Revealing the impact of ambient molecular contamination on scanning tunneling microscopy and spectroscopy of layered materials

TL;DR

This study demonstrates how ambient hydrocarbon contamination on layered material surfaces affects STM measurements by suppressing certain electronic features and altering tunneling characteristics, providing insights for better surface analysis.

Contribution

It reveals the influence of ambient alkane contamination on STM/STS measurements and offers practical detection methods for surface contamination in experiments.

Findings

Contamination suppresses the phonon-induced gap near the Fermi energy.

It alters the current-distance decay, flattening it significantly.

Provides a practical guide to detect surface contamination in STM.

Abstract

Hydrocarbon contamination is an ever-present factor to consider in surface science measurements. In the case of van der Waals material surfaces, the structure of this contamination has become known in recent years as a self-assembled layer of normal-alkanes, resulting from a few days' exposure to ambient air. Knowledge of its composition and structure enables systematic investigation of its influence on surface properties. Here, we investigate the effect of this contamination on scanning tunneling microscopy (STM) and spectroscopy measurements by comparing clean and ambient alkane-contaminated surfaces of graphite. Our results reveal that the ambient alkane layer suppresses the well-known phonon-induced gap near the Fermi energy, resolving a long-standing inconsistency in STM studies, where this feature is often absent. Furthermore, we show that the presence of the contamination layer alters the current-distance ($I(z)$) characteristics, flattening its exponential decay by a factor of 1.5 to 5 compared to the clean surface. This change arises from extra conductance channels through the alkane layer alongside the tunnel junction, as the tip penetrates the contaminant overlayer. Finally, based on the $I(z)$ characteristics, we provide a practical guide to detect the presence of surface contamination in STM measurements.