arXiv subjects

Barbara Maria Mayer

Publications and source records attributed to Barbara Maria Mayer.

6 recordsLinked to original sources

arXiv2026

Competition between vacancy creation and filling in defect-engineering of hBN

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.

cond-mat.mtrl-sci↗

arXiv2026

Quantitative Analysis of Composition and Contamination of Atomically Thin Materials by Recoil-Projectile Coincidence in Ion Transmission

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 {\deg}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 ($p_{\mathrm{base}} = 2 \times 10^{-8}$ 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.

cond-mat.mtrl-sci↗

arXiv2026

Electron-beam induced methane decomposition for in-situ carbon doping of hexagonal boron nitride

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% of carbon-rich regions are confined to the area exposed to the electron beam, while some carbon atoms are also found to diffuse up to an average distance of 4.7+-0.5 nm beyond it. The incorporated carbon atoms arrange in a hexagonal pattern within the lattice, forming patches that do not exceed ~1 nm in size. Analysis of the EELS fine structure indicates modifications to the local electronic environment within these regions, with implications for the optical properties of the resulting carbon-related defects.

cond-mat.mtrl-sci↗

arXiv2025

Single photon emitters in hBN: Limitations of atomic resolution imaging and potential sources of error

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.

cond-mat.mtrl-sci↗

arXiv2025

Origin of circular and triangular pores in electron-irradiated hexagonal boron nitride

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.

cond-mat.mtrl-sci↗

arXiv2024

Defect-engineering hexagonal boron nitride using low-energy Ar+ irradiation

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.

cond-mat.mtrl-sci↗