Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and MoS_{2} Probed by the Emission of Electrons.

We compare the ion-induced electron emission from freestanding monolayers of graphene and 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 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.

Atomic-Scale Carving of Nanopores into a van der Waals Heterostructure with Slow Highly Charged Ions.

The growing family of 2D materials led not long ago to combining different 2D layers and building artificial systems in the form of van der Waals heterostructures. Tailoring of heterostructure properties postgrowth would greatly benefit from a modification technique with a monolayer precision. However, appropriate techniques for material modification with this precision are still missing. To achieve such control, slow highly charged ions appear ideal as they carry high amounts of potential energy, which is released rapidly upon ion neutralization at the position of the ion. The resulting potential energy deposition is thus limited to just a few atomic layers (in contrast to the kinetic energy deposition). Here, we irradiated a freestanding van der Waals MoS2/graphene heterostructure with 1.3 keV/amu xenon ions in high charge states of 38, which led to nanometer-sized pores that appear only in the MoS2 facing the ion beam, but not in graphene beneath the hole. Reversing the stacking order leaves both layers undamaged, which we attribute to the high conductivity and carrier mobility in graphene acting as a shield for the MoS2 underneath. Our main focus is here on monolayer MoS2, but we also analyzed areas with few-layer structures and observed that the perforation is limited to the two topmost MoS2 layers, whereas deeper layers remain intact. Our results demonstrate that in addition to already being a valuable tool for materials processing, the usability of ion irradiation can be extended to mono- (or bi)layer manipulation of van der Waals heterostructures when the localized potential energy deposition of highly charged ions is also added to the toolbox.

Transformation and Evaporation of Surface Adsorbents on a Graphene "Hot Plate".

Dynamic surface modification of suspended graphene at high temperatures was directly observed with in situ scanning transmission electron microscopy (STEM) measurements. The suspended graphene devices were prepared on a SiN membrane substrate with a hole so that STEM observations could be conducted during Joule heating. Current-voltage characteristics of suspended graphene devices inside the STEM chamber were measured while monitoring and controlling the temperature of graphene by estimating the electrical power of the devices. During the in situ STEM observation at high temperatures, residual hydrocarbon adsorbents that had remained on graphene effectively evaporated creating large, atomically clean graphene areas. At other places, dynamic changes in the shape, position, and orientation of adsorbents could be directly observed. The temperature of the suspended graphene sample was estimated to reach up to 2000 K during the experiment, making graphene an efficient high-temperature micrometer-sized electron-transparent hot plate for future experiments in microscopes.

Silicon Substitution in Nanotubes and Graphene via Intermittent Vacancies.

The chemical and electrical properties of single-walled carbon nanotubes (SWCNTs) and graphene can be modified by the presence of covalently bound impurities. Although this can be achieved by introducing chemical additives during synthesis, it often hinders growth and leads to limited crystallite size and quality. Here, through the simultaneous formation of vacancies with low-energy argon plasma and the thermal activation of adatom diffusion by laser irradiation, silicon impurities are incorporated into the lattice of both materials. After an exposure of ∼1 ion/nm2, we find Si-substitution densities of 0.15 nm-2 in graphene and 0.05 nm-2 in nanotubes, as revealed by atomically resolved scanning transmission electron microscopy. In good agreement with predictions of Ar irradiation effects in SWCNTs, we find Si incorporated in both mono- and divacancies, with ∼2/3 being of the first type. Controlled inclusion of impurities in the quasi-1D and -2D carbon lattices may prove useful for applications such as gas sensing, and a similar approach might also be used to substitute other elements with migration barriers lower than that of carbon.