Energy harvesting technologies converting external sources (such as thermal energy, vibration and mechanical energy from the nature sources of wind, waves or animal movements) into electrical energy is recently a highly demanding issue in the materials science community for making sustainable green environments. In particular, fabrication of usable nanogenerator attract the attention of many researchers because it can scavenge even the biomechanical energy inside the human body (such as heart beat, blood flow, muscle stretching, or eye blinking) by converging harvesting technology with implantable bio-devices.
Herein, we describe two separate procedures suitable for generating and printing a lead-free BaTiO₃ based thin film nanogenerator and nanocomposite generator on plastic substrates to overcome limitations appeared in conventional flexible ferroelectric devices. First, flexible thin film nanogenerator was fabricated by transferring the BaTiO₃ thin film from bulk substrates and its piezoelectric properties of ferroelectric devices were characterized. Second, we report the nanocomposite generator (NCG) for achieving a simple, low-cost, and large area fabrication based on BaTiO₃ nanoparticles (NPs) and graphitic carbons (CNT or RGO). From the results, we demonstrate the highly efficient and stable performance of new forms of nanogenerator and the integration of bio-eco-compatible ferroelectric materials may enable innovative opportunities for artificial skin and energy harvesting system. Finally, an all-solid-state bendable LIB is demonstrated on plastic substrate using a new universal transfer approach based on sacrificial mica substrates. This flexible LIB combining with nanogenerator will realize all flexible energy harvesting and storage system in the future.
References
[1] K. Park, S. Xu, Y. Liu, G. Hwang, S. Kang, Z. L. Wang, K. Lee, Nano Letters 10(12), 4939, (2010).
[2] K. Park, M. Lee, S. Mun, G. Hwang, J. Kim, D. Kim, Z. Wang, K. Lee, Adv. Mater, 24, 2999, (2012).
[3] S. Kim, H. Jeong, S. Kim, S. Choi, K. Lee, Nano Letters 11(12), 5438, (2011).
[4] M. Koo, K. Park, S. Lee, M. Suh, D. Jeon, J. Choi, K. Kang, K. Lee, Nano Letters 12(9), 4810, (2012).
[5] S. Lee, K. Park, C. Huh, M. Koo, H. Yoo, S. Kim, C. Ah, G. Sung, K. Lee, Nano Energy 1, 145, (2012)

Flexible nanogenerators play a vitally important role in harvesting tiny and wide frequency mechanical movements in the surrounding environment to power nanodevices. In this presentation, several recently developed nanogenerators and their applications will be reported. Firstly, we talk about a synthesis method of lead zirconate titanate (PZT) textiles in which nanowires are parallel with each other, and a procedure to make them into flexible and wearable nanogenerators.[1] The nanogenerator can generate 6 V output voltage and 45 nA output current, which are large enough to power a liquid crystal display (LCD) and a UV sensor. Secondly, we report a kind of two dimensional fiber-based nanogenerator and the method to increase its robustness greatly, which is beneficial to the wearable and portable energy generation.[2] Thirdly, we talk about a flexible nanogenerator with ultrahigh output voltage of 209 V and output current density of 23.5 mu;A/cm2, which are sufficiently powerful to instantaneously lighten a commercial light-emitting diode (LED) without the energy storage process.[3] Fourthly, a new magnetic force-driven contactless nanogenerator (CLNG), with no contact between nanogenerator and mechanical movement source, will be demonstrated. The CLNG can harvest the mechanical movement energy in a non-contact mode to generate electricity. Its output voltage and current can be as large as 3.2 V and 50 nA respectively, which are sufficient to power up a liquid crystal display (LCD). [4] After that, the CLNG is implanted into a rabbit to power a sensor and wireless signal tansmission device. Finally, the biocompatibility of nanogenerator based on high piezoelectic coefficient 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) nanowires will be studied.[5]
[1] W.W. Wu, S. Bai, M.M. Yuan, Y. Qin, Z.L. Wang, T. Jing, ACS Nano, 6 (2012) 6231-6235.
[2] Two Dimensional Woven Nanogenerator, unpublished
[3] Flexible PZT Nanogenerator with 209 V Output Voltage to Directly Lighten a LED, unpublished
[4] N.Y. Cui, W.W. Wu, Y. Zhao, S. Bai, L.X. Meng, Y. Qin, Z.L. Wang, Nano Lett, 12 (2012) 3701-3705.
[5] Biocompatibility of 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 Nanogenerator, unpublished

Energy harvesting from renewable and green energy resources has attracted considerable interest due to energy crisis and global warming. Rationally designed materials and technologies have been subjects of active research and development. Likewise, in the nano-world, energy harvesting technologies based on the piezoelectric effect have been developed to convert tiny-scale mechanical energy to electricity, which are highly expected to realize self-powered micro/nano-systems. Over the years, many kinds of nanogenerators (NGs) have been demonstrated to effectively utilize the mechanical resources with variable frequencies and amplitudes in our living environment, such as light wind, body movement, vibrations and acoustic/ultrasonic waves. For a wide variety of applications, most NGs have been fabricated based on flexible substrates or piezoelectric polymer. The NGs with high flexibility have shown potential applications as not only an energy harvesting device capable of scavenging energy from light wind such as respiration, but also a sensitive sensor which can monitor the behavior of the human heart by detecting small physical motion such as pulse. Furthermore, the high flexibility can provide an opportunity applicable to a target object without any limitation of its shape and movement due to its high conformability caused by the ultra-thin thickness. However, although several research efforts have demonstrated the realization of ultra-thin NGs with high flexibility and conformability, they are still difficult to be utilized as an economic approach for super-flexible NG owing to the high cost and the low-throughput process. Thus, it is necessary to develop innovative strategies toward achieving super-flexible, conformable and cost-effective NGs applicable to any target objects regardless of their surface shape and mode of moving.
In this work, we report a super-flexible and conformable NG based on cost-effective thin Al-foil electrodes which can not only enable energy harvest from a waving flag but also detect a skin movement when attached to a human face. Using the Al-foil as a substrate and electrode, piezoelectric NG that is super-flexible in responding to the wavy motion of a tiny wind has been fabricated using ZnO nanowire arrays. The NG has been used to harvest the energy from a waving flag, demonstrating its high flexibility and excellent conformability to be integrated into fabric. The NG has also been applied to detect the wrinkling of a human face, showing its capability to serve as an active deformation sensor that needs no extra power supply. Our strategy may provide a highly promising platform as energy harvesting devices and self-powered sensors for practical use wherever movement is available.

Heat is one of the important sources of energy in our life, which can be harvested and converted into electrical energy for self-powering the electronic devices and systems. There is two type of energy harvesting systems to harvest the heat. One relies on the Seebeck effect that converts temperature difference across a pair of conductor into electrical energy. Another relies on pyroelectric effect that is based on the change of spontaneous polarization in certain anisotropic solids owing to the temperature fluctuation. The pyroelectric effect has to be a choice in an environment where the temperature is spatially uniform but time-dependent. Hence, in this work, we demonstrate a new high-output stretchable micropatterned pyroelectric nanogenerator using P(VDF-TrFE) pyroelectric polymer materials and micropatterned PDMS/CNT composite as a substrate and electrode. We fabricated three types of regular and uniform PDMS/CNT composite patterned arrays (flat, line and pyramid) to improve the efficiency of the nanogenerator. P(VDF-TrFE) pyroelectric polymer was deposited on PDMS/CNT composite with micropattered shape by spin-coating method. We observed and discussed the higher output voltage produced from pyroelectric nanogenerator with pyramid and line shape pattern compare to flat shape.

Recently, many electronic devices are developed for multifunctionality, so that the studies for high-density and high-efficient energy sources have become a big issue. Based on nano/micro structures, there are some on-going works to realize high capacity and efficiency that is hard to reach in conventional methods. To harvest the energy at anytime and in anywhere, it is important to explore innovative technologies that utilize diverse forms of available energy such as mechanical vibrations, acoustic energy, thermal gradients and electromagnetic waves, including solar energy. In this study, we introduce a novel flexible hybrid energy harvester consisting of a solar cell and a piezoelectric nanogenerator. This work establishes the methodology to harvest solar energy and low-frequency mechanical energies such as body movements, making it possible for producing a promising power generator that could be embedded in flexible architectures. Based on the hybridization, we could find a novel method that greatly enhances piezoelectric power generation by introducing a p-type polymer layer on a piezoelectric semiconducting thin film. Holes at the film surface greatly reduce the piezoelectric potential screening effect caused by free electrons in a piezoelectric semiconducting material. Furthermore, additional carriers from a conducting polymer and shift in the Fermi level help in increasing the power output. Finally, we demonstrate that such hybrid systems could be utilized as a multifunctional electronic device consisting of an energy harvester as well as a touch sensor. Therefore, our approach clearly shows the potential of hybrid approach for scavenging multi-type energies whenever and wherever they are available and of multi-type electronic devices.

By converting ambient energy into electricity, energy harvesting is capable of at least offsetting, or even replacing, the reliance of electronics on traditional power supplies, such as batteries. Here, we design and fabricate a novel and simple generator with extremely low cost for efficiently harvesting ambient mechanical energy. The electricity was generated owing to the coupling of triboelectric effect and electrostatic induction. For a fabricated generator with a dimension of 6 cm by 4 cm, the instantaneous open-circuit voltage and short-circuit current reach up to 400 V and 2 mA, respectively. By using surface modifications on the materials, we are able to further enhance the triboelectric charge density and thus the generator&’s electric output. The physical modification creates ordered nanostructures, while the chemical modification assists charge transfer. It is capable of lighting up 600 commercial LED lamps simultaneously in real time. Moreover, we have utilized this technology to develop self-powered products, such as illuminating tiles and shoes that light up upon foot stepping without additional power sources. The generator has been proved in powering portable and stand-alone electronics, with more practical applications including, but not limited to, health monitoring, wireless sensing, and security systems. This technology also opens up a new path for harvesting mechanical energy at large-scale from wind and ocean wave.

A rapid expansion of electronic devices towards wireless, portability, and multi-function desperately needs the development of independent and maintenance-free power sources. The emerging technologies for mechanical energy harvesting are effective and promising approaches for building self-powered systems, because of a great abundance of mechanical energy existing in our living environment and human body. Among all the technologies, mechanical energy scavenging based on triboelectric effect has been proven simple, cost-effective and robust. However, its output is still insufficient for sustainably driving electronic devices/systems, because the device structure has not yet been optimized based upon the factors determining the effectiveness of the electricity generation. Here, we demonstrated a rationally designed arch-shaped triboelectric nanogenerator by utilizing the contact electrification between a polymer thin film and a metal thin foil. Such an unique arch-shaped structure helps to achieve the effective periodic switching between separation and intimate contact of the two charged plates, which is vitally important to determine the nanogenerators&’ output. The working mechanism of the triboelectric nanogenerator was studied by finite element simulation. The output voltage, current density and energy volume density reached 230 V and 0.13 mA with an instantaneous maximum power density of 3.56 mW/cm2 and 128 mW/cm3. The triboelectric nanogenerator was systematically studied and demonstrated as a sustainable power source to continuously drive instantaneous operation of light-emitting diodes (LEDs). With the first realization of charging a lithium ion battery to full capacity, the triboelectric nanogenerators were combined with lithium ion batteries to form power modules for driving a wireless sensor system and a commercial cell phone, which is the unprecedented demonstration of the nanogenerator for driving personal mobile electronics, opening the chapter of impacting general people&’s life by nanogenerators.

Piezoelectric nanogenerator technology have attracted great attention after the first introduction of piezoelectric ZnO nanowire-based nanogenerator by Z. L. Wang and co-worker in 2006, because it can convert random and tiny bio-mechanical energy from inside the human body (such as the heartbeat, blood flow, muscle stretching, or eye blinking) into electric energy which can be used to power implantable biodevices, biomedical sensor, nanorobots, and even small personal electronics. Recently, there have been attempts to fabricate the nanostructure or thin film-type nanogenerators with perovskite ceramic materials (PZT and BaTiO₃), which have a high level of inherent piezoelectric properties.
Herein, we report the nanocomposite generator (NCG) achieving a simple, low-cost, and large area fabrication based on BaTiO₃ nanoparticles (NPs) synthesized via a hydrothermal reaction and graphitic carbons. The BaTiO₃ NPs and carbon nanomaterials are dispersed in polydimethylsiloxane (PDMS) by mechanical agitation to produce a piezoelectric nanocomposite (p-NC). The p-NC is spin-casted onto metal-coated plastic substrates and cured in an oven for fabricating the NCG device. Under periodic external mechanical deformation by bending stage or biomechanical movements from finger/feet of human body, electric signals are repeatedly generated from the NCG device and used to operate a commercial red LED. The amplitude of the output voltage generated from the NCG depends on the poling process, composition of nanomaterials, the angular bending strain/strain rate, and Zr-doping concentration in NPs. Moreover, the sample without any CNTs is characterized to exploit the role of the CNTs within the device. To investigate the electric and geometric effects of other graphitic carbon nanomaterials instead of the MW-CNTs, the NCG devices with SW-CNTs and reduced grapheme oxide (RGO) are also characterized.
References
[1] K. Park, S. Xu, Y. Liu, G. Hwang, S. Kang, Z. L. Wang, K. Lee, Nano Letters 10(12), 4939, 2010.
[2] K. Park, M. Lee, S. Mun, G. Hwang, J. Kim, D. Kim, Z. Wang, K. Lee Adv. Mater, 24(22), 2999, 2012.

Development of renewable and green energy based on natural resources such as solar, wind and geothermal, has been a major issue with energy crisis and global warming for long-term sustainable development of the world. Over the years, rationally designed materials and technologies have been well established, and their use has gradually increased. Meanwhile, in the nano-world, energy harvesting technologies based on piezoelectric effect have been developed to convert mechanical energy, artificially generated from vibration and movement in the environment, to electricity. Recently, hybrid cells for simultaneously harvesting multiple type energies have presented a new trend in order to synergize their output performances, and some hybrid cells have been accordingly developed for harvesting solar and mechanical energies, sound and solar energies, thermal and solar energies, and biochemical and biomechanical energies. Especially, since solar cell is strongly depend on day/night, the weather and the location, the hybrid cell for harvesting solar and mechanical energies is a representative example that two energy resources can be complementarily utilized wherever and whenever one or all of them are available. To date, although different kinds of hybrid devices have been demonstrated, hybrid cell for simultaneously harvesting thermal and mechanical energies has not been reported yet. Heat is not only a conventional energy resource in the environment that can be generated from the human body, light, and all mechanical devices, but also generally accompanied with vibration and movement. Thus, it is necessary to develop innovative approaches for harvesting both thermal and mechanical energies in order to achieve increased energy generation.
In this work, we report the first flexible hybrid cell, consisting of a thermoelectric generator (TG), for harvesting thermal energy and a nanogenerator (NG) based on ZnO nanowires (NWs) for scavenging mechanical energy. The output performance of the two processes can be complimentarily integrated without scarifying the combined output, including the high output current from TG and the high output voltage from piezoelectric NG. We also demonstrate the possibility of scavenging both thermal and mechanical energies from skin temperature and body motion. This strategy can provide a highly promising platform as hybrid cells that simultaneously harvest multitypes of energy so that the energy resources can be effectively and complementarily utilized for power sensor network and micro/nano-systems.

Piezoelectric Microelectromechanical Systems (MEMS) has been proven to be an attractive technology for harvesting small energy from the ambient vibration. This technology will eliminate the need for replacing chemical batteries or complex wiring for microsensors/microsystems. Recent advancements in piezoelectric materials and harvester structural design, individually or in combination, have improved MEMS energy harvesters to achieve enough power capacity, compactness and ultra wide bandwidth, bringing us closer towards battery-less autonomous sensors systems and networks in very near future. Especially, non-linear resonating beams for wide bandwidth resonance are the key development to enable robust operation of energy harvesters over the unpredictable and uncontrollable frequency spectra of ambient vibration. We expect that a coin size harvester will be able to harvest about 100mu;W continuous power at below 100 Hz and less than 0.5 g vibration and at reasonable cost very soon.

MEMS-scale energy harvesters have been modeled, prototyped, assembled and then tested on vibrating equipment (such as HVAC systems and water pumps) in the machine rooms of industrial buildings. The long-term practical goal is condition based monitoring of commercial equipment with wireless sensor networks; and a sub-goal is to power the nodes with inexpensive harvesting methods. Extreme low-cost is also a goal hence the focus on MEMS-scale wafer production using conventional micro-fabrication. The harvesters produced (shown below) have resonant frequencies in the range 31-232Hz. This range was designed-in based on a survey of the amplitude and frequency measurements of the equipment to be monitored. The best vibration energy harvesters produced 1.72 nano-watts rms at 232Hz and 0.29 g on a bench-top vibrometer. Following these tests, the best power output on the industrial equipment was 202 pico-watts at 31Hz on a machine that vibrated at 0.15 g. The focus of this work was to design the MEMS devices to match the relatively low frequencies of the equipment. Ongoing and future work focuses on increasing the power output.

Piezoelectric structures have been widely utilized for harvesting mechanical energy. Various forms of transducer structures have been fabricated to capture the mechanical energy with high efficiency. At micro-to-nanoscale, the design of transducer becomes challenging as the size reduction is accompanied by enhancement in the resonance frequency. In this study, we will provide the solution to the problem of low frequency resonant transducer structures. Next, we will utilize these novel transducers to develop dual phase harvesters, one that can capture mechanical energy and magnetic energy at the same time. The dual-phase harvester consists of a magnetostrictive/ piezoelectric/ magnetostrictive (M/P/M) laminate structure utilizing two mechanisms simultaneously: 1. magnetoelectric (ME) effect, where external magnetic field H can excite longitudinal strain through magnetostricitve phase and transfer to piezoelectric phase; 2. Piezoelectric effect, where induced mechanical vibration can create strain and generate charge. The first step towards achieving the desired objective was to design and fabricate high ME coefficient self-biased structures. Ferromagnetic - ferroelectric composite resonant transducers with giant magnetoelectric coefficient at ZERO bias were fabricated and utilized for the low frequency energy harvesting system. A fundamental understanding of elastic coupling in the thick film laminate structures was developed to identify the mechanisms controlling the self-biased magnetoelectric response. Transition of these structures on the micro/nano scale will be discussed and experimental results in this direction will be reported.

Nano Electromechanical Systems (NEMS) developed using piezoelectric nanowires (NWs) have gained interest in the research community as they are able to convert several different forms of mechanical energy sources into electric power and thereby, function as reliable power source for emerging ultra-low power wireless electronics. In this paper, a piezoelectric NEMS generator is fabricated using newly developed ultra-long (~45µm) aligned barium titanate (BaTiO3) nanowire (NW) arrays that exhibit piezoelectric behavior for harvesting mechanical vibrational energy. The NW NEMS generator is fabricated to have resonance at frequencies below 1 kHz for efficient power harvesting since ambient mechanical energy typically exists in the 1 Hz to 1 kHz range. The maximum power harvested from the BaTiO3 NW NEMS generator is evaluated by impedance matching at resonance. Previously, power generation from semiconducting ZnO NWs based nanogenerators has been reported and numerical comparison analysis have shown that ferroelectric BaTiO3 NWs have higher power generation capability than ZnO NWs for the same size due to their larger dielectric constant and therefore, reduce the optimal external load resistor (ZL = 1/omega;Cp) needed for maximum power transfer. Owing to this reason, NEMS generator comprised of solution grown aligned ZnO NW arrays that has a resonant frequency below 1 kHz is fabricated and direct vibration excitation experiments are performed to compare its power harvesting performance with the newly developed BaTiO3 NW arrays.

The piezoelectric energy harvester (PEH) has been investigated to generate electricity from environmental vibrations due to their higher energy conversion capabilities. Mechanical stress applied to piezoelectric materials distorts internal dipole moments and generates electrical potentials in direct proportion to the applied force. In practical applications of PEHs, narrow vibration bandwidth of a conventional PEH is considered as bottleneck in its implementation. The methods, such as the use of nonlinear harvesters, amplitude limiter or mechanical stopper, or array structure, have been reported to broaden bandwidth. A magnetic spring is commonly used to construct nonlinear generators, but factors such as nonlinear damping, higher-order coupling coefficients, and higher driving amplitudes in ferroelectrics complicates the construction of nonlinear harvesters and their analytical method.
In this study, a nonlinear harvesting device was constructed using a multiferroic layer integrated with ferromagnetic material. Simple configuration based on bistable nonlinearity was utilized by aligning a magnet to the cantilever beam direction. Under a small excitation level, large-amplitude periodic motion was obtained by applying a disturbance or equivalently an initial velocity. PZT or ZnO thin films deposited on magnetostrictive flexible foil were used to generate nonlinear motion at wider bandwidth vibration. Extensive works to characterize piezoelectric properties of the films, as well as optimal mechanical structure to generate maximum output power, have been conducted. The harvesting device demonstrates the output power from 20 to 180 Hz in the range of around a few microwatts per cubic millimeter at 0.5 g. The output power behavior was also simulated using Matlab Simulink. The issues with this approach, such as actuation energy&’s potential perturbance of the beam for small excitation amplitude and asymmetric or nonlinear response driven by the microfabrication process, will be discussed in detail.

The flexible substrates, like plastic, paper and cotton can be of interest for several reasons to generate voltage-current from piezoelectric ZnO nanowires (NWs). Zinc oxide NWs have shown very high voltage generation [1] and they are possible to grown on plastic, paper and cotton [2, 3] and work as nanogenerators. Since we with these substrates can get a new freedom to bend and also stretch the wires and to in cooperate them into new applications they are of interest. In this talk we will describe the mechanical and piezoelectric properties of nanowires/nanotubes grown on plastic, paper and cotton substrates as well as how to grow the structures.
References:
1. Zhu G, Wang AC, Liu Y, Zhou Y, Wang ZL, ‘&’Functional electrical stimulation by nano-generator with 58 V output voltage&’&’, Nano Lett. 2012 Jun 13;12(6):3086-90.
2. Soomro MY, Hussain I, Bano N, Nur O, Willander M, ‘&’Piezoelectric power generation from zinc oxide nanowires grown on paper substrate&’&’, Phys. Stat. Sol. RRL 2012; 6(2):80-82.
3. Khan A, Abbasi MA, Hussain M, Ibupoto ZH , Wissting J, Nur O, Willander M, ‘&’Piezoelectric nanogenerator based on zinc oxide (ZnO) nanorods grown on textile cotton fabric&’&’, Appl. Phys. Lett. Submitted (2012).

The anticipated increasing use of key natural energy resources, which become extremely scarce, in the coming decades have sparked significant world-wide efforts toward the search for the cost-effective renewable and green energy sources to meet global energy demands of future. In this regard, advances in self-powered nanotechnology allowing for the design of efficient energy harvesting offers an enormous potential for the creation of sustainable systems utilizing incessantly natural ambient energy sources.
Recent developments in piezoelectric power generators harvesting energy steadily from ambient mechanical vibrations without regard to time, place, or any external conditions, present innovative and emerging research topics in the area of a green energy technology. In particular, a ZnO nanowire has been intensively studied as one of the most attractive piezoelectric materials for widespread use as a clean and inexhaustible power source in future self-powered nanosystems including implantable chips, flexible/portable electronics, and environmental and heath monitoring sensors because of its unique dimensionality and superior transparent and piezoelectric semiconducting properties coupled with biocompatibility, environmental friendliness, and geometrical versatility. To date various approaches have been reported demonstrating and enhancing unique capabilities for a ZnO nanowire-based piezoelectric energy generator by introducing various nanowire arrays and device configurations, flexible/textile electrodes and substrates, and diverse types of ambient mechanical energy sources. However, despite the demonstrated ability and fascinating piezoelectric features of ZnO nanowires, there are serious impediments to the device performance as well as to a detailed understanding of the underlying physics accounting for energy harvesting mechanism because of free charge carriers in ZnO nanowires unlike ideal insulating piezoelectric materials, partially screening piezoelectrically generated immobile charges. Here we report that the various device performance of a sound-driven piezoelectric energy nanogenerator is remarkably improved by controlling and manipulating piezo-materials/structures and especially both the carrier density and the interfacial energy in a semiconducting ZnO nanowire, allowing for achieving the intrinsic efficiency limits.

Harvesting energy from our living environment is a very active research field for powering micro/nano-systems. Mechanical energy, such as vibrational energy, body movement, and heart beating, can serve as energy resources at any time and in any place. Recently, piezoelectric nanogenerators (NGs) have been developed by taking advantages of the piezoelectric property of semiconductor nanostructures to convert mechanical energy into electric energy. Owing to the large direct band gap, good thermal stability and high mobility, GaN has shown outstanding optoelectronic properties, and reveals great application potentials in a lot of fields. However, it has been rarely used for NGs especially with high output performance.
In this work, GaN nanowires (NWs) were synthesized through a vapor-liquid-solid (VLS) process. Based on structural analysis, the c-axis of the NW was confirmed to be perpendicular to the growth direction. Nanogenerators fabricated by rational assembly of the GaN NWs produced an output voltage up to 1.2 V and output current density of 0.16 mu;A/cm2. The mechanism of the NGs was proposed based on numerical calculations using finite element analysis (FEA).

Harvesting mechanical energy such as vibration, sound wave, air or water flow from the environment offers the possibility of realizing future self-powered systems. Piezoelectric nanogenerators based on ZnO nanowires have been extensively realized as sustainable energy sources due to their unique physical and chemical properties such as semiconducting, piezoelectricity, and biocompatibility. The integration of energy-harvesting devices into wearable textiles promises to provide novel forms of mechanical energy harvesting continuously and effectively from omnipresent sources in our daily life because the energy harvesting system can be always exposed to omnipresent mechanical energy sources, such as vibration from people's movement or sound noise, as well as the outside wasted mechanical energy sources. In addition to the issue of utilizing various types of natural energy sources, the creation of new concepts of higher power generation is also of particular interest for operating commercial electronic devices. The integration of piezoelectric ZnO nanowries and a charged dielectric film on a textile substrate offers the hybridized effects of electrostatic and piezoelectric generators. The output power from the sound driven textile nanogenerator has been significantly increased by electrostatic enhanced piezoelectric effect and large vibration displacement of textile substrates. The highest peak output voltage and output current of the generator were recorded to be up to 8 V and 2.5 µA, respectively, at a sonic input power of 100 dB. Using our hybrid nanogenerator, we operated a homemade green organic light-emitting diode and a 9cm x 3cm liquid crystal display panel, demonstrating its new concept of high power generation as a practical power source. Our findings provide a viable way to create a new wide range of self-powered system with a high power requirement.

We will take a walk through a variety of devices - basic diode, photodetector, nanogenerator and finally enhanced photovoltaic performance - that use ZnO nanostructures. We will see how the common, and simple structure, of a nanostructured diode can be used to enhance the performance of some interesting electronic and optoelectronic systems. Initially the talk will focus on our process developments including deposition of CuSCN as p-type layer and high pH synthesis of ZnO in aqueous environments. These developments show very clearly how the key materials properties are process dependent as we compare diodes with rectifications ranging from 24 to 20,000 for a nominally identical structure. We will then move to discuss some of the more dramatic exploitations of the piezoelectric effect in ZnO. First I will show how it is possible to use a hybrid inorganic-organic p-n diode to scavenge energy from the environment in the form of vibrations being converted into electrical energy [1]. I will then show how the energy conversion efficiency is limited by the electromagnetic coupling of the material by illustrating a series of results experimentally obtained that support the model of carrier drift and band bending at the interface[2]. Finally I will show that with careful design selection of a photovoltaic architecture it is possible to enhance the output of a hybrid - TCO:ZnO:P3HT:Au device by 50% from 1.2% ECE to 1.8 ECE using sound at 10kHz and 75 dB. This is associated with a significantly reduced, and measured, carrier recombination rate[3]. Sound of this level is consistent with that found in a typical noisy office or mode of transport. When we test ZnO:PCBM:Au devices we see no significant enhancement in photovoltaic performance. We explain our findings in terms of carrier recombination at the interface of the ZnO due to the oscillating electric field induced in the space charge layer due to the piezoelectric effect of the ZnO. In the PCBM device carrier separation occurs at locations distinct from the ZnO interface and so there is negligible piezoelectric contribution in that design.
1 Adv. Energy Mater. 2012, 2, 1261-1268
2 Appl. Phys. Lett. 101, 093902 (2012); doi: 10.1063/1.4749279
3 Submission under review

Owing to the relatively large piezoelectric coefficient, InN is considered to be one of the best candidate materials to be applied in piezotronic and nanogenerator devices, which receives escalating interests to control carrier transport and harvest energy from ambient, respectively. Besides, the performance of the devices has to be optimized by maximizing deformation. Therefore, we demonstrate to grow InN nanorod arrays aligned obliquely by incorporating glancing angle deposition with molecular beam epitaxy so that deformation can be extended merely by normal forces. These InN NR arrays are bounded by top polar {0002} surfaces and semipolar {-1102} side surfaces. The Schottky barrier height is also observed to decrease by about 58 meV for the (-1102) plane than the (0002) plane by nonlinear I-V characteristics. For the piezoelectric properties, the Schottky barrier height can be systematically tuned by varying the tip deflection forces. It is shown that the piezoelectric and electric transport properties of InN exhibit a great divergence than ZnO. The current output is harvested by deforming an obliquely aligned InN NR array with a Pt/Ir coated conductive-atomic force microscope tip to create piezoelectric potential across. The nanogenerators built on this structure can produce an average output direct current of 205.6 nA by only applying the tip deflection force of 3 nN, which can be further increased with larger forces. This work demonstrates the feasibility of using obliquely aligned InN nanorod array for harvesting electricity from ambient environment leading to the realization of self-powered nanodevices.

For solar cell design, there are two approaches to optimize the solar cell performance: developing new energy efficiency material and designing new structure. The inner-crystal piezopotential in piezoelectric semiconductor materials, such as ZnO, GaN, InN and CdS, can effectively tunes/control the carrier separations and transport processes at the vicinity of a p-n junction or metal-semiconductor contact, which is called the piezo-phototronic effect. The presence of piezoelectric charges at the interface/junction can significantly affect the performances of photovoltaic devices, especially for flexible and printed organic/inorganic solar cell fabricated by piezoelectric semiconductor nano/microwires. This is new method to improve solar cell, and offer the new way to design photovoltaic devices.
The basic structure of typical nano/microwire solar cells is a p-n junction or metal-semiconductor (M-S) contact. The working principle of the solar cell is to use the built-in electric field in the depletion region to assist the separation of electron-hole pairs generated by incident photons. The piezoelectric charges created at the junction area under strain can effectively tune/control the solar cell performance. The piezopotential significantly modify the band structure at the interface, resulting in a control over the carrier generation, separation and transport at the p-n junction or M-S interface, which is the piezo-phototronic effect.

Through a process of photoelectrochemical (PEC) water splitting, we demonstrated an effective strategy for engineering the barrier height of a heterogeneous semiconductor interface by piezoelectric polarization, known as the piezotronic effect. A consistent enhancement or reduction of photocurrent was observed when tensile or compressive strains were applied to the ZnO anode, respectively. The photocurrent variation is attributed to a changed barrier height at the ZnO/ITO interface, which is a result of the remnant piezoelectric potential across the interface due to a nonideal free charge distribution in the ITO electrode. In our system, 1.5 mV barrier height change per 0.1% applied strain was identified, and 0.21% tensile strain yielded a 10% improvement of the maximum PEC efficiency. The remnant piezopotential is dictated by the screening length of the materials in contact with piezoelectric component. The difference between this time-independent remnant piezopotential effect and time-dependent piezoelectric effect is also studied in details.

Nanowire solar cells are promising candidates for powering nanosystems and flexible electronics. The strain in the nanowires, introduced during growth, device fabrication and/or application, is an important issue for piezoelectric semiconductor (like CdS, ZnO and CdTe) based photovoltaic. In this work, we firstly demonstrated largely enhanced performance of n-CdS/p-Cu2S coaxial NW PV devices using the piezo-phototronics effect when the PV device is subjected to an external strain. Piezo-phototronics effect could control the electron-hole pair generation, transport, separation and/or recombination, thus enhanced the performance of the PV devices by as high as 70%. This effect offers a new concept for improving solar energy conversation efficiency by designing the orientation of the nanowires and the strain to be purposely introduced in the packaging of the solar cells. This study shed light on the enhanced flexible solar cells for applications in self-powered technology, environmental monitoring and even defensive technology.

Piezotronics is a new field integrating piezoelectric effect into nanoelectronics, which has attracted much attention for the fundamental research and potential applications. Firstly, we demonstrate a mechanical-electrical trigger using a ZnO piezoelectric fine-wire (PFW) (microwire, nanowire). Once subjected to mechanical impact, a bent PFW creates a voltage drop across its width, with the tensile and compressive surfaces showing positive and negative voltages, respectively. The voltage and current created by the piezoelectric effect could trigger an external electronic system, thus, the impact force/pressure can be detected. In addition, strain sensors based on individual ZnO PFWs have been demonstrated. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I-V characteristic is highly sensitive to strain due to mainly the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated.
[1] J. Zhou, P. Fei, Y. F. Gao, Y. D. Gu, G. Bao, Z. L. Wang, Mechanical-Electrical Triggers and Sensors Using Piezoelectric Micowires/nanowires, Nano Letters, 2008, 8, 2725-2730.
[2] Flexible piezotronic strain sensor, J. Zhou, Y. D. Gu, P. Fei, W. J. Mai, Y. F. Gao, R. S. Yang, G. Bao, Z. L. Wang, Nano Letters, 2008, 8, 3035.
[3] J. Zhou, P. Fei, Y. D. Gu, W. J. Mai, Y. F. Gao, R. S. Yang, G. Bao, Z. L. Wang, Piezoelectric-Potential-Controlled Polarity-Reversible Schottky Diodes and Switches of ZnO Wires, Nano Letters, 2008, 8, 3973-3977.

Mechanically coupling a piezoelectric to a material that changes resistivity with pressure forms the basis of a new class of device, the piezoelectronic transistor (PET) [1]. In this talk we will explore the materials science of piezoresistive (PR) thin films, with an emphasis on creating a viable PET. We focus on the rare-earth monochalcogenide, SmSe, which shows a continuous resistivity change of 7 orders of magnitude in bulk single crystals [2]. The PR response is caused by the pressure-sensitivity of gap 4f levels, which move toward the conduction band with strain, creating carriers. The conductivity variation spans the range from an intrinsic semiconductor to a metal.
Thin SmSe films (50nm) grown by co-sputtering at elevated temperatures are shown to undergo over 1000x change in resistivity when strained by a microindenter to loads of 2 GPa. This is the first report of piezoresistivity in chalcogenide thin films, showing a response that is nearly sufficient for low-power logic technology. Ohmic current-voltage properties are observed on unstressed films, suggesting that interfacial barriers do not play a significant role in the high-resistance phase. Structural characterization using x-ray diffraction confirms the presence of a cubic phase with a lattice constant of 6.16#8491;. Studies of temperature stability and oxidation resistance using medium energy ion scattering show that SmSe can be integrated into a device structure with a generous thermal budget and limited oxygen exposure. AFM scans of uncapped SmSe films reveal a fine-grained polycrystalline structure, with 10-25 nm grain size.
Furthermore, we have begun to explore scalability of SmSe films, and find significant piezoresponse (100x) down to thicknesses of 12nm. Although scaling and performance will continue to be important, the present material will allow several generations of PET development. Successful integration of PR films with piezoelectric layers will lead to a new generation of MEMS devices with logic applications and large market potential.
1) D. Newns, B. Elmegreen, X. H. Liu, and G. Martyna, J. Appl. Phys. 111, 084509 (2012).
2) Jaymaran et al., PRL 25, 1430, (1970).

Significant progress has been achieved in implementing flexible pixel-array pressure sensors for mimicking the tactile sensing capabilities of human skin. Notably in these demonstrated systems, external mechanical stimulations directly interface with pressure-sensitive media while electronic components like field-effect-transistors (FETs) function as read-out elements for passively detecting pressure-induced characteristic change in the media. Intensive efforts have been devoted to minimize the effect of induced strain on performance of these electronic components. This sensing scheme not only requires complicated system integration but also lacks proficiency in directly interfacing/controlling electronics with mechanical actions. Moreover, electronic components are incorporated into existing flexible systems in planar configuration, with pixel dimension around hundreds of microns to even tens of millimeters, severely limiting the pixel-density and spatial resolving capability. The concept of extending electronics into vertical dimension with wrap-gate presents an attractive approach to achieve high-density assembly of nanomaterials; whereas, it is cumbersome to fabricate the wrap-gate and manage interconnect layout for addressing and controlling individual FET effectively.
Meanwhile, the coupling of semiconducting and piezoelectric properties in ZnO NWs have been recently investigated and utilized for implementing novel applications, based on modulation of local contact characteristics as well as charge carrier transport by strain-induced piezoelectric polarization charges at the metal-semiconductor interface, which is the fundamental of piezotronics.
Driven by the above challenges and concepts, here we demonstrate by far the largest integration of 3D piezotronic NW transistors circuitry as active unit for integrated pressure-sensor, capable of monitoring profiles of applied small pressures (< 10 kPa) with the highest spatial resolution as well as tactile sensitivity. The piezotronic NW transistors array directly interface with mechanical actions while the changes in electrical characteristics of NW transistors, modulated by strain-induced polarization charges at the semiconductor/metal interface due to piezotronic effect, are simultaneously detected to reflect variations in applied deformation. Capabilities of multi-dimensional and self-adaptive sensing have also been demonstrated, showing the potential for future applications such as artificial/prosthetic skin in smart biomedical treatments. The feasibility and scalability of this platform together with its demonstrated compatibility with state-of-art microfabrication techniques paves routes towards future large-scale integration of nanomaterials for applications in functional micro/nano-systems capable for active/adaptive and self-sufficient operations.

Nano- and hetero-structures of graphene nanoplatelet (GnP) and carbon nanotube (CNT) can control piezoelectric and optoelectronic properties for acoustic actuator, flexible transparent speaker. Optimized conditions were obtained by obtaining the best dispersion and interfacial durability for the long-term use. Optical transmittance and electrical resistance were measured for GnP and CNT dip- and spraying coating on piezoelectric poly(vinylidene fluoride) (PVDF) sheets with elapsing time under cyclic loading. Uniform dip-coating was simply tried using Wilhelmy plate machine as well as spraying coating. The change in electrical resistance and optical transmittance of coated layer was dependent upon the number of dip-coating, the concentration of nano-solutions. Electric properties of coated layers were measured using four-point method and surface resistance was calculated by dual configuration method. Optical transmittance of GnP and CNT coated PVDF sheet was evaluated using UV spectrum. Surface energy and hydrophobicity were investigated by wettability test. As the elapsing time of cyclic loading passed and heating condition right after coating, the stability of surface resistance and thus comparative interfacial adhesion between coated layer and PVDF sheet was evaluated by comparing with the thermodynamic work of adhesion, Wa. As dip-coating number increased, surface resistance of coated sheet decreased, whereas the transmittance decreased steadily due to thicker nano-networking layer. Nano- and hetero-structural effects of GnP and CNT solution on the optical and acoustic response were studied. The thin film transducers showed good acoustic response over wide frequency ranges. Improved interfacial adhesion of was contributed to increase the sound pressure level (SPL) for acoustic actuator.

ZnO nanowire inorganic/organic hybrid ultraviolet (UV) light-emitting diode (LED) have been attracting growing interests as they not only combine the high flexibility of polymers with the structural and chemical stability of inorganic nanostructures, but also have the higher light extraction efficiency than thin film structures. However, UV LEDs based on ZnO nanostructures have shown very low external quantum efficiency due to the difficulties in achieving current balance between electrons and holes and high nonradiative recombination induced by surface defects. In most of the reports, there was no data about conversion efficiency or external quantum efficiency of ZnO NW-organic hybrid LEDs. Here we demonstrate that piezo-photontronic effect can largely enhance the efficiency of a hybridized inorganic/organic LED made of a ZnO nanowire/p-polymer, by trimming the electron current to match the hole current and increasing the localized hole density near the interface through a carrier channel created by piezoelectric polarization charges at the ZnO side. The external efficiency of the hybrid LED was enhanced by at least a factor of two after applying a proper strain, reaching 5.92%, which is comparable to that of inorganic pn junction LED and quantum well enhanced nanowire LED. The approach pioneered in the study can be applied to other optoelectronic devices, and may bring about significant performance improvement and energy saving.

Quasi-1D piezoelectric nanodevices are being intensively studied for sensing, energy harvesting, piezotronics, piezophototronics, and more. For this reason in literature several efforts have focused on theoretical understanding and numerical modeling of piezoelectric nanodevices; in particular, the piezopotential of laterally bent nanowires has been already investigated, even taking into account the effect of free charges inside semiconductive nanowires. However, here we focus on both theoretical understanding and numerical modeling of three important aspects of laterally bent piezo-semiconductive nanowires which have not yet been considered in existing literature.
Firstly, we give reasons for the presence, at the base of the nanowire, of two regions which exhibit a local electric potential which has an opposite sign in comparison with the piezoelectric potential within the rest of the nanowire. In fact, though in a laterally bent zinc oxide (ZnO) nanowire the extended and the compressed sides have positive and negative potentials, respectively, at the base of the nanowire there are an extended and a compressed region which have an inverted negative and positive potential, respectively. Such an inversion of the potential sign has been reported by many papers but has not been previously explained. Importantly, we also investigate the extension of the inversion region as a function of several parameters, including the length, the diameter, and the doping level of the nanowire as well as the amplitude of the input mechanical force.
Secondly, we compute, for the first time, the electrostatic stored energy in a laterally bent piezoelectric nanowire when taking into account the screening of the free charge carriers present in the semiconductive nanowire; such electrostatic energy, until now, has been computed in laterally bent nanowires only with the crude hypothesis of a purely dielectric nanowire, which is clearly not accurate for semiconductive (e.g. zinc oxide) materials. Additionally, we use two-dimensional maps for identifying the regions where the electrostatic energy is dominantly stored.
Thirdly, we compute the ratio between the total stored electrostatic energy and the total (mechanical and electrostatic) stored energy, being such ratio an upper limit to the static mechanical-to-electrical conversion efficiency which can be obtained from the laterally bent nanowire; our calculations take into account both piezoelectric (direct and inverse effect) and semiconductive properties.
Our results can provide guidelines for designing high performance devices based on laterally bent piezoelectric nanowires.

Recent advances in piezoelectric power generators open numerous doors for meaningful power generation through environmentally abundant mechanical energy harvesting for practical applications, particularly, for self-powered and low power-consuming devices. Most piezoelectric power generators are based on alternating current (AC) signal generation, so rectifier circuits should be needed to convert AC power into direct current (DC) power for storing the generated power, resulting in increased total sizes of the power packages. Furthermore, in the process of converting, power is drastically reduced. In this work, it was found that an anionic clay plays an important role in generating DC output from piezoelectric nanogenerators. Two dimensional (2D) ZnO nanosheets/anionic networks are synthesized on aluminum (Al) electrode. Subsequently, 2D ZnO nanosheet-based piezoelectric DC nanogenerators with highly efficient output performance are fabricated, which have great potential for use in portable electronics and further miniaturization of a power packages. Furthermore, compared to previous piezoelectric DC output nanogenerators, the ZnO nanosheet-based piezoelectric nanogenerator has high and stable power generation performance because of its structural stabilization.

The energy harvesting technology from environment such as solar, thermal, and mechanical sources without CO2 generation is critically important for our future life. Recently, piezoelectric nanogenerators harvesting mechanical energies from nature such as wind, vibration, and body movement have been getting huge attraction due to their potential applications in renewable energy and sensors. So far, most piezoelectric nanogenerators have been analyzed by pushing, bending, and stretching external mechanical sources. However, there are many other mechanical sources in our environment. In this study, we report the output behaviors and performance of piezoelectric nanogenerators according to the external mechanical modes such as rolling and twisting, also comparing with bending. The piezoelectric nanogenerators were simply prepared by sputtering zinc oxide (ZnO) thin film on a PET film. We could find the different output behaviors and performance of piezoelectric nanogenerators by different mechanical modes. The results were analyzed by theoretical mechanical characterization based on stress-strain analysis for each mechanical mode condition. We expect that our study will provide the cornerstone to apply the piezoelectric nanogenerators to a variety of energy harvesters and sensors.

Energy harvesting with piezoelectric material provides a compact and reliable solution for electric power converted from the irregular mechanical energy in the environment. Compared with traditional polycrystalline ceramics, single crystal piezoelectric nanostructures are more flexible and robust and they can accommodate larger strain input, which leads to a higher power density. Because of the non-toxicity and various existed synthesis methods, piezoelectric zinc oxide (ZnO) nanowire is widely studied for this application. ZnO is also a semiconducting material and the internal free electron can partially screen the generated potential, while a Schottky contact helps block the charge from flowing through the nanowire. The effect of the free charge and the Schottky contact adds on the length dependence of the output from a nanowire, while such study is still lacking. Here by depositing a series of electrodes on a single ZnO nanowire sited on a flexible substrate, we characterized the energy conversion from a single nanowire with different length and different types of contact. This study shows the influence of the dimension and the semiconducting property of a nanowire on its mechanical-electric energy conversion, and it provides a critical design guideline for a high output ZnO nanogenerator.

Understanding power generation behavior of a piezoelectric nanogenerator is essential to enhance its output power performance and to apply as a sustainable power source for wireless electronic devices. Nanogenerators based on semiconducting material zinc oxide (ZnO) can generate only a few volts intrinsic piezoelectric potential owing to free carriers that screen some parts of the piezoelectric potential1). In the present study, through modulating purity, and crystalinity of ZnO thin films, we report how these parameters have influence on piezoelectric power generation. C-axis oriented insulator-like sputtered ZnO thin films were grown in various working pressure to fabricate an optimized nanogenerator. The purity and crystalinity of ZnO were investigated with X-ray diffraction (XRD), and Photoluminescence (PL). Furthermore, by introducing a conjugated p-type polymer usually used in organic solar cell, it is discussed how free carrier passivation affects the power generation behavior.

Piezoelectric nanomaterials have attracted a lot of interest as building blocks for future nanosystems. Because of their non-central symmetric crystal structures, a piezoelectric potential (piezopotential) is created in the crystal when stress is applied. For semiconductor materials such as ZnO, the piezopotential created in the crystal has a strong effect on the carrier transport at the metal-semiconductor interface. Piezotronic devices use the piezopotential as a “gate” voltage to tune/control the charge carrier transport at a contact or junction.
As one of the most important 2-6 group semiconductor materials with wurtzite structure, CdSe has been studied extensively in the optoelectronic field. Recently, one dimensional CdSe nanostructured materials have been reported for potential applications in light emitting devices, solar cells, photo detectors and lasers. Combined with its unique optical properties, CdSe NW is an attractive potential candidate to replace ZnO NWs in hybrid energy harvesting and piezo-phototronic devices. However, little research has been reported on the field on CdSe NWs.
In this paper, we demonstrated the energy harvesting potential and piezotronic effect in CdSe NW arrays for the first time. Vertically aligned CdSe NW arrays with a wurtzite structure were synthesized using vapor-liquid-solid (VLS) process. By using the conductive AFM in contact mode, we obtained an average output voltage of 30.7 mV for the CdSe NW arrays. Then by measuring the I-V characteristics of a single CdSe NW using a Pt coated AFM tip, we found that an externally applied force can significantly tune the current through the piezotronic effect. CdSe NW arrays have promising application potentials in the fields of hybrid energy harvesting with solar cells, strain sensors, and piezo-phototronic sensors.

The power generation performance and commercialization of piezoelectric power generators with a thick and rigid template are restricted, because they are mainly acted in a low magnitude and frequency situation with very minute and irregular mechanical energy sources from the living environment, such as body movement, air flow, hydraulic pressure and acoustic vibrations, which are mostly of low magnitude and low frequency. Here we report a new type of stretchable transparent piezoelectric nanogenerator (NG) using an organic piezoelectric material consisting of poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] sandwiched with mobility-modified chemical vapor deposition-grown graphene electrodes by ferroelectric polarization into P(VDF-TrFE). This new type of NG has a very high sensitivity and mechanical durability with fully flexible, rollable, stretchable, foldable, and twistable properties. We also investigated the mobility modified graphene electrodes with ferroelectric P(VDF-TrFE) remnant polarization, and a mechanism is proposed for switching the mobility of the carriers by the ferroelectric remnant polarization. Upon the exposure to the same input sound pressure, the measured output performance of the stretchable NG with a thin polydimethylsiloxane stretchable rubber template is up to 30 times that of a normal NG with a plastic substrate. Upon the exposure to an air flow at the same speed, the measured output voltage from the stretchable NG is about 8 times larger than that of the normal NG.

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

One of the most important issues of modern society is to develop a renewable and clean energy system to surmount global warming from carbon dioxide emissions and exhaustion of fossil fuel used in various industries like electronics, transportation, manufacturing, and lots of services. Especially, a hybrid energy harvesting system used to power the electronic devices is worthy to solve environmental pollution due to its nature of multiple-type energies harvesting from the environment. A hybrid device can harvest at least two energies such as solar, mechanical, chemical energies and thermal energy simultaneously from the environment. Therefore, we report the fabrication of a hybrid architecture consist of piezoelectric nanogenerator and solar cell. This hybrid architecture is capable to produce the output of coupled piezoelectric and photovoltaic properties. We measured output voltage and current using hybridized that ZnO nanosheet-based direct current (DC) generator and organic solar cell without the use of rectifier circuit. It is found that the voltage output from the solar cell can be used to raise the output voltage of the nanogenerator, what provide an effective approach for superior storing and utilizing the power generated by the nanogenerator. Additionally, during pushing state, voltage and current enhance effect is maintained. We believe that our approach of fabricating such hybrid systems is very promising for solar and mechanical energy harvesting simultaneously from the environment.

Piezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. The most well known material that has piezoelectric effect is the provskite structured Pb(Zr, Ti)O3 (PZT), which has found huge applications in electromechanical sensors, actuators and energy generators. But PZT is an electric insulator and it is less useful for building electronic devices. Wurtzite structures, such as ZnO, GaN, InN and ZnS, also have piezoelectric properties but they are not extensively used as much as PZT in piezoelectric sensors and actuators due to their small piezoelectric coefficients. In fact, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. For materials such as ZnO, GaN, InN in the wurtzite structure family, the effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is named piezotronics, with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic units. Piezo-phototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of charge generation, separation, transport and/or recombinatioin in electro-optical processes by strain induced piezopotential. Piezotronics is likely to have important application in sensors, human-silicon technology interfacing, MEMS, nanorobotics and active flexible electronics. The role played by piezotronics in interfacing human-CMOS technology is similar to the mechanosensation in physiology. We anticipate the near future applications of piezotronics and piezo-phototronics in sensor network, bioscience, human-machine interfacing and integration, and energy sciences (LED, solar cell).
[1] Z.L. Wang and J.H. Song , Science, 312 (2006) 242.
[2] Z.L. Wang, Nano Today 5 (2010) 540.
[3] W.Z. Wu, Y.G. Wei and Z.L. Wang, Adv. Materials, 22 (2010) 4711.
[4] W.Z. Wu, Y.G. Wei and Z.L. Wang, Nano Letters, 11 (2011) 2779
[5] Y.F. Hu, Y.L. Chang, P. Fei, R.L. Snyder and Z.L. Wang, ACS Nano, 4 (2010) 1234.
[6] Q. Yang, X. Guo, W.H. Wang, Y. Zhang, S. Xu, D.H. Lien, Z.L. Wang, ACS Nano, 4 (2010) 6285.
[7] Q. Yang, W.H. Wang, S. Xu, Z.L. Wang, Nano Letters, 11 (2011) 4012.
[8] Z.L. Wang, Adv. Mater., 24 (2012) 4632.
[9] Personal website: www.nanoscience.gatech.edu

Luminescent materials have widespread applications ranging from photonic devices to biomedicine. Commonly used light-emitting diodes (LEDs) are essentially a p-n junction diode typically made from a semiconductor. The phenomenon of light-emission from electron-hole pair recombination as a result of minority carrier injection is regarded as injection electroluminescence (EL). In order to search more EL device type, a key issue is how to tailor the materials properties and therefore construct a new device structure. It should be very attractive if the light source could generate additional signal, offering a novel multi-modal functional source. Here we report on the fabrication and characteristics of strain-induced piezoelectric potential stimulated luminescence from ZnS:Mn thin-film grown on piezoelectric Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) which provides an effective and precise control over strain state. The wurtzite-type Mn2+ doped ZnS has a non-central symmetric structure. Strain-mediated luminescence of the ZnS:Mn film is realized, resulting from the piezoelectric potential. Moreover, simultaneous generation of light and ultrasound wave is observed in this system. We can tune the luminescence and ultrasound signal of the ZnS:Mn films via a converse piezoelectric effect in PMN-PT upon the application of electric-field. On the other hand, spectroscopic tuning and enhancement of photoluminescence (PL) are highly desirable for the purpose of understanding the physical processes of energy transition and widespread applications. To date, modification of the PL in metal-ion doped materials excited by a given excitation source can only be achieved by changing the composition of host materials and/or doping metal ions, which is essentially an irreversible process. Interestingly, some luminescent metal ions can be regarded as sensitive spectroscopic probes for the dopant site symmetry and structure. Therefore, it is of great interest to take advantage of the unique properties of ferroelectric host material in combination with the doping metal ions. We have demonstrated that the enhancement and modulation of upconversion PL can be realized by applying relatively low voltages to metal-ion doped ferroelectric thin-films in an in-situ and real-time manner. These results will aid further investigations of the coupling between piezoelectric/ferroelectric effects and luminescence because they provide an additional degree of freedom in the design of novel materials and devices. The works are supported by the grants from RGC GRF (PolyU5002/12P), NSFC (No. 51272218) and PolyU grant (No.A-PL50).

The concept of "Piezotronic Effect", as was proposed by Z. L. Wang, has led to the discovery of nanowire nanogenerators, which is approaching inductrial application scaling. Here we provide more evidences to support the Piezotronic Effect which is based on the bending deformation of the semiconductor nanowires. High special/energy resolution cathodoluminescence (CL) spectroscopy enables us to make precise investigation on the optical/electronic fine structures in nanostructures. The linear distribution of strain from tensile to compression in bent ZnO nano/microwires provides ideal conditions to address the modification of the electronic structures by strain in semiconductor materials. Radial line scan of the CL spectroscopy along bent ZnO wires at liquid helium temperature shows very fine excitonic emission structures, which demonstrates systematic red shift towards the increase of tensile strain, and blue shift as well as excitonic peak splitting towards the increase of compressive strain. Strain-gradient is found to dominate the overall red-shift of the emission energy at a pure bending configuration.
References:
[1] X. Han, L. Kou, X. Lang, J. Xia, N. Wang, R. Qin, J. Lu, J. Xu, Z. Liao, X. Zhang, X. Shan, X. Song, J. Gao, W. Guo, D. Yu, Adv. Mater. 2009, 21, 4937.
[2] Q. Fu, Z. Y. Zhang, L. Kou, P. Wu, X. Han, X. Zhu, J. Gao, J. Xu, Q. Zhao, W. Guo, Nano Res. 2011, 4, 308.
[3] X. Han, L. Kou, Z. Zhang, X. Zhu, J. Xu, Z. Liao, W. Guo, D. Yu, Adv. Mater. 2012, doi: 10.1002/adma.201104372.

Semiconductor nanowires (NW) are considered as one of the most useful and diverse classes of functional nanomaterials for electronics, sensors, optoelectronics and energy sciences. By surface functionalization of NWs, individual NW-based FETs can serve as ultrasensitive sensors for detecting a wide range of gas, chemicals, and biomedical species. Although the size of the devices is reduced dramatically using NWs, the sensitivity does not increase so much since the active area of the sensor decrease as well, which means the possibility of adsorbing the target molecules on the sensor surface drops a lot as well. Traditionally, in order to enhance the sensitivity, the contacts of the devices are always chosen to be ohmic for maximizing the “gating” effect from the adsorbed molecules. Also the sensor area is chosen so small that a group of molecules adsorbed on the surface may significantly change its conductance. In our previous work, we introduced a new approach of replacing the ohmic contact by the Schottky contact, in such a case, the contact area can be small although the surface area of the NW can be large. Using such a design, super-high sensitivity has been demonstrated for NW UV sensors, bio-sensors and gas sensors. The key here is that the adsorbed molecules modify the metal-semiconductor contact.
Wurtzite and zinc blend structured semiconductors, such as GaN and ZnO, usually have piezoelectric effect. Strain induced piezoelectric polarization in the material can be effectively used to tune the semiconductor characteristics of the device, which is referred as the piezotronic effect. Here, by using a metal-semiconductor-metal back-to-back Schottky contacted ZnO NW device, we have demonstrated the piezoelectric effect on the performance of a pH sensor. An externally applied strain produces a piezopotential in the NW, which tunes the effective height of the Schottky barrier at the local contact, consequently increases the sensitivity and signal level of the sensors. Furthermore, the strain produced piezopotential along the ZnO NW will lead to a non-uniform distribution of the target molecules near the NW surface owing to electrostatic interaction, which will make the sensor proactive to detect the target molecules even at extremely low concentration over all, which naturally improve the sensitivity and lower detecting limit. This prototype device offers a new concept for designing supersensitive and fast-response nanowire sensors by introducing the external strain and piezotronic effect, which may have great applications in building sensors with fast response and reset time, high selectivity, high responsively, and good signal-to-noise ratio for chemical, biochemical and gas sensing.

Photolysis, or light driven water splitting, is a promising renewable method to synthesize hydrogen. However, known catalysts are impractical either because of their high cost or low solar efficiency. This paper will describe research undertaken to test the hypothesis that by controlling charges at the interfaces of heterostructures, it is possible to build in a mechanism to separate photogenerated electron-hole pairs and thereby increase efficiency. In this talk, evidence will be provided that enhanced composite reactivity can occur when the interfacial charge is provided by a ferroelectric, a polar surface termination, or band offsets. It is first shown that polarization arising from ferroelectric domains can influence the photochemical reactivity of metal oxides. The mechanisms of photochemical reactions on BaTiO3 and BiFeO3 surfaces will then be reviewed. Next, photochemical reactions on TiO2 films supported by BaTiO3 and BiFeO3 are considered. The findings indicate that polarization within ferroelectric domains influences the motion of photogenerated charge carriers. Because electrons and holes travel in opposite directions in the same dipolar field, oxidation and reduction reactions occur in spatially selective patterns that are determined by the domain structure. Thin film heterostructure experiments have been used to show that the photochemical reactivity of TiO2 can be enhanced when it is supported on BaTiO3 and BiFeO3. The same effect has been translated to high surface area, hierarchically structured catalysts, which evolve hydrogen and degrade dyes at a greater rate than titania alone. This is attributed to electron-hole separation in the space charge region of the supporting ferroelectric that reduces recombination and makes more charge carriers available to participate in the reaction. In the final portion of the presentation, other mechanisms that can be used to create polarization at an interface will be described, including p-n junctions and polar surface terminations. For example, the reactivity of hematite (Fe2O3) films supported on SrTiO3, which supports polar terminations, it much greater than bulk hematite.

The manipulation of charge-carrier conduction characteristics is a critical attribute governing the operation and efficiency of photovoltaic, catalytic, and other energy-converting systems that are based on electrochemical principles. This manipulation is often accomplished through the application of electrical-potential gradients by an external power supply and/or the creation of electronic-state discontinuities by heterojunction-interface engineering.
Piezoelectric materials have long been used as a source of bias and mechanical displacement, relying on their mechanical to electrical coupling character for applications in sensors, actuators, and energy harvesters. This piezoelectric bias has also been used to modulate charge carrier energetics through neighboring materials. For example, straining effects in piezoelectric photoelectrochemical cells have been shown to result in performance enhancements through manipulation of interface energetics.
In principle, the piezoelectric modulation of charge carrier energetics should extend beyond the bounds of the buried electronic interfaces explored to date, thus allowing the direct enhancement or suppression of electrochemical processes that occur at the interface of a piezoelectric material and a solution (i.e., piezocatalysis). Preliminary experiments have shown an evolution of H2 and O2 from mechanically agitated piezoelectric BaTiO3 and ZnO microstructures in an aqueous sonication bath. In order to elucidate the intriguing piezocatalytic phenomenon, we report a systematic study of the piezoelectric-potential driven electrochemical H2evolution process that takes place at the electrodes located on the surface of the material. The results compliment the general trends expected from the combinatorial assemblage of piezoelectricity and electrochemistry. The H2 evolution rates were dependent upon the oscillation frequency and amplitude of the piezoelectric material, in accordance with the combination of the direct piezoelectric effect and electrochemical reactions.

Conventional CMOS based logic units are electronically triggered and driven by externally applied gate voltages, which are considered as “static” and are separated from the dynamic mechanical actuation units in nano-electromechanical systems (NEMS). We present the first piezoelectric trigged mechanical-electronic logic operation using the piezotronic effect, through which the integrated mechanical actuation and electronic logic computation are achieved using only ZnO nanowires (NWs). By utilizing the piezoelectric potential created in a ZnO NW under externally applied deformation, strain-gated transistors (SGTs) have been fabricated. Using the SGTs as building blocks, universal logic components such as inverters, NAND, NOR and XOR gates have been demonstrated for performing piezotronic logic calculations. In contrast to the conventional CMOS logic units, the SGT based logic units are driven by mechanical agitation and relies only on n-type ZnO NWs without the presence of p-type semiconductor components. The mechanical-electronic logic units can be integrated with NEMS technology to achieve advanced and complex functionalities in nanorobotics, microfluidics and micro/nano-systems.

Although Moore&’s Law scaling, the exponential increase in the number of CMOS transistors per unit area, continues unabated, computer clock speeds have been frozen since 2003. We present the development of a new digital switch, the PiezoElectronic Transistor (PET)[1,2], which is designed to circumvent the speed and power limitations of the CMOS transistor. The PET operates on a novel principle: an electrical input is transduced into an acoustic pulse by a piezoelectric element (PE) which, in turn, is used to drive a continuous insulator-to-metal transition in a piezoresistive element (PR), thus switching on the device with high ON/OFF (~10^4). The device operation involves application of pressure from the PE onto the PR in a stacked geometry (metal/PE /metal/PR/metal), the PE/PR thereby being in (continuous) mechanical contact but not sharing an interface, differentiating the PET from other piezoelectrically-driven devices. Performance is enabled by the use of key high response materials, a relaxor piezoelectric and a rare earth chalcogonide piezoresistor. We discuss the predictions of theory and simulation, assuming bulk materials properties can be approximately retained at scale, that PET&’s can operate at one-tenth the present voltage of CMOS technology and 100 times less power while running at multi-GHz clock speeds on non-trivial circuits such as adders. The development of thin film piezoresistive and piezoelectric materials and associated characterization tools is presented next, followed by description of the fabrication and characterization of 1st generation PET devices.
1. D.M. Newns, B.G. Elmegreen, X.-H. Liu, G.J. Martyna, Adv. Mat.. 24 3672 (2012).
2. D.M. Newns, B.G. Elmegreen, X.-H. Liu, G.J. Martyna, J. Appl. Phys.111, 084509 (2012).

The mechanical properties of volume-confined materials differ dramatically from those of conventional bulk samples as the nanoscale is approached. In the context of piezotronic nanogenerators, the unique mechanical properties of ZnO 1-D nanostructures represent indeed the truly “enabling factor” of this new innovative technology. Due to their ability to withstand large elastic deformations (up to 15% vs. < 1% in bulk form) without breaking, ZnO nanowires are proving decisive - in terms of energetic efficiency and durability - for the viability of nanogenerators. Accordingly, the accurate measurements of elastic (i.e. Young&’s Modulus) and failure (e.g. fracture, fatigue, buckling, etc.) properties both of individual ZnO nanowires as a function of size and of arrays made of many such nanowires are crucial for design purposes.
The first part of the talk is devoted to some notable results from experimental nanomechanics of solids, underlying the lesson learned from nanoscale metal samples. In-situ micro-compression experiments on metals nanopillars have revealed that the strengthening effect follows a power-law dependence that can be understood within a generalized Weibull-like framework. Attention to metrological issues proved also to be crucial for telling intrinsic properties from extrinsic ones when testing nanostructures, e.g. via AFM or TEM-monitored tests.
The second part turns to the characterization of mechanical properties of single ZnO nanowires, making the connection with the body of results for metal pillars. Conflicting reports in literature are pointed out and critically evaluated in the light of more modern approaches and theories. The coupling between mechanical and functional properties is emphasized, also with respect to the challenges it raises for the experimental characterization of actual ZnO nanogenerators.
The third part of the talk finally illustrates the appealing possibility to perform a collective "statistical" test on a thousand (or a million) of quasi-1D ZnO nanowires simultaneously to characterize devices for energy harvesting, which is of consequence by the viewpoints of (industrial) process control and damage tolerance.

Piezotronics is a term coined in by Prof Zhong Lin Wang (Georgia Institute of Technology, Atlanta, USA) and describes the exploitation of strain and deformation internal polarization fields in polar semiconductors. Such fields already find applications in transducers and micropositioner devices but are also know to be present in GaN based light emitting diodes and lasers. Being a property of polar semiconductors piezoelectricity is present in both III-V and II-VI compounds, such as the technologically important ZnO. For many years piezoelectricity was included in the design of devices only to first order. In recent years a great deal of evidence, both model and experimental data, has been generated that such effects need to be included to at least second order.[1] The inclusion of such non linear effects produces surprising and non intuitive results, notably the generation of fields of opposite polarity compared to the prediction of linear piezoelectricity and the possibility of enhancing the piezoelectric polarization by a factor of 5-10 under particular deformations. In this presentation we will show the evidence for non linear effects and discuss the possible applications to light emitting diodes, quantum dot emitters and energy harvesting devices. Our theory of Non Linearities, based on accurate quantum mechanical calculations and a tight binding formulation of the elastic and dielectric properties of zincblende and wurtzite crystals, is capable of highlighting and correctly predicting the polarization properties of several polar semiconductors under strain.
We observed strong non linearities in the strain dependence of both zincblende III-V&’s and wurtzite III-N and its alloys and even II-VI wurtzite semiconductors. While revealing much smaller Spontaneous Polarization effects in nitrides than previously reported, much larger values of the total (spontaneous + strain induced) polarization are observed compared to linear models, therefore highlighting the large effect of Non Linear Piezoelectricity in wurtzite semiconductors.
The significance of our work is that by incorporating our unique and well tested ability of evaluating non linear polarization in the design of composite semiconductor structures we will in the near future be able to propose layouts were the piezoelectric fields, i.e. the ‘engine&’ inside PEHs devices, can be suitably enhanced, greatly increasing, possibly by a factor of 2 or more, the ability of such devices to convert mechanical energy into an electrical potential difference.
We will also show how knowledge of non linearities in the piezoelectric field in materials such as wurtzite InGaN leads to new areas of exploitation for optical devices such as quantum sources of entangled photons or novel light emitting diodes with significantly increase efficiency.
[1] M. A. Migliorato et al, Phys. Rev. B 74, 245332 (2006); R. Garg et al, Appl. Phys. Lett. 95, 041912 (2009); J.Pal et al, Phys. Rev. B 84, 085211 (2011)

Bending Strain causes rich physical phenomena, such turning the mechanical energy into electricity in nanowire nano-generators by Z. L. Wang, significant modification of the emission energy of semiconductor micro/nanowire materials up to 100 meV, and enhancement of the light emission intensity up to 17 times of the LEDs etc.. Here, we investigate for the first time the exciton spectra evolution in bent ZnO microwires along the radial direction via high spatial/energy resolution cathodeluminescence spectroscopy at 5.5 K. Our experiments show that the exciton peak splits into multi fine peaks towards the compressive part while retains one peak in the tensile part and the emission peak displays a continuous blue-shift from tensile to compressive edges. In combination with first-principles calculations, we show that the observed near-band-edge emission splitting in compressive side is due to the valence band splitting by compressive strain and the absence of peak splitting in the tensile part maybe due to the highly localized holes in the A band and the carrier density distribution across the microwire by piezoelectric effect induced electric field. Our studies may pave the way to design nanophotonic and electronic devices using bent ZnO nanowires.
[1] Zhi-Min Liao, Han-Chun Wu, Qiang Fu, Xuewen Fu, Xinli Zhu, Jun Xu, Igor V. Shvets,
Zhuhua Zhang, Wanlin Guo, Yamin Leprince-Wang, Qing Zhao, Xiaosong Wu and Da-Peng Yu, Scientific Reports 2012, DOI: 10.1038/srep00452.
[2] X. Han, L. Kou, X. Lang, J. Xia, N. Wang, R. Qin, J. Lu, J. Xu, Z. Liao, X. Zhang, X. Shan, X. Song, J. Gao, W. Guo, D. Yu, Adv. Mater. 2009, 21, 4937.
[3] Q. Fu, Z. Y. Zhang, L. Kou, P. Wu, X. Han, X. Zhu, J. Gao, J. Xu, Q. Zhao, W. Guo, Nano Res. 2011, 4, 308.

Among the biggest challenges in harnessing the power of nanotechnology is achieving dynamic control of mechanical, chemical and electronic properties of nanoscale devices. In principle, dynamic control could be achieved by applying external electric fields to piezoelectric materials, but carbon-based nanomaterials like graphene are not intrinsically piezoelectric. In this work, we discover that piezoelectric effects can be engineered into nonpiezoelectric graphene through the selective surface adsorption of atoms. Our calculations show that chemically modifying a single sheet of graphene with atoms on one side results in the generation of piezoelectricity by breaking inversion symmetry. We demonstrate that adsorbing different atoms on graphene results in varying piezoelectric magnitudes. Despite their 2D nature, piezoelectric magnitudes are found to be comparable to those in 3D piezoelectric materials. We find that both e11 and e31 type piezoelectric properties can be chemically endowed upon graphene. Our results elucidate a designer piezoelectric phenomenon, unique to the nanoscale, that has potential to bring dynamic mechanical control and sensing to existing graphene-based devices and other technologies.

In order to achieve maximal efficiency, the charge transport mobility is a relevant factor in piezoelectric nanogenerators [1] and piezotronic devices. Calculations of mobility have been prevented by the absence of a reliable model of charge transport in nanostructures, where the mobility is dominated by short range. In such cases the Drude model, which is a successful description of transport in bulk systems, becomes inapplicable.
In recent times, however, studies of carrier motion with high resolution techniques [2] in a variety of nanostructures have indicated that a modified Drude model can be applied, by considering carrier bound motion from back-scattering mechanisms and localized oscillator modes. Such models have been verified down to very short times typical of carrier injection in the semiconductor.
Based on the results of these studies it is suggested that a model of damped harmonic oscillation modes can be used to predict the mobility in piezotronic devices. Here, the case of a system subject to static and low frequency piezoelectric fields is considered, which corresponds to typical working conditions of nanodevices. The transient mobility has been evaluated from the solutions of the damped harmonic motion, on taking into account the boundary conditions at the initial time of the drift. Besides the mobility, characteristic dynamic parameters, i.e displacement, velocity, acceleration and time-correlation functions have been calculated.
Results for a variety of ZnO structures and geometries are presented. For sufficiently large damping the d.c mobility shows biphasic structure with fast response at short length scales and slower response at longer scale, with a bi-exponential law, a distinctive feature as compared to the Drude case. This kind of behaviour has been used to discuss the relaxation dynamics of carriers studied by time-resolved spectroscopy, the conductance changes in ZnO sensors under bending and the response of ZnO nanogenerators excited by sound waves [1,3].
[1] X. Wang, J. Song, J. Liu and Z. L. Wang, “Direct-Current Nanogenerator Driven by Ultrasonic Waves”, Science, 316, 102-105(2007)
[2] J. Lloyd-Hughes, T. Jeon , “A Review of the Terahertz Conductivity of Bulk and Nano-materials”, Journal of Infrared, Millimeter, and Terahertz Waves 33, 871-925(2012)
[3] S. N. Cha, J. Seo, S. M. Kim, H. J. Kim, Y. J. Park, S. Kim, J. M. Kim, “Sound-Driven Piezoelectric Nanowire-Based Nanogenerators”, Advanced Materials, 22,4726-4730(2010)

Owing to their unique electronic, optical and piezoelectric properties, one-dimensional ZnO nanomaterials are envisioned as fundamental building blocks of future electromechanical, optoelectronic, sensing and actuation nanosystems. By employing the in situ characterization method, the fundamental principles of the piezoresistance, piezoelectric and the piezo-phototronic effects of ZnO were investigated [1-2]. Design and fabrication of novel nanowire/belt based sensors and piezotronic and optical devices will be presented in this report [3-7]. Moreover, nanoscale failure in the building blocks of a device is inevitable in nanodevice fabrication and manipulation, which can lead to malfunction and/or even failure of the entire device. So, we pioneered the investigation of nanofailure of ZnO based piezoelectric devices by scanning probe microscopy [8]. This fundamental understanding should pave the way to design optimized nanowire systems for electronic, electromechanical, and optoelectronic applications.
References:
1. Y. Zhang, X. Q. Yan, Y. Yang, et al., Adv. Mater., 2012, 24(34):4647-4655.
2. Y.Yang, J. J. Qi, Y. Zhang, et al., Nano Lett., 2012, 12 (4): 1919-1922.
3. Y. Yang, J. J. Qi, Y. Zhang, et al., Nanotechnology, 2009, 20, 125201.
4. X. M. Zhang, Y. Zhang et al., Adv. Mater. 2009, 21, 2767-2770.
5. W. Guo, Y. Yang, J. J. Qi, J. Zhao, Y. Zhang, Appl. Phys.Lett. 2010, 97, 133112.
6. Y. Lei, X. Q. Yan, Y. Yang, et al., Nanoscale, 2012, 4, 3438-3443.
7. S. W. Ma, Y. H. Huang, Y. Zhang, et al., Nanoscale, 2012, 4, 6415-6418.
8. Y. Yang, J. J. Qi, Q. L. Liao, W. Guo, Y. Zhang, Appl. Phys. Lett. 2009, 95, 123112.

For the high temperature piezoelectric application, it is essential to ask the ceramics should possess both of higher Curie Temperature TC and piezoelectric coefficient (d33). Unfortunately, it is difficult to obtain high-performance ceramics prepared via the conventional mixed oxide route because the rotation of the Ps for BLSFs ceramics is often restricted in a-b planes. A relatively high dc electric field is often used to poling the BLSFs ceramics, and the d33 value is relatively low for pure BLSFs ceramics with Aurivillius phase structure. Some studies have reported that the enhancement of piezoelectric properties of BLSFs ceramics could be obtained by chemical modification and improved sintering6.
Na0.5Bi2.5Nb2O9 (NBN) materials belong to the two layered Aurivillius phase materials with a relatively high Curie-temperature of ~780 oC.In our previous work, using Li, Ce, Sb-codoped NBN-based ceramics exhibited good piezoactivity and good thermal stability[3]. However, in these compositions, mechanical quality factor, Qm and dc conductivity were relatively low due to large amount of A-site vacancies. Therefore, in the present work, (LiCe) -substituted Na0.5Bi2.5Nb2O9 [(NaBi)0.5-x(LiCe)xBi2Nb2O9] ceramics were fabricated by the conventional mixed oxides route. The impendance properties and conductive meahcnism of the samples were studied, and underlying physical mechanisms were also discussed.

Silver-implanted aluminum nitride thin films are investigated as candidates for piezoelectric based MEMS devices with buried contact layers. Multiple source reactive ion sputtering is used to deposit asymp;1.8 mu;m of AlN on a 525 mu;m thick (100) Si substrate, followed by ion implantation into the aluminum nitride at 100 keV to achieve a dose of 1.0 × 10^16 ions/ sq. cm. and subsequent annealing at temperatures between 400 and 800°C. Computer simulation models using Transport of Ions in Matter (TRIM) and TRIDYN are applied and confirmed by Rutherford Backscattering Spectrometry (RBS). X-ray diffraction and electron diffraction establish both the epitaxy of the AlN film on the (100) Si substrate and the crystalline quality of the epilayer prior to thermal annealing. Electron microscopy reveals that the sputtered AlN films grow epitaxially in a columnar morphology with average columnar grain diameters in the 50 nanometer regime. The average Ag nanoparticle size after heat treatment is found to be 70 nanometers. It is concluded that the Ag implantation does indeed have potential as a synthesis protocol for buried contact layer generation in piezoelectric based MEMS devices.

Owing to a tremendously increasing energy consumption of modern societies, the development of green and renewable nanoenergy sources is becoming one of the most important fields of research. There are several typical physics effects, which can be used to fabricate nanognerators (NGs) for harvesting energy from the ambient environment. Both piezoelectric and triboelectric effect can be used to harvest mechanical energy. However, the NGs based on the two effects can only work under the mechanical vibration conditions. It is necessary to develop a variety of technologies for harvesting different types of nanoenergy. Pyroelectric effect is based on the change of spontaneous polarization in certain anisotropic solids due to the temperature fluctuation, which can be used to harvest the thermal energy from the change in temperature. Although the Seebeck effect can also be used to harvest thermal energy by utilizing a temperature difference between two ends of the device for driving the diffusion of charge carriers, pyroelectric effect has to be the choice in an environment where the temperature is spatially uniform without a gradient.9 This type of temperature fluctuation universally exists in outdoor in our daily life from day to night.
Self-powering nanotechnology has been developed since 2005 with the aim to build self-powered systems that can operate independently and wirelessly without the use of a battery or other energy storage/supply system. We have been developing nanogenerators for this purpose. One of the great applications for the self-powered system is to use nanogenerators to drive the personal electronics, such as LCDs, LEDs. Currently, the piezoelectric NGs have been used to drive some electrical devices by harvesting mechanical energy from environment. Although different designs of PENGs have been reported, the output voltage and current of these devices are still very low (voltage below 0.1 V, and current below 1 nA), which are not enough for driving commercial consumer electronics. To solve this problem, the performance optimization of the PENGs is desperately needed. Here, we demonstrated a pyroelectric nanogenerator (PENG), where the output voltage and current can be up to 22 V and 430 nA, respectively. A detailed theory has been developed for understanding the output performance of PENG. The obtained pyroelectric coefficient is about 80 nC/cm2K, which is 80 times larger than that of ZnO materials. Under a change in temperature of 45 K, a single output pulse of PENG can continuously drive a LCD with the time of larger than 60 s. The Li-ion battery can be charged by such PENGs under the different frequencies, which has a potential application in the wireless temperature sensors.
References:
(1) Ya Yang et al. Nano Letters, 2012, 12, 2833-2838.
(2) Ya Yang et al. ACS Nano, 2012, 6, 6984-6989.

Nobel Prize-winning work on graphene has placed atomically thin two-dimensional (2D) crystals at the focus of considerable research attention. The discovery of 2D crystals was groundbreaking because these materials possess several emergent properties that are not present in their bulk 3D parent crystal. 2D emergent properties include: exceptional mechanical strength, graphene&’s exotic electronic properties, and direct band gaps in transition metal dichalcogenides.
Our modeling work has discovered that piezoelectricity is also an emergent property of many 2D crystals: BN, MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂. These atomically thin 2D crystals are piezoelectric, whereas their bulk parent crystals are not. This radically new notion of piezoelectric monolayers being isolated from an entirely non-piezoelectric 3D crystal suggests potential for intriguing electromechanical effects in the single- and few-layer regime.
In the single-layer case, our density functional theory calculations reveal that the piezoelectric coefficients of the studied 2D crystals are on par with commonly used 3D wurtzite piezoelectrics. Piezoelectric coupling in 2D crystals could have exciting implications for nanoscale piezotronics.
Piezoelectric effects also exist in the 2-layer case: we have studied a bilayer consisting of two BN monolayers and have found that flexural electromechanical coupling is yet another emergent property that is unique to the bilayer case. A BN bilayer can be seen as a cantilever-type heterostructure where one layer&’s strain is opposite from that of the other. This hints at the possibility of electrically controlling or sensing the curvature of a membrane that is only ~3 Å thick. More detailed analysis reveals that such a bilayer of BN amplifies piezoelectric displacements by a factor on the order of 10³-10⁴.
Our work was supported in part by the U.S. Army Research Laboratory, through the Army High Performance Computing Research Center, and was also partially supported by a DARPA YFA grant.

Ferroelectric polymers, in particular polyvinylidene fluoride (PVDF) and its copolymers, offer an attractive combination of properties such as relatively high spontaneous polarization, piezoelectricity, chemically inert behavior, electrical strength, and durability. These benefits together with benign processing demands explain the attention that polyvinylidene fluoride- trifluoroethylene P(VDF-TrFE) thin films have received as promising materials for nonvolatile memories and full-organic transistors. Recently, attention of researchers has more and more often been attracted to objects simultaneously having properties of polymers and classical ferroelectrics. Such objects are composite films based on polymeric materials with addition of ferroelectrics, e. g. lead zirconate titanate (PZT), barium lead zirconate titanate (BPZT), and single crystals of the triglycine sulfate (TGS) group.
In this work, we report on local ferroelectric and piezoelectric properties of nanostructured polymer nanocomposites P(VDF-TrFE)+xBPZT (x = 0 - 50 %) studied using scanning probe microscopy techniques. Copolymer P(VDF-TrFE) samples were prepared by the solvent-cast method. Crystallization was performed for 2-3 hours at 100 °C until complete evaporation of the solvent. Film samples which thickness, d, varied from 10 to 20 mu;m were obtained. The films were not preliminary treated by stretching, high-temperature annealing, or applying a polarizing electric field. The same method was used to fabricate composite polymer films containing BPZT. The prepared preliminarily ferroelectric powder was added to the solution containing the dissolved copolymer. Composite samples with 10 - 50 % of volumetric fraction of the crystalline ferroelectrics were obtained.
Ferroelectric domains imaging, local polarization switching, and polarization relaxation dynamics were studied by piezoresponse force microscopy (PFM) using scanning probe laboratories MFP-3D (Asylum Research, USA) and NTEGRA-Prima (NT MDT, Russia). In particular, we found that BPZT inclusion usually show a strong unipolar piezoresponse signal, as compared to the polymer matrix. By scanning under high dc voltage films can be polarized uniformly under both positive and negative electric fields. Also, no obvious backswitching was observed when the remnant piezoelectric response read out again after 1.5 h.

Ceramic samples with the composition of Bi3NbTiO9(BTN) were prepared by Bi2O3,TiO2 and Nb2O5 using solid-state reaction method.The samples were analyzed using X-ray diffraction and scan electron microscopy. It was that BTN ceramics could be fabricated at lower sintering temperature and the different process conditions had the importe influence on their piezoelectric and dielectric properties. The dense and uniform BTN ceramics can be obtained by isostatic pressure,according to the experiment. The recrystallization could make the ceramic grains grow uniformly and withstand higher polarization field. The d33of BTN ceramics poled by 8kV/mm could reach 5.6pC/N.

We investigated the characteristics of Metal-ferroelectric-insulator-semiconductor (MFIS) structure using poly(vinylidene fluoride trifluorethylene) PVDF/TrFE (65/35) as a ferroelectric layer and polyvinyl alcohol (PVA)(0.4wt%) as an insulating buffer layer. In MIS structure, we found that the 0.4wt% PVA solution gave excellent insulator results at the bias sweep range ± 5V and leakage current density of the PVA film with 0.1, 0.4 and 0.7wt% were about 10^-5 A/cm², 10^-6 A/cm² and 10^-7 A/cm², respectively. With these results above, we selected the 0.4wt% PVA solution used in MFIS structure. In MFS and MFIS structures, the memory window widths of the measured for a voltage sweep ±5V, were about 2V and 3V, respectively. Hysteresis loops due to the ferroelectricity of the PVDF-TrFE with β phase were observed in C-V characteristics. The current densities ware about 10^-5 A/cm² and 10^-6 A/cm² at 5V for MFS and MFIS structures, respectively.

Due to the unique coupled piezoelectric and semiconducting properties, ZnO have been forming a new filed of piezo-phototronics, a lot of novel functional micro/nanodevices of ZnO have been developed. Recently, people have down lots of work on the bending strain effect on the ZnO micro/nanowire UV detectors and found the bending strain has great effect on the their UV response properties. Although a lot of progress has been made in the strain effect on the ZnO micro/nanowire UV detectors, there are some contradictions in the results due to the complex influence of unstable contact types and measuring environment. Here we investigate the localized ultraviolet photoresponse properties of bent ZnO microwires bridging two perfect Ohmic contacts in both atmospheric and high vacuum (8×10-6 torr) environments for the first time to explore the bending strain effect on the photoelectrical properties of ZnO micro/nanowire itself. It is found that the ZnO microwire has larger photoconductivity and faster rising speed when photo-excitation is localized at the bending region in atmospheric environment, while the rising speeds are almost the same when photo-excitations are localized at the bending and straight regions under vacuum. A possible underlying mechanism of the coupling piezoelectric and surface oxygen absorption/desorption effects on the ZnO bent microwires has been proposed and discussed for the higher UV performance at the bent region in atmospheric environment. Our study reveals a new mechanism for the coupling between piezoelectric effect and surface oxygen absorption/desorption procedure and could be potentially useful for designing and fabricating the piezo-photonic-electric micro/nanodevices.
References:
[1] S. Yang, L. Wang, X. Tian, Z. Xu, W. Wang, X. Bai, and E. Wang, Piezotronic Effect of Zinc Oxide Nanowires Studied by In Situ TEM. Advanced Materials (2012)
[2] Y. Hu, Y. Chang, P. Fei, R. L. Snyder, and Z. L. Wang. Designing the Electric Transport Characteristics of ZnO Micro/Nanowire Devices by Coupling Piezoelectric and Photoexcitation Effects. ACS Nano 4 (2010) 1234.
[3] Y. Hu, Y. Zhang, Y. Chang, R. L. Snyder, and Z. L. Wang. Optimizing the power output of a ZnO photocell by piezopotential. ACS Nano 4 (2010) 4220.
[4] Q. Yang, X. Guo, W. Wang, Y. Zhang, S. Xu, D. H. Lien, and Z. L. Wang. Enhancing sensitivity of a single ZnO micro-/nanowire photodetector by piezo-phototronic effect. ACS Nano (2010)

Due to the coupled semiconducting and piezoelectric properties, one-dimensional ZnO nanomaterials are being widely explored for electromechanical, photovoltaic and sensing nanosystems. Recently, fabricating ultrafast, high-sensitivity sensors have received extensive attention for their potential use in nanoelectromechanical system (NEMS) [1].
Here we introduce a facile and cost-effective fabrication technique of strain sensors based on individual ZnO nano/microwire. By connecting a ZnO microwire along polar growth direction with two Ag electrodes on flexible polystyrene (PS) substrate, the piezotronic strain sensor was obtained as a typical M-S-M structure [2]. When the substrate was mechanically deformed, the I-V characteristic of the device was highly sensitive to the strain caused by the obvious change in Schottky barrier height (SBH). Furthermore, a linear relationship between the strain and the SBH change was observed, and a SBH increase of 46.7 meV was realized by applying a compressive strain of 0.16%. By calculating the SBH change at source and drain side, the obtained modulation of SBH from the piezoresistive effect (ΔPhi;ps) and piezoelectric effect (ΔPhi;pz) is 43 meV and 3.7 meV, respectively [3].
Finally, a gauge factor of 500 was received; the moderately low gauge factor may be attributed to the opposite signs of ΔPhi;ps and ΔPhi;pz because the piezoelectric effect depends on the orientation of c axis of ZnO NW. Moreover, the relatively high carrier density induced by the existence of defects and vacancies may screen the piezopotential and weaken the influence of piezoelectric effect [4].
Reference:
1. Zhang, Y., Yan, X. Q., Yang, Y., et al., Adv. Mater., 2012, 24(34), 4647-4655.
2. Zhou, J.; Gu, Y.; Fei, P.; Mai, W.; Gao, Y.; Yang, R.; Bao, G.; Wang, Z. L., Nano letters 2008, 8 (9), 3035-3040.
3. Zhou, J.; Fei, P.; Gu, Y.; Mai, W.; Gao, Y.; Yang, R.; Bao, G.; Wang, Z. L., Nano letters 2008, 8 (11), 3973-3977;
4. Zhang, Y.; Liu, Y.; Wang, Z. L., Adv Mater 2011, 23 (27), 3004-13.

Using the energy harvested from the environment to power electrical nanodevices without any applied voltage has been extensively study to construct self-powered nanodevices. It is a feasible technology to integrate high performance nanodevices with a voltage power supply as a self-powered testing system. In this study, we demonstrated a facile and effective method to construct self-powered vibration detector based on ZnO nanowires array to detect frequency of mechanical vibrations.
To fabricate self-powered vibration detector with a bottom-up method, ZnO nanowires arrays covered by flexible Pt electrode on the top were vertically grown on fluorine-doped SnO2 (FTO) electrodes with a low-temperature solution method. A typical Schottky barrier was formed at the interface between Pt electrode and ZnO nanowire arrays by analyzing the current-voltage characteristic of the detector. The current responses under different frequency of vibration without any applied voltage were well confirmed with the input frequency with the error below 0.3%, and relative changes of current at 1Hz, 5Hz, 10Hz, and 15Hz were quite uniform about 10nA under compressive stress of 0.9KPa. The current responses were attributed to the piezoelectric effect and Schottky barrier change of ZnO nanowire arrays by applying compressive stresses along the polar growth direction. Fermi level upraised by the piezopotential generated from the deformation of ZnO nanowire resulted in the electrons flowing from Pt electrode to FTO electrode. The relative changes of current enhanced with increased compressive stress, and it was also effectively enhanced by connecting two detectors in parallel.
In summary, the self-powered vibration detector based on ZnO nanowires array powered by the mechanical vibration presents sensitive and accurate response to frequencies of mechanical vibration.
1. Wang, Z. L., Towards Self-Powered Nanosystems: From Nanogenerators to Nanopiezotronics. Advanced Functional Materials 2008, 18 (22), 3553-3567.
2. Xu, S.; Qin, Y.; Xu, C.; Wei, Y.; Yang, R.; Wang, Z. L., Self-powered nanowire devices. Nature nanotechnology 2010, 5 (5), 366-373.
3. Hu, Y.; Zhang, Y.; Xu, C.; Lin, L.; Snyder, R. L.; Wang, Z. L., Self-powered system with wireless data transmission. Nano letters 2011, 11 (6), 2572-7.

We introduce how a physical layer, based on a 20nm Au film, blocks a growing process of ZnO nanowires (NWs) along intended directions. Indeed, we show how by using two of the simplest methods for growing ZnO nanowires based on aqueous solution processes, i.e. the chemical and electrochemical methods at low temperature (under 90°C), synthesis of novel nano-structures is achieved. To this aim, an intermediate process step was accomplished, which consists in blocking half of the ZnO NWs walls or their tips, by evaporating gold layers at various deposition angles. By continuing the growth process, the half of nanowires develops longitudinally in trapezoidal cross section and, also, hollow nanowires are achieved. Though evaporation was preferred to sputtering method to pursue directional deposition of the gold thin film, it is not possible to achieve a totally selective covering of the desired parts due to the lack of an absolutely vertical shape of previously grown nanowires, and to their conical shape in some experiments. Accordingly, we propose a couple of strategies to overcome these challenges. Moreover, we report that specific conditions are necessary to attract all newly arrived ions in order to continue growing of the existing nanowires, and to prevent them from seeding on the gold layers and thus forming individual nanowires. Finally, it is illustrated how the technological strategies can be applied to the embedded nanowires in a polymer, in order to gain long tubular shape growing with hexagonal cross section. In fact, at last, we demonstrate how the covered tips of nanowires works as a seed to grow nanotubes on flexible or even transparent materials without requiring commonly used high temperature procedures for seeding. This concept can be considered as a bottom-up fabrication method for growing nanotubes, which brings the opportunity to build nanostructures in the hollow regions before growing of walls occur. The dimensions of the first generation of nanowires are of about 300-500 nm in diameter and 2.5 µm in length. For the hollow structures and nanotubes, the inner diameter is in the range of 150-500 nm and the outer dimension is around 400-900 nm, while the typical length for the tallest nanotubes that we developed is of 2 µm.

ZnO nanostructures are among the most important building blocks for piezotronics and for nanogenerators due to their mechanical, electrical, semiconductive, optical, piezoelectric, and pyroelectric properties, with applications ranging from optoelectronics to sensors and energy harvesting. Clearly, in most cases, a high control on the properties of the grown ZnO nanostructures is essential; as an example, some devices may require an accurate control of the nanowires length.
The wet chemistry synthesis of ZnO nanowires offers many advantages in comparison to the other methods reported in literature (e.g. PVD or CVD), including low cost, large area and low temperature processing, which is crucial for process compatibility, e.g. for integration into standard CMOS or MEMS systems. On the other hand it is also generally characterized by a rather poor control on the properties of the grown nanostructures. This disadvantage may, however, represent a serious issue for the fabrication of high performance piezophototronic devices and nanogenerators.
In order to circumvent this issue, we have developed a very low cost, in-situ control technique for continuously monitoring the impedance between a probe and the substrate during the wet-chemistry growth. In particular, we controlled, with resolution in the micrometer range, the relative position between an exchangeable stainless steel probe and the deposition substrate, e.g. a piece of n-doped silicon wafer coated by gold (with adhesion layer) and by a ZnO seed layer.
By contacting the probe and the silicon substrate, it was possible to electrically verify when the probe and the substrate came into mechanical contact directly inside the nutrient solution for the wet chemistry growth; in fact, the resistance between the substrate and the probe, when in contact and before nanostructures growth, was around 100 Omega;; the resistance between the detached probe (distance greater than 10 mu;m) and the substrate in the conductive nutrient solution was around 10 kOmega;. In our experiments, at the beginning, the probe was slowly moved down until an abrupt variation of the resistance revealed the contact between the probe and the substrate; afterwards, the probe was moved up and kept at a given distance from the substrate, e.g. at 50 mu;m, so that impedance could be continuously measured. Our preliminary results show that the probe-to-substrate impedance gradually changes as a result of the nanostructures growth and is strongly related to the characteristics of the grown nanostructures, thus confirming that the proposed method enables an in-situ on-line monitoring of wet-chemistry synthesis of ZnO nanostructures and can, therefore, represent a key tool for the fabrication of high performance piezotronic devices and nanogenerators.

Poly(vinylidene fluoride) (PVDF) copolymer has been attractive materials to be used in industrial application as energy harvester because of flexibility and lightweight. Recently reported piezoelectric electrospun PVDF copolymer nanofibers has been fabricated by far-field electrospinning since bulky size piezoelectric generator is available to apply real application. However, a gap exist about poling process between near-field and far-field electrospinning because induced electric field by far-field electrospinning is not enough strong to pole the PVDF copolymer. We introduce factor to induce poling of the Poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) nanofibers in far-field electrospinning method. Under mechanical compression, electrospun PVDF-TrFE nanofibers show repeatable and consistent electrical output. Phase conversion of PVDF-TrFE nanofiber from the initial α phase of the raw material to β phase was confirmed by X-ray diffraction. Electric field strength by residual charge in the electrospun was calculated by using Multiphysics simulation (COMSOL Multiphysics 3.5a).

Microelectromechanical systems (MEMS) incorporating active piezoelectric layers offers integrated actuation, sensing, and transduction. The broad implementation of such active MEMS has long been constrained by the inability to integrate materials with giant piezoelectric response, such as Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers using an epitaxial (001) SrTiO3 template layer, with superior piezoelectric coefficients (e31,f = -27 ± 3 C/m2) and figures of merit for piezoelectric energy harvesting systems. We have incorporated these heterostructures into microcantilevers that are actuated with extremely low drive voltage due to thin-film piezoelectric properties that rival bulk PMN-PT single crystals. These epitaxial heterostructures exhibit very large electromechanical coupling for ultrasound medical imaging, micro-fluidic control, mechanical sensing, energy harvesting and piezotronics.
This work has been done in collaboration with S. H. Baek, D. M. Kim, V. Aksyuk, R. R. Das, S. D. Bu, D. A. Felker, J. Lettieri, V. Vaithyanathan, S. S. N. Bharadwaja, N. Bassiri-Gharb, Y. B. Chen, H. P. Sun, C. M. Folkman, S. K. Streiffer, R. Ramesh, X. Q. Pan, S. Trolier-McKinstry, D. G. Schlom, M. S. Rzchowski and R. H. Blick.

Piezoelectric materials, which convert mechanical displacement to electrical energy (and vice versa), are crucial in mechanical sensing, energy harvesting, medical imaging and ultrasonic devices, etc. According to the piezoelectric theory, the piezoelectric coupling coefficient (d33), which represents the abilities of piezoelectric materials converting the mechanical deformation to electric signal, plays a key role on the device performance. The new generation of single-crystal piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) exhibits a piezoelectric effect that is ten times larger than that of conventional ceramics, and may revolutionize the application of piezoelectric material based devices. In this research, we have fabricated one-dimensional PMN-PT nanofibers and nanowires which showed very high piezoelectric coupling coefficients, and are of good potential being used as the fundamental building block for higher power nanogenerators, high sensitivity nanosensors, and large strain nanoactuators.
PMN-PT nanofibers were fabricated by an electrospinning process combined with a subsequent high temperature treatment. The diameter and alignment of electrospun nanofibers could be varied by parameters of the electrospinning process. Novel PMN-PT nanowires were also synthesized using a hydrothermal method. Detailed structural characterization of PMN-PT nanowires have showed that the PMN-PT nanowires are single crystalline with hierarchical alignments. The piezoelectric properties of single PMN-PT nanofiber and nanowire were characterized using Piezoresponse Force Microscopy (PFM). The d33 value of these PMN-PT nanostructures is much higher than the currently reported value of other piezoelectric nanostructures.
Various devices based on the PMN-PT nanostructures were fabricated and the signal generation from these devices was conducted. Our results indicate that the novel PMN-PT nanostructure based nano-devices can significantly enhance performances in energy harvesting and self-powered sensing.

We demonstrated a flexible nanogenerator made by a ZnSnO3 microbelt that generated an output power of sim;3 nW under a compressive and releasing strain of ~1%. High resolution transmission microscopy (HRTEM) image shows that the microbelt ZnSnO3 belongs to rhombohedral R3C structure. The piezoelectric properties of ZnSnO3 can be explained by the fact that the displacement of the Zn atom in the ZnO6 octahedral cell is almost two times that of the Sn atom in the SnO6 octahedral cell, resulting in a large spontaneous polarization effect that naturally forms in the crystal structure along the z-axis of ZnSnO3. An individual ZnSnO3 microbelt was bonded at its ends on a polystyrene (PS) substrate as a flexible nanogenerator. Through our detailed investigations, the flexible nanogenerator can produce a voltage and current of 100 mV and 30 nA, respectively. In addition, our nanogenerator can be applied as a self-power sensor. This discovery revealed a useful, alternative lead-free nanomaterial for piezoelectricity and energy harvesting.

As the restriction of hazardous substances is emerging as major issues, the development of lead-free piezoelectric materials has attracted considerable interest. As a strong candidate to replace lead-based piezoelectric material such as Pb(Zr,Ti)O3, alkaline niobates with perovskite structure such as (Li,Na,K)NbO3 have received great attention due to their high piezoelectricity, high Curie temperature, and electromechanical coupling constant. In this talk, we present the lead-free alkaline niobates nanostructure based piezoelectric device for high output, cost-effective, and flexible nanogenerator. For the small compressive strain, we obtained considerable open circuit voltage, closed circuit current, and successfully empowered a small LCD device. Our work implies that short and randomly aligned alkaline niobates nanostructures can be used for flexible high output nanogenerator by performing electric poling and optimizing the device structure.

Outstanding piezoelectric and dielectric properties of lead-based ferroelectrics emanate not only from strong, cooperative polar lattice distortions away from the prototype centrosymmetric structure that such compounds exhibit, but also from the flexibility of these distortions to change orientation under an applied external influence. I.e., these materials can be tuned into a specific area within their phase space — a so called morphotropic phase boundary (MPB) — where a small applied elastic or electric field triggers a phase transition accompanied by a large change of the system polarization. Although identifying and manufacturing compounds that contain MPBs is one of the most important problems related to the physics and applications of electroactive materials, it remains daunting due to their structural and compositional complexity, which obscures the workings of basic physical phenomena responsible for MPB formation.
Here, with the help of first-principles-based computational techniques applied to study the behavior of layered perovskite-oxide compounds, we establish a link between an emergence of an MPB and the existence of Goldstone-like states (collective, close to zero frequency excitations, requiring practically no consumption of energy) in such systems. Furthermore, we connect the presence of these excitations to the basic properties of the constitutive AO and BO₂ atomic planes, and the evolution of these properties as the planes are stacked together to form a crystal, thus revealing the underlying physics behind MPB formation. Specifically we find that competitiveness (rather than cooperativeness) of interactions between pseudo-Jahn-Teller distortive networks belonging to different ionic sites — e.g., lone-pair active sites, like Pb(II), and d⁰ sites, like Ti(IV) — is critical for the emergence of the Goldstone-like states in perovskite layers. This allows us to come up with a general recipe for creating new MPB-containing materials with advanced dielectric, piezoelectric and electrooptic properties that remain structurally and compositionally simple, as well as to explore options for achieving these enhanced properties in compounds that are lead-free.

The piezo-phototronic effect has attracted interest as a method of enhancing light emission and photodetection in ZnO nanowires, as well as a potential gating mechanism for electronic transport [1]. This effect relies upon local bandstructure variations induced by piezoelectric fields, which are created by mechanically straining the wire. The induced bandstructure changes have been experimentally shown to alter the light emission, light absorption, and carrier transport properties of ZnO nanowires. In order to obtain a full theoretical understanding of this effect and more fully exploit it, we use numerical modeling tools to simulate the device-scale physics. The finite-element solver COMSOL is used to obtain the strain field throughout the device for various mechanical stresses. From this, the piezoelectric polarization is easily calculated [2]. Then, the divergence of the piezoelectric polarization is calculated as an effective piezoelectric charge density. Finally, an in-house code is used to solve the Poisson equation for the electrostatic potential throughout the device, with the calculated piezoelectric charge density included as an additional term. This Poisson solver relies upon the box integration method of discretization, and it can model devices in two or three spatial dimensions or for cylindrical symmetry. The Poisson solver assumes that the Fermi level is flat (equilibrium conditions), and self-consistently calculates the mobile charge concentrations (electron and hole densities) and ionized dopant concentrations using Fermi-Dirac statistics, along with the electrostatic potential and band-edge diagram. The code also models heterojunctions and surface charges, which have a significant impact on nanowire device performance. Nonequilibrium conditions such as optically induced carrier populations and applied biases are approximately modeled using artificially split quasi-Fermi levels or discontinuities in the Fermi level (for insulators). Results will be presented showing the self-consistently calculated magnitude of local piezoelectrically induced band-edge variations, and the impact on device performance will be discussed.
[1] Z.L. Wang, Nano Today 5 (2010) 540-552.
[2] Z. Gao, J. Zhou, Y. Gu, P. Fei, Y. Hao, G. Bao, and Z.L. Wang, J. Appl. Phys. 105 (2009), 113707.

Quasi-1D piezoelectric semiconductive nanostructures are being intensively studied for their outstanding potential both for nanogenerators and for other piezotronic applications: in fact, quasi-1D nanostructures are largely deformed by tiny input mechanical forces, may have substantially higher piezoelectric coefficients, and can withstand extreme deformations without breaking (e.g. up to 15% vs. < 1% in bulk form for ZnO nanostructures), thus allowing the generation of high piezopotentials. However, taking full advantage of the potential of quasi-1D piezoelectric semiconductive nanostructures requires more accurate theoretical and numerical models. In particular, the i-v characteristics of quasi-1D piezoelectric nanostructures under a purely vertical compressive or tensile strain have only been computed for a Gaussian doping profile and Schottky contacts; moreover a standard commercial software has been used, which is likely non-ideal from both the points of view of accuracy and computational cost. For this reason here we develop an ad-hoc numerical model and also consider different doping profiles, as well as ohmic contacts. The proposed model is based on the complete drift-diffusion model that consists of a set of current and continuity equations for mobile charge carriers and the Poisson's equation, which is fully coupled in a nonlinear way with the mechanical Newton's law through the constitutive equations of the material that account for both the direct and inverse piezoelectric effects. In the continuity equations for mobile charge carriers, recombination mechanisms have been fully accounted for, including band-to-band recombination and trap-assisted (Shockley-Read-Hall) recombination. The set of nonlinear equations has been solved by the Finite Element Method which ensures the conservation of the total charge, the respect of local positive definite nature of carrier densities and the respect of monotonicity of the solution. In practice, the solution of these piezo-semiconductive nonlinear equations requires a nonlinear iteration method (e.g. Gummel's or Newton's methods) whose effectiveness and computational cost depends on the device under study and on the proper choice of variables (natural variable, quasi-Fermi level, Slotboom formulations). We choose a modified Newton method for higher effectiveness and the quasi-Fermi level approach for a more accurate modeling of charge variations and compute the i-v characteristics for various doping profiles in the nanowire, as a function of several parameters, including the length, the diameter, and the doping level of the nanowire as well as the amplitude of the input mechanical force. In addition, we compute the capacitances at the semiconductor-electrode contacts that account for the presence of accumulation or depletion regions. Our work provides insight for designing piezotronic devices with improved performance.

Flexoelectricity, which couples inhomogeneous deformation to electric polarization, has been attracting scientific attention since 1960s. Different from the case of solid crystalline dielectrics, the flexoelectricity in polymers is nonlinear and the mechanism might be more complicated. Recent studies have shown that giant flexoelectricity exists in certain polymers. We believe that this giant flexoelectricity is close related to the microstructure of these polymers. To investigate this mechanism, in this work, a series of atomistic simulations are conducted on both amorphous and crystalline polyvinylidene fluoride (PVDF) films. As an application of this study, we propose a microstructure based constitutive law for polymers with flexoelectricity. At last, the polarization-strain gradient relationship for an α-phase PVDF film is calculated using the proposed constitutive law and the results is compared with that reported in previous experiments.

The development of a method for integrating highly efficient energy conversion materials onto soft tissue could yield breakthroughs in energy harvesting systems for implantable biomedical devices. Further, the scaling of such materials down to nanometer levels may yield novel probes for studying fundamental mechanical responses of cells. Of particular interest are materials and devices which can conform to soft, curved surfaces such as skin, and operate in vital environments that may involve both flexing and stretching modes. Piezoelectric crystals are a particularly interesting category of energy conversion materials whose properties have been extensively characterized in the bulk. They are brittle inorganic crystals which are processed at high temperatures, and thus are thermally and mechanically incompatible with soft biological matter. Our group has shown advances in the fabrication and integration of highly efficient nanopiezoelectrics on flexible and stretchable substrates. Yet, questions remain about how to engineer these interfaces to be compatible with fragile biological systems. Here we propose new strategies for addressing these questions. First, we have investigated the fabrication, characterization, and device integration of new classes of nanopiezoelectrics. Next, we have interfaced these materials with cells to act as fundamental probes of mechanical deformations of cells in response to electrical excitations. Finally, we have scaled these nanopiezoelectrics to macroscopic dimensions and biointerfaced them with tissue. This research suggests exciting implications for the direct biointerfacing of nanomaterials with cells and tissue, both as fundamental probes and for bioelectromechanical energy harvesting.

Nanoscale materials have been explored in many biological applications because their novel and impressive physical and chemical properties, that offer remarkable opportunities to study and interact with complex biological processes. Despite their impressive potentials, piezoelectric nanostructures have not yet received significant attention for bio-applications. Our results suggest that the exploitation of piezoelectric nanomaterials in nanomedicine is possible and realistic, and their impressive physical properties can be most useful for several applications, that range from sensors and transducers for the detection of biomolecules, to active substrates for tissue engineering or cell stimulation (Ciofani G, et al. ACS Nano, 4:6267-6277, 2010).
Piezoelectric scaffolds have already been shown to enhance cell function without any external electrical stimulation (Seil J.T., et al. WIREs Nanomed. Nanobiotechnol. 2:635-647, 2010): for example piezoelectric polyvinylidene fluoride based matrices demonstrated to enhance neural cell differentiation and neurite outgrowth as compared with non-piezoelectric controls. The next generation of biomaterials should include combinations of electric and piezoelectric stimulation but, to date, just a few studies have combined nanomaterials with this kind of stimulatory electrical cues.
Our group, for example, investigated cardiomyocytes behaviour on poly(lactic-co-glycolic acid) scaffolds doped with different concentrations of barium titanate nanoparticles, demonstrating an increased proliferation and differentiation on the doped matrices (Ciofani G., et al. Biomed. Microdevices, 13:255-266, 2011).
Another class of smart substrates under investigation in our group is represented by zinc oxide nanorod arrays. We demonstrated their suitability in sustaining adhesion, proliferation and differentiation of electrically excitable cells (Ciofani G., et al. Mater. Sci. Eng. C, 32(2): 341-347, 2012), thus opening intriguing perspectives in tissue engineering applications.
Concluding, nanoscale piezoelectric materials have the actual potential to offer beneficial environments for cell and tissue stimulation. The nanoscale sizes of a piezoelectric material may further enhance the piezoelectric response due to the increased surface to volume ratio of the particles, and help the creation of a substrate that mimics the natural nanoroughness of tissues. Ultrasounds, finally, could potentially be applied near the area of the implanted scaffold to provide a transcutaneous stimulus, as already demonstrated by a few seminal studies (Cochran V.B., et al. J. Orthop. Res. 6:145-147, 1998).

Generation of electricity from natural phenomena such as human respiration is a promising strategy for personal systems which require flexible and lightweight devices. ZnO nanowires with piezoelectric property can convert mechanical force into electricity (nanogeneration), are suitable for such applications. To fabricate the human respiration-driven devices, ZnO nanowires and substrates must be flexible and light. To achieve such devices, polymethylglutarimide thin films were spin-coated on Si substrates followed by deposition of Au/Ti thin films. Then, ZnO nanowires were grown on an equal molar aqueous solution of Zn(NO)3 6H2O and hexamethylenetetramine at 80 °C for 24 h using hydrothermal method. Epoxy resin was spin-coated on ZnO nanowires for its flexibility, lightweight, and low thickness. ZnO nanowires and epoxy resin were lift-off from the initial Si substrates, immersing the full structure into a developer (SD-W). It introduces epoxy resin as new substrates for ZnO nanogenerators. The benefit of transferring from Si substrates to epoxy resin is making easy-triggered and conveniently portable ZnO nanogenerators, which is an important factor for flexible electronics applications. The electrical performance of ZnO nanogenerators were designed for one end-electrode output piezoelectricity. The epoxy resin forms a matrix with ZnO nanowires and fills in the gaps to prevent undesired contacts between ZnO nanowires. For normal human respiration application, a 0.65 × 1.16 cm ZnO nanogenerator was attached inside the external pipe connector of a respiratory machine (Respironics Lifecare PLV-100). The respiratory machine mimics the human respiration behavior. The external pipe connector was linked between a respiratory machine and the Test Lung. The Test Lung plays the role of normal human lungs in this work. The current density reached up to 3.65 nA cm-2 at the air flow rate of 2.0 ms-1 and tidal volume of 500 mL during inhalation and exhalation processes. For a real human respiration harvesting energy at rest, ZnO nanogenerators were placed in front of the human nose. The presented research allows fabricating flexible and lightweight ZnO nanogenerators, which have potential applications in harvesting energy by human respiration.

A closer look into fundamental challenges of the modern world reveals that increasing energy demand threatens the evolution of technology and science. The persisting problems aimed to be solved by fabricating low energy requiring devices and scavenging the waste energy. An excellent solution seems likely integrating low energy requiring devices with self powering systems which utilizes conversion of mechanical energy into electricity using piezoelectric energy harvesting circuitries. Moreover, building organic piezoelectric nanogenerators introduces the opportunity of manufacturing flexible, wearable and stretchable self powering devices where there is a greater demand for developing high performance, piezoelectric organic materials. Since Kawai discovered that poly(vinylidene floride) (PVDF) presents the highest piezoelectric coefficient among all polymers, researchers have been attempting to produce PVDF and copolymers with in different shapes, composition and size. Only “form I” crystals have piezoelectric property out of four different conformations of PVDF. Obtaining PVDF piezoelectric crystal is a process that requires polarization using high electric field strengths and is notoriously difficult to control. As a result, making PVDF at nanoscale greatly alters the polarization process. Besides, with decreasing thickness, the motion of molecular changes becomes easier where the nanoscale effects are employed. We demonstrated a new fabrication method which rely on thermal fiber drawing in order to manufacture continuous ordered nanowires with very high aspect ratio (Yaman et al., Nature Materials, July 2011: cover). In this study, we produced parallel ordered ultra-long PVDF nanoribbons using our unique iterative fiber drawing technique. As the first step, designing a preform structure which is a macroscopic copy of final fibers is crucial to adjust dimensional changes such as size reduction factor in fiber drawing steps. The preform that comprises a PVDF slab embedded in a polyether sulfone (PES) encapsulation layer is transformed into fibers as the temperature of the fiber tower&’s oven increased 250 oC above softening temperature of PES. The first step fibers bundled into a second PES preform for diminishing the PVDF ribbons into nanoscale. In conclusion, we obtained hundreds of 25 nm thick, 100 nm wide, polymer encapsulated and piezoelectric nanoribbon arrays with extraordinary aspect ratio. The total length of nanoribbons in second step is around 100 km. Embedding conducting polymer into the fiber cladding as electrodes introduces the possibility of producing in fiber piezoelectric devices. On the other hand, using dichloromethane, it is possible to dissolve PES cladding and extract free standing nanoribbons on any types of substrate. We believe that our study built a bridge between nanoscopic and macroscopic worlds and enabled to produce large area devices meanwhile utilizing the advantages of nanoscale confinement effects.

In the last two decades researchers have demonstrated that ZnO nanowires posses unique and novel unprecedented functionalities paving the way for their future employment as fundamental building blocks in a variety of applications including nanoscale interconnects, nanoelectromechanical systems (NEMS), optoelectronic and sensing nanosystems, energy harvesting for the development of self-powered devices. ZnO has several technologically relevant properties: it has a direct wide band gap, it exhibits both semiconducting and piezoelectric properties, it can be grown in a variety of nanostructures (nanorings, nanobows, nanowires, etc) which are attractive for many applications in nanotechnology. Furthermore at the nanoscale, ZnO, as many other materials, exhibits an enhancement of the material properties (e.g. size effects) which allows nanostructures to be significantly deformed by extremely small mechanical forces, have higher piezoelectric coefficients, and exhibit outstanding mechanical properties, including much higher fracture strains.
Very recently it has been shown that, in case of vertical compression, piezoelectric conical nanostructures may have important advantages over cylindrical nanostructures in terms of available open circuit voltage (piezopotential) and energy conversion (mechanical energy to electrical energy and vice versa).
In our discussion we compare the electrical and mechanical performances of cylindrical and conical ZnO nanowires when accounting for both piezoelectric and semiconductive properties. High quality conical nanowires can be obtained by means of a shape-controlled growth, which is however a very challenging task, or by a number of strategies for further modification of the shapes at the post-growth stage. Simulations, based on a fully coupled non-linear FEM model of piezo-semiconductive nanowires, consistently show that conical nanowires may provide substantially higher piezopotentials due to the much higher strain at their tip.
In addition we compute and analyze the ratio between the stored electrostatic energy and the mechanical energy as a decisive figure of merit for the design of high efficiency nanogenerators. Again, our FEM simulations show that conical nanowires may provide higher conversion efficiencies in comparison with equivalent cylindrical nanowires.
In the light of substantial computational, the conical nanowires appear to be very versatile nanostructures for fabricating high performance piezoelectric nanodevices, for possible applications in the fields of mechanical sensing, piezotronics or piezo-photo-tronics. The results discussed here can provide useful guidelines for the design of such devices.

Developing high performance lead free flexible nanogenerator is of critical importance for driving small scale self-powered nanodevices. We achieved a record high output voltage of 30 V from integrated nanogenerator (NG) based on the composite of polydimethylsiloxane (PDMS) and single crystalline piezoelectric ZnSnO3 nanocubes structures under direct application rolling tire of vehicle motion. This is about several times higher than the maximum reported value of any PDMS composite based nanogenerator. Furthermore, various nanogenerators with different concentration of ZnSnO3 were fabricated and their output piezoelectric signals were recorded in various conditions. ZnSnO3 NG showed high mechanical stability, excellent robustness behavior and could stably harvest energy from the movement of motor vehicle. Furthermore, the electric output can be modulated by changing the concentration of ZnSnO3 and scaling the size and of the device. Based on these observations, we demonstrated its application as a self-powered sensor for monitoring vehicle speed and detecting vehicle weight. These ZnSnO3 nanostructures are of good potential being used as the fundamental building block for higher power nanogenerator and ultrasensitive nanosensors/pressure sensor.

Recently, piezoelectric energy harvesting technologies have approached flexible devices using thin films or nano-structured piezoelectric materials with flexible templates to obtain large electrical power resulted from the large strain on the piezoelectric materials. Conventional synthesis methods of the piezoelectric thin films are not easily compatible with the flexible devices. This is because the synthesis of the piezoelectric thin films requires high processing temperatures (>600°C) to obtain the crystalline. Excimer laser annealing (ELA) is an attractive method for the low-temperature crystallization of amorphous ceramic thin films. However, the short absorption depth and the rapid absorption time of laser leads structural degradation as residual amorphous phases rough surface and a melt-quenched amorphous phase. These structural degradations may reduce the piezoelectric properties of the thin films. In this study, to obtain crystallized PZT thin films at room temperature, we carried out the Layer-by-Layer ELA (LBL-ELA) method. The amorphous PbZr0.52Ti0.48O3(/sub) (PZT) thin film layers were deposited by rf-sputtering at room temperature and the homogenized excimer laser was irradiated in each PZT layers. To determine the optimum thickness of the PZT layers and applied laser energy, we simulated excimer laser annealing model using COMSOL multi-physics simulation program. To evaluate the structural and piezoelectric characteristics of the films, X-ray diffraction (XRD) and piezoelectric force microscope (PFM) were used, respectively. Consequently, we obtained 200 nm-thick fully crystallized PZT thin film with the remarkably enhanced piezoelectric behavior at room temperature.

We report the direct current (DC) power generation from flexible nanogenerator (NG) based on 2D vanadium doped ZnO nanosheet networks. Interestingly, the morphologies of ZnO nanostructures changed completely from vertically aligned nanorods to vertical nanosheet network due to vanadium doping. X-Ray diffraction confirms the wurzite structures and preferred c-axis orientation of pure ZnO nanorods and V-doped ZnO nanosheet. In pure ZnO nanorod based NG, the alternative current (AC) type piezopotential and current density with average value of 9 mV and 10 nA/cm2 were observed under vertical compressive force, while in V-doped ZnO nanosheet, surprisingly the DC type power generation was observed and their piezopotential and current density is reached up to high level of 70 mV and 1.0 µA/cm2, respectively under same vertical compressive force. This work open up a new avenue for miniaturization of self-powered nanodevices and nanosystems and also promises new form of 2D based nanogenerator for multifunctional applications.

Harvesting energy using piezoelectric materials such as ZnO, at nanoscale due to geometrical effect, are highly desirable for powering (nano-)electronics, biomedcial and healthcare applications. Although one-dimensional nanostructures such as nanowires have been the most widely studied for these applications, a number of piezoelectric materials cannot be synthesized into the structures, thus, developing facile methods of preparing nanostructures from a wide variety of piezoelectric materials is essential for the advancement of self-powered devices. In this study, we present the embossed ZnO thin films nanogenerator with hollow hemispheres, achieved using soft templates composed of monolayer polystyrene beads and physical deposition onto the templates, as an effective alternative to nanowires for robust piezoelectric nanogenerators. The nanogenerators showed the piezoelectric output voltage exceeding 0.5V and output current density of ~ 100 nA/cm2. We further demonstrated that the piezoelectric properties could be enhanced by making the hemispheres asymmetric.