High temperature in-situ observations of multi-segmented metal nanowires encapsulated within carbon nanotubes by in-situ filling technique
© Hayashi et al.; licensee Springer. 2012
Received: 17 April 2012
Accepted: 29 June 2012
Published: 8 August 2012
Multi-segmented one-dimensional metal nanowires were encapsulated within carbon nanotubes (CNTs) through in-situ filling technique during plasma-enhanced chemical vapor deposition process. Transmission electron microscopy (TEM) and environmental TEM were employed to characterize the as-prepared sample at room temperature and high temperature. The selected area electron diffractions revealed that the Pd4Si nanowire and face-centered-cubic Co nanowire on top of the Pd nanowire were encapsulated within the bottom and tip parts of the multiwall CNT, respectively. Although the strain-induced deformation of graphite walls was observed, the solid-state phases of Pd4Si and Co-Pd remain even at above their expected melting temperatures and up to 1,550 ± 50°C. Finally, the encapsulated metals were melted and flowed out from the tip of the CNT after 2 h at the same temperature due to the increase of internal pressure of the CNT.
Encapsulation of one-dimensional foreign materials into carbon nanotubes (CNTs) during CNT growth has received attention because they are expected to possess new physical and chemical properties based on CNT induced by nanospace [1, 2]. Various metal nanowires have been successfully encapsulated within CNTs, employing mainly two kinds of methods. One of them is that CNTs are initially opened at their tube tips and subsequently filled with molten materials through capillary action [3–5]. The other one is an in-situ filling method, where the metals can be filled into the CNTs [6–8]. Among a variety of interesting applications, a promising application of ferromagnetic metal nanowires (such as Fe, Co, and Ni) encapsulated within a CNT is the high-density magnetic recording media due to their nanoscale size and strong anisotropic property, leading to small bit size [2–12]. Furthermore, the graphite layer provides an effective barrier against oxidation and consequently ensures a long-term stability of the metal inside CNTs .
Although it is very interesting to investigate a reaction process within the CNT due to the confined nanospace at various conditions, very few studies have been made so far on the detailed in-situ characterizations of the nanowire structure at various temperatures after encapsulation of the metal nanowire within CNTs.
Here, we present growth of self-assembled aligned Pd-Co-based multi-segmented one-dimensional metal nanowires encapsulated within multiwall CNT (MWCNT) arrays on Si by bias-enhanced microwave plasma chemical vapor deposition (MPCVD) with CH4 and H2. The metal nanowires encapsulated within MWCNTs were analyzed with transmission electron microscopy (TEM). Furthermore, in-situ microscopic environmental TEM (ETEM) was employed for in-situ observations of nanowires encapsulated within MWCNTs at a high temperature above melting points of metals.
The Pd-Co-based nanowire encapsulated within MWCNTs was grown by bias-enhanced MPCVD using a 2.45-GHz, 1.5-kW microwave power supply, as described elsewhere . A primary 6-nm-thick Pd metal thin layer and a secondary 9-nm-thick Co metal thin layers (Co/Pd: total thickness of 15 nm) were deposited on the thin barrier layer of SiO2 formed on the Si surface (Co/Pd/SiO2/Si substrate). The question arises why we chose Co/Pd bimetallic layers. Although we have previously reported Pd-based MF-CNTs using bias-enhanced MPCVD, we failed to fill the Co metal into the nanotubes using only a Co catalyst layer on the Si substrate. In combination with the Pd layer, we successfully encapsulated Co inside CNTs . The feed gas, H2, was supplied into the plasma chamber to maintain a pressure of 20 Torr. The substrate was gradually heated up to 973 K by a radio-frequency graphite heater, and a microwave plasma was turned on to 600 kW. The Pd-Co-based nanowire within MWCNTs was grown for 15 min under a negative bias of 400 V at that maintained substrate temperature.
A JEOL (JEM-3010; JEOL Ltd., Akishima, Tokyo, Japan) TEM, operated at 300 kV, was used for room temperature observations. A Hitachi (H-9000NAR; Hitachi, Ltd., Minato, Tokyo, Japan) ETEM, operated at an accelerating voltage of 300 kV and equipped with a Gatan GIF and a Gatan CCD camera (Gatan, Inc., Pleasanton, CA, USA), was used for in-situ observations. A resistance-heating tungsten wire sample holder of a TEM was used to heat the nanowire encapsulated within MWCNTs up to 1,550°C in vacuum with an accuracy of ±50°C depending on the sample position .
Results and discussion
Figure 2 shows the diffraction pattern observed at room temperature and 1,300°C. We clearly observed strain-induced deformation on the spot of graphite G(002). It is very interesting that the solid-state phases remain even at above melting points of Pd4Si (890°C) and Co-Pd (1,250°C). Based on the diffraction pattern, we estimated that the graphene layer distance varies between +0.015 and −0.025 nm at 1,300°C compared to that of RT. Therefore, both positive and negative fluctuations of interlayer spacing of graphene layers at several positions may appear to relax the residual thermal strain. The fluctuation of distance between graphene layers is metastable up to the formation of dislocations in graphene layers.
According to the experiments, the diffraction patterns indicate that both Pd4Si and Co-Pd have a crystalline structure even at the melting points. This may be due to the confined nanospace effect. Recently, Kobayashi et al. reported encapsulation of Sn, Pb, Ag, and Au within MWCNTs by capillary action. The results suggest that a confined nanospace prevents crystal growth of metals having a low melting point . Confinement of metals within a nanospace still remains an interesting question for both theoretical research and industrial application.
We synthesized multi-segmented one-dimensional metal nanowires within MWCNTs by in-situ filling technique during PECVD growth of MWCNTs. According to the TEM images and SAED of metal nanowires within the MWCNT, the Pd4Si nanowire and fcc Co nanowire on top of the Pd nanowire were encapsulated within the bottom and top parts of the MWCNT, respectively, by in-situ filling technique. The solid-state phases of Pd4Si and Co-Pd remain even at above their melting points at 890°C and 1,250°C, respectively, by ETEM. This may be due to the confined nanospace effect. We observed strain-induced deformation on the spot of graphite G(002) at 1,300°C. The accumulated internal pressure due to high temperature at 1,550°C caused the break of the CNT tip and pushed out the molten metal confirmed by electron diffraction pattern.
This work is partially supported by Grant-in-Aid for Scientific Research (B) under contract number 50314084 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. YH would like to thank Dr. N. Kishi, and Prof. T. Soga and Prof. T. Jimbo at NIT for their useful discussion. The authors would like to thank Mr. Y. Horita, Mr. T. Yanagimoto, Prof. K. Kaneko, and Prof. K. Kuroda for their assistance characterizations.
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