Light-Induced Wavelength Dependent Self Assembly Process for Targeted Synthesis of Phase Stable 1D Nanobelts and 2D Nanoplatelets of CsPbI Perovskites.

Despite black cubic phase α-CsPbI nanocrystals having an ideal bandgap of 1.73 eV for optoelectronic applications, the phase transition from α-CsPbI to non-perovskite yellow δ-CsPbI phase at room temperature remains a major obstacle for commercial applications. Since γ-CsPbI is thermodynamically stable with a bandgap of 1.75 eV, which has great potential for photovoltaic applications, herein we report a conceptually new method for the targeted design of phase stable and near unity photoluminescence quantum yield (PLQY) two-dimensional (2D) γ-CsPbI nanoplatelets (NPLs) and one-dimensional (1D) γ-CsPbI nanobelts (NBs) by wavelength dependent light-induced assembly of CsPbI cubic nanocrystals. This article demonstrates for the first time that by varying the excitation wavelengths, one can design air stable desired 2D nanoplatelets or 1D nanobelts selectively. Our experimental finding indicates that 532 nm green light-driven self-assembly produces phase stable and highly luminescent γ-CsPbI NBs from CsPbI nanocrystals. Moreover, we show that a 670 nm red light-driven self-assembly process produces stable and near unity PLQY γ-CsPbI NPLs. Systematic time-dependent microscopy and spectroscopy studies on the morphological evolution indicates that the electromagnetic field of light triggered the desorption of surface ligands from the nanocrystal surface and transformation of crystallographic phase from α to γ. Detached ligands played an important role in determining the morphologies of final structures of NBs and NPLs from nanocrystals via oriented attachment along the [110] direction initially and then the [001] direction. In addition, XRD and fluorescence imaging data indicates that both NBs and NPLs exhibit phase stability for more than 60 days in ambient conditions, whereas the cubic phase α-CsPbI nanocrystals are not stable for even 3 days. The reported light driven synthesis provides a simple and versatile approach to obtain phase pure CsPbI for possible optoelectronic applications.