Nanowires of layered van der Waals (vdW) crystals are of interest due to structural characteristics and emerging properties that have no equivalent in conventional 3D crystalline nanostructures. Here, vapor−liquid−solid growth, optoelectronics, and photonics of GaS vdW nanowires are studied. Electron microscopy and diffraction demonstrate the formation of high-quality layered nanostructures with different vdW layer orientation. GaS nanowires with vdW stacking perpendicular to the wire axis have ribbon-like morphologies with lengths up to 100 μm and uniform width. Wires with axial layer stacking show tapered morphologies and a corrugated surface due to twinning between successive few-layer GaS sheets. Layered GaS nanowires are excellent wide-bandgap optoelectronic materials with E g = 2.65 eV determined by single-nanowire absorption measurements. Nanometer-scale spectroscopy on individual nanowires shows intense blue band-edge luminescence along with longer wavelength emissions due to transitions between gap states and photonic properties such as interference of confined waveguide modes propagating within the nanowires. The combined results show promise for applications in electronics, optoelectronics, and photonics, as well as photo-or electrocatalysis owing to a high density of reactive edge sites, and intercalation-type energy storage benefiting from facile access to the interlayer vdW gaps.

High-yield synthesis of large, ultrathin GeSe ribbons combining longitudinal vapor–liquid–solid growth with lateral edge incorporation. Intense luminescence confirms high quality GeSe with low concentration of nonradiative recombination centers.

2D/layered semiconductors are of interest for fundamental studies and for applications in optoelectronics and photonics. Work to date focused on extended crystals, produced by exfoliation or growth and investigated by diffraction-limited spectroscopy. Processes such as vapor−liquid−solid (VLS) growth carry potential for mass-producing nanostructured van der Waals semiconductors with exceptionally high crystal quality and optoelectronic/photonic properties at least on par with those of extended flakes. Here, we demonstrate the synthesis, structure, morphology, and optoelectronics/photonics of GaSe van der Waals nanoribbons obtained by Au-and Ag-catalyzed VLS growth. Although all GaSe ribbons are high-quality basal-plane oriented single crystals, those grown at lower temperatures stand out with their remarkably uniform morphology and low edge roughness. Photoluminescence spectroscopy shows intense, narrow light emission at the GaSe bandgap energy. Nanophotonic experiments demonstrate traveling waveguide modes at visible/near-infrared energies and illustrate approaches for locally exciting and probing such photonic modes by cathodoluminescence in transmission electron microscopy.

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.

Defects in two-dimensional and layered materials have attracted interest for realizing properties different from those of perfect crystals. Even stronger links between defect formation, fast growth, and emerging functionality can be found in nanostructures of van der Waals crystals, but only a few prevalent morphologies and defect-controlled synthesis processes have been identified. Here, we show that in vapor–liquid–solid growth of 1D van der Waals nanostructures, the catalyst controls the selection of the predominant (fast-growing) morphologies. Growth of layered GeS over Bi catalysts leads to two coexisting nanostructure types: chiral nanowires carrying axial screw dislocations and bicrystal nanoribbons where a central twin plane facilitates rapid growth. While Au catalysts produce exclusively dislocated nanowires, their modification with an additive triggers a switch to twinned bicrystal ribbons. Nanoscale spectroscopy shows that, while supporting fast growth, the twin defects in the distinctive layered bicrystals are electronically benign and free of nonradiative recombination centers.

The chemical transformation of nanowire templates into nanotubes is a promising avenue toward hollow one-dimensional (1D) nanostructures. To date, high-quality single crystalline tubes of non-layered inorganic crystals have been obtained by solid-state reactions in diffusion couples of nanowires with deposited thin film shells, but this approach presents issues in achieving singlephase tubes with a desired stoichiometry. Chemical transformations with reactants supplied from the gas-or vapor-phase can avoid these complications, allowing single-phase nanotubes to be obtained through self-termination of the reaction once the sacrificial template has been consumed. Here, we demonstrate the realization of this scenario with the transformation of zincblende GaAs nanowires into single-crystalline cubic γ-Ga 2 S 3 nanotubes by reaction with sulfur vapor. The conversion proceeds via the formation of epitaxial GaAs-Ga 2 S 3 core-shell structures, vacancy injection and aggregation into Kirkendall voids, elastic relaxation of the detached Ga 2 S 3 shell, and finally complete incorporation of Ga in a crystalline chalcogenide tube.Absorption and luminescence spectroscopy on individual nanotubes show optoelectronic properties, notably a ~3.1 eV bandgap and intense band-edge and near band-edge emission consistent with high-quality single crystals, along with transitions between gap-states due to the inherent cation-vacancy defect structure of Ga 2 S 3 . Our work establishes the transformation of nanowires via vapor-phase reactions as a viable approach for forming single-crystalline hollow 1D nanostructures with promising properties.

Group IV (Ge, Sn) chalcogenides differ from most other two-dimensional (2D)/layered semiconductors in their ability to crystallize both as stable mono- and dichalcogenides. The associated diversity in structure and properties presents the challenge of identifying conditions for the selective growth of the different crystalline phases, as well as opportunities for phase conversion and materials integration/interface formation in heterostructures. Here, we discuss the phase-selective synthesis of free-standing GeSe and GeSe2 nanoribbons in a vapor–liquid–solid growth process over Au catalyst nanoparticles. Electron microscopy shows that the two types of ribbons adopt high-quality van der Waals structures with layering in the ribbon plane and with the ribbon axis aligned with the b-axis of GeSe and GeSe2, respectively. Nonspecific growth gives rise to a tapered morphology and, in the case of GeSe2, leads to nucleation of misoriented crystallites on the ribbon surface. The partial transformation of GeSe ribbons by selenization, finally reacts the outermost layers and edges to GeSe2, thus producing GeSe–GeSe2 core–shell heterostructures. Cathodoluminescence spectroscopy of as-grown GeSe ribbons and of GeSe–GeSe2 hybrids shows a marked enhancement of the luminescence intensity due to surface passivation by wide-band gap GeSe2 (E g = 2.5 eV). Our results support applications of germanium mono- and dichalcogenides as well as their heterostructures in areas such as optoelectronics and photovoltaics.

Many materials are known to exist in several stable polymorphs, but synthesis only provides access to a subset. This situation is exemplified by the dichalcogenide semiconductor GeSe 2 . Besides the amorphous form, which attracted intense interest, crystalline GeSe 2 in the bulk and in nanostructures such as flakes and nanobelts invariably adopts the 2D/layered monoclinic β-phase. Hence, the properties of other polymorphs such as the orthorhombic 3D GeSe 2 phase remain unknown. Here, we report the high-yield synthesis of orthorhombic GeSe 2 nanoribbons by GeSe/Se vapor transport over Au catalysts. Access to air-stable monocrystalline, single-phase ribbons enabled investigating the properties of orthorhombic GeSe 2 including its characteristic Raman spectrum. Optical absorption on ensembles and cathodoluminescence spectroscopy on individual ribbons show a wide bandgap and intense band-to-band emission in the visible, with a broad sub-bandgap emission tail. Our results establish orthorhombic GeSe 2 ribbons as a promising wide-bandgap semiconductor nanostructure for applications in optoelectronics and energy conversion.