Synthesis of metal–semiconductor heterojunctions inside carbon nanotubes

Abstract

Heterojunctions between a metal and a semiconductor are at the core of all modern electronic devices. Recently, fabrication of such structures at the nanoscale has emerged as a hot topic due to their immense potential for the next generation of nanoscale devices and electronics. Here we report a high-temperature route for the synthesis of metal (In)–semiconductor (ZnS) nano-heterojunctions inside a carbon nanotube (CNT). As In is a superconductor at low temperatures, these ‘nanocables’ are also potential superconductor–semiconductor heterojunctions, synthesized for the first time inside a CNT. A noteworthy feature is that the majority of the heterostructure surface area is involved in forming interfaces such as In||ZnS, In||CNT and ZnS||CNT. Mastering these structural relations is critical to controlling its overall properties. Several interesting facts emerged from detailed structural characterization of the heterojunctions with high-resolution transmission electron microscopy. The growth direction of the wurtzite-type ZnS encapsulated segments is along [100], while [0001] is the commonly preferred growth direction in free-standing ZnS nanowires. Following the observation of smooth In||ZnS interfaces, the orientation relationship of these two segments was analysed. Another interesting finding is the presence of a few layers of cubic ZnS near its interface with the CNT. This peculiarity is suggested to be a key contributor to the unusual encapsulated nanowire growth axis. These complex In/ZnS/CNT materials should provide opportunities for fundamental studies of heterojunctions at the nanoscale, as well as providing the basis for the development of chemical and radiation-shielded electronic nanodevices.