Copper Deposition on Au(111) in a Deep Eutectic Solvent: An In Situ STM Study**

Electrolyte composition can have significant influences on metal deposition. In this work we employ in-situ scanning tunneling microscope (STM), together with electrochemical methods to investigate Cu deposition on Au (111) in ethaline containing CuCl2. Cyclic voltammograms in the Cu overpotential deposition regime are dominated by a rapidly rising cathodic current, accompanied by a preceding shoulder. Instantaneous nucleation and three-dimensional growth are discerned for the Cu deposition at larger overpotential. In-situ STM reveals the shoulder process is associated with the formation of monoatomic-height clusters. Meanwhile, morphology evolutions of Cu nuclei along with time and potential are followed, and layered hexagonal and hexagon fortress shaped Cu nuclei and their growth are directly observed. Furthermore, atomic-resolution imaging elucidates an ordered incommensurate (6x root 79) adsorption structure on the surface of a Cu nucleus. Our observations are helpful in gaining deeper insights into the effect of deep eutectic solvents (DES) on Cu deposition at microscopic level.