Abstract
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. Cr2Ge2Te6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. 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. 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. This unusual behavior is thought to originate from the 2D nature of the crystalline phase. 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.
Highlights
Two-dimensional (2D) van der Waals materials possess a crystal structure in which a covalentlybonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces. Cr2Ge2Te6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications
X-ray (Bragg) diffraction (XRD) patterns of CrGT films taken as a function of annealing temperature are shown in Fig. 1a
It can be clearly seen that the amorphous phase is characterized by a broad XRD reflection that remains unchanged for annealing temperatures up to 250 °C, whereupon additional reflections appear after annealing at 270 °C as can be seen in the inset of Fig. 1a indicated by the black arrow
Summary
Two-dimensional (2D) van der Waals (vdW) materials possess a crystal structure in which a covalentlybonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces. Cr2Ge2Te6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. Since the presence of a second nearest neighbor peak suggests a medium- or long-range ordered atomic arrangement, it does not appear in a highly disordered system, i.e. the amorphous phase.
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