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Ga-doped Li7La3Zr2O12garnet solid electrolytes exhibit the highest Li-ion conductivities among the oxide-type garnet-structured solid electrolytes, but instabilities toward Li metal hamper their practical application. The instabilities have been assigned to direct chemical reactions between LiGaO2coexisting phases and Li metal by several groups previously. Yet, the understanding of the role of LiGaO2in the electrochemical cell and its electrochemical properties is still lacking. Here, we are investigating the electrochemical properties of LiGaO2through electrochemical tests in galvanostatic cells versus Li metal and complementary ex situstudies via confocal Raman microscopy, quantitative phase analysis based on powder X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. The results demonstrate considerable and surprising electrochemical activity, with high reversibility. A three-stage reaction mechanism is derived, including reversible electrochemical reactions that lead to the formation of highly electronically conducting products. The results have considerable implications for the use of Ga-doped Li7La3Zr2O12electrolytes in all-solid-state Li-metal battery applications and raise the need for advanced materials engineering to realize Ga-doped Li7La3Zr2O12for practical use.

Two-dimensional van der Waals (vdW) heterostructures are an attractive platform for studying exchange bias due to their defect-free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnetic material, possesses uncompensated magnetic spins in a single layer, rendering it a promising candidate for exploring exchange bias phenomena. Recent findings have highlighted that naturally oxidized vdW ferromagnetic Fe3GeTe2exhibits exchange bias, attributed to the antiferromagnetic coupling of its ultrathin surface oxide layer (O-FGT) with the underlying unoxidized Fe3GeTe2. Anomalous Hall measurements are employed to scrutinize the exchange bias within the CrPS4/(O-FGT)/Fe3GeTe2heterostructure. This analysis takes into account the contributions from both the perfectly uncompensated interfacial CrPS4layer and the interfacial oxide layer. Intriguingly, a distinct and nonmonotonic exchange bias trend is observed as a function of temperature below 140 K. The occurrence of exchange bias induced by a “preset field” implies that the prevailing phase in the polycrystalline surface oxide is ferrimagnetic Fe3O4. Moreover, the exchange bias induced by the ferrimagnetic Fe3O4is significantly modulated by the presence of the van der Waals antiferromagnetic CrPS4layer, forming a heterostructure, along with additional iron oxide phases within the oxide layer. These findings underscore the intricate and complex nature of exchange bias in van der Waals heterostructures, highlighting their potential for tailored manipulation and control.

For energy-efficient magnetic memories, switching of perpendicular magnetization by spin–orbit torque (SOT) appears to be a promising solution. This SOT switching requires the assistance of an in-plane magnetic field to break the symmetry. Here, we demonstrate the field-free SOT switching of a perpendicularly magnetized thulium iron garnet (Tm3Fe5O12, TmIG). The polarity of the switching loops, clockwise or counterclockwise, is determined by the direction of the initial current pulses, in contrast with field-assisted switching where the polarity is controlled by the direction of the magnetic field. From Brillouin light scattering, we determined the Dzyaloshinskii–Moriya interaction (DMI) induced by the Pt–TmIG interface. We will discuss the possible origins of field-free switching and the roles of the interfacial DMI and cubic magnetic anisotropy of TmIG. This discussion is substantiated by magnetotransport, Kerr microscopy, and micromagnetic simulations. Our observation of field-free electrical switching of a magnetic insulator is an important milestone for low-power spintronic devices.

Dirac materials are characterized by the emergence of massless quasiparticles in their low-energy excitation spectrum that obey the Dirac Hamiltonian. Known examples of Dirac materials are topological insulators, d-wave superconductors, graphene, and Weyl and Dirac semimetals, representing a striking range of fundamental properties with potential disruptive applications. However, none of the Dirac materials identified so far shows metallic character. Here, we present evidence for the formation of free-standing molybdenene, a two-dimensional material composed of only Mo atoms. Using MoS2as a precursor, we induced electric-field-assisted molybdenene growth under microwave irradiation. We observe the formation of millimetre-long whiskers following screw-dislocation growth, consisting of weakly bonded molybdenene sheets, which, upon exfoliation, show metallic character, with an electrical conductivity of ~940 S m−1. Molybdenene when hybridized with two-dimensional h-BN or MoS2, fetch tunable optical and electronic properties. As a proof of principle, we also demonstrate applications of molybdenene as a surface-enhanced Raman spectroscopy platform for molecular sensing, as a substrate for electron imaging and as a scanning probe microscope cantilever.

Magnetic skyrmions and hopfions are topological solitons1—well-localized field configurations that have gained considerable attention over the past decade owing to their unique particle-like properties, which make them promising objects for spintronic applications. Skyrmions2,3are two-dimensional solitons resembling vortex-like string structures that can penetrate an entire sample. Hopfions4–9are three-dimensional solitons confined within a magnetic sample volume and can be considered as closed twisted skyrmion strings that take the shape of a ring in the simplest case. Despite extensive research on magnetic skyrmions, the direct observation of magnetic hopfions is challenging10and has only been reported in a synthetic material11. Here we present direct observations of hopfions in crystals. In our experiment, we use transmission electron microscopy to observe hopfions forming coupled states with skyrmion strings in B20-type FeGe plates. We provide a protocol for nucleating such hopfion rings, which we verify using Lorentz imaging and electron holography. Our results are highly reproducible and in full agreement with micromagnetic simulations. We provide a unified skyrmion–hopfion homotopy classification and offer insight into the diversity of topological solitons in three-dimensional chiral magnets.

Cryogenic electron microscopy can provide high-resolution reconstructions of macromolecules embedded in a thin layer of ice from which atomic models can be built de novo. However, the interaction between the ionizing electron beam and the sample results in beam-induced motion and image distortion, which limit the attainable resolutions. Sample charging is one contributing factor of beam-induced motions and image distortions, which is normally alleviated by including part of the supporting conducting film within the beam-exposed region. However, routine data collection schemes avoid strategies whereby the beam is not in contact with the supporting film, whose rationale is not fully understood. Here we characterize electrostatic charging of vitreous samples, both in imaging and in diffraction mode. We mitigate sample charging by depositing a single layer of conductive graphene on top of regular EM grids. We obtained high-resolution single-particle analysis (SPA) reconstructions at 2 Å when the electron beam only irradiates the middle of the hole on graphene-coated grids, using data collection schemes that previously failed to produce sub 3 Å reconstructions without the graphene layer. We also observe that the SPA data obtained with the graphene-coated grids exhibit a higher bfactor and reduced particle movement compared to data obtained without the graphene layer. This mitigation of charging could have broad implications for various EM techniques, including SPA and cryotomography, and for the study of radiation damage and the development of future sample carriers. Furthermore, it may facilitate the exploration of more dose-efficient, scanning transmission EM based SPA techniques.

The operation of nanoscale electronic devices is related intimately to the three-dimensional (3D) charge density distributions within them. Here, we demonstrate the quantitative 3D mapping of the charge density and long-range electric field associated with an electrically biased carbon fiber nanotip with a spatial resolution of approximately 5 nm using electron holographic tomography in the transmission electron microscope combined with model-based iterative reconstruction. The approach presented here can be applied to a wide range of other nanoscale materials and devices.

Owing to their excellent mixed-ionic and electronic conductivity, fast oxygen kinetics, and cost efficiency, layered oxygen-deficient perovskite oxides hold great potential as highly efficient cathodes for solid oxide fuel cells and anodes for water oxidation. Under working conditions, cation ordering is believed to substantially enhance oxygen diffusion while maintaining structural stability owing to the formation of double perovskite (DP), thus attracting extensive research attention. In contrast, the incorporation of oxygen vacancies and the associated vacancy ordering have rarely been studied at the atomic scale, despite their decisive roles in regulating the electronic and spin structures as well as in differentiating the crystal structure from DP. Here, atomic-resolution transmission electron microscopy is used to directly image oxygen vacancies and measure their concentration in (Pr,Ba)CoO3-δfilms grown on SrTiO3substrates. We find that accompanied by the presence of oxygen vacancy ordering at Co–O planes, the A–O (A = Pr/Ba) planes also exhibit a breathing-like lattice modulation. Specifically, as confirmed by first-principle calculations, the AO–AO interplanar spacings are found to be linearly correlated with the vacancy concentration in the enclosing Co–O planes. On this basis, potential consequences of oxygen occupancy for the catalytic properties of structurally pure PBCO phases are discussed. Through establishing a simple correlation of oxygen concentration with the easily achievable lattice measurement, our results pave a way for better understanding the structure–performance relationship of oxygen-deficient complex cobaltites used for electrocatalysis.

Metal oxide nanoparticles exhibit outstanding catalytic properties, believed to be related to the presence of oxygen vacancies at the particle’s surface. However, little quantitative information is known about concentrations of point defects inside and at surfaces of these nanoparticles, due to the challenges in achieving an atomically resolved experimental access. By employing off-axis electron holography, we demonstrate, using MgO nanoparticles as an example, a methodology that discriminates between mobile charge induced by electron beam irradiation and immobile charge associated with deep traps induced by point defects as well as distinguishes between bulk and surface point defects. Counting the immobile charge provides a quantification of the concentration of F2+centers induced by oxygen vacancies at the MgO nanocube surfaces.

A fundamental property of particles and antiparticles (such as electrons and positrons, respectively) is their ability to annihilate one another. A similar behaviour is predicted for magnetic solitons1—localized spin textures that can be distinguished by their topological index Q. Theoretically, magnetic topological solitons with opposite values of Q, such as skyrmions2and their antiparticles (namely, antiskyrmions), are expected to be able to continuously merge and annihilate3. However, experimental verification of such particle–antiparticle pair production and annihilation processes has been lacking. Here we report the creation and annihilation of skyrmion–antiskyrmion pairs in an exceptionally thin film of the cubic chiral magnet of B20-type FeGe observed using transmission electron microscopy. Our observations are highly reproducible and are fully consistent with micromagnetic simulations. Our findings provide a new platform for the fundamental studies of particles and antiparticles based on magnetic solids and open new perspectives for practical applications of thin films of isotropic chiral magnets.

Magnetic skyrmions are stable topological solitons with complex non-coplanar spin structures. Their nanoscopic size and the low electric currents required to control their motion has opened a new field of research, skyrmionics, that aims for the usage of skyrmions as information carriers. Further advances in skyrmionics call for a thorough understanding of their three-dimensional (3D) spin texture, skyrmion–skyrmion interactions and the coupling to surfaces and interfaces, which crucially affect skyrmion stability and mobility. Here, we quantitatively reconstruct the 3D magnetic texture of Bloch skyrmions with sub-10-nanometre resolution using holographic vector-field electron tomography. The reconstructed textures reveal local deviations from a homogeneous Bloch character within the skyrmion tubes, details of the collapse of the skyrmion texture at surfaces and a correlated modulation of the skyrmion tubes in FeGe along their tube axes. Additionally, we confirm the fundamental principles of skyrmion formation through an evaluation of the 3D magnetic energy density across these magnetic solitons.

The construction of multiple types of active sites on the surface of a metallic catalyst can markedly enhance its catalytic activity toward specific reactions. Here, we show that heterophase gold nanowires (Au NWs) with multiple types of active surface sites can be synthesized using an etching-assisted process, yielding the highest reported turnover frequency (TOF) for Au catalysts toward the silane oxidation reaction by far. We use synchrotron powder X-ray diffraction (PXRD) and aberration-corrected (scanning) transmission electron microscopy (TEM) to show that the Au NWs contain heterophase structures, planar defects, and surface steps. Moreover, the contribution to the catalytic performance from each type of active sites was clarified. Surface steps on the Au NW catalysts, which were identified using aberration-corrected (scanning) TEM, were shown to play the most important role in enhancing the catalytic performance. By using synchrotron PXRD, it was shown that a small ratio of metastable phases within Au NWs can enhance catalytic activity by a factor of 1.35, providing a further route to improve catalytic activity. Of the three types of surface active sites, surface terminations of planar defects such as twin boundaries (TB) and stacking faults (SF) are less active than metastable phases and surface steps for Au catalysts toward the silane oxidation reaction. Such an etching-assisted synthesis of heterophase Au NWs promises to open new possibilities for catalysis, plasmonic, optics, and electrical applications.

Shape-controlled octahedral Pt–Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situanalytical techniques with in situtransmission electron microscopy (TEM), in situX-ray diffraction (XRD), and in situelectrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mgPt–1at 0.9 VRHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situelectrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situelectrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.

Concurrent structural and electronic transformations in VO2thin films are of 2-fold importance: enabling fine-tuning of the emergent electrical properties in functional devices, yet creating an intricate interfacial domain structure of transitional phases. Despite the importance of understanding the structure of VO2thin films, a detailed real-space atomic structure analysis in which the oxygen atomic columns are also resolved is lacking. Moreover, intermediate atomic structures have remained elusive due to the lack of robust atomically resolved quantitative analysis. Here, we directly resolve both V and O atomic columns and discover the presence of intermediate monoclinic (M2) phase nanolayers (less than 2 nm thick) in epitaxially grown VO2films on a TiO2(001) substrate, where the dominant part of VO2undergoes a transition from the tetragonal (rutile) phase to the monoclinic M1 phase. Strain analysis suggests that the presence of the M2 phase is related to local strain gradients near the TiO2/VO2interface. We unfold the crucial role of imaging the spatial configurations of the oxygen anions (in addition to V cations) by utilizing atomic-resolution electron microscopy. Our approach can be used to unravel the structural transitions in a wide range of correlated oxides, offering substantial implications for, e.g., optoelectronics and ferroelectrics.

Spontaneous polarization is essential for ferroelectric functionality in non-centrosymmetric crystals. High-integration-density ferroelectric devices require the stabilization of polarization in small volumes. Here, atomic-resolution transmission electron microscopy imaging reveals that twinning gives rise to multiple polarization orders without symmetry breaking in colloidal nanocrystals of HfO2. The polarization orders are associated with sub-nanometer ferroelectric and antiferroelectric phases. The minimum size limit of the ferroelectric phase is found to be ∼4 nm3. Density functional theory calculations indicate that transformations between the ferroelectric and antiferroelectric phases are energetically possible. This work provides a route toward applications of HfO2nanocrystals in information storage at densities that are more than an order of magnitude higher than the scaling limit defined by the nanocrystal size. Our results on the formation of twinning-induced polarization orders without symmetry breaking may provide general guidance for the discovery of new ferroelectric phases in ionic compounds that are not restricted to oxides.

Electron microscopy touches on nearly every aspect of modern life, underpinning materials development for quantum computing, energy and medicine. We discuss the open, highly integrated and data-driven microscopy architecture needed to realize transformative discoveries in the coming decade.

Pyrrhotites, characterized by the chemical formula Fe1–δS (0 < δ ≤ 1/8), represent an extended group of minerals that are derived from the NiAs-type FeS aristotype. They contain layered arrangements of ordered Fe vacancies, which are at the origin of the various magnetic signals registered from certain natural rocks and can act as efficient electrocatalysts in oxygen evolution reactions in ultrathin form. Despite extensive studies over the past century, the local structural details of pyrrhotite superstructures formed by different arrangements of Fe vacancies remain unclear, in particular at the atomic scale. Here, atomic-resolution high-angle annular dark-field imaging and nanobeam electron diffraction in the scanning transmission electron microscope are used to study natural pyrrhotite samples that contain commensurate 4C and incommensurate 4.91 ± 0.02C constituents. Local measurements of both the intensities and the picometer-scale shifts of individual Fe atomic columns are shown to be consistent with a model for the structure of 4C pyrrhotite, which was derived using X-ray diffraction by Tokonami et al. (1972). In 4.91 ± 0.02C pyrrhotite, 5C-like unequally sized nano-regions are found to join at anti-phase-like boundaries, leading to the incommensurability observed in the present pyrrhotite sample. This conclusion is supported by computer simulations. The local magnetic properties of each phase are inferred from the measurements. A discussion of perspectives for the quantitative counting of Fe vacancies at the atomic scale is presented.

Hydroalkoxycarbonylation of olefins has been considered to be one of the most attractive methods to synthesize esters. Controlling the regioselectivities of linear esters (L) and branched esters (B) is a challenging project for researchers working in this reaction. Although most of the attention has been paid to control the regioselectivity through ligand design in homogeneous catalytic systems, study in the area is still limited. Herein, Ru-clusters/CeO2is employed as a heterogeneous catalyst for the hydromethoxycarbonylation of styrene without any additives. After optimization of the reaction conditions, the conversion of styrene is > 99% with 83% and 12% regioselectivity of linear and branched ester, respectively. By using different supports (CeO2(nanoparticle), CeO2-rod, and CeO2-cube), three catalysts including Ru-clusters/CeO2, Ru/CeO2-rod, and Ru/CeO2-cube are prepared and applied in the reaction. Structural characterizations demonstrate that the L/Bratio is related to the Ru size of supported Ru catalysts. Further Raman characterization and NH3-TPD demonstrate that the metal-support interaction and the concentration of oxygen vacancy of the catalyst have a great influence on the Ru size. The mechanism and kinetic analysis for this reaction are also investigated in this work.

Arrays of interacting 2D nanomagnets display unprecedented electromagnetic properties via collective effects, demonstrated in artificial spin ices and magnonic crystals. Progress toward 3D magnetic metamaterials is hampered by two challenges: fabricating 3D structures near intrinsic magnetic length scales (sub-100 nm) and visualizing their magnetic configurations. Here, we fabricate and measure nanoscale magnetic gyroids, periodic chiral networks comprising nanowire-like struts forming three-connected vertices. Via block copolymer templating, we produce Ni75Fe25single-gyroid and double-gyroid (an inversion pair of single-gyroids) nanostructures with a 42 nm unit cell and 11 nm diameter struts, comparable to the exchange length in Ni–Fe. We visualize their magnetization distributions via off-axis electron holography with nanometer spatial resolution and interpret the patterns using finite-element micromagnetic simulations. Our results suggest an intricate, frustrated remanent state which is ferromagnetic but without a unique equilibrium configuration, opening new possibilities for collective phenomena in magnetism, including 3D magnonic crystals and unconventional computing.

Surface atomic, compositional, and electronic structures play decisive roles in governing the performance of catalysts during electrochemical reactions. Nevertheless, for efficient and cheap transition-metal phosphides used for water splitting, such atomic-scale structural information is largely missing. Despite much effort being made so far, there is still a long way to go for establishing a precise structure–activity relationship. Here, in combination with electron-beam bombardment and compositional analysis, our atomic-scale transmission electron microscopy study on Ni5P4nanosheets, with a preferential (001) orientation, directly reveals the coverage of a self-epitaxial Ni2P nanolayer on the phosphide surface. Apart from the presence of nickel vacancies in the Ni5P4phase, our quantum-mechanical image simulations also suggest the existence of an additional NiPx(0 < x< 0.5) nanolayer, characteristic of complex surface atom reconstruction, on the outermost surface of the phosphides. The surface chemical gradient and the core–shell scenario, probably responsible for the passivated catalytic activity, provide a novel insight to understand the catalytic performance of transition-metal catalysts used for electrochemical energy conversion.

Core–shell particles with earth-abundant cores represent an effective design strategy for improving the performance of noble metal catalysts, while simultaneously reducing the content of expensive noble metals1–4. However, the structural and catalytic stabilities of these materials often suffer during the harsh conditions encountered in important reactions, such as the oxygen reduction reaction (ORR)3–5. Here, we demonstrate that atomically thin Pt shells stabilize titanium tungsten carbide cores, even at highly oxidizing potentials. In situ, time-resolved experiments showed how the Pt coating protects the normally labile core against oxidation and dissolution, and detailed microscopy studies revealed the dynamics of partially and fully coated core–shell nanoparticles during potential cycling. Particles with complete Pt coverage precisely maintained their core–shell structure and atomic composition during accelerated electrochemical ageing studies consisting of over 10,000 potential cycles. The exceptional durability of fully coated materials highlights the potential of core–shell architectures using earth-abundant transition metal carbide (TMC) and nitride (TMN) cores for future catalytic applications.

The self-assembly of nanoparticles into highly ordered crystals is largely influenced by variations in the size and shape of the constituent particles, with crystallization generally not observed if their polydispersity is too large. Here, we report on size selectivity in the self-assembly of rounded cubic maghemite nanoparticles into three-dimensional mesocrystals. Different X-ray scattering techniques are used to study and compare a nanoparticle dispersion that is used later for self-assembly, an ensemble of mesocrystals grown on a substrate, as well as an individual mesocrystal. The individual μm-sized mesocrystal is isolated using a focused-ion-beam-based technique and investigated by the diffraction of a micro-focused X-ray beam. Structural analysis reveals that individual mesocrystals have a drastically smaller size dispersity of nanoparticles than that in the initial dispersion, implying very strong size selectivity during self-assembly. The small size dispersity of the nanoparticles within individual mesocrystals is accompanied by a very narrow lattice parameter distribution. In contrast, the lattice parameter distribution within all mesocrystals of an ensemble is about four times wider than that of individual mesocrystals, indicating significant size fractionalization between mesocrystals during self-assembly. The small size dispersity within each mesocrystal has important implications for their physical properties.

Platinum is the most active anode and cathode catalyst in next-generation fuel cells using methanol as liquid source of hydrogen. Its catalytic activity can be significantly improved by alloying with 3d metals, although a precise tuning of its surface architecture is still required. Herein, we report the design of a highly active low-temperature (below 0 °C) methanol dehydrogenation anode catalyst with reduced CO poisoning based on ultralow amount of precisely defined PtxNi1–x(x= 0 to 1) bimetallic clusters (BCs) deposited on inert flat oxides by cluster beam deposition. These BCs feature clear composition-dependent atomic arrangements and electronic structures stemming from their nucleation mechanism, which are responsible for a volcano-type activity trend peaking at the Pt0.7Ni0.3composition. Our calculations reveal that at this composition, a cluster skin of Pt atoms with d-band centers downshifted by subsurface Ni atoms weakens the CO interaction that in turn triggers a significant increase in the methanol dehydrogenation activity.

Magnetite (Fe3O4) nanoaggregates with a flower-like morphology are considered promising materials in the field of magnetically induced hyperthermia in cancer therapy due to their good heating efficiency at low applied alternating magnetic fields. Although the structure and the magnetic state of such flower-like aggregates have been investigated previously, the mechanism that leads to the hierarchical morphology is still poorly understood. Here, we study the formation mechanism of Fe3O4aggregates synthesized through the partial oxidation of ferrous hydroxide in the presence of poly(acrylic acid) by using cryogenic electron microscopy. The aggregates are formed through a multistep process involving first the conversion of ferrous hydroxide precursors in ∼5 nm primary particles that aggregate into ∼10 nm primary Fe3O4crystals that finally arrange into the secondary mesocrystal structure. High-resolution electron tomography is used to show that the Fe3O4mesocrystals are composed of ∼10 nm subunits, often showing a uniform crystallographic orientation resulting in single-crystal-like diffraction patterns. Furthermore, electron holography reveals that mesocrystals have a single magnetic domain despite polymeric interfaces between subunits being present throughout the mesocrystal. Our findings could be used to design materials with specific properties by modulating the morphology and/or magnetic state that is suitable for biomedical application.

We use off-axis electron holography to measure the electrostatic charge density distributions on graphene-based nanogap devices that have thicknesses of between 1 and 10 monolayers and separations of between 8 and 58 nm with a precision of better than a single unit charge. Our experimental measurements, which are compared with finite element simulations, show that wider graphene tips, which have thicknesses of a single monolayer at their ends, exhibit charge accumulation along their edges. The results are relevant for both fundamental research on graphene electrostatics and applications of graphene nanogaps to single nucleotide detection in DNA sequencing, single molecule electronics, plasmonic antennae, and cold field emission sources.

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron–hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials’ properties.

Increasing the catalyst’s stability and activity are one of the main quests in catalysis. Tailoring crystal surfaces to a specific reaction has demonstrated to be a very effective way in increasing the catalyst’s specific activity. Shape controlled nanoparticles with specific crystal facets are usually grown kinetically and are highly susceptible to morphological changes during the reaction due to agglomeration, metal dissolution, or Ostwald ripening. A strong interaction of the catalytic material to the support is thus crucial for successful stabilization. Taken both points into account, a general catalyst design is proposed, combining the enhanced activity of shape-controlled nanoparticles with a pore-confinement approach for high stability. Hollow graphitic spheres with narrow and uniform bimodal mesopores serve as model system and were used as support material. As catalyst, different kinds of particles, such as pure platinum (Pt), platinum/nickel (Pt3Ni) and Pt3Ni doped with molybdenum (Pt3Ni–Mo), have exemplarily been synthesized. The advantages, limits and challenges of the proposed concept are discussed and elaborated by means of time-resolved, inand ex situmeasurements. It will be shown that during catalysis, the potential boundaries are crucial especially for the proposed catalyst design, resulting in either retention of the initial activity or drastic loss in shape, size and elemental composition. The synthesis and catalyst design can be adapted to a wide range of catalytic reactions where stabilization of shape-controlled particles is a focus.

Octahedral faceted nanoparticles are highly attractive fuel cell catalysts as a result of their activity for the oxygen reduction reaction (ORR). However, their surface compositional and morphological stability currently limits their long-term performance in real membrane electrode assemblies. Here, we perform in situheating of compositionally segregated PtNi1.5octahedral nanoparticles inside a transmission electron microscope, in order to study their compositional and morphological changes. The starting PtNi1.5octahedra have Pt-rich edges and concave Ni-rich {111} facets. We reveal a morphological evolution sequence, which involves transformation from concave octahedra to particles with atomically flat {100} and {111} facets, ideally representing truncated octahedra or cuboctahedra. The flat {100} and {111} facets are thought to comprise a thin Pt layer with a Ni-rich subsurface, which may boost catalytic activity. However, the transformation to truncated octahedra/cuboctahedra also decreases the area of the highly active {111} facets. The morphological and surface compositional evolution, therefore, results in a compromise between catalytic activity and morphological stability. Our findings are important for the design of more stable faceted PtNi nanoparticles with high activities for the ORR.

In order to obtain a fundamental understanding of the interplay between charge, spin, orbital and lattice degrees of freedom in magnetic materials and to predict and control their physical properties1–3, experimental techniques are required that are capable of accessing local magnetic information with atomic-scale spatial resolution. Here, we show that a combination of electron energy-loss magnetic chiral dichroism4and chromatic-aberration-corrected transmission electron microscopy, which reduces the focal spread of inelastically scattered electrons by orders of magnitude when compared with the use of spherical aberration correction alone, can achieve atomic-scale imaging of magnetic circular dichroism and provide element-selective orbital and spin magnetic moments atomic plane by atomic plane. This unique capability, which we demonstrate for Sr2FeMoO6, opens the door to local atomic-level studies of spin configurations in a multitude of materials that exhibit different types of magnetic coupling, thereby contributing to a detailed understanding of the physical origins of magnetic properties of materials at the highest spatial resolution.

We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO3/La0.7Sr0.3MnO3interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.

In the presence of disorder, superconductivity exhibits short-range characteristics linked to localized Cooper pairs which are responsible for anomalous phase transitions and the emergence of quantum states such as the bosonic insulating state. Complementary to well-studied homogeneously disordered superconductors, superconductor-normal hybrid arrays provide tunable realizations of the degree of granular disorder for studying anomalous quantum phase transitions. Here, we investigate the superconductor–bosonic dirty metal transition in disordered nanodiamond arrays as a function of the dispersion of intergrain spacing, which ranges from angstroms to micrometers. By monitoring the evolved superconducting gaps and diminished coherence peaks in the single-quasiparticle density of states, we link the destruction of the superconducting state and the emergence of bosonic dirty metallic state to breaking of the global phase coherence and persistence of the localized Cooper pairs. The observed resistive bosonic phase transitions are well modeled using a series–parallel circuit in the framework of bosonic confinement and coherence.

This work investigates the effect of copper substitution on the magnetic properties of SmCo5thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experimental and theoretical studies show that Cu substitution at the Co3gsites not only stabilizes the formation of the SmCo5structure but also enhances magnetic anisotropy and coercivity. Density functional theory calculations indicate that Sm(Co4Cu3g)5possesses a higher single-ion anisotropy as compared to pure SmCo5. In addition, X-ray magnetic circular dichroism reveals that Cu substitution causes an increasing decoupling of the Sm 4f and Co 3d moments. Scanning transmission electron microscopy confirms predominantly SmCo5phase formation and reveals nanoscale inhomogeneities in the Cu and Co distribution. Our study based on thin film model systems and advanced characterization as well as modeling reveals novel aspects of the complex interplay of intrinsic and extrinsic contributions to magnetic hysteresis in rare-earth-based magnets, i.e., the combination of increased intrinsic anisotropy due to Cu substitution and the extrinsic effect of inhomogeneous elemental distribution of Cu and Co.

The hallmark of spintronics has been the ability of spin–orbit interactions to convert a charge current into a spin current and vice versa, mainly in the bulk of heavy metal thin films. Here, we demonstrate how a light metal interface profoundly affects both the nature of spin–orbit torques and its efficiency in terms of damping-like (HDL) and field-like (HFL) effective fields in ultrathin Co films. We measure unexpectedly HFL/HDLratios much larger than 1 by inserting a nanometer-thin Al metallic layer in Pt|Co|Al|Pt as compared to a similar stacking, including Cu as a reference. From our modeling, these results evidence the existence of large Rashba interaction at the Co|Al interface generating a giant HFL, which is not expected from a metallic interface. The occurrence of such enhanced torques from an interfacial origin is further validated by demonstrating current-induced magnetization reversal showing a significant decrease of the critical current for switching.

Catalytic and optical properties can be coupled by combining different metals into nanoscale architectures in which both the shape and the composition provide fine-tuning of functionality. Here, discrete, small Pt nanoparticles (diameter = 3–6 nm) were grown in linear arrays on Au nanoprisms, and the resulting structures are shown to retain strong localized surface plasmon resonances. Multidimensional electron microscopy and spectroscopy techniques (energy-dispersive X-ray spectroscopy, electron tomography, and electron energy-loss spectroscopy) were used to unravel their local composition, three-dimensional morphology, growth patterns, and optical properties. The composition and tomographic analyses disclose otherwise ambiguous details of the Pt-decorated Au nanoprisms, revealing that both pseudospherical protrusions and dendritic Pt nanoparticles grow on all faces of the nanoprisms (the faceted or occasionally twisted morphologies of which are also revealed), and shed light on the alignment of the Pt nanoparticles. The electron energy-loss spectroscopy investigations show that the Au nanoprisms support multiple localized surface plasmon resonances despite the presence of pendant Pt nanoparticles. The plasmonic fields at the surface of the nanoprisms indeed extend into the Pt nanoparticles, opening possibilities for combined optical and catalytic applications. These insights pave the way toward comprehensive nanoengineering of multifunctional bimetallic nanostructures, with potential applications in plasmon-enhanced catalysis and in situ monitoring of chemical processes via surface-enhanced spectroscopy.

Carrier collection in silicon heterojunction (SHJ) solar cells is usually achieved by doped amorphous silicon layers of a few nanometers, deposited at opposite sides of the crystalline silicon wafer. These layers are often defect-rich, resulting in modest doping efficiencies, parasitic optical absorption when applied at the front of solar cells, and high contact resistivities with the adjacent transparent electrodes. Their substitution by equally thin doped nanocrystalline silicon layers has often been argued to resolve these drawbacks. However, low-temperature deposition of highly crystalline doped layers of such thickness on amorphous surfaces demands sophisticated deposition engineering. In this paper, we review and discuss different strategies to facilitate the nucleation of nanocrystalline silicon layers and assess their compatibility with SHJ solar cell fabrication. We also implement the obtained layers into devices, yielding solar cells with fill factor values of over 79% and efficiencies of over 21.1%, clearly underlining the promise this material holds for SHJ solar cell applications.

The effect of interfacial pH during the surface cleaning of shape-selected PtNi nanoparticles was investigated. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) elemental mapping techniques were used to analyze the morphology and composition of the particles at the nanoscale. The particles show similar atomic compositions for both treated samples but different elemental distribution on the surface of the nanooctahedra. X-ray photoelectron spectroscopy (XPS) analysis confirmed different surface compositions and the presence of different oxidation states species at the outer part of the nanoparticles. In addition, we compare characteristic voltammetric profiles of these nanocatalysts when immersed in three different aqueous supporting electrolytes (H2SO4, HClO4and NaOH). The behavior of the bimetallic nanoparticles towards adsorbed CO oxidation has been analyzed and compared with that observed after surface disordering of the same catalysts. The electrocatalytic activity of these nanoparticles has been also tested for the electroreduction of oxygen showing high specific and mass activity and better catalytic performance than pure Pt shaped nanoparticles. The different treatments applied to the surface of the nanocatalysts have led to remarkably different catalytic responses, pointing out the outstanding importance of the control of the surface of the alloyed shape-selected nanoparticles after their synthesis and before their use as electrocatalysts.

The high CO2selectivity of ZnPd/ZnO, an outstanding catalyst in methanol steam reforming, was recently attributed to the interaction between small ZnO patches and ZnPd particles. Yet, the detailed microstructure of this catalytic system and the formation mechanism of the ZnO patches inducing the high catalytic selectivity are unknown. In this work, we uncover the formation mechanism of ZnO patches using aberration-corrected electron microscopy, electron energy loss spectroscopy, X-ray spectroscopy, and in situheating experiments. We show that Zn-rich regions in chemically inhomogeneous ZnPd particles, penetrating the particle surface, are capped by ZnO. It is demonstrated that the ZnO patches form by direct oxidation of the particles rather than by transport from the ZnO substrate, thus ruling out a classical strong metal–support interaction (SMSI).

The thermomagnetic behavior of remanence‐induced magnetite (Fe3O4) particles in the pseudo‐single‐domain (PSD) size range (~0.1–10 µm), which dominate the magnetic signature of many rock lithologies, is investigated using off‐axis electron holography. Construction of magnetic induction maps allowed for the visualization of the vortex domain state in an individual Fe3O4grain (~200 nm in diameter) as a function of temperature. Acquisition of a series of electron holograms at 100°C intervals during in situ heating up to 700°C demonstrates the vortex state of the Fe3O4grain, in this instance, remains thermally stable close to its unblocking temperature and exhibits a similar in‐plane remanent state upon cooling; i.e., the particle is effectively behaving like a uniaxial single‐domain particle to temperatures near TC. Such particles are thought to be robust magnetic recorders. It is suggested that evidence for PSD behavior should therefore not preclude paleomagnetic investigation. Thermomagnetic stability of nonideal magnetite particlesVisualization of stability of vortex domain state with heatingParticles behave like uniaxial SD particles near to Curie temperature

An appropriate way of realizing property nanoengineering in complex quaternary chalcogenide nanocrystals is presented for Cu2CdxSnSey(CCTSe) polypods. The pivotal role of the polarity in determining morphology, growth, and the polytypic branching mechanism is demonstrated. Polarity is considered to be responsible for the formation of an initial seed that takes the form of a tetrahedron with four cation-polar facets. Size and shape confinement of the intermediate pentatetrahedral seed is also attributed to polarity, as their external facets are anion-polar. The final polypod extensions also branch out as a result of a cation-polarity-driven mechanism. Aberration-corrected scanning transmission electron microscopy is used to identify stannite cation ordering, while ab initiostudies are used to show the influence of cation ordering/distortion, stoichiometry, and polytypic structural change on the electronic band structure.

Over the past decade, advances in the colloidal syntheses of octahedral-shaped Pt–Ni alloy nanocatalysts for use in fuel cell cathodes have raised our atomic-scale control of particle morphology and surface composition, which, in turn, helped raise their catalytic activity far above that of benchmark Pt catalysts. Future fuel cell deployment in heavy-duty vehicles caused the scientific priorities to shift from alloy particle activity to stability. Larger particles generally offer enhanced thermodynamic stability, yet synthetic approaches toward larger octahedral Pt–Ni alloy nanoparticles have remained elusive. In this study, we show how a simple manipulation of solvothermal synthesis reaction kinetics involving depressurization of the gas phase at different stages of the reaction allows tuning the size of the resulting octahedral nanocatalysts to previously unachieved scales. We then link the underlying mechanism of our approach to the classical “LaMer” model of nucleation and growth. We focus on large, annealed Mo-doped Pt–Ni octahedra and investigate their synthesis, post-synthesis treatments, and elemental distribution using advanced electron microscopy. We evaluate the electrocatalytic ORR performance and stability and succeed to obtain a deeper understanding of the enhanced stability of a new class of relatively large, active, and long-lived Mo-doped Pt–Ni octahedral catalysts for the cathode of PEMFCs.

We show that by operating a scanning transmission electron microscope (STEM) with a 0.1 nm 300 kV electron beam, one can sculpt free-standing monolayer graphene with close-to-atomic precision at 600 °C. The same electron beam that is used for destructive sculpting can be used to image the sculpted monolayer graphene nondestructively. For imaging, a scanning dwell time is used that is about 1000 times shorter than for the sculpting. This approach allows for instantaneous switching between sculpting and imaging and thus fine-tuning the shape of the sculpted lattice. Furthermore, the sculpting process can be automated using a script. In this way, free-standing monolayer graphene can be controllably sculpted into patterns that are predefined in position, size, and orientation while maintaining defect-free crystallinity of the adjacent lattice. The sculpting and imaging processes can be fully computer-controlled to fabricate complex assemblies of ribbons or other shapes.

We report that the shape, orientation, edge geometry, and thickness of chemical vapor deposition graphene domains can be controlled by the crystallographic orientations of Cu substrates. Under low-pressure conditions, single-layer graphene domains align with zigzag edges parallel to a single ⟨101⟩ direction on Cu(111) and Cu(101), while bilayer domains align to two directions on Cu(001). Under atmospheric pressure conditions, hexagonal domains also preferentially align. This discovery can be exploited to generate high-quality, tailored graphene with controlled domain thickness, orientations, edge geometries, and grain boundaries.

Cable‐to‐tube transition: Core–shell sulfide–carbon nanocables have been oxidised to hollow polycrystalline nanowires and further assembled to crystallites inside a transmission electron microscope. The full experimental life cycle (see graphic) was followed for a single nanostructure; this provided extended structural and chemical information that supports a nanocable‐to‐nanotube transition and the presence of the Kirkendall effect.

When suspended in solution, clay platelets coated with nanometer-scale magnetite particles behave as magnetorheologic fluids that are important to a variety of industrial applications. Such dual-phase assemblages are also similar to natural aggregates that record the direction and intensity of the Earth’s magnetic field in lake and marine depositional environments. This study characterizes the mineralogical structure and magnetic behavior of montmorillonite platelets coated with aggregates of nanometerscale magnetite crystals. The distribution of magnetite crystal sizes in three different clay-magnetite assemblages was directly measured using conventional transmission electron microscopy and agrees within error with estimates derived from magnetic hysteresis measurements. Magnetic hysteresis and low field susceptibility measurements combined with electron holography experiments indicate that all three samples behave superparamagnetically at room temperature, and show increasing levels of single domain behavior as the samples are cooled to liquid nitrogen temperatures. At such low temperatures, magnetostatic interactions are observed to partially stabilize otherwise superparamagnetic grains in flux closure structures.

Lorentz transmission electron microscopy has been used to study fine-scale exsolution microstructures in ilmenite-hematite, as part of a wider investigation of the lamellar magnetism hypothesis. Pronounced asymmetric contrast is visible in out-of-focus Lorentz images of ilmenite lamellae in hematite. The likelihood that lamellar magnetism may be responsible for this contrast is assessed using simulations that incorporate interfacial magnetic moments on the (001) basal planes of hematite and ilmenite. The simulations suggest qualitatively that the asymmetric contrast is magnetic in origin. However, the magnitude of the experimental contrast is higher than that in the simulations, suggesting that an alternative origin for the observed asymmetry cannot be ruled out. Electron tomography was used to show that the lamellae have lens-like shapes and that (001) planes make up a significant proportion of the interfacial surface that they share with their host.

The need to determine the electrical properties of semiconductor junctions with high spatial resolution is as pressing now as ever. One technique that offers the possibility of quantitative high‐resolution mapping of two‐ and three‐dimensional electrostatic potential distributions is off‐axis electron holography. In this study, we review some of the issues associated with interpreting phase shifts measured using off‐axis electron holography, and we describe how a quantitative determination of the dopant‐related electrostatic potential can be achieved for device structures. Issues that include the presence of surface “dead” layers, external electrostatic fringing fields, variations in specimen thickness and dynamical diffraction are discussed, and their impact on the quantification of results obtained using off‐axis electron holography is examined. SCANNING 30: 299–309, 2008. © 2008 Wiley Periodicals, Inc.

Magnetotactic bacteria comprise several aquatic species that orient and migrate along geomagnetic field lines. This behavior is based on the presence of intracellular ferrimagnetic grains of the minerals magnetite (Fe3O4) or greigite (Fe3S4). Whereas the structural and magnetic properties of magnetite magnetosomes have been studied extensively, the properties of greigite magnetosomes are less well known. Here we present a study of the magnetic microstructures, chemical compositions, and threedimensional morphologies and positions of Fe-sulfide crystals in air-dried cells of magnetotactic bacteria. Data were obtained using several transmission electron microscopy techniques that include electron holography, energy-filtered imaging, electron tomography, selected-area electron diffraction, and high-resolution imaging. The studied rod-shaped cells typically contain multiple chains of greigite magnetosomes that have random shapes and orientations. Many of the greigite crystals appear to be only weakly magnetic, because the direction of their magnetic induction is almost parallel to the electron beam. Nevertheless, the magnetosomes collectively comprise a permanent magnetic dipole moment that is sufficient for magnetotaxis. One of the cells, which is imaged at the point of dividing, contains multiple chains of both equidimensional Fe-sulfide and elongated Fe-oxide crystals. The equidimensional and elongated crystals have magnetic properties that are consistent with those of greigite and magnetite, respectively. These results can be useful for obtaining a better understanding of the function of magnetotaxis in sulfide-producing cells, and they have implications for the interpretation of the paleomagnetic signals of greigite-bearing sedimentary rocks.

Off-axis electron holography is used to measure electrostatic potential profiles across a silicon p-n junction, which has been prepared for examination in the transmission electron microscope (TEM) in two different specimen geometries using focused ion beam (FIB) milling. Results are obtained both from a conventional unbiased FIB-milled sample and using a novel sample geometry that allows a reverse bias to be applied to an FIB-milled sample in situ in the TEM. Computer simulations are fitted to the results to assess the effect of TEM specimen preparation on the charge density and the electrostatic potential in the thin sample.

Fe, Co, and Ni magnetic nanoparticles have been characterized using energy-selected imaging in a high-resolution transmission electron microscope. The samples comprised Fe/FeOx and Co/CoOx nanoparticles synthesized by inert gas evaporation and a Ni/C nano-composite prepared by a sonochemical method. All of the particles examined were found to be between 5 and 30 nm in size, with the Fe and Co crystals coated in 5–10 nm of metal oxide layer and the Ni metallic crystallites embedded in an amorphous carbon spherical matrix.