Below-gap excitation of semiconducting single-wall carbon nanotubes

Abstract

aWe investigate the optoelectronic properties of the semiconducting (6,5) species of single-walled carbon nanotubes by measuring ultrafast transient transmission changes with 20 fs time resolution. We demonstrate that photons with energy below the lowest exciton resonance efficiently lead to linear excitation of electronic states. This finding challenges the established picture of a vanishing optical absorption below the fundamental excitonic resonance. Our result points towards below-gap electronic states as an intrinsic property of semiconducting nanotubes.

Ultrafast Photophysics of Single‐Walled Carbon Nanotubes

“…It has in fact been predicted theoretically that the band edge exciton is split into two states, one bright (of odd parity) and one dark (of even parity), located a few meV below the bright one; the two states thermalize via interaction with acoustic phonons. [140] The existence of a broad and featureless low-energy (below the optical bandgap) absorption band, that we have recently measured experimentally, [141] might further explain the importance of the non-radiative recombination channels.…”

Nanoscale Excitation Dynamics of Carbon Nanotubes Probed with Photoinduced Force Microscopy

“…Among the signatures in the transient absorption spectrum are bleach-type profiles at energies higher than the excitation energy. − Figure a shows the transient response near 2.19 eV, at energies close to the E 22 transition and the region of interest in the current study. The bleach of the transient absorption is seen to reduce in magnitude on the tens of picosecond time scale.…”

“…In addition, the minimum PiFM signal is reached at ∼1 ps after the pump pulse. Such delayed dynamics on this time scale are not commonly observed in highly purified (6,5) SWCNT samples in which inter-CNT interactions are minimized, 17 suggesting that the delayed formation of charge carriers might stem from direct interactions between CNTs. Another possibility is the transfer of charges, generated within the much larger illumination area (∼400 nm) into the probed area (∼10 nm), producing a delayed response.…”

“…In this context, bleach-type signatures near E 22 following excitation at lower energies have been attributed to the generation of E 11 excitons, which may lead to a bleach near E 22 due to phase space filling and the presence of longer lived charge carriers, which can induce dispersive lineshapes in the transient absorption spectrum at higher exciton energies due to the Stark effect. 17 Some additional insight into the nature of the photoexcitation under the current experimental conditions can be obtained from the power dependence of the TAS signal. Before studying the power dependence of the TAS signal at 2.19 eV following nonresonant excitation, we first focus on the photobleach signal near E 11 following resonant excitation.…”

Semiconducting carbon nanotubes in photovoltaic blends: The case of pTB7:PC60BM:(6,5) SWNT

“…7,8 Such higher excitons, in particular, can serve as probes for photogenerated charge carriers in the highly enriched (6,5) SWNT (see figure 1a). [9][10][11] The employment of SWNTs as active materials in organic photovoltaic diodes (OPVs) is very promising because of their optical absorption in the near infrared region and the relatively high carrier mobility. [12][13][14][15][16] Given the excitonic character of OPVs it is necessary a proper electron donor/acceptor interface to achieve effective excitons splitting and charge generation.…”