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Highly ordered liquid crystalline (LC) phases have important potential for organic electronics. We studied the molecular alignment and domain structure in a columnar LC thin film with nanometer resolution during in situ heating using four-dimensional scanning transmission electron microscopy (4D STEM). The initial disordered vapor-deposited LC glass thin film rapidly ordered at its glass transition temperature into a hexagonal columnar phase with small (<10 nm), well-aligned, planar domains (columns oriented parallel to the surface). Upon further heating, the domains coarsen via bulk diffusion, then the film crystallizes, then finally transforms back to an LC phase at an even higher temperature. The LC phase at high temperature shows straight columns of molecules, which we attribute to structure inherited from the intermediate crystalline phase. Nanoscale 4D STEM offers direct insight into the mechanisms of domain reorganization, and intermediate crystallization is a potential approach to manipulate orientational order and texture at the nano- to mesoscale in LC thin films.

We describe a method for identifying and clustering diffraction vectors in four-dimensional (4-D) scanning transmission electron microscopy data to determine characteristic diffraction patterns from overlapping structures in projection. First, the data is convolved with a 4-D kernel, then diffraction vectors are identified and clustered using both density-based clustering and a metric that emphasizes rotational symmetries. The method works well for both crystalline and amorphous samples and in high- and low-dose experiments. A simulated dataset of overlapping aluminum nanocrystals provides performance metrics as a function of Poisson noise and the number of overlapping structures. Experimental data from an aluminum nanocrystal sample shows similar performance. For an amorphous Pd 77.5 Cu 6 Si 16.5 thin film, experiments measuring glassy structure show strong evidence of 4- and 6-fold symmetry structures. A significant background arises from the diffraction of overlapping structures. Quantifying this background helps to separate contributions from single, rotationally symmetric structures vs. apparent symmetries arising from overlapping structures in projection., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Remote and van der Waals epitaxy are promising approaches for synthesizing single crystalline membranes for flexible electronics and discovery of new properties via extreme strain; however, a fundamental challenge is that most materials do not wet the graphene surface. We develop a cold seed approach for synthesizing smooth intermetallic films on graphene that can be exfoliated to form few nanometer thick single crystalline membranes. Our seeded GdAuGe films have narrow X-ray rocking curve widths of 9-24 arc seconds, which is 2 orders of magnitude lower than their counterparts grown by typical high temperature methods, and have atomically sharp interfaces observed by transmission electron microscopy. Upon exfoliation and rippling, strain gradients in GdAuGe membranes induce an antiferromagnetic to ferri/ferromagnetic transition. Our smooth, ultrathin membranes provide a clean platform for discovering new flexomagnetic effects in quantum materials.

Electron correlation microscopy (ECM) characterizes local structural relaxation dynamics in fluctuating systems like supercooled liquids with nanometer spatial resolution. We have developed a new type of ECM technique that provides moderate resolution in momentum transfer or k space using five-dimensional scanning transmission electron microscopy. k-resolved ECM on a Pt 57.5 Cu 14.7 Ni 5. 3 P 22.5 metallic supercooled liquids measures rich spatial and momentum structure in the relaxation time data τ(r,k). Relaxation time maps τ(r) at each azimuthal k are independent samples of the material's underlying relaxation time distribution, and τ of radial k shows more complex behavior than the de Gennes narrowing observed in analogous X-ray experiments. We have determined the requirements for electron counts per k-pixel, number of k-pixels per speckle, and time sampling to obtain reliable k-resolved ECM data., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Two-dimensional (2D) magnets exhibit unique physical properties for potential applications in spintronics. To date, most 2D ferromagnets are obtained by mechanical exfoliation of bulk materials with van der Waals interlayer interactions, and the synthesis of single- or few-layer 2D ferromagnets with strong interlayer coupling remains experimentally challenging. Here, we report the epitaxial growth of 2D non-van der Waals ferromagnetic bilayer FeSb on SrTiO 3 (001) substrates stabilized by strong coupling to the substrate, which exhibits in-plane magnetic anisotropy and a Curie temperature above 390 K. In situ low-temperature scanning tunneling microscopy/spectroscopy and density-functional theory calculations further reveal that an Fe Kagome layer terminates the bilayer FeSb. Our results open a new avenue for further exploring emergent quantum phenomena from the interplay of ferromagnetism and topology for application in spintronics.

Electron counting can be performed algorithmically for monolithic active pixel sensor direct electron detectors to eliminate readout noise and Landau noise arising from the variability in the amount of deposited energy for each electron. Errors in existing counting algorithms include mistakenly counting a multielectron strike as a single electron event, and inaccurately locating the incident position of the electron due to lateral spread of deposited energy and dark noise. Here, we report a supervised deep learning (DL) approach based on Faster region-based convolutional neural network (R-CNN) to recognize single electron events at varying electron doses and voltages. The DL approach shows high accuracy according to the near-ideal modulation transfer function (MTF) and detector quantum efficiency for sparse images. It predicts, on average, 0.47 pixel deviation from the incident positions for 200 kV electrons versus 0.59 pixel using the conventional counting method. The DL approach also shows better robustness against coincidence loss as the electron dose increases, maintaining the MTF at half Nyquist frequency above 0.83 as the electron density increases to 0.06 e-/pixel. Thus, the DL model extends the advantages of counting analysis to higher dose rates than conventional methods., Competing Interests: Conflict of Interest The authors declare that they have no competing interest., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Microscopy Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180 K below the clearing point TLC-iso (and 50 K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

The information content of atomic-resolution scanning transmission electron microscopy (STEM) images can often be reduced to a handful of parameters describing each atomic column, chief among which is the column position. Neural networks (NNs) are high performance, computationally efficient methods to automatically locate atomic columns in images, which has led to a profusion of NN models and associated training datasets. We have developed a benchmark dataset of simulated and experimental STEM images and used it to evaluate the performance of two sets of recent NN models for atom location in STEM images. Both models exhibit high performance for images of varying quality from several different crystal lattices. However, there are important differences in performance as a function of image quality, and both models perform poorly for images outside the training data, such as interfaces with large difference in background intensity. Both the benchmark dataset and the models are available using the Foundry service for dissemination, discovery, and reuse of machine learning models., Competing Interests: Conflict of interest The authors declare that they have no competing interest., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Microscopy Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor-deposited and liquid-cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface-diffusion-mediated ordering during film deposition. These results demonstrate the ability of low-dose 4D STEM to characterize a mesoscale structure in a molecular glass system which may be relevant to organic electronics.

The high density of aluminum nanocrystals (>10 21  m -3 ) that develop during the primary crystallization in Al-based metallic glasses indicates a high nucleation rate (∼10 18  m -3  s -1 ). Several studies have been advanced to account for the primary crystallization behavior, but none have been developed to completely describe the reaction kinetics. Recently, structural analysis by fluctuation electron microscopy has demonstrated the presence of the Al-like medium range order (MRO) regions as a spatial heterogeneity in as-spun Al 88 Y 7 Fe 5 metallic glass that is representative for the class of Al-based amorphous alloys that develop Al nanocrystals during primary crystallization. From the structural characterization, an MRO seeded nucleation configuration is established, whereby the Al nanocrystals are catalyzed by the MRO core to decrease the nucleation barrier. The MRO seeded nucleation model and the kinetic data from the delay time (τ) measurement provide a full accounting of the evolution of the Al nanocrystal density (N v ) during the primary crystallization under isothermal annealing treatments. Moreover, the calculated values of the steady state nucleation rates (J ss ) predicted by the nucleation model agree with the experimental results. Moreover, the model satisfies constraints on the structural, thermodynamic, and kinetic parameters, such as the critical nucleus size, the interface energy, and the volume-free energy driving force that are essential for a fully self-consistent nucleation kinetics analysis. The nucleation kinetics model can be applied more broadly to materials that are characterized by the presence of spatial heterogeneities.

Densely spaced four-dimensional scanning transmission electron microscopy (4D STEM) analyzed using correlation symmetry coefficients enables large area mapping of approximate rotational symmetries in amorphous materials. Here, we report the effects of Poisson noise, limited electron counts, probe coherence, reciprocal space sampling, and the probe-sample interaction volume on 4D STEM symmetry mapping experiments. These results lead to an experiment parameter envelope for high quality, high confidence 4D STEM symmetry mapping. We also establish a direct link between the symmetry coefficients and approximate rotational symmetries of nearest-neighbor atomic clusters using electron diffraction simulations from atomic models of a metallic glass. Experiments on a Pd 77.5 Cu 6 Si 16.5 metallic glass thin film demonstrate the ability to image the types, sizes, volume fractions, and spatial correlations amongst local rotationally symmetry regions in the glass., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Surface diffusion is vastly faster than bulk diffusion in some glasses, but only moderately enhanced in others. We show that this variation is closely linked to bulk fragility, a common measure of how quickly dynamics is excited when a glass is heated to become a liquid. In fragile molecular glasses, surface diffusion can be a factor of 10^{8} faster than bulk diffusion at the glass transition temperature, while in the strong system SiO&#95;{2}, the enhancement is a factor of 10. Between these two extremes lie systems of intermediate fragility, including metallic glasses and amorphous selenium and silicon. This indicates that stronger liquids have greater resistance to dynamic excitation from bulk to surface and enables prediction of surface diffusion, surface crystallization, and formation of stable glasses by vapor deposition.

Angular symmetry in diffraction reflects rotational symmetry in the sample. We introduce the angular symmetry coefficient as a method to extract local symmetry information from electron nanodiffraction patterns of amorphous materials. Symmetry coefficients are the average of the angular autocorrelation function at the characteristic angles of a particular rotational symmetry. The symmetry coefficients avoid non-structural features arising from Fourier transformation and Friedel symmetry breakdown that affect the angular power spectrum approach to determining angular symmetries in amorphous nanodiffraction. Both methods require thin samples to avoid overlapping diffraction from clusters of atoms separated in the thickness of the sample, but symmetry coefficients are more forgiving. Electron nanodiffraction experiments on a Pd-based metallic glass sample demonstrate both potentially misleading information in angular power spectrum and the utility of symmetry coefficients., (Copyright © 2021. Published by Elsevier B.V.)

Recent machine learning models for bandgap prediction that explicitly encode the structure information to the model feature set significantly improve the model accuracy compared to both traditional machine learning and non-graph-based deep learning methods. The ongoing rapid growth of open-access bandgap databases can benefit such model construction not only by expanding their domain of applicability but also by requiring constant updating of the model. Here, we build a new state-of-the-art multi-fidelity graph network model for bandgap prediction of crystalline compounds from a large bandgap database of experimental and density functional theory (DFT) computed bandgaps with over 806 600 entries (1500 experimental, 775 700 low-fidelity DFT, and 29 400 high-fidelity DFT). The model predicts bandgaps with a 0.23 eV mean absolute error in cross validation for high-fidelity data, and including the mixed data from all different fidelities improves the prediction of the high-fidelity data. The prediction error is smaller for high-symmetry crystals than for low symmetry crystals. Our data are published through a new cloud-based computing environment, called the "Foundry," which supports easy creation and revision of standardized data structures and will enable cloud accessible containerized models, allowing for continuous model development and data accumulation in the future.

The dynamics near the surface of glasses can be much faster than in the bulk. We studied the surface dynamics of a Pt-based metallic glass using electron correlation microscopy with sub-nanometer resolution. Our studies show an ∼20 K suppression of the glass transition temperature at the surface. The enhancement in surface dynamics is suppressed by coating the metallic glass with a thin layer of amorphous carbon. Parallel molecular dynamics simulations on Ni 80 P 20 show a similar temperature suppression of the surface glass transition temperature and that the enhanced surface dynamics are arrested by a capping layer that chemically binds to the glass surface. Mobility in the near-surface region occurs via atomic caging and hopping, with a strong correlation between slow dynamics and high cage-breaking barriers and stringlike cooperative motion. Surface and bulk dynamics collapse together as a function of temperature rescaled by their respective glass transition temperatures.

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al 2 O 3 substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetism and magnetostriction) and strain gradients (flexomagnetism).

The enhancement of surface diffusion (D S ) over the bulk (D V ) in metallic glasses (MGs) is well documented and likely to strongly influence the properties of glasses grown by vapor deposition. Here, we use classical molecular dynamics (MD) simulations to identify different factors influencing the enhancement of surface diffusion in MGs. MGs have a simple atomic structure and belong to the category of moderately fragile glasses that undergo pronounced slowdown of bulk dynamics with cooling close to the glass transition temperature (T g ). We observe that D S exhibits a much more moderate slowdown compared to D V when approaching T g , and D S /D V at T g varies by two orders of magnitude among the MGs investigated. We demonstrate that both the surface energy and the fraction of missing bonds for surface atoms show good correlation to D S /D V , implying that the loss of nearest neighbors at the surface directly translates into higher mobility, unlike the behavior of network-bonded and hydrogen-bonded organic glasses. Fragility, a measure of the slowdown of bulk dynamics close to T g , also correlates to D S /D V , with more fragile systems having larger surface enhancement of mobility. The deviations observed in the fragility-D S /D V relationship are shown to be correlated to the extent of segregation or depletion of the mobile element at the surface. Finally, we explore the relationship between the diffusion pre-exponential factor (D 0 ) and the activation energy (Q) and compare it to a ln(D 0 )-Q correlation previously established for bulk glasses, demonstrating similar correlations from MD as in the experiments and that the surface and bulk have very similar ln(D 0 )-Q correlations.

Achieving sub-picometer precision measurements of atomic column positions in high-resolution scanning transmission electron microscope images using nonrigid registration (NRR) and averaging of image series requires careful optimization of experimental conditions and the parameters of the registration algorithm. On experimental data from SrTiO3 [100], sub-pm precision requires alignment of the sample to the zone axis to within 1 mrad tilt and sample drift of less than 1 nm/min. At fixed total electron dose for the series, precision in the fast scan direction improves with shorter pixel dwell time to the limit of our microscope hardware, but the best precision along the slow scan direction occurs at 6 μs/px dwell time. Within the NRR algorithm, the “smoothness factor” that penalizes large estimated shifts is the most important parameter for sub-pm precision, but in general, the precision of NRR images is robust over a wide range of parameters.

Tensor singular value decomposition (SVD) is a method to find a low-dimensional representation of data with meaningful structure in three or more dimensions. Tensor SVD has been applied to denoise atomic-resolution 4D scanning transmission electron microscopy (4D STEM) data. On data simulated from a SrTiO 3 [100] perfect crystal and a Si [110] edge dislocation, tensor SVD achieved an average peak signal-to-noise ratio (PSNR) of ~40 dB, which matches or exceeds the performance of other denoising methods, with processing times at least 100 times shorter. On experimental data from SrTiO 3 [100] and LiZnSb [112¯0]/GaSb [110] samples, tensor SVD denoises multiple GB 4D STEM data sets in ten minutes on a typical personal computer. Denoising with tensor SVD improves both convergent beam electron diffraction patterns and virtual-aperture annular dark field images., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Euclidean geometry is the fundamental mathematical framework of classical crystallography. Traditionally, layered materials are grown on flat substrates; growing Euclidean crystals on non-Euclidean surfaces has rarely been studied. We present a general model describing the growth of layered materials with screw-dislocation spirals on non-Euclidean surfaces and show that it leads to continuously twisted multilayer superstructures. This model is experimentally demonstrated by growing supertwisted spirals of tungsten disulfide (WS 2 ) and tungsten diselenide (WSe 2 ) draped over nanoparticles near the centers of spirals. Microscopic structural analysis shows that the crystal lattice twist is consistent with the geometric twist of the layers, leading to moiré superlattices between the atomic layers., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Radiation-induced segregation is well known in metals, but has been rarely studied in ceramics. We discover that radiation can induce notable segregation of one of the constituent elements to grain boundaries in a ceramic, despite the fact that the ceramic forms a line compound and therefore has a strong thermodynamic driving force to resist off-stoichiometry. Specifically, irradiation of silicon carbide at 300 °C leads to carbon enrichment near grain boundaries, whereas the enrichment diminishes for irradiation at 600 °C. The temperature dependence of this radiation-induced segregation is different from that shown in metallic systems. Using an ab initio informed rate theory model, we demonstrate that this difference is introduced by the unique defect energy landscapes present in the covalent system. Additionally, we discover that grain boundaries in unirradiated silicon carbide grown by chemical vapour deposition are intrinsically carbon-depleted. The inherent grain boundary chemistry and its evolution under radiation are both critical for understanding the many properties of ceramics associated with grain boundaries.

The emergence of memristive behavior in amorphous-crystalline 2D oxide heterostructures, which are synthesized by atomic layer deposition (ALD) of a few-nanometer amorphous Al 2 O 3 layers onto atomically thin single-crystalline ZnO nanosheets, is demonstrated. The conduction mechanism is identified based on classic oxygen vacancy conductive channels. ZnO nanosheets provide a 2D host for oxygen vacancies, while the amorphous Al 2 O 3 facilitates the generation and stabilization of the oxygen vacancies. The conduction mechanism in the high-resistance state follows Poole-Frenkel emission, and in the the low-resistance state is fitted by the Mott-Gurney law. From the slope of the fitting curve, the mobility in the low-resistance state is estimated to be ≈2400 cm 2 V -1 s -1 , which is the highest value reported in semiconductor oxides. When annealed at high temperature to eliminate oxygen vacancies, Al is doped into the ZnO nanosheet, and the memristive behavior disappears, further confirming the oxygen vacancies as being responsible for the memristive behavior. The 2D heterointerface offers opportunities for new design of high-performance memristor devices., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two types of convolutional neural network (CNN) models, a discrete classification network and a continuous regression network, were trained to determine local sample thickness from convergent beam diffraction (CBED) patterns of SrTiO 3 collected in a scanning transmission electron microscope (STEM) at atomic column resolution. Acquisition of atomic resolution CBED patterns for this purpose requires careful balancing of CBED feature size in pixels, acquisition speed, and detector dynamic range. The training datasets were derived from multislice simulations, which must be convolved with incoherent source broadening. Sample thicknesses were also determined using quantitative high-angle annular dark-field (HAADF) STEM images acquired simultaneously. The regression CNN performed well on sample thinner than 35 nm, with 70% of the CNN results within 1 nm of HAADF thickness, and 1.0 nm overall root mean square error between the two measurements. The classification CNN was trained for a thicknesses up to 100 nm and yielded 66% of CNN results within one classification increment of 2 nm of HAADF thickness. Our approach depends on methods from computer vision including transfer learning and image augmentation., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Room-temperature ferromagnetism in two-dimensional (2D) oxide materials is an intriguing phenomenon for spintronic applications. Here, we report significantly enhanced room-temperature ferromagnetism observed from ultrathin cerium oxide nanosheets hybridized with organic surfactant molecules. The hybrid nanosheets were synthesized by ionic layer epitaxy over a large area at the water-air interface. The nanosheets exhibited a saturation magnetization of 0.149 emu/g as their thickness reduced to 0.67 nm. This value was 5 times higher than that for CeO 2 thin films and more than 20 times higher than that for CeO 2 nanoparticles. The magnetization was attributed to the high concentration (15.5%) of oxygen vacancies stabilized by surfactant hybridization as well as electron transfer between organic and oxide layers. This work brings an effective strategy of introducing strong ferromagnetism to functional oxide materials, which leads to a promising route toward exploring new physical properties in 2D hybrid nanomaterials.

Cryptomelane-type α-MnO 2 has been demonstrated as a promising anode material for high-energy Li-ion batteries because of its high capacity and intriguing [2 × 2] tunnel structure. However, applications of MnO 2 electrode, especially at high current rates and mass active material loading, are limited by the poor mechanical stability, unstable solid electrolyte interphase layer, and low reversibility of conversion reactions. Here, we report a design of homogeneous core-shell MnO 2 nanowires (NWs) created by near-surface gradient Ti doping (Ti-MnO 2 NWs). Such a structurally coherent core-shell configuration endowed gradient volume expansion from the inner core to the outer shell, which could effectively release the stress of the NW lattice during cycling and avoid pulverization of the electrode. Moreover, the gradiently doped Ti is able to avoid the Mn metal coarsening, reducing the metal particle size and improving the reversibility of the conversion reaction. In this way, the Ti-MnO 2 NWs achieved both high reversible areal and volumetrical capacities (2.3 mA h cm -2 and 991.3 mA h cm -3 at 200 mA g -1 , respectively), a superior round-trip efficiency (Coulombic efficiency achieved above 99.5% after only 30 cycles), and a long lifetime (a high capacity of 742 mA h g -1 retained after 3000 cycle at 10 A g -1 ) at a high mass loading level of 3 mg cm -2 . In addition, the detailed conversion reaction mechanism was investigated through in situ transmission electron microscopy, which further evidenced that the unique homogeneous core-shell structure could largely suppress the separation of core and shell upon charging and discharging. This new NW configuration could benefit the design of other large-volume-change lithium battery anode materials.

The structures and properties of intermetallic phases are intimately connected to electron count; unfavorable electron counts can result in structural rearrangements or new electrical or magnetic behavior when no such transformation is available. The compound PtGa 2 appears to teeter on the border between these two scenarios with its two polymorphs: a cubic fluorite type form (c-PtGa 2 ) and a complex tetragonal superstructure (t-PtGa 2 ) whose Pt-Pt connectivity aligns with the 18- n electron counting rule. Here, we investigate the factors underlying this polymorphism. Electronic structure calculations show that the transition to t-PtGa 2 opens a pseudogap at the Fermi energy that can be traced to Pt-Pt isolobal bond formation, in line with the 18- n bonding scheme. Conversely, DFT-chemical pressure (CP) analysis reveals a network of positive local pressures along Pt-Ga contacts, requiring that the c-PtGa 2 to t-PtGa 2 transition follows tightly concerted atomic motions. Experimentally, a series of samples with varying Pt:Ga ratios were synthesized to examine the stability ranges of the polymorphs. Ga-poor samples yield exclusively the cubic polymorph over the full range of temperatures studied, which can be correlated to the enhanced incorporation of interstitial Pt atoms (at points of negative pressure in the CP scheme). At more Ga-rich compositions, however, t-PtGa 2 emerges as a low-temperature form. In these samples, the t-PtGa 2 to c-PtGa 2 transition is found to be reversible, but with a large hysteresis that in single crystals can exceed 100 °C. Together, the theoretical and experimental results indicate that the c-PtGa 2 phase is buttressed at its unfavorable electron count by the interstitial atoms and networks of positive CPs that restrict atomic motion, suggesting more general strategies for achieving exotic electronic structures in intermetallic materials.

Metastable intermediates represent a non-equilibrium state of matter that may impose profound impacts to materials properties beyond our understandings of monolithic and equilibrium systems. Here, we report a discovery of hidden metastable intermediates in amorphous TiO 2 thin films and their critical role in electrochemical damage. These intermediates have a non-bulk crystal-like structure and exhibit significantly higher electrical conductivity than both the amorphous and the crystalline phases. When these TiO 2 films are applied to protect Si photoelectrochemical (PEC) photoanodes, the intermediates can induce localized high electrical currents that largely accelerate the etching of the TiO 2 film and the Si electrode underneath. The intermediates can be effectively suppressed by raising their nucleation barrier via reducing the film thickness from 24 to 2.5 nm. The homogeneous amorphous TiO 2 -film-coated Si photoanodes achieved more than 500 h of PEC water oxidation at a steady photocurrent density of over 30 mA·cm -2 .

Development of novel magnetic materials is of interest for fundamental studies and applications such as spintronics, permanent magnetics, and sensors. We report on the first experimental realization of single element ferromagnetism, since Fe, Co, and Ni, in metastable tetragonal Ru, which has been predicted. Body-centered tetragonal Ru phase is realized by use of strain via seed layer engineering. X-ray diffraction and electron microscopy confirm the epitaxial mechanism to obtain tetragonal phase Ru. We observed a saturation magnetization of 148 and 160 emu cm -3 at room temperature and 10 K, respectively. Control samples ensure the ferromagnetism we report on is from tetragonal Ru and not from magnetic contamination. The effect of thickness on the magnetic properties is also studied, and it is observed that increasing thickness results in strain relaxation, and thus diluting the magnetization. Anomalous Hall measurements are used to confirm its ferromagnetic behavior.

Supercooled liquids exhibit spatial heterogeneity in the dynamics of their fluctuating atomic arrangements. The length and time scales of the heterogeneous dynamics are central to the glass transition and influence nucleation and growth of crystals from the liquid. Here, we report direct experimental visualization of the spatially heterogeneous dynamics as a function of temperature in the supercooled liquid state of a Pt-based metallic glass, using electron correlation microscopy with sub-nanometer resolution. An experimental four-point space-time correlation function demonstrates a growing dynamic correlation length, ξ, upon cooling of the liquid toward the glass transition temperature. ξ as a function of the relaxation time τ are in good agreement with Adam-Gibbs theory, inhomogeneous mode-coupling theory and random first-order transition theory of the glass transition. The same experiments demonstrate the existence of a nanometer thickness near-surface layer with order of magnitude shorter relaxation time than inside the bulk.

Electron correlation microscopy (ECM) is a way to measure structural relaxation times, τ, of liquids with nanometer-scale spatial resolution using coherent electron scattering equivalent of photon correlation spectroscopy. We have applied ECM with a 3.5nm diameter probe to Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 amorphous nanorods and Pd 40 Ni 40 P 20 bulk metallic glass (BMG) heated inside the STEM into the supercooled liquid region. These data demonstrate that the ECM technique is limited by the characteristics of the time series, which must be at least 40τ to obtain a well-converged correlation function g 2 (t), and the time per frame, which must be less than 0.1τ to obtain sufficient sampling. A high-speed direct electron camera enables fast acquisition and affords reliable g 2 (t) data even with low signal per frame., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Understanding the structure of ZnO surface reconstructions and their resultant properties is crucial to the rational design of ZnO-containing devices ranging from optoelectronics to catalysts. Here, we are motivated by recent experimental work that showed a new surface reconstruction containing Zn vacancies ordered in a Zn(3 × 3) pattern in the subsurface of (0001)-O-terminated ZnO. Reconstruction with Zn vacancies on (0001)-O is surprising and counterintuitive because Zn vacancies enhance the surface dipole rather than reduce it. In this work, we show using density functional theory (DFT) that subsurface Zn vacancies can form on (0001)-O when coupled with adsorption of surface H and are in fact stable under a wide range of common conditions. We also show that these vacancies have a significant ordering tendency and that Sb-doping-created subsurface inversion domain boundaries (IDBs) enhance the driving force of Zn vacancy alignment into large domains of the Zn(3 × 3) reconstruction.

Image registration and non-local Poisson principal component analysis (PCA) denoising improve the quality of characteristic x-ray (EDS) spectrum imaging of Ca-stabilized Nd2/3TiO3 acquired at atomic resolution in a scanning transmission electron microscope. Image registration based on the simultaneously acquired high angle annular dark field image significantly outperforms acquisition with a long pixel dwell time or drift correction using a reference image. Non-local Poisson PCA denoising reduces noise more strongly than conventional weighted PCA while preserving atomic structure more faithfully. The reliability of and optimal internal parameters for non-local Poisson PCA denoising of EDS spectrum images is assessed using tests on phantom data.

Perpendicular magnetic tunnel junctions (p-MTJs) were patterned into nanopillars using electron-beam lithography to study their scaling and switching behaviour. Magnetoresistance measurements of annealed and unannealed p-MTJ films using scanning probe microscopy showed good agreement with Monte Carlo modeling. p-MTJ pillars demonstrated clear parallel magnetic states, both 'up' or both 'down' following AC-demagnetization. Significant variability in the resistance of p-MTJ pillars was observed and attributed to edge features generated during patterning or local inhomogeneity in the MgO layer.

Determining the three-dimensional (3D) atomic structure of nanoparticles is critical to identifying the structures controlling their properties. Here, we demonstrate an integrated genetic algorithm (GA) optimization tool that refines the 3D structure of a nanoparticle by matching forward modeling to experimental scanning transmission electron microscopy (STEM) data and simultaneously minimizing the particle energy. We use the tool to create a refined 3D structural model of an experimentally observed ∼6000 atom Au nanoparticle.

The direct growth of hexagonal boron nitride (h-BN) by industrially scalable methods is of broad interest for spintronic and nanoelectronic device applications. Such applications often require atomically precise control of film thickness and azimuthal registry between layers and substrate. We report the formation, by atomic layer epitaxy (ALE), of multilayer h-BN(0001) films (up to 7 monolayers) on Co(0001). The ALE process employs BCl3/NH3 cycles at 600 K substrate temperature. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) data show that this process yields an increase in h-BN average film thickness linearly proportional to the number of BCl3/NH3 cycles, with BN layers in azimuthal registry with each other and with the Co(0001) substrate. LEED diffraction spot profile data indicate an average BN domain size of at least 1900 Å. Optical microscopy data indicate the presence of some domains as large as ∼20 μm. Transmission electron microscopy (TEM) and ambient exposure studies demonstrate macroscopic and microscopic continuity of the h-BN film, with the h-BN film highly conformal to the Co substrate. Photoemission data show that the h-BN(0001) film is p-type, with band bending near the Co/h-BN interface. Growth of graphene by molecular beam epitaxy (MBE) is observed on the surface of multilayer h-BN(0001) at temperatures of 800 K. LEED data indicate azimuthal graphene alignment with the h-BN and Co(0001) lattices, with domain size similar to BN. The evidence of multilayer BN and graphene azimuthal alignment with the lattice of the Co(0001) substrate demonstrates that this procedure is suitable for scalable production of heterojunctions for spintronic applications.

Perpendicular magnetic anisotropy [Co/Pd]15 and L10-FePt nanowire arrays of period 63 nm with linewidths 38 nm and 27 nm and film thickness 27 nm and 20 nm respectively were fabricated using a self-assembled PS-b-PDMS diblock copolymer film as a lithographic mask. The wires are predicted to support Néel walls in the Co/Pd and Bloch walls in the FePt. Magnetostatic interactions from nearest neighbor nanowires promote a ground state configuration consisting of alternating up and down magnetization in adjacent wires. This was observed over ∼75% of the Co/Pd wires after ac-demagnetization but was less prevalent in the FePt because the ratio of interaction field to switching field was much smaller. Interactions also led to correlations in the domain wall positions in adjacent Co/Pd nanowires. The reversal process was characterized by nucleation of reverse domains, followed at higher fields by propagation of the domains along the nanowires. These narrow wires provide model system for exploring domain wall structure and dynamics in perpendicular anisotropy systems.

Electron correlation microscopy (ECM) is a new technique that utilizes time-resolved coherent electron nanodiffraction to study dynamic atomic rearrangements in materials. It is the electron scattering equivalent of photon correlation spectroscopy with the added advantage of nanometer-scale spatial resolution. We have applied ECM to a Pd40Ni40P20 metallic glass, heated inside a scanning transmission electron microscope into a supercooled liquid to measure the structural relaxation time τ between the glass transition temperature T g and the crystallization temperature, T x . τ determined from the mean diffraction intensity autocorrelation function g 2(t) decreases with temperature following an Arrhenius relationship between T g and T g +25 K, and then increases as temperature approaches T x . The distribution of τ determined from the g 2(t) of single speckles is broad and changes significantly with temperature.

Quantitative density measurements from electron scattering show that shear bands in deformed Al88Y7Fe5 metallic glass exhibit alternating high and low density regions, ranging from -9% to +6% relative to the undeformed matrix. Small deflections of the shear band from the main propagation direction coincide with switches in density from higher to lower than the matrix and vice versa, indicating that faster and slower motion (stick slip) occurs during the propagation. Nanobeam diffraction analyses provide clear evidence that the density changes are accompanied by structural changes, suggesting that shear alters the packing of tightly bound short- or medium-range atomic clusters. This bears a striking resemblance to the packing behavior in granular shear bands formed upon deformation of granular media.

Measuring picometre-scale shifts in the positions of individual atoms in materials provides new insight into the structure of surfaces, defects and interfaces that influence a broad variety of materials' behaviour. Here we demonstrate sub-picometre precision measurements of atom positions in aberration-corrected Z-contrast scanning transmission electron microscopy images based on the non-rigid registration and averaging of an image series. Non-rigid registration achieves five to seven times better precision than previous methods. Non-rigidly registered images of a silica-supported platinum nanocatalyst show pm-scale contraction of atoms at a (111)/(111) corner towards the particle centre and expansion of a flat (111) facet. Sub-picometre precision and standardless atom counting with <1 atom uncertainty in the same scanning transmission electron microscopy image provide new insight into the three-dimensional atomic structure of catalyst nanoparticle surfaces, which contain the active sites controlling catalytic reactions.

Aberration-corrected scanning transmission electron microscopy images of the In(0.15)Ga(0.85)N active region of a blue light-emitting diode, acquired at ~0.1% of the electron dose known to cause electron beam damage, show no lateral compositional fluctuations, but do exhibit one to four atomic plane steps in the active layer's upper boundary. The area imaged was measured to be 2.9 nm thick using position averaged convergent beam electron diffraction, ensuring the sample was thin enough to capture compositional variation if it was present. A focused ion beam prepared sample with a very large thin area provides the possibility to directly observe large fluctuations in the active layer thickness that constrict the active layer at an average lateral length scale of 430 nm.

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2 O3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nanoparticles composed of a magnetic iron oxide core surrounded by a metal shell have utility in a broad range of biomedical applications. However, the presence of surface energy differences between the two components makes wetting of oxide with metal unfavorable, precluding a "core-shell" structure of an oxide core completely surrounded by a thin metal shell. Three-dimensional island growth followed by island coalescence into thick shells is favored over the two-dimensional layer-by-layer growth of a thin, continuous metal coating of a true core-shell. Aqueous synthesis of gold-coated magnetite nanoparticles with analysis by infrared, energy-dispersive X-ray, and electron energy loss spectroscopies; high-resolution transmission electron microscopy; selected area electron diffraction; and high-angle annular dark-field scanning transmission electron microscopy showed two distinct morphologies that are inconsistent with an idealized core-shell. The majority were isolated ~16-22-nm-diameter nanoparticles consisting of ~7-nm-diameter magnetite and a thick deposition of gold, most often discontinuous, with some potentially "sandwiched" morphologies. A minority were aggregates of agglomerated magnetite decorated with gold but displaying significant bare magnetite. Both populations were successfully conjugated to fibrinogen and targeted to surface-activated platelets, demonstrating that iron oxide-gold nanoparticles produced by aqueous synthesis do not require an ideal core-shell structure for biological activity in cell labeling and targeting applications.

Fast flexible electronics operating at radio frequencies (>1 GHz) are more attractive than traditional flexible electronics because of their versatile capabilities, dramatic power savings when operating at reduced speed and broader spectrum of applications. Transferrable single-crystalline Si nanomembranes (SiNMs) are preferred to other materials for flexible electronics owing to their unique advantages. Further improvement of Si-based device speed implies significant technical and economic advantages. While the mobility of bulk Si can be enhanced using strain techniques, implementing these techniques into transferrable single-crystalline SiNMs has been challenging and not demonstrated. The past approach presents severe challenges to achieve effective doping and desired material topology. Here we demonstrate the combination of strained- NM-compatible doping techniques with self-sustained-strain sharing by applying a strain-sharing scheme between Si and SiGe multiple epitaxial layers, to create strained print-transferrable SiNMs. We demonstrate a new speed record of Si-based flexible electronics without using aggressively scaled critical device dimensions.

We report that Sb-decorated head-to-head (H-H) basal plane inversion domain boundaries (b-IDBs) lead to stable p-type conduction in Sb-doped ZnO nanowires (NWs) due to Sb and O codoping. Aberration-corrected Z-contrast scanning transmission electron microscopy shows that all of the Sb in the NWs is incorporated into H-H b-IDBs just under the (0001) NW growth surfaces and the (0001) bottom facets of interior voids. Density functional theory calculations show that the extra basal plane of O per H-H b-IDB makes them electron acceptors. NWs containing these defects exhibited stable p-type behavior in a single NW FET over 18 months. This new mechanism for p-type conduction in ZnO offers the potential of ZnO NW based p-n homojunction devices., (© 2012 American Chemical Society)

We have used geometric optics calculations and experiments to investigate the probe-forming capability of an aberration-corrected, three-condenser scanning transmission electron microscope (STEM). Large, minimally convergent and coherent electron probes are useful for a variety of electron diffraction measurements. A three-condenser lens STEM can form a probe either using a virtual aperture below the sample and a virtual source on the sample plane or using a virtual aperture on the sample and a virtual source in the front focal plane. Adding a hexapole probe aberration corrector greatly increases the range of aperture demagnification and thus probe size and convergence angle. We have created probes 0.1 to approximately 12 nm in diameter in the simplest operating mode of our STEM, and we calculate that probes as large as 5000 nm that are almost perfectly parallel may be possible in more exotic lens configurations. We have also measured the spatial coherence of some of these probes.