Two-Step Growth of Oriented Quasi-2D Perovskite Nanocrystal Arrays with Composition Gradients

<jats:p> In this work, we demonstrate the oriented growth of quasi-2D metal halide perovskite nanowire and nanoplate arrays with continuous corner-sharing metal halide octahedra sheets normal to the substrate surface, the optimal orientation for device stacks. Oriented perovskite growth is achieved through two-step deposition in TiO <jats:sub>2</jats:sub> nanoporous scaffolds in which PbI <jats:sub>2</jats:sub> films are immersed in co-solutions of formamidinium iodide (FAI) and n-butylammonium iodide (BAI) in isopropyl alcohol to form metal halide perovskites. At 50 mol% FAI or higher relative to BAI, only 3D FAPbI <jats:sub>3</jats:sub> ( <jats:italic toggle="yes">n</jats:italic> = ∞) nanowires form at room temperature. BAI does not incorporate into the crystal lattice at these concentrations but stabilizes semiconducting α-FAPbI <jats:sub>3</jats:sub> from converting to insulating δ-FAPbI <jats:sub>3</jats:sub> , the thermodynamically stable phase at room temperature. Immersion in solutions with 33 mol% FAI or lower results in quasi-2D (BA) <jats:sub>2</jats:sub> (FA)Pb <jats:sub>2</jats:sub> I <jats:sub>7</jats:sub> ( <jats:italic toggle="yes">n</jats:italic> = 2) or 2D (BA) <jats:sub>2</jats:sub> PbI <jats:sub>4</jats:sub> ( <jats:italic toggle="yes">n</jats:italic> = 1) nanoplate growth. In situ photoluminescence spectra collected during crystal growth revealed the presence of intermediate <jats:italic toggle="yes">n</jats:italic> &gt;2 phases before conversion to the thermodynamically favored phase at some compositions. Vertical gradients in <jats:italic toggle="yes">n</jats:italic> number were also identified by monitoring photoluminescence from the tops and bottoms of films, with α-FAPbI <jats:sub>3</jats:sub> and low <jats:italic toggle="yes">n</jats:italic> phases preferentially forming at the film surfaces and buried interfaces, respectively. Proof-of-concept conductivity mapping via atomic force microscopy demonstrates that these compositionally-graded nanocrystalline arrays are photoconductive, with highest conductivity normal to the substrate. </jats:p>