Charge dynamics in transparent single-walled carbon nanotube films from optical transmission measurements

We report on the transmission over a wide frequency range (from the far infrared through the visible) of pristine and hole-doped, free-standing carbon nanotube films at temperatures between 50 and $300\mathrm{K}$. Optical constants are estimated by Kramers-Kronig analysis of the transmittance. We see evidence in the far infrared for a gap below $10\mathrm{meV}$. Hole doping causes a shift of spectral weight from the first interband transition into the far infrared. Temperature dependence in both the doped and undoped samples is restricted to the far-infrared region.

Figure 1

(Color online) Transmittance, optical conductivity, and absorption coefficient of two SWNT transparent films of different thickness.

Figure 2

(Color online) The results of Drude-Lorentz fits (least-squares fit to transmission curves) compared to measured room-temperature transmission data and to conductivity obtained by Kramers-Kronig transformation. Numbers refer to oscillator labels in Table .

Figure 3

(Color online) Temperature dependence of the optical conductivity of the two samples.

Figure 4

(Color online) Top panel: sum rule for baked and unbaked sample at $300\mathrm{K}$. Middle panel: added carriers per carbon as function of frequency at selected temperatures. Bottom panel: difference in conductivity upon doping. For the definition of regions I, II, and III, see text.