Hybrid carbon nanotubes: Strategy, progress, and perspectives

We introduce the concept of meta-nanotubes, among which are hybrid carbon nanotubes (X@CNTs), which are CNTs whose hollow core is filled—fully or partially—with foreign atoms, molecules, or compounds. The article focuses on the latter, describing their potential interest and the various ways currently available to synthesize them, while providing examples of the resulting materials mainly taken from the author’s works but also from literature, as characterized by means of high-resolution microscopy and related techniques. We discuss advantages and drawbacks of the various synthesis routes to help willing scientists and engineers to define a strategy for X@CNT synthesis with respect to their specific goals and expectations. Some examples of peculiar properties and behaviors of X@CNTs will be provided as well, although such related investigations are still scarcely reported because we are dealing with quite new nanomaterials.