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

Recognition of unusual optoelectronic properties for two-dimensional (2D) layered organic-inorganic lead(II) halide materials (CnH2n+1NH3)(2)PbX4 (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C10H21NH3)(2)PbBr4 and pseudo-alloy (C10H21NH3)(2)PbI2Br2 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 (C10F12NH3)(2)PbBr4 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 (C10H12NH3)(2)PbBr4 with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C10H21NH3)(2)PbBr4 and (C10H21NH3)(2)PbI4 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)(2)PbBr2I2 give an excitonic absorption peak close to that of the tetrabromide (C10H21NH3)(2)PbBr4 and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C10H21NH3)(2)PbI4.