Energy has emerged to be a global concern as the world confronts the challenges of population growth, climate change, economic recovery, energy affordability. To overcome the energy crisis, people need to work with both hands: use one hand to make a transition from fossil fuels to clean, renewable energy and the other hand to cut down energy consumption by developing energy efficient devices and systems. In this dissertation, I focus my research to tackle these problems using wide bandgap semiconductors, III-nitride (GaN and InGaN) and zinc oxide (ZnO) as basic materials due to their unique optoelectronic properties. Material growth -- III-nitride using MOCVD and ZnO using hydrothermal solution synthesis are studied. For renewable energy application, I developed InGaN/GaN MQW photoelectrode for spontaneous photoelectrochemical (PEC) water splitting and hydrogen fuel generation. The InGaN/ GaN MQW structure provides sufficient photovoltage to split water. Using this single photoelectrode, a current density of 0.16 mA·cm⁻² and a peak solar-to-hydrogen conversion efficiency of 0.2% are obtained at zero external bias. Two schemes are studied to improve the photoelectrode efficiency and stability. By adding NiOx oxygen evolution catalyst, the overall solar-to-hydrogen conversion efficiency is improved to 0.64% at zero bias. The stability of photoelectrode is also much improved and photocurrent reduction of 4.5% in 12 hours is achieved. In order to further boost the performance of the photoelectrode, plasmonic metal nanostructures are created for enhanced light absorption. The conversion efficiency is improved up to 0.93%. For energy-saving application, I demonstrated a novel lighting technology based on nano- materials--ZnO nanowires and carbon nanotubes. The novel design, junctionless light emitting device, consists of a carbon nanotube array cathode for electron field emission and a ZnO nanowire array light-emitting anode. High energy electrons directly bombard onto nanowires, generate electron-hole pairs, which subsequently recombine to emit light. Strong near band edge emission is obtained with a peak at 390 nm. The use of vertical nanowire array as anode offers advantages in increasing the junction area and carrier recombination/photon generation and enhancing the light extraction efficiency. This approach utilizes the unique properties of nanoscale materials and opens an area for efficient lighting technologies in the future
