AbstractsPhysics

Single-walled carbon nanotubes (SWNTs) are hollow cylinders of graphene with unique mechanical, electrical, and optical properties that are directly influenced by their exact diameter and chirality. Stable near-infrared fluorescence from semiconducting SWNTs could generate significant advances in the fields of biological imaging, single molecule sensing, telecommunications, and quantum optics. Still, one factor potentially limiting the development of applications based on SWNT fluorescence is their extremely low fluorescence quantum yield (QY), which is typically <0.1%. Surprisingly, the fluorescence from individual SWNTs can be detected with relatively high signal-to-noise, making it unclear if the low luminescence efficiency is an intrinsic property of all nanotubes. Further, elucidating the intrinsic QY of carbon nanotubes could provide important insights into their fundamental photophysics. SWNTs were suspended in a suite of surfactants and biomolecules using an ultrasonic technique, and the procedure was optimized for subsequent single molecule investigations. Epifluorescence microscopy revealed important features of individual SWNT fluorescence such as Lorentzian line shapes, narrow line widths, and the absence of blinking at room temperature. To determine the luminescence efficiency of individual SWNTs, the fluorescence from individual nanotubes was compared to single CdTe/ZnS quantum dots with a well-defined QY. For the brightest SWNTs, the QY was ~3 ± 1%, suggesting that the intrinsic nanotube QY is much higher than previously believed. To quantify the population of emissive particles and further clarify the role of extrinsic factors on the QY, correlated measurements of fluorescence and topography were performed for individual SWNTs. These studies elucidated that only ~6% of all isolated SWNTs are emissive and that approximately one-third of SWNTs are bundled, suggesting that the QY is low because only a small population of SWNTs is bright. Further, SWNT photoluminescence exhibited a strong dependence on the local environment. Thus, with appropriate tailoring of the SWNT surroundings, it may be possible to engineer carbon nanotubes that have brighter luminescence than current samples. In the future, the availability of higher-quality SWNT starting materials will enable further clarification of the intrinsic photophysical properties of carbon nanotubes and will inform the rational development of applications based on these fluorophores.