Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates. Here, the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated. Graphene was directly prepared on SiC substrates by thermal decomposition. Its pre-treatment with nitrogen-plasma can introduce C–N dangling bonds, which provides nucleation sites for subsequent epitaxial growth. The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma. We study the growth behavior on different areas of graphene surface after pre-treatment, and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene. Significantly, graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells (MQWs) active region, which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs. This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

Surface-enhanced Raman scattering (SERS) is a supersensitive technique for monitoring catalytic reactions. However, building a SERS-kinetics model to investigate catalytic efficiency on the surface or interface of the catalyst remains a great challenge. In the present study, we successfully obtained an excellent semiconducting SERS substrate, reduced MnCo2O4 (R-MnCo2O4) nanotubes, whose favorable SERS sensitivity is mainly related to the promoted interfacial charge transfer caused by the introduction of oxygen vacancies as well as the electromagnetic enhancement effect. Furthermore, the R-MnCo2O4 nanotubes showed a favorable oxidase-like activity toward oxidation with the aid of molecular oxygen. It was also showed the oxidase-like catalytic process could be monitored using the SERS technique. A new SERS-kinetics model to monitor the catalytic efficiency of the oxidase-like reaction was developed, and the results demonstrate that the V m values measured by the SERS-kinetics method are close to that obtained by the UV–vis approach, while the K m values measured by the SERS-kinetics method are much lower, demonstrating the better affinity between the enzyme and the substrate from SERS results and further confirming the high sensitivity of the SERS-kinetics approach and the actual enzyme-like reaction on the surface of nanozymes, which provides guidance in understanding the kinetics process and catalytic mechanism of natural enzymatic and other artificial enzymatic reactions. This work demonstrated the improved SERS sensitivity of defective semiconductors for the application of enzyme mimicking, providing a new frontier to construct highly sensitive biosensors.

Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar Al x Ga 1-x N (x ¼ 0-0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by $17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 Â 10 17 cm À3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.