Electron Correlation Effects on the Femtosecond Dephasing Dynamics ofE₂₂Excitons in (6,5) Carbon Nanotubes

Abstract

Highly nonlinear pump fluence dependence was observed in the ultrafast one-color pump-probe responses excited by 38 fs pulses resonant with the E(22) transition in a room-temperature solution of (6,5) carbon nanotubes. The differential probe transmission (ΔT/T) at the peak of the pump-probe response (τ = 20 fs) was measured for pump fluences from ∼10(13) to 10(17) photons/pulse cm(2). The onset of saturation is observed at ∼2 × 10(15) photons/pulse cm(2) (∼8 × 10(5) excitons/cm). At pump fluences >4 × 10(16) photons/pulse cm(2) (∼1.6 × 10(6) excitons/cm), ΔT/T decreases as the pump fluence increases. Analogous signal saturation behavior was observed for all measured probe delays. Despite the high exciton density at saturation, no change in the E(22) population decay rate was observed at short times (<300 fs). The pump probe signal was modeled by a third-order perturbation theory treatment that includes the effects of inhomogeneous broadening. The observed ΔT/T signal is well-fit by a pump-fluence-dependent dephasing rate linearly dependent on the number of excitons created by the pump pulse. Therefore, the observed nonlinear pump intensity dependence is attributed to the effects of quasi-elastic exciton-exciton interactions on the dephasing rates of single carbon nanotubes. The low fluence total dephasing time is 36 fs, corresponding to a homogeneous width of 36 meV (290 cm(-1)), and the derived E(22) inhomogeneous width is 68 meV (545 cm(-1)). These results are contrasted with photon-echo-derived parameters for the E(11) transition.

Below-gap excitation of semiconducting single-wall carbon nanotubes

“…4b) and resonant S 22 excitation from ref. 35 with that of below-gap excitation, we are able to estimate a below-gap optical density (in the energy region between 0.8 and 1 eV investigated here) of approximately 0.07-0.012. Our experiments also indicate that below-gap excitation produces the same transient spectra (inset Fig.…”

Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer

“…The exciton dynamics of the Hybrid in tetrahydrofuran and SWCNT in water are investigated upon the E 11 , E 22 , and E 33 excitation with variable excitation fluences, and PCT products in the Hybrid are analyzed. We demonstrate that the PCT from photoexcited (6,5) SWCNT to PFO−BPy is driven by the energetically favorable E 33 excitation and may be also driven by the energetically unfavorable E 22 excitation. The latter process occurs via Auger recombination of excitons and charge carriers at high excitation fluences.…”

“…The highly selective wrapping of semiconducting single-walled carbon nanotubes (SWCNTs) with conjugated polymers, e.g., polyfluorenes and polythiophenes, has enabled monochiral samples with high purity 1−3 to be available for a wide range of optoelectronic applications from field-effect transistors, 4 lightemitting diodes, 5 and electrochromic cells 6 to photovoltaic cells 7,8 and photodiodes. 9 The interaction of nanotubes with their immediate environment, i.e., the wrapping polymer or matrix, with regard to energy and charge transfer depending on the energy level alignment 10,11 is crucial for their functionality and not yet fully understood.…”

“…2 The energy level alignment of (6,5) SWCNTs and PFO−BPy (Figure 1) indicates that the PFO−BPywrapped (6,5) SWCNT hybrid system (hereafter referred to as Hybrid) constitutes a Type-I heterojunction. When regarding the narrow-bandgap (6,5) SWCNT as a charge donor, the PCT from excited (6,5) SWCNT toward wide-bandgap acceptor PFO−BPy is energetically unfavorable, at least for the E 11 and E 22 transitions. However, the established exciton photophysics of SWCNTs suggests that low-energy photon excitation is able to promote populations of high-energy excitonic states via, e.g., Auger recombination of excitons.…”

“…35 Here, we analyze this SWCNT-based Type-I heterojunction by femtosecond TA spectroscopy. A comparison with surfactant-dispersed (6,5) SWCNTs in water (hereafter referred to as SWCNT), which allows for observations of exciton dynamics without energy or charge transfer, 37−40 is conducted throughout this work. The exciton dynamics of the Hybrid in tetrahydrofuran and SWCNT in water are investigated upon the E 11 , E 22 , and E 33 excitation with variable excitation fluences, and PCT products in the Hybrid are analyzed.…”