Amorphization and Directional Crystallization of Metals Confined in

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Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy Daiming Tang, Cui-Lan Ren, Ruitao Lv, Wan-Jing Yu, Peng-Xiang Hou, Ming-Sheng Wang, Xianlong Wei, Zhi Xu, Naoyuki Kawamoto, Yoshio Bando, Masanori Mitome, Chang Liu, Hui-Ming Cheng, and Dmitri Golberg Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00664 • Publication Date (Web): 26 Jun 2015 Downloaded from http://pubs.acs.org on July 4, 2015

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Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy Dai-Ming Tang1,2*, Cui-Lan Ren3,4, Ruitao Lv5, Wan-Jing Yu3, Peng-Xiang Hou3, Ming-Sheng Wang2, Xianlong Wei2, Zhi Xu2, Naoyuki Kawamoto2, Yoshio Bando2, Masanori Mitome2, Chang Liu3*, HuiMing Cheng3, and Dmitri Golberg2* 1

International Center for Young Scientists, National Institute for Materials Science, Namiki 1-1,

Tsukuba, Ibaraki, 305-0044, Japan 2

World Premier International Center for Materials Nanoarchitectonics, National Institute for Materials

Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan 3

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of

Sciences, 72 Wenhua Road, Shenyang 110016, China 4

Division of Nuclear Materials Science and Engineering, and Key Laboratory of Interfacial Physics and

Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 5

Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua

University, Beijing 100084, China *Corresponding authors: E-mail: [email protected], [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology and metallurgy of different materials confined in such nano-space. Here, we investigate the non-equilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Due to nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (~1013 K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-tomedium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nanowire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.

KEYWORDS: In situ electron microscopy, molecular dynamics simulations, carbon nanotubes, phase transition, amorphization


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Non-equilibrium processes under extreme conditions are vital for the exploration of new materials with unique microstructures and superb properties. For example, a fast cooling rate of ~106 K/s was realized on micrometer-sized samples by splat-quench solidification. This led to the revolutionary discoveries of metallic glasses by Duwez et al. and quasicrystals by Shechtman et al.1, 2 Recently, it was reported that under an extremely high cooling rate even monatomic metallic glasses could be formed.3-8 Another example is the severe plastic deformation (SPD). By heavy deformations under confined conditions, grain refinement could be realized and nanostructured metals with superior mechanical properties could be fabricated.9, 10 Carbon nanotubes (CNTs) possess characteristic nanoscale channels which can be filled with various materials.11 This provides a unique platform for the investigation of size related phenomena, including phase transformations, chemical reactions, mass transport, and mechanical, electrical and thermal properties.11-18 Because the system is so small, it is only possible to thoroughly explore the processes using an instrument with high spatial resolution, i.e. a high-resolution transmission electron microscope (HRTEM). Modern TEM facilities can be used not only as an improved powerful human “eye” but also as a dedicated electro-mechanical system: in situ TEM allows one to reveal kinetic processes under atomic resolution.7,

17-22

In the present work, we used multiwalled CNTs as nanocrucibles and

investigated the ultrafast nano-metallurgical processes by an in situ TEM approach. It is demonstrated that various metals can be transformed into amorphous-like phases under heating/cooling cycles due to the efficient heat transfer within the CNT nanocrucibles. In addition, we show that subsequent amorphous-to-crystalline transition can be precisely controlled using delicate in situ electrical biasing and Joule heating of the metal@CNT systems. Intermediate states during amorphization and severe


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deformation-induced defects during crystallization were captured, providing valuable insights into peculiar nano-metallurgical processes. Heat transfer is strongly size dependent. This applies to both thermal radiation and thermal conduction. Thermal radiation from a black body follows the Stefan-Boltzmann law: q = σ AT 4 , where σ is the Stefan–Boltzmann constant, A is the surface area, T is the temperature and q is the thermal radiation power. Therefore, the radiation is proportional to the surface area, and the radiation power per volume is inversely proportional to the size of the radiator. The heat conduction along a one-dimensional CNT could be described by the Fourier equation:

∂T ∂ 2T = α 2 , where T, t, x, and α are the temperature, ∂t ∂x

time, position coordinate and thermal diffusion coefficient, respectively. Assuming a linear temperature gradient, an approximate solution could be obtained as follows: T=

8

π

e 2

−

απ 2 l2

t

sin

π l

x . And thus, the

cooling rate is approximately inversely proportional to l2. In addition, the interfaces between the filled metals and CNTs are atomically smooth. With the aid of electron beam nanowelding,23, 24 all walls of the CNT could be tightly connected with the metal electrodes prior to the discussed pulse experiments. Therefore, an efficient thermal transfer through the filled metal-CNT and CNT-metal electrode pathways should be expected. Based on such considerations, we designed the metal-filled CNTs as nanocrucibles for the investigation of the non-equilibrium processes during ultrafast bias-induced Joule heating/cooling cycles. The experimental configuration is illustrated in Fig. 1a. A metal-filled CNT was manipulated by a scanning tunneling microscopy (STM) probe inside TEM to connect it to the counterpart Au electrode. The structures of the metal-filled CNTs were characterized by XRD and HRTEM. One typical XRD


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spectrum of Fe-filled CNTs is shown in Figure S1, besides the graphite (002) peaks, major peaks could be ascribed to body-centered cubic (BCC) and face-centered cubic (FCC) phases of iron. In Figure 1b-c, a Fe nanowire in the CNT was indexed as a single-crystalline BCC phase, according to the well-resolved (110) lattice fringes, and the selective area electron diffraction (SAED) pattern corresponding to the BCC Fe [-111] zone axis. The diffraction arcs associated with some spots suggest that the Fe crystal is slightly bent. Besides BCC phase, FCC phase was also identified in some samples, as demonstrated in Figure S2. For the FCC-structured Fe, twinning on (111) plane was frequently observed (Figure S2 b-c). The Co nanowires filled in CNTs, as shown in Figure S3, typically have polycrystalline FCC structures, as revealed by the HRTEM image and corresponding SAED pattern. Importantly, in cases of different metals with distinctive structures, it is common for CNTs walls to be straight, indicating a high degree of graphitization. And the interfaces between CNTs and metals are tight and smooth down to the atomic level, ensuring efficient heat transfer during heating/cooling cycles. Such metal-filled CNTs were used as nanoscale Joule-heated crucibles for the observation of the phase transitions within the confined metals. A bias pulse was programmed and applied to the CNT to induce a short Joule heating pulse. Structural changes during heating, melting and subsequent solidification were observed in real time under HRTEM imaging and video recording. This concept has firstly been tested under finite element method (FEM) simulations, as demonstrated in Figure 1d-f and Movie S1. A model was constructed by clamping a metal-filled CNT between two bulk-sized Au electrodes. Initially, the temperature was set at room temperature. When a bias was applied, the Fe nanowire inside the CNT was heated to ~1840 K within ~347 ps. The temperature distribution is uniform in the metal part, with large temperature gradient closer to the electrode (Figure 1e). When the


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bias was turned off, the maximum temperature dropped quickly to room temperature within ~286 ps, with the calculated average cooling rate of ~6.3×1012 K/s. Actually, the cooling rate was not constant. Due to the larger temperature difference at the hot state, the cooling rate was in the range of 1.5~2.5×1013 K/s, when the temperature was decreased from the maximum temperature to ~1000 K. The ultrafast cooling rate at a high temperature range (above the glass transition temperature) is critical for the formation of amorphous structures. Experimentally, bias pulses were applied to the metal-filled CNTs. The results are presented in Figure 2, Figure S4-10 and in situ recorded Movies S2-4. The structures documented before and after the bias pulse for a Fe-filled CNT are demonstrated in Figure 2. Initially, the Fe nanowire exhibits a FCC phase, with twinning on (111) plane (Figure 2a-b), with the orientation near [110] zone axis. It was transformed into a fully amorphous structure after the bias pulse, as revealed by the disappearance of lattice fringes and appearance of the typical “salt and pepper” contrast characteristic of an amorphous phase (Figure 2c). In addition, the consecutive SAED patterns before and after the bias pulse confirmed the amorphization. Sharp diffraction spots natural for the crystalline phase had been replaced by the halo-featured diffused rings, peculiar to the amorphous phases (Figure 2d). Besides the FCC- structured Fe, different types of metals with different structures were transformed into amorphous substances through the regarded pulsed bias processing. Figure S4a-d demonstrates the amorphization of a FCCstructured Fe, presenting the initial single-crystalline phase along the [-111] zone axis and resulted fully amorphous structure. Polycrystalline Co filled in CNTs was also converted into an amorphous phase under a bias pulse (Figure S5-6). In some cases, the metal particle was split during the transformation. At the beginning, the Co particle had a rod shape, consisting of many small grains with a size of around


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5 nm. After the bias pulse, the particle was transformed into two amorphous particles. The separation and flow of the particles during the process indicates that the metals were melted and subsequently quenched from the liquid state to form the amorphous phases. The chemical compositions of the amorphous metals were examined by electron energy loss spectroscopy (EELS). An example of an amorphous Co-filled CNT is shown in Figure S11. C-K and Co-L edges were clearly identified, and no impurity edges were detected. In addition, spatially-resolved EELS elemental maps were constructed from amorphous Co-filled and Fe-filled CNTs (Figure S12-13). These results show that only two spatially separated species (i.e. metal and carbon) are present. In addition to the metals encapsulated inside CNTs, we analyzed the composition of one Fe particle driven out of the CNT via electromigration. As shown in Figure S14a, the Fe particle is attached to the end of the CNT. We carefully selected the tip for EELS analysis (Figure S14b). Under the same conditions, carbon and oxygen edges could not be observed, while Fe-L edge could be clearly detected. The EEL spectra prove that the CNT could provide effective protection against oxidation and the Fe particle is free of carbon contamination. Molecular dynamic (MD) simulations were carried out to estimate the required cooling rates for the formation of amorphous metals during ultrafast quenching inside CNTs (Figure 3). A Fe-filled CNT (Figure 3a) was heated until Fe was melted (Figure 3b), and then quenched at different constant cooling rates (Figure 3c). Besides cooling at constant rate, more realistic processes with varying cooling rates at different temperature ranges obtained from FEM (Figure 1f) was also adopted in the MD simulations to control the temperature. When the Fe melt was cooled at a lower rate ~ 4.9×1011 K/s, a sharp potential drop was observed along with liquid-crystalline phase transition during the solidification (Figure 3d).


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When the cooling rate was high enough (~2.0×1013 K/s), the potential curve became smooth and an amorphous structure was retained down to room temperature. The calculated radial distribution functions (RDFs) for different cooling rates demonstrate that sharp peaks are observed for the lower rates, and broad peaks are evident for the highest cooling rate, indicating the disappearance of longrange crystalline order (Figure 3e). Therefore, through the MD simulations it is confirmed that the proposed CNT nanocrucibles are effective for the efficient heat transfer and for the investigation of the ultrafast nano-metallurgical processes. It is interesting to note that amorphization may take different routes and the as-prepared amorphous structures may show different features. Fe usually forms a homogeneous amorphous substance exhibiting complete and uniform halo-like rings in the SAED patterns (Figure 2d and Figure S4d), while the FFT pattern intensities of Co and Au are sometimes non-uniformly distributed, indicating the formation of an inhomogeneous structure (Figure S6b and S10g). The initial structures of Co and Au particles were polycrystalline and single-crystalline, respectively. After the bias pulse, the lattice fringes and diffraction spots disappeared, indicating the absence of long-range crystalline order. However, from the HRTEM images, local ordering over up to three atomic planes could be resolved, indicating the preservation of local, short-to-medium-range order.25, 26 The shapes of these FFT patterns are largely similar to those of the original structures, therefore it is believed that the non-crystalline structures have inherited the distribution of local orientation order from the initial crystalline states, and they are intermediate states between crystalline and amorphous phases. The inhomogeneous non-crystalline phase could be transformed into a homogeneous amorphous phase by an additional bias pulse, as shown in Figure S6b-c. Among various parameters, the applied bias pulse and energy are very important for the


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path of a phase transition. Depending on the contact and electrical/thermal resistivity, the applied bias varies from 0.5 V to 1.5 V, with the same pulse conditions. However, it was found that even under the same experimental condition, the resulted products could be different. Besides the crystalline-amorphous transition, crystalline-crystalline status change due to the bias pulse was also observed, as demonstrated in Figure S8. The Co particle was observed to switch from one orientation to the other after applying the bias pulse. In addition, during our MD simulations, it was found that under the same cooling conditions (initial temperature, cooling rate) the solidified structure could be different. The results of ten simulations are summarized in Table S1. While the amorphous phase with undefined local structure dominates in all cases, the percentage varies from ~72 % to ~93 %. Therefore, besides the important factors such as the applied bias pulse, thermal contact, the actual phase transition path is also determined by the probability of the “random jump” among the non-equilibrium structures. The uncovered intermediate state during crystalline-to-amorphous phase transformations fully demonstrates the unique advantage of the present in situ HRTEM approach. After the accomplished amorphization, the stability of the amorphous metals was further investigated. Electron irradiation is an important issue for in situ TEM investigations, especially for the crystallization process, where the electron irradiation could lead to increased temperature and decreased activation energy. Due to the high thermal conductivity of the CNTs and metal electrodes, a temperature increase of only a few degrees could be expected.27 Under investigations of the amorphous structures stability we turned off the electron beam within the intervals between taking consecutive TEM images to minimize the electron irradiation influence. It was found that Fe and Co amorphous phases confined in the CNTs are quite stable at room temperature, while spontaneous crystallization was observed for amorphous Au.


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An example of amorphous Fe is demonstrated in Figure S9. The Fe particle was transformed into amorphous structure and then kept at room temperature (Figure S9a-b). Within one hour, there was no detectable change (Figure S9c). After three hours, local structure relaxation was observed (Figure S9c). In contrast, crystallization of the non-crystalline Au particle occurred quite fast (Figure S10c). After keeping the system at room temperature for only ~1 min, lattice fringes corresponding to Au (111) planes start to appear in the central area, as seen on the HRTEM image and FFT pattern. The spontaneous crystallization of amorphous Au is consistent with the theoretical prediction that amorphous Au is unstable and its crystallization does not need thermal activation.28 At higher temperatures, Fe and Co amorphous phases were both transformed to crystalline phases. For example, in Figure S4e, when a constant bias was applied to the amorphous Fe-filled CNT, the uniform contrast of the amorphous phase was gradually replaced by the specific diffraction contrast indicating the appearance of a crystalline phase. And the halo-ring shaped amorphous SAED pattern was replaced by the sharp dotted-ring pattern, indicating that the amorphous phase had been transformed into a polycrystal. Similarly, after the amorphous transition due to a bias pulse applied to the CNT filled with a polycrystalline Co particle (Figure S7a-b), the particle was transformed into a single crystal under constant bias and Joule heating (Figure S7c). In addition to the homogeneous nucleation and formation of polycrystalline structures, directional crystallization was realized for a Fe amorphous nanowire in the CNT (Figure 4a and Movie S5). Due to the local cooling effect from the metal electrode, the Fe nanowire close to the electrode retained the original crystalline structure after heating and quenching (Figure 4b). Under a constant bias and corresponding resistive heating, the remaining crystalline phase acted as a nucleation seed for the


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crystallization of an amorphous phase within the CNT. Figure 4c-e shows the directional crystallization process for the Fe amorphous nanowire at a constant current of ~ 8 µA. The rough interface between the crystalline and amorphous dominating regions is marked by a white line. The crystallized Fe structure inherits the orientation of the crystalline seed, with the orientation close to the BCC [-111] zone axis. The crystallization was completed within 287 s, with the crystallization rate of ~0.1 nm/s, i.e. less than a single atomic plane per second, indicating the good controllability of the present nano-metallurgical approach. It is noticed that during the crystallization process a high density of crystalline defects has been produced, as revealed by the dark line-contrast in Figure 4e and streaking in the FFT pattern (inset of Figure 4e). Along with the appearance of crystalline defects in Fe, the CNT was bent and its initially straight graphitic walls became locally curved. The severe plastic deformation could be caused by the high compressive stress originated from the CNT’s wall, due to the volume change during crystallization and knock-on effect of the electrons. It was predicted that inside graphitic nanoscale vessels a pressure as high as 40 GPa could be reached, leading to the transformation from graphite to diamond, extrusion of Fe3C, and plastic deformation of nanosized metals.15, 29, 30 Normally, metal nanowires deform by dislocation glide and slip at the crystalline plane with the highest resolved shear stress.31, 32 However, due to the confinement from the CNT wall, the dislocation activities meet a high energy barrier. Such confinement is the origin of the high degree of lattice distortion and high density of crystalline defects discovered during the crystallization and severe deformation. Currently, we are working on further optimization of such nano-SPD processing to understand the SPD mechanism and structural modification for nanostructured metals. In summary, the rapid solidification, amorphization and crystallization of various metals, e.g. Fe, Co


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and Au, confined in CNTs were investigated by in situ TEM. With an ultrafast cooling rate of the order of ~1013 K/s, amorphous metals were obtained. By passing a constant current, directional crystallization of amorphous Fe was demonstrated. The nano-space confinement of the CNTs is the key for achieving the ultrafast cooling rate during amorphization and generating high density crystalline defects during subsequent crystallization. Our work enables a direct insight into the non-equilibrium phase transitions at the atomic level, and enlightens the potential for the nano-metallurgical processing of metals. Acknowledgements The work was supported by JSPS Kakenhi Grant Number 25820336; International Center for Young Scientists (ICYS), World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS), Tsukuba, Japan; Ministry of Science and Technology of China (Grant 2011CB932601); National Natural Science Foundation of China (Grants 51221264, 51272257, 51102242); Chinese Academy of Sciences (Grant KGZD-EW-T06); Shenyang Supercomputing Center, Chinese Academy of Sciences. The authors thank Dr. Isamu Yamada for a technical support; Profs. Koichi Tsuchiya, Takahito Ohmura, Dmitri V. Louzguine, Drs. Li-Chang Yin, and Hao Wang for fruitful discussions; and Prof. Peter Thrower for reading the manuscript and providing valuable editorial suggestions. Supporting Information Available. In situ TEM videos of the amorphization and crystallization processes; detailed methods for sample preparation, in situ TEM probing, finite element method simulations, and molecular dynamics simulations; structure of the initial metal-filled CNTs; TEM characterization of the amorphization and crystallization of various metals with different structures and transformation routes; electron energy loss


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Figure 1. Design of a metal-filled CNT as nanoscale crucible for the ultrafast thermal experiments. (a) TEM image of a Fe-filled CNT connected to an Au electrode and a STM probe. A controlled bias is applied to generate the desired current, either a short pulse or a constant current. (b) HRTEM image of a Fe-filled CNT, revealing the intimate interface between the highly crystallized CNT and single-crystal BCC-Fe phase. (c) Selective area electron diffraction (SAED) pattern of the Fe-filled CNT. Along with the diffraction of CNT (002), the Fe shows a single-crystalline pattern on the [-111] zone axis. (d) Finite element model showing the dimensions of a Fe nanowire in the CNT, and electrodes. The temperature distribution under Joule heating is marked in color. (e) Temperature distribution along the Fe-C interface. (f) Time-dependent temperature changes during a heating-cooling cycle.


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Nano Letters

Figure 2. Crystalline-to-amorphous phase transition in a Fe@CNT system. (a, b) HRTEM image and SAED pattern of the original crystalline Fe. The initial filling is a twinned crystalline FCC Fe nanowire close to the [110] zone axis. (c, d) HRTEM image and SAED pattern of transformed amorphous Fe after flash heat treatment followed by rapid natural quenching.


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Nano Letters

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Figure 3. Molecular dynamics simulations of the heating/cooling processes of a Fe-filled CNT. (a-c) Snapshots of the simulated Fe@CNT at the (a) initial, (b) molten and (c) quenched states under high cooling rate. The common neighbor analysis pattern for each atom is used to show their local crystal structures. Carbon atoms are shown in dark blue. Fe atoms with BCC, icosahedral and undefined local structures are marked in yellow, orange and dark red, respectively. (d) Potential energy per Fe atom as a function of temperature under different cooling rates. With a low cooling rate, a sharp drop corresponding to crystallization is observed during solidification, while a smooth transition is seen for the high cooling rates and FEM cooling style corresponding to the formation of an amorphous phase. (e) Radial distribution functions of solidified Fe at different cooling rates. As the cooling rate increases, the peaks become lower and broader, indicating the appearance of an amorphous phase.


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Nano Letters

Figure 4. Directional crystallization of an amorphous Fe nanowire in CNT. (a) Schematic of the directional crystallization. (b) HRTEM image of the crystalline seed (close to the electrode) and amorphous phase (near central region of the CNT). (c-e) HRTEM images of the different stages of crystallization. The rough interfaces between the crystalline and amorphous phase dominated regions are marked with the white lines. The whole crystallization process was completed in 287 s. High density of crystalline defects due to severe plastic deformation was observed, as demonstrated by the numerous dark contrast lines and streaking in the corresponding FFT pattern.


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Nano Letters

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Amorphization and Directional Crystallization of Metals Confined in

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