Abstract
The temperature-induced structural changes of Fe–, Co–, and Ni–Au core–shell nanoparticles with diameters around 5 nm are studied via atomically resolved transmission electron microscopy. We observe structural transitions from local toward global energy minima induced by elevated temperatures. The experimental observations are accompanied by a computational modeling of all core–shell particles with either centralized or decentralized core positions. The embedded atom model is employed and further supported by density functional theory calculations. We provide a detailed comparison of vacancy formation energies obtained for all materials involved in order to explain the variations in the restructuring processes which we observe in temperature-programmed TEM studies of the particles.
Highlights
Bimetallic systems at the nanoscale have recently received increased attention as the combination of intermetallic interactions and surface size effects can trigger unexpected physical behavior and new phenomena
Magnetic core nanoparticles have been suggested for the activation of stem cells,[16,17] as enhancers of supercapacitors,[18] or for the optomagnetic fine-tuning of semiconductors.[19]. Due to their synergistic effects, bimetallic nanoparticles formed by a combination of magnetic and noble metals are interesting for various physicochemical applications such as bifunctional catalysis.[20,21]
A surface coverage of 1.1 ± 0.2% was chosen in order to avoid contact between individual clusters on the substrate. The temperature of the latter was increased in steps of 50 °C, starting at room temperature, and high-angle annular dark-field (HAADF) images were taken at each step
Summary
Bimetallic systems at the nanoscale have recently received increased attention as the combination of intermetallic interactions and surface size effects can trigger unexpected physical behavior and new phenomena. Potential applications cover a wide range of different fields, including biomedical applications,[1−5] optics,[6−9] heterogeneous catalysis,[10−13] electrochemistry,[14] and electronics.[14,15] magnetic core nanoparticles have been suggested for the activation of stem cells,[16,17] as enhancers of supercapacitors,[18] or for the optomagnetic fine-tuning of semiconductors.[19] Due to their synergistic effects, bimetallic nanoparticles formed by a combination of magnetic and noble metals are interesting for various physicochemical applications such as bifunctional catalysis.[20,21] Understanding the features of structural stability and metastability in local energetic minima within this novel class of materials would provide us with new possibilities in material design but necessitates an adequate modeling of metallic interactions in large, yet finite systems where surface and interface effects play an important role as atomic diffusion.[22,23] Metastable off-equilibrium structures are strongly affected by thermal rearrangement processes such as surface diffusion, which makes them very interesting for the investigation of thermodynamically induced structural changes but renders their behavior rather challenging to predict. Fe, Co, and Ni are rather similar to each other in their interaction energies as well as their bond lengths, the slight deviations result in a substantially different thermodynamic behavior
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