- Scanning tunneling microscopy (STM) and optical spectroscopy measurements were performed on InAs nanorods 7 to 25 nm long. Both methods reveal a clear dependence of the band-gap on length, with a red shift for longer rods. This (zero-dimension like) behavior is different than that of CdSe rods, where the band-gap is nearly insensitive to length, a signature of quasi one-dimensionality. The transition between these two regimes is governed by the ratio between the Bohr radius and the nanorod length. The gaps measured by tunneling spectroscopy are larger than the optical gaps by a factor that depends on the tunneling configuration. This is attributed to a combination of the Coulomb interaction and the voltage division between the two tunnel junctions in the STM experiment.