arXiv subjects

Geert Brocks

Publications and source records attributed to Geert Brocks.

At least 19 recordsLinked to original sources

arXiv2026

Operando Raman probing of mode selective electron phonon coupling in two dimensional halide perovskites

Electron phonon coupling governs charge transport, carrier relaxation, and polaron formation in halide perovskites, yet its microscopic origin in low dimensional systems remains poorly understood. Here, we combine operando, bias dependent Raman spectroscopy with density functional theory (DFT) calculations to directly probe carrier lattice interactions in two-dimensional Ruddlesden Popper perovskites,(PEA)$_2$PbI$_4$ and its fluorinated analogue, (PEA-F)$_2$PbI$_4$. Under applied electric fields, both systems exhibit mode-selective Raman linewidth broadening predominantly near 100 cm$^{-1}$, whereas other phonon modes remain largely unaffected, revealing highly selective coupling between injected carriers and specific lattice vibrations. DFT calculations identify these modes as hybrid organic inorganic vibrations involving coupled motion of the organic spacer and symmetric Pb I equatorial stretching, rather than purely inorganic phonons. Fluorination fundamentally reconstructs the vibrational landscape by modifying molecular packing, crystal symmetry, and organic inorganic coupling, resulting in changes to the phonon density of states, longer phonon lifetimes, and an enhanced carrier mediated lattice response. Notably, electrical bias produces opposite phonon lifetime evolution in thin films and single crystals: the phonon lifetime decreases by approximately 17 to 22% in thin films but increases 20 to 26% in single crystals. These contrasting responses demonstrate that structural order plays a fundamental role in determining carrier phonon interactions and phonon relaxation pathways in two-dimensional halide perovskites.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic Defect Diffusion in Layered CsPbBr$_\mathrm{x}$I$_\mathrm{3-x}$ Perovskites

Mixed-halide perovskites offer a route to enhance phase stability and modify optoelectronic properties. Here, we use large-scale molecular dynamics simulations with a reactive force field to investigate defects in CsPbBr$_\mathrm{x}$I$_\mathrm{3-x}$ perovskites, focusing on how defect mobility can be controlled and the stability of the material improved by layered ordering of Br and I anions. Our results show that layered halide ordering induces strongly anisotropic defect diffusion: migration proceeds readily along the layers, whereas diffusion across them is strongly suppressed. For Cs defects, this anisotropy originates from directional lattice strain and the associated octahedral tilting, whereas halide migration is governed by an interplay between strain and preferential local halide bonding configurations.

cond-mat.mtrl-sci↗

arXiv2026

A Unified microscopic picture of cation and anion migration in MAPbI$_3$

Passivating defects and restricting defect mobilities in halide perovskites to increase device lifetimes has become a main field of research. Modeling structure and mobility of point defects is an essential contribution to this endeavor. We employ molecular dynamics, based on neural network potentials trained on density functional theory data, to model ion migration in MAPbI$_3$ triggered by I and MA vacancies or interstitials. Most of these species diffuse rapidly at room temperature, with migration barriers between 0.15 and 0.20 eV. MA interstitials are highly mobile despite their molecular nature, owing to a concerted migration mechanism involving multiple MA ions. No evidence of MA vacancy migration is obtained. Whereas diffusion of I-related defects appreciably depends on their charge state, diffusion of MA defects does not. These results revise the conventional picture of ion transport in hybrid perovskites and highlight the role of collective molecular motion in enabling fast ionic migration.

cond-mat.mtrl-sci↗

arXiv2026

Chemical Origin of Exciton Self-trapping in Cs$_3$Cu$_2$X$_5$ Cesium Copper Halides

Copper halides Cs3Cu2X5 (X=Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts. In this work, we uncover the chemical bonding origin of the Stokes shift in these materials using density functional theory calculations. Upon excitation, one [Cu2X5]3- anion undergoes sizeable local distortions, driven by Cu-X and Cu-Cu bond formation. These structural changes coincide with the formation of a self-trapped exciton, where particularly the hole is strongly localized on one anion. Analysis of the electronic structure and bonding reveals reduced antibonding interactions and enhanced bonding character in the excited state, stabilizing the distorted geometry. Our results establish a direct link between orbital-specific hole localization and bond formation. It provides a fundamental understanding of the excitation mechanism in Cs3Cu2X5 and offers design principles to tune optical properties in 0D copper halides.

cond-mat.mtrl-sci↗

arXiv2026

Halide diffusion in mixed-halide perovskites and heterojunctions

Migration of halide defects guides ion transport in metal halide perovskites and controls the kinetics of halide mixing and phase separation. We study the diffusion of halide vacancies and interstitials in \ce{CsPb(I_{x}Br_{1-x})_{3}} and \ce{CsPbI_{3}}/\ce{CsPbBr_{3}} heterojunctions by molecular dynamics simulations using neural network potentials trained on density functional theory calculations. We observe enhanced diffusion of both vacancies and interstitials in the mixed halide compounds compared to the single halide ones, as well as a difference in mobility between Br and I ions in the mixed compound. Diffusion across heterojunctions is governed by the interface structure, where a Br-rich interface blocks migration of vacancies in particular, but an I-rich interface is permeable.

cond-mat.mtrl-sci↗

arXiv2025

Impact of Metal Cation on Chiral Properties of 2D Halide Perovskites

Chiral two-dimensional (2D) halide perovskites are formed by embedding chiral organic cations in a perovskite crystal structure. The chirality arises from distortions of the 2D metal halide layers induced by the packing of these organic cations. Sn-based octahedra spontaneously distort, but it remains unclear whether this intrinsic structural instability enhances the chirality. We investigate the effect of the metal cation on structural and phonon chirality in MBA$_{2}$Sn$_{\mathrm{x}}$Pb$_{1-\mathrm{x}}$I$_{4}$ (x = 0, 1/2, and 1). Incorporating Sn does distort the metal halide octehedra, yet it only has a minor impact on the structural chirality. In contrast, the phonons in MBA$_{2}$SnI$_{4}$ are substantially more chiral than in MBA$_{2}$PbI$_{4}$, especially the in-plane acoustic modes. However, this enhanced phonon chirality does not lead to a generation of a larger angular momentum under a temperature gradient, because the contributions of different chiral phonons tend to compensate one another.

cond-mat.mtrl-sci↗

arXiv2024

Chiral Phonons in 2D Halide Perovskites

Phonons in chiral crystal structures can be circularly polarized, making them chiral. Chiral phonons carry angular momentum, which is observable in heat currents, and, via coupling to electron spin, in spin currents. Two-dimensional (2D) halide perovskites, versatile direct band gap semiconductors, can easily form chiral structures by incorporating chiral organic cations. As a result, they exhibit phenomena such as chirality-induced spin selectivity (CISS) and the spin Seebeck effect, although the underlying mechanisms remain unclear. Using on-the-fly machine-learning force fields trained against density functional theory calculations, we confirm the presence of chiral phonons, a potential key factor for these effects. Our analysis reveals that low-energy phonons, originating from the inorganic framework, primarily exhibit chirality. Under a temperature gradient, these chiral phonons generate substantial angular momentum, leading to experimentally observable effects. These findings position chiral 2D perovskites as a promising platform for exploring the interplay between phononic, electronic, spintronic, and thermal properties.

cond-mat.mtrl-sci↗

arXiv2024

Tracing Ion Migration in Halide Perovskites with Machine Learned Force Fields

Halide perovskite optoelectronic devices suffer from chemical degradation and current-voltage hysteresis induced by migration of highly mobile charged defects. Atomic scale molecular dynamics simulations can capture the motion of these ionic defects, but classical force fields are too inflexible to describe their dynamical charge states. Using CsPbI3 as a case study, we train machine learned force fields from density functional theory calculations and study the diffusion of charged halide interstitial and vacancy defects in bulk CsPbI3. We find that negative iodide interstitials and positive iodide vacancies, the most stable charge states for their respective defect type, migrate at similar rates at room temperature. Neutral interstitials are faster, but neutral vacancies are one order of magnitude slower. Oppositely charged interstitials and vacancies, as they can occur in device operation or reverse bias conditions, are significantly slower and can be considered relatively immobile.

cond-mat.mtrl-sci↗

arXiv2024

Temperature-Dependent Chirality in Halide Perovskites

With the use of chiral organic cations in two-dimensional metal halide perovskites, chirality can be induced in the metal halide layers, which results in semiconductors with intriguing chiral optical and spin-selective transport properties. The chiral properties strongly depend upon the temperature, despite the basic crystal symmetry not changing fundamentally. We identify a set of descriptors that characterize the chirality of metal halide perovskites such as MBA$_{2}$PbI$_{4}$, and study their temperature dependence using molecular dynamics simulations with on-the-fly machine-learning force fields obtained from density functional theory calculations. We find that, whereas the arrangement of organic cations remains chiral upon increasing the temperature, the inorganic framework loses this property more rapidly. We ascribe this to the breaking of hydrogen bonds that link the organic with the inorganic substructures, which leads to a loss of chirality transfer.

cond-mat.mtrl-sci↗

arXiv2023

Phase transitions of LaMnO$_3$ and SrRuO$_3$ from DFT + U based machine learning force fields simulations

Perovskite oxides are known to exhibit many magnetic, electronic and structural phases as function of doping and temperature. These materials are theoretically frequently investigated by the DFT+U method, typically in their ground state structure at $T=0$. We show that by combining machine learning force fields (MLFFs) and DFT+U based molecular dynamics, it becomes possible to investigate the crystal structure of complex oxides as function of temperature and $U$. Here, we apply this method to the magnetic transition metal compounds LaMnO$_3$ and SrRuO$_3$. We show that the structural phase transition from orthorhombic to cubic in LaMnO$_3$, which is accompanied by the suppression of a Jahn-Teller distortion, can be simulated with an appropriate choice of $U$. For SrRuO$_3$, we show that the sequence of orthorhombic to tetragonal to cubic crystal phase transitions can be described with great accuracy. We propose that the $U$ values that correctly capture the temperature-dependent structures of these complex oxides, can be identified by comparison of the MLFF simulated and experimentally determined structures.

cond-mat.mtrl-sci↗

arXiv2023

Calculating the Circular Dichroism of Chiral Halide Perovskites: A Tight-Binding Approach

Chiral metal halide perovskites have emerged as promising optoelectronic materials for emission and detection of circular polarized visible light. Despite chirality being realized by adding chiral organic cations or ligands, the chiroptical activity originates from the metal halide framework. The mechanism is not well understood, as an overarching modeling framework is lacking. Capturing chirality requires going beyond electric dipole transitions, the common approximation in condensed matter calculations. We present a density functional theory (DFT) parameterized tight-binding (TB) model, which allows us to calculate optical properties including circular dichroism (CD) at low computational cost. Comparing Pb-based chiral perovskites with different organic cations and halide anions, we find that the structural helicity within the metal halide layers determines the size of the CD. Our results mark an important step in understanding the complex correlations of structural, electronic and optical properties of chiral perovskites, and provide a useful tool to predict new compounds with desired properties for novel optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2022

Defects in Halide Perovskites: Does It Help to Switch from 3D to 2D?

Ruddlesden-Popper hybrid iodide 2D perovskites are put forward as stable alternatives to their 3D counterparts. Using first-principles calculations, we demonstrate that equilibrium concentrations of point defects in the 2D perovskites PEA$_2$PbI$_4$, BA$_2$PbI$_4$, and PEA$_2$SnI$_4$ (PEA: phenethyl ammonium, BA: butylammonium), are much lower than in comparable 3D perovskites. Bonding disruptions by defects are more detrimental in 2D than in 3D networks, making defect formation energetically more costly. The stability of 2D Sn iodide perovskites can be further enhanced by alloying with Pb. Should, however, point defects emerge in sizable concentrations as a result of nonequilibrium growth conditions, for instance, then those defects hamper the optoelectronic performance of the 2D perovskites, as they introduce deep traps. We suggest that trap levels are responsible for the broad sub-bandgap emission in 2D perovskites observed in experiments.

cond-mat.mtrl-sci↗

arXiv2022

Compound Defects in Halide Perovskites: A First-Principles Study of CsPbI$_3$

Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI$_3$, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs forms in a concentration comparable to those of the most prominent point defects, whereas the other compound defects are negligible. However, under nonequilibrium thermal and operating conditions, other complexes also become as important as the point defects. Those are the Cs substituting Pb antisite, and, to a lesser extent, the compound vacancies of PbI$_2$ or CsPbI$_3$ units, and the I substituting Cs antisite. These compound defects only lead to shallow or inactive charge carrier traps, which testifies to the electronic stability of the halide perovskites. Under operating conditions with a quasi Fermi level very close to the valence band, deeper traps can develop.

cond-mat.mtrl-sci↗

arXiv2022

Light-tunable three-phase coexistence in mixed halide perovskites

Mixed iodine-bromine perovskites used in solar cells undergo below a critical temperature an intrinsic demixing into phases with different iodine-bromine compositions. In addition, under illumination they show nucleation of an iodine-rich phase. We predict from thermodynamic considerations that in mixed iodine-bromine perovskites like MAPb(I$_{1-x}$Br$_x$)$_3$ the interplay of these effects can lead to coexistence of a bromine-rich, iodine-rich, and nearly iodine-pure nucleated phase. This three-phase coexistence occurs in a region in the composition-temperature phase diagram near the critical point for intrinsic demixing. We investigate the hysteresis in the evolution of this coexistence when temperature or illumination intensity are cycled. Depending on the particular way the coexistence is established, nearly iodine-pure nuclei should form either in the iodine-rich phase only or both in the bromine-rich and iodine-rich phases. Experimental verification of this fundamentally novel type of light-tunable three-phase coexistence should be possible by a combination of absorption and photoluminescence experiments.

cond-mat.mtrl-sci↗

arXiv2022

Evidence of Spin Frustration in Vanadium Diselenide Monolayer Magnet

Monolayer VSe2, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic two-dimensional transition-metal dichalcogenides (2D-TMDs). Herein, by means of in-situ microscopic and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

cond-mat.mtrl-sci↗

arXiv2021

The thermodynamic trends of intrinsic defects in primary halide perovskites: A first-principles study

Defects in halide perovskites play an essential role in determining the efficiency and stability of the resulting optoelectronic devices. Here, we present a systematic study of intrinsic point defects in six primary metal halide perovskites, MAPbI$_3$, MAPbBr$_3$, MAPbCl$_3$, FAPbI$_3$, CsPbI$_3$ and MASnI$_3$, using density functional theory calculations with the SCAN+rVV10 functional. We analyse the impact of changing anions and cations on the defect formation energies and the charge state transitions levels and identify the physical origins underlying the observed trends. Dominant defects in the lead-iodide compounds are the A$^+$ cation interstitials (A = Cs, MA, FA), charge-compensated by I$^-$ interstitials or lead $({2-})$ vacancies. In the lead-bromide and -chloride compounds, halide vacancies become relatively more prominent, and for MAPbBr$_3$, the Pb$^{2+}$ interstitial also becomes important. The trends can be explained in terms of the changes in electrostatic interactions and chemical bonding upon replacing cations and anions. Defect physics in MASnI$_3$ is strongly dominated by tin $({2-})$ vacancies, promoted by the easy oxidation of the tin perovskite. Intrinsically, all compounds are mildly p-doped, except for MASnI$_3$, which is strongly p-doped. All acceptor levels created by defects in the six perovskites are shallow. Some defects, halide vacancies and Pb or Sn interstitials in particular, create deep donor traps. Although these traps might hamper the electronic behavior of MAPbBr$_3$ and MAPbCl$_3$, in iodine-based perovskites their equilibrium concentrations are too small to affect the materials' properties.

cond-mat.mtrl-sci↗

arXiv2021

A ReaxFF molecular dynamics study of hydrogen diffusion in ruthenium -- the role of grain boundaries

Ruthenium thin films can serve as protective caps for multi-layer extreme ultraviolet mirrors exposed to atomic hydrogen. Hydrogen permeation through ruthenium is problematic as it leads to blisters on the mirrors. H has been shown to exhibit low solubility in bulk Ru, and rapidly diffuses in and out of Ru. Therefore, the underlying mechanisms of the blistering effect remains unknown. This work makes use of reactive molecular dynamics simulations to study the influence of imperfections in a Ru film on the behaviour of H. For the Ru/H system, a ReaxFF force field was parametrised which reproduces structures and energies obtained from quantum-mechanical calculations. Molecular dynamics simulations have been performed with the newly-developed force field, to study the effect of tilt and twist grain boundaries on the overall diffusion behaviour of H in Ru. Our simulations show the tilt and twist grain boundaries provide energetically favourable sites for hydrogen atoms and act as sinks and highways for H. They therefore block H transport across their planes, and favour diffusion along their planes. This results in the accumulation of hydrogen at the grain boundaries. The strong effect of the grain boundaries on the hydrogen diffusion suggests tailoring the morphology of ruthenium thin films as a means to curb the rate of hydrogen permeation.

cond-mat.mtrl-sci↗

arXiv2021

First-principles calculations of defects in metal halide perovskites: a performance comparison of density functionals

Metal halide perovskite semiconductors have outstanding optoelectronic properties. Although these perovskites are defect-tolerant electronically, defects hamper their long-term stability and cause degradation. Density functional theory (DFT) calculations are an important tool to unravel the microscopic structures of defects, but results suffer from the different approximations used in the DFT functionals. In the case of metal halide perovskites, qualitatively different results have been reported with different functionals, either predicting vacancy or interstitial point defects to be most dominant. Here, we conduct a comprehensive comparison of a wide range of functionals for calculating the equilibrium defect formation energies and concentrations of point defects in the archetype metal halide perovskite, MAPbI$_3$. We find that it is essential to include long-range Van der Waals interactions in the functional, and that it is vital to self-consistently optimize structure and volume of all compounds involved in the defect formation. For calculating equilibrium formation energies of point defects in MAPbI$_3$ and similar metal halide perovskites, we argue that the exact values of the chemical potentials of the species involved, or of the intrinsic Fermi level, are not important. In contrast to the simple Schottky or Frenkel pictures, we find that the dominant defects are MA and I interstitials, and Pb vacancies.

cond-mat.mtrl-sci↗