Twins in Pb nanocrystals by swap motion movements

Nanotwinned materials have been studied extensively over the past decade after discovering their much higher hardness and strength than their untwinned counterparts. Despite frequent encounters of twins in nanocrystals during chemical, mechanical, and thermal processes, their nucleation and propagation mechanisms remain elusive.

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Study: Swap motion-directed twins of nanocrystals. Credit: Yurchanka Siarhei/Shutterstock.com

A paper published in Science Advances demonstrated twinning in single lead (Pb) nanocrystals through use on site Transmission electron microscopy (TEM) with atomic resolution. The results showed that twinning occurred due to the shifting of atomic layers relative to each other through double-layer exchange motion. Twin nucleation induced by the swap movement was the crucial step in twinning.

Density functional theory (DFT) calculations revealed that the swap motion was the phonon eigenmode of Pb with its face-centered cubic (FCC) crystal structure amplified due to the quantum size effect of the nanocrystals according to theoretical simulations.

Twins of nanocrystals

Twins are a fundamental mode of deformation that competes with dislocation slip in crystalline materials. Under high stress, twinning was preferable for dislocation slip. Strain twins have 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 twinned nanocrystals exhibit greater mechanical strength, increased thermal stability, high electrical conductivity, excellent light emission, and enhanced catalytic activity compared to their individual counterparts.

Therefore, structural modulation of nanomaterials with preferred properties is enabled by understanding the transformation twin mechanisms in nanocrystals. Traditionally, transformation twinning occurs via layer-by-layer partial dislocations in the neighboring atomic planes.

Among stimuli-induced twinning, only mechanical stress-induced twinning is well documented. Under external mechanical loading, unusual processes such as synchronous partial dislocation activation, random partial dislocation activation, and a shuffling mechanism are involved in deformation twinning.

Although the traditional deformation twin mechanism was thought to perform transformation twins of nanocrystals, concrete support was lacking. In addition, external energy is required to break the energy barrier during twinning.

The formation of twinning in nanocrystals can occur by supplying external energy during thermal annealing or ion or electron beam irradiation, suggesting that the transformation twins of nanocrystals can show unusual pathways under the influence of kinetics. Technically, it is difficult to realize twin excitation and atomic imaging at the same time, since the speed of the partial slip or dislocation is presumably as fast as the speed of sound.

​​​​​​Direct observation of the structural fluctuation between single 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¯] shows the view zone axis, which consists of four {111} 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 Movie S3 show the structural fluctuation between the single-crystal (“S”) and twin (“T”) structures of a single Pb nanocrystal. TB, twin boundary. Scale bar, 2 nm. (D) Corresponding FFT of nanostructures confirms the single-crystal and twin-crystal structure transformation. (E) Trajectories of structural transitions between single-crystal and nanotwinned states during the twinning and detwinning process. (F) The retention time of single crystal and nanotwin states in Movie S3. © Zhang, Q et al. (2022).

Analysis of twinning of nanocrystals through experimental and theoretical studies

Previously, researchers confirmed the superiority of Pb nanocrystals over other materials due to their low melting point and resistance to oxidation, making them ideal candidates for studying structural changes with regulated electron beam irradiation.

In this study, the research team focused on transformation twins in single Pb nanocrystals with an FCC crystal structure through the insert on site TEM with atomic resolution and temporal resolution in the millisecond range. Two sophisticated aberration-corrected TEMs were equipped with a high-speed 40 fps Thermo Fisher Scientific Ceta camera and a 400 fps Gatan K2 IS camera to achieve high temporal resolution.

On site Imaging revealed electron-beam-induced structural transformations in the Pb nanocrystals. The structural fluctuations between the twinned and the individual nanocrystals were tuned by varying the electron beam intensity and the corresponding temperature, demonstrating the influence of the electron beam current density on the formation of twinned nanocrystals.

On the other hand, irradiation of the Pb nanocrystals with an electron beam of the same current density and at cryogenic temperatures did not induce any structural variations, suggesting that the transition twins were generated by electron beam-induced thermal oscillations.

Previous reports have mentioned that nanocrystals dissipate additional energy through phonon vibrations, leading to reversible transformations between twinned and single nanocrystals. In addition, the retention time of a single nanocrystal is higher than that of its twinned counterpart. Therefore, the single crystal structure lasts longer.

Furthermore, the transformation dynamics of Pb nanocrystals were studied theoretically using DFT-based phonon calculations, which suggested that the enhancement of the nanocrystals’ swap mode was due to the decrease in nanocrystal size.

The swap phonon has the lowest energy of any short-wavelength phonon, making the swap mode the most likely mode of motion to occur. As a result, attenuation can cause an exchange model phonon to become a twin. Finally, it was observed that the experimental results agreed with the DFT calculations.

Conclusion

In summary, this research showed that the interchange motion of two adjacent atomic layers moving relative to each other caused transform twinning of Pb nanocrystals. Twin nucleation and twin propagation in the nanocrystals were caused by swap motion. The discoveries of the present study on previously unknown twin mechanisms offered new potential for the design and fabrication of nanoscale materials.

Relation

Zhang, Q et al. (2022). Swap motion-directed twins of nanocrystals. scientific advances. https://www.science.org/doi/10.1126/sciadv.abp9970

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