Abstract
Strong quantum confinement and low dielectric screening impart single-walled carbon nanotubes with exciton-binding energies substantially exceeding kBT at room temperature. Despite these large binding energies, reported photoluminescence quantum yields are typically low and some studies suggest that photoexcitation of carbon nanotube excitonic transitions can produce free charge carriers. Here we report the direct measurement of long-lived free-carrier generation in chirality-pure, single-walled carbon nanotubes in a low dielectric solvent. Time-resolved microwave conductivity enables contactless and quantitative measurement of the real and imaginary photoconductance of individually suspended nanotubes. The conditions of the microwave conductivity measurement allow us to avoid the complications of most previous measurements of nanotube free-carrier generation, including tube–tube/tube–electrode contact, dielectric screening by nearby excitons and many-body interactions. Even at low photon fluence (approximately 0.05 excitons per μm length of tubes), we directly observe free carriers on excitation of the first and second carbon nanotube exciton transitions.
Highlights
Strong quantum confinement and low dielectric screening impart single-walled carbon nanotubes with exciton-binding energies substantially exceeding kBT at room temperature
In this study we demonstrate that free charge generation takes place in individual single-walled carbon nanotubes (SWCNTs) suspended in toluene, even at ultra-low excitation fluences, which rules out both high dielectric and multi-exciton effects
The energy-level diagram shown in Figure 1b illustrates that the PFO polymers22,23 and the [7,5]-SWCNTs24 form a type-I heterostructure and, it is evident that the [7,5]-SWCNT* state generated via S11 excitation should not exhibit electron/energy transfer from [7,5]-SWCNTs to PFO polymers, which is consistent with previous literature results25
Summary
Strong quantum confinement and low dielectric screening impart single-walled carbon nanotubes with exciton-binding energies substantially exceeding kBT at room temperature. Most of these examples of carrier generation have been observed in solid-state samples featuring either tube–tube contacts or tube–electrode contacts As these interfaces can likely serve as carrier-generation sites, they obscure the intrinsic properties of the individual nanotubes. Examples of these potential solid-state artefacts include heterogeneous chiralities of SWCNTs that may form type-I or type-II energylevel alignments in SWCNT bundles, electrostatic screening effects in SWCNT aggregates that can enhance free-carrier generation, SWCNTs on substrates in air that often become pdoped or potential morphological defects or contacts with electrodes in which electric fields can dissociate excitons. We find that the low-fluence yield mobility product F m, which is thePproduct of charge-carrier generation efficiency F and the sum m of electron and hole mobilities me þ mh 0.4 cm in VÀ isolated 1sÀ1
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