Co nanoparticle hybridization with single-crystalline Bi nanowires
© Noh et al; licensee Springer. 2011
Received: 15 July 2011
Accepted: 21 November 2011
Published: 21 November 2011
Crystalline Co nanoparticles were hybridized with single-crystalline Bi nanowires simply by annealing Co-coated Bi nanowires at elevated temperatures. An initially near-amorphous Co film of 2-7 nm in thickness began to disrupt its morphology and to be locally transformed into crystallites in the early stage of annealing. The Co film became discontinuous after prolonged annealing, finally leading to isolated, crystalline Co nanoparticles of 8-27 nm in size. This process spontaneously proceeds to reduce the high surface tension and total energy of Co film. The annealing time required for Co nanoparticle formation decreased as annealing temperature increased, reflecting that this transformation occurs by the diffusional flow of Co atoms. The Co nanoparticle formation process was explained by a hole agglomeration and growth mechanism, which is similar to the model suggested by Brandon and Bradshaw, followed by the nanoparticle refinement.
Magnetic nanoparticles have unique size effects that may provide insights into potential applications in various fields, such as ultra-high density information storage, color imaging, bioprocessing, and ferrofluids [1–3]. Specifically, cobalt (Co) nanoparticles have been a subject of intensive research because of its high magnetocrystalline anisotropy (7 × 106 erg/cm3) and large estimated critical size for single domains (approx. 70 nm) . To synthesize Co nanoparticles with a controlled size and size distribution, various techniques have been utilized, including evaporation in an inert gas , chemical vapor condensation by either heating or laser-irradiating Co2(CO)8 precursor [6, 7], and solution phase reduction of CoCl2 in stabilizing agents . Although these techniques have demonstrated monodisperse arrays of Co nanoparticles with sizes down to 2 nm, elaborate temperature control and/or the use of complex chemical species are in demand, limiting their widespread use.
Metallic and semiconducting nanowires are another class of nanostructures that have attracted a great deal of interest because of their intriguing quantum properties and potential use for advanced nanodevices. Bismuth (Bi) is a semimetal widely explored for understanding physics in nanowire systems, because of its highly anisotropic Fermi surface, low carrier concentrations, small carrier effective mass [9–11], and long carrier mean-free path . In particular, Bi nanowires can be good building blocks for thermoelectric applications, since good thermoelectric properties  of bulk Bi such as the large thermoelectric power (-50 to -100 μV/K) and small thermal conductivity (approx. 8 W/mK) have been demonstrated to be further improved in nanowire systems . The quality of Bi nanowires is a critical requisite for success in both fundamental study and applications. We previously demonstrated that high-quality single-crystalline Bi nanowires could be synthesized using the unique on-film formation of nanowires (OFF-ON) method [12, 15].
Hybridizing Bi nanowires with Co nanoparticles may be an interesting research topic. That is not only a combination of 0D nanoparticle and 1D nanowire, but it can also provide fundamental understanding of mutual interaction between thermoelectrics and magnetism. Recently, the thermoelectrics has sought a link to spintronics via groundbreaking works performed by several research groups worldwide. The studies on the spin-Seebeck effect  and magneto-Seebeck effect  were typical. The spin-Seebeck effect refers to power generation from a magnetic material under a temperature gradient, while the magneto-Seebeck effect concerns a change in Seebeck coefficient of a magnetic multilayer structure with insulting barrier inside, depending on the relative magnetizations. Although these works laid cornerstones for the investigation of interactions between thermoelectrics and spintronics, none of them included a thermoelectric material in their experiments. In contrast, we try to combine a magnetic material with a thermoelectric material at the nanoscale toward an eventual elucidation of the effects of magnetic nanostructures on the thermoelectric performance in this study.
The first step of this research is to establish a simple and reliable platform for incorporating Co nanoparticles into Bi nanowires. In this article, we report a simple method to synthesize Co-nanoparticles-embedding Bi nanowires, using a combination of the OFF-ON growth of Bi nanowires, sputter-deposition of a thin Co film, and post-annealing. The synthesis method of our nano-heterostructures is simpler than that of the subtle magneto-Seebeck structures and the thermoelectric performance of our heterostructures are expected to be more pronounced than that from the spin-Seebeck structures because of the use of thermoelectric material as a backbone.
2. Experimental details
3. Results and discussion
The single-crystalline Co nanoparticles are also observed from a Bi-Co core/shell nanowire annealed at 200°C for the same period of time (see Figure 3c). However, the degree of shape completion of the 200°C-formed Co nanoparticles is worse than those from 240°C annealing. Considering that a 1-nm-thicker Co film did not evolve into Co nanoparticles after annealing at 200°C for 3 h (Figure 2b), these results indicate that annealing temperature is indeed a key control parameter in nanoparticle formation. To further investigate the crystallinities and compositions of the above-mentioned Bi nanowire and Co nanoparticles, SAED and EDX analyses were performed on Co nanoparticle area (named "1") and Bi core area (named "2"), respectively. From two SAED patterns shown in Figure 3d, e, it is found that the area "1" contains extra spots (circled ones) other than characteristic Bi spots, which represent major crystal planes of FCC Co, while the area "2" shows only clear Bi spots. This indicates that the nanoparticles are really crystalline Co in accord with a TEM image in Figure 3b. The gray background of Figure 3d may come from the oxide layers on Bi core and Co nanoparticle. In addition, the EDX spectra (Figure 3f) from both areas show that significant Co peaks come out of the nanoparticles, whereas no meaningful Co peaks are observed on Bi core, reflecting that the identity of the nanoparticle is Co. The Co concentration (< 20 at.%) from "1" and non-zero concentration (0.5-1.5 at.%) from "2" are presumably caused by the limited spot size (approx. 20 nm) of electron beam.
We hybridized single-crystalline Bi nanowires with crystalline Co nanoparticles, using a combination of the OFF-ON nanowire growth, thin film deposition, and post-annealing. A Co thin film coated on a Bi nanowire began to deform its morphology via thermal annealing at elevated temperatures, which is driven by the high surface tension of the film. Local valleys developed in the Co film after a short time of annealing, and Co nanoparticles finally appeared on the surface of Bi nanowire through annealing for a time longer than a critical value, leaving behind Co-free Bi surface in between them. The time required for Co nanoparticle formation was shorter at a higher annealing temperature, suggesting that this process is governed by the diffusional flow of Co atoms. Interestingly, the crystalline Co nanoparticles were obtained from an initially near-amorphous Co film using our method. The whole process of Co nanoparticle hybridization with Bi nanowire was explained by the hole agglomeration/growth and nanoparticle refinement mechanism. The hybrid nanostructure would be a good testbed for exploiting multidisciplinary nanophysics. Various nanoparticles made of materials with high surface tension could be hybridized with a variety of nanowires, employing this simple method.
energy dispersive X-ray spectroscopy
on-film formation of nanowires
selected area electron diffraction.
This research was supported by a grant from the Priority Research Centers Program (2009-0093823), a grant (2011K000198) from 'Center for Nanostructured Materials Technology' under '21st Century Frontier R&D Programs' and the Pioneer Research Center Program (2010-0019313) through the National Research Foundation of Korea.
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