Tunneling Electrons Stimulate Individual Molecules

Using a scanning tunneling microscope, a research team at the University of California, Irvine, has induced photoemission in individual porphyrin molecules on an aluminum oxide film, yielding fluorescence patterns that depend on the molecular conformations. The technique, which was reported in the Jan. 24 issue of Science, may find applications in the study of electron dynamics in conjugated polymers and organic molecules and of the influence of local plasmon modes on intermolecular fluorescence resonance energy transfer.

In the experiments, the researchers employed a homebuilt ultrahigh-vacuum scanning tunneling microscope with silver or tungsten tips. They found that the silver tips enabled higher photon emission efficiencies and revealed greater details in the resulting emission spectra and intensities. Sample bias voltages were 2 to 2.4 V, and the tunneling current was 0.5 nA.