Time-resolved investigation of Co-Ag and Ni-Ag nanoalloy formation: effect of the metal deposition sequence P. Andreazza 1* , A. Coati 2 , Y. Garreau 2 , R. Ferrando 3 , D. Forster 1 , A. Hizi 1 , J. Creuze 4 , C. Andreazza-Vignolle 1 1 Interfaces, Confinement, Matériaux et Nanostructures, ICMN, Université d’Orléans, France, 2 Synchrotron Soleil, L’Orme de Merisiers, France, 3 Università di Genova, Physics Department, Italy, 4 ICMMO, Univ. Paris-Saclay, France As lot of Ag-based binary metallic alloys, Ag-Co and Ni-Ag systems are a very weakly miscible system in wide ranges of temperature and composition. We can distinguish three main driving forces in order to predict the nature and the quantity of the segregating species in a binary alloy: the differences in surface energies and radii of the two elements (mismatch-effect), and their ability to mix in the bulk. In Co-Ag and Ni-Ag systems, we can predict a surface segregation of silver since Ag presents lower surface energy and larger size (to minimize elastic-energy), in addition to the immiscibility behavior [1,2]. At nanoscale, the size reduction favors a segregation behavior by surface and core contraction effects that can be opposed to kinetic trapping effects induced by the growth mode [1]. In this case of UHV atomic deposition and condensation on the surface substrate, the mobility of Ag and Co (or Ni) atoms on the substrate and on the particles must be considered. Consequently, the effect of substrate, growth kinetics and annealing was followed in several configurations of deposition (simultaneous and sequential deposition of metals). Experimentally, morphological and structural evolutions of clusters are studied by HRTEM/EFTEM/HAADF techniques combined with in situ and real time wide- and small-angle X-ray scattering in grazing incidence simultaneously (GIWAXS and GISAXS): the metastable deposition mode at room temperature, i.e. by depositing Co or Ni above an Ag core, and the more stable reverse deposition mode until their equilibrium state obtained by thermal activation. In addition, the quantitative structural analysis of experimental data was facilitated and consolidated using atomistic simulations of the cluster growth by molecular dynamics (MD) to reveal the atom migration leading to phase separation or core-shell formation. In a metastable deposition mode, i.e. by depositing Co above an Ag core, the configuration is complex: Co atoms incorporate the initial Ag core in sub-layer position leading to Janus then core-shell configuration with the Co content [5]. In the more stable reverse deposition mode, unexpected, Ag nucleate in domains on Co core, rather than in shell configuration, due to a combined substrate and kinetic effect. In addition, the Ni-Ag case reveals that Janus configuration is favored rather than core-shell with respect to the same experiment in the Co-Ag case due to several mechanisms. References
1. I. Parsina, F. Baletto, J. Phys. Chem. C (2010) 114, 1504 2. D. Bochicchio, R. Ferrando, Phys. Rev. B (2013) 87, 165435 3. P. Andreazza, in Nanoalloys , Éd. Springer London, (2012) , p69-112
4. P. Andreazza, V. Pierron-Bohnes, F. Tournus, C. Andreazza-Vignolle, V. Dupuis, Surf. Sci. Rep., (2015) 70, 2, 188 5. P. Andreazza, A. Lemoine, A. Coati, D. Nelli, R. Ferrando, Y. Garreau, J. Creuze, C. Andreazza-Vignolle, Nanoscale (2021) 13, 6096
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