Stereochemically Active Lone Pairs and Nonlinear Optical Properties of Two-Dimensional Multilayered Tin and Germanium Iodide Perovskites

Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn- and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable ns2 lone-pair electrons. Herein, we report the synthesis and structures of 2D tin iodide perovskites (BA)2(A)Sn2I7, where BA = n-butylammonium and A = methylammonium, formamidinium, dimethylammonium, guanidinium, or acetamidinium, and those of 2D germanium iodide perovskites (BA)2(A)Ge2I7, where A = methylammonium or formamidinium. By comparing these structures along with their Pb counterparts, we establish correlations between the effect of group IV-cation’s lone-pair stereochemical activity on the perovskite crystal structures and the resulting semiconducting properties such as bandgaps and carrier–phonon interactions and nonlinear optical properties. We find that the strength of carrier–phonon interaction increases with increasing lone-pair activity, leading to a more prominent photoluminescence tail on the low-energy side. Moreover, (BA)2(A)Ge2I7 exhibit strong second harmonic generation with second-order nonlinear coefficients of ∼10 pm V–1 that are at least 10 times those of Sn counterparts and 100 times those of Pb counterparts. We also report the third-order two-photon absorption coefficients of (BA)2(A)Sn2I7 to be ∼10 cm MW–1, which are one order of magnitude larger than those of the Pb counterparts and traditional inorganic semiconductors. These results not only highlight the role of lone-pair activity in linking the compositions and physical properties of 2D halide perovskites but also demonstrate 2D tin and germanium iodide perovskites as promising lead-free alternatives for nonlinear optoelectronic devices

Visualization and Studies of Ion-Diffusion Kinetics in Cesium Lead Bromide Perovskite Nanowires

The facile chemical transformation of metal halide perovskites via ion exchange has been attributed to their “soft” crystal lattices that enable fast ion migration. Kinetic studies of such processes could provide mechanistic insights on the ion migration dynamics. Herein, by using aligned single-crystal nanowires of cesium lead bromide (CsPbBr₃) perovskite on epitaxial substrates as platforms, we visualize and investigate the cation or anion interdiffusion kinetics via spatially resolved photoluminescence measurement on heterostructures fabricated by stacking CsPbCl₃, MAPbI₃, or MAPbBr₃ microplates on top of CsPbBr₃ nanowires. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion readily occurs to result in halide concentration gradients along CsPbBr_{3–3<i>x</i>}Cl_3<i>x</i> (<i>x</i> = 0–1) nanowires. Quantitative analysis of such composition profiles using Fick’s law allowed us, for the first time, to extract interdiffusion coefficients of the chloride-bromide couple and an activation energy of 0.44 ± 0.02 eV for ion diffusion from temperature-dependent studies. In contrast, iodide-bromide interdiffusion is limited, likely due to the complex phase behaviors of mixed alloys of CsPb­(Br,I)₃. In contrast to the relatively mobile anions, A-site cation interdiffusion across the MAPbBr₃/CsPbBr₃ junctions was barely observed at room temperature. Our results present a general method to investigate the kinetics of the solid-state ion migration, and the gained insights on ion diffusion can provide guidelines for rationally designing perovskite heterostructures that could lead to new properties for fundamental studies and technological applications

Visualization and Studies of Ion-Diffusion Kinetics in Cesium Lead Bromide Perovskite Nanowires

The facile chemical transformation of metal halide perovskites via ion exchange has been attributed to their “soft” crystal lattices that enable fast ion migration. Kinetic studies of such processes could provide mechanistic insights on the ion migration dynamics. Herein, by using aligned single-crystal nanowires of cesium lead bromide (CsPbBr₃) perovskite on epitaxial substrates as platforms, we visualize and investigate the cation or anion interdiffusion kinetics via spatially resolved photoluminescence measurement on heterostructures fabricated by stacking CsPbCl₃, MAPbI₃, or MAPbBr₃ microplates on top of CsPbBr₃ nanowires. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion readily occurs to result in halide concentration gradients along CsPbBr_{3–3<i>x</i>}Cl_3<i>x</i> (<i>x</i> = 0–1) nanowires. Quantitative analysis of such composition profiles using Fick’s law allowed us, for the first time, to extract interdiffusion coefficients of the chloride-bromide couple and an activation energy of 0.44 ± 0.02 eV for ion diffusion from temperature-dependent studies. In contrast, iodide-bromide interdiffusion is limited, likely due to the complex phase behaviors of mixed alloys of CsPb­(Br,I)₃. In contrast to the relatively mobile anions, A-site cation interdiffusion across the MAPbBr₃/CsPbBr₃ junctions was barely observed at room temperature. Our results present a general method to investigate the kinetics of the solid-state ion migration, and the gained insights on ion diffusion can provide guidelines for rationally designing perovskite heterostructures that could lead to new properties for fundamental studies and technological applications

Carrier Decay Properties of Mixed Cation Formamidinium–Methylammonium Lead Iodide Perovskite [HC(NH₂)₂]_1–<i>x</i>[CH₃NH₃]_<i>x</i>PbI₃ Nanorods

Organic–inorganic lead iodide perovskites are efficient materials for photovoltaics and light-emitting diodes. We report carrier decay dynamics of nanorods of mixed cation formamidinium and methylammonium lead iodide perovskites [HC­(NH₂)₂]_1–<i>x</i>[CH₃NH₃]_<i>x</i>PbI₃ (FA_1–<i>x</i>MA_<i>x</i>PbI₃) synthesized through a simple solution method. The structure and FA/MA composition ratio of the single-crystal FA_1–<i>x</i>MA_<i>x</i>PbI₃ nanorods are fully characterized, which shows that the mixed cation FA_1–<i>x</i>MA_<i>x</i>PbI₃ nanorods are stabilized in the perovskite structure. The photoluminescence (PL) emission from FA_1–<i>x</i>MA_<i>x</i>PbI₃ nanorods continuously shifts from 821 to 782 nm as the MA ratio (<i>x</i>) increases from 0 to 1 and is shown to be inhomogeneously broadened. Time-resolved PL from individual FA_1–<i>x</i>MA_<i>x</i>PbI₃ nanorods demonstrates that lifetimes of mixed cation FA_1–<i>x</i>MA_<i>x</i>PbI₃ nanorods are longer than those of the pure FAPbI₃ or MAPbI₃ nanorods, and the FA_0.4MA_0.6PbI₃ displays the longest average PL lifetime of about 2 μs. These results suggest that mixed cation FA_1–<i>x</i>MA_<i>x</i>PbI₃ perovskites are promising for high-efficiency photovoltaics and other optoelectronic applications

Photocurrent Mapping in Single-Crystal Methylammonium Lead Iodide Perovskite Nanostructures

We investigate solution-grown single-crystal methylammonium lead iodide (MAPbI₃) nanowires and nanoplates with spatially resolved photocurrent mapping. Sensitive perovskite photodetectors with Schottky contacts are fabricated by directly transferring the nanostructures on top of prepatterned gold electrodes. Scanning photocurrent microscopy (SPCM) measurements on these single-crystal nanostructures reveal a minority charge carrier diffusion length up to 21 μm, which is significantly longer than the values observed in polycrystalline MAPbI₃ thin films. When the excitation energy is close to the bandgap, the photocurrent becomes substantially stronger at the edges of nanostructures, which can be understood by the enhancement of light coupling to the nanostructures. These perovskite nanostructures with long carrier diffusion lengths and strong photonic enhancement not only provide an excellent platform for studying their intrinsic properties but may also boost the performance of perovskite-based optoelectronic devices

High-Performance Electrocatalysis for Hydrogen Evolution Reaction Using Se-Doped Pyrite-Phase Nickel Diphosphide Nanostructures

The study of efficient, robust, and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is essential for hydrogen-based energy technologies. Previous works have demonstrated that pyrite-structure materials (e.g., CoS₂, NiSe₂) are efficient HER catalysts. Here, we first systematically investigate the nanostructure synthesis of a series of pyrite-phase nickel phosphoselenide materialsNiP₂, Se-doped NiP₂ (NiP_1.93Se_0.07), P-doped NiSe₂ (NiP_0.09Se_1.91), and NiSe₂through a facile thermal conversion of Ni­(OH)₂ nanoflakes. The similar nanostructures enable a systematic and fair comparison of their structural properties and catalytic activities for HER. We found that NiP_1.93Se_0.07 shows the best HER performance, followed by NiP₂, NiP_0.09Se_1.91, and NiSe₂. Se-doped NiP₂ grown on carbon fiber paper can achieve an electrocatalytic current density of 10 mA cm^–2 at an overpotential as low as 84 mV and a small Tafel slope of 41 mV decade^–1. This study not only estabilishes Se-doped NiP₂ as a competitive HER catalyst, but also demonstrates that doping or alloying of developed catalysts (especially doping with anions from another group; e.g., selenium to phosphorus) can improve the HER catalytic activity, which provides a general strategy to improve catalytic efficiencies of existing electrocatalysts for HER

Broad Wavelength Tunable Robust Lasing from Single-Crystal Nanowires of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, I)

Lead halide perovskite nanowires (NWs) are emerging as a class of inexpensive semiconductors with broad bandgap tunability for optoelectronics, such as tunable NW lasers. Despite exciting progress, the current organic–inorganic hybrid perovskite NW lasers suffer from limited tunable wavelength range and poor material stability. Herein, we report facile solution growth of single-crystal NWs of inorganic perovskite CsPbX₃ (X = Br, Cl) and their alloys [CsPb­(Br,Cl)₃] and a low-temperature vapor-phase halide exchange method to convert CsPbBr₃ NWs into perovskite phase CsPb­(Br,I)₃ alloys and metastable CsPbI₃ with well-preserved perovskite crystal lattice and NW morphology. These single crystalline NWs with smooth end facets and subwavelength dimensions are ideal Fabry–Perot cavities for NW lasers. Optically pumped tunable lasing across the entire visible spectrum (420–710 nm) is demonstrated at room temperature from these NWs with low lasing thresholds and high-quality factors. Such highly efficient lasing similar to what can be achieved with organic–inorganic hybrid perovskites indicates that organic cation is not essential for light emission application from these lead halide perovskite materials. Furthermore, the CsPbBr₃ NW lasers show stable lasing emission with no measurable degradation after at least 8 h or 7.2 × 10⁹ laser shots under continuous illumination, which are substantially more robust than their organic–inorganic counterparts. The Cs-based perovskites offer a stable material platform for tunable NW lasers and other nanoscale optoelectronic devices

Additional file 1 of Multi-faceted analysis reveals the characteristics of silk fabrics on a Liao Dynasty DieXie belt

Additional file 1. Supplementary material

Two-Dimensional Lead Halide Perovskites Templated by a Conjugated Asymmetric Diammonium

We report novel two-dimensional lead halide perovskite structures templated by a unique conjugated aromatic dication, <i>N</i>,<i>N</i>-dimethylphenylene-<i>p</i>-diammonium (DPDA). The asymmetrically substituted primary and tertiary ammoniums in DPDA facilitate the formation of two-dimensional network (2DN) perovskite structures incorporating a conjugated dication between the PbX₄^2– (X = Br, I) layers. These 2DN structures of (DPDA)­PbI₄ and (DPDA)­PbBr₄ were characterized by single-crystal X-ray diffraction, showing uniquely low distortions in the Pb–X–Pb bond angle for 2D perovskites. The Pb–I–Pb bond angle is very close to ideal (180°) for a 2DN lead iodide perovskite, which can be attributed to the ability of the rigid diammonium DPDA to insert into the PbX₆^2– octahedral pockets. Optical characterization of (DPDA)­PbI₄ shows an excitonic absorption peak at 2.29 eV (541 nm), which is red-shifted in comparison to similar 2DN lead iodide structures. Temperature-dependent photoluminescence of both compounds reveals both a self-trapped exciton and free exciton emission feature. The reduced exciton absorption energy and emission properties are attributed to the dication-induced structural order of the inorganic PbX₄^2– layers. DFT calculation results suggest mixing of the conjugated organic orbital component in the valence band of these 2DN perovskites. These results demonstrate a rational new strategy to incorporate conjugated organic dications into hybrid perovskites and will spur spectroscopic investigations of these compounds as well as optoelectronic applications