Two-sphere model of photon emission from the scanning tunneling microscope

Photon emission from the scanning tunneling microscope (STM) has been shown to produce spectra that are dependent on the dielectric function of the surface. This presents the exciting possibility of chemical information from the STM, previously impossible albeit with a few exceptions. However, the spectra are also critically dependent on the geometry of the tunnel junction-changes in either tip or sample curvature can have dramatic effects on the spectra. A model is presented here that allows for curvature in the sample as well as the tip, and the results compare favorably with existing experimental data. It predicts that under certain conditions, spectra can be made almost invariant to changes in the geometry, leaving them dependent solely on the sample material-making the STM a chemical probe. The model also predicts that gold and silver particles may be differentiated by comparing their variations of photon intensity with bias, and this prediction is confirmed experimentally. Finally, a strategy for chemical identification of any metal surface is presented.