Ultrafast photoresponse of metallic and semiconducting single-wall carbon nanotubes

Utilizing a transient absorption (TA) technique based on a chirp-corrected broadband probe, we have studied the ultrafast photoresponse of dispersed HiPco single-wall carbon nanotubes (SWNTs) over the range of 440–1050 nm for excitation in the range of 430–1700 nm. While both metallic and semiconducting SWNTs show transient bleaching at their $M_{11}$ and $S_{11}$ energies for excitation above these energies, metallic SWNTs uniquely exhibit a photoresponse to sub-$M_{11}$ excitation. We observe a TA spectral response for metallic SWNTs which is consistent with thermal broadening of the $M_{11}$ transition bands. In contrast to metallic SWNTs, specific semiconducting SWNTs exhibit transparency for sub-$S_{11}$ excitation, in accord with the expected zero density of states below the first interband transition. We report the observation of a long-lived $(\tau >1\mathrm{ns})$ transient bleach component for three semiconducting SWNT species.

Figure 1

(Color) (a) Absorption and photoluminescence spectra for HiPco SWNTs dispersed in ${\mathrm{D}}_2\mathrm{O}$ with SDS surfactant. Emission spectra were measured using 532 nm laser excitation. Labeled features correspond to semiconducting SWNT emission peaks. (b) Near-infrared photoluminescence excitation spectrum, detected using a liquid-nitrogen-cooled CCD.

Figure 2

(Color) (a) Transient absorption spectra for excitation at 430, 1300, and 1700 nm, acquired at 1.0 ps delay relative to peak signal and for an absorbed photon density held constant at $2.5\times {10}^{14}{\mathrm{cm}}^{-2}$. The dotted line shows the linear absorption spectrum. (b) Transient absorption spectra for 430 and 1700 nm, at 1.0 ps delay, for which the excitation intensity was adjusted to achieve the same total excitation pulse energy absorbed $(1.15\times {10}^{-4}\mathrm{J}∕{\mathrm{cm}}^2)$.

Figure 3

(Color) Chirp-corrected transient absorption spectra for excitation at 532, 1300, and 1700 nm, acquired at 1.0 ps delay. Peaks $A\text{–}E$ indicate peaks correlated to semiconductor SWNT emission peaks observed in the PL spectrum. The dotted line shows the linear absorption spectrum.

Figure 4

(Color) Chirp-corrected transient absorption spectra for four pump wavelengths, acquired at 1.0 ps delay. Narrow peaks (dashed lines) show the pump spectra. Peaks $A\text{–}F$ denote those features correlated to band-gap photoluminescence emission wavelengths.

Figure 5

(Color) Transient absorption dynamics measured at 975 nm for pump wavelengths varying between 520 and 1300 nm. Data have been normalized to the peak positive-going signal.

Figure 6

Dynamics of the photoinduced bleach at the $S_{11}$ for the (8,3), (6,5), and (7,5) semiconducting SWNTs, for ${\lambda }_{\text{pump}}=650\mathrm{nm}$ and an absorbed photon density of $8\times {10}^{14}{\mathrm{cm}}^{-2}$. Solid lines show single-exponential fits to long-lived components for $t_{\text{delay}}>200\mathrm{ps}$.

Figure 7

(Color) (a) Intensity dependence of the photoinduced bleach signal at 975 nm, at the peak amplitude and at 30 ps delay, for 1300 nm excitation. The linear dependence indicates that the signal arises from linear absorption at the sub-band-gap energy, and not from two- or three-photon excitation. (b) Normalized transient absorption dynamics at 975 nm as a function of pump intensity at ${\lambda }_{\text{pump}}=1300\mathrm{nm}$. The triangles show the 975 nm dynamics for above-$S_{11}$ excitation at ${\lambda }_{\text{pump}}=579\mathrm{nm}$.