ABSTRACT
Figure 5.3 TEM images and diffraction pattern of the cross section of a Bi nanowire — Bi thin film. (a) TEM image of the cross section of the Bi nanowire grown on a Bi thin film. (b) Magnified image of the Bi nanowire grown at a grain of the Bi thin film [4]. 5.3.2 Bismuth Telluride NanowiresBismuth telluride (Bi2Te3) is the most widely used thermoelectric material whose thermoelectric figure-of-merit, ZT, is approximately 1 at room temperature. It is also recognized as an important topological insulator, which shows nearly perfect electrical conduction on the surface in contrast to insulator-like behavior in the bulk [6]. Similar to Bi, low dimensional Bi2Te3 exhibits greatly enhanced thermoelectric performance. Venkatasubramanian et al., for example, demonstrated ZT ~ 2.3 for Bi2Te3/Sb2Te3 superlattices [7]. Low-dimensional Bi2Te3 structures are equally important for the study of topological insulators because surface states in low-dimensional structures become significant as the surface-to-volume ratio increases. The OFF-ON method can be used for the growth of high quality Bi2Te3 nanowires. Two different approaches are used: a one-step process and a two-step process. The one-step process refers to the use of the same experimental protocol as for the Bi nanowire growth, as represented in Fig. 5.4a [8]. First, a BiTe thin film is deposited onto a Si substrate with a SiO2 overlayer, normally by sputtering. Either sputtering from a BiTe compound target or co-sputtering from separate Bi and Te elemental targets can be used
for this purpose. The BiTe film thickness is controlled in the range of 40 to 100 nm. In the next step, the BiTe film on the substrate undergoes vacuum-annealing at a high temperature. The annealing temperature should be set higher than that for Bi nanowire growth because of the higher melting point of Bi2Te3 (585°C). The annealing temperatures should be between 300°C and 400°C. The BiTe film expands more than the substrate does, as indicated by its thermal expansion coefficient (~13.4 × 10-6/°C) which is larger than the thermal expansion coefficient of the substrate ((0.5 × 10-6/°C)/(2.4 × 10-6/°C)). The compressive stress induced in the BiTe film is the driving force for Bi2Te3 nanowire growth, as it is for Bi nanowire growth. It has been demonstrated that Bi2Te3 nanowires grown by this method are highly single-crystalline and uniform in diameter without tapering. The growth mechanism and characterization of Bi2Te3 nanowires grown by the OFF-ON method is described in Ref. [8].
