Unraveling Interdiffusion\nPhenomena and the Role of\nNanoscale Diffusion Barriers in the Copper–Gold System

Diffusion is one of the most fundamental concepts in\nmaterials\nscience, playing a pivotal role in materials synthesis, forming, and\ndegradation. Of particular importance is solid state interdiffusion\nof metals which defines the usable parameter space for material combinations\nin the form of alloys. This parameter space can be explored on the\nmacroscopic scale by using diffusion couples. However, this method\nreaches its limit when going to low temperatures, small scales, and\nwhen testing ultrathin diffusion barriers. Therefore, this work transfers\nthe principle of the diffusion couples to small scales by using core–shell\nnanowires and <i>in situ</i> heating. This allows us to\ndelve into the interdiffusion dynamics of copper and gold, revealing\nthe interplay between diffusion and the disorder–order phase\ntransition. Our <i>in situ</i> TEM experiments in combination\nwith chemical mapping reveal the interdiffusion coefficients of Cu\nand Au at low temperatures and highlight the impact of ordering processes\non the diffusion behavior. The formation of ordered domains within\nthe solid-solution is examined using high-resolution imaging and nanodiffraction\nincluding strain mapping. In addition, we examine the effectiveness\nof ultrathin Al<sub>2</sub>O<sub>3</sub> barrier layers to control\ninterdiffusion of the diffusion couple. Our findings indicate that\na 5 nm thick layer serves as an efficient diffusion barrier. This\nresearch provides valuable insights into the interdiffusion behavior\nof Cu and Au on the nanoscale, offering potential applications in\nthe development of miniaturized integrated circuits and nanodevices.