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Angew Chem Int Ed Engl ; 61(6): e202112204, 2022 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-34860450


The role of intermediate phases in CeO2 mesocrystal formation from aqueous CeIII solutions subjected to γ-radiation was studied. Radiolytically formed hydroxyl radicals convert soluble CeIII into less soluble CeIV . Transmission electron microscopy (TEM) and X-ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to CeIV : (1) formation of hydrated CeIV hydroxides, serving as intermediates in the liquid-to-solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into "pre-mesocrystals" and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of "mineral bridges". Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.

J Phys Condens Matter ; 34(5)2021 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-34783682


The mineral franckeite is a naturally occurring van der Waals superlattice which has recently attracted attention for future applications in optoelectronics, biosensors and beyond. Furthermore, its stacking of incommensurately modulated 2D layers, the pseudo tetragonal Q-layer and the pseudo hexagonal H-layer, is an experimentally accessible prototype for the development of synthetic van der Waals materials and of advanced characterization methods to reveal new insights in their structure and chemistry at the atomic scale that is crucial for deep understanding of its properties. While some experimental studies have been undertaken in the past, much is still unknown on the correlation between local atomic structure and chemical composition within the layers. Here we present an investigation of the atomic structure of franckeite using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT). With atomic-number image contrast in HAADF STEM direct information about both the geometric structure and its chemistry is provided. By imaging samples under different zone axes within the van der Waals plane, we propose refinements to the structure of the Q-layer and H-layer, including several chemical ordering effects that are expected to impact electronic structure calculations. Additionally, we observe and characterize stacking faults which are possible sources of differences between experimentally determined properties and calculations. Furthermore, we demonstrate advantages and discuss current limitations and perspectives of combining TEM and APT for the atomic scale characterization of incommensurately modulated von der Waals materials.

Nano Lett ; 20(11): 8258-8266, 2020 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-33026227


With their ns2 np3 valence electronic configuration, pnictogens are the only system to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light pnictogens crystallize in the A17 phase, and bulk heavier elements prefer the A7 phase. Herein, we demonstrate that the A17 of heavy pnictogens can be stabilized in antimonene grown on weakly interacting surfaces and that it undergoes a spontaneous thickness-driven transformation to the stable A7 phase. At a critical thickness of ∼4 nm, A17 antimony transforms from AB- to AA-stacked α-antimonene by a diffusionless shuffle transition followed by a gradual relaxation to the A7 phase. Furthermore, the competition between A7- and A17-like bonding affects the electronic structure of the intermediate phase. These results highlight the critical role of the atomic structure and substrate-layer interactions in shaping the stability and properties of layered materials, thus enabling a new degree of freedom to engineer their performance.