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# Considerable knock-on displacement of metal atoms under a low energy electron beam

• Scientific Reports 7, Article number: 184 (2017)
• doi:10.1038/s41598-017-00251-3
Accepted:
Published online:

## Abstract

Under electron beam irradiation, knock-on atomic displacement is commonly thought to occur only when the incident electron energy is above the incident-energy threshold of the material in question. However, we report that when exposed to intense electrons at room temperature at a low incident energy of 30 keV, which is far below the theoretically predicted incident-energy threshold of zirconium, Zircaloy-4 (Zr-1.50Sn-0.25Fe-0.15Cr (wt.%)) surfaces can undergo considerable displacement damage. We demonstrate that electron beam irradiation of the bulk Zircaloy-4 surface resulted in a striking radiation effect that nanoscale precipitates within the surface layer gradually emerged and became clearly visible with increasing the irradiation time. Our transmission electron microscope (TEM) observations further reveal that electron beam irradiation of the thin-film Zircaly-4 surface caused the sputtering of surface α-Zr atoms, the nanoscale atomic restructuring in the α-Zr matrix, and the amorphization of precipitates. These results are the first direct evidences suggesting that displacement of metal atoms can be induced by a low incident electron energy below threshold. The presented way to irradiate may be extended to other materials aiming at producing appealing properties for applications in fields of nanotechnology, surface technology, and others.

## Introduction

With the aim of studying how electron beam irradiation changes the structures and properties of thin film solids or nanosystems, an increasing number of irradiation experiments were carried out in a TEM using a high energy electron beam. In general, radiation damage of electron beam is undesirable, however, recent experiments have demonstrated that it can have beneficial effects6, 7, 9,10,11,12. Examples are precise cutting of single-walled carbon nanotubes using electron beam13, electron-beam-assisted coalescence or joining of single-walled carbon nanotubes14,15,16 and metallic nanowires17, interesting phenomena due to electron beam irradiation such as phase transformation in graphite18, 19, α-FeSi220, and Sn-based nanowires21, controlled growth-reversal of catalytic carbon nanotubes22, and extreme pressure inside carbon nanotubes23, 24, electron-beam-induced formation of nanostructures like carbon onions25, double-walled nanotubes26, nanopores27, alumina nanocapsules28, silicon nanocrystals29, 30, and crystalline aluminum borate nanowires31, restructuring of NaREF4 nanocrystals under electron beam irradiation32, just to mention a few. In these examples, the applied electron energies are all equal to or greater than 200 keV and even up to MeV, which makes possible the atomic displacement. However, to date research of electron irradiation of inorganic solids with a low energy (<100 keV) electron beam is still limited33,34,35,36,37,38,39 and the reported studies mainly focus on graphene33,34,35,36,37. Also, our literature survey has reflected that there has been no attempt to investigate radiation damage or radiation effect of low energy electron beam on metals.

In this study, we irradiated surfaces of recrystallized α-type Zircaloy-4 (Zr-1.50Sn-0.25Fe-0.15Cr (wt.%)) at room temperature using stationary electron beam with a small diameter at 30 kV accelerating voltage in a FEI Inspect F50 field-emission scanning electron microscope (FE-SEM). It was striking to find that under irradiation nanoscale precipitates within the surface layer of bulk Zircaloy-4 gradually emerged and became clearly visible with increasing the irradiation time. Furthermore, TEM investigations using a combination of bright field (BF) TEM imaging, selected area electron diffraction (SAED), fast Fourier transformation (FFT) diffraction, and inverse fast Fourier transformation (IFFT) imaging reveal that under irradiation with 30 keV electrons the displacement of zirconium atoms at the surface of thin-film Zircaloy-4 indeed occurred, exhibiting in the forms of sputtering of surface α-Zr atoms, nanoscale atomic reconstructions in the α-Zr matrix and disorder formation in precipitates. These results are beyond the common expectation as the incident electron energy under study is much lower than the theoretically predicted incident-energy threshold of zirconium for knock-on atomic displacement and we attribute them to a considerably high specimen current density and a relatively high energy deposition rate in the specimens.

## Results and Discussion

We begin by preparing bulk specimens (5 mm in thickness) of recrystallized pure Zr (>99.9 wt.%) and Zricaloy-4 (Zr-1.50Sn-0.25Fe-0.15Cr (wt.%)) with α-Zr phase of P63/mmc space group40, and polishing their surfaces (see Methods for detail). On these two polished surfaces irradiation with focused electron beam at a 30 kV accelerating voltage in the FE-SEM was performed at room temperature for 32 electron beam scans (imaging was simultaneously carried out and every scan lasts 35 s to obtain a SEM image), respectively (see Methods for detail). Their SEM morphological evolutions under irradiation are shown in Fig. 1a–e and f–j, respectively. By comparison, it can be found that as irradiation continued up to 32 scans, an increasing number of ball-shaped nanoparticles with bright contrast and various diameters (~35–500 nm) gradually emerged on the Zircaloy-4 surface and their profiles became clearly visible (see Supplementary video), whereas the surface of pure Zr remained unchanged. These nanoparticles should be assigned to the precipitates in Zircaloy-4, as nanoscale precipitates resulting from addition of alloying elements exist in Zircaloy-4 rather than in pure Zr. To further confirm this assignment, compositional analysis was carried out. Energy dispersive X-ray spectrum (EDS) results (Fig. 2a–c) reveal that the newly presented nanoparticles (Point 2 in Fig. 2a) after irradiation on the zircaloy-4 surface possess a higher content of alloying elements Fe and Cr compared with the matrix of Zircaloy-4 (Point 1 in Fig. 2a). It is in line with the fact that the precipitates in Zircaloy-4 (ball-shaped nanoparticles with dark contrast in Fig. 2d) ~30–450 nm in diameter are rich in Fe and Cr (Fig. 2g,h). Thus, it can be concluded that performing focused and stationary electron beam at a low incident energy of 30 keV in the FE-SEM enables the precipitates in Zircaloy-4 to emerge on its surface with clear profiles. This phenomenon can be interpreted as a radiation effect, as it is a macroscopically observable change of surface morphology due to electron beam irradiation. We noticed that this radiation effect can be clearly observed only when the diameter of electron beam in the FE-SEM is small enough like our experimental condition (the magnification should be at 20,000× or larger and the irradiation area is ~14.9 × 12.8 μm2 or smaller, correspondingly).

To investigate the atomic-scale radiation damage of electron beam on the precipitates as well as the α-Zr matrix, a TEM specimen of Zircaloy-4 was prepared (see Methods for detail), and then in this specimen a thin film region containing a precipitate of interest (yellow arrows in Fig. 1k–q) was selected to be irradiated at room temperature for 32 scans in the FE-SEM (like the case for the bulk Zircaloy-4 surface) and subsequently observed in the TEM. Such alternative treatments of electron beam irradiation in the FE-SEM and microstructure observation in the TEM were repeated for 4 times (TEM observations were carried out after irradiation for 32, 64, 96, and 128 scans, respectively).

Prior to irradiation, detailed pre-observations of the selected precipitate were carried out in the TEM. Figure 3(A-a-1)–(A-a-3) show the BF TEM morphology of the precipitate before irradiation viewed along the [11$2 ̄$3]α-Zr direction. The FFT image (Fig. 3(A-c-2)) corresponding to the central area of the precipitate (Fig. 3(A-c-1)) shows a clear periodic FFT diffraction pattern. Through indexing, it can be identified that the precipitate is Zr(Fe,Cr)2 phase of P63/mmc space group41 and the zone axis in Fig. 3(A-c-2) is along the [110]Zr(Fe,Cr)2 direction. Thus, the orientation relationship between the matrix (α-Zr phase) and the precipitate (Zr(Fe,Cr)2 phase) here can be written as:

$[ 11 2 ̄ 3 ] α - Zr ∣∣ [ 110 ] Zr Fe,Cr 2$

A close observation indicates that not only stacking faults but also moiré fringes existed on the precipitate as described below. Figure 3(A-a-1) shows that the right half of the precipitate exhibiting lamellar morphology (orange arrow in Fig. 3(A-a-1)). These lamellar contrasts are almost parallel to each other and also parallel to the (001)Zr(Fe,Cr)2 planes, but their spacings are not uniform. Seen from the composite SAED pattern of the precipitate and its surrounding matrix (Fig. 3(A-b)), it is apparent that bright streamlines (yellow arrow in Fig. 3(A-b)) existed between the diffraction spots corresponding to the (001)Zr(Fe,Cr)2 planes (making undiscernible the [110]Zr(Fe,Cr)2 SAED pattern), suggesting that the lamellar contrasts are attributed to stacking faults in the precipitate42, 43. Apart from these stacking faults, the perimeter of the precipitate exhibited morphology of parallel fringes with an equal spacing (red arrow in Fig. 3(A-a-1)). These fringes are found to, on one hand, terminate at the interface of the precipitate and the matrix, and, on the other hand, disappear at the interior of the precipitate. To investigate these fringes, a local area where they disappeared at the interior of the precipitate (Fig. 4a) was selected to analyze. The IFFT image (Fig. 4f) corresponding to Fig. 4a, which reduces the noise, shows that parallel fringes can be clearly observed on the up-right part of the image. A detailed look indicates the up-right and down-left parts of the image exhibited the atomic structures of α-Zr phase along [11$2 ̄$3]α-Zr and Zr(Fe,Cr)2 phase along [110]Zr(Fe,Cr)2, respectively, as confirmed by their corresponding FFT diffraction patterns (Fig. 4c and d), respectively. These results reveal that the matrix covered on the perimeter of the precipitate and that the fringes only appeared on where the matrix and the precipitate were overlapped. Thus, these fringes can be assigned to moiré fringes, which are formed under electron beam if two crystals are superimposed at a suitable mutual orientation44. The moiré fringes together with the stacking faults provide two interesting characteristics of morphology to study radiation damage of electron beam.

The electron-beam-induced radiation damage occurred not only on the precipitate but also on its surrounding matrix, which have some typical manifestations of atomic structure change as shown in Fig. 5a and k. Seen from Fig. 5a, in spite of the remaining [11$2 ̄$3]α-Zr atomic structure (the insert of Fig. 5g), localized areas exhibited a new atomic structure (the insert of Fig. 5i), corresponding to a new FFT diffraction pattern (Fig. 5d). Its formation is geometrically due to <10$1 ̄$1> displacement (red arrow in the inset of Fig. 5g) of atoms within the (10$1 ̄ 1 ̄$)α-Zr planes. Apart from this new atomic structure, another irradiation-induced atomic feature in Fig. 5a is that every two (10$1 ̄ 1 ̄$)α-Zr planes exhibited the same stronger (orange arrows in the inserts of Fig. 5h and j) or weaker (green arrows in the inserts of Fig. 5h and j) contrasts, forming a sort of planar fault. It is like a sinusoidal vibration of (10$1 ̄ 1 ̄$)α-Zr planes along the [11$2 ̄$3]α-Zr direction. These planar faults result in new periodic atomic structures (red circles in the inserts of Fig. 5h and j), corresponding to new FFT diffraction patterns (Fig. 5c and e). Such an atomic feature can also be seen in Fig. 5k, which is more apparent. In Fig. 5k, moreover, strain fringes can be observed (red arrows in Fig. 5k), among which dislocations are visible (yellow ‘T's in Fig. 5p). The appearance of strain fringes is more likely because of irradiation-induced stress relaxation at the interface of the precipitate and the matrix. The formation of new atomic structures along with the appearance of strain fringes suggest that within the matrix surrounding the precipitate atomic displacement took place upon electron beam irradiation.

Above experiment results have confirmed the occurrence of knock-on displacement of zirconium atoms under electron beam at a low incident energy of 30 keV. Theoretically, a simple expression given by Hobbs for the displacement energy (Ed) allows us to determine the incident-energy threshold (Et) for displacement of atoms of atomic weight A within a lattice network3, 4

$E t = 100 + A E d 5 1 2 - 10 20$
(1)

where Et is in MeV and Ed is in eV. For zirconium, which has an atomic weight of 91.22 40, the minimum energy for atomic displacement within its crystal is ~18 eV2, so accordingly the incident-energy threshold for knock-on damage calculated according to Hobbs’ expression is ~433 keV. In the case of sputtering, Hobbs’ expression remains valid, but the energy for displacing a surface atom is much lower because there are fewer other atoms to disrupt. In general, the incident-energy threshold for sputtering is ~50% less than that for knock-on damage4. As for zirconium, it was reported to be ~210 keV2. By comparison, it is evident that the 30 keV incident electron energy that caused knock-on displacement of zirconium atoms in our experiments is far below the incident-energy threshold of zirconium for knock-on damage (~433 keV) and sputtering (~210 keV). Such a finding leads to our reconsidering of the criteria for knock-on atomic displacement to occur under electron irradiation. Literature indicates that calculating the incident-energy threshold according to Hobbs’ expression assumes that a nucleus is only exposed to one electron, so this assumption is suitable for the situation when the specimen current density is very low, in which a nucleus interacts with one electron at the same time. In this case, the incident electron energy should be the dominate factor that influences atomic displacement and displacement damage cannot occur when the incident electron energy below threshold. This is in line with our TEM observation experience that under electron beam in the TEM, which was maintained at a very low specimen current density of ~40 pA/cm2 at a high accelerating voltage of 200 kV, the Zircaloy-4 specimen remained stable during our experimental time (in the scale of hours). However, in the case for the FE-SEM a high specimen current density at the level of mA/cm2 and even A/cm2 can result from electron probes of very small diameter5, like the case for irradiation in our experiments. As our SEM observation indicates that the displacement of zirconium atoms under electron beam at a low accelerating voltage of 30 kV can take place significantly only when the diameter of electron beam in the FE-SEM is small enough, that is only when the specimen current density in the FE-SEM is large enough. Thus, a high specimen current density should mainly contribute to considerable knock-on displacement of zirconium atoms under a low energy electron beam in the FE-SEM. Under such electron beam at a high specimen current density (even though the incident electron energy is very low), a nucleus is likely to simultaneously undergo elastic collisions with multiple electrons and the small momentum that it acquires through the impact of every electron can be accumulated to a large one, making possible its displacement. This might be one explanation, which still needs to be further confirmed. Moreover, electron energy deposition rate increases with decreasing the incident electron energy3, so it is higher under electron beam in the FE-SEM compared with that in the TEM, which also possibly contributes to the radiation damage on the Zircaloy-4 surfaces under irradiation in the FE-SEM. The other possible damage mechanism is through heating induced by electron beam irradiation. A higher specimen current density results in a larger increase of specimen temperature5, however, the temperature rise is insignificant for metals4, 5, in particular for the bulk, in which heat flow is radial in three dimensions5, so the irradiation-induced thermal effect cannot be the dominant factor that accounts for atomic displacement. Thus, we believe that a considerably high specimen current density together with a relatively high energy deposition rate are the main reasons for the knock-on displacement of zirconium atoms under focused and stationary electron beam at a low incident energy in the FE-SEM.

## Materials

Recrystallized pure Zr (>99.9 wt.%) and Zricaloy-4 (Zr-1.50Sn-0.25Fe-0.15Cr (wt.%)) with α-Zr phase of P63/mmc space group40 were prepared. Both of them were cut into bulk specimens 5 mm in thickness by electrical discharge machining and their surfaces were wet ground through P150, P320, P800 and P2000 silicon carbide abrasive papers and polished using liquid SiO2 (~50 nm in diameter) suspension solution until the ‘mirror-like’ surface finish. In addition, a transmission electron microscope (TEM) specimen of Zircaloy-4 was prepared by a standard preparation technique, which includes cutting of a disk 3 mm in diameter, grinding, and electro-polishing in a twin-jet electropolisher with a chemical solution of 10 vol.% perchloric acid (HClO4) and 90 vol.% ethanol (C2H5OH) at −30 °C until perforation occurred.

## Electron beam irradiation

Electron beam irradiation was conducted at room temperature in a FEI Inspect F50 field-emission scanning electron microscope (FE-SEM) under high vacuum using electron beam at 30 kV accelerating voltage at 204 μA emission current. During irradiation, the FE-SEM was operated in line scan mode at 20,000× magnification with a selected area of ~14.9 × 12.8 μm2 focused by electron beam. The electron irradiation dosage was controlled by the number of sustained electron beam scans (every scan lasts 35 s to obtain a SEM image), that is, the irradiation time, as the specimen current density was constant during irradiation. 32 sustained electron beam scans were performed on the polished surfaces of bulk pure Zr and Zircaloy-4 specimens, respectively. In addition, a thin film region of the TEM specimen of Zircaloy-4 was irradiated for 32 sustained electron beam scans in the FE-SEM and subsequently observed in a JEM 2100 F TEM. Such alternative treatments of electron beam irradiation in the FE-SEM and microstructure observation in the TEM were repeated for 4 times (TEM observations were carried out after irradiation for 32, 64, 96, and 128 scans, respectively).

## Characterization

Morphology observations during electron beam irradiation were performed in the FEI Inspect F50 FE-SEM under high vacuum at 30 kV electron accelerating voltage at 204 μA electron emission current. During FE-SEM observations, chemical compositions were analyzed with energy dispersive X-ray spectrometer (EDS) attached to the FE-SEM. TEM observation was conducted in a JEM 2100 F TEM under high vacuum operating at 200 kV electron accelerating voltage at 204 μA electron emission current at ~40 pA/cm2 specimen current density. During TEM observations, bright field (BF) TEM images, selected area electron diffraction (SAED) patterns, high-resolution transmission electron microscope (HRTEM) images, scanning transmission electron microscope (STEM) images, and elemental maps were taken for analysis. In order to analyze HRTEM images, Digital Micrograph (Gatan Inc.) software was used to obtain fast Fourier transformation (FFT) diffraction patterns and inverse fast Fourier transformation (IFFT) images.

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## References

1. 1.

Was, G. S. Fundamentals of radiation materials science: metals and alloys (Springer, New York, 2007).

2. 2.

Murty, K. L., Charit, I. An introduction to nuclear materials: fundamentals and applications (Wiley-VCH, Weinheim, 2013).

3. 3.

Hren, J. J., Goldstein, J. I. & Joy, D. C. Introduction to analytical electron microscopy (Springer, New York, 1979).

4. 4.

Williams, D. B. & Carter C. B. Transmission electron microscopy: a textbook for materials science (Springer, New York, 2009).

5. 5.

Egerton, R. F., Li, P. & Malac, M. Radiation damage in the TEM and SEM. Micron 35, 399–409 (2004).

6. 6.

Banhart, F. Irradiation effects in carbon nanostructures. Rep. Prog. Phys 62, 1181–1221 (1999).

7. 7.

Krasheninnikov, A. V. & Banhart, F. Engineering of nanostructured carbon materials with electron or ion beams. Nature Mater 6, 723–733 (2007).

8. 8.

Cook, I. Materials research for fusion energy. Nature Mater 5, 77–80 (2006).

9. 9.

Banhart, F. Formation and transformation of carbon nanoparticles under electron irradiation. Phil. Trans. R. Soc. Lond. A 362, 2205–2222 (2004).

10. 10.

Krasheninnikov, A. V. & Nordlund, K. Ion and electron irradiation-induced effects in nanostructured materials. J. Appl. Phys. 107, 071301 (2010).

11. 11.

Sun, L., Banhart, F. & Warner, J. Two-dimensional materials under electron irradiation. MRS Bull 40, 29–37 (2015).

12. 12.

Mel, A.-A. E. & Bittencourt, C. In situ conversion of nanostructures from solid to hollow in transmission electron microscopes using electron beam. Nanoscale 8, 10876–10884 (2016).

13. 13.

Banhart, F., Li, J. & Terrones, M. Cutting single-walled carbon nanotubes with an electron beam: evidence for atom migration inside nanotubes. Small 1, 953–956 (2005).

14. 14.

Terrones, M., Terrones, H., Banhart, F., Charlier, J.-C. & Ajayan, P. M. Coalescence of single-walled carbon nanotubes. Science 288, 1226–1229 (2000).

15. 15.

Terrones, M. et al. Molecular junctions by joining single-walled carbon nanotubes. Phys. Rev. Lett 89, 075505 (2002).

16. 16.

Kis, A. et al. Reinforcement of single-walled carbon nanotube bundles by intertube bridging. Nature Mater 3, 153–157 (2004).

17. 17.

Xu, S. et al. Nanometer-scale modification and welding of silicon and metallic nanowires with a high-intensity electron beam. Small 1, 1221–1229 (2005).

18. 18.

Banhart, F. & Ajayan, P. M. Carbon onions as nanoscopic pressure cells for diamond formation. Nature 382, 433–435 (1996).

19. 19.

Lyutovich, Y. & Banhart, F. Low-pressure transformation of graphite to diamond under irradiation. Appl. Phys. Lett 74, 659–660 (1999).

20. 20.

Naito, M., Ishimaru, M., Valdez, J. A. & Sickafus, K. E. Electron irradiation-induced phase transformation in α-FeSi2. J. Appl. Phys. 104, 073524 (2008).

21. 21.

Zhang, H. et al. Phase transformation of Sn-based nanowires under electron beam irradiation. J. Mater. Chem. C 3, 5389–5397 (2015).

22. 22.

Stolojan, V., Tison, Y., Chen, G. & Silva, R. Controlled growth-reversal of catalytic carbon nanotubes under electron-beam irradiation. Nano Lett 6, 1837–1841 (2006).

23. 23.

Sun, L. et al. Carbon nanotubes as high-pressure cylinders and nanoextruders. Science 312, 1199–1202 (2006).

24. 24.

Rodríguez-Manzo, J. A. et al. In situ nucleation of carbon nanotubes by the injection of carbon atoms into metal particles. Nature Nanotechnol 2, 307–311 (2007).

25. 25.

Ugarte, D. Curling and closure of graphitic networks under electron-beam irradiation. Nature 359, 707–709 (1992).

26. 26.

Smith, B. W., Monthioux, M. & Luzzi, D. E. Encapsulated C60 in carbon nanotubes. Nature 396, 323–324 (1998).

27. 27.

Xu, T., Xie, X. & Sun, L. Fabrication of nanopores using electron beam. 8th nano/micro engineered and molecular systems 637-640 (2013).

28. 28.

Duan, X. et al. Formation of alumina nanocapsules by high-energy-electron irradiation of na-dawsonite nanorods. Sci. Rep 3, 3218 (2013).

29. 29.

Huang, W. et al. Magic electron affection in preparation process of silicon nanocrystal. Sci. Rep 4, 9932 (2015).

30. 30.

Huang, W. et al. Silicon nanocrystal growth under irradiation of electron beam. Sci. Rep 5, 16682 (2015).

31. 31.

Gonzalez-Martinez, I. G. et al. In-situ quasi-instantaneous e-beam driven catalyst-free formation of crystalline aluminum borate nanowires. Sci. Rep 6, 22524 (2016).

32. 32.

Su, X. et al. Restructuring and remodeling of NaREF4 nanocrystals by electron irradiation. Small 10, 4711–4717 (2014).

33. 33.

Yuzvinsky, T. D. et al. Precision cutting of nanotubes with a low-energy electron beam. Appl. Phys. Lett 86, 053109 (2005).

34. 34.

He, H. et al. Modifying electronic transport properties of graphene by electron beam irradiation. Appl. Phys. Lett 99, 033109 (2011).

35. 35.

Dharmaraj, P. et al. Controlled and selective area growth of monolayer graphene on 4H-SiC substrate by electron-beam-assisted rapid heating. J. Phys. Chem. C 117, 19195–19202 (2013).

36. 36.

Hossain, M. Z., Rumyantsev, S., Shur, M. S. & Balandin, A. A. Reduction of 1/f noise in graphene after electron-beam irradiation. Appl. Phys. Lett 102, 153512 (2013).

37. 37.

Yu, X. et al. Photocurrent generation in lateral graphene p-n junction created by electron-beam irradiation. Sci. Rep 5, 12014 (2015).

38. 38.

Hong, Y. K. et al. Fine characteristics tailoring of organic and inorganic nanowires using focused electron-beam irradiation. Angew. Chem. Int. Ed 50, 3734–3738 (2011).

39. 39.

Hong, C.-H. et al. Electron beam irradiated silver nanowires for a highly transparent heater. Sci. Rep 5, 17716 (2015).

40. 40.

Banerjee, S. & Mukhopadhyay, P. Phase transformations: examples from titanium and zirconium alloys (Elsevier, Oxford, 2007).

41. 41.

Toffolon-Masclet, C., Brachet, J.-C. & Jago, G. Studies of second phase particles in different zirconium alloys using extractive carbon replica and an electrolytic anodic dissolution procedure. J. Nucl. Mater. 305, 224–231 (2002).

42. 42.

Hirth, J. P. & Lothe, J. Theory of dislocations. (Wiley, New York, 1982).

43. 43.

Phillips, R. Crystals, defects and microstructure modeling across scales. (Cambridge University Press, Cambridge, 2001).

44. 44.

Amidror, I. The theory of the moiré phenomenon. (Springer, London, 2009).

## Acknowledgements

We thank Dr. Feixue Yang and Dr. Xiaolan Wang for FE-SEM and TEM measurements, respectively.

## Affiliations

1. ### Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, People’s Republic of China

• Hengfei Gu
• Geping Li
• Chengze Liu
• Fusen Yuan
• Fuzhou Han
•  & Lifeng Zhang
2. ### University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing, 100049, People’s Republic of China

• Hengfei Gu
•  & Lifeng Zhang
3. ### University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, People’s Republic of China

• Geping Li
• Chengze Liu
• Fusen Yuan
•  & Fuzhou Han
4. ### Key Laboratory of Optoelectronic Materials Chemical and Physics, Fujian Institute of Research on The Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, 350002, People’s Republic of China

• Songquan Wu

## Contributions

G.L. and H.G. contributed to the design of the experiments. H.G., C.L. and F.Y. carried out the experiments. G.L. and H.G. wrote the first draft of the manuscript and all authors assisted in the writing process and data analysis.

## Competing Interests

The authors declare that they have no competing interests.

## Corresponding author

Correspondence to Geping Li.

1. 1.

2. 2.