| 1. |
- Löfgren, Joakim, 1989, et al.
(författare)
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Computational assessment of the efficacy of halides as shape-directing agents in nanoparticle growth
- 2020
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Ingår i: Physical Review Materials. - 2475-9953. ; 4:9
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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.
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| 2. |
- De Knoop, Ludvig, 1972, et al.
(författare)
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Electric-field-controlled reversible order-disorder switching of a metal tip surface
- 2018
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Ingår i: Physical Review Materials. - 2475-9953. ; 2:8
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Tidskriftsartikel (refereegranskat)abstract
- While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.
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| 3. |
- Eklöf, Johnas, 1988, et al.
(författare)
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Understanding Interactions Driving the Template-Directed Self-Assembly of Colloidal Nanoparticles at Surfaces
- 2020
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:8, s. 4660-4667
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Tidskriftsartikel (refereegranskat)abstract
- Controlled deposition of colloidal nanoparticles using self-assembly is a promising technique for, for example, manufacturing of miniaturized electronics, and it bridges the gap between top-down and bottom-up methods. However, selecting materials and geometry of the target surface for optimal deposition results presents a significant challenge. Here, we describe a predictive framework based on the Derjaguin-Landau-Verwey-Overbeek theory that allows rational design of colloidal nanoparticle deposition setups. The framework is demonstrated for a model system consisting of gold nanoparticles stabilized by trisodium citrate that are directed toward prefabricated sub-100 nm features on a silicon substrate. Experimental results for the model system are presented in conjunction with theoretical analysis to assess its reliability. It is shown that three-dimensional, nickel-coated structures are well suited for attracting gold nanoparticles and that optimization of the feature geometry based on the proposed framework leads to a systematic improvement in the number of successfully deposited particles.
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| 4. |
- Larsen, A. H., et al.
(författare)
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libvdwxc: a library for exchange-correlation functionals in the vdW-DF family
- 2017
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Ingår i: Modelling and Simulation in Materials Science and Engineering. - 1361-651X .- 0965-0393. ; 25:6
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Tidskriftsartikel (refereegranskat)abstract
- We present libvdwxc, a general library for evaluating the energy and potential for the family of vdW-DF exchange-correlation functionals. libvdwxc is written in C and provides an efficient implementation of the vdW-DF method and can be interfaced with various general-purpose DFT codes. Currently, the GPAW and Octopus codes implement interfaces to libvdwxc. The present implementation emphasizes scalability and parallel performance, and thereby enables ab initio calculations of nanometer-scale complexes. The numerical accuracy is bench-marked on the S22 test set whereas parallel performance is benchmarked on ligand-protected gold nanoparticles (Au-144(SC11NH25)(60)) up to 9696 atoms.
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| 5. |
- Löfgren, Joakim, 1989
(författare)
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Elements of modeling nanoparticle growth: Surface thermodynamics and dispersive interactions
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metal nanoparticles have in recent decades been the subject of intense research owing to their wide range of size and shape-dependent properties, which makes them interesting candidates for a variety of applications. Gold nanorods represent a particularly intriguing type of particle due to their tunable plasmonic properties. To benefit from these features, access to a large supply of high-quality, monodisperse nanorods is required, but has yet to be realized. The most facile route to obtaining nanorods is through seeded-mediated growth, a family of wet-chemical synthesis protocols. While such protocols have the potential of delivering high quantities of rods with finely tuned properties, progress is being hampered by a lack of theoretical understanding regarding the growth mechanism. The aim of this thesis is to lay the foundation for a comprehensive picture of the gold nanorod growth process. In particular, the role of the CTAB surfactant layer covering the nanorod surfaces is investigated. The discussion is based on first-principles density functional theory calculations, molecular dynamics simulations carried out in our research group, and a critical review of ideas found in literature. A framework for explaining experimental observations and predicting the outcome of synthesis processes as a function of their parameters is presented based on the surface phase diagram of CTAB. A key feature of the phase diagram, in addition to the conventional bilayer structure that is often assumed, is the existence of cylindral and spherical micelles. These micelles expose the underlying gold surface to incoming gold ions that fuel the growth. The importance of dispersive interactions in understanding transitions between the different phases is emphasized. Furthermore, different models for the anistropic growth are reviewed within the newly established framework, and are complemented by a discussion of the impact of the seed particle structure.
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| 6. |
- Löfgren, Joakim, 1989
(författare)
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Modeling colloidal nanoparticles: From growth to deposition
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent decades metal nanoparticles (NPs) have been the subject of intense research. The interest stems from the NPs physicochemical properties that can be conveniently tuned through, e.g., their size, shape or composition. A good example is the selective absorption of electromagnetic radiation exhibited by gold nanorods, which is leveraged for applications in sensing and medicine. In order to realize the full potential of technologies reliant on NPs and ensure fitness for commercial use, facile fabrication methods that allow for a high degree of shape and size control are required. For this purpose, wet-chemistry-based syntheses in which colloidal NPs self-assemble into a targeted morphology have emerged as promising candidates. Development and refinement of synthesis protocols is, however, hampered by a lack of theoretical understanding of the complex chemical environment in NP solutions. As a result, experimental workers are often left to rely on intuition. This applies not only to the growth process itself, but also later down the processing chain, e.g., during NP deposition. This thesis aims to address two problem areas relating to NP growth and deposition where current models need improvement. The first such area concerns the description of ionic and molecular adsorption on the surface of metal NPs. We show how combining thermodynamic modeling, density functional theory and experimental data can lead to more realistic NP shape predictions. A closely related subject is the growth mechanism of anisotropic gold nanorods, which has been a subject of debate for almost the three decades. Here, we consider possible avenues through which shape anisotropy can arise using insight from molecular dynamics simulations. The second problem area is the description of forces between NPs and nearby surfaces, which is relevant, e.g, for applications reliant on NP deposition. A model based on Derjaguin-Landau-Verwey-Overbeek theory is developed that describes how the shape and composition of a surface affects particle deposition.
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| 7. |
- Löfgren, Joakim, 1989, et al.
(författare)
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Understanding the Phase Diagram of Self-Assembled Monolayers of Alkanethiolates on Gold
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:22, s. 12059-12067
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Tidskriftsartikel (refereegranskat)abstract
- Alkanethiolate monolayers on gold are important both for applications in nanoscience as well as fundamental studies of adsorption and self-assembly at metal surfaces. While considerable experimental effort has been put into understanding the phase diagram of these systems, theoretical work based on density functional theory (DFT) has long been hampered by the inability of conventional exchange-correlation functionals to describe dispersive interactions. In this work, we combine dispersion-corrected DFT calculations using the new vdW-DF-CX functional with the ab initio thermodynamics method to study the stability of dense standing-up and low-coverage lying-down phases on Au(111). We demonstrate that the lying-down phase has a thermodynamic region of stability starting from thiolates with alkyl chains consisting of n ? 3 methylene units. This phase emerges as a consequence of a competition between dispersive chain-chain and chain-substrate interactions, where the strength of the latter varies more strongly with n. A phase diagram is derived under ultrahigh-vacuum conditions, detailing the phase transition temperatures of the system as a function of the chain length. The present work illustrates that accurate ab initio modeling of dispersive interactions is both feasible and essential for describing self-assembled monolayers.
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| 8. |
- Rahm, Magnus, 1990, et al.
(författare)
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A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd–Au–H
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 211
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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.
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| 9. |
- Rahm, Magnus, 1990, et al.
(författare)
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Quantitative predictions of thermodynamic hysteresis: Temperature-dependent character of the phase transition in Pd–H
- 2022
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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.
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