Nanostructured semiconductors exhibit promising optoelectronic properties, such as high photoluminescence quantum yield and efficient charge separation, making them attractive materials for applications including photovoltaics, LEDs, and lasers. These superior properties are often due to improved crystallinity and reduced charge separation distance compared to bulk semiconductors. As a result, it is desirable to synthesize uniform and highly crystalline nanostructures of arbitrary composition and placement relative to adjacent materials. Metal halide perovskites have emerged as a new class of solution-processable, high-performance semiconductors; however, complex compositions and poorly understood surface chemistry have precluded the synthesis of uniform perovskite nanostructures. Furthermore, controlled nanostructured interfaces (e.g. core-shell nanowires), which can dramatically reduce the distance excited charges must travel to be extracted for useful energy, have not been realized for perovskite-based systems.

Anodic aluminum oxide (AAO) templates allow one to overcome these synthetic challenges, as they enable the materials general synthesis of both oriented nanowires, as well as nanotubes that can encapsulate them, with uniform and tunable size. AAO templates are characterized by a dense array of oriented cylindrical nanopores with tunable diameter, length, and spacing; herein, their implementation for nanowire synthesis facilitated the study of diameter-dependent crystallinity in perovskites using powder X-ray diffraction (XRD) and time-resolved photoluminescence spectroscopy (TRPL). Compositional generality was also explored by synthesizing CH3NH3PbI3, CH3NH 3PbBr3, and Cs2SnI6 nanowires, all using a simple spin coating method. Moreover, perovskite-based nanostructured interfaces were designed and synthesized within AAO pores, wherein desired materials were precisely placed in a core-shell architecture. This was accomplished using coaxial lithography (COAL), a powerful electrodeposition-based technique for the synthesis of nanorings and nanotubes with controllable dimensions and compositions. Combined with perovskite nanowire synthesis, COAL enabled the preparation of CH3NH3PbI3 perovskite core--copper thiocyanate shell nanowires. One can use TRPL to monitor excited charge lifetimes and quantify the advantage of short charge separation distance for charge extraction; TRPL decay curves showed that the rate of charge extraction from perovskite by an adjacent semiconductor was accelerated by three orders of magnitude when a radial heterojunction (as opposed to an axial heterojunction only) was introduced.

Successful synthesis and characterization of core-shell nanowire arrays composed of complex materials validates AAO as a versatile template for nanomaterials by design. By combining multiple strategies for nanostructure synthesis within AAO (i.e. electrodeposition and solution-casting), this work sets the foundation for the development of a diverse library of composite nanomaterials with compositions, dimensions, and interfaces that can be tailored for a desired application.