Fast in Situ Ultrahigh-Voltage Electron Microscopy Observation of

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Fast in situ ultra-high voltage electron microscopy observation of crystal nucleation and growth in amorphous antimony nanoparticles Hidehiro Yasuda Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01626 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Crystal Growth & Design

Fast in situ ultra-high voltage electron microscopy observation of crystal nucleation and growth in amorphous antimony nanoparticles

Hidehiro Yasuda Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567-0047, Japan Corresponding author; Tel.: +81 6 6879 7941; fax: +81 6 6879 7942 E-mail: [email protected]

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Abstract: Electron-irradiation-induced crystallization processes and the mechanisms in amorphous antimony nanoparticles have been investigated by microsecond temporal and pico-meter spatial resolution in situ observations. Electron irradiation experiments and the simultaneous in situ observations were carried out by ultra-high voltage electron microscope operating at an accelerating voltage of 1 MV, which has temporal resolution of 625 µs per frame. At the early stage of the crystallization in approximately 20 nm-sized amorphous nanoparticles, small crystal nucleus on the surface repeats between formation and annihilation. When the nucleus size becomes more than the critical size of 6.3 nm in diameter, the crystal growth takes place in the whole nanoparticle. The crystal growth rate estimated was approximately 20 µm s-1. The growth rate depends on the particle size, and it was confirmed that the smaller the particle size is, the faster the growth rate is. It was suggested that the crystallization driven by long-range elastic interaction due to small crystal nucleus formation in amorphous nanoparticles is induced by short-range atomic rearrangements.

Keywords:

fast in situ TEM observation, phase transformation, amorphous nanoparticle

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1. Introduction Fast in situ observation by transmission electron microscope is one of useful technique in researches on phase transformations in materials. This technique plays an important role not only in basic scientific researches but also in developments of recording materials utilizing phase transformations. In the previous in situ observation technique, video rate imaging of 30 frames per second is popular.1 Recently, transmission electron microscopy (TEM) with high temporal resolution has been developed. TEM with high temporal resolution has found many applications of materials science, in the studies of materials dynamics such as phase transformations, transient states, or chemical reactions.2 For example, pulse electron beams make the timescale from nanoseconds to femtoseconds accessible to TEM with nanometer spatial resolution. At Lawrence Livermore National Laboratory, the technique are applied to observe dynamic phenomena with snapshots of diffraction and imaging by nanosecond temporal resolution, such as crystallization of amorphous germanium film or metallic glass,3, 4 the α−β phase transformation in titanium nanocrystals,5 solidification in aluminum thin film,6 mixing in a multilayer foil of Al/Ni0.91V0.09,7 and so on. On the other hand, using continuous electron beams, atomic scale observations by microsecond temporal resolution were achieved by a combination with direct electron detection systems. Those recording systems are consisting of both supersensitive imaging system by position-sensitive electron detector and high speed recording system by data processing utilizing mass storage. Using one of the systems developed, image recording of 1600 frames per second is possible. The high temporal resolution ultra-high voltage electron microscope (UHVEM) equipped with a direct electron detection system (JEM-1000EES) developed recently at Osaka University has a great advantage to carry out in situ observations by microsecond temporal and pico-meter spatial resolution. This UHVEM can be applied to observe a crystallization process of an amorphous material. A semi-metallic antimony material, which is utilized as a component of recording materials for optical disk and so on, has been focused here. On the assumption that the recording region is nm-sized region, 3



amorphous antimony nanoparticles were adopted as an analytic sample. It was previously evident by our research group that amorphous antimony nanoparticles crystallize with ease not only by an annealing but also by stimulation from the outside. For example, it was revealed that when a crystalline clusters consisting of lead atoms which are not reactive with antimony is attached on the surface of an amorphous antimony nanoparticle, crystallization is abruptly induced.8, 9 On the other hand, researches on phase transformations using knock-on displacements by high energy electrons in the UHVEM have been carried out in our group. In this technique, electron irradiation controls crystalline-to-amorphous or its reverse phase transformations.10, 11 Knock-on displacements also become one of the stimulations for the crystallization of the amorphous nanoparticles. In amorphous antimony materials, knock-on displacements of atoms by 1MeV electrons occur with ease, and the atomic displacements induce the crystallization.12 From the above overview, in the present study, electron-irradiation-induced crystallization in amorphous antimony nanoparticles has been studied by fast in situ UHVEM. Both of crystallization rates and mechanisms will be discussed in detail.

2. Experimental procedures Preparations of amorphous antimony nanoparticles were carried out using an evaporator installed in the specimen chamber of a conventional transmission electron microscope. An amorphous carbon film mounted on a copper grid was used as a supporting film. Using the evaporator consisting of a spiral-shaped tungsten filament, antimony atoms were evaporated to produce nm-sized amorphous nanoparticles on the supporting film. Electron irradiation experiments and the simultaneous in situ observations were carried out by JEM-1000EES UHVEM operating at an accelerating voltage of 1 MV, and the electron flux of the order of approximately 1024 e m-2 s-1. The Gatan K2-IS direct electron detection camera equipped has the temporal 4


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resolution of 625 µs per one frame. In this irradiation condition, knocked-on displacements of antimony atoms take place. Additionally, when an incident electron passes through a nanoparticle, most of the electron energies lost in the nanoparticle are converted into heat. The temperature of the nanoparticle becomes stationary when the heat generated in the nanoparticle by electron irradiation balances with the heat dissipated by conduction. The heat dissipation by thermal conduction takes place on the nanoparticle/amorphous carbon interface. The temperature rise in an amorphous carbon film mounted on a metal frame of electron microscope grid is approximately 10-2 K at the center of the beam, based on the energy loss given by Bethe's stopping power equation.13 The heat flowed from a nanoparticle will distribute in the amorphous carbon film. The heat distribution was calculated by a finite difference method, and it was assumed that the free surface of an amorphous carbon film is adiabatic. In this case, the maximum temperature rise of amorphous carbon film due to the electron beam heating was estimated to be approximately 10 K.14 Then, it seems reasonable to estimate that the magnitude of the temperature rise of nanoparticles due to the beam heating is of the order of 10 K. The value of 10 K is in agreement with the conventional observation by Takayanagi et al. that the beam heating of lead nanoparticles on a graphite substrate is below 20 K.15 The base pressure in the specimen chamber was below 1×10-5 Pa, and is enough low to keep oxidation in the specimens negligibly slow.

3. Results and discussion Figure 1 shows diameter d from the nucleation site to the crystal/amorphous interface as a function of time t. The diameter indicated by the left inset in the graph were measured from the in situ TEM images inset on the right side of the graph. The images are 12 cross-sectional snapshots of bright field images captured at the time interval of 625 µs during crystal growth in an about 80 nm-sized particle. The crystallization proceeds from a sharp-pointed portion with a small surface curvature of the nanoparticle 5



indicated by an arrow in Figure 1 (a). The traces of the nanoparticle surface with crystal/amorphous interface are shown by black arrows from Figure 1(b) to (k). The crystal growth gradually proceeds from a nucleation site of the surface and it is estimated that the interface migration rate is approximately 5 µm s-1. The relationship between the diameter of the crystal nucleus formed in the amorphous nanoparticles and the time is indicated by a curve fitting using an equation of d [nm] = 0.74√t [µs]. This result corresponds to a parabolic law in which the growth rate of crystal nucleus is inversely proportional to the diameter. In this case, the atomic displacement becomes rate-determining step in the growth of crystal nucleus. When the size of the crystal nucleus becomes larger, it takes long time to displace atoms, and this kinetics is consistently approved. In the parabolic law of the crystallization process, the atomic displacement at the amorphous/crystal interface is the rate-determining step. From Fick's law, assuming (d/2)2=kD’t, where the crystal nucleus radius is d/2, the atomic displacement which is the rate-determining step, that is, the diffusion coefficient is D’, a constant is k, and the time is t, then d = 2√kD’t. When the constants are rounded into diffusion coefficient D, a nucleus diameter d is indicated by the equation of d=√Dt. It is estimated that a value of D is 5.2×10-13 [m2 s-1] by fitting of the data at the early stage in Figure 1. On the other hand, self-diffusion coefficient of bulk antimony is calculated by the equation DBulk = 2.21×10-3 exp(-2.37×104 / T) ,where T is temperature.16 Extrapolation of this equation to 300 K gives a value of 1.08×10-37 [m2 s-1]. From the above calculation, it was evaluated that the antimony self-diffusivity in nanoparticles observed here is at least 1024 larger than that in bulk antimony. This fact on the fast diffusivity by a nano-size effect suggests that crystallization in pure elemental amorphous nanoparticles such as amorphous antimony nanoparticles takes place not by long-range atomic diffusion but by short-range atomic displacements. This diffusion is a rearrangement by local atomic displacement and is extraordinarily faster than the self-diffusion estimated in the previous experiments. The mobility of surface atoms in a direction parallel to the surface is large at the portion with a small surface curvature. In this experiment, it is suggested that crystal nucleation occurs preferentially at the 6


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portion where the knock-on displacement due to electron irradiation is induced with ease, and the crystal growth proceeds toward the inside of the nanoparticle. Additionally, nucleation on the surface is also advantageous from the viewpoint of the free energy change of the system. The nucleation in the interior of the particle forms an amorphous/ crystalline interface in the nucleus embedded in the amorphous nanoparticle, and the interfacial energy increases. However, nucleation of the same size on the surface brings about small increase of the interfacial energy, because the interface area of the nucleus embedded decreases. The free energy reduction obtained by the surface nucleation is larger than that obtained by nucleation inside. Figure 2 reveals 8 snapshots of phase contrast images during nucleation and growth in an approximately 20 nm-sized amorphous antimony nanoparticle. The difference in the contrast between the nanoparticle and substrate is small because of small number of electrons to capture an image. A bandpass Fourier transform filtering of the snapshots was carried out in order to remove the background noise and to enhance the difference in the contrast between the amorphous and crystalline phase. An amorphous nanoparticle before crystallization indicates a random black and white contrast as shown in (a). In the fast Fourier transformation (FFT) pattern from the particle, a halo ring originated from an amorphous structure is recognized. In a part of the same particle after 8125 µs under electron irradiation in Figure 2(b), an approximately 2 nm-sized crystalline nucleus appears as encircled by a yellow broken line. In the corresponding FFT pattern, week two spots superimposed on the halo ring appear as indicated by yellow arrows. From Figure 2(c) to (e), the crystal nucleus in the particle repeats to form and annihilate at irregular time intervals from 16875 to 31875 µs. In the individual FFT patterns, weak two or four spots corresponding to the crystal nucleation sometimes appear as indicated by yellow arrows. The phase contrast image is sensitive to defocus, but the repeats of formation and annihilation of the small crystal nucleus are not an artifact by defocusing due to the fluctuation of specimen position during observation. The crystal growth process in the particle is indicated from (f) to (h). In this case, when the diameter of the 7



crystal nucleus becomes larger than 6.3 nm as encircled by a red broken lines, crystallization takes place in the whole amorphous nanoparticle. The growth rate is roughly estimated to be 20 µm s-1. In the corresponding FFT pattern, the weak four spots pattern, as indicated by red arrows, changes to a net pattern of [2-21] zone axis, which is indexed by the hexagonal structure with the lattice constants of a0=0.43 nm and c0=1.13 nm. From the observation, it cannot be determined whether the nucleation site is inside the surface, since the crystallization is observed by projection, but it will be sufficient to determine that the crystal nucleation and growth from the surface is observed from the experimental result in Figure 1 and a large gain of free energy is obtained by the surface nucleation mentioned above. The schematic illustrations of the crystal nucleation and growth process attached reveal that a small crystal nucleus on the particle surface repeats the formation and annihilation at the early stage, and the growth quickly occurs after the formation of the 6.3 nm-sized critical nucleus. Compared with the result on the growth rate in Figure 1, the crystal growth rate depends on the size. Although two representative examples of experiments are shown in the paper, more than 10 experiments were carried out. For example, the crystallization rate of approximately 60 nm-sized particle is estimated to be about 5 µm/s, and is almost consistent with the result on the crystallization in 80 nm-sized particle of Figure 1. Additionally, the crystallization rate in the 30 nm-sized particle was evaluated to be less than 24 µm/s. The result is consistent with the result that the crystallization rate of the approximately 20 nm particle in Figure 2 is about 20 µm/s. From those results, it was confirmed that the smaller the particle size is, the faster the growth rate is. In the present experiments, in situ observations of crystallization in amorphous antimony nanoparticles have been achieved by the UHVEM with fast temporal and high spatial resolution. Knock-on displacements by high energy electrons trigger the crystal nucleation at room temperature, but the crystallization by knock-on displacements occurs by enhancement of atomic diffusion as well as 8


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crystallization by annealing. Therefore, the candidate mechanisms applied to the general crystallization behavior in amorphous nanoparticles could be proposed based on the present experiment. Crystallization of amorphous materials proceeds by the processes of crystal nucleation and the consequent growth. The bulk and interface free energies of a crystal nucleus compete in the crystallization process. In the three dimensional system, the interface free energy is an increasing function of the square of a nucleus radius, on the contrary the bulk free energy is a decreasing function of the cube of a nucleus radius. The competition of the interface and bulk free energies gives a barrier, that is, an activation energy. A nucleus, which has a size corresponding to a maximum of the barrier, is called a critical nucleus. When the size of nucleus is larger than that of the critical nucleus by a thermal fluctuation or knock-on displacement in the present experiment, the crystal growth proceeds.17 This classical nucleation model stands up in a short-range interaction system, but can not be applied in a long-range interaction system, in which the interface free energy can not be distinguished from the bulk free energy. Recently, a research focused on a nucleation theory, that long-range interaction by an elastic interaction is associated with a nucleation process from a non-equilibrium phase has been published.18 In this research, the critical nucleus size in a nucleation process from a non-equilibrium phase to a stable phase was demonstrated by molecular dynamics simulations in the system with different sizes. Consequently, it was confirmed that the size of the critical nucleus is proportional to the total system size. A schematic picture of crystal nucleus in the amorphous nanoparticle is indicated by the right inset in the graph of Figure 1. As a parameter to characterize the nucleus size, the central angle θ is defined. With the contact angle of π/2, lens-shaped nucleus is defined for any θ, where the interface is given by the circle of the radius r=R tan (θ/2), whose center is the crossing point of the two triangle lines. The relative energy density between energy density of crystal nucleus and that of the complete amorphous nanoparticle is described as a function of θ for several system size 2R. For the small value of θ , the relative energy density is almost constant and then that increases with θ. In this region, the nucleus is expected to shrink in 9



the relaxation process. Around θ =2.3π/10, the relative energy density indicates the maximum value and it decreases for larger θ. At this θ, the nucleus fraction is equal to approximately 5 %, which agrees with the threshold value of the fraction. When the nucleus size exceeds the critical size, the nucleus grows. For the different system size, this critical size r changes in proportion to the system size R. Namely, the critical angle exists, but not a specific critical size. This fact was demonstrated that the nucleus shape is almost the same for systems of different size. This nucleation theory has been applied to the present experiments. In Figure 1, the radius of the nanoparticle is R=10nm. From the relation of r=R tan (θ/2), the value of nucleus radius is estimated to be r=3.8 nm. This value is consistent with the experimental value of the critical nucleus radius, 3.2 nm, measured in Figure 1(f). On the other hand, as the size of the nanoparticle is R=40nm in Figure 2, the critical nucleus size will be expected to be r=15 nm from the equation mentioned above. In photo (b) of Figure 2, the fraction of the nucleus is approximately 20 %, which is larger than that of the critical size, and the growth is going on. Phase stability of the non-equilibrium state is improved with increasing the system size. The bulk and interface contributions to the potential barrier cannot be distinguished in this long-range interaction system, in which the crystal nucleus elastically affects to the amorphous matrix. This fact suggests that the critical size changes depending on the system size. Figure 3 shows a schematic illustration of the free energy difference during the crystal nucleation and growth processes as a function of the reaction coordinate. The free energy in a crystalline phase ECry is lower than that in an amorphous phase EAm, but the activation energy for crystallization ∆E puts a barrier between the two phases, that is, the activation energy. A driving force to overcome ∆E is supplied by the free energy increasing due to the elastic interaction ∆Ee in an amorphous matrix near the crystal/amorphous interface in the nanoparticle. It is suggested from the above discussion that the elastic interaction, as colored by green circles, relatively affects from a short range to a long range with increasing the crystal nucleus size in a nanoparticle with a fixed size. As soon as the long-range elastic interaction 10


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energy ∆Ee is larger than the activation energy ∆E, crystal growth may take place abruptly by short range atomic displacements like a domino toppling. Generally, in amorphous materials, an activation energy ΔE during incubation at an early nucleation stage of crystallization is equivalent to that during the consequent crystal growth, and accordingly the atomic diffusion during incubation and growth are involved with the same diffusion mechanism.19 Especially, in pure elemental amorphous materials, short range atomic displacements mentioned above are dominant in the nucleation and growth.

4. Conclusions The processes and mechanisms of crystallization in amorphous antimony nanoparticles were first elucidated by microsecond temporal and pico-meter spatial resolution in situ observations using the UHVEM. It was suggested that the crystallization driven by long-range elastic interaction due to small crystal nucleus formation in amorphous nanoparticles is induced by short-range atomic rearrangements. This new finding obtained from fast in situ UHVEM observations may contribute developments of recording materials by utilizing phase transformations.

Supporting information The supporting is available on the ACS Publication website. The movies of Fig. 1 and Fig. 2

Acknowledgements The author is grateful to kind support to construct the Materials- and Bio-Science Ultra-High Voltage Electron Microscope by Shigemasa Ohta, Akihiro Ohsaki, Yusuke Agatsuma, Sadayuki Takakuwa, Mitsuaki Ohsaki and the JEM-1000EES UHVEM project members at JEOL Ltd.

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References (1) Jacobsson, D.; Panciera, F.; Tersoff, J.; Reuter, M. C.; Lehmann, S.; Hofmann, S.; Dick, K. A.; Ross, F. M. Interface Dynamics and Crystal Phase Switching in GaAs Nanowires. Nature 2016, 531, 317-322. (2) Zewail, A.; Thomas, J. 4D Electron Microscopy; Imperial College Press, 2010. (3) Nikolova, L.; LaGrange, T.; Reed, B. W.; Stern, M. J.; Browning, N. D.; Campbell, G. H.; Kieffer, J. C.; Siwick, J.; Rosei1, F. In Situ Laser Crystallization of Amorphous Silicon: Controlled Nanosecond Studies in the Dynamic Transmission Electron Microscope. Appl. Phys. Lett. 2010, 97, 203102-1-3. (4) LaGrange, T.; Grummon, D. S.; Reed, B. W.; Browning, N. D.; King, W. E.; Campbell, G. H. Strongly Driven Crystallization Processes in a Metallic Glass. Appl. Phys. Lett. 2009, 94, 184101-1-3. (5) LaGrange, T.; Armstrong, M. R.; Boyden, K.; Brown, C. G.; Campbell, G. H.; Colvin,J. D.; DeHope, W. J.; Frank, A. M.; Gibson, D. J.; Hartemann, F. V.; Kim, J. S.; King, W. E.; Pyke, B. J.; Reed, B. W.; Shirk, M. D.; Shuttlesworth, R. M.; Stuart, B. C.; Torralva, B. R.; Browning, N. D. Single-Shot Dynamic Transmission Electron Microscopy. Appl. Phys. Lett. 2006, 89, 044105-1-3. (6) Kulovits, A.; Wiezorek, J. M. K.; LaGrange, T.; Reed, B. W.; Campbell, G. H. Revealing the Transient States of Rapid Solidification in Aluminum Thin Films Using Ultrafast In Situ Transmission Electron Microscopy. Philos. Mag. Lett. 2011, 91, 287-296. (7) Kim, J. S.; LaGrange, T.; Reed, B. W.; Taheri, M. L.; Armstrong, M. R.; King, W. E.; Browning, N. D.; Campbell, G. H. Imaging of Transient Structures Using Nanosecond in Situ TEM. Science 2008, 321, 1472-1475. (8) Yasuda, H.; Mori, H. Spontaneous Alloying of Zinc Atoms into Gold Clusters and Formation of Compound Clusters. Phys. Rev. Lett. 1992, 69, 3747-3750. (9) Yasuda, H.; Mori, H. Spontaneous Alloying and Crystallization in Nanometer Sized Amorphous Antimony Clusters. Thin Solid Films 1997, 298, 143-146. (10) Thomas, G.; Mori, H.; Fujita, H.; Sinclair, R. Electron Irradiation Induced Crystalline Amorphous 12


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Transitions in NiTi Alloys. Scripta Met. 1982, 16, 589-592. (11) Anada, S.; Nagase, T.; Kobayashi, K.; Yasuda, H.; Mori, H. Phase Stability of σ-CrFe Intermetallic Compound under Fast Electron Irradiation. Acta Mater. 2014, 71, 195-205. (12) Yasuda, H.; Furuya, K. Irradiation-Temperature Dependence of Chemical and Topological Disordering Induced by High-Energy Electron Irradiation in GaSb. Philos. Mag. A 2000, 80, 2355-2363. (13) Bethe, H. A.; Ashkin, J. Experimental Nuclear Physics Vol.1; John Wiley and Sons: NY, 1952. (14) Holman, J. P. Heat Transfer; McGraw-Hill Kogakusha: Tokyo, 1976. (15) Mitome,M.; Tanishiro, Y.; Takayanagi, K. On the Structure and Stability of Small Metal Particles: High-Resolution UHV Electron Microscope Study. Z. Phys. D 1989, 12, 45-51. (16) Gilifalco, L. A. Atomic Migration in Crystals; Blaisdell Waltham: MA, 1964. (17) Abraham, F. F. Homogeneous nucleation theory; Academic Press: NY, 1974. (18) Nishino, M.; Enachescu, C.; Miyashita, S.; Rikvold, P. A.; Boukheddaden, K.; Varret, F. Macroscopic nucleation phenomena in continuum media with long-range interactions. Sci. rep. 2011, 162, 1-5. (19) Varshneya, A. K. Fundamentals of Inorganic Glasses; Academic Press: NY, 1994.

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Figure 1. Changes in a distance from the crystal nucleation site to the interface as a function of time. The distances were measured from 12 snapshots of bright field images captured at the time interval of 625 µs during crystalline growth in an about 80 nm-sized particle shown in the right side.

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Figure 2. Snapshots of phase contrast images during nucleation and growth in an approximately 20 nm-sized amorphous antimony nanoparticle, and the schematic illustrations of the crystal nucleation and growth process.

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Figure 3. A schematic illustration of the free energy difference during the crystal nucleation and growth processes as a function of the reaction coordinate.

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For Table of Contents Use Only Manuscript title Fast in situ ultra-high voltage electron microscopy observation of crystal nucleation and growth in amorphous antimony nanoparticles

Author list Hidehiro Yasuda

Synopsis Fast in situ observations by ultra-high voltage electron microscope exhibits that crystallization of amorphous antimony nanoparticles occurs in critical nucleus size and that the smaller the particle size is, 17



the faster the growth rate is.

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Fast in Situ Ultrahigh-Voltage Electron Microscopy Observation of

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