In situ Observation of Structural Evolution and Phase Engineering of Amorphous Materials during Crystal Nucleation

Abstract

The nucleation pathway determines the structures and thus properties of formed nanomaterials, which is governed by the free energy of the intermediate phase during nucleation. The amorphous structure, as one of the intermediate phases during nucleation, plays an important role in modulating the nucleation pathway. However, the process and mechanism of crystal nucleation from amorphous structures still need to be fully investigated. Here, in situ aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) is employed to conduct real‐time imaging of the nucleation of ultrathin amorphous nanosheets (NSs). The results indicate that their nucleation contains three distinct stages, i.e., aggregation of atoms, crystallization to form lattice‐expanded nanocrystals, and relaxation of the lattice‐expanded nanocrystals to form final nanocrystals. In particular, the crystallization processes of various amorphous materials are investigated systematically to form corresponding nanocrystals with unconventional crystalline phases, including face‐centered‐cubic (fcc) Ru, hexagonal‐close‐packed (hcp) Rh, and a new intermetallic IrCo alloy. In situ electron energy‐loss spectroscopy (EELS) analysis unveils that the doped carbon in the original amorphous NSs can migrate to the surface during the nucleation process, stabilizing the obtained unconventional crystal phases transformed from the amorphous structures, which is also proven by density functional theory (DFT) calculations.

Recent Progress of Amorphous Nanomaterials

“…Similarly, Han et al investigated the nucleation pathway of amorphous phases using carbon-doped Ru, Rh, and IrCo nanosheets (ultrathin amorphous metal nanosheets) as starting materials. By employing in situ atomic-resolution STEM, they revealed three distinct stages in the formation of atomic nuclei: atom clustering, crystallization leading to lattice-expanded nanocrystals, and relaxation of lattice-expanded nanocrystals resulting in the formation of final nanocrystals (Figure a–c) . The researchers initiated nucleation from precursor/starting materials with intermediate phases (such as the amorphous phase) and manipulated the nucleation pathway by altering the activation energy.…”

“…The insets show the corresponding FFT patterns, respectively. (a–c) are adapted with permission from ref . Copyright 2022, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.…”

“…By employing in situ atomic-resolution STEM, they revealed three distinct stages in the formation of atomic nuclei: atom clustering, crystallization leading to lattice-expanded nanocrystals, and relaxation of lattice-expanded nanocrystals resulting in the formation of final nanocrystals (Figure 57a− c). 346 The researchers initiated nucleation from precursor/ starting materials with intermediate phases (such as the amorphous phase) and manipulated the nucleation pathway by altering the activation energy. This approach enabled them to obtain various metastable crystalline phases, including facecentered cubic (fcc) Ru, hexagonal close-packed (hcp) Rh, and a novel intermetallic compound IrCo alloy.…”

Recent Progress on Phase Engineering of Nanomaterials

“…Very recently, in cooperation with Hong's and Lin's groups, our group demonstrated that amorphous metal nanosheets could crystallize into the unconventional crystal phases upon electron-beam irradiation via in situ STEM characterizations. 163 Specifically, when amorphous Ru nanosheets doped with C (Figure 21b) were exposed under an electron beam, crystalline Ru domains with the unconventional fcc phase were gradually generated upon irradiation (Figure 21c,d). It was revealed that the formation of fcc-Ru could be ascribed to the migration and accumulation of C atoms on the surface of Ru nanocrystals during the nucleation, which reduced the surface energy of fcc-Ru compared to the thermodynamically stable hcp-Ru.…”

“…(f) HAADF-STEM image and (g) corresponding FFT pattern of Rh nanosheets after electron-beam irradiation. Reproduced with permission from ref . Copyright 2022 Wiley-VCH.…”