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.

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 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.

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 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.

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.

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.

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.

Influence of external electric field on piezotronic effect in ZnO nanowires

In this work, the piezotronic effect is investigated for the first time in external electric fields ranging from 0 V·cm-I to 2,000 V·cm-1 by using n-type ZnO nanowires supported by a flexible substrate. In the presence of an external electric field, the Schottky barrier height （SBH） is lowered by the image force, allowing more free carriers to pass through the metal-semiconductor junction and enhancing the screening effect on positive piezoelectric polarization charges. As the strength of the external electric field increases, the piezotronic effect is significantly suppressed and the metal-semiconductor contact finally exhibits Ohmic behavior. The experimental results show that devices can be classified into three groups, corresponding to low, moderate, and high carrier densities of the nanowires used. This work not only helps us to explicate the basic physical mechanism of the piezotronic effect in a harsh environment in an electric field but also provides guidelines for future design and fabrication of piezotronic devices.

Electrical Response of a Multiferroic Composite Semiconductor Fiber Under a Local Magnetic Field

We study the electrical response of a multiferroic composite semiconductor fiber consisting of a piezoelectric semiconductor layer and two piezomagnetic layers under a transverse magnetic field applied locally to a finite part of the fiber.The phenomenological theory of piezomagnetic-piezoelectric semiconductors is employed.A one-dimensional model is derived for magnetically induced extension of the fiber.For open-circuit boundary conditions at the two ends of the fiber,an analytical solution is obtained from the model linearized for small carrier perturbations.The solution shows a local electric polarization and a pair of local electric potential barrier-well.When the two ends of the fiber are under a voltage,a nonlinear numerical solution shows that the potential barrier and well forbid the passage of currents when the voltage is low.The results have potential applications in piezotronic devices when magnetic fields are involved for manipulating the devices or sensing and transduction.

压电电子学金属-绝缘体-半导体晶体管的电容-电压特性

压电半导体具有压电效应和半导体电学特性,可以用于压电电子学器件和压电光电子学器件.压电电子学器件和压电光电子学器件利用外加应变产生的压电电荷调控压电电子学pn结、金属半导体(M-S)接触的载流子传输、分离、复合等过程.压电电子学器件具有灵敏度高、响应速度快和功耗超低的优点,非常适合在自驱动系统中应用,因此在物联网、可穿戴系统、植入式传感系统具有巨大的应用价值.压电电荷的分布宽度能够显著影响压电电子学器件的传感性能,是压电电子学器件重要参数之一.压电电荷的分布宽度则可以通过压电电子学器件的电容电压(C-V)特性获得.本文以压电电子学金属-绝缘体-半导体(MIS)晶体管为例,提出对压电电荷分布影响进行测量和计算的理论模型,并进行了数值模拟,为以C-V测量为基础的结区压电电荷分布宽度实验设计给出了可行的理论方案.

Piezotronic effect on the luminescence of quantum dots for micro/nano-newton force measurement

The luminescence of semiconductor quantum dots （QDs） can be adjusted using the piezotronic effect. An external mechanical force applied on the QD generates a piezoelectric potential, which alters the luminescence of the QD. A small mechanical force may induce a significant change on the emission spectrum. In the case of InN QDs, it is demonstrated that the unforced emission wavelength is more than doubled by a force of 1 μN. The strategy of using the piezotronic effect to tune the color of the emission leads to promising noncontact force- measurement applications in biological and medical sensors and force-sensitive displays. Several piezoelectric semiconductor materials have been investigated in terms of the tunability of the emission wavelength in the presence of an external applied force. It is found that CdS and CdSe demonstrate much higher tunability δλ/δF, which makes them suitable for micro/nano-newton force measurement applications.

Ballistic transport in single-layer MoS2 piezotronic transistors

Because of the coupling between semiconducting and piezoelectric properties in wurtzite materials, strain-induced piezo-charges can tune the charge transport across the interface or junction, which is referred to as the piezotronic effect. For devices whose dimension is much smaller than the mean free path of carriers （such as a single atomic layer of MoS2）, ballistic transport occurs. In this study, transport in the monolayer MoS2 piezotronic transistor is studied by presenting analytical solutions for two-dimensional （2D） MoS2. Furthermore, a numerical simulation for guiding future 2D piezotronic nanodevice design is presented.