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Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer. 

A three‐dimensional porous MnOOH hierarchical nanostructure grown on flexible carbon fiber fabric is successfully synthesized by a facile hydrothermal method. Monoclinic MnOOH has a crystalline structure layered in the [010] direction. The layered structure facilitates intercalation/deintercalation of Li⁺into the interlayer space as well as the adsorption/desorption of Li⁺onto the surface of each layer during fast charging/discharging process. This porous hierarchical structure provides many electroactive sites and efficient paths for ion transport. When used as flexible and lightweight electrodes for supercapacitors, the porous MnOOH electrode manifests good electrochemical performance with a high capacitance of 160 F g⁻¹at a current density of 0.2 mA cm⁻², a high energy density of 14 Wh kg⁻¹at a power density of 162 W kg⁻¹, and great cycling stability. Moreover, the application for the low‐cost and highly flexible supercapacitors in powering light‐emitting diodes (LEDs) highlights its enormous commercial potential in next‐generation energy storage systems.