Leveraging Machine Learning for Advanced Nanoscale X-ray Analysis: Unmixing Multicomponent Signals and Enhancing Chemical Quantification.

Energy dispersive X-ray (EDX) spectroscopy in the transmission electron microscope is a key tool for nanomaterials analysis, providing a direct link between spatial and chemical information. However, using it for precisely determining chemical compositions presents challenges of noisy data from low X-ray yields and mixed signals from phases that overlap along the electron beam trajectory. Here, we introduce a novel method, non-negative matrix factorization based pan-sharpening (PSNMF), to address these limitations. Leveraging the Poisson nature of EDX spectral noise and binning operations, PSNMF retrieves high-quality phase spectral and spatial signatures via consecutive factorizations. After validating PSNMF with synthetic data sets of different noise levels, we illustrate its effectiveness on two distinct experimental cases: a nanomineralogical lamella, and supported catalytic nanoparticles. Not only does PSNMF obtain accurate phase signatures, but data sets reconstructed from the outputs have demonstrably lower noise and better fidelity than from the benchmark denoising method of principle component analysis.

Near-Atomic-Scale Mapping of Electronic Phases in Rare Earth Nickelate Superlattices.

Nanoscale mapping of the distinct electronic phases characterizing the metal-insulator transition displayed by most of the rare-earth nickelate compounds is fundamental for discovering the true nature of this transition and the possible couplings that are established at the interfaces of nickelate-based heterostructures. Here, we demonstrate that this can be accomplished by using scanning transmission electron microscopy in combination with electron energy-loss spectroscopy. By tracking how the O K and Ni L edge fine structures evolve across two different NdNiO3/SmNiO3 superlattices, displaying either one or two metal-insulator transitions depending on the individual layer thickness, we are able to determine the electronic state of each of the individual constituent materials. We further map the spatial configuration associated with their metallic/insulating regions, reaching unit cell spatial resolution. With this, we estimate the width of the metallic/insulating boundaries at the NdNiO3/SmNiO3 interfaces, which is measured to be on the order of four unit cells.

Imaging Nonradiative Point Defects Buried in Quantum Wells Using Cathodoluminescence.

Crystallographic point defects (PDs) can dramatically decrease the efficiency of optoelectronic semiconductor devices, many of which are based on quantum well (QW) heterostructures. However, spatially resolving individual nonradiative PDs buried in such QWs has so far not been demonstrated. Here, using high-resolution cathodoluminescence (CL) and a specific sample design, we spatially resolve, image, and analyze nonradiative PDs in InGaN/GaN QWs at the nanoscale. We identify two different types of PDs by their contrasting behavior with temperature and measure their densities from 1014 cm-3 to as high as 1016 cm-3. Our CL images clearly illustrate the interplay between PDs and carrier dynamics in the well: increasing PD concentration severely limits carrier diffusion lengths, while a higher carrier density suppresses the nonradiative behavior of PDs. The results in this study are readily interpreted directly from CL images and represent a significant advancement in nanoscale PD analysis.

Length scales of interfacial coupling between metal and insulator phases in oxides.

Controlling phase transitions in transition metal oxides remains a central feature of both technological and fundamental scientific relevance. A well-known example is the metal-insulator transition, which has been shown to be highly controllable. However, the length scale over which these phases can be established is not yet well understood. To gain insight into this issue, we atomically engineered an artificially phase-separated system through fabricating epitaxial superlattices that consist of SmNiO3 and NdNiO3, two materials that undergo a metal-to-insulator transition at different temperatures. We demonstrate that the length scale of the interfacial coupling between metal and insulator phases is determined by balancing the energy cost of the boundary between a metal and an insulator and the bulk phase energies. Notably, we show that the length scale of this effect exceeds that of the physical coupling of structural motifs, which introduces a new framework for interface-engineering properties at temperatures against the bulk energetics.

Electronic Structure-Dependent Surface Plasmon Resonance in Single Au-Fe Nanoalloys.

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au-Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au-Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

A Cobalt-Iron Double-Atom Catalyst for the Oxygen Evolution Reaction.

Single-atom catalysts exhibit well-defined active sites and potentially maximum atomic efficiency. However, they are unsuitable for reactions that benefit from bimetallic promotion such as the oxygen evolution reaction (OER) in an alkaline medium. Here we show that a single-atom Co precatalyst can be in situ transformed into a Co-Fe double-atom catalyst for the OER. This catalyst exhibits one of the highest turnover frequencies among metal oxides. Electrochemical, microscopic, and spectroscopic data, including those from operando X-ray absorption spectroscopy, reveal a dimeric Co-Fe moiety as the active site of the catalyst. This work demonstrates double-atom catalysis as a promising approach for the development of defined and highly active OER catalysts.

Centimeter-Sized Single-Orientation Monolayer Hexagonal Boron Nitride With or Without Nanovoids.

Large-area hexagonal boron nitride (h-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer h-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO2, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of h-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single h-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids.

In-Plane Plasmonic Antenna Arrays with Surface Nanogaps for Giant Fluorescence Enhancement.

Optical nanoantennas have a great potential for enhancing light-matter interactions at the nanometer scale, yet fabrication accuracy and lack of scalability currently limit ultimate antenna performance and applications. In most designs, the region of maximum field localization and enhancement (i.e., hotspot) is not readily accessible to the sample because it is buried into the nanostructure. Moreover, current large-scale fabrication techniques lack reproducible geometrical control below 20 nm. Here, we describe a new nanofabrication technique that applies planarization, etch back, and template stripping to expose the excitation hotspot at the surface, providing a major improvement over conventional electron beam lithography methods. We present large flat surface arrays of in-plane nanoantennas, featuring gaps as small as 10 nm with sharp edges, excellent reproducibility and full surface accessibility of the hotspot confined region. The novel fabrication approach drastically improves the optical performance of plasmonic nanoantennas to yield giant fluorescence enhancement factors up to 104-105 times, together with nanoscale detection volumes in the 20 zL range. The method is fully scalable and adaptable to a wide range of antenna designs. We foresee broad applications by the use of these in-plane antenna geometries ranging from large-scale ultrasensitive sensor chips to microfluidics and live cell membrane investigations.

Geometrical Effect in 2D Nanopores.

A long-standing problem in the application of solid-state nanopores is the lack of the precise control over the geometry of artificially formed pores compared to the well-defined geometry in their biological counterpart, that is, protein nanopores. To date, experimentally investigated solid-state nanopores have been shown to adopt an approximately circular shape. In this Letter, we investigate the geometrical effect of the nanopore shape on ionic blockage induced by DNA translocation using triangular h-BN nanopores and approximately circular molybdenum disulfide (MoS2) nanopores. We observe a striking geometry-dependent ion scattering effect, which is further corroborated by a modified ionic blockage model. The well-acknowledged ionic blockage model is derived from uniform ion permeability through the 2D nanopore plane and hemisphere like access region in the nanopore vicinity. On the basis of our experimental results, we propose a modified ionic blockage model, which is highly related to the ionic profile caused by geometrical variations. Our findings shed light on the rational design of 2D nanopores and should be applicable to arbitrary nanopore shapes.

Protein Corona: Impact of Lymph Versus Blood in a Complex In Vitro Environment.

In biological environments, the surface of nanoparticles (NPs) are modified by protein corona (PC) that determines their biological behavior. Unfortunately, in vitro tests still give different PC than in vivo tests causing in vitro-in vivo discrepancy; hence, in vitro studies are not indicative for the NPs' behavior in vivo. Here is demonstrated that PC in vitro is strongly influenced by the type of extracellular fluid (ECF), blood or lymph, by their high and low flow conditions and transitions between ECFs, and a combination of these parameters. As a result, this in vitro study approaches fluidic and dynamic variations to which NPs are exposed in vivo: different ECF that NPs encounter first in different injection routes, different transitions in-between ECFs during circulation, and simultaneous change in the exposed flow in these transitions. The most-abundant proteins in PCs are found to be not the most abundant in ECFs, but those having high affinity for binding to the surface of NPs. Moreover, some proteins are differently abundant in PCs at different flows, which indicate force-promoted binding, catch bonds. These results suggest that future in vitro studies should consider more complex incubation conditions to improve the in vitro-in vivo consistency necessary for translational research.

Synthesis of Responsive Two-Dimensional Polymers via Self-Assembled DNA Networks.

Despite a growing interest in two-dimensional polymers, their rational synthesis remains a challenge. The solution-phase synthesis of a two-dimensional polymer is reported. A DNA-based monomer self-assembles into a supramolecular network, which is further converted into the covalently linked two-dimensional polymer by anthracene dimerization. The polymers appear as uniform monolayers, as shown by AFM and TEM imaging. Furthermore, they exhibit a pronounced solvent responsivity. The results demonstrate the value of DNA-controlled self-assembly for the formation of two-dimensional polymers in solution.

CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water, Light, and Heat.

Herein, the assembly of CsPbBr3 QD/AlOx inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlOx ), is described as a novel protection scheme for such QDs. The nucleation and growth of AlOx on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBrx I3-x QD/AlOx and CsPbI3 QD/AlOx nanocomposites, which were more stable than the pristine QD films.

Non-negative matrix factorization-aided phase unmixing and trace element quantification of STEM-EDXS data.

Energy-dispersive X-ray spectroscopy (EDXS) mapping with a scanning transmission electron microscope (STEM) is commonly used for chemical characterization of materials. However, STEM-EDXS quantification becomes challenging when the phases constituting the sample under investigation share common elements and overlap spatially. In this paper, we present a methodology to identify, segment, and unmix phases with a substantial spectral and spatial overlap in a semi-automated fashion through combining non-negative matrix factorization with a priori knowledge of the sample. We illustrate the methodology using a sample taken from an electron beam-sensitive mineral assemblage representing Earth's deep mantle. With it, we retrieve the true EDX spectra of the constituent phases and their corresponding phase abundance maps. It further enables us to achieve a reliable quantification for trace elements having concentration levels of ∼100 ppm. Our approach can be adapted to aid the analysis of many materials systems that produce STEM-EDXS datasets having phase overlap and/or limited signal-to-noise ratio (SNR) in spatially-integrated spectra.

Operando insights into correlating CO coverage and Cu-Au alloying with the selectivity of Au NP-decorated Cu2O nanocubes during the electrocatalytic CO2 reduction.

Electrochemical reduction of CO2 (CO2RR) is an attractive technology to reintegrate the anthropogenic CO2 back into the carbon cycle driven by a suitable catalyst. This study employs highly efficient multi-carbon (C2+) producing Cu2O nanocubes (NCs) decorated with CO-selective Au nanoparticles (NPs) to investigate the correlation between a high CO surface concentration microenvironment and the catalytic performance. Structure, morphology and near-surface composition are studied via operando X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy, operando high-energy X-ray diffraction as well as quasi in situ X-ray photoelectron spectroscopy. These operando studies show the continuous evolution of the local structure and chemical environment of our catalysts during reaction conditions. Along with its alloy formation, a CO-rich microenvironment as well as weakened average CO binding on the catalyst surface during CO2RR is detected. Linking these findings to the catalytic function, a complex compositional interplay between Au and Cu is revealed in which higher Au loadings primarily facilitate CO formation. Nonetheless, the strongest improvement in C2+ formation appears for the lowest Au loadings, suggesting a beneficial role of the Au-Cu atomic interaction for the catalytic function in CO2RR. This study highlights the importance of site engineering and operando investigations to unveil the electrocatalyst's adaptations to the reaction conditions, which is a prerequisite to understand its catalytic behavior.

espm: A Python library for the simulation of STEM-EDXS datasets.

We present two open-source Python packages: "electron spectro-microscopy" (espm) and "electron microscopy tables" (emtables). The espm software enables the simulation of scanning transmission electron microscopy energy-dispersive X-ray spectroscopy datacubes, based on user-defined chemical compositions and spatial abundance maps of constituent phases. The simulation process uses X-ray emission cross-sections generated via state-of-the-art calculations made with emtables. These tables are designed to be easily modifiable, either manually or using espm. The simulation framework is designed to test the application of decomposition algorithms for the analysis of STEM-EDX spectrum images with access to a known ground truth. We validate our approach using the case of a complex geology-related sample, comparing raw simulated and experimental datasets and the outputs of their non-negative matrix factorization. In addition to testing machine learning algorithms, our packages will also help experimental design, for instance, predicting dataset characteristics or establishing minimum counts needed to measure nanoscale features.

Metal-insulator transition in composition-tuned nickel oxide films.

Thin films of the solid solution Nd1-xLaxNiO3are grown in order to study the expected 0 K phase transitions at a specific composition. We experimentally map out the structural, electronic and magnetic properties as a function ofxand a discontinuous, possibly first order, insulator-metal transition is observed at low temperature whenx= 0.2. Raman spectroscopy and scanning transmission electron microscopy show that this is not associated with a correspondingly discontinuous global structural change. On the other hand, results from density functional theory (DFT) and combined DFT and dynamical mean field theory calculations produce a 0 K first order transition at around this composition. We further estimate the temperature-dependence of the transition from thermodynamic considerations and find that a discontinuous insulator-metal transition can be reproduced theoretically and implies a narrow insulator-metal phase coexistence withx. Finally, muon spin rotation (µSR) measurements suggest that there are non-static magnetic moments in the system that may be understood in the context of the first order nature of the 0 K transition and its associated phase coexistence regime.

Investigation of the Impact of Point Defects in InGaN/GaN Quantum Wells with High Dislocation Densities.

In this work, we report on the efficiency of single InGaN/GaN quantum wells (QWs) grown on thin (<1 µm) GaN buffer layers on silicon (111) substrates exhibiting very high threading dislocation (TD) densities. Despite this high defect density, we show that QW emission efficiency significantly increases upon the insertion of an In-containing underlayer, whose role is to prevent the introduction of point defects during the growth of InGaN QWs. Hence, we demonstrate that point defects play a key role in limiting InGaN QW efficiency, even in samples where their density (2-3 × 109 cm-2) is much lower than that of TD (2-3 × 1010 cm-2). Time-resolved photoluminescence and cathodoluminescence studies confirm the prevalence of point defects over TDs in QW efficiency. Interestingly, TD terminations lead to the formation of independent domains for carriers, thanks to V-pits and step bunching phenomena.

Influence of the sintering atmosphere on the physico-chemical properties and the osteoclastic resorption of ß-tricalcium phosphate cylinders.

One of the most widely used materials for bone graft substitution is ß-Tricalcium phosphate (ß-TCP; ß-Ca3(PO4)2). ß-TCP is typically produced by sintering in air or vacuum. During this process, evaporation of phosphorus (P) species occurs, leading to the formation of a calcium-rich alkaline layer. It was recently shown that the evaporation of P species could be prevented by co-sintering ß-TCP with dicalcium phosphate (DCPA; CaHPO4; mineral name: monetite). The aim of this study was to see how a change of sintering atmosphere could affect the physico-chemical and biological properties of ß-TCP. For this purpose, three experimental groups were considered: ß-TCP cylinders sintered in air and subsequently polished to remove the surface layer (control group); the same polished cylinders after subsequent annealing at 500 °C in air to generate a calcium-rich alkaline layer (annealed group); and finally, ß-TCP cylinders sintered in a monetite-rich atmosphere and subsequently polished (monetite group). XPS analysis confirmed that cylinders from the annealed group had a significantly higher Ca/P molar ratio at their surface than that of the control group while this ratio was significantly lower for the cylinders from the monetite group. Sintering ß-TCP in the monetite-rich atmosphere significantly reduced the grain size and increased the density. Changes of surface composition affected the activity of osteoclasts seeded onto the surfaces, since annealed ß-TCP cylinders were significantly less resorbed than ß-TCP cylinders sintered in the monetite-rich atmosphere. This suggests that an increase of the surface Ca/P molar ratio leads to a decrease of osteoclastic resorption. STATEMENT OF SIGNIFICANCE: Minimal changes of surface and bulk (< 1%) composition have major effects on the ability of osteoclasts to resorb ß-tricalcium phosphate (ß-TCP), one of the most widely used ceramics for bone substitution. The results presented in this study are thus important for the calcium phosphate community because (i) ß-TCP may have up to 5% impurities according to ISO and ASTM standards and still be considered to be "pure ß-TCP", (ii) ß-TCP surface properties are generally not considered during biocompatibility assessment and (iii) a rationale can be proposed to explain the various inconsistencies reported in the literature on the biological properties of ß-TCP.

Ripples and layers in ultrathin MoS2 membranes.

Single-layer molybdenum disulfide (MoS2) is a newly emerging two-dimensional semiconductor with a potentially wide range of applications in the fields of nanoelectronics and energy harvesting. The fact that it can be exfoliated down to single-layer thickness makes MoS2 interesting both for practical applications and for fundamental research, where the structure and crystalline order of ultrathin MoS2 will have a strong influence on electronic, mechanical, and other properties. Here, we report on the transmission electron microscopy study of suspended single- and few-layer MoS2 membranes with thicknesses previously determined using both optical identification and atomic force microscopy. Electron microscopy shows that monolayer MoS2 displays long-range crystalline order, although surface roughening has been observed with ripples which can reach 1 nm in height, just as in the case of graphene, implying that similar mechanisms are responsible for the stability of both two-dimensional materials. The observed ripples could explain the degradation of mobility in MoS2 due to exfoliation. We also find that symmetry breaking due to the reduction of the number of layers results in distinctive features in electron-beam diffraction patterns of single- and multilayer MoS2, which could be used as a method for identifying single layers using only electron microscopy. The isolation of suspended single-layer MoS2 membranes will improve our understanding of two-dimensional systems, their stability, and the interplay between their structures, morphologies, and electrical and mechanical properties.

Near-Field Mapping of Photonic Eigenmodes in Patterned Silicon Nanocavities by Electron Energy-Loss Spectroscopy.

Recently, there has been significant interest in using dielectric nanocavities for the controlled scattering of light, owing to the diverse electromagnetic modes that they support. For plasmonic systems, electron energy-loss spectroscopy (EELS) is now an established method enabling structure-optical property analysis at the scale of the nanostructure. Here, we instead test its potential for the near-field mapping of photonic eigenmodes supported in planar dielectric nanocavities, which are lithographically patterned from amorphous silicon according to standard photonic principles. By correlating results with finite element simulations, we demonstrate how many of the EELS excitations can be directly corresponded to various optical eigenmodes of interest for photonic engineering. The EELS maps present a high spatial definition, displaying intensity features that correlate precisely to the impact parameters giving the highest probability of modal excitation. Further, eigenmode characteristics translate into their EELS signatures, such as the spatially and energetically extended signal of the low Q-factor electric dipole and nodal intensity patterns emerging from excitation of toroidal and second-order magnetic modes within the nanocavity volumes. Overall, the spatial-spectral nature of the data, combined with our experimental-simulation toolbox, enables interpretation of subtle changes in the EELS response across a range of nanocavity dimensions and forms, with certain simulated resonances matching the excitation energies within ±0.01 eV. By connecting results to far-field simulations, perspectives are offered for tailoring the nanophotonic resonances via manipulating nanocavity size and shape.