Nanowires sheathed inside nanotubes: Manipulation, properties and applications

Since the discovery of metals encapsulated into multi-walled carbon nanotubes (CNTs), such sheathed structures attracted extensive interest with respect to the development of various synthetic strategies for producing the unique structure of nanowires sheathed inside nanotubes. The nanowire materials varied from metals to alloys, from semiconductors to insulators, and even metal–semiconductor heterojunctions were tried. In recent years, the studies on these nanostructures have been mainly focused on in-situ manipulation, property analysis and applications. Exploration of on-demand nano-engineering of the regarded structures toward practical device design and fabrication was mainly guided by high-resolution transmission electron microscopy (TEM) technique combining new capabilities of implementation of atomic force (AFM) or scanning tunneling microscopy (STM) holders, and heating/cooling holders. Such novel in-situ TEM techniques have rapidly developed to a stage where they truly become a very powerful tool for the studies of core/shell nanowire heterostructures. In this review, we summarize the significant developments and achievements in regards of manipulation, property measurements and device applications of inorganic nanowires sheathed inside nanotubes according to different categories of the filling materials, i.e., metals, alloys, compounds and semiconductor–metal heterojunction nanowires. We also highlight the irreplaceable value of in-situ TEM technology in this field, compare different fillings for so-called nanothermometers, discuss mass transportation mechanism in nanotubes, and conclude with an outlook of future developments and challenging issues that are still in the premature stage.