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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

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.

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a lot 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 (IR) region using the 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. Further analysis indicates that the observed polariton anisotropy is originated from the different optical bandgaps and exciton binding energies along the two principal crystal axes of SnS., 21 pages

Borophene, a two-dimensional (2D) layer of elemental boron, has attracted attention as a metallic 2D material with rich polymorphism, as well as potentially interesting electronic, optical, chemica...

Many materials are known to exist in several stable polymorphs, but synthesis only provides access to a subset. This situation is exemplified by the dichalcogenide semiconductor GeSe

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.

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.

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.

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.

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

Tin sulfide (SnS) is a part of the group IV chalcogenides (SnX, GeX; X:S, Se), a family of anisotropic layered materials considered for thin-film photovoltaics, optoelectronics, and valleytronics a...

Oligonucleotide–nanoparticle conjugates are promising building blocks for stimuli responsive, reconfigurable functional materials in wet environments. Here, we demonstrate the self-assembly of DNA-...

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.

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 moire 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 moire 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 moire registry along the nanowires. The findings demonstrate a step towards scalable fabrication of van der Waals structures with defined twist angles, in which interlayer moire patterns are realized along a helical path on a nanowire instead of a planar interface.

Tin monosulfide (SnS) is a layered two-dimensional semiconductor that has attracted attention because of its potential applications, for instance, photovoltaics, optoelectronics, and valleytronics. However, these applications require synthesis of SnS flakes with a controlled thickness and shape. Here, we demonstrate the possibility of using Au nanoparticles to seed the nucleation, determine the position, and control the thickness of the growing SnS plates. Further, we use in situ transmission electron microscopy on individual ultrathin SnS flakes during annealing at high temperatures to establish their thermal stability and decomposition pathways, which is important due to the large surface areas of ultrathin flakes. We find that ultrathin SnS plates decompose rapidly at temperatures above 400 °C. The decomposition invariably starts at the edges of the flakes and is highly anisotropic. Real-time observations show that while the {010} and {110} facets are relatively stable and retract slowly, rapid removal...

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.

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.

Group IV monochalcogenides, anisotropic van der Waals crystals (and black phosphorus analogues), are attracting increasing interest due to a number of exceptional properties including anisotropic optoelectronics and multiferroicity predicted and in part realized in the ultrathin limit. Due to their enhanced chemical reactivity, both exfoliation and synthesis of monolayer and few-layer crystals pose challenges not found in other 2D/layered materials, such as the transition metal dichalcogenides. Here, we show that SnS synthesis on SnS2 van der Waals substrates can address these challenges and consistently produces few-layer flakes, a capability that is explained via analysis of real-time microscopy of the growth process. Raman spectroscopy combined with efficient computations of the Raman-active modes across an extended thickness range enables a comprehensive understanding of the evolution of the vibrational properties of SnS with number of layers. Lateral piezoresponse force microscopy provides unprecedented insight into the stacking-dependent polarization and ferroelectric domain structures in large few-layer SnS flakes. The combined results establish a basis for further fundamental studies and applications of SnS and other group IV monochalcogenides in the few-layer regime.

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

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

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.

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

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.

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical ...

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.

We considered the formation of one-dimensional (1D) nanowires from the two-dimensional (2D) layered material GeS. Growth by a low-temperature vapor–liquid–solid process over a Au catalyst gives rise to single-crystalline nanowires with typical diameters of ∼100 nm and length exceeding 10 μm that consist of a crystalline GeS core with layering along the nanowire axis, surrounded by an ultrathin, sulfur-rich GeSx shell with larger bandgap that provides chemical and electronic passivation of the nanowires. Combined variable-temperature in situ electron microscopy, chemical mapping, and single-nanowire cathodoluminescence spectroscopy are used to uncover key elements of the growth process and to identify the optoelectronic properties of the GeS/GeSx core–shell nanowires. Our results suggest that vapor–liquid–solid processes can readily be extended from conventional three-dimensional crystals to the growth of 1D wires from 2D layered crystals and may provide novel low-dimensional materials for electronic, opto...

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., Multilayer heterostructures with localized and sharp lateral interfaces across hundreds of individual van der Waals layers are realized from the anisotropic layered semiconductors SnS and GeS. Cathodoluminescence spectroscopy shows the transfer of electron–hole pairs across the lateral interfaces, demonstrating excellent connectivity without interfacial recombination. The results are promising for applications benefiting from thick layered crystals combined with in‐plane carrier manipulation.

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.

Corundum structured VTiO3 has been grown as epitaxial films on c-cut sapphire by laser molecular beam epitaxy. The properties of the film were characterized by reflection high energy electron diffraction, X-ray diffraction, transmission electron microscopy (TEM), and photoemission spectroscopy. All the structural probes clearly indicate the corundum structure of the film. X-ray photoemission spectroscopy (XPS) indicates that V is in a 3 + charge state implying that Ti also should adopt a 3 + charge state in order for the corundum structure to form. However, the Ti-2p XPS, while clearly broadened to the lower binding energy side compared to TiO2, also exhibits a pronounced Ti4 + component. Similarly TEM-electron energy loss spectroscopy indicates a mixture of Ti3 + and Ti4 +. This is tentatively assigned to a combination of final state effects in XPS measurements and existence of excess (interstitial) oxygen. The valence band spectra show occupation of 3d metal states that resemble more closely those of Ti2O3 than for V2O3, suggesting that mostly the a1g molecular states are occupied.

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.

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-Ga

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

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 (Ge1-x Snx 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.

Black phosphorus (BP) has attracted significant interest as a monolayer or few-layer material with extraordinary electrical and optoelectronic properties. Chemical reactions with different ambient species, notably oxygen and water, are important as they govern key properties such as stability in air, electronic structure and charge transport, wetting by aqueous solutions, and so on. Here, we report experiments combined with ab initio calculations that address the effects of oxygen and water in contact with BP. Our results show that the reaction with oxygen is primarily responsible for changing properties of BP. Oxidation involving the dissociative chemisorption of O2 causes the decomposition of BP and continuously lowers the conductance of BP field-effect transistors (FETs). In contrast, BP is stable in contact with deaerated (i.e., O2 depleted) water and the carrier mobility in BP FETs gated by H2O increases significantly due to efficient dielectric screening of scattering centers by the high-k dielectri...

International audience; Here, we show that the residual presence of reaction products markedly influences the growth mode of 3D superlattices made of silver nanocrystals. The silver nanocrystals (5 or 6 nm in diameter) are synthesized via the reduction of a silver triphenylphosphine complex in the presence of thiols. The presence of triphenylphosphine, coming from the decomposition of the silver precursor, is shown to change the superlattice growth mode in hexane from heterogeneous to homogeneous. In toluene, it remains homogeneous. This is in agreement with simulations based on a Flory-type model, which shows that the interaction potential between the silver nanocrystals in the presence of triphenylphosphine changes from repulsive to attractive in hexane and becomes even more attractive in toluene. Ex situ investigation of the different superlattices following solvent evaporation by scanning electron microscopy reveals different morphologies depending on the growth mode. A transition from films to individual hexagonal-shaped superlattices is observed when the growth mode changes from heterogeneous to homogeneous. Direct in situ observations of the 3D superlattice growth by liquid cell transmission electron microscopy follow in real time the formation process and movement of the superlattices in both hexane and toluene solvents.

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.