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1.
Nature ; 577(7790): 359-363, 2020 01.
Artigo em Inglês | MEDLINE | ID: mdl-31942056

RESUMO

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials1,2 is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations3-5. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co3O4 nanocube core that carries a Mn3O4 shell on each facet. The individual shells are symmetry-related interconnected grains6, and the large geometric misfit between adjacent tetragonal Mn3O4 grains results in tilt boundaries at the sharp edges of the Co3O4 nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase7. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation8-10. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.

2.
Ultramicroscopy ; 209: 112890, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31743883

RESUMO

Nanoscale strain mapping by four-dimensional scanning transmission electron microscopy (4D-STEM) relies on determining the precise locations of Bragg-scattered electrons in a sequence of diffraction patterns, a task which is complicated by dynamical scattering, inelastic scattering, and shot noise. These features hinder accurate automated computational detection and position measurement of the diffracted disks, limiting the precision of measurements of local deformation. Here, we investigate the use of patterned probes to improve the precision of strain mapping. We imprint a "bullseye" pattern onto the probe, by using a binary mask in the probe-forming aperture, to improve the robustness of the peak finding algorithm to intensity modulations inside the diffracted disks. We show that this imprinting leads to substantially improved strain-mapping precision at the expense of a slight decrease in spatial resolution. In experiments on an unstrained silicon reference sample, we observe an improvement in strain measurement precision from 2.7% of the reciprocal lattice vectors with standard probes to 0.3% using bullseye probes for a thin sample, and an improvement from 4.7% to 0.8% for a thick sample. We also use multislice simulations to explore how sample thickness and electron dose limit the attainable accuracy and precision for 4D-STEM strain measurements.

3.
Ultramicroscopy ; 208: 112860, 2020 01.
Artigo em Inglês | MEDLINE | ID: mdl-31704623

RESUMO

Electron tomography is used in both materials science and structural biology to image features well below the optical resolution limit. Here, we present a new method for high-resolution 3D transmission electron microscopy (TEM) which approximately reconstructs the electrostatic potential of a sample at atomic resolution in all three dimensions. We use phase contrast images captured through-focus and at varying tilt angles, along with an implicit phase retrieval algorithm that accounts for dynamical and strong scattering, providing more accurate results with much lower electron doses than current atomic electron tomography methods. We test our algorithm using simulated images of a synthetic needle geometry dataset composed of an amorphous silicon dioxide shell around a silicon core. By simulating various levels of electron dose, tilt and defocus, missing projections, and regularization methods, we identify a configuration that allows us to accurately determine both atomic positions and species. We also test the ability of our method to recover randomly positioned vacancies in light elements such as silicon, and to accurately reconstruct strongly-scattering elements such as tungsten.

4.
Sci Adv ; 5(12): eaax2799, 2019 12.
Artigo em Inglês | MEDLINE | ID: mdl-31853495

RESUMO

Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history.

5.
Adv Mater ; : e1906105, 2019 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-31746516

RESUMO

The operating conditions of functional materials usually involve varying stress fields, resulting in structural changes, whether intentional or undesirable. Complex multiscale microstructures including defects, domains, and new phases, can be induced by mechanical loading in functional materials, providing fundamental insight into the deformation process of the involved materials. On the other hand, these microstructures, if induced in a controllable fashion, can be used to tune the functional properties or to enhance certain performance. In situ nanomechanical tests conducted in scanning/transmission electron microscopes (STEM/TEM) provide a critical tool for understanding the microstructural evolution in functional materials. Here, select results on a variety of functional material systems in the field are presented, with a brief introduction into some newly developed multichannel experimental capabilities to demonstrate the impact of these techniques.

6.
Proc Natl Acad Sci U S A ; 116(35): 17181-17186, 2019 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-31399548

RESUMO

Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in [Formula: see text] surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first-principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering additional insight into defect formation pathways and kinetics during [Formula: see text] surface oxidation.

7.
Nano Lett ; 19(9): 6482-6491, 2019 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-31430158

RESUMO

Simultaneous imaging of individual low and high atomic number atoms using annular dark field scanning transmission electron microscopy (ADF-STEM) is often challenging due to substantial differences in their scattering cross sections. This often leads to contrast from only the high atomic number species when imaged using ADF-STEM such as the Mo and 2S sites in monolayer MoS2 crystals, without detection of lighter atoms such as C, O, or N. Here, we show that by capturing an array of convergent beam electron diffraction patterns using a 2D pixelated electron detector (2D-PED) in a 4D STEM geometry enables identification of individual low and high atomic number atoms in 2D materials by multicomponent imaging. We have used ptychographic phase reconstructions, combined with angular dependent ADF-STEM reconstructions, to image light elements at lateral (nanopores) and vertical interfaces (surface dopants) within 2D monolayer MoS2. Differential phase contrast imaging (Div(DPC)) using quadrant segmentation of the 2D pixelated direct electron detector data not only qualitatively matches the ptychographic phase reconstructions in both resolution and contrast but also offers the additional potential for real time display. Using 4D-STEM, we have identified surface adatoms on MoS2 monolayers and have separated atomic columns with similar total atomic number into their relative combinations of low and high atomic number elements. These results demonstrate the rich information present in the data obtained during 4D-STEM imaging of ultrathin 2D materials and the ability of this approach to extract unique insights beyond conventional imaging.

8.
Nat Mater ; 18(9): 970-976, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31285617

RESUMO

Two-dimensional (2D) materials have attracted significant interest because of their large surface-to-volume ratios and electron confinement. Compared to common 2D materials such as graphene or metal hydroxides, with their intrinsic layered atomic structures, the formation mechanisms of 2D metal oxides with a rocksalt structure are not well understood. Here, we report the formation process for 2D cobalt oxide and cobalt nickel oxide nanosheets, after analysis by in situ liquid-phase transmission electron microscopy. Our observations reveal that three-dimensional (3D) nanoparticles are initially formed from the molecular precursor solution and then transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the transformation of the 3D nanocrystal into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for future material design and synthesis in solution.

9.
Small ; 15(33): e1901966, 2019 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-31225719

RESUMO

Nanoparticle (NP) superlattices have attracted increasing attention due to their unique physicochemical properties. However, key questions persist regarding the correlation between short- and long-range driving forces for nanoparticle assembly and resultant capability to predict the transient and final superlattice structure. Here the self-assembly of Ag NPs in aqueous solutions is investigated by employing in situ liquid cell transmission electron microscopy, combined with atomic force microscopy-based force measurements, and theoretical calculations. Despite the NPs exhibiting instantaneous Brownian motion, it is found that the dynamic behavior of NPs is correlated with the van der Waals force, sometimes unexpectedly over relatively large particle separations. After the NPs assemble into clusters, a delicate balance between the hydration and van der Waals forces results in a distinct distribution of particle separation, which is ascribed to layers of hydrated ions adsorbed on the NP surface. The study demonstrates pivotal roles of the complicated correlation between interparticle forces; potentially enabling the control of particle separation, which is critical for tailoring the properties of NP superlattices.

10.
Sci Adv ; 5(6): eaaw5623, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-31187062

RESUMO

The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole-dipole interactions.

11.
Nat Commun ; 10(1): 2445, 2019 06 04.
Artigo em Inglês | MEDLINE | ID: mdl-31164643

RESUMO

To date, there has not yet been a direct observation of the initiation and propagation of individual defects in metallic glasses during deformation at the nanoscale. Here, we show through a combination of in situ nanobeam electron diffraction and large-scale molecular dynamics simulations that we can directly observe changes to the local short to medium range atomic ordering during the formation of a shear band. We observe experimentally a spatially resolved reduction of order prior to shear banding due to increased strain. We compare this to molecular dynamics simulations, in which a similar reduction in local order is seen, and caused by shear transformation zone activation, providing direct experimental evidence for this proposed nucleation mechanism for shear bands in amorphous solids. Our observation serves as a link between the atomistic molecular dynamics simulation and the bulk mechanical properties, providing insight into how one could increase ductility in glassy materials.

12.
Nat Mater ; 18(8): 860-865, 2019 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-31160799

RESUMO

The properties of organic solids depend on their structure and morphology, yet direct imaging using conventional electron microscopy methods is hampered by the complex internal structure of these materials and their sensitivity to electron beams. Here, we manage to observe the nanocrystalline structure of two organic molecular thin-film systems using transmission electron microscopy by employing a scanning nanodiffraction method that allows for full access to reciprocal space over the size of a spatially localized probe (~2 nm). The morphologies revealed by this technique vary from grains with pronounced segmentation of the structure-characterized by sharp grain boundaries and overlapping domains-to liquid-crystal structures with crystalline orientations varying smoothly over all possible rotations that contain disclinations representing singularities in the director field. The results show how structure-property relationships can be visualized in organic systems using techniques previously only available for hard materials such as metals and ceramics.

13.
Nature ; 570(7762): 500-503, 2019 06.
Artigo em Inglês | MEDLINE | ID: mdl-31243385

RESUMO

Nucleation plays a critical role in many physical and biological phenomena that range from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases1-3. However, nucleation is a challenging process to study experimentally, especially in its early stages, when several atoms or molecules start to form a new phase from a parent phase. A number of experimental and computational methods have been used to investigate nucleation processes4-17, but experimental determination of the three-dimensional atomic structure and the dynamics of early-stage nuclei has been unachievable. Here we use atomic electron tomography to study early-stage nucleation in four dimensions (that is, including time) at atomic resolution. Using FePt nanoparticles as a model system, we find that early-stage nuclei are irregularly shaped, each has a core of one to a few atoms with the maximum order parameter, and the order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter distribution and its gradient. These experimental observations are corroborated by molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. Our experimental and molecular dynamics results indicate that a theory beyond classical nucleation theory1,2,18 is needed to describe early-stage nucleation at the atomic scale. We anticipate that the reported approach will open the door to the study of many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry, such as phase transition, atomic diffusion, grain boundary dynamics, interface motion, defect dynamics and surface reconstruction with four-dimensional atomic resolution.

14.
Microsc Microanal ; 25(3): 563-582, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-31084643

RESUMO

Scanning transmission electron microscopy (STEM) is widely used for imaging, diffraction, and spectroscopy of materials down to atomic resolution. Recent advances in detector technology and computational methods have enabled many experiments that record a full image of the STEM probe for many probe positions, either in diffraction space or real space. In this paper, we review the use of these four-dimensional STEM experiments for virtual diffraction imaging, phase, orientation and strain mapping, measurements of medium-range order, thickness and tilt of samples, and phase contrast imaging methods, including differential phase contrast, ptychography, and others.

15.
Nat Commun ; 10(1): 1127, 2019 03 08.
Artigo em Inglês | MEDLINE | ID: mdl-30850616

RESUMO

Defects in materials give rise to fluctuations in electrostatic fields that reflect the local charge density, but imaging this with single atom sensitivity is challenging. However, if possible, this provides information about the energetics of adatom binding, localized conduction channels, molecular functionality and their relationship to individual bonds. Here, ultrastable electron-optics are combined with a high-speed 2D electron detector to map electrostatic fields around individual atoms in 2D monolayers using 4D scanning transmission electron microscopy. Simultaneous imaging of the electric field, phase, annular dark field and the total charge in 2D MoS2 and WS2 is demonstrated for pristine areas and regions with 1D wires. The in-gap states in sulphur line vacancies cause 1D electron-rich channels that are mapped experimentally and confirmed using density functional theory calculations. We show how electrostatic fields are sensitive in defective areas to changes of atomic bonding and structural determination beyond conventional imaging.

16.
Nat Commun ; 10(1): 592, 2019 02 05.
Artigo em Inglês | MEDLINE | ID: mdl-30723202

RESUMO

Structure plays a vital role in determining materials properties. In lithium ion cathode materials, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining lithium ion diffusion kinetics. In most conventional cathode materials that are well-ordered, the average structure as seen in diffraction dictates the lithium ion diffusion pathways. Here, we show that this is not the case in a class of recently discovered high-capacity lithium-excess rocksalts. An average structure picture is no longer satisfactory to understand the performance of such disordered materials. Cation short-range order, hidden in diffraction, is not only ubiquitous in these long-range disordered materials, but fully controls the local and macroscopic environments for lithium ion transport. Our discovery identifies a crucial property that has previously been overlooked and provides guidelines for designing and engineering cation-disordered cathode materials.

17.
Commun Biol ; 2: 26, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-30675524

RESUMO

Changes in lattice structure across sub-regions of protein crystals are challenging to assess when relying on whole crystal measurements. Because of this difficulty, macromolecular structure determination from protein micro and nanocrystals requires assumptions of bulk crystallinity and domain block substructure. Here we map lattice structure across micron size areas of cryogenically preserved three-dimensional peptide crystals using a nano-focused electron beam. This approach produces diffraction from as few as 1500 molecules in a crystal, is sensitive to crystal thickness and three-dimensional lattice orientation. Real-space maps reconstructed from unsupervised classification of diffraction patterns across a crystal reveal regions of crystal order/disorder and three-dimensional lattice tilts on the sub-100nm scale. The nanoscale lattice reorientation observed in the micron-sized peptide crystal lattices studied here provides a direct view of their plasticity. Knowledge of these features facilitates an improved understanding of peptide assemblies that could aid in the determination of structures from nano- and microcrystals by single or serial crystal electron diffraction.

18.
J Phys Chem B ; 123(5): 1195-1205, 2019 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-30629439

RESUMO

Vesicle formation in a series of amphiphilic sequence-defined polypeptoid block co-polymers comprising a phosphonated hydrophilic block and an amorphous hydrophobic block, poly- N-(2-ethyl)hexylglycine- block-poly- N-phosphonomethylglycine (pNeh- b-pNpm), is studied. The hydrophobic/hydrophilic block ratio was varied keeping the total chain length of the co-polymers constant. A new approach for characterizing the vesicle membrane morphology based on low-dose cryogenic electron microscopy (cryo-EM) is described. The individual low-dose micrographs cannot be interpreted directly due to low signal-to-noise ratio. Sorting and averaging techniques, developed in the context of protein structure determination, were thus applied to vesicle micrographs. Molecular dynamic simulations of the vesicles were used to establish the relationship between membrane morphology and averaged cryo-EM images. This approach enables resolution of the local thickness of the hydrophobic membrane core at the 1 nm length scale. The thickness of the hydrophobic core of the pNeh- b-pNpm membranes increases linearly with the length of the hydrophobic block.

19.
Sci Adv ; 5(9): eaaw5519, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-32047855

RESUMO

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed.

20.
Nano Lett ; 18(10): 6427-6433, 2018 10 10.
Artigo em Inglês | MEDLINE | ID: mdl-30256644

RESUMO

Formation mechanisms of dendrite structures have been extensively explored theoretically, and many theoretical predictions have been validated for micro- or macroscale dendrites. However, it is challenging to determine whether classical dendrite growth theories are applicable at the nanoscale due to the lack of detailed information on the nanodendrite growth dynamics. Here, we study iron oxide nanodendrite formation using liquid cell transmission electron microscopy (TEM). We observe "seaweed"-like iron oxide nanodendrites growing predominantly in two dimensions on the membrane of a liquid cell. By tracking the trajectories of their morphology development with high spatial and temporal resolution, it is possible to explore the relationship between the tip curvature and growth rate, tip splitting mechanisms, and the effects of precursor diffusion and depletion on the morphology evolution. We show that the growth of iron oxide nanodendrites is remarkably consistent with the existing theoretical predictions on dendritic morphology evolution during growth, despite occurring at the nanoscale.

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