While dislocations are traditionally perceived as detrimental to electronic materials, recent theories predicted emerging functional properties associated with dislocations. To identify and ultimately harness such functionality, approaches are needed for embedding single dislocations with tunable geometry (edge/screw ratio) in small-volume host crystals. Here, we address this challenge by mixing screw- and edge-dislocations in Ge1-xSnxS layered (van der Waals) core-shell nanowires. Phase separation during vapor-liquid-solid growth yields the required core-shell structure, and the growth process also produces a homojunction between a defect-free layered segment near the tip and a base segment containing a single mixed (helical) dislocation. Nanometer-scale cathodoluminescence (CL) spectroscopy sheds light on the effects of single helical dislocations on optoelectronics. CL shows that the luminescence intensity depends on the distance from the dislocation line. Efficient radiative recombination is found for the pristine material near the nanowire tip whereas the dislocated part of the nanowires shows a sharp reduction of the spontaneous emission quantum efficiency, attributed to the edge component of the helical dislocation. The results demonstrate control over the geometry of single mixed dislocations and the ability of probing their effect on functional properties, important steps toward the ultimate use of dislocations as active elements in devices.

2D and layered van der Waals semiconductors have attracted interest as alternatives to conventional oxide ferroelectrics, combining attributes such as facile materials integration, distinct symmetry-breaking and polarization mechanisms, and bandgaps in the visible and near-infrared spectral region. Often, growth processes that produce high-quality ferroelectrics of a layered crystal are not easily adaptable to other materials, even within the same family. Here, we show that in such cases, a nonequilibrium ferroelectric crystal phase can be templated from one material into another across interfaces in heterostructures. Growing two types of layered SnS-GeS heterostructures, using seed crystals of equilibrium (centrosymmetric) SnS or a metastable (distorted) phase of SnS, respectively, we find that the distorted noncentrosymmetric phase can be templated from SnS to GeS. This templating effect is identified both across vertical (van der Waals) interfaces and at lateral (covalent) interfaces of the synthetic heterostructures. Furthermore, in vertically stacked regions, the GeS layers also inherit the ferroelectric stripe domain patterns from the underlying SnS seed. Heterostructures using centrosymmetric SnS seeds, on the other hand, incorporate the nonferroelectric equilibrium phase of GeS throughout. Within the studied materials system, the findings point to a rational route for obtaining ferroelectric Ge monochalcogenides. More broadly, the results indicate that interfaces in heterostructures between van der Waals crystals allow transcribing nonequilibrium crystal structures between dissimilar materials, which can be used to access distinct structures and functional properties.

Doping is a primary design parameter in semiconductor devices. Whereas traditional materials, such as Si, Ge, and GaAs, benefit from facile ambipolar substitutional doping by shallow donors and acceptors, many compound semiconductors are strongly auto‐doped by native point defects. Layered Sn monochalcogenides, for example, show ubiquitous p‐type conduction due to Sn vacancy defects with low formation energy. Realization of robust n‐type doping would support numerous applications, e.g., in energy conversion as well as conventional and neuromorphic computing. Here, we demonstrate n‐type doping of large single‐crystalline SnS flakes by substitutional incorporation of Bi. Optical and electron microscopy show well‐formed SnS:Bi flakes. X‐ray photoelectron spectroscopy confirms the controllable incorporation of Bi in two oxidation states, corresponding to ionized substitutional donors as well as non‐ionized impurities that fill Sn vacancies and thus suppress p‐type conduction. Field‐effect transport and Hall effect confirm electrons as majority carriers with mobility exceeding 110 cm2/V s. Crystallization of thin SnS:Bi flakes in a distorted ferroelectric phase with stripe domains, similar to undoped SnS flakes, implies that ambipolar doping can be combined with in‐plane ferroelectricity in support of emerging applications of this versatile van der Waals semiconductor.

Junctions between p ‐ and n ‐doped semiconductors are ubiquitous in modern electronic devices and circuits. However, the tendency toward ‘natural’ defect doping (i.e., a fixed majority carrier polarity) has made the realization of pn ‐junctions between 2D/layered chalcogenide semiconductors challenging. Here, the formation of high‐quality, electrically active lateral junctions between Bi‐doped n ‐type SnSe and p ‐type SnS is demonstrated via a two‐step growth process, building on the successful integration of single‐crystalline ( p ‐type) SnSe and SnS in multilayer lateral heterostructures. The growth of single‐crystalline n ‐type SnSe:Bi flakes is established using vapor transport with in situ Bi doping. Subsequent SnS growth yields heterostructures between the SnSe:Bi seeds and a laterally stitched edge band of p ‐type SnS. Combined optical microscopy, Raman spectroscopy, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy demonstrate the formation of purely lateral SnS/SnSe:Bi heterostructures from standing, Bi‐doped SnSe seeds on mica substrates. Electron beam induced current measurements on individual heterostructures provide evidence for successful n ‐type doping of the SnSe:Bi seeds and the formation of an electrically active pn ‐junction at the lateral SnS/SnSe interface. The realization of sharp lateral pn ‐junctions in single‐crystalline layered chalcogenide semiconductors paves the way for applications in electronics, photovoltaics, thermoelectrics, etc.

Van der Waals bilayers assembled by mechanical stacking of 2D layers have been shown to realize symmetry-breaking mechanisms, such as sliding- or moiré ferroelectricity, which are without equivalent in 3D ferroelectrics. Here, we discuss emergent, highly unconventional behavior in thicker semiconducting few-layer SnS(e) van der Waals ferroelectrics crystallizing in a distorted polar structure, obtained by bottom-up synthesis. Electron diffraction at variable temperature demonstrates a gradual displacive transformation between the polar low-temperature and symmetric high-temperature phase, with significant variability in both the lattice constants and the Curie temperature (TC) where the transition is completed. Strikingly, such variations are even found among stripe domains in individual crystals, where lattice constants and distortion angles change periodically between adjacent domains. The resulting spontaneously formed homo-materials superlattices imply that such ferroelectrics adopt domain-specific Curie temperatures, in sharp contrast with the long-held notion of TC as a global materials parameter for a given ferroelectric.

Single-layer monochalcogenides are predicted to be in-plane ferroelectrics but are challenging to obtain in the 2D limit. Recent work showed that synthetic few-layer SnSe and SnS flakes also support ferroelectricity. Key properties such as the Curie temperature may become tunable via anion substitution in SnS1-xSex alloys. Hence, protocols need to be developed that produce ferroelectric few-layer alloy crystals. Here, we report SnS1-xSex alloy flakes across the entire composition range obtained by a highly reproducible growth process using mixed SnS/SnSe precursors. Characterization by electron microscopy and diffraction, X-ray dispersive spectroscopy, and Raman spectroscopy shows the flakes to be high-quality single crystals whose phonon modes and optical bandgaps interpolate between SnSe and SnS. Thin SnS1-xSex flakes across all compositions carry ubiquitous stripe domain patterns, i.e., the flakes are ferroelectric with twin domain walls. Such composition-tunable alloy flakes can support research on the fundamental mechanisms of in-plane ferroelectricity in few-layer monochalcogenide van der Waals semiconductors.

Alloying is one of the main tools of bandgap engineering, allowing the tuning of crystal structure, lattice parameters, and electronic structure of 3D and 2D/layered semiconductors. Among the latter, it can play a key role in tailoring the properties of tin monochalcogenides, a class of van der Waals semiconductors of interest for optoelectronics, thermoelectrics, ferroelectrics, and valleytronics. Here, the study investigates the synthesis and properties of large flakes of the anion substitution alloys SnSe 1‐x S x . Alloy flakes across a wide range of compositions are obtained systematically by repeated growth from the same mixed (SnS, SnSe) powder precursor. Combined experiment and theory show full miscibility for all compositions, along with tunable lattice constants, bandgaps, and vibrational modes. Atomic resolution imaging demonstrates the accumulation of S and Se in alternating layers in the SnSe 1‐x S x unit cell, attributed to growth kinetics. Polarized Raman spectroscopy confirms anisotropic vibrational modes; the calculated and measured band structure shows systematic changes in the band edge energies and anisotropic electronic structure due to the anisotropic in‐plane lattice of the monochalcogenides. Cathodoluminescence, finally, indicates that a unique configuration of two non‐degenerate, direct valleys along orthogonal k ‐space directions persists all the way from SnS to SnSe, making SnSe 1‐x S x alloys interesting for valleytronics.

While symmetry breaking in 2D ferroelectrics is obviously linked to the single-layer structure, layered (van der Waals) ferroelectrics can have a multitude of underlying mechanisms, making their identification nontrivial and often controversial. This complexity is exemplified by tin chalcogenides whose equilibrium structure, the orthorhombic α-phase with space group Pnma, includes an inversion center and which therefore should not be ferroelectric. Yet, recent work demonstrated polarization switching and ferroelectric domains in few-layer SnS and SnSe. Here, we use in situ electron microscopy and diffraction to determine the mechanism and characteristics of ferroelectricity across the SnSe1-xSx system. We identify two distinct phases of synthetic SnSe1-xSx: nonpolar (centrosymmetric) equilibrium (α-phase) crystals and metastable crystals adopting a distorted monoclinic structure, which are in-plane ferroelectrics with Curie temperatures of 320 to 420 °C. A surprising structural plasticity of the ferroelectric crystals during heating/cooling indicates a shallow energy landscape. This in turn suggests absence of a pronounced driving force for conversion to the α-phase that can explain the formation of the nonequilibrium crystals and their stability even after transfer to other supports. Our results highlight opportunities for the discovery of novel ferroelectrics among nonequilibrium van der Waals crystals.

Epitaxial vapor–liquid–solid growth on Au/van der Waals SnSe substrates yields complete coverage with mixed-dimensional GeS heterostructures combining 1D nanowires and 2D plates with intense luminescence and size-tunable emission wavelengths.

Alloying is one of the main tools of bandgap engineering, allowing the tuning of crystal structure, lattice parameters, and electronic structure of 3D and 2D/layered semiconductors. Among the latter, it can play a key role in tailoring the properties of tin monochalcogenides, a class of van der Waals semiconductors of interest for optoelectronics, thermoelectrics, ferroelectrics, and valleytronics. Here, the study investigates the synthesis and properties of large flakes of the anion substitution alloys SnSe1-xSx. Alloy flakes across a wide range of compositions are obtained systematically by repeated growth from the same mixed (SnS, SnSe) powder precursor. Combined experiment and theory show full miscibility for all compositions, along with tunable lattice constants, bandgaps, and vibrational modes. Atomic resolution imaging demonstrates the accumulation of S and Se in alternating layers in the SnSe1-xSx unit cell, attributed to growth kinetics. Polarized Raman spectroscopy confirms anisotropic vibrational modes; the calculated and measured band structure shows systematic changes in the band edge energies and anisotropic electronic structure due to the anisotropic in-plane lattice of the monochalcogenides. Cathodoluminescence, finally, indicates that a unique configuration of two non-degenerate, direct valleys along orthogonal k-space directions persists all the way from SnS to SnSe, making SnSe1-xSx alloys interesting for valleytronics.

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 SnSe 2-x S x dichalcogenide shell. In a second growth step, such SnSe - SnSe 2-x S x heterostructures 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.

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-xSx dichalcogenide shell. In a second growth step, such SnSe–SnSe2-xSx heterostructures 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.

Abstract2D and layered semiconductors are considered as promising electronic materials, particularly for applications that require high carrier mobility and efficient field‐effect switching combined with mechanical flexibility. To date, however, the highest mobility has been realized primarily at low carrier concentration. Here, it is shown that few‐layer/multilayer SnSe2 gated by a solution top gate combines very high room‐temperature electron mobility (up to 800 cm2 V−1s−1), along with large on‐off current ratios (>105) and a subthreshold swing below the thermodynamic limit (50 mV per decade) in field‐effect devices, at exceptionally large sheet carrier concentrations of ≈1013 cm−2. Observed mobility enhancements upon partial depletion of the channel point to near‐surface defects or impurities as the mobility‐limiting scattering centers. Under illumination, the resulting gap states give rise to gate‐controlled switching between positive and negative photoconductance. The results qualify SnSe2 as a promising layered semiconductor for flexible and wearable electronics, as well as for the realization of advanced approaches to photodetection.

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.

Performance of the group IV monochalcogenide GeSe in solar cells, electronic, and optoelectronic devices is expected to improve when high-quality single crystalline material is used rather than polycrystalline films. Crystalline flakes represent an attractive alternative to bulk single crystals as their synthesis may be developed to be scalable, faster, and with higher overall yield. However, large - and especially large and thin - single crystal flakes are notoriously hard to synthesize. Here it is demonstrated that vapor-liquid-solid growth combined with direct lateral vapor-solid incorporation produces high-quality single crystalline GeSe ribbons with tens of micrometers size and controllable thickness. Electron microscopy shows that the ribbons exhibit perfect equilibrium (AB) van der Waals stacking order without extended defects across the entire thickness, in contrast to the conventional case of substrate-supported flakes where material is added via layer-by-layer nucleation and growth on the basal plane. Electrical measurements show anisotropic transport and a high Hall mobility of 85 cm2 V-1 s-1, on par with the best single crystals to date. Growth from mixed GeSe and SnSe vapors, finally, yields ribbons with unchanged structure and composition but with jagged edges, promising for applications that rely on ample chemically active edge sites, such as catalysis or photocatalysis.

Valleytronics, i.e., use of the valley degree of freedom in semiconductors as an information carrier, is a promising alternative to conventional approaches for information processing. Transition metal dichalcogenides with degenerate K/K ' valleys have received attention as prototype 2D/layered semiconductors for valleytronics, but these systems rely on exotic effects such as the valley-Hall effect for electrical readout of the valley occupancy. Non-traditional valleytronic systems hosting sets of addressable non-degenerate valleys could overcome this limitation. In the van der Waals semiconductor Sn(ii) sulfide (SnS), for instance, different bandgaps and band edges may allow manipulating the population of the X- and Y-valleys via charge transfer across interfaces to other layered semiconductors. Here, we establish this concept by comparing SnS flakes and SnS-based heterostructures. Cathodoluminescence spectroscopy shows a striking reversal of the luminescence intensity of the two valleys in SnS-GeS van der Waals stacks, which stems from a selective electron transfer from the Y-valley into GeS while X-valley electrons remain confined to SnS. Our results suggest that non-traditional systems, embodied here by SnS-based van der Waals heterostructures, open avenues for valley-selective readout relying on design parameters such as heterostructure band offsets that are among the core concepts of semiconductor technology. Valley-selective carrier transfers across interfaces in heterostructures pave the way for valleytronics with electrical readout of the valley population.

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 moire 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.

Alloyingof two-dimensional (2D)/layered chalcogenide semiconductorsby forming ternaries with properties that span the range between thebinary constituents allows tuning of the electronic and optical propertiesand achieving the full potential of these materials. While the focusso far has been on transition-metal dichalcogenides, alloying in layeredgroup IV chalcogenides promising for optoelectronics, photovoltaics,ferroelectrics, etc.remains less understood. Here, we investigatealloying in the GeSe-GeS system and its effect on the fundamentalband gap. We synthesize single-crystalline layered GeS (x) Se1-x alloy micro-and nanowires whose compositions are tunable over the entire rangeof S content, x, via the GeS and GeSe precursor temperatures.Cathodoluminescence in scanning transmission electron microscopy isused to investigate the composition dependence of the band gaps ofGeS (x) Se1-x alloy micro- and nanowires. The band gaps of bulk-like microwiresincrease systematically with the sulfur content of the alloys, therebycovering the entire range between GeSe (1.27 eV) and GeS (1.6 eV).The composition dependence of the fundamental band gap is close tolinear with a bowing coefficient b = 0.173 eV. Densityfunctional theory calculations support the isomorphous behavior ofGeSe-GeS solid solutions and demonstrate that the band gapsare indirect and have similar small bowing as determined experimentally.Finally, we establish pronounced size effects in GeS (x) Se1-x alloy nanowiresthat provide access to higher-energy optoelectronic transitions thancan be realized in bulk alloys of the same composition. Our resultssupport applications of germanium monochalcogenide alloys in areassuch as optoelectronics and photovoltaics.

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. To date, realizing such heteromaterials mixed-dimensional heterostructures has required sequential multi-step growth processes. Here, it is shown that differences in precursor incorporation rates between vapor-liquid-solid growth of 1D nanowires and direct vapor-solid growth of 2D plates attached to the wires can be harnessed to synthesize heteromaterials mixed-dimensional heterostructures in a single-step growth process. Exposure to mixed GeS and GeSe vapors produces GeS1-x Sex van der Waals nanowires whose S:Se ratio is considerably larger than that of attached layered plates. Cathodoluminescence spectroscopy on single heterostructures confirms that the bandgap contrast between the components is determined by both composition and carrier confinement. These results demonstrate an avenue toward complex heteroarchitectures using single-step synthesis processes.

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. SnS-GeS1-xSex-GeSe-GeS1-xSex heterostructures with a GeS1-xSex active layer sandwiched laterally between GeSe and SnS, semiconductors with similar optical constants but smaller bandgaps, were designed and realized via sequential vapor transport synthesis. Raman spectroscopy, electron microscopy/diffraction, and energy-dispersive X-ray spectroscopy confirm a high crystal quality of the laterally stitched components with sharp interfaces. Nanometer-scale cathodoluminescence spectroscopy provides evidence for a facile transfer of electron-hole pairs across the lateral interfaces and demonstrates the selection of photon emission at discrete energies in the laterally embedded active (GeS1- xSex) part of the heterostructure.