Abstract
Understanding classical and nonclassical mechanisms of crystal nucleation and growth at the atomic scale is of great interest to scientists in many disciplines. However, fulfilling direct atomic‐scale observation still poses a significant challenge. Here, by taking a thin amorphous bismuth (Bi) metal nanosheet as a model system, direct atomic resolution of the crystal nucleation and growth initiated from an amorphous state of Bi metal under electron beam inside an aberration‐corrected transmission electron microscope is provided. It is shown that the crystal nucleation and growth in the phase transformation of Bi metal from amorphous to crystalline structure takes place via the particle‐mediated nonclassical mechanism instead of the classical atom‐mediated mechanism. The dimension of the smaller particles in two contacted nanoparticles and their mutual orientation relationship are critical to governing several coalescence pathways: total rearrangement pathway, grain boundary migration‐dominated pathway, and surface migration‐dominated pathway. Sequential strain analyses imply that migration of the grain boundary is driven by the strain difference in two Bi nanocrystals and the coalescence of nanocrystals is a defect reduction process. The findings may provide useful information to clarify the nanocrystal growth mechanisms of other materials on the atomic scale.
Highlights
Understanding classical and nonclassical mechanisms of crystal nucleation and recognized that this mechanism is not suitable to all aspects of nucleation and growth at the atomic scale is of great interest to scientists in many disciplines
Low-magnification transmission electron microscopy (TEM), selected area electron diffraction, high resolution TEM (HRTEM), and fast Fourier transformation (FFT) characterizations indicate that the nanosheet is of amorphous nature (Figure S2, Supporting Information)
The as-obtained thin amorphous Bi nanosheet can act as an ideal model system to probe the crystal nucleation and growth mechanism.[30,31,32,33,34]
Summary
Amorphous Bi nanosheets were synthesized using a simple ultrasonic route.[28]. Atomic force microscope image shows that the nanosheet has a thickness of ≈10 nm (Figure S1, Supporting Information). Figure 3m–r shows coalescence of two nanocrystals with a size of ≈13 nm (larger than dc), leading to the formation of a small high-angle grain boundary with a rotation angle of 20° (see Video S4 in the Supporting Information), as confirmed by the corresponding FFT analyses (Figures S7m–r and S10, Supporting Information). To further shed light on the migration of dislocation in the coalescence process, we show an atomic-scale sequential HRTEM image of dislocation arrays at a grain boundary with a rotation angle of ≈16° viewed along [213] direction and the corresponding sequential strain tensor analyses in Figure 6a–i (see Video S6 in the Supporting Information). This emphasizes that the coalescence of nanocrystals is a defect (dislocation, grain boundary) reduction process.[37,50]
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