Soon after the identification of carbon nanotubes, produced by arc-discharge techniques, the possibility of introducing metals into the inner core of the tubes was considered (Figure 1). This idea followed logically from the successful observation of fullerenes containing endohedral metals. The introduction of metals or metal carbides and oxides into multiwalled nanotubes [usually 2–70 nm outside diameter (OD), <60 microns in length] may significantly alter their conducting, electronic, and mechanical properties, arising from the internal framework within these structures. Early theoretical work predicted that all carbon open nanotubes could behave as “molecular straws.” In this context, Ajayan and lijima were the first to introduce Pb by heating the metal with open-ended nanotubes (sometimes in the presence of oxygen). Capillarity, wetting, and surface tension play an important role in the process; other elements such as Bi, Cs, S, and Se were also introduced into nanotubes in this way. Generally, the filling of open nanotubes by capillary action is confined to low surface-tension substances. Alternative methods of encapsulating other materials within nanotubes have now been developed, including chemical treatment, arc discharge, catalyzed hydrocarbon pyrolysis, and electrochemical techniques. In addition, carbon nanotubes can be used as reactive templates to generate novel metal carbide and nitride nanorods. These developments represent only the tip of the iceberg, because numerous noncarbon materials (e.g., Si, SiOx, SiC, MoS2, WS2, etc.) are able to form novel nanowires and fullerene-like morphologies. This review discusses methods for generating these fascinating structures and evaluates their possible applications in materials science and engineering.
