Hybrid Technique Enables Single-Molecule Absorption Spectroscopy

By combining rear-illumination, rapid excitation modulation and lock-in amplifier detection, investigators at the University of Illinois at Urbana-Champaign have used scanning tunneling microscopy to detect laser absorption by single molecules. The technique, which they reported in the online edition of Nano Letters, may have applications in analytical chemistry and in the study of the electrical properties of nanostructures and of energy transfer on surfaces where single-molecule fluorescence spectroscopy is unsuitable, they suggest.

To demonstrate the approach, the researchers focused 1250-nm radiation from a custom InGaAs diode laser onto the back of a hydrogen-passivated silicon wafer, machined to enable total internal reflection excitation, on top of which they had deposited single-walled carbon nanotubes. A mechanical optical chopper modulated the excitation. The lock-in amplifier, tied to the chopper, filtered the system noise from the resulting microscope images, which were collected by raster scanning the sample in constant-current mode.