| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 97 === Bismuth telluride (Bi2Te3) is the most well known thermoelectric material at near-room-temperature regime. Due to the anisotropic nature of the material, thermoelectric properties will be strongly dependent on its crystal orientation. Electrodeposition, one of many methods of preparing Bi2Te3, has the advantages of being low cost, high efficiency, and simple process, with the additional attribute of grain growth under a distinct crystallization mechanism. Findings in this research provide a thorough understanding of the structural evolution during the electrodeposition process.
In this study, structural features and growth mechanism of electrodeposited Bi2Te3 grains are carefully investigated. From scanning electron microscopy (SEM), the interpenetrating discs exhibit a distinctly pyramidal shape with combined triangular and square bases. Further analysis with transmission tunneling electron microscopy (TEM) shows that each individual disk is a single crystal, with the c-direction [00.1] perpendicular to the flat surface of the crystal. The interpenetrating structure is speculated to be related to the twin structure of Bi2Te3 crystals with and twin plane, based on the similarities between the measured angles 65o and 77o from SEM micrographs and the theoretical calculated included angles 63.82o and 77.5o. Finally, the XRD diffraction patterns indicate that the intensity of {11.0} plane increases with increasing electrodeposition time. Such preferential growth plane is the result of electrons preferably traveling through the basal plane, which has the lowest electrical resistivity. Subsequently, bismuth and tellurium ions would likewise deposit along the same path near the basal plane, leading to the plane perpendicular to the basal plane {11.0} being the preferred orientation.
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