Anomalous contraction of theC−Cbond length in semiconducting carbon nanotubes observed during Cs doping

In situ first-order Raman scattering was used to study the time evolution of the intratube $\mathrm{C}-\mathrm{C}$ bond length of single-walled carbon nanotubes (SWNTs) during slow vapor phase Cs doping. The evolution was followed via changes observed in the scattering from the tangential $\left(T\right)$ and radial $\left(R\right)$ phonon modes. After the initial exposure to Cs vapor, a very long induction interval was observed, followed by three distinct intervals where different phonon mode behavior with doping could be identified. The second interval was marked by a strong decrease in the scattering intensity; the $R$ band was observed to have a $2.5{\mathrm{cm}}^{-1}$ upshift before its disappearance, while no apparent shift in the $T$ band was observed. The third interval is marked by a continued loss in $T$ band intensity, while the shoulders on the $T$ band are gradually lost; the final $T$ band shape in this interval is well described by a single broad Lorentzian. Also during the third interval, an anomalous upshift in the $T$ band is observed $(\sim 4{\mathrm{cm}}^{-1})$. In the fourth, and final, interval, the Lorentzian $T$ band first downshifts and then assumes an asymmetrically broadened Fano interference line shape. During the evolution of this Fano line shape, the renormalized phonon frequency continued to downshift. The results are discussed in terms of current theories for doping-induced changes in the $\mathrm{C}-\mathrm{C}$ bond length of SWNTs.