Distance dependence of tunneling thermovoltage on metal surfaces

Thermovoltage in a tunnel junction of a scanning tunneling microscope is a valuable observable that is known to have chemical sensitivity. Tunneling thermovoltage is typically measured at a fixed tip-surface distance. Here the author presents distance-dependent thermovoltage measurements that were systematically acquired over Ag(111) surface with a small coverage of adsorbates. The author confirms that the distance dependence can be used to obtain thermovoltage contrast due to the difference in the tunneling barrier height, and reveals that adjusting the width of the tunneling gap can significantly enhance thermoelectronic contrast. Furthermore, comparing tunneling thermovoltage and a synchronously acquired tunneling current as a function of gap width provided a direct verification of the Stovneng-Lipavsky model, which is often used to interpret tunneling thermovoltage experiments. Finally, the author shows that the distance-dependent thermovoltage can potentially be used to evaluate a thermal gradient in an atomic-scale tip-surface junction without explicit temperature calibration, which has so far not been achievable experimentally.