Starts 28 Feb 2013 15:30
Ends 28 Feb 2013 20:00
Central European Time
SISSA, Santorio Building, Room 131 (1st floor)
The structures of metallic nanoparticles are searched for by a computational methodology combining global optimization searches within an atomistic model and density-functional local relaxation. Different systems are considered, from single-metal nanoparticles adsorbed on magnesium oxide, to free and supported nanoalloys. Metallic nanoparticles adsorbed on MgO are of great interest for applications in catalysis. Here we show that a variety of different morphologies can be obtained depending on the magnitude of the lattice mismatch and on the interaction strength with the substrate. We consider Au, Ag and Ni clusters on MgO(100) and compare our results with the experimental findings. Nanoalloys with core-shell arrangement are of special interest in applications, such as in optics, catalysis, magnetism and biomedicine. Despite this wide interest, the physical factors stabilizing the structures of these nanoparticles are still unclear to a great extent, especially for what concerns the relationship between geometric structure and chemical ordering. Here global-optimization searches are performed in order to determine the most stable chemical ordering patterns corresponding to the most important geometric structures, for a series of weakly miscible systems, including AgCu, AgNi, AgCo andC AuCo. Our calculations show that a) the overall geometric structure of the nanoalloy and the shape and placement of its inner core are strictly correlated; b) centered cores can be obtained in icosahedral nanoparticles but not in crystalline or decahedral ones, in which asymmetric quasi-Janus morphologies form; c) in icosahedral nanoparticles, when the core exceeds a critical size, a new type of morphological instability develops, making the core asymmetric and extending it towards the nanoparticle; d) multi-center patterns can be obtained in large polyicosahedral nanoalloys. Analogies and differences between the instability of the core in icosahedral nanoalloys and the Stranski-Krastanov instability occurring in thin-film growth are discussed. All these issues are crucial for designing strategies to achieve effective coatings of the cores.
  • M. Poropat