Macroscopic Monochalcogenide van der Waals Ferroics: Growth, Domain Structures, and Curie Temperature.

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.

Directing Charge Carriers and Ferroelectric Domains at Lateral Interfaces in van der Waals Heterostructures.

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.

Photonics in Multimaterial Lateral Heterostructures Combining Group IV Chalcogenide van der Waals Semiconductors.

Lateral heterostructures combining two multilayer group IV chalcogenide van der Waals semiconductors have attracted interest for optoelectronics, twistronics, and valleytronics, owing to their structural anisotropy, bulk-like electronic properties, enhanced optical thickness, and vertical interfaces enabling in-plane charge manipulation/separation, perpendicular to the trajectory of incident light. Group IV monochalcogenides support propagating photonic waveguide modes, but their interference gives rise to complex light emission patterns throughout the visible/near-infrared range both in uniform flakes and single-interface lateral heterostructures. Here, this work demonstrates the judicious integration of pure and alloyed monochalcogenide crystals into multimaterial heterostructures with unique photonic properties, notably the ability to select photonic modes with targeted discrete energies through geometric factors rather than band engineering.

Tunable 1D van der Waals Nanostructures by Vapor-Liquid-Solid Growth.

ConspectusVapor-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.

Tuning of Single Mixed (Helical) Dislocations in Core-Shell van der Waals Nanowires.

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.

Chirality and dislocation effects in single nanostructures probed by whispering gallery modes.

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.

Self-Assembly of Mixed-Dimensional GeS Se (1D Nanowire)-(2D Plate) Van der Waals Heterostructures.

The integration of dissimilar materials into heterostructures is a mainstay of modern materials science and technology. An alternative strategy of joining components with different electronic structure involves mixed-dimensional heterostructures, that is, architectures consisting of elements with different dimensionality, for example, 1D nanowires and 2D plates. Combining the two approaches can result in hybrid architectures in which both the dimensionality and composition vary between the components, potentially offering even larger contrast between their electronic structures.

Lateral Integration of SnS and GeSe van der Waals Semiconductors: Interface Formation, Electronic Structure, and Nanoscale Optoelectronics.

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.

Stacking Fault Induced Symmetry Breaking in van der Waals Nanowires.

While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks.

Germanium Diselenide Ribbons with Orthorhombic Crystal Structure.

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. Besides the amorphous form, which attracted intense interest, crystalline GeSe in the bulk and in nanostructures such as flakes and nanobelts invariably adopts the 2D/layered monoclinic β-phase.

Plasmonic Gold Trimers and Dimers with Air-Filled Nanogaps.

The subwavelength confinement of light energy in the nanogaps formed between adjacent plasmonic nanostructures provides the foundational basis for nanophotonic applications. Within this realm, air-filled nanogaps are of central importance because they present a cavity where application-specific nanoscale objects can reside. When forming such configurations on substrate surfaces, there is an inherent difficulty in that the most technologically relevant nanogap widths require closely spaced nanostructures separated by distances that are inaccessible through standard electron-beam lithography techniques.

Free-standing large, ultrathin germanium selenide van der Waals ribbons by combined vapor-liquid-solid growth and edge attachment.

Among group IV monochalcogenides, layered GeSe is of interest for its anisotropic properties, 1.3 eV direct band gap, ferroelectricity, high mobility, and excellent environmental stability. Electronic, optoelectronic and photovoltaic applications depend on the development of synthesis approaches that yield large quantities of crystalline flakes with controllable size and thickness.

1D Germanium Sulfide van der Waals Bicrystals by Vapor-Liquid-Solid Growth.

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.

Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide.

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a great deal of research interest due to their capability of shaping nanoscale field distributions and controlling nanophotonic energy flows. Here we report a nano-optical imaging study of waveguide exciton polaritons (EPs) in tin sulfide (SnS) in the near-infrared (near-IR) region using scattering-type scanning near-field optical microscopy (s-SNOM). With s-SNOM, we mapped in real space the propagative EPs in SnS, which show sensitive dependence on the excitation energy and sample thickness.

Multilayer Lateral Heterostructures of Van Der Waals Crystals with Sharp, Carrier-Transparent Interfaces.

Research on engineered materials that integrate different 2D crystals has largely focused on two prototypical heterostructures: Vertical van der Waals stacks and lateral heterostructures of covalently stitched monolayers. Extending lateral integration to few layer or even multilayer van der Waals crystals could enable architectures that combine the superior light absorption and photonic properties of thicker crystals with close proximity to interfaces and efficient carrier separation within the layers, potentially benefiting applications such as photovoltaics. Here, the realization of multilayer heterstructures of the van der Waals semiconductors SnS and GeS with lateral interfaces spanning up to several hundred individual layers is demonstrated.

Ultrathin Twisted Germanium Sulfide van der Waals Nanowires by Bismuth Catalyzed Vapor-Liquid-Solid Growth.

1D nanowires of 2D layered crystals are emerging nanostructures synthesized by combining van der Waals (vdW) epitaxy and vapor-liquid-solid (VLS) growth. Nanowires of the group IV monochalcogenide germanium sulfide (GeS) are of particular interest for twistronics due to axial screw dislocations giving rise to Eshelby twist and precision interlayer twist at helical vdW interfaces. Ultrathin vdW nanowires have not been realized, and it is not clear if confining layered crystals into extremely thin wires is even possible.

Unconventional van der Waals heterostructures beyond stacking.

Two-dimensional crystals provide exceptional opportunities for integrating dissimilar materials and forming interfaces where distinct properties and phenomena emerge. To date, research has focused on two basic heterostructure types: vertical van der Waals stacks and laterally joined monolayer crystals with in-plane line interfaces. Much more diverse architectures and interface configurations can be realized in the few-layer and multilayer regime, and if mechanical stacking and single-layer growth are replaced by processes taking advantage of self-organization, conversions between polymorphs, phase separation, strain effects, and shaping into the third dimension.

Optoelectronics and Nanophotonics of Vapor-Liquid-Solid Grown GaSe van der Waals Nanoribbons.

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.

Sequential Symmetry-Breaking Events as a Synthetic Pathway for Chiral Gold Nanostructures with Spiral Geometries.

Symmetry-breaking synthetic controls allow for nanostructure geometries that are counter to the underlying crystal symmetry of a material. If suitably applied, such controls provide the means to drive an isotropic metal along a growth pathway yielding a three-dimensional chiral geometry. Herein, we present a light-driven solution-based synthesis yielding helical gold spirals from substrate-bound seeds.

Real-Time Electron Microscopy of Nanocrystal Synthesis, Transformations, and Self-Assembly in Solution.

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.

Cathodoluminescence of Ultrathin Twisted Ge Sn S van der Waals Nanoribbon Waveguides.

Ultrathin van der Waals semiconductors have shown extraordinary optoelectronic and photonic properties. Propagating photonic modes make layered crystal waveguides attractive for photonic circuitry and for studying hybrid light-matter states. Accessing guided modes by conventional optics is challenging due to the limited spatial resolution and poor out-of-plane far-field coupling.

Single-strand DNA-nanorod conjugates - tunable anisotropic colloids for on-demand self-assembly.

Directed self-assembly uses different stimuli to initiate and control the interaction between nanocrystals. Protonation at reduced pH represents a convenient stimulus for initiating self-assembly. Prior work has focused on protonation-induced hydrogen bonding between peptide or amino acid functionalized nanocrystals for reversible cycling between dispersed and aggregated states.

Lateral Heterostructures of Multilayer GeS and SnS van der Waals Crystals.

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.

Author Correction: Universal mechanical exfoliation of large-area 2D crystals.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.