Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures.

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Two-dimensional lead halide perovskite lateral homojunctions enabled by phase pinning.

Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.

Broadband emission originating from the stereochemical expression of 6s2 lone pairs in two-dimensional lead bromide perovskites.

The stereochemical expression of the 6s2 lone pair on the lead atom has a significant impact on the crystal structures and physical properties of lead halide perovskites. Two-dimensional (2D) lead bromide perovskites often exhibit a broadband emission, yet the structural origin of the broadband emission has been under debate. Here, we report the synthesis and characterization of a 2D lead bromide hybrid (4-chlorophenylammonium)2PbBr4 that consists of a combination of the octahedral unit PbBr6 and the rarely observed capped octahedral unit PbBr7 through corner-sharing and edge-sharing linkages. The seven-coordination geometry indicates a strong stereo-active lone pair on the Pb2+ cation. By comparing this structure with two representative 2D perovskites, (benzylammonium)2PbBr4 and (4-aminotetrahydropyran)2PbBr4, we establish how the lone pair expression affects the local coordination geometry of the Pb2+ cation and the resulting optical and electronic properties. As the Pb-Br bond length increases, the lone pair expression leads to off-centering displacement of Pb2+ within the octahedra and then the formation of seven-coordination capped octahedra. Density functional theory calculations indicate that the off-centering distorted octahedra and capped octahedra are due to the asymmetric distribution of the Pb electrons that have both s and p orbital characteristics. Spectroscopic studies show the photoluminescence spectra evolving from narrowband emission to broadband emission with increasing LPE, as well as softer and more anharmonic lattice vibrations that facilitate exciton self-trapping. Our results demonstrate that lone pairs could be a powerful design rule for developing light emitting materials.

Correlating Photophysical Properties with Stereochemical Expression of 6s2 Lone Pairs in Two-dimensional Lead Halide Perovskites.

Two-dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light-emitting applications and excitonic devices. Fulfilling these promises demands an in-depth understanding on the relationships between the structural dynamics and exciton-phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out-of-plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb-I bond length, resulting in Pb2+ off-center displacement driven by stereochemical expression of the Pb2+ 6s2 lone pair electrons. Density functional theory calculations indicate that the Pb2+ cation is off-center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb2+ off-centering lead to a broad Raman central peak background and phonon softening, which increase the non-radiative recombination loss via exciton-phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs.

Chirality-Dependent Structural Transformation in Chiral 2D Perovskites under High Pressure.

Chiral perovskites have attracted considerable attention owing to their potential applications in spintronic- and polarization-based optoelectronic devices. However, the structural chirality/asymmetry transfer mechanism between chiral organic ammoniums and achiral inorganic frameworks is still equivocal, especially under extreme conditions, as the systematic structural differences between chiral and achiral perovskites have been rarely explored. Herein, we successfully synthesized a pair of new enantiomeric chiral perovskite (S/R-3PYEA)PbI4 (3PYEA2+ = C5NH5C2H4NH32+) and an achiral perovskite (rac-3PYEA)PbI4. Hydrostatic pressure was used, for the first time, to systematically investigate the differences in the structural evolution and optical behavior between (S/R-3PYEA)PbI4 and (rac-3PYEA)PbI4. At approximately 7.0 GPa, (S/R-3PYEA)PbI4 exhibits a chirality-dependent structural transformation with a bandgap "red jump" and dramatic piezochromism from translucent red to opaque black. Upon further compression, a previously unreported chirality-induced negative linear compressibility (NLC) is achieved in (S/R-3PYEA)PbI4. High-pressure structural characterizations and first-principles calculations demonstrate that pressure-driven homodirectional tilting of homochiral ammonium cations strengthens the interactions between S/R-3PYEA2+ and Pb-I frameworks, inducing the formation of new asymmetric hydrogen bonds N-H···I-Pb in (S/R-3PYEA)PbI4. The enhanced asymmetric H-bonding interactions further break the symmetry of (S/R-3PYEA)PbI4 and trigger a greater degree of in-plane and out-of-plane distortion of [PbI6]4- octahedra, which are responsible for chirality-dependent structural phase transition and NLC, respectively. Nevertheless, the balanced H-bonds incurred by equal proportions of S-3PYEA2+ and R-3PYEA2+ counteract the tilting force, leading to the absence of chirality-dependent structural transition, spectral "red jump", and NLC in (rac-3PYEA)PbI4.

Understanding Electron-Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs.

The electron-phonon (e-ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e-ph interaction in LHPs. Herein, we advance a fundamental understanding of the e-ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure-property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e-ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e-ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.

Antioxidant therapy with Salvia miltiorrhiza decreases plasma endothelin-1 and thromboxane B2 after cardiopulmonary bypass in patients with congenital heart disease.

OBJECTIVE: The endothelium-derived vasoconstrictor endothelin-1 is increased after cardiopulmonary bypass in children with congenital heart defects. This study determines whether antioxidant therapy with Salvia miltiorrhiza injection, an herb extract containing phenolic compounds, prevents the postoperative increase of endothelin-1. The relationship between endothelin-1 and the endothelium-derived prostacyclin (prostaglandin I2) and thromboxane A2 postoperatively is also investigated. METHODS: Twenty children with congenital heart defects and pulmonary hypertension were randomly assigned to group A (placebo control, n=10) or B (200 mg/kg Salvia miltiorrhiza intravenously after anesthesia induction and at the time of rewarming, respectively; n =10) before cardiac surgery. Central venous blood samples were taken before operation (T(0)), 10 (T(1)) and 30 minutes (T(2)) after starting cardiopulmonary bypass, 10 (T(3)) and 30 minutes (T(4)) after aortic declamping, and 30 minutes (T(5)) and 24 hours (T(6)) after termination of cardiopulmonary bypass. Plasma lipid peroxidation product malondialdehyde, myocardial specific creatine kinase-MB activity, thromboxane B2, and 6-keto-prostaglandin F(1 alpha) (stable metabolites of thromboxane A2 and prostaglandin I2) were measured. RESULTS: Malondialdehyde increased significantly at T(1) in group A and remained significantly higher than in group B thereafter (P <.05). Malondialdehyde in group B did not significantly increase over time. At T(5), plasma creatine kinase-MB, thromboxane B2, and endothelin-1 in group B were lower than in group A (P <.05); malondialdehyde correlated significantly with creatine kinase-MB (r = 0.71, P =.0005). At T(6), endothelin-1 negatively correlated with the 6-keto-prostaglandin F(1 alpha)/thromboxane B2 ratio (r = -0.64, P =.0025). CONCLUSION: Antioxidant therapy reduces myocardial damage and attenuates postoperative vasoactive mediator imbalance.