【Research Background】

Two-dimensional (2D) atomic layer crystals have some unique physical and chemical properties, so they have a wide range of applications in electronics, sensors, catalysis, and batteries. Generally speaking, some materials with 2D structure, such as graphene, boron nitride, and transition metal sulfide, can be peeled from the corresponding van der Waals directly from top to bottom by mechanical, liquid and electrochemical methods Heterojunction.

In the past ten years, some special non-Vander Waals solid-state 2D nanocrystals have appeared, such as hematite and layered transition metal carbon / nitride, known as the MAX phase, which greatly expanded the system. In particular, this non-Vandervals MAX phase -M represents a transition metal element, A usually represents elements of groups 13-16 in the periodic table, and X is carbon or nitrogen, mainly some mixed covalent bonds, ionic MX key and metal type MA key. Because the MA bond is usually more chemically active than the MX bond, the A atom in the MAX phase can be etched by a highly reactive solvent (hydrofluoric acid or a strong base). As a result, a compound called MXene is produced. A few layers of 2D transition metal carbides, carbonitrides or nitrides. These 2D nanocrystals usually have some defects and functional groups on the surface, such as -OH, -O, -F or -Cl.Since in this non-van der Waals solids too closely stacked with a strong chemical bond atoms, to convert it to having a more exposed surface 2D and determining the phase of nanocrystalline material it remains a great challenge.

[ Achievement Profile]

Recently, the Beijing University of Aeronautics and AstronauticsProfessor Yang Shubin research group and Rice University Pulickel M. Ajayan professor cooperate in top international academic journal Naturepublished an article entitled on : Conversion of non-Van der Waals 2D Solids to Transition-Metal chalcogenides research papers, This article reports a topological transformation method to transform a van der Waals solid into a 2D van der Waals transition metal chalcogenide with a 2H / 1T phase. This conversion is achieved by exposing non-Vandervals solids to chalcogen vapors, which can be controlled by controlling the entropy and vapor pressure of the reaction products. Heteroatom-substituted (such as yttrium and phosphorus) transition metal chalcogenides can also be prepared in this way. Therefore, the phase selection of 2D transition metal chalcogenides obtained by this general method at high temperatures (1373 Kelvin) has good stability, and at the same time, it can also achieve large-scale production of single-layer materials, which has far-reaching significance for the development of single-layer two-dimensional materials.

[Picture and text guide]

Figure 1. Schematic representation of non-Vandervals solids and 2D transition metal chalcogenide conversion

Figure 2. Structural characterization of a MAX phase-derived 2D transition metal chalcogenide

Figure 3. Structural characterization of a quaternary MAX phase material-derived heteroatom-doped 2H phase 2D transition metal chalcogenide.

Figure 4. Structural characterization and electrical testing of a 1T phase 2D transition metal chalcogenide with heteroatom co-doping (Y and P) derived from a quaternary MAX phase material.

Literature link:

https://doi.org/10.1038/s41586-019-1904-x