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

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 the atmospheric contamination is severely limited, even after the last decades of intense research in the field. 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.

Effect of alkane adsorption on the electrochemical properties of graphene

“…Previous studies have pointed to the presence of a self-organized alkane layer on the surface of graphene-like materials, which is formed during a few days of air exposure. 16,19 Further studies showed that the alkane layer adsorbed on the surface of highly oriented pyrolytic graphite remained stable in aqueous electrolyte and led to a reduction in capacitance. 26 So, it is very likely that the alkane molecules are responsible for our results.…”

“…The surface of graphene is covered by saturated alkane molecules, which consist of a carbon chain containing 23 carbon atoms. This conguration is inspired by the research conducted by Palinkas et al 19 The electried graphene-alkane/water interfaces were modeled by fully lling the vacuum region with water molecules (the water density of the bulk region was kept close to 1 g cm −3 ). To represent Helmholtz layers in the models, Na atoms were initially placed in the vicinity of both graphene-alkane surfaces, as shown in Fig.…”

“…Subsequent studies showed that the hydrophilicity of fresh graphene decreased with the adsorption of alkanes, with no relationship to the substrate type and number of graphene layers. 17,18 In 2022, Palinkas et al 19 directly observed alkanes adsorbed on the graphene surface by scanning tunneling microscopy, and they pointed out that the common contamination on the surface of graphene-like materials consists of a self-organizing layer of alkane molecules, which is formed over several days of air exposure. This study supports previous research on the wettability of graphene.…”