Depends on the functionlisation process adopted.....

But most often if acid functionalization is used to covalently bond to CNTs, this will also induce defects which give rise to the D band ....

Thanks Sir...If u have any paper on this. .kindly suggest me..

This paper will give a brief review about D and G band of CNTs.

Perspectives on Carbon Nanotubes and

Graphene Raman Spectroscopy, Nano Lett. 2010, 10, 751–758

http://pubs.acs.org/doi/pdf/10.1021/nl904286r

The D+G peak, unlike the 2D peak, needs to be activated by a defect scattering site in order to provide momentum conservation in the Raman scattering process. Whenever a high wavevector phonon is emitted during the Raman process, there will be quasi-momentum missmatch among the hole in the valence band and the excited electron in the conduction band. This prevents radiative recombination unless a scattering site provides elastic scattering to bridge the missmatch. The pure D-band is the most simple example for this - however, as the phonon responsible for the G-band has very little wavevector (its a so-called zone-center or Gamma-point phonon) and hence is recorded without the presence of a defect, the D+G-band will also need a defect for activation. The appearance of the 2D-band at the double frequency of the D-band is a little different in this respect, as it will not require a defect for action - it just uses the same phonon twice to move the electron as well as the hole through the Brillouin-zone and hence there will be no more missmatch (you could also say the electron gets shuttled forward and backward using the same phonon twice).

Hope this answers your question!

Best Regards,

JME