Insights into the Heterogeneous Nuclei of an Ultrafast-Crystallizing Glassy Solid

Crystallization determines the programming speed of phase-change memory devices; while, the nucleation phenomenon of many phase-change materials (PCMs) is not entirely understood, especially concerning the atomic structures and dynamic behaviors of the subcritical nuclei. This is undoubtedly an insurmountable challenge for scandium antimony telluride (ScxSb2Te3) PCM as its subnanosecond-crystallization nature impedes the real-time observation of the transient nucleation process. To solve the puzzle, atomic probe tomography and transmission electron microscopy are employed to circumvent the technical difficulties; for the first time, the atomistic information of the heterogeneous nuclei in ScxSb2Te3 is unveiled, such as enriched Sc approximate to 25 at% in core composition, approximate to 1.0 nm in geometric size, and approximate to 1023-1024 m-3 in spatial density. The unique nanoscale chemical inhomogeneity ensures the unusual stabilities and dynamics of the early-stage nuclei, reinforcing them to survive the melt-quenching action and greatly suppressing the nucleating randomness, thereby facilitating simultaneous and prompt crystal growth throughout the amorphous phase to achieve ultrafast crystallization. The present study offers a new insight into the nonclassical pathways, which will improve understanding and promote better regulation of the nucleation phenomenon in functional materials. Nanometer-size Sc-rich domains with the approximate Sc1Sb1Te2 composition act as intrinsic and robust heterogeneous nuclei in the amorphous ScxSb2Te3 phase, greatly suppressing nucleation randomness; and thereby, facilitating subnanosecond ultrafast crystallization.image