Fluorination of graphene leads to susceptibility for nanopore formation by highly charged ion impact

The formation of nanopores by highly charged ion impacts on freestanding fluorine-functionalized graphene is demonstrated. The process is driven by potential sputtering, which becomes active by changing the semimetallic property of graphene into a strongly insulating state by fluorination. The interaction of fluorographene with highly charged ions is also studied in terms of charge exchange and kinetic energy loss. A higher number of captured electrons and a larger kinetic energy loss than in pristine graphene are observed, which can be well explained by an increase in the ion neutralization length and in the atomic areal density of the target, respectively. Using a computer code based on a time-dependent scattering potential model, a connection between the efficiency of charge exchange and the fluorine coverage is revealed. Our results suggest a competition of two distinct nanostructure formation processes, leading either to pore formation or fluorine desorption.

Figure 1

A highly charged xenon ion with initial charge state $q_{\text{in}}$ is transmitted through fluorographene. During neutralization, the ion captures and stabilizes a number of $n_{\text{cap}}$ electrons, leading to a reduction of the ion charge state accordingly. Corresponding potential energy deposition in the target and subsequent energy dissipation lead to nanopore formation. Fluorine atoms are shown in blue-gray, carbon atoms in brown.

Figure 2

Raman spectra as a function of fluorination time. Spectra are stacked for better visualization.

Figure 3

Examples of STEM images of fluorographene before (a) and after (b) irradiations with highly charged xenon ions. White spots denote iron-containing clusters left from transfer. Darker areas indicate pores. Both images have the same scale bar.

Figure 4

Distributions of the pore radii before and after highly charged ion irradiations.

Figure 5

(a) Exit charge-state spectra of ${\mathrm{Xe}}^{30+}$ ions with an initial kinetic energy of 112 keV transmitted through fluorographene and graphene. Mean exit charge states of 11 and 15, respectively, are indicated. (b) The kinetic energy loss determined from the spectra in (a) is shown for both target materials as a function of the exit charge state. Dashed lines mark the mean exit charge states. The diameter of the symbols indicates the ion abundance.

Figure 6

Number of electrons captured by ${\mathrm{Xe}}^{30+}$ ions over the ion inverse velocity after transmission through fluorographene and graphene. Evaluated spectra were measured within the first 48 h of consecutive irradiations. Lines are added and indicate fits (see text) to the values obtained from tdpot simulations assuming a fluorine coverage of 0%, 40%, and 100%, respectively.

Figure 7

(a) Experimentally obtained exit charge-state spectra of ${\mathrm{Xe}}^{30+}$ ions at ${\mathrm{E}}_{\text{kin}}=88$ keV transmitted through fluorographene measured on day 1 and on day 10 of consecutive irradiations. The mean exit charge states of 9 and 12, respectively, are indicated. (b) Dependence of the mean exit charge on the fluorine coverage obtained from tdpot simulations for the same irradiation conditions as given in (a).

Figure 8

Time dependence of the relative fluorine coverage estimated by charge-exchange measurements, showing a continuous decrease over time. The solid line was obtained by an exponential fit for eye guiding (see text). On day 10 of the consecutive irradiations, the beam position on the sample was changed to a different target spot, for which a fluorine coverage of $\approx 50\%$ was determined. Compared to the already irradiated region revealing a coverage of $\approx 25\%$, the lower value indicates enhanced fluorine loss due to ion irradiation.

Figure 9

Velocity dependence of the kinetic energy loss (averaged over the exit charge state) for ${\mathrm{Xe}}^{30+}$ ions transmitted through fluorographene, which was measured within the first 48 h and after 10 days of consecutive irradiations. Values for pristine graphene are shown as reference. The solid line is a fit to the data for graphene, assuming a proportional dependence on the velocity. Dashed lines were obtained by multiplying the fit function with a factor of 1.2 and 1.7, respectively (see text).