Atomic scale characterization of the ferroic orders in 2D hybrid perovskites using combined STM/ncAFM techniques

The two-dimensional (2D) organic-inorganic hybrid perovskites have emerged as a new class of materials exhibiting a variety of ferroic orders, dependent on their chemical composition. Despite extensive effort on the design and synthesis of these materials, there remains a notable gap in understanding of the nanoscale structure of the ferroic domains. Here, we present a systematic atomic-scale characterization of the 2D Ruddlesden-Popper perovskites (RPPs)—a prototypical class of 2D organic-inorganic perovskites—as function of their dimensionality, specifically the number of metal-halide layers (n) and halogen type (X) (X = Br, I). We utilize a combination of scanning tunneling microscopy (STM) and non-contact atomic force microscopy (ncAFM) for atomically resolved imaging of the RPPs. STM measurements enable probing the atomic reconstruction of the inorganic lead-halide lattice, while ncAFM provides nonperturbative visualization of the cooperative ordering of surface organic cations, driven by hydrogen bonding interactions with the inorganic lattice. Our findings reveal distinctly different atomic arrangements in I-based and Br-based RPPs. Based on these nanoscale insights, we establish a direct correlation between the micron-scale ferroelastic properties (for I-based RPPs, n > 1) and ferroelectric characteristics (for Br-based RPPs, n > 1) of the crystals and their nanoscale reconstructions.