Excitonic and Confinement Effects of 2D Layered (C10H21NH3)2PbBr4 Single Crystals

Abstract

Recognition of unusual optoelectronic properties for two-dimensional (2D) layered organic–inorganic lead(II) halide materials (CnH2n+1NH3)2PbX4 (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C10H21NH3)2PbBr4 and pseudo-alloy (C10H21NH3)2PbI2Br2 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 (PbX42–) semiconductor layer. Single crystals of the 2D layered (C10H21NH3)2PbBr4 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 PbBr4-based layered material has only singlet character. Sheets of (C10H21NH3)2PbBr4 with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C10H21NH3)2PbBr4 and (C10H21NH3)2PbI4 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 (C10H21NH3)2PbBr2I2 give an excitonic absorption peak close to that of the tetrabromide (C10H21NH3)2PbBr4 and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C10H21NH3)2PbI4.