Temperature-Dependent Chirality in Halide Perovskites

Abstract

With the use of chiral organic cations in two-dimensional metal halide perovskites, chirality can be induced in the metal halide layers, which results in semiconductors with intriguing chiral optical and spin-selective transport properties. The chiral properties strongly depend upon the temperature, despite the basic crystal symmetry not changing fundamentally. We identify a set of descriptors that characterize the chirality of metal halide perovskites, such as MBA₂PbI₄, and study their temperature dependence using molecular dynamics simulations with on-the-fly machine-learning force fields obtained from density functional theory calculations. We find that, whereas the arrangement of organic cations remains chiral upon increasing the temperature, the inorganic framework loses this property more rapidly. We ascribe this to the breaking of hydrogen bonds that link the organic with the inorganic substructures, which leads to a loss of chirality transfer.

Figure 1

Overview of structural descriptors for 2D metal halide perovskites. (a) Unit cell of the chiral (S-MBA)₂PbI₄ perovskite. (b) Chirality of organic cations (ϵ_A2). (c) Out-of-plane chirality of the inorganic framework (ϵ_MX4^⊥). (d) In-plane chirality of the inorganic framework (ϵ_MX4^∥). (e) Hydrogen bond asymmetry (Δr_HB). The various structural vectors used in the vector pairs and projection directions are shown in the panels.

Figure 2

Finite-temperature distributions of structural descriptors and corresponding degree of chirality for MBA⁺-based perovskites at 50 K. Distributions of (a) cation chirality, (b) in-plane framework chirality, (c) out-of-plane framework chirality, and (d) hydrogen bond asymmetry of (S-MBA)₂PbI₄ and (R-MBA)₂PbI₄. Distributions of (e) cation chirality, (f) in-plane framework chirality, (g) out-of-plane framework chirality, and (h) hydrogen bond asymmetry of (rac-MBA)₂PbI₄. Degree of chirality for (i) (S-MBA)₂PbI₄ and (j) (rac-MBA)₂PbI₄. In panels a–d, the dark-colored distributions belong to (S-MBA)₂PbI₄, whereas the light-colored distributions correspond to (R-MBA)₂PbI₄.

Figure 3

Temperature dependence of the degree of chirality in MBA⁺-based perovskites. Temperature-dependent descriptor distributions for (a) cation chirality, (b) in-plane framework chirality, (c) out-of-plane framework chirality, and (d) hydrogen bond asymmetry in (S-MBA)₂PbI₄. Temperature-dependent descriptor distributions for (e) cation chirality, (f) in-plane framework chirality, (g) out-of-plane framework chirality, and (h) hydrogen bond asymmetry in (rac-MBA)₂PbI₄. Temperature-dependent degree of chirality for (i) (S-MBA)₂PbI₄ and (j) (rac-MBA)₂PbI₄.

Figure 4

Orientational autocorrelation of the NH₃⁺ headgroup of organic cations. (a) Schematic overview of the N–H bond vectors r_i used to determine the orientation of the headgroups of organic cations. Temporal autocorrelation of the headgroup orientation A_NH3⁺ for the cations in (b) (S-MBA)₂PbI₄ and (c) (rac-MBA)₂PbI₄ at temperatures ranging from 50 to 400 K. The dashed gray line indicates where A_NH3⁺ = e^–1.