Longfei Wang

Piezotronic Effect Enhanced Photocatalysis in Strained Anisotropic ZnO/TiO2 Nanoplatelets via Thermal Stress

Effective piezoelectric semiconductor based hybrid photocatalysts are successfully developed by assembling TiO2 nanoparticles on ZnO monocrystalline nanoplatelets. The piezopotential can be introduced and tuned by thermal stress on the piezoelectric material of ZnO monocrystalline nanoplatelets through cooling hybrid photocatalysts from high temperature to room temperature with different rates based on the mismatched thermal expansion coefficient of the two materials, which can be used to engineer the heterojunction band structure and significantly enhance the photocatalytic performance in a wide range by improving charge separation. It is proposed that the piezotronic effect enhanced photocatalyst will provide a strategy for high-performance photocatalysis applications.

p‐Type MoS2 and n‐Type ZnO Diode and Its Performance Enhancement by the Piezophototronic Effect

A plasma-induced p-type MoS2 flake and n-type ZnO film diode, which exhibits an excellent rectification ratio, is demonstrated. Under 365 nm optical irradiation, this p-n diode shows a strong photoresponse with an external quantum efficiency of 52.7% and a response time of 66 ms. By increasing the pressure on the junction to 23 MPa, the photocurrent can be enhanced by a factor of four through the piezophototronic effect.

Strain-Gated Field Effect Transistor of a MoS2–ZnO 2D–1D Hybrid Structure

Two-dimensional (2D) molybdenum disulfide (MoS2) is an exciting material due to its unique electrical, optical, and piezoelectric properties. Owing to an intrinsic band gap of 1.2-1.9 eV, monolayer or a-few-layer MoS2 is used for fabricating field effect transistors (FETs) with high electron mobility and on/off ratio. However, the traditional FETs are controlled by an externally supplied gate voltage, which may not be sensitive enough to directly interface with a mechanical stimulus for applications in electronic skin. Here we report a type of top-pressure/force-gated field effect transistors (PGFETs) based on a hybrid structure of a 2D MoS2 flake and 1D ZnO nanowire (NW) array. Once an external pressure is applied, the piezoelectric polarization charges created at the tips of ZnO NWs grown on MoS2 act as a gate voltage to tune/control the source-drain transport property in MoS2. At a 6.25 MPa applied stimulus on a packaged device, the source-drain current can be tuned for ∼25%, equivalent to the results of applying an extra -5 V back gate voltage. Another type of PGFET with a dielectric layer (Al2O3) sandwiched between MoS2 and ZnO also shows consistent results. A theoretical model is proposed to interpret the received data. This study sets the foundation for applying the 2D material-based FETs in the field of artificial intelligence.

Piezo‐Phototronic Effect Enhanced Flexible Solar Cells Based on n‐ZnO/p‐SnS Core–Shell Nanowire Array

The piezo‐phototronic effect is about the enhanced separation, transport, and recombination of the photogenerated carriers using the piezoelectric polarization charges present in piezoelectric‐semiconductor materials. Here, it is presented that the piezo‐phototronic effect can be effectively applied to improve the relative conversion efficiency of a flexible solar cell based on n‐ZnO/p‐SnS core–shell nanowire array for 37.3% under a moderate vertical pressure. The performance of the solar cell can be effectively enhanced by a gentle bending of the device, showing its potential for application in curly geometries. This study not only adds further understanding about the concept of increasing solar energy conversion efficiency via piezo‐phototronic effect, but also demonstrates the great potential of piezo‐phototronic effect in the application of large‐scale, flexible, and lightweight nanowire array solar cells.

Ultrasensitive 2D ZnO Piezotronic Transistor Array for High Resolution Tactile Imaging

In this paper, a new kind of 2D piezotronic transistor (PT) with the highest sensitivity till date has been designed and demonstrated, and the 2DPT array with ultrahigh spatial resolution has been developed through assembling ZnO nanoplatelets into ordered nanoplatelet array. As active sensors by directly converting applied mechanical actuations into electrical control signals without applying gate voltage, the ZnO 2DPT array has a great advantage as a fundamental component of piezotronics. The 2DPT array paves the way for a large-scale and integrated production of two terminal vertical transistors, which will contribute to its application in many fields such as human-machine interfacing, smart sensor, and processor systems.

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.

Ultrasensitive Vertical Piezotronic Transistor Based on ZnO Twin Nanoplatelet

High sensitivity of pressure/strain sensors is the key to accurately evaluating external mechanical stimuli and could become more important in future generations of human-machine interfaces and artificial skin. Here we report the study of a two-terminal piezotronic transistor based on ZnO twin nanoplatelets (TNPT). Owing to the mirror symmetrical structure of ZnO twin nanplatelet, compressive pressure-induced positive piezoelectric polarization charges created at both metal-semiconductor interfaces can simultaneously lower both Schottky barrier heights and thus significantly modulate the carrier transport. Our device exhibits the highest pressure sensitivity of 1448.08-1677.53 meV/MPa, which is more than ∼20 times larger than the highest value reported previously, and a fast response time of <5 ms. In addition, it can be used as a photodector with an ultrahigh external photoresponsivity of ∼1.45 × 104 AW-1, which is ∼105 times larger in magnitude than that of commercial UV photodetectors. The coupling between the mirror symmetrical structure and strong piezotronic effect in ZnO twin nanoplatelets may enable the development of ultrasensitive pressure/strain sensors for various applications such as artificial skin, health monitoring, and adaptive biomedical probes.

Piezotronic Effect on Rashba Spin–Orbit Coupling in a ZnO/P3HT Nanowire Array Structure

A key concept in the emerging field of spintronics is the voltage-gate control of spin precession via the effective magnetic field generated by the Rashba spin-orbit coupling (SOC). Traditional external gate voltage usually needs a power supply, which can easily bring about background noise or lead to a short circuit in measurement, especially for nanoscale spintronic devices. Here, we present a study on the circular photogalvanic effect (CPGE) in a ZnO/P3HT nanowire array structure with the device excited under oblique incidence. We demonstrate that a strong Rashba SOC is induced by the structure inversion asymmetry of the ZnO/P3HT heterointerface. We show that the Rashba SOC can be effectively tuned by inner-crystal piezo-potential created inside the ZnO nanowires instead of an externally applied voltage. The piezo-potential can not only ensure the stability of future spin-devices under a static pressure or strain but also work without the need of extra energy; hence this room-temperature generation and piezotronic effect control of spin photocurrent demonstrate a potential application in large-scale flexible spintronics in piezoelectric nanowire systems.

Tunable Tribotronic Dual‐Gate Logic Devices Based on 2D MoS2 and Black Phosphorus

With the Moores law hitting the bottleneck of scaling-down in size (below 10 nm), personalized and multifunctional electronics with an integration of 2D materials and self-powering technology emerge as a new direction of scientific research. Here, a tunable tribotronic dual-gate logic device based on a MoS2 field-effect transistor (FET), a black phosphorus FET and a sliding mode triboelectric nanogenerator (TENG) is reported. The triboelectric potential produced from the TENG can efficiently drive the transistors and logic devices without applying gate voltages. High performance tribotronic transistors are achieved with on/off ratio exceeding 106 and cutoff current below 1 pA μm-1 . Tunable electrical behaviors of the logic device are also realized, including tunable gains (improved to ≈13.8) and power consumptions (≈1 nW). This work offers an active, low-power-consuming, and universal approach to modulate semiconductor devices and logic circuits based on 2D materials with TENG, which can be used in microelectromechanical systems, human-machine interfacing, data processing and transmission.

Double-Channel Piezotronic Transistors for Highly Sensitive Pressure Sensing

Piezotronic transistors (PTs) that utilize inner crystal potential generated by interface piezoelectric polarization charges as the gate voltage have great potential applications in force/pressure-triggered or controlled electronic devices, sensors, human-machine communication, and microelectromechanical systems. Although the performance of PTs has been partially enhanced by exploring special materials with different geometries or high piezoelectricity, few studies have been focused on the structure design of PT itself to more effectively enhance the performance and structural reliability. Here, an integrated double-channel plane piezotronic transistor is invented as a high-performance pressure-sensing technology. Owing to the double-channel modulation and the plane structure, the PT has the merits of high pressure sensitivity (84.2-104.4 meV/MPa) and high structural reliability, which provides the opportunity for great applications, such as human-computer interfacing, biosensing, and health monitoring.