The Role of Length and Defects on Optical Quantum Efficiency and Exciton Decay Dynamics in Single-Walled Carbon Nanotubes

Abstract

). The existence of a dark exciton level below the bright exciton band has been proposed as an explanation for low quantum efficiency.9 Spataru et al. showed that this splitting has minimal effect at room temperature, and the more important effect is the thermal momentum blocking of the radiative transition. 10The relatively low values of the quantum efficiency show that it is the nonradiative decay rate that dominates over the radiative decay rate. There are several nonradiative decay mechanisms proposed in the literature. At high fluences, excitonϪexciton Auger deexcitation has been suggested. 11,12 Furthermore, excitons in doped semiconducting nanotubes can decay via electronϪphonon interactions. 13 However, for undoped nanotubes in the linear regime, the main nonradiative decay mechanism is exciton diffusion to quenching sites, such as structural defects, adsorbate molecules, or the ends of the nanotube. 14Ϫ16 Stepwise fluorescence quenching in carbon nanotubes in response to changes to the nanotube's environment 16Ϫ18 can clearly also be utilized in sensor applications, such as biological sensors, ideally capable of detecting single molecules. 19The diffusion constant is the main parameter of the diffusion model, but it is not well-known. Experimentally extracted values of the diffusion constant range over 3 orders of magnitude, from 0.1 to 100 cm 2 /s. 12,16,18,20,21 Environment, chirality variation, length distributions, sample quality, and the presence of bundling introduce complexities in the behavior of the optical properties of nanotubes. Therefore, it is important to first examine the expected behavior of an individual nanotube with wellcontrolled static or dynamic quenching defects as the only interaction with the environment.In this paper, we assume that the observed diffusive behavior arises from random walks by excitons and perform Monte Carlo simulations of these random walks to model the fluorescence from nanotubes under uniform excitation (see Detailed Methods). From these simulations, we obtain the time-resolved, spatially resolved, and integrated quantum efficiency for nanotubes in the presence of perfectly quenching defects and of varying lengths. We show

Applicability of Birth–Death Markov Modeling for Single-Molecule Counting Using Single-Walled Carbon Nanotube Fluorescent Sensor Arrays

“…The most recent theory of exciton dynamics on a pristine SWNT assumes that there is no limit to the number of adsorption/quenching events, and these can happen anywhere along the length. 19 In practice, it is believed that either the matrix or tethering chemistry divides the SWNT into isolated segments, each of which can be quenched completely through an adsorption event, in agreement with recent experimental studies. 14À17 Further work is necessary to reconcile these two physical models.…”

Room-Temperature Single-Photon Emission from Micrometer-Long Air-Suspended Carbon Nanotubes

“…1(a) and 1(b)]. The PL image shows a smooth spatial profile, indicating that the suspended nanotube is defect-free and does not contain any quenching sites or trapping sites [30,31]. Next, we perform PL excitation spectroscopy to identify the chirality (n, m) and to confirm that the nanotube is not bundled [ Fig.…”

Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes

“…Evidence of EEA has been observed in the form of pump-dependent variations in PL decay dynamics [11][12][13] as well as saturation behaviors of PL [14,15] and transient absorption signals [20]. Many attempts have also been made to determine the exciton diffusion length, L D , in SWCNTs [8,[16][17][18][19], revealing values ranging from nanometers [9] to hundreds of nanometers [10,16,19]. The large discrepancies in these measurements have been attributed to effects of pump power [10], differences in fabrication methods (CoMoCAT or HiPco) [8,10], and the environment (surfactant-or DNA-wrapped or air-suspended) [8,10] of the SWCNTs.…”