Low Threshold Fabry-Pérot Mode Lasing from Lead Iodide Trapezoidal Nanoplatelets

Edge Raman enhancement at layered PbI2platelets induced by laser waveguide effect

As a two-dimensional (2D) layered semiconductor, lead iodide (PbI₂) has been widely used in optoelectronics owing to its unique crystal structure and distinctive optical and electrical properties. A comprehensive understanding of its optical performance is essential for further application and progress. Here, we synthesized regularly shaped PbI₂platelets using the chemical vapor deposition method. Raman scattering spectroscopy of PbI₂platelets was predominantly enhanced when the laser radiated at the edge according to Raman mapping spectroscopy. Combining the outcome of polarized Raman scattering spectroscopy and finite-difference time domain simulation analysis, the Raman enhancement was proven to be the consequence of the enhancement effects inherent to the high refractive index contrast waveguide, which is naturally formed in well-defined PbI₂platelets. Because of the enlarged excited area determined by the increased propagation length of the laser in the PbI₂platelet formed waveguide, the total Raman enhancements are acquired rather than a localized point enhancement. Finally, the Raman enhancement factor is directly related to the thickness of the PbI₂platelet, which further confirms the waveguide-enhanced edge Raman. Our investigation of the optical properties of PbI₂platelets offers reference for potential 2D layered-related optoelectronic applications.

Orientation-Controlled Large-Area Epitaxial PbI2 Thin Films with Tunable Optical Properties

Lead iodide (PbI₂) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI₂ with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI₂ remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI₂ thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI₂ thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI₂ on other substrates and heterostructures of PbI₂ on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI₂ and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

Efficient Quasi-Two-Dimensional Perovskite Light-Emitting Diodes with Improved Multiple Quantum Well Structure

Quasi-two-dimensional (quasi-2D) perovskites with a multiple quantum well structure can enhance the exciton binding energy and controllable quantum confine effect, which are attractive materials for efficient perovskite light-emitting diodes (PeLEDs). However, the multiphase mixtures contained in these materials would cause nonradiative recombination at the perovskite film surface. Here, a facile solution surface treatment is adopted to improve the multiple quantum well structure of the quasi-2D perovskite emitting layer, which can reduce the influence of defectinduced nonradiative recombination and the electric-field-induced dissociation of excitons for the PeLEDs. The improved multiple quantum well structure is verified by UV absorption spectra and temperature-dependent photoluminescence spectra measurements. The photoluminescence quantum yield of the quasi-2D perovskite film with surface treatment has been approximately increased by 200%. Meanwhile, the electroluminescence device achieves a current efficiency of 45.9 cd/A.

Highly stable and Pb-free bismuth-based perovskites for photodetector applications

Herein, we report synthesis of highly stable, Pb-free bismuth iodide, stoichiometric methylammonium bismuth iodide and non-stoichiometric methylammonium bismuth iodide perovskite thin films for photodetector applications.

Near-Infrared Microlasers from Self-Assembled Spiropyrane-Based Microsphercial Caps

Near-infrared (NIR) microlasers play a significant role in telecommunication and biomedical tissue imaging. However, it remains a big challenge to realize NIR microlasers because of the difficulty in preparing highly efficient NIR luminescent materials and perfect optical resonators. Here, we propose a molecular design strategy to creatively realize the first spiropyrane (SP)-based NIR microlasers with low threshold from self-assembled microsphercial caps. The tetraphenylethylene (TPE) moiety with a highly twisted conformation provides a large free volume to facilitate the photoisomerization process of SP and enhance NIR emission of merocyanine in the solid state. Moreover, self-assembled TPE-SP microsphercial caps simultaneously serve as gain media and resonant microcavities, providing optical gain and feedback for NIR laser oscillations with a low threshold (3.68 μJ/cm²). These results are beneficial for deeply understanding the SP microstructures-lasing emission characteristic relationship and provide a useful guideline for the rational molecular design of NIR microlasers with special functionalities.

Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted Planar FA-Cs Mixed-Cation Perovskite Solar Cells

Depth-dependent growth of perovskite crystals remains challenging for high-performance perovskite solar cells made by a two-step spin-coating method. Effective morphology engineering approaches that enable depth-independent perovskite crystals growth and facile characterization technique to monitor subtle yet influential accompanying changes are urgently required. Here, a porous and intercrossed PbI₂-(CsI)_0.15 nanorods scaffold is prepared by integrating CsI incorporation with toluene dripping in ambient air, and the underlying mechanism is uncovered. With this porous scaffold and moisture-assisted thermal annealing, depth-independent growth of FA_0.85Cs_0.15PbI₃ is achieved, as evidenced in the photoluminescent (PL) spectra acquired by exciting the perovskite film from the top and bottom individually. It is of broad interest that PL spectroscopy is demonstrated as a sensitive technique to monitor the depth-dependent growth of perovskite. Moreover, the resulting inverted planar FA_0.85Cs_0.15PbI₃ perovskite solar cells deliver an efficiency of 16.85%, along with superior thermal and photostability. By incorporating 2% large-sized diammonium cation, propane-1,3-diammonium, the efficiency is further increased to 17.74%. Our work not only proposes a unique porous PbI₂-(CsI)_0.15 nanorods scaffold to achieve high-quality perovskite films in a two-step method but also highlights the distinctive advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films.