Orientation-Dependent Hybrid Perovskite Conversion of VLS-Grown Lead Halide Nanowires

초록

Organic-inorganic hybrid perovskites, such as methylammonium lead iodide (CH3NH3PbI3) have shown outstanding optoelectronic properties, promising extensive application in solar harvesting. Although most hybrid perovskite materials have been synthesized under solution process, recent demonstrations of the vapor phase synthesis of the hybrid perovskite nanostructures suggest there is an emerging interest in controlling their properties under highly-confined structures. In particular, single-crystalline 1D lead halide nanowires can be prepared via the vapor-liquid-solid (VLS) growth mechanism through the self-catalyzed growth mechanism, which then converted to hybrid perovskite. Since the conversion occurs on the preformed nanowire, electronic properties 1D perovskite depend on the original nanowire structure. In this study, we report the VLS growth of lead iodide (PbI2) nanowires on a c-sapphire (0001) substrate followed by conversion to CH3NH3PbI3 using methylammonium iodide, and confirm that the degree of perovskite conversion depends on the growth orientations of PbI2 nanowires. We observe two different growth directions; vertically-oriented [0001] nanowires and kinked [1 ?21 ?0] nanowires. Photoluminescence (PL) measurements on each growth direction suggest that the [1 ?21 ?0] oriented nanowires exhibit a higher degree of conversion compared to the [0001] oriented nanowires. In addition, [0001] oriented nanowires exhibit the position-dependent degree of conversion, depending on the presence of the catalyst tip on top of the nanowire. In particular, the conversion is observed both on the catalyst tip and the base of nanowire, suggesting methylammonium iodide incorporates into the nanowire either by vapor transport and surface diffusion. Our observation indicates that the vapor phase conversion of PbI2 to CH3NH3PbI3 is a diffusion-limited process. This finding is an important step towards a structural engineering of perovskite nanowires.