Nanomesh materials have been widely studied over the past decade following the discovery of their much higher hardness and strength than their twinless counterparts. Despite the frequent encounter of twins in nanocrystals during chemical, mechanical and thermal processes, their nucleation and propagation mechanisms remain elusive.
Study: twinning of nanocrystals directed by the exchange movement. Image Credit: Yurchanka Siarhei/Shutterstock.com
A paper published in Science Advances demonstrated twinning in individual lead (Pb) nanocrystals using on the spot atomic resolution transmission electron microscopy (TEM). The results revealed that the twinning occurred due to the displacement of the atomic layers relative to each other by a double-layer exchange motion. The exchange motion-induced twin nucleation was the crucial step in the propagation of twins.
Density functional theory (DFT) calculations revealed that the exchange motion was the phonon eigenmode of Pb with its face-centered cubic (FCC) crystal structure, which became amplified due to the quantum size effect of nanocrystals according to theoretical simulations.
Nanocrystal pairing
Twinning is a fundamental deformation mode that competes with dislocation slip in crystalline materials. Under high tension, the twin was preferable to sliding dislocations. Strain pairing has been well documented in FCC nanocrystals.
Twinning occurs in materials in response to heating, laser shock, mechanical stress, electron beam density, and other external stimuli. Previous studies have reported that structure-twin nanocrystals exhibit greater mechanical strength, increased thermal stability, high electrical conductivity, exceptional light emission, and enhanced catalytic activity compared to their single counterparts.
Therefore, the structural modulation of nanomaterials with preferred characteristics is made possible by understanding the mechanisms of transformation twins in nanocrystals. Classically, transform twinning occurs via layer-by-layer partial dislocations in adjacent atomic planes.
Among the twinning induced by an external stimulus, only the twinning induced by a mechanical load is well documented. Under external mechanical stress, unusual processes such as synchronous partial dislocation activation, random partial dislocation activation, and a shuffling mechanism are involved in strain twinning.
Although the conventional deformation twinning mechanism was supposed to realize transformation twinning of nanocrystals, it lacked concrete support. Additionally, external energy is needed to break the energy barrier during pairing.
The creation of twins in nanocrystals can occur by injecting external energy during thermal annealing or ionic or electron irradiation, suggesting that nanocrystal transformation twinning can display unusual paths under the influence of kinetics. Technically, it is difficult to perform twinning excitation and atomic imaging simultaneously due to the partial slip or dislocation velocity, which is believed to occur as fast as the speed of sound.
![](https://oponame.com/wp-content/uploads/2022/10/1666541943_696_Twinning-in-Pb-nanocrystals-via-interchange-motion-movements.jpg)
Direct observation of the structural fluctuation between single crystal and twin structures of a Pb nanocrystal. (A) Reconstructed 3D atomic model of a truncated Pb nanocrystal and a two-dimensional (2D) projection along the [011¯] showing the axis of the viewing area composed of four {111} planes and two {200} planes. (B) Histograms of the number of Pb layers along the 〈111〉 and 〈200〉 directions obtained from the analysis of 36 nanocrystals. (C) Sequential images extracted from the S3 movie show the structural fluctuation between single crystal (“S”) and paired (“T”) structures of an individual Pb nanocrystal. TB, twin border. Scale bar, 2 nm. (D) The corresponding FFT of the nanostructures confirms the monocrystalline and bicrystalline structural transformation. (E) Trajectories of structural transitions between single crystal and nanotwin states during the process of pairing and unpairing. (F) The retention time of single crystal and nanotwin states in the S3 film. © Zhang, Q et al. (2022).
Analyze nanocrystal twinning through experimental and theoretical studies
Previously, researchers confirmed the superiority of Pb nanocrystals over other materials due to their low melting point and oxidation resistance, making them ideal candidates to study structural changes with Pb-regulated irradiation. electron beam.
In this study, the research team focused on transformation twinning in individual Pb nanocrystals with an FCC crystal structure by employing on the spot Atomic resolution TEM with millisecond time resolution. Two sophisticated aberration-corrected TEMs were equipped with a Thermo Fisher Scientific Ceta high-speed camera at 40 frames per second and a Gatan K2 IS camera at 400 frames per second to achieve high temporal resolution.
On the spot imaging revealed electron beam-induced structural transformations in Pb nanocrystals. Structural fluctuations between twinned and single nanocrystals were tuned by varying the electron beam intensity and corresponding temperature, demonstrating the impact of electron beam current density on the formation of twin nanocrystals.
On the other hand, subjecting the Pb nanocrystals to an electron beam of the same current density and at cryogenic temperatures did not induce any structural fluctuation, suggesting that the transformation twinning was generated by thermally induced vibration. by an electron beam.
Earlier reports mentioned that nanocrystals dissipate additional energy through phonon vibrations, resulting in reversible transformations between paired and single nanocrystals. Moreover, the retention time of a single nanocrystal is greater than that of its twinned counterpart. Therefore, the monocrystalline structure lasts longer.
In addition, the transformation dynamics of Pb nanocrystals were theoretically investigated using DFT-based phonon calculations, which indicated that the enhancement of the exchange mode of the nanocrystals was due to the decrease in the size of the nanocrystals.
The exchange phonon has the lowest energy of all shortwave phonons, making the exchange mode the mode of motion most likely to occur. As a result, damping can cause an exchange model phonon to become a twin. Finally, the experimental results were observed to be in agreement with the DFT calculations.
Conclusion
In conclusion, this research demonstrated that the interchange motion of two neighboring atomic layers moving relative to each other caused transformational twining of Pb nanocrystals. The nucleation and propagation of twins in nanocrystals has been caused by an exchange movement. The findings of the present study on previously unknown twinning mechanisms offered new potential for the design and fabrication of nanoscale materials.
Reference
Zhang, Q. et al. (2022). Twinning of nanocrystals directed by the movement of exchange. Scientists progress. https://www.science.org/doi/10.1126/sciadv.abp9970
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