Caihong Liu

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.

High-Resolution Dynamic Pressure Sensor Array Based on Piezo-phototronic Effect Tuned Photoluminescence Imaging

A high-resolution dynamic tactile/pressure display is indispensable to the comprehensive perception of force/mechanical stimulations such as electronic skin, biomechanical imaging/analysis, or personalized signatures. Here, we present a dynamic pressure sensor array based on pressure/strain tuned photoluminescence imaging without the need for electricity. Each sensor is a nanopillar that consists of InGaN/GaN multiple quantum wells. Its photoluminescence intensity can be modulated dramatically and linearly by small strain (0-0.15%) owing to the piezo-phototronic effect. The sensor array has a high pixel density of 6350 dpi and exceptional small standard deviation of photoluminescence. High-quality tactile/pressure sensing distribution can be real-time recorded by parallel photoluminescence imaging without any cross-talk. The sensor array can be inexpensively fabricated over large areas by semiconductor product lines. The proposed dynamic all-optical pressure imaging with excellent resolution, high sensitivity, good uniformity, and ultrafast response time offers a suitable way for smart sensing, micro/nano-opto-electromechanical systems.

Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators

In this work, by employing halogen elements (fluorine, chlorine, bromine, and iodine) as dopant we demonstrate a unique strategy to enhance the output performance of ZnO-based flexible piezoelectric nanogenerators. For a halogen-doped ZnO nanowire film, dopants and doping concentration dependent lattice strain along the ZnO c-axis are established and confirmed by the EDS, XRD, and HRTEM analysis. Although lattice strain induced charge separation was theoretically proposed, it has not been experimentally investigated for wurtzite structured ZnO nanomaterials. Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to other halogen elements due to the ionic size difference between dopants and oxygen. With its focus on a group of nonmetal element induced lattice strain in ZnO-based nanomaterials, this work paves the way for enhancing the performance of wurtzite-type piezoelectric semiconductor nanomaterials via lattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effective manner.

Raman study of 2D anatase TiO2 nanosheets

Herein, we present for the first time a spectroscopic study of two-dimensional (2D) anatase TiO2 nanosheets. Previous publications demonstrated that Raman spectroscopy was mostly employed to characterize the TiO2 nanoparticle size and the phase transition of amorphous-anatase and anatase-rutile. In this study, TiO2 nanosheets were characterized by XRD, AFM and Raman spectroscopy, which demonstrated a shift toward higher frequency and broadening in the full width at half maximum of the characteristic Eg mode by decreasing the thickness of anatase TiO2 with a 2D nanostructure. In contrast to the study of TiO2 nanoparticles, the Raman vibrations can be attributed to phonon confinement in 2D TiO2 nanosheets which can be employed to characterize the thickness of TiO2 nanosheets. In order to effectively identify the thickness of the 2D TiO2 nanostructure, we established a reliable method for the examination by characterizing the shifts of the Eg mode.

Piezotronic-effect-enhanced Ag2S/ZnO photocatalyst for organic dye degradation

Taking advantage of the piezotronic effect, enhanced photocatalytic performance of a Ag2S@ZnO hybrid photocatalyst was achieved under sonication. Decorating Ag2S (∼1.4 eV bandgap) nanoparticles on ZnO (∼3.3 eV bandgap) nanowire surfaces successfully extended the light response to the visible light range. The formation of type II band alignment at the Ag2S/ZnO heterointerface facilitated the separation of photoexcited electrons and holes, promoting utilization of photoexcited charge carriers in the photocatalytic process. Furthermore, the strain-generated positive piezocharges effectively lowered the barrier height, considerably motivating charge transport across the Ag2S/ZnO heterointerface, which further boosted the hybrid photocatalyst performance. The high reproducibility of the photocatalytic activities indicated that the modification of ZnO nanowires with Ag2S nanoparticles effectively stabilized the photocatalyst in reactions. The degradation rate of Ag2S@ZnO nanowires remained at C/C0 = 18.6% after eight cycles, while bare ZnO nanowires exhibited poorer performance (C/C0 = 48.1%). This study showed the effectiveness of using the piezotronic effect in water purification and recovery by combined use of solar and mechanical energy.

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.

Enhancement of piezoelectric perfermance of ZnO based nanogenerator and related applications

The pursuit of harvesting renewably generated electricity and then powering electronic devices may be a strategy to decline the over reliance of fossil fuel. ZnO nanomaterial is one of idea candidates for converting mechanical energy to electrical energy. Here we demonstrate by doping halogen ions and interfacial engineering, the output voltage and current can be dramatically enhanced.