Photon emission in scanning tunneling microscopy: Interpretation of photon maps of metallic systems

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Abstract
We analyze maps of the integral photon intensity emitted from the tunneling gap of a scanning tunneling microscope obtained simultaneously with topography from a variety of metal films and single-crystal surfaces in ultrahigh vacuum. The effects of adsorbates and structures created with the scanning tunneling microscope on their local photon emission properties are investigated to explore the potential of the technique for chemical mapping. It is proposed that contrasts in photon maps on a scale of some tens of nanometers are attributable to local variations in the field strength of tip-induced plasmon modes which are determined by the surface geometry of the junction and its dielectric properties. On a (sub)nanometer scale, a second contrast mechanism is observed to occur, consistent with geometry-induced variations in the matrix element for inelastic tunneling. A comparison of electron spectroscopic data with bias-dependent photon maps indicates that contrasts on a subnanometer scale are further mediated by local modifications of the density of final states positioned one quantum of energy (hν) below the bottom of the elastic tunneling channel with respect to the Fermi level. These three mechanisms provide a framework for the interpretation of photon maps obtained on metallic systems. DOI: http://dx.doi.org/10.1103/PhysRevB.48.4746 © 1993 The American Physical Society