The electronic structures of single-walled carbon nanotubes on Ag(100) and on ultrathin insulating NaCl(100)/Ag(100) were studied using low-temperature scanning tunneling microscopy. The Fermi level of the nanotubes was shifted toward the conduction band on Ag(100), while it was shifted toward the valence band on NaCl films. We explain this opposite behavior by different basic mechanisms accounting for the Fermi level shifts. On the metal surface, the work function difference between the tube and the substrate determines the direction of the Fermi level shift. In the case of carbon nanotubes on insulating films, the electric field resulting from the dipole moment formed at the interface between the insulating film and the metal plays a decisive role in determining the Fermi level.
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