Layer-dependent characterization of two-dimensional lead-halide perovskite crystals

Abstract

This study investigates the layer-dependent terahertz (THz) characteristics of two-dimensional (2D) Ruddlesden[sbnd]Popper organic halide perovskite (RP-OHP) single crystals, BA₂MAn₋₁PbnI₃n₊₁ (n = 1[sbnd]3). The objective is to better understand how the number of layers in these 2D materials influences their THz properties. Samples with area of approximately 1.5 mm2, exfoliated mechanically to a few micrometers in thickness, were analyzed using THz time-domain spectroscopy (THz-TDS), a technique chosen for its ability to provide detailed insights into the phonon dynamics and non-contact conductivity at THz frequencies. Two distinct phonon modes were identified: one corresponding to the bending of the Pb-I-Pb angle (around 1 THz) and another to the stretching of the Pb-I bond (near 2 THz). Both the refractive index and conductivity increased with the number of layers, with a notable suppression of the Pb-I-Pb bending mode in the 2D structures compared to 3D materials. This suppression suggests a lattice properties of single layer RP-OHPs. The fitting analysis of plasma frequency and scattering rate highlights the role of dimensionality in shaping the material's THz response. These findings contribute to the development of high-performance THz materials and offer valuable insight into the broader understanding of the physical properties of 2D RP-OHPs in the THz regime.