1D Wires of 2D Layered Materials: Germanium Sulfide Nanowires as Efficient Light Emitters

We considered the formation of one-dimensional (1D) nanowires from the two-dimensional (2D) layered material GeS. Growth by a low-temperature vapor–liquid–solid process over a Au catalyst gives rise to single-crystalline nanowires with typical diameters of ∼100 nm and length exceeding 10 μm that consist of a crystalline GeS core with layering along the nanowire axis, surrounded by an ultrathin, sulfur-rich GeS_<i>x</i> shell with larger bandgap that provides chemical and electronic passivation of the nanowires. Combined variable-temperature in situ electron microscopy, chemical mapping, and single-nanowire cathodoluminescence spectroscopy are used to uncover key elements of the growth process and to identify the optoelectronic properties of the GeS/GeS_<i>x</i> core–shell nanowires. Our results suggest that vapor–liquid–solid processes can readily be extended from conventional three-dimensional crystals to the growth of 1D wires from 2D layered crystals and may provide novel low-dimensional materials for electronic, optoelectronic, or energy conversion applications that benefit from the wide-ranging properties of van der Waals materials

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. The fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. The fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. The fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. The fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. The fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. The fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics

Phase-Specific Vapor–Liquid–Solid Growth of GeSe and GeSe₂ van der Waals Nanoribbons and Formation of GeSe–GeSe₂ Heterostructures

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 (Eg = 2.5 eV). Our results support applications of germanium mono- and dichalcogenides as well as their heterostructures in areas such as optoelectronics and photovoltaics

Germanium Sulfide Nano-Optics Probed by STEM-Cathodoluminescence Spectroscopy

Nano-optical studies of confined modes in planar waveguides have attracted significant interest as a means to probe exciton-polaritons and other hybrid light-matter quasiparticles in layered semiconductors, such as transition metal dichalcogenides or boron nitride. There is a need to broaden such studies to other materials and to identify alternatives to scanning near-field optical microscopy for exciting and measuring confined waveguide modes. Here, we establish an approach for probing the dispersion of traveling waveguide modes by cathodoluminescence spectroscopy excited by the focused electron beam in scanning transmission electron microscopy (STEM-CL) and apply it to solid-state resonators consisting of mesoscale monocrystalline prisms and plates composed of GeS, an anisotropic layered semiconductor with direct bandgap in the near-infrared spectral range. Structure, crystallography, and chemical composition of the mesostructures are analyzed by analytical electron microscopy. STEM-CL maps and spectra show pronounced interference effects and sharp emission peaks at photon energies below the fundamental bandgap of GeS. Our analysis demonstrates that locally excited light emission in STEM-CL launches in-plane waveguide modes in the mesoscale GeS structures, which are internally reflected by highly specular GeS edges to cause interference of the waveguide modes. Reabsorption and secondary luminescence give rise to the intensity modulations detected in the far field. Our results highlight avenues for probing light-matter interactions below the diffraction limit in a wide range of quantum materials and open up the possibility of tuning light emission geometrically using interference rather than by the conventional bandgap engineering

Nanoscale Integration of Two-Dimensional Materials by Lateral Heteroepitaxy

Materials integration in heterostructures with novel properties different from those of the constituents has become one of the most powerful concepts of modern materials science. Two-dimensional (2D) crystals represent a new class of materials from which such engineered structures can be envisioned. Calculations have predicted emergent properties in 2D heterostructures with nanoscale feature sizes, but methods for their controlled fabrication have been lacking. Here, we use sequential graphene and boron nitride growth on Ru(0001) to show that lateral heteroepitaxy, the joining of 2D materials by preferential incorporation of different atomic species into exposed 1D edges during chemical vapor deposition on a metal substrate, can be used for the bottom-up synthesis of 2D heterostructures with characteristic dimensions on the nanoscale. Our results suggest that on a proper substrate, this method lends itself to building nanoheterostructures from a wide range of 2D materials