arXiv subjects

Stephan Barcikowski

Publications and source records attributed to Stephan Barcikowski.

9 recordsLinked to original sources

arXiv2026

Laser-generated CuPdAgPtAu High-Entropy Alloy Nanoparticles -- Thermal Segregation Threshold and Elemental Segregation

High-entropy alloy nanoparticles synthesized via laser ablation in liquid are promising for catalysis due to their ability to form simple solid solutions despite chemical complexity. In this study, noble metal HEA NPs (CuPdAgPtAu) are produced from equimolar and Cu- or Ag-enriched bulk targets. Advanced electron microscopy, XRD, and atomistic simulations are used for structural and compositional analysis. Equimolar targets and NPs exhibit a single fcc phase. In contrast, Cu- or Ag-enriched targets show phase segregation into two fcc phases, which is not observed in the synthesized NPs. Simulations predict segregation tendencies, including Ag surface enrichment and Pt core enrichment due to surface energy differences. However, experimentally, individual NPs remain compositionally homogeneous. Thermal stability studies reveal that phase segregation can be induced post-synthesis. Upon heating, Cu-Ag segregation occurs, forming a second fcc phase similar to bulk targets. These findings demonstrate that rapid quenching during laser ablation suppresses thermodynamically driven segregation and stabilizes metastable solid solutions under kinetic control. Subsequent slow heating overcomes kinetic barriers, enabling equilibrium phase formation at higher temperatures. The thermal stability of these NPs and their tunable composition, including Cu enrichment beyond equilibrium limits, make them promising for high-temperature catalytic applications while reducing noble metal usage.

cond-mat.mtrl-sci↗

arXiv2025

Microparticle laser fragmentation in liquids: mechanisms, energetics, and efficiency quantified with single-pulse, single-particle precision

Microparticle laser fragmentation in liquids has emerged as a promising approach to generate nanoparticles with high efficiency. Despite its advantages, the underlying fragmentation mechanisms, their connection to the nanoparticle size distribution, and the energy efficiency of the process remain poorly understood. In this study for the first time, microparticle fragmentation is investigated in single-pulse, single-particle experiments on Au microparticles. Determining the absorbed peak fluence enables assessment of the process energetics. Pump-probe microscopy identifies photomechanical fracture of the molten microparticle volume and photothermal phase explosion of its superheated surface as the fragmentation mechanisms. We find that 83% of the absorbed laser energy is converted into cavitation bubble formation, while only 1% contributes to the surface energy of the generated nanoparticles. Despite this small fraction, MP-LFL outperforms laser ablation in liquids. The surface energy generated per absorbed energy is 10 times higher, and the overall energy efficiency is 14 times higher. This gain originates from the confined microparticle geometry, which minimizes energy losses and enhances photomechanical fragmentation via pressure focusing. These results position microparticle fragmentation in liquids as a fundamentally more energy-efficient approach for scalable, laser-based nanoparticle production than laser ablation in liquids.

physics.optics↗

arXiv2025

Controlling Metastability through Annealing of High-Entropy Nanoalloy Electrocatalysts to Boost Performance towards the Oxygen Evolution Reaction

Low-cost transition metal high-entropy nanoalloys are emerging as sustainable alternatives to platinum group electrocatalysts. Synthesis conditions of single-phase solid solutions can alter phase stability, causing surface composition changes that affect electrocatalytic performance. Here, we propose to exploit the metastability of carbon-doped Cantor alloy-based amorphous high-entropy alloy nanoparticles produced by nanosecond-pulsed laser synthesis in organic solvents. In situ electron microscopy reveals crystallization and partitioning of elements upon heating to 600 {\deg}C, forming heterostructured nanoparticles with reinforced carbon shells that exhibit a 5- to 7-fold enhancement of the electrocatalytic activity compared to the as-synthesized counterparts for the oxygen evolution reaction. We demonstrate the strategic utilization of phase metastability in high-entropy nanoalloys through post-synthesis annealing to enhance the electrochemical activity of laser-generated nanoparticles.

cond-mat.mtrl-sci↗

arXiv2025

Specimen preparation for atom probe tomography analysis of complex multifunctional nanoparticles and nanostructures: state-of-the-art and challenges

Atom probe tomography (APT) provides the three-dimensional composition of materials at near-atomic length scales, achieving detection limits in the range of tens of atomic parts-per-million regardless of element type. APT requires the specimen to be shaped as a needle with a tip radius of ~100 nm. The development of site-specific lift-out procedures using focused ion beam-scanning electron microscopy (FIB-SEM) has enabled APT analysis of multifunctional materials, advancing our understanding of their structure-composition-property relationships. Yet these approaches are not readily suitable for analyzing many nanomaterials. Co-electrodeposition of metallic films forms a composite containing the nanomaterials of interest, thereby facilitating APT specimen preparation and enabling analysis of nanowires, nanosheets, and nano- and microparticles, etc. In this perspective article, we showcase diverse examples from simple elementary to compositionally complex alloys of varying dimensionalities, from individual nanoparticles to aerogel structures. We emphasize the challenges encountered with specific material classes during co-electrodeposition procedures and provide recommendations for improving specimen preparation protocols to enhance measurement yield, thereby advancing APT analysis capabilities for optimizing the performance of functional nanomaterials.

cond-mat.mtrl-sci↗

arXiv2025

Kinetically controlling surface atom arrangements in thermally robust, amorphous high-entropy alloy nanoparticles by solvent selection

The ability to tailor nanoscale surface atom arrangements through multi-elemental compositional control provides high-entropy nanoalloys with promising functional properties. Developing a fundamental understanding of nanoalloy formation mechanisms during synthesis is therefore essential for effectively engineering the surface composition and resulting functional properties. Using the Cantor alloy (CrMnFeCoNi) as a model system, we investigate how solvent selection during reactive, nanosecond-pulsed laser synthesis influences carbon doping and the resulting changes in nanoparticle morphology, structure, and composition. Supersaturated carbon incorporation, partitioned from the organic solvent molecules, produces amorphous nanoparticles with distinctive carbon shells, thermally stable up to 350 {\deg}C. We propose kinetically controlled particle formation mechanisms and rationalize the criticality of the time scales between the competing reactions of carbon doping, carbon shell formation, and coalescence of metallic fragments, ruling compositional and morphological characteristics. This work demonstrates effective solvent-driven surface-compositional control in amorphous high-entropy nanoalloys. It introduces a novel synthesis approach for tailoring surface atom arrangements through carbon incorporation via reactive, pulsed laser synthesis.

cond-mat.mtrl-sci↗

arXiv2025

Effect of Ag nano-additivation on microstructure formation in Nd-Fe-B magnets built by laser powder bed fusion

Laser powder bed fusion (PBF-LB/M) enables the near-net shape production of permanent magnets with complex geometry while reducing material waste. However, controlling the microstructure and optimizing magnetic properties remain challenging due to rapid solidification and intrinsic heat treatment effects occurring during both inter-layer and intra-layer processing. Surface additivation of the feedstock powder with Ag nanoparticles (NPs) is a concept that has been shown to increase the coercivity of PBF-LB/M-produced Nd-Fe-B magnets. Using atom probe tomography (APT) and transmission electron microscopy (TEM), we reveal that Ag nano-additivation promotes heterogeneous nucleation of the Nd2Fe14B phase, leading to refined, equiaxed grains and increased stability of the Ti-Zr-B-rich intergranular phase. The intrinsic heat treatment, influenced by layer-wise processing, further affects the distribution of Ag-rich regions, impacting grain growth and intergranular phase composition across different regions of the melt pool. Compared to the unadditivated sample, the Ag-additivated sample exhibits a significantly finer grain structure and a changed intergranular phase, which contribute to enhanced domain wall pinning and coercivity. These microstructural changes directly modify the magnetic domain structure, as evidenced by Lorentz transmission electron microscopy (TEM). Our results highlight that the interplay between nano-additivation and in-process heat treatment provides a novel pathway for tailoring the microstructure and enhancing the magnetic performance of permanent magnets.

cond-mat.mtrl-sci↗

arXiv2023

Particle-Based Simulations of Electrophoretic Deposition with Adaptive Physics Models

This work represents an extension of mesoscale particle-based modeling of electrophoretic deposition (EPD), which has relied exclusively on pairwise interparticle interactions described by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. With this standard treatment, particles continuously move and interact via excluded volume and electrostatic pair potentials under the influence of external fields throughout the EPD process. The physics imposed by DLVO theory may not be appropriate to describe all systems, considering the vast material, operational, and application space available to EPD. As such, we present three modifications to standard particle-based models, each rooted in the ability to dynamically change interparticle interactions as simulated deposition progresses. This approach allows simulations to capture charge transfer and/or irreversible adsorption based on tunable parameters. We evaluate and compare simulated deposits formed under new physical assumptions, demonstrating the range of systems that these adaptive physics models may capture.

cond-mat.mes-hall↗

arXiv2021

Multidimensional thermally-induced transformation of nest-structured complex Au-Fe nanoalloys towards equilibrium

Bimetallic nanoparticles are often superior candidates for a wide range of technological and biomedical applications, thanks to their enhanced catalytic, optical, and magnetic properties, which are often better than their monometallic counterparts. Most of their properties strongly depend on their chemical composition, crystallographic structure, and phase distribution. However, little is known of how their crystal structure, on the nanoscale, transforms over time at elevated temperatures, even though this knowledge is highly relevant in case nanoparticles are used in, e.g., high-temperature catalysis. Au-Fe is a promising bimetallic system where the low-cost and magnetic Fe is combined with catalytically active and plasmonic Au. Here, we report on the in situ temporal evolution of the crystalline ordering in Au-Fe nanoparticles, obtained from a modern laser ablation in liquids synthesis. Our in-depth analysis, complemented by dedicated atomistic simulations, includes a detailed structural characterization by X-ray diffraction and transmission electron microscopy as well as atom probe tomography to reveal elemental distributions down to a single atom resolution. We show that the Au-Fe nanoparticles initially exhibit highly complex internal nested nanostructures with a wide range of compositions, phase distributions, and size-depended microstrains. The elevated temperature induces a diffusion-controlled recrystallization and phase merging, resulting in the formation of a single face-centered-cubic ultrastructure in contact with a body-centered cubic phase, which demonstrates the metastability of these structures. Uncovering these unique nanostructures with nested features could be highly attractive from a fundamental viewpoint as they could give further insights into the nanoparticle formation mechanism under non-equilibrium conditions.

cond-mat.mtrl-sci↗

arXiv2020

Size-Selective Optical Printing of Silicon Nanoparticles through Their Dipolar Magnetic Resonance

Silicon nanoparticles possess unique size-dependent optical properties due to their strong electric and magnetic resonances in the visible range. However, their widespread application has been limited, in comparison to other (e.g. metallic) nanoparticles, because their preparation on monodisperse colloids remains challenging. Exploiting the unique properties of Si nanoparticles in nano- and micro-devices calls for methods able to sort and organize them from a colloidal suspension onto specific positions of solid substrates with nanometric precision. Here, we demonstrate that surfactant-free Silicon nanoparticles of a predefined and narrow ($\sigma$ < 10 nm) size range can be selectively immobilized on a substrate by optical printing from a polydisperse colloidal suspension. The size selectivity is based on differential optical forces that can be applied on nanoparticles of different sizes by tuning the light wavelength to the size-dependent magnetic dipolar resonance of the nanoparticles.

physics.optics↗