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

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.