Surface contamination not only influences but in some cases even dominates the measured properties of two-dimensional (2D) materials. Although different cleaning methods are often used for contamination removal, commonly used spectroscopic cleanliness assessment methods can leave the level of achieved cleanliness ambiguous. Therefore, despite two decades of research on 2D materials, the true cleanliness of the used samples is often left open to interpretation. In this work, free-standing monolayer graphene and hexagonal boron nitride are annealed at different temperatures in a custom-built ultra-high vacuum heating chamber, connected to a scanning transmission electron microscope via a vacuum transfer line, enabling atomically resolved cleanliness characterization as a function of annealing temperature, while eliminating the introduction of airborne contamination during sample transport. While annealing at 200 degrees C already reduces contamination significantly, it is not until 400 degrees C or higher, where over 90% of the free-standing monolayer areas are atomically clean. At this point, further contamination removal is mainly limited by defects in the material and metal contamination introduced during the sample transfer or growth. The achieved large, atomically clean areas can then be used for further nanoscale engineering steps or device processing, facilitating interaction with the material rather than contamination.

There is a growing interest in identifying the origin of single-photon emission in hexagonal boron nitride (hBN), with proposed candidates including boron and nitrogen vacancies as well as carbon substitutional dopants. Because photon emission intensity often increases with sample thickness, hBN flakes used in these studies commonly exceed 30 atomic layers. To identify potential emitters at the atomic scale, annular dark-field scanning transmission electron microscopy (ADF-STEM) is frequently employed. However, due to the intrinsic AA' stacking of hBN with vertically alternating boron and nitrogen atoms, this approach is complicated even in few-layer systems. Here, we demonstrate using STEM image simulations and experiments that, even under idealized conditions, the intensity differences between boron- and nitrogen-dominated columns and carbon substitutions become indistinguishable at thicknesses beyond 17 atomic layers (ca. 6 nm). While vacancy-type defects can remain detectable at somewhat larger thicknesses, also their detection becomes unreliable at thicknesses typically used in photonic studies. We further show that common residual aberrations, particularly threefold astigmatism, can lead to artificial contrast differences between columns, which may result in misidentification of atomic defects. We systematically study the effects of non-radially symmetric aberrations on multilayer hBN and demonstrate that even small residual threefold astigmatism can significantly distort the STEM contrast, leading to misleading interpretations.

For nearly two decades, it has been known that electron irradiation of hexagonal boron nitride (hBN) in a transmission electron microscope leads to the formation of triangular pores. This has been attributed to the lower displacement threshold energy of boron, with or without the assistance of an inelastic scattering event, typically assuming that chemical processes caused by residual gases can be neglected. In this study, in contrast to previous high-vacuum experiments, we show that electron irradiation in ultra-high vacuum leads to circular pores, whereas even small amounts of oxygen in the atmosphere during the experiment drive the pores to grow into triangle shapes with nitrogen-terminated edges. This result is shown to hold for samples of different types and from different manufacturers, and at different electron energies as well as focused scanning and defocused stationary beams. Our results explain the chemical origin of triangular pores in hBN and demonstrate a deterministic way to create atomically defined pores in this important 2D material.

Mixed-dimensionality heterostructures of low-dimensional bismuth (Bi) with two-dimensional (2D) graphene are of interest in a variety of application fields ranging from nanoelectronics, next-generation batteries, and (photo)catalysis to plasmonics. We here explore the evolution of the morphology and structure of low-dimensional Bi/graphene heterostructures by high-resolution (scanning) transmission electron microscopy ((S)TEM). To this end, we deposit low-dimensional Bi nanostructures onto suspended monolayer graphene membranes via physical vapor deposition (PVD). This enables us to study intrinsic Bi-graphene interactions, in contrast to prior work that utilized Bi on supported graphene. We find that Bi deposited onto room temperature graphene consists of grains formed by irregularly shaped β-Bi crystals with a β-Bi[001]⊥graphene(001) texture and β-Bi nanorods with a β-Bi[2-21]⊥graphene(001) texture. Importantly, both texture types show rotational van der Waals epitaxy with the supporting graphene. The room temperature depositions grow via an initial amorphous β-Bi[2-21]-like state into a closed film of β-Bi structure. For higher graphene temperatures of 150 to 250 °C during deposition, we find the formation of amorphous Bi nanoparticles (NPs) at much reduced coverage due to Bi reverse desorption at these temperatures. While the room temperature deposited Bi films remain static under the electron beam in (S)TEM, the amorphous Bi NPs from higher temperature depositions exhibit electron beam induced in situ crystallization in TEM. In parallel to observing their structural evolution during this crystallization, this also enables us to probe the evolution of plasmonic features of Bi NPs via (valence) electron energy loss spectroscopy ((V)EELS), suggesting a link between crystallization state and Bi NP surface plasmon (SP) energy.

Hexagonal boron nitride (hBN) has recently become the focus of intense research as a material that can host quantum emitters. It is known that such emission is related to point defects, but in order to conclusively correlate specific defects to their spectra, having control over the defect creation mechanism is required. Here, we prepare freestanding, monolayer hBN samples and irradiate them with ultra-low-energy (150 eV) Ar+ ions. The samples are characterized before and after irradiation via scanning transmission electron microscopy to assess the defect density and distribution. Contrary to what analytical potential molecular dynamics simulations have predicted, we predominantly observe boron single vacancies after ion irradiation, followed by double vacancies at half the count. Moreover, we also observe that vacancy filling with Si and C impurity atoms plays a more significant role in the created defects than previously assumed, potentially posing a problem for selective creation of quantum emitters in hBN.

According to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory, the transition from a solid to liquid in two dimensions proceeds through an orientationally ordered liquid-like hexatic phase. However, alternative mixed melting scenarios, in which melting proceeds through the hexatic phase with both continuous and discontinuous transitions, have also been observed in some two-dimensional systems. In this study, we imaged silver iodide embedded in multilayer graphene using time- and temperature-resolved in situ atomic-resolution scanning transmission electron microscopy and nanobeam electron diffraction. We observed the hexatic phase and provide evidence supporting a mixed melting scenario.

We substitute individual Pt atoms into monolayer MoS2 and study the resulting atomic structures with single-sideband ptychography (SSB) supported by ab initio simulations. We demonstrate that while high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging provides excellent Z-contrast, distinguishing some defect types such as single and double sulfur vacancies remains challenging due to their low relative contrast difference. However, SSB with its nearly linear Z-contrast and high phase sensitivity enables reliable identification of these defect configurations, as well as various Pt dopant structures at significantly lower electron doses. Our findings uncover the precise atomic placement and highlight the potential of SSB for detailed structural analysis of dopant-modified 2D materials while minimizing beam-induced damage, offering new pathways for understanding and engineering atomic-scale features in 2D systems.

We measure the two-dimensional elastic modulus E^{2D} of atomically clean defect-engineered graphene with a known vacancy distribution and density in correlated ultrahigh vacuum experiments. The vacancies are introduced via low-energy (<200  eV) Ar ion irradiation, and the atomic structure is obtained via semiautonomous scanning transmission electron microscopy and image analysis. Based on atomic force microscopy nanoindentation measurements, a decrease of E^{2D} from 286 to 158  N/m is observed when measuring the same graphene membrane before and after introducing vacancies at a density of 1.0×10^{13}  cm^{-2}. This decrease is significantly greater than what is predicted by most theoretical studies and in stark contrast to some measurements presented in the literature. With the assistance of atomistic simulations, we show that this softening is mostly due to corrugations caused by local strain at vacancies with two or more missing atoms, while the influence of single vacancies is negligible. We further demonstrate that the opposite effect can be measured when surface contamination is not removed before defect engineering.

The effect of electron irradiation on two-dimensional (2D) materials is an important topic, both for the correct interpretation of electron microscopy experiments and for possible applications in electron lithography. After the importance of including inelastic scattering damage in theoretical models describing beam damage and the lack of oxygen sensitivity under electron irradiation in 2D MoS2 were recently shown, the role of temperature has remained unexplored on a quantitative level. Here we show the effect of temperature on the creation of individual defects and the effect of temperature on defect dynamics. Based on the measured displacement cross section of sulfur atoms in MoS2 by atomic resolution scanning transmission electron microscopy, we find an increased probability for defect creation for temperatures up to 150∘C, in accordance with theoretical predictions. However, higher temperatures lead to a decrease of the observed cross sections. Despite this apparent decrease, we find that the elevated temperature does not mitigate the creation of defects as this observation would suggest, but rather hides the created damage due to rapid thermal diffusion of the created vacancies before their detection, leading to the formation of vacancy lines and pores outside the measurement's field of view. Using the experimental data in combination with previously reported theoretical models for the displacement cross section, we estimate the migration energy barrier of sulfur vacancies in MoS2 to be 0.26±0.13eV. These results mark another step towards the complete understanding of electron beam damage in MoS2. Published by the American Physical Society 2025

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.

Defect-engineered and even amorphous two-dimensional (2D) materials have recently gained interest due to properties that differ from their pristine counterparts. Since these properties are highly sensitive to the exact atomic structure, it is crucial to be able to characterize them at atomic resolution over large areas. This is only possible when the imaging process is automated to reduce the time spent on manual imaging, which at the same time reduces the observer bias in selecting the imaged areas. Since the necessary datasets include at least hundreds if not thousands of images, the analysis process similarly needs to be automated. Here, we introduce disorder into graphene and monolayer hexagonal boron nitride (hBN) using low-energy argon ion irradiation, and characterize the resulting disordered structures using automated scanning transmission electron microscopy annular dark field imaging combined with convolutional neural network-based analysis techniques. We show that disorder manifests in these materials in a markedly different way, where graphene accommodates vacancy-type defects by transforming hexagonal carbon rings into other polygonal shapes, whereas in hBN the disorder is observed simply as vacant lattice sites with very little rearrangement of the remaining atoms. Correspondingly, in the case of graphene, the highest introduced disorder leads to an amorphous membrane, whereas in hBN, the highly defective lattice contains a large number of vacancies and small pores with no indication of amorphisation. Overall, our study demonstrates that combining automated imaging and image analysis is a powerful way to characterize the structure of disordered and amorphous 2D materials, while also illustrating some of the remaining shortcomings with this methodology.

AbstractPhosphorene, a puckered 2D allotrope of phosphorus, has sparked considerable interest in recent years due to its potential especially for optoelectronic applications with its layer‐number‐dependant direct band gap and strongly bound excitons. However, detailed experimentalcharacterization of its intrinsic defects as well as its defect creation characteristics under electron irradiation are scarce. Here, the creation and stability of a variety of defect configurations under 60 kV electron irradiation in mono‐ and bilayer phosphorene are reported including the first experimental reports of stable adatom‐vacancy‐complexes. Displacement cross section measurements in bilayer phosphorene yield a value of 7.7 ± 1.4 barn with an estimated lifetime of adatom‐vacancy‐complexes of 19.9 ± 0.7 s, while some are stable for up to 68 s under continuous electron irradiation. Surprisingly, ab initio‐based simulations indicate that the complexes should readily recombine, even in structures strained by up to 3%. The presented results will help to improve the understanding of the wide variety of defects in phosphorene, their creation, and their stability, which may enable new pathways for defect engineered phosphorene devices.

While the deformation behavior of nanocrystalline ductile metals and alloys is extensively studied, there is little understanding for brittle intermetallic alloys with very small grain sizes. Herein, B2‐ordered FeAl with different grain sizes is produced and deformed by high‐pressure torsion. At a grain size of 120 nm, conventional dislocation processes remain dominant, resulting in a disordered saturation structure with highly defected grains of around 100 nm. The situation is different for an initial grain size of 30 nm; grain‐boundary‐mediated processes appear along with dislocation processes and deformation shows a tendency toward extreme localization in the form of thin bands. Interestingly, the saturation structure is not reached after severe plastic deformation. The nanocrystals remain ordered with a grain size of 30 nm; only within the deformation bands, some degree of disordering and an increase of the dislocation density are revealed by profile analysis using selected electron diffraction. This result demonstrates an extreme stability of ordered FeAl at very small grain sizes, and indicates that the deformation behavior in brittle intermetallics can strongly depend on the grain size.

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.

Monolayer hexagonal boron nitride (hBN) has recently become the focus of intense research as a material to host quantum emitters. Although it is well known that such emission is associated with point defects, so far no conclusive correlation between the spectra and specific defects has been demonstrated. Here, we prepare atomically clean suspended hBN samples and subject them to low-energy ion irradiation. The samples are characterized before and after irradiation via automated scanning transmission electron microscopy imaging to assess the defect concentrations and distributions. We find an intrinsic defect concentration of ca. 0.03/nm2 (with ca. 55% boron and 8% nitrogen single vacancies, 20% double vacancies and 16% more complex vacancy structures). To be able to differentiate between these and irradiation-induced defects, we create a significantly higher (but still moderate) concentration of defects with the ions (0.30/nm2), and now find ca. 55% boron and 12% nitrogen single vacancies, 14% double vacancies, and 18% more complex vacancy structures. The results demonstrate that already the simplest irradiation provides selectivity for the defect types, and open the way for future experiments to explore changing the selectivity by modifying the irradiation parameters.