arXiv subjects

Bart J. Kooi

Publications and source records attributed to Bart J. Kooi.

23 records · Page 2Linked to original sources

arXiv2020

Single-source, solvent-free, room temperature deposition of black $γ$-CsSnI$_3$ films

The presence of a non-optically active polymorph (yellow-phase) competing with the optically active polymorph (black $γ$-phase) at room temperature in CsSnI3 and the susceptibility of Sn to oxidation, represent two of the biggest obstacles for the exploitation of CsSnI3 in optoelectronic devices. Here room-temperature single-source in vacuum deposition of smooth black $γ$ - CsSnI3 thin films is reported. This has been done by fabricating a solid target by completely solvent-free mixing of CsI and SnI2 powders and isostatic pressing. By controlled laser ablation of the solid target on an arbitrary substrate at room temperature, the formation of CsSnI3 thin films with optimal optical properties is demonstrated. The films present a band gap of 1.32 eV, a sharp absorption edge and near-infrared photoluminescence emission. These properties and X-ray diffraction of the thin films confirmed the formation of the orthorhombic (B-$γ$) perovskite phase. The thermal stability of the phase was ensured by applying in situ an Al2O$_3$ capping layer. This work demonstrates the potential of pulsed laser deposition as a volatility-insensitive single-source growth technique of halide perovskites and represents a critical step forward in the development and future scalability of inorganic lead-free halide perovskites.

physics.app-ph↗

arXiv2019

Resolving hydrogen atoms at metal-metal hydride interfaces

Hydrogen as a fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. Understanding fundamental behavior of hydrogen at atomic scale is key to improve the properties of metal-metal hydride systems. However, currently, there is no robust technique capable of visualizing hydrogen atoms. Here, we demonstrate that hydrogen atoms can be imaged unprecedentedly with integrated differential phase contrast, a recently developed technique performed in a scanning transmission electron microscope. Images of the titanium-titanium monohydride interface reveal remarkable stability of the hydride phase, originating from the interplay between compressive stress and interfacial coherence. We also uncovered, thirty years after three models were proposed, which one describes the position of the hydrogen atoms with respect to the interface. Our work enables novel research on hydrides and is extendable to all materials containing light and heavy elements, including oxides, nitrides, carbides and borides.

cond-mat.mtrl-sci↗

arXiv2018

A rhombohedral ferroelectric phase in epitaxially-strained Hf0.5Zr0.5O2 thin films

After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 (STO) substrates. The films, which are under epitaxial compressive strain and are predominantly (111)-oriented, display large FE polarization values up to 34 μC/cm2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2016

Insights into ultrafast Ge-Te bond dynamics in a phase-change superlattice

A long-standing question for avant-grade data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved X-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The novel approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process, meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in phase-change materials.

cond-mat.mtrl-sci↗

arXiv2014

Domain wall magnetism in thin films of orthorhombic manganites

Thin films of orthorhombic TbMnO3, as well as other orthorhombic manganites, epitaxially grown on cubic SrTiO3 substrates display an induced magnetic moment that is absent in the bulk (antiferromagnetic) counterpart. Here we show that there is a clear correlation between the domain microstructure and the induced magnetic moment in TbMnO3 films on SrTiO3. In addition, the distinct dependence of the magnetization with the film thickness is not consistent with domain magnetism and indicates that the domain walls, rather than the domains, are the origin of the net magnetic moment. Since the orientation of the domain walls can be designed by the film-substrate relationship and its density can be tuned with the film thickness, these results represent a significant step forward towards the design of devices based on domain wall functionality.

cond-mat.mtrl-sci↗