Mengxiao Chen

Progress in Piezo‐Phototronic‐Effect‐Enhanced Light‐Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non-central symmetric crystal structures. The three-way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo-phototronics. This effect can efficiently manipulate the emission intensity of light-emitting diodes (LEDs) by utilizing the piezo-polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo-phototronic-effect-enhanced LEDs is reviewed; following their development from single-nanowire pressure-sensitive devices to high-resolution array matrices for pressure-distribution mapping applications. The piezo-phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.

Enhancing Light Emission of ZnO‐Nanofilm/Si‐Micropillar Heterostructure Arrays by Piezo‐Phototronic Effect

n-ZnO nanofilm/p-Si micropillar heterostructure light-emitting diode (LED) arrays for white light emissions are achieved and the light emission intensity of LED array is enhanced by 120% under -0.05% compressive strains. These results indicate a promising approach to fabricate Si-based light-emitting components with high performances enhanced by the piezo-phototronic effect, with potential applications in touchpad technology, personalized signatures, smart skin, and silicon-based photonic integrated circuits.

Full Dynamic-Range Pressure Sensor Matrix Based on Optical and Electrical Dual-Mode Sensing

A pressure-sensor matrix (PSM) with full dynamic range can accurately detect and spatially map pressure profiles. A 100 × 100 large-scale PSM gives both electrical and optical signals by itself without applying an external power source. The device represents a major step toward digital imaging, and the visible display of the pressure distribution covers a large dynamic range.

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.

Tuning Light Emission of a Pressure-Sensitive Silicon/ZnO Nanowires Heterostructure Matrix through Piezo-phototronic Effects

Based on white light emission at silicon (Si)/ZnO hetrerojunction, a pressure-sensitive Si/ZnO nanowires heterostructure matrix light emitting diode (LED) array is developed. The light emission intensity of a single heterostructure LED is tuned by external strain: when the applied stress keeps increasing, the emission intensity first increases and then decreases with a maximum value at a compressive strain of 0.15-0.2%. This result is attributed to the piezo-phototronic effect, which can efficiently modulate the LED emission intensity by utilizing the strain-induced piezo-polarization charges. It could tune the energy band diagrams at the junction area and regulate the optoelectronic processes such as charge carriers generation, separation, recombination, and transport. This study achieves tuning silicon based devices through piezo-phototronic effect.

Visualization Recording and Storage of Pressure Distribution through a Smart Matrix Based on the Piezotronic Effect

Pressure sensors that can both directly visualize and record applied pressure/stress are essential for e-skin and medical/health monitoring. Here, using a WO3 -film electrochromic device (ECD) array (10 × 10 pixels) and a ZnO-nanowire-matrix pressure sensor (ZPS), a pressure visualization and recording (PVR) system with a spatial resolution of 500 µm is developed. The distribution of external pressures can be recorded through the piezotronic effect from the ZPS and directly expressed by color changes in the ECD. Applying a local pressure can generate piezoelectric polarization charges at the two ends of the ZnO nanowires, which leads to the tuning of the current to be transported through the system and thus the color of the WO3 film. The coloration and bleaching process in the ECD component show good cyclic stability, and over 85% of the color contrast is maintained after 300 cycles. In this PVR system, the applied pressure can be recorded without the assistance of a computer because of the color memory effect of the WO3 material. Such systems are promising for applications in human-electronic interfaces, military applications, and smart robots.

A self-powered system based on triboelectric nanogenerators and supercapacitors for metal corrosion prevention

As water covers most of the earths surface, the energy of the ocean is abundant and almost unexplored, which can be one of the most environmentally friendly forms of energy. Prevention of metal corrosion plays an important role in national economic development and daily life. Here, we report a network of triboelectric nanogenerators (TENGs) and supercapacitors (SCs), which is also called the self-powered system, to harvest a huge amount of water energy for preventing metal corrosion. When the TENG is integrated with a SC, the output current is stable and continuous. The corrosion results indicate that the TENG-SC self-powered system can prevent about 80% degree of corrosion for Q235 steel in 0.5 M NaCl solution. This work demonstrates that the TENG-SC system, which is self-powered, flexible and environmentally friendly, can harvest and store large-scale blue energy from the ocean, and also renders an innovative approach toward preventing the metal corrosion without other power sources.

Progress in piezo-phototronic effect enhanced photodetectors

Wurtzite structured materials such as InN, CaN, ZnO, and CdSe simultaneously possess piezoelectric, semiconducting, and photoexcitation properties. The piezo-phototronic effect utilizes the piezo-polarization charges induced in the vicinity of the interface/junction to regulate the energy band diagrams and modulate charge carriers in the optoelectronic processes, such as transport, generation, recombination, and separation. This article reviews recent progress in piezo-phototronic effect enhanced photodetectors, starting from the fundamental physics, following the development from a single nanowire device to a large-scale photodetector array for illumination imaging. The piezo-phototronic effect provides a promising approach to improve the performance of the wurtzite structured material-based photodetectors. It may have potential applications in optical communication, optoelectronic devices, and multifunctional computing systems.