Yongliang Shao

Gallium Nitride Crystals: Novel Supercapacitor Electrode Materials.

A type of single-crystal gallium nitride mesoporous membrane is fabricated and its supercapacitor properties are demonstrated for the first time. The supercapacitors exhibit high-rate capability, stable cycling life at high rates, and ultrahigh power density. This study may expand the range of crystals as high-performance electrode materials in the field of energy storage.

Three-Dimensional MoS2@CNT/RGO Network Composites for High-Performance Flexible Supercapacitors

Two-dimensional atomically thick materials, reduced graphene oxide (RGO), and layered molybdenum disulfide (MoS2 ) have been investigated as potential novel energy storage materials because of their distinct physicochemical properties. These materials suffer, however, from rapid capacity decay and low rate capability. This study describes a facile, binder-free approach to fabricate large-scale, 3D network structured MoS2 @carbon nanotube (CNT)/RGO composites for application in flexible supercapacitor devices. The as-obtained composites possess a hierarchical porosity, and an interconnected framework. The electrochemical supercapacitive measurements of the MoS2 @CNT/RGO electrode show a high specific capacitance of 129 mF cm-2 at 0.1 mA cm-2 . The symmetric supercapacitor devices based on the as-obtained composites exhibit a long lifetime (94.7 % capacitance retention after 10 000 cycles), and a high electrochemical performance (29.7 mF cm-2 ). The present experimental findings will lead to scalable, binder-free synthesis of MoS2 @CNT/RGO hybrid electrodes, with enhanced, flexible, supercapacitive performance, in portable and wearable energy storage devices.

Improving the Quality of GaN Crystals by Using Graphene or Hexagonal Boron Nitride Nanosheets Substrate

The progress in nitrides technology is widely believed to be limited and hampered by the lack of high-quality gallium nitride wafers. Though various epitaxial techniques like epitaxial lateral overgrowth and its derivatives have been used to reduce defect density, there is still plenty of room for the improvement of gallium nitride crystal. Here, we report graphene or hexagonal boron nitride nanosheets can be used to improve the quality of GaN crystal using hydride vapor phase epitaxy methods. These nanosheets were directly deposited on the substrate that is used for the epitaxial growth of GaN crystal. Systematic characterizations of the as-obtained crystal show that quality of GaN crystal is greatly improved. The fabricated light-emitting diodes using the as-obtained GaN crystals emit strong electroluminescence under room illumination. This simple yet effective technique is believed to be applicable in metal-organic chemical vapor deposition systems and will find wide applications on other crystal growth.

Self‐Supporting GaN Nanowires/Graphite Paper: Novel High‐Performance Flexible Supercapacitor Electrodes

Flexible supercapacitors have attracted great interest as energy storage devices because of their promise in applications such as wearable and smart electronic devices. Herein, a novel flexible supercapacitor electrode based on gallium nitride nanowire (GaN NW)/graphite paper (GP) nanocomposites is reported. The outstanding electrical conductivities of the GaN NW (6.36 × 102 S m-1 ) and GP (7.5 × 104 S m-1 ) deliver a synergistically enhanced electrochemical performance that cannot be achieved by either of the components alone. The composite electrode exhibits excellent specific capacitance (237 mF cm-2 at 0.1 mA cm-2 ) and outstanding cycling performance (98% capacitance retention after 10 000 cycles). The flexible symmetric supercapacitor also manifests high energy and power densities (0.30 mW h cm-3 and 1000 mW cm-3 ). These findings demonstrate that the GaN/GP composite electrode has significant potential as a candidate for the flexible energy storage devices.

Improvement of crystal quality HVPE grown GaN on an H3PO4 etched template

In this paper, GaN films were grown on an H3PO4 etched MOCVD-GaN/Al2O3 template (EMGA) by hydride vapor phase epitaxy (HVPE). The GaN film grown on the EMAG template showed a smaller full width at half maximum (FWHM) of the (002) and (102) reflections in the XRD measurement than that grown on the MOCVD-GaN/Al2O3 (MGA) template. A stronger PL band edge emission was observed for the GaN grown on the EMGA template compared to the MGA template. Defect related yellow luminescence was observed for GaN grown on the MGA template, which did not appear for the EMGA template. The Raman results showed that the stress of the GaN grown on the EMGA is much smaller than that on the MGA template. The penetrated etch pits played an important role in the reduction of threading dislocations. These results indicate that the quality of the GaN films is improved by using an EMGA template.

Influence of stress in GaN crystals grown by HVPE on MOCVD-GaN/6H-SiC substrate

GaN crystals without cracks were successfully grown on a MOCVD-GaN/6H-SiC (MGS) substrate with a low V/III ratio of 20 at initial growth. With a high V/III ratio of 80 at initial growth, opaque GaN polycrystals were obtained. The structural analysis and optical characterization reveal that stress has a great influence on the growth of the epitaxial films. An atomic level model is used to explain these phenomena during crystal growth. It is found that atomic mobility is retarded by compressive stress and enhanced by tensile stress.

Direct growth of freestanding GaN on C-face SiC by HVPE.

In this work, high quality GaN crystal was successfully grown on C-face 6H-SiC by HVPE using a two steps growth process. Due to the small interaction stress between the GaN and the SiC substrate, the GaN was self-separated from the SiC substrate even with a small thickness of about 100 μm. Moreover, the SiC substrate was excellent without damage after the whole process so that it can be repeatedly used in the GaN growth. Hot phosphoric acid etching (at 240 °C for 30 min) was employed to identify the polarity of the GaN layer. According to the etching results, the obtained layer was Ga-polar GaN. High-resolution X-ray diffraction (HRXRD) and electron backscatter diffraction (EBSD) were done to characterize the quality of the freestanding GaN. The Raman measurements showed that the freestanding GaN film grown on the C-face 6H-SiC was stress-free. The optical properties of the freestanding GaN layer were determined by photoluminescence (PL) spectra.

A Novel Mild Phase-Transition to Prepare Black Phosphorus Nanosheets with Excellent Energy Applications

Based on the phase transformation of phosphorus and Gibbs free energy theory, a new mild method to fabricate black phosphorus nanosheets from their red phosphorus microsphere counterparts is proposed. Interestingly, the as-prepared black phosphorus nanosheets, as a kind of novel metal-free photocatalyst, exhibit excellent photocatalytic H2 production performance owing to their intrinsic layered polycrystalline structure. Besides, the nanosheet is also a kind of potential anode material in lithium-ion batteries and shows good electrochemical performance.

Growth of high quality GaN on a novel designed bonding-thinned template by HVPE

In this paper, GaN films were grown on a novel designed template via hydride vapour phase epitaxy (HVPE). The template was fabricated by bonding a thinned MOCVD-GaN/sapphire template to a Si wafer (BTMG). Both the lattice deformation and thermal mismatch stress in the film can be decreased. The GaN film grown on the BTMG template displayed a smaller full width at half maximum (FWHM) for the (002) and (102) reflections than the film grown on the MOCVD-GaN/sapphire template (MG), characterized by high-resolution X-ray diffraction (HRXRD). The Raman measurements showed that the residual stress of the HVPE GaN film grown on the BTMG template was greatly released. The better optical quality of the GaN film grown on the BTMG template was detected in comparison to the film grown on the MG template by photoluminescence (PL) spectra. These results indicated that high quality GaN films can be obtained by using the new template.

One-step exfoliation and fluorination of g-C3N4 nanosheets with enhanced photocatalytic activities

Exfoliation and functionalisation of two-dimensional (2D) nanosheets are highly desired for the improvement of their chemical and physical properties. Herein, we propose a novel and efficient method for the preparation of fluorinated graphite carbon nitride (F-C3N4) nanosheets with enhanced photocatalytic activities. A one-step route was used to exfoliate and fluorinate the graphite carbon nitride (g-C3N4) nanosheets by using g-C3N4 powder combined with ammonium fluoride (NH4F). Through related instrumental characterisation, we confirmed that few-layered F-C3N4 nanosheets were produced successfully. Meanwhile, the enhanced hydrogen evolution rate of F-C3N4 nanosheets (477.6 µmol h−1 g−1) also confirmed the feasibility of this exfoliation method. This report not only opens up new possibilities for the rational design of metal-free F-C3N4 nanosheets with stable photochemical applications but also provides a potential method for producing functionalised 2D materials.