S. Bals

Investigation of the Octahedral Network Structure in Formamidinium Lead Bromide Nanocrystals by Low-Dose Scanning Transmission Electron Microscopy

N. J. Schrenker, T. Braeckevelt, A. De Backer, N. Livakas, C.-P. Yu, T. Friedrich, M.B.J. Roeffaers, J. Hofkens, J. Verbeeck, L. Manna, V. Van Speybroeck, S. Van Aert, S. Bals
Nano Letters
24, 35, 10936-10942
2024
A1

Abstract 

Metal halide perovskites (MHP) are highly promising semiconductors. In this study, we focus on FAPbBr3 nanocrystals, which are of great interest for green light-emitting diodes. Structural parameters significantly impact the properties of MHPs and are linked to phase instability, which hampers long-term applications. Clearly, there is a need for local and precise characterization techniques at the atomic scale, such as transmission electron microscopy. Because of the high electron beam sensitivity of MHPs, these investigations are extremely challenging. Here, we applied a low-dose method based on four-dimensional scanning transmission electron microscopy. We quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb–Br–Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure. Additionally, this study provides an approach to prospectively investigating the fundamental degradation mechanisms of MHPs.

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DOI 

https://doi.org/10.1021/acs.nanolett.4c02811

Additivity of atomic strain fields as a tool to strain-engineering phase-stabilized CsPbI3 perovskites

J. Teunissen, T. Braeckevelt, I. Skvortsova, J. Guo, B. Pradhan, E. Debroye, M. Roeffaers, J. Hofkens, S. Van Aert, S. Bals, S.M.J. Rogge, V. Van Speybroeck
The Journal of Physical Chemistry C
127, 48, 23400-23411
2023
A1

Abstract 

CsPbI3 is a promising perovskite material for photovoltaic applications in its photoactive perovskite or black phase. However, the material degrades to a photovoltaically inactive or yellow phase at room temperature. Various mitigation strategies are currently being developed to increase the lifetime of the black phase, many of which rely on inducing strains in the material that hinder the black-to-yellow phase transition. Physical insight into how these strategies exactly induce strain as well as knowledge of the spatial extent over which these strains impact the material is crucial to optimize these approaches but is still lacking. Herein, we combine machine learning potential-based molecular dynamics simulations with our in silico strain engineering approach to accurately quantify strained large-scale atomic structures on a nanosecond time scale. To this end, we first model the strain fields introduced by atomic substitutions as they form the most elementary strain sources. We demonstrate that the magnitude of the induced strain fields decays exponentially with the distance from the strain source, following a decay rate that is largely independent of the specific substitution. Second, we show that the total strain field induced by multiple strain sources can be predicted to an excellent approximation by summing the strain fields of each individual source. Finally, through a case study, we illustrate how this additive character allows us to explain how complex strain fields, induced by spatially extended strain sources, can be predicted by adequately combining the strain fields caused by local strain sources. Hence, the strain additivity proposed here can be adopted to further our insight into the complex strain behavior in perovskites and to design strain from the atomic level onward to enhance their sought-after phase stability.

Open Access version available at UGent repository
Gold Open Access

DOI 

http://dx.doi.org/10.1021/acs.jpcc.3c05770

Catalytic and molecular separation properties of Zeogrids and Zeotiles

J.A. Martens, J.W. Thybaut, J.F.M. Denayer, S. Pulinthanathu Sree, A. Aerts, M-F. Reyniers, V. Van Speybroeck, M. Waroquier, A. Buekenhoudt, I. Vankelecom, W. Buijs, J. Persoons, G.V. Baron, S. Bals, G. Van Tendeloo, G.B. Marin, P.A. Jacobs, C. Kirschhock
Catalysis Today
168, 17-27
2011
A1

Abstract 

Zeogrids and Zeotiles are hierarchical materials built from assembled MFI zeolite precursor units. Permanent secondary porosity in these materials is obtained through self assembly of nanoparticles encountered in MFI zeolite synthesis in the presence of supramolecular templates. Hereon, the aggregated species are termed nanoslabs. Zeogrids are layered materials with lateral spacings between nanoslabs creating galleries qualifying as supermicropores. Zeotiles present a diversity of tridimensional nanoslab assemblies with mesopores. Zeotile-1, -4 and -6 are hexagonal mesostructures. Zeotile-1 has triangular and hexagonal channels; Zeotile-4 has hexagonal channels interconnected via slits. Zeotile-2 has a cubic structure with gyroid type mesoporosity. The behavior of Zeogrids and Zeotiles in adsorption, membrane and chromatographic separation and catalysis has been characterized and compared with zeolites and mesoporous materials derived from unstructured silica sources. Shape selectivity was detected via adsorption of n- and iso-alkanes. The mesoporosity of Zeotiles can be exploited in chromatographic separation of biomolecules. Zeotiles present attractive separation properties relevant to CO2 sequestration. Because of its facile synthesis procedure without hydrothermal steps Zeogrid is convenient for membrane synthesis. The performance of Zeogrid membrane in gas separation, nanofiltration and pervaporation is reported. In the Beckmann rearrangement of cyclohexanone oxime Zeogrids and Zeotiles display a catalytic activity characteristic of silicalite-1 zeolites. Introduction of acidity and redox catalytic activity can be achieved via incorporation of Al and Ti atoms in the nanoslabs during synthesis. Zeogrids are active in hydrocracking, catalytic cracking, alkylation and epoxidation reactions. Zeogrids and Zeotiles often behave differently from ordered mesoporous materials as well as from zeolites and present a valuable extension of the family of hierarchical silicate based materials.

DOI 

http://dx.doi.org/10.1016/j.cattod.2011.01.036

Design of zeolite by inverse sigma transformation

E. Verheyen, L. Joos, K. Van Havenbergh, N. Kasian, E. Gobechiya, K. Houthoofd, M. Hinterstein, E. Breynaerts, V. Van Speybroeck, M. Waroquier, S. Bals, G. Van Tendeloo, C. Kirschhock, J.A. Martens
Nature Materials
11 (12), 1059-1064
2012
A1

Abstract 

Zeolites are silicon materials, that have channels and pores on the nanoscale. This paper reports the synthesis of a new zeolite, in which the pores were widened using a revolutionary synthesis method. The final material has a series of unique and special properties, useful for industrial processes. Molecular modeling was used to determine the structure of the material.

Zeolieten zijn materialen opgebouwd uit silicium, die op nanoschaal kanalen en poriën bevatten. Deze paper rapporteert de synthese van een nieuw type zeoliet, waarbij de kanalen op een revolutionaire manier werden vergroot. Het eindmateriaal heeft daarom een hele reeks aan unieke en bijzonder interessante eigenschappen voor een aantal industriële processen. Moleculaire modelering werd gebruikt om de structuur van het materiaal te bepalen.

A graphical representation of COK14:

DOI 

http://dx.doi.org/10.1038/nmat3455

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