Thermovoltage in scanning tunneling microscopy

Abstract

The thermovoltage which will be discussed arises if the tunneling tip and the sample are at different temperatures. For zero external bias voltage the difference of the Fermi distribution for both electrodes will lead to a thermally driven current since, in general, the balance between forward and backward tunneling of electrons will be disturbed. If the external current is zero a bias voltage is found leading to a tunneling current of the same magnitude but opposite direction. This voltage will be referred to as thermovoltage. It provides an analytical tool with excellent lateral resolution which is very sensitive to minor variations of the local electronic density of states of tip and sample in the vicinity of the Fermi level. The investigation of the (111) surfaces of the noble metals reveals several details of the electronic structure. Au(111) shows a local variation with atomic periodicity. For the first time, this provides evidence that the energy dependence of the electronic density of states varies between a position right above an atom or in between atoms of a metallic surface. The thermovoltage may also be used for the investigation of chemically heterogeneous surfaces. As an example the growth of Ag on Au(111) is analyzed. It demonstrates that the measurement of the thermovoltage should be especially useful for the study of alloying processes.