Excitonic and Confinement Effects of 2D Layered (C 10 H 21 NH 3 ) 2 PbBr 4 Single Crystals

Recognition of unusual optoelectronic properties for two-dimensional (2D) layered organic–inorganic lead(II) halide materials (C n H 2 n +1 NH 3 ) 2 PbX 4 (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C 10 H 21 NH 3 ) 2 PbBr 4 and pseudo-alloy (C 10 H 21 NH 3 ) 2 PbI 2 Br 2 were grown for photophysical evaluation. A 10-carbon alkylammonium cation was selected for investigation to provide strong dielectric screening in order to highlight quantum confinement effects of the anionic (PbX 4 2– ) semiconductor layer. Single crystals of the 2D layered (C 10 H 21 NH 3 ) 2 PbBr 4 compound display a characteristic free exciton with a binding energy of ca. 280 meV. Observation of a short photoluminescence lifetime of 2.8 ± 0.2 ns suggests that this electronic transition for the PbBr 4 -based layered material has only singlet character. Sheets of (C 10 H 21 NH 3 ) 2 PbBr 4 with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C 10 H 21 NH 3 ) 2 PbBr 4 and (C 10 H 21 NH 3 ) 2 PbI 4 exhibit relatively small spectral shifts from the bulk down to a thickness of five layers indicative of the strong confinement effect of the 10-carbon alkylammonium spacers. Single crystals of the pseudo-alloy (C 10 H 21 NH 3 ) 2 PbBr 2 I 2 give an excitonic absorption peak close to that of the tetrabromide (C 10 H 21 NH 3 ) 2 PbBr 4 and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C 10 H 21 NH 3 ) 2 PbI 4 .