Emergence of Chirality in an Optically Active Two-Dimensional Crystal with a Spiral Surface Pattern

Some one-dimensional (1D) crystals containing a screw dislocation along their longer axis exhibit a helical twist due to lattice strain. These chiral structures have been thoroughly investigated by using transmission electron microscopy. However, whether two-dimensional (2D) crystals with a spiral surface pattern, presumably containing a screw dislocation, are structurally chiral remains unclear because their internal structures are not visible. Here, we report that a particular 2D crystal with a spiral surface pattern emits circularly polarized luminescence despite comprising achiral building blocks. Comprehensive X-ray structural analysis of this 2D crystal reveals a helical structure with an achiral space group featuring distinctive twinning. This twinned 2D crystal consists of four centrosymmetric components. Applying the twin law to these four achiral components results in a spirally arranged topology, leading to the emergence of chirality in the 2D crystals.