Creation of a two-dimensional layered Zintl phase by dimensional manipulation of the crystalline structure

The discovery of new families of two-dimensional (2D) materials beyond graphene has always attracted attention, but the artificial restitution of the atomic honeycomb network structure with multiple components such as hexagonal boron nitride remains difficult. In a new study now published on Progress of scienceJunseong Song and colleagues from the Departments of Energy Science, Nanostructural Physics, Environmental Science and Materials in the Republic of Korea have developed an unprecedented Zintl phase structure.

They built the material by staking sp²Hybrid honeycomb ZnSb layers and by the dimensional manipulation of a crystalline structure³-hybrid 3-D-ZnSb. Material scientists have combined structural analysis 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 ambient pressure in the Zintl families. Therefore, the new work provides a rational design strategy for searching and creating new 2D layered materials in various compounds. The new results will allow unlimited expansion of 2D libraries and their corresponding physical properties.

The advent of graphene physics by Dirac has sparked an explosive interest in two-dimensional (2D) materials research with diverse applications in electronics, magnetism, energy, and chemistry in quantum physics. At present, the 2D research focuses mainly on a few 2D materials containing one or more exfoliated atomic layers of their parent compounds, as opposed to 2D atomic crystals such as silicone. This can limit the method of developing 2D materials to two approaches to exfoliation and chemical vapor deposition. It is therefore highly desirable to develop research on 2D materials in order to artificially create a new 2D material with a new synthesis approach and to form a variety of material groups.

In the discovery of new materials, the transformation of a crystalline structure is a widely recognized key factor. Structural phase transitions induced by temperature, pressure and electrostatic doping are essential for exploring a new crystalline structure or for modifying the properties of 2D materials. For example, most transition metal dichalcogenides exhibit a polymorphic phase transition that provides access to inherently diverse properties, including superconducting and topological states. The transition has led to promising applications, including electronic homo junction, photonic memory devices and catalytic energy materials.

These polymorphic transitions occurred only between different layered structures in the same two dimensions and remain to be achieved between different dimensions of a crystalline structure at ambient pressure. Achieving the ultimate crystalline engineering and changing the structural dimension of multicomponent compounds is a promising new frontier in the science of materials beyond carbon allotropes.

In this work, Song et al. Two-dimensional polymorphism established via the discovery of two-dimensional layer structures in Zintl phases containing a large number of chemical compositions. Because of the sp² Hybrid orbital bonding of honeycomb structured 2D atomic crystals such as graphene and hexagonal boron nitride, scientists expected to 3D-structured Zintl phases (with³ hybrid orbital link) to transform into sp² 2-D structured honeycomb layered materials, also, by electron transfer. As proof of concept, Song et al. selected a 3-D orthorhombic ZintS ZnSb (3-D-ZnSb) phase and created the unprecedented two-dimensional ZnSb layer structure (2-D-ZnSb).

In the new method, Song et al. the first ternary compounds in AZnSb (2-D-AZnSb) in synthesized layers; A relates to an alkali metal such as Na, Li and K. The materials contain a ZnSb layered structure by transformation of 3-D-ZnSb via an alloy A, although the phases can be synthesized independently. Song et al. performed selective etching of A-ions to create 2-D-ZnSb in two different processes, including (1) a chemical reaction in solutions containing demineralized water, and (2) an electrochemical ion etching reaction in an electrolyte based on alkali.

For example, they synthesized the polycrystalline and monocrystalline intermediate 2-D-LiZnSb substrate by first combining Li into polycrystalline 3-D-ZnSb followed by ionic etching to form a 2-D crystal. -ZnSb. Scientists easily cleaned Li-etched 2-D-ZnSb crystals using adhesive tape exfoliation as a mechanical split to present a flat surface typical of two-dimensional materials.

To understand the effect of the manufacturing process, they examined the role of Li alloying and etching on structural transformations using X-ray photoelectron spectroscopy (XPS) measurements. to reveal the difference between two-dimensional and three-dimensional crystals. To further validate their conclusions, Song et al. X-ray diffraction spectroscopy (XRD) models used, transmission electron microscopy (TEM) and tunneling tunneling electron microscopy (STEM) observations, combined with elemental energy dispersive spectroscopy mapping ( EDS) to confirm the atomic structure of 2-D-ZnSb.

On the basis of the results, the scientists interpreted the interlayer stretchable distances between the Zn-Zn and Sb-Sb atoms as weak inter-layer bonds and verified that 2-D-ZnSb could be exfoliated in the form of of laminated material. The newly developed layered structure of 2-D-ZnSb in the present work completed the first discovery of two-dimensional polymorphism in Zintl phases at ambient pressure.

As a result, Song et al. handled the sp³-hybridized binding state in 3-D-ZnSb in the sp² state in the honeycomb network 2-D-ZnSb. Previous studies on polymorphic transitions between 3D and 2D structures in Zintl phases have only been observed under high pressure. The present results on the two-dimensional polymorphism between 3-D-ZnSb and 2-D-ZnSb have highlighted the potential and wide availability of such an electron transfer to transform the crystal structure.

Song et al. We then studied the electrical transport properties of two-dimensional ZnSb polymorphs and 2-D-LiZnSb crystals, as well as calculations according to the basic principles of their electronic energy band structure. In contrast to the semiconductor nature of 3-D-ZnSb, 2-D-LiZnSb and 2-D-ZnSb both exhibited metallic conduction behavior. When they decreased the temperature, the electrical mobilities of 2-D-LiZnSb and 2-D-ZnSb increased to a higher value than 3-D-ZnSb. Scientists have attributed the increase in observed bandwidths for 2-D-ZnSb² nature of structured honeycomb layers with weak interactions between the layers that formed the semi-metal. They used theoretical calculations to confirm that 2-D-ZnSb could be exfoliated mechanically in the bilayer to exist in an energetically stable form as a 2-D material, whereas the 2-D-ZnSb monolayer was energetically unfavorable .

To demonstrate the structural transformation of two-dimensional ZnSb polymorphs during the formation of 2-D-LiZnSb, the scientists performed a synchrotron XRD – during the electrochemical reaction. They observed peaks corresponding to the 3-D-ZnSb Li alloy to 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 state of hybrid binding has changed from³ in 3-D-ZnSb with sp² in 2-D-LiZnSb to form the folded honeycomb network.

The result of the Li-alloy-based 2-D-LiZnSb transformation gave the product 2-D-ZnSb, which did not return to its 3-D form. Song et al. showed that once formed, the layered 2-D-ZnSb was a stable material in honeycomb architecture, validating the stable two-dimensional polymorphic transition. Scientists are anticipating applications of the new material in sustainable alkaline batteries.

Thus, Junseong Song and his collaborators have carried out rigorous experimental and theoretical studies to demonstrate the creation of Zintl phases in 2D layers by manipulating the structural dimensionality. The new method is a first to establish the two-dimensional polymorphic family in the Zintl phases at ambient pressure to allow new phase transformations as a general synthetic route. This work provides a rational design strategy for exploring new two-dimensional layer materials and unlocking other interesting properties, such as 2D magnetism, ferroelectricity, thermoelectricity, and topological states for other applications.

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