Graphene oxide (GO) powder processed by Hummer's method is mixed with p-type Bi2Te3 based thermoelectric materials by a high-energy ball milling process. The synthesized GO-dispersed p-type Bi2Te3 composite powder has a composition of Bi0.5Sb1.5Te3 (BSbT), and the powder is consolidated into composites with different contents of GO powder by using the spark plasma sintering (SPS) process. It is found that the addition of GO powder significantly decreases the thermal conductivity of the pure BSbT material through active phonon scattering at the newly formed interfaces. In addition, the electrical properties of the GO/BSbT composites are degraded by the addition of GO powder except in the case of the 0.1 wt% GO/BSbT composite. It is found that defects on the surface of GO powder hinder the electrical transport properties. As a result, the maximum thermoelectric performance (ZT value of 0.91) is achieved from the 0.1% GO/BSbT composite at 398 K. These results indicate that introducing GO powder into thermoelectric materials is a promising method to achieve enhanced thermoelectric performance due to the reduction in thermal conductivity.

We report the structural characterization of BixZn1-xO thin films grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. By increasing the Bi flux during the growth process, BixZn1-xO thin films with various Bi contents (x = 0~13.17 atomic %) were prepared. X-ray diffraction (XRD) measurements revealed the formation of Bi-oxide phase in (Bi)ZnO after increasing the Bi content. However, it was impossible to determine whether the formed Bi-oxide phase was the monoclinic structure α-Bi2O3 or the tetragonal structure β-Bi2O3 by means of XRD θ-2θ measurements, as the observed diffraction peaks of the 2θ value at ~28 were very close to reflection of the (012) plane for the monoclinic structure α-Bi2O3 at 28.064 and the reflection of the (201) plane for the tetragonal structure β-Bi2O3 at 27.946. By means of transmission electron microscopy (TEM) using a diffraction pattern analysis and a high-resolution lattice image, it was finally determined as the monoclinic structure α-Bi2O3 phase. To investigate the distribution of the Bi and Bi-oxide phases in BiZnO films, elemental mapping using energy dispersive spectroscopy equipped with TEM was performed. Considering both the XRD and the elemental mapping results, it was concluded that hexagonal-structure wurtzite BixZn1-xO thin films were grown at a low Bi content (x = ~2.37 atomic %) without the formation of α-Bi2O3. However, the increased Bi content (x = 4.63~13.17 atomic %) resulted in the formation of the α-Bi2O3 phase in the wurtzite (Bi)ZnO matrix.