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 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.

Adsorbate Formation/Removal and Plasma‐Induced Evolution of Defects in Graphitic Materials

“…They showed that the pattern is formed by adsorption of linear alkanes with a length of 20–26 carbon atoms aligned along the zigzag direction of the graphene surface. According to Pálinkás et al, [ 39 ] the periodicity of these stripes (5 nm) at room temperature is equal to the length of the linear alkanes. Consequently, the stripes reflect the orientation of the armchair direction of the graphene lattice.…”

“…In addition to the adsorbates forming different domains, we also expect stripe-like patterns to form on graphitic surfaces upon storage. [29][30][31][32][33][34][35][36][37][38][39] Based on the results of our previous study, [40] we analyzed the effect of plasma treatment on the stripe-like patterns by using the second flexural phase images, as shown in Figure 4. The corresponding lateral drive-amplitude images, in which the stripe-like patterns are also visible, are shown in Figure S6 (Supporting Information).…”

“…[29] The periodicity of the stripes is ≈5 nm, which is in agreement with our recent work [40] and within the range of 4-6 nm mentioned by others. [29][30][31][32][33][34][35][36][37][38][39] Pálinkás et al [39] performed scanning tunneling microscopy (STM) measurements at a temperature of 9 K to clarify the origin of this stripe-like pattern. They showed that the pattern is formed by adsorption of linear alkanes with a length of 20-26 carbon atoms aligned along the zigzag direction of the graphene surface.…”

“…These both techniques were used in this study for the identification of the adsorbate species on μm-sized, substratesupported few-layer graphene/graphite flakes because conventional methods for adsorbate analysis, e.g., X-ray photoelectron spectroscopy (XPS) exhibit major issues, such as detection limits caused by the X-ray beam diameter, hydrogen atoms, and desorption of adsorbates from the sample surface in ultra-high vacuum during analysis. A detailed study about XPS on highly oriented pyrolytic graphite (HOPG) has been conducted by Pálinkás et al [39] Graphene samples have a high tendency to become covered by adsorbates, which, depending on the duration of storage and surrounding conditions, form stripe-like structures with periodicities of 4-6 nm. [29][30][31][32][33][34][35][36][37][38][39] Recently, we demonstrated (using AFM) that different types of adsorbates covering graphitic surfaces could be distinguished with high resolution by analyzing the flexural and torsional/lateral cantilever vibrational modes simultaneously (AMFlex2-OLTor1-FMLat1-FMFlex3 mode), when the tip interacted with the adsorbates.…”

Influence of Atmospheric Contaminants on the Work Function of Graphite

“…33,34 A recent study identified, using low-temperature scanning tunneling microscopy (STM) and other measurements, that the contaminants are normal alkanes with lengths of 20−26 carbon atoms when freshly-cleaved samples of two-dimensional (2D) materials were stored in plastic containers. 35 from the plastic container; the contaminants from ambient are likely much more complex. All these findings suggest that the airborne hydrocarbon contaminates are likely one of the sources of inconsistent surface potentials of graphite previously reported in the literature.…”