Ultrafast electronic response of graphene to a strong and localized electric field

Abstract

The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 1012 A cm−2, the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.

Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS2 Films

“…A key drawback toward their elucidation via the data presented in Figures 1–5 is however that the lattice of the supporting graphene is not resolved in these images due to nonoptimal imaging conditions for the lighter carbon ( Z C = 6) as well as static residual adventitious carbon contamination which is typical 5,36,68 for graphene samples from sample transport and storage in air. This precludes the assessment of orientational relations between the underlying graphene and the crystallizing MoS 2 in Figures 1–4, despite the interesting observation that after 17 min electron beam exposure, the FT in Figure 1b shows signs of one discrete hexagonal spot pattern across several nonconnected MoS 2 crystallites.…”

Unraveling energy loss processes of low energy heavy ions in 2D materials

“…Recent experimental work showed that heavy ions, especially in high initial charge states, undergo ultrafast neutralization and de-excitation within less than 10 fs when transmitted through a freestanding monolayer of graphene 23 or 1 nm thick polymeric carbon nanomembranes 24 . The neutralization is accompanied by a strong enhancement of the ion stopping 25 , which depends on both, the ion incident charge state Q in and the amount of charge exchange ΔQ = Q in − Q out .…”

Proton energy loss in multilayer graphene and carbon nanotubes

“…of velocity), the energy loss in graphene predicted for by F. Mao is around 11 eV/Å, meanwhile, Echenique gives 3.3 eV/Å. Also, recent experiments of slow highly charged ions interacting with graphene layers report that the energy loss and charge exchange of ions in two-dimensional materials show significant differences with respect to bulk solids (24,25). The high energy losses differ by an order of magnitude with respect to results obtained by TRIM (12).…”

“…Q11 The higher energy loss for protons, found in our experiments and which recent calculations would fairly explain, can be traced to the unusual properties of graphene. Assuming that the measured energy loss is due to electronic excitations, and graphene with a high electron mobility can react very fast to the presence of the intruder, near the surface, with its subsequent fast screening (25). This phenomenon produces a very high and fast flux of electrons against the energetic proton producing a high momentum transfer which translates into a high graphene stopping power on the particle.…”