We report the cooling-induced crystallization of layered two-dimensional lead halide perovskites with controllable inorganic quantum-well thickness (n = 1, 2, 3, 4), organic spacer chain length (butyl-, pentyl-, hexylammonium), A-site cation (methylammonium, formamidinium), and halide anion (iodide, bromide). We report crystal structures for the iodide family as a function of these compositional parameters, and across their temperature dependent phase transitions. In general, lower symmetry crystal structures, increasing extents of organic-spacer interdigitation, and increasing organic-spacer corrugation tilts are observed at low temperature. In addition, greater structural distortions are seen in lead halide octahedra closest to the organic spacer layer, and larger-n structures exhibit periodic variation in Pb-I bond lengths. We also provide detailed guidance regarding the combination of synthetic parameters needed to achieve phase-pure crystals of each composition, and discuss difficulties encountered when trying to synthesize particular members of the 2D perovskite family containing formamidinium or cesium as the A-site cation. These results provide a foundation for understanding structural trends in 2D lead halide perovskites and the effect these trends have on their thermal, electrical, and optical properties.

Chiral metallic nanostructures can generate evanescent fields which are more highly twisted than circularly polarized light. However, it remains unclear how best to exploit this phenomenon, hindering the optimal utilization of chiral electromagnetic fields. Here, inspired by optical antenna theory, we address this challenge by introducing chiral antenna parameters: the chirality flux efficiency and the chiral antenna aperture. These quantities, which are based on chirality conservation, quantify the generation and dissipation of chiral light. We then present a label-free experimental technique, chirality flux spectroscopy, which measures the chirality flux efficiency, providing valuable information on chiral near fields in the far field. This principle is verified theoretically and experimentally with two-dimensionally chiral coupled nanorod antennas, for which we show that chiral near and far fields are linearly dependent on the magnetoelectric polarizability. This elementary system confirms our concept to quantify chiral electromagnetic fields and paves the way toward broadly tunable chiral optical applications including ultrasensitive detection of molecular chirality or optical information storage and transfer.

We investigate the phase behavior of two-dimensional (C x H 2x+1 NH 3 ) 2 [(MA,FA)PbI 3 ] n−1 PbI 4 layered perovskites near room temperature (−20 °C to +100 °C) as a function of the octahedral layer thickness (n = 1, 2, 3, 4), alkylammonium chain length (butyl, pentyl, and hexyl), and identity of the small organic cation (methylammonium and formamidinium). Using differential scanning calorimetry and X-ray diffraction, we observe a reversible first-order phase transition corresponding to a partial melting transition of the alkylammonium chains separating the perovskite layers. The melting temperature, T m , increases from 10 to 77.9 to 95.9 °C as the carbon chain length increases from C 4 to C 5 to C 6 , but it is insensitive to octahedral layer thickness, n. The latent heat of melting, ΔH m , was in the range of 3− 5 kJ/mol-spacer, indicating only partial disordering of the carbon chain. We discuss these findings and their implications in the context of melting in other twodimensional molecular systems.

Layered hybrid perovskites exhibit emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and their interaction with the molecular moieties. Here, we address this question by using a combination of spontaneous Raman scattering, terahertz spectroscopy, and molecular dynamics simulations. This approach reveals the structural dynamics in and out of equilibrium and provides unexpected observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layered perovskites, an off-resonant terahertz pulse can drive a long-lived coherent phonon mode in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced, and the findings pave the way for ultrafast structural engineering and high-speed optical modulators based on layered perovskites.