Dual modulation STM: Simultaneous high-resolution mapping of the differential conductivity and local tunnel barrier height demonstrated on Au(111)

TL;DR

This paper introduces a dual modulation STM technique that enables simultaneous high-resolution mapping of topography, differential conductivity, and local tunnel barrier height, validated on Au(111) surfaces.

Contribution

A novel STM method that concurrently measures multiple electronic and topographic properties with high resolution, avoiding measurement interference.

Findings

Successfully mapped Friedel oscillations on Au(111).

Detected tunnel barrier differences between hcp and fcc regions.

Demonstrated applicability to open feedback loop measurements.

Abstract

We present a scanning tunneling microscopy (STM) technique to simultaneously measure the topography, the local tunnel barrier height (dI/dz) and the differential conductivity (dI/dV). We modulate the voltage and tip piezo with small sinusoidal signals that exceed the cut-off frequency of the STM electronics and feed the tunneling current into two lock-in amplifiers (LIAs). We derive and follow a set of criteria for the modulation frequencies to avoid any interference between the LIA measurements. To validate the technique, we measure Friedel oscillations and the subtle tunnel barrier difference between the hcp and fcc stacked regions of the Au(111) herringbone reconstruction. Finally, we show that our method is also applicable to open feedback loop measurements by performing grid I(V) spectroscopy.