Ossi Lehtinen

Two-Dimensional Transition Metal Dichalcogenides under Electron Irradiation: Defect Production and Doping

Using first-principles atomistic simulations, we study the response of atomically thin layers of transition metal dichalcogenides (TMDs)--a new class of two-dimensional inorganic materials with unique electronic properties--to electron irradiation. We calculate displacement threshold energies for atoms in 21 different compounds and estimate the corresponding electron energies required to produce defects. For a representative structure of MoS2, we carry out high-resolution transmission electron microscopy experiments and validate our theoretical predictions via observations of vacancy formation under exposure to an 80 keV electron beam. We further show that TMDs can be doped by filling the vacancies created by the electron beam with impurity atoms. Thereby, our results not only shed light on the radiation response of a system with reduced dimensionality, but also suggest new ways for engineering the electronic structure of TMDs.

Spin-half paramagnetism in graphene induced by point defects

T he possibility to induce a magnetic response in graphene by the introduction of defects has been generating much interest, as this would expand the already impressive list of its special properties and allow novel devices where charge and spin manipulation could be combined. So far there have been many theoretical studies (for reviews, see refs 1‐3) predicting that point defects in graphene should carry magnetic moments B and these can in principle couple (anti)ferromagnetically 1‐12 . However, experimental evidence for such magnetism remains both scarce and controversial 13‐16 . Here we show that point defects in graphene—(1) fluorine adatoms in concentrations x gradually increasing to stoichiometric fluorographene CFxD1:0 (ref. 17) and (2) irradiation defects (vacancies)—carry magnetic moments with spin 1=2. Both types of defect lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene’s low-temperature magnetic properties, despite the fact that the maximum response we could achieve was limited to one moment per approximately 1,000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by the loss of graphene’s structural stability. Our work clarifies the controversial issue of graphene’s magnetism and sets limits for other graphitic compounds. The emerging consensus that magnetism in carbon-based systems can exist is based mostly on a large body of work on magnetic measurements of highly-oriented pyrolytic graphite (HOPG) and carbon films, with many reports of weak ferromagnetic signals at room temperature (T) observed in both pristine HOPG and after itsionirradiation(see,forexample,refs18,19).However,thewhole subject remains controversial, especially concerning (1) the role of possible contamination and (2) the mechanism responsible for the strong interaction required to lead to ferromagnetic ordering at room temperature. Some observations of ferromagnetism are probably artefacts, doing little justice to the subject (one frequent artefact is identified and described in the Supplementary Information, where we show that commonly used HOPG crystals contain micrometre-sized magnetic particles). Adatom magnetism in graphite is also contentious and, for example, different studies of fluorinatedgraphitehavereportedinconsistentresults 20,21 .

Atom-by-Atom Observation of Grain Boundary Migration in Graphene

Grain boundary (GB) migration in polycrystalline solids is a materials science manifestation of survival of the fittest, with adjacent grains competing to add atoms to their outer surfaces at each others expense. This process is thermodynamically favored when it lowers the total GB area in the sample, thereby reducing the excess free energy contributed by the boundaries. In this picture, a curved boundary is expected to migrate toward its center of curvature with a velocity proportional to the local radius of boundary curvature (R). Investigating the underlying mechanism of boundary migration in a 3D material, however, has been reserved for computer simulation or analytical theory, as capturing the dynamics of individual atoms in the core region of a GB is well beyond the spatial and temporal resolution limits of current characterization techniques. Here, we similarly overcome the conventional experimental limits by investigating a 2D material, polycrystalline graphene, in an aberration-corrected transmission electron microscope, exploiting the energy of the imaging electrons to stimulate individual bond rotations in the GB core region. The resulting morphological changes are followed in situ, atom-by-atom, revealing configurational fluctuations that take on a time-averaged preferential direction only in the presence of significant boundary curvature, as confirmed by Monte Carlo simulations. Remarkably, in the extreme case of a small graphene grain enclosed within a larger one, we follow its shrinkage to the point of complete disappearance.

Dual origin of defect magnetism in graphene and its reversible switching by molecular doping

Control of magnetism by applied voltage is desirable for spintronics applications. Finding a suitable material remains an elusive goal, with only a few candidates found so far. Graphene is one of them and attracts interest because of its weak spin-orbit interaction, the ability to control electronic properties by the electric field effect and the possibility to introduce paramagnetic centres such as vacancies and adatoms. Here we show that the magnetism of adatoms in graphene is itinerant and can be controlled by doping, so that magnetic moments are switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions; one from itinerant magnetism and the other from dangling bonds. Our work suggests that graphenes spin transport can be controlled by the field effect, similar to its electronic and optical properties, and that spin diffusion can be significantly enhanced above a certain carrier density.

Atomic scale study of the life cycle of a dislocation in graphene from birth to annihilation

Dislocations, one of the key entities in materials science, govern the properties of any crystalline material. Thus, understanding their life cycle, from creation to annihilation via motion and interaction with other dislocations, point defects and surfaces, is of fundamental importance. Unfortunately, atomic-scale investigations of dislocation evolution in a bulk object are well beyond the spatial and temporal resolution limits of current characterization techniques. Here we overcome the experimental limits by investigating the two-dimensional graphene in an aberration-corrected transmission electron microscope, exploiting the impinging energetic electrons both to image and stimulate atomic-scale morphological changes in the material. The resulting transformations are followed in situ, atom-by-atom, showing the full life cycle of a dislocation from birth to annihilation. Our experiments, combined with atomistic simulations, reveal the evolution of dislocations in two-dimensional systems to be governed by markedly long-ranging out-of-plane buckling.

Atomic scale microstructure and properties of Se-deficient two-dimensional MoSe2.

We study the atomic scale microstructure of nonstoichiometric two-dimensional (2D) transition metal dichalcogenide MoSe2-x by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe2 grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during in situ removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire. Our results on Se-deficient MoSe2 suggest routes toward engineering the properties of 2D transition metal dichalcogenides by deviations from the stoichiometric composition.

The pristine atomic structure of MoS2 monolayer protected from electron radiation damage by graphene

Materials can, in principle, be imaged at the level of individual atoms with aberration-corrected transmission electron microscopy. However, such resolution can be attained only with very high electron doses. Consequently, radiation damage is often the limiting factor when characterizing sensitive materials. Here, we demonstrate a simple and an effective method to increase the electron radiation tolerance of materials by using graphene as protective coating. This leads to an improvement of three orders of magnitude in the radiation tolerance of monolayer MoS2. Further on, we construct samples in different heterostructure configurations to separate the contributions of different radiation damage mechanisms.

Ion‐Irradiation‐Induced Defects in Isotopically‐Labeled Two Layered Graphene: Enhanced In‐Situ Annealing of the Damage

Contrary to theoretical estimates based on the conventional binary collision model, experimental results indicate that the number of defects in the lower layer of the bi-layer graphene sample is smaller than in the upper layer. This observation is explained by in situ self-annealing of the defects.

Ion impacts on graphene/Ir(111): interface channeling, vacancy funnels, and a nanomesh.

By combining ion beam experiments and atomistic simulations we study the production of defects in graphene on Ir(111) under grazing incidence of low energy Xe ions. We demonstrate that the ions are channeled in between graphene and the substrate, giving rise to chains of vacancy clusters with their edges bending down toward the substrate. These clusters self-organize to a graphene nanomesh via thermally activated diffusion as their formation energy varies within the graphene moiré supercell.

Dry-cleaning of graphene

Studies of the structural and electronic properties of graphene in its pristine state are hindered by hydrocarbon contamination on the surfaces. Also, in many applications, contamination reduces the performance of graphene. Contamination is introduced during sample preparation and is adsorbed also directly from air. Here, we report on the development of a simple dry-cleaning method for producing large atomically clean areas in free-standing graphene. The cleanness of graphene is proven using aberration-corrected high-resolution transmission electron microscopy and electron spectroscopy.