Anomalous contraction of the $\mathrm{C}-\mathrm{C}$ bond 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.

Figure 1

Time evolution of the SWNT Raman spectra with in situ Cs doping (intervals I and II). The inset shows the schematic drawing of the experimental apparatus, where ${\mathrm{H}}_1$ and ${\mathrm{H}}_2$ are two band heaters. $\mathrm{T}{\mathrm{C}}_1$ and $\mathrm{T}{\mathrm{C}}_2$ are thermocouples used to measure the temperature at the SWNT and Cs ends of the ampoule. The SWNT sample and Cs source were set to $160°\mathrm{C}$ and $140°\mathrm{C}$, respectively, during the doping. The length of the glass ampoule is $\sim 25\mathrm{cm}$. Laser excitation of $514.5\mathrm{nm}$ $\left(2.41\mathrm{eV}\right)$ was used for the Raman study.

Figure 2

Time evolution of the SWNT $T$ band Raman spectra with in situ Cs doping (interval III). The circles are the experiment data, and the thin solid lines are results of a Lorentzian line shape fit to the data. The curves marked with $a$, $b$, $c$ in the top spectrum are three individual Lorentzian components.

Figure 3

Time evolution of the SWNT $T$ band Raman spectra during interval IV of Cs doping. The dots are the experiment data, and the thin solid lines are the results of a Breit-Wigner-Fano (BWF) line-shape analysis. The peak marked with Hg is from the low-pressure Hg lamp used for frequency calibration.

Figure 4

Time evolution of the fitting parameters ${\omega }_T$ and $\Gamma$ used to describe the dominant $T$ band. The squares represent fitted data of ${\omega }_T$, and the stars are fitted data of $\Gamma$. Polynomial fitted curves (solid line) are used as a guide to the eye. In intervals I–III, ${\omega }_T$ is the center frequency. For interval IV, ${\omega }_T$ and $\Gamma$ are the renormalized phonon frequency and the Breit-Wigner-Fano line-broadening parameter [Eq. (1)], respectively.