Microsecond time‐ and subnanometer spatial‐scale in situ observations of crystallization process in amorphous antimony nanoparticles by the UHVEM newly developed at O saka U niversity

Abstract

Fast in situ observation by TEM is one of useful techniques in researches on phase transitions of nanoparticles. In our previous study, it was evident that amorphous antimony nanoparticles can be crystallized with ease by stimulation from the outside. For example, when lead atoms are vapour‐deposited onto amorphous antimony nanoparticles kept at room temperature, crystallization of the amorphous antimony nanoparticles is abruptly induced by an interfacial strain between an antimony nanoparticle and crystalline lead nanoparticles attached. On the other hand, knock‐on displacements by high energy electron irradiation also become one of the stimulations for the crystallization of the amorphous nanoparticles. In the present study, electron‐irradiation‐induced crystallization processes of amorphous antimony nanoparticles have been studied by microsecond time‐ and subnanometer spatial‐scale in situ observations by ultra‐high voltage electron microscope developed with JEOL Ltd. at Osaka University recently. Amorphous antimony nanoparticles supported on thin amorphous carbon substrates were prepared by a vapour‐deposition method. 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 10 24 e m −2 s −1 , which was equipped with Gatan K2‐IS electron direct detection CMOS camera. The time for one frame was 625 μs. The figure 1 shows a typical example of migration of interface between an amorphous and crystalline phase during crystallization in an approximately 60 nm‐sized nanoparticle as indicated by arrows. As indicated in fig. 1(a), the nucleation site of the crystalline phase is located on the particle surface. At the early stage of the crystallization, the interface has a small curvature as shown in (b) ˜ (f), but at the steady state of (g) ˜ (j), the interface becomes flat. The velocity of the interface migration is estimated to be approximately 10 μm s −1 . Atomic scale observations by HREM were carried out. The figure 2 shows the snapshots during crystal growth in about 20 nm sized nanoparticle. In fig, 2(a), 2 nm‐sized crystalline nucleus appears on the surface of the particle, and the FFT pattern from the particle is in set. Week four spots are recognized as indicated by four arrows in the FFT pattern, and correspond to nucleation of the small crystal. In fig. 2(b), the nucleus grows up to approximately 5 nm in diameter, after that the amorphous nanoparticle is crystallized in the whole nanoparticle. In the FFT pattern, the week four spots change to an obvious net pattern, which is indexed as the [2–21] zone axis pattern of an antimony crystal. In this case of the 20 nm‐sized nanoparticle, the velocity of interface migration is estimated to be approximately 20 μm s −1 . The velocity of the interface migration depends on the particle size, and it was confirmed that the smaller the particle size is, the faster the velocity is. From the observation, the critical particle size for crystallization all over the nanoparticle is estimated to be approximately 5 nm. A strain on the interface between this crystalline nucleus and the amorphous nanoparticle may induce the crystallization all over the nanoparticle. A schematic illustration of crystallization mechanism in amorphous antimony nanoparticles is shown in the bottom of figure 2. The amorphous nanoparticle has to jump beyond the activation energy for the crystallization. At the early stage of the crystallization, small nucleus fluctuates between an appearance and a disappearance. However, when the size of the nucleus is larger than the critical size for crystallization, the strain energy of interface between this crystalline nucleus and the amorphous nanoparticle will be larger than the activation energy. It is suggested that the strain energy is a trigger for crystallization in amorphous antimony nanoparticles.