Length-Dependent Optical Effects in Single-Wall Carbon Nanotubes

Abstract

Among the novel chemical and physical attributes of single-wall carbon nanotubes (SWCNTs), the optical properties are perhaps the most compelling. Although much is known about how such characteristics depend on nanotube chirality and diameter, relatively little is known about how the optical response depends on length, the next most obvious and fundamental nanotube trait. We show here that the intrinsic optical response of single-wall carbon nanotubes exhibits a strong dependence on nanotube length, and we offer a simple explanation that relates this behavior to the localization of a bound exciton along the length of a nanotube. The results presented here suggest that, for a given volume fraction, the longest nanotubes display significantly enhanced absorption, near-infrared fluorescence, and Raman scattering, which has important practical implications for potential applications that seek to exploit the unique optical characteristics of SWCNTs.

Photophysics of Individual Single-Walled Carbon Nanotubes

“…91 Samples that have endured ultrasonic processing are expected to display shorter average lengths for large-diameter species, which we did not observe here, likely due to limited statistical data for (9,1) and (8,3) SWNTs. 137,156 In addition, unlike previous measurements performed for ensembles of nanotubes, 212 longer SWNT lengths did not correlate with brighter luminescence intensities, although direct comparison was again complicated by variations in the orientation of the SWNTs on the substrate with respect to the excitation polarization.…”

“…It is also unclear how the QY varies with SWNT length, which is a subject of current debate. For example, Fagan et al have observed a strong dependence of fluorescence intensity on nanotube length 212 that is supported by another study showing an exciton diffusion length of ~90 nm along the SWNT. 116 These studies imply that the fluorescence efficiency for short SWNTs may lowered by efficient quenching of bound excitons at the cut ends of nanotubes.…”

“…154,189 Attenuation of nanotube fluorescence also accompanies dispersal in polar solvents, 218 encapsulation in certain surfactant systems, 149 protonation at low pH, 167 and shortening of the nanotube length. 212 In addition, the presence of defect sites, impurities, and oxygen is expected to decrease the QY via hole doping, which effectively removes an electron from the valence band and provides a trap for the excited state. 213,219 Another conjecture is that these processes may accelerate intersystem crossing to triplet states, which sequesters excited state carriers and lowers the observed QY.…”

Theoretical studies on the electronic structures and spectra of single silicon-doped SWCNTs

“…The first absorption peak of (4,4) (C 48 ) was located at 977.8 nm, and that of (5,5) (C 60 ) was at 937.5 nm. It had been observed in the experiment that the first absorption peak of (6,5) SWCNT was near 990 nm [11], which supported our calculation results. On one hand, the first absorptions for most of the doped SWCNTs compared with those of the pristine SWCNTs were redshifted (Table 2).…”

“…This duality depends on the diameter of the SWCNTs and arrangement of the hexagons [10]. Besides the electrical conductivities, a tunable optical property of the SWCNTs has been observed in Jeffrey's experiments [11]. In addition, the dispersed SWCNTs have also played an important role in the enhancement of the effective thermal conductivity of fluids [12].…”