FORMATION OF SINGLE CRYSTALS OF GOLD WITH INTRICATE SHAPES AND MORPHOLOGIES BY EUTECTIC DECOMPOSITION

Maria Koifman Khristosov Alex Katsman Yaron Kauffmann Boaz Pokroy
Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa, Israel

Single crystals in nature often demonstrate fascinating intricate porous morphologies rather than classical faceted surfaces. Inspired by these biogenic crystals, we show that nanoporous and curved single crystals of gold can be grown with no need for any elaborate fabrication steps. We use the Au-Ge eutectic phase diagram for this purpose. After evaporating thin films of Au and Ge on a non-reactive surface, annealing is performed and due to deweting, micron-size droplets of Au-Ge melt are obtained on the surface. Then, after cooling to room temperature, these droplets solidify, and intricate and curved shapes of gold single crystals are obtained inside these droplets. The shape of the gold single crystals depends first of all on the relative ratio of Au-Ge inside the droplets. By using the exact eutectic concentration, the droplets have a eutectic nanostructure, and after selective etching of Ge, a nanoporous gold single crystal droplets are revealed. By using hypo-eutectic concentration (with Au access), curved single crystal of gold, wrapped by nanoporous gold, is obtained, when all the structure is a single crystal. By using hyper-eutectic concentration, Ge curved crystals are obtained inside the droplets, wrapped by gold-germanium eutectic structure. We investigated this gold intricate structure by TEM, sub-micron scanning synchrotron diffractometry and EBSD, and showed that these gold structures are indeed single crystals. We also showed that nanoporous single crystal gold, prepared by eutectic decomposition, demonstrated superior thermal stability as compared to their counterpart nanoporous gold prepared by dealloying, which is essential for catalysis processes.

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