Abstract
Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S33 transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S11. The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.
Highlights
Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally
We apply ultrafast optical spectroscopy to the semiconducting (6,5) Single-walled carbon nanotubes (SWNTs) and show that charge-carriers can be identified by their effect on excitonic resonances, in particular the large energy shift that they induce on the third excitonic subband (S33) transition
Our assignment of a red-shift induced by Stark effect is based on the following chain of reasoning: i) charge-carriers are photogenerated in SWNTs; ii) each charge-carrier is a source of a strong local electric field; iii) the amplitude of the Stark signal has a distinct kinetics from that of the exciton PB, being in particular much longer lived; iv) any other source of modulation that could explain the first derivative shape of the DT/T spectra for S33 has been ruled out
Summary
Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. The study of photo-excitation dynamics in one dimension has been prompted by theoretical predictions of a wealth of singular properties, such as the giant oscillator strength and non-linear response of confined states, the large Coulomb interaction, the sharply-peaked density of states and peculiar excited-state recombination kinetics[1,2,3]. In this respect, SWNTs represent a very close approximation to a 1D solid, achieving aspect ratio as high as 103. This kinetics is consistent with an initial electron-hole separation of the same order as the exciton correlation length
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