Photostriction-Driven Phase Transition in Layered Chiral NbOX$_2$ Crystals: Electrical-Field-Controlled Enantiomer Selectivity

TL;DR

This work presents a photostriction-enabled, two-step scheme to achieve enantiomer selectivity in layered NbOX$_2$ crystals. By photoexciting the material, a chiral $C2$ ground state is driven into the achiral $C2/m$ phase, after which an applied electric field biases relaxation toward a chosen enantiomer; the two resulting enantiomers are confirmed to be mirror-related via opposite shift-current responses. Ab initio calculations detail the necessary photocarrier densities and laser fluences (with NbOI$_2$ being the most accessible) to reach the transition, and show that the electric-field–assisted relaxation dramatically lowers the coercive field required for polarization reversal compared to the bulk value ($E_c \sim 100$ kV/cm to ~$0.52$ kV/cm). This optoelectrical control of chirality enables fast, reversible, non-contact manipulation of enantiomers with potential applications in ultrafast memory and chiral optoelectronics.

Abstract

Chiral crystals offer an unique platform for controlling structural handedness through external stimuli. However, the ability to select between structural enantiomers remains challenging, both theoretically and experimentally. In this work, we demonstrate a two-step pathway for enantiomer selectivity in layered chiral NbOX$_2$ (X = Cl, Br, I) crystals based on photostriction-driven phase transitions. Ab-initio simulations reveal that optical excitation is capable of inducing a structural phase transition in NbOX$_2$ from the monoclinic ($C2$) ground state to the higher-symmetry ($C2/m$) structure. In the resulting transient high-symmetry state, an applied electric field breaks the residual inversion-symmetry degeneracy, selectively stabilizing one enantiomeric final state configuration over the other. Our results establish a combined optical-electrical control scheme for chiral materials, enabling reversible and non-contact enantiomer selection with potential applications in ultrafast switching, optoelectronics, and chiral information storage.

Figures (2)

• Figure 1: (a) Schematic view of the two-step crystal chirality selection mechanism proposed: (1) light-induced nonthermal phase transition and (2) electric-field–driven enantiomer stabilization. (b) Layered structure of NbOX$_2$ (X = Cl, Br, I), with Nb (blue), O (red), and X (yellow) atoms; $d_i$ and $d_j$ denote Nb–O bond lengths. (c–e) Evolution of lattice constants (upper panels) and space group (lower panels) as a function of the photocarrier concentration for (c): NbOCl$_2$, (d): NbOBr$_2$, and (e): NbOI$_2$, respectively. Purple line represent the quantity required for a symmetry-phase transition.
• Figure 2: (a) Schematic illustration (along the c-axis) of the electric-field–selective mechanism, represented in terms of right- and left-handed structures defined by the Nb–O bond lengths ($d_i$, $d_j$). (b) Calculated shift-current spectra as a function of photon energy for the structures in (a): achiral (orange), right-handed (dashed red), and left-handed (solid blue). (c) Energy–polarization double-well profiles for ferroelectric bulk NbO$X_2$. Dots denote the calculated data, while solid lines represent fits based on the Landau model (see Supplementary V).