Synthesis of metal-semiconductor heterojunctions inside carbon nanotubes

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 InZnS, InCNT and ZnSCNT. 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 [1010], while [0001] is the commonly preferred growth direction in free-standing ZnS nanowires. Following the observation of smooth InZnS 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.