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

Abstract

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.

Single-layer graphene on epitaxial FeRh thin films

“…LEIS is known for its extreme surface sensitivity and possibility to provide quantitative composition of the outermost surface layer [40]. It has also proved its ability in detailed characterization of graphene on silica substrate [41]. We have used a 3-keV He ion beam at a scattering angle of 145° to obtain a spectrum of the FeRh sample covered by graphene ( Fig.…”

Double matrix effect in Low Energy Ion Scattering from La surfaces

“…The observed change of the La signal by 2.5 times in absence of other elements is very intriguing and is unusual for LEIS. There are only a few of matrix effects which have a stronger influence on the signal, for example matrix effect in graphitic carbon [10]. Therefore a logical check if this behavior can be explained by small quantities of other elements was performed, shown in Fig.…”

“…Matrix effects can only exist if more than one element is present (carbon with its vastly different valence zone structure of allotropes is again an exception [10]). But LEIS spectra of fresh La film show no sign of contamination.…”

“…An example of such interaction is the He + -Ga resonance [24]. When He 1s is (quasi-)resonant with two or more electronic states, a band of energy states, or has a large mismatch with the interacting level, quasiresonant neutralization (qRN) becomes much stronger than quasiresonant reionization, which leads to damping of the oscillations [25,26,10]. In the case when qRCT becomes mostly qRN, as a one-directional process it will also have a characteristic velocity of its own.…”

Growth, thermal desorption and low dose ion bombardment damage of C60 films deposited on Cu(111)

“…In particular, this is important for certain carbonaceous nanostructured surfaces, wherein the irradiation damage can be important for certain irradiation conditions. Recently [32], it has been shown that the technique is capable of checking the purity of graphene surfaces, demonstrating the benefits of the technique in screening the quality of the graphene during its fabrication process.…”