Alkali-metal-doping dynamics and anomalous lattice contraction of individual debundled carbon nanotubes

The dynamics of K doping of thin films of semiconducting debundled single-walled carbon nanotubes (SWNTs) on a $\mathrm{Si}∕{\mathrm{SiO}}_2$ substrate is followed using in situ Raman scattering. Similar to the behavior we observed in the Cs doping of bundled SWNTs, four distinct intervals during doping can be distinguished via the behavior of the high frequency tangential band (or G-band) scattering. We therefore find that the character of the time evolution of the frequency and linewidth of the most prominent G band with alkali-metal doping is almost identical for debundled and bundled tubes. This includes the observation of an anomalous upturn in the G-band frequency during the third interval of doping that is consistent with a contraction of the $\mathrm{C}\mathrm{C}$ bond within the tube wall. We conclude that this anomalous contraction is not associated with doping-induced changes in the tube-tube interaction within a bundle, but is an intrinsic, and yet unexplained, consequence of an upshifting Fermi level in debundled $n$-doped SWNTs. A long induction period between when doping conditions are established and when changes in the Raman spectrum occur is observed and remains unexplained.