Electron Energy Loss Spectroscopy Measurements Research Articles

4H-SiC/SiO2 Interface Degradation in 1.2 kV 4H-SiC MOSFETs due to Power Cycling Tests

Power cycling tests (PCTs) assess the reliability of power devices by closely simulating their operating conditions. A PCT was performed on commercially available 1.2 kV 4H-SiC power metal–oxide–semiconductor field-effect transistors to observe its impact on the 4H-SiC/SiO2 interface. High-resolution transmission electron microscopy and electron energy loss spectroscopy measurements showed variations in the length of the 4H-SiC/SiO2 transition layer, depending on whether the device was power cycled. Moreover, the total resistance at Vg >> Vt in Rtot − (Vg-Vt)−1 graph increased to 16.5%, while it changed more radically to 47.3% at Vg ≈ Vt. The threshold voltage shifted negatively. These variations cannot be expected solely through the wearout of the package.

Local defect and mid-gap state analysis of GaN using monochromated EELS combined with nanodiffraction and atomic-resolution imaging

Defects in semiconductor materials significantly impact their inherent properties, making the evaluation of local defects and their energy levels crucial for controlling device performance. With advancements in monochromators, electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) has emerged as a promising approach for assessing crystal and band structures of semiconductor materials at the nanoscale. However, there is limited experimental research investigating the relationship between actual defective structures in local regions and mid-gap states. In this study, we conducted high-energy resolution EELS (HR-EELS) measurements with an energy resolution of less than 100 meV to detect the mid-gap states of GaN. Various defects were intentionally induced through Ga-ion implantation, and the defect concentration varied in the depth direction. To understand the origin of the mid-gap states, we performed 4D-STEM analysis and atomic-resolution STEM observations. The HR-EELS measurements provided insights into the depth-dependent valence-loss spectra, revealing that the intensities corresponding to mid-gap states gradually increased toward the surface, whereas the slope at the onsets corresponding to interband transition decreased. Furthermore, local structural analysis unveiled the presence of structural disorder and defective structures, indicating the existence of extended defects such as stacking faults and domain boundaries. Observably, these defective structures were abundant near the surface and less pronounced in deeper regions. Based on these experimental findings, we concluded that the variations in valence-loss spectra can be utilized to qualitatively evaluate the crystal imperfections at the nanoscale.

Next Level of 1/1 μm Line and Space Copper Redistribution Layer Fabrication with Organic Dielectric Materials

An electrical reliability of fine copper redistribution layer (RDL) with 1/1 μm line and space fabricated by using positive photosensitive polyimide (PSPI) has been demonstrated. The polyimide containing flexible molecular units in PSPI was introduced, in assumption to increase the entanglement of each polymer chain with a design concept of polymer backbone to enhance mechanical and thermal reliability. To conduct a series of tests, test vehicles with semi-additive copper RDL with 1/1 μm line and space were fabricated with Toray’s PSPI for electrical reliability test. Through a series of electrical reliability tests utilizing our test vehicles, PSPI out-performed the phenol-based resin, which has been proven to have excellent mechanical and thermal stabilities when compared with other resins. 1/1 μm line and space electrodes after electrical reliability tests were also observed and cross-sectioned. The observation of electrodes clearly showed oxidation-reduction of the electrodes on anode and cathode, respectively. On the cathode, copper particles of several nm in diameter were formed on the PI film surrounding the copper wiring. On the anode, copper oxide layers of about 100 nm thickness were observed on the copper wiring surface, which turned out to be Cu2O when analyzed with Electron Energy-Loss Spectroscopy (EELS) measurement.

The atomic and electronic band structures in Ca/Si-doped and Ca/Si/Ti-doped Al2O3 grain boundaries

Grain boundary (GB) segregation plays a crucial role in determining structures, chemistries and resultant properties of polycrystalline ceramic materials. Although GB segregation behaviors of single-element dopants have been extensively investigated, GB segregation in practical polycrystalline ceramics often involves multiple elements. In this study, we conducted experimental investigations to explore the atomic and electronic band structures of Ca/Si-doped and Ca/Si/Ti-doped Al2O3 Σ745̅10 GBs. By employing atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations, we elucidated the interactions between different elements at the GBs. Our findings reveal that Ca and Si were synergistically co-segregated in both GBs, maintaining charge neutrality within the GB core, while the site competition between Ca and Ti occurred in the Ca/Si/Ti-doped GB. Furthermore, valence electron energy loss spectroscopy measurements provided insights into the band gaps of doped GBs, demonstrating that they are dependent on doping species but independent of interactions between different elements.

MnEdgeNet for accurate decomposition of mixed oxidation states for Mn XAS and EELS L2,3 edges without reference and calibration

Accurate decomposition of the mixed Mn oxidation states is highly important for characterizing the electronic structures, charge transfer and redox centers for electronic, and electrocatalytic and energy storage materials that contain Mn. Electron energy loss spectroscopy (EELS) and soft X-ray absorption spectroscopy (XAS) measurements of the Mn L2,3 edges are widely used for this purpose. To date, although the measurements of the Mn L2,3 edges are straightforward given the sample is prepared properly, an accurate decomposition of the mix valence states of Mn remains non-trivial. For both EELS and XAS, 2+, 3+, and 4+ reference spectra need to be taken on the same instrument/beamline and preferably in the same experimental session because the instrumental resolution and the energy axis offset could vary from one session to another. To circumvent this hurdle, in this study, we adopted a deep learning approach and developed a calibration-free and reference-free method to decompose the oxidation state of Mn L2,3 edges for both EELS and XAS. A deep learning regression model is trained to accurately predict the composition of the mix valence state of Mn. To synthesize physics-informed and ground-truth labeled training datasets, we created a forward model that takes into account plural scattering, instrumentation broadening, noise, and energy axis offset. With that, we created a 1.2 million-spectrum database with 1-by-3 oxidation state composition ground truth vectors. The library includes a sufficient variety of data including both EELS and XAS spectra. By training on this large database, our convolutional neural network achieves 85% accuracy on the validation dataset. We tested the model and found it is robust against noise (down to PSNR of 10) and plural scattering (up to t/λ = 1). We further validated the model against spectral data that were not used in training. In particular, the model shows high accuracy and high sensitivity for the decomposition of Mn3O4, MnO, Mn2O3, and MnO2. The accurate decomposition of Mn3O4 experimental data shows the model is quantitatively correct and can be deployed for real experimental data. Our model will not only be a valuable tool to researchers and material scientists but also can assist experienced electron microscopists and synchrotron scientists in the automated analysis of Mn L edge data.

How Pt Influences H2 Reactions on High Surface-Area Pt/CeO2 Powder Catalyst Surfaces.

The addition of platinum-group metals (PGMs, e.g., Pt) to CeO2 is used in heterogeneous catalysis to promote the rate of redox surface reactions. Well-defined model system studies have shown that PGMs facilitate H2 dissociation, H-spillover onto CeO2 surfaces, and CeO2 surface reduction. However, it remains unclear how the heterogeneous structures and interfaces that exist on powder catalysts influence the mechanistic picture of PGM-promoted H2 reactions on CeO2 surfaces developed from model system studies. Here, controlled catalyst synthesis, temperature-programmed reduction (TPR), in situ infrared spectroscopy (IR), and in situ electron energy loss spectroscopy (EELS) were used to interrogate the mechanisms of how Pt nanoclusters and single atoms influence H2 reactions on high-surface area Pt/CeO2 powder catalysts. TPR showed that Pt promotes H2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles, suggesting that H-spillover proceeds far from Pt-CeO2 interfaces and across CeO2-CeO2 particle interfaces. IR and EELS measurements provided evidence that Pt changes the mechanism of H2 activation and the rate limiting step for Ce3+, oxygen vacancy, and water formation as compared to pure CeO2. As a result, higher-saturation surface hydroxyl coverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt is heterogeneously distributed and localized at and around interparticle CeO2-CeO2 boundaries, while activated H2 on pure CeO2 results in homogeneously distributed Ce3+. Ce3+ localization at and around CeO2-CeO2 boundaries for Pt/CeO2 is accompanied by surface reconstruction that enables faster rates of H2 consumption. This study reconciles the materials gap between model structures and powder catalysts for H2 reactions with Pt/CeO2 and highlights how the spatial heterogeneity of powder catalysts dictates the influence of Pt on H2 reactions at CeO2 surfaces.

Using electron energy-loss spectroscopy to measure nanoscale electronic and vibrational dynamics in a TEM.

Electron energy-loss spectroscopy (EELS) can measure similar information to x-ray, UV-Vis, and IR spectroscopies but with atomic resolution and increased scattering cross-sections. Recent advances in electron monochromators have expanded EELS capabilities from chemical identification to the realms of synchrotron-level core-loss measurements and to low-loss, 10-100meV excitations, such as phonons, excitons, and valence structures. EELS measurements are easily correlated with electron diffraction and atomic-scale real-space imaging in a transmission electron microscope (TEM) to provide detailed local pictures of quasiparticle and bonding states. This perspective provides an overview of existing high-resolution EELS (HR-EELS) capabilities while also motivating the powerful next step in the field-ultrafast EELS in a TEM. Ultrafast EELS aims to combine atomic-level, element-specific, and correlated temporal measurements to better understand spatially specific excited-state phenomena. Ultrafast EELS measurements also add to the abilities of steady-state HR-EELS by being able to image the electromagnetic field and use electrons to excite photon-forbidden and momentum-specific transitions. We discuss the technical challenges ultrafast HR-EELS currently faces, as well as how integration with in situ and cryo measurements could expand the technique to new systems of interest, especially molecular and biological samples.

Vanadium in phyllosilicate ores: Occurrence, crystal chemistry, and leaching behavior

This article examines the nanoscale occurrence patterns of vanadium in phyllosilicates and correlates them with amenability to leaching. A large fraction of the global vanadium resource is hosted in phyllosilicate ores such as roscoelite and V-bearing chlorites, clays, and micas. Recovery of V from these minerals in acid leaching is typically low but varies widely for unclear reasons. Transmission electron microscopy (TEM) of V-illites, V-chlorites, and roscoelites in leach heads and tails from the La Sal mine (Colorado Plateau) shows that the observed variability in recoveries correlates with intercalations of nanoscale V oxide phases within the phyllosilicate lattice. These are present in some of the V-illites in this study, which displayed higher leach recovery than V-illites without the V oxides. In the latter, as well as in V-chlorites, the V occurs as crystallographic substitutions without oxide intercalations. Electron energy loss spectroscopy (EELS) measurements in the scanning transmission electron microscope (STEM) show that this crystallographic V is mixed-valence, averaging 3.3 in V-chlorite, 3.6 in roscoelite, and 3.8 in V-illite. Unlike the oxide intercalations, this crystallographic type of V corresponds to negligible leaching recoveries irrespective of V siting, oxidation state, or type of phyllosilicate.These results shed light on the causes of a central problem in geometallurgy, which is attempting quantitative predictions of process outcomes from geological inputs. These results show that what appears to be the same mineral (in this case V-illite) can yield wildly disparate leach recoveries depending on nanoscale metal occurrence factors. Similar under-recognized variation in other ore minerals, within or between sites, may explain much of the notorious variability in their leaching behavior.

Oxygen vacancy region formation in BaTiO3 adjacent to the interface between the internal electrode and the dielectric layer in Ni–Sn internal electrode multilayer ceramic capacitor exhibiting leakage current suppression

This study underlines the ceramic BaTiO3 dielectric layer adjacent to the electrode of long-term reliability-improved Ni–Sn alloy internal electrode BaTiO3-based multilayer ceramic capacitor to clarify the cause of electric barrier formation. Electron energy loss spectroscopy measurements of the Ti L 3,2 near the edges and the O K near the edge structure changes to characterize the existence of an oxygen vacancy region, approximately 60 nm in width, and generated in BaTiO3 adjacent to the interface. Accordingly, the n-type semiconductor layer of BaTiO3 that originated from the oxygen vacancies, led to the formation of a rigid Schottky barrier at the interface.

Mechanistic Understanding of Ring-Opening of Tetrahydrofurfuryl Alcohol over WOx-Modified Pt Model Surfaces and Powder Catalysts

The upgrading of heterocyclic biomass-derived oxygenates such as tetrahydrofurfuryl alcohol (THFA) via ring-opening is a promising pathway to produce value-added diol molecules using renewable carbon sources. This study combines model surface experiments, first-principles calculations, and powder catalyst characterization and activity evaluation to unravel the nature of the Pt and WOx active sites and the reaction mechanism of the THFA ring-opening reaction on a WOx/Pt inverse oxide catalyst. Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) measurements on model surfaces demonstrated that THFA ring opened on Pt(111) but underwent further decomposition due to its strong bonding with the surface. However, WOx deposited on Pt(111) altered the interaction strength between the ring-opened intermediate and the surface to a proper extent to facilitate the facile desorption of the desired 1,5-pentanediol (1,5-PeD) product. Density functional theory (DFT) calculations showed that WOx/Pt(111) could promote ring opening of THFA via an oxocarbenium ion-like transition state, which was stabilized by hydrogen bonding with the hydroxyl groups of WOx. The hydrogenation of the ring-opened 5-hydroxyvaleraldehyde intermediate to 1,5-PeD was then feasible via Brønsted acid sites present on WOx. Steady-state activity studies on the corresponding powder catalysts showed that the 1,5-PeD selectivity increased from 20% on Pt/SiO2 to 65% on WOx/Pt/SiO2 with 1 wt % WOx loading, consistent with model surface experiments and DFT calculations. This study demonstrates the feasibility of using model surface experiments and first-principles calculations to guide practical catalyst design, and provides a design strategy that can be applied to the selective ring-opening of relevant heterocyclic biomass-derived oxygenates.

Interrogating Quantum Nonlocal Effects in Nanoplasmonics through Electron-Beam Spectroscopy.

A rigorous account of quantum nonlocal effects is paramount for understanding the optical response of metal nanostructures and for designing plasmonic devices at the nanoscale. Here, we present a scheme for retrieving the quantum surface response of metals, encapsulated in the Feibelman d-parameters, from electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We theoretically demonstrate that quantum nonlocal effects have a dramatic impact on EELS and CL spectra, in the guise of spectral shifts and nonlocal damping, when either the system size or the inverse wave vector in extended structures approaches the nanometer scale. Our concept capitalizes on the unparalleled ability of free electrons to supply deeply subwavelength near-fields and, thus, probe the optical response of metals at length scales in which quantum-mechanical effects are apparent. These results pave the way for a widespread use of the d-parameter formalism, thereby facilitating a rigorous yet practical inclusion of nonclassical effects in nanoplasmonics.

Two-Dimensional Violet Phosphorus P11: A Large Band Gap Phosphorus Allotrope.

The discovery of novel large band gap two-dimensional (2D) materials with good stability and high carrier mobility will innovate the next generation of electronics and optoelectronics. A new allotrope of 2D violet phosphorus P11 was synthesized via a salt flux method in the presence of bismuth. Millimeter-sized crystals of violet-P11 were collected after removing the salt flux with DI water. From single-crystal X-ray diffraction, the crystal structure of violet-P11 was determined to be in the monoclinic space group C2/c (no. 15) with unit cell parameters of a = 9.166(6) Å, b = 9.121(6) Å, c = 21.803(14)Å, β = 97.638(17)°, and a unit cell volume of 1807(2) Å3. The structure differences between violet-P11, violet-P21, and fibrous-P21 are discussed. The violet-P11 crystals can be mechanically exfoliated down to a few layers (∼6 nm). Photoluminescence and Raman measurements reveal the thickness-dependent nature of violet-P11, and exfoliated violet-P11 flakes were stable in ambient air for at least 1 h, exhibiting moderate ambient stability. The bulk violet-P11 crystals exhibit excellent stability, being stable in ambient air for many days. The optical band gap of violet-P11 bulk crystals is 2.0(1) eV measured by UV-Vis and electron energy-loss spectroscopy measurements, in agreement with density functional theory calculations which predict that violet-P11 is a direct band gap semiconductor with band gaps of 1.8 and 1.9 eV for bulk and monolayer, respectively, and with a high carrier mobility. This band gap is the largest among the known single-element 2D layered bulk crystals and thus attractive for various optoelectronic devices.

Efficient calculation of k -integrated electron energy loss spectra: Application to monolayers of MoS2, hBN, and graphene

The theoretical scattering cross section of electron energy loss spectroscopy (EELS) is essentially given by $\ensuremath{-}\text{Im}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\varepsilon}}^{\ensuremath{-}1}(\mathbf{k},\ensuremath{\omega})$ with the energy loss $\ensuremath{\hbar}\ensuremath{\omega}$ and the momentum transfer $\ensuremath{\hbar}\mathbf{k}$. The macroscopic dielectric function $\ensuremath{\varepsilon}(\mathbf{k},\ensuremath{\omega})$ can be calculated from first principles using time-dependent density-functional theory. However, experimental EELS measurements have a finite $\mathbf{k}$ resolution or, when operated in spatial resolution mode, yield a $\mathbf{k}$-integrated loss spectrum, which deviates significantly from EEL spectra calculated for specific $\mathbf{k}$ momenta. On the other hand, integrating the theoretical spectra over $\mathbf{k}$ is complicated by the fact that the integrand varies over several (typically six) orders of magnitude around $k=0$. In this article, we present a stable technique for integrating EEL spectra over an adjustable range of momentum transfers. The important region around $k=0$, where the integrand is nearly divergent, is treated partially analytically, allowing an analytic integration of the near divergence. The scheme is applied to three prototypical two-dimensional systems: monolayers of ${\mathrm{MoS}}_{2}$ (semiconductor), hexagonal BN (insulator), and graphene (semimetal). Here, we are confronted with the added difficulty that the long-range Coulomb interaction leads to a very slow supercell (vacuum size) convergence. We address this difficulty by employing an extrapolation scheme, enabling an efficient reduction of the supercell size and thus a considerable speedup in computation time. The calculated $\mathbf{k}$-integrated spectra are in very favorable agreement with experimental EEL spectra.

Enhancing the Catalytic Activity of Mo(110) Surface via Its Alloying with Submonolayer to Multilayer Boron Films and Oxidation of the Alloy: A Case of (CO + O2) to CO2 Conversion

In-situ formation of boron thin films on the Mo(110) surface, as well as the formation of the molybdenum boride and its oxide and the trends of carbon monoxide catalytic oxidation on the substrates formed, have been studied in an ultra-high vacuum (UHV) by a set of surface-sensitive characterization techniques: Auger and X-ray photoelectron spectroscopy (AES, XPS), low-energy ion scattering (LEIS), reflection-absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), electron energy loss spectroscopy (EELS) and work function measurements using the Anderson method. The boron deposited at Mo(110) via electron-beam deposition at a substrate temperature of 300 K grows as a 2D layer, at least in submonolayer coverage. Such a film is bound to the Mo(110) via polarized chemisorption bonds, dramatically changing the charge density at the substrate surface manifested by the Mo(110) surface plasmon damping. Upon annealing of the B-Mo(110) system, the boron diffuses into the Mo(110) bulk following a two-mode regime: (1) quite easy dissolution, starting at a temperature of about 450 K with an activation energy of 0.4 eV; and (2) formation of molybdenum boride at a temperature higher than 700 K with M-B interatomic bonding energy of 3.8 eV. The feature of the formed molybdenum boride is that there is quite notable carbon monoxide oxidation activity on its surface. A further dramatic increase of such an activity is achieved when the molybdenum boride is oxidized. The latter is attributed to more activated states of molecular orbitals of coadsorbed carbon monoxide and oxygen due to their enhanced interaction with both boron and oxygen species for MoxByOz ternary compound, compared to only boron for the Mox'By' double alloy.

Cation Segregation in Alloyed Thiophosphates Fe2–xCoxP2S6

Metal thiophosphates are a versatile class of van der Waals materials that can exhibit drastically different properties depending on the metal cations that are incorporated. Solid solutions between different members of this class are of interest to study due to the evolution of the magnetic, topological, and semiconductor properties in these materials. However, there has not yet been a thorough examination of the homogeneity of the cation species within the alloyed system of interest that may prove vital if these thiophosphates are to be incorporated into devices in conjunction with other van der Waals materials. Herein, we report on the structural and chemical differences between measurements of bulk and exfoliated Fe2–xCoxP2S6, which were synthesized with a P2S5 flux method. In the bulk, powder X-ray diffraction and scanning electron microscopy energy-dispersive X-ray spectroscopy indicate a homogeneous stoichiometry. This is corroborated by Mössbauer spectroscopy, which in addition suggests that as the quantity of Co introduced into the Fe2–xCoxP2S6 structure increases, the probability of having the nearest neighbors of any given Fe2+ cation to be Co2+ cations is substantially increased. However, upon exfoliation of Fe2–xCoxP2S6, Raman spectroscopy and electron energy loss spectroscopy (EELS) measurements suggest that the composition of individual exfoliated flakes deviates significantly from the expected average stoichiometry, and the distribution of Fe2+ and Co2+ is not perfectly homogeneous.

Nanoscale Characterization of Individual Three-Dimensional Split Ring Resonator Systems

In this work, we investigate the hybridization of three-dimensional plasmonic split ring resonator (SRR) pairs using focused electron beam nanospectroscopy and model their combined electric and magnetic responses using electromagnetic theory and numerical calculations. Specifically, we fabricate three-dimensional (3D) SRR dimers with varying in-plane rotations and out-of-plane tilts and perform electron energy loss spectroscopy (EELS) measurements to elucidate the impact of their 3D electric and magnetic interactions on the EEL spectrum of each individual SRR dimer. On the basis of the competition between the SRR’s electric and magnetic interactions within our model, we find that varying the 3D tilt angle diminishes the magnetic coupling but increases the level of overall mode mixing. Through further modeling of the experimental data, we determine the system’s electric and magnetic coupling constants, as well as the overall effective coupling constant, a useful metric for characterizing the strength of light–matter interaction. We additionally explore the potential for geometric frustration in the magnetic mode ordering of a coupled SRR trimer using both EELS experiment and companion theoretical modeling. Taken together, the insight gained into the behavior of coupled 3D SRRs serves as a stepping stone to the rational design of 3D photonic metamaterials endowed with even richer optical functionality.

Vanadium Electronic Configuration Determination From L2,3 Transition in V-oxide Compounds and Roscoelite.

We report on the electronic structure of vanadium in synthetic V-oxides and in natural roscoelite (V-rich phyllosilicate). This study applied electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM), combined with first-principle calculations, to (1) establish relationships between the V oxidation state and EELS L2,3 features and (2) better constrain the oxidation state and crystallographic siting of V in roscoelite, with implications for other V-bearing phyllosilicates. Both EELS measurements and band structure calculations show that the EELS L2/L3 ratio increases as the oxidation state of V increases. We establish a quantitative relationship between the V L2,3 near-edge structure and the V oxidation state by normalizing the L2 maximum peak intensity to the L3 peak intensity. By applying this method to roscoelite, we find that it hosts a mix of trivalent and tetravalent V distributed between the octahedral and tetrahedral sites with a V4+/ΣV = 0.6 ± 0.1. This relationship is applicable to measurements of V oxidation states in oxide and phyllosilicate minerals, which is useful for constraining the conditions of rock and mineral formation and has potential implications for metal extraction from phyllosilicate ores.

Constructing N-Coordinated Co and Cu Single-Atomic-Pair Sites toward Boosted CO2 Photoreduction

The visible-light-driven photocatalytic reduction reaction of carbon dioxide (CO2) (CO2RR) to value-added fuels presents a feasible approach to curb anthropogenic CO2 emissions and mitigate the increasing energy crisis. However, developing photocatalysts with excellent performance still remains a great challenge in this field. Herein Co,Cu,N-codoped carbon nanoparticles (Co1Cu1/NC) were fabricated through the pyrolysis of zeolitic imidazolate framework (ZIF-8) with Cu(NO3)2 adsorbed inside the cavities and CoTBPP decorated over the surface of ZIF. Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy measurements disclose the dual-metal single-atomic nature of Co1Cu1/NC consisting of atomically dispersed Co–Cu pair sites on a nitrogen-doped carbon support. Extended X-ray absorption fine-structure analysis reveals the tetra-N-coordinated nature of each metal in Co1Cu1/NC (N2–Co–N2–Cu–N2). For the purpose of comparative study, Co,N- and Cu,N-codoped carbon nanoparticles (Co1/NC and Cu1/NC) also with single atomic site nature have been fabricated following the same route. The as-prepared Co1Cu1/NC exhibits highly effective photocatalytic CO2-to-CO reduction with a considerably high CO-generating yield of 22.46 mmol g–1 and a CO selectivity of 83.4% after 2 h of visible-light irradiation. Experimental characterizations and in particular theoretical calculations disclose the close association of the remarkable CO2RR catalytic activity of Co1Cu1/NC with the synergetic effect of the Co–Cu atomic-pair sites, which facilitate the conversion of CO2 to CO via lowering the energy barrier for the formation of the *COOH intermediate. This work paves a new avenue for the rational design and construction of atomic-pair photocatalysts with boosted performance.

Simultaneous Imaging of Dopants and Free Charge Carriers by Monochromated EELS.

Doping inhomogeneities in solids are not uncommon, but their microscopic observation and understanding are limited due to the lack of bulk-sensitive experimental techniques with high enough spatial and spectral resolution. Here, we demonstrate nanoscale imaging of both dopants and free charge carriers in La-doped BaSnO3 (BLSO) using high-resolution electron energy-loss spectroscopy (EELS). By analyzing high- and low-energy excitations in EELS, we reveal chemical and electronic inhomogeneities within a single BLSO nanocrystal. The inhomogeneous doping leads to distinctive localized infrared surface plasmons, including a previously unobserved plasmon mode that is highly confined between high- and low-doping regions. We further quantify the carrier density, effective mass, and dopant activation percentage by EELS and transport measurements on the bulk single crystals of BLSO. These results not only represent a practical approach for studying heterogeneities in solids and understanding structure-property relationships at the nanoscale, but also demonstrate the possibility of infrared plasmon tuning by leveraging nanoscale doping texture.

Helium-induced morphology evolution in tungsten under thermal treatment

Surface and near-surface morphology evolution of helium-irradiated tungsten due to thermal cycling up to the ITER-relevant temperature of 1350 K was studied using electron microscopy and positron annihilation spectroscopy techniques. Holes at the surface and bubbles in the near-surface of recrystallized polycrystalline tungsten samples were created by 75 eV helium plasma irradiation with the fluence of 3 × 1023 He m−2 at the surface temperature of 1073 K. Subsequent annealing experiments were combined with a detailed electron microscopy analysis to investigate the shape and density changes of holes and helium bubbles with respect to grain orientation. We show that the initially circular holes and round bubbles became faceted upon heating with 1 K s−1 ramp up to 870 K. Annealing cycles up to 1350 K induced resulted in bubbles removal in the first 5 nm below the surface and surface smoothing. Electron energy loss spectroscopy measurements allowed estimation of helium gas density and pressure inside bubbles. Positron annihilation spectroscopy allowed to investigate the nature of defects and their evolution in the helium-irradiated tungsten with thermal cycling.