Computer keyboard is one of the most common, reliable, accessible and effective approaches used for human-machine interfacing and information exchange. 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 keys, which converts mechanical stimuli applied onto the keyboard into local electronic signals without applying an external power. The intelligent keyboard (IKB) 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.Science, under review.
2.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
3.W. Yang*, J. Chen*, G. Zhu, J. Yang, P. Bai, Y. Su, Q. Jing and Z. L. Wang.ACS Nano7 (2013),11317-11324
4.G. Zhu*, J. Chen*, Y Liu, P Bai, Y Zhou, Q Jing, C Pan, ZL Wang. Nano letters 13 (2013), 2282-2289
5.G. Zhu*, J. Chen*, T. Zhang, Q. Jing and Z. L. Wang. Nat. Commun.5 (2014), 3426
6.J. Yang*, J. Chen*, Y. Yang, H. Zhang, W. Yang, P. Bai, Y. Su and Z. L. Wang. Adv. Energy Mater.4 (2014), 1301322

Wearable sensors, implantable biosensors, things of internet and other micro/nano sensors network have the pressing needs for energy harvesting technology. Integrating sensor and energy harvesting technology into a self-powered system will has wide applications in many fields. In this presentation, several flexible piezoelectric nanogenerators, high sensitivity nanowire UV sensor and flexible self-powered systems based on them will be reported. Firstly, we will talk about wearable nanogenerators [1,2], a flexible nanogenerator with 209 V output [3], non-contact nanogenerators [4] and biocompatible nanogenerator based on high piezoelectic coefficient 0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-BCT) nanowires [5]. After that, a kind of ultrahigh sensitivity ZnO nanowire UV sensor through utilizing the interface will be presented [6]. Finally, some self-powered systems to detect UV light or in-vivo biodetection will be demonstrated [7,8].
[1] "Lead Zirconate Titanate Nanowire Textile Nanogenerator for Wearable Energy-Harvesting and Self-Powered Devices" W. W. Wu, S. Bai, M, M, Yuan, Y. Qin*, Z. L. Wang* and T. Jing, ACS Nano. 2012, 6(7), 6231-6235.
[2] "Two Dimensional Woven Nanogenerator" S. Bai, L. Zhang, Q. Xu, Y. B. Zheng, Y. Qin*, Z. L. Wang*, Nano Energy. 2013, http://dx.doi.org/10.1016/j.nanoen.2013.01.001.
[3] "Flexible Fiber Nanogenerator with 209 V Output Voltage Directly Powers a Light-Emitting Diode" L. Gu, N. Y. Cui, L. Cheng, Q. Xu, S. Bai, M. M. Yuan, W. W. Wu, J. M. Liu, Y. Zhao, F. Ma, Y. Qin*, Z. L. Wang*, Nano Letters. 2013, 13(1), 91-94.
[4] "Magnetic Force Driven Nanogenerators as a Noncontact Energy Harvester and Sensor" N. Y. Cui, W. W. Wu, S. Bai, L. X. Meng, Y. Qin*, Z. L. Wang*, Nano Letters. 2012, 12(7), 3701-3705.
[5] "Biocompatibility of 0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ Nanogenerator" M.M. Yuan, L. Cheng, Q. Xu, W. W. Wu, S. Bai, L. Gu, Z. Wang, J. Lu, H. P. Li, Y. Qin*, T. Jing*, Z. L. Wang*, 2014, http://dx.doi.org/10.1002/adma.201402868.
[6] "Self-powered nanosystem for all time, wireless and in-vivo biodetection" L. Cheng, M.M. Yuan, Y. Qin*, et al. to be submitted.
[7] "An electrospun nanowire-based triboelectric nanogenerator and its application in a fully self-powered UV detector" Y. B. Zheng, L. Cheng, M. M. Yuan, Z. Wang, L. Zhang, Y. Qin*, T. Jing, Nanoscale. 2014, 6, 7842-7846.
[8] "Ultrahigh sensitivity ZnO nanowire UV sensor" L. X. Meng, Y. Qin*, et al. to be submitted.

We present triboelectric behavior of graphene on top of different metal substrates including aluminum, copper, and platinum (Al, Cu, and Pt). Notably, tribo-induced charge on metal-supported graphene have long life time. Graphene layers transferred on Al, Cu, and Pt have different work functions of 4.32, 4.41 and 4.45 eV, respectively, with different triboelectric potential as well (48.98 mV for Al, 41.82 mV for Cu, 39.65 mV for Pt). In addition, this induced tribo-potential is observed to sustain for a couple of hours at least that indicates towards hitgh charge holding capacity of graphene. These results show the first reliable quantitative measurement of very unique triboelectric characteristics of graphene which can be controlled by substrate, and the induced tribo-charge sustains for quite long times.

One dimensional ZnO nanostructures are envisioned as fundamental building blocks of future electronic, electromechanical, electro-optomechanical nanodevices. Meanwhile, the service behavior and damage in nanomaterials and nanodevices it vital for practical applications. The recent developments in our group for designs, fabrication of energy conversion devices based on ZnO nanomaterials, and some effective approaches for performance enhancement will be presented in this talk. We will focus on the major progress in three area. Firstly, we present the fabrication of wurtzite nanostructures and the property modulation. Then, we introduce the progresses on the piezotronic properties of ZnO nano-materials and prototype piezotronic devices, including piezoelectric field effect transistors, piezoelectric diodes, strain sensors and nanogenerators. Finally, the investigations of property degrading and damage of nanomaterials, and functional degrading and failure of these nanodevices will be presented for future electronic applications.

Mechanical energy is one of the most pervasive sources of energy in our environment and is thus a clear target for energy harvesting. Besides, a wide variety of sensors are also based on the detection of movements, whether originating from vibrations, sounds, pressure variations, shocks, imprints, flows, body movement or others. While there already exist stand-alone devices that can be integrated at board level, cost, efficiency and power issues are motivating the investigation of new paths that could lead to on-chip or above-IC fully integrated solutions. The fabrication of composite materials based on the combination of vertically grown piezoelectric nanowires with an insulating embedding material is one of them.[1] The aim of this presentation is to show what can be expected from these composite materials, with respect to a more standard solution which would consist in using a thin film instead.
Multi-physics simulations were performed in our group in order to evaluate the theoretical performance of such piezoelectric nanowire-based composites. We focused on ZnO piezoelectric nanowires with dimensions typical of what can be obtained with chemical bath deposition. Two device configurations were considered, corresponding to devices (i) integrated on a rigid substrate [2] and (ii) integrated on a flexible substrate [3]. In both cases, isolated contacts were considered, resulting in capacitive operation, with displacement currents driven by strain variations. Several issues were discussed, among which the choice of the embedding dielectric materials, of nanowires dimensions and areal density or the influence of statistical variations in nanowire diameter or orientation. For performance evaluation, thin films of the same piezoelectric material were used as a reference in order to identify the real assets of the nanocomposite and its preferential fields of application. As a whole, it was found that nanowire composites could improve piezopotential and output power when used in compression on a rigid substrate and sensitivity to low deformations when strained by the bending of a flexible substrate. Additional improvements are expected as a result of lateral isolation by the dielectric matrix material.
[1] Sheng Xu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Yang and Zhong Lin Wang, "Self-powered nanowire devices", Nature Nanotechnology, Vol. 5, pp. 366-373 (May 2010)
[2] R. Hinchet, S. Lee, G. Ardila, L. Montès, M. Mouis, Z.L. Wang, "Performance Optimization of Vertical Nanowire-Based Piezoelectric Nanogenerators", Advanced Functional Materials, Vol. 24, no. 7, pp. 971-977, (Feb. 2014)
[3] G. Ardila, R. Tao, L. Montes, M. Mouis, "Multiphysics modeling of thin piezoelectric transducers integrated in flexible substrates", International Conference on Ultimate Integration on Silicon (ULIS), pp. 77-80 (IEEE Conference Publications, 2014)

The rapid development of graphene has increased the demand for next generation electronics. The application of graphene as an exciting transparent and flexible mechanical energy scavenging device is an effective approach for sustainable and green power source for wireless, portable and implanted electronics.
We demonstrate the first application of chemical vapour deposition (CVD)-grown graphene to transparent, flexible triboelectric nanogenerators (TNGs) and show that it has large electrical power generation capabilities under external mechanical strain. TNGs based on monolayer (1L), bilayer (2L), trilayer (3L), quadlayer (4L) and few-layer graphene grown on copper (Cu) and nickel (Ni) foils by CVD were fabricated, and their electrical output were measured. 1L graphene-based TNGs produce a larger power output than other randomly stacked, misoriented graphene-based TNGs, and the output voltage and current density from regularly stacked, few-layer graphene-based TNGs produced a voltage and current of approximately 9.0 V and 1.2 µA/cm², respectively. The variations in the electrical power output of 1L graphene, randomly stacked and regularly stacked, few-layer graphene-based TNGs are explained in terms of the work function and friction. This study provides a simple and novel method for harvesting mechanical energy from transparent, flexible graphene TNGs.

Nanogenerators are considered to have substantial application potential to solve the problems of energy depletion and serious environmental pollution. We present two nanogenerators based on graphene oxide (GO) and zinc-oxide (ZnO) nanoparticles (NPs). A flexible nanogenerator based on graphene oxide film was designed. Its peak output voltage and current reaches up to 2 V and 30 nA, respectively, upon repetitive application of a 15 N force with a 1 Hz frequency. Furthermore, the output voltage increased to 34.4 V when the frequency was increased to 10 Hz compared with the control samples. We found the output voltage clearly increased from 0.1 V to 2.0 V when the GO film was embedded into the device. Besides, the mechanism of our nanogenerator can be explained by an electrostatic effect, which is fundamentally different from that of previously reported piezoelectric and triboelectric generators. We also present a nanogenerator based on ZnO NPs and multiwall-carbon nanotubes (MW-CNTs). In this device, the ZnO NPs and MW-CNTs were mixed with polydimethylsiloxane (PDMS) to form a completely flexible nanogenerator(FNG). The output voltage and power density could reach up to 7.5 V and 18.75 mW per cycle, respectively. Moreover, a peak voltage of 30 V can be reached by stamping one&’s foot on the FNG. As compared with the ZnO control samples, we found that the output voltage can be significantly increased from 0.8 to 7.5 V by adding MW-CNTs into the matrix. In the updated work, two flexible layers were combined by four springs. In this device, the voltage and power density can reach up to 9 V and 27mW per cycle, respectively. This work may bring out some important and interesting applications in energy harvesting.

Mechanical energy is one of the most abundant energies in the human body. Harvesting biomechanical energy could provide a sustainable power source for most implanted medical devices and potentially solve the problems caused by the limited life time of a battery. In this work, we report the first application of an implanted triboelectric nanogenerator (iTENG) that enable harvesting energy from in-vivo mechanical movement in breathing for directly driving a pacemaker. With the energy harvested by TENG from animal breathing, the pacemaker can be stimulated once using the energy harvested for about 5 breaths of a rat. Once extrapolated to the lung volume change of a human, it is largely possible to directly power a pacemaker by breathing. This work presents a successful initial attempt in applying TENG for in-vivo mechanical energy harvesting, which will have a broad impact on the development of lifetime self-powered implantable medical devices in the near future.

CdSe quantum dots when treated with SbCl₃ undergo a series of chemical and physical changes that result in the generation of ferroelectric particles. These changes include surface destabilization through ligand disruption, leeching of Cd to produce CdCl₂, and infiltration of Sb into remaing particles of Se, assessed by STEM-EDS, XRD, ATR-IR, and TGA. Correlation of these changes with polarization switching and changes in the saturation polarization during Olsen cycling allows confirmation of the ferroelectric nature of the particles and determination of the waste heat regeneration efficiency of the particles.

Natural cellulose nanofibers(CNF) possess a great promise for the development of high-performance energy harvesting and storage devices, due to their crystalline structure, great abundance, easy processability, and bio-compatibility. In this work, we report the piezoelectric properties of uniaxially oriented CNF films, and show that three-point bending of these films produces oscillating piezoelectric voltage output. 4cm x 1cm x 0.34um CNF stripe with 1mm strain displacement generates a peak output of 400mv. Multiple CNF films can be readily integrated and operate synchronically to raise the output power for the operation of electronic devices.

Over the past decades, increasing research efforts have been devoted to harvest ambient environmental energy owing to the rapid-growing worldwide energy consumptions. Searching for clean and renewable energy with reduced carbon emission is urgent to the sustainable development of human civilization. Nowadays people become increasingly relying on mobile electronic devices, such as for the purposes of medical, communication, and positioning systems as one moves around cities or in the remote areas. Presently, all of these devices are powered by batteries, which have a limited energy storage capacity and also add considerable additional weight. More importantly, a battery has limited life time, and once it is out of charges, it is “dead”. Meanwhile, human body has numerous potential mechanical energy sources, thus, so far, a lot of portable and renewable human motion-driven self-powdered systems were developed.
In this work, we developed a self-powered backpack based on the coupling effect of contact electrification and electrostatic induction. Under the circumstance of natural human walking with loads of 2.0 kilograms, the power generated by one unit cell is high enough to simultaneously light up more than 40 commercial LEDs, rendered a vibration-to-electric energy conversion efficiency up to 10.6%. The newly designed self-powered backpack is capable of acting as a mobile power source for field engineers, explorers, and disaster-relief workers.
References:
(* indicate co-first author).
J. Chen*, G. Zhu*, W. Yang, Q. Jing, P. Bai, Y. Yang, T. C. Hou and Z. L. Wang. “Harmonic- Resonator-Based Triboelectric Nanogenerator as a Sustainable Power Source and a Self-Powered Active Vibration Sensor”. Adv. Mater.25 (2013), 6094-6099.
W. Yang*, J. Chen*, G. Zhu, J. Yang, P. Bai, Y. Su, Q. Jing and Z. L. Wang. “Harvesting Energy from the Natural Vibration of Human Walking”. ACS Nano7 (2013),11317-11324,.
G. Zhu*, J. Chen*, T. Zhang, Q. Jing and Z. L. Wang. “Radial-Arrayed Rotary Electrification for High Performance Triboelectric Generator”. Nat. Commun.5 (2014), 3426.
J. Yang*, J. Chen*, Y. Yang, H. Zhang, W. Yang, P. Bai, Y. Su and Z. L. Wang. “Broadband Vibrational Energy Harvesting Based on a Triboelectric Nanogenerator”. Adv. Energy Mater.4, (2014), 1301322.
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.

Emerging applications in wearable technology, pervasive computing, human-machine interfacing, implantable surgical instruments and biomedical diagnostics demand active and adaptive interactions between electronics and ambient/host (e.g. human body). Direct detecting, processing and controlling of the information encoded in environmental stimuli by logic units may therefore be necessary. Here we first demonstrate the strain-gated transistors (SGTs) based logic gates and half-adder computation by utilizing mechanical strains as controlling inputs via piezotronic effect² on gallium nitride (GaN) nanotelts; furthermore, we report implementations of piezo-phototronic² binary computations, such as half/full addition and subtraction, over optical and mechanical dual-inputs through cascaded logic circuits in cadmium sulfide (CdS) nanowire networks. Using polarization charges created at metal-semiconductor interface under strain to gate/modulate electrical transport and optoelectronic processes of local charge carriers, both the piezotronic and piezo-phototronic effects have been applied to design two-terminal transistors, which process mechanical and/or optical stimuli on the devices into electronic controlling signals. The nanowire networks have been further demonstrated for achieving gated D latch to store information carried by these stimuli. The piezotroinc/piezo-phototronic logic devices may have applications in human-machine interfacing, active flexible/stretchable electronics, optical micro-/nano-electromechanical systems, tunable bio-optoelectronic probes, adaptive optical computing and communication.
References:
1. Yu, R. M.; Wu, W. Z.; Ding, Y.; Wang, Z. L. GaN Nanobelt-Based Strain-Gated Piezotronic Logic Devices and Computation. Acs Nano2013, 7, (7), 6403-6409.
2. Wang, Z. L. Progress in Piezotronics and Piezo-Phototronics. Adv Mater2012, 24, (34), 4632-4646.

We report novel near-field microwave excitation and readout of piezo-activated mechanical transducers and resonators in both lead zirconium titanate (PZT) and aluminium nitride (AlN). This approach will enable ultra-small and fast piezo based sensors, amplifiers and memory elements to be developed. Disc-shaped silicon resonators incorporating a thin-film piezo layer have been fabricated with sizes ranging from 2 mm down to 50 mm. Thin film metal electrodes are provided on top and bottom of the piezo layer to provide activation or readout. The microwave drive and readout is derived from a high Q dielectric coaxial resonator (4GHz). This is a quarter wavelength structure with the centre conductor at the open end sharpened to a small radius tungsten tip. This can be brought into close proximity to a piezo resonator using a 3-axis translation stage with minimum step size of 100nm. The interferometric microwave readout is made using a homodyne detection system in which the reflected microwave signal (excited at 4GHz) is modulated by movement of the piezo resonator and the sideband on the microwave carrier is detected. Alternatively the microwave system can provide an excitation force for the piezo resonator by amplitude modulating the 4GHz signal at the resonant frequency of the piezo element. Due to the finite capacitance between tip and piezo electrode the time varying voltage will cause a time varying force which drives the resonator. The reflected signal carries the information on relative movement and use of a microwave bridge to null out the non-resonant part of the reflected signal improves the signal to noise. Sub-nm sensitivity is readily achieved in this way. The inverse piezo effect can also be observed by driving the piezo element with amplitude modulated microwaves and reading out the movement by measuring the induced a.c. voltage between the piezo electrodes. We report results on these approaches for several different samples.

Harvesting energy in our living environment is a viable way to cope with the increasing energy consumption of numerous mobile electronic devices. One route that can be considered is to convert various mechanical energy sources into electrical energy in order to power nano/micro-devices. The piezoelectric nanogenerators (NGs) has been actively investigated in this aspect. In particular, the NGs based on ZnO have been of great interest due to its abundance, biocompatibility, and low permittivity. However, intrinsic ZnO shows defect-induced n-type behavior, which limits high piezoelectric output power generation for practical applications. Although consistent efforts have been made to date, however, the output performances of the intrinsic ZnO NGs are still relatively low due to its innately low piezoelectric coefficient by comparison to ferroelectric materials such as BaTiO₃ and PZT.
In this work, we present a novel method to extend the performance limit of the ZnO NGs by ferroelectric phase transition. The ZnO NWs were doped with lithium (Li) by substituting Zn atoms during hydrothermal synthesis of ZnO NWs, inducing non-symmetric crystal structure and thereby ferroelectric phase transition. The NGs fabricated with Li-doped ZnO demonstrated enhanced output power generation due to spontaneous polarization. Besides, the polarization domain of NWs can be aligned by high voltage poling process even if they are not well aligned initially, relaxing the c-axis alignment required for high output power generation in intrinsic ZnO NG. The NG with an active area 10 cm × 10 cm generates output voltage up to ~180 V_oc and short-circuit current up to ~50 mA. In addition, the device shows the superior durability and reproducibility owing to the NWs embedded in PDMS. The method introduced in our work is cost-effective and also suitable for large-scale production.

Motion tracking is of great importance in a wide range of fields such as automation, robotics, security, sports and entertainment. Here, we report a self-powered, single-electrode-based triboelectric sensor (TES) to accurately detect the movement of a moving object/body in two dimensions. Based on the coupling of triboelectric effect and electrostatic induction, the movement of an object on the top surface of a polytetrafluoroethylene (PTFE) layer induces changes in the electrical potential of the patterned aluminum electrodes underneath. From the measurements of the output performance (open-circuit voltage and short-circuit current), the motion information about the object, such as trajectory, velocity, and acceleration is derived in conformity with the preset values. Moreover, the TES can detect motions of more than one objects moving at the same time. In addition, applications of the TES are demonstrated by using LED illuminations as real-time indicators to visualize the movement of a sliding object and the walking steps of a person.

A rapid growing demand of portable electronic devices and wireless sensor networks has spurred great interest in robust and independent energy harvesting techniques. Among all mechanical energy harvesting technologies, triboelectric nanogenerators (TENGs), which are recently invented based on coupling of contact electrification and electrostatic induction, show prospect of a promising technology due to their numerous advantages: high output power, high energy conversion efficiency, and inexpensive fabrication. Single-electrode triboelectric nanogenerators (SETENGs) that eliminate the moving electrode significantly expand the application of triboelectric nanogenerators in various circumstances, such as touch-pad technologies. Although the basic function of SETENGs has been demonstrated, there still lacks a theoretical model to systematically provide an in-depth understanding of their working principle. Further, the influence of their structural parameters on their performance is not clear yet, which, however, is essential for the optimization of their output characteristics.
In this work, a theoretical model of SETENGs is presented with in-depth interpretation and analysis of their working principle. Electrostatic shield effect from the primary electrode is the main consideration in the design of such SETENGs, which limits the maximum charge transfer efficiency to 50%. On the basis of this analysis, the impacts of two important structural parameters, that is, the electrode gap distance and the area size, on the output performance are theoretically investigated. Increasing the electrode gap will rise up the open-circuit voltage but reduce the short-circuit transferred charges, so an optimized electrode gap distance to provide a maximum transit output power. Increase the area size will enhance the electrostatic shield effect and reduce the charge transfer efficiency. Thus, an optimum area size is observed to generate the largest total power and parallel connection of multiple SETENGs with micro-scale size and relatively larger spacing should be utilized as the scaling-up strategy. The discussion of the basic working principle and the influence of structural parameters on the whole performance of the device can serve as an important guidance for rational design of the device structure towards the optimum output in specific applications. [1]
Reference:
1. S. Niu, Y. Liu, S. Wang, L. Lin, Y. S. Zhou, Y. Hu, Z. L. Wang, Advanced Functional Materials 24 (22), 3332-3340

Piezotronics serves as the foundation for a highly sensitive tactile sensor for measuring contact forces using zinc oxide (ZnO) nanowires. Previously, a piezotronic strain sensor with high gauge factor was fabricated based on ZnO nanowire arrays. Microfabrication techniques including sputtering, photolithography and reactive ion etching are used along with hydrothermal growth of ZnO nanowires to create the device. The Schottky contact that can form between ZnO and gold leads to rectifying, nonlinear I-V behavior. Such devices have demonstrated high sensitivity with gauge factors as high as 1813 ^[1]. ZnO nanowire tactile sensors based on the same principles can be fabricated on silicon or flexible substrates to measure pressure and contact forces. Multichannel sensor configurations show increased versatility over single channel designs ^[2]. There are many important potential applications for piezotronic ZnO nanowire tactile sensors including prosthetics and surgical robotics.
[1] Zhang, Wengui, et al. "Highly sensitive and flexible strain sensors based on vertical zinc oxide nanowire arrays." Sensors and Actuators A: Physical 205 (2014): 164-169.
[2] Wu, Wenzhuo, Xiaonan Wen, and Zhong Lin Wang. "Taxel-addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging." Science 340.6135 (2013): 952-957.

Ambient vibrations are major source of wasted energy, exploiting properly such vibration can be converted to valuable energy and harvested to power up devices, i.e. electronic devices. Accordingly, energy harvesting using smart structures with active piezoelectric ceramics has gained wide interest over the past few years as a method for converting such wasted energy. This paper provides numerical and experimental analysis of piezoelectric fiber based composites for energy harvesting applications proposing a multi-scale modeling approach coupled with experimental verification.
The multi-scale approach suggested to predict the behavior of piezoelectric fiber-based composites use micromechanical model based on Transformation Field Analysis (TFA) to calculate the overall material properties of electrically active composite structure. Capitalizing on the calculated properties, single-phase analysis of a homogeneous structure is conducted using finite element method. The experimental work approach involves running dynamic tests on piezoelectric fiber-based composites to simulate mechanical vibrations experienced by real structures with embedded piezoelectric filaments . Experimental results agree well with the numerical results both for static and dynamic tests.

Many forms of mechanical energy harvesters generate voltage and current pulses on the order of 50-100 milliseconds, making them ideal for vibration energy harvesters or high-frequency motion harvesters. Energy harvesters with longer current pulse duration, on the order of 1 second, remain to be developed to efficiently harvest mechanical energy from low frequency motions such as human walking or every-day activities. In this work, we introduce a type of mechanical energy harvester based on electrochemically alloyed electrodes. The device consists of two identical lithiated silicon electrodes on flexible substrate, separated by a layer of electrolyte. When different stress states are introduced between the two electrodes by mechanical input such as bending, electrochemical potential difference develops between the two electrodes. This potential difference drives lithium ion migration from the relatively more compressed electrode to the other electrode, generating current pulses that lasts over 5 seconds per bending, continuously generating current over 10µA. This working principle is in analogous to Nabarro-Herring creep in metallurgy and by carefully engineering the electrode, the device cycles over 300 times. We also argue that the fundamental efficiency of this device is limited only by the amount of deviatoric stress introduced to electrodes.

The ability to arbitrarily regulate semiconductor interface provides the most effective way to modulate performance of optoelectronic devices. However, relatively less work were reported on piezo-modulated interface engineering in all-oxide system. In this paper, an enhanced photoresponse of all-oxide Cu₂O/ZnO heterojunction was obtained by taking advantage of the piezotronic effect. The illumination-density-dependent piezoelectric modulation ability was also comprehensively investigated. An 18.6% enhancement of photoresponse has been achieved when applying a -0.88% compressive strain. Comparative experiment confirmed that this enhancement could be interpreted from the band modification induced by interfacial piezoelectric polarization. Positive piezopotential generated at ZnO side produces an augment of space charge region in Cu₂O, thus providing an extra driving force to separate the excitons apart more efficiently under illumination. Our research provides a promising method to boost the performance of optoelectronics without altering the interface structure and could be extended to other metal oxide devices.

Ferroelectric films have been offered a number of advantages in the field of microelectromechinical systems (MEMS) due to its low power requirements and large motion that is easily created. Among ferroelectric films, lead zinconate titanate (PZT) films have been widely used as the on-chip transducer in micromechanical sensors. The PZT thin films have high piezoelectric response to generate sufficient energy for operating microstructures such as cantilever, membrane. These piezoelectric-driven micromechanical sensors show excellent sensitivity in air or vacuum condition, while viscous damping leads degradation of sensing capability in liquid where most biological interactions occur. It is critical issue for mechanical sensors to detect small molecules in liquid that strongly demand in-situ monitoring.
To address the viscous damping issue, we developed a novel piezoelectric-driven micromechanical cantilever sensor with micro-slit. The cantilever sensor was vibrated by embedded piezoelectric material, PZT (Pb(Zr_0.52Ti_0.48)O₃) thin film on the sensor chip. PZT film with 2 µm-thickness was deposited by sol-gel method. The film was placed between upper and bottom Pt electrodes to form a Pt-PZT-Pt capacitor structure having polarization value of 14 µC/cm² when electric field of 100 kV/cm was applied. A 1 µm-thick Si₃N₄ cantilever which has size of 50 ×150 µm² (width × length) was released by a micro-slit of 5 µm-gap surrounding. The structure with micro-slit allows separating liquid sample on one side of surface against the opposite side to form liquid-air interface due to meniscus of fluid. By this structural feature with micro-slit, resonating characteristics (quality factor) of cantilever sensor could be improved by reducing viscous damping.
The resonant frequency of cantilever sensor was measured by laser Doppler vibrometer. The resonant frequencies and quality factor in air condition were measured at 488 ~ 490 kHz and 200 where the range was less than 5 % error of theoretical prediction. When the liquid buffer injected in the sensing chamber, resonant frequency and quality factor of cantilever sensor decreased at 120 kHz and 25 by liquid viscosity and density. However, the quality factor value is ten times better than that of conventional mechanical sensors. In order to evaluate cantilever sensors as an in-situ biomolecule detection tool, we prepared the extracellular vesicles (EVs) samples that were ultra-centrifuged from intact cells and diluted in buffer solution. Because EVs are damaged in out of liquid, measurement in liquid is highly recommended. When EVs with diluted concentrations of 10^-6, 10^-4, and 10^-2 from original sample (240 µg/mL) were introduced, resonant frequency was shifted by -110, -340, and -1,200 Hz respectively. These results showed promising aspect of reliable in-situ monitoring of specific biomolecules with our piezoelectric-driven cantilever sensors.

Recently triboelectric nanogenerator (TENG) devices that transform environmental mechanical energy to electric power have been demonstrated as a renewable, clean and usable power source. In this talk, we will demonstrate several novel high-performance TENGs for implantable devices. First is a novel sandwich-shape triboelectric nanogenerator to convert low-frequency mechanical energy to electric energy with double frequency which can be directly used to drive an implantable 3-D microelectrode array for neural prosthesis without any energy storage unit or rectification circuit. Second is a hybrid NanoGenerator which has Micro/nanostructures on the PDMS surface for triboelectric generator and PVDF thin film as piezoelectric generator to enhance the output performance, the power density of this device can be reach to 50mW/cm3, and demonstrate the application in self-powering active implantable sensor. Third, a single-friction-surface triboelectric generator (STEG) has been developed and characterized as self-powered touch sensor could potentially be used to develop self-powered supersensitive artificial skin, 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 BaTiO3 thin film from bulk substrates. Second, we report the nanocomposite generator (NCG) for achieving a simple, low-cost, and large area fabrication based on BaTiO3 (or PZT) nanoparticles and graphitic carbons (CNT or RGO). The second part will introduce flexible electronics including memory and large scale integration (LSI). Flexible memory is an essential part of electronics for data processing, storage, and radio frequency (RF) communication. To fabricate a fully functional flexible memory, we integrated flexible single crystal silicon transistors with an amorphous titanium oxide (a-TiO2) based memristor to control the logic state of memory. The third part will discuss the flexible GaN 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 thin film LED enable the dramatic extension of not only consumer electronic applications but also the biosensing scale. A water-resist and a biocompatible PTFE-coated flexible LED biosensor can detect PSA at a detection limit of 1 ng/mL. Finally, we will discuss laser material interaction for flexible 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] Adv. Mater. 24, 2999, 2012.
[5] Adv. Mater, 26, 2514, 2014. [6] Adv. Mater. 26, 4880, 2014
[7] Adv. Mater, 10.1002/adma.201402472 [8] ACS Nano 7, 4545, 2013.
[9] ACS Nano 7, 11016, 2013 [10] ACS Nano 7, 2651, 2013.
[11] ACS Nano 8, 7671, 2014 [12] ACS Nano 8, 9492, 2014
[13] Adv. Energy Mater. 3, 1539, 2013 [14] Adv. Funct. Mater. 24, 2620, 2014.
[15] Energy Environ. Sci., 10.1039/C4EE02435D [16] Nano Energy, 1, 145, 2012

Establishing the optimal morphology of piezoelectric film is critical for improvement of power performance of nanogenerator (NG). Herein, we fabricated line and pyramid-like micropatterned piezoelectric flexible P (VDF-TrFE) polymer films based nanogenerator for effectively converting mechanical energy into electrical energy under vertical compression and for highly sensitive self-powered pressure sensor. The output voltage and current reached up to high value of 3.8 V, 2.4µA and 4.4 V, 3.3µA for line and pyramid-like micropatterned P (VDF-TrFE) based NGs, respectively, while non-patterned P (VDF-TrFE) based NG exhibited low output power under same vertical compressive force. The micro patterning of P (VDF-TrFE) polymer made it ultra-sensitive in response of mechanical deformation, and we successfully demonstrated their practical application as self-powered pressure sensor in which mechanical energy came from water droplet and wind. The mechanism of the high performance was intensively discussed and illustrated in terms of strain developed from flat and micropatterned films. The impact derived from the structuring on the output performance was studied in term of effective pressure using COMSOL simulation

Developing wireless nanodevices and nanosystems is of critical importance for sensing, medical science, environmental/infrastructure monitoring, defense technology and even personal electronics. It is highly desirable for wireless devices to be self-powered without using battery. Nanogenerators (NGs) have been developed based on piezoelectric, trioboelectric and pyroelectric effects, aiming at building self-sufficient power sources for mico/nano-systems. The output of the nanogenerators now is high enough to drive a wireless sensor system and charge a battery for a cell phone, and they are becoming a vital technology for sustainable, independent and maintenance free operation of micro/nano-systems and mobile/portable electronics. An energy conversion efficiency of 50% and an output power density of 1200 W/m² have been demonstrated. This technology is now not only capable of driving portable electronics, but also has the potential for harvesting wind and ocean wave energy for large-scale power application. This talk will focus on the updated progress in NGs.
For Wurtzite and zinc blend structures that have non-central symmetry, such as ZnO, GaN and InN, a piezoelectric potential (piezopotential) is created in the crystal by applying a strain. Such piezopotential can serve as a “gate” voltage that can effectively tune/control the charge transport across an interface/junction; electronics fabricated based on such a mechanism is coined as piezotronics, with applications in force/pressure triggered/controlled electronic devices, sensors, logic units and memory. By using the piezotronic effect, we show that the optoelectronc devices fabricated using wurtzite materials can have superior performance as solar cell, photon detector and light emitting diode. Piezotronics is likely to serve as a “mechanosensation” for directly interfacing biomechanical action with silicon based technology and active flexible electronics. This lecture will focus on the updated progress in the field and its expansion to 2D materials.
References
G. Zhu#, J. Chen#, T.J. Zhang, Q.S. Jing, Z.L. Wang* “Radial-arrayed rotary electrification for high-performance triboelectric generator”, Nature Communication, 5 (2014) 3456.
W.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.
C.F. Pan, L. Dong, G. Zhu, S. Niu, R. 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.
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, 2014, DOI: 10.1038/nature13792.

Soft ferroelectric nanostructures have promising potential in energy harvesting devices, pressure sensors and printed nanoelectronics due to their multi functionality, such as ferroelectricity, piezoelectricity and pyroelectricity. In the present work, we report synthesis of polymeric ferroelectric PVDF-TrFE nanotubes with improved piezoelectric properties and their energy harvesting performance. PVDF-TrFE nanotubes are synthesized by melt wetting of anodized aluminum oxide porous templates. X- ray diffraction measurements confirm the formation of ferroelectric beta phase with (110)/(200) planes along the length of the PVDF-TrFE nanotubes. The domain structure of the as-grown PVDF-TrFE nanotubes, studied using piezoresponse force microscopy (PFM), exhibits mono domain behavior corresponding to the polarization component perpendicular to the nanotubes axis (Pperp;) and poly domain behavior observed for the polarization component parallel to the axis (P#449;) of the nanotubes. The PFM switching spectroscopy measurements reveal that the effective polarization orientation in PVDF-TrFE nanotubes is at an inclination to the long axis. Switching of the Pperp; and P#449; polarization components by applying a field in the direction perpendicular to the nanotubes axis further supports the inclined polarization orientation in PVDF-TrFE nanotubes. The nanotubes exhibit a reduced coercive field of 18.6 MVm^-1 along the axis and 40 MVm^-1 (13.2 MVm^-1 by considering the air gap) in a direction perpendicular to the axis, compared to film counterparts 50 MVm^-1. The poled nanotubes, with 40% reduction in poling field, exhibit larger piezoelectric d₃₃ coefficient values of 44 pmV^-1, compared to poled films (~ 20 pmV^-1). Superior energy harvesting performance of PVDF-TrFE nanotubes observed over films with an output voltage of ~ 4.8 V and power density of 2.2 mu;Wcm^-2, under a dynamic compression pressure of 0.075 MPa at 1 Hz. ^[1] These PVDF-TrFE nanotubes are expected to find application in the functional devices, such as pressure sensors, electronic skin.
Reference:
1. V. Bhavanasi, D. Y. Kusuma, P. S. Lee, Adv. Energy Mater., DOI: 10.1002/aenm.201400723

Self-powered piezoelectric systems are vital components to harvest ambient waste energy for applications such as autonomous self-powered sensors. ZnO nanorod-based devices are gaining wide attention for energy harvesters as they are easily synthesized at low temperature onto a range of substrates - including flexible ones. However, losses related to screening of piezoelectric polarisation charges by free carriers in ZnO nanorods can significantly reduce the output of these devices. The surface chemisorbed and physisorbed species on ZnO reduces the piezoelectric voltage generation and reliability by increasing the carrier concentration and therefore the internal screening. In this presentation we discuss approaches to reduce this internal screening through surface passivation of the ZnO nanorods. The electrical field which can be delivered using a strained ZnO nanorod energy harvester is related to the rate at which the depolarising field is set up and the rate at which piezoelectric polarisation charges are separated in the material. This balance between charge leakage and stored charge is related to the surface and bulk conductivities of the materials system and we show how this may be both modelled and optimised.
Though there is still some controversy in the community concerning the origins of the charge measured in many ZnO systems and by many labs, in our work, we demonstrate that the effect is very likely to be piezoelectric in nature. Controlled vibration testing of the devices provides strong evidence that the effect results from a piezoelectric response in the material. The generated voltage increases linearly with the pressure applied to the sample, which is expected for a piezoelectric effect. When the devices are illuminated the voltage output drops significantly. This is attributed to the photo-induced conductivity of the ZnO, which is known to reduce the piezoelectric coefficient due to screening by conduction electrons. The input force to the hybrid device is measured using controlled bending. By simultaneously measuring the output power of the devices the energy conversion efficiency is calculated to be 0.0067 % with slow bending, increasing exponentially with bending rate up to ~8.5 % with much faster bending. This increase agrees with our proposed model of screening-limited energy output. These results not only demonstrate an alternative approach to the design of a ZnO nanorod energy harvesting design, but also contribute to the understanding of the factors that limit the device performance. Finally, we summarise some of the issues surrounding the importance of how measurements are made in these nanosystem energy harvesters.

Presented are the initial findings on a new piezotronic device, which makes use of a piezoelectric enhanced thermoelectric effect. We constructed TPEG by integrating insulating layers of polyvinylidene fluoride (PVDF) piezoelectric films between flexible thin-film p-type and n-type thermoelectrics. The piezoelectric bound surface charge modifies the thermoelectric properties of the semiconductor electrodes which facilitates an increase in voltage. The TPEG voltage output has three contributions: traditional thermoelectric and piezoelectric terms, and a unique coupling term. A combined thermoelectric and piezoelectric model can be used to quantify the expected coupling voltage as a function of stress and thermal gradient. The fabrication, placement, and configuration of this interface allows for different device designs and affects overall performance. Under easily achievable stress and thermal gradient this new coupling effect can increase voltage output by 20%. TPEG are ideal to capture waste heat and vibrational energy while creating larger voltages and minimizing space when compared with similar thermoelectric or piezoelectric generators.

MEMS-scale vibration energy harvesting has been investigated for more than a decade to enable autonomous systems such as batteryless wireless sensor networks. Toward this goal, a fully assembled energy harvester at a size of a quarter dollar coin should be able to generate robustly about 100mW continuous power from ambient vibration (mostly less than 100Hz and 0.5g acceleration) with reasonably wide bandwidth (>20%). We are inching close toward this goal in terms of power density and bandwidth, but not in terms of low frequency operations.
Most of the reported vibration energy harvesters use a linear cantilever resonator structure to amplify small ambient vibration. While such structures are easy to model, design and build, they typically have a very narrow bandwidth. In contrast, nonlinear resonators have different dynamic response and greatly increase the bandwidth by hardening the resonance characteristic of the beam structure. Our previous research with non-linear resonating stretching energy harvesters achieved 2.0 mW/mm³ power density with >50% power bandwidth. But it was operated with input vibrations of >1 KHz and 4.0 g acceleration, which practically limits the use of this technology, harvesting energy from real environmentally available vibrations. Many believe high frequency resonance is an inherent limitation imposed on the MEMS scale structures.
We approached this problem with a bi-stable nonlinear resonating buckled beam. Compared to mono-stable nonlinear resonance, we found bi-stable resonance could bring more dynamics phenomena to help reduce the operating frequency as well as the g-requirement. Electromechanical lumped model has been built for simulating the dynamics of buckled clamped-clamped beam structure. The two oscillation modes, intra- well and inter-well with respect to the double energy well potential of the bi-stable system, have been simulated where we found characteristic spring softening and spring stiffening responses, which were associated with the small-amplitude intra-well and large-amplitude inter-well oscillations, respectively. In order to validate the simulated models, a meso-scale prototype has been built and tested on an electromagnetic shaker with controlled and monitored input vibration frequency and amplitude. The testing results verify the theoretical predictions, showing softening response generating a decent amount of power at lower frequencies compared to the same testing setup without bi-stability. Hysteresis also exists when varying the vibration amplitude at fixed frequency, so that at low g input, the bi-stable energy harvesters still generate a significant amount of power. The buckled bi-stable beam structure is believed to solve the last remaining challenge of the MEMS energy harvester: power generation at low frequency and low-g input vibrations.

Vast amount of excess heat are generated by industrial processes, power plants, and residential and commercial buildings. Although the total energy of the low-temperature waste heat (ambient to 100^°C) is more than half of the energy used of industry, the wasted energy usually goes unclaimed[1]. One of the methods currently used to harvest the low-grade thermal energy is thermal-to-mechanical-to-electrical conversion. Common thermomechanical heat engines[1] such as external combustion heat engines, thermoacoustic heat engines, and thermomagnetic generators exhibit large volume and heavy weight. To integrate heat engines to next generation nano/micro energy harvesting devices, light weight, downscale, and low-temperature operation are strongly required.
In this work, we have developed novel micro heat engines based on carbon nanotube (CNT) free-standing film (FSF), of which weight and thickness are less than several hundred mu;g/cm² and less than several ten mu;m, respectively. The FSF shows continuous mechanical stretching on a hot plate at constant temperature less than 100°C.
The heat engine we fabricated is composed of multi-walled CNT (MWNT) FSF with various thin films of different materials. The MWNT FSFs were fabricated by spray coating. Various thin films, X, were deposited on MWNT FSFs. The X/MWNT FSFs act as bimorphs because thermal expansion coefficients of material X and MWNT are different. We call them “bimorph heat engines (BHEs)”. Thus, the BHE on the hot plate becomes bent. The maximum height of the inverted-U-shaped BHE was monitored by a laser displacement sensor.
We have found that the strip of the BHE exhibits continuous mechanical stretching on a hot plate at the temperature less than 100°C when the ambient temperature was 27°C. Minimum temperature difference at which the BHE can be actuated was found to be approximately 5°C. Thus, the BHE can be utilized for thermal-to-mechanical conversion devices, scavenging low-grade thermal energy.
In the session, experimental detail of mechanical stretching, and the mechanism of the mechanical stretching will be also discussed with results of simulation.
[1] S. Percy et al. “Thermal Energy Harvesting for Application at MEMS Scale”, Springer (2014).

Zinc oxide nanowire has wide applications in nanogenerators and piezotronics due to its piezoelectricity and semiconductivity. Because the piezoelectric property is anisotropic, proper alignment and orientation of nanowire arrays are necessary for an effective device integration. Bottom-up synthesis of ZnO nanowires requires a highly engineered substrate to achieve alignment and orientation control. We have developed a textured ZnO film as an inexpensive substrate to fulfill the requirement. The textured film is coated conformally on various surface topographies and allows the epitaxial growth of ZnO nanowires with vertical, tilted, or lateral orientations. The textured film can also be formed into three-dimensional structure for growing novel nanostructures. The growth flexibility can potentially simplify device fabrication and optimize device performance in nanogenerators and piezotronics.

Numerous engineering feats are being performed with the increasing array of single-layer and few-layer materials including MoS₂. Some of the most dramatic accomplishments are enabled by properties that emerge only at the single or few-layer limit and are not found in bulk forms. I will discuss our efforts to elucidate several new and useful emergent electromechanical properties of monolayer and few-layer materials that are not found in bulk forms. Using a variety of atomistic modeling methods, we have predicted that many of the commonly studied single-layer and few-layer transition metal dichalcogenide (TMD) materials (e.g. MoS₂) exhibit strong electromechanical coupling in the form of piezoelectric and flexoelectric-like effects, unlike their bulk parent crystals.¹ We further discover that single layer materials that are not intrinsically piezoelectric may be endowed with piezoelectricity through specific forms of chemical adsorption.² The latter is a consequence of the atomically-thin nature of these materials. I will discuss the first recently reported observations of some of these effects in the laboratory.

¹ Karel-Alexander N. Duerloo, Mitchell T. Ong, and Evan J. Reed, Journal of Physical Chemistry Letters 3 (19), 2871 (2012); Karel-Alexander N. Duerloo and Evan J. Reed, Nano Letters 13 (4), 1681 (2013).
² Mitchell T. Ong and Evan J. Reed, ACS Nano 6 (2), 1387 (2012); Mitchell T. Ong, Karel-Alexander N. Duerloo, and Evan J. Reed, J. Phys. Chem. C 117 (7), 3615 (2013).

Shape Memory Alloys (SMA) can exist in different phases based on the temperature and stress. When thermo-mechanically loaded, they can deform to a different shape. This feature makes them good candidates for sensors and actuators. A novel idea is to combine them with a piezoelectric material for harvesting thermal energy [1]. When a piezoelectric matrix embedded with SMA fibers is subjected to a heating-cooling cycle, the SMA converts the temperature gradient into a strain change stimulating the piezoelectric matrix to produce a voltage. To verify the feasibility of the idea using a computational model, the first step is to predict the behavior of the SMA accurately. We focus on NiTi since its transformation temperatures lie close to room temperature.
Experimental evidence shows that SMA&’s oscillate between two shapes during thermal cycling [2]. This phenomenon, known as two-way shape memory effect, occurs due to a transformation between the austenite and the single-variant martensite phase. This property of the SMA will be utilized in the harvester. Recent tests revealed the occurrence of length scale effects on martensitic transformations in NiTi at the nano-scale [3]. The harvester we intend to prototype uses SMA fibers with diameters in the order of 500 nm, making size effects relevant. In this study, the transformation temperatures, stress and size effects in the two-way shape memory phenomenon of cylindrical NiTi nano-wires are, for the first time, simulated and analyzed using molecular dynamics (MD) simulations.
Preliminary results from MD simulations of the one-way shape memory effect indicate the formation of a higher density of martensite variants and variant interfaces and a reduction in the de-twinning stress, as the diameter of the nano-wire increases. Size dependency in the transformation to austenite, proved in nano-spheres [4], has also been observed in nano-wires. An asymmetry in the stress-induced transformation properties and mechanism during tension and compression is also seen. This analysis will be extended to the two-way shape memory effect. The results obtained from this atomistic study will be used in a larger scale model, to predict the thermo-electro-mechanical behavior of the harvester.
References:
[1] N. Nersessian, G. P. Carman, and H. B. Radousky. Energy harvesting using a thermoelectric material, US Patent 7,397,169 B2, 2008.
[2] N. G. Jones, and D. Dye. Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load. Intermetallics 19, 1348-1358, 2011.
[3] T. Waitz, T. Antretter, F. D. Fischer, N. K. Simha, and H. P. Karnthaler. Size effects on the martensitic phase transformation of NiTi nanograins. Journal of the Mechanics of Physics of Solids 55, 419-444, 2007.
[4] D. Mutter, and P. Nielaba. Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles. Eur. Phys. J. B 84, 109-113, 2011.

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.

Oxides based on Ln₂O₃-MO₃ (A = Bi³⁺, Ln³⁺ M = Mo and W) system are of significant technological interest for their laser applications¹, ionic conduction², catalytic³, photoluminescence⁴ and ferroelectric⁵ properties.
In this work, rare earth tungstate La₂WO₆ grown by pulsed laser deposition for the first time as thin film at room temperature, has been in situ characterized. The experiments are carried out in a JEOL 2010F (200 kV) Transmission Electron Microscope (TEM) equipped with a scanning tunneling microscope (STM) holder for in situ electrical probing. A sharp platinum tip is attached to the movable part of the STM holder controlled with piezoelectrics, and both sample and tip are oriented perpendicular to the electron beam of the TEM. The probe can be positioned in a millimeter-scale workspace with subnanometer resolution enabling the selection of a specific nanostructure and to perform electrical measurements.
The characterization of conductivity through the La₂WO₆ and a qualitative characterization of its piezoelectric behavior are performed. An I-V characteristic has been taken for each step of the tip controlled with the piezoelectrics of the holder. During the experiment a decrease of the resistance is observed while more pressure is applied from the tip to the sample from 8M#8486; to 800K#8486;. Besides, ferroelectricity characterizations are carried out from the cycle of the hysteresis switching current using the I-V method. The measured current is maximum when the applied voltage is close to the coercive voltage of the ferroelectric films and these current maxima correspond to the dipole reorientation contribution of the ferroelectric material⁶. For La₂WO₆ the measured coercive voltage is 620mV. Thus, in this work we show the piezoelectric and ferroelectric behavior of La₂WO₆ through TEM-STM in situ characterization. Moreover, this system also allows us to determine its coercive voltage.
¹ Kumaran, A. S. et al. Crystal growth and characterization of KY(WO₄)₂ and KGd(WO₄)₂ for laser applications. J. Cryst. Growth 2006, 292 (2),368-372.
² Lacorre, P et al. Designing fast oxide-ion conductors based on La₂Mo₂O₉. Nature 2000, 404 (6780), 856-858.
³ Alonso, J. A. et al. Preparation and structural study from neutron diffraction data of R₂MoO₆ (R = Dy, Ho, Er, Tm, Yb, Y). J. Solid State Chem. 2004, 177 (7), 2470-2476.
⁴ Ishigaki,T. et al. Melt synthesis of oxide red phosphors La₂WO₆: Eu³⁺. Physics Procedia. 2009, 2, 587-601.
⁵ Brixner, L. H.; Sleight, A. W.; Licis, M. S. Ln₂MoO₆-type rare-earth molybdates -preparation and lattice-parameters. J. Solid State Chem. 1972, 5 (2), 186-190.
⁶ K. Svensson et al. Compact design of a transmission electron microscopescanning tunneling microscope holder with three-dimensional coarse motion. Rev. Sci. 2003, 74, 4945-4947.

Recently, strain gradient-induced polarization--flexoelectric effect, has been attracting a lot of attention. Flexoelectric effect can be used in devising next-generation electromechanical transducers, nanogenerators and optimizing piezotronics. By employing finite element method, we numerically simulate the potential distribution on BaTiO3 NWs(rectangular) and ZnO NWs(circular), and study the relationship between the strain gradient and the maximum potential based on three conditions, cantilever beam, three-point bending beam and four-point pure bending beam. The comparison between piezoelectric effect and flexoelectric effect was made. When a NW is under three-point bending condition, the maximum potential induced by flexoelectric effect locates exactly at the loading point, only negligible potential is induced by piezoelectric effect. When a NW is under four-point bending condition, the potential induced by flexoelectric effect regularly distributes along the pure bending part of the NW, where there is no potential is induce by piezoelectric effect. The results show obvious difference between the flexoelectric effect and piezoelectric effect, and can provide valuable reference to the design of novel nanogenerators and the measurement of the flexoelectric coefficients.

With the development of society and energy crisis, searching for new energy materials and developing new energy have been one of the eternal subjects. In addition to natural energy materials and new energy materials, such as solar cell, Li-ion battery, thermoelectric conversion materials, etc., nanogenerator is also a hot spot which is based upon a certain nana-material and nano-effect. The most famous nanogenerators are the piezoelectric nanogenerator which is based on piezoelectric effect of ZnO nanorods in 2006, and triboelectric nanogenerator which is based upon the triboelectrification and electrostatic induction effects. Up to now, these two kinds of nanogenerators are closer to practical applications.
In recent years, several researches have revealed that under certain conditions, graphene exhibited an ability for energy conversion and harvesting. For example, flow of a liquid, gas flow, acoustic and cyclical force, etc. In our research, monolayer graphene sheets were deposited on a transparent and flexible polydimethylsiloxane (PDMS) substrate, and a tensile strain was loaded by stretching the substrate in one direction. It was found that an electric potential difference between stretched and static monolayer graphene sheets reached 8 mV when the strain was 5%. Theoretical calculations for the band structure and total energy revealed an alternative way to experimentally tune the band gap of monolayer graphene, and induce the generation of electricity. Comparing with other nanogenerators, the graphene-based nanogenerator provides advantages, such as simple assemble, flexibility and high structural stability. It is expected this nanogenerator is of potential applications in active sensors and sustainable power source.

The magnitude of piezoresistance phenomena in p-type Silicon nanowires reported in the literature varies enormously [1] - from the known bulk Silicon values [2] up to giant piezoresistance values 100 times larger [3]. As such, the origin (and even the very existence) of the giant effect have been questioned.
Despite this uncertainty, giant piezoresistance has been proposed as the basis for a wide range of force transduction applications requiring the sensitive electrical detection of MEMS and NEMS motion. This, and the fact that mechanical stress is a key ingredient in the semiconductor technology roadmap for the improvement of microelectronic device performance, suggests that an understanding of the microscopic origin of the phenomena should be a priority.
This talk will present an overview of the published observations and suggested explanations [4, 5], including some of our own most recent results, and will suggest possible future research directions.
[1] A.C.H. Rowe, J. Mater. Res. 29, 731 (2014)
[2] J.S. Milne et al., Phys. Rev. Lett. 105, 226802 (2010); Phys. Rev. Lett. 108, 256801 (2012)
[3] R.R. He et al., Nature Nanotech. 1, 42 (2006)
[4] J.X. Cao et al., Phys. Rev. B 75, 233302 (2007)
[5] A.C.H. Rowe, Nature Nanotech. 3, 312 (2008)

Energy harvesting through piezoelectric and triboelectric methods have been explored as a prospective solution to gather the irregular mechanical energy into electricity used in our daily life. These techniques were able to power the LEDs, active sensors, and other low-powered consumption devices. Among them, the active sensors have successfully analyzed the object motions, for instance, muscle-driven nanogenerator, wind-velocity detector, and human-machine interfacing devices. Today, mobile technologies, especially personal and mobile electronics, are becoming very popular. However, it is important to address the electric-waste issue (ie., non-recyclable garbage that contains toxic heavy metals such as lead.) by adoption of miniaturized energy-harvesting and/or self-powered systems. Thus, lead-free electronic materials are a topic of intense interest. In this report, we will demonstrate a flexible nanogenerator made from lead-free nanomaterials, for instance, ZnS, TeO₂, and ZnSnO₃ etc. It can be further applied on the wrist-worn devices for detecting the human pulse, tiny physical motions, and touchless control of smart devices.

Lithium Niobate (LN, LiNbO₃) is commonly used crystal in variety of applications like guided-wave optics, electro-optics and photonics. It is one of the fastest electro-optical materials in use due to its high electro-optic coefficient and good dielectric properties. Most optical modulators are fabricated using LN. It displays pyroelectric, piezoelectric and Pockels effects. Due to its attractive electro-optic properties and piezoelectric effect, these two properties can be merged to fabricate an opto-mechanical sensor where sensors are needed with dielectric components only. In this work we demonstrated a piezoelectric sensor made out of LN and tested its dynamic properties. Although strain gauge and piezoelectric sensors work with different cut-off frequencies and are sensitive to different amounts of forces, we tested our LN sensors within the dynamic range of commercial sensors to compare the applied conditions.
In this talk I will first give a background on the photonic and acousto-optic applications of LN, then will explain the differences in piezoelectric and piezoresistive transducers. I will present the electronic circuitries we built for both type of transducers. Finally, I will present the specs and the test results of LN based piezoelectric transducer and the possibility of its use as an opto-mechanical transducer.

With the increasing demand for sustainable and reliable energy for wireless nanosystems and personal electronics, it has become a promising approach to harvest energy from the ambient sources including body movements, air flow and even thermal fluctuations which are available in most of the circumstances. Nanogenerators (NGs) are emerging as novel devices which can convert various kinds of ambient energy into electric power both in nano and macro scale. It is imperative to further improve the performance of NGs and develop scalable manufacturing techniques for practical applications. In this work, we report a novel high-performance nanogenerator based on piezoelectric nanocomposite (p-NC) of Pb-free BaTiO₃ nanoparticles (BTO NPs) and bacterial cellulose (BC). BC pellicles are disintegrated and dispersed into aqueous solution by a high speed homogenizer, followed by mixing with BTO NPs to form a uniform suspension. A facile vacuum filtrating method was exploited to remove water from the BTO/BC p-NC suspension, during which BC fibers are integrated naturally by strong hydrogen bonds and all the BTO NPs are bounded tightly and uniformly in the BC matrix, resulting in a wet BTO NPs/BC p-NC membrane. The wet membrane was then pressed and dried followed by integration with Au coated Kapton films as top and bottom electrodes to form a NG device. Upon poling, the output voltage of ~14 V in the open circuit condition and the maximum output power density of as high as 0.65 mu;W cm^-2 was achieved. In addition, we also fabricated p-NC film using polydimethylsiloxane (PDMS) as the matrix. The resultant open circuit voltage and maximum output power density was only ~3 V and 0.039 mu;W cm^-2 respectively, which is much lower than the BTO NPs/BC based devices. SEM result exhibited that BTO NPs can be distributed uniformly in BC easily while serious sedimentation of BTO NPs was occurred in the PDMS matrix. COMSOL software simulation shows that the piezoelectric voltage can be drastically enhanced when BTO NPs are more uniformly dispersed in the p-NC film, which is consistent with the results. The BTO NPs/BC based NG we proposed here is cheap, scalable and environmental friendly which demonstrates unique merits for powering personal electronics.

Graphene/ZnO nanorod (NR) hybrid structure was fabricated by by mechanical exfoliation of hydrothermally-grown ZnO NRs on graphite substrate. ZnO NRs were grown on HOPG substrates by a two-step hydrothermal method involving the formation of a ZnO seed layer. The existence of the graphene sheets on the hybrid structure was confirmed by using the Raman spectra and current-voltage (I-V) characteristics. The Raman spectra of the exfoliated graphene/ZnO NR hybrid structure shows G and 2D band peaks that are shifted to lower wave numbers, indicating that the exfoliated graphene layer exists under a significant amount of strain. The I-V characteristics of the graphene/ZnO NR hybrid structure show current flow through the graphene layer, while no current flow is observed on the ZnO NR/polydimethylsiloxane (PDMS) composite without graphene, thereby indicating that the few-layer graphene was successfully transferred onto the hybrid structure. The shear stress required to exfoliate the graphene from the graphite is sufficient to overcome the weak van der Waals interactions between the graphene layers of the graphite substrate. The ZnO NRs on graphene have been known to be mechanically stable on an atomic level which has been verified through the first principles calculations. The exfoliated ZnO NR/PDMS with graphene was transparent and highly flexible, which enabled us to extend the benefits with using a ZnO NR/graphene composite in many applications such as optoelectronic and energy conversion devices. A piezoelectric nanogenerator is demonstrated by using the fabricated graphene/ZnO NR hybrid structure. The piezoelectric nanogenerator exhibited stable output voltage above 3 V with an alternating current output signal and the output signal showed frequency dependent increase. Our results showed the validity of potential device applications using the proposed flexible and transparent graphene/ZnO NR hybrid structures.

CdSe is an important II-VI direct band gap (1.74 eV) semiconductor with attractive optical properties and potential applications in optoelectronic devices, such as photodetectors, solar cells, light emitting diodes (LEDs), etc. Aditionally, CdSe is piezoelectric owing to its non-centrosymmetric wurtzitic structure, and its nanowire based mechanical to electrical energy generator has also been demonstrated. The excellent optical and piezoelectric properties make CdSe nanowires a suitable candidate for piezo-phototronic device fabrication. To date, little work has been done to explore three dimensional (3D) CdSe nanowire array devices based on piezo-phototronic effect. In the present work, we use 3D CdSe nanowire array, synthesized by simple one-step chemical-vapor-deposition (CVD) technique on low cost and semitransparent flexible mica substrate, to integrate a prototype photodetector. It is found that the responsivity of photodetector based on 3D nanowire array can be greatly enhanced by applying external load over wide spectral and illumination intensity range, clearly indicating that the nanowire array is a promising nanoarchitecture for piezo-phototronic device.

In our environment, there is abundance of energy in the forms of solar, thermal, mechanical, chemical, biological, and so on. In order to satisfy the long-term energy needs and to achieve the sustainable and maintenance-free operation of micro/nanosystems, energy harvesting has been developed as a part of technologies to generate electricity from the ambient environment. Recently, piezoelectric nanogenerators (NGs) harvesting mechanical energies have been getting attraction due to their applications in renewable energy and sensors. So far, most piezoelectric NGs have been analyzed by bending, and stretching conditions. Depending on the load conditions, strain distributions are critically different, resulting in different output performance. In this study, we characterize the efficiency dependency of piezoelectric NGs on the mechanical modes such as bending and twisting. The piezoelectric NGs were prepared by sputtering ZnO thin film on an ITO-coated PET film. We could find the different output behaviors and performance of piezoelectric NGs under different mechanical modes. The results were analyzed by theoretical mechanical characterization based on stress-strain analysis for each condition. Depending on the strain, the efficiency of piezoelectric NGs was critically affected. We expect that our study will provide the way to apply the piezoelectric NGs to a variety of energy harvesters and sensors.

Nanogenerators (NGs) are able to convert energy from various mechanical sources that is wasted around us to electric energy such as kinds of pushing, sliding and rotating mode. It is widely studied to use triboelectric nanogenerators with vibration of various types [1-4]. We especially studied a rotary motion system for triboelectric energy harvesting. In the case of papers published previously regarding rotation mode, the durability problem is caused due to frictional rubbing by rotation. In this study, in order to overcome this limitation and take advantage of innumerable rotational motion, we switch rotary motion to up-and-down motion and investigate various characteristics for higher power. For example, there are power changes by the number of contact per unit time between the upper and lower electrodes based on the number of bumper spring. In conclusion, we present a rotary motion mechanism that can be converted into vertical motion and provide some output behaviors depending on control parameters such as input power and contact time.
References
[1] FR Fan, ZQ Tian, Z Lin Wang “Flexible triboelectric generator” Nano Energy (2012) Volume 1, Issue 2, March 2012, Pages 328-334
[2] G Zhu, C Pan, W Guo, CY Chen, Y Zhou, R Yu, Z.L. Wang * “Triboelectric-Generator-Driven Pulse Electrodeposition for Micropatterning”, Nano Lett., (2012), 12 (9), pp 4960-4965
[3] G Zhu, J Chen, T Zhang, Q Jing, ZL Wang “Radial-arrayed rotary electrification for high performance triboelectric generator”, Nature Communications 5, Article number: 3426, Published 04 March 2014
[4] C Zhang, T Zhou, W Tang, C Han, L Zhang, ZL Wang “Rotating-Disk-Based Direct-Current Triboelectric Nanogenerator” Advanced Energy Materials Volume 4, Issue 9, June 24, 2014

Development of flexible pressure sensors that can detect the environment through touch is of profound interest for various applications such as artificial skin, wearable electronic sensors, robotics, etc. Typically, these sensors are fabricated by integrating pressure-sensitive component with an active-matrix backplane with organic or inorganic transistors on the flexible substrate.^1,2 Thus, it complicates the fabrication process and increases overall fabrication cost. Recently, the tactile and flexible pressure sensor has been demonstrated using ZnO NW.³ Using the piezoelectric polarization effect, the backplane was simplified as two-terminal transistor sensor array, greatly simplifying the fabrication process and enhancing sensor performance.
In this work, we present the performance of flexible pressure sensor realized by exploiting piezoelectric materials and semiconducting carbon nanotubes (s-CNTs). The two-terminal resistor array was first prepared by forming s-CNT network matrix on a flexible substrate. Next, the pressure-sensing layer was subsequently integrated over the CNT array, where the pressure sensitive layer consists of the composite with clustered BaTiO₃ naoparticles (NPs) and PDMS. When the force is applied on the pressure sensitive layer, piezoelectric polarization is induced and it directly modulates the resistance CNT network without applying any external voltage to the s-CNT channel. Thus, our method also simplifies overall fabrication processes, as it requires only two terminal device configurations. Besides, the CNT array provides advantageous for realizing ultrathin and flexible sensor array. Consequently, our flexible sensor may have potential applications in artificial skin, wearable electronics, and nanoelectromechanical systems.
Reference
¹. T. Sekitani et al., Science 326, 1516-1519 (2009)
². K. Takei et al., Nature Mater. 9, 821-8216 (2010)
³. W. Wu et al., Science 340. 952-957 (2013)

Triboelectric generators offer huge potential in scavenging energy out of mundane but plentiful mechanical motions. Much progress has been made in improving their power output and a number of potential applications have been proposed in the literature.
The operation of triboelectric generators relies on repeated contact and separation between materials. Thus some of these devices can be modelled as spring-mass systems. Simulating vibration behaviors of such a system, under motion, within constraints can give insight into design parameters. This can be further used to optimize the design and improve the overall performance of the system.
In our presentation we will introduce how a spring mass analogy can be made for triboelectric generators. We will provide simulation results using MATLAB and Simulink for selected example systems with specific constraints. Finally, we will show how this analysis can help in the design and optimization process while building a real triboelectric generator device.

Piezoelectric ceramics have been widely used in sensors, actuators and ultrasonic transducers due to their ability to achieve efficient conversion between electric and mechanical vibrations. There is an urgent desire to move to lead-free materials achieving comparable piezoelectric performance to lead-based materials. The pseudobinary system xBa_0.7Ca_0.3TiO₃-(1-x)BaTi_0.8Zr_0.2O₃ (BCZT) has been reported to achieve a piezoelectric charge coefficient (d₃₃) of 620 pC/N when x=0.5 due to the presence of a morphotropic phase boundary ¹. BCZT can be synthesised by combining Ba_0.7Ca_0.3TiO₃ (BCT) and BaTi_0.8Zr_0.2O₃ (BZT) in the solid state, requiring substitution of Ca and Zr on the Ba-site and Ti-site respectively, and where the rate of Ca substitution has been reported to be a limitation ². Therefore, a detailed understanding of the Ca-Ba substitution mechanism is necessary to enable a fundamental structural study into the synthesis and properties of BCZT. An intermediate Ba₂TiO₄ phase has been observed in the formation of BaTiO₃ from the reaction of BaCO₃ and TiO₂³. However, studies into the formation mechanism of CaTiO₃ from CaCO₃ and TiO₂ have shown that CaTiO₃ is formed once the carbonate has decomposed ⁴.
The aim of this work is to understand the Ca-Ba substitution mechanism by studying the reaction between BaTiO₃ and CaTiO₃ using DSC-TGA, complimented by in situ XRD to investigate the synthesis mechanisms and Raman spectroscopy to probe the diffusion of Ca-Ba between CaTiO₃ and BaTiO₃.
The reaction between 0.7BaCO₃, 0.3CaCO₃ and TiO₂ reveals two weight losses at 5750C and 7200C attributed to the decomposition of CaCO₃ (CT) and BaCO₃ (BT). There is an endothermic peak due to the Ba₂TiO₄ formation at 8250C and a broad exothermic peak from the formation of BaTiO₃ and CaTiO₃ is also observed at10000C. The XRD patterns up to 8000C only show the formation of CaO, Ba₂TiO₄ and BaTiO₃. The absence of CaTiO₃ in the diffraction patterns suggests that BCT may be formed directly, rather than via the formation of CT and BT and a subsequent homogenation process.
A diffusion couple of BaTiO₃-CaTiO₃ was prepared for testing, using Raman spectroscopy. Before sintering, Raman mapping shows two descrete BaTiO₃ and CaTiO₃ phases. After sintering at 15000C for 4h, a 50mu;m-region of interdiffusion was observed between BaTiO₃ and CaTiO₃, suggesting a Ba-Ca gradient through the interface of diffusion couple. The implications of these findings for the formation of phase pure BCZT will be discussed.
1. W. Liu and X. Ren, Phys. Rev. Lett. 103 (25), 257602 (2009).
2. V.S. Puli, D.K. Pradhan, B.C. Riggs, D.B. Chrisey, R.S. Katiyar, J. Alloy. Compd. 584, 369-373 (2014).
3. K. Tsuzuku and M. Couzi, J. Mater. Sci. 47, 4481-4487 (2012).
4. V. Berbenni and A. Marini, J. Mater. Res. 39, 5279-5282 (2004).

Electrospinning process has been widely used in various ways due to its simplicity and versatility. Generally, in the process of fiber fabrication using electrospinning, the nanofibers are randomly distributed on the plat collector. However, we demonstrated that polymer nanofibers could be highly aligned by changing the shape of the plat collector and successfully explained the responsible alignment mechanism using FDTD simulation. Among the various electrospun materials, polyvinylidenefluoride (PVDF) has been investigated because of its unique properties such as piezoelectric, pyroelectric and ferroelectric activities. We have electrospun PVDF nano-fibers which have high β-phase fraction and diameters about 200~ 600nm and fabricated devices based on well-aligned electrospun PVDF NF by step inducing alignment method. Piezoelectric energy harvesting devices based on well-aligned electrospun PVDF nanofibers have remarkable performance compared with those based on randomly distributed electrospun PVDF nano-fibers.

Lots of efforts, recently, have been focused on developing new structures with the purpose of harvesting mechanical energy based on triboelectric effect produced by friction between two kinds of materials. However, movable machine elements, as the devices which usually waste energy by friction, have not been considered. In fact, a part of such wasted energy can be recovered by the same mechanical components providing that ad hoc modification is applied on the structure of those elements. This approach is followed in this work by selecting and improving the structure of three versatile machine elements as bearing, gears and belt-pulley systems. A real application of this kind of smart element is illustrated by developing a self-sustainable encoder, which is able to detect and display the rotation speed of the pulley.
We demonstrate our concept by the development of a bearing made of Teflon rollers and parallel aluminium electrodes connected to the inner side of the outer ring of the bearing which is made of Plexiglas. Two gears made of Teflon and Plexiglas were used to show the smart gear system mechanism, where the latter teeth were covered by the aluminium electrodes separately. A belt-pulley system also is developed using polypropylene /Teflon tape as a belt and Plexiglas pulleys, where one of them is covered completely by aluminium and the other is covered partly by means of parallel aluminium electrodes. In all aforementioned devices, the frequent contact and release between those parts made of Teflon and Aluminium electrodes serve the power generation mechanism. They were studied by Comsol mutiphysics software, and validated by a set of IF laser/ photodetector diode. The generated power also in different external loads and the rotational speed have been investigated.
Furthermore, the signals to noise ratio of components were evaluated to confirm that such smart components can be used as reliable sensors. To this aim, a similar belt-pulley system was fabricated by substituting the belt with a commonly used material in industry. The effect of the number of electrodes on the generated power was investigated and a self-powered system is developed. Indeed, the generated power, stored in the capacitor, is used to run a micro controller and a LCD, which is able to show the rotational speed of the pulley every 2 seconds.

Developing of technique for digitalized motion tracking recognition and fast strain monitoring is an important research area in human-machine interacting, artificial skin and micro-electromechanical systems.^1,2 The potential of triboelectric sensor array (TESA) as tactile or tracking sensor has been extensively recognized in recent years due to its lower cost, self-powered ability, compared with traditional motion sensors which are usually based on optical, microwave, or acoustic sound, etc.^3-5 However, it still remains unsolved for high-speed recording with high spatial-resolution, high-sensitive and large pixel size.
Here, we report a single-electrode-based TESA that can accurately and fast record the strain distributions on the device and can also track and map two-dimensional trajectory of moving objects at first hand. The electrical responses of the TESA are obtained by the charge transfer between the electrodes and the ground, when the object moves on the top surface of a polydimethylsiloxane (PDMS) layer, based on the coupling of triboelectric effect and electrostatic induction. A large-scale, flexible and stable TESA with 1616 pixels was fabricated, utilized to statically and dynamically detect motion information with high-speed recording function. In addition, the moving object can be instead by any materials that are commonly found for daily usage, even by human hands, indicating that the device has a widespread potential applicability in tactile sensor and touchpad technology.
Reference:
1 Cao, Q. et al. Medium-scale carbon nanotube thin-film integrated circuits on flexible plastic substrates. Nature454, 495-U494, doi:10.1038/nature07110 (2008).
2 Sekitani, T. et al. Organic Nonvolatile Memory Transistors for Flexible Sensor Arrays. Science326, 1516-1519, doi:10.1126/science.1179963 (2009).
3 Lin, L. , Wang, Z.L.et al. Triboelectric Active Sensor Array for Self-Powered Static and Dynamic Pressure Detection and Tactile Imaging. ACS Nano7, 8266-8274, doi:10.1021/nn4037514 (2013).
4 Yi, F. , Wang, Z.L.et al. Self-Powered Trajectory, Velocity, and Acceleration Tracking of a Moving Object/Body using a Triboelectric Sensor. Advanced Functional Materials, n/a-n/a, doi:10.1002/adfm.201402703 (2014).
5 Zhu, G. , Wang, Z.L.et al. Self-Powered, Ultrasensitive, Flexible Tactile Sensors Based on Contact Electrification. Nano Letters14, 3208-3213, doi:10.1021/nl5005652 (2014).

Damage sensing was one of the important methods for evaluation of micro-cracking in such as, airplane, vehicle, vessel and building. Damage sensing paste was made as paint type and the optimum paste condition was evaluated using nanocomposites with shape and concentration of filler. Bisphenol A type epoxy was used as composites matrix. Curing agents were used as two types of hardener like as normal adhesive type amino hardner and construction adhesive type amide hardener. Two kinds of nanofillers were using CNT and graphene nanopowder. Dispersion condition of nanoparticles in epoxy matrix was compared with CNT/epoxy and graphene/epoxy using electrical resistance measurement and FE-SEM. Damage sensing property was investigated using uni-directional tensile test to evaluate electrical resistance signal. Results of electrical resistance measurement were compared and correlated with those of tensile elongation and strength. Durability of CNT nanocomposites paste was evaluated using dynamic fatigue test. Graphene nanopowder showed lower cohesive effect with each other than CNT case. At last, nanocomposites paste for damage sensing was investigated in fiber reinforced composites for the prediction of micro-crack and delamination. Interfacial adhesion and interface condition were analyzed using electrical resistance measurement of nanocomposites under compression loading. Damage sensing property of normal adhesive was better than the constructional adhesive, whereas mechanical durability of construction adhesive type was better. Acknowledgement: This work was supported financially by Basic Project from Korea Research Foundation (KRF, 2013R1A1A2058093), 2013-2016.

Flexible sensors from piezoelectric and ferroelectric polymers are lightweight, cost-effective, easy-to-fabricate, biocompatible, and versatile sensing. Thus, they are attractive for applications like electronic skin and health monitoring. Particularly, detection of our physiological signals such as pulse waves and body temperature can provide much useful information for disease diagnosis and medical treatment. Moreover, to help our body collect information from environment such as light, voice, or heat can assist physically handicapped person. Typically, such energy sensors are mostly based on piezoelectricity or pyroelectricity. As most promising electroactive materials, ferroelectric polymers are of interest for high sensitive and versatile detections. They can also be easily fabricated into miniaturized devices by various techniques including nanoimprint lithography.
Here we described thin-film transistors on flexible substrates with active semiconducting polymer layer and using ferroelectric polymer thin film as gate dielectrics. The sensitivity of the OTFTs was 0.7kPa^-1, which is much higher than most ferroelectric transistors and piezoelectric sensors, and their detection limits are comparable to other OFET. When attached directly on the wrist, the flexible sensors can detect pulse wave non-invasively and in situ, and thus are suitable for mobile health monitoring. High-resolution picture of one pulse wave was available for providing two most common parameters for arterial stiffness diagnosis. Moreover, different laryngeal muscle movements can be distinguished by the sensors, which might be usefully for those who are vocal cords damaged to recover their speech ability. We also fabricated ferroelectric polymer array with dots of well-defined pyramid-shaped structure by lithography to increase the sensitivity. This special structure can lead to the enhancement of flexoelectricity in ferroelectrics, which has a high efficiency in converting mechanical force to electricity than typical piezoelectricity.
To simulate the sense of vision, we designed a biological affinity artificial retina by coupling ferroelectric polymer with azo-compound containing liquid crystal polymer. The device can convert visual light into electric signal, which plays similar roles as photoreceptor cells. Through integrating the devices into arrays, they can clearly observe the movement of a light source.

We have extended the functionality of ion gels in order to demonstrate the use of electrical energy-harvesting triboelectric nanogenerators (TENG). In order to enhance the performance of the power output, we have employed a nanofiber-structured ion gel as a triboelectric material. A conventional electrospinning technique was used to fabricate a large-area (16 cm × 16 cm) ion gel nanofiber mat with a thickness ranging from 10 - 1000 mu;m. We have designed the TENG by using a bare polymer (poly(vinylidene fluoride-co-hexafluoropropylene), P(VDF-HFP)) and ion gel nanofibers. When compared to a device with spin-coated film-based TENG (3 V and 5 mu;A/cm²), the device with a P(VDF-HFP) nanofiber structure fabricated via electrospinning showed an improved power output (20 V, 19 mu;A/cm²). By doping ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)amide [EMIM][TFSI] into P(VDF-HFP), we could fabricate ion gel nanofibers, and the output voltage and current from the TENG increases to a high value of 45 V and 49 mu;A/cm², respectively, under the same cycled compressive force. When a compressive force is applied to the TENG with ion gels, positive and negative charges are generated by the triboelectric effect and attract counterions in the ion gels, and electrical double layers with a large capacitance (> 10 mu;F/cm²) formed at the ion gel/electrode interfaces, resulting in an improvement in the power output. We also evaluated the device stability, and found that the ion gel nanofiber-based TENGs exhibited good stability under continuous operation over 10,000 cycles. We could also power 15-LEDs using the electrical power output of the TENG, without any other external energy source or rectifier. Overall, the results indicate that the nanofiber-structured ion gel provides a convenient route to incorporate an active layer in energy harvesting devices. Future work will focus on achieving fabric based energy devices and on developing practical power sources for electronic applications.

Lowering the overpotential required for driving oxygen evolution reaction (OER) is crucial for practically generating hydrogen from water splitting process. Besides developing effective and non-noble electrocatalysts, integrating the catalyst with silicon and III-IV semiconductors based photoanodes is a popular strategy to achieve this goal. However, they are always suffering from the poor stability and induce light limited anodic current, which is substantially smaller than the diffusion limited anodic current. Piezotronics have been frequently adopted to engineer the interfacial electronic band structure of heterojunctions to enhance charge generation, separation and transportation. Here, we propose to use the piezoelectric potential generated by sputtering ZnO film to decrease the external overpotential required for the OER with e-beam evaporated nickel film as the electrocatalyst. The piezo-induced positive charge on the adjacent surface of nickel film can presumably lower the electron energy level of Ni/NiO_x film and thus facilitate the oxidizing reaction. It has been found that piezoelectric potential could also enhance charge delivering between ZnO film and ITO. Significantly larger current density and long term stability are expected by replacing ZnO with other ceramic piezomaterials like PMNPT or PZT. This approach could render a new pathway for efficient and practical water splitting.

The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics. With their high crystallinity and ability to withstand enormous strain, two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer MoS₂ is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers.
Here we report the first experimental study of the piezoelectric properties of two-dimensional MoS₂ and show that cyclic stretching and releasing of thin MoS₂ flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW m^-2 and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer MoS₂, but not in bilayer and bulk MoS₂. The coupling between piezoelectricity and semiconducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.
References
Wu WZ*, Wang L*, Li YL, Zhang F, Lin L, Niu SM, Chenet D, Zhang X, Hao YF, Heinz TF, Hone J, Wang ZL, 2014. Piezoelectricity of single-atomic-layer MoS₂ for energy conversion and piezotronics Nature DOI: 10.1038/nature13792

Rod-shaped semiconductor nanowires provide today highly promising materials for applications in areas like energy scavenging as well as in energy conservation. The ability to form each nanowire into a designed three-dimensional device structure, and ways in which such units can be combined into ideal arrays thus forming a complete device, lends great promise for many areas of applications. The ability to control the crystalline structure in-between the cubic zincblende and the hexagonal wurtzite structures lends another handle to optimize performance, and may sometime also be an issue in the ability to control things. In this talk I will concentrate on two such examples: (a) one in which III-V NWs, like InP or GaAs and ternaries based on these, can form the basis for a highly efficient photovoltaic technology, one that can easily constitute an add-on to standard silicon solar cells, and (b) one based on the GaN family of hexagonal materials, in which one aims to master and control the effect of piezo-electric effects in axial and radial LED device structures. I will conclude by discussing how one may take advantage of the built-in piezo-electric fields in specially designed tunnel device structures to increase the interband tunneling probability.

As a clean, ubiquitous and sustainable energy source, acoustic waves are one of the wasted energies that are abundant in our daily life. Sound energy is usually taken as unwanted noise that is polluting our living environment. Acoustic energy harvesting was not as popular as other types of energy harvesting, such as solar energy and thermal electric energy, possibly because not only sound waves having much lower power density but also the lacking of effective technology for harvesting such energy. Harvesting acoustic energy has been demonstrated using approaches based on piezoelectric effect and electrostatic effect, but their performances are limited by low energy conversion efficiency, low power density, structure complexity and requirement of high-quality materials. Therefore, harvesting sound wave energy remains as a challenge.
In this work, we developed a new principle in ambient acoustic energy harvesting by using of contact electrification, a ubiquitous phenomenon in daily life. Using a polytetrafluoroethylene thin film and a holey aluminum film electrode under carefully designed straining condition, the organic film nanogenerator is capable of converting acoustic energy into electric energy via triboelectric transduction. With an acoustic sensitivity of 9.54 V Pa^-1 in a pressure range from 70 dB to 110 dB and a directivity angle of 52⁰, the nanogenerator produced a maximum electric power density of 60.2 mW m^-2, which directly lighted up 17 commercial light emitting diodes. Furthermore, the nanogenerator can also act as a self-powered active sensor for automatically detecting the location of an acoustic source with an error less than 7 cm. In addition, an array of devices with varying resonance frequencies was employed to widen the overall bandwidth from 10 Hz to 1700 Hz, so that the nanogenerator was used as a superior self-powered microphone for sound recording. Our approach presents an adaptable, mobile, and cost-effective technology for harvesting acoustic energy from ambient environment, with applications in infrastructure monitoring, sensor networks, military surveillance and environmental noise reduction.
References: (* indicate co-first author).
G. Zhu, C. F. Pan, W. X. Guo, C. Y. Chen, Y. S. Zhou, R. M. Yu and Z. L. Wang .Nano Lett. 12 (2012) 4960-4965.
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.
J. Yang*, J. Chen*, Y. Liu, W. Yang, Y. Su and Z. L. Wang. ACS Nano, 8 (2014), 2649-2657.
G. Zhu*, J. Chen*, T. Zhang, Q. Jing and Z. L. Wang. Nat. Commun.5 (2014), 3426.

We used a phase field model of ferroelectric/ferroelastic switching in monocrystalline barium titanate to devise configurations of a nanodot on a substrate capable of harvesting energy through reversible switching effects. The device converts incoming mechanical energy manifested as substrate strain into electrical energy. The major advantage of the proposed concept is that the polarisation switching currents generated are much greater than charge flows due to the piezoelectric effect alone. This allows greater energy density to be achieved. However, a problem with using ferroelastic/ferroelectric switching effects for energy harvesting is that, unlike piezoelectric effects, the process is not readily reversible. We addressed this by using the substrate to provide electrical and mechanical bias towards a preferred polarization state, while deformation of the substrate, for example by bending, stabilizes a different polarization state. Models indicate that an electromechanical hysteresis suitable for energy harvesting can then be achieved, providing a conceptual design for a nanogenerator. The concept of further stabilizing the switching cycle by engineering the domain state is explored. This is achieved by using a shaped nanodot that traps domain walls in key positions, so that the switching cycle repeats reliably. Simulations indicate that such designs could be better able to cope with overloads or variation in duty cycle. We discuss methods for manufacture and scaling-up to produce practically useful electrical energy, which is ongoing research.
One issue that arises associated with the designs presented is the effect of surface energy on ferroelectric properties at the scale envisaged. To explore this we modelled generic configurations of ferroelectric nanodot/nanowire that are expected to form minimum energy flux closure states, monodomains, or non-ferroelectric states, depending on size and surface conditions. Using the phase-field approach, we show that surface energy can be expected to have a significant or even dominant effect on the nanodots. The domain structure or ferroelectric state is mapped out as a function of surface energy and nanodot size. The results are consistent with and help to explain experimental observations of phase, tetragonality and domain pattern in barium titanate nanowires and nanodots. Specifically, for small size (less than about 30nm) and high surface energy, non-ferroelectric states are prevalent, while for larger sizes complex multi-domain patterns become the most stable state. Only at specific combinations of size and surface energy do the vortex states, much desired for favourable device properties, become energetically favourable. By taking surface energy into account, we propose conceptual designs for nano-scale energy harvesting devices which are expected to have optimum stability and functionality.

A piezoelectric can create enough pressure to drive a piezoresistive film from an insulating to a conductive state. If we combine these components into a monolithic device, the result is a PiezoElectronic Transistor (PET). This talk will focus on the experimental realization of the PET, describing both progress and challenges. I will discuss the high-performance materials needed to achieve optimum device results, as well as strategies for integrating these incompatible substances. For a piezoresistive element, we have used a rare-earth monochalocogenide, samarium selenide. In thin film form, this gives a large dynamic range with pressures of a few GPa (1). Lead titanates were the basis for the piezoelectric films. Initial results have been obtained using commercial PZT films. Capabilities for more advanced piezoelectrics are underway, with a recently developed pathway to wafer-scale fabrication of PMN-PT by chemical solution deposition (2). Of course, combining an oxide piezoelectric with a rare-earth piezoresistor in one device creates an additional level of difficulty. This talk will also report electrical results for preliminary devices.
1) M. Copel et al, Nano Letters 13 (10), 4650-4653 (2013).
2) R. Keech et al, J. Appl. Phys. 115, 234106 (2014).

Piezoelectric nanowire based nanogenerator is a promising technology to harvest ambient mechanical energy. It is essential to experimentally quantify the strain-piezopotential relationship on NWs for the development of high-output nanogenerators. In this paper, 3D Kelvin probe microscopy (3DKPM) is applied to precisely mapping the piezopotential along a bent ZnO microwire (MW). In order to remove the charge screening effect and recover the actual piezopotential generated by the MW, an external DC bias was applied along the axial direction of the bent MW. This external field drives charged species in and outside of the MW to the two oppositely-biased ends, respectively, and thus minimizes the screening effect. We also developed a numerical method to calculate the strain distribution along the bent ZnO wire based on its scanning electron microscopy (SEM) image, with which the strain-piezopotential relationship is obtained. The overall theoretical and experimental relationships showed a good match, indicating 3DKPM under biased condition can be an effective approach for quantifying piezopotential from strained nanomaterials. The detected piezopotential is independent of screening charge and external screening effect, and is not affected by the sharp topography variation along the edge of wires. It could serve as an important methodology for revealing nanoscale piezoelectric and flexoelectric properties.

With the advancement of MEMS technology employing PZT thin films for micro actuator, energy harvesters, sensors, and piezo-electronic applications a quest for the best possible film is going on. Each operation mode will have its own optimal film. Sensors and energy harvesters need a film with high remanence, and a relatively low dielectric constant, whereas actuators such as for inkjet printing need large forces generated in the plane of the film as the result of large applied electric fields. Remanence is not such an issue. Both rely on the transverse e_31,f coefficient. Recently another application came up: nano actuators for electronic applications, counting on the longitudinal thickness mode effect. Very tiny (10-20 nm) cube-like elements should generate huge stresses by very large electric fields. So it is clear that in many cases the performance limit is given by the product of piezoelectric coefficient times the break-down field.
In this talk, the case of flexural actuators with piezoelectric films in parallel plate geometry is presented in more details. These are governed by the maximal in-plane film stress that can be generated: T₁(max)= - e_31,fE₃(max). This coefficient is negative, and thus will lead to tensile piezoelectric stresses. It was found that the break-down was related to cracking of the film. This is rather obvious for a piezoelectric thin film, because the result of the electric field is a tensile stress. The total stress is composed of three contributions: a residual one originating from the process conditions and microstructure, a “ferroelectric” stress at zero electric field originating from the domain pattern after eventual poling, also including the piezoelectric effect of an internal electric field, and finally the stress directly caused by applying an electric field. In piezoelectric films, a large voltage sweep is at the same time also a crack propagation test. The stress response is reduced as soon as cracking occurs. Furthermore one might also consider that there is a dynamic effect, i.e. that failure occurs after a critical number of stress cycles. The latter effect would also eventually include adhesion failures, and other issues related to the stability of the electrode interfaces. We determined film stresses based on optically measured excursions of beams having full wafer thickness and coated with a PZT parallel plate structure. We shall compare different films processed by sputter deposition and by sol-gel deposition. The latter already have a residual tensile stress resulting from the shrinkage during crystallization. We shall discuss the impact of fracture to the large signal response and the fatigue of films during unipolar cycling. The observed stress values leading to crack propagation are well compatible with known toughness coefficients of bulk ceramics. It is attempted to make predictions for downscaling of the thickness.

Recently, a new concept about built-in electric field (internal field) have been used for enhancement of photo-induced carrier separation in semiconductor during photo-electric conversion to realize high performance photocatalysis or solar cells. However, in most cases, the carrier separation could be terminated because of the neutralization of built-in electric field by accumulation of separated photo-induced carriers at the poles of the built-in field with photocatalysis process continuing. Therefore, it is the urgent to develop a practical approach to renew the built-in electric field momently to keep the high efficient separation of photo-induced carriers during photocatalysis process. In our approach, instead of using a static mechanical stress that creates a DC field, the built-in field in Ag2O-BaTiO3 hybrid nanocubes can be altered by applying a periodic stress on the BaTiO3 nanocube as provided by the sonic wave, so that the piezoelectric charges will not be fully screened due to the varying piezoelectric field in responding to the excitation of the sonic wave. As a result, the Ag2O-BaTiO3 hybrid nanocubes can remain active for photodegradation as long as both light and ultrasonic irradiation are supplied. And the complete Rh B or MO degradation time for Ag2O-PbTiO3 hybrid photocatalysts under UV and visible light irradiation with ultrasonic irradiation decrease 1/3 to 1/2 compared with that without ultrasonic irradiation. This methodology opened a new door for design and manufacture of high performance photocatalysis system. We are confident that this paper will attract much attention of materials scientists, inorganic chemists, applied physicians, and scientists and engineers in environmental and energy science and engineering.

Emulation of human senses via electronic means has long been a grand challenge in research of artificial intelligence as well as prosthetics, and is of pivotal importance for developing intelligently accessible and natural interfaces between human/environment and machine. Unlike other senses (seeing, hearing, smelling and tasting), capability of skin for touch sensing remains stubbornly difficult to be mimicked, which necessitates the development of large-scale pressure sensor arrays with high spatial-resolution, high-sensitivity and fast response. In this talk, we present a novel design of nanowire LED arrays, which can be used to directly record the strain distribution by piezo-phototronic effect. This work is published on Nature Photonics.¹
In our previous work, we have demonstrated how the piezo-phototronic effect can be effectively utilized to enhance the emission intensity of an n-ZnO/p-GaN NW LED.^{2, 3} The emission light intensity and injection current at a fixed applied voltage has been enhanced by a factor of 17 and 4 after applying a 0.093% compressive strain, respectively. Here, we extend the single NW device to NW LEDs array, for pressure/force sensor arrays for mapping strain with a resolution as high as 2.7 mu;m. Such sensors are capable of recording spatial profiles of pressure distribution, and the tactile pixel area density of our device array is 6250000/cm², which is much higher than the number of tactile sensors in recent reports (~ 6-27/cm²) and mechanoreceptors embedded in the human fingertip skins (~ 240/cm²).
When the device is under pressure, the images unambiguously show that the change in LED intensity occurred apparently at the pixels that were being compressed by the molded pattern, while those were off the molded characters showed almost no change in LED intensity. Instead of using the cross-bar electrodes for sequential data output, the pressure image is read out in parallel for all of the pixels at a response and recovery time-resolution of 90 ms. Furthermore, our recent studies achieve such piezo-phototronic effect induced strain mapping in a flexible n-ZnO NWs/p-polymer LEDs array system. This may be a major step toward digital imaging of mechanical signals by optical means, with potential applications in touch pad technology, personalized signatures, bio-imaging and optical MEMS.
Reference:
1. Pan, C.F. et al. High-resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire led array. Nature Photonics7, 752-758 (2013).
2. Yang, Q. et al. Largely enhanced efficiency in zno nanowire/p-polymer hybridized inorganic/organic ultraviolet light-emitting diode by piezo-phototronic effect. Nano Lett.13, 607-613 (2013).
3. Yang, Q., Wang, W.H., Xu, S. & Wang, Z.L. Enhancing light emission of zno microwire-based diodes by piezo-phototronic effect. Nano Lett.11, 4012-4017 (2011).

When fabricating nano structures and devices using ZnO nanowires (NWs), strain is unavoidable and often modifies the NW&’s electronic, optical and mechanical properties. Previous studies have revealed that strain can induce a significant redshift and broadening of the near-band-edge (NBE) emission in the cathodoluminescence and the photoluminescence (PL) spectra of a bent ZnO NW. Based on the study of near-field scanning optical microscopy at low temperatures, we find that the net redshift of the free exciton emission in a bent ZnO NW is mainly caused by strain-induced changes in the energy band structure together with the piezotronic effects, which modifies the spatial distribution of photoexcited carriers. We show that the strain in bent ZnO NWs can also induce phonon energy variations besides the change in the bandgap and the piezotronic effects.
Due to the piezotronic effects in bent ZnO NWs, the electromechanical properties of ZnO NWs could be greatly modified. For example, ZnO NW cantilevers in a single-clamed configuration can be driven by an AC or DC potential and work in the self-oscillating regime. Self-oscillations are highly required in nanoelectromechanical systems (NEMS), because it provides an effective way to realize truly nanoscale integrated systems by removing the external components. We demonstrate that sustained self-oscillations can be achieved in singe-clamped ZnO NWs during field emission by simply applying a DC bias voltage. The field emission of ZnO NWs is significantly enhanced during the self-oscillations due to piezoelectric effects in bent ZnO NWs. The enhanced field emission provides an internal positive feedback, resulting in the increase of oscillation amplitudes. The amplifying self-oscillations offer a new route for the development of self-excited and integratable signal generators in NEMS. (This work is supported by Research Grants Council of Hong Kong. Project Nos. PD13SC02/GMGS14SC02, 604112/N_HKUST613/12).

ZnO based nanowires and thin films are widely utilized in piezotronics, piezo-phototronics and nanogenerators. The performance of piezotronics and piezo-phototronic strongly depends on the magnitude of piezopotential. By alloying Mg into ZnO, Mg_xZn_1-xO of wider band gaps has been regarded as a good candidate for solar-blind UV sensors. In this study, high-performance self-powered ZnO-based photodetectors are developed by enhancing piezopotential through alloying with Mg. A series of n-Mg_xZn_1-xO thin films (from x = 0 to 0.2) with strong c-axis preferred orientation in the wurtzite phase are grown on p-type Si substrate by magnetron co-sputtering. The Mg content is determined by energy dispersive spectroscopy (EDS) analysis in conjunction with photoluminescence (PL) where the results confirm the increasing trend on band gap with increasing Mg. The piezotronics and piezo-phototronics properties of photodetectors made of selective preferentially orientated Mg_xZn_1-xO thin films are studied by investigating the coupling of electrical and optical characteristics on strain, which reveals different piezotronic phenomena with different Mg contents.
The Mg_xZn_1-xO thin films are of wurtzite structure with highly preferred 0002 orientation and the photodetectors exhibit unique characteristics of self-power, ultrafast response, and superior stability over time. The performance of the self-powered photodetectors enhances with Mg content due to the increase of the piezoelectric coefficient by the alloying process, and thus an enhanced piezopotential. An enhancement of up to 100% in the output current and voltage can be achieved through the piezo-phototronic effect. More notably, the sensitivity of the Mg_0.20Zn_0.80O self-powered photodetector is more than 6-fold higher than that of the Mg_0.05Zn_0.95O photodetector as a result of alloying with more Mg. The higher sensitivity with the higher Mg content in the MgZnO compound is attributed to the increased piezoelectric coefficient.
The results demonstrate that the piezoelectric coefficient of Mg_xZn_1-xO increases with Mg content, resulting in an enhancement of the effective piezopotential, which is responsible for the higher sensitivity of self-powered photodetectors. The developed material is thus a promising candidate for ultrahigh sensitive photodetectors.

Piezoelectrics with their permanent polarization and its change with temperature can be used to enhance optoelectronic properties of semiconductor devices. Here we present several exemplary devices to illustrate the benefits of piezoelectric substrates. The first example is an optothermal field effect transistor (FET) based on the pyroelectric effect of the piezoelectric substrates. The device is a graphene-lead zirconate titanate (PZT) system utilizing the high optical transparency and conductance of graphene.¹ Under the incidence of an infrared (IR) laser beam, the drain current can be increased or decreased depending on the direction of the polarization of the PZT substrate. The drain current sensitivity of the optothermal FET can reach up to 360 nA/mW at a drain field of 6.7 kV/m more than 5 orders of magnitude higher than that of the photogating transistors based on carbon nanotube on SiO2/Si substrate. A similar device using single zinc oxide nanowire and PZT (ZnO NW-PZT) was also demonstrated.²
Recently, we combined the transparent and conductive properties of graphene with the optical and photovoltaic properties of Poly(3-hexylthiophene) (P3HT) as a hybrid composite.³ Based on the inherent nature of the band alignment between graphene and P3HT, the photogenerated holes are able to transfer to the graphene layer and improve the photoresponse. When the graphene was deposited on a piezoelectric Pb(Zr_0.2Ti_0.8)O₃ (PZT) substrate, the photoresponse of such composite photodetectors was found to be ten times larger than on SiO₂ substrate. It was demonstrated that the electric field of the polarization of piezoelectric substrate helped the spatial separation of photogenerated electrons and holes and promoted the hole doping of graphene to enhance the photoconduction. Moreover, with the replacement of P3HT by a thin layer of bulk heterojunction of polymer and fullerene, the photosensitivity can be further increased by more than one order of magnitude.
More recently, we replaced the P3HT with graphene quantum dots (GQDs). Preliminary results showed that the photoresponse of the GQD-graphene composite on PZT was even higher than that of P3HT-graphene system. More updated results will also be presented.
References
C.-Y. Hsieh, Y.-T. Chen, W.-J. Tan, Y.-F. Chen, W. Y. Shih, and W.-H. Shih, “Graphene-PZT Optothermal Field Effect Transistors,” Appl. Phys. Lett., 100, 113507 (2012)
C.-Y. Hsieh, M.-L. Lu, J.-Y. Chen, Y.-T. Chen, Y.-F. Chen, W. Shih, and W.-H. Shih, "Single ZnO nanowire-PZT Optothermal Field Effect Transistors," Nanotechnology, 23, 355201 (2012)
W.-C. Tan, W.-H. Shih, and Y.-F. Chen, “Highly Sensitive Graphene-Organic Hybrid Photodetector with Piezoelectric Substrate,” Advanced Functional Materials, DOI: 10.1002/adfm.201401421

Energy losses through the coupling between photodetector (PD) and optical fiber based communicating system have been an inevitable obstacle to improving the efficiency, lowering the costs and enhancing the quality of information communications. Newly designed PDs that can be coupled with standard optical fibers in energy-efficient manners may therefore be necessary. Here we introduce an optical fiber (OF)-based, dual-mode nanowire PD to implement zero-loss integration with optical communication system without utilizing couplers via the well-developed fiber welding technology. ZnO/CdS nanowires (NWs) heterojunctions are synthesized coaxially around an OF to fulfill ultraviolet and visible light detections. The PD works in two modes: axial and off-axial illumination modes. Utilizing strain-induced piezoelectric polarization charges at ZnO/CdS heterojunction interface to gate/modulate electronic transport and optoelectronic processes of local carriers, the piezo-phototronic effect¹ has been employed to enhance/optimize the performances of these PDs by up to 718% in sensitivity and 2067% in photoresponsivity. Upon straining, the response time for UV detections are improved by 3% and 74% at rise and fall edges, respectively. The best response time under visible illuminations are obtained as rise time of 20 ms and fall time of 43 ms. The OF-NWs hybridized structure can be extended to other material combinations of infrared range and applications in energy-economical information communication, damage-free mapping of electromagnetic field distribution in isolated systems and bio-compatible optoelectronic probes.
References:
1. Wang, Z. L. Progress in Piezotronics and Piezo-Phototronics. Adv Mater. 2012, 24, (34), 4632-4646.

Nanomaterials majorly fall into three categories according to the nano-confinment dimension: two dimensional (2D), one dimensional (1D), zero dimensional (0D). Among them, 1D and 0D nanomaterials attract most research interest because the multi nano-confinment effects enable novel properties.
Up to present, all the studies on 1D and 0D nanomaterials are limited in their own dimensions. There is no research about the vast blank area that connects 1D and 0D nanomaterials. When reducing the dimension of 1D nanomaterial, previously not within nanometer scale, the property will undergo significantly changes. The research field that covers the intermediate scale between 1D and 0D nanomaterials will attract much more research interest and cover most of the materials.
Here, we named the intermediate nanoscale materials between 1D and 0D Half-Dimensional (0.5D) nanomaterials and we systematically investigate the photoelectric property change of ZnO in that dimension and found the photoelectric property does not follow the Ohm&’s Law. [i] We build a theoretical model based on semi-classical physics and well explained this unique phenomena. This is the first time that 0.5D nanomaterial concept is defined and the first preliminary result has ever been reported. The research in this paper initiates a brand new research field, which covers all properties for 0.5 nanomaterials and is applied to all materials, such as semiconducting materials, carbon nanotube, grapheme, etc.
[i] "Significant Photoelectric Property Change Caused by Additional Nano-confinement: A Study of Half-Dimensional Nanomaterials" Chengming Jiang and Jinhui Song Small, DOI: 10.1002/smll.201400704.

Recently, the invention of a triboelectric nanogenerator (TENG) has provided an effective approach to convert ambient mechanical energy into electricity. The working principle of the TENG is based on the coupling of contact electrification and electrostatic induction. Contact-induced charge transfer between two materials with opposite tribopolarity results in a potential difference when they are separated. This potential difference is an inner electrical signal created by the external mechanical force, which could be used as a gate signal to tune/control the carrier transport characteristics in FET as the same effect as applying a gate voltage.
Utilizing the coupled metal oxide semiconductor field-effect transistor and triboelectric nanogenerator, we demonstrate an external force triggered/controlled contact electrification field-effect transistor (CE-FET), in which an electrostatic potential across the gate and source is created by a vertical contact electrification between the gate material and a “foreign” object, and the carrier transport between drain and source can be tuned/controlled by the contact-induced electrostatic potential instead of the traditional gate voltage. With the two contacted frictional layers vertically separated by 80 mu;m, the drain current is decreased from 13.4 to 1.9 mu;A in depletion mode and increased from 2.4 to 12.1 mu;A in enhancement mode at a drain voltage of 5 V. Compared with the piezotronic devices that are controlled by the strain-induced piezoelectric polarization charged at an interface/junction, the CE-FET has greatly expanded the sensing range and choices of materials in conjunction with semiconductors. The CE-FET is likely to have important applications in sensors, human silicon technology interfacing, MEMS, nanorobotics, and active flexible electronics.
The CE-FET is based on the MOSFET and contact electrification effect, which can be used as fundamental components in novel triboelectronic devices and systems. We call this tribotronics, a new field of research and applications in flexible electronics. Tribotronics is about the devices fabricated using the electrostatic potential created by triboelectrification as a “gate” voltage to tune/control charge carrier transport in the semiconductor. Tribotronics is a field by coupling the semiconductor with triboelectricity, which is an extension of piezotronics first proposed in 2007, and also a profound application of TENG first proposed in 2012. By the three-way coupling among triboelectricity, semiconductor, and photoexcitation, plenty of potentially important research fields are expected to be explored in the near future.

Harvesting energy from ambient mechanical and solar energy sources is highly desirable for powering portable electronics, biomedical, and healthcare applications Most recently, a new type of power generating device, named as triboelectric nanogenerator(TENG) based on triboelectric effects coupled with electrostatic effects have been demonstrated as powerful means of harvesting mechanical energy from living environment. Here, we can demonstrate significantly improved the output performance for organic solar cell (OSC) structure-based triboelectric nanogenerator. The fabricated TENG is based on the contact and separation between the TiO_x on the OSC and the polyimide film. In OSC structure under the light illumination, excitons are broken up, leading to free electron-hole pairs by effective fields. The excited electrons are injected into the conduction band of the TiO_x layer. Surface charge density was changed by the injected electron. By controlling the periodic contact and separation between a TiO_x layer on the OSC and the polyimide film. The output voltage and current density were increased by a factor of 2.0 and 1.8, respectively, with photon absorption of OSC, because of the additional charge transfer from TiO_x to polyimide film. To maximize the surface charge density in triboelectric nanogenerators, a new method of injecting electron into TiO_x surfaces is introduced for the generation of surface charges. This study demonstrates the possibility of improving the electrical output of TENG through Photon management.

Hydroelectric power is the most important and wildly-used renewable energy source in the environment. In this paper, we propose the concept of using a multi-layered triboelectric nanogenerator (TENG) to effectively harvest the water wave and water drop energy. For a single-layered TENG, interdigital electrodes are incorporated in order to generate multiple electric outputs under one water wave or water drop impact. For the collection of water wave energy, a polyurethane (PU) coated copper rod is used to roll and contact with the polytetrafluoroethylene (PTFE) film covered interdigital electrodes. The surface of the PU and PTFE film are fabricated as porous structures and nanowire arrays, which provide the advantage of large contact area. Under one water wave impact, the single-layered TENG composed of 9 pairs of interdigital electrodes can provide 9 pulses of electric outputs (voltage can reach 52V). The output current density and instantaneous output power density of a 5-layered TENG are 15.3 mA/m2 and 1.5 W/m2, respectively. The rectified electric outputs have been applied to drive light emitting diodes and charge commercial capacitors. In addition, the part of the polytetrafluoroethylene (PTFE) film covered interdigital electrodes has been successfully used to harvest water drop energy, whcih can also generate 9 pulses of electric outputs upon one water drop falling. All these results show the developed TENG has great potential to harvest the hydroelectric power of ocean wave and raindrop in the near future.
References:
1. Lin, Z.-H.; Cheng, G.; Lin, L.; Lee, S.; Wang, Z. L. Angew. Chem. Int. Ed. 2013, 52, 12545-12549.
2. Lin, Z.-H.; Cheng, G.; Lee, S.; Wang, Z. L. Adv. Mater. 2014, 26, 4690-4698.
3. Lin, Z.-H.; Cheng, G.; Wu, W.; Pradel, K. C.; Wang, Z. L. ACS Nano 2014, 8, 6440-6448.
4. Cheng, G.; Lin, Z.-H.; Du, Z.; Wang, Z. L. ACS Nano 2014, 8, 1932-1939.

There has been increasing demand for highly efficient portable energy harvesters due to the development and mass consumption of portable electronic devices. Photovoltaic, thermoelectric, piezoelectric and triboelectric energy scavengers have become strong candidates for portable energy harvesters in future self-powered portable electronic device applications.Among these, Triboelectric energy harvesters that can convert mechanical energy into electrical energy have recently attracted attention. Compared to time-constrained photovoltaic, piezoelectric energy harvesters are not limited in time and space because mechanical energy sources such as body movement, heartbeat, blood #64258;ow, wind, tide, low-frequency seismic vibrations, and sound from the surrounding environment are prevalent.
Textile based wearable electric power generating device that is capable to harvest mechanical energy from irregular available energy in the variable environments is promising for smart clothing and powering small scale electronic devices in our daily life. Herein, we demonstrate the fabrication of high performance textile based wearable triboelectric nanogenerators (WTNG) by using flexible and foldable silver (Ag) coated textile substrate and nanopatterned polydimethylsiloxane (PDMS). Vertically aligned ZnO nanowires (NWs) grown on textile substrate were used as template for PDMS nanopatterning.
A record high value of output voltage up to 175 V and current up to 118 µA were obtained, under compressive force of 15 kgf. We embedded WTNG with commercial suit and successfully operated light emitting diodes (LEDs), liquid crystal display (LCD) and a vehicle keyless entry system by the output power of multi-layered stacked WTNGs

Ferroelectric generators are used to generate large magnitude current pulse by impacting a polarized ferroelectric material like PZT (lead zirconate titanate) 95/5. The impact induced shock wave induces a ferroelectric to anti-ferroelectric phase transition in the material causing depolarization. At high impact speeds, the material undergoes breakdown leading to charges to appear inside the material. Depending on the loading conditions and the electromechanical boundary conditions, the current or voltage profiles obtained vary. The exact physics of this process is largely unknown.
In this paper, we explore the large deformation dynamic response of a ferroelectric material. Using the Maxwell&’s equations, conservation laws and the second law of thermodynamics, we derive the governing equations for the phase boundary propagation as well as the driving force acting on it. We allow for the phase boundary to contain surface charges which introduces the contribution of curvature of phase boundary in the governing equations and the driving force. This type of analysis accounts for the dielectric breakdown of the material and resulting conduction in the ferroelectric.
Next, we implement the equations derived to solve a one dimensional impact problem on a ferroelectric material under different electrical boundary conditions. The constitutive law is chosen to be piecewise quadratic in polarization and quadratic in the strain. We adopt a shock capturing finite volume scheme to solve the phase boundary propagation problem. We solve for the current profile generated in short circuit case and for voltage profile in open circuited case.

Mechanical energy sources are available almost everywhere and at all the time. It can come from gentle airflow, ambient noise, vibration, human body activity, etc., which is super suitable to be utilized for the purpose of constructing a self-powered system. For the recently developed mechanical energy harvesting technology TENG, the basic structure is simple, which just includes two friction surfaces composed of different materials. When there is a friction between these two surfaces, due to the combination of triboelectric effect and electrostatic induction, electrons can be driven to flow in the external circuit. The first demonstrated TENG in 2012 has already shown very good output performance. Then very quick progress has been made, and now the output power density can reach as high as 50 mW/cm². But in most cases, the devices are designed for harvesting mechanical energy sources at normal or high level intensity. Here we will introduce devices specially designed for weak vibration energy harvesting and detection. When a mechanical structure is set at the critical state of a mechanical equilibrium, it will be unstable and very sensitive to the disturbance in the environment. Further, if such a structure can balance back to the equilibrium by itself when it is disturbed, it can be a superior choice for weak vibration energy harvesting and detection. TENGs constructed on 3D spiral structures and magnetic suspension structures are picked as two examples. Applications of TENGs including active sensor system for vibration positioning, and a self-powered logic circuits to operate mechanical inputs are demonstrated.

With improvements in microelectronics technology, portable electronics are indispensable in our daily life, which bring an increasing demand in power supply. At the same time, more and more feature-rich electronics largely increase their power consumption. To meet the energy need for portable electronics and sensor networks, harvesting mechanical energy from our surroundings is becoming a powerful approach. Recently, the triboelectric nanogenerator (TENG) has been demonstrated as an effective means for harvesting all kinds of mechanical energy, such as wind power, wave energy, and walk energy, which is likely to be a parallel important technology as the traditional generator for power generation at large scale. In different kinds of TENG, the contact mode and the sliding mode represent the two basic types of TENGs. Here, the siding mode, designed with micro-sized grating structure, offers a unique and straightforward solution in harvesting energy from the relative sliding between two surfaces. For example, a planar-structured TENG, composed of radial-arrayed gratings with a central angle of 3°, generated a high output power of 1.5 W (corresponding to the power density of 19 mW/cm² at the rotation speed of 3000 rpm) and the theoretical calculation revealed that the narrow gratings will be more effective for high output.
In this work, an industrialization technology of printed circuit board (PCB) was introduced to prepare TENG with composite disk structure. Two central angles of 3° and 1° for the gratings were integrated on a disk TENG to improve the space utilization and output power of the device. By means of PCB technology, TENG can be manufactured in quantity with high yield and lifetime. Operating at a rotation speed of 1000 rpm, the TENG generates a short-circuit current (I_SC) of ~5.3 mA and an output power of 25.7 W at a matched load of ~0.93 MW. Through a transformer, an open-circuit voltage (V_OC) of 5 V and a maximum I_SCof 45 mA were obtained. This work not only realized a high power output, it also paves the way for the large-scale production and application of TENGs.

Urbanization, major influx of people from rural areas to the cities, marks one of the most significant global trends in the 21st century.This drastically increased logistic burden demands smarter ways to manage complexity, increase efficiency, reduce expenses, and improve quality of life, or “smart city”. As the backbone of the implementation of the "smart city" concepts, wireless sensor networksdes have attracted increasing attention in the past decade due to the continuous development of intelligent cities.The conventional wireless sensor nodes are usually powered by an external power source such as a Li-ion battery. This has forced all sensors to exercise painful trade off between costly Li-ion battery replacements and data transmission duty cycle. Therefore, a perpetual power source, such as an energy harvester, is the main bottleneck preventing and the ideal solution for large area adoption of many “smart city” concepts. Wireless gas meter reading is an integral part of the “smart city” concept due to the prevalence of natural gas in modern metropolitan life as well as demand ventilation control system that is indispensable to people&’s daily life. Scavenging fluid mechanical energy in the environments like metering system and ventilation control system shows its promising features prior other energy scavenging solutions.
Here, we report a triboelectric generator (TEG) that can scavenge flow-driven mechanical energy for directly powering a wireless sensor node for the first time. The mechanism of TEG is based on the flow-driven vibration of a kapton film in an acrylic tube, which can induce the periodic contactparation between a triboelectric polytetrafluoroethylene (PTFE) film and an Al electrode on the kapton film. As a result, the triboelectric charges can be transferred between two electrodes, resulting in the flow of electrons in the external circuit as an alternating current. By systematically investigating the output of TEGs with different dimensions, the device with a size of 22 mm×10 mm×67 mm has the best output performance, where it delivers an open-circuit voltage up to 400 V, a short-circuit current of about 60 mu;A, and a corresponding output power of about 3.7 mW under an external load of 3 MOmega;, which can directly light up tens of commercial LEDs.By using a transformer and a power management circuit, the TEG can produce a continuous direct-current (DC) source with a constant voltage of about 1.8 V for directly and sustainably powering a wireless sensor node with an inductor signal frequency of about 436 MHz. This work is an important progress toward the practical applications of TEG for harvesting the flow-driven mechanical energy for directly powering wireless sensor nodes.
References and Notes
(1)Ya Yang, et al, ACS Nano, 2013, 7, 9461-9468.

Water-related energy is an inexhaustible and renewable energy resource in our environment, which has huge amount of energy and is not largely dictated by daytime and sunlight. The transparent characteristic plays a key role in practical applications for some devices designed for harvesting water-related energy. In this paper, a transparent triboelectric nanogenerator (T-TENG) was designed to harvest the electrostatic energy from flowing water for the first time. The output peak-to-peak open-circuit voltage and current density of the T-TENG could reach 10 V and 2 mu;A/cm², respectively, with the flow rate of the tap water of 93 ml/s. The instantaneous output power density of the T-TENG was 11.56 mW/m² when connecting to a load resistor of 0.5 MOmega;. Moreover, the transmittance of the as-prepared T-TENG was 87.41% with the organic film thickness of 1mu;m, larger than that of individual glass substrate of 83.41%, which was caused by the organic film acting as an antireflection coating. These results illustrated the potential applications of the T-TENG for harvesting wastewater energy in our living environment and on smart home system and smart car system.

The tremendous development of portable electronics makes it an urgent demand for sustainable energy sources. Recently, triboelectric nanogenerators (TENGs) have shown unique merits including large output power, high efficiency, and cost effective materials. Sliding-mode TENG structure based on in-plane charge separation are the most promising design, especially because grating structure can be integrated into it. Maximizing the energy output is always designers&’ only target. However, the inherent complexity in the physics of sliding-mode TENGs and their mismatch with other downstream energy storage and load components greatly limit their power output, which require thorough fundamental understanding and careful optimization of the TENG system. However, this system couples the complex effect of both electrostatics and circuit theory. There is neither previous theoretical understanding nor numerical models to deal with that.
To address this issue, we proposed the first governing equation and equivalent circuit model for TENGs, leading to the demonstration of the first TENG simulation tool, in which the coupling simulation of both electrostatic and circuit part is realized for the first time. Utilizing this tool, we clearly uncover the output characteristics of sliding-mode TENGs to maximize their power output.
Sliding-mode TENGs without grating structures are first studied to unveil their fundamental physics and verification of our method. We derive their first analytical model and resistive load characteristics. The “three-working-region” behavior is interpreted with the impedance match mechanism. A corresponding experiment is then performed as validation of theory, which shows good agreement with theoretical anticipation.[1]
Next, we move to grating TENGs with multiple sliding units. To optimize the energy output, we perform an in-depth discussion on the influence of electrode structure, thickness of the dielectric layers, and number of grating units. As for the electrode structure, grating electrodes always lead to a better performance than plate electrodes. As for the dielectric thickness, the thickness of the dielectric of the longer plate should be much smaller than that of the shorter plate. As for the most important parameters of grating TENGs—the number of grating units, our calculation clearly indicates that increasing the number of grating units will generally improve the output performance. However, when the pitch is very fine, the edge effect begins to dominate, resulting in degradation of performance when the number of units continues to increase. Thus, there exists an optimum number of grating units and an optimum unit aspect ratio that mainly depends on the materials dielectric constant and the motion type. The optimization strategy provided here can serve as guidance of experiments towards practical application.[2]
Reference
1. S. Niu, Z. L. Wang et al Adv. Mater. 25, 6184.
2. S. Niu, Z. L. Wang et al Energy Environ. Sci. 7, 2339.

Sound recording is a common and well-developed technology, which has been widely used and promoted our living standards. For example, recording the music played through a piano can benefit both the musician and the audience. However, traditional sound recording technology cannot avoid the noise sound in the environment, which will affect the quality of the recorded music.
To achieve noise-free recording, we developed r-shaped triboelectric generators (TEGs) which can be integrated into the keyboards. Each TEG contains a bended PET film, an Al layer as the top electrode and friction material, a PDMS layer as the bottom friction material, and Cu as the bottom electrode, which is fixed under the keyboard of a piano. By applying an external force on the piano keyboard, electric signal will be generated based on the combined effect of contact electrification and electrostatic induction.
Through analysis, we observe that the electric signal can reflect the movement of each key. In detail, when pressing a key, the corresponding TEG will produce an electric signal which can be used to determine the tone. Based on the working principle, pressing the key will cause a positive peak in the electric signal, and the start of the positive peak indicates the pressing time of a note. Meanwhile, the voltage value of the positive peak can reflect the intensity of the music. When releasing the key, charges will flow back in the external load producing a negative peak. Similarly, the start of the negative peak indicates the releasing time of a note. Therefore, the music information (frequency, amplitude, time) can be obtained from the electric signal. A software has been developed to convert the electric signal back into music information can be obtained from the electric signal, thus realizing noise-free recording.
As a demonstration, we played a music clip from the piano integrated with our TEG, and a multi-channel oscilloscope with 100 MOmega; probe was utilized to measure the electric signal. Each channel was connected to one generator to measure the movement of the corresponding key. Through pressing and releasing the key, electric signal was generated. Then, using the developed software, we have successfully converted the recorded electric signal back into music information, and the recorded music can be directly played from the software.
In summary, we integrated an r-shaped TEG into the keyboard of a piano for music recording. Based on triboelectrification effect, the device can function without external power supply. In the application of music recording, only the movement of the keyboard can cause the generator to produce electric signal. In other words, other noise sounds in the environment can be filtered automatically, which realizes the noise-free recording.

With the advantage of a high output voltage, triboelectric nanogenerator is very suitable for actuating capacitive devices. In this paper, a triboelectric nanogenerator (TENG) in planar sliding mode with dual-output voltages is proposed and firstly used for driving and controlling micromechanics. The TENG consists of a freestanding triboelectric-layer and two pairs of orthogonal electrodes. When the triboelectric-layer slides in plane, the dual-output voltages are proportional to the displacements in X and Y directions, respectively and independently. For the capacitive device actuated by the TENG, the driving voltage applied on the device is large if the load capacitance is small. Based on the TENG, an active piezoelectric micro-actuator with two orthogonal positioned piezoelectric bimorphs is developed for two-dimensional direction modulation. The piezoelectric bimorphs are actuated by the TENG and the movements of the light-spot in screen can be controlled and in proportion to the sliding of the triboelectric-layer in plane. Besides, an active electrostatic micro-actuator with two MEMS optical attenuators is also developed for double-channel power modulation. The electrostatic MEMS mirrors are actuated by the TENG and the double-channel attenuations can be controlled by the sliding of the triboelectric-layer in plane, respectively. This work presents the first active micro-actuators driven by the mechanical energy without an external power or mechanical joint, which has demonstrated the TENG&’s great capabilities and broad prospects in independent and sustainable self-powered MEMS/NEMS, and opened up a new application of the TENG as the triboelectric-voltage-controlled devices.

Wearable and self-powered devices have represented the emerging technology necessary to new life pattern of human. Textile structures are desirable for wearable device that are stretchable and flexible. In this article, a new textile-structured triboelectric nanogenerators are demonstrated for powering to the wearable devices. The tube-type triboelectric nanogenerator consists of PDMS tube, copper wire in center of device, and aluminum wrapped around the outer surface of tube, which are utilized to generate and harvest triboelectricity. We also fabricate the cable-type triboelectric nanogenerator, which is composed of several tube-type nanogenerator with series connection, resulting in 3 times higher output voltage than that of a tube-type nanogenerator. Additionally, textile-type triboelectric nanogenerator is fabricated with weaving from numerous tube-type triboelectric nanogenerator with parallel connection to generate high output current. A maximum output voltages and currents of 50 V and 250 µA for the textile-structured nanogenerator are obtained under the vertical stress of pushing tester. Moreover, the relationship between output performance and the amount of deformation at stretchable loading in the stretchable textile-type triboelectric nanogenerator is also investigated. This work should play an important role in new direction to wearable electronics.