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Carbon nanotubes give out a steady glow
Researchers from the University of Rochester in the US and Siegen University in Germany have used single-molecule fluorescence spectroscopy to study single-walled carbon nanotubes. Unlike most other molecules and nanocrystals such as quantum dots, the nanotubes fluoresced with a steady intensity and frequency, bringing the possibility of their application as stable infrared photon sources. “Due to the unavoidable sample inhomogeneities present in macroscopic-sized samples, previous optical studies have missed important nanotube characteristics because of ensemble averaging,” Rochester researcher Todd Krauss told nanotechweb.org. “We observed these in our single nanotube measurements, including the true single-walled carbon nanotube line shape and the important fact that nanotubes with identical structure will emit with different frequencies, line widths and intensities.” The scientists attributed these different emission properties for identically structured nanotubes to defects or the local environment. According to Krauss, the main challenge was to prove that the emission the team was measuring was coming from a single nanotube, and to identify the structure of that particular tube. The researchers overcame this by simultaneously meas uring the Raman spectrum of the nanotube that was fluorescing, and by simultaneous measurements of the excitation dipole of the Raman and fluorescence. “We also observed a narrow luminescence line width that is temperature-limited and thus has the potential to be ultra-narrow,” said Krauss. As nanotubes emit in the near infrared, they could potentially have applications as single SWNT fluorescence-based biosensors, single nanotube integrated electroluminescent nanophotonic devices, and single photon quantum optics sources for fields such as quantum cryptography. “The fact that the tubes do not blink is crucial for all these applications,” added Krauss. And what’s next? The scientists are currently conducting experiments to measure the ultimate line width of the fluorescing transition, as it “has important implications regarding the strength of linear and non-linear optical processes”. They will also develop some of the applications and look at the effect of environmental perturbations on the fluorescence spectra and intensity. The researchers reported their work in Science. Liz Kalaugher is editor of nanotechweb.org. |
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