Zhenfu Zhao

Wearable Self‐Charging Power Textile Based on Flexible Yarn Supercapacitors and Fabric Nanogenerators

A novel and scalable self-charging power textile is realized by combining yarn supercapacitors and fabric triboelectric nanogenerators as energy-harvesting devices.

Freestanding Flag-Type Triboelectric Nanogenerator for Harvesting High-Altitude Wind Energy from Arbitrary Directions

Wind energy at a high altitude is far more stable and stronger than that near the ground, but it is out of reach of the wind turbine. Herein, we develop an innovative freestanding woven triboelectric nanogenerator flag (WTENG-flag) that can harvest high-altitude wind energy from arbitrary directions. The wind-driven fluttering of the woven unit leads to the current generation by a coupled effect of contact electrification and electrostatic induction. Systematic study is conducted to optimize the structure/material parameters of the WTENG-flag to improve the power output. This 2D WTENG-flag can also be stacked in parallel connections in many layers for a linearly increased output. Finally, a self-powered high-altitude platform with temperature/humidity sensing/telecommunicating capability is demonstrated with the WTENG-flag as a power source. Due to the light weight, low cost, and easy scale-up, this WTENG-flag has great potential for applications in weather/environmental sensing/monitoring systems.

Relaxed Ge0.9Si0.1 alloy layers with low threading dislocation densities grown on low-temperature Si buffers

Relaxed GexSi1−x epilayers with high Ge fractions but low threading dislocation densities have been successfully grown on Si (001) substrate by employing a stepped-up strategy and a set of low-temperature GeySi1−y buffers. We show that even if the Ge fraction rises up to 90%, the threading dislocation density can be kept lower than 5×106 cm−2 in the top layers, while the total thickness of the structure is no more than 1.7 μm.

Piezotronic Effect in Polarity-Controlled GaN Nanowires.

Using high-quality and polarity-controlled GaN nanowires (NWs), we studied the piezotronic effect in crystal orientation defined wurtzite structures. By applying a normal compressive force on c-plane GaN NWs with an atomic force microscopy tip, the Schottky barrier between the Pt tip and GaN can be effectively tuned by the piezotronic effect. In contrast, the normal compressive force cannot change the electron transport characteristics in m-plane GaN NWs whose piezoelectric polarization axis is turned in the transverse direction. This observation provided solid evidence for clarifying the difference between the piezotronic effect and the piezoresistive effect. We further demonstrated a high sensitivity of the m-plane GaN piezotronic transistor to collect the transverse force. The integration of c-plane GaN and m-plane GaN indicates an overall response to an external force in any direction.

Piezo-Phototronic Effect Controlled Dual-Channel Visible light Communication (PVLC) Using InGaN/GaN Multiquantum Well Nanopillars

Visible light communication (VLC) simultaneously provides illumination and communication via light emitting diodes (LEDs). Keeping a low bit error rate is essential to communication quality, and holding a stable brightness level is pivotal for illumination function. For the first time, a piezo-phototronic effect controlled visible light communication (PVLC) system based on InGaN/GaN multiquantum wells nanopillars is demonstrated, in which the information is coded by mechanical straining. This approach of force coding is also instrumental to avoid LED blinks, which has less impact on illumination and is much safer to eyes than electrical on/off VLC. The two-channel transmission mode of the system here shows great superiority in error self-validation and error self-elimination in comparison to VLC. This two-channel PVLC system provides a suitable way to carry out noncontact, reliable communication under complex circumstances.

Tuning carrier lifetime in InGaN/GaN LEDs via strain compensation for high-speed visible light communication

In recent years, visible light communication (VLC) technology has attracted intensive attention due to its huge potential in superior processing ability and fast data transmission. The transmission rate relies on the modulation bandwidth, which is predominantly determined by the minority-carrier lifetime in III-group nitride semiconductors. In this paper, the carrier dynamic process under a stress field was studied for the first time, and the carrier recombination lifetime was calculated within the framework of quantum perturbation theory. Owing to the intrinsic strain due to the lattice mismatch between InGaN and GaN, the wave functions for the holes and electrons are misaligned in an InGaN/GaN device. By applying an external strain that “cancels” the internal strain, the overlap between the wave functions can be maximized so that the lifetime of the carrier is greatly reduced. As a result, the maximum speed of a single chip was increased from 54 MHz up to 117 MHz in a blue LED chip under 0.14% compressive strain. Finally, a bandwidth contour plot depending on the stress and operating wavelength was calculated to guide VLC chip design and stress optimization.

Moiré fringes characterization of surface plasmon transmission and filtering in multi metal-dielectric films

We use moire fringes to experimentally characterize the transmission and filtering effect of surface plasmons (SPs) in the alternatively stacked metal-dielectric layers (ASMDLs). Two gratings with unequal periods are introduced at the two sides of films structure to excite SPs and generate moire fringes observable in the far-field. The observed moire fringes with specific period agree well with simulations and clearly demonstrate SP with corresponding wave vector transmission and filtering in ASMDLs. By applying variant incidence angle, the fringes contrast fluctuation further illustrate the transmission of SP in a specific wide wave vector band and with an uneven amplitude transmission profile.

Robust Pb2+ sensor based on flexible ZnO/ZnS core-shell nanoarrays

We designed a flexible robust sensor with ZnO/ZnS core-shell nanoarrays to detect Pb2+ ions. This device is powered by electrical energy transferred from the environmental mechanical energy and senses Pb2+ ions with the cation exchange reaction between ZnS shell and Pb2+ ions (ZnS (s) + Pb2+ (aq) ↔ PbS (s) + Zn2+(aq)). The high density intrinsic carriers in PbS diffuse into the ZnO core to partly screen the piezopotential, which results in an exponential relationship between the concentrations of Pb2+ ions and the piezo-voltages. The detected limit is as low as 1 ppm. This sensor also exhibits a very high selectivity towards Pb2+ ions due to the limitation of energy band and solubility, which has potential applications in environmental protection and pollutant surveillance.

Piezotronic effect tuned AlGaN/GaN high electron mobility transistor

The piezotronic effect is about utilizing strain-induced piezoelectric polarization charges to tune the carrier transportation across the interface/junction. We fabricated a high performance AlGaN/GaN High Electron Mobility Transistor (HEMT), and the transport property was proven to be enhanced by applying an external stress for the first time. The enhanced source-drain current was also observed at any gate voltage and the maximum enhancement of the saturation current was up to 21 % with 15 N applied stress (0.18 GPa at center) at -1 V gate voltage. The physical mechanism of HEMT with/without external compressive stress conditions was carefully illustrated and further confirmed by a self-consistent solution of the Schrödinger-Poisson equations. This study proves the cause-and-effect relationship between the piezoelectric polarization effect and two-dimensional electron gas formation, which provides a tunable solution to enhance the device performance. The strain tuned HEMT has potential applications in human-machine interface and the security control of the power system.The piezotronic effect utilizes strain-induced piezoelectric polarization charges to tune the carrier transportation across the interface/junction. We fabricated a high-performance AlGaN/GaN high electron mobility transistor (HEMT), and the transport property was proven to be enhanced by applying an external stress for the first time. The enhanced source-drain current was also observed at any gate voltage and the maximum enhancement of the saturation current was up to 21% with 15 N applied stress (0.18 GPa at center) at -1 V gate voltage. The physical mechanism of HEMT with/without external compressive stress conditions was carefully illustrated and further confirmed by a self-consistent solution of the Schrödinger-Poisson equations. This study proves the cause-and-effect relationship between the piezoelectric polarization effect and 2D electron gas formation, which provides a tunable solution to enhance the device performance. The strain tuned HEMT has potential applications in human-machine interface and the security control of the power system.