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Thermoelectric (TE) energy converters are solid-state devices that can generate electricity by harvesting waste thermal energy, thereby improving the efficiency of a system. The many advantages of TE devices include solid-state operation, zero-emissions, vast scalability, no maintenance, and a long operating lifetime. Nonetheless, because of their limited energy conversion efficiencies, thermoelectric devices currently have a rather limited set of applications. However, there is a reinvigorated interest in the field of thermoelectrics by identifying classical and quantum mechanical size effects, which provide additional ways to enhance energy conversion efficiencies in nanostructured materials 1,2 including superlattice thin films 3 and quantum dots. 4 For example, thermoelectric figure of the merit (ZT) up to 2.5 was achieved by synthesizing two-dimensional Sb 2 Te 3 /Bi 2 Te 3 superlattice thin films, exceeding pervious limits of ∼1 from bulk counterpart. 5 Even more exciting are the theoretical predictions for one-dimensional nanostructures including nanowires and nanotubes, which are thought to have ZT exceeding 5. 1,6 In the case of nanotubes, theoretical calculation predicts a further reduction in the thermal conductivity, because of a stronger phonon-surface scattering, compared to solid nanowire. 7 Limited works have been reported on the synthesis of thermoelectric nanotubes including hydrothermally grown Bi, Bi 2 Se 3 , and Bi 2 Te 3 nanotubes 8 and electrodeposited Bi nanotubes. 9 However, these processes have some limitations. For example, the nanotube production yield is very low (<30%). 8 In the template-directed method it is difficult to restrict the proceeding electrodeposition along the walls without filling up the whole pores. 9 The galvanic displacement reaction is an electrochemical process, which is induced by the difference in redox potentials between materials. Various metallic nanotubes have been synthesized via this reaction (e.g., gold nanotubes from silver nanowires); 10 however, no one to-date has demonstrated the synthesis of semiconducting thermoelectric nanotubes. In this paper, we demonstrate the synthesis of high-aspect ratio Bi 2 Te 3 nanotubes with controlled composition by galvanic displacement of nickel nanowires in acidic nitric electrolyte containing Bi 3+ and HTeO 2 + ions. Bi 2 Te 3 nanotubes were synthesized because Bi 2 Te 3 and its derivative compounds are considered to be the best materials used in thermoelectric refrigeration at room temperature. In addition to synthesis, we also demonstrate the fabrication method to create individual Bi 2 Te 3 nanotube-based devices by combining the magnetic assembly of single nickel nanowire across microfabricated electrodes, followed by a galvanic displacement reaction. Figure 1A shows the schematic illustration of the galvanic displacement reaction of Ni nanowires to Bi 2 Te 3 nanotubes. The detailed experimental conditions are provided in the Supporting Information. When Ni nanowires are immersed into an acidic nitric solution con...

There is a renewed interest in the field of thermoelectrics because of the remarkable efficiency improvement that can be achieved in nanostructured materials, [1] for example, superlattice thin films [2,3] and quantum dots. [4] Theoretical calculations predict that further enhancement of the thermoelectric figure of merit can be achieved in superlattice nanowires (zero-dimensional) [5] rather than conventional nanowires (1D) or superlattice thin-films (2D). In superlattice nanowires, a reduction of the thermal conductivity, which is related to phonon scattering at the interfaces between periodically alternating materials, could be achieved by controlling the diameter and length of the individual segments of the nanowires. [6] In addition, the band offset between segments provides both higher quantum confinement as well as a sharper density of electronic states compared to nanowires. Recently, superlattice nanowires have been fabricated by a variety of methods. [7][8][9] For example, Si/SiGe [7] and GaAs/ GaP [8] superlattice nanowires were synthesized by a laser-assisted catalytic growth method. Electrodeposition is a promising alternative technique for the fabrication of superlattice nanowires because it is simple, inexpensive, fast, operates at near room-temperature, and is able to tailor the properties of the deposit by adjusting the deposition conditions. For example, Fert et al. [10] fabricated Co/Cu superlattice nanowires from a single sulfate bath containing both metal ions by using a template-directed method that utilized the different deposition potentials of Co and Cu. By alternating the deposition potentials and deposition times, superlattice nanowires with controlled compositions and lengths were fabricated. Using a similar method, Co/Pt superlattice nanowires were also reported.[11] Even though there are several studies on the electrodeposition of thermoelectric nanowires, [12][13][14][15][16] thermoelectric superlattice nanowires have not yet been reported. In this study, we electrodeposited Bi 2 Te 3 /(Bi 0.3 Sb 0.7 ) 2 Te 3 superlattice nanowires by using a template-directed method, where the composition of the segments was controlled by manipulation of the deposition potential. BiTe/BiSbTe superlattice nanowires were chosen because BiTe and its derivative compounds are considered to be the best materials for thermoelectric refrigeration at room temperature. In the electrochemical deposition of alloys from aqueous electrolytes, the composition of the deposit can be manipulated by varying the applied potential, because the reduction potential of each ion in the solution is different. Therefore, a periodically modulated composition can be obtained, with two different phases forming the nanowire (e.g., Sb-rich BiSbTe and BiTe with minimal incorporation of Sb), by applying an alternating potential. Characterization of the composition and microstructure was performed to verify the synthesis of Bi 2 Te 3 / (Bi 0.3 Sb 0.7 ) 2 Te 3 superlattice nanowires.Based on previous work, BiSbTe ternary alloys can b...

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