Comparing photoconductivity measurements, using p-n diodes formed along individual single-walled carbon nanotubes (SWNT), with modeling results, allows determination of the quantum efficiency, optical capture cross section, and oscillator strength of the first (E11) and second (E22) excitonic transitions of SWNTs. This is in the infrared region of the spectrum, where little experimental work on SWNT optical absorption has been reported to date. We estimate quantum efficiency (η) ~1-5% and provide a correlation of η, capture cross section, and oscillator strength for E11 and E22 with nanotube diameter. This study uses the spectral weight of the exciton resonances as the determining parameter in optical measurements.
The band gap of a semiconductor is one of its most fundamental properties. It is one of the defining parameters for applications, including nanoelectronic and nanophotonic devices. Measuring the band gap, however, has received little attention for quasi-one-dimensional materials, including single-walled carbon nanotubes. Here we show that the current-voltage characteristics of p-n diodes fabricated with semiconducting carbon nanotubes can be used along with the excitonic transitions of the nanotubes to measure both the fundamental (intrinsic) and renormalized nanotube band-gaps.
It is often assumed that single-walled carbon nanotubes grown via catalytic chemical vapor deposition are free of defects, particularly when suspended over a trench. Here we show that semiconducting nanotubes grown in this manner can contain a surprisingly large number of states within the band gap. High-resolution photocurrent spectroscopy is used to probe these states in individual nanotubes. We observe a series of band-gap states in suspended nanotubes, resulting in long trapping and detrapping times. These states significantly alter the exciton spectra, resulting in a manifold of strongly localized exciton states with narrow linewidths (<1 meV at room temperature) within a broader exciton envelope.
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