Understanding Anisotropic Growth of Au Penta-Twinned Nanorods by

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Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter

Understanding Anisotropic Growth of Au Penta-Twinned Nanorods by Liquid Cell Transmission Electron Microscopy Biao Jin, Maria L. Sushko, Zhaoming Liu, Xiaoxiao Cao, Chuanhong Jin, and Ruikang Tang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00164 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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The Journal of Physical Chemistry Letters

Understanding Anisotropic Growth of Au PentaTwinned Nanorods by Liquid Cell Transmission Electron Microscopy Biao Jin, † Maria L. Sushko, § Zhaoming Liu, † Xiaoxiao Cao,¶ Chuanhong Jin, *‡ and Ruikang Tang *†‡ † Department

of Chemistry, ‡ State Key Laboratory of Silicon Materials, School of Materials

Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China § Physical

and Computational Sciences Directorate, Pacific Northwest National Laboratory,

Richland, Washington 99354, United States ¶ Gatan,

Inc. 5794 W. Las Positas Blvd. Pleasanton, CA 94588, United States

Corresponding Author *[email protected] *[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Morphology control of anisotropic nanocrystals is important for tuning shapedependent physicochemical properties, but anisotropic growth mechanism remains unclear. Here, we investigate the formation of Au penta-twinned nanorod by liquid cell transmission electron microscopy (LC-TEM). It is found that a truncated-decahedron forms in the absence of cetyltrimethyl ammonium bromide (CTAB), whereas in the presence of CTAB a penta-twinned nanorod forms by producing {100} facets via reentrant groove and selectively inhibiting atoms addition to {100} facets. Density functional theory (DFT) simulations show that the partial relieve of strain energy of decahedron is caused by the adsorption of Br- ions. Moreover, the selective adsorption of CTAB lowers the surface energy of {100} facets, thus enhancing the nanorod growth. Our work points out the importance of the synergy of strain and surface energy, which provides an in-depth insight into the anisotropic growth of nanorods and lays foundations for controlled synthesis of nano-materials.

TOC GRAPHIC

KEYWORDS: anisotropic growth, Au nanorod, CTAB, LC-TEM, penta-twinned structure, surface and strain energy


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Metal nanocrystals have great potentials in various applications due to their facet and shapedependent physical and chemical properties.1-5 Among them, the penta-twinned face centered cubic (fcc) metal nanorods/nanowires have received extensive attentions because of their remarkable mechanical properties6,7 and applications in energy storage,8 catalysis9,10 and surface enhanced Raman spectroscopy.11 The synthetic approaches to nanorod growth (e.g. Au nanorods) are largely emperical,12-16 and suffer from limited reproducibility and low yield. Notably, these studies reveal that additives can promote anisotropic growth of nanorods via altering the surface energy of different facets (thermodynamic control)17 or by blocking the growth of specific crystal facets (kinetic control).18 It has proven difficult to elucidate the mechanisms governing anisotropic growth at the nanoscale partly due to the inherent difficulty in providing key information regarding possible growth trajectories and intermediated structures,19 although there are some theoretical modeling predictions.20-27 For example, cetyl-trimethyl ammonium bromide (CTAB) can be used to modulate anisotropic growth of Au nanorod by preferential adsorption, formation of wider water/ions channels or production of distinguished electronic potential difference.21,23-25 However, these mechanisms do not provide a straightforward insight into when and why {100} facets start to appear and grow, and how additives affect anisotropic growth of nanorods in solution phase syntheses.19 Thereby, revealing the microscopic physical process in solution is important to understand the anisotropic growth mechanism and control the aspect ratio of rod,19 which necessitates the insight from advanced characteristic techniques. Liquid cell transmission electron microscopy (LC-TEM) enables tracking individual nanoparticles in solution28 and providing information on the role of surfactants in nanocrystals growth.29-31 Here, we use LC-TEM to investigate the anisotropic growth mechanism of Au pentatwinned nanorod by capturing its growth trajectories in aqueous solution. This work provides


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direct experimental evidence that the formation of Au penta-twinned nanorod in water is a multistage process that starts with the isotropic growth of regular decahedron and then, proceeds to the reentrant groove induced anisotropic growth of a truncated decahedron and finally, to a penta-twinned nanorod with higher aspect ratio. It reveals a new mechanism by which strain and surface energy synergistically promote anisotropic growth of Au penta-twinned nanorod.

Figure 1. The growth dynamics of decahedron to truncated Au decahedron. (a) Time-lapse sequence of in situ TEM images showing the growth process of Au decahedron. (b) Analysis of the measured distances from the crystal center to {100} and mean gray value. (c, d) A tilted decahedron with ten {111} planes (blue) and five {100} side planes (green). The electron dose rate is 1.33 e-/Å2·s. Sequential TEM images in Figure 1a display the growth process in the absence of CTAB of a decahedron aligned along the zone axis (Movie S1). The detailed growth process is


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revealed by tracking the shape and size evolution of the particle. Change in the distance from the center of nanocrystal to its facet is quantified by tracking the propagation of {111} facets (Figure 1b). The growth dynamics along [100] direction is referred to as the height changes of nanocrystal, which is evaluated by measuring the gray value (intensity transmitted through the nanocrystal).32 Based on the difference in growth rates, the whole process is divided into three stages (Figure 1b). During the stage I, the growth rate of the decahedron is 0.64 nm/s, while the mean gray value remains almost unchanged, following an isotropic enlargement of the particle. Then (stage II), the lateral growth rate reduces to 0.45 nm/s, and the gray value of the nanocrystal decreases linearly. This process corresponds to the decahedron not only enlarging but also probably starting to elongate along the [110] direction. At stage III, the decahedron slightly elongates along the [110] direction with slightly decreasing in gray value. Noticeably, only a truncated decahedron forms from the initial regular decahedron, as evidenced by the ex situ TEM micrographs and the corresponding schematic showing some decahedrons with five {100} side faces (Figure 1c, d). Assuming the constant rate of reduction of gold ions, we estimate the reaction rate per unit area to be around 12.49 atom·s-1·nm-2 for Au decahedron, which is comparable to the reaction rate for Au nanoplates synthesized in liquid cell. Considering that the atomic density of (111) planes is 13.87 atom·nm-2, this reaction rate is roughly equal to one layer per second.33 The slow reduction kinetics caused by low electron dose rate is favorable for Au decahedron formation and growth, which is in line with the formation and growth of nanocrystals reported in previous publications.18


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50 nm

Figure 2. The growth dynamics of Au penta-twinned nanorod. (a) Time-lapse sequence of in situ TEM images showing the growth process of penta-twinned decahedron and truncated decahedron and resulted penta-twinned nanorod. The electron dose rate is 1.55 e-/Å2·s. (b) The corresponding schematic illustrations of growth process in (a). (c) The projected area of Au nanocrystal as a function of time. (d) Analysis of the measured distances, including the width ([100] direction) and length ([110] direction) of Au nanorod. (e) The decahedron with re-entrant groove during in situ LC-TEM experiments. Insets represent the structure schematic. To understand the roles of CTAB in anisotropic growth, the growth process of Au decahedron in the presence of CTAB is investigated (Movie S2). Sequential TEM images in Figure 2a reveal the formation and growth process of Au penta-twinned nanorod. The corresponding schematic illustration is presented in Figure 2b. Initially, a regular decahedron is observed along the zone axis. During the first 40.0 s of reaction the decahedron continuously enlarges with ten {111} facets (isotropic growth), as evidenced by the increasing projected area of the nanocrystal (Figure 2c). Then the projected area of the nanocrystal remains almost unchanged for the next 15.0 s (time


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period between 40.0 ~ 55.0 s). During this period, a reentrant groove forms at one of the five twin boundaries (Figure 2e and at 43.0 s in Figure S1), which matches previous observations that reentrant groove most likely appears between two adjacent twin boundaries for relieving the strain.17 It is also noted that the planes of reentrant groove are parallel to the twin boundaries (insets in Figure 2e), showing that they should belong to the (111) planes.17 This new reentrant groove causes {100} facets to appear, which shows that the strain created by the gap between the five twins may favor the formation of side faces via the reentrant groove. Then the reentrant groove disappeared rapidly due to the attachment of atoms on it under growth condition (Figure S1). Subsequently, the projected area of the nanocrystal increases dramatically after 55.0 s (marked by arrows in Figure 2b), corresponding to the rotation process. The rotation makes a truncated decahedron structure with five new {100} facets is clearly visible (58.0 ~ 63.0 s). The results indicate that the nanoparticle begins to elongate parallel to the twin planes when the size of decahedron is ~ 50 nm. Then the truncated decahedron lies on a side as a result of elongation (63.8 s), as demonstrated by the electron beam direction in viewed zone combinations of / zone axis. In order to investigate the growth dynamics of penta-twinned nanorod, the growth rate along [110] direction is quantified. It is found that the truncated decahedron continuously grows along [110] direction with the growth rate of 0.46 nm/s, whereas the growth rate along [100] direction remains almost unchanged (Figure 2d). Due to the selective growth along the [110] direction the truncated decahedron evolves into a longer penta-twinned nanorod (105.2 s in Figure 2a), which is also demonstrated by ex situ TEM (Figure S2). Such a selective growth has been also confirmed when a penta-twinned nanorod grows in aqueous solution in the presence of CTAB (Figure S3) where the growth rate along [110] direction is significantly higher than that along [100] direction


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(Figure S3). This growth dynamic indicates that CTAB molecules can promote anisotropic growth of penta-twinned nanorod by selectively inhibiting atomic addition to the {100} facets. But it should be noted that the growth rates of different nanorods are different (Figure S4a-d), which may depend on electron dose rate or local precursor concentration. During our LC-TEM experiments, electron beam induced Au penta-twinned nanorod could be understood by seedless growth method.34,35 The Au decahedrons were first produced by higher electron dose rate (Figure S5a, b), which can be correlated to strong reducing agents (e.g. NaBH4) in a typical colloidal synthesis. The subsequent growth of the decahedron to Au penta-twinned nanorod is achieved at low electron dose rates (~ 1.55 e-/Å2·s), which corresponds to weak reducing gents (e.g. ascorbic acid). But it is also found that decahedron transformed into complex polyhedron at high electron dose rate (~ 23.36 e-/Å2·s) in the presence of CTAB (Figure S6), which is consistent with the fact that scientists tend to use weak reducing agents to synthesize Au nanorods.16 Generally, our in situ observation reveal that decahedron forms first, then elongates to truncated nanorod, and finally evolves into penta-twinned nanorod. Such structural transformation is consistent with previous observations during colloidal seed-mediated growth of penta-twinned nanorod.36,37 Therefore, the efforts to elucidate the growth mechanisms of penta-twinned nanorods under electron beam irradiation are highly valuable for understanding their growth in general. Ex situ high-resolution TEM shows that the seeds have a decahedral shape with five-folded twins (Figure S5a, b), indicating that the penta-twinned nanostructure can be produced from these initial seeds. It also shows that the penta-twinned structure is the key for the anisotropic growth of nanorod.38 Given that {111} facets have the lowest surface energy, the multiply twinned decahedron with most of the surfaces consisting of {111} facets are thermodynamically most stabilized. The growth rates of these {111} facets are equal, even if in the presence of CTAB,


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which may result in the isotropic growth of decahedron. After the particle grows isotopically and approaches a critical size, five new {100} facets become exposed (Figure 1c), transforming the particle into a truncated decahedron with an aspect ratio of ~ 1-1.2. The shape transformation of decahedron to truncated decahedron can be understood by preferential atom attachment to the twin defect sites that have the highest surface energy. These areas usually are the disclination wedges between facets formed due to an intrinsic geometric misfit between five tetrahedral constituent units of a decahedron (as shown in the HR-TEM image in Figure S5c).12 However, it should be noted that the intermediates-reentrant groove and growth dynamics of decahedron in the presence of CTAB are very different from the case in the absence of CTAB. For example, the decahedron with reentrant groove structure appears during the growth process of regular-to-truncated decahedron in the presence of CTAB (Figure S1). Although the reentrant groove has been found in previous work,17 it has not been reported as an intermediated structure of penta-twinned nanorod. This important phenomenon unraveled by LC-TEM provides a reliable evidence of the relaxation of internal strain by reentrant groove. Furthermore, the growth dynamics of pentatwinned nanorod (Figure 2d) implies that surface properties of {100} and {111} facets may be altered by CTAB. These experimental observations furnish a hypothesis that the synergy effect of intrinsic strain and surface energy may play a key role in the morphology evolution of pentatwinned nanocrystals, as reported by previous works.17,38


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Figure 3. DFT simulations for the (111)-(111) and (111)-(100)-(111) edge. Structure of {111}{111} junction (a) and {111}-{100}-{111} junction without (b) and with adsorbed Br (c). Br are shown in brown, Au atom displaced from their equilibrium bulk positions by less than 0.30 Å are shown in gold, those displaced by 0.35 ± 0.05 Å are shown in grey, and those displaced by over 0.40 Å are shown in blue. Average Au atom displacement is (a) 0.25 Å, (b) 0.22 Å, and (c) 0.19 Å, respectively. To test this hypothesis, computational simulations were performed. Density functional theory (DFT) simulations reveal that the top layer of Au atoms on a {111}-{111} junction experiences a significant force directed towards the center of the particle leading to their displacement from their bulk-like positions (Figure 3a). The displacement and the associated strain are the largest at the tip of the junction (Figure 3a). The associated surface strain increases with the length of the {111}{111} junction and can be partially relieved by the creation of the {100} surface (Figure 3b). However, in the absence of CTAB the growth of the extended {100} surface come at the penalty of the increased surface energy.39 These competing effects of strain and surface energy cost impose thermodynamic limits on the maximum particle size and the area of the {100} facets. In particular, the free energy of decahedron can be calculated as:26 𝐺(𝑇) = 𝑉(𝑊𝐷𝑉 + 𝛥𝐺𝑓) + ∑𝑖𝐴𝑖(𝛾𝑖 + 𝑊𝑆𝑖) + 𝛾𝑡𝐴𝑡

(1)


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where V is the volume of the nanoparticle, 𝑊𝐷𝑉is the volume strain energy, ΔG̅f is the change in free energy per unit volume (from liquid to solid), γ is the surface free energy, WS is the surface strain energy density term and A is the area of the crystallographic facet. The subscript i refers to various crystallographic facets present on the surface and subscript t refers to the {111} twin boundary in the penta-twinned nanoparticles. Based on the equation (1), the free energy of decahedron depends on surface energy and strain energy. The 𝑊𝐷𝑉 contribution is negligible compared to the surface energy term at very small sizes, so the decahedron tends to grow isotopically into larger decahedron with lower surface energy {111} facets, along with the decreasing of total surface energy. Whereas 𝑊𝐷𝑉 starts to dominate the free energy as particle size increases. Only when the size of decahedron increases to a critical size, {100} facets begin to appear. Although the surface energy of {100} facets is lowered to less than that of {111} facets in the presence of CTAB,39 the precondition is that {100} facets need to form since the decreasing of surface energy is achieved by selective absorption of CTAB. Therefore, to reduce strain energy, the truncated decahedron with {100} facets appears when the particle grows to a critical size both in the absence and presence of CTAB. But it is unclear that what factors control the critical size, speculated by additives or the precursor/seeds concentrations ratio.16,40 And the measurement of exact critical size is still lacked since LC-TEM is difficult to capture statistical cases about structural transformation in homogeneous penta-twinned nanostructures nucleation and growth, which needs a new exploration for us in future. CTAB has a two-fold effect on the decahedron growth. Br- ions adsorbed on the surface relieve surface strain on {100} and {111} facets and drive the surface atoms to relax closer to their bulk positions thereby reducing the strain energy (Figure 3c) and facilitating initial stages of particle growth. This explains the elongation of decahedron at smaller size in the presence of


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CTAB. In the presence of Br- ions, the regular decahedron produces new {100} facets by creating reentrant groove for relieving the strain (Figure 2e), which is in line with previously proposed growth mechanism of metal rods.41 Upon the formation of extended {100} facets, CTAB starts playing the dominant role in directing the asymmetric nanorod growth. Previous DFT simulations predicted that when the area of {100} facet is sufficiently large to support a dense monolayer of adsorbed CTAB, the surface energy of the {100} facets becomes smaller than that of {111} facets. In particular, without CTAB γ100/γ111 = 1.20 and with CTAB γ100/γ111 = 0.71.39 It means that when CTAB is present {100} facets are more stable than {111} facets, resulting in the preferential atom attachment on {111} facets. In agreement with the theoretical predictions,25 the adsorption density of CTAB is different on these two facets (Figure S7 and S8), which promotes growth anisotropy of the penta-twinned nanorod. Although these phenomena can be simply understood by the selective capping model, it is worth noting that the involvement of CTAB also changes the formation pathway of {100} facets. Another report showed that only truncated decahedron or a thicker rod forms without or with low concentration of additives, whereas thinner nanowire forms in solutions with high concentrations of additives.17 In other words, if the elongation of decahedron is only related to the strain, the width of the resulting truncated decahedron, nanorod and nanowire should be identical. However, the experimental observations reported here contradict this conclusion, that is the width of the nanorod depends on the different critical size (~ 120 nm in Figure 1a and ~ 50 nm in Figure 2a) of the decahedron particle when it starts to elongate. In this regard, the width of nanorod/nanowire is likely affected by the additives. For example, the sub-20 nm Ag nanowires were prepared with the assistance of Br-.42 Therefore, CTAB not only promotes anisotropic growth of penta-twinned nanorod but may also affect its critical size of the particle when it starts to elongate, resulting in the controlled aspect-ratio of the nanorod.


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It has been shown based on the modified Wulff construction,27 that the elongated pentatwinned nanorod could be obtained by lowing the surface energy of the {100} facets to γ111/γ100 = 2/√3. The prediction was verified for Ag penta-twinned rod.17 Lowing the surface energy of {100} facets will decrease the total excess free energy of penta-twinned nanorod described by equation (2):43

∆𝐸𝐹𝑇𝑅 =

0.5

2𝛾111𝑡𝑎𝑛 (𝜃1)

(15𝑡𝑎𝑛 (𝜃1)) (10𝛾100 + 𝑠𝑖𝑛 (𝜃 ))𝐿𝐴0.5 𝐶𝑆 +2𝑈𝑀𝐿𝐴𝐶𝑆 + 𝑠𝑖𝑛 (𝜃 )𝑐𝑜𝑠 (𝜃 )𝐴𝐶𝑆 5𝛾𝑡

1

1

2

(2)

where ΔEFTR is the total excess energy of the penta-twinned nanorod, γ is the surface energy, γt is the twin energy, L is length of rod, Acs is the cross-sectional area, UM is the elastic energy term, θ1= 0.628 rad and θ2= 0.523 rad. With γ100/γ111 = 0.7139 in the presence of CTAB the above condition for the nanorod growth is satisfied. Based on equation (2) and the critical projected area in Figure 2c, the critical excess energy is approximately 10-14 J and is of the same order of magnitude as the change in volumetric strain energy at the critical size, when decahedron starts to elongate.26 In our experiments, the concentration (0.5 mM) of CTAB is lower than the critical micelle concentration (~ 1.0 mM),44 which prevents the formation of CTAB micelles and water channels at the surface of the decahedron. Moreover, the elemental analysis (Figure S8) demonstrates that the adsorption density of Br- (~ 5 number/nm2) is significantly higher than the maximum density of CTAB on Au lattice facets (~ 2.44 number/nm2).22 This underscores the role of Br- ions in promoting the appearance of high-energy {100} facets at smaller nanoparticle sizes in the presence of CTAB. The origin of this effect can be understood by considering that at smaller particle sizes the area of the {111} facets is relatively small, and the strain energy dominates the total free energy. Simulations predict that Br- adsorption at the {111}-{100}-{111} junction reduces 𝑊𝐷𝑉(Figure 3c), which can explain the reentrant groove structure (Figure 2e) related to


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strain stress relaxation.17 Only at the later stages of growth CTAB enhances the anisotropic growth along the axial direction via modifying the surface energy of {100} and {111} facets.

Lower the γ100

Figure 4. Schematic diagram describing the different growth process of Au penta-twinned nanocrystal under two conditions. (a) The growth of decahedron in the absence of CTAB. (b) The growth process of decahedron in the presence of CTAB. State I, II and III represent regular decahedron, IV and V represent truncated decahedron, VI represents penta-twinned nanorod. Using this mechanistic understanding of the interplay of the strain and surface energies tuned by an additive, it is possible to control the final products shapes in which either a truncated decahedron (without CTAB, Figure 4a) or a penta-twinned nanorod forms (with CTAB, Figure 4b). Following this mechanism, the formation of penta-twinned nanorod structure starts with the initial anisotropic growth (symmetry breaking) and is followed by the enhanced anisotropic growth. Both stages can be tuned by specific additives. Although the unfavorable sharp atomic edges at the intersection of {111} facets and intrinsic stacking faults can lead to truncated decahedron with some level of {100} faceting, the appearance of {100} facets can also be regulated by additive-induced strain energy reduction (Figure 3c). Most importantly, once a truncated decahedron structure is established, surface energy changes as a result of the distinct additives distribution on {100} versus {111} facets provides an intrinsic driving force for


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preferential growth in [110] direction. The regulation of these two processes can control the final aspect ratio of penta-twinned nanorod, which is important for tuning their mechanical flexibility and localized surface plasmon resonance properties.42 In summary, we report an in situ study of anisotropic growth process of Au penta-twinned nanorods with the assistance of CTAB in aqueous solution to reveal the formation and anisotropic growth mechanism. The present study reveals the intermediate structure during the anisotropic growth process of penta-twinned nanorods, a decahedron with reentrant groove, and identifies the roles of the strain and surface energies through DFT simulations. It is suggested that partial relieve of strain energy of decahedral nanoparticle by Br- ions adsorption causes the {100} facets to appear possibly via reentrant groove at smaller size. The asymmetric selective adsorption of CTAB onto the Au {100} facets results in the decrease in the surface energy of {100} facets below that of the {111} facets and drives the formation and anisotropic growth of penta-twinned nanorod. Strain and surface energy-tuning strategies are proposed to play a key role in regulating penta-twinned nanostructure, providing a new insight for controlled synthesis of metallic nanorods. Generally, such an understanding of anisotropic growth mechanism can be used to improve materials engineering and nano-fabrication tailored for applications.

ASSOCIATED CONTENT Supporting information. The following files are available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett. Details for materials and methods and supplementary figures. (PDF)


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In situ TEM movies showing the growth process of Au penta-twinned nanostructures in aqueous solution. (AVI) AUTHOR INFORMATION Notes The authors declare no competing financial interests. Author Contributions C. Jin and R. Tang conceived and designed the experiments. B. Jin performed LC-TEM imaging and analyses. M. L. Sushko performed the DFT simulations. B. Jin, Z. Liu, C. Jin and R. Tang analyzed the data. X. Cao provided Gatan OneView camera and supported data acquisition. B. Jin and M. L. Sushko co-wrote the paper. All authors revised the manuscript. ACKNOWLEDGMENTS We thank X. Ma, H. Wang and Y. Jiang for helpful discussions. The work on microscopy was carried out in the Center of Electron Microscopy of Zhejiang University. This work was funded by the National Natural Science Foundation of China (21625105, 51772265 and 61721005). Theoretical work by M. L. Sushko was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award KC020105-FWP12152. Simulations were performed using Pacific Northwest National Laboratory (PNNL) Institutional Computing Resources. PNNL is operated by Battelle for the Department of Energy under contract No. DE-AC05-76RLO1830.


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