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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. [ABSTRACT FROM AUTHOR]

Moiré patterns at van der Waals interfaces between twisted 2D crystals give rise to distinct optoelectronic excitations, as well as, narrowly dispersive bands responsible for correlated electron phenomena. Contrasting with the conventional, mechanically stacked planar twist moirés, recent work shows twisted van der Waals interfaces spontaneously formed in nanowires of layered crystals, where Eshelby twist due to axial screw dislocations stabilizes a chiral structure with small interlayer rotation. Here, the realization of tunable twist in germanium(II) sulfide (GeS) van der Waals nanowires is reported. Tapered nanowires host continuously variable interlayer twist. Homojunctions between dislocated (chiral) and defect‐free (achiral) segments are obtained by triggering the emission of axial dislocations during growth. Measurements across such junctions, implemented here using local absorption and luminescence spectroscopy, provide a convenient tool for detecting twist effects. The results identify a versatile system for 3D twistronics, probing moiré physics, and for realizing moiré architectures without equivalent in planar systems. [ABSTRACT FROM AUTHOR]

Integration of graphene with other materials by direct growth, i.e., not using mechanical transfer procedures, is investigated on the example of metal/graphene/dielectric heterostructures. Such structures may become useful in spintronics applications using graphene as a spin-filter. Here, we systematically discuss the optimization of synthesis procedures for every layer of the heterostructure and characterize the material by imaging and diffraction methods. 300 nm thick contiguous (111) Ni-films are grown by physical vapor deposition on YSZ(111) or Al2O3(0001) substrates. Subsequently, chemical vapor deposition growth of graphene in ultra-high vacuum (UHV) is compared to tube-furnace synthesis. Only under UHV conditions, monolayer graphene in registry with Ni(111) has been obtained. In the tube furnace, mono- and bilayer graphene is obtained at growth temperatures of ∼800 °C, while at 900 °C, non-uniform thick graphene multilayers are formed. Y2O3 films grown by reactive molecular beam epitaxy in UHV covers the graphene/Ni(111) surface uniformly. Annealing to 500 °C results in crystallization of the yttria with a (111) surface orientation. [ABSTRACT FROM AUTHOR]

Gallium oxide (Ga2O3) and its most stable modification, monoclinic β-Ga2O3, is emerging as a primary material for power electronic devices, gas sensors and optical devices due to a high breakdown voltage, large bandgap, and optical transparency combined with electrical conductivity. Growth of β-Ga2O3 is challenging and most methods require very high temperatures. Nanowires of β-Ga2O3 have been investigated extensively as they might be advantageous for devices such as nanowire field effect transistors, and gas sensors benefiting from a large surface to volume ratio, among others. Here, we report a synthesis approach using a sulfide precursor (Ga2S3), which requires relatively low substrate temperatures and short growth times to produce high-quality single crystalline β-Ga2O3 nanowires in high yields. Even though Au- or Ag-rich nanoparticles are invariably observed at the nanowire tips, they merely serve as nucleation seeds while the nanowire growth proceeds via supply and local oxidation of gallium at the substrate interface. Absorption and cathodoluminescence spectroscopy on individual nanowires confirms a wide bandgap of 4.63 eV and strong luminescence with a maximum ∼2.7 eV. Determining the growth process, morphology, composition and optoelectronic properties on the single nanowire level is key to further application of the β-Ga2O3 nanowires in electronic devices. [ABSTRACT FROM AUTHOR]

Vertical van der Waals (vdW) heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials. To date, twist-heterostructures have been realized by micromechanical stacking. Direct synthesis is hindered by the tendency toward equilibrium stacking without interlayer twist. Here, we demonstrate that growing a 2D crystal with fixed azimuthal alignment to the substrate followed by transformation of this intermediate enables a potentially scalable synthesis of twisted heterostructures. Microscopy during growth of ultrathin orthorhombic SnS on trigonal SnS2 shows that vdW epitaxy yields azimuthal order even for non-isotypic 2D crystals. Excess sulfur drives a spontaneous transformation of the few-layer SnS to SnS2, whose orientation – rotated 30° against the underlying SnS2 crystal – is defined by the SnS intermediate rather than the substrate. Preferential nucleation of additional SnS on such twisted domains repeats the process, promising the realization of complex twisted stacks by bottom-up synthesis. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures with small misalignment between adjacent layers ('interlayer twist') are of interest because of electronic structure and correlation phenomena (such as superconductivity) that are determined by both the atomic lattice and long-range superlattice potentials arising in interlayer moiré patterns1–7. Previously, such twisted heterostructures have involved a single planar interface between layers isolated by exfoliation and micromechanically stacked in the desired relative orientation1,8–12. Here we demonstrate a class of materials—van der Waals nanowires of layered crystals—in which a tunable interlayer twist evolves naturally during synthesis. In vapour–liquid–solid growth, nanowires of germanium(ii) sulfide, an anisotropic layered semiconductor, crystallize with layering along the wire axis13 and have a strong propensity for forming axial screw dislocations. Nanometre-resolved electron diffraction shows that Eshelby twist, induced by a torque on the ends of a cylindrical solid due to the stress field of an axial dislocation14,15, causes a chiral structure in the van der Waals nanowires. The in-plane germanium sulfide crystal axes progressively rotate along the wire, and germanium sulfide layers in adjacent turns of the helix naturally form a moiré pattern because of their interlayer twist. The axial rotation and the twist are tunable by varying the nanowire thickness. Combined electron diffraction and cathodoluminescence spectroscopy show the correlation between the interlayer twist and locally excited light emission that is due to progressive changes in the lattice orientation and in the interlayer moiré registry along the nanowires. The findings demonstrate a step towards scalable fabrication of van der Waals structures with defined twist angles, in which interlayer moiré patterns are realized along a helical path on a nanowire instead of a planar interface. A tunable interlayer twist that evolves naturally during synthesis of van der Waals nanowires made from layered crystals of germanium sulfide could produce new electronic structure and correlation phenomena. [ABSTRACT FROM AUTHOR]

Germanium nanowires (NWs) have attractive properties for a variety of applications, including micro- and optoelectronics, memory devices, solar energy conversion, and energy storage, among others. For applications that involve exposure to air, the poor chemical stability and electronic surface passivation of native oxides have remained a long-standing concern. Termination by sulfur-rich surface layers has emerged as a promising strategy for passivation of planar Ge surfaces. Here we discuss experiments on solid-state sulfurization of Ge nanowires in sulfur vapor at near-ambient pressures and at different temperatures. Combined transmission electron microscopy imaging and chemical mapping establishes that Ge NWs remain intact during vapor-phase reaction with S at elevated temperatures, and show the formation of sulfur-rich shells with T-dependent morphology and thickness on the Ge NW surface. Photoluminescence of ensembles of such core–shell nanowires is dominated by strong emission at ∼1.85 eV, consistent with luminescence of GeS. Cathodoluminescence spectroscopy on individual NWs establishes that this luminescence originates in thin GeS shells formed by sulfurization of the NWs. Our work establishes direct sulfurization as a viable approach for forming stable, wide-bandgap surface terminations on Ge NWs. [ABSTRACT FROM AUTHOR]

Reactions of Ge with S vapor, of interest as a potential approach for forming thin passivation layers on Ge surfaces, have been studied by photoelectron spectroscopy and Raman spectroscopy. Exposure of Ge(100) and Ge(111) to S drives the formation of Ge sulfide near-surface layers. At low temperatures, the reaction products comprise a thin GeS interlayer terminated by near-surface GeS2. Above 400 °C, exposure to sulfur gives rise to single-phase GeS2 layers whose thickness increases with temperature. Arrhenius analysis of the GeS2 thickness yields an activation energy (0.63 ± 0.08) eV, close to the barrier that controls Ge oxidation by O radicals. XPS measurements after extended ambient exposure show a stable, ultrathin near-surface GeS2 without significant oxidation, indicating that Ge–sulfides may provide an effective surface passivation for Ge surfaces. [ABSTRACT FROM AUTHOR]

Electron-beam (e-beam) irradiation damage is often regarded as a severe limitation to atomic-scale study of two-dimensional (2D) materials using electron microscopy techniques. However, energy transferred from the e-beam can also provide a way to modify 2D materials via defect engineering when the interaction of the beam with the sample is precisely controlled. In this article, we discuss the atomic geometry, formation mechanism, and properties of several types of structural defects, ranging from zero-dimensional point defects to extended domains, induced by an e-beam in a few representative 2D materials, including graphene, hexagonal boron nitride, transition-metal dichalcogenides, and phosphorene. We show that atomic as well as line defects and even novel nanostructures can be created and manipulated in 2D materials by an e-beam in a controllable manner. Phase transitions can also be induced. The e-beam in a (scanning) transmission electron microscope not only resolves the intrinsic atomic structure of materials with defects, but also provides new opportunities to modify the structure with subnanometer precision. [ABSTRACT FROM PUBLISHER]

We report an improved photosensitivity in few-layer tin disulfide (SnS2) field-effect transistors (FETs) following doping with CdSe/ZnS core/shell quantum dots (QDs). The hybrid QD-SnS2 FET devices achieve more than 500% increase in the photocurrent response compared with the starting SnS2-only FET device and a spectral responsivity reaching over 650 A/W at 400 nm wavelength. The negligible electrical conductance in a control QD-only FET device suggests that the energy transfer between QDs and SnS2 is the main mechanism responsible for the sensitization effect, which is consistent with the strong spectral overlap between QD photoluminescence and SnS2 optical absorption as well as the large nominal donor-acceptor interspacing between QD core and SnS2. We also find enhanced charge carrier mobility in hybrid QD-SnS2 FETs which we attribute to a reduced contact Schottky barrier width due to an elevated background charge carrier density. [ABSTRACT FROM AUTHOR]

CoxPt100−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. [ABSTRACT FROM AUTHOR]

Using transmission electron microscopy, the size-dependent room temperature oxidation of tin nanoparticles is studied. The oxide that forms during room temperature oxidation of Sn particles is amorphous SnO, and it retains this stoichiometry and structure over extended time periods. From the investigation of arrays of Sn nanoparticles with broad size distribution, under identical conditions, the Sn oxide thickness is evaluated as a function of size and oxidation time. The oxide thickness depends strongly on the size of the Sn nanoparticles, which is in excellent agreement with predictions for a Mott-Cabrera model corrected for a non-uniform electric field. The results demonstrate the accelerated oxidation kinetics of nanoscale particles with high curvature, due to the amplified electric field at the interface to a continuously shrinking metal core. [ABSTRACT FROM AUTHOR]

The growth and properties of two-dimensional (2D) materials-graphene as well as related monolayer systems, such as hexagonal boron nitride-on metals are topics of high scientific and technological interest. Real-time low-energy electron microscopy (LEEM) can provide unique insight into the fundamental growth mechanisms of 2D materials on metal substrates. In combination with in situ spectroscopic measurements, LEEM can greatly facilitate the search for synthesis and processing protocols that produce 2D materials with desired properties for applications. Here, progress is reviewed in understanding the scalable growth of high-quality graphene on metals, novel processing strategies based on selective chemical reactions at the graphene/metal interface, and important materials properties (structure, electronic properties, work function, etc.) by surface microscopy and complementary methods, using graphene/ruthenium as a model system. The body of work shows that in situ microscopy can be used as a powerful tool for achieving and probing a wide range of functionalities in 2D materials. [ABSTRACT FROM AUTHOR]

The oxidation of bimetallic alloy nanoparticles comprising a noble and a nonnoble metal is expected to cause the formation of a single-component surface oxide of the nonnoble metal, surrounding a core enriched with the noble metal. Studying the room temperature oxidation of Au–In nanoparticles, we show that this simple picture does not apply to an important class of bimetallic alloys, in which the oxidation proceeds via predominant oxygen diffusion. Instead of a crystalline In2O3 shell, such oxidation leads to an amorphous shell of mixed Au–In oxide that remains stable to high temperatures and whose surface layer is enriched with Au. The Au-rich mixed oxide is capable of adsorbing both CO and O2 and converting them to CO2, which desorbs near room temperature. The oxidation of Au–In alloys to a mixed Au–In oxide shows significant promise as a viable approach toward Au-based oxidation catalysts, which do not require any complex synthesis processes and resist deactivation up to at least 300 °C. [ABSTRACT FROM AUTHOR]

Segmented noble-metal nanowires (NWs) represent an exciting, multifunctional, one-dimensional, structural architecture with a variety of potential applications. However, the widespread use of electrodeposition in the preparation of these systems has limited their potential to be produced on a large scale, since this protocol is costly and requires complex processes and caustic reaction media. Given the inherent limitations of electrodeposition, we report, for the first time, an ambient, surfactantless, template-based approach that is not only sustainable but also efficient for the reliable production of Pd/Pt and Pd/Au segmented NWs, possessing two spatially separated, chemically distinctive, but elementally pure, axial subunits. Our simple two-step synthetic approach allows for direct and predictable control over the relative segment lengths in these nanomaterials. Moreover, thorough structural characterization of these as-prepared samples confirms that our segmented NWs maintain high-quality, crystalline, elementally pure subunits with a well-defined interface between the constituent metals. In the context of preparing segmented NWs as multifunctional nanostructures, we demonstrate that these as-prepared NWs achieve high levels of performance when employed as both electrocatalysts and nanomotors. [ABSTRACT FROM AUTHOR]

A eutectic AuGe target immersed in distilled water was ablated by pulsed ultraviolet laser light. The structure of the ablated material was investigated by high-resolution transmission electron microscopy (HRTEM). The images show formation of nanowire structures of AuGe up to 100 nm in length, with widths of 5-10 nm. These nanostructures have Ge content significantly lower than the target material. Electron diffraction demonstrates that they crystallize in the α-AuGe structure. For comparison, laser ablation of pure Au and pure Ge targets was also performed under the same conditions. HRTEM shows that Ge forms spherical nanoparticles with a characteristic size of ~30 nm. Au forms spherical nanoparticles with diameters of ~10 nm. Similar to AuGe, it also forms chainlike structures with substantially lower aspect ratio. [ABSTRACT FROM AUTHOR]

Variable temperature transmission electron microscopy of individual 5 nm Au nanoparticles shows a striking increase in the particle size on raising the temperature from room temperature to 500 °C in the presence of carbon from amorphous carbon support. Using the assembly of ordered graphene shells on the surface of individual nanoparticles at elevated temperatures-and the high pressures induced by such shells-as an experimental tool to study the origins of this swelling, we find that the volume increase is associated with the uptake of carbon to concentrations exceeding the bulk solubility by more than four orders of magnitude. The formation of stable metal-carbon nanostructures that have no bulk equivalent may have important implications on the functional properties of metal nanoparticles. [ABSTRACT FROM AUTHOR]

Single-crystalline Sb-doped Ge nanowires (NWs) with excellent structural properties and uniform composition have been synthesized with high yield by vapor–liquid–solid (VLS) growth by low-temperature thermal evaporation from a mixture of Ge and Sb powders. During deposition, both the Ge and the Sb dopant became incorporated in the VLS seed nanoparticle. In situ annealing experiments during transmission electron microscopy establish that a liquid ternary Au-Sb-Ge alloy constitutes the active phase of the VLS seed drop at high temperatures, which governs the growth of the one-dimensional Ge NW and its doping by Sb. [ABSTRACT FROM AUTHOR]

Graphene has been used to explore the fascinating electronic properties of ideal two-dimensional carbon, and shows great promise for quantum device architectures. The primary method for isolating graphene, micromechanical cleavage of graphite, is difficult to scale up for applications. Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly affect the electronic properties of epitaxial graphene layers. Here, we show that epitaxy on Ru(0001) produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the first graphene layer indeed interacts strongly with the metal substrate, the second layer is almost completely detached, shows weak electronic coupling to the metal, and hence retains the inherent electronic structure of graphene. Our findings demonstrate a route towards rational graphene synthesis on transition-metal templates for applications in electronics, sensing or catalysis. [ABSTRACT FROM AUTHOR]

The controlled delivery of fluids is a key process in nature and in many areas of science and technology, where pipettes or related devices are used for dispensing well-defined fluid volumes. Existing pipettes are capable of delivering fluids with attolitre (10−18 l) accuracy at best. Studies on phase transformations of nanoscale objects would benefit from the controlled dispensing and manipulation of much smaller droplets. In contrast to nanoparticle melting whose fundamental pathway was studied extensively, experiments on crystallization, testing classical nucleation theory, are hindered by the influence of support interfaces. Experiments on free-standing fluid drops are extremely challenging. Here, we demonstrate the operation of a pipette, which, observed by transmission electron microscopy, delivers a metal-alloy melt with zeptolitre (10−21 l) resolution. We use this exquisite control to produce nearly free-standing Au72Ge28 drops suspended by an atomic-scale meniscus at the pipette tip, and to image their phase transformations with near-atomic resolution. Our observations of the liquid–solid transition challenge classical nucleation theory by providing experimental evidence for an intrinsic crystallization pathway of nanometre-sized fluid drops that avoids nucleation in the interior, but instead proceeds via liquid-state surface faceting as a precursor to surface-induced crystallization. [ABSTRACT FROM AUTHOR]

We report on a one-step process based on thermal evaporation at moderate temperatures that yields single-crystalline Ge nanowires (NWs) encapsulated in SiO2 shells. The dielectric shell forms around the Ge NW core during the NW growth process itself, an advantage in the assembly of NW devices such as surround-gate NW field-effect transistors (FETs). The formation of the core-shell structures proceeds via an unconventional vapor-liquid-solid process involving root growth of SiGe NWs and selective Si oxidation by background oxygen in the reactor. Electrical measurements of the p-type Ge–SiO2 FET devices show efficient gate control and hole mobilities of 20 cm2/V s. [ABSTRACT FROM AUTHOR]

In situ annealing experiments on individual group III-nitride nanowires (NWs) in a transmission electron microscope demonstrate the encapsulation of GaN wires in crystalline carbon shells in the presence of C at moderate temperatures. The complete encapsulation of GaN wires in carbon shells can be achieved when small indium metal clusters are introduced on the wire surface. No encapsulation is observed on pure GaN wires under the same conditions. The observations suggest a general processing route for the formation of semiconductor core/C-shell NW structures based on surface decoration with small metal clusters. [ABSTRACT FROM AUTHOR]

The ability to dynamically reconfigure superlattices in response to external stimuli is an intriguing prospect for programmable DNA‐guided nanoparticle (NP) assemblies, which promises the realization of "smart" materials with dynamically adjustable interparticle spacing and real‐time tunable properties. Existing in situ probes of reconfiguration processes have been limited mostly to reciprocal space methods, which can follow larger ordered ensembles but do not provide access to real‐space pathways and dynamics. Here, in situ atomic force microscopy is used to investigate DNA‐linked NP assemblies and their response to external stimuli, specifically the contraction and expansion of on‐surface self‐assembled monolayer superlattices upon reversible DNA condensation induced by ethanol. In situ microscopy allows observation and quantification of key processes in solution, e.g., lattice parameter changes, defects, and monomer displacements in small groups of NPs. The analysis of imaging data uncovers important boundary conditions due to DNA bonding of NP superlattices to a substrate. Tension in the NP–substrate DNA bonds, which can elastically extend, break, and re‐form during contraction/expansion cycles, counteracts the changes in lattice parameter and causes hysteresis in the response of the system. The results provide insight into the behavior of supported DNA‐linked NP superlattices and establish a foundation for designing and probing tunable nanocrystal‐based materials in solution. The reconfiguration of DNA‐linked substrate‐supported nanoparticle assemblies in response to ethanol added to the aqueous solution is studied using in situ atomic force microscopy. In situ imaging directly shows the changes to the monolayer superlattice during ethanol‐induced contraction and expansion. A statistical analysis of the interparticle spacings reveals hysteresis in the lattice parameter due to the nanoparticle–substrate DNA bonding. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]