Gas-Induced Confinement-Deconfinement Interplay in Organic-Inorganic Hybrid Perovskite Thin Film Results in Systematic Band Modulation

Gas-induced growth of organic-inorganic hybrid perovskites, especially methylammonium lead iodide (MAPbI₃), has shown interesting properties and applications in the area of optoelectronics. In this report, we introduce a method of gas-induced band gap engineering of thin films of MAPbI₃ due to systematic dimensional confinement-deconfinement along the crystallographic c axis of growing MAPbI₃. Interestingly, such a restricted growth phenomenon was observed when the hexylammonium lead iodide (two-dimensional hybrid perovskite) film was exposed to methylamine gas instead of the conventional PbI₂ film-methylamine gas precursor pair. Hexylamine, formed due to the cation exchange reaction, interacts selectively with the Pb centers of growing MAPbI₃ crystals, and this induces an enormous restriction in the growth of MAPbI₃ along the crystallographic c direction, leading to a unique sheet-type MAPbI₃ film having a much higher band gap (2.18 eV) compared to conventional bulk MAPbI₃. However, careful control of exposure timing gradually evaporates the hexylamine, leading to systematic dimensional deconfinement, enabling modulation of the band gap from 2.18 to 1.69 eV. An interplay of adsorption and desorption of hexylamine is also utilized for generating patterns of two different fluorescent hybrid perovskite materials in a single pixel. This new mechanistic investigation highlighting gas-induced interplay of dimensional confinement-deconfinement associated with band gap tuning provides smooth thin films, which can be used to develop optoelectronic devices.