Search

Emergent phenomena in traditional ferroelectrics are frequently observed at heterointerfaces. Accessing such functionalities in van der Waals ferroelectrics requires the formation of layered heterostructures, either vertically stacked (similar to oxide ferroelectrics) or laterally stitched (without equivalent in 3D-crystals). Here, we investigate lateral heterostructures of the ferroelectric van der Waals semiconductors SnSe and SnS. A two-step process produces ultrathin crystals comprising an SnSe core laterally joined to an SnS edge-band, as confirmed by Raman spectroscopy, transmission electron microscopy (TEM) imaging, and electron diffraction. TEM shows a moiré pattern across the SnSe core due to coverage by an ultrathin SnS layer. The ability of the lateral interface (IF) to direct excited carriers, probed by cathodoluminescence, shows electron transfer over 560 nm diffusion length from the SnS edge-band. Large, thin flakes supporting ferroelectricity allow investigating domains and domain wall interactions in uniform crystals and lateral heterostructures. Polarized optical microscopy of sub-20 nm flakes consistently shows ⟨110⟩ oriented stripe domains with mirror-twin domain walls. Heterostructures adopt two domain configurations, with domains either constrained to the SnSe core or propagating across the entire SnSe–SnS flakes. The combined results demonstrate multifunctional van der Waals heterostructures with high-quality IFs presenting extraordinary opportunities for manipulating carrier flows and ferroelectric domain patterns.

Vapor–liquid–solid (VLS) growth using molten metal catalysts has traditionally been used to synthesize nanowires from different 3D-crystalline semiconductors. With their anisotropic structure and properties, 2D/layered semiconductors create additional opportunities for materials design when shaped into 1D nanostructures. In contrast to hexagonal 2D crystals such as graphene, h-BN, and transition metal dichalcogenides, which tend to roll up into nanotubes, VLS growth of layered group III and group IV monochalcogenides produces diverse nanowire and nanoribbon morphologies that crystallize in a bulk-like layered structure with nanometer-scale footprint and lengths exceeding tens of micrometers. In this Account, we discuss the achievable morphologies, the mechanisms governing key structural features, and the emerging functional properties of these 1D van der Waals (vdW) architectures. Recent results highlight rich sets of phenomena that qualify these materials as a distinct class of nanostructures, far beyond a mere extension of 3D-crystalline VLS nanowires to vdW crystals.

Linear defects (dislocations) not only govern the mechanical properties of crystalline solids but they can also produce distinct electronic, thermal, and topological effects. Accessing this functionality requires control over the placement and geometry of single dislocations embedded in a small host volume to maximize emerging effects. Here we identify a synthetic route for rational dislocation placement and tuning in van der Waals nanowires, where the layered crystal limits the possible defect configurations and the nanowire architecture puts single dislocations in close proximity to the entire host volume. While homogeneous layered nanowires host single screw dislocations, the synthesis of radial nanowire heterostructures (here exemplified by GeS-Ge1–xSnxS monochalcogenide core–shell nanowires) transforms the defect into a mixed (helical) dislocation whose edge/screw ratio is tunable via the core–shell lattice mismatch. The ability to design nanomaterials with control over individual mixed dislocations paves the way for identifying the functional properties of dislocations and harnessing them in technology.

Alloying of two-dimensional (2D)/layered chalcogenide semiconductors by forming ternaries with properties that span the range between the binary constituents allows tuning of the electronic and optical properties and achieving the full potential of these materials. While the focus so far has been on transition-metal dichalcogenides, alloying in layered group IV chalcogenides─promising for optoelectronics, photovoltaics, ferroelectrics, etc.─remains less understood. Here, we investigate alloying in the GeSe–GeS system and its effect on the fundamental band gap. We synthesize single-crystalline layered GeSxSe1–xalloy micro- and nanowires whose compositions are tunable over the entire range of S content, x, via the GeS and GeSe precursor temperatures. Cathodoluminescence in scanning transmission electron microscopy is used to investigate the composition dependence of the band gaps of GeSxSe1–xalloy micro- and nanowires. The band gaps of bulk-like microwires increase systematically with the sulfur content of the alloys, thereby covering the entire range between GeSe (1.27 eV) and GeS (1.6 eV). The composition dependence of the fundamental band gap is close to linear with a bowing coefficient b= 0.173 eV. Density functional theory calculations support the isomorphous behavior of GeSe–GeS solid solutions and demonstrate that the band gaps are indirect and have similar small bowing as determined experimentally. Finally, we establish pronounced size effects in GeSxSe1–xalloy nanowires that provide access to higher-energy optoelectronic transitions than can be realized in bulk alloys of the same composition. Our results support applications of germanium monochalcogenide alloys in areas such as optoelectronics and photovoltaics.

The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS2precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initiocalculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. The results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge- and thermal transport.

Nanostructures such as nanoribbons and -wires are of interest as components for building integrated photonic systems, especially if their basic functionality as dielectric waveguides can be extended by chiroptical phenomena or modifications of their optoelectronic properties by extended defects, such as dislocations. However, conventional optical measurements typically require monodisperse (and chiral) ensembles, and identifying emerging chiral optical activity or dislocation effects in single nanostructures has remained an unmet challenge. Here we show that whispering gallery modes can probe chirality and dislocation effects in single nanowires. Wires of the van der Waals semiconductor germanium(ii) sulfide (GeS), obtained by vapor–liquid–solid growth, invariably form as growth spirals around a single screw dislocation that gives rise to a chiral structure and can modify the electronic properties. Cathodoluminescence spectroscopy on single tapered GeS nanowires containing joined dislocated and defect-free segments, augmented by numerical simulations and ab-initiocalculations, identifies chiral whispering gallery modes as well as a pronounced modulation of the electronic structure attributed to the screw dislocation. Our results establish chiral light-matter interactions and dislocation-induced electronic modifications in single nanostructures, paving the way for their application in multifunctional photonic architectures.

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 GeSe2. Besides the amorphous form, which attracted intense interest, crystalline GeSe2in 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 GeSe2phase remain unknown. Here, we report the high-yield synthesis of orthorhombic GeSe2nanoribbons by GeSe/Se vapor transport over Au catalysts. Access to air-stable monocrystalline, single-phase ribbons enabled investigating the properties of orthorhombic GeSe2including 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 GeSe2ribbons as a promising wide-bandgap semiconductor nanostructure for applications in optoelectronics and energy conversion.

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.

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.

The layered semiconductor tin selenide (SnSe) has received extensive interest due to its promising thermoelectric and ferroelectric properties. Integrating SnSe with other layered crystals in heterostructures can enable the modification of charge- and thermal transport, electrical polarization, and other properties such as chemical stability, optoelectronics, and photonics. Here, we demonstrate the vapor transport synthesis of single-crystalline SnSe monochalcogenide flakes that are spontaneously encapsulated in a thin layered SnSe2-xSxdichalcogenide shell. In a second growth step, such SnSe–SnSe2-xSxheterostructures are integrated with the monochalcogenide GeSe, which is laterally stitched to the SnSe side facets while preserving the dichalcogenide shell across the basal facets. This architecture is confirmed by optical microscopy, electron microscopy and diffraction, energy dispersive X-ray and Raman spectroscopies, as well as cathodoluminescence spectroscopy. The results extend our capabilities for materials integration by forming complex heterostructures with both vertical van der Waals interfaces and covalent lateral interfaces between layered semiconductors.

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.

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.

Solution-phase processes such as colloidal synthesis and transformations have enabled the formation of nanocrystals with exquisite control over size, shape, and composition. Self-assembly, in solution or at phase boundaries, can arrange such nanocrystal building blocks into ordered superlattices and dynamically reconfigurable “smart” materials. Ultimately, continued improvements in our ability to direct nanocrystal matter depend on progress in understanding colloidal chemistry and self-assembly in solution. The traditional approach for investigating the underlying, inherently dynamic processes involves sampling at different stages combined with ex situcharacterization, for example, using electron microscopy. In situstudies have been restricted to a few methods capable of measuring in bulk liquids, either in reciprocal space by diffraction or scattering or using spatially averaging (e.g., optical) measurements. These strategies face clear limitations in obtaining mechanistic information, and they are unable to address heterogeneous systems that may harbor rich sets of configurations with different local properties. The development of microfabricated cells that hermetically encapsulate bulk solutions between ultrathin (electron transparent) membranes has paved the way for studying processes in liquids in real time by electron microscopy at resolution down to the atomic scale. Electrons interact much more strongly with matter than other probes, for example, X-rays. In ordinary inorganic samples, the main effects are atom displacements and defect formation via knock-on and ionization damage. In liquid-cell electron microscopy, the interaction of the beam with both the suspended nanostructures and the solution creates more diverse effects, so the straightforward scenario of imaging unperturbed nanocrystal chemistry in solution is rarely realized.

Abstract

Engineered heterostructures derive distinct properties from materials integration and interface formation. Two-dimensional crystals have been combined to form vertical stacks and lateral heterostuctures with covalent line interfaces. While thicker vertical stacks have been realized, lateral heterostructures from multilayer van der Waals crystals, which could bring the benefits of high-quality interfaces to bulk-like layered materials, have remained much less explored. Here, we demonstrate the integration of anisotropic layered Sn and Ge monosulfides into complex heterostructures with seamless lateral interfaces and tunable vertical design using a two-step growth process. The anisotropic lattice mismatch at the lateral interfaces between GeS and SnS is relaxed viadislocations and interfacial alloying. Nanoscale optoelectronic measurements by cathodoluminescence spectroscopy show the characteristic light emission of joined high-quality van der Waals crystals. Spectroscopy across the lateral interface indicates valley-selective luminescence in the bulk SnS component that arises due to anisotropic electron transfer across the interface. The results demonstrate the ability to realize high-quality lateral heterostructures of multilayer van der Waals crystals for diverse applications, e.g., in optoelectronics or valleytronics.

Tin sulfide (SnS) is a part of the group IV chalcogenides (SnX, GeX; X:S, Se), a family of anisotropic layered materials considered for thin-film photovoltaics, optoelectronics, and valleytronics and predicted narrow band gap multiferroic materials. Large ultrathin SnS flakes, suitable for a variety of applications, are challenging to synthesize because of the enhanced surface reactivity due to the open layer structure of SnS, which gives rise to a competition between lateral and vertical growth. Here, we investigate the effects of added sulfur on passivating the surface and modifying the balance between lateral expansion and thickening of SnS flakes in chemical vapor transport from a SnS precursor. We investigate the growth of SnS flakes and compare the results of synthesis from a pure SnS precursor with growth, in which a slight excess of sulfur is supplied in different ways. Our results demonstrate that small amounts of excess sulfur can profoundly affect the size and thickness distributions of SnS flakes. The largest and thinnest flakes are obtained if (i) traces of sulfur are added and (ii) the sulfur source consists of either small Sxfragments released at high temperatures from the reactor walls or atomic S supplied by sublimation from SnS2. The likely mechanism for the observed growth modifications is a transient surface passivation of SnS flakes during growth, which reduces the reactivity of the top facet of the flakes, limits vertical growth, and thus gives rise to ensembles with increased lateral size and reduced thickness.

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 Eg= 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.

The physical properties of two-dimensional (2D) materials depend strongly on the number of layers. Hence, methods for controlling their thickness with atomic layer precision are highly desirable, yet still too rare, and demonstrated for only a limited number of 2D materials. Here, we present a simple and scalable method for the continuous layer-by-layer thinning that works for a large class of 2D materials, notably layered germanium pnictides and chalcogenides. It is based on a simple oxidation/etching process, which selectively occurs on the topmost layers. Through a combination of atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction experiments we demonstrate the thinning method on germanium arsenide (GeAs), germanium sulfide (GeS), and germanium disulfide (GeS2). We use first-principles simulation to provide insights into the oxidation mechanism. Our strategy, which could be applied to other classes of 2D materials upon proper choice of the oxidation/etching reagent, supports 2D material-based device applications, e.g., in electronics or optoelectronics, where a precise control over the number of layers (hence over the material’s physical properties) is needed. Finally, we also show that when used in combination with lithography, our method can be used to make precise patterns in the 2D materials.

The light-stimulated transformation of ensembles of spherical nanoparticles into anisotropic metal nanostructures mediated by localized surface plasmon resonance (LSPR) excitation is an elegant way of synthesizing triangular silver nanoprisms with extraordinary control over size and shape. Generally, the transformation occurs in oxidizing environments along a pathway that involves the oxidative etching of small preexisting Ag seeds, followed by plasmon-mediated reduction of the resulting Ag ions and Ag0incorporation into the anisotropic nanocrystals. Here, we investigate pathways toward Ag nanoprisms from initially homogeneous AgNO3solutions held under reducing conditions. Observations using in situ electron microscopy show that reducing environments and high Ag precursor concentrations in the presence of sodium citrate favor two alternative transformation routes of initial spherical nuclei into anisotropic nanoprisms: (i) the aggregation of spherical nanoparticles and plasmon-mediated conversion of small clusters into triangular prisms; (ii) shape fluctuations of individual small nanoparticles. Simulated field distributions confirm that the coupling of the LSPR excitation between closely spaced nanoparticles causes significant field enhancements near the local plasmonic hot spots, which facilitates accelerated Ag incorporation and thus supports the transformation into nanoprisms.

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 nonlayered 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 single-phase 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-Ga2S3nanotubes by reaction with sulfur vapor. The conversion proceeds via the formation of epitaxial GaAs-Ga2S3core–shell structures, vacancy injection and aggregation into Kirkendall voids, elastic relaxation of the detached Ga2S3shell, 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 Ga2S3. 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.

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.

Among 2D/layered semiconductors, group IV monochalcogenides such as SnS(e) and GeS(e) have attracted attention as phosphorene/black phosphorus analogues with anisotropic structures and predicted unusual properties. In contrast to SnS, for which bottom-up synthesis has been reported, few-layer GeS has been realized primarily viaexfoliation from bulk crystals. Here, we report the synthesis of large (up to >20 μm), faceted single crystalline GeS flakes with anisotropic properties using a vapor transport process. In situelectron microscopy is used to identify the thermal stability and sublimation pathways, and demonstrates that the GeS flakes are self-encapsulated in a thin, sulfur-rich amorphous GeSxshell during growth. The shell provides exceptional chemical stability to the layered GeS core. In contrast to exfoliated GeS, which rapidly degrades during exposure to air, the synthesized GeS–GeSxcore–shell structures show no signs of chemical attack and remain unchanged in air for extended time periods. Measurements of the optoelectronic properties by photoluminescence spectroscopy show a tunable bandgap due to out-of-plane quantum confinement in flakes with thickness below 100 nm. Cathodoluminescence (CL) spectroscopy with nanoscale excitation provides evidence for interfacial charge transfer due to a type II heterojunction between the crystalline core and amorphous shell. Measurements by locally excited CL yield a minority carrier (electron) diffusion length in the p-type GeS core ldiff= 0.27 μm, on par with diffusion lengths in the highest-quality layered chalcogenide semiconductors.

Van der Waals heterostructures with small misalignment between adjacent layers (‘interlayer twist’) are of interest because of electronic structure and correlation phenomena (such as superconductivity) that are determined by both the atomic lattice and long-range superlattice potentials arising in interlayer moiré patterns1–7. Previously, such twisted heterostructures have involved a single planar interface between layers isolated by exfoliation and micromechanically stacked in the desired relative orientation1,8–12. Here we demonstrate a class of materials—van der Waals nanowires of layered crystals—in which a tunable interlayer twist evolves naturally during synthesis. In vapour–liquid–solid growth, nanowires of germanium(ii) sulfide, an anisotropic layered semiconductor, crystallize with layering along the wire axis13and have a strong propensity for forming axial screw dislocations. Nanometre-resolved electron diffraction shows that Eshelby twist, induced by a torque on the ends of a cylindrical solid due to the stress field of an axial dislocation14,15, causes a chiral structure in the van der Waals nanowires. The in-plane germanium sulfide crystal axes progressively rotate along the wire, and germanium sulfide layers in adjacent turns of the helix naturally form a moiré pattern because of their interlayer twist. The axial rotation and the twist are tunable by varying the nanowire thickness. Combined electron diffraction and cathodoluminescence spectroscopy show the correlation between the interlayer twist and locally excited light emission that is due to progressive changes in the lattice orientation and in the interlayer moiré registry along the nanowires. The findings demonstrate a step towards scalable fabrication of van der Waals structures with defined twist angles, in which interlayer moiré patterns are realized along a helical path on a nanowire instead of a planar interface. A tunable interlayer twist that evolves naturally during synthesis of van der Waals nanowires made from layered crystals of germanium sulfide could produce new electronic structure and correlation phenomena.

Tin monosulfide (SnS) is a layered two-dimensional semiconductor that has attracted attention because of its potential applications, for instance, photovoltaics, optoelectronics, and valleytronics. However, these applications require synthesis of SnS flakes with a controlled thickness and shape. Here, we demonstrate the possibility of using Au nanoparticles to seed the nucleation, determine the position, and control the thickness of the growing SnS plates. Further, we use in situ transmission electron microscopy on individual ultrathin SnS flakes during annealing at high temperatures to establish their thermal stability and decomposition pathways, which is important due to the large surface areas of ultrathin flakes. We find that ultrathin SnS plates decompose rapidly at temperatures above 400 °C. The decomposition invariably starts at the edges of the flakes and is highly anisotropic. Real-time observations show that while the {010} and {110} facets are relatively stable and retract slowly, rapid removal of material along the <100> directions leads to the fast decay of the {100} facets, which are thus replaced by the extended {110} facets. The comparison of shapes developed during growth and sublimation establishes that the typical crystal habits observed for SnS and related group IV monochalcogenides are indeed kinetic growth shapes rather than equilibrium (Wulff) shapes determined by different facet surface free energies. These findings can further guide the design of growth experiments for achieving particular shapes of flakes of SnS and related layered crystals.

Two-dimensional and layered van der Waals materials promise to overcome the limitations of conventional ferroelectrics in terms of miniaturization and material integration, but synthesis has produced only small (up to few micrometer-sized) ferroic crystals. Here, we report the realization of in-plane ferroelectric few-layer crystals of the monochalcogenides tin(II) sulfide and selenide (SnS, SnSe) whose linear dimensions exceed the current state of the art by up to 1 order of magnitude. Such large crystals allow the investigation of ferroic domain patterns that are unaffected by edges and finite-size effects. Analysis of the abundant stripe domains by electron microscopy and nanobeam electron diffraction shows two distinct domain types: twin domains separated by positively charged walls with alternating head-to-head and tail-to-tail polarization as well as not previously observed purely rotational domains connected by neutral domain walls with head-to-tail dipoles. Access to large crystals allowed the determination of the Curie temperature of few-layer SnSe van der Waals ferroelectrics, and it enables the investigation of this class of ferroelectrics by widely available methods such as polarized optical microscopy. The combination with layer transfer protocols promises uniform materials for exploring fundamentals and for implementing devices for information processing and energy conversion.

Black phosphorus (BP) has attracted significant interest as a monolayer or few-layer material with extraordinary electrical and optoelectronic properties. Chemical reactions with different ambient species, notably oxygen and water, are important as they govern key properties such as stability in air, electronic structure and charge transport, wetting by aqueous solutions, and so on. Here, we report experiments combined with ab initio calculations that address the effects of oxygen and water in contact with BP. Our results show that the reaction with oxygen is primarily responsible for changing properties of BP. Oxidation involving the dissociative chemisorption of O2causes the decomposition of BP and continuously lowers the conductance of BP field-effect transistors (FETs). In contrast, BP is stable in contact with deaerated (i.e., O2depleted) water and the carrier mobility in BP FETs gated by H2O increases significantly due to efficient dielectric screening of scattering centers by the high-k dielectric. Isotope labeling experiments, contact angle measurements, and calculations show that the pristine BP surface is hydrophobic but is turned progressively hydrophilic by oxidation. Our results open new avenues for exploring applications that require contact of BP with aqueous solutions including solution gating, electrochemistry, and solution-phase approaches for exfoliation, dispersion, and delivery of BP.

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.

Layered semiconductors show promise as channel materials for field-effect transistors (FETs). Usually, such devices incorporate solid back or top gate dielectrics. Here, we explore deionized (DI) water as a solution top-gate for field-effect switching of layered semiconductors including SnS2, MoS2, and black phosphorus. The DI water gate is easily fabricated, can sustain rapid bias changes, and its efficient coupling to layered materials provides high on–off current ratios, near-ideal subthreshold swing, and enhanced short-channel behavior even for FETs with thick, bulk-like channels, where such  control is difficult to realize with conventional back gating. Screening by the high-k solution gate eliminates hysteresis due to surface and interface trap states and substantially enhances the field-effect mobility. The onset of water electrolysis sets the ultimate limit to DI water gating at large negative gate bias. Measurements in this regime show promise for aqueous sensing, demonstrated here by the amperometric detection of glucose in aqueous solution. DI water gating of layered semiconductors can be harnessed in research on novel materials and devices, and it may with further development find broad applications in microelectronics and sensing.

By temperature-, time-, and pressure-resolved imaging of the dynamics of surface steps on NiAl(100) during its oxidation, we provide direct evidence of the significant effects of atomic steps in controlling the local thermodynamic driving force for oxidation. Our results show that the inherent barriers associated with step crossing by surface species of oxygen cause a heterogeneous oxygen concentration across the crystal surface, giving rise to localnonequilibrium effects governing oxidation even for surfaces that are globallyin equilibrium. The asymmetry in the step-crossing barriers for oxygen atoms crossing up or down steps is such that descendant steps exert a local driving force that favors oxidation, whereas ascendant steps locally destabilize the surface oxide in their vicinity. The local differences in the thermodynamic driving force for oxidation due to atomic steps and step bunches give rise to novel phenomena, such as nonmonotonous oxide growth and the net translation motion of surface oxide stripes by growing on one end while receding on the other end.

Abstract