Behavior of the high frequency Raman modes of double-wall carbon nanotubes after doping with bromine or iodine vapors

Abstract

The effects of two different halogen dopants (bromine and iodine) at different concentrations on the higher frequency modes (the so-called G and G′ bands) of the Raman spectra of double-wall carbon nanotube (DWCNT) “buckypaper” are investigated. The effects of dopants on different DWCNT configurations (metallic inner/semiconducting outer and vice versa) are studied by changing the laser excitation energy. The doping causes the loss of the Breit–Wigner–Fano line shape and the appearance of less metallic behavior. An increase of the relative intensity of the G + band, which is more sensitive for the outer metallic tubes, is clearly observed with increasing Br 2 concentration in the sample. By analysis of the G + band and the G′ band it is possible to measure the changes in the electron–phonon coupling, due to the charge-transfer between the dopant (Br 2 or I 2) and the tubes in the DWCNT. The doping effect causes an upshift of the G + band and a suppression of the contribution of the inner tubes to the G′ band signal and as a consequence, the observed G′ band is dominated by the contribution from the outer tubes.