Toward Discrete Multilayered Composite Structures: Do Hollow Networks Form in a Polycrystalline Infinite Nanoplane by the Kirkendall Effect?

Spatial confinement in nanostructures is of critical importance in the fabrication of tubular and hollow spherical objects using void formation via the Kirkendall effect. For both core-shell nanowires and nanospheres, generated vacancies are trapped within an area that is confined either in two (nanowires) or all three (nanospheres) spatial dimensions. When the void formation is extended to multilayered thin films where only one dimension (thickness) is in the nanoscale and the other dimensions are infinitely extended, the final morphology of the formed voids can be significantly different. Using a multilayered system consisting of alternating layers of ZnO and Al2O3 grown by atomic layer deposition (ALD), we investigate the effects of annealing temperature, annealing duration, layer thickness, polycrystallinity, grain size, and reaction space on the solid-state diffusion process and final morphology of the produced Kirkendall voids. As opposed to single-crystal ZnO nanowires coated with an amorphous Al2O3 shell, which involves only a one-way transfer of ZnO into Al2O3, polycrystalline ZnO layers in the multilayered films also cause diffusion of Al2O3 into ZnO layers via grain boundary diffusion. Temperature treatment at 700 degrees C generally yielded layered voids for multilayered films with thin component sublayers. This morphology was well-preserved even at 800 degrees C. In contrast, partially continuous nanogap morphologies were formed for multilayered films with thick sublayers at 700 degrees C. However, only interlaced voids were produced at 800 degrees C in this case, because of grain boundary migration induced oriented attachment. The mechanisms revealed here allow precise fabrication and design of porous multilayered composite films with controlled void morphology.