Piezotronic Effect Research Articles

Temperature dependence of the piezotronic and piezophototronic effects in flexible GaN thin films

Flexible devices operated at a cryogenic temperature are required for significant applications, such as sensors in thermal imaging and infrared and nuclear particle detectors. Hence, in this study, comprehensive temperature dependence was performed on flexible GaN thin films from 225 K to 325 K under 0.0–0.3% strains to investigate the piezotronic and piezophototronic effects. The GaN thin-film device strongly depends on the applied temperature, and the piezotronic effect is enriched by above 360% under 0.3% applied strain at a low chamber temperature. This type of behavior is caused by the increased essential piezocharges at the interface/surface, which results from the less screening effect of the reduced charge carriers in the GaN thin film. To support this behavior, we investigate the piezophototronic effect in a GaN thin film at various temperatures under different strains. The study results will provide an in-depth understanding of the piezotronic and piezo-photonic effects and pave the way for forthcoming device applications in various fields, including human–machine interface, health monitoring, artificial intelligence, and surgical robotic operations.

Piezotronic effect and hierarchical Z-scheme heterostructure stimulated photocatalytic H2 evolution integrated with C-N coupling of benzylamine

Photocatalytic hydrogen generation represents a potential approach to address global energy and environmental issues for artificial photosynthesis. Generally, the inevitable use of the holes scavengers leads to increased cost and production of waste. The integration of H2-producing half-reaction with value-added organic molecule oxidation reaction provides a promising strategy to cope with this issue. However, owing to the low separation efficiency of photo-generated charge carriers and sluggish kinetics of the surface reaction, the photocatalytic activity without a co-catalyst is unsatisfying. Herein, the hierarchical and piezoelectric Z-scheme BaTiO3@ZnIn2S4 heterostructure without any co-catalyst were dexterously constructed. The Z-scheme electron transfer paths maintain the strong redox ability of the photo-generated electrons and holes, also offer spatial separation of both charge carriers and the surface redox regions. The well-designed redox regions endow BaTiO3@ZnIn2S4 with the lowest energy barriers of hydrogen production and C-N coupling of benzylamine than that of pure BaTiO3 or ZnIn2S4. Significantly, piezotronic effect can further accelerate the separation and transfer of photo-generated charge carriers in Z-scheme BaTiO3@ZnIn2S4 heterostructure. Thus, by the feat of the multiple advantages of the piezotronic effect and Z-scheme heterostructure, the high photocatalytic activity for the co-production of C-N coupling products (5593 umol g−1) and H2 (8041 umol g−1) was achieved through coupling utilization of mechanical energy and solar energy.

Piezotronic-enhanced photocatalytic performance of heterostructured BaTiO3/SrTiO3 nanofibers

The piezoelectric effect can effectively modulate the energy band structure of the optoelectronic semiconductor and the transport behavior of carriers. Here, we propose a BaTiO 3 /SrTiO 3 nanocomposite, and effectively enhance its photocatalytic performance through piezoelectric effect. The heterostructured BaTiO 3 /SrTiO 3 nanofibers were prepared by electrospinning, and they were polarized by a high-voltage electric field. Under the co-excitation of ultrasonic and UV irradiation, the BaTiO 3 /SrTiO 3 nanofibers presented excellent degradation ability of pollutants in water. The BaTiO 3 /SrTiO 3 nanofibers can degrade Rhodamine B (RhB) dye by ~97.4% within 30 min under the combined action of ultrasonic and ultraviolet irradiation, which was 2.2 times that of pure SrTiO 3 nanofibers. The piezoelectric effect provides a built-in polarization field to improve the separation efficiency of photo-generated carriers. Moreover, DFT calculation results showed that piezoelectric potential can reduce the band gap width of the SrTiO 3 . This article provides a promising strategy for improving photocatalytic performance of nanocomposites through the use of mechanical energy. Here, we propose a heterostructured BaTiO 3 /SrTiO 3 nanocomposites that present enhanced photocatalytic performance through a piezotronic effect. The piezopotential generated in the BaTiO 3 /SrTiO 3 nanofibers under ultrasonic significantly improves the photocatalytic performance of the catalysts. • Heterostructured BaTiO 3 /SrTiO 3 nanofibers were prepared by electrospinning technique for piezo-photocatalysis. • Photocatalytic performance of BaTiO 3 /SrTiO 3 nanofibers were significantly improved through piezotronic effect. • Density functional theory (DFT) calculations prove that the band gap of SrTiO 3 can be modulated by an electric field.

Piezotronic effect determined neuron-like differentiation of adult stem cells driven by ultrasound

Electrical stimulation is an efficient approach to inducing neural differentiation of stem cells. However, most demonstrations of conventional electrical stimulation for the regulation of stem cell differentiation generally involve three components—an electrical signal generator, conductive culture substrate, and pair of lines, which limits its use in clinical applications for neural degeneration treatments. Herein, we proposed a facile method to generate localized electrical signals on the surface of a piezoelectric poly (vinylidene fluoride) (PVDF) film with a well-designed nanopillar array driven by ultrasound irradiation based on a piezotronic effect, which proved to induce neuronal differentiation of rat bone marrow mesenchymal stem cells (rBMSCs) without any biological or chemical neural inducing factors. The assessment of rBMSCs on the surface of the PVDF nanopillar array at the gene and protein levels confirmed that rBMSCs could differentiate into neuron-like cells. This demonstration provides a practical approach for the regulation of adult stem cells to differentiate into neurons, which will be a great achievement to overcome the shortage of neural stem cells in the adult human body and realize the autologous stem cell treatment of neurodegeneration. This work creates a new therapeutic avenue for contactless, controlled neuroregenerative therapies.

Strain-induced piezotronic effects in nano-sized GaN thin films

Flexible devices have attracted considerable attention because of their very productive applications in the diverse areas such as healthcare, artificial intelligence, and robotics. Accordingly, in this study, we fabricated a flexible nano-sized GaN thin-film device via a double-transfer method using a laser lift-off process from sapphire with thermal tape and then with carbon tape onto a polycarbonate substrate. With the application of compressive/tensile strains, the flexible nano-sized GaN thin-film device can be mechanically controlled by the piezotronic effect. The device has excellent electromechanical performance with a gauge factor of 2206 at a compressive strain of 0.32%, which is nearly 11.11 times greater than that of a conventional strain-gauge value. Furthermore, the device shows an effective ON condition in the compressive direction and OFF condition in the tensile direction at an applied bias voltage of 0.5 V. The fabricated device has potential for innovative and advanced applications such as human–machine interfaces, biomedical diagnoses, prostheses, intelligently controlled rooms, and active flexible electronics.

Polarization-induced ultrahigh Rashba spin-orbit interaction in ZnO/CdO quantum well

Spin-orbit interaction (SOI) connecting an electronic spin with its momentum is crucial for numerous fundamental physical researches and their applications, including quantum spin Hall effect, Majorana Fermions and spin-orbit qubits. By breaking structural inversion symmetry, Rashba spin-orbit interaction (RSOI) provides an available method for the manipulation of spin by controlling electronic movement within external potential field. In this study, we demonstrate the RSOI of conduction electron modulated by stress-induced polarization field in ZnO/CdO quantum well (QW). The polarization field exactly triggers band inversion between the electron and light hole. The peak of RSOI coefficient can reach approximately up to 83 meV ⋅ nm, almost three orders of magnitude higher than the conventional GaAs-based QWs. This study can be beneficial to sufficient manipulation of spin qubits by strong RSOI quantum piezotronic effect, and will stimulate an intense researching interest in low-dimensional quantum piezotronic devices. • Rashba spin-orbit interaction (RSOI) of ZnO/CdO quantum well (QW) has been investigated. • Strain induced polarization of the QW adjusts the RSOI coefficient. • A maximum RSOI coefficient appears when varying external stress. • Ultrahigh RSOI coefficient has been unveiled by comparing a set of heterostructural QWs.

Low frequency hydromechanics-driven generation of superoxide radicals via optimized piezotronic effect for water disinfection

Water transmission of pathogenic microorganisms can lead to infectious diseases and potential pandemics. Piezocatalysis is an emerging technology for water disinfection, however, current piezocatalysts often have low catalytic performance due to limited charge utilization and require the use of high-frequency ultrasound. Herein, low-frequency hydromechanics is employed as mechanical energy resource to achieve efficient disinfection via a rationally-designed piezocatalyst, BaTiO 3 @Au. The interfacial position of Au cocatalyst was optimized by in situ piezodeposition instead of random chemical reduction, which facilitated charge separation and promoted the generation of reactive oxygen species (ROS). By simply vibrating (e.g., stirring) the piezocatalysts dispersed in aqueous solution, efficient degradation of organic molecules as well as inactivation of model bacteria and virus could be obtained. This approach allowed us to take advantage of low-frequency hydromechanical energy to significantly reduce intestinal pathogenic bacteria and total organic carbon content in hospital wastewater, which could provide a green and feasible way for water disinfection. • Efficient water disinfection was achieved by simply stirring a rationally-designed piezocatalyst. • The hydromechanics-driven piezocatalysis was enabled by optimized piezotronic effect. • A simple in situ piezodeposition of gold on piezocatalysts was introduced. • This strategy was successfully applied for the treatment of wastewater from a local hospital.

Visualizing Intrinsic 3D‐Strain Distribution in Gold Coated ZnO Microstructures by Bragg Coherent X‐Ray Diffraction Imaging and Transmission Electron Microscopy with Respect to Piezotronic Applications

AbstractNovel devices ranging from bio magnetic field sensors to energy harvesting nano machines utilize the piezotronic effect. For optimal function, understanding the interaction of electrical and strain phenomena within the semiconductor crystal is necessary. Here, studies of a model piezotronic system are presented, consisting of a ZnO microrod coated by a thin layer of gold, which forms a Schottky contact with the piezoelectric ZnO material. Coherent X‐ray diffraction imaging (CXDI) and transmission electron microscopy (TEM) are used to visualize the structure and strain distribution, showing that the ZnO microrod exhibits strains of multiple origins in the bulk and at the interface. Strain values of −6 × 10‐4 have been measured by CXDI at the ZnO/Au interface. The origin is shown to be a combination of an interface strain, possibly caused by the Schottky contact formation, and distinct, localized electrical fields inside the crystal which are assigned to electron depletion and screening in a bent ZnO/Au piezotronic rod. These findings will contribute to sensor development and to a better understanding of piezotronic applications.

Effect of flexoelectricity on piezotronic responses of a piezoelectric semiconductor bilayer

Based on three-dimensional equations of piezoelectric semiconductors, we take flexoelectricity into consideration to develop a deformation–polarization–carrier coupling analysis model for the bending of a piezoelectric semiconductor (PS) composite bilayer, which is composed of a piezoelectric semiconductor layer and an elastic layer. Using the derived equations, we investigate the macroscopic responses, such as the distribution of electromechanical field and carrier concentration, of the PS composite bilayer with bending deformation. The induced polarization in the PS composite bilayer exhibits an apparent size-dependent property due to the flexoelectric coupling effect, and thus has a remarkable influence on the piezotronic effect in the PS composite bilayer with nano-thickness. The obtained results are useful for designing novel piezoelectric semiconductor devices.

Piezotronic effect in a normally off p-GaN/AlGaN/GaN HEMT toward highly sensitive pressure sensor

We report the effect of stress or strain on the electronic characteristics of a normally off AlGaN/GaN high electron mobility transistor (HEMT) and demonstrate its role as a highly sensitive pressure sensor. We observe that the HEMT drain current exhibits a linear change of 2.5%/bar upon the application of pressure, which is translated to a strain sensitivity of 1250 ppm−1. This is the highest strain sensitivity ever reported on HEMTs and many other conventional strain sensing configurations. The relative change of drain current is largest when the gate bias is near-threshold and drain bias is slightly larger than the saturation bias. The electron sheet density and mobility changes in the AlGaN/GaN heterointerface under the applied pressure or mechanical strain are explained qualitatively. The spontaneous and piezoelectric-polarization-induced surface and interface charges in the AlGaN/GaN heterojunction can be used to develop very sensitive and robust pressure sensors. The results demonstrate a considerable potential of normally off AlGaN/GaN HEMTs for highly sensitive and reliable mechanical sensing applications with low energy consumption.

Photoelectric dual-mode strain sensing based on piezoelectric effect

Realizing high-sensitivity, high-precision, non-contact strain perception units, and making it flexible to integrate with existing systems is an important research topic in the field of sensing technology. In this paper, we constructed one type of dual-mode strain sensing device, combining the non-contact and easy integration characteristics of optical and electrical signals. On one hand, the resonant wavelength of lasing mode can be tuned using strain-induced changes in the refractive index of ZnO microresonator with a detection sensitivity of 17.25 meV/% based on optical signal; on the other hand, the Schottky contact barrier also can be modulated using the piezotronics effect to control the output current with a detection sensitivity of 22.41 μA/% through electrical signals. These dual-mode strain-sensing mechanisms indicates that the device integrates the advantages of both optical and electrical signals, which is expected to expand its scope of application in complex environments, such as intelligence systems, in-vivo and underwater.

Measurement of the piezotronic effect on single grain boundaries in zinc oxide Varistors

In this work we propose a new way of applying biaxial strain on an electrically conductive material to investigate stress-induced change of properties, namely the piezotronic effect, at a microscopic level. A piezoelectric stage is actuated to certain biaxial compressive strain levels. The microsection of the sample attached to it experiences the same amount of strain. The external strain translates to inner stresses in the material which can be controlled with high accuracy.This method was applied on zinc oxide-based varistor ceramics which is known as a functional material with a pronounced piezotronic effect on its highly non-linear current-voltage characteristics. By setting up a micro four-point probe measurement it was possible to record the current-voltage characteristics at single grain boundaries at certain stress levels. The results lead to an understanding how mechanical stresses modify the overall electrical behavior of zinc oxide varistors.

Ultrasonic-driven electrical signal-iron ion synergistic stimulation based on piezotronics induced neural differentiation of mesenchymal stem cells on FeOOH/PVDF nanofibrous hybrid membrane

Neural stem cells (NSCs) are thought to be the best seed cells for neurodegenerative diseases therapy due to their potential to differentiate into either neurons or glial cells. However, the lack of resources for autologous NSCs prohibits their clinical applications. Although mesenchymal stem cells can be conveniently obtained from patients, neural differentiation is difficult to be realized, which becomes the great challenge for stem cell-based neurodegenerative diseases therapy. Herein, we report a novel method to induce neural differentiation of rat bone-marrow-derived mesenchymal stem cells (rBMSCs) based on piezotronics without any neural inducing factors (NIFs). In this work, a hybrid nanofibrous membrane was prepared by assembling a layer of FeOOH nanorods on the surface of polyvinylidene fluoride (PVDF) electrospun nanofibers. Under ultrasonic irradiation, piezoelectric-driven localized electric potential and electrical-driven iron ion (Fe 3+ ) release based on piezotronics induced neural differentiation of rBMSCs cultured on the surface of these membranes. Genetic and molecular assays on the differentiated cells demonstrated that the synergistic effects of electrical signals and iron ion release induced the differentiation of rBMSCs into neurons (γ-aminobutyric acid (GABA)ergic, cholinergic and aminergic neurons) without any NIFs. Intracellular calcium imaging experiments showed that the differentiated cells generated fast peaking spontaneous [Ca 2+ ]i-transients under the effects of neurotransmitters, especially GABA, indicating that rBMSCs-derived neurons had neuronal function. This study provides a novel strategy for inducing neural differentiation of rBMSCs via wireless stimulation with piezotronic effect, which is of great significance in clinical and neural tissue engineering. The localized electrical signals derived from ultrasound-driven PVDF and the piezoelectric potential-accelerated controlled release of iron based on piezotronics can cause the differentiation and development of rBMSCs into neurons. • A novel strategy for inducing neural differentiation based on piezotronics. • Neural differentiation via wireless stimulation without neural inducing factors. • Synergistic effects of electrical signals and Fe 3+ release under ultrasound. • The differentiation of rBMSCs into functional neurons.

The piezotronic effect on carrier recombination processes in InGaN/GaN multiple quantum wells microwire

Understanding piezotronic correlated carrier recombination behavior in quantum wells is essential for their applications. In this work, we have studied the influence of piezotronics on carrier recombination processes in single InGaN/GaN multiple quantum wells microwire (MQW-MW) by using steady-state and time-resolved spectroscopies. We conclude that mechanical strain induced piezotronics promotes the charge separation of excitons in space, and slows down the recombination rate of free carriers. The proposed model is supported by three independent experiments: photoluminescence experiment of MQW-MW before and after peel off, strain dependent TRPL experiment, and excitation fluency dependent PL intensity experiment. Our study could provide a guideline for the application of piezotronic in MQW-MW-based optoelectronic devices.

Local Strain Distribution in ZnO Microstructures Visualized with Scanning Nano X‐Ray Diffraction and Impact on Electrical Properties

The fast and contact‐free detection of biomagnetic vital signs can benefit clinical diagnostics in medical care, emergency services, and scientific studies, hugely. A highly sensitive magnetoelectric sensor for the detection of biomagnetic signals combined with the piezotronic effect is a promising path to increase the signal detection limit. Herein, the results of three ZnO microrods examined by nano X‐ray diffraction and current–voltage curves to investigate the crystalline structure influence on the Schottky contact properties are presented. The measurements reveal different strain distributions for the three rods and that these are linked with the electrical properties, showing that the crystalline quality has a direct influence on the Schottky contact properties. An analytical model is created to determine the influence of the stress. Although rotation of the strain orientation changes the strain appearance in the measurement, it does not affect the Schottky contact properties.

Edge-state transport in circular quantum point contact quantum piezotronic transistors

Quantum piezotronic transistor is studied based on HgTe/CdTe topological insulator with a circular quantum point contact. The radius of the circular region is modulated by strain-induced piezoelectric potential. The electronic transport behavior of the edge and bulk states is explored by calculating the conductance and electronic density distribution under different Fermi energies and strains. Transport property of edge states is studied by machine learning method and the transport conductance can be effectively predicted. These results show that the neural network can be used for obtaining electronic transport properties, and it has great potential for optimizing and designing high-performance quantum piezotronic devices. A quantum piezotronic transistor is proposed based on a circular quantum point contact of HgTe/CdTe topological insulator. Edge states and bulk states can be effectively controlled by strain-induced piezoelectric potential. Machine learning is employed to study edge-state transport, which can effectively predict transport conductance. • Piezotronic effect can effectively control bulk and edge states. • The radius of a circular quantum point contact is modulated by strain-induced piezoelectric potential. • Machine learning is applied to predict transport conductance of quantum piezotronic devices.

Combining triboelectric nanogenerator with piezoelectric effect for optimizing Schottky barrier height modulation

Schottky-contacted sensors have been demonstrated to show high sensitivity and fast response time in various sensing systems. In order to improve their sensing performance, the Schottky barriers height (SBH) at the interface of semiconductor and metal electrode should be adjusted to appropriate range to avoid low output or low sensitivity, which was induced by excessively high or low SBH, respectively. In this work, a simple and effective SBH tuning method by triboelectric generator (TENG) is proposed, the SBH can be effectively lowered by voltage pulses generated by TENG and gradually recover over time after withdrawing the TENG. Through combining the TENG treatment with piezotronic effect, a synergistic effect on lowering SBH was achieved. The change of SBH is increased by 3.8 to 12.8 times, compared with dependent TENG treatment and piezotronic effect, respectively. Furthermore, the recovery time of the TENG-lowered SBH can be greatly shortened from 1.5 h to 40 s by piezotronic effect. This work demonstrated a flexible and feasible SBH tuning method, which can be used to effectively improve the sensitivity of Schottky-contact sensor and sensing system. Our study also shows great potential in broadening the application scenarios of Schottky-contacted electronic devices.

Temperature dependence of the piezotronic effect in CdS nanospheres

We used simple graphene transfer and microwave synthesis to deposite wurtzite-structured CdS nanospheres with gold nanoparticles (AuNPs) on a flexible polyethylene terephthalate (PET) substrate. The temperature dependence of the piezotronic effect of the CdS nanospheres was studied in the temperature range 240–320 K under tensile strains of 0.00–0.40%, the effect was found to significantly depend on the device temperature. Moreover, the piezotronic effect was enhanced by over 590% at a strain of 0.40% by cooling temperature owing to the improved efficiency of piezoelectric potential charges. This is a result of the decreased screening effect by reduced carrier generation in the CdS. In addition, the consistent charge transfer mechanism in the investigated piezotronic device is explained by the possible energy band diagrams. The results of this analysis provide a comprehensive understanding of the piezotronic effect and provide a direction for future device applications in fields such as health monitoring, human-machine communication, surgical robotics, and artificial intelligence.

Study on the GaN/AlGaN Piezotronic Effect Applied in Pressure Sensors

The GaN piezotronic effect is known as the coupling of its semiconductor and piezoelectric properties. In this work, with FEM simulation we study this effect applied in the piezoresistive, capacitive, and piezoelectric pressure sensors. The piezoresistive and capacitive sensing elements which are directly integrated in a single 2 thick microcantilever show a gauge factor of 251 and 1600, respectively. The piezoelectric sensing element of 2 GaN film anchored at the end of a thick suspended cantilever shows significant output voltage depending on the donor doping concentration in GaN. The capacitive sensing element of a graded AlGaN/GaN heterostructure with linear change of Al mole fraction which is anchored on the thick suspended cantilever shows a gauge factor of 122. This sensing mechanism can be explained that the pressure applied on the cantilever changes the distribution of space piezoelectric polarization charge and further the width of depletion region in the heterostructure.

Piezoelectric nanocomposites for sonodynamic bacterial elimination and wound healing

• Au@BTO nanocomposites are used as novel sonosensitizers to generate considerable ROS under US irradiation. • Au@BTO nanocomposites have significant antibacterial efficiency under US irradiation in vitro and in vivo . • Au@BTO nanocomposites also promote the migration of fibroblasts to promote wound healing. Sonodynamic therapy is considered as a minimally invasive method for pathogen elimination and cancer therapy with high spatial and temporal accuracy. However, sonosensitizers with good biocompatibility and high efficiency are still urgently needed. Herein, we have developed a piezoelectric nanocomposite, barium titanate (BaTiO 3 , BTO) nanocubes with Schottky junction modified by Au nanoparticles (Au@BTO) as a new kind of sonosensitizer for high-efficient sonodynamic therapy. Ultrasound as an exogenous mechanical wave can trigger the piezotronic effect of Au@BTO to facilitate the separation and migration of charge carriers at the piezoelectric/metal interface, which further effectively increases reactive oxygen species (ROS) generation via redox reaction. Due to the high ROS generation efficiency, Au@BTO as the sonosensitizer shows a high antibacterial efficiency against representative Gram-negative and Gram-positive bacteria. Furthermore, the in vitro and in vivo results illustrate that the sonodynamic process also promotes fibroblast migration, which contributes to dermal wound healing in mice. The proposed piezoelectric nanocomposite as a new kind of sonosensitizer has great potential for sonodynamic therapy.