Jinzong Kou

Flexible Self-Powered GaN Ultraviolet Photoswitch with Piezo-Phototronic Effect Enhanced On/Off Ratio

Flexible self-powered sensing is urgently needed for wearable, portable, sustainable, maintenance-free and long-term applications. Here, we developed a flexible and self-powered GaN membrane-based ultraviolet (UV) photoswitch with high on/off ratio and excellent sensitivity. Even without any power supply, the driving force of UV photogenerated carriers can be well boosted by the combination of both built-in electric field and piezoelectric polarization field. The asymmetric metal-semiconductor-metal structure has been elaborately utilized to enhance the carrier separation and transport for highly sensitive UV photoresponse. Its UV on/off ratio and detection sensitivity reach to 4.67 × 10(5) and 1.78 × 10(12) cm·Hz(0.5) W(1-), respectively. Due to its excellent mechanical flexibility, the piezoelectric polarization field in GaN membrane can be easily tuned/controlled based on piezo-phototronic effect. Under 1% strain, a stronger and broader depletion region can be obtained to further enhance UV on/off ratio up to 154%. As a result, our research can not only provide a deep understanding of local electric field effects on self-powered optoelectronic detection, but also promote the development of self-powered flexible optoelectronic devices and integrated systems.

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.

A flexible p-CuO/n-MoS2 heterojunction photodetector with enhanced photoresponse by the piezo-phototronic effect

Flexible functional devices based on two dimensional (2D) materials are extremely suitable for malleable, portable and sustainable applications, such as health monitoring, electronic skin and optoelectronics. In this work, we developed a flexible photodetector based on a p-CuO/n-MoS2 heterojunction with an enhancement in photocurrent and detection sensitivity. Because of the non-centrosymmetric structure in monolayer MoS2, the piezo-potential induced by applied strain adjusts the band structure at the heterojunction interface and broadens the depletion region based on the piezo-phototronic effect. The border depletion can be discreetly used to improve the photo-generated carrier separation and transport to enhance photoresponse performance. When illuminated by a 532 nm laser, the photocurrent of the heterojunction can be enhanced 27 times under a tensile strain of 0.65% compared to strain free conditions and the detection sensitivity can reach up to 3.27 × 108 Jones. As a result, our research provides a new strategy for novel design and performance optimization of 2D material heterostructures in the application of optoelectronics.

Piezo-phototronic effect enhanced UV photodetector based on CuI/ZnO double-shell grown on flexible copper microwire

In this work, we present a facile, low-cost, and effective approach to fabricate the UV photodetector with a CuI/ZnO double-shell nanostructure which was grown on common copper microwire. The enhanced performances of Cu/CuI/ZnO core/double-shell microwire photodetector resulted from the formation of heterojunction. Benefiting from the piezo-phototronic effect, the presentation of piezocharges can lower the barrier height and facilitate the charge transport across heterojunction. The photosensing abilities of the Cu/CuI/ZnO core/double-shell microwire detector are investigated under different UV light densities and strain conditions. We demonstrate the I-V characteristic of the as-prepared core/double-shell device; it is quite sensitive to applied strain, which indicates that the piezo-phototronic effect plays an essential role in facilitating charge carrier transport across the CuI/ZnO heterojunction, then the performance of the device is further boosted under external strain.

Piezotronic Effect Enhanced Flexible Humidity Sensing of Monolayer MoS2

We report the piezotronic effect on the performance of humidity detection based on a back-to-back Schottky contacted monolayer MoS2 device. By introducing an upswept mechanical strain, the in-plane electrical polarization can be induced at the MoS2/metal junction region. The polarization charges can modify the Schottky barrier height at the interface of MoS2/metal junction, subsequently improving the sensitivity of the humidity sensing. An energy band diagram is proposed to explain the experiment phenomenon of the humidity sensor. This work provides a simple way to enhance the sensitivity of ultrathin two-dimensional-materials-based sensors by the piezotronic effect, which has great potential applications in electronic skin, human-computer interfacing, gas sensing, and environment monitoring.

Magnetic‐Induced‐Piezopotential Gated MoS2 Field‐Effect Transistor at Room Temperature

Utilizing magnetic field directly modulating/turning the charge carrier transport behavior of field-effect transistor (FET) at ambient conditions is an enormous challenge in the field of micro-nanoelectronics. Here, a new type of magnetic-induced-piezopotential gated field-effect-transistor (MIPG-FET) base on laminate composites is proposed, which consists of Terfenol-D, a ferroelectric single crystal (PMNPT), and MoS2 flake. When applying an external magnetic field to the MIPG-FET, the piezopotential of PMNPT triggered by magnetostriction of the Terfenol-D can serve as the gate voltage to effectively modulate/control the carrier transport process and the corresponding drain current at room temperature. Considering the two polarization states of PMNPT, the drain current is diminished from 9.56 to 2.9 µA in the Pup state under a magnetic field of 33 mT, and increases from 1.41 to 4.93 µA in the Pdown state under a magnetic field of 42 mT and at a drain voltage of 3 V. The current on/off ratios in these states are 330% and 432%, respectively. This work provides a novel noncontact coupling method among magnetism, piezoelectricity, and semiconductor properties, which may have extremely important applications in magnetic sensors, memory and logic devices, micro-electromechanical systems, and human-machine interfacing.