Abstract
As surface-only materials, freestanding 2D materials are known to have a high level of contamination-mostly in the form of hydrocarbons, water, and residuals from production and exfoliation. For well-designed experiments, it is of particular importance to develop effective cleaning procedures, especially since standard surface science techniques are typically not applicable. We perform ion spectroscopy with highly charged ions transmitted through freestanding atomically thin materials and present two techniques to achieve clean samples, both based on thermal treatment. Ion charge exchange and energy loss are used to analyze the degree of sample contamination. We find that even after cleaning, heavily contaminated spots remain on single layer graphene. The contamination coverage, however, clusters in strand-like structures leaving large clean areas. We present a way to discriminate clean from contaminated areas with our ion beam spectroscopy if the heterogeneity of the surface is increased sufficiently enough. We expect a similar discrimination to be necessary in most other experimental techniques.
Highlights
In the past decade, two-dimensional (2D) materials evolved to a major field of research in physics, chemistry, and materials science
We perform ion spectroscopy with highly charged ions transmitted through freestanding atomically thin materials and present two techniques to achieve clean samples, both based on thermal treatment
We present a way to discriminate clean from contaminated areas with our ion beam spectroscopy if the heterogeneity of the surface is increased sufficiently enough
Summary
Two-dimensional (2D) materials evolved to a major field of research in physics, chemistry, and materials science. The prime candidates are semi-metal graphene, semi-conducting transition metal dichalcogenides (TMDs), and insulating hexagonal boron nitride.. The prime candidates are semi-metal graphene, semi-conducting transition metal dichalcogenides (TMDs), and insulating hexagonal boron nitride.2 This variety makes 2D materials promising candidates for future applications, e.g., in electronics, optoelectronics, or molecular sieving.. Often, the properties of 2D materials are significantly worse in real applications than they should be based on their intrinsic material characteristics. The latter are sometimes even hard to determine in a controlled laboratory environment.. The main reason is that any surface and, especially, surface-only 2D materials are densely covered with contaminations made of mostly hydrocarbons, which are adsorbed as adventitious carbon from ambient air exposure during sample handling, and possibly residuals from transfer and/or production processes.. The latter are sometimes even hard to determine in a controlled laboratory environment. The main reason is that any surface and, especially, surface-only 2D materials are densely covered with contaminations made of mostly hydrocarbons, which are adsorbed as adventitious carbon from ambient air exposure during sample handling, and possibly residuals from transfer and/or production processes. Adsorbates may change material properties, e.g., due to electronic doping or by unintentional chemical functionalization and can facilitate particular processes such as self-healing of pores observed for graphene.
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