Electrodeposition of Thermoelectric Superlattice Nanowires

There is a renewed interest in the field of thermoelectrics because of the remarkable efficiency improvement that can be achieved in nanostructured materials, for example, superlattice thin films and quantum dots. 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. 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. For example, Si/SiGe and GaAs/ GaP 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. 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. Even though there are several studies on the electrodeposition of thermoelectric nanowires, thermoelectric superlattice nanowires have not yet been reported. In this study, we electrodeposited Bi2Te3/(Bi0.3Sb0.7)2Te3 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 Bi2Te3/ (Bi0.3Sb0.7)2Te3 superlattice nanowires. Based on previous work, BiSbTe ternary alloys can be deposited from various acidic baths with the use of complexing agents such as tartaric acid or ethylenediaminetetraacetic acid (EDTA) to solubilize Sb in water. In this study, tartaric acid (33 mM) was used as a complexing agent. Because of the low-pH operating conditions, the effect of tartaric acid on the electrodeposition was minimal. To successfully achieve compositional modulation in the BiTe/BiSbTe nanowires, the composition as a function of applied potential was first studied in order to determine the deposition potential necessary for obtaining BiSbTe alloys with different compositions. Initially, linear-sweep voltammetry (LSV) was performed to determine the deposition potential range. As shown in Figure 1a, three reduction waves were observed. The first reduction wave (C1) extends from 100 mV to –40 mV (versus the saturated calomel electrode (SCE)), and it involved the reduction of HTeO to Te as well as the deposition of Bi2Te3. [19] This is expected because electrodeposition of Te and Bi2Te3 is more noble than reduction of other possible products, such as SbxTey, Bi, and BixSbyTez. [16] The composition of the electrodeposit obtained at –20 mV could be evidence that the formation of Te and BixTey was the preferred reaction in this potential range. (Fig. 1b) The intermediate reduction wave (C2) extends from –40 mV to –100 mV, and involved the formation of BixSbyTe3 ternary alloys. The third reduction wave (C3) extends from –100 mV to –280 mV, and involved the formation of (Bi0.3Sb0.7)2Te3. As shown in Figure 1b, the amount of Sb in C O M M U N IC A TI O N