Substitutional Si impurities in monolayer hexagonal boron nitride

We report the first observation of substitutional silicon atoms in single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning transmission electron microscopy (STEM). The medium angle annular dark field (MAADF) images reveal silicon atoms exclusively filling boron vacancies. This structure is stable enough under electron beam for repeated imaging. Density functional theory (DFT) is used to study the energetics, structure and properties of the experimentally observed structure. The formation energies of all possible charge states of the different silicon substitutions (Si$_\mathrm{B}$, Si$_\mathrm{N}$ and Si$_\mathrm{{BN}}$) are calculated. The results reveal Si$_\mathrm{B}^{+1}$ as the most stable substitutional configuration. In this case, silicon atom elevates by 0.66{\AA} out of the lattice with unoccupied defect levels in the electronic band gap above the Fermi level. The formation energy shows a slightly exothermic process. Our results unequivocally show that heteroatoms can be incorporated into the h-BN lattice opening way for applications ranging from single-atom catalysis to atomically precise magnetic structures

Efficient first principles simulation of electron scattering factors for transmission electron microscopy

Electron microscopy is a powerful tool for studying the properties of materials down to their atomic structure. In many cases, the quantitative interpretation of images requires simulations based on atomistic structure models. These typically use the independent atom approximation that neglects bonding effects, which may, however, be measurable and of physical interest. Since all electrons and the nuclear cores contribute to the scattering potential, simulations that go beyond this approximation have relied on computationally highly demanding all-electron calculations. Here, we describe a new method to generate ab initio electrostatic potentials when describing the core electrons by projector functions. Combined with an interface to quantitative image simulations, this implementation enables an easy and fast means to model electron microscopy images. We compare simulated transmission electron microscopy images and diffraction patterns to experimental data, showing an accuracy equivalent to earlier all-electron calculations at a much lower computational cost.Comment: 10 pages, 5 figures, 2 table

Perforating freestanding molybdenum disulfide monolayers with highly charged ions

Porous single layer molybdenum disulfide (MoS$_2$) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS$_2$. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process, and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.Comment: 22 pages, 4 figure

Atomic structure of low dimensional materials and their in-situ transformation in the transmission electron microscope

Transmissionselektronenmikroskopie ist eine vielseitige Methode um Proben auf atomarer Ebene zu erforschen. Im Speziellen 2D Materialien, wie Graphen, hBN, MoS2 und weitere Nanostrukturierte Materialien sind für diese Art der Untersuchung besonders geeignet. Doch nicht nur statische Untersuchungen lassen sich in TEMs durchführen, sondern auch die Messung dynamischer Vorgänge, wie Zugversuche aber auch Umbauprozesse im Material, aufgrund der Bestrahlung mit hochenergetischen Elektronen. In dieser Arbeit wurden verschiedene Bauformen von TEMs verwendet. Ein modernes STEM wurde verwendet um die Dichte von Si Verunreinigungen in monolagigem hBN, zu untersuchen. Diese wurden aufgrund von chemical vapour deposition (CVD) Wachstum in das Material eingebracht. Doch hBN ist ein nicht einfach zu untersuchendes Material in einem Elektronenmikroskop, da es zu chemischen Ätzprozessen während der Elektronenbestrahlung kommt. Hier eignen sich insbesonders Methoden, die nur kuze Verweildauern des Strahls auf der Probe benötigen, wie zum Beispiel schnelle STEM Rastertechniken. Hier büßt man allerdings einen Teil der Bildqualität ein. Für die quantitative Untersuchung von Kohlenstoff Nanozwiebeln wurden verschiedene TEM Techniken angewendet, allen voran Z-Kontrast STEM Aufnahmen und EELS. Mit letztgenannter Technik können zum Beispiel so genannte Spektrum Abbildungen aufgenommen werden. Hierbei werden, mithilfe eines EELS Spektrometers, nur Elektronen mit bestimmten Energieverlussten zur Bildgebung verwendet. Dadurch lässt sich die Verteilung von Kaliumatomen in Kohlenstoff-Nanozwiebeln bestimmen. Ein weiterer Modus in der Elektronenmikroskopie, ist der Beugungsmodus, welcher Einblicke in den reziproken Raum gewährleistet. Hierbei können weitere Aspekte des zu Untersuchenden Materials enthüllt werden. Betrachtung der Intensitätsverteilung innerhalb eines Beugungsbildes, lässt Rückschlüsse auf die drei-dimensionale Struktur eines 2D Materials zu. Es zeigt sich, das diese nicht flach sind, sondern verschieden ausgeprägte Korrugationen aufweisen. Die im unbelasteten Material anfänglich randomisierte Verteilung der Wellen, lässt sich gezielt durch anbringen einer äußeren Zugkraft, mithilfe eines Spezial TEM Probenhalters, beeinflussen. Durch die, notwendigerweise hohe Energie, der in einem TEM eingestrahlten Elektronen, kommt es häufg zu unvermeidbaren Schäden an der Probe. Einer dieser Strahlungsschäden ist der so genannte Anstoßeffekt, welcher zu Umbaueffekten in 2D Materialien führt. Diese verlieren dabei zunehmend ihre geordnete Struktur und werden in ungeordnete, 2D Strukturen umgewandelt. Auch diese Prozesse lassen sich mithilfe des Beugungsmodus beobachten. Anf änglich scharfe Beugungspunkte, werden schwächer in ihrer Intensität und es kommt zu einer allmählichen Ausbildung eines ringförmigen Beugungsbildes. Zusammenfassend sei gesagt, dass die in dieser Arbeit demonstrierten TEM Methoden, Grundlagen für die Erzeugung von 2D Materialien mit massgeschneiderten Eigenschaften, aufzeigt.Transmission electron microscopy is a powerful method for investigating the atomic structure of thin specimens, and especially two-dimensional materials, such as graphene, hexagonal boron nitride (hBN), molybdenum disulfide molybdenium disulfide (MoS2) and other structures at the nanometer length scale. However, a transmission electron microscope (TEM) is not limited to simple investigation of these structures in a static form, but can also be used to study their deformation under external manipulation, such as mechanical strain, or due to electron irradiation induced structure altering processes during the observation. In this thesis, TEM has been used as a purely investigatory tool for a number of studies. For example, studying the density of Si impurities in monolayered hBN, introduced during chemical vapour deposition (CVD) growth. As hBN is prone to chemical etching processes due to its chemical structure, investigating this material with high energy electrons can be rather challenging. Using fast techniques in a scanning transmission electron microscope (STEM), enabled imaging of Si impurities not caused by electron irradiation, with only a minor trade-o_ in resolution. For investigating and quantifying the amount of intercalated potassium within carbon nano-onions, different TEM techniques were applied, including Z-contrast STEM imaging with atomic resolution and electron energy loss spectrometry (EELS). The latter is a powerful tool to generate spectrum images in which only electrons of a certain energy loss are used for image formation, leading to the characterization of the elemental distribution in a specimen. These measurements revealed the spatial distribution of intercalated potassium within and around the carbon nano-onions By switching to reciprocal space via diffraction mode in a TEM, additional details of the studied structures can be revealed. It is demonstrated that the intensity distribution within the diffraction pattern depends on the chemical elements building up the materials, and that the three-dimensional structure of nominally two dimensional (2D) materials can be revealed. It is shown that a pristine 2D material tends to exhibit ripples in the third dimension, which can be oriented by applying mechanical strain providing means for controlling the material properties inside the microscope. Due to the high-energy electron irradiation during TEM investigation, structural damage of the studied materials is often inevitable. One manifestation of this is knock-on damage that can lead to increasing disorder of an initially crystalline graphene sample, assisted by in situ heating of the sample during the experiment. The lattice disorder can also be observed through changes in the diffraction pattern; starting from a pattern with distinct spots that corresponds to the ordered structure, it gradually changes through the spreading of the spots along the azimuthal angle until a structure with a closed ring, corresponding to a completely disordered 2D structure, emerges. In conclusion, the work presented in this thesis both demonstrates the use of TEM for the structural characterization of low-dimensional materials as well as its capability for tracking structural changes while they occur inside the microscope, which provides new means for tailoring novel materials for applications

Corrugations in Free-Standing Graphene

Although both the tendency of 2D materials to bend out of plane as well as its effect on materials’ properties are well known, the factors influencing this phenomenon have not been extensively studied. Graphene, the one-atom-thick membrane of carbon atoms, is both arguably the best known 2D material, as well as the most prone to spontaneous corrugations. Here, we use electron diffraction to systematically study the factors influencing corrugations in graphene, including the size of the free-standing area, the preparation method, the amount of surface contamination, and electron-beam-induced structural disorder. We find that mechanically exfoliated graphene is less corrugated than graphene grown via chemical vapor deposition (corrugation amplitude of (0.83±0.10) Å compared to (1.33±0.20) Å for a free-standing area with a diameter of 1.7μm). Similarly, corrugation amplitude grows by more than a factor of two when the diameter of the free- standing area is increased from 1.7μm to ca. 3.0μm. Electron beam irradiation affects the corrugation in two ways, firstly by removing the hydrocarbon contamination, which decreases corrugation, and secondly by creating increasing amounts of disorder into the material, which again increases corrugation. Overall, our results show that control over the sample during both initial preparation and post-preparation treatment allows for a change in the amount of corrugation in free-standing 2D materials, which may lead to new advances in their use in applications

Imaging and structure analysis of ferroelectric domains, domain walls, and vortices by scanning electron diffraction

Direct electron detectors in scanning transmission electron microscopy give unprecedented possibilities for structure analysis at the nanoscale. In electronic and quantum materials, this new capability gives access to, for example, emergent chiral structures and symmetry-breaking distortions that underpin functional properties. Quantifying nanoscale structural features with statistical significance, however, is complicated by the subtleties of dynamic diffraction and coexisting contrast mechanisms, which often results in low signal-to-noise and the superposition of multiple signals that are challenging to deconvolute. Here we apply scanning electron diffraction to explore local polar distortions in the uniaxial ferroelectric Er(Mn,Ti)O$_3$. Using a custom-designed convolutional autoencoder with bespoke regularization, we demonstrate that subtle variations in the scattering signatures of ferroelectric domains, domain walls, and vortex textures can readily be disentangled with statistical significance and separated from extrinsic contributions due to, e.g., variations in specimen thickness or bending. The work demonstrates a pathway to quantitatively measure symmetry-breaking distortions across large areas, mapping structural changes at interfaces and topological structures with nanoscale spatial resolution

Efficient first principles simulation of electron scattering factors for transmission electron microscopy

Electron microscopy is a powerful tool for studying the properties of materials down to their atomic structure. In many cases, the quantitative interpretation of images requires simulations based on atomistic structure models. These typically use the independent atom approximation that neglects bonding effects, which may, however, be measurable and of physical interest. Since all electrons and the nuclear cores contribute to the scattering potential, simulations that go beyond this approximation have relied on computationally highly demanding all-electron calculations. Here, we describe a new method to generate ab initio electrostatic potentials when describing the core electrons by projector functions. Combined with an interface to quantitative image simulations, this implementation enables an easy and fast means to model electron scattering. We compare simulated transmission electron microscopy images and diffraction patterns to experimental data, showing an accuracy equivalent to earlier all-electron calculations at a much lower computational cost.© 2018 Elsevier B.V