties. Pertinent to this proposed research, helicenes have been shown both computationally and experimentally to be piezoelectric. It is the intent of this research to utilize functionalized helicenes as monomers in the backbone of existing polymer systems. In this way, a new multi-functional class of polymers will be developed which possess the inherent properties of helicenes. These molecules derive their piezoelectric properties from the inherent strain in the molecule originating from their helical shape. This is unlike existing piezoelectric polymers such as poly- (vinylidene fluoride) (PVDF) whose piezoelectric properties originate from interaction between dipoles in the polymer structure. This research will begin with the synthesis of a diol-functionalized, nine-ringed helicene for application in a proof-of-concept polyester system. Initial characterization of this polymer will provide insight the as-synthesized properties of this novel polymer, and later optimization of piezoelectric properties will show the presence and extent of the functional activity of the polymer. The capability of these molecules to be adapted for use in various polymer chemistries give this new class of polymers potential for use in applications where material properties have previously limited the versatility of piezoelectric polymers.

We report for increase the surface charge density of the TENG using a surface plasma resonance. Resonant energy transfer (RET) generates electron–hole pairs in the semiconductor by the dipole–dipole interaction between the plasmonic metal and semiconductor. A wide range of enhancement phenomena have been observed from enhanced light trapping in semiconductor loaded with plasmonic nanoparticles to plasmonically assisted hot carrier generation for TENG.

As a sustainable power source, triboelectric nanogenerators (TENGs) have attractedgot extensive attention from both academia and industry to solve the growing energy crisis. Herein, a high voltage output contact-seperation mode triboelectric nanogenerator is demonstrated by using commercial-available polyurethane foam and ebonite as triboelectric materials, which presents flexibility, durability and reliabiltiy, simultaneously. The dependence of electric outputs on the different parameters of the TENGs, including triboeletric affinity, gap distance between frictional layers, and the dimension of devices, has been systematically investigated. The optimized device can generate an open-circuit voltage of above 1000 V and a short-circuit current of above 100 μA. The TENG can operate continously for 500 k cycles under large pressuring force with stable output performance. Meanwhile it can harvest mechanical energy from various human motions, such as stepping and tapping. Arising from its reliable high voltage output, the device can be utilized as self-powered spark plug. This study provides a feasible method for developing power sources in simple fabrication and low-cost way. The work was supported by the grant of Research Grants Council of Hong Kong.

With the advent of the IoT era, the demand for sensors has increased significantly. We note that these sensor systems can communicate with each other over wireless networks in the IoT era. Especially, in particular sites where access to power is difficult, batteries have been installed as power source for wireless sensor systems. However, these batteries have a limited lifetime and require periodic replacement. So, various energy harvesting technologies have received research attention to realize an autonomous wireless sensor network by replacing the batteries. Due to relatively low power generation capability and energy conversion efficiency, however, these harvesting technologies have been applied only to limited applications.
In order to address these issues, here, we design and demonstrate a piezoelectric/electromagnetic hybrid energy harvester that can generate significant amount of electricity at vibration environments(Fig 1). Oval-like configuration using two curved piezoelectric energy harvesters based on MFC(Macro-fiber composite) piezoelectric material induces large displacement of a permanent magnet installed at the end of the top surface which allows efficient power generation both piezoelectric and electromagnetic energy harvesters. Compared to output average power (1.09 mW) of cantilever type piezoelectric energy harvester, piezoelectric part of Oval-shaped harvester shows about two times higher output average power (2.58 mW) at 0.5 g and 60 Hz vibration due to its enhanced vibration characteristics. Also, we provide the resonance frequency tuning guidelines and the optimization strategies of the Oval-shaped hybrid energy harvester using Finite Element Analysis (FEA) and mechanical equivalent system. The hybrid energy harvester contains two magnets facing same polarity on both side of Oval-shaped structure which shows improved displacement of the magnet due to magnet induced repulsive force. This force improves the spring constant of Oval-shaped harvester, reducing the damping ratio, efficiently. With two magnets, the Oval-shaped hybrid energy harvester with MFC achieves up to 1.03 mm displacement at 60 Hz and 0.5 g vibration conditions. Also, considerable improvement of output power is observed at both piezoelectric and electromagnetic parts. The output average power of piezoelectric part and electromagnetic part are 7.66 mW (239.4 μW/cm²) and 0.60 mW (18.8 μW/cm²), respectively. Total hybrid output power is 8.26 mW which is sufficient power for operating commercialized Bluetooth module (5 mW), continuously. This milliwatt-scale (~mW) hybrid energy harvester provides us the opportunity to realize autonomous sensor system to come in the near future.

Many aggregation-induced emission (AIE) luminogens exhibit mechanochromic effect through grinding the luminogen powdery. The AIE luminogens undergo bright-dark switching between crystalline and amorphous states. However, a relatively high pressure is required to trigger the phase transfer. Meanwhile, the color manipulation cannot be realized in an in situ and reversible manner, which limits some of their applications. Smart materials including piezoelectrics and magnetostriction materials provide the opportunities to couple external field to the AIE materials and realize more flexible controllable AIE. In this work, magnetic alloy nanoparticles iron cobalt nickel and piezoelectric poly(vinylidene fluoride) (PVDF) have been utilized to tune two kinds of aggregation-induced emission luminogens. Precise control over the emission intensity of luminogens under the external applied magnetic field or electric field has been demonstrated. The AIE has been decreased with the magnetostrictive stress increasing, which is contributed to the looser molecular structure of AIE luminogens under the stimulating of a time-varying magnetic field. While, the piezoelectric strain induced an enhancement of emission intensity under electric field, which originates from the contact of PVDF lattice constant and restriction of intramolecular rotations of AIE luminogens. Thus, these results are expected to enrich the understanding of the luminescence process of AIE luminogens and promise more flexibility of AIE luminogens for the magnetic-optical and optoelectronic applications.

Lead Zirconium Titanate (PZT) is one of the most widely used piezoelectric materials due to its high piezoelectric constant, great piezoelectric response, operating temperature relative to other used piezoelectrics and is one of the industry standard materials in DRAMs and MEMs due to its ferroelectric properties. In addition, PZT is of great interest in piezotronics applications but achieving spatially ordered arrays are a challenge for device construction. This paper presents the characteristics of hexagonal closed-pack PZT nanocolumns grown by a Glancing Angle Pulsed Laser Deposition (GAPLD) technique on a template silicon (Si) substrates. The substrate is patterned with a self-assembled monolayer of Silica Nanospheres (SNSs) of sphere sizes of 3.5 μm, 1.18 μm, 850 nm, 500 nm, 250 nm and 150 nm in diameter by Langmuir-Blodgett technique. Subsequently, PZT was deposited by GAPLD technique at an oblique angle to the substrate to promote ballistic shadowing. In this study, we pursued an investigation of the columnar angle’s relationship with the deposition angle, and its effect on the crystallinity of the PZT nanostructured films. Results pertaining to the morphology, structure, and composition investigated for varying template sphere sizes by SEM, EDS, AFM, and XRD will be presented.

Mechanical force-driven power generation can be achieved by the utilization of piezoelectric materials that produce electricity in response to external mechanical stress. Diverse piezoelectric crystal structures such as perovskite and wurtzite have been studied for piezoelectric nanogenerator applications. In particular, ZnO is extensively researched due to its semiconducting properties, along with its directional preferential crystal growth nature. The piezoelectric applications of ZnO have been demonstrated mostly through the bending stress of one-dimensional nanostructures such as nanowires. The utilization of ZnO in piezoelectric applications can be significantly improved by using the piezoelectric operation mode d_33, via the structural engineering (morphology) of crystalline ZnO from one-dimensional to pyramidal nanostructures.
In this work, our modified CVD synthesis technique is applied to synthesize the mechanically stable and highly piezoelectric ZnO hexagonal nanopyramids array. The superior mechanical stability was achieved through the morphology engineering of ZnO as a step-wise pyramidal structure. The single crystal pyramidal ZnO was grown in vertically-aligned pyramidal structures by controlling the supply of Zn vapor precursor during synthesis. The piezoelectric performance was measured by piezoelectric force microscopy under the highest piezoelectric operation mode d₃₃. The piezoelectric performance was improved by increasing the crystallinity and reducing planar and point crystal defects, as confirmed by transmission electron microscopy and X-ray diffraction analysis. The mechanical, optical, and physical characteristics of synthesized ZnO nanopyramids were characterized by piezoelectric force microscopy, photoluminescence spectroscopy, and Raman spectroscopy.

Higher requirements have been put forward for self-powered systems operated with weak energy in the environment. Sensitivity, efficiency and power management design of the energy harvesting module play key roles on the sustainability of the system. In this work, we present an ultrasensitive triboelectric nanogenerator (TENG) with Cu coated fluorinated ethylene propylene films stacked in a unipolar manner and suspended by springs. Triboelectrification and electrostatic induction take place between every adjacent film surfaces, during which transferred charges are increased by at least 50%, leading to an increased efficiency. Also, the reduced framework weight makes soft spring can be adopted for greatly improved sensitivity. Depending on the investigation of mechanical motion in the system, TENG’s energy harvesting efficiency is boosted by bringing down the moment of inertia. The linear region characteristic of an n-type junction field-effect transistor is utilized for a low-loss power management design with greatly reduced power consumption and start-up time for superior weak energy harvesting applications. Ultrasensitive and high efficient energy harvester with low-loss power management demonstrated in this work promises an outlook for an effective approach to achieve more powerful and more sustainable self-powered systems working with comprehensive ambient weak mechanical energy fitted in diverse application circumstances.

As silicon transistors rapidly approaching their scaling limit because of severe short channel effects, alternative technologies are being urgently needed for the next-generation electronics. Here, we demonstrate ultrathin zinc oxide piezotronic transistors with a ~2-nm physical channel length using the inner crystal self-generated out-of-plane piezopotential created at metal-semiconductor interface as the gate voltage to control the transport process of local carriers. This design removes the need for external gate electrodes that are challenging at nanometer scale. These ultrathin devices exhibit a strong piezotronic effect and excellent pressure-switching characteristics. As active nanodevices by directly converting applied mechanical actuations into electrical control signals without applying an external gate voltage, ultrathin piezotronic devices could be used to construct next generation of electromechanical devices for human-machine interfacing, energy harvesting and self-powered nanosystems.

Recently, a novel catalysis technology, which named piezocatalysis, has received significant attention due to independence of light irradiation. Here, we report the new advances in the piezocatalysis of BaTiO₃ and further investigate the relationship between piezoelectric potential and piezocatalysis. In this work, we successfully synthesized BaTiO₃ nanowires and nanoparticles by a two-step hydrothermal method. It was found that the BaTiO₃ nanowires exhibit effectively enhanced piezocatalytic activity under ultrasonic vibration compared with the BaTiO₃ nanoparticle. To explore the origin of the excellent piezocatalysis performance of BaTiO₃ nanowires, the distribution of piezoelectric potential in these nanomaterials was simulated by the finite element method (FEM) with the aid of COMSOL multiphysics software package. On the basis of the piezoelectric potential analysis by FEM stimulation, the enhanced piezocatalytic activity of the BaTiO₃ nanowires can be attributed to the larger piezoelectric potential along the polar axis. A relatively larger piezoelectric potential of the catalyst surface can induce a greater shift of conduction band and valance band, resulting in easier and faster immigration of the electrons and holes, during reacting with dissolved oxygen and hydroxyl to form superoxide radicals and hydroxyl radicals. Furthermore, we demonstrate that the intrinsic charge carriers (not piezoelectric charges) in piezoelectric crystallites play the role of charge transfer in the catalysis process through regulating the concentration of charge carriers in catalyst. This study provides further understanding of piezocatalysis of piezoelectric nano-materials as well as insights on the relationship between piezoelectric potential and piezocatalysis.

Display technology has not only presented information traditionally but also multifunctional capability, flexibility, lightness and slimness design are desired attributes for being adopted by artificial intelligent (AI) to interact with consumers. In this regard, ferroelectric poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) is of great interest as potentially applicable for display power saving material with having mechanical stability, transparency, and electrically controllable polarization. Here, we report the experimental study of a luminescent light harvesting property of CdSe QD embedded P(VDF-TrFE) nanocomposite, whereas the concentration of QD is not as much high as to interfere presenting information in visual. In agreement with theoretical calculation, the converted power from luminescent light guided to the c-Si solar cell is enhanced by 42% based on the quantum-confined Stark effect (QCSE) owing to ferroelectric polarization of P(VDF-TrFE). The resulting nanocomposite shows enhanced mechanical sensitivity (~40%) by aligned dipole moment of P(VDF-TrFE) upon mechanical contact. This nanocomposite material may potentially be applicable as multimodal display element not only enable to power itself but also detect given mechanical touch.

Triboelectric nanogenerators (TENGs) offer a promising approach for mechanical energy harvesting, but an effective strategy for enhancing the output performance is a crucial requirement. Though many studies have been performed to maximize the triboelectrification phenomenon on the surface of dielectric materials, studies for improving electrostatic induction phenomenon inside the dielectric materials have not yet been conducted. Here, we propose a high output performance TENG with a composite of butylated melamine formaldehyde (BMF) matrix and the high dielectric constant particles of CaCu₃Ti₄O₁₂ (CCTO) as an excellent dielectric layer for enhancing the electrostatic induction phenomenon. The high dielectric constant CCTO particles result into a strong polarization of the dielectric materials under the triboelectric field. As a consequence, the charge induction in the bottom electrode is enhanced thereby increasing the triboelectric output power. A rotation-type freestanding mode TENG based on BMF-CCTO 1wt% composite has produced an RMS voltage and a current density (268 V and 25.8 mA/m²) which is about 3 times higher than that (113.2 V and 8.7 mA/m²) of the TENG based on pure BMF. The strategy of incorporating the high dielectric constant CCTO particles is completely general and can be applied to any polymer matrix in order to enhance the output performance of TENGs.

Domain walls in ferroelectric materials act as functionalized interfaces. The atomic structures in domain wall areas have a significant impact on the properties of the domain walls. Both theoretical simulations and experimental measurements demonstrated that dense charged domain walls (CDWs) can effectively enhance the piezoelectric response of ferroelectrics. Usually CDWs do not steadily exist due to the high electrostatic energy. In some cases, the formation of CDWs can be realized via electrical bias. However, applying an electric field to ferroelectrics could induce current leakage. In this presentation, we demonstrate that CDWs can be created via mechanical stress, which effectively avoids the potential problem of current leakage.

Here, an in-situ straining transmission electron microscopy technique was used to explore the structural evolution of ferroelectric domains introduced by mechanical stressing. Barium titanate micro-pillars were prepared using the focused ion beam technique. Mechanical loading was applied along a <100> direction. A high density of fine nanoscale domains formed when a pillar was compressed. Electron diffraction patterns of pillars with and without mechanical loading confirmed that all the newly created domain walls were CDWs. The original domain structure was restored with the full retraction of the external stress. The mechanism of the stress-induced evolution of domain wall structures will be discussed in detail. Our discovery is very important for the investigation of electromechanical properties of ferroelectrics.

Experimental studies on the physical properties of two-dimensional (2D) materials have grown exponentially since 2D materials offer unique advantages for use in such next-generation devices.^[1-3] Various semiconducting 2D materials have been studied, including transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten diselenide (WSe₂), which are likely to bring breakthroughs in future electronic and optoelectronic devices.^[4-7] The physical properties of 2D MoS₂ nanosheets have been actively explored particularly as a result of their possible integration in both nano/microelectromechanical devices and energy harvesting devices. Monolayer MoS₂ has a direct band gap and high mobility and has been used to successfully fabricate field-effect transistors so it has emerged as an interesting complement to graphene in various semiconducting applications. In additions it is expected that piezoelectric properties of 2D layered materials are very useful to realize high sensitive, high impact resistive, self-powered sensor.^[8] However, studies on the suspended model of 2D MoS₂ have been done mainly on studies on the measurement method or photosensitivity, and studies using the piezoelectric characteristics of suspended MoS₂ have not been actively done. Here we report a new way to use 2D MoS₂ for high sensitive 2D piezoelectric acoustic sensor. The acoustic sensor fabricated by using suspended structure of MoS₂ that generate piezoelectric output potential due to vibration transmitted by sound. We have achieved high sensitivity and high impact resistance by using single layer or several layers of MoS₂, and by using piezoelectric properties of MoS₂, we have produced an acoustic sensor that does not require standby power. This study demonstrates a new way of studying 2D MoS₂ research and acoustic sensors by developing a high-sensitivity and high-durability acoustic sensor using suspended MoS₂.

A Triboelectric generator (TEG) is one of the most promising energy harvesters for powering electronics or sensors, conversing abundant kinetic energy to electrical energy. TEG generates displacement current according to triboelectric surface charge and electrostatic induction by periodic motion. Fundamentally, the surface charge density on triboelectric layer greatly affects to output performance of TEG and there have been numerous intensive studies to increase surface charge density. The surface charge density has been increased dramatically using advanced materials such as nanocomposite, ferroelectric materials or fluorinated materials. However, high surface charge density induces the microdischarge generation and upper limitation of available surface charge density in certain TEG mode becomes new challenge for next-generation high performance TEG. Here, we propose direct current generator based on microdischarge via accumulation of triboelectric charge in atmospheric condition. Microdischarge based direct current generator TEG (MD-TEG) employs in-plane configuration of each layer, electrode layer, rotator and segment layer. Also there is gap of sub-millimeter scale between electrode and rotator. These structural characteristics enable accumulation of huge amount of triboelectric charge in atmospheric condition and efficient charge-conveying those who are essential for high output performance. Based on the novel working mechanism and structural characteristics of CSTEG, we found out that it can generate maximum output current of 12.2 mA/m² in RMS value under 1000 rpm.

Mechanoluminescence (ML) from purely inorganic materials is basically triggered by a pre-UV-irradiation process, which largely limits its practical applications. Here, by designing a pining-trap structure, a novel sustainable ML is developed from Ruddlesden–Popper perovskite Sr₃Sn₂O₇:Sm³⁺ even without the pre-UV-irradiation process. This ML of Sr₃Sn₂O₇:Sm³⁺ could keep stable and remain a certain intensity even extending the wait time to 50 h. ML mechanism is also investigated by analysis of trap structure through the thermoluminescence method. A thermoluminescence peak at 420 K showed no attenuation after 50 h waiting time, which could be ascribed to a high-density pining trap, result in the increasement of ML intensity and ML intensity maintenance ratio.

Advanced medical diagnostics tools like magnetoencephalography (MEG) and magnetocardiography (MCG) require sensors that can detect low frequency magnetic fields of very small amplitudes in the nano- to sub-picotesla range. Superconducting quantum interference device (SQUID) sensors, which are state of the art in these applications, have several drawbacks including the need of cooling, operation in shielded chambers and high cost. Magnetoelectric (ME) sensors are currently developed as a promising alternative. The working principle of ME composite sensors relies on a magnetostrictive layer, which translates the change in magnetic field into mechanical deformation. It is mechanically coupled to a piezoelectric layer to convert the mechanical strain into electric voltage.
The piezotronic effect can be employed for reading out the ME sensor signal to improve the limit of detection (LoD, which is the smallest measurable magnetic field normalized to an equivalent noise bandwidth of 1 Hz). One of the two electrodes attached to the piezoelectric component of the sensor is chosen to be a material which forms a Schottky type contact with the piezoelectric material, while the second electrode forms an Ohmic contact. The electric field generated in the piezoelectric material when it is deformed changes the charge carrier transport characteristics across the Schottky contact drastically. This effect replaces an external amplifier for the sensor signal (in this case a piezotronic current), leading to lower noise and reduced power consumption compared to conventional voltage read-out.
Two kinds of ME sensors have been investigated here: one is a cleanroom-fabricated MEMS cantilever type sensor that consists of magnetostrictive FeCoSiB and piezoelectric AlN. The other sensor type consists of a single crystalline ZnO needle mounted on a piece of magnetostrictive foil. The variation of parameters such as doping, defects and stress in the material and geometry, contact configuration, electric and magnetic bias fields and read-out process of the sensor setup leads to a deeper understanding of the piezotronic effect and is expected to increase the performance of various ME sensor types.
In the present work, we have analyzed the time signal of the ME sensors to further understand the noise sources and reduce the noise floor of the sensor system by customized digital signal processing.

The auditory system is the most efficient and straightforward communication strategy for connecting the human beings and robots in the upcoming artificial intelligent (AI) era. Here, we design a self-powered triboelectric auditory sensor (TAS) for constructing a new electronic auditory system and further applying as the architecture of external hearing aid in intelligent robotic applications. Based on the unique working mechanism of newly developed triboelectric nanogenerator (TENG) technology, the TAS shows ultra-high sensitivity (110 mV/dB). TAS with the broadband response from 100 to 5000 Hz is achieved by designing the annular or sectorial inner-boundary architecture with systematical optimization. By incorporating with intelligent robotic devices, TAS demonstrates a capability of high-quality music recording and accurate voice recognition for realizing intelligent human-robot interaction. Furthermore, the fascinating feature of TAS with tunable resonant frequency can be achieved by adjusting the geometric design of inner-boundary architecture, which can be used to amplify a specific sound wave naturally. Based on this unique property, we propose a new type of hearing aid with TENG technic, which can simplify the signal processing circuit and reduce the power consuming. This work expresses significant advantages of using TENG technology to build a new generation of auditory systems for meeting the challenges in social robotics.

With the fast development of Internet of Things, the requirements of system miniaturization and integration have accelerated research on multifunctional sensors. Based on the triboelectric nanogenerator, a self-powered multifunctional motion sensor (MFMS) is proposed in this work, which is capable of detecting the motion parameters, including direction, speed, and acceleration of linear and rotary motions simultaneously. The MFMS consists of a triboelectric nanogenerator (TENG) module, a magnetic regulation module, and an acrylic shell. The TENG module is formed by placing a free-standing magnetic disk (MD) on a polytetrafluorethylene (PTFE) plate with six copper electrodes. The movement of the MFMS causes the MD to slide on the PTFE plate and hence excites the electrodes to produce voltage output. The carefully designed six copper electrodes (an inner circle electrode, an outer circle electrode, and four arc electrodes between them) can distinguish eight directions of movement with the acceleration and determine the rotational speed and direction as well. Besides, the magnetic regulation module is applied here by fixing a magnetic cylinder (MC) in the shell, right under the center of the PTFE plate. Due to the magnetic attraction applied by the MC, the MD will automatically return to the center to prepare for the next round of detection, which makes the proposed sensor much more applicable in practice.

With increasing vehicle ownership and environment pollution, the release of nitrogen dioxide (NO₂) and its derivatives have considerably posed a threat to human health and public security. By far, various approaches have been applied to develop a reliable, accurate and sensitive approach for NO₂ detection, including field effect transistor (FET), surface acoustic wave (SAW), bulk acoustic wave (BAW), resistance, quartz crystal microbalance (QCM). However, all of these sensing techniques are demanded for a power supply which undoubtedly renders incredible high cost and complexity. In addition, considering the tremendous number and wide distribution of gas sensor networks, it is unrealistic to continuously sustain the maintenance of the external power source and replace the battery for each device especially in wilderness and extreme environment. Therefore, a simple, robust and low-cost gas sensor operated by scavenging ambient energy is desperately desired.

In this work, a triboelectric nanogenerator (TENG) driven self-powered gas sensor has been developed for room temperature nitrogen dioxide (NO₂) detection under UV illumination (365 nm). Powered by TENG, the concentration of NO₂ can be spontaneously detected via chemoresistive gas sensor based on Zinc Oxide-Reduced graphene oxide (ZnO-RGO) hybrid films, where the voltage drop across the interdigital electrodes (IDEs) has a proportional relationship with NO₂ concentration. The self-powered triboelectric gas sensor (TGS) composed by ZnO-RGO composite films exhibit superior response (~16.8) and sensitivity as well as selectivity than that for pure ZnO film. With respect to selectivity, the response of ZnO-RGO composite film based TGS to NO₂ was at least 49-fold higher than those for other gases. Furthermore, through a signal processing circuit, a self-powered NO₂ gas alert system was developed for real-time monitoring the ambient NO₂ concentration. This work not only promotes the applicability of TENGs as self-powered sensors but also pushes forwards the active sensing network node for environmental monitoring by harvesting ambient energy.


References:( *indicate corresponding author)
[1] Yuanjie Su, Guangzhong Xie, Huiling Tai*, Shuangding Li, Boxi Yang, Si Wang, Qiuping Zhang, Hongfei Du, Hulin Zhang, Xiaosong Du, Yadong Jiang, Nano Energy, 2018, 47, 316-324.
[2] Yuanjie Su, Guangzhong Xie, Si Wang, Huiling Tai*, Qiuping Zhang, Hongfei Du, Hulin Zhang, Xiaosong Du, Yadong Jiang, Sensors and Actuators B: Chemical, 2017, 251,144-152.
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Emerging technologies in wearable systems demand that functional devices can adaptively interact with the human body, where mechanical stimuli are ubiquitous and abundant. However, the electrical manipulation of charge carriers underpins the operations of state-of-the-art devices, and the effective control of interfacial energetics for charge carriers by the dynamic mechanical stimuli is still a relatively unexplored degree of freedom for semiconductor nanodevices. Piezotronic effect in nanostructured piezoelectric semiconductors offers exciting opportunities in addressing the above challenges. Here we report the first experimental exploration of piezotronic effect in 1D solid of p-type tellurium nanobelt and systematically investigate the strain-gated charge carriers transport properties. The strain-induced polarization charges at the [1010] surfaces of Te nanobelt can modulate the electronic transport through the interfacial effect on the Schottky contacts and the volumetric effect on the conducting channel. The competing phenomenon between interfacial and volumetric effects has been studied for the first time in piezotronics. Our research allows the access to a broad range of characterization and application of Te nanomaterials for piezotronics and could guide the future study of piezotronic effect in other materials. This progress in piezotronics, together with emerging methods for deterministic production and assembly of nanomaterials, leads to compelling opportunities for research from basic studies of piezoelectricity and semiconductor properties in functional nanomaterials to the development of ‘smarter’ electronics and optoelectronics.

Triboelectric nanogenerator (TENG) is regarded as a promising technology to replace traditional power source by converting mechanical energy to electricity. However, most TENGs are made from materials that are not biocompatible, which limits their applications in biomedical and implanted applications. Here we report for the first time the development of low-cost, biodegradable and flexible triboelectric nanogenerators based on chitosan, which is one of the most abundant biopolymers on earth and can be derived from the shells of shrimp or other sea crustaceans. Tunable electrical outputs were achieved by either mixing the chitosan with other natural materials such as starch and lignin or through laser processing. The chitosan-based TENGs present efficient energy conversion performance and the optimized chitosan-based power source can be used for water purification by taking advantage of the electrostatic attraction effect and chelation with contaminates in the water. The new class of TENGs derived from natural materials may pave the way towards the economically viable production of flexible TENGs for self-powered nanosystems in biomedical and environmental applications.

Smart sensing devices with high stretchability and self-powered characteristics are essential in future generation wearable human-integrated applications. Here we report for the first time scalable synthesis and integration of selenium (Se) nanowires into wearable piezoelectric devices, and explore the feasibility of such devices for self-powered sensing applications, e.g., physiological monitoring. The ultrathin device can be conformably worn onto the human body, effectively converting the imperceptible time-variant mechanical vibration from human body into distinguishable electrical signals, e.g., gesture, vocal movement, and radial artery pulse, through straining the piezoelectric Se nanowires. Our results suggest the potential of solution-synthesized Se nanowire a new class of piezoelectric nanomaterial for self-powered biomedical devices, and opens doors to new technologies in energy, electronics, and sensor applications.

Wearable and stretchable electronic devices have attracted great interest because they cannot only extend the application scope of electronic systems but also provide compliant user experience. Driving those devices inevitably need power sources. However, conventional batteries suffer from not only heavy weight and bulky volume but also limited capacity and lifetime, handicapping the progress and practical uses of those emerging devices. In this present, super-stretchable and mechanically-durable triboelectric nanogenerators (SD-TENGs) will be demonstrated by using composed of intrinsic stretchable components. The newly-designed SD-TENGs can generate electricity from contact with other materials (such as skins, plastic, metal, etc.) regardless of various extreme deformations required from uses, such as extreme stretch of over 300% strain, and multiple twists and folds. Especially, even experiencing severe tearing damages, the device can retain its functionality. With the perfect flexibility, the device can be fully conformal on various nonplanar or irregular objects, including human bodies, spheres, and tubes, etc., to act as power sources for other components. The device can also be introduced into fabric materials for wearable energy and fabric-based self-powered sensing uses. By designing a surface pressure-sensitive structure, the SD-TENGs can further use as a stretchable self-powered electronic skins. We will comprehensively demonstrate their potentials in various kinds of self-powered sensing applications including smart keyboards, gesture sensing, physiological signal sensing, and so on. These works can open the crucial doors for wearable/stretchable/deformable electronics and artificial skins.

[Ref]
[1] Ying-Chih Lai, Jianan Deng, Ruiyuan Liu, Yung-Chi Hsiao, Steven L. Zhang, Wenbo Peng, Hsing-Mei Wu, Xingfu Wang, and Zhong Lin Wang, Advanced Materials, 2018, 30,1801114
[2] Ying-Chih Lai, Jianan Deng, Steven L. Zhang, Simiao Niu, Hengyu Guo, Zhong Lin Wang, Advanced Functional Materials, 2017, 27, 1604462.
[3] Ying-Chih Lai, Jianan Deng, Simiao Niu, Wenbo Peng, Changsheng Wu, Ruiyuan Liu, Zhen Wen, Zhong Lin Wang, Advanced Materials, 2016, 28, 10024.

The role of silicon in spintronic applications is often neglected due to its small spin-orbit interactions from the centrosymmetric crystal structure. In this study of a Ni80Fe20/MgO/p-Si freestanding structure, we created Rashba-Dresselhaus spin-orbit coupling in silicon with a strain gradient. We induced the strain gradient with thermal expansion compressive stresses and piezoresistivity of silicon. The strain gradient broke the structural inversion symmetry, and induced flexoelectric effects and charge separation. This led to cubic Rashba-Dresselhaus spin-orbit coupling throughout silicon and the interface, which lifted the spin degeneracy in band structures and introduced intrinsic spin-Hall effect in n-type and p-type silicon. We confirmed the significant spin-Hall effect through spin-Hall magnetoresistance measurements. Density functional theory calculations confirm that the flexoelectric effect mediates the spin splitting and that the strain-mediated spin splitting is ASDF meV. This work demonstrates that silicon can be used not only for spin channel but also spin manipulation, which substantiates silicon-based spintronics.

As a revolutionary energy harvesting technology, triboelecric nanogenerator (TENG) has attracted the worldwide attention for its distinguished properties and wide application as self-powered sensors since it was invented. A common challenge for sliding-mode TENG was surface friction induced materials abrasion and electrical output performance degradation in mechanical energy harvesting under long-term continuous work. Clarify the effect of friction force on the electrical output performance of sliding-mode TENG not only can improve its output performance but also provide a guideline in the structure design of TENG. In this work, a series of experiments were designed and conducted to reveal the effects of the friction force on the electrical output performance of three kinds of sliding-mode TENG, namely, metal-dielectric, semiconductor-dielectric and dielectric-dielectric. The results showed that the short-circuit output current and the open-circuit voltage of the metal-dielectric sliding-mode TENG increased with the increase of sliding friction force. More importantly, we found that with an excitation induced by friction force the short-circuit output current of the metal-dielectric sliding-mode TENG showed an inverse tendency compared with the short-circuit output current under a changeless friction force. In addition, it was found that a wave friction force has great benefit in enhancing the electrical output performance of metal-dielectric sliding-mode TENG. These findings not only reveal the effect of friction force on the electrical output performance of sliding-mode TENG but also provide a guideline for designing and optimizing the structure of TENG to efficiently harvesting mechanical energy.

High-power devices, such as ultrasonic motors, underwater acoustic transducers and piezoelectric transformers, require piezoelectric ceramics with low dielectric loss tan δ, high mechanical quality factor Q_m (low mechanical loss), and simultaneously large piezoelectric constant d₃₃ and electromechanical coupling factor k_p [1]. Many ternary and quaternary piezoelectric ceramics, such as Pb(Mg_1/3Nb_2/3)O₃-Pb(Zr,Ti)O₃, Pb(Mn_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr,Ti)O₃, etc. [2, 3] have been synthesized and modified by MnO₂, Co₂O₃, ect. to satisfy these requirements. However, it is still difficult to improve piezoelectric properties and reduce dielectric loss simultaneously.
Herein we report our research work on the Fe₂O₃ doped Pb(Mn_1/3Sb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr,Ti)O₃ (PMnS-PZN-PZT) high-power piezoelectric ceramics. These ceramics possess a single-phase perovskite structure, and the addition of Fe₂O₃ helps to acquire a dense morphology. Fe₂O₃ doping also tailors the phase structure of the PMnS-PZN-PZT ceramics. Ceramics with Fe₂O₃ content of 0.45 wt% exhibit a relatively large piezoelectric responses and extremely low losses, showing that the ceramics is suitable for the high-power applications. The electric field dependence of dielectric and piezoelectric properties in subswitching field range, and the effect of temperature on the nonlinearity of dielectric property are also investigated. The results show that the dielectric and piezoelectric constants gradually increase with the increase of electric field level. Rayleigh analysis reveals the contribution from lossless reversible domain wall motion to the high-field nonlinear dielectric and piezoelectric properties [4]. This behavior is associated with the orderly distribution of defect pinning centers, and is thought to be responsible for the low losses of the ceramics. At elevated temperatures, the mobility of the oxygen vacancies increases, so that the distributions of the defect pinning centers are gradually randomized, which consequently lead to the enhancement of high-field nonlinearity.
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Recently, piezoelectricity in 2D van der Waals (vdW) materials has attracted great interest owing to promising unique applications in nanoscale electromechnical systems, ultrasensitive sensors, high-precision actuators, piezotronics and piezo-piezotronics. Monolayer transition metal dichalcogenide MoS₂ with intrinsic in-plane piezoelectrcity has been extensively addressed in recent years, which leads to great potential applications. However, the vertical piezoelectric response of MoS₂ is theoretically absent due to the inversion symmetry. Here, we report out-of-plane piezoelectricity and ferroelectricity in 2D layered material In₂Se₃. Specially, the piezoelectricity based on the In₂Se₃ is not limited by edge effect and layer-dependence. Moreover, the out-of-plane ferroelectricicity in In₂Se₃ is also investigated by piezoresponse force microscope, which can retain the ferroelectric behaviour down to 6 nm. This work opens the door to explore ferroelectricity and piezoelectricity to the family of 2D layered materials which will create novel functionalities of both materials and atomic-scale electronic devices. The work was supported by the grant from Research Grants Council of Hong Kong (GRF No. PolyU 153033/17P).

Wearable systems integrated with fiber-based electronic and photonic devices are soft, ubiquitous, flexible, stretchable, light, permeable, having the most favorite features of human-friendliness. It is extremely desirable to replace rechargeable batteries or prolong their life by harvesting energy from ambient or our body for the wearable systems to work for us anywhere at any time.

In this paper, our recent progress is presented on a number of issues of fiber-based hybrid energy conversion systems. Theoretical models for the fiber-based piezoelectric, triboelectric and their hybrid generators are proposed and verified to predict the output performance of the devices in terms of materials, device structure, harvesting circuits and operating conditions. The interaction between the cascaded PENG and TENG units in a hybrid nanogenerator and its effect on output are disclosed. Furthermore, the theoretical upper limits of output from contact-mode FTENG are revealed and verified experimentally by considering the electric breakdown due to field-induced- emission and gas-ionization. In addition, an experimental platform developed is reported for reliable triboelectric charge measurement of highly deformable and porous materials like fabrics. An extended triboelectric series is described by us including twenty-one types of commercial and new fibers. On the other hand, fiber-based flexible thermoelectric energy conversion devices are being developed. The materials, fabrication processes and characterization are discussed. The study sheds light on the scope and focus for further improvement of the devices. Based upon the findings, we have made significant enhancements of the performance.

Acknowledgement
The work has been partially supported by Research Grants Council of Hong Kong SAR Government (Grant No. 525113, 15215214, 15204715, 1521016) and Hong Kong Polytechnic University (Grants. 1A-BB3).

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Recently, piezoelectric nanogenerator（PENG） and triboelectric nanogenerator (TENG) have attracted much attention and been considered as another potential solution for harvesting mechanical energy. With its high output performance, outstanding biocompatibility and low cost, nanogenerator（NG） has been studied for powering implantable medical electronic devices.
Here, we demonstrated an in vivo biomechanical-energy harvesting using a NG. An implantable triboelectric nanogenerator (iTENG) in a living animal has been developed to harvest energy from its periodic breathing. The energy generated from breathing and body moving was used to power a prototype pacemaker and a low-level laser cure (SPLC) system, respectively. It was found that the self-powered system could regulate the heart rate of a rat and significantly accelerated the mouse embryonic osteoblasts' proliferation and differentiation. Real-time acquisition and wireless transmission of self-powered cardiac monitoring data was demonstrated for the first time. It showed broad clinical applications of implantable self-powered medical systems for disease detection and health care. These works are concentrated on live-powered implantable medical devices. The NGs can convert the mechanical energy from human motion into electricity and drive the implanted long-term self-powered medical devices or biosensors. These are significant progress for fabricating implantable self-powered medical electronic devices using NGs as a power source and an active sensor.

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[10] Z. Li*, et al, ACS Appl. Mater. Interfaces, 2016, 8, 26697−26703
[11] Z. Li*, et al, ACS nano, 2015, 9(8), 7867-7873.
[12] Z. Li*, et al, Adv. Mater. 2014, 26, 5851–5856.
[13] Z. Li, et al, Z.L. Wang, Advanced Materials, 2010, 22, 23, 2534–2537.

Energetic motions like wind and tide are already well handled with existing technologies on large industrial equipment for examples. Meanwhile, mechanical movements with small amplitudes and low frequencies are underexplored, which take place in a wider range of both time and space scales by random excitation in the environment. Fully utilizing such abundant weak energy sources ensures a more powerful and more sustainable self-powered system. However, higher requirements are put forward for weak energy harvesting technologies, especially in two terms of sensitivity and efficiency. Several recent progresses of weak mechanical energy harvesting based on triboelectric nanogenerators are summarized here, including designing the device with unstable mechanical structure, utilizing soft textile to capture subtle body movement, investigating mechanical motion in the system for efficiency optimization, realizing low-loss power management, etc.

There have been profound research works aiming at monitoring humanity physiological signals for disease prophylaxis and treatment, health assessment,tactile (or touch) sensing, and even security application by developing wearable/portable and sensitive nanogenerator-based sensors. More often, sensors focusing on diagnosis or precaution could meet practical demand by discovering bioelectric signals from special point, such as blood pressure from the radial artery revealing arterial sclerosis, movement of body and eyeballs telling sleep disorder, and motion of respiratory muscles for diagnosis of respiratory disease, respectively. Meanwhile, the foot pressure distribution not only can suggest footwear design and sport biomechanics information, but also indicate possible injury and even predict ulceration in the feet of patients with type 2 diabetes. Specifically, plantar pressure values from a lot of areas, including heel, lateral mid foot, metatarsal, and so on, can serve as a necessary medical indication.
In this paper, we present two our recent works on utilizing nanogenerators as the sensing component and then constructing wearable physical monitoring systems for real-time foot pressure mapping and ankle rotation angle sensing.
As for the foot pressure mapping, a flexible printed circuit board (FPC) with 32 pieces of PVDF as the PENG pressure sensors (PENG-PS) is employed, which can be large quantity manufactured at low cost. To convert the raw sensor signal to digital signal, the FPC with PENG-PS attached is connected to a designed data acquisition circuit (DAQ). These converted results can be transmitted to mobile terminal, such as laptop, pad, and smart phone through Bluetooth 4.2 protocol, then the real-time pressure distribution is reconstructed. More importantly, through driving by a hybridized triboelectric–electromagnetic nanogenerator, a self-powered and real-time pressure mapping system is demonstrated.
As for the ankle rotation angle sensing, a free-standing rotary TENG is utilized. It achieved a rotation resolution less than 1 degree. Moreover, by employing two sets of TENGs, the rotation direction can be detected. We integrated the device with embedded circuits, which can convert the electric signal of TENG into digital data, and transmit to the laptop or smart phone through Bluetooth, forming the real-time monitoring.
These works show the promising future of constructing wearable sport medical monitoring system by utilizing TENG.

This seminar introduces three recent progresses of self-powered flexible electronic systems beyond thermal limits. The first part will introduce self-powered systems for IoT sensors and flexible energy source. Flexible nanogenerator converts external bio-mechanical movement into electrical energy for self-powered IoT and biomedical devices such as pacemaker and transportation. In addition, flexible piezoelectric materials detects the minute vibration of membrane or human skin that expands the application of self-powered acoustic sensor and healthcare monitor. The second part will introduce laser material interaction for flexible applications. Laser technology of ultra-short pulse duration becomes important for future flexible electronics since high temperature process can be adopted on plastic substrates, which is essential for high performance electronics. Exciting results of flexible laser material interaction will be explored from both material and device perspectives including nanomaterial synthesis, inorganic laser liftoff and plasmonic material reaction.
The third part will discuss flexible large scale integration (f-LSI) for flexible CPU and high density memory. Flexible LSI is an essential part of future electronics for data processing, storage, and radio frequency (RF) communication. To fabricate f-LSI, we integrated 0.18 CMOS process of single crystal silicon nano-transistors with flexible electronics. Simultaneous roll transfer and interconnection of flexible NAND Flash memory was achieved using anisotropic conductive film (ACF). Finally, we introduce the highly efficient and long-term stable flexible vertical micro LED (f-VLED) for full color displays, wearable and biomedical applications. Using optogenetic mouse models, f-VLED stimulated motor neurons deep below layer III from the brain surface and induced mouse behavior changes. These f-VLED can be also used as tools of skin research and phototherapy.

With the challenge of powering networks of trillions of sensors and devices for the Internet of Things the need for effective generators harvesting low-frequency vibrational energy from the ambient is expected to increase dramatically. This type of energy should be mobile and ubiquitously accessible entailing generators that are compact, effective over a large bandwidth, and of small physical dimensions. Triboelectric nanogenerators (TENGs) are ideal candidates [1] out-competing traditional electromagnetic generators as the latter is ineffective at the low frequencies characterizing ambient vibrational energy from wind, human activity etc. Further, TENGs may provide high power densities up to 500 W/m² or 15 MW/m³. The latter characteristics of TENGs have fuelled a huge interest in exploiting and designing effective generators experimentally. Despite triboelectric energy and charge transport in TENGs is an effect known since ancient times, a detailed microscopic understanding is still not available [2,3].

In this work, we present a quantum-mechanical model of contact electrification between atomic systems and then proceed to analyze the more complicated case involving crystalline solids [4]. The transfer mechanism in atomic systems, that we consider, is based on electron transfer from an atomic state with energy E_A (material A) to a lower-energy (E_B) state associated with material B and the emission of a photon with energy E_A - E_B. The spontaneous emission rate is determined using Fermi’s Golden Rule in the dipole approximation based on the one-electron Schroedinger equation. The process requires spatial overlap of the initial and final state wavefunctions, hence the electron transfer is a sensitive function of the distance between the two materials A and B as well as the symmetries of the atomic states. We start out by determining the spontaneous emission rate for a fixed distance between atoms and then analyze the temporal variation of level occupations, i.e., the time dependence of electron transfer, using the Einstein rate equations and assuming the higher-energy level is occupied initially. Next, we determine the transfer of electrons in a dynamic setup where the distance of the two materials changes in an oscillatory manner.

Inspired by this simple quantum-mechanical description of contact electrification between atomic systems we generalize the idea to crystalline solids. Using a one-band k.p envelope function approximation of electronic states [5], the dipole matrix elements between surface states in dielectric materials A and B are computed. Since solids are quasi-continuum systems the transfer rate depends intricately on the conduction and valence bands of the two interacting materials, Fermi levels of the two materials, temperature, the vacuum energy (since vacuum separates the two materials), effective masses, and the distance between materials A and B. We calculate the spontaneous emission rate as a function of distance by varying the Fermi levels, temperature, and the effective masses. The model can easily be generalized to describe contact electrification between conductors and/or dielectrics. Other transfer mechanisms such as phonon emission through, e.g., the Froehlich coupling can be described in a similar way.

References
[1] Z. L. Wang, L. Lin, J. Chen, S. Niu, and Y. Zi, Triboelectric Nanogenerators, Green Energy
and Technology, Springer (2016).
[2] J. Henniker, Nature 196, 474 (1962).
[3] C. Xu, Y. Zi, A. C. Wang, H. Zou, X. He, P. Wang, Y.-C. Wang, P. Feng, D. Li, and Z. L.
Wang, Adv. Mater. 1706790 (2018).
[4] M. Willatzen and Z. L. Wang, Nano Energy 52, 517 (2018).
[5] M. Willatzen and Z. L. Wang, to be submitted (2018).

In the Internet of Things era, both self-powered nanodevices and nanogenerators (NGs) that harvest energy from mechanical vibrations are highly attractive, prompting a rapid surge in research on enhancing their performance. To this end, it becomes nature to search for new materials with higher piezoelectric coefficients. However, it is not always a simple task to fabricate large-scale and high-quality new materials as considering the limited synthesis techniques. Alternatively, to optimize the microstructure characteristics of a given material may be more beneficial and effective. To alter the properties of a given material, we have demonstrated several strategies including doping, oblique orientation and alloying and the output performances of various piezotronic devices as well as piezoelectric nanogenerators have been significantly improved. However, some of the strategies are specific to materials and those material-dependent characteristics may have limitations for universal utilization. We demonstrate a versatile approach for diverse materials, by altering microstructure with discontinuous nano-pores in one-dimensional nanostructures, which can be readily extended into higher-dimensional single crystal materials, such as epitaxial thin film. This novel approach is demonstrated for two applications, i.e., direct-current (DC) NGs and piezotronics by taking ZnO as a model, where remarkably enhanced performance is depicted in theoretical simulations and confirmed by experiments. Here, we report that porous ZnO nanowires-based DC-NGs demonstrate ~23 times enhancement in output performance, and strain-gated transistors exhibit ~6 times enhancement in force-sensitivity. This can potentially augment energy harvesting and pressure sensing for many applications, such as, self-powered nano-devices and touch-panels.

Triboelectric nanogenerators (TENGs) are promising innovative energy conversion devices that convert mechanical energy to electricity based on triboelectric friction. We developed a series of TENGs based on sponge-like porous polymer thin films such as polydimethylsiloxane (PDMS) and polytetrafluoroethylene (PTFE). The porosity effect on the output performance of the porous TENG was investigated under mechanical impacts. The output voltage of the porous TENG is obvious higher than that of the solid polymer thin film based TENG under the same condition. The porous TENG can also generate considerable electricity by harvesting mechanical energy from human motions. The generated electric energy could instantaneously power some light emitting diodes (LEDs) and other small electronics without any energy storage process. The development of the porous TENGs could open a new avenue toward developing self-powered personal electronics, owing to their flexibility, simple structure, and the ability to harvest mechanical energy from human motions.

References:
1. Chuan Ning, Lan Tian, Xinya Zhao, Shengxin Xiang, Yingjie Tang, Erjun Liang, Yanchao Mao*, Washable Textile-Structured Single-Electrode Triboelectric Nanogenerator for Self-Powered Wearable Electronics, J. Mater. Chem. A, 2018, DOI: 10.1039/C8TA07784C.
2. Meng Wang, Jiahao Zhang, Yingjie Tang, Jun Li, Baosen Zhang, Erjun Liang, Yanchao Mao*, Xudong Wang*, Air-Flow-Driven Triboelectric Nanogenerators for Self-Powered Real-Time Respiratory Monitoring, ACS Nano, 2018, 12, 6156-6162.
3. Meng Wang, Nan Zhang, Yingjie Tang, Heng Zhang, Chuan Ning, Lan Tian, Weihan Li, Jiahao Zhang, Yanchao Mao*, Erjun Liang, Single-Electrode Triboelectric Nanogenerators based on Sponge-Like Porous PTFE Thin Films for Mechanical Energy Harvesting and Self-Powered Electronics, J. Mater. Chem. A, 2017, 5, 12252.
4. Yanchao Mao*, Nan Zhang, Yingjie Tang, Meng Wang, Mingju Chao, Erjun Liang*, A Paper Triboelectric Nanogenerator for Self-Powered Electronic Systems, Nanoscale, 2017, 9, 14499.
5. Yanchao Mao, Dalong Geng, Erjun Liang, Xudong Wang*, Single-Electrode Triboelectric Nanogenerator for Scavenging Friction Energy from Rolling Tires, Nano Energy, 2015, 15, 22.
6. Yanchao Mao, Ping Zhao, Geoffrey McConohy, Hao Yang, Yexiang Tong, Xudong Wang*, Sponge-Like Piezoelectric Polymer Film for Scalable and Integratable Nanogenerators and Self-Powered Electronic Systems, Adv. Energy Mater., 2014, 4, 1301624.

The coupling between piezoelectric and semiconducting properties opens the pathway to the development of electronic devices with additional functionality. ^[1] In the past, the changes in potential barrier, which were achieved via application of mechanical stress, are only in the order of meV or a few percentage of the total height of the potential barrier. Similarly, charge screening is deemed critical and not necessarily the full piezoelectric potential of ZnO can be harvested.
The latter is addressed by conducting an experimental study using undoped intrinsically conductive ZnO single crystals with both Ohmic and Schottky contacts ^[2]. The effective piezoelectric response was then quantified at temperatures from -140°C and 20°C and frequencies from 0.5 Hz to 160 Hz. The formation of an electrostatic potential barrier at the metal-semiconductor interface was found to increase electrical resistance and hence delivers full utility of unbiased piezoelectric coefficients of ZnO single crystals even at room temperature. This can also be seen as an illustration and quantification of the piezotronic effect on an ideal macroscopic single crystal. Due to the Schottky barrier the piezoelectric charge cannot be fully screened by electronic charge carriers.
An almost complete elimination of the potential barrier at the interface was achieved by utilizing a bicrystal ZnO / ZnO interface with well-defined polarization conditions. The methodology is based on bonding two well-aligned single crystals of ZnO with an intermediate thin polycrystalline sacrificial layer of doped ZnO. Subsequent high temperature treatment grows the single crystals into the sacrificial layer and consumes the polycrystalline materials either partially or fully. The doping content is optimized so that a double Schottky barrier develops at the ZnO-ZnO interface. For full growth, a single interface evolves. Based on this structure, we will show how stress can tune the potential barrier in head-to-head and tail-to-tail orientation of the polarization vector to enhance/lower the conductivity across individual ZnO bicrystal interfaces. Surpassing the stress sensitivity of previous piezotronic systems.^[3]


[1] Wen, X. et al. Development and progress in piezotronics. Nano Energy 14, 276-295 (2015)
[2] Novak N. et al. Influence of metal/semiconductor interface on attainable piezoelectric and energy harvesting properties of ZnO. Acta Mater. 162, 277-283 (2019)
[3] Keil, P. et al. Piezotronic Tuning of Potential Barriers in ZnO Bicrystals. Advanced Materials, 1705573 (2018)

Multifunctional materials have received increasing attention in recent years. Owing to their inherent biological nature as well as piezoelectric properties, piezoelectric biomaterials are considered as promising candidates for application in fields ranging from electrochemical energy storage to biological systems. The rational material design is important to enhance their piezoelectric and biological activity. Herein, recent advancements to piezoelectric biomaterials like peptide-based micro/nanostructures are provided. Synthetic methods, morphological features, and piezoelectric performance of piezoelectric biomaterials are presented. The effect of growth direction, phase and structure of piezoelectric biomaterials on their piezoelectric activity are discussed. The applications of piezoelectric biomaterials in the field of nanogenerators are provided at the end.

Reference:
1. H. Yuan, T. Lei, Y. Qin, R. Yang, Flexible electronic skins based on nanogenerators and piezotronics, Nano Energy, submitted
2. V. Nguyen, S. Kelly, R. Yang, Piezoelectric peptide-based nanogenerator enhanced by single-electrode triboelectric nanogenerator, APL Materials 5, 074108 (2017)
3. V. Nguyen, R. Zhu, K. Jenkins, R. Yang, Self-assembly of diphenylalanine peptide with controlled polarization for power generation, Nature Communications 7, 13566 (2016)
4. V. Nguyen, K. Jenkins, R. Yang, Epitaxial growth of vertically aligned piezoelectric diphenylalanine peptide microrods with uniform polarization, Nano Energy 17, 323 (2015)

Aiming at environmental renewable and sustainable energy sources, energy harvesting technology has been developing rapidly in the recent years. Emerging as significant applications of Maxwell’s displacement current in the energy field, triboelectric nanogenerator (TENG) has received worldwide attention and undergone speedy progress. In the field of TENG, structural design is an important research topic due to the diverse application circumstances. In our woks, several device structures have been carefully investigated, including angle-shaped wind-driven TENG with enhanced output performance and very low start-up wind speed, kinesio-tape based TENG with compact structure for self-powered human motion sensing, and so on. We will also discuss the applications of TENG as flexible electronics for wearable device. Our works may provide design strategies for TENG and put forward its practical applications as newly energy harvesters and self-powered sensors.

Harvesting energy from the environment offers great promise for the application of distributed sensor networks. Ideally, highly effective energy conversion processes should be environmentally friendly, spontaneous and maintenance-free. Recently, we have reported that evaporation-driven water flow within a porous carbon film can reliably generate sustainable electricity with open-circuit voltage (V_oc) up to 1 V and short-circuit current (I_sc) of 100 nA, respectively[1]. The output performance of the device can be easily scaled up by using all print method [2]. A device based on freestanding and flexible carbon-based hybrid film with size of 400*45*0.13 mm³ produce an I_sc ~ 28 μA [3]. The V_oc of the device can be tuned from -3 V to 3 V by manipulating the surface functional groups on the carbon nanoparticles. In addition, a hybrid energy utilization technology was also developed by using solar energy for desalination (solar thermal efficiency of ~75%) and extracting electricity (power of ~ 1 W m^-2) from the evaporation induced salinity gradient [4]. Because of the ubiquity of water evaporation in nature and the low cost of materials involved, our study presents a novel avenue to harvest ambient energy and has potential applications in low-cost, green, self-powered devices and systems [5-6].

Reference
[1] G. B. Xue, Y. Xu, T. P. Ding, J. Li, J. Yin, W. W. Fei, Y. Z. Cao, J. Yu, L. Y. Yuan, L. Gong, J. Chen, S. Z. Deng, J. Zhou, W. L. Guo, Water-evaporation-induced electricity with nanostructured carbon materials, Nat. Nanotechnol. 2017, 12, 317-321
[2] T. P. Ding, K. Liu, J. Li, G. B. Xue, Q. Chen, L. Huang, B. Hu, J. Zhou, All-Printed Porous Carbon Film for Electricity Generation from Evaporation-driven Water Flow, Adv. Funct. Mater. 2017, 27, 1700551.
[3] T. P. Ding, K. Liu, J. Li, P. Yang, Q. Chen, G. B. Xue, J. Zhou, Chin. Sci. Bull. 2018, 63, 2846-2852.
[4] P. H. Yang, K. Liu, Q. Chen, J. Li, J. J. Duan, G. B. Xue, Z. S. Xu, W. K. Xie, J. Zhou, Solar-driven simultaneous steam production and electricity generation from salinity, Energy Environ. Sci. 2017, 10, 1923-1927.
[5] J. Li, K. Liu, G. B. Xue, T. P. Ding, P. H. Yang, Q. Chen, Y. Shen, S. Li, G. Feng, A. G. Shen, M. Xu and J. Zhou, Electricity generation from water droplets via capillary infiltrating, Nano Energy 2018, 48, 211-216.
[6] K. Liu, T. P. Ding, J. Li, Q. Chen, G. B. Xue, P. H. Yang, M. Xu, Z. L. Wang, J. Zhou, Thermal-Electric Nanogenerator Based on the Electrokinetic Effect in Porous Carbon Film, Adv. Energy Mater. 2018, 8, 1702481.

We have examined the piezoelectric single crystal PMN-PT (Pb(Mg1/3Nb2/3)O3-PbTiO3) as a piezoelectric energy harvester to convert the mechanical energy to electrical energy in various forms. One of those applications is the push-button-type energy harvester, which can directly generate the electrical energy enough to send a wireless signal to the nearby receiver without battery. The quantitative analysis of the voltage and the power generation during the push-button operation had been done, and it could be doubled when we employed the idea of snap-through buckling. The other is the vibration energy harvester that is designed for the status-monitoring purpose of a pole-mounted electrical power transformer. From the basic vibration originating from the AC voltage, it could permanently generate the electricity required for operating an event logger of partial discharge and sending the wireless message containing that data. From those two works, we demonstrate that the piezoelectric single-crystal PMN-PT can be one of the most reliable and powerful materials for the piezoelectric power generation combined with wireless communication capability. (This project is supported by the Korea Electric Power Corporation (KEPCO).)

The rapid development of the Internet of Things (IoT) and the increasing energy crisis call for a new type of energy sources. Triboelectric nanogenerators (TENGs), which can convert mechanical energies to be electricity, develop quickly in recent years and catch more and more people’s eyes in the world. Meanwhile the importance of the standardization for TENG has been emphasized, and developing the standardized theoretical models and experimental methods to evaluate the performance quantitatively becomes more and more important for the commercialization and industrialization of the TENG technology. This presentation will systematically review the origin of the V-Q plot based standardized characterization methods and the figure-of-merits (FOM) based the standard for quantitatively charactering the energy output. This presentation will also discuss several essential factors, including the input energy, environmental factors, and life-time assessment, to be potentially involved in standards for TENGs in future. The standardization of TENG will be great helpful to understand and improve this emerging energy harvester towards further applications. We believe the standardization of TENG will greatly facilitate the commercialization and industrialization of TENGs in future.

Photoelectrochemical (PEC) reactions are a promising pathway for the direct transformation of solar energy to chemical fuels.^[1–3] The past decade has produced significant progress in PEC reactions for this type of conversion, especially PEC water splitting for H₂ generation.^[4–6] Despite these advancements, the ultimate industry-level solar-to-hydrogen productions through PEC still remains unreachable due to challenges in the stability, light absorption and charge separation.^[7] In regards to charge separation, the piezotronic effect has arisen as a new paradigm to break the forgoing capstone. Applying ferro- and piezoelectric materials an their polarization trigger considerable free carrier redistribution in the adjacent semiconductor. As a result, the charge separation and overall PEC performance can be effectively tuned.
Electronic energy band diagrams provide useful and illustrative information on how materials stacking might affect electronic properties and charge transport throughout a multi-junction device. However, schematic diagrams, which are often used in many publications, lack quantified changes in potential across junction interfaces. This is especially true as the energetics become increasingly unintuitive when incorporating the effects of dielectric layers and applying ferro- and piezoelectric dipoles, such as the case of piezotronics. By applying the Poisson-Boltzmann equation and idealizing interfaces, we can produce first approximation, quantitative electronic energy band diagrams for solid/solid and solid/electrolyte heterojunctions that incorporate ferro- and piezoelectric elements.^[8] Using a Mathematica code that we developed, we can show that band bending at semiconductor/electrolyte interface can vary over several meV by adjusting piezotronic polarization.
Aside from the semiconductor/electrolyte interface, piezotronics have also shown great promises in promoting the efficiency of PEC devices by other means. Full-gear functionalities of these devices are constrained by the longstanding contradiction between the charge collection of semiconductors and the screening effect of polarization materials. We tackled this issue by decoupling the collecting and screening trajectories using graphene as the charge collector.^[9] The moderate charge density of graphene ensures minimal screening of the adjacent electrical polarizations and concurrently delivers photogenerated free carriers toward the counter electrode. Based on a PMN-PT/graphene/TiO2 piezotronic PEC system, substantial performance gains were received through tuning the interfacial electronic energy level via ferroelectric polarizations, obtaining a favorably shifted onset potential. Both calculation and experimental results suggest that this outcome was beneficial from the low screening effect from graphene. In contrast, a gold electrode will fully screen the polarization from PMN-PT and yield similar PEC performance despite of the poling conditions. This work reveals graphene could be an ideal conductive electrode selection for freeing piezotronic PEC devices from the performance capstone imposed by the trade-off between carrier collecting and charge screening.

References:
[1] M. Grätzel, Nature 2001, 414, 338.
[2] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446.
[3] M. S. Dresselhaus, I. L. Thomas, Nature 2001, 414, 332.
[4] Y. Yu, C. Sun, X. Yin, J. Li, S. Cao, C. Zhang, P. M. Voyles, X. Wang, Nano Lett. 2018, 18, 5335.
[5] Y. Yu, Z. Zhang, X. Yin, A. Kvit, Q. Liao, Z. Kang, X. Yan, Y. Zhang, X. Wang, Nat. Energy 2017, 2, 17045.
[6] H. Li, Y. Yu, M. B. Starr, Z. Li, X. Wang, J. Phys. Chem. Lett. 2015, 6, 3410.
[7] N. S. Lewis, Science 2016, 351, aad1920.
[8] L. N. German, M. B. Starr, X. Wang, Adv. Electron. Mater. 2018, 4, 1700395.
[9] X. Chen, L. German, J. Bong, Y. Yu, M. Starr, Y. Qin, Z. Ma, X. Wang, Nano Energy 2018, 48, 377.

Converting ambient mechanical energy into electricity provides a viable solution to a sustained power source for distributed small electronic devices. It is particularly significant for the development of wireless sensor networks. In the past five years, we innovatively utilized contract electrification on the surface of flexible thin-film materials, proposing a novel mechanism for the conversion of mechanical energy through the coupling of triboelectrification and electrostatic induction. This so-called triboelectric nanogenerator has a considerable advantage in terms of high energy density. Based on this mechanism, we tuned the processes of triboelectric charge generation as well as free electron transporting frequency and direction. As a result, a family of high-performance miniaturized triboelectric nanogenerators was developed. These devices could convert and harvest diverse types of mechanical motions, such as human body and water wave movement. Upon energy storage and management, the generated electricity could provide maintenance-free power supply for diverse types of small sensors. In this talk, the self-powered sensing systems for the use of on-site diagnosis, security surveillance, and Internet of Things will be introduced.

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, strain) and the introduced optical properties. Herein, the tuning strategies, fundamental mechanisms, recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, two-dimensional materials, and wurtzite-type semiconductors are presented. The future outlook and challenges of this exciting field are suggested.

Chip-based 3D subdiffraction microscopy with easy-configuration, fast-speed is highly desired. We propose wide-field far-field super-resolution imaging chip based on frequency shift effect. Our previous work demonstrates wide-field label free super-resolution imaging can be achieved utilizing a combination of a spatial frequency shift and a Stokes frequency shift [1, 2]. Recently, combining the broad spectra of polymer fluorescent film and well-designed polygon shape, full-angle wide wave vector coverage illumination was achieved for complete imaging reconstruction to eliminate distortion. By frequency shift and iteratively stitching different spatial frequency range together in Fourier space, reconstruction of two dimensional (2D) samples without distortion was achieved in a wide FOV. Furthermore, frequency loss free 3D super-resolution circuits was designed. The spatial-frequency of surface wave illumination can be tuned flexibly through wave vector modulation and operation, to achieve complete spatial-frequency detection for deep subwavelength imaging. Piezophototronic effect can be used to precise control the phase and frequency shift in the circuit.

References:
X. Liu, C. Kuang, X. Hao, C. Pang, P. Xu, H. Li, Y. Liu, C. Yu, Y. Xu, D. Nan, W. Shen, Y. Fang, L. He, X. Liu, and Q. Yang^*, “Fluorescent Nanowire Ring Illumination for Wide-Field Far-Field Subdiffraction Imaging”, Phys. Rev. Lett. 118, 076101 (2017).
C. Pang, X.W. Liu, M. Zhuge, X. Liu, M. G. Somekh, Y. Zhao, D. Jin, W. Shen, H. Li, L. Wu, C. Wang, C. Kuang, Q. Yang, High-contrast wide-field evanescent wave illuminated subdiffraction imaging, Opt. Lett. 42, 4569-4572 (2017)

Research on piezoelectric polymers is mostly concentrated on polyvinylidene difluoride, PVDF, and its random copolymer, poly(vinylidene difluoride-trifluoroethylene), P(VDF-TrFE). Among engineering polymers, odd-nylons are ferroelectric. Since the discovery of ferroelectricity in polymers, nearly half a century ago, a solution processed ferroelectric nylon thin-film has not been demonstrated due to the strong tendency of nylon chains to form hydrogen bonds. In this contribution, we show that solution quenching allows for the fabrication of ultra-smooth and optically transparent nylon-11 thin-films which are crystallized in the polar piezoelectric/ferroelectric phase. The ferroelectric properties are on-par with those of PVDF and P(VDF-TrFE). However, nylon ferroelectric capacitors show superior fatigue-free behavior upon cycling in sharp contrast to PVDF and P(VDF-TrFE) capacitors.
Nylons are among the basic materials for textile industry. Hence realization of piezo/ferroelectricity in nylon fibers can readily lead to textiles that are responsive to electrical or mechanical stimuli, thereby enabling e- or smart-textiles. We discuss fabrication of piezoelectric nylon fibers and nanogenerators thereof for energy harvesting.

Two-dimensional orthorhombic SnS has been recently attracted interests in the application to nanogenerators for its remarkable piezoelectric coefficient, d~145 pm/V for monolayer, which is much larger than MoS₂ and comparable to PZT [1]. However, there has not been demonstrated any observation of piezoelectric response for SnS, because strong interlayer ionic bonding by lone pair electrons in Sn atoms prevents the mechanical isolation of monolayer [2]. We have successfully realized the growth of few-to-monolayer SnS on mica substrate via physical vapor deposition (PVD), where the growth temperature was precisely controlled to balance adsorption/desorption of SnS. Synthetic mica, KMg₃AlSi₃O₁₀F₂, is an attractive platform with atomically flat surface, thermal tolerance up to 1100°C, and flexibility, which enable a straightforward process from crystal growth to fabrication of flexible devices. In this work, for the first time, electromechanical response of SnS is demonstrated with using ultra-thin SnS layers grown on mica. SnS is a semiconductor material unlike the traditional piezoelectric ceramics; therefore, understanding of piezoresistive effect in SnS is essential as well as piezoelectric effect, toward the nanogenerator application. In order to observe the piezoresistive effect, electromechanical response was investigated for ~10 layers SnS, which is supposed to be non-piezoelectric.
Flexible device was fabricated with PVD-grown SnS on mica substrate directly bonded to PC or PET film. I_d–time measurements at constant V_d were performed with applying in-plain tensile/compressive strain of ε = ±0.5 % repeatedly. Electromechanical responses were observed reproducibly; the resistance R increased with tensile strain and decreased with compressive strain. These results are consistent with the theoretical study on bandgap E_g of SnS with different strain; E_g increases with tensile strain and vise versa [3]. Based on these piezoresistive responses, the typical gauge factor was estimated as ΔR/εR = 12.4, which is comparable to that of metals. The present SnS on flexible mica platform will be able to extract the piezoelectric effect for monolayer SnS with negligible noise level.
[1] R. Fei et al., Appl. Phys. Lett. 107, 173104 (2015). [2] N. Higashitarumizu et al., MRS Adv. 3, 2809 (2018). [3] L. Huang et al., J. Chem. Phys. 144, 114708 (2016).

Majority of current IMDs are powered by conventional primary or secondary batteries that contribute up to 90% weight and volume of the entire device. [1.2] While replacement of or recharging the batteries requires substantial surgical or technical efforts, introducing additional suffering and complexity to the patients, other batteries potential issues such as overheat and leakage of toxic electrolyte further prohibit the advancement and miniaturization of IMDs. Therefore, increasing efforts are now being focused on the innovation of designated IMD power sources. Implantable nanogenerators (i-NGs) have been designed to convert biomechanical energy into electricity. [3,4] In spite of their numerous merits, the outputs of state-of-the-art i-NGs are always in a form of largely discrete pulses. Although their theoretical output power could be sufficient for IMDs, battery component is still needed in the i-NG design to produce a steady and useable direct current (DC) output. Moreover, most i-NGs are non-stretchable with incompatible mechanical properties compared to soft biological tissues, which further challenges their practical applications.
In this work, we reported an ultra-soft stretchable i-NG system that could function as a battery-free DC micro-power supply. The i-NG consisted of ultrafine micro-scale interdigital electrodes (IDEs) and multi tribo-active layers with a small working area (approximately 2 cm²), packaged with biocompatible silicone elastomer. While the micro IDEs support the output of high frequency electricity (1 μA at 70 Hz) driven by slow mechanical stimulation, the silicone elastomer and an embedded cavity design enable i-NGs with extremely low Young’s Modulus (46 kPa), exactly matching the mechanical property range of most soft biological tissues. By implanted inside the abdominal cavity of Sprague Dawley (SD) adult rats, the i-NG could convert slow diaphragm movement during normal breath into stable high-frequency electrical spikes, which were readily transmitted into a continuous ~2.2 V DC output on a LED load after being integrated with a basic electrical circuit (rectifier and capacitor) for a relatively long period of time.
This electric output could continuously power the LED without any observable power decay, successfully demonstrating a constant operation of small electronics DC power free of the battery component.This solely biomechanical-energy driven DC micro power supply offers a very promising solution for the development of self-powered IMDs in the near future.
References:
[1] A. B. Amar, A. B. Kouki and H. Cao, Sensors, 2015, 15, 28889-28914.
[2] D. C. Bock, A. C. Marschilok, K. J. Takeuchi and E. S. Takeuchi, Electrochimica Acta, 2012, 84, 155-164.
[3] J. Li and X. Wang, APL Materials, 2017, 5, 073801.
[4] M. Parvez Mahmud, N. Huda, S.H. Farjana, M. Asadnia, C. Lang, Advanced Energy Materials, 2018, 8, 1701210.

With the growing popularity of various internet of things sensors and portable devices, such as smartphones and smartwatches, the demand for power generation sources for driving such electronic devices is becoming increasingly important and challenging. If we rely on batteries to drive all of the sensors related to Internet of things, most of the IoT would be impossible. Here, we report the exciting possibilities for the use of triboelectric nanogenerators (TENGs) toward real-time self-powered electronics, such as a smartphone-to-smartwatch telecommunication over Bluetooth via the capacitors charged by the TENGs, a self-powered pulse sensor, and a real-time self-powered calculator. To achieve the high-output power of the TENGs, we focused on multiple strategic points such as device structures, contacted materials, and mechanical systems, as well as circuit design methods of enhancing the charging efficiency to the energy storage device. The novel integration scheme of TENGs made it generate areal output power of approximately 3 mW/cm² under a low frequency of 3 Hz through a gear-cam system. Based on these results, fast-chargeable portable power-supplying systems for the continuous self-powered electronic systems were successfully developed.

Triboelectric nanogenerator (TENG) is one of alternative energy harvesting technologies of ambient mechanical energy source in nature environments. Here, a new-type of sliding-mode TENG with the enhanced potential through the direct metal-to-metal contact with the ground connection is reported. When the positively charged Al layer is contacted with the grounded Al, the electrons flow is promoted to the ground by large potential difference between the two metals. This induces the charge transfer between bottom electrodes, enhancing the electric potential, confirmed by the COMSOL simulations. This new type of TENG generates a maximum power of 0.13 W under low frequency of 3 Hz on an external load of 1 MΩ, corresponding to energy conversion efficiency of approximately 42 %. Performance of the working-mode are also systematically investigated in free-rotating disk-type and patch-type TENGs, which offered a lot of convenience and feasibility for energy harvesting from the wearable energy sources.

Polymeric piezoelectric nanogenerators (PNGs) have emerged as a suitable candidates to harvest waste mechanical energy to power up portable electronic devices. Smart textiles, with piezoelectric functionalities integrated in the fabrics have been envisioned. In this regards, numerous piezoelectric (nano-) generators based on PVDF or P(VDF-TrFE) nanofibers have been reported.
However, the piezoelectric polymer (nano-)generators have typically shown low output energy densities; A the common issue hindering their application.
Introduction of porosity into the piezoelectric polymer has been proposed to increase the voltage output of PNGs. However, designing a process that allows introduction of pores in polymer fibers with typical diameter of the order 100 nm, would be a breakthrough in the field of PNGs. In this contribution, we discuss an elegant approach to tailor porosity in electrospun P(VDF-TrFE) nanofibers. The approach is based on the thermodynamics of polymer solutions, and solvent/non-solvent interactions with the polymer. We calculated the ternary phase diagram of P(VDF-TrFE)/ non-solvent (water)/solvent, and experimentally verified it. Based on the phase diagram, a conscious amount of water is intentionally added into the P(VDF-TrFE) solution to induce porosity in the fiber. PNGs based on the porous electrospun P(VDF-TrFE) nanofibers show systematic increases of the output voltage with porosity. The output power increased from 0.1 mW/cm³ for PNGs with zero porosity to 7 mW/cm³ for PNGs with 50% porosity. Dielectric spectroscopy of the nanofibers attributes the enhanced output to the reduced dielectric permittivity of the fibers and that the voltage generation in the porous fibers is of the same origin as in neat piezoelectric P(VDF-TrFE) films and is due to the relaxation of segments within the restricted amorphous phase.

The triboelectric nanogenerator (TENG) has been investigated intensively during the last decade in terms of power improvement and application as sensors. However, its voltage is usually too high and spike-like to be used directly as a power source for electronic devices. Although there was an effort to adjust the voltage through the circuit design, the circuit board has limitations with the complexity and the bulkiness. Here, we propose a simple approach to change the spike-like voltage profiles to the square-like profiles and adjust the output voltage to be in the suitable range for electronics. We used printed ion gel electrolyte patterns as capacitors and investigated the effects of the dimension of the capacitor, the connection types (serial, parallel) of multiple capacitors, and the electrochemical conditions. The voltage profile of the TENG-ion gel system was modulated by the contact frequency applied to the TENG, the contact area of the ion gel with the electrode, the type of ions in the electrolyte, reduction/oxidation reaction in the gel, and the connection type (parallel, series) between ion gel patterns. We successfully demonstrated the light emission of a large number of printed electrochemiluminescence patterns.

Portable electronic devices have gained tremendous attention of research community owing to their promising properties of lightweight and miniaturization. The further development of the wearable electronics has triggered growing demands for high power supply with multifunctionality, such as transparency and flexibility. Rechargeable battery as the traditional power supply restricts its application in next-generation wearable electronics due to the limitations, such as lifetime, size and weight.

The advent of triboelectric nanogenerator (TENG) has demonstrated itself as a promising technique to harvest ubiquitous mechanical energy and convert it to electrical energy toward the development of the self-powered smart electronics. The main concern of the replacement of batteries with TENG lies in the low output power, where surface functionalization/morphology modification has been applied, to enhance the surface charge density. To avoid wear of the surface properties during long-term operation, bulk property engineering of materials has proved to be an efficient approach to enhance the output performance. However, the energy-efficiency of engineered materials largely depends not only on the property of nanofiller but also its interfacial interactions with the host.

Herein, in this study, we propose a crosslinked polyethyleneimine/poly(vinyl alcohol) polymer and fine tune its bulk dielectric property through incorporation of different amount of Au nanoparticles. The electrostatic interactions between the COO- capped Au nanoparticles and protonated amine groups facilitate the distribution of the conducting nanofillers. Employing the transparent and flexible composite polymer as positive triboelectric material, a flexible device with a sandwiched structure is developed. Due to the enhanced surface charge density resulting from the increase of dielectric constant of the bulk composite, the device delivers a high output power with long-term stability. In addition, the charge transfer mechanism involved during the contact electrification is discussed in detail. This device shows high potential as a power supply for future flexible electronics.

Reference:
L.Y. Wang, X.Y. Yang, W.A. Daoud. High power-output mechanical energy harvester based on flexible and transparent Au nanoparticle-embedded polymer matrix. Nano Energy (2018), DOI, https://doi.org/10.1016/j.nanoen.2018.10.030.

Advances in next-generation soft electronic devices rely on the development of highly deformable, and healable energy generators to power these electronics. Triboelectric nanogenerators (TENG) have emerged as a promising power source for portable and stretchable electronic devices. However, the development of deformable energy generators that can attain extreme stretchability with superior healability, and transparency simultaneously is difficult to achieve due to the use of metallic electrodes. We address this issue by the use of an ionic conductor as the current collector in a triboelectric nanogenerator, resulting in a highly transparent, stretchable and self-healing device. The energy harvesting performance of ionic triboelectric nanogenerator is 12 times higher than that of the metallic based triboelectric nanogenerator due to the formation of an electrical double layer (EDL). The developed slime-based ionic conductor owing to the non-Newtonian behaviour of the current collector, an extreme stretchability as high as 700% was achieved without degrading the device performance, thus enables the triboelectric device to act as an a power source for highly deformable electronics. The device has a transparency of 92% transmittance, while transparency helps in the visual transmission of information, which can be potentially utilized in user-interactive displays, biomedical imaging, therapeutic optogenetics, and touch screens. Due to the weak hydrogen bonding interactions of the ionic conductor, the TENG can autonomously self-heal and can recover its performance even after 300 times of complete bifurcation. The resulting device demonstrates an extremely stretchable, highly transparent self-heal power source to be used as a power supplies for sensors, wearable electronics and soft robotics.

Triboelectric charging is an electrical charging phenomenon that occurs when different materials come into contact and separate spontaneously. Irrespective of the magnitude, all materials could be charged by contact. Some of the common materials consistently exhibit triboelectric charging patterns, and based on the charging characteristics, empirical “triboelectric series between materials” was constructed by arranging them according to the relative polarity of the contact charge acquired.
Recently, a triboelectric nanogenerator, which offers a unique solution to convert mechanical energy into electricity via combining the concept of triboelectric charging and electrostatic induction, was invented as a new technology of energy harvesting. Furthermore, using a similar principle, triboelectric sensors and tribotronics that utilize triboelectric output to drive and control electronic devices were also introduced. One of the important factors that determine the performance of the device is the material. However, materials known to exhibit triboelectric charging behavior are only limited to some polymers and a few metals already located in the triboelectric series. Further investigating the triboelectric charging behaviors of new materials and widening the material library of the triboelectric series are required.
Here, we investigated the triboelectric charging behaviors of various two-dimensional (2D) materials (MoS₂, MoSe₂, WS₂, WSe₂, graphene, and graphene oxide), which have received great attention for decades owing to their distinct electronic, optical, mechanical, and thermal properties. The triboelectric charging behaviors were investigated using the concept of the triboelectric nanogenerator. Consistent results of contact-charging polarities were observed, using which, a modified triboelectric series including 2D materials was developed. For further verification, the effective work functions of the 2D materials, which are one of the main factors deciding their triboelectric charging behaviors, were estimated via Kelvin-probe force microscopy and calculated via first principles simulations as well. The charging polarity indicated the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical-vapor-deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing the thin and flexible device fabrication