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Organic-inorganic hybrid perovskite semiconductors are under investigation for many applications owing to their excellent optoelectronic properties and relatively simple processing. Two-dimensional (2D) halide perovskites are an attractive class of hybrid perovskites that have additional optoelectronic tunability due to their accommodation of relatively large organic ligands. Nevertheless, contemporary ligand design depends on either expensive trial-and-error testing of whether a ligand can be integrated within the lattice or conservative heuristics that unduly limit the scope of ligand chemistries. Here, the structural determinants of ligand incorporation and perovskite stability are established by molecular dynamics (MD) simulations of over ten thousand perovskites, including an algorithmically generated set of prospective ligand chemistries and all previously reported experimental primary ammonium ligands based on C, H, O, S, and N. The simulation results show near-perfect predictions of positive and negative literature examples, predict trade-offs between several ligand features and perovskite stability, and ultimately predict an inexhaustibly large 2D-compatible ligand design space. This dataset is used to train machine learning classifiers capable of predicting the geometric stability of perovskite structures based solely on generalizable ligand features, thus providing an inexpensive tool for screening putative ligands. As a demonstration, the model was used to down-select five new ligands that were successfully synthesized and incorporated into 2D perovskites. This work provides a new paradigm for future low-dimensional perovskite design.

Force-field development has undergone a revolution in the past decade with the proliferation of quantum chemistry based parametrizations and the introduction of machine learning approximations of the atomistic potential energy surface. Nevertheless, transferable force fields with broad coverage of organic chemical space remain necessary for applications in materials and chemical discovery where throughput, consistency, and computational cost are paramount. Here, we introduce a force-field development framework called Topology Automated Force-Field Interactions (TAFFI) for developing transferable force fields of varying complexity against an extensible database of quantum chemistry calculations. TAFFI formalizes the concept of atom typing and makes it the basis for generating systematic training data that maintains a one-to-one correspondence with force-field terms. This feature makes TAFFI arbitrarily extensible to new chemistries while maintaining internal consistency and transferability. As a demonstration of TAFFI, we have developed a fixed-charge force-field, TAFFI-gen, from scratch that includes coverage for common organic functional groups that is comparable to established transferable force fields. The performance of TAFFI-gen was benchmarked against OPLS and GAFF for reproducing several experimental properties of 87 organic liquids. The consistent performance of these force fields, despite their distinct origins, validates the TAFFI framework while also providing evidence of the representability limitations of fixed-charge force fields.

Molecular dynamics simulations were conducted to study the equilibrium and transport properties of methane/water two-phase systems with sodium dodecyl sulfate (SDS) at the interface. In particular, the properties were determined at different SDS packing fractions, 0%, 25%, 50%, 75%, and 100%. The calculated interfacial tension, determined from the difference of normal and transverse components of the pressure tensor, was found to decrease with increasing SDS concentration at the methane/water interface, whereas the equilibrium partition of methane in the gas and water phases remained nearly unchanged. Both of these results were in quantitative agreement with experimental observations. The presence of SDS at the interface increased the free energy barrier of methane transport through the interface. Therefore, it was found that the rate of methane transport across the interface slowed down with increasing SDS concentrations. In other words, the thermodynamic driving force was unchanged, but the kinetic barr...

Commercialization of halide perovskites in the semiconductor industry is hindered by their short-term stability. The instability of perovskites is closely interlinked with ionic diffusion. Historically, attempts to study diffusion in 2D perovskites mostly utilized electrical characterizations, but these characterizations pose a challenge in deconvoluting the impact of device architecture, interlayers, and ionic species. In this Perspective, we focus our attention on simple optical characterizations employed in the literature to investigate halide diffusion in 2D perovskites using lateral and vertical heterostructure platforms. We review the various synthesis techniques used for fabrication of halide perovskite heterostructures and discuss the qualitative and quantitative diffusion studies performed using these platforms. We discuss the numerical methods used to validate and supplement the experimental halide diffusion kinetics. Finally, we highlight the need to conduct further research on the impact of device operating conditions, lattice structure, and vacancy concentration on halide diffusion. Through this Perspective, we aim to emphasize the need of developing a comprehensive understanding of halide diffusion in perovskites for their successful deployment in optoelectronics.