Tunable Layer Orientation and Morphology in Vapor–Liquid–Solid Growth of One-Dimensional GeS van der Waals Nanostructures

Abstract

Vapor–liquid–solid (VLS) growth of layered crystals produces van der Waals nanowires, an emerging class of one-dimensional (1D) nanostructures. The crystal structure of van der Waals materials, covalently bonded within and weakly interacting between the layers, not only gives rise to highly anisotropic properties but also provides opportunities for controlling the layer orientation in VLS growth processes. Here, we show that such control can be realized via additives to the VLS catalyst, using 1D GeS van der Waals nanostructures as a model system. Au-catalyzed VLS growth from a pure GeS precursor invariably yields GeS nanowires with layering along the nanowire axis. Adding a small amount of SnS to the GeS vapor reproducibly switches the morphology to high-quality ribbonlike GeS nanostructures with two different layer stacking geometries, “angled ribbons” consisting of two misaligned halves with layer stacking parallel to the ribbon axis, and “tilted-layer ribbons” in which a single set of GeS sheets is tilted away from the nanowire axis. TEM imaging and chemical analysis show that SnS is not incorporated in the growing ribbons but remains confined to the VLS drop where its likely role is to change the wetting of GeS sheets by the catalyst drop, thus modifying the morphology and layer orientation. Cathodoluminescence measurements on individual GeS nanoribbons demonstrate small modifications of the emission between the two types of ribbons, while the overall optoelectronic properties remain those of GeS. Our results demonstrate opportunities for tailoring the morphology of 1D van der Waals nanostructures by modifications to the catalyst used in vapor–liquid–solid growth processes.

1D Germanium Sulfide van der Waals Bicrystals by Vapor–Liquid–Solid Growth

“…We find that Bi as a low-melting single-component VLS catalyst simultaneously yields chiral twisted van der Waals nanowires with axial screw dislocations and ultrathin bicrystal nanoribbons with a characteristic central twin plane. While Au-catalyzed VLS growth consistently yields chiral twisted nanowires, modifying the Au catalyst by introducing small amounts of SnS vapor along with the primary GeS precursor switches the growth to the formation of bicrystal van der Waals ribbons, analogous to those obtained over Bi catalysts, along with a small number of other fast-growing nanoribbon morphologies. , Transmission electron microscopy (TEM) and electron diffraction establish the morphology, structure and stacking of the 1D bicrystals observed here. Cathodoluminescence spectroscopy on individual bicrystal ribbons demonstrates that, while enabling fast growth rates, the central twin defect is electronically benign, i.e.…”

“…VLS growth using a pure Au catalyst invariably results in the synthesis of GeS nanowires with axial screw dislocations as a fast-growing defect morphology. Small amounts of co-evaporated SnS act as a growth modifier without being significantly incorporated (see Methods for details), and leads to the formation of GeS bicrystals at all substrate temperatures between 265 and 300 °C, along with tilted ribbon-like 1D GeS nanostructures reported previously . Growth at lower temperatures generally produces thinner bicrystals.…”

“…To show that the VLS catalyst controls the formation of bicrystals, we demonstrate the switching between chiral twisted GeS nanowires and of GeS bicrystals by modification of a Au VLS catalyst. In prior work, we obtained different GeS nanoribbon geometries by evaporation of GeS and SnS powders at low ratio between SnS and GeS vapor pressures ( p (SnS): p (GeS) ≈ 0.04) and substrate temperatures between 265 and 300 °C . VLS growth using a pure Au catalyst invariably results in the synthesis of GeS nanowires with axial screw dislocations as a fast-growing defect morphology.…”

Br-Induced Orientation Control of PbI2 van der Waals Nanowires and Their Optoelectronics

“…23 Thus, the development of the strategies of modulating vdW-layered crystal structures during the VLS growth would be beneficial, as recently demonstrated for the GeS nanostructures. 31 Here, we report an in situ control of PbI 2 vdW layer stacking orientations during VLS growth and their corresponding anisotropic luminescence characteristics. Our previous study has demonstrated that the transverse stack of 2H phase PbI 2 vdW layers along the [0001] c-axis can be realized by the selfcatalyzed, homoepitaxial growth on PbI 2 layers.…”

“…These examples suggest that there is a unique design space for the vdW-layered material systems when the VLS approach is applied, including variable stacking orientation (i.e., vertical/horizontal) or stacking sequence, in which a high level of control is critical for investigating new functionalities in optoelectronics or nanophotonics . Thus, the development of the strategies of modulating vdW-layered crystal structures during the VLS growth would be beneficial, as recently demonstrated for the GeS nanostructures …”