Unoccupied electronic states of Multiwall Carbon Nanotubes Arrays

We have grown multiwall carbon nanotube (MWCNT) arrays by CVD both from pyrolysis of Fe-Phthalocyanine and decomposition of Acetylene on Fe covered SiO$_{2 }$/Si(111) substrates. The characteristic diameter of the tubes is 50 nm for both type of samples. Even though the films show good alignment in the bulk, they do present some disorder of the tubes at the top of the films. Inverse photoemission spectra from these samples are similar to those obtained from HOPG. The main differences are in: the non existence of what has been recognized as an image charge state on graphite and some additional intensity very close to the Fermi level ($\varepsilon _{F})$. A similar intensity has been measured previously by photoemission in a symmetrical position with respect to $\varepsilon _{F}$. This increased metallic character could, at this point, be interpreted as defects from the closure of the tubes or as a manifestation of van Hove oscillations in the unoccupied density of states.