Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure



Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure

Creation of ZnSb in 2D layers. (A) Schematic illustration of the dimensional manipulation of a crystal structure from 3D-ZnSb to 2D-ZnSb through the processes of alloy and Li etching. The Li alloy in 3D-ZnSb was made by thermal and electrochemical (ER) reactions. The selective etching of the Li ions was carried out by reaction with the reaction of polar solvent solution (SR). A reversible process of alloying and etching occurs at the average of the electrochemical reaction (ER). (B) 3D-ZnSb and 2D-LiZnSb DRX patterns. The polycrystal 2D-LiZnSb and the single crystal were synthesized using the 3D-ZnSb synthesized as a precursor. All the patterns combine well with the simulated patterns of the corresponding compounds. a.u., arbitrary units. (C) XRD patterns of 2D-ZnSb crystals obtained by reaction in solution and electrochemical reaction processes. For the reaction process in solution, water-based solutions. [DI water and dimethyl sulfoxide (DMSO) with 1 volume % of DI water, and hexamethyl phosphoric triamide (HMPA) with 1 volume % of DI water] They were used. For the electrochemical reaction process, 1 M LiPF6 dissolved in a 1: 1 mixture of ethylene carbonate and diethyl carbonate solution as electrolyte was used. The distances between layers were calculated from the angle of greatest intensity. (D to I) Scanning electron microscopy (D to F) and optical images (G to I) of 2D-LiZnSb and 2D-ZnSb created by the reaction in solution and the electrochemical reaction processes. The 2D-ZnSb scales were exfoliated by mechanical excision using 3M tape. (From J to L) X-ray photoelectron (XPS) spectroscopy of Li 1s (J), Zn 2p (K) and Sb 3d (L) for 3D-ZnSb, 2D-LiZnSb and 2D-ZnSb, respectively. The peak Li 1s (54.6 eV) of 2D-LiZnSb indicates the Li1 + state. While the binding energies of Zn 2p3 / 2 (1019.8 eV) and Sb 3d5 / 2 (525.8 eV) are significantly lower than those of Zn 2p3 / 2 (1021.5 eV) and Sb 3d5 / 2 (527.6 eV) in 3D -ZnSb, the binding energies of Zn 2p3 / 2 (1022.1 eV) and Sb 3d5 / 2 (528.2 eV) of 2D-ZnSb are slightly higher than those of 3D-ZnSb. Credit: Science Advances, doi: 10.1126 / sciadv.aax0390

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.

Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure

Crystal structure of ZnSb in 2D layers. (A and B) Images with atomic resolution STEM-HAADF (dark angle annular field of high angle) of 2D-LiZnSb along the [110] (A) and [001] (B) zone axes, respectively. (C) Elementary mapping of atomic resolution STEM-EDS for 2D-LiZnSb along the [110] (up and [001] Area axes (below). (D and E) STEM-HAADF atomic resolution images of 2D-ZnSb along the [110] (D) and [211] (E) zone axes. The determined crystal structure of 2D-ZnSb. The atomic distances of 2D-ZnSb are compared with those of 3D-ZnSb and 2D-LiZnSb. From observation in [211] On the axis of the 2D-ZnSb zone, the honeycomb network is slightly inclined. For the detection of lithium, the STEM-EELS (Electron Energy Loss Spectroscopy) technique was used, which shows the clear existence and absence of lithium in 2D-LiZnSb and 2D-ZnSb. (G) Calculation of cohesive energy (ΔEcoh) of predictable 2D-ZnSb structures. The structure I that is determined from the STEM observations shows the lowest energy compared to other candidates, showing an excellent concordance between the experiments and the calculations. Credit: Science Advances, doi: 10.1126 / sciadv.aax0390

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.

Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure

Electronic properties of ZnSb in 2D layers. (A to C) Dependence on the temperature of the electrical resistivity (A), Hall mobility (B) and carrier concentration (C) for 3D-ZnSb, 2D-LiZnSb and 2D-ZnSb. The two-dimensional polymorphs of 3D-ZnSb and 2D-ZnSb show the transition of the metal insulator. (D to F) Electronic band structures of 3D-ZnSb (D), 2D-LiZnSb (E) and 2D-ZnSb (F). The band structures of 3D-ZnSb (D) and 2D-LiZnSb (E) indicate that both are semiconductors with a well-defined indirect band gap of 0.05 and 0.29 eV, respectively. A low electrical resistivity and a high concentration of 2D-LiZnSb carriers indicate a strongly doped semiconductor behavior. Credit: Science Advances, doi: 10.1126 / sciadv.aax0390

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.

Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure

Behavior in 2D layers of 2D-ZnSb. (A) [100] View of 3D-ZnSb. (SECOND) [100] View of 2D-ZnSb. (C) Cohesive energy calculation (ΔEcoh) of 3D-ZnSb, 2D-ZnSb. From the calculation of cohesive energy, 3D-ZnSb is more stable, but the cohesive energy of 2D-ZnSb is sufficiently large, which indicates that 2D-ZnSb exists as a stable material. (D) The calculation of the energy of the Li alloy (ΔELi of the alloy) of 3D-ZnSb and 2D-ZnSb indicating the process of alloying Li in 2D-ZnSb and 3D-ZnSb is an energy booster. Compare two ΔELi alloys, Li ions that are mixed with 2D-ZnSb are favorable than 3D-ZnSb. (E) Binding energy between layers (Einter) of 3D-ZnSb and 2D-ZnSb. The big difference between Einter between 3D-ZnSb and 2DZnSb indicates the characteristics of the materials in 2D layers for 2D-ZnSb. (F) Calculation of exfoliation energy (Eexf) of 2D-ZnSb and other 2D materials. The Eexf of 2D-ZnSb is much higher than that of the conventional 2D adhered materials of van der Waals (vdW), such as graphene and h-BN, which indicates that 2D-ZnSb is not a stratified material of the vdW type . However, the Eexf of 2D-ZnSb is lower than that of antimony, which can be exfoliated or become monolayer, indicating that the independent monolayer or some layers of 2D-ZnSb may be possible as materials in conventional 2D vdW layers. Credit: Science Advances, doi: 10.1126 / sciadv.aax0390.

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.

Creation of a Zintl phase in two dimensions through the dimensional manipulation of the crystalline structure

Dimensional manipulation of a crystalline structure for the two-dimensional polymorphic ZnSb. (A and B) XRD patterns in situ synchrotron powder using 3D-ZnSb (A) and 2D-ZnSb (B) through the electrochemical reaction. The alloying and etching processes were controlled by reducing and increasing the voltage potential, respectively. The box (bottom left) of (A) shows the peak change of the plane (002) for 3D-ZnSb. The box (top left) of (A) shows the disappearance of the planes of diffraction peaks (002) and (101) planes in 11.1 ° and 11.7 ° of 2D-LiZnSb with Li etching, which indicates the transformation to 2D-ZnSb. The box (center) shows the appearance and disappearance of the byproduct Li1 + xZnSb with discharge and charge reactions, respectively. The inserts of (B) show the same changes observed in the inserts (upper left and middle) of (A). No 3D-ZnSb diffraction peaks were observed during the reversible structural transformation by alloying and Li etching processes. (C) Schematic illustration of the dimensional manipulation of a crystal structure, together with the transition of spliced ​​link characters from sp3 from 3D-ZnSb to sp2 from 2D-LiZnSb and 2D-ZnSb. The displacement of the blue arrow in Sb fifth to Zn fourth orbital represents the character of covalent bond between Zn and Sb in the net of the honeycomb. The transfer of electrons from the hybridized state of Li to sp3 of 3D-ZnSb allows the transition to the state hybridized with sp2 of the ZnSb honeycomb network in 2D-LiZnSb and 2D-ZnSb. Credit: Science Advances, doi: 10.1126 / sciadv.aax0390.

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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More information:
Paul F. McMillan. New materials of high pressure experiments, Materials of nature (2003). DOI: 10.1038 / nmat716

Manish Chhowalla et al. The chemistry of transition metal dicalcogenide nanosheets in two-dimensional layers, Chemistry of nature (2013). DOI: 10.1038 / nchem.1589.

Kenneth S. Burch et al. Magnetism in van der waals two-dimensional materials, Nature (2018). DOI: 10.1038 / s41586-018-0631-z

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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