Triboelectrification is an effect that is known to each and every one probably ever since the ancient Greek time, but it is usually taken as a negative effect and is avoided in many technologies. We have recently invented a triboelectric nanogenerator (TENG) that is used to convert mechanical energy into electricity by a conjunction of triboelectrification and electrostatic induction. As for this power generation unit, in the inner circuit, a potential is created by the triboelectric effect due to the charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity; in the outer circuit, electrons are driven to flow between two electrodes attached on the back sides of the films in order to balance the potential. Ever since the first report of the TENG in January 2012, the output power density of TENG has been improved for five orders of magnitude within 12 months. The area power density reaches 500 W/m², volume density reaches 490 kW/m³, and a conversion efficiency of ~50% has been demonstrated. The TENG can be applied to harvest all kind mechanical energy that is available but wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more. Alternatively, TENG can also be used as a self-powered sensor for actively detecting the static and dynamic processes arising from mechanical agitation using the voltage and current output signals of the TENG, respectively, with potential applications for touch pad and smart skin technologies. The TENG is possible not only for self-powered portable electronics, but also as a new energy technology with a potential of contributing to the world energy in the near future.
Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano 7 (2013) 9533-9557.
G. Zhu, J. Chen, T. Zhang, Q. Jing, Z. L. Wang* “Radial-arrayed rotary electrification for high-performance triboelectric generator”, Nature Communication, 5 (2014) 3456.

Piezoelectric unimorphs are made of laminating a piezoelectric layer with an inert flexible substrate. They have found wide applications in sensing, energy harvesting, and actuation. We have developed a comprehensive understanding of the effects of unimorph thickness ratios on mechanical load induced voltages, charges, and energies, as well as voltage induced displacements of eight different boundary conditions, with both analytical and numerical means. By adopting an average s₃₃ stress method, the effect of d₃₃ for both point and distributed loads is taken care of and our analytical equations are able to fully capture the FEM results which are obtained by COMSOL Piezoelectric Devices Module. Non-monotonic voltage and energy generation versus thickness ratio curves have been found for load-controlled energy generation scenarios. When the unimorph is actuated by applied voltage, non-uniform maximum deflection versus thickness ratio curves are also found. Our results reveal that the optimum thickness ratios for actuation is an order higher than the optimum thickness ratios for energy generation. In conclusion, closed-form analytical solutions are now available for the thickness optimization of piezoelectric unimorphs generators and actuators, under different boundary conditions.

We report a novel class of energy harvesters for generating AC power through repeated mechanical bending and unbending. The device consists of two identical Li-alloyed Si as the electrodes, separated by electrolyte-soaked polymer separator. The device is in soft thin-film form and is highly flexible and durable. Bending induced asymmetric stresses generate chemical potential difference, driving the lithium ion flux from the compressed to the tensed electrode to generate electrical current. Removing the bending reverses ion flux and electrical current. Our thermodynamic analysis reveals that the ideal energy-harvesting efficiency of this device is dictated by the Poisson&’s ratio of the electrodes. For the thin-film based energy harvester used in this study, we show that the device has an ideal efficiency of 27.8% and achieves a generating capacity of 10%. The device is well-suited for harvesting low-frequency motion energy because of its finite current width. We provide kinetic models to explain the current width and the possibility of timescale tunability. The device demonstrates a practical use of stress-composition coupling in electrochemically active alloys to harvest low-grade mechanical energies from various low-frequency motions, such as everyday human activities that generate inhomogeneous stress.

Energy harvesting systems based on piezoelectric and 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 piezoelectric and triboelectric nanogenerators based on various kinds of nanomaterials. Flexible nanogenerators 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 nanogenerators, including their four basic operation modes. Then the different improvement parameters will be discussed. As main topics, I will also address a couple of recent achievements regarding highly stretchable piezoelectric-pyroelectric hybrid nanogenerators, transparent flexible graphene triboelectric nanogenerators, textile-based wearable triboelectric nanogenerators, etc.

This seminar introduces three recent progresses that can extend the application of self-powered flexible inorganic electronics. The first part will introduce self-powered flexible piezoelectric energy harvesting technology. Energy harvesting technologies converting external sources (such as vibration and bio-mechanical energy) into electrical energy is recently a highly demanding issue. The high performance flexible thin film nanogenerator was fabricated by transferring the perovskite thin film from bulk substrates for self-powered biomedical devices such as pacemaker and brain stimulation. The second part will introduce flexible electronics including large scale integration (LSI) and high density memory. Flexible memory is an essential part of electronics for data processing, storage, and radio frequency (RF) communication. To fabricate flexible large scale integration and fully functional memory, we integrated flexible single crystal silicon transistors with 0.18 CMOS process and memristor devices. The third part will discuss the flexible GaN/GaAs LED for implantable biomedical applications. Inorganic III-V light emitting diodes (LEDs) have superior characteristics, such as long-term stability, high efficiency, and strong brightness. Our flexible GaN/GaAs thin film LED enable the dramatic extension of not only consumer electronic applications but also the biomedical devices such as biosensor or optogenetics. Finally, we will discuss laser material interaction for flexible and nanomaterial applications. Laser technology is extremely important for future flexible electronics since it can adopt high temperature process on plastics, which is essential for high performance electronics, due to ultra-short pulse duration. (e.g. LTPS process over 1000 °C) We will explore our new exciting results of this field from both material and device perspective.
Related References (from Keon&’s group as corresponding authors)
[1] Nano Letters 11, 5438, 2011. [2] Nano Letters 10, 4939, 2010.
[3] Nano Letters 12, 4810, 2012. [4] Nano Letters 14, 7031, 2014
[5] Adv. Mater, 26, 2514, 2014. [6] Adv. Mater. 26, 4880, 2014
[7] Adv. Mater, 26, 7480, 2014 [8] Adv. Mater. 24, 2999, 2012.
[9] Adv. Mater. 10.1002/adma.201501592 [10] Adv. Mater. 27, 2866, 2015
[11] Energy Environ. Sci. 10.1039/C5EE01593F [12] Energy Environ. Sci., 7, 4035, 2014
[13] ACS Nano 7, 11016, 2013 [14] ACS Nano 9, 4120, 2015
[15] ACS Nano 7, 4545, 2013 [16] ACS Nano 7, 2651, 2013.
[17] ACS Nano 8, 9492, 2014 [18] ACS Nano 8, 7671, 2014
[19] ACS Nano, 10.1021/acsnano.5b02579 [20] Adv. Energy Mater. 3, 1539, 2013
[21] Adv. Energy Mater. 5, 1500051, 2015 [22] Adv. Func. Mater. 24, 2620, 2014
[23] Adv. Func. Mater. 24, 6914, 2014 [24] Nano Energy, 14, 111, 2015
[25] Nano Energy, 1, 145, 2012

Triboelectric nanogenerators (TENG) have been developed to harvest mechanical energy through contact/friction motions. However, the efficiency of the TENG might be limited by the energy loss during power generation process, such as heat dissipation, wearing and plastic deformation. In this work, a hybrid cell consisting of a TENG and a pyroelectric-piezoelectric nanogenerator (PPENG) was developed for high-efficient mechanical energy harvesting through multiple mechanisms. Outstanding output performances were demonstrated for both the TENG and PPENG, and the hybridized output power could provide a sustainable power source for driving a light-emitting diode with extended illumination time and charging a supercapacitor. Furthermore, the hybrid cell was also applied as a multi-functional self-powered sensor for measuring both the temperature and the normal force. These excellent performances of the triboelectric-pyroelectric-piezoelectric hybrid cell enhance the energy-harvesting efficiency significantly (by 26.2% at 1 k#8486; load resistance), improve the charging rate greatly (by 81.3%), and enable the self-powered sensing, which will lead to a variety of advanced applications.

Recently, triboelectric nanogenerators (TENG) have received increasing interest due to their potential for mechanical energy harvesting. Important advancements have been achieved to increase the output power and efficiency, even as new structures have emerged. Their robustness and endurance have increased, but there still remain some concerns about the degradation and lifetime of TENG. How will TENGs age under intensive use in daily life? To address this issue, we propose in this paper to use shape memory polymer (SMP) to extend TENG&’s lifetime and better guarantee their performance. For this purpose we have introduced a new smart SMP-TENG structure which has the capacity to be healed and recover good performance after degradation of its triboelectric layer. We studied the degradation and the self-healing process of SMP-TENG, improving their endurance, life time and thus demonstrating the huge potential of self-healing SMP-TENGs.

Computer keyboard is one of the most common, reliable, accessible and effective approaches used for human-machine interfacing. Accessing the information provided by computer from internet dictates the quality, efficiency and happiness of our work and life. A keyboard, an indispensable component of the system, is the only means for information input and control for many purposes such as information recording/outputting, financial management, bill payment, personal communications and many more. With this regard, the heavy reliance on computer incurs a major concern for its security issue. Although keyboard has been used for hundreds of years for advancing human civilization, studying human behavior by keystroke dynamics using smart keyboard remains a great challenge.
Here we report the first intelligent, self-powered, non-mechanical-punching keyboard enabled by contact electrification between human fingers and a flexible transparent thin-film layer. The intelligent keyboard (IKB) converts mechanical stimuli applied onto the keyboard into local electronic signals without applying an external power. It can not only sensitively trigger a wireless alarm system once gentle finger tapping occurs but also be capable of tracing and recording typing contents by detecting both the dynamic time intervals between and during inputting letters and the force used for each typing action. Such features promise its use as a smart security system that can realize detection, alert, recording, and identification. Moreover, the IKB is able to identify personal characteristics from different individuals if assisted by behavioral biometric of keystroke dynamics. Furthermore, the IKB can effectively harness typing motions for electricity to charge commercial electronics at arbitrary typing speed larger than 100 characters per min, with an area power density of 69.6 mWcm^-2. Given the above features, the IKB can be potentially applied not only to self-powered electronics but also to artificial intelligence, cyber security, and computer or network access control. The justified concepts and demonstrations in this work can be immediately and extensively adopted in a variety of applications, and come into effect of improving the way of our living.
References: (* indicate co-first author).
1.J. Chen*, G. Zhu*, J. Yang, Q. Jing, P. Bai, W. Yang, X. Qi, Y. Su and Z. L. Wang. ACS Nano9 (2015), 105-116.
2.J.Chen*, G. Zhu*, T. Zhang, Q. Jing and Z. L. Wang. Nat. Commun.5 (2014), 3426.
3.J. Chen*, G. Zhu*, W. Yang, Q. Jing, P. Bai, Y. Yang, T. C. Hou and Z. L. Wang. Adv. Mater.25(2013), 6094-6099.
4.J. Chen*, J. Yang*, Z. Li, X. Fan, Y. Zi, Q. Jing, H. Guo, Z. Wen, K. C. Pradel, S. Niu and Z. L. Wang. ACS Nano 9 (2015), 3324-3331.
5.J. Yang*, J. Chen*, Y. Su, Q. Jing, Z. Li, F. Yi, X. Wen, Z. Wang and Z. L. Wang. Adv. Mater. 27(2015), 1316-1326.

Transparent electronic devices built on flexible substrates are expected to meet emerging technological demands for the next generation of flexible electronic and optoelectronic devices. A suitable energy source is a vital part for realizing fully self-powered systems. Energy harvesting and conversion technology from environment in which the system will be deployed is a promising route. Recently, we have introduced a new concept on the basis of triboelectrification and electrostatic induction for successfully converting mechanical energy into electric energy at an energy conversion efficiency as high as 50%. Subsequently, transparent triboelectric nanogenerators (TENGs) using indium tin oxide (ITO) or graphene as electrodes have been demonstrated, which are the only possible power source that light can penetrate through. However, there is a need for a low-cost and large-area compatible technology for producing transparent TENGs with high-output power supply, aiming for applications such as touch screens.
In our previous work, we fabricated a transparent TENG and pressure sensor by using polyester (PET) and polydimethylsiloxane (PDMS) films as flexible substrates to integrate ITO electrodes. It has also been demonstrated that the patterned surface is an effective method to improve the output performance of the TENG. However, realizing such a structure is not trivial in mass production and the patterned structure will reduce the transparency of the device due to the light scattering effect. Here, we present a simple, cost-effective and large-scalable method for fabrication of highly transparent TENGs.
Although triboelectric effect has been known for thousands years, the underlying mechanism is actually very complex and still under debating. The competing possible mechanisms appear to include electron transfer, ion transfer, bond dissociation, chemical changes, and material transfer, and it is likely that different mechanisms may be involved depending on the specific materials and environmental conditions. [18-21] Usually, it needs specialized equipment to characterize the charged surface and study the mechanism. It is worth noting that the output performance of TENG can directly reflect the capacity of the triboelectrification. Herein, we use this feature to investigate the mechanism of the triboelectrification by examining various factors on the performance of TENG. Furthermore, another meaningful outcome of our study is that we explore the mechanism from the microscopic and molecular perspective using the methods for surface characterization, which would be beneficial to the improvement and application of TENG at a large scale.
References
F. R. Fan, Z. Q. Tian, Z. L. Wang, Nano Energy 2012, 1, 328-334.
F. R. Fan, L. Lin, G. Zhu, W. Z. Wu, R. Zhang, Z. L. Wang, Nano Lett. 2012, 12, 3109-3114.
F. R. Fan, J. J. Luo, W. Tang, C. Y. Li, C. P. Zhang, Z. Q. Tian, Z. L. Wang, J. Mater. Chem. A 2014, 2, 13219-13225.

Triboelectric nanogenerators (TENGs) have been invented as a high efficient, cost-effective and easy scalable energy harvesting technology for converting ambient mechanical energy into electricity at a total energy conversion efficiency as high as 85%. The four basic working modes of the TENG have been demonstrated, each of which has different structural design and choice of materials to accommodate the corresponding mechanical triggering conditions. A common standard is thus required to quantify the performance of the TENGs so that their outputs can be compared and evaluated. Here, starting from the plot of built-up voltage V - total transferred charges Q, we propose a general figure-of-merit (FOM) for defining the performance of a TENG, which is composed of a structural FOM related to the design of the TENG and a material FOM that is the square of the surface charge density. The structural FOM is derived and simulated to compare the TENGs with different configurations. A standard method is introduced to quantify the material FOM for a general surface. This study is likely to establish the standards for developing TENGs toward practical applications and industrialization.

Effectively harvesting energy from ambient environment as a renewable energy source has attracted much interest. But as one rich source of water, the rain drop energy still need to be exploited deeply. Previous studies employed triboelectric generator have successfully harvested its energy. However, the output performance and energy conversion efficiency were in very low level. Besides, the hydrophobic surface with micro textured structure made the device not transparent or flexible, which limits its practical applications.
To improve the energy harvesting efficiency, we developed interdigital-electrodes-based triboelectric generators (TEGs) which is totally transparent. It is composed of PET substrate, interdigital-shaped ITO electrode, PDMS dielectric layer, and ZXL-SS-02 hydrophobic layer. The contact angle of its surface with a 2 mu;L was about 120°. Compared with PET-ITO film, the fabricated device exhibits a better transmittance (more than 80%) in the visible and near infrared region.
The working principal of the device is based on the triboelectricity and electrostatic induction. The surface of the hydrophobic layer will possess negative charges once contacts with a water drop, while the water drop will be charged positive. Along with the droplet flowing, the surface will be charged successively, which can drive the current to flow between the interdigital electrodes according to the electrostatic induction effect.
The device was characterized by a 30 mu;L droplet flowing on its surface while the inclined angle theta; was fixed at 60°. The obtained peak-to-peak voltage and current were about 50 V and 800 nA separately. The maximum RMS power was 1.1 mu;W under a 70 M optimal resistance. The output electricity by a water drop could charge a 30 nF capacitor to 1.2 V, corresponding to 36 nC transferred charge. By adjusting the theta;, a maximum output energy of about 2.67 mu;J was obtained at 80°. The average energy conversion efficiency was about 4%, and a highest efficiency of about 8.7% was achieved when the angle was 10°. Compared with previous works, our device shows much better output performance and higher efficiency.
In summary, this work presents a transparent and flexible generator based on contact-electrification effect for harvesting water drop energy. The generator shown a transmittance of more than 80% in the visible and near infrared region. Facilitated by the utilization of interdigital-shaped electrode, this generator could harvest a water drop&’s energy for multiple time. Employed the simple structure and method, considerate output performance was observed. With a 30 mu;L water drop flowing on the device, 22 V effective voltage and 2.67 mu;J maximum output energy were achieved, while a maximum energy conversion efficiency of about 8.7% was observed. Thanks to its highly transparency, simple manufacturing process, and high output performance, the generator has much potential to harvest the energy from rain drop or liquid flow in microfluidics.

Recently, several interesting reports suggest a moving droplet of sea water or ionic solution over monolayer graphene using pseudocapacitive effects between graphene and a single droplet, producing electrical signals of about 19 nW as a maximum power output. This power output is very low, thus limited device application, indicating increasing power output is much challenging. Here, we demonstrate anomalous large power generation from a single moving water droplet on the basis of triboelectrification-induced pseudocapacitance between graphene and a droplet. The output power of around 2 mu;W achieved from a single moving droplet onto a monolayer graphene, which is one hundred-folds larger than that in the previous research. Very strong negative triboelectric potential on the surface of a PTFE template is generated during a graphene transfer process. The triboelectric potential induces positive and negative charge accumulation on the bottom and top surfaces of graphene, respectively. The negative charges accumulated onto the graphene top surface are driven forward by the moving droplet, charging and discharging at the front and rear of the droplet.

Micrometer-size [110] (Na_1-xK_x)NbO₃ (NKN) platelets were synthesized through the annealing of (K_8-8xNa_8x)Nb₆O₁₉middot;nH₂O (KNNH) precursors at 500^oC. The plate-like KNNH precursors were produced from (1-y)NaOH-yKOH + Nb₂O₅ specimens using the hydrothermal process at 160^oC. The size of the NKN platelets was similar to that of the KNNH precursor, but the surfaces of the NKN platelets were rough while the KNNH precursor had a smooth surface. Formation of a rough surface is related to the vigorous evaporation of the H₂O from the KNNH platelets during the annealing process at high temperature. NKN platelets with the smooth surfaces can be synthesized using KNNH platelets, which were heated to 150^oC to evaporate H₂O before annealing at 500^oC. These NKN platelets can be used for the fabrication of textured NKN ceramics. The nanogenerator was synthesized using NKN platelets and their output power will be presented in this work.

Graphene has a unique combination of electrical, mechanical, and optical properties, is being actively explored for future electronic applications. Specially, graphene has an excellent optical transparency, mechanical flexibility, high mechanical elasticity. These fascinating properties make graphene an ideal material for photodetectors, transparent, flexible electrodes in solar cells and nanogenerators.
For graphene triboelectric nanogenerator, device need high-k dielectric material for insulating layer. However high-k dielectric such as Al₂O₃ or HfO₂ doesn&’t deposit on graphene surface because graphene has sp² atomic configuration. Hence, in this present work we demonstrated the deposition of high-k dielectric material on graphene using hexagonal boron nitride (h-BN) as a buffer layer. Transmission electron and atomic farce microscopy studies show that presence h-BN layer on the top of graphene facilitates the growth of high-quality aluminum oxide (Al₂O₃) layer by atomic layer deposition (ALD). Simulation results also support the experimental observations and provide explanation for suitability of h-BN as buffer layer. The analysis of Raman and X-ray photoelectron spectroscopy (XPS) data also confirms the importance of h-BN for growth of good-quality oxide dielectric materials. Also, h-BN works as protective shield to prevent graphene from oxidation during ALD of Al₂O₃ for the fabrication of triboelectric nanogenerator.

Recently, silver nanowires (AgNWs) have had a great interest as a conducting material for flexible and transparent electrode. In this paper, we studied the network structure formation of AgNWs by stamping transfer process using polydimethylsiloxane (PDMS) with various surface treatments. Surface treatment could control the transfer yield (thickness) of AgNWs. We have designed various conditions of AgNWs surface using silane-coupling modifiers. Flexible PDMS substrate, with embedded AgNWs, was fabricated by the following processes; spraying AgNWs layer and network film formation on silicon wafer, transferring and curing of PDMS, and releasing AgNWs/ PDMS from the original substrate (glass). Transfer yield of AgNWs was adjusted by the variation of surface treatment condition (different concentration and exposure time) with silane-based modifier coating such as Methacryloxypropyltrimethoxysilane (MPS) and Octadecyltrichlorosilane (OTS) on top of AgNWs. Transparency of embedded AgNWs film has increased up to 20% at the optimum surface treatment condition. Especially, MPS treatment process was appropriate for thin film organic device substrate due to its generation of transferred AgNWs with low surface roughness profile. As a result, polymer bulk-hetero junction (BHJ) solar cells with anodes composed of AgNWs have been successfully fabricated with the following conventional device structure: embedded AgNWs film (Anode)/ poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) (45nm)/ poly[(4,8-bis-(2-ethylhexyloxy)-venzo[1,2-b;4.5-b&’]dithiophen)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophen))-2,6-diyl] (PBDTTT-c): phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) (110nm)/ LiF (0.7nm)/ Al (100nm). The organic solar cell showed 3.2% of power conversion efficiency (Jsc=10.333 mA/cm2, Fill factor=43.8%, Voc=0.71V) at the condition of the optimized MPS treatment, and this AgNWs/ PDMS embedded substrate provided a working range of substrate stretching within 15~25 %.

There is an urgent need to look for alternate and renewable energy source as fossil fuel based energy sources are depleting at an alarming rate. Recently piezoelectric materials based renewable energy source by utilizing stray vibrational energy have emerged as an alternative for this demand. Here we report facile, low temperature synthesis of large area self assembled Lead Zirconate Titanate (PZT) based micro/nanostructures using hydrothermal method for omni-directional vibrational energy harvesting. Growth morphologies are found to be dependent on concentration and pressure inside the reaction chamber. Extensive characterizations using SEM, TEM, XRD, raman spectroscopy, absorption spectroscopy etc have been performed to identify quality of synthesized material. These synthesized micro/nanostructures film were used for making functional miniaturized device for omni-directional energy harvesting.
Keywords: Lead Zirconate Titanate (PZT), Vibrational Energy, piezoelectric materials, Hydrothermal, Energy harvesting.

With the rapid growth of portable electronics and sensor networks, mobile and sustainable energy sources for these devices becomes indispensable in modern society. Recently, triboelectric nanogenerators (TENGs) based on contact electrification and electrostatic induction emerge as a promising mechanical energy harvesting technique because of their unique figure of merits, including high output power density, ultra-high energy conversion efficiency, low weight, cost-effective materials, and high adaptability design to different applications. However, because of the inherent uncontrollable and unstable characteristics of the environmental mechanical energy source, the converted electrical energy from TENG is thus unstable and hardly utilized to directly power electronic devices. Thus, understanding the integration performance of TENGs with an energy storage unit is critical for designing a practical energy harvesting system. However, until now there are still many mysteries about the charging behavior of TENGs.
In this work, the characteristics of utilizing a TENG to charge a capacitor are thoroughly studied. Through rigorous mathematical derivation, TENGs have the saturation charging behavior, which is completely analogous to utilizing a DC voltage source with an internal resistance to charge a load capacitor. The TENG with larger short circuit transferred charges and smaller inherent TENG capacitance can charge a load capacitor to a higher saturation voltage under unlimited charging cycles. Besides, the TENG with larger inherent capacitance can provide a smaller charging time constant. An optimum load capacitance that matches the TENG&’s impedance is observed for the maximum stored energy as well. However, different from unidirectional charging, this optimum load capacitance is linearly proportional to the charging cycle numbers and the inherent capacitance of the TENG. Finally, corresponding experiments were performed to further validate the above theoretical prediction to show its application in guiding experiments. [1]
With the above thorough understanding of TENG charging characteristics, we developed the first practical exclusively TENG self-powered systems through systematical design and optimization. A power management board was designed for TENG to realize maximum charging efficiency. With solely human motion as the energy source, we can continuously provide DC electrical energy for the use of practical portable el