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Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with subpicometer precision and the ability to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector-the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space data sets contain the full spatially resolved lattice information on the sample. By using this technique on tungsten disulfide (WS2) and tungsten diselenide (WSe2) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multimicron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally epitaxial structures. [ABSTRACT FROM AUTHOR]

Diverse topological defects arise in hexagonal manganites, such as ferroelectric vortices, as well as neutral and charged domain walls. The topological defects are intriguing because their low symmetry enables unusual couplings between structural, charge, and spin degrees of freedom, holding great potential for novel types of functional 2D and 1D systems. Despite the considerable advances in analyzing the different topological defects in hexagonal manganites, the understanding of their key intrinsic properties is still rather limited and disconnected. In particular, a rapidly increasing number of structural variants is reported without clarifying their relation, leading to a zoo of seemingly unrelated topological textures. Here, we combine picometer-precise scanning-transmission-electron microscopy with Landau theory modeling to clarify the inner structure of topological defects in Er1-xZrxMnO3. By performing a comprehensive parametrization of the inner atomic defect structure, we demonstrate that one primary length scale drives the morphology of both vortices and domain walls. Our findings lead to a unifying general picture of this type of structural topological defects. We further derive novel fundamental and universal properties, such as unusual bound-charge distributions and electrostatics at the ferroelectric vortex cores with emergent U(1) symmetry. [ABSTRACT FROM AUTHOR]

Scanning electron microscopes (SEMs) require a high vacuum environment to generate and shape an electron beam for imaging; however, the vacuum conditions greatly limit the nature of specimens that can be examined. From a purely scattering physics perspective, it is not necessary to place the specimen inside the vacuum chamber-the mean free paths (MFPs) for electron scattering in air at typical SEM beam voltages are 50-100 μm. This is the idea behind the airSEM, which removes the specimen vacuum chamber from the SEM and places the sample in air. The thickness of the gas layer is less than a MFP from an electron-transparent window to preserve the shape and resolution of the incident beam, resulting in comparable imaging quality to an all-vacuum SEM. Present silicon nitride windows scatter far more strongly than the air gap and are currently the contrast and resolution limiting factor in the airSEM. Graphene windows have been used previously to wrap or seal samples in vacuum for imaging. Here we demonstrate the use of a robust bilayer graphene window for sealing the electron optics from the room environment, providing an electron transparent window with only a 2% drop in contrast. There is a 5-fold-increase in signal/noise ratio for imaging compared to multi-MFP-thick silicon nitride windows, enabling high contrast in backscattered, transmission, and surface imaging modes for the new airSEM geometry. [ABSTRACT FROM AUTHOR]

Improving the catalytic activityof Pt-based bimetallic nanoparticles is a key challenge in the applicationof proton-exchange membrane fuel cells. Electrochemical dealloyingrepresents a powerful approach for tuning the surface structure andmorphology of these catalyst nanoparticles. We present a comprehensivestudy of using electrochemical dealloying methods to control the morphologyof ordered Cu3Pt/C intermetallic nanoparticles, which coulddramatically affect their electrocatalytic activity for the oxygenreduction reaction (ORR). Depending on the electrochemical dealloyingconditions, the nanoparticles with Pt-rich core–shell or porousstructures were formed. We further demonstrate that the core–shelland porous morphologies can be combined to achieve the highest ORRactivity. This strategy provides new guidelines for optimizing nanoparticlessynthesis and improving electrocatalytic activity. [ABSTRACT FROM AUTHOR]

Wedemonstrate dual interface formation in nanocrystals (NCs) throughcation exchange, creating epitaxial heterostructures within sphericalNCs. The thickness of the inner-disk layer can be tuned to form two-dimensional(2D), single atomic layers (<1 nm). During the cation exchangereaction from copper sulfide to zinc sulfide (ZnS), we observe a solid–solidphase transformation of the copper sulfide phase in heterostructuredNCs. As the cation exchange reaction is initiated, Cu ions replacedby Zn ions at the interfaces are accommodated in intrinsic Cu vacancysites present in the initial roxbyite (Cu1.81S) phase ofcopper sulfide, inducing a full phase transition to djurleite (Cu1.94S)/low chalcocite (Cu2S), a more thermodynamicallystable phase than roxbyite. As the reaction proceeds and reduces thesize of the copper sulfide layer, the epitaxial strain at the interfacesbetween copper sulfide and ZnS increases and is maximized for a coppersulfide disk ∼5 nm thick. To minimize this strain energy, asecond phase transformation occurs back to the roxbyite phase, whichshares a similar sulfur sublattice to wurtzite ZnS. The observationof a solid–solid phase transformation in our unique heterostructuredNCs provides a new pathway to control desired phases and an insightinto the influence of cation exchange on nanoscale phase transitionsin heterostructured materials. [ABSTRACT FROM AUTHOR]

A major challenge in the developmentof new battery materials isunderstanding their fundamental mechanisms of operation and degradation.Their microscopically inhomogeneous nature calls for characterizationtools that provide operando and localized information from individualgrains and particles. Here, we describe an approach that enables imagingthe nanoscale distribution of ions during electrochemical chargingof a battery in a transmission electron microscope liquid flow cell.We use valence energy-loss spectroscopy to track both solvated andintercalated ions, with electronic structure fingerprints of the solvatedions identified using an ab initio nonlinear response theory. Equippedwith the new electrochemical cell holder, nanoscale spectroscopy andtheory, we have been able to determine the lithiation state of a LiFePO4electrode and surrounding aqueous electrolyte in real timewith nanoscale resolution during electrochemical charge and discharge.We follow lithium transfer between electrode and electrolyte and imagecharging dynamics in the cathode. We observe competing delithiationmechanisms such as core–shell and anisotropic growth occurringin parallel for different particles under the same conditions. Thistechnique represents a general approach for the operando nanoscaleimaging of electrochemically active ions in the electrode and electrolytein a wide range of electrical energy storage systems. [ABSTRACT FROM AUTHOR]

Theability to control the stacking structure in layered materialscould provide an exciting approach to tuning their optical and electronicproperties. Because of the lower symmetry of each constituent monolayer,hexagonal boron nitride (h-BN) allows more structuralvariations in multiple layers than graphene; however, the structure–propertyrelationships in this system remain largely unexplored. Here, we reporta strong correlation between the interlayer stacking structures andoptical and topological properties in chemically grown h-BN bilayers, measured mainly by using dark-field transmission electronmicroscopy (DF-TEM) and optical second harmonic generation (SHG) mapping.Our data show that there exist two distinct h-BNbilayer structures with different interlayer symmetries that giverise to a distinct difference in their SHG intensities. In particular,the SHG signal in h-BN bilayers is observed onlyfor structures with broken inversion symmetry, with an intensity muchlarger than that of single layer h-BN. In addition,our DF-TEM data identify the formation of interlayer topological defectsin h-BN bilayers, likely induced by local strain,whose properties are determined by the interlayer symmetry and thedifferent interlayer potential landscapes. [ABSTRACT FROM AUTHOR]

A promising electrocatalyst prototype of low Pt mole fraction, intermetallic nanoparticles of Cu3Pt, has been prepared using a simple impregnation-reduction method, followed by a post heat-treatment. Two dealloying methods (electrochemical and chemical) were implemented to control the atomic-level morphology and improve performance for the oxygen reduction reaction (ORR). The morphology and elemental composition of the dealloyed nanoparticles were characterized at angstrom resolution using an aberration-corrected scanning transmission electron microscope equipped with an electron energy loss spectrometer. We found that the electrochemical dealloying method led to the formation of a thin Pt skin of ca. 1 nm in thickness with an ordered Cu3Pt core structure, while chemical leaching gave rise to a "spongy" structure with no ordered structure being preserved. A three-dimensional tomographic reconstruction indicated that numerous voids were formed in the chemically dealloyed nanoparticles. Both dealloying methods yielded enhanced specific and mass activities toward the ORR and higher stability relative to Pt/C. The spongy nanoparticles exhibited better mass activity with a slightly lower specific activity than the electrochemically dealloyed nanoparticles after 50 potential cycles. In both cases, the mass activity was still enhanced after 5000 potential cycles. [ABSTRACT FROM AUTHOR]

We present an electron tomography method that allows for the identification of hundreds of electrocatalyst nanoparticles with one-to-one correspondence before and after electrochemical aging. This method allows us to track, in three-dimensions, the trajectories and morphologies of each Pt−Co nanocatalyst on a fuel cell carbon support. In conjunction with the use of atomic-scale electron energy loss spectroscopic imaging, our experiment enables the correlation of performance degradation of the catalyst with changes in particle/interparticle morphologies, particle−support interactions, and the near-surface chemical composition. We found that aging of the catalysts under normal fuel cell operating conditions (potential scans from +0.6 to +1.0 V for 30 000 cycles) gives rise to coarsening of the nanoparticles, mainly through coalescence, which in turn leads to the loss of performance. The observed coalescence events were found to be the result of nanoparticle migration on the carbon support during potential cycling. This method provides detailed insights into how nanocatalyst degradation occurs in proton exchange membrane fuel cells (PEMFCs) and suggests that minimization of particle movement can potentially slow down the coarsening of the particles and the corresponding performance degradation. [ABSTRACT FROM AUTHOR]

While chemical vapor deposition (CVD) promises a scalablemethodto produce large-area graphene, CVD-grown graphene has heretoforeexhibited inferior electronic properties in comparison with exfoliatedsamples. Here we test the electrical transport properties of CVD-growngraphene in which two important sources of disorder, namely grainboundaries and processing-induced contamination, are substantiallyreduced. We grow CVD graphene with grain sizes up to 250 μmto abate grain boundaries, and we transfer graphene utilizing a novel,dry-transfer method to minimize chemical contamination. We fabricatedevices on both silicon dioxide and hexagonal boron nitride (h-BN)dielectrics to probe the effects of substrate-induced disorder. Onboth substrate types, the large-grain CVD graphene samples are comparablein quality to the best reported exfoliated samples, as determinedby low-temperature electrical transport and magnetotransport measurements.Small-grain samples exhibit much greater variation in quality andinferior performance by multiple measures, even in samples exhibitinghigh field-effect mobility. These results confirm the possibilityof achieving high-performance graphene devices based on a scalablesynthesis process. [ABSTRACT FROM AUTHOR]