Microstructural analysis of ZnO-ZnS Heterostructures fabricated by a low temperature sulfidation process

Abstract

Oxide semiconductors have unique properties in various applications such as in transparent electrodes, smart windows, solar cells, and light-emitting diodes (LED). Especially, zinc oxide (ZnO) has been widely studied owing to outstanding electrical and optical properties. In general, ZnO adapts to a hexagonal wurtzite structure. ZnO has interesting properties such as a wide and direct bandgap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. In particular, low dimensional ZnO nanostructures like nanowires, nanotubes, nanorings, and nanowalls show exceptional properties due to quantum confinement effects. Moreover, as low dimensional nanostructures have high surface-to-volume ratios, it is good for optoelectronic devices and chemical sensors. Recently, ZnO-ZnS core-shell or heterostructures have been widely studied because of enhanced or newly designed properties. As the band gap of ZnS is 3.7 eV, which is higher than ZnO, the photoluminescence quantum yields can be improved. Moreover, light and gas sensing properties is also enhanced. One of the most facile methods to make ZnO-ZnS nanostructures is using thioacetamide (TAA) and ZnO sacrificial templates in water. This method is not only simple but also versatile to make ZnS-ZnO coreshell and ZnS hollow structures. For these reasons, this method has been widely used and studied for a long time. However, most researches are concentrated on the synthesis of core-shell or hollow structures and little is known about phase formation and structural transformation during ZnO sulfidation process. In addition, the sulfidation process could be affected by the morphology of ZnO structures. ZnS has two basic structures such as cubic sphalerite, stable at room temperature, and hexagonal wurtzite, high temperature form. In addition, many polytype structures such as 8H, and 15R are observed and have similar interplanar spacing. Therefore, the classification of crystalline phase is difficult. Neverth...