We investigate thermoelectric properties of Cu-doped Bi₂Te_2.7Se_0.3 fabricated using a simple doping process and spark plasma sintering. Through precise control of Cu doping, it is found that Cu atoms preferentially occupied Bi sites and then intercalated into the van der Waals gap with an increasing Cu content. Electrical transport properties of Cu-doped samples were systemically controlled using this mechanism. At the same time, thermal conductivities of the Cu-doped samples were reduced by the enhancement of point defect phonon scattering due to the Cu atoms. Compared to that of pristine samples, the dimensionless thermoelectric figure of merit (“ZT”) of 0.98 at 323 K for the Cu-doped sample was increased by more than 92% owing to these synergetic effects. Furthermore, the shift of maximum ZT to room temperature provides advantages for enlarging the applications of thermoelectric effects at room temperature.

Schematic of the NiCu(CO₃)(OH)₂, NiZn(CO₃)(OH)₂, and Ni_xCu_yZn_z(CO₃)(OH)₂ compounds prepared on a Ni foam substrate.

The anisotropy in the thermoelectric-transport properties of single-crystalline Bi nanowires was systematically investigated. Bi nanowires were grown along the crystal orientations of the [-102] and [100] directions using the on-film formation of nanowires method. The electrical conductivity and Seebeck coefficient of Bi nanowires with different diameters were measured with respect to temperature in both directions. The temperature dependence in electrical conductivity exhibited a significant diameter dependence, and significant anisotropy was observed in the Seebeck coefficient. Anisotropy was also observed in the thermoelectric power factor. The thermoelectric figure of merit was estimated using our previously reported thermal-conductivity data, which indicate that [100]-oriented Bi nanowires are more suitable for thermoelectric applications than [-102] nanowires. This is the first report of the anisotropy investigating all the thermoelectric-transport properties of single-crystalline Bi nanowires.

A schematic of (a) a NiCo(CO₃)(OH)₂ compound fabricated with Ni foam and (b) a NiCo(CO₃)(OH)₂ compound synthesized without a Ni foam substrate.