RELATION BETWEEN METAL ELECTRONIC-STRUCTURE AND MORPHOLOGY OF METAL-COMPOUNDS INSIDE CARBON NANOTUBES

SEVERAL attempts have been made to fill carbon nanotubes(1) with metal s or metallic compounds to obtain nanocomposite materials with potenti ally interesting properties. Capillary action, predicted(2) to be a fi lling mechanism, has been used(3,4) to encapsulate lead and bismuth in open tubes. Compounds of yttrium(5), manganese(6) and gadolinium(7) h ave also been encapsulated by formation of the nanotubes in an are dis charge with the metals present in situ. Very recently, Tsang et al.(8) showed that oxides of nickel, cobalt, iron and uranium can be encapsu lated by opening the tubes and depositing the filling material using w et chemical techniques. Here we report a search for general principles relating to the nature and structure of the filling material, using t he are-discharge method to fill tubes with fifteen metals and/or their compounds: Ti, Cr, Fe, Co, Ni, Cu, Zn, Mo, Pd, Sn, Ta, W, Gd, Dy and Yb. We find that the propensity for forming continuous 'nanowires' thr oughout the length of the tubes seems to be strongly correlated with t he existence of an incomplete electronic shell in the most stable ioni c state of the metal. We also find that the interplay between growth o f the nanotube and growth of the filling results, in one case, in the formation of an unusual helical filling morphology.