3D networks of nanopores in alumina: Structural and optical properties

Abstract

The production of spatially ordered arrays of nanopores in alumina is attracting considerable attention due to their valuable filtering and sensing applications. Here we investigate the production conditions of thin films consisting of a continuous 3D network of nanopores in alumina, with the network nodes arranged in a body-centered tetragonal lattice, and the pore size about 1 nm. The emphasis is on controlling the nanopores length, radius, and geometry of their network by the preparation parameters. The material is produced from 3D network of 1D Ge nanowires in alumina by a dedicated annealing treatment that causes Ge to evaporate. We show that the main limiting factors for the production of the nanopore networks are the fraction of Ge nanowires before annealing combined with the unit cell parameters of their network. The 3D networks of nanopores form easily when the Ge atomic fraction in the film is less than 40 %. For larger Ge percentages, the alumina matrix collapses because the interconnection to the neighbouring unit cells in the vertical direction is lost. The refractive index of the materials can be tuned in a wide range by varying the porosity, i.e. the parameters of the Ge nanowire network.