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Disintegration of nanostructured metals with the formation of nanoparticles in electron microscope
Nano-Structures & Nano-Objects 16 (2018) 104–109
Contents lists available at ScienceDirect
Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Disintegration of nanostructured metals with the formation of nanoparticles in electron microscope G.K. Strukova a, *, G.V. Strukov a , I.I. Khodos b , S.A. Vitkalov c a b c
Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, 142432 Chernogolovka, Russia Physics Department, City College of City University of New York, NY 10031, USA
highlights
graphical abstract
• 3D hierarchical metallic mesostructures disintegrate in transition electron microscope. • Samples containing FeNi3 nanowires, silver nanowafers or Pb–In nanorods were irradiated. • Explosive disintegration leads to form of metallic nanoparticles on a holding substrate. • Direct atomic resolution images exhibit the single crystal structure of nanoparticles.
article
info
Article history: Received 12 November 2017 Received in revised form 13 March 2018 Accepted 18 April 2018 Keywords: Nanostructured mesostructure Disintegration Metallic nanoparticles
a b s t r a c t In this paper we report on an explosive disintegration of metallic 3D samples with built-in nanoscaled hierarchical order under electron beam irradiation in a transmission electron microscope. The objects of our investigation are novel 3D mesostructures containing either FeNi3 intermetallic nanowires, or silver wafers of nanoscale thickness, or Pb–In nanorods. These structures were fabricated via a selforganization of metallic nanowires growing on templates during the pulsed electro-deposition process. The disintegration of 3D mesostructures yields an array of 2–50 nm metallic crystalline nanoparticles scattered on a holding substrate in the vicinity of the contact of the electron beam with samples. Direct atomic resolution images made in-situ reveal the monocrystalline structure of the nanoparticles. The observed rapid disintegration of 3D mesostructures in the electron beam is related to the internal energy significantly enhanced in the nanostructured samples. Possible applications of the phenomenon are © 2018 Published by Elsevier B.V. discussed.
1. Introduction
* Corresponding author.
E-mail address: [email protected] (G.K. Strukova).
https://doi.org/10.1016/j.nanoso.2018.04.004 2352-507X/© 2018 Published by Elsevier B.V.
Recently we have reported on 3D mesostructures obtained by a self-organization of nanowires growing on templates in electrolytes under the action of programmed electric pulses. These
G.K. Strukova et al. / Nano-Structures & Nano-Objects 16 (2018) 104–109
novel objects, having size of few microns to few hundred microns, demonstrate intriguing structural similarity with biological objects such as plants, fungi, seashells (biomimetics) [1]. SEM and TEM studies of Pd–Ni seashell-shaped samples revealed a hierarchic order of densely packed conical bundles of metallic nanowires. The nanowires themselves consist of 4–15 nm nanocrystals embedded in an amorphous matrix [2]. During the TEM study of branchshaped metallic mesostructures with nanoscale needles we have observed a rapid disintegration of the samples, which appear locally in the vicinity of the contact with a focused electron beam. The disintegration results in an array of nanoscale metallic particles arises near the contact on a substrate supporting samples. This fascinating behavior of 3D nanostructured metallic samples in an electron microscope, i.e. their disintegration and an appearance of the nanoparticles, to the best of our knowledge, was not described in the literature. This issue has initiated the study presented below. The usefulness of metallic nanoparticles in modern technologies stimulates the development of new methods for their production [3]. Accomplishments in this field have been demonstrated recently [4,5]. An explosion of wires in a liquid media produces nanoparticles of s- and p-type metals [4]. A microwave assisted synthesis of Pt-nanoparticles on graphene oxide facilitates a creation of effective electrodes for dye sensitized solar cells [5]. A wide variety of metal nanoparticles obtained by methods of a chemical reduction of metal ions in solutions as well as microbiological methods have been utilized in a diversity of applications [6]. The current state of art in the nanocrystal synthesis, including metal growth in a non-thermal plasma, as well as examples of applications, using the nanocrystals, are presented in a recent review [7]. The synthesis of powders of metal nanoparticles via action of a powerful focused electron beam on a massive metal target in an industrial scale is described in Ref. [8]. Furthermore during last 15 years a technique of metal deposition by a focused electron beam (FEBID) is developed, that allows to synthesize nanostructures using metallic atoms produced in decomposition processes with gaseous precursors [9,10]. Organometallic compounds (metal carbonyls) used as precursor are thermodynamically unstable under the e-beam irradiation. This property allows to conduct FEBID process in an electron microscope in-situ, since the microscope beam power is sufficient for a rapid decomposition of the precursor. As mentioned above, our nanostructured samples obtained via a pulsed electrodeposition on templates have a complex multiscale architecture with building blocks containing both amorphous and nanocrystalline phases. This leads to an increased internal energy and, as a consequence, may enhance significantly the disintegration of the samples under the electron beam irradiation. Presented below study of the novel 3D nanostructured materials under a relatively weak electron beam irradiation in an electron microscope both sheds a light on the stability of these structures and demonstrates an effective and clean method of the synthesis of crystalline metallic nanoparticles in the electron microscope. 2. Experiment The fabrication of the studied 3D metallic nanostructured materials is described in detail in [1,2]. The nanostructured metal samples are obtained onto templates by an electroplating from aqueous solutions of metal salts using programmable electric current pulses. In such conditions, the electroplating leads to both the metal deposition and a self-organization of growing nanowires, yielding nanostructured samples with a built in 3D hierarchical order propagating up to several hundred microns. Shapes of the obtained structures are often similar to the shapes of natural objects such as mushrooms, plants, seashells. In the presented work we used three different sets of samples: (1) ‘‘shells’’, obtained as
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a result of self-organization of nanowires of intermetallic FeNi3 , (2) ‘‘branches’’, consisting of silver nano-plates, and (3) ‘‘branches’’ of the Pb–In alloy nanorods. The SEM and TEM studies of the samples were performed using JEM-2100 electron microscope. For these studies the grown samples were rinsed in an ultrasonic bath twice. The first rinse was in acetone. The second was in alcohol to make a suspension of the grown structures. A droplet of the alcohol suspension was then placed on a holding substrate (a carbon-coated copper grid) and after the alcohol evaporation the substrate with the nanostructured samples was placed into the electron microscope column. All experiments were carried in the electron microscope at 200 kV accelerating voltage. An electron beam carrying total electric current (4 pA) was focused on the samples area of 20–50 nm2 . The disintegration of the sample occurs promptly upon a contact of the focused electron beam with the sample. Metal nanoparticles were observed on holding substrate in the vicinity of the contact of the sample with the focused electron beam. High resolution images of individual nanoparticles obtained via the sample disintegration were made in-situ in the same electron microscope. 3. Results All studied samples have demonstrated the explosive disintegration under irradiation by the focused electron beam. Fig. 1a, b present a hierarchical 3D mesostructure composed of nanowires of magnetic intermetallic compound FeNi3 (permalloy). The sample is shaped as a seashell. The studied sample was a single-phase FeNi3 intermetallide, that has been confirmed by an analysis of the X-ray diffraction (not shown). Fig. 1c demonstrates the edge of the sample and the array of nanoparticles disjected onto the substrate during the electron beam irradiation of the sample. The inset to Fig. 1c shows the electron diffraction pattern indicating the crystalline structure of these particles. Fig. 1c, d show that the particles have sizes less than 20 nm. The particle shape is identical to rounded polyhedrons. The nanoparticles form single crystals. This is seen from the direct high resolution images presented in Fig. 1e, f. A single-crystal FeNi3 -flake with a nano-sized thickness was found in a mortar produced by grounding of the permalloy mesostructures (Fig. 2a). After the explosive disintegration in the focused electron beam, this sample was dispersed into nanoparticles of several nanometers in size shown in Fig. 2b, c. Fig. 3 presents the image of a hierarchical 3D structure made of silver. The structure looks as a conifer branch with elements of fractal self-similarity. It is a single crystal as shown in the inset to Fig. 3a presenting the electron diffraction image of the structure. A localization of the focused electron beam on a ‘‘branch’’ of the structure induces the explosive evaporation of the metal producing an array of nanoparticles placed on the substrate near the beam. Fig. 3b shows the substrate with the array of nanoparticles scattered near the contact between the electron beam with the sample. Fig. 3c demonstrates that the scattered particles are shaped as polyhedrons with different linear sizes less than 50 nm. The electron diffraction images of the scattered particles reveal that these particles are nanocrystals. This is presented in the inset to Fig. 3c. Finally, Fig. 3d shows the direct resolution image of a single nanoparticle indicating the ordered arrangement of the atomic planes and confirming the single crystal structure of the nanoparticle. In a similar experiment, using a mesostructure consisting of gold wafers of nanoscale thickness as a target for a focused electron beam in the microscope, we have obtained gold nanoparticles on a substrate.
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Fig. 1. SEM images: the overall view of the FeNi3 3D mesostructure (a), the bundles of nanowires forming the 3D structure (b); TEM images of FeNi3 nanoparticles formed on the substrate after the electron beam irradiation of the sample demonstrate fcc crystal lattice (c–f); the lattice-resolution images of the individual nanoparticles (e, f).
Fig. 2. A single crystal fragment of the FeNi3 ‘‘seashell’’ before irradiation (a), nanoparticles formed on the substrate after irradiation (b, c). Electron diffraction patterns indicate a crystalline structure studied objects: (a) original sample before irradiation; (b) nanocrystals produced by irradiation.
Fig. 4a shows a fractal 3D structure composed of nanorods of Pb–In alloy. EDX indicates composition of 85% of Pb and 15% of In.
Fig. 4b presents a sample exposed to the electron beam. The central trunk of the ‘‘branch’’ is completely melted and shown in
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Fig. 3. 3D mesostructure of silver (a) and its enlarged fragment in ⟨001⟩ zone orientation along the electron beam direction (b); overview of Ag nanoparticles with fcc crystal structure (as it follows from the inset) on the substrate in the vicinity of the mesostructure exposed to the electron beam [c]; the TEM high-resolution image indicates the single crystal structure of disjected nanoparticle (d).
Fig. 4. Overall view of hierarchical 3D mesostructures made of Pb–In alloy (a). Figs. (b, c) show e-beam melted trunk and array of scattered nanoparticles after electron beam irradiation of the mesostructure. Selected area diffraction pattern in Fig. c (inset) corresponds to fcc structure of InPb nanoparticles. Fig. (d) presents TEM high-resolution image of an individual nanoparticle produced by e-beam disintegration.
Fig. 4a sharp needles are completely evaporated. The substrate is covered with nanoparticles of different nanometer sizes. In contrast to the e-beam disintegration shown in Figs. 1 and 3, Fig. 4b indicates that the further away from the decomposed structure the smaller the particle size. The picture suggests that this disintegration proceeds largely via melting and evaporation of the material. Fig. 4c shows that the 2D images of small particles resemble circles or rounded polygons, suggesting that the particle shape is similar
to rounded polyhedrons. The electron diffraction image of the scattered particles reveals that the particles are nanocrystals. This is presented in the inset to Fig. 4c. Fig. 4d presents the direct highresolution image of the nanoparticle with hexagonal polyhedron faces. The ordered atomic layers indicate the single crystal structure of the nanoparticle.
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Fig. 5. A fragment of the nanowire taken from the FeNi3 ‘‘seashell’’ (a); a view of the marked section indicating presence of nanoparticles (b); the electronic diffraction pattern of the FeNi3 nanowire indicating an amorphous-nanocrystalline structure (c).
4. Discussion To understand the observed phenomenon we examine obtained results together with the data on structural analysis of the samples and known mechanisms of the interaction of a focused electron beam with metal targets. In all cases studied in this work, the interaction of a sample with the electron beam results in an disintegration of its nano-sized elements and the emergence of nanoparticles. This explosion process is accompanied by a melting of more massive fragments of sample during the absorption of the e-beam energy. The studied samples are 3D tree-like fractals with self-similar elements of nano- and micro-sizes. In such nanostructured porous constructions most of atoms sit near the surface, increasing significantly the free energy and decreasing, therefore, the stability of the structures. The internal energy is further enhanced due to contributions of a large number of defects and the amorphous content between the nanocrystals that is associated with a pulsed deposition regime. As noted in the introduction, the Pd–Ni nanowires, which are building blocks of the ‘‘seashell’’ samples, have an amorphous-nanocrystalline structure [2]. The FeNi3 nanowires taken from similar ‘‘seashell’’ samples and studied in this work (Fig. 1a, b) also have an amorphous-nanocrystalline structure. This property is shown on TEM-images and electron diffraction pattern shown in Fig. 5a, b and Fig. 5c correspondingly. The large surface of the porous nanostructured samples provides an instantaneous access of the electron beam to virtually entire enhanced internal energy stored in the samples, inducing the explosive disintegration of its nano-sized elements. Below we consider the increased internal energy and the large surface of the nanostructured samples as the main sources of the high rate of the observed processes. Recently, we have reported that volumetric mesoporous structures of self-organized of Pd–Ni nanowires interact actively with a low temperature non-equilibrium oxygen plasma leading to an intense red incandescence [11]. This active interaction is another independent indicator of the increased internal energy of the studied 3D nanostructured porous materials. To understand the mechanism leading to the phenomenon, we used data and a model presented in paper [8]. The model describes a formation of ultra-dispersed particles in a plasma cloud, which emerge during an evaporation of a metallic target induced by a high-power electron beam. The model describes also the dynamics of the process, taking into account the equations of heat conduction, heat transfer and friction between plasma components, relaxation of the components to an equilibrium, condensation, evaporation and coagulation of droplets as a result of their collisions. The particle size distribution for various irradiation and cooling regimes is obtained via numerical simulation of the process. Following paper [8], we assume that presented in Fig. 3 formation of nanoparticles from nano-sized fragments of a singlecrystal Ag-nanostructure is due to a rapid evaporation of these fragments followed by a condensation of the vapor on the substrate and a subsequent crystallization into metal nanoparticles. The disintegration of the presented in Fig. 2(b) single-crystal FeNi3 -flake
and the formation of metal nanoparticles may proceed via similar scenario (Fig. 2). We have noted that the e-beam irradiation of this monocrystalline fragment produces nanoparticles with sizes of only few nanometers. This result could be explained by a small original concentration of the evaporated atoms due to the small amount of the material stored in this sample with a nanosized thickness. Subsequent aggregation of the evaporated atoms into nanoparticles proceeds, most likely, on the holding substrate and goes similar to the early stages of epitaxial thin film growth [12]. Results of the e-beam irradiation of Pb–In-mesostructure suggest again a similar mechanism (Fig. 4). The reduction of the particle size with the distance from the irradiated sample is due to the decrease of the concentration of metallic atoms in the gas cloud spreading above the substrate and, hence, the number of the deposited atoms. Shown in Fig. 1 FeNi3 -mesostructure is self-organized from nanowires containing nanocrystals embedded in an amorphous matrix. In contrast to the flake shown in Fig. 2, case the e-beam disintegration of the FeNi3 -mesostructure is accomplished by a rapid evaporation of the amorphous content and a spreading out, perhaps, solid crystalline nanoparticles (Fig. 1). This is supported by the fact that the size of nanoparticles after e-beam disintegration is comparable with the ones embedded in the amorphous content of the original structure. The presented above consideration regarding mechanisms leading to the phenomenon agrees conceptually with the model presented in [8]. A quantitative description of the nanoparticle formation and distribution requires a special investigation and is beyond the scope of this paper. The presented results suggest that metallic mesoporous structures, obtained by other methods enhancing internal energy, will also be capable for the disintegration and the formation of nanoparticles under the action of a focused electron beam in an electron microscope. From this point of view, various nanostructured aggregates, including mesoporous Pt-nanospheres, synthesized by the original chemical method of the metal reduction in a solution, are of undoubted interest [13,14], The presented method does not only yield metallic nanoparticles under conditions of high vacuum but provides a comprehensive study their physical properties in situ. This technique allows an effective access to nano-sized objects not exposed to the atmosphere or other reactive environments. This could be important for studies of artificial objects sensitive, in particular, to the oxidation or other chemical reactions. For example, mentioned above method FEBID, that was noted in Introduction, yields metallic particles which, as a rule, contain a significant amount of the carbon as an impurity because an irradiated precursor itself is a compound of carbon. 5. Conclusion Explosive disintegration of 3D hierarchical metallic mesostructures, obtained on templates in electrolytes under the action of
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programmed electric pulses, is observed after interaction with a focused electron beam in electron microscope. Mesostructures containing FeNi3 intermetallic nanowires, silver wafers of nanoscale thickness, and Pb–In nanorods were irradiated. Disintegration of these samples yields metallic nanoparticles of size 2–50 nm dispersed on a holding substrate. Obtained in-situ direct atomic resolution images exhibit the single crystal structure of nanoparticles. Presented results open new possibilities for the fabrication of metallic nanoparticles and for study them in situ in electron microscope. 6. Outlook The observed disintegration in electron microscope of 3D nanoscaled structures is a promising experimental tool due to both the simplicity and technological effectiveness of the fabricating method of the broad spectrum of metallic 3D nanostructured materials [1,2] and the accessibility of the electron microscopes. The developed pulse current electro-deposition on templates allows the controllable synthesis of a variety of metallic 3D samples with different hierarchical order built-in from nanometer to micrometer ranges and containing the fractal branching and elements of the self-similarity [1,2]. Metallic mesostructures composed of nanowires contain nanocrystals embedded into an amorphous matrix [2]. Presented in this paper study suggest that the electron beam-induced explosive disintegration in electron microscope of the nanostructured samples composed of metal, metal alloy or intermetallide may lead to the formation of the 2–50 nm crystalline nanoparticles. Metallic nanocrystals with 3–5 nm in size are considered to be of paramount importance for modern nanoelectronics and are the basic block for single quantum operations at the room temperature [15]. The distribution of the superconducting nanocrystals of the fractal Pb–In mesostructure observed after the electron beam disintegration (Fig. 4) opens a way to study single electron quantum transport between nanoscaled superconducting objects [16,17] of the sizes, which are still unattainable for the fabrication using modern lithography. Recently, a new patterning technique for gold nanoparticles placed on substrates is proposed [18]. According to [18], particular nanoparticles of their array were fixed on the substrate by irradiation with a focused electron beam in an electron microscope to produce a desired pattern. The unfixed nanoparticles can be then removed. The technique [18] in a combination with the method presented in this article opens a perspective for a fabrication of effective nanoscaled devices such as plasmonic waveguides in one technological line in situ. Acknowledgments We are grateful to I.V. Kukushkin and V.V. Ryazanov for fruitful discussions.
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References [1] G.K. Strukova, G.V. Strukov, E.Yu Postnova, A.Yu Rusanov, I.S. Veshchunov, Mesoscopic models of plants composed of metallic nanowires, J. Bionic Eng. 10 (2013) 368–376 arXiv:14014444. [2] G.K. Strukova, G.V. Strukov, S.V. Egorov, A.A. Mazilkin, I.I. Khodos, S.A. Vitkalov, 3D-mesostructures obtained by self-organization of metallic nanowires, Mater. Lett. 128 (2014) 212–215. [3] D. Talapin, E. Shevchenko, Introduction: Nanoparticle chemistry, Chem. Rev. 116 (2016) 10343–10345. [4] E. Abdelkader, P. Jelliss, S. Buckner, Main group nanoparticle synthesis using electrical explosion of wires, Nano-Struct. Nano-Objects 7 (2016) 23–31. [5] E. Demir, A. Savk, B. Sen, F. Sen, A novel monodisperse metal nanoparticles anchored graphene oxide as counter electrode for dye-sensitized solar cells, Nano-Struct. Nano-Objects 12 (2017) 41–45. [6] P. Vikram, B. Arpit, G. Rinki, J. Navin, P. Jitendra, Synthesis and applications of noble metal nanoparticles: A review, Adv. Sci. Eng. Med. 9 (2017) 527–544. [7] U. Kortshagen, R. Sankaran, R. Pereira, S. Girshick, J. Wu, E. Aydil, Nonthermal plasma synthesis of nanocrystals: Fundamental principles, materials, and applications, Chem. Rev. 116 (2016) 11061–11127. [8] N.B. Volkov, E.L. Fen’ko, A.P. Yalovets, Simulation of generation of ultradisperse particles upon irradiation of metals by a high-power electron beam, Zh. Tekh. Fiz. 80 (2010) 1–11; A. P. Tech. Phys. 55 (2010) 1389-1399. [9] M. Huth, F. Porrati, C. Schwalb, M. Winhold, R. Sachser, M. Dukic, J. Adams, G. Fantner, Focused electron beam induced deposition: A perspective, Beilstein J. Nanotechnol. 3 (2012) 597–619. [10] F. Porrati, M. Pohlit, J. Müller, S. Barth, F. Biegger, C. Gspan, H. Plank, M. Huth, Direct writing of CoFe alloy nanostructures by focused electron beam induced deposition from a heteronuclear precursor, Nanotechnology 26 (2015) 475701. [11] G.K. Strukova, G.V. Strukov, S.V. Egorov, A.A. Rossolenko, D.V. Matveyev, V.S. Stolyarov, S.A. Vitkalov, Interaction of 3D mesostructures composed of Pd-Ni alloy nanowires with low-temperature oxygen plasma, Mater. Lett. 203 (2017) 68–72. [12] D.W. Pashley, M.J. Stowell, M.H. Jacobs, T.J. Law, The growth and structure of gold and silver deposits formed by evaporation inside an electron microscope, Phil. Mag. 10 (1964) 127–158. [13] Y. Li, B.P. Bastakoti, V. Malgras, C. Li, J. Tang, J. Ho Kim, Y. Yamauchi, Polymeric micelle assembly for the smart synthesis of mesoporous platinum nanospheres with tunable pore sizes, Angew. Chem. Int. Ed. 54 (2015) 11073– 11077. [14] V. Malgras, H. Ataee-Esfahani, H. Wang, B. Jiang, C. Li, K.C.-W. Wu, J. Ho Kim, Yu Yamauchi, Nanoarchitectures for mesoporous metals, Adv. Mater. 28 (2016) 993–1010. [15] N. Okabayashi, K. Maeda, T. Muraki, D. Tanaka, M. Sakamoto, T. Teranishi, Y. Majima, Uniform charging energy of single-electron transistors by using sizecontrolled Au nanoparticles, Appl. Phys. Lett. 100 (2012) 033101. [16] M. Feigelman, M. Skvortsov, K. Tikhonov, Proximity-induced superconductivity in graphene, Pis’ma Zh. Eksp. Teor. Fiz. 88 (2008) 862–866. [17] Z. Han, A. Allain, H. Arjmandi-Tash, K. Tikhonov, M. Feigelman, B. Sacepe, V. Bouchiat, Collapse of superconductivity in a hybrid tin–graphene Josephson junction array, Nat. Phys. 10 (2014) 380–386. [18] T. Noriki, S. Abe, K. Kajikawa, M. Shimojo, Patterning technique for gold nanoparticles on substrates using a focused electron beam, Beilstein J. Nanotechnol. 6 (2015) 1010–1015.
Contents lists available at ScienceDirect
Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Disintegration of nanostructured metals with the formation of nanoparticles in electron microscope G.K. Strukova a, *, G.V. Strukov a , I.I. Khodos b , S.A. Vitkalov c a b c
Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, 142432 Chernogolovka, Russia Physics Department, City College of City University of New York, NY 10031, USA
highlights
graphical abstract
• 3D hierarchical metallic mesostructures disintegrate in transition electron microscope. • Samples containing FeNi3 nanowires, silver nanowafers or Pb–In nanorods were irradiated. • Explosive disintegration leads to form of metallic nanoparticles on a holding substrate. • Direct atomic resolution images exhibit the single crystal structure of nanoparticles.
article
info
Article history: Received 12 November 2017 Received in revised form 13 March 2018 Accepted 18 April 2018 Keywords: Nanostructured mesostructure Disintegration Metallic nanoparticles
a b s t r a c t In this paper we report on an explosive disintegration of metallic 3D samples with built-in nanoscaled hierarchical order under electron beam irradiation in a transmission electron microscope. The objects of our investigation are novel 3D mesostructures containing either FeNi3 intermetallic nanowires, or silver wafers of nanoscale thickness, or Pb–In nanorods. These structures were fabricated via a selforganization of metallic nanowires growing on templates during the pulsed electro-deposition process. The disintegration of 3D mesostructures yields an array of 2–50 nm metallic crystalline nanoparticles scattered on a holding substrate in the vicinity of the contact of the electron beam with samples. Direct atomic resolution images made in-situ reveal the monocrystalline structure of the nanoparticles. The observed rapid disintegration of 3D mesostructures in the electron beam is related to the internal energy significantly enhanced in the nanostructured samples. Possible applications of the phenomenon are © 2018 Published by Elsevier B.V. discussed.
1. Introduction
* Corresponding author.
E-mail address: [email protected] (G.K. Strukova).
https://doi.org/10.1016/j.nanoso.2018.04.004 2352-507X/© 2018 Published by Elsevier B.V.
Recently we have reported on 3D mesostructures obtained by a self-organization of nanowires growing on templates in electrolytes under the action of programmed electric pulses. These
G.K. Strukova et al. / Nano-Structures & Nano-Objects 16 (2018) 104–109
novel objects, having size of few microns to few hundred microns, demonstrate intriguing structural similarity with biological objects such as plants, fungi, seashells (biomimetics) [1]. SEM and TEM studies of Pd–Ni seashell-shaped samples revealed a hierarchic order of densely packed conical bundles of metallic nanowires. The nanowires themselves consist of 4–15 nm nanocrystals embedded in an amorphous matrix [2]. During the TEM study of branchshaped metallic mesostructures with nanoscale needles we have observed a rapid disintegration of the samples, which appear locally in the vicinity of the contact with a focused electron beam. The disintegration results in an array of nanoscale metallic particles arises near the contact on a substrate supporting samples. This fascinating behavior of 3D nanostructured metallic samples in an electron microscope, i.e. their disintegration and an appearance of the nanoparticles, to the best of our knowledge, was not described in the literature. This issue has initiated the study presented below. The usefulness of metallic nanoparticles in modern technologies stimulates the development of new methods for their production [3]. Accomplishments in this field have been demonstrated recently [4,5]. An explosion of wires in a liquid media produces nanoparticles of s- and p-type metals [4]. A microwave assisted synthesis of Pt-nanoparticles on graphene oxide facilitates a creation of effective electrodes for dye sensitized solar cells [5]. A wide variety of metal nanoparticles obtained by methods of a chemical reduction of metal ions in solutions as well as microbiological methods have been utilized in a diversity of applications [6]. The current state of art in the nanocrystal synthesis, including metal growth in a non-thermal plasma, as well as examples of applications, using the nanocrystals, are presented in a recent review [7]. The synthesis of powders of metal nanoparticles via action of a powerful focused electron beam on a massive metal target in an industrial scale is described in Ref. [8]. Furthermore during last 15 years a technique of metal deposition by a focused electron beam (FEBID) is developed, that allows to synthesize nanostructures using metallic atoms produced in decomposition processes with gaseous precursors [9,10]. Organometallic compounds (metal carbonyls) used as precursor are thermodynamically unstable under the e-beam irradiation. This property allows to conduct FEBID process in an electron microscope in-situ, since the microscope beam power is sufficient for a rapid decomposition of the precursor. As mentioned above, our nanostructured samples obtained via a pulsed electrodeposition on templates have a complex multiscale architecture with building blocks containing both amorphous and nanocrystalline phases. This leads to an increased internal energy and, as a consequence, may enhance significantly the disintegration of the samples under the electron beam irradiation. Presented below study of the novel 3D nanostructured materials under a relatively weak electron beam irradiation in an electron microscope both sheds a light on the stability of these structures and demonstrates an effective and clean method of the synthesis of crystalline metallic nanoparticles in the electron microscope. 2. Experiment The fabrication of the studied 3D metallic nanostructured materials is described in detail in [1,2]. The nanostructured metal samples are obtained onto templates by an electroplating from aqueous solutions of metal salts using programmable electric current pulses. In such conditions, the electroplating leads to both the metal deposition and a self-organization of growing nanowires, yielding nanostructured samples with a built in 3D hierarchical order propagating up to several hundred microns. Shapes of the obtained structures are often similar to the shapes of natural objects such as mushrooms, plants, seashells. In the presented work we used three different sets of samples: (1) ‘‘shells’’, obtained as
105
a result of self-organization of nanowires of intermetallic FeNi3 , (2) ‘‘branches’’, consisting of silver nano-plates, and (3) ‘‘branches’’ of the Pb–In alloy nanorods. The SEM and TEM studies of the samples were performed using JEM-2100 electron microscope. For these studies the grown samples were rinsed in an ultrasonic bath twice. The first rinse was in acetone. The second was in alcohol to make a suspension of the grown structures. A droplet of the alcohol suspension was then placed on a holding substrate (a carbon-coated copper grid) and after the alcohol evaporation the substrate with the nanostructured samples was placed into the electron microscope column. All experiments were carried in the electron microscope at 200 kV accelerating voltage. An electron beam carrying total electric current (4 pA) was focused on the samples area of 20–50 nm2 . The disintegration of the sample occurs promptly upon a contact of the focused electron beam with the sample. Metal nanoparticles were observed on holding substrate in the vicinity of the contact of the sample with the focused electron beam. High resolution images of individual nanoparticles obtained via the sample disintegration were made in-situ in the same electron microscope. 3. Results All studied samples have demonstrated the explosive disintegration under irradiation by the focused electron beam. Fig. 1a, b present a hierarchical 3D mesostructure composed of nanowires of magnetic intermetallic compound FeNi3 (permalloy). The sample is shaped as a seashell. The studied sample was a single-phase FeNi3 intermetallide, that has been confirmed by an analysis of the X-ray diffraction (not shown). Fig. 1c demonstrates the edge of the sample and the array of nanoparticles disjected onto the substrate during the electron beam irradiation of the sample. The inset to Fig. 1c shows the electron diffraction pattern indicating the crystalline structure of these particles. Fig. 1c, d show that the particles have sizes less than 20 nm. The particle shape is identical to rounded polyhedrons. The nanoparticles form single crystals. This is seen from the direct high resolution images presented in Fig. 1e, f. A single-crystal FeNi3 -flake with a nano-sized thickness was found in a mortar produced by grounding of the permalloy mesostructures (Fig. 2a). After the explosive disintegration in the focused electron beam, this sample was dispersed into nanoparticles of several nanometers in size shown in Fig. 2b, c. Fig. 3 presents the image of a hierarchical 3D structure made of silver. The structure looks as a conifer branch with elements of fractal self-similarity. It is a single crystal as shown in the inset to Fig. 3a presenting the electron diffraction image of the structure. A localization of the focused electron beam on a ‘‘branch’’ of the structure induces the explosive evaporation of the metal producing an array of nanoparticles placed on the substrate near the beam. Fig. 3b shows the substrate with the array of nanoparticles scattered near the contact between the electron beam with the sample. Fig. 3c demonstrates that the scattered particles are shaped as polyhedrons with different linear sizes less than 50 nm. The electron diffraction images of the scattered particles reveal that these particles are nanocrystals. This is presented in the inset to Fig. 3c. Finally, Fig. 3d shows the direct resolution image of a single nanoparticle indicating the ordered arrangement of the atomic planes and confirming the single crystal structure of the nanoparticle. In a similar experiment, using a mesostructure consisting of gold wafers of nanoscale thickness as a target for a focused electron beam in the microscope, we have obtained gold nanoparticles on a substrate.
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Fig. 1. SEM images: the overall view of the FeNi3 3D mesostructure (a), the bundles of nanowires forming the 3D structure (b); TEM images of FeNi3 nanoparticles formed on the substrate after the electron beam irradiation of the sample demonstrate fcc crystal lattice (c–f); the lattice-resolution images of the individual nanoparticles (e, f).
Fig. 2. A single crystal fragment of the FeNi3 ‘‘seashell’’ before irradiation (a), nanoparticles formed on the substrate after irradiation (b, c). Electron diffraction patterns indicate a crystalline structure studied objects: (a) original sample before irradiation; (b) nanocrystals produced by irradiation.
Fig. 4a shows a fractal 3D structure composed of nanorods of Pb–In alloy. EDX indicates composition of 85% of Pb and 15% of In.
Fig. 4b presents a sample exposed to the electron beam. The central trunk of the ‘‘branch’’ is completely melted and shown in
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Fig. 3. 3D mesostructure of silver (a) and its enlarged fragment in ⟨001⟩ zone orientation along the electron beam direction (b); overview of Ag nanoparticles with fcc crystal structure (as it follows from the inset) on the substrate in the vicinity of the mesostructure exposed to the electron beam [c]; the TEM high-resolution image indicates the single crystal structure of disjected nanoparticle (d).
Fig. 4. Overall view of hierarchical 3D mesostructures made of Pb–In alloy (a). Figs. (b, c) show e-beam melted trunk and array of scattered nanoparticles after electron beam irradiation of the mesostructure. Selected area diffraction pattern in Fig. c (inset) corresponds to fcc structure of InPb nanoparticles. Fig. (d) presents TEM high-resolution image of an individual nanoparticle produced by e-beam disintegration.
Fig. 4a sharp needles are completely evaporated. The substrate is covered with nanoparticles of different nanometer sizes. In contrast to the e-beam disintegration shown in Figs. 1 and 3, Fig. 4b indicates that the further away from the decomposed structure the smaller the particle size. The picture suggests that this disintegration proceeds largely via melting and evaporation of the material. Fig. 4c shows that the 2D images of small particles resemble circles or rounded polygons, suggesting that the particle shape is similar
to rounded polyhedrons. The electron diffraction image of the scattered particles reveals that the particles are nanocrystals. This is presented in the inset to Fig. 4c. Fig. 4d presents the direct highresolution image of the nanoparticle with hexagonal polyhedron faces. The ordered atomic layers indicate the single crystal structure of the nanoparticle.
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Fig. 5. A fragment of the nanowire taken from the FeNi3 ‘‘seashell’’ (a); a view of the marked section indicating presence of nanoparticles (b); the electronic diffraction pattern of the FeNi3 nanowire indicating an amorphous-nanocrystalline structure (c).
4. Discussion To understand the observed phenomenon we examine obtained results together with the data on structural analysis of the samples and known mechanisms of the interaction of a focused electron beam with metal targets. In all cases studied in this work, the interaction of a sample with the electron beam results in an disintegration of its nano-sized elements and the emergence of nanoparticles. This explosion process is accompanied by a melting of more massive fragments of sample during the absorption of the e-beam energy. The studied samples are 3D tree-like fractals with self-similar elements of nano- and micro-sizes. In such nanostructured porous constructions most of atoms sit near the surface, increasing significantly the free energy and decreasing, therefore, the stability of the structures. The internal energy is further enhanced due to contributions of a large number of defects and the amorphous content between the nanocrystals that is associated with a pulsed deposition regime. As noted in the introduction, the Pd–Ni nanowires, which are building blocks of the ‘‘seashell’’ samples, have an amorphous-nanocrystalline structure [2]. The FeNi3 nanowires taken from similar ‘‘seashell’’ samples and studied in this work (Fig. 1a, b) also have an amorphous-nanocrystalline structure. This property is shown on TEM-images and electron diffraction pattern shown in Fig. 5a, b and Fig. 5c correspondingly. The large surface of the porous nanostructured samples provides an instantaneous access of the electron beam to virtually entire enhanced internal energy stored in the samples, inducing the explosive disintegration of its nano-sized elements. Below we consider the increased internal energy and the large surface of the nanostructured samples as the main sources of the high rate of the observed processes. Recently, we have reported that volumetric mesoporous structures of self-organized of Pd–Ni nanowires interact actively with a low temperature non-equilibrium oxygen plasma leading to an intense red incandescence [11]. This active interaction is another independent indicator of the increased internal energy of the studied 3D nanostructured porous materials. To understand the mechanism leading to the phenomenon, we used data and a model presented in paper [8]. The model describes a formation of ultra-dispersed particles in a plasma cloud, which emerge during an evaporation of a metallic target induced by a high-power electron beam. The model describes also the dynamics of the process, taking into account the equations of heat conduction, heat transfer and friction between plasma components, relaxation of the components to an equilibrium, condensation, evaporation and coagulation of droplets as a result of their collisions. The particle size distribution for various irradiation and cooling regimes is obtained via numerical simulation of the process. Following paper [8], we assume that presented in Fig. 3 formation of nanoparticles from nano-sized fragments of a singlecrystal Ag-nanostructure is due to a rapid evaporation of these fragments followed by a condensation of the vapor on the substrate and a subsequent crystallization into metal nanoparticles. The disintegration of the presented in Fig. 2(b) single-crystal FeNi3 -flake
and the formation of metal nanoparticles may proceed via similar scenario (Fig. 2). We have noted that the e-beam irradiation of this monocrystalline fragment produces nanoparticles with sizes of only few nanometers. This result could be explained by a small original concentration of the evaporated atoms due to the small amount of the material stored in this sample with a nanosized thickness. Subsequent aggregation of the evaporated atoms into nanoparticles proceeds, most likely, on the holding substrate and goes similar to the early stages of epitaxial thin film growth [12]. Results of the e-beam irradiation of Pb–In-mesostructure suggest again a similar mechanism (Fig. 4). The reduction of the particle size with the distance from the irradiated sample is due to the decrease of the concentration of metallic atoms in the gas cloud spreading above the substrate and, hence, the number of the deposited atoms. Shown in Fig. 1 FeNi3 -mesostructure is self-organized from nanowires containing nanocrystals embedded in an amorphous matrix. In contrast to the flake shown in Fig. 2, case the e-beam disintegration of the FeNi3 -mesostructure is accomplished by a rapid evaporation of the amorphous content and a spreading out, perhaps, solid crystalline nanoparticles (Fig. 1). This is supported by the fact that the size of nanoparticles after e-beam disintegration is comparable with the ones embedded in the amorphous content of the original structure. The presented above consideration regarding mechanisms leading to the phenomenon agrees conceptually with the model presented in [8]. A quantitative description of the nanoparticle formation and distribution requires a special investigation and is beyond the scope of this paper. The presented results suggest that metallic mesoporous structures, obtained by other methods enhancing internal energy, will also be capable for the disintegration and the formation of nanoparticles under the action of a focused electron beam in an electron microscope. From this point of view, various nanostructured aggregates, including mesoporous Pt-nanospheres, synthesized by the original chemical method of the metal reduction in a solution, are of undoubted interest [13,14], The presented method does not only yield metallic nanoparticles under conditions of high vacuum but provides a comprehensive study their physical properties in situ. This technique allows an effective access to nano-sized objects not exposed to the atmosphere or other reactive environments. This could be important for studies of artificial objects sensitive, in particular, to the oxidation or other chemical reactions. For example, mentioned above method FEBID, that was noted in Introduction, yields metallic particles which, as a rule, contain a significant amount of the carbon as an impurity because an irradiated precursor itself is a compound of carbon. 5. Conclusion Explosive disintegration of 3D hierarchical metallic mesostructures, obtained on templates in electrolytes under the action of
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programmed electric pulses, is observed after interaction with a focused electron beam in electron microscope. Mesostructures containing FeNi3 intermetallic nanowires, silver wafers of nanoscale thickness, and Pb–In nanorods were irradiated. Disintegration of these samples yields metallic nanoparticles of size 2–50 nm dispersed on a holding substrate. Obtained in-situ direct atomic resolution images exhibit the single crystal structure of nanoparticles. Presented results open new possibilities for the fabrication of metallic nanoparticles and for study them in situ in electron microscope. 6. Outlook The observed disintegration in electron microscope of 3D nanoscaled structures is a promising experimental tool due to both the simplicity and technological effectiveness of the fabricating method of the broad spectrum of metallic 3D nanostructured materials [1,2] and the accessibility of the electron microscopes. The developed pulse current electro-deposition on templates allows the controllable synthesis of a variety of metallic 3D samples with different hierarchical order built-in from nanometer to micrometer ranges and containing the fractal branching and elements of the self-similarity [1,2]. Metallic mesostructures composed of nanowires contain nanocrystals embedded into an amorphous matrix [2]. Presented in this paper study suggest that the electron beam-induced explosive disintegration in electron microscope of the nanostructured samples composed of metal, metal alloy or intermetallide may lead to the formation of the 2–50 nm crystalline nanoparticles. Metallic nanocrystals with 3–5 nm in size are considered to be of paramount importance for modern nanoelectronics and are the basic block for single quantum operations at the room temperature [15]. The distribution of the superconducting nanocrystals of the fractal Pb–In mesostructure observed after the electron beam disintegration (Fig. 4) opens a way to study single electron quantum transport between nanoscaled superconducting objects [16,17] of the sizes, which are still unattainable for the fabrication using modern lithography. Recently, a new patterning technique for gold nanoparticles placed on substrates is proposed [18]. According to [18], particular nanoparticles of their array were fixed on the substrate by irradiation with a focused electron beam in an electron microscope to produce a desired pattern. The unfixed nanoparticles can be then removed. The technique [18] in a combination with the method presented in this article opens a perspective for a fabrication of effective nanoscaled devices such as plasmonic waveguides in one technological line in situ. Acknowledgments We are grateful to I.V. Kukushkin and V.V. Ryazanov for fruitful discussions.
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