Search

Electrode materials for Li⁺-ion batteries require optimization along several disparate axes related to cost, performance, and sustainability. One of the important performance axes is the ability to retain structural integrity though cycles of charge/discharge. Metal–metal bonding is a distinct feature of some refractory metal oxides that has been largely underutilized in electrochemical energy storage, but that could potentially impact structural integrity. Here LiScMo₃O₈, a compound containing triangular clusters of metal–metal bonded Mo atoms, is studied as a potential anode material in Li⁺-ion batteries. Electrons inserted though lithiation are localized across rigid Mo₃ triangles (rather than on individual metal ions), resulting in minimal structural change as suggested by operando diffraction. The unusual chemical bonding allows this compound to be cycled with Mo atoms below a formally +4 valence state, resulting in an acceptable voltage regime that is appropriate for an anode material. Several characterization methods including potentiometric entropy measurements indicate two-phase regions, which are attributed through extensive first-principles modeling to Li⁺ ordering. This study of LiScMo₃O₈ provides valuable insights for design principles for structural motifs that stably and reversibly permit Li⁺ (de)insertion.

An advanced and highly scalable approach for determining the number of layers of two-dimensional (2D) materials via optical spectroscopy is introduced. Based on appropriate subjacent layer stacks, the reflectance spectra of the 2D material assemblies exhibit wavelength shifts in distinct minima, which are linearly related to the number of layers. A linear correlation enables straightforward data interpretation, which is essential for implementing simple and comparatively fast measurement routines for process control on wafer scale. The structure of the optical layer stacks as well as the complex refractive indices of 2D materials were found to strongly influence the spectral position of the reflectance minima as well as the magnitude and the linearity of the wavelength shifts. We experimentally prove this method to be applicable for large-area layer counting of subsequently stacked chemical vapor deposition graphene films on a layer stack consisting of silicon nitride and silicon oxide. The measurement results confirm the calculated wavelength shift of the reflection minimum around 540 nm equaling approx. 3 nm per layer. Numerical analysis shows that comparable behavior is also achievable by the tailored design of subjacent layer stacks for graphene oxide, hexagonal boron nitride, and more complex 2D materials like transition-metal dichalcogenides. For the achievement of linear relations between wavelength shifts of the respective minimum and the layer count of the 2D material, analytical design rules are derived considering the optical properties of the underlying layer stack as well as oscillator frequencies within the complex refractive index of the 2D material. The largest signal response of 12 nm per layer was calculated for MoSe2 on an optimized layer stack.

Defects in crystalline materials govern the materials’ overall physical properties and can be used to tune them for specific applications. Dislocations are found among the most ubiquitous and fundamental defects in crystalline materials. Further defects include doping with individual heteroatoms, grain boundaries or vacancies. All these defects strongly influence the optical, electric and/or other physical properties. The effects of crystalline defects become more pronounced for thin materials, as the defective volume in comparison to the non-defective will grow relatively. For the thinnest 2D (or pseudo 3D) crystalline material, the carbon allotrope (bilayer) graphene, the properties are expected to be heavily influenced by those crystalline defects. This work is therefore dedicated to the investigation, manipulation and real-time imaging of defects and dislocations in (bilayer) graphene. To explore the structural peculiarities of dislocations, dark field investigations at low electron energies are conducted. In-depth investigation of epitaxial bi- and trilayer graphene grown on the Si-face of 6H-SiC bearing an enormous amount of dislocations was carried out using dark field transmission electron microscopy in a Titan Themis 300 operated at 80 kV. The strain relaxation resulting from the ultra-thin material leads to a wavy topography for parallel partial dislocations separated by a distance of ca. 25 nm. By evaluation of directed dark-field tilt series a technique for the topographic reconstruction of thin 2D materials was elaborated and tested on freestanding graphite and graphene membranes. The observed partial dislocations were not only observed statically but can be imaged and directly manipulated to trigger dislocation reactions, using a state-of-the-art scanning electron microscope with a STEM detector and precisely controllable micromanipulators. To follow the in situ manipulation of individual dislocations in CVD grown bilayer graphene, the electron trajectory for an imaging mode with an additional magnetic field (immersion mode, IM) was elaborated. The recording of diffraction patterns in the SEM leads to the deduction of image formation principles on the STEM detector and advanced the establishment of a novel diffraction technique for ultra-low energies in a SEM (1-30 kV). Based on the mechanical manipulation experiments of individual partial dislocations, a platform-independent large-area cleaning approach for an exhaustive removal of surface adsorbents in vacuo was devised. The mechanical contact and movement of manipulator tips can be used to remove adsorbed contamination while imaging it in situ. The cleaning procedure boosts the resolution in the SEM at ultra-low energies and is able to discriminate between physisorbed contamination and covalently bound alkyl chains. After cleaning dodecyl functionalized monolayer graphene directly in the TEM, atomic resolved imaging of alkyl chains on graphene could be carried out. The obtained data can be compared to a large dataset obtained from molecular dynamic simulations to assign the molecular arrangement of individual carbon chains bound to sp³ defects on monolayer graphene. Defekte in kristallinen Materialien bestimmen deren physikalische Eigenschaften und können genutzt werden, diese Eigenschaften für ausgewählte Anwendungen zu nutzen. Innerhalb dieser Kristalldefekte spielen Versetzungen eine zentrale Rolle. Daneben sind unter anderem Einzelatomdoping, Korngrenzen und Leerstellen als weitere Defekte zu erwähnen. Alle diese Kristalldefekte beeinflussen optische, elektrische und weitere physikalische Eigenschaften von Materialien. Im Falle von immer dünneren Materialien nimmt der Einfluss dieser Defekte auf das gesamte Material enorm zu. Dies ist mit dem Rückgang des ungestörten Kristallvolumens im Vergleich zu defekten Bereichen zu erklären. Die Auswirkungen von Kristalldefekten zeigen sich besonders stark bei Untersuchungen des dünnst möglichen 2D (bzw. pseudo-3D) Materials, (Zweilagen-) Graphen. In dieser Arbeit werden daher Defekte und Versetzungen in (Zweilagen-) Graphen untersucht, manipuliert und abgebildet. Zur strukturellen Untersuchung von Partialversetzungen wurden hierzu Dunkelfelduntersuchungen in einem Transmissionselektronenmikroskop bei niedriger Energie durchgeführt. Auf der Si Seite von 6H-SiC epitaktisch gewachsenes Zwei- und Dreilagengraphen mit einer enormen Dichte an Partialversetzungen wurde untersucht. Die Relaxation von Verspannungen führt hier in Netzwerken mit abwechselnden Partialverstzungspaaren (im Abstand von ca. 25 nm) zur Ausbildung einer welligen Topographie. Dunkelfeldkippserien wurden zudem genutzt um die Topographie von freistehenden Graphit- und Graphenmembranen quantitativ zu rekonstruieren. Vereinzelte Versetzungen in CVD gewachsenem Zweilagengraphen wurden manipuliert und Reaktionen zwischen einzelnen Versetzungen untersucht, während diese zeitgleich abgebildet werden können. Hierfür ist ein Rasterelektronenmikroskop mit genauestens steuerbaren Mikromanipulatoren genutzt worden. Zum besseren Verständnis der erhaltenen Kontraste in Durchstrahlabbildungen sind die Elektronenbahnen bei lokal angeregtem magnetischen Feld (Immersionsmodus) untersucht und klassifiziert worden. Basierend auf den erstellten Beugungsdaten ist ein nachrüstbares Modul zur direkten Aufnahme von Beugungsmustern im REM bei Spannungen von 1-30 kV Beschleunigungsspannung etabliert worden. Die mechanischen Manipulationen einzelner Versetzungen eröffnete die Möglichkeit freistehende dünne Membranen umfassend von adsorbierten Kontaminationen zu befreien. Dabei wird eine dünne Spitze in physischem Kontakt zur Membran bewegt und ermöglicht die Entfernung adsorbierter Kontaminationen. Dadurch lässt sich die erreichbare Auflösung für niederenergetische Mikroskopie drastisch erhöhen. Die großflächige Reinigung direkt im TEM erlaubte die Untersuchung von, mit einzelnen Alkylketten, kovalent funktionalisiertem Graphen. In Verbindung mit Molekulardynamik Simulationen und neuartiger Bildsimulation konnten atomar aufgelöste Aufnahmen einzelner, an sp³ Defekte in Graphen gebundener Alkylketten erstellt werden.