Despite advances in single-atom doping, the controlled assembly of multi-atom defects in 2D materials remains difficult, limiting the ability to tailor their electronic, magnetic, or catalytic functionality. Here, we demonstrate that ultralow-energy (82 eV) implantation of mass-selected Mn2 cluster ions into monolayer graphene enables the formation of substitutional Mn dimers. Low-temperature scanning tunneling microscopy and spectroscopy, supported by density-functional theory calculations, reveal that these Mn–Mn pairs exhibit a characteristic four-lobe signature and distinct spectroscopic fingerprints. Molecular dynamics simulations show that Mn2 dimers fragment upon impact, displacing several carbon atoms and enabling recombination events that yield substitutional dimers. These results show that cluster-ion implantation can create multi-atom defects in 2D materials, extending ultralow-energy implantation beyond single-atom doping. This provides access to a broader range of defects relevant to spintronics, quantum technologies, and catalysis.

Controlling the spatial incorporation of carbon into hexagonal boron nitride (hBN) is essential for engineering optically active defects, yet existing approaches lack nanoscale precision and control over the carbon supply. Here, we demonstrate a method for carbon doping of hBN using electron-beam irradiation in a low-pressure methane atmosphere, where the beam simultaneously generates vacancies and decomposes methane into individual carbon and hydrogen atoms. Using annular dark-field scanning transmission electron microscopy, we show that increasing the methane partial pressure suppresses pore growth and drives the formation of triangular boron-terminated pores through preferential hydrogen etching of nitrogen. Time-resolved electron energy-loss spectroscopy (EELS) mapping reveals progressive carbon incorporation into the lattice, accompanied by boron and nitrogen depletion. Carbon clustering occurs predominantly within the irradiated area: 84+-7

Strain is a key tuning parameter in solid-state systems, but most studies focus on strain fields extending over tens of nanometers or more. Here we investigate the extreme limit of ultra-localized strain in graphene, introduced through bond defects generated by ultralow-energy implantation of noble gas ions. Using molecular dynamics simulations, Raman spectroscopy, and scanning tunneling microscopy, we identify the formation and thermal stability of bond defects that locally stretch only a few C-C bonds without removing or substituting atoms. Tight-binding calculations reveal that such bond defects induce local charge trapping, leading to substantial Fermi-level shifts. Synchrotron-based angle-resolved photoemission spectroscopy directly confirms these predictions: even at modest defect densities (similar to 1012 cm-2), the graphene Fermi level shifts by up to 0.3 eV. This strong effect is remarkable given that it is achieved without altering graphene's composition, in contrast to conventional impurity doping or vacancy formation. Upon thermal annealing, the electronic structure recovers towards the pristine state, showing that these effects can be tuned and reversed. Our results establish bond defects as a new class of functional disorder in graphene, capable of strongly modifying its electronic properties solely by bond rearrangement.

Compared to bulk solids, defects in low-dimensional materials and, specifically, 2D systems are expected to have a stronger effect, detrimental or beneficial, on their properties. Owing to their geometry, defects in 2D materials can easily be formed due to the interaction with the environment or under impacts of energetic particles, such as ions and electrons. At the same time, many concepts of defect production under irradiation in bulk systems are not applicable for 2D materials or require substantial modifications. Various aspects of the physics and chemistry of defects in 2D materials have been addressed, and the results of these investigations are presented in hundreds of research papers and review articles. However, the challenges and open questions that still remain in the field have received relatively little attention. These topics were recently addressed at the symposium “Defect-mediated engineering of nanomaterials for energy and quantum applications” organized by the Beilstein-Institut. Following the discussions at the symposium, here, we present the challenges and open questions in our understanding of the behavior of defective 2D materials, interaction of energetic particles with low-dimensional targets, and defect-mediated engineering of the properties of 2D systems. We further discuss possible solutions to these problems or suggest “work-arounds”, which should accelerate the progress in the field.

Surface contamination strongly affects the intrinsic properties of nanoscale materials, making its reliable identification and quantification crucial for both accurate experimental interpretation and nanofabrication. Although scanning transmission electron microscopy can resolve contaminants at atomic resolution within nanometer-scale regions, it cannot easily provide a quantitative, large-area contamination measure. Here, we introduce a minimally destructive recoil-projectile coincidence method for ion transmission experiments that enables element-specific identification and quantification of surface contaminants with isotopic resolution. We demonstrate this approach by comparing self-supporting graphene samples prepared using either a polymethylmethacrylate (PMMA)-based or a PMMA-free transfer process. Carbon and hydrogen are identified as the dominant surface contaminants. PMMA-free transferred graphene exhibits the lowest native contamination levels. Following in-situ thermal annealing at 400 °C for 1 h, the measured carbon areal density approaches the value expected for atomically clean single-layer graphene within the experimental uncertainty, while hydrogen coverage is strongly reduced. Unlike PMMA-transferred graphene, which rapidly recontaminates after annealing, PMMA-free transferred graphene remains nearly contamination-free for at least 140 min under ultra-high vacuum conditions (p_base = 2 × 10^-8 mbar). Beyond graphene, the presented method establishes a quantitative characterization platform for ultrathin materials, enabling studies of surface cleanliness, adsorption, implantation and surface interaction dynamics in such systems.

In this work, a general route to creating two-dimensional, one-atom-thick metal layers, metallene, on functionalized graphene is proposed. To explore its viability, low-energy ion irradiation is performed to introduce vacancies into initially pristine graphene, followed by ultralow-energy gold irradiation to deposit individual gold atoms onto it. While gold freely migrates on pristine graphene, vacancies provide anchoring points where gold atoms gather and promote the growth of atomically thin nanoplatelets. The physical and chemical structure of the gold flakes is confirmed through atomic-resolution scanning transmission electron microscopy and electron energy loss spectroscopy, while their formation is investigated using ab initio simulations. The thickness and diameter of the gold flakes are directly proportional to gold ion fluence during ultralow-energy ion irradiation. Gold atoms in small gold structures are arranged in a one-atom-thick hexagonal lattice. Larger goldene platelets with lateral sizes in the range of tens of nanometers contain multiple gold layers. Mono- and few-layer flakes are metastable under continuous 60 keV electron irradiation during imaging but occasionally rotate and take small jumps as the atoms at the edges move. A reversible transformation between a flat monolayer and an amorphous three-dimensional gold cluster is observed in the experiments and is also seen in the simulations.

Fabricating dispersed single atoms and size-controlled metal nanoclusters remains a difficult challenge due to sintering. Here, we demonstrate that atoms and clusters can be immobilized using atomically clean defect-engineered graphene as the matrix. The graphene is first cleaned of surface contamination with laser heating, after which low-energy Ar irradiation is used to create spatially well-separated vacancies into it. Metal atoms are then evaporated either via thermal or ebeam evaporation onto graphene, where they diffuse until being trapped into a vacancy. The density of embedded structures can be controlled through irradiation dose, and the size of the structures through evaporation time. The resulting structures are confirmed through atomic-resolution scanning transmission electron microscopy and electron energy loss spectroscopy. We demonstrate here incorporation of Al, Ti, Fe, Ag and Au single atoms or nanoclusters, but the method should work equally well for other elements.

Carbon nano-onions (CNOs) are a unique class of carbon nanomaterials, with a concentric fullerene-like structure and sp2-hybridized carbon atoms. Onions present high mechanical strength, good conductivity, the capability to intercalate alkali metals, showing promising applications in diverse fields, such as tribology, energy storage, and nanomedicine. Their production has been developed by several techniques, with the most interesting being the thermal annealing of carbon nanodiamonds because of the easy scalability (gram-scale) and morphologic control (size and shape form perfectly spherical to polygonal). However, this synthesis is commonly affected by the formation of strong aggregates larger than 100 nm, attributed to carbon soot formation or strong van der Waals interactions, hindering the work with small and individual particles, leading to an altered yield of functionalization. In this study we propose a method for CNO individualization based on strong acid treatment, yielding highly water dispersible nanoparticles. Analytical ultracentrifugation (AUC) analysis is employed as a technique to investigate the particle dimensions directly in dispersion, permitting an ensemble analysis free from further sample preparation bias. Moreover, we also provide a comprehensive evaluation of the common post-synthetic treatment methods, and their effects. By means of scanning transmission electron microscopy with medium angle annular dark field detector and electron energy loss spectroscopy (STEM-MAADF and EELS) we have shown images of individual CNOs. Their successful separation achieved in this study is significant for future research and applications in nanomedicine, electrochemistry, and materials composites, where sample homogeneity is critical.

The van der Waals atomic solids of noble gases on metals at cryogenic temperatures were the first experimental examples of two-dimensional systems. Recently, such structures have also been created on surfaces under encapsulation by graphene, allowing studies at elevated temperatures through scanning tunnelling microscopy. However, for this technique, the encapsulation layer often obscures the arrangement of the noble gas atoms. Here we create Kr and Xe clusters in between two suspended graphene layers, and uncover their atomic structure through transmission electron microscopy. We show that small crystals (N < 9) arrange on the basis of the simple non-directional van der Waals interaction. Larger crystals show some deviations, possibly enabled by deformations in the encapsulating graphene lattice. We further discuss the dynamics of the clusters within the graphene sandwich, and show that although all the Xe clusters with up to N ≈ 100 remain solid, Kr clusters with already N ≈ 16 turn occasionally fluid under our experimental conditions (under a pressure of ~0.3 GPa). This study opens a way for the so-far unexplored frontier of encapsulated two-dimensional van der Waals solids with exciting possibilities for fundamental condensed-matter physics research and possible applications in quantum information technology.

Journal Article Single Heteroatom Configurations in Graphene and Diamond Get access A Trentino, A Trentino University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar G Zagler, G Zagler University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar M Längle, M Längle University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar D Propst, D Propst University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar E H Ahlgren, E H Ahlgren University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar C Mangler, C Mangler University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar K Mustonen, K Mustonen University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar T Susi, T Susi University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar J Kotakoski J Kotakoski University of Vienna, Faculty of Physics, Vienna, Austria Corresponding author: jani.kotakoski@univie.ac.at Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Page 1364, https://doi.org/10.1093/micmic/ozad067.700 Published: 22 July 2023

Dispersed impurities in diamond present a flourishing platform for research in quantum informatics, spintronics and single phonon emitters. Based on the vast pool of experimental and theoretical work describing impurity atoms in diamond, we review the configurations by the chemical element discussing the relevant atomic configurations and most important properties. Dopant structures expand from single to co-doping configurations, also combined with carbon vacancies. Despite of their importance, not much is known about the exact atomic configurations associated with the dopant structures beyond computational models, partially due to difficulties in their microscopic observation. To assess the visibility of these structures, we carry out image simulations to show that the heavier dopants may be easily discernible in scanning transmission electron microscopy annular dark field images, with a window of visibility of up to over ± 10 nm in defocus. We further present the first atomic resolution images of an impurity atom configuration (substitutional Er atom) in the diamond lattice, confirmed by a comparison to the simulated images. Overall, our results demonstrate that there is a vast research field waiting for the microscopy community in resolving the exact atomic structure of various impurity atom configurations in diamond.

As a one-atom thick, mechanically strong, and chemically stable material with unique electronic properties, graphene can serve as the basis for a large number of applications. One way to tailor its properties is the controlled introduction of covalently bound heteroatoms into the lattice. In this study, we demonstrate efficient implantation of individual gold atoms into graphene up to a concentration of 1.7 × 1011 atoms cm−2 via a two-step low-energy ion implantation technique that overcomes the limitation posed by momentum conservation on the mass of the implanted species. Atomic resolution scanning transmission electron microscopy imaging and electron energy-loss spectroscopy reveal gold atoms occupying double vacancy sites in the graphene lattice. The covalently bound gold atoms can sustain intense electron irradiation at 60 kV during the microscopy experiments. At best, only limited indication of plasmonic enhancement is observed. The method demonstrated here can be used to introduce a controlled concentration of gold atoms into graphene, and should also work for other heavier elements with similar electronic structure.

Oxidation is the main cause of degradation of many 2D materials, including transition metal dichalcogenides (TMDs), under ambient conditions. Some of the materials are more affected by oxidation than others. To elucidate the oxidation‐induced degradation mechanisms in TMDs, the chemical effects in single layer MoS 2 and MoTe 2 are studied in situ in an electron microscope under controlled low‐pressure oxygen environments at room temperature. MoTe 2 is found to be reactive to oxygen, leading to significant degradation above a pressure of 1 × 10 −7 torr. Curiously, the common hydrocarbon contamination found on practically all surfaces accelerates the damage rate significantly, by up to a factor of forty. In contrast to MoTe 2 , MoS 2 is found to be inert under oxygen environment, with all observed structural changes being caused by electron irradiation only, leading to well‐defined pores with high proportion of molybdenum nanowire‐terminated edges. Using density functional theory calculations, a further atomic‐scale mechanism leading to the observed oxygen‐related degradation in MoTe 2 is proposed, and the role of the carbon in the etching is explored. Together, the results provide an important insight into the oxygen‐related deterioration of 2D materials under ambient conditions relevant in many fields.

Ultralow energy (ULE) ion implantation is being increasingly applied to the modification of 2D materials, in particular, for substitutional doping and intercalation of graphene. Implantation-induced defects, whether desired or not, have a strong impact on the properties of graphene. Significant research has been devoted to vacancy-related defects however, disorder induced by ion irradiation in the ULE limit, that is, for energies below the vacancy-formation threshold, remains poorly understood. Here, we focus on that regime and report the formation of defects resulting from the breaking of C-C sp2 bonds and formation of C-substrate bonds. As a model system we used epitaxial graphene grown on Cu(111) and on Pt(111), subsequently implanted with He, Ne and Ar at energies between 15 eV and 40 eV. The bond defect density is found to increase with increasing energy and atomic number of the implanted element. These findings significantly advance our understanding of disorder induced in graphene by ULE ion implantation, while simultaneously revealing the potential for exploiting such bond defects for physical or chemical functionalization. In particular, these bond defects can be generated with a high degree of selectivity since they occur in the low-energy limit (at least down to 15 eV), significantly below the energies required to form stable vacancies.

Substituting heteroatoms into graphene can tune its properties for applications ranging from catalysis to spintronics. The further recent discovery that covalent impurities in graphene can be manipulated at atomic precision using a focused electron beam may open avenues towards sub-nanometer device architectures. However, the preparation of clean samples with a high density of dopants is still very challenging. Here, we report vacancy-mediated substitution of aluminium into laser-cleaned graphene, and without removal from our ultra-high vacuum apparatus, study their dynamics under 60 keV electron irradiation using aberration-corrected scanning transmission electron microscopy and spectroscopy. Three- and four-coordinated Al sites are identified, showing excellent agreement with ab initio predictions including binding energies and electron energy-loss spectrum simulations. We show that the direct exchange of carbon and aluminium atoms predicted earlier occurs under electron irradiation, although unexpectedly it is less probable than the same process for silicon. We also observe a previously unknown nitrogen-aluminium exchange that occurs at Al─N double-dopant sites at graphene divacancies created by our plasma treatment.

Using a magnetron sputtering approach that allows size-controlled formation of nanoclusters, we have created palladium nanoclusters that combine the features of both heterogeneous and homogeneous catalysts. Here we report the atomic structures and electronic environments of a series of metal nanoclusters in ionic liquids at different stages of formation, leading to the discovery of Pd nanoclusters with a core of ca. 2 nm surrounded by a diffuse dynamic shell of atoms in [C4C1Im][NTf2]. Comparison of the catalytic activity of Pd nanoclusters in alkene cyclopropanation reveals that the atomically dynamic surface is critically important, increasing the activity by a factor of ca. 2 when compared to compact nanoclusters of similar size. Catalyst poisoning tests using mercury and dibenzo[a,e]cyclooctene show that dynamic Pd nanoclusters maintain their catalytic activity, which demonstrate their combined features of homogeneous and heterogeneous catalysts within the same material. Additionally, kinetic studies of cyclopropanation of alkenes mediated by the dynamic Pd nanoclusters reveal an observed catalyst order of 1, underpinning the pseudo-homogeneous character of the dynamic Pd nanoclusters. Establishing a structure-property relationship for nanoclusters and the link with their catalytic performance remain challenging. Here the authors show palladium nanocluster with a core of 2 nm surrounded by a diffuse dynamic shell of Pd atoms exhibit features of heterogeneous and homogenous catalyst at the same time.

We report the formation of nanobubbles on graphene with a radius of the order of 1 nm, using ultralow energy implantation of noble gas ions (He, Ne, Ar) into graphene grown on a Pt(111) surface. We show that the universal scaling of the aspect ratio, which has previously been established for larger bubbles, breaks down when the bubble radius approaches 1 nm, resulting in much larger aspect ratios. Moreover, we observe that the bubble stability and aspect ratio depend on the substrate onto which the graphene is grown (bubbles are stable for Pt but not for Cu) and trapped element. We interpret these dependencies in terms of the atomic compressibility of the noble gas as well as of the adhesion energies between graphene, the substrate, and trapped atoms.

Recent success in the direct implantation of Ge-74(+) ion, the heaviest atomic impurity to date, into monolayer graphene presents a general question of the efficiency of low-energy ion implantation technique for heavy atoms. A comparative computational study, using classical molecular dynamics, of low-energy Ge and Pt ions implantation into single- and double-layer graphene is presented. It confirms that the highest probability for the perfect substitutional doping of single-layer graphene, i.e., direct implanting of ion into monovacancy, can be achieved 80 eV and it reaches the value of 64% for Ge ions directed at 45 degrees angle to graphene plane and 21% for Pt ion beam perpendicular to graphene. Implantation efficiency is strongly dependent on the angle of ion beam. The sputtering yield of carbon atoms is found to be lower for double layer of graphene, which has better protective properties against low-energy ion irradiation damage than a single graphene layer. In double-layer graphene, incident ions traveling in the direction perpendicular to graphene can be trapped between the layers with the highest efficiency above or equal to 80% in the energy range of 40-90 eV for Ge ions and above 90% in the energy range of 40-70 eV for Pt ions. The energy range corresponding to the efficient trapping of ions in double-layer graphene is shifted toward higher energies upon tilting of the angle of incident ion beam.