Large-Scale Layer-by-Layer Synthesis of Borophene on Ru(0001)

Borophene, a two-dimensional (2D) layer of elemental boron, has attracted attention as a metallic 2D material with rich polymorphism, as well as potentially interesting electronic, optical, chemical, and catalytic properties. While controllable adjustments of the thickness beyond a single layer promise a further tunability of the properties, the growth of borophene on metal substrates has largely remained limited to monolayers. Recently, bilayer borophene has been realized on Ag(111), albeit so far limited to domains a few tens of nanometers in size. The broader exploration of the properties of borophene calls for the development of synthesis methods that produce borophene with controlled thickness beyond a single layer. Here, we report layer-by-layer borophene growth on Ru(0001) substrates up to a coverage of two atomic layers using surface segregation of interstitial boron during cooling from high temperatures. Real-time surface microscopy follows borophene nucleation and growth, establishes the growth mode of the first and second layers, and demonstrates coverage across large surface areas. Diffraction shows highly crystalline borophene layers that grow in a high-order superstructure on the Ru(0001) surface lattice. Our results add to the capabilities of synthesizing borophene and point out avenues for growing high-quality few-layer borophene on metal substrates

Axial Heterostructures with Phase-Controlled Metastable Segments via Post-Growth Reactions of Ge Nanowires

Metastable crystal phases promise new functionalities by providing access to material properties beyond those of thermodynamically stable crystalline solids. While realizing metastable phases in the bulk requires nonequilibrium processes such as rapid quenching or high pressures, size effects at the nanometer scale can promote their formation and stabilization during mild processing. Here, we demonstrate the controlled formation of axial heterostructures of alternating stable semiconductor (Ge) and metastable metallic (AuGe) segments via post-growth processing by decorating Ge nanowires with Au, encapsulating them in graphitic carbon shells, annealing at moderate temperatures to induce alloying of Au and Ge in the melt, followed by slow cooling and crystallization. The process is extended to form heterostructures of binary AgGe and ternary AuAgGe alloy segments in Ge nanowires. The metallic alloys adopt different metastable crystal phases that are selected via the nanowire diameter. This phase selection, explained by a size-dependent solubility of Ge in liquid AuGe (and AgGe) and distinct structural motifs in Ge-rich AuGe melts, allows establishing a previously unknown metastable γ-AgGe phase in thin axially segmented Ge–AgGe nanowires. The findings demonstrate an avenue for synthesizing axial heterostructures of stable and metastable solid phases in nanowires

Phase Diagram of Nanoscale Alloy Particles Used for Vapor−Liquid−Solid Growth of Semiconductor Nanowires

We use transmission electron microscopy observations to establish the parts of the phase diagram of nanometer sized Au−Ge alloy drops at the tips of Ge nanowires (NWs) that determine their temperature-dependent equilibrium composition and, hence, their exchange of semiconductor material with the NWs. We find that the phase diagram of the nanoscale drop deviates significantly from that of the bulk alloy, which explains discrepancies between actual growth results and predictions on the basis of the bulk-phase equilibria. Our findings provide the basis for tailoring vapor−liquid−solid growth to achieve complex one-dimensional materials geometries

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

Large-Scale Layer-by-Layer Synthesis of Borophene on Ru(0001)

Borophene, a two-dimensional (2D) layer of elemental boron, has attracted attention as a metallic 2D material with rich polymorphism, as well as potentially interesting electronic, optical, chemical, and catalytic properties. While controllable adjustments of the thickness beyond a single layer promise a further tunability of the properties, the growth of borophene on metal substrates has largely remained limited to monolayers. Recently, bilayer borophene has been realized on Ag(111), albeit so far limited to domains a few tens of nanometers in size. The broader exploration of the properties of borophene calls for the development of synthesis methods that produce borophene with controlled thickness beyond a single layer. Here, we report layer-by-layer borophene growth on Ru(0001) substrates up to a coverage of two atomic layers using surface segregation of interstitial boron during cooling from high temperatures. Real-time surface microscopy follows borophene nucleation and growth, establishes the growth mode of the first and second layers, and demonstrates coverage across large surface areas. Diffraction shows highly crystalline borophene layers that grow in a high-order superstructure on the Ru(0001) surface lattice. Our results add to the capabilities of synthesizing borophene and point out avenues for growing high-quality few-layer borophene on metal substrates

Large-Scale Layer-by-Layer Synthesis of Borophene on Ru(0001)

Borophene, a two-dimensional (2D) layer of elemental boron, has attracted attention as a metallic 2D material with rich polymorphism, as well as potentially interesting electronic, optical, chemical, and catalytic properties. While controllable adjustments of the thickness beyond a single layer promise a further tunability of the properties, the growth of borophene on metal substrates has largely remained limited to monolayers. Recently, bilayer borophene has been realized on Ag(111), albeit so far limited to domains a few tens of nanometers in size. The broader exploration of the properties of borophene calls for the development of synthesis methods that produce borophene with controlled thickness beyond a single layer. Here, we report layer-by-layer borophene growth on Ru(0001) substrates up to a coverage of two atomic layers using surface segregation of interstitial boron during cooling from high temperatures. Real-time surface microscopy follows borophene nucleation and growth, establishes the growth mode of the first and second layers, and demonstrates coverage across large surface areas. Diffraction shows highly crystalline borophene layers that grow in a high-order superstructure on the Ru(0001) surface lattice. Our results add to the capabilities of synthesizing borophene and point out avenues for growing high-quality few-layer borophene on metal substrates

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