Issue Date: January 2, 2006
Absorption spectroscopy on a single molecule
Single-molecule absorption spectroscopy is a measurement tool that, until now, has eluded analytical chemists wishing to probe molecular structure on surfaces. That's because single molecules don't absorb much light, making detection difficult, and light-induced heating of the sample and of the microscope tip tends to produce enough noise to swamp out the signal. Researchers in the chemistry department and the Beckman Institute at the University of Illinois, Urbana-Champaign, have overcome these problems and have reported single-molecule laser absorption by single-walled carbon nanotubes, as in the 7-nm-long specimen shown (Nano Lett. 2006, 6, 45). According to Martin Gruebele, who spearheaded the research, the new measurement method combines the chemical selectivity of optical absorption spectroscopy and the atomic-scale resolution of a scanning tunneling microscope (STM). When the sample molecule absorbs energy, its electron density changes shape. The STM then measures that shape change. The team cut down on the heat generated, and therefore the excess noise, by using a transparent silicon substrate and by backlighting the area where the STM tip interacts with the sample. Gruebele says the technique is extremely sensitive for measuring molecules' optoelectronic properties and for studying energy transfer on surfaces.
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