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. SnS-GeS1-x Sex -GeSe-GeS1-x Sex heterostructures with a GeS1-x Sex 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- x Sex ) part of the heterostructure.

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. 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.The main difference between 3D- and vdW crystals, the pronounced in-plane/cross-plane anisotropy of layered materials, motivates investigating the factors governing the layer orientation. Recent research suggests that the VLS catalyst plays a key role, and that its modification via the choice of chalcogens or through modifiers added to the growth precursor can switch both the nanostructure morphology and vdW layering. In many instances, ordinary layered structures are not formed but VLS growth is dominated by morphologies─often containing a crystal defect─that present reduced or vanishing layer nucleation barriers, thus achieving fast growth and emerging as the principal synthesis product. Prominent defect morphologies include vdW bicrystals growing by a twin-plane reentrant process and chiral nanowires formed by spiral growth around an axial screw dislocation. The latter carry particular promise, e.g., for twistronics. In vdW nanowires, Eshelby twist─a progressive crystal rotation caused by the dislocation stress field─translates into interlayer twist that is precisely tunable via the wire diameter. Projected onto a helicoid vdW interface, the resulting twist moirés not only modify the electronic structure but also realize configurations without equivalent in planar systems, such as continuously variable twist and twist homojunctions.1D vdW nanostructures derive distinct functionality from both their layered structure and embedded defects. Correlated electron microscopy methods including imaging, nanobeam diffraction, as well as electron-stimulated local absorption and luminescence spectroscopies combine to an exceptionally powerful probe of this emerging functionality, identifying twist-moiré induced electronic modulations and chiral photonic modes, demonstrating the benign nature of defects in optoelectronics, and uncovering ferroelectricity via symmetry-breaking by single-layer stacking faults in vdW nanowires. Far-reaching possibilities for tuning crystal structure, morphology, and defects create a rich playground for the discovery of new functional nanomaterials based on vdW crystals. Given the prominence of defects and extensive prospects for controlling their character and placement during synthesis, 1D vdW nanostructures have the potential to cause a paradigm shift in the science of electronic materials, replacing the traditional strategy of suppressing crystal imperfections with an alternative philosophy that embraces the use of individual defects with designed properties as drivers of technology.

Self-Assembly of Mixed-Dimensional GeS1- x Sex (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. 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.

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. 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.

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. 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-initio calculations, 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.

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. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS₂ precursor 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 initio calculations 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.

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. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe_1-xS_x) alloys, obtained by vapor-liquid-solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe_1-xS_x alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.

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

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. Herein, we demonstrate an assembly route for the fabrication of aligned plasmonic gold trimers with air-filled vertical nanogaps having widths that are defined with spatial controls that exceed those of lithographic processes. The devised procedure uses a sacrificial oxide layer to define the nanogap, a glancing angle deposition to impose a directionality on trimer formation, and a sacrificial antimony layer whose sublimation regulates the gold assembly process. By further implementing a benchtop nanoimprint lithography process and a glancing angle ion milling procedure as additional controls over the assembly, it is possible to deterministically position trimers in periodic arrays and extend the assembly process to dimer formation. The optical response of the structures, which is characterized using polarization-dependent spectroscopy, surface-enhanced Raman scattering, and refractive index sensitivity measurements, shows properties that are consistent with simulation. This work, hence, forwards the wafer-based processing techniques needed to form air-filled nanogaps and place plasmonic energy at site-specific locations.

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. Here, we demonstrate the growth of single-crystalline GeSe nanoribbons by a vapor-liquid-solid process over Au catalyst on different substrates at low thermal budget. The nanoribbons crystallize in a layered structure, with ribbon axis along the armchair direction of the van der Waals layers. The ribbon morphology is determined by catalyst driven fast longitudinal growth accompanied by lateral expansion via edge-specific incorporation into the basal planes. This combined growth mechanism enables temperature controlled realization of ribbons with typical widths of up to 30 μm and lengths exceeding 100 μm, while maintaining sub-50 nm thickness. Nanoscale cathodoluminescence spectroscopy on individual GeSe nanoribbons demonstrates intense temperature-dependent band-edge emission up to room temperature, with fundamental bandgap and temperature coefficient of E_g(0) = 1.29 eV and α = 3.0 × 10^-4 eV K^-1, respectively, confirming high quality GeSe with low concentration of non-radiative recombination centers promising for optoelectronic applications including light emitters, photodetectors, and solar cells.

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. 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.

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. Moreover, we found that both the polariton wavelength and propagation length are anisotropic in the sample plane. In particular, in a narrow spectral range from 1.32 to 1.44 eV, the EPs demonstrate quasi-one-dimensional propagation, which is rarely seen in natural polaritonic materials. A further analysis indicates that the observed polariton anisotropy originates from the different optical band gaps and exciton binding energies along the two principal crystal axes of SnS.

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. Structural and chemical imaging identifies {110} interfaces that are perpendicular to the (001) layer plane and are laterally localized and sharp on a 10 nm scale across the entire thickness. Cathodoluminescence spectroscopy provides evidence for a facile transfer of electron-hole pairs across the lateral interfaces, indicating covalent stitching with high electronic quality and a low density of recombination centers.

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. If axial screw dislocations are still stable, ultrathin vdW nanowires can reach large twists and should display significant quantum confinement. Here it is shown that VLS growth over Bi catalysts yields vdW nanowires down to ≈15 nm diameter while maintaining tens of µm length. Combined electron microscopy and diffraction demonstrate that ultrathin GeS nanowires crystallize in the orthorhombic bulk structure but can realize nonequilibrium stacking that may lead to 1D ferroelectricity. Ultrathin nanowires carry screw dislocations, remain chiral, and achieve very high twist rates. Whenever the dislocation extends to the nanowire tip, it continues into the Bi catalyst. Eshelby twist analysis demonstrates that the ultrathin nanowires follow continuum predictions. Cathodoluminescence on individual nanowires, finally, shows pronounced emission blue shifts consistent with quantum confinement.

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. 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.

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. The devised growth mode relies on three separate symmetry-breaking events ushered in by seeds lined with planar defects, a capping agent that severely frustrates early stage growth, and the Coulombic repulsion that occurs when identically charged growth fronts collide. Together they combine to advance a growth pathway in which planar growth emanates from one side of the seed, advances to encircle the seed from both clockwise and counterclockwise directions, and then, upon collision of the two growth fronts, sees one front rise above the other to realize a self-perpetuating three-dimensional spiral structure.

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. Here, we discuss a fundamentally different approach, in which changes in pH modify the nonspecific interparticle interaction between Au nanorods conjugated with single-stranded (ss) DNA. While electrostatic repulsion stabilizes dispersed suspensions at neutral pH, protonation in acidic solution modifies the DNA corona, turning the interaction between the rods attractive and triggering their self-assembly. Analysis of in-situ electron microscopy of ssDNA-Au nanorods in solution is consistent with a van der Waals attraction of charge-neutral monomers at acidic pH. The results demonstrate ssDNA-conjugated anisotropic nanostructures as versatile building blocks with stimuli-programmable interactions for on-demand self-assembly.

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. The traditional approach for investigating the underlying, inherently dynamic processes involves sampling at different stages combined with ex situ characterization, for example, using electron microscopy. In situ studies 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.In this Account, we discuss applications of real-time electron microscopy to the analysis of nanocrystal synthesis, transformations, and self-assembly in solution. While in the simplest case the effects of the electron beam are negligible, the interaction with high-energy electrons often provides excitation or stimulus for solution-phase processes or opens up competing chemical pathways. Real-time observations of self-assembly demonstrate particularly clearly the power of in situ microscopy in identifying key nucleation and growth mechanisms and providing information about preferred structural motifs that can be analyzed to quantify the balance of forces and the role of entropy in stabilizing ordered assemblies. Modifications of the solution by the electron beam can provide stimuli for on-demand self-assembly, for example, via an acid spike due to water radiolysis that locally lowers the pH in the imaged area. While in this and other cases (e.g., colloidal synthesis), beam-induced radicals become part of the experimental design, in imaging redox reactions such as galvanic transformations of nanocrystal templates, radicals need to be managed and if possible eliminated by suitable scavengers. Finally, excitation by the imaging electron beam can transfer energy to individual nanocrystals in solution, thus driving nonthermal (e.g., plasmon-mediated) synthesis or other chemistry while following the reaction progress with high resolution. Overall, with validation by ex situ control experiments, the unique ability of observing processes in solution at the nanometer scale should make liquid-cell electron microscopy an integral part of the toolkit for designing novel inorganic nanocrystal architectures.

Cathodoluminescence of Ultrathin Twisted Ge1- x Snx 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. Scanning near-field optical microscopy can overcome these issues and can characterize waveguide modes down to a resolution of tens of nanometers, albeit for planar samples or nanostructures with moderate height variations. Electron microscopy provides atomic-scale localization also for more complex geometries, and recent advances have extended the accessible excitations from interband transitions to phonons. Here, bottom-up synthesized layered semiconductor (Ge_{1- x} Sn_x S) nanoribbons with an axial twist and deep subwavelength thickness are demonstrated as a platform for realizing waveguide modes, and cathodoluminescence spectroscopy is introduced as a tool to characterize them. Combined experiments and simulations show the excitation of guided modes by the electron beam and their efficient detection via photons emitted in the ribbon plane, which enables the measurement of key properties such as the evanescent field into the vacuum cladding with nanometer resolution. The results identify van der Waals waveguides operating in the infrared and highlight an electron-microscopy-based approach for probing complex-shaped nanophotonic structures.

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. 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 via dislocations 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.

Vapor-Liquid-Solid Growth and Optoelectronics of Gallium Sulfide van der Waals Nanowires

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 E_g = 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.

Universal mechanical exfoliation of large-area 2D crystals

Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.

Plasmonic Effects on the Growth of Ag Nanocrystals in Solution

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 Ag⁰ incorporation into the anisotropic nanocrystals. Here, we investigate pathways toward Ag nanoprisms from initially homogeneous AgNO₃ solutions 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.

Single-Crystalline γ-Ga2S3 Nanotubes via Epitaxial Conversion of GaAs Nanowires

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-Ga₂S₃ nanotubes by reaction with sulfur vapor. The conversion proceeds via the formation of epitaxial GaAs-Ga₂S₃ core-shell structures, vacancy injection and aggregation into Kirkendall voids, elastic relaxation of the detached Ga₂S₃ shell, 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.

Few-Layer to Multilayer Germanium(II) Sulfide: Synthesis, Structure, Stability, and Optoelectronics

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 via exfoliation 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 situ electron 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 GeS_x shell 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-GeS_x core-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.

Wrap-Around Core-Shell Heterostructures of Layered Crystals

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one-pot synthesis of layered tin chalcogenides spontaneously creates core-shell structures in which large orthorhombic SnS crystals are enclosed in a wrap-around shell of trigonal SnS₂ , forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core-shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core-shell interfaces and extended long-wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core-shell structures attractive for energy-conversion applications.

In situ electron microscopy of the self-assembly of single-stranded DNA-functionalized Au nanoparticles in aqueous solution

Solution-phase self-assembly of DNA-functionalized nanoparticles into mesoscale structures is a promising strategy for creating functional materials from nanocrystal building blocks. The predominant approach has been the use of Watson-Crick base pairing between complementary bases in designated 'sticky ends' to trigger programmable self-assembly into ordered superlattices. Here we demonstrate the ordered self-assembly of Au nanoparticles conjugated with single-stranded (ss) DNA in acidic solutions. Au nanoparticles functionalized with thiolated ssDNA are protected against coalescence and the DNA conformation undergoes significant modifications at low pH, which can be associated with the protonation of adenine bases and the formation of a parallel poly-adenine duplex, which govern the interaction between ssDNA-Au nanoparticle conjugates. In situ liquid cell electron microscopy enables real-time imaging of the self-assembly process and the identification of key characteristics, such as the preferred structural motifs and interparticle separations in the native solution environment. Our results highlight alternatives to conventional base-pairing interactions for building DNA-directed nanoparticle superlattices.

Thick Layered Semiconductor Devices with Water Top-Gates: High On-Off Ratio Field-Effect Transistors and Aqueous Sensors

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.

Germanium Sulfide Nano-Optics Probed by STEM-Cathodoluminescence Spectroscopy

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

Radiation damage during in situ electron microscopy of DNA-mediated nanoparticle assemblies in solution

Oligonucleotide-nanoparticle conjugates, also called programmable atom equivalents, carry promise as building blocks for self-assembled colloidal crystals, reconfigurable or stimuli responsive functional materials, as well as bio-inspired hierarchical architectures in wet environments. In situ studies of the DNA-mediated self-assembly of nanoparticles have so far been limited to reciprocal space techniques. Liquid-cell electron microscopy could enable imaging such systems with high resolution in their native environment but to realize this potential, radiation damage to the oligonucleotide linkages needs to be understood and conditions for damage-free electron microscopy identified. Here, we analyze in situ observations of DNA-linked two-dimensional nanoparticle arrays, along with control experiments for different oligonucleotide configurations, to identify the mechanisms of radiation damage for ordered superlattices of DNA-nanoparticle conjugates. In a biological context, the results point to new avenues for studying direct and indirect radiation effects for small ensembles of DNA in solution by tracking conjugated nanoparticles. By establishing low-dose conditions suitable for extended in situ imaging of programmable atom equivalents, our work paves the way for real-space observations of DNA-mediated self-assembly processes.

Formation of Ge–GeS core–shell nanostructures via solid-state sulfurization of Ge nanowires

Solid-state sulfurization of Ge nanowires results in formation of layered GeS shells that provide surface passivation and oxidation protection.

Defect-Laden MoSe2 Quantum Dots Made by Turbulent Shear Mixing as Enhanced Electrocatalysts

A high density of edge sites and other defects can significantly improve the catalytic activity of layered 2D materials. Herein, this study demonstrates a novel top-down strategy to maximize catalytic edge sites of MoSe₂ by breaking up bulk MoSe₂ into quantum dots (QDs) via "turbulent shear mixing" (TSM). The ultrasmall size of the MoSe₂ QDs provides a high fraction of atoms in reactive edge sites, thus significantly improving the catalytic activities. The violent TSM further introduces abundant defects as additional active sites for electrocatalytic reactions. These edge-proliferated and defect-laden MoSe₂ QDs are found to be efficient electrocatalysts for the hydrogen evolution reaction, and useful as counter electrodes in dye-sensitized solar cells. The work provides a new paradigm for creating edge-proliferated and defect-rich QDs from bulk layered materials.

Termination of Ge surfaces with ultrathin GeS and GeS2 layers via solid-state sulfurization

Thermally activated solid-state reactions of germanium with sulfur give rise to passivating germanium sulfide surface layers.

Correction: Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix

Correction for 'Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix' by P. Sutter et al., Nanoscale Horiz., 2016, 1, 212-219.

Electron-Beam Induced Transformations of Layered Tin Dichalcogenides

By combining high-resolution transmission electron microscopy and associated analytical methods with first-principles calculations, we study the behavior of layered tin dichalcogenides under electron beam irradiation. We demonstrate that the controllable removal of chalcogen atoms due to electron irradiation, at both room and elevated temperatures, gives rise to transformations in the atomic structure of Sn-S and Sn-Se systems so that new phases with different properties can be induced. In particular, rhombohedral layered SnS2 and SnSe2 can be transformed via electron beam induced loss of chalcogen atoms into highly anisotropic orthorhombic layered SnS and SnSe. A striking dependence of the layer orientation of the resulting SnS-parallel to the layers of ultrathin SnS2 starting material, but slanted for transformations of thicker few-layer SnS2-is rationalized by a transformation pathway in which vacancies group into ordered S-vacancy lines, which convert via a Sn2S3 intermediate to SnS. Absence of a stable Sn2Se3 intermediate precludes this pathway for the selenides, hence SnSe2 always transforms into basal plane oriented SnSe. Our results provide microscopic insights into the transformation mechanism and show how irradiation can be used to tune the properties of layered tin chalcogenides for applications in electronics, catalysis, or energy storage.

Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix

While nanoparticles are being pursued actively for a number of applications, dispersed atomic species have been explored far less in functional materials architectures, primarily because composites comprising dispersed atoms are challenging to synthesize and difficult to stabilize against sintering or coarsening. Here we show that room temperature oxidation of Au-Sn alloys produces nanostructures whose surface is terminated by a reducible amorphous oxide that contains atomically dispersed Au. Analysis of the oxidation process shows that the dispersal of Au in the oxide can be explained by predominant oxygen anion diffusion and kinetically limited metal mass transport, which restrict phase separation due to a preferential oxidation of Sn. Nanostructures prepared by oxidation of nanoscale Au-Sn alloys with intermediate Au content (30-50%) show high activity in a CO-oxidation probe reaction due to a cooperative mechanism involving Au atoms as sites for CO adsorption and reaction to CO₂ embedded in a reducible oxide that serves as a renewable oxygen reservoir. Our results demonstrate a reliable approach toward nanocomposites involving oxide-embedded, atomically dispersed noble metal species.

Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to Single-Layer and Few-Layer Tin Disulfide

The combination of zero-dimensional (0D) colloidal CdSe/ZnS quantum dots with tin disulfide (SnS2), a two-dimensional (2D)-layered metal dichalcogenide, results in 0D-2D hybrids with enhanced light absorption properties. These 0D-2D hybrids, when exposed to light, exhibit intrahybrid nonradiative energy transfer from photoexcited CdSe/ZnS quantum dots to SnS2. Using single nanocrystal spectroscopy, we find that the rate for energy transfer in 0D-2D hybrids increases with added number of SnS2 layers, a positive manifestation toward the potential functionality of such 2D-based hybrids in applications such as photovoltaics and photon sensing.

Reliable Exfoliation of Large-Area High-Quality Flakes of Graphene and Other Two-Dimensional Materials

Mechanical exfoliation has been a key enabler of the exploration of the properties of two-dimensional materials, such as graphene, by providing routine access to high-quality material. The original exfoliation method, which remained largely unchanged during the past decade, provides relatively small flakes with moderate yield. Here, we report a modified approach for exfoliating thin monolayer and few-layer flakes from layered crystals. Our method introduces two process steps that enhance and homogenize the adhesion force between the outermost sheet in contact with a substrate: Prior to exfoliation, ambient adsorbates are effectively removed from the substrate by oxygen plasma cleaning, and an additional heat treatment maximizes the uniform contact area at the interface between the source crystal and the substrate. For graphene exfoliation, these simple process steps increased the yield and the area of the transferred flakes by more than 50 times compared to the established exfoliation methods. Raman and AFM characterization shows that the graphene flakes are of similar high quality as those obtained in previous reports. Graphene field-effect devices were fabricated and measured with back-gating and solution top-gating, yielding mobilities of ∼4000 and 12,000 cm(2)/(V s), respectively, and thus demonstrating excellent electrical properties. Experiments with other layered crystals, e.g., a bismuth strontium calcium copper oxide (BSCCO) superconductor, show enhancements in exfoliation yield and flake area similar to those for graphene, suggesting that our modified exfoliation method provides an effective way for producing large area, high-quality flakes of a wide range of 2D materials.

Role of the nanocrystallinity on the chemical ordering of Co(x)Pt(100-x) nanocrystals synthesized by wet chemistry

Co(x)Pt(100-x) nanoalloys have been synthesized by two different chemical processes either at high or at low temperature. Their physical properties and the order/disorder phase transition induced by annealing have been investigated depending on the route of synthesis. It is demonstrated that the chemical synthesis at high temperature allows stabilization of the fcc structure of the native nanoalloys while the soft chemical approach yields mainly poly or non crystalline structure. As a result the approach of the order/disorder phase transition is strongly modified as observed by high-resolution transmission electron microscopy (HR-TEM) studies performed during in situ annealing of the different nanoalloys. The control of the nanocrystallinity leads to significant decrease in the chemical ordering temperature as the ordered structure is observed at temperatures as low as 420 °C. This in turn preserves the individual nanocrystals and prevents their coalescence usually observed during the annealing necessary for the transition to an ordered phase.

Shear stress-induced atherosclerotic plaque composition in ApoE(-/-) mice is modulated by connexin37

OBJECTIVE

Shear stress patterns influence atherogenesis and plaque stability; low laminar shear stress (LLSS) promotes unstable plaques whereas oscillatory shear stress (OSS) induces more stable plaques. Endothelial connexin37 (Cx37) expression is also regulated by shear stress, which may contribute to localization of atherosclerotic disease. Moreover, Cx37 reduces initiation of atherosclerosis by inhibiting monocyte adhesion. The present work investigates the effect of Cx37 on the phenotype of plaques induced by LLSS or OSS.

METHODS

Shear stress-modifying casts were placed around the common carotid artery of ApoE(-/-) or ApoE(-/-)Cx37(-/-) mice, and animals were placed on a high-cholesterol diet for 6 or 9 weeks. Atherosclerotic plaque size and composition were assessed by immunohistochemistry.

RESULTS

Plaque size in response to OSS was increased in ApoE(-/-)Cx37(-/-) mice compared to ApoE(-/-) animals. Most plaques contained high lipid and macrophage content and a low amount of collagen. In ApoE(-/-) mice, macrophages were more prominent in LLSS than OSS plaques. This difference was reversed in ApoE(-/-)Cx37(-/-) animals, with a predominance of macrophages in OSS plaques. The increase in macrophage content in ApoE(-/-)Cx37(-/-) OSS plaques was mainly due to increased accumulation of M1 and Mox macrophage subtypes. Cx37 expression in macrophages did not affect their proliferation or their polarization in vitro.

CONCLUSION

Cx37 deletion increased the size of atherosclerotic lesions in OSS regions and abrogated the development of a stable plaque phenotype under OSS in ApoE(-/-) mice. Hence, local hemodynamic factors may modify the risk for adverse atherosclerotic disease outcomes associated to a polymorphism in the human Cx37 gene.

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

Tin disulfide-an emerging layered metal dichalcogenide semiconductor: materials properties and device characteristics

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on-off current ratios >10(6), as well as carrier mobilities up to 230 cm(2)/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.

Nanoscale integration of two-dimensional materials by lateral heteroepitaxy

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

Evolution and characteristics of GaN nanowires produced via maskless reactive ion etching

The formation of nanowires (NWs) by reactive ion etching (RIE) of maskless GaN layers was investigated. The morphological, structural and optical characteristics of the NWs were studied and compared to those of the layer they evolve from. It is shown that the NWs are the result of a defect selective etching process. The evolution of density and length with etching time is discussed. Densely packed NWs with a length of more than 1 μm and a diameter of ∼60 nm were obtained by RIE of a ∼2.5 μm thick GaN layer. The NWs are predominantly free of threading dislocations and show an improvement of optical properties compared to their layer counterpart. The production of NWs via a top down process on non-masked group III-nitride layers is assessed to be very promising for photovoltaic applications.

Über die Ultrarotabsorption freier Ladungsträger bei nichtparabolischer Bandstruktur am Beispiel InSb

Bei nichtparabolischem Bandverlauf ist wegen der Energieabhängigkeit der effektiven Masse der Absorptionskoeffizient der „Leitungsabsorption“ freier Träger nicht mehr proportional zur Trägerdichte. Die Meßergebnisse an 16 n-leitenden InSb-Proben wachsender Trägerdichte [N=2 · 1016 (Eigenleitungsdichte bei 20°C) bis 1,24 · 1018 cm-3] bestätigen die Erwartung. Die Theorie des Absorptionskoeffizienten bei nichtparabolischem Band wird diskutiert, um umgekehrt aus der gemessenen Abhängigkeit des Absorptionskoeffizienten von der Trägerdichte den Bandverlauf E(k) zu ermitteln. Unter Verwendung der quantentheoretisch adaptierten DRUDESchen Theorie gelingt dies. Der Verlauf des Leitungsbandes von InSb wird auf diese Weise von k=0 bis k=4 · 108m-1 (bzw. vom Bandrand bis 0,2 eV Abstand vom Bandrand) quantitativ angegeben, womit auch die effektive Masse eines einzelnen Elektrons als Funktion von k bzw. E im genannten Bereich bekannt ist. Am Bandrand beträgt m*=0,016 m0 und wird mit wachsendem Abstand größer.