Fluorescence Efficiency of Individual Carbon Nanotubes

Abstract

The luminescence efficiency of individual single-walled carbon nanotubes was determined by comparing the fluorescence from individual nanotubes to single CdTe/ZnS quantum dots with a well-defined fluorescence quantum yield (QY). The single carbon nanotube QY was determined to be 3 +/- 1%, nearly 100 times greater than values previously reported for ensembles. The intrinsic nanotube QY is potentially much higher than previously believed and appears lower in ensembles due to defective nanotubes and residual bundles.

Photophysics of Individual Single-Walled Carbon Nanotubes

“…170 Ensemble studies benefit significantly from the clarification of fundamental photophysical behavior that is exposed on the single particle level, which has catalyzed single molecule studies for many fluorescent systems including dyes, 171-174 polymer chains, 175,176 proteins, [177][178][179] semiconductor quantum dots, 170,180,181 and single-walled carbon nanotubes. 72,77,182 The observation of fluorescence from individual SWNTs was first reported in 2003 and, importantly, confirmed spectral assignments of (n,m) made on the ensemble level. 72 Importantly, single molecule spectroscopy intrinsically avoids the need for difficult separation techniques by enabling the examination of one molecule at a time, which is particularly useful for studying non-uniform SWNT samples.…”

“…This high fraction of bundles corroborates previous studies suggesting that residual bundles are present in typical samples and significantly lower the ensemble QY, as typical samples contain metallic SWNTs that quench fluorescence from semiconducting species in bundles. 77 Also, it is expected that the percentage of aggregated SWNTs in this (6,5)-enriched sample was lower than that for a typical surfactant-suspended ensemble, as the HPLC purification process removes some aggregates; thus, the fraction of residual bundles in a typical sample likely exceeds ~30%. Finally, correlated measurements indicate that some spectra usually assigned to individual SWNTs may actually result from bundles or nanotubes that are situated too close together to be resolved with most optical microscopes.…”

“…72,75,76 Still, the luminescence efficiency, or fluorescence quantum yield (QY), for ensembles of SWNTs is extremely poor, typically <0.1%. 77 Due to their unique optical properties, SWNTs may satisfy a long-standing need for superior fluorophores required for in vivo single biomolecule imaging and single particle tracking (Figure 1 -5). Coupled with their relative ease of uptake into cells and infrared emission at wavelengths most transparent to biological tissue, SWNTs hold great promise in this area because NIR radiation passes through tissue without causing significant heating, scattering, or damage to live cells.…”

“…The estimated lengths of the SWNTs used in single molecule studies in this dissertation were 100-500 nm, and the average length measured for DNAwrapped samples used in Chapter 5 was ~380 nm. 77 Ultracentrifugation is required because complete separation of bundled and isolated SWNTs is never achieved by the sonication procedure, as a dynamic equilibrium is established between these species during the process. 131 After centrifugation, it was extremely important not to extract too much supernatant liquid from the centrifuge tube.…”

“…77 Previously, Credo and co-workers performed measurements of fluorescence and shear force topography for luminescent porous silicon, and clarified that the 1-10% ensemble QY was caused by a small population of bright emitters instead of a large population of weak emitters. 217 Similarly, Banin and co-workers used simultaneous measurements of fluorescence and AFM topography to examine single molecule versus ensemble QYs of CdSe/ZnS semiconductor nanocrystals.…”