Highly Sensitive Detection of Surface and Intercalated Impurities in Graphene by LEIS

Low-energy ion scattering (LEIS) is known for its extreme surface sensitivity, as it yields a quantitative analysis of the outermost surface as well as highly resolved in-depth information for ultrathin surface layers. Hence, it could have been generally considered to be a suitable technique for the analysis of graphene samples. However, due to the low scattering cross section for light elements such as carbon, LEIS has not become a common technique for the characterization of graphene. In the present study we use a high-sensitivity LEIS instrument with parallel energy analysis for the characterization of CVD graphene transferred to thermal silica/silicon substrates. Thanks to its high sensitivity and the exceptional depth resolution typical of LEIS, the graphene layer closure was verified, and different kinds of contaminants were detected, quantified, and localized within the graphene structure. Utilizing the extraordinarily strong neutralization of helium by carbon atoms in graphene, LEIS experiments performed at several primary ion energies permit us to distinguish carbon in graphene from that in nongraphitic forms (e.g., the remains of a resist). Furthermore, metal impurities such as Fe, Sn, and Na located at the graphene-silica interface (intercalated) are detected, and the coverages of Fe and Sn are determined. Hence, high-resolution LEIS is capable of both checking the purity of graphene surfaces and detecting impurities incorporated into graphene layers or their interfaces. Thus, it is a suitable method for monitoring the quality of the whole fabrication process of graphene, including its transfer on various substrates.