(n,m) Assignments of Metallic Single-Walled Carbon Nanotubes by Raman Spectroscopy: The Importance of Electronic Raman Scattering

In this work, we report an accurate and convenient method that can be used to assign the chirality of all metallic single-walled carbon nanotubes (M-SWNTs). This method is designed based on the electronic Raman scattering (ERS) features, which are resonantly enhanced at the corresponding excitonic transition energies (M_ii⁺ and M_ii^-). Using this method, we are able to accurately determine the electronic property M_ii with the resolution of a vibrational Raman spectroscopy (∼0.3 meV), which is significantly higher than that of the electronic spectroscopies (∼3 meV). We use the M_ii splitting value, which is found insensitive to environmental changes, as a universal criteria for (n,m) assignments in various environments. As an illustrative example, simply using a commercialized Raman spectrometer with two laser lines (1.959 and 2.330 eV), we are able to unambiguously assign 18 metallic chiralities with M₁₁ in the 1.6-2.3 eV range in our samples. This method provides an accurate database of M_ii's in a similar way as photoluminescence excitation spectroscopy does for S_ii's. It can facilitate further systematic studies on the properties of M-SWNTs with defined chirality.