Influence of ZnO encapsulation on the luminescence property of GeO2 nanowires

GeO2-core/ZnO-shell nanowires were synthesized on (100) Si substrates by thermal evaporation of Ge powders, followed by atomic layer deposition of ZnO. X-ray diffraction, scanning electron microscopy and transmission electron microscopy analyses showed that the mean diameter and lengths of the core-shell nanowires were approximately 100 nm and from a few tens to a few hundreds of micrometers, respectively. Photoluminescence measurements showed that pure GeO2 nanowires had a violet emission band centered at approximately 430 nm. In contrast, GeO2-core/ZnO-shell nanowires had both a sharp near-band edge (NBE) emission band centered at approximately 380 nm and a broad deep-level (DL) emission band centered at approximately 590 nm, which is characteristic of ZnO. GeO2-core/ZnO-shell nanowires showed a higher intensity ratio of NBE emission to DL emission than either GeO2 or ZnO nanowires. In addition, the origin of the enhancement of luminescence in GeO2 nanowires by ZnO encapsulation is discussed.