  The Al-Au binary diffusion couple is a classic example of the system
exhibiting Kirkendall voiding during interdiffusion. We demonstrate that this
effect, which is a major reason for failures of the wire bonds in
microelectronics, can be utilized for producing hollow AlAu2 nanoparticles
attached to sapphire substrate. To this end, we produced the core-shell Al-Au
nanoparticles by performing a solid state dewetting treatment of Al thin film
deposited on sapphire substrate, followed by the deposition of thin Au layer on
the top of dewetted sample. Annealing of the core-shell nanoparticles in air
resulted in outdiffusion of Al from the particles, formation of pores, and
growth of the AlAu2 intermetallic phase in the particles. We demonstrated that
the driving force for hollowing is the oxidation reaction of the Al atoms at
the Au-sapphire interface, leading to the homoepitaxial growth of newly formed
alumina at the interface. We developed a kinetic model of hollowing controlled
by diffusion of oxygen through the Au thin film, and estimated the solubility
of oxygen in solid Au. Our work demonstrates that the core-shell nanoparticles
attached to the substrate can be hollowed by the Kirkendall effect in the thin
film spatially separated from the particles.
