14 research outputs found

    Direct Atomic-Scale Observation of Intermediate Pathways of Melting and Crystallization in Supported Bi Nanoparticles

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    Uncovering the evolutional pathways of melting and crystallization atomically is critical to understanding complex microscopic mechanism of first-order phase transformation. We conduct in situ atomic-scale observations of melting and crystallization in supported Bi nanoparticles under heating and cooling within an aberration-corrected TEM. We provide direct evidence of the multiple intermediate state events in melting and crystallization. The melting of the supported nanocrystal involves the formation and migration of domain boundaries and dislocations due to the atomic rearrangement under heating, which occurs through a size-dependent multiple intermediate state. A critical size, which is key to inducing the transition pathway in melting from two to four barriers, is identified for the nanocrystal. In contrast, crystallization of a Bi droplet involves three stages. These findings demonstrate that the phase transformations cannot be viewed as a simple single barrier-crossing event but as a complex multiple intermediate state phenomenon, highlighting the importance of nonlocal behaviors

    Direct Atomic-Scale Observation of Intermediate Pathways of Melting and Crystallization in Supported Bi Nanoparticles

    No full text
    Uncovering the evolutional pathways of melting and crystallization atomically is critical to understanding complex microscopic mechanism of first-order phase transformation. We conduct in situ atomic-scale observations of melting and crystallization in supported Bi nanoparticles under heating and cooling within an aberration-corrected TEM. We provide direct evidence of the multiple intermediate state events in melting and crystallization. The melting of the supported nanocrystal involves the formation and migration of domain boundaries and dislocations due to the atomic rearrangement under heating, which occurs through a size-dependent multiple intermediate state. A critical size, which is key to inducing the transition pathway in melting from two to four barriers, is identified for the nanocrystal. In contrast, crystallization of a Bi droplet involves three stages. These findings demonstrate that the phase transformations cannot be viewed as a simple single barrier-crossing event but as a complex multiple intermediate state phenomenon, highlighting the importance of nonlocal behaviors

    <i>In Situ</i> Atomic-Scale Observation of Droplet Coalescence Driven Nucleation and Growth at Liquid/Solid Interfaces

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    Unraveling dynamical processes of liquid droplets at liquid/solid interfaces and the interfacial ordering is critical to understanding solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication processes, all of which are linked to different important applications in material science. In this work, we observe direct <i>in situ</i> atomic-scale behavior of Bi droplets segregated on SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub> by using aberration-corrected transmission electron microscopy and demonstrate ordered interface and surface structures for the droplets on the oxide at the atomic scale and unravel a nucleation mechanism involving droplet coalescence at the liquid/solid interface. We identify a critical diameter of the formed nanocrystal in stabilizing the crystalline phase and reveal lattice-induced fast crystallization of the droplet at the initial stage of the coalescence of the nanocrystal with the droplet. Further sequential observations show the stepped coalescence and growth mechanism of the nanocrystals at the atomic scale. These results offer insights into the dynamic process at liquid/solid interfaces, which may have implications for many functionalities of materials and their applications

    Real-Time Dynamical Observation of Lattice Induced Nucleation and Growth in Interfacial Solid–Solid Phase Transitions

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    Uncovering dynamical processes of lattice induced epitaxial growth of nanocrystal on the support is critical to understanding crystallization, solid-phase epitaxial growth, Oswald ripening process, and advanced nanofabrication, all of which are linked to different important applications in the materials field. Here, we conduct direct <i>in situ</i> atomic-scale dynamical observation of segregated Bi layers on SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub> support under low dose electron irradiation to explore the nucleation and growth from an initial disordered solid state to a stable faceted crystal by using aberration-corrected transmission electron microscopy. We provide, for the first time, atomic-scale insights into the initial prenucleation stage of lattice induced interfacial nucleation, size-dependent crystalline fluctuation, and stepped-growth stage of the formed nanocrystal on the oxide support at the atomic scale. We identify a critical diameter in forming a stable faceted configuration and find interestingly that the stable nanocrystal presents a size-dependent coalescence mechanism. These results offer an atomic-scale view into the dynamic process at solid/solid interfaces, which has implications for thin film growth and advanced nanofabrication

    Patterning Oxide Nanopillars at the Atomic Scale by Phase Transformation

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    Phase transformations in crystalline materials are common in nature and often modify dramatically properties of materials. The ability to precisely control them with a high spatial precision represents a significant step forward in realizing new functionalities in confined dimensions. However, such control is extremely challenging particularly at the atomic scale due to the intricacies in governing thermodynamic conditions with a high spatial accuracy. Here, we apply focused electron beam of a scanning transmission electron microscope to irradiate SrNbO<sub>3.4</sub> crystals and demonstrate a precise control of a phase transformation from layered SrNbO<sub>3.4</sub> to perovskite SrNbO<sub>3</sub> at the atomic scale. By purposely squeezing O atoms out of the vertex-sharing NbO<sub>6</sub> octahedral slabs, their neighboring slabs zip together, resulting in a patterning of SrNbO<sub>3</sub> nanopillars in SrNbO<sub>3.4</sub> matrix. Such phase transformations can be spatially manipulated with an atomic precision, opening up a novel avenue for materials design and processing and also for advanced nanodevice fabrication

    Full Determination of Individual Reconstructed Atomic Columns in Intermixed Heterojunctions

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    Heterojunctions offer a tremendous opportunity for fundamental as well as applied research, ranging from the unique electronic phases in between oxides to the contact issues in semiconductor devices. Despite their pivotal roles, determining individual building atom of matter in heterojunctions is still challenging, especially for those between highly dissimilar structures, in which breaking of symmetry, chemistry, and bonds may give rise to complex reconstruction and intermixing at the junction. Here, we combine electron microscopy, spectroscopy, and first-principles calculations to determine individual reconstructed atomic columns and their charge states in a complex, multicomponent heterojunction between the delafossite CuScO<sub>2</sub> and spinel MgAl<sub>2</sub>O<sub>4</sub>. The high resolution enables us to demonstrate that the reconstructed region can accommodate a highly selective intermixing of Cu cations at specific Sc cation sites with half atomic density, forming a complex ordered superstructure. Such ability to resolve reconstructed heterojunctions to the atomic dimensions helps elucidate atomistic mechanisms and discover novel properties with applications in a diverse range of scientific disciplines

    Atomic-Scale Structure and Local Chemistry of CoFeB–MgO Magnetic Tunnel Junctions

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    Magnetic tunnel junctions (MTJs) constitute a promising building block for future nonvolatile memories and logic circuits. Despite their pivotal role, spatially resolving and chemically identifying each individual stacking layer remains challenging due to spatially localized features that complicate characterizations limiting understanding of the physics of MTJs. Here, we combine advanced electron microscopy, spectroscopy, and first-principles calculations to obtain a direct structural and chemical imaging of the atomically confined layers in a CoFeB–MgO MTJ, and clarify atom diffusion and interface structures in the MTJ following annealing. The combined techniques demonstrate that B diffuses out of CoFeB electrodes into Ta interstitial sites rather than MgO after annealing, and CoFe bonds atomically to MgO grains with an epitaxial orientation relationship by forming Fe­(Co)-O bonds, yet without incorporation of CoFe in MgO. These findings afford a comprehensive perspective on structure and chemistry of MTJs, helping to develop high-performance spintronic devices by atomistic design

    Planar Vacancies in Sn<sub>1–<i>x</i></sub>Bi<sub><i>x</i></sub>Te Nanoribbons

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    Vacancy engineering is a crucial approach to manipulate physical properties of semiconductors. Here, we demonstrate that planar vacancies are formed in Sn<sub>1–<i>x</i></sub>Bi<sub><i>x</i></sub>Te nanoribbons by using Bi dopants <i>via</i> a facile chemical vapor deposition. Through combination of sub-angstrom-resolution imaging and density functional theory calculations, these planar vacancies are found to be associated with Bi segregations, which significantly lower their formation energies. The planar vacancies exhibit polymorphic structures with local variations in the lattice relaxation level, determined by their proximity to the nanoribbon surface. Such polymorphic planar vacancies, in conjunction with Bi dopants, trigger distinct localized electronic states, offering platforms for device applications of ternary chalcogenide materials

    A Single-Atom-Thick TiO<sub>2</sub> Nanomesh on an Insulating Oxide

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    The electronic structures and macroscopic functionalities of two-dimensional (2D) materials are often controlled according to their size, atomic structures, and associated defects. This controllability is particularly important in ultrathin 2D nanosheets of transition-metal oxides because these materials exhibit extraordinary multifunctionalities that cannot be realized in their bulk constituents. To expand the variety of materials with exotic properties that can be used in 2D transition-metal-oxide nanosheets, it is essential to investigate fabrication processes for 2D materials. However, it remains challenging to fabricate such 2D nanosheets, as they are often forbidden because of the crystal structure and nature of their host oxides. In this study, we demonstrate the synthesis of a single-atom-thick TiO<sub>2</sub> 2D nanosheet with a periodic array of holes, that is, a TiO<sub>2</sub> nanomesh, by depositing a LaAlO<sub>3</sub> thin film on a SrTiO<sub>3</sub>(001)-(√13×√13)-<i>R</i>33.7° reconstructed substrate. In-depth investigations of the detailed structures, local density of states, and Ti valency of the TiO<sub>2</sub> nanomesh using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, and density functional theory calculations reveal an unexpected upward migration of the Ti atoms of the substrate surface onto the LaAlO<sub>3</sub> surface. These results indicate that the truncated TiO<sub>5</sub> octahedra on the surface of perovskite oxides are very stable, leading to semiconducting TiO<sub>2</sub> nanomesh formation. This nanomesh material can be potentially used to control the physical and chemical properties of the surfaces of perovskite oxides. Furthermore, this study provides an avenue for building functional atomic-scale oxide 2D structures and reveals the thin-film growth processes of complex oxides

    A Single-Atom-Thick TiO<sub>2</sub> Nanomesh on an Insulating Oxide

    No full text
    The electronic structures and macroscopic functionalities of two-dimensional (2D) materials are often controlled according to their size, atomic structures, and associated defects. This controllability is particularly important in ultrathin 2D nanosheets of transition-metal oxides because these materials exhibit extraordinary multifunctionalities that cannot be realized in their bulk constituents. To expand the variety of materials with exotic properties that can be used in 2D transition-metal-oxide nanosheets, it is essential to investigate fabrication processes for 2D materials. However, it remains challenging to fabricate such 2D nanosheets, as they are often forbidden because of the crystal structure and nature of their host oxides. In this study, we demonstrate the synthesis of a single-atom-thick TiO<sub>2</sub> 2D nanosheet with a periodic array of holes, that is, a TiO<sub>2</sub> nanomesh, by depositing a LaAlO<sub>3</sub> thin film on a SrTiO<sub>3</sub>(001)-(√13×√13)-<i>R</i>33.7° reconstructed substrate. In-depth investigations of the detailed structures, local density of states, and Ti valency of the TiO<sub>2</sub> nanomesh using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, and density functional theory calculations reveal an unexpected upward migration of the Ti atoms of the substrate surface onto the LaAlO<sub>3</sub> surface. These results indicate that the truncated TiO<sub>5</sub> octahedra on the surface of perovskite oxides are very stable, leading to semiconducting TiO<sub>2</sub> nanomesh formation. This nanomesh material can be potentially used to control the physical and chemical properties of the surfaces of perovskite oxides. Furthermore, this study provides an avenue for building functional atomic-scale oxide 2D structures and reveals the thin-film growth processes of complex oxides
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