Scanning tunneling microscopy and spectroscopy of sodium-chloride overlayers on the stepped Cu(311) surface: Experimental and theoretical study

TL;DR

This study combines experimental STM/STS measurements with density functional theory calculations to analyze ultrathin NaCl overlayers on Cu(311), revealing detailed electronic and geometric properties, including work function changes and tunneling behavior.

Contribution

It provides a comprehensive experimental and theoretical analysis of NaCl overlayers on Cu(311), including STM image simulations and interpretation of tunneling spectra.

Findings

Measured apparent barrier heights match calculations.

Large reduction in tunneling conductance indicates a band gap.

Positive voltage onset corresponds to valence band edge.

Abstract

The physical properties of ultrathin NaCl overlayers on the stepped Cu(311) surface have been characterized using scanning tunneling microscopy (STM) and spectroscopy, and density functional calculations. Simulations of STM images and differential conductance spectrum were based on the Tersoff-Hamann approximation for tunneling with corrections for the modified tunneling barrier at larger voltages and calculated Kohn-Sham states. Characteristic features observed in the STM images can be directly related to calculated electronic and geometric properties of the overlayers. The measured apparent barrier heights for the mono-, bi-, and trilayers of NaCl and the corresponding adsorption-induced changes in the work function, as obtained from the distance dependence of the tunneling current, are well reproduced by and understood from the calculated results. The measurements revealed a large reduction of the tunneling conductance in a wide voltage region, resembling a band gap. However, the simulated spectrum showed that only the onset at positive sample voltages may be viewed as a valence band edge, whereas the onset at negative voltages is caused by the drastic effect of the electric field from the tip on the tunneling barrier.