Removing fluoride-terminations from multilayered V2CTx MXene by gas hydrolyzation

Two-dimensional MXenes have shown great promise for many different applications, but in order to fully utilize their potential, control of their termination groups is essential. Here we demonstrate hydrolyzation with a continuous gas flow as a method to remove F-terminations from multilayered V2CTx particles, in order to prepare nearly F-free and partly bare vanadium carbide MXene. Density functional theory calculations demonstrate that the substitution of F-terminations is thermodynamically feasible and presents partly nonterminated V2CO as the dominating hydrolyzation product. Hydrolyzation at elevated temperatures reduced the F content but only subtly changed the O content, as inferred from spectroscopic data. The ideal hydrolyzation temperature was found to be 300 °C, as a degradation of the V2CTx phase and a transition to vanadium oxycarbides and V2O3 were observed at higher temperature. When tested as electrodes in Li-ion batteries, the hydrolyzed MXene demonstrated a reduced polarization compared with the pristine MXene, but no change in intercalation voltage was observed. Annealing in dry Ar did not result in the same F reduction, and the importance of water vapor was concluded, demonstrating hydrolyzation as a new and efficient method to control the surface terminations of multilayered V2CTx post etching. These results also provide new insights on the thermal stability of V2CTx MXene in hydrated atmospheres.publishedVersio

Low-energy properties of electrons and holes in CuFeS2

The antiferromagnetic semiconductor CuFeS2 belongs to a magnetic symmetry class that is of interest for spintronics applications. In addition, its crystal lattice is compatible with Si, making it possible to integrate it with nonmagnetic semiconducting structures. Therefore, we investigate this material by finding the effective k⋅p Hamiltonian for the electron and hole bands. We base this description on ab initio calculations and classify the electronic bands by their symmetry. As a result, we find that CuFeS2 exhibits spin-polarized bands. We also find that the crystal symmetry allows for the anomalous Hall effect. Finally, we suggest using cyclotron resonance to verify our proposed effective mass tensors at the conduction band minimum and valence band maximum.acceptedVersio

Origin of ferroelectric polarization in tetragonal tungsten-bronze-type oxides

The origin of ferroelectric polarization in tetragonal tungsten-bronze- (TTB-) type oxide strontium barium niobate (SBN) is investigated using first-principles density functional calculations. We study in particular the relationship between the polarization and the cation and vacancy ordering on alkali-earth metal lattice sites. Lattice dynamical calculations for paraelectric structures demonstrate that all cation configurations that can be accommodated in a 1×1×2 supercell result in a single unstable polar phonon, composed primarily of relative Nb-O displacements along the polar axis, as their dominant instability. The majority of the configurations also have a second octahedral tilt-mode instability which couples weakly to the polar mode. The existence of the tilt mode is strongly dependent on the local cation ordering, consistent with the fact that it is not found experimentally. Our results suggest that ferroelectricity in the SBN system is driven by a conventional second-order Jahn-Teller mechanism caused by the d 0 Nb 5 + cations, and demonstrate the strong influence of the size of Sr and Ba on the lattice distortions associated with polarization and octahedral tilting. Finally, we suggest a mechanism for the relaxor behavior in Sr-rich SBN based on Sr displacement inside pentagonal channels in the TTB structure

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena.publishedVersio

Local and average structure of Mn- and La-substituted BiFeO3

The local and average structure of solid solutions of the multiferroic perovskite BiFeO3 is investigated by synchrotron X-ray diffraction (XRD) and electron density functional theory (DFT) calculations. The average experimental structure is determined by Rietveld refinement and the local structure by total scattering data analyzed in real space with the pair distribution function (PDF) method. With equal concentrations of La on the Bi site or Mn on the Fe site, La causes larger structural distortions than Mn. Structural models based on DFT relaxed geometry give an improved fit to experimental PDFs compared to models constrained by the space group symmetry. Berry phase calculations predict a higher ferroelectric polarization than the experimental literature values, reflecting that structural disorder is not captured in either average structure space group models or DFT calculations with artificial long range order imposed by periodic boundary conditions. Only by including point defects in a supercell, here Bi vacancies, can DFT calculations reproduce the literature results on the structure and ferroelectric polarization of Mn-substituted BiFeO3. The combination of local and average structure sensitive experimental methods with DFT calculations is useful for illuminating the structure-property-composition relationships in complex functional oxides with local structural distortions

Role of Lone Pair Cations in Ferroelectric Tungsten Bronzes

acceptedVersio

Ferroelectric domain walls for nanotechnology

Ferroelectric domain walls have emerged as a new type of interface in which the dynamic characteristics of ferroelectricity introduce the element of spatial mobility, allowing real-time adjustment of position, density and orientation of the walls. Because of electronic confinement, and of their distinct symmetry and chemical environment, the spatially mobile domain walls offer a wide range of functional electric and magnetic properties, representing excellent 2D components for the development of more agile next-generation nanotechnology. In this Review, we discuss how the field of domain-wall nanoelectronics evolved from classical device ideas to advanced concepts for multilevel resistance control in memristive and synaptic devices. Recent advances in modelling and atomic-scale characterization provide insight into the interaction of ferroelectric domain walls and point defects, offering additional routes for local property design. We also explore the discovery of functional domain walls in improper ferroelectrics and the intriguing possibility of developing the walls themselves into ultra-small electronic components, controlling electronic signals through their intrinsic physical properties. We conclude with a discussion of open experimental challenges and newly discovered domain-wall phenomena that may play an important role in future directions of the field