Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and ${\mathrm{MoS}}_2$ Probed by the Emission of Electrons

We compare the ion-induced electron emission from freestanding monolayers of graphene and ${\mathrm{MoS}}_2$ to find a sixfold higher number of emitted electrons for graphene even though both materials have similar work functions. An effective single-band Hubbard model explains this finding by a charge-up in ${\mathrm{MoS}}_2$ that prevents low energy electrons from escaping the surface within a period of a few femtoseconds after ion impact. We support these results by measuring the electron energy distribution for correlated pairs of electrons and transmitted ions. The majority of emitted primary electrons have an energy below 10 eV and are therefore subject to the dynamic charge-up effects at surfaces.

Figure 1

(a) Schematic of the experimental setup including an electron statistics (PIPS) detector, an HEA as well as a pair of deflection plates for charge state separation on the DLD. (b) Electron emission spectra for 130 keV ${\mathrm{Xe}}^{35+}$ ions transmitted through ${\mathrm{MoS}}_2$, SLG, and the targets’ QF support. (c) Mean number of emitted electrons from ${\mathrm{MoS}}_2$ and SLG for 87 and 130 keV Xe ions. (d) Potential and kinetic energy loss of 130 keV Xe ions in ${\mathrm{MoS}}_2$ and SLG. Lines are shown to guide the eye.

Figure 2

HCI-monolayer interaction for 113 keV ${\mathrm{Xe}}^{32+}$ centrally passing through SLG (top, blue) and ${\mathrm{MoS}}_2$ (bottom, violet). (a) Ion-induced change of local charge neutrality in the target, obtained from the model’s electron density averaged over equally colored lattice sites (cf. inset). (b) Electrostatic potential, $V(\mathbf{r},t)$, induced by the target’s charge density at $t=2\mathrm{fs}$, versus radial coordinate $x$, at four distances $z$ from the monolayer. The arrows indicate the position of the innermost honeycomb from which electron emission is expected to occur primarily.

Figure 3

(a) Computed detection ratio of electrons emitted from SLG and ${\mathrm{MoS}}_2$ as a function of their initial kinetic energy at a distance of $0.8\AA$ from the monolayer, following from a classical trajectory analysis using the time-dependent potential presented in Fig. 2. The time stamps define the emission time of the electrons (cf. Fig. 2). (b) Experimentally determined electron energy distribution of SLG and ${\mathrm{MoS}}_2$ induced by 190 keV ${\mathrm{Xe}}^{20+}$ ion impacts.

Figure 4

(a) Molecular dynamics simulations for computation of the ratio of detected ${\mathrm{MoS}}_2/\text{graphene}$ electrons in dependence of the electron energy using the potentials shown in Fig. 2 for $t=0$. The emission origin stems from averaging probability distributions for various orbitals and a charge density obtained from STM measurements [72], respectively. Experimental data points are added in black circles. (b) Energy spectrum of graphene and ${\mathrm{MoS}}_2$ for the ground state and $t=-1\mathrm{fs}$ before the impact of a 113 keV ${\mathrm{Xe}}^{32+}$ ion. The latter is a highly nonequilibrium state from which complex charge transfer processes and electron emission start. Vertical lines indicate the work functions $\varphi$ of the respective materials. G1–G2 simulations with second order Born self-energies [5] for honeycomb targets containing 96 sites.