The discovery of new families of materials in two-dimensional (2-D) layers beyond graphene has always attracted great attention, but it remains challenging to artificially recreate the structure of the honeycomb atomic network with multiple components such as boron nitride. hexagonal in the laboratory. In a new study now published in Scientific advancesJunseong Song and his colleagues from the departments of Energy Science, Physics of Nanostructure, Environmental Science and Materials Science in the Republic of Korea developed an unprecedented structure of the Zintl phase.
They built the material by staking sp.twonest of ZnSb honeycomb and by dimensional manipulation of a crystal structure from the sp3– Hybridized 3-D-ZnSb state. Material scientists combined structural badysis with theoretical calculations to form a stable and robust layered structure of 2-D-ZnSb. This phenomenon of two-dimensional polymorphism was not previously observed at environmental pressure in the Zintl families. Therefore, the new work provides a rational design strategy to search and create new materials in 2-D layers in various compounds. The new results will allow the unlimited expansion of 2-D libraries and their corresponding physical properties.
The advent of Dirac's graphene physics sparked an explosive interest in the investigation of two-dimensional (2-D) materials with diverse applications in electronics, magnetism, energy and chemistry for quantum physics. Currently, 2-D research focuses primarily on a few 2-D materials that contain one or more atomic layers exfoliated from their parent compounds, in contrast to 2-D atomic crystals such as silicone. This may restrict the method of developing 2-D materials to two methods of exfoliation and chemical vapor deposition. Therefore, it is highly desirable to expand the research into 2-D materials to artificially create a new 2-D material with a new synthetic approach and form a variety of material groups.
In the discovery of new materials, the transformation of a crystal structure is a widely recognized key factor. Where structural phase transitions induced by temperature and pressure and electrostatic doping are critical to explore a new crystalline structure or to change the properties of 2D materials. For example, most transition metal dicalcogenides exhibit a polymorphic phase transition to access inherently diverse properties, including superconductors and topological states. The transition has led to promising applications that include electronic homojunction, photonic memory devices and catalytic energy materials.
These polymorphic transitions only occurred between different structures in layers in the same two dimensions and are still made between different dimensions of a glbad structure at ambient pressure. Achieving definitive crystal engineering and altering the structural dimension of multicomponent compounds is a promising next frontier in the science of materials beyond the allotropes of carbon.
In the present work, Song et al. Established two-dimensional polymorphism through the discovery of structures in 2-D layers in Zintl phases containing a large number of chemical compositions. Due to the sptwo the hybrid orbital junction of 2D atomic crystals structured in honeycomb, such as graphene and hexagonal boron nitride, scientists expected the 3D structured Zintl phases (with sp3 hybrid orbital link) to transform sptwo Honeycomb honeycomb materials in 2-D layers, too, through electron transfer. As proof of concept, Song et al. selected a 3-D orthorhombic Zintl phase ZnSb (3-D-ZnSb) and created the unprecedented 2D layer structure of ZnSb (2-D-ZnSb).
In the new method, Song et al. the first ternary compounds AZnSb (2-D-AZnSb) in synthesized layers; where A referred to an alkali metal such as Na, Li and K. The materials contained a layered structure of ZnSb by transforming 3-D-ZnSb through an alloy, although the phases could be synthesized independently. Song et al. he made the selective etching of the A ions to create the 2-D-ZnSb in two different processes, which include (1) chemical reaction in deionized solutions incorporated in water, and (2) chemical attack reaction by electrochemical ions in electrolytes based of alkalis.
For example, they synthesized the polycrystalline and monocrystalline intermediate substrate 2-D-LiZnSb by first enrolling Li in polycrystalline 3-D-ZnSb, followed by etching with Li-ion to form a 2-D-ZnSb crystal. Scientists easily cleaned 2-D-ZnSb crystals etched with Li using an adhesive tape exfoliation such as mechanical excision to exhibit a typical flat surface as reported for 2-D materials.
To understand the effect of the manufacturing process, they examined the role of alloy and Li etching in structural transformations using X-ray photoelectron spectroscopy (XPS) measurements to reveal the difference between 2-D and 3-D crystals. To further validate their findings, Song et al. used X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (STEM) combined with energy dispersion spectroscopy (EDS) elementary cartography to confirm the atomic structure of 2-D- ZnSb.
Based on the results, the scientists interpreted the stretchable intermediate layer distances between the Zn-Zn and Sb-Sb atoms as weak bonds of the intermediate layer and verified that 2-D-ZnSb could exfoliate as a layered material. The newly evolved layered structure of 2-D-ZnSb in the present work completed the first discovery of two-dimensional polymorphism in Zintl phases at ambient pressure.
Consequently, Song et al. manipulated the sp3Hybridized link state in 3-D-ZnSb in the sptwo Status in 2-D-ZnSb honeycomb network. Previous studies on polymorphic transitions between 3-D and 2-D structures in Zintl phases were only observed under high pressure. Current results on the two-dimensional polymorphism between 3-D-ZnSb and 2-D-ZnSb emphasized the potential and wide availability of such electron transfer to transform the crystal structure.
Song et al. Next, the electric transport properties of the two-dimensional ZnSb polymorphs and the 2-D-LiZnSb crystals were investigated along with the first calculations of the principles of their electronic energy band structure. In contrast to the semiconductor nature of 3-D-ZnSb, both 2-D-LiZnSb and 2-D-ZnSb showed a metallic conduction behavior. When the temperature decreased, the electric mobilities of 2-D-LiZnSb and 2-D-ZnSb increased to a higher value than that of 3-D-ZnSb. The scientists attributed the increased bandwidths observed for 2-D-ZnSb to the improved sptwo Nature of the honeycomb structured layers with interactions between weakened layers that formed the semi-metal. They used theoretical calculations to confirm that 2-D-ZnSb could mechanically exfoliate in the bilayer to exist in an energetically stable form as 2-D material, whereas the 2-D-ZnSb monolayer was energetically unfavorable.
To demonstrate the structural transformation of the two-dimensional polymorphs of ZnSb during the formation of 2-D-LiZnSb, the scientists performed the XRD synchrotron during the electrochemical reaction. They observed peaks corresponding to the Li alloy of 3-D-ZnSb in the pure formation of 2-D-LiZnSb, followed by the final product of 2-D-ZnSb. During the electrochemical reaction, the Li atoms selectively penetrated 3-D-ZnSb to break the Zn-Sb and Sb-Sb bonds. At the level of electron transfer, the hybridized link state changed sp3 in 3-D-ZnSb a sptwo in 2-D-LiZnSb to form the wrinkled honeycomb network.
The result of the transformation of 2-D-LiZnSb based on the Li alloy produced the product 2-D-ZnSb, which did not return to its 3-D form. Song et al. showed that once formed, the 2-D-ZnSb layer was a stable material with a honeycomb architecture, which validated the stable two-dimensional polymorphic transition. Scientists anticipate applications of the new material in sustainable alkaline ion batteries.
In this way, Junseong Song and his collaborators made rigorous experimental and theoretical studies to demonstrate the creation of Zintl phases in two dimensions through the manipulation of structural dimensionality. The new method is the first to establish the two-dimensional polymorphic family in Zintl phases at environmental pressure, to allow new phase transformations as a general synthesis route. This work provides a rational design strategy to explore new materials in 2-D layers and unlock other properties of interest within materials, such as 2-D magnetism, ferroelectricity, thermoelectricity and topological states for other applications.
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Junseong Song et al. Creation of the Zintl phase in two-dimensional layers through the dimensional manipulation of the crystalline structure, Scientific advances (2019). DOI: 10.1126 / sciadv.aax0390
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Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure (2019, July 12)
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