Photoluminescence/Raman System Images Nanotubes

Daniel S. Burgess

Using a setup that enables simultaneous high-resolution photoluminescence and Raman imaging, scientists at Universität Tübingen in Germany and at the University of Rochester in New York are probing single-walled carbon nanotubes. Coupled with the work of investigators at Columbia University in New York, IBM’s T.J. Watson Research Center in Yorktown Heights, N.Y., and Rice University in Houston, the findings may offer insight into the role of exciton trapping and nonradiative decay in these structures, said Achim Hartschuh of Universität Tübingen.

Simultaneous high-resolution near-field photoluminescence (shown) and Raman imaging offer insight regarding the properties of single-walled carbon nanotubes. Courtesy of Achim Hartschuh, Universität Tübingen.

Such an understanding is more than academic, as nonradiative decay is responsible for energy losses in the tubes that result in their low effective photoluminescence quantum yield. This yield, in turn, is a potential barrier to the use of the nano-structures in photonic applications.

The imaging system, which has a spatial resolution of approximately 12 nm, is based on an inverted optical microscope incorporating a Physik Instrumente X-Y-Z scanning stage with an RHK controller. In operation, 632.8-nm light from a HeNe laser, focused by a 1.25-NA objective, stimulates photoluminescence and Raman scattering in the sample. A 20- to 30-nm-diameter gold tip positioned at the focus of the radially polarized laser beam and maintained at 2 nm above the sample surface acts as a local excitation source. The 950-nm photoluminescence and 700-nm Raman signals return through the objective and are collected continuously by a Roper Scientific CCD camera and an Acton Research spectrometer as the sample is raster scanned.

In a demonstration of the approach, the researchers collected photoluminescence and Raman images of single-walled nanotubes on mica and on glass. Beyond serving as a proof-of-principle experiment, the work revealed that the photoluminescence is composed of multiple emission bands, varying in energy with their location on a tube. The scientists suggest that the phenomenon may be the result of structural or environmental perturbations.

Moreover, Hartschuh said, the demonstration suggests methods of improving the photoluminescence quantum yield of single-walled carbon nanotubes. In the experiment, the field enhancement of the gold tip boosted the photoluminescence signal by a factor of approximately 85.

Nano Letters, online Oct. 19, 2005, doi:10.1021/nl051775e.