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1.
Microsc Microanal ; : 1-7, 2020 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-32519630

RESUMO

Multiple electron scattering and the nonintuitive nature of image formation with coherent radiation complicate the interpretation of conventional transmission electron microscopy images. Precession of the illuminating beam in transmission electron microscopy (TEM) can lead to more robust and interpretable images with some penalty to image contrast, a technique known as dynamic hollow-cone illumination TEM. We demonstrate direct and robust imaging of light and heavy atoms in a crystalline environment with this technique. This method is similar to the annular bright-field technique in scanning transmission electron microscopy, via the principle of reciprocity. Dynamic hollow-cone illumination TEM is challenging in practice due to sensitivity to the misalignment of the precession axis, microscope objective aperture, and crystal zone axis.

3.
Nature ; 580(7804): 478-482, 2020 04.
Artigo em Inglês | MEDLINE | ID: mdl-32322080

RESUMO

Ultrathin ferroelectric materials could potentially enable low-power perovskite ferroelectric tetragonality logic and nonvolatile memories1,2. As ferroelectric materials are made thinner, however, the ferroelectricity is usually suppressed. Size effects in ferroelectrics have been thoroughly investigated in perovskite oxides-the archetypal ferroelectric system3. Perovskites, however, have so far proved unsuitable for thickness scaling and integration with modern semiconductor processes4. Here we report ferroelectricity in ultrathin doped hafnium oxide (HfO2), a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to a thickness of one nanometre. Our results indicate not only the absence of a ferroelectric critical thickness but also enhanced polar distortions as film thickness is reduced, unlike in perovskite ferroelectrics. This approach to enhancing ferroelectricity in ultrathin layers could provide a route towards polarization-driven memories and ferroelectric-based advanced transistors. This work shifts the search for the fundamental limits of ferroelectricity to simpler transition-metal oxide systems-that is, from perovskite-derived complex oxides to fluorite-structure binary oxides-in which 'reverse' size effects counterintuitively stabilize polar symmetry in the ultrathin regime.

4.
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.

5.
J Am Chem Soc ; 141(33): 13028-13032, 2019 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-31386354

RESUMO

The self-assembly of nanoparticles, a process whereby nanocrystal building blocks organize into even more ordered superstructures, is of great interest to nanoscience. Here we report the layer-by-layer assembly of 2D perovskite nanosheet building blocks. Structural analysis reveals that the assembled superlattice nanocrystals match with the layered Ruddlesden-Popper perovskite phase. This assembly proves reversible, as these superlattice nanocrystals can be reversibly exfoliated back into their building blocks via sonication. This study demonstrates the opportunity to further understand and exploit thermodynamics to increase order in a system of nanoparticles and to study emergent optical properties of a superlattice from 2D, weakly attracted, perovskite building blocks.

6.
Sci Adv ; 5(8): eaaw2398, 2019 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-31467973

RESUMO

Cooling, nucleation, and phase growth are ubiquitous processes in nature. Effective control of nucleation and phase growth is of significance to yield refined microstructures with enhanced performance for materials. Recent studies reveal that ultrafine grained (UFG)/nanocrystalline metals exhibit extraordinary properties. However, conventional microstructure refinement methods, such as fast cooling and inoculation, have reached certain fundamental limits. It has been considered impossible to fabricate bulk UFG/nanocrystalline metals via slow cooling. Here, we report a new discovery that nanoparticles can refine metal grains to ultrafine/nanoscale by instilling a continuous nucleation and growth control mechanism during slow cooling. The bulk UFG/nanocrystalline metal with nanoparticles also reveals an unprecedented thermal stability. This method overcomes the grain refinement limits and may be extended to any other processes that involve cooling, nucleation, and phase growth for widespread applications.

7.
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.


Assuntos
Nanopartículas/química , Nanopartículas/ultraestrutura , Peptídeos/química , Modelos Teóricos
8.
Chem Commun (Camb) ; 54(78): 11005-11008, 2018 Sep 27.
Artigo em Inglês | MEDLINE | ID: mdl-30215089

RESUMO

Solution-based growth of magnetic FePt-FeCo (core-shell) nanoparticles with a controllable shell thickness has been demonstrated. The transition from spin canting to exchange coupling of FePt-FeCo core-shell nanostructures leads to a 28% increase in the coercivity (12.8 KOe) and a two-fold enhancement in the energy product (9.11 MGOe).

9.
Nat Commun ; 9(1): 2998, 2018 07 31.
Artigo em Inglês | MEDLINE | ID: mdl-30065278

RESUMO

The growing interest in two-dimensional imine-based covalent organic frameworks (COFs) is inspired by their crystalline porous structures and the potential for extensive π-electron delocalization. The intrinsic reversibility and strong polarization of imine linkages, however, leads to insufficient chemical stability and optoelectronic properties. Developing COFs with improved robustness and π-delocalization is highly desirable but remains an unsettled challenge. Here we report a facile strategy that transforms imine-linked COFs into ultrastable porous aromatic frameworks by kinetically fixing the reversible imine linkage via an aza-Diels-Alder cycloaddition reaction. The as-formed, quinoline-linked COFs not only retain crystallinity and porosity, but also display dramatically enhanced chemical stability over their imine-based COF precursors, rendering them among the most robust COFs up-to-date that can withstand strong acidic, basic and redox environment. Owing to the chemical diversity of the cycloaddition reaction and structural tunability of COFs, the pores of COFs can be readily engineered to realize pre-designed surface functionality.

10.
ACS Nano ; 11(8): 8401-8412, 2017 08 22.
Artigo em Inglês | MEDLINE | ID: mdl-28759200

RESUMO

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga1-xZnx)(N1-xOx) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa2O4 NCs. (Ga1-xZnx)(N1-xOx) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga1-xZnx)(N1-xOx) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga1-xZnx)(N1-xOx) NCs in the solid state that involves phase transformation of cubic spinel ZnGa2O4 to wurtzite (Ga1-xZnx)(N1-xOx) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga1-xZnx)(N1-xOx) NCs varies with composition and elemental distribution.

11.
Nano Lett ; 17(6): 3421-3428, 2017 06 14.
Artigo em Inglês | MEDLINE | ID: mdl-28485598

RESUMO

Assembly of anisotropic nanocrystals into ordered superstructures is an area of intense research interest due to its relevance to bring nanocrystal properties to macroscopic length scales and to impart additional collective properties owing to the superstructure. Numerous routes have been explored to assemble such nanocrystal superstructures ranging from self-directed to external field-directed methods. Most of the approaches require sensitive control of experimental parameters that are largely environmental and require extra processing steps, increasing complexity and limiting reproducibility. Here, we demonstrate a simple approach to assemble colloidal nanorods in situ, wherein dopant incorporation during the particle synthesis results in the formation of preassembled 2D sheets of close-packed ordered arrays of vertically oriented nanorods in solution.

12.
Nat Mater ; 16(5): 532-536, 2017 05.
Artigo em Inglês | MEDLINE | ID: mdl-28218922

RESUMO

Metal-organic frameworks (MOFs) are crystalline porous materials with designable topology, porosity and functionality, having promising applications in gas storage and separation, ion conduction and catalysis. It is challenging to observe MOFs with transmission electron microscopy (TEM) due to the extreme instability of MOFs upon electron beam irradiation. Here, we use a direct-detection electron-counting camera to acquire TEM images of the MOF ZIF-8 with an ultralow dose of 4.1 electrons per square ångström to retain the structural integrity. The obtained image involves structural information transferred up to 2.1 Å, allowing the resolution of individual atomic columns of Zn and organic linkers in the framework. Furthermore, TEM reveals important local structural features of ZIF-8 crystals that cannot be identified by diffraction techniques, including armchair-type surface terminations and coherent interfaces between assembled crystals. These observations allow us to understand how ZIF-8 crystals self-assemble and the subsequent influence of interfacial cavities on mass transport of guest molecules.

13.
Ultramicroscopy ; 176: 170-176, 2017 05.
Artigo em Inglês | MEDLINE | ID: mdl-28162833

RESUMO

Scanning nanobeam electron diffraction strain mapping is a technique by which the positions of diffracted disks sampled at the nanoscale over a crystalline sample can be used to reconstruct a strain map over a large area. However, it is important that the disk positions are measured accurately, as their positions relative to a reference are directly used to calculate strain. In this study, we compare several correlation methods using both simulated and experimental data in order to directly probe susceptibility to measurement error due to non-uniform diffracted disk illumination structure. We found that prefiltering the diffraction patterns with a Sobel filter before performing cross correlation or performing a square-root magnitude weighted phase correlation returned the best results when inner disk structure was present. We have tested these methods both on simulated datasets, and experimental data from unstrained silicon as well as a twin grain boundary in 304 stainless steel.

14.
Adv Struct Chem Imaging ; 2(1): 15, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-28003952

RESUMO

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a ß-Si3N4 wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and lattice fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.

15.
Nat Commun ; 7: 12572, 2016 08 31.
Artigo em Inglês | MEDLINE | ID: mdl-27578622

RESUMO

Layered complex oxides offer an unusually rich materials platform for emergent phenomena through many built-in design knobs such as varied topologies, chemical ordering schemes and geometric tuning of the structure. A multitude of polar phases are predicted to compete in Ruddlesden-Popper (RP), An+1BnO3n+1, thin films by tuning layer dimension (n) and strain; however, direct atomic-scale evidence for such competing states is currently absent. Using aberration-corrected scanning transmission electron microscopy with sub-Ångstrom resolution in Srn+1TinO3n+1 thin films, we demonstrate the coexistence of antiferroelectric, ferroelectric and new ordered and low-symmetry phases. We also directly image the atomic rumpling of the rock salt layer, a critical feature in RP structures that is responsible for the competing phases; exceptional quantitative agreement between electron microscopy and density functional theory is demonstrated. The study shows that layered topologies can enable multifunctionality through highly competitive phases exhibiting diverse phenomena in a single structure.

16.
Ultramicroscopy ; 170: 35-42, 2016 11.
Artigo em Inglês | MEDLINE | ID: mdl-27526257

RESUMO

Surface structures that are different from the corresponding bulk, reconstructions, are exceedingly difficult to characterize with most experimental methods. Scanning tunneling microscopy, the workhorse for imaging complex surface structures of metals and semiconductors, is not as effective for oxides and other insulating materials. This paper details the use of transmission electron microscopy plan view imaging in conjunction with image processing for solving complex surface structures. We address the issue of extracting the surface structure from a weak signal with a large bulk contribution. This method requires the sample to be thin enough for kinematical assumptions to be valid. The analysis was performed on two sets of data, c(6×2) on the (100) surface and (3×3) on the (111) surface of SrTiO3, and was unsuccessful in the latter due to the thickness of the sample and a lack of inversion symmetry. The limits and the functionality of this method are discussed.

17.
Ultramicroscopy ; 169: 22-29, 2016 10.
Artigo em Inglês | MEDLINE | ID: mdl-27421079

RESUMO

High-resolution transmission electron microscopy (HRTEM) examination of nanoparticles requires their placement on some manner of support - either TEM grid membranes or part of the material itself, as in many heterogeneous catalyst systems - but a systematic quantification of the practical imaging limits of this approach has been lacking. Here we address this issue through a statistical evaluation of how nanoparticle size and substrate thickness affects the ability to resolve structural features of interest in HRTEM images of metallic nanoparticles on common support membranes. The visibility of lattice fringes from crystalline Au nanoparticles on amorphous carbon and silicon supports of varying thickness was investigated with both conventional and aberration-corrected TEM. Over the 1-4nm nanoparticle size range examined, the probability of successfully resolving lattice fringes differed significantly as a function both of nanoparticle size and support thickness. Statistical analysis was used to formulate guidelines for the selection of supports and to quantify the impact a given support would have on HRTEM imaging of crystalline structure. For nanoparticles ≥1nm, aberration-correction was found to provide limited benefit for the purpose of visualizing lattice fringes; electron dose is more predictive of lattice fringe visibility than aberration correction. These results confirm that the ability to visualize lattice fringes is ultimately dependent on the signal-to-noise ratio of the HRTEM images, rather than the point-to-point resolving power of the microscope. This study provides a benchmark for HRTEM imaging of crystalline supported metal nanoparticles and is extensible to a wide variety of supports and nanostructures.

18.
Nat Commun ; 7: 10719, 2016 Feb 29.
Artigo em Inglês | MEDLINE | ID: mdl-26923483

RESUMO

The ability to image light elements in soft matter at atomic resolution enables unprecedented insight into the structure and properties of molecular heterostructures and beam-sensitive nanomaterials. In this study, we introduce a scanning transmission electron microscopy technique combining a pre-specimen phase plate designed to produce a probe with structured phase with a high-speed direct electron detector to generate nearly linear contrast images with high efficiency. We demonstrate this method by using both experiment and simulation to simultaneously image the atomic-scale structure of weakly scattering amorphous carbon and strongly scattering gold nanoparticles. Our method demonstrates strong contrast for both materials, making it a promising candidate for structural determination of heterogeneous soft/hard matter samples even at low electron doses comparable to traditional phase-contrast transmission electron microscopy. Simulated images demonstrate the extension of this technique to the challenging problem of structural determination of biological material at the surface of inorganic crystals.

19.
Ultramicroscopy ; 162: 1-9, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26716724

RESUMO

Unwanted motion of the probe with respect to the sample is a ubiquitous problem in scanning probe and scanning transmission electron microscopies, causing both linear and nonlinear artifacts in experimental images. We have designed a procedure to correct these artifacts by using orthogonal scan pairs to align each measurement line-by-line along the slow scan direction, by fitting contrast variation along the lines. We demonstrate the accuracy of our algorithm on both synthetic and experimental data and provide an implementation of our method.

20.
J Am Chem Soc ; 137(20): 6464-7, 2015 May 27.
Artigo em Inglês | MEDLINE | ID: mdl-25974290

RESUMO

The presence of antimony, as a dopant in the colloidal growth reaction for CuIn(1-x)Ga(x)S2 (CIGS) nanocrystals, causes end-to-end fusion of nanorod pairs into nanodumbbells at high yield. The influence of the dopant on shape is indirect; antimony catalyzes the incorporation of gallium, which is found in high concentration at the junction between the fused nanorods.

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