TOGETHER was structured into three main parts: the synthesis of 2DLPs and their heterostructures, the in-depth characterization of their formation mechanisms, optical and structural properties, and the demonstration of light-driven reactions.
The project developed innovative synthesis strategies for 2DLPs and their lateral heterostructures. TOGETHER introduced three distinct synthesis techniques for fabricating regularly shaped pristine 2DLPs, achieving lateral sizes from 100 nm to 100 µm, tailored to the scale requirements of various characterization methods. To create lateral heterostructures, a solution-based anion exchange process was developed, involving the exposure of pristine 2DLPs to halide salts of the spacer cation dissolved in alcohols such as octanol. Both Br-to-I and I-to-Br exchanges were performed on PEA2PbX4 microcrystals, forming core-frame lateral heterostructures. These structures feature at least two distinct phases with unique optical properties within a single crystal. More complex heterostructures with high-quality interfaces were synthesized using a developed sequential crystallization technique, leveraging the solubility differences of various 2DLPs in polar solvents. These advancements enabled the rational design of lateral heterostructures in 2DLPs, leading to the successful fabrication of triple-halide heterostructures incorporating three distinct phases within a single crystal, as well as lateral heterostructures featuring different metal cations, such as PEA2PbBr4–PEA4AgBiBr8 and PEA4AgBiBr8–PEA2SnBr4.
In situ imaging and in situ spectroscopic techniques were combined with advanced electron microscopy to elucidate the formation mechanism of lateral heterostructures. These studies revealed that anion exchange in 2DLPs is strongly influenced by the initial halide composition of the parent 2DLP, which can be explained by the preferential occupation of halides within the octahedral framework. The stability of anion-exchanged heterostructures was assessed by storing microcrystals under ambient conditions. Both Br-to-I and I-to-Br exchanged microcrystals remained stable without any signs of interdiffusion between the two phases. However, while the iodine-rich phase in Br-to-I exchanged crystals degraded over several days, the I-to-Br heterostructures remained fully stable throughout the experiment. Time-resolved PL and exciton diffusion measurements on anion-exchanged samples revealed indications of directional energy flow toward the lower bandgap material within the lateral heterostructures. Advanced electron microscopy was used to evaluate the interface quality between the two phases after the exchange process, revealing interfaces dominated by pores and overgrowth. As a consequence of the low interface quality, a one-pot sequential crystallization technique was developed. In situ photoluminescence spectroscopy of the sequential crystallization technique showed a stepwise growth of heterostructures, driven by the varying solubility of 2DLPs in polar solvents upon antisolvent injection. Whether PEA2PbBr4 and PEA2PbI4 were mixed from the beginning or introduced at different stages of the crystallization process determined the formation of either alloyed phases or distinct pure-phase heterostructures. Structural analysis of the triple-halide heterostructures confirmed an almost strain-free epitaxial connection between the different phases.
The deposition of metal nanoparticles on 2DLPs with the bifunctional spacer molecule cysteamine was used as a benchmark reaction to demonstrate the potential of 2DLPs for light-driven chemical reactions. In line with TOGETHER’s green energy character, 2DLPs were also explored for osmotic power generation. Solid-state nanopores were fabricated using focused ion beam drilling, with pore diameters tunable by exposing 2DLP microcrystals to a precursor solution of PbBr2 and PEABr. The pore size depended on the dilution factor of the precursor solution. To enhance stability in aqueous solutions, 2DLPs were treated with halide salts of the spacer cations (e.g. PEABr). Conductivity measurements, performed as a function of salt concentration and pH value, revealed that the pore sidewalls carried a positive charge. Under a 1000× concentration gradient, the osmotic power output reached a value of nearly 6000 pW.
