An electrochemical scanning tunneling microscopy study of tunneling at the solid-liquid interface.

摘要

Electrochemical scanning tunneling microscopy (ECSTM) has been used to study quantum mechanical tunneling in an aqueous environment. Barrier heights of Au(111) surface in various electrolytes were measured with a tip bias of 0.1V. The measured barrier heights are comparable to that obtained in vacuum. However, in contrast to vacuum tunneling, the data show a strong dependence on the bias applied between the tip and the substrate. Asymmetry exists such that it requires more energy to tunnel from the tip to the substrate than vice versa which can be attributed to permanent water dipole alignment in the tunneling gap. A sharp dip of tunnel barrier near zero bias was observed. It is not observed in a non-polar liquid and it is attributed to induced polarization in the tunnel gap. Combined with the results of calculation of conductance for tunneling through organic molecules, a tunneling picture in liquid environment was proposed. Both tip and substrate are covered by a layer of oriented water molecules with the substrate also having the adsorbed organic molecule. Liquid water may also be present in the gap and the tip will follow the contour of quantum-point-contact at the set-point conductance.; Cytosine adsorption on Au(111) surface was studied using ECSTM. Cytosine adlayer prefers unprotonated flat-lying structure and unpaired stacking structure at high positive potentials ("high-pH"). All the adsorbates are paired into a stacked base-paired structure at high negative potentials ("low-pH"). Spontaneous adsorption of cytosine onto Au(111) surface occurs when the rest potential is slightly positive of PZC. Domains of planar hydrogen bonded networks and stacked cytosine molecules were observed simultaneously which are compromise structures between the "high-pH" and "low-pH" structures. Surface potential-induced structural phase transitions confirms the proposed protonation and deprotonation model.