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–x Se x 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–x Se x 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–x Se x has good potential to tune the optoelectronic property.

Emerging Nonplanar van der Waals Nanoarchitectures from 2D Allotropes for Optoelectronics

“…The initial diameter of spiral nanowires is equivalent to the droplet size, which could be adjusted by controlling the concentration and the composition of precursors to tune the twist rate and period length, as the entry of precursor affect the surface tension of the alloy droplet. [ 22,138,141 ] At higher temperatures, the droplet solubility becomes higher, corresponding to a larger initial diameter and period length of spiral nanowires. [ 142 ] Other growth mechanisms, such as LBL growth and twin‐driven growth, sometimes occur, for the orientation and facets of initial whiskers in contact with the droplet are determined by the alloy components.…”

Growth and Properties of Dislocated Two-dimensional Layered Materials

“…The twist rates and periods of the structures are determined by the radii of the dislocated nanowires that are defined by the size of the Au-Ge alloy droplets catalyzing the vapor-liquid-solid (VLS) process. Through introducing germanium selenide (GeSe) into the growth, the droplet size was chemically tuned and thereby the twist rate and period of the twisted structures 52 . The chemical modulation demonstrates good potential to tailor the twist rate and period of helical vdW crystals, enabling a new degree of freedom to modulate optical, electrical, thermal, mechanical and catalytic properties.…”

Controllable Edge Epitaxy of Helical GeSe/GeS Heterostructures

“…In our previous study, , we found that an axial dislocation in GeS nanowires (NWs) can induce a continuous twist along the substrate, known as Eshelby twist. Further growth along the radical direction leads to helical MWs, consisting of discretely twisted nanoplates.…”

“…Figure e shows the Raman spectra of helical GeS MWs and GeSe/GeS heterostructures, respectively. The characteristic Raman modes are labeled, which can be categorized to GeS and GeSe. ,, We also perform detailed miro-X-ray diffraction measurements on helical GeS and GeSe/GeS MWs (see Figure S6 for details), showing that the epitaxial GeSe is of the same crystallinity and orientation as the helical GeS template. All these characterizations suggest that our facile edge epitaxy method effectively serves for preparing high-quality GeSe/GeS heterostructures with helicoidal morphology.…”