Optical properties of AgAu clusters: effects of mixing, configuration, and size Hans-Christian Weissker 1 and Florent Calvo 2 1 Centre Interdisciplinaire de nanoscience de Marseille, Aix-Marseille University and CNRS, Marseille, France, 2 Laboratoire Interdisciplinaire de Physique, CNRS & Université Grenoble Alpes, France Gold and silver are, for all their chemical similarities, optically very different. In particular, small Ag clusters show a localized surface-plasmon resonance (LSPR), whereas in Au clusters smaller than about 300 atoms the resonance is absent due to coupling with the interband transitions from the d electrons. This opens the possibility to tune cluster properties depending on composition and chemical configuration. Past work on AgAu alloy clusters has shown that the outermost shell of atoms is crucial for the overall optical properties [1]. This is intuitively natural because the size effects on the plasmon energy are due to the intricate equilibrium between the electron spill-out, the reduced screening from the d electrons, and the interfaces with the matrix [2], all taking place at the surface. However, the translation between these effective concepts and the results of atomistic calculations is not straightforward. In this work, we use, in particular, a realistic multistep rearrangement pathway [3] in which the Ag and Au atoms change interior and surface positions in a 55-atom AgAu nanoalloy. In addition, we consider larger core-shell clusters of up to 561 atoms where the plasmon is already present in gold and where the numbers of surface and interior atoms are comparable. Calculations using Time-dependent Density-Functional Theory are, in particular, analyzed in view of the nature of transitions so as to elucidate the contributions of surface and interior atoms for the interband transitions, thus providing a clearer picture of the nature of the nanoalloys' optical properties. References 1. Xochitl Lopez-Lozano, Christine Mottet, and Hans-Christian Weissker; Effect of Alloying on the Optical Properties of Ag-Au Nanoparticles; J. Phys. Chem. C 117, 3062-3068 (2013). 2. Alfredo Campos, Nicolas Troc, Emmanuel Cottancin, Michel Pellarin, Hans-Christian Weissker, Jean Lerme, Mathieu Kociak and Matthias Hillenkamp; Plasmonic quantum size effects in silver nanoparticles are dominated by interfaces and local environments. Nature Physics 15, 275-280 (2019). 3. F. Calvo, A. Fortunelli, F. Negreiros, and D.J. Wales; Kinetics of chemical ordering in Ag-Au and Ag-Ni nanoalloys, J. Chem. Phys. 139, 111102 (2013).
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