Title: Excitons in halide double perovskites and the limits of the Wannier-Mott model in heterogeneous semiconductors

Speaker: Linn Leppert (UT, Computational Chemical Physics)
Time: April 21, 2022, 10:00–11:00
Location: Hybrid: TU/e (Flux 1.124) and online (MS Teams)

Abstract: Halide double perovskites are an emerging class of photoactive materials with considerable structural and electronic diversity and reliable stability towards heat and moisture under ambient conditions. However, little is known about their excitonic properties; whether simple physically motivated exciton models like the Wannier-Mott model are reliable for these quaternary, heterogeneous materials, is hard to predict a priori. In this talk I will discuss how first principles numerical modelling techniques can provide an atomistic understanding of excitons in halide double perovskites. I will show that these materials can feature a wide range of excitons, with binding energies spanning several orders of magnitude. Whether these excitons fall into the tenets of the Wannier-Mott model is determined by the symmetry and orbital character of their band edges – a consequence of their chemical heterogeneity.

The CCER seminars are aimed at researchers interested in computational approaches to (energy) research. The seminar is small-scale, typically 15 participants, and interactive, offering lots of room for discussion. If you would like to attend, just This email address is being protected from spambots. You need JavaScript enabled to view it..

Title: On-the-fly machine learning force fields with near first principles precision: Predicting phase transitions in complex Dynamic Solids

Speaker: Menno Bokdam (UT, MESA+)
Time: March 31, 2022, 10:00–11:00
Location: Hybrid: TU/e (Flux 1.124) and online (MS Teams)

Abstract: Lattice dynamics at the atomic scale is often well described by phonons in the harmonic approximation. However, it does not always suffice. For example, it does not explain a crystal’s phase transitions or why some materials have ultra-low thermal conductivity. While a more accurate method, ab-initio molecular-dynamics (MD), captures the anharmonicity of the atomic interactions correctly, it is computationally orders of magnitude too expensive to describe the effects of phonon scattering related to the "rattling" and "flipping" of atoms and molecules. In this talk I will present a recently developed method that can account for these effects based on an on-the-fly Machine- Learning Force-Field (MLFF) approach [1]. It allows us to automatically ‘train’ smooth and ‘cheap’ models of the potential energy surface based on density functional theory calculations. The MLFF gives access to the nanosecond time- and tens of nanometer length-scales and opens up the possibility to predict complex phase transitions, capture the formation and breaking of weak bonds, ion diffusion and simulate lattice thermal conductivity in complex ‘Dynamic Solids’. It enables linking to experiments such as NMR dipolar coupling [2] and momentum resolved phonon spectroscopy [3]. I will illustrate the capabilities of the approach with several examples from the halide perovskites.

1. Jinnouchi, Lahnsteiner, Karsai, Kresse and Bokdam, Phase transitions of hybrid perovskites simulated by machine- learning force fields trained on the fly with Bayesian inference, Phys. Rev. Lett. 122, 225701 (2019)
2. Grueninger, Bokdam, Leupold, Tinnemans, Moos, de Wijs, Panzer and Kentgens, Microscopic (dis)order and dynamics of cations in mixed FA/MA lead halide perovskites, J. Phys. Chem. C, 125, 1742-1753 (2021)
3. Lahnsteiner and Bokdam, Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: CsPbBr₃ a phonon liquid with Cs rattlers, Phys. Rev. B 105, 024302 (2022)

The CCER seminars are aimed at researchers interested in computational approaches to (energy) research. The seminar is small-scale, typically 15 participants, and interactive, offering lots of room for discussion. If you would like to attend, just This email address is being protected from spambots. You need JavaScript enabled to view it..

Title: Classical simulations for water splitting: Development of models and force fields for the hematite-water interface

Speaker: Juan Jose Gutierrez Sevillano
Time: March 10, 2022, 10:00–11:00
Location: Hybrid: TU/e (Flux 0.300) and online (MS Teams)

Abstract: To study the water splitting by using classical force fields, we first need to accurately describe the interaction between water and the hematite surface. For this purpose, we develop several models and force fields for this system. The development is based on geometrical aspects that have an influence on the water-hematite interaction, such as the proximity to different parts of the surface (figure 1), the molecule-surface distance, and the orientation of the molecule. A critical analysis of the functional forms of the force fields, the modelling of the interaction sites, the partial charges as well as a full parametrization is performed. This analysis allows us to obtain an accurate description of the interactions of the water molecule with pure and doped hematite.

Figure 1: On the left, interaction energy profile of one water molecule and the pure hematite surface. On the right, a snapshot of the system.

The CCER seminars are aimed at researchers interested in computational approaches to (energy) research. The seminar is small-scale, typically 15 participants, and interactive, offering lots of room for discussion. If you would like to attend, just This email address is being protected from spambots. You need JavaScript enabled to view it..