Frequency-modulated, single-molecule absorption detected by scanning tunneling microscopy

We image frequency-modulated single-molecule optical absorption using a scanning tunneling microscope as the detector (SMA-STM). As a first example of the technique, a semiconducting carbon nanotube adsorbed on a silicon surface is studied. Excitation is achieved using laser amplitude as well as frequency modulation, and these two complementary approaches are compared. Detection is achieved via the resulting change in tunneling current through the excited molecule. We distinguish three mechanisms, direct, relaxed, and bolometric, for detecting single-molecule absorption spectra. Kinetic models for these mechanisms as well as for surface heating are presented. The latter effect can be eliminated by frequency modulation, keeping the laser power density on the surface constant.