Tuning the Structure and Dynamics of 2D Hybrid Perovskites Through the Structure of the Organic Cation

Two-dimensional hybrid perovskites are promising materials for a wide range of opto-electronic applications. Their properties markedly differ from those of their three-dimensional analogues, but they also offer additional routes to engineer their properties by introducing specific functionality in the organic component. One of the key features of 2D hybrid perovskites is their relatively soft structure that allows substantial structural fluctuations, both during synthesis and after deposition. The softness of the material leaves considerable room to manipulate the structure of the inorganic layer through external stimuli such as pressure and adsorption of molecules on the surface, or internal effects such as strain cause by the organic separating layers. The nature of the organic component directly affects the structural and dynamic properties of the inorganic lattice. [1] Such changes in the structure and dynamics translate to changes in the electronic properties of the materials, for instance leading to changes in optical and conductive properties or the introduction of chirality in the materials. Therefore, by controlling the structure and interactions in the organic part it is possible to tune important physical parameters of the materials, including the charge carrier mobility, exciton binding energy, carrier lifetime and the decay pathways of charges and excited states.<br/><br/>In this work we computationally explore approaches to engineer the structural and dynamics properties of the 2D perovskite materials by introducing specific organic moieties that have additional functionality. The layer of organic molecules influences the structure and dynamics of the inorganic layer, which also has an immediate effect on the electronic structure. We have performed classical molecular dynamics simulations using custom forcefields for a variety of 2D halide perovskites, including Ruddlesden-Popper and Dion-Jacobson structures. It is shown that the nature of the organic cation determines the average structure and the structural dynamics to a large extent. This indicates that these properties can be tuned significantly by exploiting specific intermolecular interactions in the organic layer. The structure and dynamics of the organic layer directly affect the structure and dynamics of the inorganic layer, for instance the disorder in the octahedral tilt angles. This should lead to marked effect on the steady state electronic properties but also offers a handle to influence the strength and nature of the electron-phonon coupling in these materials.<br/><br/>The soft nature of of 2D hybrid perovskites suggest a marked dependence of the electronic properties on external stimuli, for instance the strain in the material. Strain can be introduced by applying external pressure, either uniformly of by bending of the material. Using pressure dependent molecular dynamics simulations we also show that the response to external pressure depends strongly on the nature of the organic layer and also here substantial tuning is possible through structural engineering. This can eventually be exploited for sensing applications.<br/><br/>[1] M.B. Fridrikson, N. van der Meer, J. de Haas and F.C. Grozema<i>, J. Phys. Chem. C </i><b>124</b> (2020) 28201