Pnictogens Allotropy and Phase Transformation during van der Waals Growth

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes

Correction: Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins.

Correction for 'Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins' by M. Ślęzak et al., Nanoscale, 2020, DOI: 10.1039/d0nr04193a

Ghost anti-crossings caused by interlayer umklapp hybridization of bands in 2D heterostructures

In two-dimensional heterostructures, crystalline atomic layers with differing lattice parameters can stack directly one on another. The resultant close proximity of atomic lattices with differing periodicity can lead to new phenomena. For umklapp processes, this opens the possibility for interlayer umklapp scattering, where interactions are mediated by the transfer of momenta to or from the lattice in the neighbouring layer. Using angle-resolved photoemission spectroscopy to study a graphene on InSe heterostructure, we present evidence that interlayer umklapp processes can cause hybridization between bands from neighbouring layers in regions of the Brillouin zone where bands from only one layer are expected, despite no evidence for Moiré-induced replica bands. This phenomenon manifests itself as ‘ghost’ anti-crossings in the InSe electronic dispersion. Applied to a range of suitable two-dimensional material pairs, this phenomenon of interlayer umklapp hybridization can be used to create strong mixing of their electronic states, giving a new tool for twist-controlled band structure engineering

In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe2_2 van der Waals ferromagnet

The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe$_2$ van der Waals ferromagnet. Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back down the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have N\'eel texture, are preferentially oriented along directions separated by 120 degrees, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few mT only, showing that the CrTe$_2$ compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases

Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices

AbstractResistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure

Spectro-microscopic investigation of Fe-oxide based model catalysts and instrumental development

Diese Arbeit untersucht Fe-Oxid-Systeme mit Hilfe einer Kombination aus Mikroskopie (LEEM, Röntgen PEEMs), Beugung (LEED) und Spektroskopie (XPS) und berichtet über die elektronenoptische Entwicklung adaptiver Optiken und Aberrationskorrekturen für einen elektrostatischen abbildenden Energieanalysator. Experimentell untersuchten wir Magnetit und Hämatit Dünnschichten. Ihre Kristallstruktur, Stöchiometrie sowie deren Oberflächenterminierung können durch spezielle Herstellungsverfahren eingestellt werden. Unter Ausnutzung der Echtzeit-Beobachtung mit Mikroskopie, Beugung und Spektroskopie untersuchten wir (a) die Oberflächenmodifikationen von Fe3O4 und α-Fe2O3-Dünnschichten durch Fe Ablagerung; (b) die reversible Phasenumwandlung Fe3O4 ↔ α-Fe2O3 unter verschiedenen Oxidationsbedingungen; (c) die Bildung der metastabilen γ-Fe2O3-Phase und (d) die Wechselwirkung von Fe3O4 und α-Fe2O3 Oberflächen mit unterstützten Pt-Nanopartikeln. Es wurde ein Algorithmus entwickelt, um den LEEM Bildkontrast für inhomogene 2D Oberflächen zu simulieren. Abschließend wird das Design eines Energiefilter-System vorgestellt, das in ein PEEM/LEEM Mikroskop der neuen Generation eingebaut werden wird. Das System basiert auf dem gleichen Abbildungsprinzip wie der magnetische Ω-Filter, der erfolgreich im aktuellen SMART Mikroskop eingesetzt wird. Das neue Instrument zielt auf die Verbesserung der Orts- und Energieauflösung im XPEEM (5 nm und 70 meV). Die Mehrzahl der möglichen Aberrationen zweiter Ordnung wird durch die intrinsische Symmetrie selbstkompensiert. Die Wirkung der anderen Aberrationen wird durch ein geeignetes Design der Verzögerungs- und Beschleunigungsoptiken kombiniert mit einer optimierten Passenergie reduziert. Darüber hinaus kompensieren zusätzliche Hexapole die restlichen dominierenden Aberrationen, wodurch eine Orts- und Energieauflösung besser als 2 nm bzw. 75 meV erreicht wird.This work presents the investigation of Fe-oxide systems, combining microscopy (LEEM, X-PEEM), diffraction (LEED) and spectroscopy (XPS), and the electron-optical development of adaptive optics and aberration corrections for an electrostatic imaging energy analyzer. Experimentally, we studied magnetite (Fe3O4) and hematite (α-Fe2O3) thin films. Their crystal structure, stoichiometry as well as their surface termination can be tuned by special preparation procedures. Taking advantage of real time observation with microscopy, diffraction and spectroscopy, we investigated (a) the surface modifications of Fe3O4 and α-Fe2O3 thin films by Fe deposition; (b) the reversible phase transformation Fe3O4 ↔ α-Fe2O3 under different oxidation conditions; (c) the formation of the metastable γ-Fe2O3 phase and (d) the interaction of Fe3O4 and α-Fe2O3 surfaces with supported Pt nanoparticles . An algorithm was developed to simulate the LEEM image contrast for inhomogeneous 2D surfaces. The possible application to experimental data and the limitation will be discussed. Finally, the design of an energy filtering system is presented, which will be implemented in a new generation PEEM/LEEM microscope. The system bases on the same imaging principle as the magnetic Ω-filter, successfully implemented in the actual SMART microscope. The new instrument aims for the improvement of lateral and energy resolution in X-PEEM (5 nm and 70 meV, respectively). The majority of the possible second order aberrations are self-compensated by intrinsic symmetry. The effect of the other aberrations is reduced by an adequate design for the deceleration-acceleration optics in combination with optimized pass energy. Furthermore, additional hexapole multipoles compensate for the residual dominating aberrations, yielding in the lateral resolution and energy resolution better than 2 nm and 75 meV, respectively