Giant Apparent Optical Circular Dichroism in Thin Films of Bi-Based Hybrid Organic-Inorganic Metal Halide Semiconductor Through Preferred Orientation

Introducing chirality into organic/inorganic hybrid materials can impart chiroptical properties such as circular dichroism. The ability to tune chiroptical properties in self-assembled materials could have important implications for spintronic and optoelectronic applications. Here, we incorporate a chiral organic cation, (R/S)-4-methoxy-α-methylbenzylammonium, to synthesize the bismuth-based hybrid organic-inorganic metal halide semiconductor, (R/S-MeOMePMA)BiI₄. Thin films of this Bi-based compound demonstrate large chiroptical responses, with circular dichroism anisotropy (g_CD) values up to ~0.1, close to the highest value observed in another chiral metal-halide semiconductor, (R-MBA₂CuCl₄). Further investigation discloses that this large g_CD in (R/S-MeOMePMA)BiI₄ is caused by the apparent CD effect. Careful selection of deposition conditions and the concomitant thin-film orientation enable the control of g_CD, with maximum value observed when its thin film has a preferred (001) orientation parallel to the substrate. Together with other recent reports, it appears that the low symmetry plays an important role in achieving unusually large g_CD in these chiral metal-halide materials.