Photon emission from polycrystalline Ag induced by scanning tunneling microscopy: comparison of different tip materials

Photon emission from polycrystalline silver induced by scanning tunneling microscopy is studied for three different tip materials (Au, PtIr and W). Photon emission intensity curves as a function of the tip voltage are observed to be almost identical for platinum–iridium alloy and gold tips (and more than 10 times enhanced as compared with the tungsten tip). An evolution in topography and photon map for different applied voltages is investigated along with the study of the spatial distribution of photon emission in dependence upon the surface local differential height. It turns out that no clear correlation between a local curvature and enhancement of light emission can be found. Simultaneous measurements of tunneling current and photon intensity as a function of vertical tip displacement confirm the earlier observation, namely, that similar apparent barrier heights exist for both elastic and inelastic tunneling channels. The role of the tip material as well as its shape is discussed.