Equilibrium potential of free charge carriers in a bent piezoelectric semiconductive nanowire

Growth of Wide-Bandgap Monolayer Molybdenum Disulfide for a Highly Sensitive Micro-Displacement Sensor

Two-dimensional (2D) piezoelectric semiconductor materials are garnering significant attention in applications such as intelligent sensing and energy harvesting due to their exceptional physical and chemical properties. Among these, molybdenum disulfide (MoS2), a 2D wide-bandgap semiconductor, exhibits piezoelectricity in odd-layered structures due to the absence of an inversion symmetry center. In this study, we present a straightforward chemical vapor deposition (CVD) technique to synthesize monolayer MoS2 on a Si/SiO2 substrate, achieving a lateral size of approximately 50 µm. Second-harmonic generation (SHG) characterization confirms the non-centrosymmetric crystal structure of the wide-bandgap MoS2, indicative of its piezoelectric properties. We successfully transferred the triangular MoS2 to a polyethylene terephthalate (PET) flexible substrate using a wet-transfer method and developed a wide-bandgap MoS2-based micro-displacement sensor employing maskless lithography and hot evaporation techniques. Our testing revealed a piezoelectric response current of 5.12 nA in the sensor under a strain of 0.003% along the armchair direction of the monolayer MoS2. Furthermore, the sensor exhibited a near-linear relationship between the piezoelectric response current and the strain within a displacement range of 40-100 µm, with a calculated response sensitivity of 1.154 µA/%. This research introduces a novel micro-displacement sensor, offering potential for advanced surface texture sensing in various applications.

Transition metal dichalcogenides nanomaterials based piezocatalytic activity: recent progresses and outlook

Piezoelectric materials have drawn significant attention from researchers in the recent past as the piezo-potential, induced by applied external stress, generates an electric field, which paves the way for the creation and transfer of electrons and holes. After the theoretical prediction of the existence of the piezoelectric effect in transition metal dichalcogenides (TMDCs) semiconductors, intense research efforts have been made by various researchers to demonstrate the effect experimentally. In addition 2D TMDCs exhibit layer-dependent tunable electronic structure, strongly bound excitons, enhanced catalytic activity at their edges, and novel spin/pseudospin degrees of freedom. The edge sites and activated basal planes of 2D TMDCs are shown to be highly active toward catalysis of the hydrogen evolution reaction (HER). However, as compared to electrocatalytic or even photocatalytic performances, TMDC materials exhibit poorer piezocatalytic activity, in general. Therefore, a numbers of research strategies have been made to intensify the piezoelectric effect by synthesizing different types of TMDC nanostructures, by coupling the piezoelectric effect with the photocatalytic effect, by doping with other materials, etc. This review discusses various techniques of synthesis of TMDCs nanostructures and the recent progresses in applications of TMDC nanomaterials in piezocatalysis. In the present article, the piezocatalytic dye degradation performances and HER activity using different TMDCs have been reviewed in detail. Different methods of increasing the piezocatalytic activity of various TMDCs nanostructures have been illustrated. Here, it has also been attempted to systematically summarize and provide an outlook of the charge transfer behaviour and catalytic mechanisms in large varieties of TMDC piezocatalysts and piezo-photocatalysts. In addition, advanced applications of TMDC piezocatalytic materials as piezoelectric nanogenerator, piezocatalytic dye degradation, piezo-phototronic dye degradation and HER studies have been highlighted.

Pick-up strategies for and electrical characterization of ZnO nanowires with a SEM-based nanomanipulator

Because of their unique mechanical and electrical properties, zinc oxide (ZnO) nanowires are used widely in microscopic and nanoscopic devices and structures, but characterizing them remains challenging. In this paper, two pick-up strategies are proposed for characterizing the electrical properties of ZnO nanowires using SEM equipped with a nanomanipulator. To pick up nanowires efficiently, direct sampling is compared with electrification fusing, and experiments show that direct sampling is more stable while electrification fusing is more efficient. ZnO nanowires have cut-off properties, and good Schottky contact with the tungsten probes was established. In piezoelectric experiments, the maximum piezoelectric voltage generated by an individual ZnO nanowire was 0.07 V, and its impedance decreased with increasing input signal frequency until it became stable. This work offers a technical reference for the pick-up and construction of nanomaterials and nanogeneration technology.

In Situ Quantification of Strain-Induced Piezoelectric Potential of Dynamically Bending ZnO Microwires

The piezoelectric properties of semiconductor micro/nanowires (M/NWs) are crucial for optimizing semiconductors' electronic structure and carrier dynamics. However, the dynamic characterization of the piezoelectric properties of M/NWs remains challenging. Here, a Kelvin probe force microscopy technique based on a dual-probe atomic force microscope is developed to achieve in situ piezoelectric potential measurements of dynamic bending MWs. This technique can not only characterize the surface potential on different crystal faces of ZnO MWs in a natural state through controllable axial rotation, but also investigate the piezoelectric potential of the dynamically bending flake-like ZnO MW at different points and under different strain loads. The results show that the surface potentials of different faces/positions of the ZnO MWs are varied significantly, and determine that the quasi-static conditions piezo-strain factor of the flake-like ZnO MW is 0.28 V/%, while the factor was 0.14 V/% under low-frequency (⩽5 Hz) sinusoidal strain loading. This work provides a significant methodology to further study piezoelectric materials, and it aims to facilitate their applications in piezoelectric devices and systems.

From Piezoelectric Nanogenerator to Non-Invasive Medical Sensor: A Review

Piezoelectric nanogenerators (PENGs) not only are able to harvest mechanical energy from the ambient environment or body and convert mechanical signals into electricity but can also inform us about pathophysiological changes and communicate this information using electrical signals, thus acting as medical sensors to provide personalized medical solutions to patients. In this review, we aim to present the latest advances in PENG-based non-invasive sensors for clinical diagnosis and medical treatment. While we begin with the basic principles of PENGs and their applications in energy harvesting, this review focuses on the medical sensing applications of PENGs, including detection mechanisms, material selection, and adaptive design, which are oriented toward disease diagnosis. Considering the non-invasive in vitro application scenario, discussions about the individualized designs that are intended to balance a high performance, durability, comfortability, and skin-friendliness are mainly divided into two types: mechanical sensors and biosensors, according to the key role of piezoelectric effects in disease diagnosis. The shortcomings, challenges, and possible corresponding solutions of PENG-based medical sensing devices are also highlighted, promoting the development of robust, reliable, scalable, and cost-effective medical systems that are helpful for the public.

Effects of Group-I Elements on Output Voltage Generation of ZnO Nanowires Based Nanogenerator; Degradation of Screening Effects by Oxidation of Nanowires

Here, we report the successful incorporation of group I elements (K, Na, Li) to ZnO nanowires. Three distinct (2, 4, and 6 wt.%) doping concentrations of group I elements have been used to generate high piezoelectric voltage by employing a vertically integrated nanowire generator (VING) structure. X-ray photoelectron spectra (XPS) indicated the seepage of dopants in ZnO nanowires by substitution of Zn. Shallow acceptor levels (Li_Zn, Na_Zn, K_Zn) worked as electron trapping centers for intrinsically n-type ZnO nanowires. Free moving electrons caused a leakage current through the nanowires and depleted their piezoelectric potential. Reverse leakage current is a negative factor for piezoelectric nanogenerators. A reduction in reverse leakage current signifies the rise in output voltage. A gradual rise in output voltage has been witnessed which was in accordance with various doping concentrations. K-doped ZnO nanowires have generated voltages of 0.85 V, 1.48 V, and 1.95 V. For Na-doped ZnO nanowires, the voltages were 1.23 V, 1.73 V, and 2.34 V and the voltages yeilded for Li-doped ZnO nanowires were 1.87 V, 2.63 V, and 3.54 V, respectively. Maximum voltage range has been further enhanced by the surface enrichment (oxidized with O₂ molecules) of ZnO nanowires. Technique has been opted to mitigate the screening effect during an external stress. After 5 h of oxidation in a sealed chamber at 100 ppm, maximum voltage peaks were pronounced to 2.48 V, 3.19 V, and 4.57 V for K, Na, and Li, respectively. A low-cost, high performance mechanical transducer is proposed for self-powered devices.

Fatigue and its effect on the piezopotential properties of gallium nitride nanowires

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.

Piezoelectric A15B16C17 Compounds and Their Nanocomposites for Energy Harvesting and Sensors: A Review

Interest in pyroelectrics and piezoelectrics has increased worldwide on account of their unique properties. Applications based on these phenomena include piezo- and pyroelectric nanogenerators, piezoelectric sensors, and piezocatalysis. One of the most interesting materials used in this growing field are A¹⁵B¹⁶C¹⁷ nanowires, an example of which is SbSI. The latter has an electromechanical coupling coefficient of 0.8, a piezoelectric module of 2000 pC/N, and a pyroelectric coefficient of 12 × 10⁻³ C/m²K. In this review, we examine the production and properties of these nanowires and their composites, such as PAN/SbSI and PVDF/SbSI. The generated electrical response from 11 different structures under various excitations, such as an impact or a pressure shock, are presented. It is shown, for example, that the PVDF/SbSI and PAN/SbSI composites have well-arranged nanowires, the orientation of which greatly affects the value of its output power. The power density for all the nanogenerators based upon A¹⁵B¹⁶C¹⁷ nanowires (and their composites) are recalculated by use of the same key equation. This enables an accurate comparison of the efficiency of all the configurations. The piezo- and photocatalytic properties of SbSI nanowires are also presented; their excellent ability is shown by the high reaction kinetic rate constant (7.6 min⁻¹).

Spontaneous flexoelectricity and band engineering in MS2 (M = Mo, W) nanotubes

Spontaneous flexoelectricity in transition metal dichalcogenide (TMD) nanotubes is critical to the design of new energy devices. However, the electronic properties adjusted by the flexoelectric effect in TMD nanotubes remain vague. In this work, we investigate the effect of flexoelectricity on band engineering in single- and double-wall MS₂ (M = Mo, W) nanotubes with different diameters based on first-principles calculations and an atomic-bond-relaxation method. We find that the energy bandgap reduces and the polarization and flexoelectric voltage increase with decreasing diameter of single-wall MS₂ nanotubes. The polarization charges promoted by the flexoelectric effect can lead to a straddling-to-staggered bandgap transition in the double-wall MS₂ nanotubes. The critical diameters for bandgap transition are about 3.1 and 3.6 nm for double-wall MoS₂ and WS₂ nanotubes, respectively, which is independent of chirality. Our results provide guidance for the design of new energy devices based on spontaneous flexoelectricity.

Dimensional Roadmap for Maximizing the Piezoelectrical Response of ZnO Nanowire-Based Transducers: Impact of Growth Method

ZnO nanowires are excellent candidates for energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. The key parameters governing the general performance of the integrated devices include the dimensions of the ZnO nanowires used, their doping level, and surface trap density. However, although the method used to grow these nanowires has a strong impact on these parameters, its influence on the performance of the devices has been neither elucidated nor optimized yet. In this paper, we implement numerical simulations based on the finite element method combining the mechanical, piezoelectric, and semiconducting characteristic of the devices to reveal the influence of the growth method of ZnO nanowires. The electrical response of vertically integrated piezoelectric nanogenerators (VING) based on ZnO nanowire arrays operating in compression mode is investigated in detail. The properties of ZnO nanowires grown by the most widely used methods are taken into account on the basis of a thorough and comprehensive analysis of the experimental data found in the literature. Our results show that the performance of VING devices should be drastically affected by growth method. Important optimization guidelines are found. In particular, the optimal nanowire radius that would lead to best device performance is deduced for each growth method.

Emerging Pyroelectric Nanogenerators to Convert Thermal Energy into Electrical Energy

Pyroelectric energy harvesting systems have recently received substantial attention for their potential applications as power generators. In particular, the pyroelectric effect, which converts thermal energy into electrical energy, has been utilized as an infrared (IR) sensor, but upcoming sensor technology that requires a miniscule amount of power is able to utilize pyroelectric nanogenerators (PyNGs) as a power source. Herein, an overview of the progress in the development of PyNGs for an energy harvesting system that uses environmental or artificial energies such as the sun, body heat, and heaters, is provided. It begins with a brief introduction of the pyroelectric effect, and various polymer and ceramic materials based PyNGs are reviewed in detail. Various approaches for developing polymer-based PyNGs and various ceramic materials-based PyNGs are summarized in particular. Finally, challenges and perspectives regarding the PyNGs are described.

Mechanical/Electrical Characterization of ZnO Nanomaterial Based on AFM/Nanomanipulator Embedded in SEM

ZnO nanomaterials have been widely used in micro/nano devices and structure due to special mechanical/electrical properties, and its characterization is still deficient and challenging. In this paper, ZnO nanomaterials, including nanorod and nanowire are characterized by atomic force microscope (AFM) and nanomanipulator embedded in scanning electron microscope (SEM) respectively, which can manipulate and observe simultaneously, and is efficient and cost effective. Surface morphology and mechanical properties were observed by AFM. Results showed that the average Young's modulus of ZnO nanorods is 1.40 MPa and the average spring rate is 0.08 N/m. Electrical properties were characterized with nanomanipulator, which showed that the ZnO nanomaterial have cut-off characteristics and good schottky contact with the tungsten probes. A two-probe strategy was proposed for piezoelectric property measurement, which is easy to operate and adaptable to multiple nanomaterials. Experiments showed maximum voltage of a single ZnO nanowire is around 0.74 mV. Experiment criteria for ZnO manipulation and characterization were also studied, such as acceleration voltage, operation duration, sample preparation. Our work provides useful references for nanomaterial characterization and also theoretical basis for nanomaterials application.

Phase transformation and its effect on the piezopotential in a bent zinc oxide nanowire

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.

Fundamental Definitions for Axially-Strained Piezo-Semiconductive Nanostructures

Piezoelectric nanotransducers may offer key advantages in comparison with conventional piezoelectrics, including more choices for types of mechanical input, positions of the contacts, dimensionalities and shapes. However, since most piezoelectric nanostructures are also semiconductive, modeling becomes significantly more intricate and, therefore, the effects of free charges have been considered only in a few studies. Moreover, the available reports are complicated by the absence of proper nomenclature and figures of merit. Besides, some of the previous analyses are incomplete. For instance, the local piezopotential and free charges within axially strained conical piezo-semiconductive nanowires have only been systematically investigated for very low doping (10¹⁶ cm⁻³) and under compression. Here we give the definitions for the enhancement, depletion, base and tip piezopotentials, their characteristic lengths and both the tip-to-base and the depletion-to-enhancement piezopotential-ratios. As an example, we use these definitions for analyzing the local piezopotential and free charges in n-type ZnO truncated conical nanostructures with different doping levels (intrinsic, 10¹⁶ cm⁻³, 10¹⁷ cm⁻³) for both axial compression and traction. The definitions and concepts presented here may offer insight for designing high performance piezosemiconductive nanotransducers.

Free carrier enhanced depletion in ZnO nanorods decorated with bimetallic AuPt nanoclusters

The decoration of semiconductor nanostructures with small metallic clusters usually leads to an improvement of their properties in sensing or catalysis. Bimetallic cluster decoration typically is claimed to be even more effective. Here, we report a detailed investigation of the effects of Au, Pt or AuPt nanocluster decoration of ZnO nanorods on charge transport, photoluminescence and UV sensitivity. ZnO nanorods were synthesized by chemical bath deposition while decoration with small nanoclusters (2-3 nm in size) was achieved by a laser-ablation based cluster beam deposition technology. The structural properties were investigated by scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, and the optoelectronic properties by current-voltage and photoluminescence measurements. The extent of band bending at the cluster-ZnO interface was quantitatively modeled through numerical simulations. The decoration of ZnO nanorods with monometallic Au or Pt nanoclusters causes a significant depletion of free electrons below the surface, leading to a reduction of UV photoluminescence, an increase of ZnO nanorod dark resistance (up to 200 times) and, as a consequence, an improved sensitivity (up to 6 times) to UV light. These effects are strongly enhanced (up to 450 and 10 times, respectively) when ZnO nanorods are decorated with bimetallic AuPt nanoclusters that substantially augment the depletion of free carriers likely due to a more efficient absorption of the gas molecules on the surface of the bimetallic AuPt nanoclusters than on that of their monometallic counterparts. The depletion of free carriers in cluster decorated ZnO nanorods is quantitatively investigated and modelled, allowing the application of these composite materials in UV sensing and light induced catalysis.

Flexoelectronics of centrosymmetric semiconductors

Interface engineering by local polarization using piezoelectric^1-4, pyroelectric^5,6 and ferroelectric^7-9 effects has attracted considerable attention as a promising approach for tunable electronics/optoelectronics, human-machine interfacing and artificial intelligence. However, this approach has mainly been applied to non-centrosymmetric semiconductors, such as wurtzite-structured ZnO and GaN, limiting its practical applications. Here we demonstrate an electronic regulation mechanism, the flexoelectronics, which is applicable to any semiconductor type, expanding flexoelectricity^10-13 to conventional semiconductors such as Si, Ge and GaAs. The inner-crystal polarization potential generated by the flexoelectric field serving as a 'gate' can be used to modulate the metal-semiconductor interface Schottky barrier and further tune charge-carrier transport. We observe a giant flexoelectronic effect in bulk centrosymmetric semiconductors of Si, TiO₂ and Nb-SrTiO₃ with high strain sensitivity (>2,650), largely outperforming state-of-the-art Si-nanowire strain sensors and even piezoresistive, piezoelectric and ferroelectric nanodevices¹⁴. The effect can be used to mechanically switch the electronics in the nanoscale with fast response (<4 ms) and high resolution (~0.78 nm). This opens up the possibility of realizing strain-modulated electronics in centrosymmetric semiconductors, paving the way for local polarization field-controlled electronics and high-performance electromechanical applications.

Magnetically Induced Carrier Distribution in a Composite Rod of Piezoelectric Semiconductors and Piezomagnetics

In this work, we study the behavior of a composite rod consisting of a piezoelectric semiconductor layer and two piezomagnetic layers under an applied axial magnetic field. Based on the phenomenological theories of piezoelectric semiconductors and piezomagnetics, a one-dimensional model is developed from which an analytical solution is obtained. The explicit expressions of the coupled fields and the numerical results show that an axially applied magnetic field produces extensional deformation through piezomagnetic coupling, the extension then produces polarization through piezoelectric coupling, and the polarization then causes the redistribution of mobile charges. Thus, the composite rod exhibits a coupling between the applied magnetic field and carrier distribution through combined piezomagnetic and piezoelectric effects. The results have potential applications in piezotronics when magnetic fields are relevant.

On the piezotronic behaviours of wurtzite core-shell nanowires

The piezotronic behaviours of wurtzite core-shell nanowires (NWs) are studied in this paper by using a multiscale modelling technique. A difference between piezopotentials obtained from molecular dynamics simulations and finite element calculations indicates that due to the influence of small-scale effects the widely used conventional electromechanical theory is not accurate in describing the piezopotential properties of the present core-shell NWs. Although the residual strains intrinsically existing in core-shell NWs and the structural reconstruction at their surface and interface both account for these small-scale effects, the latter is found to play the dominate role, which makes the material properties of core-shell NWs significantly depend on their geometric size. A novel core-interface-shell-surface model is proposed here to analytically describe the size dependence of the material properties and thus the small-scale effects on the piezopotential of core-shell NWs. Besides possessing a good piezoelectric performance, our density functional theory calculations also show that the core-shell NWs under external loading can retain the semiconducting properties, which confirms the existence of piezotronic effects in them. However, owing to the intrinsic asymmetric Schottky barriers at the source and drain contacts induced by residual piezopotentials in core-shell NWs, the piezotronic effects of core-shell NWs are different to those of their conventional single-component counterparts. The superb piezopotential properties and unique piezotronic behaviours observed in wurtzite core-shell NWs make them good candidates for high performance components in novel piezotronic nanodevices.

A facile hot-pressing process for fabricating flexible top electrodes of piezoelectric ZnO nanowire nanogenerators

We report a facile and effective hot-pressing strategy for fabricating the flexible top electrode for a piezoelectric nanogenerator (PENG). Flexible stainless steel (SUS) foil was employed as the bottom electrode and substrate of the device. Zinc oxide nanowires (ZnO NWs) were grown on SUS substrate through the hydrothermal synthesis method. The top electrode of Zn foil was combined with dielectric polydimethylsiloxane (PDMS) film using the hot-pressing process. The resulting top electrode is thick enough to enable the device to generate piezoelectric output differently under bending conditions. The PENG devices generated an output voltage of about 2.2 V and an output current of 8 nA under the optimum operating conditions. The devices fabricated by the hot-pressing process were robust enough to retain their generating ability after thousands of bending and releasing cycles.

Influence of Doping Concentration on the Outputs of a Bent ZnO Nanowire

In this article, the governing equation on electric potential is established by coupling both the semiconducting characteristics and the piezo-effect property of a ZnO nanowire (ZNW). Carrier redistributions are solved for different doping concentrations while considering the effect of both types of charge carriers. The reason for the semiconducting characteristics of a ZNW to induce electric leakage in energy-harvesting is illustrated in detail and a proper initial carrier concentration, n₀ or p₀ , is obtained as follows: for a n-type ZnO fiber and for a p-type one under the intrinsic carrier concentration n_i = 1.0×10¹⁶(1/m³) , which is of significance in both the design and the practical applications of piezotronics and piezo-phototropic devices.

Study on ultra-high sensitivity piezoelectric effect of GaN micro/nano columns

High-quality GaN micro/nano columns were prepared with self-organized catalytic-free method. Young's modulus of GaN nanocolumns were measured under both compressive stress and tensile stress. It was found that the Young's modulus decreases with the increasing of nanocolumn diameter due to the increase of face defect density. Furthermore, we measured the piezoelectric properties and found that there was a 1000-fold current increase under a strain of 1% with a fixed bias voltage of 10 mV. Based on the Schottky Barrier Diode model, we modified it with the effect of polarization charge, image charge and interface state to analyze the experiment results which reveals that the strong piezopolarization effect plays an important role in this phenomenon. Therefore, the GaN nanocolumns has a great prospect to be applied in high-efficiency nanogenerators and high-sensitivity nanosensors.

Nonlinear effect of carrier drift on the performance of an n-type ZnO nanowire nanogenerator by coupling piezoelectric effect and semiconduction

In piezoelectric semiconductors, electric fields drive carriers into motion/redistribution, and in turn the carrier motion/redistribution has an opposite effect on the electric field itself. Thus, carrier drift in a piezoelectric semiconducting structure is essentially nonlinear unless the induced fluctuation of carrier concentration is very small. In this paper, the nonlinear governing equation of carrier concentration was established by coupling both piezoelectric effect and semiconduction. A nonlinear carrier-drift effect on the performance of a ZnO nanogenerator was investigated in detail and it was elucidated that carrier motion/redistribution occurs in the ZnO nanowire (ZNW) cross section while there is no carrier motion in the axial direction. At the same time, we noted that the amplitude of boundary electric charge grows with increasing deformation, but the peaks of boundary electric charge do not appear at the cross-section endpoints. Thus, in order to effectively improve the performance of the ZNW nanogenerator, the effect of electrode configuration on the piezoelectric potential difference and output power was analyzed in detail. The electrode size for the optimal performance of a ZnO nanowire generator was proposed. This analysis that couples electromechanical fields and carrier concentration as a whole has some referential significance to piezotronics.

The Influence of Shape on the Output Potential of ZnO Nanostructures: Sensitivity to Parallel versus Perpendicular Forces

With the consistent shrinking of devices, micro-systems are, nowadays, widely used in areas such as biomedics, electronics, automobiles, and measurement devices. As devices shrunk, so too did their energy consumptions, opening the way for the use of nanogenerators (NGs) as power sources. In particular, to harvest energy from an object's motion (mechanical vibrations, torsional forces, or pressure), present NGs are mainly composed of piezoelectric materials in which, upon an applied compressive or strain force, an electrical field is produced that can be used to power a device. The focus of this work is to simulate the piezoelectric effect in different ZnO nanostructures to optimize the output potential generated by a nanodevice. In these simulations, cylindrical nanowires, nanomushrooms, and nanotrees were created, and the influence of the nanostructures' shape on the output potential was studied as a function of applied parallel and perpendicular forces. The obtained results demonstrated that the output potential is linearly proportional to the applied force and that perpendicular forces are more efficient in all structures. However, nanotrees were found to have an increased sensitivity to parallel applied forces, which resulted in a large enhancement of the output efficiency. These results could then open a new path to increase the efficiency of piezoelectric nanogenerators.

Piezoelectric Potential in Single-Crystalline ZnO Nanohelices Based on Finite Element Analysis

Electric potential produced in deformed piezoelectric nanostructures is of significance for both fundamental study and practical applications. To reveal the piezoelectric property of ZnO nanohelices, the piezoelectric potential in single-crystal nanohelices was simulated by finite element method calculations. For a nanohelix with a length of 1200 nm, a mean coil radius of 150 nm, five active coils, and a hexagonal coiled wire with a side length 100 nm, a compressing force of 100 nN results in a potential of 1.85 V. This potential is significantly higher than the potential produced in a straight nanowire with the same length and applied force. Maintaining the length and increasing the number of coils or mean coil radius leads to higher piezoelectric potential in the nanohelix. Appling a force along the axial direction produces higher piezoelectric potential than in other directions. Adding lateral forces to an existing axial force can change the piezoelectric potential distribution in the nanohelix, while the maximum piezoelectric potential remains largely unchanged in some cases. This research demonstrates the promising potential of ZnO nanohelices for applications in sensors, micro-electromechanical systems (MEMS) devices, nanorobotics, and energy sciences.

Piezo-Phototronic Matrix via a Nanowire Array

Piezoelectric semiconductors, such as ZnO and GaN, demonstrate multiproperty coupling effects toward various aspects of mechanical, electrical, and optical excitation. In particular, the three-way coupling among semiconducting, photoexcitation, and piezoelectric characteristics in wurtzite-structured semiconductors is established as a new field, which was first coined as piezo-phototronics by Wang in 2010. The piezo-phototronic effect can controllably modulate the charge-carrier generation, separation, transport, and/or recombination in optical-electronic processes by modifying the band structure at the metal-semiconductor or semiconductor-semiconductor heterojunction/interface. Here, the progress made in using the piezo-phototronic effect for enhancing photodetectors, pressure sensors, light-emitting diodes, and solar cells is reviewed. In comparison with previous works on a single piezoelectric semiconducting nanowire, piezo-phototronic nanodevices built using nanowire arrays provide a promising platform for fabricating integrated optoelectronics with the realization of high-spatial-resolution imaging and fast responsivity.

Optimal geometrical design of inertial vibration DC piezoelectric nanogenerators based on obliquely aligned InN nanowire arrays

Piezoelectric nanogenerators have been investigated to generate electricity from environmental vibrations due to their energy conversion capabilities. In this study, we demonstrate an optimal geometrical design of inertial vibration direct-current piezoelectric nanogenerators based on obliquely aligned InN nanowire (NW) arrays with an optimized oblique angle of ∼58°, and driven by the inertial force of their own weight, using a mechanical shaker without any AC/DC converters. The nanogenerator device manifests potential applications not only as a unique energy harvesting device capable of scavenging energy from weak mechanical vibrations, but also as a sensitive strain sensor. The maximum output power density of the nanogenerator is estimated to be 2.9 nW cm^-2, leading to an improvement of about 3-12 times that of vertically aligned ZnO NW DC nanogenerators. Integration of two nanogenerators also exhibits a linear increase in the output power, offering an enormous potential for the creation of self-powered sustainable nanosystems utilizing incessantly natural ambient energy sources.

Surface/Interface Carrier-Transport Modulation for Constructing Photon-Alternative Ultraviolet Detectors Based on Self-Bending-Assembled ZnO Nanowires

Surface/interface charge-carrier generation, diffusion, and recombination/transport modulation are especially important in the construction of photodetectors with high efficiency in the field of nanoscience. In the paper, a kind of ultraviolet (UV) detector is designed based on ZnO nanostructures considering photon-trapping, surface plasmonic resonance (SPR), piezophototronic effects, interface carrier-trapping/transport control, and collection. Through carefully optimized surface/interface carrier-transport modulation, a designed device with detectivity as high as 1.69 × 10¹⁶/1.71 × 10¹⁶ cm·Hz^1/2/W irradiating with 380 nm photons under ultralow bias of 0.2 V is realized by alternating nanoparticle/nanowire active layers, respectively, and the designed UV photodetectors show fast and slow recovery processes of 0.27 and 4.52 ms, respectively, which well-satisfy practical needs. Further, it is observed that UV photodetection could be performed within an alternative response by varying correlated key parameters, through efficient surface/interface carrier-transport modulation, spectrally resolved photoresponse of the detector revealing controlled detection in the UV region based on the ZnO nanomaterial, photodetection allowed or limited by varying the active layers, irradiation distance from one of the electrodes, standing states, or electric field. The detailed carrier generation, diffusion, and recombination/transport processes are well illustrated to explain charge-carrier dynamics contributing to the photoresponse behavior.

Energy harvesting efficiency in GaN nanowire-based nanogenerators: the critical influence of the Schottky nanocontact

The performances of 1D-nanostructure based nanogenerators are governed by the ability of nanostructures to efficiently convert mechanical deformation into electrical energy, and by the efficiency with which this piezo-generated energy is harvested. In this paper, we highlight the crucial influence of the GaN nanowire-metal Schottky nanocontact on the energy harvesting efficiency. Three different metals, p-type doped diamond, PtSi and Pt/Ir, have been investigated. By using an atomic force microscope equipped with a Resiscope module, we demonstrate that the harvesting of piezo-generated energy is up to 2.4 times more efficient using a platinum-based Schottky nanocontact compared to a doped diamond-based nanocontact. In light of Schottky contact characteristics, we evidence that the conventional description of the Schottky diode cannot be applied. The contact is governed by its nanometer size. This specific behaviour induces notably a lowering of the Schottky barrier height, which gives rise to an enhanced conduction. We especially demonstrate that this effective thinning is directly correlated with the improvement of the energy harvesting efficiency, which is much pronounced for Pt-based Schottky diodes. These results constitute a building block to the overall improvement of NW-based nanogenerator devices.

Enhancing the Efficiency of Silicon-Based Solar Cells by the Piezo-Phototronic Effect

Although there are numerous approaches for fabricating solar cells, the silicon-based photovoltaics are still the most widely used in industry and around the world. A small increase in the efficiency of silicon-based solar cells has a huge economic impact and practical importance. We fabricate a silicon-based nanoheterostructure (p⁺-Si/p-Si/n⁺-Si (and n-Si)/n-ZnO nanowire (NW) array) photovoltaic device and demonstrate the enhanced device performance through significantly enhanced light absorption by NW array and effective charge carrier separation by the piezo-phototronic effect. The strain-induced piezoelectric polarization charges created at n-doped Si-ZnO interfaces can effectively modulate the corresponding band structure and electron gas trapped in the n⁺-Si/n-ZnO NW nanoheterostructure and thus enhance the transport process of local charge carriers. The efficiency of the solar cell was improved from 8.97% to 9.51% by simply applying a static compress strain. This study indicates that the piezo-phototronic effect can enhance the performance of a large-scale silicon-based solar cell, with great potential for industrial applications.

High-efficiency piezoelectric micro harvester for collecting low-frequency mechanical energy

A single-layer zinc oxide (ZnO) nanorod array-based micro energy harvester was designed and integrated with a piezoelectric metacapacitor. The device presents outstanding low-frequency (1-10 Hz) mechanical energy harvesting capabilities. When compared with conventional pristine ZnO nanostructured piezoelectric harvesters or generators, both open-circuit potential and short-circuit current are significantly enhanced (up to 3.1 V and 124 nA cm^-2) for a single mechanical knock (∼34 kPa). Higher electromechanical conversion efficiency (1.3 pC/Pa) is also observed. The results indicate that the integration of the piezoelectric metacapacitor is a crucial factor for improving the low-frequency energy harvesting performance. A double piezoelectric-driven mechanism is proposed to explain current higher output power, in which the metacapacitor plays the multiple roles of charge pumping, storing and transferring. An as-fabricated prototype device for lighting an LED demonstrates high power transference capability, with over 95% transference efficiency to the external load.

Density functional studies on wurtzite piezotronic transistors: influence of different semiconductors and metals on piezoelectric charge distribution and Schottky barrier

The mechanical-electrical coupling properties of piezoelectric semiconductors endow these materials with novel device applications in microelectromechanical systems, sensors, human-computer interfaces, etc. When an applied strain is exerted on a piezoelectric semiconductor, piezoelectric charges are generated at the surface or interface of the semiconductor, which can be utilized to control the electronic transport characteristics. This is the fundamental working mechanism of piezotronic devices, called the piezotronic effect. In the present report, a series of piezotronic transistors composed of different electrode metals and semiconductors is examined using density functional theory calculation. It is found that the influence of semiconductors on the piezotronic effect is larger than the impact of metals, and GaN and CdS are promising candidates for piezotronic and piezo-phototronic devices, respectively. The width of the piezoelectric charge distribution obtained in the present study can be used as a parameter in classical finite-element-method based simulations, which provide guidance on designing high-performance piezotronic devices.

In Situ Characterization of the Local Work Function along Individual Free Standing Nanowire by Electrostatic Deflection

In situ characterization of the work function of quasi one dimensional nanomaterials is essential for exploring their applications. Here we proposed to use the electrostatic deflection induced by work function difference between nanoprobe and nanowire for in situ measuring the local work function along a free standing nanowire. The physical mechanism for the measurement was discussed in details and a parabolic relationship between the deflection and the potential difference was derived. As a demonstration, measurement of the local work functions on the tip and the sidewall of a ZnO nanowire with Au catalyst at its end and a LaB₆ nanowire have been achieved with good accuracy.

Nanogenerators based on vertically aligned InN nanowires

Piezoelectric nanogenerators (NGs) based on vertically aligned InN nanowires (NWs) are fabricated, characterized, and evaluated. In these NGs, arrays of p-type and intrinsic InN NWs prepared by plasma-assisted molecular beam epitaxy (MBE) demonstrate similar piezoelectric properties. The p-type NGs show 160% more output current and 70% more output power product than the intrinsic NGs. The features driving performance enhancement are reduced electrostatic losses due to better NW array morphology, improved electromechanical energy conversion efficiency due to smaller NW diameters, and the higher impedance of intrinsic NGs due to elevated NW surface charge levels. These findings highlight the potential of InN based NGs as a power source for self-powered systems and the importance of NW morphology and surface state in overall NG performance.

Temperature Dependence of the Piezotronic and Piezophototronic Effects in a-axis GaN Nanobelts

The temperature dependence of the piezotronic and piezophototronic effects in a-axis GaN nanobelts from 77 to 300 K is investigated. The piezotronic effect is enhanced by over 440% under lower temp-eratures. Two independent processes are discovered to form a competing mechanism through the investigation of the temperature dependence of the piezophototronic effect in a-axis GaN nanobelts.

Recent Advancements in Nanogenerators for Energy Harvesting

Nanomaterial-based generators are a highly promising power supply for micro/nanoscale devices, capable of directly harvesting energy from ambient sources without the need for batteries. These generators have been designed within four main types: piezoelectric, triboelectric, thermoelectric, and electret effects, and consist of ZnO-based, silicon-based, ferroelectric-material-based, polymer-based, and graphene-based examples. The representative achievements, current challenges, and future prospects of these nanogenerators are discussed.

Piezotronic Effect in Strain-Gated Transistor of a-Axis GaN Nanobelt

Due to the non-centrosymmetric crystal structures, wurtzite family semiconducting materials possess piezoelectric properties and exhibit polarizations along certain directions upon straining. Utilizing strain-induced piezoelectric polarization charges to modulate the energy band structures and thus to tune/control the transport processes of charge carriers is referred to as the piezotronic effect. Distinct from the previous studies of c-axis GaN nanowires, here we systematically study the piezotronic-effect-induced modifications of energy band structures and the corresponding influence on electronic transport properties of a-axis GaN nanobelts. The physical mechanism is carefully illustrated and further confirmed by theoretical simulations via finite element analysis. The spatial distributions of local carrier concentration and the energy band diagrams of a-axis GaN under various straining conditions are calculated. This work provides a thorough understanding of strain-gated transport properties of a-axis GaN piezotronic transistors and its future applications in semiconductor devices.

Defect related emission versus intersystem crossing: blue emitting ZnO/graphene oxide quantum dots

In ref. [Nat. Nanotechnol., 2012, 7, 465-471] interesting optoelectronic properties of ZnO/graphene oxide (GO) composite were presented. Essentially, in the luminescence spectrum indirect optical transitions were identified to be from the epoxy group of GO (GOepoxy) to the valance band (Ev) of ZnO. Viz. 406 nm, L1: (LUMO+2)GOepoxy→Ev and 436 nm, L2: (LUMO)GOepoxy→Ev. Furthermore, the emission peak at ∼550 nm was attributed to zinc interstitials (Znis) or oxygen vacancies (VOs) and shown to span from 350-650 nm (equivalent to a width of ∼0.8 eV). In this report we accentuate two vital though largely ignored concerns as itemized in the following. (i) By considering the growth mechanism of ZnO in the composite, there is a certain possibility that these two bands (L1 and L2) may originate from intrinsic defects of ZnO such as Znis and extended Znis (ex-Znis). Or L1 and L2 might be intrinsic to GO. (ii) The 550 nm emission involves VOs and consists of two components with a typical width of ∼0.3 eV. Here we present the results of a thorough investigation confirming the presence of Znis, ex-Znis and intrinsic emission from GO. We also note that during the synthesis the presence of dimethyl formamide significantly affected the emission from GO in addition to some chemical modifications. Apart from these, we have discussed other crucial factors which require deeper attention in the context of luminescence from complex systems such as those present.

Novel Two-Dimensional Mechano-Electric Generators and Sensors Based on Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, provide two-dimensional atomic crystals with semiconductor band gap. In this work, we present a design of new mechano-electric generators and sensors based on transition metal dichalcogenide nanoribbon PN junctions and heterojunctions. The mechano-electric conversion was simulated by using a first-principle calculation. The output voltage of MoS2 nanoribbon PN junction increases with strain, reaching 0.036 V at 1% strain and 0.31 V at 8% strain, much larger than the reported results. Our study indicates that the length, width and layer number of TMDC nanoribbon PN junctions have an interesting but different impact on the voltage output. Also, the results indicate that doping position and concentration only cause a small fluctuation in the output voltage. These results have been compared with the mechano-electric conversion of TMDC heterojunctions. Such novel mechano-electric generators and sensors are very attractive for applications in future self-powered, wearable electronics and systems.

Effects of mechanical deformation on energy conversion efficiency of piezoelectric nanogenerators

Piezoelectric nanogenerators (PNGs) are capable of converting energy from various mechanical sources into electric energy and have many attractive features such as continuous operation, replenishment and low cost. However, many researchers still have studied novel material synthesis and interfacial controls to improve the power production from PNGs. In this study, we report the energy conversion efficiency (ECE) of PNGs dependent on mechanical deformations such as bending and twisting. Since the output power of PNGs is caused by the mechanical strain of the piezoelectric material, the power production and their ECE is critically dependent on the types of external mechanical deformations. Thus, we examine the output power from PNGs according to bending and twisting. In order to clearly understand the ECE of PNGs in the presence of those external mechanical deformations, we determine the ECE of PNGs by the ratio of output electrical energy and input mechanical energy, where we suggest that the input energy is based only on the strain energy of the piezoelectric layer. We calculate the strain energy of the piezoelectric layer using numerical simulation of bending and twisting of the PNG. Finally, we demonstrate that the ECE of the PNG caused by twisting is much higher than that caused by bending due to the multiple effects of normal and lateral piezoelectric coefficients. Our results thus provide a design direction for PNG systems as high-performance power generators.

Thermal-electric model for piezoelectric ZnO nanowires

The behavior of ZnO nanowires under uniaxial loading is characterized by means of a numerical model that accounts for all coupled mechanical, electrical, and thermal effects. The paper shows that thermal effects in the nanowires may greatly impact the predicted performance of piezoelectric and piezotronic nanodevices. The pyroelectric effect introduces new equivalent volumic charge in the body of the nanowire and surface charges at the boundaries, where Kapitza resistances are located, that act together with the piezoelectric charges to improve the predicted performance. It is shown that the proposed model is able to reproduce several effects experimentally observed by other research groups, and is a promising tool for the design of ultra-high efficient nanodevices.

Piezo-phototronic Effect Enhanced UV/Visible Photodetector Based on Fully Wide Band Gap Type-II ZnO/ZnS Core/Shell Nanowire Array

A high-performance broad band UV/visible photodetector has been successfully fabricated on a fully wide bandgap ZnO/ZnS type-II heterojunction core/shell nanowire array. The device can detect photons with energies significantly smaller (2.2 eV) than the band gap of ZnO (3.2 eV) and ZnS (3.7 eV), which is mainly attributed to spatially indirect type-II transition facilitated by the abrupt interface between the ZnO core and ZnS shell. The performance of the device was further enhanced through the piezo-phototronic effect induced lowering of the barrier height to allow charge carrier transport across the ZnO/ZnS interface, resulting in three orders of relative responsivity change measured at three different excitation wavelengths (385, 465, and 520 nm). This work demonstrates a prototype UV/visible photodetector based on the truly wide band gap semiconducting 3D core/shell nanowire array with enhanced performance through the piezo-phototronic effect.

Detecting Liquefied Petroleum Gas (LPG) at Room Temperature Using ZnSnO3/ZnO Nanowire Piezo-Nanogenerator as Self-Powered Gas Sensor

High sensitivity, selectivity, and reliability have been achieved from ZnSnO3/ZnO nanowire (NW) piezo-nanogenerator (NG) as self-powered gas sensor (SPGS) for detecting liquefied petroleum gas (LPG) at room temperature (RT). After being exposed to 8000 ppm LPG, the output piezo-voltage of ZnSnO3/ZnO NW SPGS under compressive deformation is 0.089 V, much smaller than that in air ambience (0.533 V). The sensitivity of the SPGS against 8000 ppm LPG is up to 83.23, and the low limit of detection is 600 ppm. The SPGS has lower sensitivity against H2S, H2, ethanol, methanol and saturated water vapor than LPG, indicating good selectivity for detecting LPG. After two months, the decline of the sensing performance is less than 6%. Such piezo-LPG sensing at RT can be ascribed to the new piezo-surface coupling effect of ZnSnO3/ZnO nanocomposites. The practical application of the device driven by human motion has also been simply demonstrated. This work provides a novel approach to fabricate RT-LPG sensors and promotes the development of self-powered sensing system.

Influence of the carrier concentration on the piezotronic effect in a ZnO/Au Schottky junction

The piezotronic effect, which utilizes the piezopotential to engineer the interface characteristics, has been widely exploited to design novel functional device or to optimize the device performance, which is intimately related to the carrier concentration. Here, by constructing a general Schottky diode, the piezotronic effect dependence on the carrier concentration was investigated systematically using ultraviolet (UV) illumination. Scanning Kelvin Probe Microscopy was employed to quantify the carrier concentration in ZnO nanorods under UV illumination. The results showed that the carrier concentration increases with increasing light intensity and an average value of up to 5.6 × 10(18) cm(-3) under 1.2 mW cm(-2) light illumination was obtained. Furthermore, with increasing UV light intensity, an increasingly imperceptible variation in the current-voltage characteristics under strain was observed, which finally disappeared under 1.2 mW cm(-2) light illumination. This phenomenon was attributed to the weakened modulation ability of the piezopotential due to the strengthened screening effect. In addition, the gradual disappearing in the barrier also contributed to the gradual disappearance of the piezotronic effect. This study provides an in-depth understanding of piezotronics, which could be extended to other piezoelectric devices and guide the design and optimization of piezotronic and even piezophototronic devices.

Highly stable piezo-immunoglobulin-biosensing of a SiO2/ZnO nanogenerator as a self-powered/active biosensor arising from the field effect influenced piezoelectric screening effect

Highly stable piezo-immunoglobulin-biosensing has been realized from a SiO2/ZnO nanowire (NW) nanogenerator (NG) as a self-powered/active biosensor. The piezoelectric output generated by the SiO2/ZnO NW NG can act not only as a power source for driving the device, but also as a sensing signal for detecting immunoglobulin G (IgG). The stability of the device is very high, and the relative standard deviation (RSD) ranges from 1.20% to 4.20%. The limit of detection (LOD) of IgG on the device can reach 5.7 ng mL(-1). The response of the device is in a linear relationship with IgG concentration. The biosensing performance of SiO2/ZnO NWs is much higher than that of bare ZnO NWs. A SiO2 layer uniformly coated on the surface of the ZnO NW acts as the gate insulation layer, which increases mechanical robustness and protects it from the electrical leakages and short circuits. The IgG biomolecules modified on the surface of the SiO2/ZnO NW act as a gate potential, and the field effect can influence the surface electron density of ZnO NWs, which varies the screening effect of free-carriers on the piezoelectric output. The present results demonstrate a feasible approach for a highly stable self-powered/active biosensor.

Elastic properties of GaN nanowires: revealing the influence of planar defects on young's modulus at nanoscale

The elastic properties of gallium nitride (GaN) nanowires with different structures were investigated by in situ electron microscopy in this work. The electric-field-induced resonance method was utilized to reveal that the single crystalline GaN nanowires, along [120] direction, had the similar Young's modulus as the bulk value at the diameter ranging 92-110 nm. Meanwhile, the elastic behavior of the obtuse-angle twin (OT) GaN nanowires was disclosed both by the in situ SEM resonance technique and in situ transmission electron microscopy tensile test for the first time. Our results showed that the average Young's modulus of these OT nanowires was greatly decreased to about 66 GPa and indicated no size dependence at the diameter ranging 98-171 nm. A quantitative explanation for this phenomenon is proposed based on the rules of mixtures in classical mechanics. It is revealed that the elastic modulus of one-dimensional nanomaterials is dependent on the relative orientations and the volume fractions of the planar defects.

Flexible piezoelectric nanogenerator based on Cu2O–ZnO p–n junction for energy harvesting

A nanogenerator based on Cu₂O–ZnO p–n junction has been fabricated on a Cu wire substrate for harvesting mechanical energy from the environment.

Optimizing performance of silicon-based p-n junction photodetectors by the piezo-phototronic effect

Silicon-based p-n junction photodetectors (PDs) play an essential role in optoelectronic applications for photosensing due to their outstanding compatibility with well-developed integrated circuit technology. The piezo-phototronic effect, a three-way coupling effect among semiconductor properties, piezoelectric polarizations, and photon excitation, has been demonstrated as an effective approach to tune/modulate the generation, separation, and recombination of photogenerated electron-hole pairs during optoelectronic processes in piezoelectric-semiconductor materials. Here, we utilize the strain-induced piezo-polarization charges in a piezoelectric n-ZnO layer to modulate the optoelectronic process initiated in a p-Si layer and thus optimize the performances of p-Si/ZnO NWs hybridized photodetectors for visible sensing via tuning the transport property of charge carriers across the Si/ZnO heterojunction interface. The maximum photoresponsivity R of 7.1 A/W and fastest rising time of 101 ms were obtained from these PDs when applying an external compressive strain of -0.10‰ on the ZnO NWs, corresponding to relative enhancement of 177% in R and shortening to 87% in response time, respectively. These results indicate a promising method to enhance/optimize the performances of non-piezoelectric semiconductor material (e.g., Si) based optoelectronic devices by the piezo-phototronic effect.

Design Concepts, Fabrication and Advanced Characterization Methods of Innovative Piezoelectric Sensors Based on ZnO Nanowires

Micro- and nano-scale materials and systems based on zinc oxide are expected to explode in their applications in the electronics and photonics, including nano-arrays of addressable optoelectronic devices and sensors, due to their outstanding properties, including semiconductivity and the presence of a direct bandgap, piezoelectricity, pyroelectricity and biocompatibility. Most applications are based on the cooperative and average response of a large number of ZnO micro/nanostructures. However, in order to assess the quality of the materials and their performance, it is fundamental to characterize and then accurately model the specific electrical and piezoelectric properties of single ZnO structures. In this paper, we report on focused ion beam machined high aspect ratio nanowires and their mechanical and electrical (by means of conductive atomic force microscopy) characterization. Then, we investigate the suitability of new power-law design concepts to accurately model the relevant electrical and mechanical size-effects, whose existence has been emphasized in recent reviews.

Effects of free carriers on piezoelectric nanogenerators and piezotronic devices made of GaN nanowire arrays

This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 10(17) (unintentionally doped) to 10(19) cm(-3) (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 10(18) cm(-3) and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN(-1) and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current-voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.

Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics

The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics. With their high crystallinity and ability to withstand enormous strain, two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers. Here we report the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW m(-2) and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. The coupling between piezoelectricity and semiconducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.