Spark Ablation—A Novel Technique for Generation and Self-Assembly of Multifunctional Magnetic Nanoparticles

Smart nanomaterials with designed properties based on nanoparticles have the potential to revolutionize applications in magnetics, catalysis, and optoelectronics. However, implementing nanoparticles’ potential for such applications requires realizing and understanding nanoparticles with controllable size, morphology, crystal structure, and chemical composition on a large scale, at low costs, and in a safe and environmentally friendly way. Aerosol technology, using spark ablation, is a promising route that fulfills the requirements above.<br/>Spark ablation is based on heating and evaporation of material by forming a plasma channel between two conducting electrodes and subsequent condensation of the resulting vapor into nanoparticles when transported away by an inert carrier gas. Due to its simple design, it was recently shown that spark ablation could be easily scaled up at low cost and with minimal impact on the environment due to high energy efficiency in converting electrodes into particles and the possibility of recycling the non-heated carrier gas. Unlike nanoparticles produced by wet chemical synthesis, aerosol-generated nanoparticles will have a surface free from chemical residues that can affect their magnetic and/or optoelectronic performance, a monodisperse size distribution can be obtained, and the particle production is continuous. A final advantage with spark ablation is the possibility of generating alloy and mixed metal particles easily; even the production of mixed metal particles consisting of materials immiscible in bulk has been reported, as well as the generation of single particles consisting of up to six different elements.<br/>Here, we present the generation of magnetic single-element and multi-component nanoparticles and their self-assembly into larger magnetic structures. A combined electric and magnetic field guides the self-assembly, and the nanoparticles can be assembled one by one along any direction to form 1D, 2D, or 3D structures. In these systems, both shape and magneto-crystalline contributions play a key role in the magnetic anisotropy resulting in enhanced magnetic properties such as higher coercivity. The self-assembly allows for direct integration of the magnetic structures onto suitable substrates for, e.g., transport measurements, offering another advantage compared to standard solution-based methods.<br/>Furthermore, we show the possibility of combining both magnetic and catalytic elements in single nanoparticles to create multifunctional particles. Also, the combination of different single-element particle segments in 1D chain structures will be presented. Advanced characterization of the magnetic/multifunctional nanoparticle-based structures include high resolution transmission electron microscopy (HRTEM), X-ray energy dispersive spectroscopy (XEDS) in scanning (S)TEM mode, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) performed at different synchrotrons, superconducting quantum interference device (SQUID), X-ray magnetic circular dichroism (XMCD), and planar laser-induced fluorescence (PLIF). Finally, the experimental results are supported by simulations.<br/>.