Preparation, structural characteristics and optical parameters of the synthesized nano-crystalline sulphur-doped Bi₂Te_2.85Se_0.15 thermoelectric materials

In the present work, a systematic preparation of Bi₂Te_2.85Se_0.15−xS_x (x = 0.0, 0.02, 0.04 and 0.06) compositions was carried out by solvothermal method. The materials were characterized by XRD, SEM, EDX, TEM and Raman spectroscopy. XRD, as well as TEM, to confirm the nanostructure of samples. The electrical and thermal properties were investigated in the temperature range from room temperature to 600 K. The highest power factor was 1.3 × 10⁻² mW/K² m at 600 K for (x = 0.06) sample. This improvement in the thermoelectric properties may be due to the ordered atomic arrangement of Bi, Te, and Se induced by sulphur in the Bi₂Te_2.85Se_0.15−xS_x nanocomposites, which was confirmed by X-ray diffraction and Raman spectral analysis. The optical properties, such as energy gap, refractive index, extinction coefficient and dielectric constants, were obtained from the diffused reflectance data in the range of 200 to 800 nm. Sulphur doping influences the optical energy gap, reaching the lowest value for x = 0.04 samples (0.14 eV), while the highest allowed energy gap was found in x = 0.06 ones (0.41 eV). Also, It should be mentioned that dielectric constants (ε_r and ε_i) were affected due to Sulphur doping, whereas the samples with x = 0.04 and 0.02 have the highest and lowest ε_r, respectively, and the opposite for ε_i when increasing the wavelength.