Simple two-step fabrication method of Bi2Te3 nanowires
© Kang et al; licensee Springer. 2011
Received: 1 November 2010
Accepted: 4 April 2011
Published: 4 April 2011
Bismuth telluride (Bi2Te3) is an attractive material for both thermoelectric and topological insulator applications. Its performance is expected to be greatly improved when the material takes nanowire structures. However, it is very difficult to grow high-quality Bi2Te3 nanowires. In this study, a simple and reliable method for the growth of Bi2Te3 nanowires is reported, which uses post-sputtering and annealing in combination with the conventional method involving on-film formation of nanowires. Transmission electron microscopy study shows that Bi2Te3 nanowires grown by our technique are highly single-crystalline and oriented along  direction.
Low-dimensional nanostructures have received great attention due to their unique and unusual properties in many research fields related to nanoscience and nanotechnology . One of the low-dimensional nanostructures, namely the one-dimensional (1D) nanowire, has a high aspect-ratio, making it suitable for future electronic and thermoelectric devices and new types of sensors [2, 3]. In particular, it is believed that the classical size effect and quantum confinement effect in 1D nanowire play a crucial role in enhancing thermoelectric performance [1, 4, 5]. Bismuth telluride (Bi2Te3) is well known for its high thermoelectric figure-of-merit (ZT ~ 1) in bulk. Moreover, its thermoelectric performance is expected to be remarkably improved for nanowire structures as a consequence of the high thermoelectric power (S 2 σ) and suppressed thermal conductivity (κ) in the low-dimensional structures [6, 7]. More recently, Bi2Te3 has also been intensively investigated for the search of an efficient topological insulator since the observation of the quantum-spin-Hall-like phenomenon on the surface of a material even without the applied magnetic fields. Topological insulator materials show almost dissipationless surface conduction because of the high spin degeneracy caused by the spin--orbit coupling, although they behave like an insulator in bulk. Unlike the bulk Bi2Te3, the existence of the surface states in 1D Bi2Te3 nanowires has been predicted only by theory [8, 9]. Since the theoretical expectation, numerous synthesis methods of Bi2Te3 nanowires have been developed over the past several years [10–16]. As part of such efforts, we have already reported the simple Bi2Te3 nanowire growth using a stress-induced method with no catalysts, starting materials, and templates, which is called the on-film formation of nanowires (OFF-ON) [17, 18]. However, the one-step compound nanowire growth using this method is hard to establish the optimum conditions because diffusivity difference between multiple components often leads to nanowires grown with compositions different from a nominal stoichiometry in the thermal annealing step. In this article, a more reliable Bi2Te3 nanowire growth method is reported based on the OFF-ON process. Our approach is a two-step OFF-ON process. The first step involves pure Bi nanowire growth by the conventional OFF-ON method . The second step is the in situ deposition of Bi2Te3 thin film onto a substrate including pure Bi nanowires, followed by thermal annealing. Bi2Te3 nanowires are synthesized through the inter-diffusion of constituent elements between the Bi nanowire core and the Bi2Te3 shell during this second step. Here, the reliability of this Bi2Te3 nanowire growth process and the quality of single-crystalline Bi2Te3 nanowires thus grown will be presented.
Results and discussion
A simple and new synthesis method of quality single-crystalline Bi2Te3 nanowires combining the OFF-ON method with post-sputtering and annealing is demonstrated. In step one, Bi nanowires are grown by the conventional OFF-ON method. In step two, a Bi2Te3 thin film is in situ deposited onto the Bi nanowire-including substrate by RF sputtering, followed by the post-annealing at a high temperature well above the melting point of Bi. Bi2Te3 nanowires are synthesized during the high-temperature annealing by the atomic inter-diffusion between the Bi core and the Bi2Te3 shell. Indeed, our two-step growth method yielded homogeneous, stoichiometric Bi2Te3 nanowires with high single-crystallinity and no observable defects, which were hard to achieve using the conventional OFF-ON growth from a single compound source. These results are expected to facilitate the studies on high-efficiency thermoelectric devices and topological insulators taking advantage of Bi2Te3 nanowires.
energy dispersive X-ray spectroscopy
high-angle angular dark field
high-resolution transmission electron microscopy
on-film formation of nanowires
selected area electron diffraction
This study was supported by the Priority Research Centers Program (2009-0093823) through the National Research Foundation of Korea (NRF), a grant from the "Center for Nanostructured Materials Technology," under the "21st Century Frontier R&D Programs" of the Ministry of Education, Science, and by the Pioneer Research Center Program (2010-0019313) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.
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