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We study the response of a composite beam (unimorph) with a piezomagnetic layer and a piezoelectric semiconductor layer under a transverse magnetic field. A one-dimensional model for coupled bending and extension of the beam is derived from the three-dimensional theory. A simple and analytical solution of the model is obtained, showing that various electromechanical fields develop in the beam through piezomagnetic and piezoelectric couplings as well as semiconduction. In particular, the mobile charges in the semiconductor layer redistribute themselves under the magnetic field. A coupling coefficient is defined to describe the strength of this magneto-semiconduction coupling. The effects of physical and geometric parameters on the fields and the coupling coefficient are examined. The results are fundamental to piezotronics when magnetic effects are involved.

Two-dimensional (2D) piezoelectric materials are mechanically flexible and atomically thin, enabling new emerging applications, such as wearable electronics, human–machine interfacing and strain sensors. The main obstacles for 2D piezoelectric materials are their low in-plane polarization and weak piezoelectric effect. In this study, we found piezoelectric effect in monolayer WS2 for the first time. Further, the in-plane uniaxial deformation was applied along the polarized armchair direction of WS2, giving rise to an enlarged in-plane polarization as evidenced from the higher Raman anisotropy of angle-resolved polarized Raman spectra. As a result, engineered WS2 exhibited a significantly improved dynamic piezoelectric signal by almost one order of magnitude. This study demonstrates a new strategy to artificially engineer monolayer WS2 by applying in-plane deformation for the larger anisotropy, higher polarization and more superior piezoelectricity, which can open up a new way for applications in 2D-based piezotronics and angle-dependent electronics.

Metal-semiconductor piezo-photocatalysts are generally investigated due to their high photocatalytic performances by the coupling effect of piezotronics and localized surface plasmon resonance (LSPR). However, the mechanism is still indistinct, even the charge migration route is disputable. Here, the electron migration from Ag to BaTiO3 has been confirmed by electron spin resonance (ESR) and radical trapping experiments. Furthermore, this work first proposes that piezoelectric field can promote the photocatalytic properties through electrical modulation of plasmon-exciton interaction. To exactly reveal the mechanism, the piezo-photocatalysis process is fully simulated by numerical analysis and finite element method (FEM). The unabridged mechanism of piezotronics boosted photocatalysis of LSPR-BaTiO3 shows that piezoelectric field can modulate LSPR to generate more hot electrons, suppress Schottky barrier for improved hot electron injection from Ag to BaTiO3 and restrain the recombination of electron–hole pairs by spatial separation of opposite charges.

Two dimensional (2D) chalcogenide monolayers have diversified applications in optoelectronics, piezotronics, sensors and energy harvesting. The group-IV tellurene monolayer is one such emerging material in the 2D family owing to its piezoelectric, thermoelectric and optoelectronic properties. In this paper, the mechanical and piezoelectric properties of 2D tellurene in centrosymmetric β and non-centrosymmetric β′ phases are investigated using density functional theory. β′-Te has shown a negative Poisson's ratio of −0.024 along the zigzag direction. Giant in-plane piezoelectric coefficients of −83.89 × 10−10 C m−1 and −42.58 × 10−10 C m−1 are observed for β′-Te under biaxial and uniaxial strains, respectively. The predicted values are remarkably higher, that is 23 and 12 times the piezoelectric coefficient of a MoS2 monolayer with biaxial and uniaxial strain in the zigzag direction, respectively. A large thermal expansion coefficient of tellurene is also estimated using quasi harmonic approximation. High piezoelectricity combined with exotic mechanical and thermal properties makes tellurene a very promising candidate in nanoelectronics.

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.

The electric band energy variation in a bent piezoelectric semiconductor (PSC) nanowire of circular cross-section induced by the mechanical force is analyzed based on a six-band k · p method. The electric-mechanical fields are first obtained analytically in a cantilever bent PSC nanowire by solving the fully-coupled electro-mechanical equations. Then, the band energy is acquired numerically via the six-band Hamiltonian. By considering further the nonlinear coupling between the piezoelectric and semiconducting quantities, the contribution of the redistribution carriers to the electric field is analyzed from the Gauss’s law. Numerical examples are carried out for an n-type ZnO nanowire in different locations induced by an applied concentrated end force. These include the electric potential, heavy hole (HH), light hole (LH), spin-orbit split-off (SO), and conduction band (CB) edges along the axial and thickness directions. Our results show that the applied force has a significant effect on the band energies. For instance, on the bottom surface along the axial direction, the bandgaps near the fixed end are greater than those near the loading end, and this trend is reversed on the top surface. Moreover, at a fixed axial location, the energy level of the lower side can be enhanced by applying a bending force at the end. The present results could be of significant guidance to the electronic devices and piezotronics.

A full-coupling model on the current-voltage (J-V) characteristics of PN junctions is put forward in the paper by taking into account both the whole junction and the two electrode regions consisting of metal/semiconductor (M/S) contacts. The depletion layer assumption proposed by the Shockley model is discarded. Gauss’ law on the electric potential and the electric field is applied in the whole junction region such that the majority-carrier currents inside and outside the P/N barrier region are able to be exactly defined and clearly calculated. Then, the stable continuity equations of the electron and hole currents are established to show the current conversion between minority- and majority-carriers inside the whole PN junction region. By analyzing all the conversion procedure, the J-V characteristics of a PN junction are obtained with good agreement to the experimental results, which are closely dependent on the minority-carrier lifetime and doping concentrations. Obviously, the study on this topic possesses referential significance to mechanically tuning the performance of piezoelectric PN junctions and piezotronics devices.

Recent research on hybrid organic–inorganic perovskites has greatly advanced the fields of photovoltaics, photodetection, and light emission. The emergence of piezotronics and piezo-phototronics has led to tremendous high-performance devices that are based on piezoelectric materials. Although many previous research studies were centered around single crystal hybrid perovskites, polycrystalline materials are easier to fabricate, such as by using a solution-process, and have many other advantages, e.g. low cost, low environmental requirements, and high conversion efficiency. So far, there are very few reports of piezotronically modulated polycrystalline perovskite devices. Here, a novel piezo-phototronic effect enhanced photodetector based on MAPbI3 polycrystalline perovskite is designed, fabricated, and subsequently characterized. With polycrystalline materials, it is seen that the device performances can be significantly enhanced using the piezo-phototronic effect. Moreover, the polycrystalline perovskite introduces unprecedented potential to fine tune the devices from weak to strong piezoelectric performance. Our study explores the possibility of using polycrystalline perovskites to create high performance strain-controlled piezo-phototronic devices, which will have promising applications in the internet of things, multifunctional micro/nanoelectromechanical devices and sensor networks.

With the demand for accurately recognizing human actions and environmental situations, multifunctional sensors are essential elements for smart applications in various emerging technologies, such as smart robots, human-machine interface, and wearable electronics. Low-dimensional materials provide fertile soil for multifunction-integrated devices. This review focuses on the multifunctional sensors for mechanical stimulus and environmental information, such as strain, pressure, light, temperature, and gas, which are fabricated from low-dimensional materials. The material characteristics, device architecture, transmission mechanisms, and sensing functions are comprehensively and systematically introduced. Besides multiple sensing functions, the integrated potential ability of supplying energy and expressing and storing information are also demonstrated. Some new process technologies and emerging research areas are highlighted. It is presented that optimization of device structures, appropriate material selection for synergy effect, and application of piezotronics and piezo-phototronics are effective approaches for constructing and improving the performance of multifunctional sensors. Finally, the current challenges and direction of future development are proposed.

The gallium nitride (GaN) nanowires (NWs) in piezotronic applications are usually under cyclic loading, which thus may inevitably suffer the mechanical fatigue. In this paper, the fatigue behaviours of defective GaN NWs are investigated by using molecular dynamics (MD) simulations. Our results show no significant changes in the molecular structures of GaN NWs until their final failure during the fatigue process. The final fracture occurring in the GaN NWs under fatigue loading is triggered by the crack that unusually initiates from the NW surface. The GaN NW with a smaller defect concentration or under the fatigue load with a smaller amplitude is found to possess a longer fatigue life. In addition, the ultimate fatigue strain of GaN NWs can be significantly increased by reducing the defect concentration of NWs. The material parameters including elastic constants, piezoelectric coefficients, and dielectric constants of GaN NWs in the fatigue test are evaluated through MD simulations, all of which are found to keep almost unchanged during the fatigue process. These material parameters together with the band gaps of GaN NWs extracted from first-principles calculations are employed in finite element calculations to investigate the piezopotential properties of GaN NWs under fatigue loading. No significant changes are found in the piezopotential properties of GaN NWs during the fatigue process, which indicates the long-term dynamic reliability of GaN NWs in piezotronic applications.

In this work, we present a novel force-sensing device with zinc oxide nanorods (ZnO NRs) integrated with a metal-oxide-semiconductor (MOS) capacitor and encapsulated with Kapton tape. The details of the fabrication process and working principle of the integrated ZnO NRs-MOS capacitor as a force sensor and nanogenerator have been discussed. The fabricated ZnO-MOS device is tested for both the open-circuit and resistor-connected mode. For an input force in the range of 1–32 N, the open-circuit output voltage of the device is measured to be in the range of 60–100 mV for different device configurations. In the resistor-connected mode, the maximum output power of 0.6 pW is obtained with a 1 MΩ external resistor and input force of 8 N. In addition, the influence of different seed layers (Ag and ZnO) and the patterning geometry of the ZnO nanorods on the output voltage of ZnO-MOS device have been investigated by experiments. An equivalent circuit model of the device has been developed to study the influence of the geometry of ZnO NRs and Kapton tape on the ZnO-MOS device voltage output. This study could be an example of integrating piezoelectric nanomaterials on traditional electronic devices and could inspire novel designs and fabrication methods for nanoscale self-powered force sensors and nanogenerators.

Compared with the first-generation semiconductors (such as silicon, germanium) and the second-generation semiconductors (such as gallium arsenide, indium antimonide), the third-generation semiconductor materials represented by silicon carbide (SiC), zinc oxide (ZnO), gallium nitride (GaN) and cadmium sulfide (CdS) usually possess wider band gap, higher thermal conductivity, bigger electron saturation rate and better radiation resistance properties, and thus draw intensive attentions in high temperature and high frequency applications in recent years. Most of the third-generation semiconductors are wurtzite structures, which have piezoelectric effects due to their lack of symmetry in certain directions. This feature serves as a good bridge of transferring mechanical stress signals between the flexible semiconductor electronic devices and the surrounding environment or the host (e.g., the human body). Conventional piezoelectric effects are mainly found in barium titanate and lead zirconate titanate type perovskite materials, but such materials do not have semiconductor properties, thus limiting their use in electronics and optoelectronic devices. Our group has pioneered a new research field, i.e., piezoelectric nanogenerator, by utilizing third-generation semiconductor nanowires (such as ZnO nanowires) under a dynamic force. The piezoelectric potential generated by the dynamic strain on the nanowire can drive electrons to flow in the external load circuit, which is the basic principle of the piezoelectric nanogenerator. Piezoelectric nanogenerator based on ZnO nanowires was firstly proposed in 2006. When a uniform strain is applied on a non-centrosymmetric semiconductor, a piezo-potential will be induced in the semiconductor, accompanied with static piezoelectric charges distributed on the surfaces. This piezoelectric effect is commonly existed in the third generation semiconductors. The strain-induced piezo-potential and piezoelectric polarization charges can also statically and/or dynamically tune the transportation and photoelectric processes of carriers at the interfaces or junctions. The three ways coupling among piezoelectric, photoexcitation and semiconducting properties coined two new research fields, that is, the piezotronics and piezo-phototronics. In order to systematically explain the coupling properties of piezoelectric and semiconductor transport properties in such materials, our group first coined the term piezotronics in a paper published in 2007. The piezotronics is about the modulation of the metal-semiconductor interface barrier via piezoelectric potential acting as a “gate” voltage, which has been used to explain the transistor behavior observed in metal-ZnO-metal structures and the strain-gated diode effect of metal-ZnO structures. The basic core of piezo-phototronics is to regulate the generation, separation, transport and/or recombination of photogenerated carriers at interfaces or heterojunctions by piezoelectric polarization charges, which was first proposed by our group in 2010. The performance of many optoelectronic devices can be effectively enhanced by piezo-phototronic effect. Over the past decade, these two areas have received extensive attentions and made great progresses in basic science and device applications. The combinations of piezotronics and piezo-phototronics with the current popular electronics, optoelectronics and spintronics have great significances, which will bring in revolutionized impacts for future sensor networks, artificial intelligence, micro-nano energy and human-machine interaction applications. This paper gives a brief review of the experimental progress in device application in these two fields in recent years, and looks forward to the future development of these two subjects.

A model is proposed to study the diffusion of non-equilibrium minority carriers under the influence of a piezo potential and to calculate the corresponding current–voltage (I–V) characteristics of a piezoelectric p–n junction exposed to mechanical loading. An effective solution to describe this non-equilibrium process has been put forward including two concepts: the influence of prestress loading on p–n junctions in a quasi-electrostatic thermal equilibrium and the perturbation of small fields superposed on the obtained quasi-electrostatic solutions. A few useful results are obtained through this loaded p–n junction model. Under a forward-bias voltage, a tensile (compressive) loading raises (reduces) the potential barrier of the space charge zone (SCZ), i.e., produces an equivalent reverse- (forward-) electric voltage on the SCZ. When a piezoelectric p–n junction is exposed to a reverse-bias voltage, the current density monotonically decreases with increasing reverse voltage and gradually approaches saturation. A bigger tensile (compressive) loading produces a smaller (larger) saturation current density. The appearance of an equivalent voltage on the SCZ induced by prestress indicates that the performance of a p–n junction with the piezo effect can be effectively tuned and controlled by mechanical loadings. Meanwhile, numerical results show that a loading location closer to the SCZ produces a stronger effect on the I–V characteristics of a piezoelectric p–n junction, implying that the tuning effect of mechanical loadings depends on how much influence of the deformation-induced electric field can reach the SCZ. Furthermore, it is also found that the deformation-induced electric field becomes weak with increasing doping because the higher doping is corresponding to the stronger electric leakage. Thus, the higher mechanical tuning performance on higher doped piezoelectric p–n junctions requires the prestress loadings to be applied closer to the interface of p- and n-zone. This study on a non-equilibrium process of piezoelectric p–n junctions has significance for piezotronics.

Ag-BaTiO3 piezo-photocatalysts were fabricated by precipitating Ag nanoparticles on BaTiO3 nano-piezoelectric through a photochemical reducing approach. The mechanism of piezo-photocatalysis and the effect of the size and distribution of Ag nanoparticles on the properties of the photocatalyst were investigated. The surface plasmon resonance (SPR) of Ag nanoparticles endows the catalyst an absorption in the visible light region. The piezoelectric field originated from the deformation of BaTiO3 can further enhance the separation of the photogenerated carriers of SPR and promote the formation of oxidizing radicals that could accelerate the degradation of organic dyes. The 1mAg-BaTiO3 showed an excellent photocatalytic performance of degrading 83% Rh B in 75 min under full-spectrum light irradiation with ultrasonic excitation. The piezoelectric charges on the surfaces of the BaTiO3 and formed piezoelectric potential in the nanocrystal have been confirmed to express an increment of the catalytic activity more than 20% (compared with the sole photocatalysis). This work provides an effective technology for environment purification and could be extended to other piezoelectric materials.

Piezotronic and piezophototronic devices have attracted increasing interests in self-powered sensor, piezotronic logic units and flexible energy-harvesting devices. The carrier transport characteristics can be effectively tuned by the piezoelectric charges induced by an externally applied strain, which has been extensively investigated for a uniform strain case. However, a non-uniform strain can induce a giant piezoelectric field at the interface to significantly enhance the piezotronic effect. In this paper, we propose a model that a mechanical stress is gradient distributed along a piezoelectric-semiconductor material. The electric properties of piezotronic p-n junction with non-uniform strain has been simulated by using the finite element method, including the current-voltage characteristics and carrier concentration. The piezotronic p-n junction involving strain gradient possesses ultrahigh sensitivity (the gauge factor over 2500) due to the enhancement of piezoelectric constant. This investigation further broadens the fundamental theory of piezotronics and offers guidance for designing ultrahigh performance piezotronic devices.

Electronic skins have received substantial attention in recent years and found great potential in prosthetics, robots, wearable devices, medical equipment, and many other areas. The development of the emerging field of nanogenerators and piezotronics unveiled new types of flexible electronic skins. These devices demonstrated high sensitivity, rapid response, and self-power capability. This review gives the research progress of electronic skins based on principles found in nanogenerators and piezotronics. The role of the piezoelectric effect and the piezotronic effect in sensors is explored, materials used in devices are discussed, and different sensors for electronic skins are provided in this review. The review concludes with an outlook of electronic skins enabled by nanogenerators and piezotronics.

The performance of a piecewise-stressed ZnO piezoelectric semiconductor nanofiber is studied with the multi-field coupling theory. The fields produced by equal and opposite forces as well as sinusoidally distributed forces are examined. Specific distributions of potential barriers, wells, and regions with effective polarization charges are found. The results are fundamental for the mechanical tuning on piezoelectric semiconductor devices and piezotronics.

The development of recently discovered two-dimensional (2D) piezotronics will benefit from the comprehensive understanding of the piezopotential properties of 2D materials. Taking two most widely studied 2D piezoelectric nanomaterials (PNMs), hexagonal boron nitride and molybdenum disulfide for examples, in this paper we investigate the piezopotential properties of 2D PNMs based on numerical simulations and analytical modelling. Our molecular dynamics (MD) simulations show that the piezopotential coefficients of 2D PNMs can be significantly enhanced by small-scale effects at the nanoscale, which becomes more aggressive as the length of 2D PNMs decreases. In addition, a complicated piezopotential distribution is also observed in the stretched 2D PNMs due to the influence of small-scale effects. The influence of small-scale effects on piezopotential properties of 2D PNMs is beyond the reach of previously widely used classical electromechanical theories. To consider the small-scale effects in the analytical modelling of 2D PNMs, we propose here a novel nonlocal electromechanical model based on Eringen's nonlocal theory. Analytical expressions of the piezopotential distribution and piezopotential coefficient of intrinsic 2D PNMs are obtained from the proposed nonlocal models, which show good agreements with MD simulation results. In addition to the electromechanical properties, small-scale effects can also affect the piezopotential of 2D PNMs by changing their semiconducting properties, since it is observed in our density functional theory calculations that the conduction band edges of 2D PNMs can be greatly downwards shifted by small-scale effects.

Inspired by recent developments in the field of nanogenerators and piezotronics, two different approaches for the improvement of magnetic field sensing are discussed and their feasibility has been experimentally demonstrated. In the first concept, it could be shown that a piezotronic read-out eliminates the need for an external pre-amplifier in MEMS type cantilever sensors. This leads to reduced noise and lower power consumption, which makes the sensors interesting for self-powered applications. Furthermore, the lower noise is measured at short integration times, which reveals further application strategies for piezotronic sensor read-out. In the second concept, a new nanogenerator inspired utilization of electret film allows a reduction of the resonance frequency of cantilever sensors. This reduction is accompanied by higher amplitudes and lower Q-factors allowing energy efficient broadband measurements. A theoretical model has been developed based on an anharmonic oscillator, which is in good agreement with the observed experimental results. Both the demonstrated strategies can help to improve the energy efficiency of the presented magnetic field sensors, allowing lower power consumption and a decreased noise level.

In the Internet of Things era, both self-powered nanodevices and nanogenerators (NGs) that harvest energy from mechanical vibrations are highly attractive, prompting a rapid surge in research on enhancing their performance. We demonstrate a versatile approach for diverse materials, by altering the microstructure with discontinuous nano-pores in one-dimensional nanostructures, which can be readily extended to higher-dimensional single crystal materials, such as epitaxial thin films. This novel approach is demonstrated for two applications, direct-current (DC) NGs and piezotronics, by taking ZnO as a model, where remarkably enhanced performance is demonstrated in theoretical simulations and confirmed by experiments. Here, we report that porous ZnO nanowire based DC-NGs demonstrated ∼23 times enhancement of output performance, and strain-gated transistors exhibited ∼6 times enhancement of force sensitivity. This can potentially augment energy harvesting and pressure sensing for many applications, such as self-powered nano-devices and touch panels.

Yapay zeka (AI) ve makine öğrenimi (ML), uzaktan sağlık izleme ve önleyici bakımda yeni bir döneme öncülük ederken, ZnO tabanlı gerinim sensörünü ve nanojeneratörleri çok çekici bir veri toplama aracı haline getiriyor. Burada, insan eli / parmak hareketlerini ve yürüyüş asimetrilerini izlemek için kimyasal olarak modifiye edilmiş grafen aerojellere dayanan esnek piezotronik gerinim sensörü / nanojeneratör gösteriyoruz. Sensörün I-V özelliği, insan hareketinin algılanmasına karşı yüksek hassasiyet , 95 kadar, gösterge faktörü bulunmuştur., Artificial intelligence (AI) and machine learning (ML) lead a new era in remote health monitoring and preventive care, while making ZnO based strain sensor and nanogenerators a very attractive data collection tool. Here, we demonstrate flexible piezotronics strain sensor/nanogenerator, based on chemically modified graphene aerogels to monitor human hand/finger motions as well as gait asymmetries. The I-V characteristic of the sensor shows high sensitivity towards detection of human motion with a good gauge factor of as high as 95 has been demonstrated.

In this work, the dynamical analysis of carrier transportation in a typical piezoelectric device working in the ballistic regime is conducted. Based on the quantum scatting theory, the Poisson and Schrodinger equations are combined for calculating the dynamical transmission coefficient of the metal–piezoelectric ZnO–metal structure subject to sinusoidal and rectangular external stresses. The roles played by the spanning width of induced piezopotential and incident electron energy in affecting transmission probability are further discussed and clarified, respectively. The cutoff frequency of the piezoelectric device is also studied. Moreover, MEMS/NEMS hybrids specified by double/single-clamped ZnO quantum wire micro/nano-electromechanical resonator hybrids are proposed. Through a comprehensive numerical simulation, it is revealed that the rich nonlinear dynamics of the resonator can be subtly transferred to the piezoelectric device, and the chaotic transmission in the piezoelectric device can arise in two-dimensional parameter space with respect to time and incident electron energy. Therefore, the hybrids are endowed with the ability to detect amplitude changes by measuring ultra-sensitive quantum tunneling current. The study sheds light on developing quantum piezotronics and its related MEMS/NEMS integrations.

Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal–semiconductor–metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor–metal junctions. By directly measuring the experimental I–V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d33eff ∼ 8.6 pm V−1–12.3 pm V−1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics., Financial support was obtained under projects from the Spanish Ministerio de Economía y Competitividad (MINECO) under project FIS2015-73932-JIN, H2020 ECSEL-JU under EnSO project (Energy for Smart Objetcs) (Contract no. 692482) and La Caixa Foundation under the Junior Leader Retaining program (LCF/BQ/PR19/11700010). In addition, this work was partially funded by 2017-SGR-579 from the Generalitat de Catalunya. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant Nos. SEV-2017-0706).

Most piezotronic nanodevices rely on the piezopotential generated by the bending of their component piezoelectric nanowires (NWs). The mechanical behaviours and piezopotential properties of zinc oxide (ZnO) NWs under lateral bending are investigated in this paper by using a multiscale modelling technique combining first-principles calculations, molecular dynamics simulations and finite-element calculations. Two phase transformation processes are successively found in ZnO NWs by increasing the bending force. As a result, the inner and outer surfaces of bent ZnO NWs transform from the parent wurtzite (WZ) structure to a hexagonal (HX) structure and a body-centred-tetragonal (BCT-4) structure, respectively. Different material properties are found among the WZ, BCT-4, and HX structures, which result in a significant change in the piezopotential distribution in bent ZnO NWs after the phase transformation. Meanwhile, the piezopotential generated in bent ZnO NWs can be enhanced by an order of magnitude due to the phase transformation. Moreover, a significant increase in the electronic band gap is found in the transformed HX structure, which implies that the phase transformation may also affect the piezopotential in bent ZnO NWs by modifying their semiconducting properties especially when the doping level of NWs is large.

As a promising technology for sustainable hydrogen generation, photocatalytic (PC) and photoelectrochemical (PEC) water splitting have gathered immense attention over a half-century. While many review articles have covered extensive research achievements and technology innovations in water splitting, this article focuses on illustrating how the ferroelectric polarization influences charge separation and transportation in photocatalyst heterostructures during PC and PEC water splitting. This article first discusses the fundamentals of PC and PEC water splitting and how these electrochemical processes interact with the ferroelectric polarization-induced interfacial band bending, known as piezotronics. A few representative ferroelectric material-based heterogeneous photocatalyst systems are then discussed in detail to illustrate the effects of polarization, space charge region, and free charge concentration, which are critical factors determining the ferroelectric influences. Finally, a forward looking statement is provided to point out the research challenges and opportunities in this promising interdisciplinary research field between ferroelectrics and electrochemistry for clean energy applications.

Piezotronics is an emerging field, which exploits strain to control the transport properties in condensed matters. At present, piezotronics research majorly focuses on insulators with tunable electric dipole by strain. Metals are excluded in this type of application due to the absence of the electric dipole. The recently discovered Berry-curvature dipole can exist in metals, consequently introduces the possibility of the piezoelectric phenomena in them. In this paper, we predict that strain can switch the Berry-curvature dipole, and lead to the nonlinear Hall effect in the two-dimensional (2D) $1H\text{\ensuremath{-}}{\mathrm{MX}}_{2}$ ($\mathrm{M}=\mathrm{Nb},\mathrm{Ta}$; $\mathrm{X}=\mathrm{S},\mathrm{Se}$). Based on symmetry analysis and first-principles calculations, we show these 2D monolayer metals have the desired piezoelectriclike property: without strain, the Berry-curvature dipole is eliminated by symmetry, prohibiting the nonlinear Hall effect; while uniaxial strain can effectively reduce the symmetry to introduce sizable Berry-curvature dipole, and it can generate observable Hall voltage under a reasonable experimental condition. Due to the nonlinear and topological properties, the piezoelectriclike property here is quite different from the traditional one based on the electric dipole. Compared with the traditional piezoelectric materials, which can only exist in insulators, we manifest that the 2D metallic $1H\text{\ensuremath{-}}{\mathrm{MX}}_{2}$ ($\mathrm{M}=\mathrm{Nb},\mathrm{Ta}$; $\mathrm{X}=\mathrm{S},\mathrm{Se}$) can be applied in the platform for piezoelectriclike devices such as strain sensors, terahertz detections, energy harvesters etc.

PVDF sensor is a sensor that is often used to measure force, strain, vibration and heat. In this study, PVDF sensors with surface polarization are used to detect cutting forces on the machine. The PVDF sensor that has been polarized on the surface is placed in the chuck part of the engine. Measuring instrumen for testing and calibrating PVDF sensors is oscilloscope with increased loading and reduced axial and tangential directions. After the calibration process, the PVDF sensor was used to measure cutting force on drilling machine, and then the results were compared with the PCB piezotronics force sensor. The PVDF sensor output signal is measured and studied for its voltage using an oscilloscope, where the output signal is compared to the weight given to the PVDF sensor. From the results of these tests indicate that the maximum deviation in axial loading is 0.32V while the tangential loading is 0.31VKeywords. PVDF sensor, Surface polarization, Drilling machine, Cutting force

Piezotronics and piezo-phototronics based on the third generation semiconductors are two novel fields for low power consumption, self-powered technology and internet of things. Strain-induced piezoelectric field plays a key role to modulate not only charge-carrier transport but also quantum transport properties in piezoelectric semiconductors, especially for two-dimensional materials which can withstand large strain. In this paper, we theoretically study piezotronic effect on the modulation of spin and valley properties in single-layered MoS2. Spin- and valley-dependent conductance, electronic density distribution and polarization ratio are investigated by quantum transport calculation. Because of piezotronic effect, strain-gated spin and valley transistors have excellent quantum state selectivity using by strain. Our work provides not only piezotronic effect on spin and valley quantum states, but also a guidance for designing novel quantum piezotronic devices.

Two-dimensional (2D) materials are promising candidates for atomic-scale piezotronics and piezophototronics. Quantum edge states show fascinating fundamental physics such as nontrivial topological behavior and hold promising practical applications for low-power electronic devices. Here, using the tight-binding approach and quantum transport simulations, we investigate the piezotronic effect on the spin polarization of edge states in a zigzag-terminated monolayer MoS2 nanoribbon. We find that the strain-induced piezoelectric potential induces a phase transition of edge states from metal to semiconductor. However, in the presence of exchange field, edge states become semi-metallic with significant spin splitting and polarization that can be tuned by external strain. We show that quantum transport conductance exhibits a 100% spin polarization over a wide range of strain magnitudes. This effect is used in a propose prototype of piezotronic spin transistor. Our results provide a fundamental understanding of the piezotronic effect on edge states in zigzag monolayer MoS2 nanoribbons and are relevant for designing high-performance piezotronic spin devices.

Third generation semiconductors for piezotronics and piezo-phototronics, such as ZnO and GaN, have both piezoelectric and semiconducting properties. Piezotronic devices normally exhibit high strain sensitivity because strain-induced piezoelectric charges control or tune the carrier transport at junctions, contacts and interfaces. The distribution width of piezoelectric charges in a junction is one of important parameters. Capacitance-voltage (C-V) characteristics can be used to estimate the distribution width of strain-induced piezoelectric charges. Piezotronic metal–insulator-semiconductor (MIS) has been modelled by analytical solutions and numerical simulations in this paper, which can serve as guidance for C-V measurements and experimental designs of piezotronic devices.

Piezoactive biomaterials are currently in the forefront of the worldwide research due to the multitude of applications ranging from implantable biosensors to biocompatible energy harvesters. Among them, biomolecular piezoelectrics based on amino acids and dipeptides (as exemplified by diphenylalanine, FF) are the most studied. Major problem is an inability to control the self-assembly process to produce dense films with controlled orientation and thickness. To overcome this, we propose a novel method of the formation of crystalline piezoactive FF films via solid phase crystallization directly from the amorphous phase. The process starts from the spin-coating of FF monomers in an organic solution. These layers are then exposed to a controlled humidity that triggers nucleation and growth of highly oriented piezoactive areas (domains). The crystallization process proceeds without changing the morphology and results in dense films with controlled thickness. Large ferroelectric-like domains possess uniform piezoresponse of about 30 pm/V with the in-plane polarization. The growth kinetics is controlled by the temperature and humidity, suggesting that fully in-plane oriented films can be obtained. It is hypothetized that the solid-phase crystallization can be applied to other bioorganic piezoelectrics and thus open an avenue for further use of these materials in implantable piezotronics and beyond.

The piezotronic effect links the mechanical stimuli with various semiconductor devices, promising for low-power-consuming electronic devices, sensitive sensors, and interactive control systems. The persistent requirement for external strains in piezotronic modulation may hinder its application in some circumstances (such as devices on rigid substrate or complicated synergistic piezoelectric modulation on multidevice). Here, we propose an efficient method to realize piezoelectric modulation of optical and electrical properties of MoS2 FET in both static and dynamic manner, expanding the application of piezotronics. Through capacitive coupling between piezo-electret and MoS2 FET, the remanent piezo-potential can efficiently tune the Fermi level of MoS2, programming the initial electrical property for subsequent fabrication of sophisticated devices. The external strain can induce enhanced piezo-potentials to further affect the energy band bending of MoS2 channel, giving rise to high-performance strain sensor...

In my research in the last 20 years, a few fields have been coined in the area of energy and electronics during the course of studying piezoelectric-semiconductor nanostructures. These fields are now attracting a worldwide interest and become the focuses of a few communities. This paper reviews the background and initial ideas based on which we introduced the following original concepts and effects: piezoelectric nanogenerators; self-powered systems; hybrid cell; nano energy; triboelectric nanogenerators; pyroelectric nanogenerators; piezotronics; piezo-phototronics; piezophotonics; tribotronics; blue energy; energy for the new era, and entropy for energy utilization. This paper serves as a record of how these fields were started. Finally, a perspective is given regarding to the energy for the new era, application of nanogenerators, piezotronics and self-powered systems.

This article discusses recent studies of piezotronics and piezo-phototronics of two-dimensional (2D) materials. Two-dimensional semiconductor materials have demonstrated excellent electronic and optoelectronic properties, and these ultrathin materials are candidates for next-generation devices. Among 2D semiconductors, transition-metal dichalcogenides in particular have large in-place piezoelectricity due to the noncentrosymmetry along the armchair direction. A strong coupling of piezoelectric and semiconducting properties has been reported for Schottky contacts and p–n junctions, even in single-layer materials. Since the carrier concentration of ultrathin 2D materials can be easily modulated by external piezocharges, layered composites of ferroelectric/2D materials also show promising piezotronic and piezo-phototronic properties.

Because of the novel properties owing to two-dimensional (2D) confinement, research on 2D nanomaterials has become one of the leading topics in condensed matter physics and materials science. From the viewpoint of crystallography, the 2D morphology embodies spontaneous breakdown of three-dimensional symmetry, which means that the inversion symmetry preserved in some bulk materials can be broken in their corresponding 2D ones, possibly resulting in intrinsic piezoelectric property. Such 2D materials in conjunction with their semiconductor properties are good candidates for novel ultrathin piezotronics and piezophototronics. In one hand, the 2D piezoelectric materials are easy to integrate with the state-of-the-art semiconductor process and conventional electronic technologies. On the other hand, the possible combination of piezoelectricity with other unusual properties in 2D materials such as ferromagnetism or topological insulator may give birth to new physics and innovative devices design for novel applications. Here, we present an overview of recent breakthroughs in 2D piezotronics and piezophototronics, covering from the fundamental principles to their vast applications in energy harvesting and adaptive electronics/optoelectronics. Considering the potential scientific and device developments, we conclude with an in-depth discussion of possible future directions in this active research field.