ZnO nanotubes: Controllable synthesis and tunable UV emission modulated by the wall thickness

The ZnO nanotubes (ZNTs) with different wall thickness were fabricated by a simple wet chemical approach. Both indirect and direct transitions contribute to the UV emission of ZNTs. With the decrease of wall thickness, not only the indirect transition becomes more and more important during the emission process due to the stronger surface band bending effects and even turns into the main emission when the wall thickness decreases to 25 nm, but also the PL intensity is enhanced step by step due to the less defect density and higher carrier concentration caused by the introduction of chlorine in ZNTs during the etching process.