# Twisted Tin‐Chloride Perovskite Single‐Crystal Heterostructures

## TL;DR

Replacing lead with tin in a perovskite heterostructure causes a twist between layers, leading to structural and electronic changes that affect optical properties.

## Contribution

The paper introduces twisted tin-chloride perovskite heterostructures with unique structural and electronic properties.

## Key findings

- Tin-based heterostructures exhibit a twist between perovskite and intergrowth layers due to local distortions at Sn centers.
- Structural differences, not composition, drive the distinct band structures in tin and lead analogs.
- Interfacial strain in tin heterostructures allows differential Pb incorporation and redshifted optical absorption.

## Abstract

Self‐assembly affords simpler synthetic routes to heterostructures compared with manual layer‐by‐layer stacking, yet controlling interlayer twist angles in a bulk solid remains an outstanding challenge. We report two new single‐crystal heterostructures: (Sn2Cl2)(CYS)2SnCl4 (CYS = +NH3(CH2)2S–; Sn_CYS) and (Sn2Cl2)(SeCYS)2SnCl4 (SeCYS = +NH3(CH2)2Se–; Sn_SeCYS) synthesized in solution, with alternating perovskite and intergrowth layers. Notably, compared to the recently reported lead analog, (Pb2Cl2)(CYS)2PbCl4 (Pb_CYS), the tin heterostructures feature a twist between the perovskite and intergrowth layers. We trace this twist to local distortions at the Sn centers, which change the interfacial lattice‐matching requirements compared to those of the Pb analog. Electronic band structure calculations show that the striking differences in the relative energies of perovskite‐ and intergrowth‐derived bands in Sn_CYS and Pb_CYS arise from structural and not compositional differences. The structural anisotropy of Sn_CYS is also reflected in a large in‐plane photoluminescence linear anisotropy ratio. Interfacial strain further affords differential incorporation of Pb into the perovskite and intergrowth layers of the Sn heterostructures, resulting in redshifted optical absorption onsets. Thus, we posit that local structural distortions may be exploited to manipulate the twist angle and interfacial strain in bulk heterostructures, providing a new handle for tuning the band alignments of bulk quantum‐well electronic structures.

Replacing lead with tin in a single‐crystal halide perovskite heterostructure drives a twist between the perovskite (gray) and intergrowth (blue) layers. The accompanying structural distortions and interfacial strain change the calculated orbital composition of the band edges and enable high in‐plane optical anisotropy in the Sn analog. (VBT = valence band top; CBB = conduction band bottom).