The composition and structure of the ubiquitous hydrocarbon contamination on van der Waals materials

TL;DR

This study reveals that ambient hydrocarbon contamination on van der Waals materials forms a self-organized alkane layer within days, significantly affecting surface properties and environmental interactions.

Contribution

It provides the first detailed atomic and molecular characterization of ambient hydrocarbon contamination on vdW materials, highlighting its self-organizing nature and composition.

Findings

Contamination forms a self-organized alkane layer within days.

The layer consists of normal alkanes with 20-26 carbons.

This layer displaces other airborne contaminants on vdW surfaces.

Abstract

The behavior of single layer van der Waals (vdW) materials is profoundly influenced by the immediate atomic environment at their surface, a prime example being the myriad of emergent properties in artificial heterostructures. Equally significant are adsorbates deposited onto their surface from ambient. While vdW interfaces are well understood, our knowledge regarding atmospheric contamination is severely limited. Here we show that the common ambient contamination on the surface of: graphene, graphite, hBN and MoS2 is composed of a self-organized molecular layer, which forms during a few days of ambient exposure. Using low-temperature STM measurements we image the atomic structure of this adlayer and in combination with infrared spectroscopy identify the contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms. Through its ability to self-organize, the alkane layer displaces the manifold other airborne contaminant species, capping the surface of vdW materials and possibly dominating their interaction with the environment.