Formation of Single Crystals of Gold with Intricate Shapes and Morphologies

Seminars
08.12.2016
14:30
David Wang Auditorium, 3rd floor Dalia Maydan Bldg.
Maria Koifman Khristosov, PhD candidate

Maria Koifman Khristosov, PhD candidate

The Norman Seiden Multidisciplinary Graduate program in Nanotechnology & Nanoscience and Department of Materials Science and Engineering
Technion – Israel Institute of Technology, 32000 Haifa, Israel
Single crystals in nature often demonstrate fascinating intricate porous morphologies rather than classical faceted surfaces. Drawing inspiration from such 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. Following cooling to room temperature, these droplets solidify and intricate and curved shapes of gold single crystals are obtained within the droplets.
The morphology of the gold single crystals depends mainly 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, nanoporous gold single crystal droplets are revealed. By using a hypo-eutectic concentration (with access of gold), curved single crystal of gold, wrapped by nanoporous gold, is obtained, while all the structure behaves as one single crystal. By using hyper-eutectic concentration, Ge curved crystals are obtained inside the droplets, wrapped by gold-germanium eutectic structure.
We investigated these intricate structures of single crystalline gold utilizing HRTEM, sub-micron scanning synchrotron diffractometry and EBSD, and showed that these gold structures are indeed single crystals. In addition, we present a kinetic model that shows how nanoporous gold grows into a single crystal. Additionally, we explain how curved crystals can be grown inside a droplet.
Last but not least we show that nanoporous single crystals prepared via eutectic decomposition demonstrate superior thermal stability as compared to their counterpart nanoporous gold prepared by dealloying, which is essential for catalysis.

Supervisor: Prof. Boaz Pokroy

References:
1. Koifman Khristosov, M.; Bloch, L.; Burghammer, M.; Kauffmann, Y.; Katsman, A.; Pokroy, B., Nat Commun 2015, 6
2. Koifman Khristosov, M.; Kabalah-Amitai, L.; Burghammer, M.; Katsman, A.; Pokroy, B., ACS nano 2014, 8, 4747-4753