Date Published: March 27, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Junjie Li, Jiangchun Chen, Hua Wang, Na Chen, Zhongchang Wang, Lin Guo, Francis Leonard Deepak.
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.
Crystallization represents an important process in chemistry, materials science, and condensed‐matter physics.1, 2, 3, 4 Based on classical homogeneous nucleation and growth mechanism, a crystal nucleus is first generated by spontaneous random aggregation of species from liquid or solution, followed by atomic or molecular attachment to form a stable crystal structure, a usual process for crystallization.5, 6, 7 However, it is increasingly recognized that this mechanism is not suitable to all aspects of nucleation and growth process.8, 9, 10, 11 Recently, a prenucleation stage has been proposed and validated experimentally. In light of this new theory, an amorphous dense phase is first formed and subsequently reorders to form a thermodynamically stable crystal nucleus.11, 12, 13 Other nonclassical mechanisms are related to particle‐mediated growth and assembly mechanism, including the oriented attachment and mesocrystal formation process.14, 15, 16, 17
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). Low‐magnification 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 corresponding energy‐dispersive X‐ray spectroscopy (EDS) mapping and spectrum verify their chemical composition (Figure S3, Supporting Information). It is well‐known that bulk Bi metal has a low melting point of 544.4 K and the melting temperature of its nanoparticles can be as low as room temperature due to the size effect.29 As such, 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
The ability to perform in situ atomic‐resolution observation of dynamic process of crystallization and growth represents a significant step forward in understanding crystal nucleation and growth mechanisms at the atomic scale. We show that Bi crystal nucleation and growth in the transformation process from amorphous to crystalline phase take place via the nonclassical mechanism mediated by particle coalescence and that the coalescence pathway of two nanoparticles is governed by the dimension of the smaller particle and their orientation relationship. When the size of the smaller one in the two nanoparticles is less than the critical size, coalescence takes a thorough rearrangement and the rotation mechanism as well as the orientation has a negligible effect on the pathway. The coalescence of two crystals takes place via a grain boundary migration‐dominated pathway if the formed grain boundary is a small high‐angle grain boundary, while via a surface migration‐dominated pathway if a large high‐angle grain boundary is formed. Further sequential strain analyses imply that migration of a grain boundary is driven by strain difference in the two nanocrystals. These findings reveal the atomic‐scale dynamic information on the nonclassical particle‐mediated crystal nucleation and growth mechanism, which helps to advance our general understanding of dynamic process of phase transformation and nucleation.
Sample Preparation and Characterization: The ultrasonic method was used to fabricate amorphous Bi nanosheets.28 In a typical synthesis process, a 100 mL ethanol solution containing bismuth powder of 300 mesh (350 mg) was sonicated for 5 h at 30 °C under an ultrahigh powder of 700 W. After that, the gray solution was concentrated at 1000 rpm for 30 min. The resultant transparent supernatant was used for the following experiments. The morphology and composition were confirmed by atomic force microscope, transmission electron microscopy, and corresponding energy‐dispersive X‐ray spectroscopy.
The authors declare no conflict of interest.