# Neutralization Dynamics of Slow Highly Charged Ions in 2D Materials
## 2 Experimental Results on Highly Charged Ion Transmission
### Our Work
#### 2.2.2 Single Layer Graphene  
Graphene is not only thinner than CNMs, it is in fact the thinnest solid state material. It also has a well defined crystallographic structure and, as a zero band gap semi-metal, a high electrical conductivity [52]. In this section, we show how HCI neutralization proceeds in a freestanding single layer of graphene (SLG). Samples are produced by standard chemical vapor deposition with a carbon containing gas (e.g., methane) on a Cu foil. The self-limiting growth on catalytic surfaces leads to the formation of a single layer of graphene [53]. By using a Cu foil, which is polycrystalline, several graphene domains are formed, differing in their rotational lattice alignment. As a result, the SLG we used has single crystalline domain sizes of a few hundred nanometers in diameter and grain boundaries between them. The grain boundaries limit the mechanical stress our SLG can withstand. The as-grown SLG is transferred from the Cu foil without the use of a spin-coated polymer film onto a Quantifoil TEM grid. The Quantifoil support structure is a 20-50 nm thick carbon material with regularly sized (\(d\) = 1.2-2 \(\upmu\)m) and spaced holes, which are covered by our SLG. The absence of a polymer layer during transfer is essential for our HCI transmission studies. PMMA as the most commonly used polymer cannot be entirely dissolved by acetone afterwards, leaving nm-thick residuals on the graphene layer [54]. Our samples are additionally cleaned using a method described in [54]. Samples brought into the HCI irradiation chambers were heated up to 200 \({}^{\circ}\)C, whereas no change in measured charge exchange was observed with increasing target temperature. We cannot entirely exclude remaining contamination of our freestanding SLG samples. Ex-situ performed scanning TEM (STEM) measurements indicate clean, uncontaminated areas of about 50-70% of the sheet. Due to the lack of in-situ atomically resolved microscopy, we do not know if contaminations are always present or arise again from the sample transport in air. Figure 6a,b shows an angle resolved charge exchange spectrum of Xe\({}^{20+}\) at 180 keV kinetic energy transmitted through SLG. The experimental conditions are the same as for the measurement in Figure 4c. Differences between CNM and SLG become immediately clear. First, there is only one  
Figure 5: (Color online) TEM image of a CNM after 176 keV Xe\({}^{40+}\) irradiation. Individual nanopores can be observed as a result of single ion impacts [20]. charge state distribution visible between \(Q=0\) and 8 (note the log-scale in Figure 6b and compare to Figure 4c). Secondly, the distribution peaks at about \(Q=5\), instead of 2 for CNM. This results from the reduced thickness of SLG and subsequently from a shorter interaction time of the HCIs with SLG. Varying the kinetic energy of our ions allows us to adjust the interaction time in the order of 1-10 fs [2]. Figure 6c shows the relative neutralization \(\left(\frac{Z-Q_{\text{exit}}}{Z}\right)\) of Xe\({}^{20+}\) and Xe\({}^{30+}\) for different velocities. \(Q_{\text{exit}}\) is here the mean of the exit charge state distribution (e.g., 5 in Figure 6b). For low velocities (\(v<2\times 10^{5}\) m/s) ions are almost neutralized (>90%). For higher velocities, neutralization is incomplete, but still the charge exchange is large (\(\approx\)22 captured electrons for Xe\({}^{30+}\) at \(6\times 10^{5}\) m/s). As a comparison, the equilibrium charge state \(Q_{\text{eq}}\) is also shown in Figure 6c, indicating that our HCIs are not yet in charge equilibrium. In light of the large charge exchange and potential energy release as well as the two-dimensional confinement of the ion-induced electronic excitations, visible modifications could be expected on (semi-)metallic graphene. To our surprise, SLG remains intact without observable ion-induced modifications. Figure 7 shows an atomically resolved TEM image with a field-of-view (FoV) of \(37\times 37\) nm\({}^{2}\).