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

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 Br2 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 (Br2 or I2) 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.