Chemically Modulating the Twist Rate of Helical van der Waals Crystals

Abstract

Twisted van der Waals (vdW) materials with a controllable twist are of great interest because the twist offers new opportunities to modify the optoelectronic properties of the materials, giving rise to exotic phenomena, such as superconductivity, moiré excitons, and chiroptical response. Recently, we have synthesized helical vdW crystals with a periodic twist via the vapor–liquid–solid (VLS) growth of dislocated germanium sulfide nanowires with an Eshelby twist. The twist rates and periods of these structures are determined by the radii of the dislocated nanowires, which are defined by the size of the droplets catalyzing the VLS process. In this work, we tailor the twist rates and periods of the structures via chemically modulating the droplet size and the diameter of dislocated vdW nanowires. Our chemical analysis reveals that the growth of twisted GeS nanowires is catalyzed by droplets of an Au–Ge alloy. The size of the catalyst droplets was tailored by introducing GeSe into the growth. The addition of GeSe significantly increases the surface energy of the droplets, increasing the size of the droplets. This results in the growth of GeS1–xSex with decreased twist rates. The chemical modulation of the droplet size is correlated with the change of germanium concentration (supersaturation) in the alloy droplets, consistent with the Gibbs–Thomson effect. Increasing the selenium concentration in the GeS1–xSex from x = 0 to x = 0.11 decreases the twist rate from 0.59 to 0.22 rad/μm, increasing the period from 8 to 15 μm. The chemical modulation demonstrates good potential to tailor the twist rate and period of helical vdW crystals, providing more flexibility to modulate the optoelectronic properties and chiral light–matter interactions. Moreover, adding GeS into the source powder provides a means to tune the composition of the nanowires and mesoscale twisted crystals as GeS1-xSex alloy structures are produced. Our Raman spectroscopy and photoluminescence spectroscopy studies suggest that the compositional engineering in GeS1–xSex has good potential to tune the optoelectronic property.