Controlling Chirality of Entropic Crystals

TL;DR

This paper demonstrates how the chirality of entropic colloidal crystals can be controlled by particle design and external conditions, enabling the formation of homochiral structures relevant for photonic applications.

Contribution

It introduces a method to control and switch the chirality of colloidal crystals through particle faceting and entropic force tuning, advancing the design of chiral photonic materials.

Findings

Stronger faceting increases chiral crystal selectivity.

Particle rounding and depletants enable reconfiguration between achiral and chiral crystals.

Chirality strength can be quantified from geometry and force diagrams.

Abstract

Colloidal crystal structures with complexity and diversity rivaling atomic and molecular crystals have been predicted and obtained for hard particles by entropy maximization. However, so far homochiral colloidal crystals, which are candidates for photonic metamaterials, are absent. Using Monte Carlo simulations we show that chiral polyhedra exhibiting weak directional entropic forces self-assemble either an achiral crystal or a chiral crystal with limited control over the crystal handedness. Building blocks with stronger faceting exhibit higher selectivity and assemble a chiral crystal with handedness uniquely determined by the particle chirality. Tuning the strength of directional entropic forces by means of particle rounding or the use of depletants allows for reconfiguration between achiral and homochiral crystals. We rationalize our findings by quantifying the chirality strength of each particle, both from particle geometry and potential of mean force and torque diagrams.