“…Additionally, nanoscale Kirkendall voids enable great control of their size and position by manipulating temperature or multilayer structure and thickness . However, challenges still remains for the formation of controllable void size over 100 nm in planar films that are essential for various applications, for instance, optical devices including light extraction layer for organic light emitting devices (OLEDs), absorption enhancement film for photovolotics and functional devices for biological and chemical process because thermal diffusion of atoms in metal oxide such as ZnO has limits in their transport length in nanometer scale . Some diffusion enhancement strategies have been explored to obtain sizable voids of submicrometer scale, including applying electric field, introducing dopants to core species (e.g., Al in ZnO), or adopting structural stress …”