Epitaxial Electrodeposition of BiI3 and Topotactic Conversion to Highly Ordered Solar Light-Absorbing Perovskite (CH3NH3)3Bi2I9

Highly ordered, epitaxial semiconductor thin films have low defect densities and excellent photophysical properties that are desired for optoelectronic devices. In this work, the epitaxial films of BiI3 are electrodeposited onto Au(111) substrates, and these films are removed by epitaxial lift-off to produce free-standing, single-crystal-like foils of BiI3. The two-dimensional (2D) layering of the BiI3 van der Waals solid allows for the epitaxial lift-off, and it provides a pathway to producing other important materials. The epitaxial films of the solar light-absorbing perovskite (CH3NH3)(3)Bi2I9 are produced in a solution from BiI3 by topotactic transformation. Epitaxially grown BiI3 shows a 2D-nanodisc-like morphology and highly ordered [001] out-of-plane orientation having the epitaxial relationship with the substrate as BiI3 (0001)[(11) over bar 20 +/- 3 degrees]//Au(111)[(11) over bar2], determined by X-ray pole figures. The lattice mismatch is minimized by the (+/-)3 degrees rotation of the film relative to the substrate. The X-ray rocking curve shows that single crystal like BiI 3 has a mosaic spread with full width at half-maximum (FWHM) of about similar to 0.69 degrees. The topotactic transformation of BiI 3 produces a highly ordered [001]-oriented epitaxial perovskite (CH3NH3)(3)Bi2I9.