Self-powered systems by nanogenerators and smart MEMS by piezotronics

Developing wireless nanodevices and nanosystems is of critical importance for sensing, medical science, environmental/infrastructure monitoring, defense technology and even personal electronics. It is highly desirable for wireless devices to be self-powered without using battery, without which most of the sensor network may be impossible. The pie- zoelectric nanogenerators have the potential to serve as self-sufficient power sources for micro/nano systems. For wurtzite structures that have non-central symmetry, such as ZnO, GaN and InN, a piezoelectric potential （piezopotential） is created in the crystal by applying a strain. The nanogenerator is invented by using the piezopotential as the driving force for electrons to flow in respond to a dynamic straining of piezoelectric nanowires. A gentle straining can produce an output voltage of up to 20 - 50 V from an integrated nanogenerator. Furthermore, piezopotential in the wurtzite structure can serve as gate voltage that can effectively tune/control the charge transport across an interface/junction; electronics fabricated based on such a mechanism is coined as piezotronics, with applications in force/pressure triggercd/controlled electronic devices, sensors, logic units and memory. By using the piezotronic effect, it is showed that the optoelectronic devices fabricated using wurtzite materials can have superior performance as solar cell, photon detector and light emitting diode. Piezotronie is likely to serve as ＂mechanosensation＂ for directly interfacing biomechanieal action with silicon based technology and active flexible electronics. The paper gives a brief review about the basis of nanogenertors and piezotronics and their potential applications in smart MEMS （micro-electro-mechanical systems）.

Piezotronics in two-dimensional materials

The fascinating two-dimensional(2D)materials are being potentially applied in various fields from science to engineering benefitting from the charming physical and chemical properties on optics,electronics,and magnetism,compared with the bulk crystal,while piezotronics is a universal and pervasive phenomenon in the materials with broking center symmetry,promoting the new field and notable achievements of piezotronics in 2D materials with higher accuracy and sensitivity.For example,20 parts per billion of the detecting limitations in NO_(2)sensor,500μm of spatial strain resolution in flexible devices,and 0.363 eV output voltage in nanogenerators.In this review,three categories of 2D piezotronics materials are first introduced ranging from organic to inorganic data,among which six types of 2D inorganic materials are emphasized based on the geometrical arrangement of different atoms.Then,the microscopic mechanism of carrier transport and separation in 2D piezotronic materials is highlighted,accompanied with the presentation of four measured methods.Subsequently,the developed applications of 2D piezotronics are discussed comprehensively including different kinds of sensors,piezo-catalysis,nanogenerators and information storage.Ultimately,we suggest the challenges and provide the ideas for qualitative-quantitative research of microscopic mechanism and large-scale integrated applications of 2D piezotronics.

Piezotronic neuromorphic devices:principle,manufacture,and applications

With the arrival of the era of artificial intelligence(AI)and big data,the explosive growth of data has raised higher demands on computer hardware and systems.Neuromorphic techniques inspired by biological nervous systems are expected to be one of the approaches to breaking the von Neumann bottleneck.Piezotronic neuromorphic devices modulate electrical transport characteristics by piezopotential and directly associate external mechanical motion with electrical output signals in an active manner,with the capability to sense/store/process information of external stimuli.In this review,we have presented the piezotronic neuromorphic devices(which are classified into strain-gated piezotronic transistors and piezoelectric nanogenerator-gated field effect transistors based on device structure)and discussed their operating mechanisms and related manufacture techniques.Secondly,we summarized the research progress of piezotronic neuromorphic devices in recent years and provided a detailed discussion on multifunctional applications,including bionic sensing,information storage,logic computing,and electrical/optical artificial synapses.Finally,in the context of future development,challenges,and perspectives,we have discussed how to modulate novel neuromorphic devices with piezotronic effects more effectively.It is believed that the piezotronic neuromorphic devices have great potential for the next generation of interactive sensation/memory/computation to facilitate the development of the Internet of Things,AI,biomedical engineering,etc.

Analysis of piezoelectric semiconductor fibers under gradient temperature changes

Piezoelectric semiconductors(PSs)possess both semiconducting properties and piezoelectric coupling effects,making them optimal building blocks for semiconductor devices.PS fiber-like structures have wide applications in multi-functional semiconductor devices.In this paper,a one-dimensional(1D)theoretical model is established to describe the piezotronic responses of a PS fiber under gradient temperature changes.The theoretical model aims to explain the mechanism behind the resistance change caused by such gradient temperature changes.Numerical results demonstrate that a gradient temperature change significantly affects the physical fields within the PS fiber,and can induce changes in its surface resistance.It provides important theoretical guidance on the development of piezotronic devices that are sensitive to temperature effects.

The action mechanism of the work done by the electric field force on moving charges to stimulate the emergence of carrier generation/recombination in a PN junction

It is discovered that the product of the current and the electric field in a PN junction should be regarded as the rate of work(power)done by the electric field force on moving charges(hole current and electron current),which was previously misinterpreted as solely a Joule heating effect.We clarify that it is exactly the work done by the electric field force on the moving charges to stimulate the emergence of non-equilibrium carriers,which triggers the novel physical phenomena.As regards to Joule heat,we point out that it should be calculated from Ohm’s law,rather than simply from the product of the current and the electric field.Based on this understanding,we conduct thorough discussion on the role of the electric field force in the process of carrier recombination and carrier generation.The thermal effects of carrier recombination and carrier generation followed are incorporated into the thermal equation of energy.The present study shows that the exothermic effect of carrier recombination leads to a temperature rise at the PN interface,while the endothermic effect of carrier generation causes a temperature reduction at the interface.These two opposite effects cause opposite heat flow directions in the PN junction under forward and backward bias voltages,highlighting the significance of managing device heating phenomena in design considerations.Therefore,this study possesses referential significance for the design and tuning on the performance of piezotronic devices.

Interaction between bending and mobile charges in a piezoelectric semiconductor bimorph

We study the bending of a two-layer piezoelectric semiconductor plate(bimorph).The macroscopic theory of piezoelectric semiconductors is employed.A set of two-dimensional plate equations is derived from the three-dimensional equations.The plate equations exhibit direct couplings among bending,electric polarization along the plate thickness,and mobile charges.In the case of pure bending,a combination of physical and geometric parameters is identified which characterizes the strength of the interaction between the mechanical load and the distribution of mobile charges.In the bending of a rectangular plate under a distributed transverse mechanical load,it is shown that mobile charge distributions and potential barriers/wells develop in the plate.When the mechanical load is local and self-balanced,the induced carrier distributions and potential barriers/wells are also localized near the loading area.The results are fundamentally useful for mechanically manipulating mobile charges in piezoelectric semiconductor devices.

Epitaxial Lift-Off of Flexible GaN‑Based HEMT Arrays with Performances Optimization by the Piezotronic Effect

High-electron-mobility transistors(HEMTs)are a promising device in the field of radio frequency and wireless communication.However,to unlock the full potential of HEMTs,the fabrication of large-size flexible HEMTs is required.Herein,a large-sized(>2 cm^(2))of AlGaN/AlN/GaN heterostructure-based HEMTs were successfully stripped from sapphire substrate to a flexible polyethylene terephthalate substrate by an electrochemical lift-off technique.The piezotronic effect was then induced to optimize the electron transport performance by modulating/tuning the physical properties of two-dimensional electron gas(2DEG)and phonons.The saturation current of the flexible HEMT is enhanced by 3.15%under the 0.547%tensile condition,and the thermal degradation of the HEMT was also obviously suppressed under compressive straining.The corresponding electrical performance changes and energy diagrams systematically illustrate the intrinsic mechanism.This work not only provides in-depth understanding of the piezotronic effect in tuning 2DEG and phonon properties in GaN HEMTs,but also demonstrates a low-cost method to optimize its electronic and thermal properties.

The mechanism to reform dynamic performance of an elastic wave-front in a piezoelectric semiconductor by the wave-carrier interaction induced from static biasing fields

The propagation of an elastic wave(EW)in a piezoelectric semiconductor(PSC)subjected to static biasing fields is investigated.It is found that there exist two coupling waves between electric field and charge carriers.One is stimulated by the action of the polarized electric field in the EW-front on charge carriers(EFC),and the other is stimulated by the action of initial electric field in biasing fields on dynamic carriers(IEC).Obviously,the latter is a man-made and tunable wave-carrier interaction.A careful study shows that IEC can play a leading role in remaking dynamic performance of the wave-front and an inter-medium role in transferring energy from biasing fields to EW-fronts.Hence,a method is proposed to reform the EW performance by biasing-fields:reforming the dispersivity of EW-fronts by promoting competition between IEC and EFC and inverting the dissipation by the IEC to transfer energy from biasing fields to EWfronts.The corresponding tuning laws on the phase-frequency characteristics of an EW show that the wave velocity can be regulated smaller than the pure EW velocity at a lowfrequency and larger than the pure piezoelectric wave velocity at a high-frequency.As for regulating the amplitude-frequency characteristics of the EW by the IEC,analyses show that EWs can obtain amplification only for those with relatively high vibration frequencies(small wave lengths).The studies will provide guidance for theoretical analysis of waves propagating in PSCs and practical application and design of piezotronic devices.

Giant piezotronic effect in ferroelectric field effect transistor

The piezotronics effect utilizes a piezopotential to modulate and control current in piezo-semiconductors.Ferroelectric materials,as a type of piezoelectric materials,possess piezoelectric coefficients that are significantly larger than those found in conventional piezoelectric materials.Here,we propose a strain modulated ferroelectric field-effect transistor(St-FeFET)utilizing external strain instead of gate voltage to achieve ferroelectric modulation,which eliminates the need for gate voltage.By applying a very small strain(0.01%),the St-FeFET can achieve a maximum on-off current ratio of 1250%and realizes a gauge factor(GF)of 1.19×10^(6),which is much higher than that of conventional strain sensors.This work proposes a new method for realizing highly sensitive strain sensors and presents innovative approaches to the operation methods of ferroelectric field-effect transistors as well as potential applications for coupling of strain sensors and various devices across different fields.

A switchable high-sensitivity strain sensor based on piezotronic resonant tunneling junctions

Developing emerging technologies in Internet of Things and artificial intelligence requires high-speed, low-power, high-sensitivity, and switchable-functionality strain sensors capable of sensing subtle mechanical stimuli in complex ambience. Resonant tunneling diodes (RTDs) are the good candidate for such sensing applications due to the ultrafast transport process, lower tunneling current, and negative differential resistance. However, notably enhancing sensing sensitivity remains one of the greatest challenges for RTD-related strain sensors. Here, we use piezotronic effect to improve sensing performance of strain sensors in double-barrier ZnO nanowire RTDs. This strain sensor not only possesses an ultrahigh gauge factor (GF) 390 GPa^(−1), two orders of magnitude higher than these reported RTD-based strain sensors, but also can switch the sensitivity with a GF ratio of 160 by adjusting bias voltage in a small range of 0.2 V. By employing Landauer–Büttiker quantum transport theory, we uncover two primary factors governing piezotronic modulation of resonant tunneling transport, i.e., the strain-mediated polarization field for manipulation of quantized subband levels, and the interfacial polarization charges for adjustment of space charge region. These two mechanisms enable strain to induce the negative differential resistance, amplify the peak-valley current ratio, and diminish the resonant bias voltage. These performances can be engineered by the regulation of bias voltage, temperature, and device architectures. Moreover, a strain sensor capable of electrically switching sensing performance within sensitive and insensitive regimes is proposed. This study not only offers a deep insight into piezotronic modulation of resonant tunneling physics, but also advances the RTD towards highly sensitive and multifunctional sensor applications.

Enhanced performance of GaN nanobelt-based photodetectors by means of piezotronic effects

GaN ultraviolet （UV） photodetectors （PDs） have attracted tremendous attention due to their chemical stability in harsh environments. Although Schottky- contacted GaN-based UV PDs have been implemented with better performance than that of ohmic contacts, it remains unknown how the barrier height at local Schottky contacts controls the sensors＇ performance. In this work, the piezotronic effect was employed to tune the Schottky barrier height （SBH） at local contacts and hence enhance the performances of Schottky-contacted metal-semiconductor- metal （MSM） structured GaN nanobelt （NB）-based PDs. In general, the response level of the PDs was obviously enhanced by the piezotronic effect when applying a strain on devices. The responsivity of the PD was increased by 18%, and the sensitivity was enhanced by from 22% to 31%, when illuminated by a 325 nm laser with light intensity ranging from 12 to 2 W/cm2. Carefully studying the mechanism using band structure diagrams reveals that the observed enhancement of the PD performance resulted from the change in SBH caused by external strain as well as light intensity. Using piezotronic effects thus provides a practical way to enhance the performance of PDs made not only of GaN, but also other wurtzite and zinc blende family materials.

Remarkably enhanced piezo-photocatalytic performance in BaTiO_(3)/CuO heterostructures for organic pollutant degradation

Introducing polarization field of piezoelectric materials is an effective strategy to improve photocatalytic performance.In this study,a new type of BaTiO_(3)/CuO heterostructure catalyst was designed and synthesized to achieve high piezo-photocatalytic activity through the synergy of heterojunction and piezoelectric effect.The BaTiO_(3)/CuO heterostructure shows a significantly enhanced piezo-photocatalytic degradation efficiency of organic pollutants compared with the individual BaTiO_(3) nanowires(NWs)and CuO nanoparticles(NPs).Under the co-excitation of ultrasonic vibration and ultraviolet radiation,the optimal degradation reaction rate constant k of polarized BaTiO_(3)/CuO heterostructure on methyl orange(MO)dye can reach 0.05 min^(−1),which is 6.1 times of photocatalytic rate and 7 times of piezocatalytic rate.The BaTiO_(3)/CuO heterostructure with remarkable piezo-photocatalytic behavior provides a promising strategy for the development of high-efficiency catalysts for wastewater purification,and it also helps understand the coupling mechanism between piezoelectric effect and photocatalysis.

Illumination-dependent free carrier screening effect on the performance evolution of ZnO piezotronic strain sensor

Performance modulation of ZnO optoelectronic devices in the presence of proper piezoelectric polarization charges has been widely reported, whereas relatively less work has been performed about the influence of photoexcitation on piezotronics. In this stud~ we experimentally investigated the performance evolution of ZnO piezotronic strain sensor under various 365 nm UV irradiation densities. The device demonstrated a response ratio of -200 under no illumination and under -0.53% compressive strain, and the response time is approximately 0.3 s. However, tremendous performance degradation was observed with the increase in the illumination densi~, which is attributed to the W-modulated change in the free electron concentration and Schottky barrier height. It was observed that increased carrier density intensifies the screening effect and thus, the modulation ability of piezo-polarization charges weakens. Meanwhile, the deterioration of rectifying behavior at the interface under UV illumination also jeopardizes the device performance.

Theoretical study of piezotronic heterojunction

Due to the coupling of piezoelectric and semiconducting dual properties,much attention has been focused on the piezoelectric semiconductor materials,such as ZnO,ZnS,CdS and GaN.With the usage of these piezoelectric semiconductor materials,novel nanodevices have been demonstrated,from which a new field called piezotronics was formulated.The core of piezotronics is to study the mechanism of the piezoelectric effect on tuning the charge transport behavior across various junctions or interfaces,with potential applications in sensors,microelectromechanical systems,and force/pressure triggered electric devices.Here following the theoretical frame work of piezotronic effect,analytical solutions of piezoelectric heterojunction are presented to investigate the electrical transport behavior at a p-n junction.Numerical simulation is given for guiding future experimental measurements.

Mechanical Manipulation of Electrical Behaviors of Piezoelectric Semiconductor Nanofibers by Time-Dependent Stresses

We study electric currents in a piezoelectric semiconductor fiber under a constant voltage and time-dependent axial stresses applied locally.From a nonlinear numerical analysis based on a one-dimensional phenomenological model using the commercial software COMSOL,it is found that pulse electric currents can be produced by periodic or time-harmonic stresses.The pulse currents can be tuned by the amplitude and frequency of the applied stress.The result obtained provides a new approach for the mechanical control of electric currents in piezoelectric semiconductor fibers and has potential applications in piezotronics.

Synthesis and enhanced piezoelectric response of CVD-grown SnSe layered nanosheets for flexible nanogenerators

Piezoelectricity is the electric charge which accumulates in certain materials in response to mechanical stimuli,while piezoelectric nanogenerators(PENGs)converting mechanical energy into electricity can be widely used for energy harvesting and self-powered systems.The group IV-VI monochalcogenides may exhibit strong piezoelectricity because of their puckered C_(2v)symmetry and electronic structure,making them promising for flexible PENG.Herein,we investigated the synthesis and piezoelectric properties of multilayer SnSe nanosheets grown by chemical vapor deposition(CVD).The SnSe nanosheets exhibited high single-crystallinity,large area,and good stability.The strong layer-dependent in-plane piezoelectric coefficient of SnSe nanosheets showed a saturated trend to be~110 pm/V,which overcomes the weak piezoelectric response or odd-even effects in other layered nanosheets.A high energy conversion efficiency of 9.3%and a maximum power density of 538 mW/cm^(2)at 1.03%strain have been demonstrated in a SnSe-based PENG.Based on the enhanced piezoelectricity of SnSe and attractive output performance of the nanogenerator,a self-powered sensor for human motion monitoring is further developed.These results demonstrate the strong piezoelectricity in high quality CVD-grown SnSe nanosheets,allowing for application in flexible smart piezoelectric sensors and advanced microelectromechanical devices.

Flexible piezoelectric nanogenerators based on a fiber/ZnO nanowires/paper hybrid structure for energy harvesting

We present a novel, low-cost approach to fabricate flexible piezoelectric nano- generators （NGs） consisting of ZnO nanowires （NWs） on carbon fibers and foldable Au-coated ZnO NWs on paper. By using such designed structure of the NGs, the radial ZnO NWs on a cylindrical fiber can be utilized fully and the electrical output of the NG is improved. The electrical output behavior of the NGs can be optionally controlled by increasing the fiber number, adjusting the strain rate and connection modes. For the single-fiber based NGs, the output voltage is 17 mV and the current density is about 0.09 μA·cm^-2, and the electrical output is enhanced greatly compared to that of previous similar micro-fiber based NGs. Compared with the single-fiber based NGs, the output current of the multi-fiber based NGs made of 200 carbon fibers increased 100-fold. An output voltage of 18 mV and current of 35 nA are generated from the multi-fiber based NGs. The electrical energy generated by the NGs is enough to power a practical device. The developed novel NGs can be used for smart textile structures, wearable and self-powered nanodevices.

Piezotronic effect enhanced Schottky-contact ZnO micro/nanowire humidity sensors

A ZnO micro/nanowire has been utilized to fabricate Schottky-contacted humidity sensors based on a metal-semiconductor-metal （M-S-M） structure. By means of the piezotronic effect, the signal level, sensitivity and sensing resolution of the humidity sensor were significantly enhanced when applying an external strain. Since a higher Schottky barrier markedly reduces the signal level, while a lower Schottky barrier decreases the sensor sensitivity due to increased ohmic transport, a 0.22% compressive strain was found to optimize the performance of the humidity sensor, with the largest responsivity being 1,240%. The physical mechanism behind the observed mechanical-electrical behavior was carefully studied by using band structure diagrams. This work provides a promising way to significantly enhance the overall performance of a Schottky-contact structured micro/nanowire sensor.