Preparation Of BI-TE Thin Films Via Electrodeposition And Investigation On Thermoelectric Properties

For thermoelectric alloy thin films, the methods of synthesis like vapor deposition and molecular beam epitaxy etc. remains some disadvantages such as device-dependence, expensive target materials cost and difficulty in large-scaled fabrication. In this dissertation, by regulating the coating bath compositions and electrodeposition parameters, the low-cost simple method of electrodeposition was introduced to synthesis large-area n type single-phase Bi2Te3 and p type BixTey thermoelectric thin films on gold seed modified indium tin oxide glass (Au/ITO), which exhibited excellent thermoelectric properties.In the coating bath of 10 mM Bi3++ 15 mM HTeO2++1 M HNO3, n type single-phase Bi2Te3 thermoelectric thin films with different morphologies were controllably fabricated. At the applied potential of-0.20 V and time of 120 s, the obtained film demonstrated density wheat-like structure. The electrical resistance, carrier density, Seebeck coefficient and power factor of the obtained film was 8.060×106 Ω·m,5.029×1020cm-3,-64.949μV·K-1, and 5.234× 10-4 W·m-1·K2, respectively。In the coating bath of 12.5 mM Bi3++ 12.5 mM HTeO2++1 M HNO3, Bi-rich p type thermoelectric thin films of Bi2Te3 hexagonal structure were synthesized. At the applied potential of-0.25 V and time of 60 s, the electrical resistance, carrier density and Seebeck coefficient of the synthesized film with dense wheat-like morphology was 4.525×10-6 Ω·m,1.093×1021 cm-3,57.168 μV·K-1, respectively. The PF value increased to 7.222×10-4 W·m-1·K2. The type of carriers was changed and thermoelectric property improved via self-doped procedure.The mechanism of co-deposition of Bi-Te thin film was also investigated: Bi3+ was reduced into Bi0 in advance meanwhile an under potential deposition (UPD) reaction that Bi0 induced HTeO2+ to co-deposition occurred. Under circumstance of preferred potential, the nucleation process of dense thin films was to form the dispersive nuclei on Au/ITO substrate; the constant growth was co-deposition reaction of Bi3+ and HTeO2+, which was controlled by ion-diffusion process. Therefore, the size of the nuclei continued to develop in the direction parallel to the substrate to form the density coatings.Inspired by the above mechanism, the uniform Bi-Te nanorod arrays were fabricated via template-free periodic pulsed bi-potentiostatic deposition. The diameter and length of individual nanorod was 80-120 nm and 250 nm respectively. The process of co-deposition was demonstrated as the following procedures:a instantaneous reductive potential was applied to form dispersive nuclei, then a reversal oxidic potential to strip partial Bi atoms in the amorphous site to prevent further cross-growth. Repeatedly, the nanorod array film was obtained.