| 1. |
- Rahm, Martin, 1982, et al.
(författare)
-
Non-Bonded Radii of the Atoms Under Compression
- 2020
-
Ingår i: ChemPhysChem. - : Wiley. - 1439-7641 .- 1439-4235. ; 21:21, s. 2441-2453
-
Tidskriftsartikel (refereegranskat)abstract
- Abstract: We present quantum mechanical estimates for non-bonded, van der Waals-like, radii of 93 atoms in a pressure range from 0 to 300 gigapascal. Trends in radii are largely maintained under pressure, but atoms also change place in their relative size ordering. Multiple isobaric contractions of radii are predicted and are explained by pressure-induced changes to the electronic ground state configurations of the atoms. The presented radii are predictive of drastically different chemistry under high pressure and permit an extension of chemical thinking to different thermodynamic regimes. For example, they can aid in assignment of bonded and non-bonded contacts, for distinguishing molecular entities, and for estimating available space inside compressed materials. All data has been made available in an interactive web application.
|
|
| 2. |
- Ekborg-Tanner, Pernilla, 1994, et al.
(författare)
-
Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen
- 2022
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 5:8, s. 10225-10236
-
Tidskriftsartikel (refereegranskat)abstract
- Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multiscale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical" sensitivity) is approximately constant at 180 nm/c(H) for nanodisk diameters of greater than or similar to 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic" sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures.
|
|
| 3. |
- Erhart, Paul, 1978, et al.
(författare)
-
The Wulff construction goes low-symmetry
- 2023
-
Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-4660 .- 1476-1122. ; 22:8, s. 941-942
-
Tidskriftsartikel (refereegranskat)
|
|
| 4. |
- Fan, Zheyong, et al.
(författare)
-
GPUMD: A package for constructing accurate machine-learned potentials and performing highly efficient atomistic simulations
- 2022
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 157:11
-
Tidskriftsartikel (refereegranskat)abstract
- We present our latest advancements of machine-learned potentials (MLPs) based on the neuroevolution potential (NEP) framework introduced in Fan et al. [Phys. Rev. B 104, 104309 (2021)] and their implementation in the open-source package gpumd. We increase the accuracy of NEP models both by improving the radial functions in the atomic-environment descriptor using a linear combination of Chebyshev basis functions and by extending the angular descriptor with some four-body and five-body contributions as in the atomic cluster expansion approach. We also detail our efficient implementation of the NEP approach in graphics processing units as well as our workflow for the construction of NEP models and demonstrate their application in large-scale atomistic simulations. By comparing to state-of-the-art MLPs, we show that the NEP approach not only achieves above-average accuracy but also is far more computationally efficient. These results demonstrate that the gpumd package is a promising tool for solving challenging problems requiring highly accurate, large-scale atomistic simulations. To enable the construction of MLPs using a minimal training set, we propose an active-learning scheme based on the latent space of a pre-trained NEP model. Finally, we introduce three separate Python packages, viz., gpyumd, calorine, and pynep, that enable the integration of gpumd into Python workflows.
|
|
| 5. |
- Fransson, Erik, 1990, et al.
(författare)
-
Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites
- 2023
-
Ingår i: Communications Physics. - 2399-3650. ; 6:1
-
Tidskriftsartikel (refereegranskat)abstract
- The soft modes associated with continuous-order phase transitions are associated with strong anharmonicity. This leads to the overdamped limit where the phonon quasi-particle picture can break down. However, this limit is commonly restricted to a narrow temperature range, making it difficult to observe its signature feature, namely the breakdown of the inverse relationship between the relaxation time and damping. Here we present a physically intuitive picture based on the relaxation times of the mode coordinate and its conjugate momentum, which at the instability approach infinity and the inverse damping factor, respectively. We demonstrate this behavior for the cubic-to-tetragonal phase transition of the inorganic halide perovskite CsPbBr3 via molecular dynamics simulations, and show that the overdamped region extends almost 200 K above the transition temperature. Further, we investigate how the dynamics of these soft phonon modes change when crossing the phase transition.
|
|
| 6. |
- Fransson, Erik, 1990, et al.
(författare)
-
Revealing the Free Energy Landscape of Halide Perovskites: Metastability and Transition Characters in CsPbBr₃ and MAPbI₃
- 2023
-
Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 8229-
-
Tidskriftsartikel (refereegranskat)abstract
- Halide perovskites have emerged as a promising class of materials for photovoltaic applications. A challenge of these applications is preventing the crystal structure from degrading to photovoltaically inactive phases, which requires an understanding of the free energy landscape of these materials. Here, we uncover the free energy landscape of two prototypical halide perovskites, CsPbBr3 and MAPbI3, via atomic-scale simulations using umbrella sampling and machine-learned potentials. For CsPbBr3, we find very small free energy differences and barriers close to the transition temperatures for both the tetragonal-to-cubic and orthorhombic-to-tetragonal transitions. For MAPbI3, however, the situation is more intricate. In particular, the orthorhombic-to-tetragonal transition exhibits a large free energy barrier, and there are several competing tetragonal phases. Using large-scale molecular dynamics simulations, we explore the character of these transitions and observe the latent heat and a discrete change in the structural parameters for the tetragonal-to-cubic phase transitions in both CsPbBr3 and MAPbI3, indicating first-order transitions. We find that in MAPbI3, the orthorhombic phase has an extended metastability range, and we identify a second metastable tetragonal phase. Finally, we compile a phase diagram for MAPbI3 that includes potential metastable phases.
|
|
| 7. |
|
|
| 8. |
- Linderälv, Christopher, 1986, et al.
(författare)
-
High-Throughput Characterization of Transition Metal Dichalcogenide Alloys: Thermodynamic Stability and Electronic Band Alignment
- 2022
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 34, s. 9364-9372
-
Tidskriftsartikel (refereegranskat)abstract
- Alloying offers a way to tune many of the properties of the transition metal dichalcogenide (TMD) monolayers. While these systems in many cases have been thoroughly investigated previously, the fundamental understanding of critical temperatures, phase diagrams, and band edge alignment is still incomplete. Based on first-principles calculations and alloy cluster expansions, we compute the phase diagrams of 72 TMD monolayer alloys and classify the mixing behavior. We show that ordered phases in general are absent at room temperature but that there exist some alloys that have a stable Janus phase at room temperature. Furthermore, for a subset of these alloys, we quantify the band edge bowing and show that the band edge positions for the mixing alloys can be continuously tuned in the range set by the boundary phases.
|
|
| 9. |
- Lindgren, Eric, 1997, et al.
(författare)
-
calorine: A Python package for constructing and sampling neuroevolution potential models
- 2024
-
Ingår i: Journal of Open Source Software. - 2475-9066. ; 9:95, s. 6264-6264
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics (MD) simulations are a key tool in computational chemistry, physics, and materials science, aiding the understanding of microscopic processes but also guiding the development of novel materials. A MD simulation requires a model for the interatomic interactions. To this end, one traditionally often uses empirical interatomic potentials or force fields, which are fast but inaccurate, or ab-initio methods based on electronic structure theory such as density functional theory, which are accurate but computationally very expensive (Müser et al., 2023). Machine-learned interatomic potentials (MLIPs) have in recent years emerged as an alternative to these approaches, combining the speed of heuristic force fields with the accuracy of ab-initio techniques (Unke et al., 2021). Neuroevolution potentials (NEPs), implemented in the GPUMD package, in particular, are a highly accurate and efficient class of MLIPs (Fan et al., 2021, 2022; Fan, 2022). NEP models have already been used to study a variety of properties in a range of materials, with recent examples including radiation damage in tungsten (Liu et al., 2023), phase transitions (Fransson, Wiktor, et al., 2023) and dynamics of halide perovskites (Fransson, Rosander, et al., 2023) as well as thermal transport in two-dimensional materials (Sha et al., 2023). Here, we present calorine, a Python package that simplifies the construction, analysis and use of NEP models via GPUMD.
|
|
| 10. |
- Löfgren, Joakim, 1989, et al.
(författare)
-
Computational assessment of the efficacy of halides as shape-directing agents in nanoparticle growth
- 2020
-
Ingår i: Physical Review Materials. - 2475-9953. ; 4:9
-
Tidskriftsartikel (refereegranskat)abstract
- We report a comprehensive study of aqueous halide adsorption on nanoparticles of gold and palladium that addresses several limitations hampering the use of atomistic modeling as a tool for understanding and improving wet-chemical synthesis and related applications. A combination of thermodynamic modeling with density functional theory (DFT) calculations and experimental data is used to predict equilibrium shapes of halide-covered nanoparticles as a function of the chemical environment. To ensure realistic and experimentally relevant results, we account for solvent effects and include a large set of vicinal surfaces, several adsorbate coverages, as well as decahedral particles. While the observed stabilization is not significant enough to result in thermodynamic stability of anisotropic shapes such as nanocubes, nonuniformity in the halide coverage indicates the possibility of obtaining such shapes as kinetic products. With regard to technical challenges, we show that inclusion of surface-solvent interactions leads to qualitative changes in the predicted shape. Furthermore, accounting for nonlocal interactions on the functional level yields a more accurate description of surface systems.
|
|
| 11. |
- Rahm, Magnus, 1990, et al.
(författare)
-
A Library of Late Transition Metal Alloy Dielectric Functions for Nanophotonic Applications
- 2020
-
Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 30:35
-
Tidskriftsartikel (refereegranskat)abstract
- Accurate complex dielectric functions are critical to accelerate the development of rationally designed metal alloy systems for nanophotonic applications, and to thereby unlock the potential of alloying for tailoring nanostructure optical properties. To date, however, accurate alloy dielectric functions are widely lacking. Here, a time-dependent density-functional theory computational framework is employed to compute a comprehensive binary alloy dielectric function library for the late transition metals most commonly employed in plasmonics (Ag, Au, Cu, Pd, Pt). Excellent agreement is found between electrodynamic simulations based on these dielectric functions and selected alloy systems experimentally scrutinized in 10 at% composition intervals. Furthermore, it is demonstrated that the dielectric functions can vary in very non-linear fashion with composition, which paves the way for non-trivial optical response optimization by tailoring material composition. The presented dielectric function library is thus a key resource for the development of alloy nanomaterials for applications in nanophotonics, optical sensors, and photocatalysis.
|
|
| 12. |
- Rahm, Magnus, 1990, et al.
(författare)
-
A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd–Au–H
- 2021
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 211
-
Tidskriftsartikel (refereegranskat)abstract
- Due to their ability to reversibly absorb/desorb hydrogen without hysteresis, Pd–Au nanoalloys have been proposed as materials for hydrogen sensing. For sensing, it is important that absorption/desorption isotherms are reproducible and stable over time. A few studies have pointed to the influence of short and long range chemical order on these isotherms, but many aspects of the impact of chemical order have remained unexplored. Here, we use alloy cluster expansions to describe the thermodynamics of hydrogen in Pd–Au in a wide concentration range. We investigate how different chemical orderings, corresponding to annealing at different temperatures as well as different external pressures of hydrogen, impact the behavior of the material with focus on its hydrogen absorption/desorption isotherms. In particular, we find that a long-range ordered L12 phase is expected to form if the H2 pressure is sufficiently high. Furthermore, we construct the phase diagram at temperatures from 250 K to 500 K, showing that if full equilibrium is reached in the presence of hydrogen, phase separation can often be expected to occur, in stark contrast to the phase diagram in para-equilibrium. Our results explain the experimental observation that absorption/desorption isotherms in Pd–Au are often stable over time, but also reveal pitfalls for when this may not be the case.
|
|
| 13. |
- Rahm, Magnus, 1990, et al.
(författare)
-
Quantitative predictions of thermodynamic hysteresis: Temperature-dependent character of the phase transition in Pd–H
- 2022
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 227
-
Tidskriftsartikel (refereegranskat)abstract
- The thermodynamics of phase transitions between phases that are size-mismatched but coherent differs from conventional stress-free thermodynamics. Most notably, in open systems such phase transitions are always associated with hysteresis. In spite of experimental evidence for the relevance of these effects in technologically important materials such as Pd hydride, a recipe for first-principles-based atomic-scale modeling of coherent, open systems has been lacking. Here, we develop a methodology for quantifying phase boundaries, hysteresis, and coherent interface free energies using density-functional theory, alloy cluster expansions, and Monte Carlo simulations in a constrained ensemble. We apply this approach to Pd–H and show that the phase transition changes character above approximately 400 K, occurring with an at all times spatially homogeneous hydrogen concentration, i.e., without coexistence between the two phases. Our results are consistent with experimental observations but reveal aspects of hydride formation in Pd nanoparticles that have not yet been accessible in experiment.
|
|
| 14. |
- Rahm, Magnus, 1990
(författare)
-
There Is an Alloy at the End of the Rainbow: Structure and Optical Properties From Bulk to Nano
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mixing different chemical species and decreasing dimensions to the nanoscale are two powerful approaches for improving materials. In both cases new properties emerge, and structure, composition, and chemical ordering can be tuned to tailor materials for specific purposes. To exploit the potential of these materials, it is crucial that they are fundamentally understood, and to this end, computational methods have emerged as an important complement to experiment. This thesis presents the development and application of methods for modeling alloys, nanoparticles, and nanoalloys on the atomic scale, with the purpose of guiding the search for new materials, in particular those related to plasmonic sensing of hydrogen. A software for creating and sampling alloy cluster expansion has been developed partially in connection to this thesis, and is applied to hydrogenation of Pd and Pd--Au. For Pd--Au, the impact of chemical order on hydrogen uptake is studied, and two kinds of phase diagrams are calculated; one in which the Pd/Au atoms are fixed, and one in which they rearrange in response to hydrogen. These phase diagrams are constructed under the assumption that phase separation occurs with incoherent interfaces. This is not always the case, in particular not during hydrogenation of small Pd nanoparticles. Coherent interfaces lead to strain, and a methodology for studying this significantly more complex case is developed and applied to Pd--H, showing that there are three temperature intervals with qualitatively distinct hydrogenation behaviors. Moreover, a software for creating Wulff constructions for the prediction of equilibrium nanoparticle shapes has been developed as part of this thesis and is used to study the impact of halides on the shapes of Au and Pd nanoparticles. Furthermore, an algorithm for finding equilibrium shapes of nanoparticles on the atomic scale is detailed, and the results indicate that an ensemble of nanoparticles in thermodynamic equilibrium in general should be expected to contain multiple different shapes. Moreover, nanoalloys of Ag--Cu and Pd--Au are studied on the atomic scale with the aim to understand how chemical ordering is impacted on the nanoscale, which reveals an interplay between chemistry and strain that can give rise to a rather complex distribution of the components throughout a nanoalloy. Finally, the dielectric functions of ten metallic alloys are calculated with first-principles methods and benchmarked with experiment, providing a library of reference data to aid modeling of nanoplasmonic systems. The latter results have also been made available in the form of a web application.
|
|
| 15. |
- Rahm, Magnus, 1990, et al.
(författare)
-
WulffPack: A Python package for Wulff constructions
- 2021
-
Ingår i: Journal of Open Source Software. - : The Open Journal. - 2475-9066. ; 5:45
-
Tidskriftsartikel (refereegranskat)abstract
- WulffPack is a Python package that carries out the Wulff construction and its generalizations using an efficient algorithm based on calculation of the convex hull of the vertices of the dual of the Wulff polyhedron. The user provides surface energies and crystal symmetry and WulffPack returns a versatile object that, at its core, contains the coordinates of the Wulff shape. Extraction of symmetry operations is handled internally with spglib. WulffPack includes functionality for visualizing the constructed shapes using Matplotlib. There are also functions for analyzing the constructed shape, most notably in terms of area fraction of symmetrically inequivalent facets. This quantity is important in applications where properties of the material are facet-dependent, such as in catalysis. Finally, using the Atomic Simulation Environment, an atomistic representation of the Wulff shape can also be extracted. An extensive user guide including a documentation of the API is available at http://wulffpack.materialsmodeling.org/.
|
|
