Research Update: Fast and tunable nanoionics in vertically aligned nanostructured films

This review provides the design principles to develop new nanoionic applications usingvertically aligned nanostructured (VAN) thin films, incorporating two phases whichself-assemble in one film. Tunable nanoionics has attracted great attention for energy anddeviceapplications, such as ion batteries, solid oxide fuel cells, catalysts, memories, andneuromorphic devices. Among many proposed device architectures, VAN films have strong potential fornanoionic applications since they show enhanced ionic conductivity and tunability. Here, we will review therecent progress on state-of-the-art nanoionic applications, which have been realized byusing VAN films.In many VAN systems made by the inclusion of an oxygen ionic insulator, it is found thations flow through the vertical heterointerfaces. The observation is consistent with structuralincompatibility at the vertical heteroepitaxial interfaces resulting in oxygen deficiency in one of thephases and hence to oxygen ion conducting pathways. In other VAN systems where one of thephases is an ionic conductor, ions flow much faster within the ionic conducting phase thanwithin the corresponding plain film. The improved ionic conduction coincides with much improvedcrystallinity in the ionically conducting nanocolumnar phase, induced by use of the VANstructure. Furthermore, for both cases Joule heating effects induced by localized ioniccurrent flow also play a role for enhanced ionic conductivity. Nanocolumn stoichiometry and strain areother important parameters for tuning ionic conductivity in VAN films. Finally, double-layeredVAN filmarchitectures are discussed from the perspective of stabilizing VAN structures which wouldbe less stable and hence less perfect when grown on standard substrates