![]() Therefore, the fundamental question arises as to how the amorphous-to-crystal transition (or simply crystallization) occurs between two phases with different chemical bonding dimensionality. In other words, the chemical bonding dimensionality of the amorphous phase is 3D. Generally, chemical bonding in the amorphous phase is random and isotropic. In particular, the amorphous phase of 2D vdW materials has been overlooked. Although disorder in 2D vdW materials such as point defects or grain boundaries has been explored, structural information on the disordered state of 2D vdW materials has yet to be understood 10. Most research studies on 2D vdW materials have focused on the crystalline phase. The emerging functionality arising from the atomic-layer scale structure of these materials holds the promise of the ultimate nanotechnology for future devices. In addition to typical composition dependent electronic material properties such as insulating, semiconductor, semimetal, or metallic behaviors, the unusual electronic, optical, and magnetic properties of 2D vdW materials have generated interest in a wide range of research fields from fundamental to applied science 7, 8, 9. Representative 2D vdW materials include graphene 2, transition metal dichalcogenides (TMDs) 3, boron nitride 4, black phosphorus 5, and topological insulators such as Sb 2Te 3 6. Interlayer interactions along the z-axis are weak and such materials are typically stable even in monolayer form (Angstrom-scale) resulting in the epithet 2D materials. Consequently, there is inherent chemical bonding anisotropy in the crystal structure along the x, y, and z axes. There is, of course, interatomic interaction along the z-axis in the form of vdW bonding also known as secondary bonds, but this form of bonding is much weaker in terms of bonding energy than primary bonds i.e. Two-dimensional (2D) van der Waals (vdW) materials are characterized by covalently bonded Angstrom-length scale atomic layers located within the 2D x–y planes of the crystal structure 1. ![]() These observations provide insight into the crystallization mechanism of layered materials in general, and consequently, will be useful for the realization of 2D vdW material-based functional nanoelectronic device applications. This unusual behavior is thought to originate from the 2D nature of the crystalline phase. A counterintuitive metastable “quasi-layered” state during crystallization that exhibits both “long-range order and short-range disorder” with respect to atomic alignment clearly distinguishes the system from conventional materials. Here, we provide evidence for a dimensional transformation in the chemical bonding from a randomly bonded three-dimensional (3D) disordered amorphous phase to a 2D bonded vdW crystalline phase. Cr 2Ge 2Te 6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. Two-dimensional (2D) van der Waals (vdW) materials possess a crystal structure in which a covalently-bonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces.
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