ANISOTROPIC INTERMIXING IN Au-Fe BIMETALLIC NANOWHISKERS

The classical laws of diffusion in metals fail at the nanoscale because of the steep concentration gradients and scarcity of vacancy sources. Here, we conducted a series of systematic diffusion measurements on Au-Fe bimetallic nanowhiskers. Defect-free -oriented Au nanowhiskers having {001} and {111} side facets were grown by molecular beam epitaxy technique, followed by another deposition of Fe layers. The Fe layers deposited on {001} and {111} Au facets were single- and polycrystalline, respectively. Focused ion beam (FIB) milling and lift-out technique was used to harvest individual Au-Fe nanowhiskers for heat treatments and subsequent transmission electron microscopy (TEM) sample preparation in cross section geometry with the zone axis parallel to the nanowhiskers axis. Scanning transmission electron microscopy (STEM) with atomic resolution high-angle angular dark-field (HAADF) imaging and electron energy loss spectroscopy (EELS) scanning were carried out for the characterization of the atomic structure of interfaces and composition distribution in their vicinity. We uncovered anisotropic penetration of Fe in Au along and directions, even though the diffusion in cubic crystal should be isotropic. The penetration depth of Fe in Au along was much shorter than it was along direction and both were shorter than the values estimated from the literature data on bulk diffusion of Fe in Au. We attribute this anisotropic intermixing to the difference in mobility of single- and polycrystalline Fe-Au interfaces. We developed a kinetic model allowing determination of the interface mobilities from our interdiffusion data. The slow mobility of the Fe-{001} Au interface was correlated with its atomic structure.