Quantum yield of individual nanotubes determined

Single-walled carbon nanotubes have interesting optical properties, including fluorescence in the near-infrared range. Unfortunately, they tend to clump together during production, quenching their fluorescence to less-than-useful levels. In fact, the quantum yield of heterogeneous mixtures of isolated nanotubes is less than 0.05 percent.

To find out whether the same poor performance is true of all individual nanotubes, investigators at DuPont Central Research and Development in Wilmington, Del., and at the University of Rochester and at Cornell University in Ithaca, both in New York, studied the particles using single-molecule spectroscopy.

Their setup included an inverted microscope, a laser emitting at 632.8 nm and a 100× oil-immersion objective with an NA of 1.3 to deliver the beam and to collect fluorescence. Before exciting individual nanotubes and collecting the subsequent emissions, the scientists performed the same steps with CdTe/ZnS quantum dots. The data from these particles provided a benchmark for comparison with the nanotubes’ fluorescence.

They also imaged the quantum dots using a scanning transmission electron microscope and an electron energy loss spectroscopy system, which enabled them to measure accurately the absorption cross sections of the quantum dots. The absorption cross section of the nanotubes was extracted from absorbance measurements. In turn, this information allowed them to determine the excitation rates of both types of particles.

By combining the cross sections and the fluorescence intensities from individual nanotubes and quantum dots, the investigators found that the average quantum yield of the brightest nanotubes in the sample was approximately 3 ±1 percent — about two orders of magnitude higher than the quantum yield of nanotube ensembles. They believe that the yield could be even higher if manufacturing flaws in the nanotubes could be avoided, thus enabling the use of the particles in biological imaging and biosensing applications.

They report their work in the Nov. 13 ASAP edition of Nano Letters.