Local Tunneling Conductivity and Surface Activity by Scanning Tunneling Spectroscopy

Regularities that govern the way the tunneling current I_t, measured in air on highly oriented graphite; polycrystalline rhodium and gold; single-crystal platinum with axial indexes (111), (110), and (311); and on textured Pt and Ir coatings on titanium with axial structures (111), depends on the tunneling voltage U_t are analyzed. The last sample is also studied at a potential monitored in a 0.05 M H₂SO₄ solution. An I_t vs. U_t curve is shown to comprise two oppositely directed tunneling currents, each of which nicely follows an exponential dependence on U_t and depends on the coefficient of the electron transfer to the needle tip . The notion of a tunneling electron exchange current at U_t = 0, I_oo, is introduced. The ratio of I_oo to a given tunneling current I_o may vary by many orders of magnitude. Coefficient is practically independent of the tunneling distance and depends on the physicochemical characteristics of materials and the needle. For metals with electronic properties close to those of the needle material, it equals nearly 0.50, otherwise it is different. At the sample's potential monitored in an electrolytic solution, parameters depend on the sample's potential and the U_t scan direction. An assumption is made as to the nature of the tunneling exchange current and the tunneling conduction at U_t = 0.