Tuning colour centres at a twisted hexagonal boron nitride interface.

The colour centre platform holds promise for quantum technologies, and hexagonal boron nitride has attracted attention due to the high brightness and stability, optically addressable spin states and wide wavelength coverage discovered in its emitters. However, its application is hindered by the typically random defect distribution and complex mesoscopic environment. Here, employing cathodoluminescence, we demonstrate on-demand activation and control of colour centre emission at the twisted interface of two hexagonal boron nitride flakes. Further, we show that colour centre emission brightness can be enhanced by two orders of magnitude by tuning the twist angle. Additionally, by applying an external voltage, nearly 100% brightness modulation is achieved. Our ab initio GW and GW plus Bethe-Salpeter equation calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron-hole recombination. This mechanism is further exploited to draw nanoscale colour centre patterns using electron beams.

Electrochemically scrambled nanocrystals are catalytically active for CO2-to-multicarbons.

Promotion of C-C bonds is one of the key fundamental questions in the field of CO2 electroreduction. Much progress has occurred in developing bulk-derived Cu-based electrodes for CO2-to-multicarbons (CO2-to-C2+), especially in the widely studied class of high-surface-area "oxide-derived" copper. However, fundamental understanding into the structural characteristics responsible for efficient C-C formation is restricted by the intrinsic activity of these catalysts often being comparable to polycrystalline copper foil. By closely probing a Cu nanoparticle (NP) ensemble catalyst active for CO2-to-C2+, we show that bias-induced rapid fusion or "electrochemical scrambling" of Cu NPs creates disordered structures intrinsically active for low overpotential C2+ formation, exhibiting around sevenfold enhancement in C2+ turnover over crystalline Cu. Integrating ex situ, passivated ex situ, and in situ analyses reveals that the scrambled state exhibits several structural signatures: a distinct transition to single-crystal Cu2O cubes upon air exposure, low crystallinity upon passivation, and high mobility under bias. These findings suggest that disordered copper structures facilitate C-C bond formation from CO2 and that electrochemical nanocrystal scrambling is an avenue toward creating such catalysts.

Hard Ferromagnetism Down to the Thinnest Limit of Iron-Intercalated Tantalum Disulfide.

Two-dimensional (2D) magnetic crystals hold promise for miniaturized and ultralow power electronic devices that exploit spin manipulation. In these materials, large, controllable magnetocrystalline anisotropy (MCA) is a prerequisite for the stabilization and manipulation of long-range magnetic order. In known 2D magnetic crystals, relatively weak MCA typically results in soft ferromagnetism. Here, we demonstrate that ferromagnetic order persists down to the thinnest limit of FexTaS2 (Fe-intercalated bilayer 2H-TaS2) with giant coercivities up to 3 T. We prepare Fe-intercalated TaS2 by chemical intercalation of van der Waals-layered 2H-TaS2 crystals and perform variable-temperature transport, transmission electron microscopy, and confocal Raman spectroscopy measurements to shed new light on the coupled effects of dimensionality, degree of intercalation, and intercalant order/disorder on the hard ferromagnetic behavior of FexTaS2. More generally, we show that chemical intercalation gives access to a rich synthetic parameter space for low-dimensional magnets, in which magnetic properties can be tailored by the choice of the host material and intercalant identity/amount, in addition to the manifold distinctive degrees of freedom available in atomically thin, van der Waals crystals.

Large Spin-Orbit Splitting of Deep In-Gap Defect States of Engineered Sulfur Vacancies in Monolayer WS_{2}.

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping. We report on the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS_{2} by a combination of CO-tip noncontact atomic force microscopy and scanning tunneling microscopy. Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states followed by vibronic sidebands provide a unique fingerprint of this defect. Direct imaging of the defect orbitals, together with ab initio GW calculations, reveal that the large splitting of 252±4 meV between these defect states is induced by spin-orbit coupling.

Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites.

The distinct physical properties of hybrid organic-inorganic materials can lead to unexpected nonequilibrium phenomena that are difficult to characterize due to the broad range of length and time scales involved. For instance, mixed halide hybrid perovskites are promising materials for optoelectronics, yet bulk measurements suggest the halides reversibly phase separate upon photoexcitation. By combining nanoscale imaging and multiscale modeling, we find that the nature of halide demixing in these materials is distinct from macroscopic phase separation. We propose that the localized strain induced by a single photoexcited charge interacting with the soft, ionic lattice is sufficient to promote halide phase separation and nucleate a light-stabilized, low-bandgap, ∼8 nm iodide-rich cluster. The limited extent of this polaron is essential to promote demixing because by contrast bulk strain would simply be relaxed. Photoinduced phase separation is therefore a consequence of the unique electromechanical properties of this hybrid class of materials. Exploiting photoinduced phase separation and other nonequilibrium phenomena in hybrid materials more generally could expand applications in sensing, switching, memory, and energy storage.

Statistical Study on the Schottky Barrier Reduction of Tunneling Contacts to CVD Synthesized MoS2.

Creating high-quality, low-resistance contacts is essential for the development of electronic applications using two-dimensional (2D) layered materials. Many previously reported methods for lowering the contact resistance rely on volatile chemistry that either oxidize or degrade in ambient air. Nearly all reported efforts have been conducted on only a few devices with mechanically exfoliated flakes which is not amenable to large scale manufacturing. In this work, Schottky barrier heights of metal-MoS2 contacts to devices fabricated from CVD synthesized MoS2 films were reduced by inserting a thin tunneling Ta2O5 layer between MoS2 and metal contacts. Schottky barrier height reductions directly correlate with exponential reductions in contact resistance. Over two hundred devices were tested and contact resistances extracted for large scale statistical analysis. As compared to metal-MoS2 Schottky contacts without an insulator layer, the specific contact resistivity has been lowered by up to 3 orders of magnitude and current values increased by 2 orders of magnitude over large area (>4 cm(2)) films.

A Universal Wet-Chemistry Route to Metal Filling of Boron Nitride Nanotubes.

We present a facile wet-chemistry method for efficient metal filling of the hollow inner cores of boron nitride nanotubes (BNNTs). The fillers conform to the cross-section of the tube cavity and extend in length from a few nm to hundreds of nm. The methodology is robust and is demonstrated for noble metals (Au, Pt, Pd, and Ag), transition metals (Co), and post-transition elements (In). Transmission electron microscopy and related electron spectroscopy confirm the composition and morphology of the filler nanoparticles. Up to 60% of BNNTs of a given preparation batch have some degree of metal encapsulation, and individual tubes can have up to 10% of their core volume filled during initial loading. The growth, movement, and fusing of metal nanoparticles within the BNNTs are also examined.

Carrier Scattering at Alloy Nanointerfaces Enhances Power Factor in PEDOT:PSS Hybrid Thermoelectrics.

UNLABELLED: This work demonstrates the first method for controlled growth of heterostructures within hybrid organic/inorganic nanocomposite thermoelectrics. Using a facile, aqueous technique, semimetal-alloy nanointerfaces are patterned within a hybrid thermoelectric system consisting of tellurium (Te) nanowires and the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) ( PEDOT: PSS). Specifically, this method is used to grow nanoscale islands of Cu1.75Te alloy subphases within hybrid PEDOT: PSS-Te nanowires. This technique is shown to provide tunability of thermoelectric and electronic properties, providing up to 22% enhancement of the system's power factor in the low-doping regime, consistent with preferential scattering of low energy carriers. This work provides an exciting platform for rational design of multiphase nanocomposites and highlights the potential for engineering of carrier filtering within hybrid thermoelectrics via introduction of interfaces with controlled structural and energetic properties.

Short- and Long-Range Attractive Forces That Influence the Structure of Montmorillonite Osmotic Hydrates.

Clay swelling is a colloidal phenomenon that has a large influence on flow and solute migration in soils and sediments. While models for clay swelling have been proposed over many years, debate remains as to the interaction forces that combine to produce the observed swelling behavior. Using cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering, we study the influence of salinity, in combination with layer charge, interlayer cation, and particle size, on montmorillonite swelling. We observe a decrease in swelling with increased layer charge, increased cation charge, and decreased cation hydration, each indicative of the critical influence of Coulombic attraction between the negatively charged layers and interlayer cations. Cryo-TEM images of individual montmorillonite particles also reveal that swelling is dependent upon the number of layers in a particle. Calculations of the van der Waals (vdW) interaction based on new measurements of Hamaker coefficients confirm that long-range vdW interactions extend beyond near-neighbor layer interactions and result in a decrease in layer spacing with a larger number of layers. This work clarifies the short- and long-range attractive interactions that govern clay structure and ultimately the stability and permeability of hydrated clays in the environment.

Catalyst-directed crystallographic orientation control of GaN nanowire growth.

In this work, we demonstrate that catalyst composition can be used to direct the crystallographic growth axis of GaN nanowires. By adjusting the ratio of gold to nickel in a bimetallic catalyst, we achieved selective growth of dense, uniform nanowire arrays along two nonpolar directions. A gold-rich catalyst resulted in single-crystalline nanowire growth along the ⟨11̅00⟩ or m axis, whereas a nickel-rich catalyst resulted in nanowire growth along the ⟨112̅0⟩ or a axis. The same growth control was demonstrated on two different epitaxial substrates. Using proper conditions, many of the nanowires were observed to switch direction midgrowth, resulting in monolithic single-crystal structures with segments of two distinct orientations. Cathodoluminescence spectra revealed significant differences in the optical properties of these nanowire segments, which we attribute to the electronic structures of their semipolar {112̅2} or {11̅01} sidewalls.

Modular synthesis of a dual metal-dual semiconductor nano-heterostructure.

Reported is the design and modular synthesis of a dual metal-dual semiconductor heterostructure with control over the dimensions and placement of its individual components. Analogous to molecular synthesis, colloidal synthesis is now evolving into a series of sequential synthetic procedures with separately optimized steps. We detail the challenges and parameters that must be considered when assembling such a multicomponent nanoparticle, and their solutions. This multicomponent nanosystem, Ru-CdSe@CdS-Pt, was designed to achieve charge carrier separation and directional transfer across different interfaces toward two separate redox catalysts. This heterostructure may potentially serve as a nanometric closed circuit photoelectrochemical cell.

Investigating processes of nanocrystal formation and transformation via liquid cell TEM.

Recent ex situ observations of crystallization in both natural and synthetic systems indicate that the classical models of nucleation and growth are inaccurate. However, in situ observations that can provide direct evidence for alternative models have been lacking due to the limited temporal and spatial resolution of experimental techniques that can observe dynamic processes in a bulk solution. Here we report results from liquid cell transmission electron microscopy studies of nucleation and growth of Au, CaCO3, and iron oxide nanoparticles. We show how these in situ data can be used to obtain direct evidence for the mechanisms underlying nanoparticle crystallization as well as dynamic information that provide constraints on important energetic parameters not available through ex situ methods.

Electron microscopy reveals structure and morphology of one molecule thin organic films.

Transmission electron microscopy was used to determine the structure of molecular films of self-assembled monolayers of pentathiophene derivatives supported on various electron transparent substrates. Despite the extreme beam sensitivity of the monolayers, structural crystallographic maps were obtained that revealed the nanoscale structure of the film. The image resolution is determined by the minimum beam diameter that the radiation hardness of the monolayer can support, which in our case is about 90 nm for a beam current of 5 × 10(6) e(-)/s. Electron diffraction patterns were collected while scanning a parallel electron beam over the film. These maps contain uncompromised information of the size, symmetry and orientation of the unit cell, orientation and structure of the domains, degree of crystallinity, and their variation on the micrometer scale, which are crucial to understand the electrical transport properties of the organic films. This information allowed us to track small changes in the unit cell size driven by the chemical modification of the support film.

Organic Matrix Derived from Host-Microbe Interplay Contributes to Pathological Renal Biomineralization.

Matrix stones are a rare form of kidney stones. They feature a high percentage of hydrogel-like organic matter, and their formation is closely associated with urinary tract infections. Herein, comprehensive materials and biochemical approaches were taken to map the organic-inorganic interface and gather insights into the host-microbe interplay in pathological renal biomineralization. Surgically extracted soft and slimy matrix stones were examined using micro-X-ray computed tomography and various microspectroscopy techniques. Higher-mineral-density laminae were positive for calcium-bound Alizarin red. Lower-mineral-density laminae revealed periodic acid-Schiff-positive organic filamentous networks of varied thickness. These organic filamentous networks, which featured a high polysaccharide content, were enriched with zinc, carbon, and sulfur elements. Neutrophil extracellular traps (NETs) along with immune response-related proteins, including calprotectin, myeloperoxidase, CD63, and CD86, also were identified in the filamentous networks. Expressions of NETs and upregulation of polysaccharide-rich mucin secretion are proposed as a part of the host immune defense to "trap" pathogens. These host-microbe derived organic matrices can facilitate heterogeneous nucleation and precipitation of inorganic particulates, resulting in macroscale aggregates known as "matrix stones". These insights into the plausible aggregation of constituents through host-microbe interplay underscore the unique "double-edged sword" effect of the host immune response to pathogens and the resulting renal biominerals.

Elucidating the Mechanism of Large Phosphate Molecule Intercalation Through Graphene-Substrate Heterointerfaces.

Intercalation is the process of inserting chemical species into the heterointerfaces of two-dimensional (2D) layered materials. While much research has focused on the intercalation of metals and small gas molecules into graphene, the intercalation of larger molecules through the basal plane of graphene remains challenging. In this work, we present a new mechanism for intercalating large molecules through monolayer graphene to form confined oxide materials at the graphene-substrate heterointerface. We investigate the intercalation of phosphorus pentoxide (P2O5) molecules directly from the vapor phase and confirm the formation of confined P2O5 at the graphene-substrate heterointerface using various techniques. Density functional theory (DFT) corroborates the experimental results and reveals the intercalation mechanism, whereby P2O5 dissociates into small fragments catalyzed by defects in the graphene that then permeates through lattice defects and reacts at the heterointerface to form P2O5. This process can also be used to form new confined metal phosphates (e.g., 2D InPO4). While the focus of this study is on P2O5 intercalation, the possibility of intercalation from predissociated molecules catalyzed by defects in graphene may exist for other types of molecules as well. This in-depth study advances our understanding of intercalation routes of large molecules via the basal plane of graphene as well as heterointerface chemical reactions leading to the formation of distinctive confined complex oxide compounds.

Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals.

The development of probes for single-molecule imaging has dramatically facilitated the study of individual molecules in cells and other complex environments. Single-molecule probes ideally exhibit good brightness, uninterrupted emission, resistance to photobleaching, and minimal spectral overlap with cellular autofluorescence. However, most single-molecule probes are imperfect in several of these aspects, and none have been shown to possess all of these characteristics. Here we show that individual lanthanide-doped upconverting nanoparticles (UCNPs)--specifically, hexagonal phase NaYF(4) (beta-NaYF(4)) nanocrystals with multiple Yb(3+) and Er(3+) dopants--emit bright anti-Stokes visible upconverted luminescence with exceptional photostability when excited by a 980-nm continuous wave laser. Individual UCNPs exhibit no on/off emission behavior, or "blinking," down to the millisecond timescale, and no loss of intensity following an hour of continuous excitation. Amphiphilic polymer coatings permit the transfer of hydrophobic UCNPs into water, resulting in individual water-soluble nanoparticles with undiminished photophysical characteristics. These UCNPs are endocytosed by cells and show strong upconverted luminescence, with no measurable anti-Stokes background autofluorescence, suggesting that UCNPs are ideally suited for single-molecule imaging experiments.

Tunable infrared absorption and visible transparency of colloidal aluminum-doped zinc oxide nanocrystals.

Plasmonic nanocrystals have been attracting a lot of attention both for fundamental studies and different applications, from sensing to imaging and optoelectronic devices. Transparent conductive oxides represent an interesting class of plasmonic materials in addition to metals and vacancy-doped semiconductor quantum dots. Herein, we report a rational synthetic strategy of high-quality colloidal aluminum-doped zinc oxide nanocrystals. The presence of substitutional aluminum in the zinc oxide lattice accompanied by the generation of free electrons is proved for the first time by tunable surface plasmon absorption in the infrared region both in solution and in thin films.

Electrical transport properties of oligothiophene-based molecular films studied by current sensing atomic force microscopy.

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.

Optimizing cathodoluminescence microscopy of buried interfaces through nanoscale heterostructure design.

Mapping the optical response of buried interfaces with nanoscale spatial resolution is crucial in several systems where an active component is embedded within a buffer layer for structural or functional reasons. Here, we demonstrate that cathodoluminescence microscopy is not only an ideal tool for visualizing buried interfaces, but can be optimized through heterostructure design. We focus on the prototypical system of monolayers of semiconducting transition metal dichalcogenide sandwiched between hexagonal boron nitride layers. We leverage the encapsulating layers to tune the nanoscale spatial resolution achievable in cathodoluminescence mapping while also controlling the brightness of the emission. Thicker encapsulation layers result in a brighter emission while thinner ones enhance the spatial resolution at the expense of the signal intensity. We find that a favorable trade-off between brightness and resolution is achievable up to about ∼100 nm of total encapsulation. Beyond this value, the brightness gain is marginal, while the spatial resolution enters a regime that is achievable by diffraction-limited optical microscopy. By preparing samples of varying encapsulation thickness, we are able to determine a surprisingly isotropic exciton diffusion length of >200 nm within the hexagonal boron nitride which is the dominant factor that determines spatial resolution. We further demonstrate that we can overcome the exciton diffusion-limited spatial resolution by using spectrally distinct signals, which is the case for nanoscale inhomogeneities within monolayer transition metal dichalcogenides.

Optical cavity characterization in nanowires via self-generated broad-band emission.

Broadband white light is of great spectroscopic value and would be a powerful tool for nanoscale spectroscopy, however, generation and direction of white light on this length scale remains challenging. Here, we demonstrate the generation of broadband white light in sub-wavelength diameter Gallium Nitride (GaN) wires by coincident one- and two-photon absorption mediated via defect states. This generation of broadband, "white" light enables single-nanowire interferometric measurements of the nanowires themselves via analysis of the Fabry-Pérot fringes that overlay the entirety of the emission spectrum. The quality factor and finesse of individual nanowire cavities were measured and calculated to be 186 ± 88 and 3.05 ±0.6 respectively, averaged over 20 individual wires. This work presents a new, simple approach for the generation and direction of broad band white light at sub-diffraction limit length scales, ideal for translating classical white light spectroscopies to higher spatial resolutions then previously achieved.