No abstract

Motivated by the recent experimental synthesis of atomic-thick SnTe [Liu et al., Science 353(6296), 274 2016] exhibiting a layered orthorhombic phase similar to SnSe, we carried out systematic investigations on its electronic, thermoelectric, and phonon transport properties based on a combination of density functional theory and Boltzmann transport theory. Our results indicate that the monolayer is dynamically stable with a band gap of 1.05 eV. A considerable figure of merit (ZT) is predicted to be 2.9 for n-type doping and 2.2 for p-type doping along the armchair direction at a moderate carrier concentration of 1020 cm−3. The electronic band structure and the Fermi surface with multi-valleys lead to band convergence and anisotropic transport behavior. The synergistic optimization of Seebeck coefficient and electrical conductivity is achieved in anisotropic monolayer SnTe, due to the independence of carrier relaxation time and directional effective mass. A maximum power factor of 37 mW/(mK2) can be achieved for the n-type SnTe monolayer along the armchair direction, almost two times as high as that in the zigzag direction. However, the anisotropy of intrinsic lattice thermal conductivity is relatively low and strong phonon anharmonicity is found due to the coexistence of weak bonding and resonant bonding.

Serials of Mn doping by substituting Cd sites on Cu2CdSnSe4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu2Cd1−xMnxSnSe4. Our experimental and theoretical studies show that the moderate Mn doping by substituting Cd sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4. The electrical resistivity is decreased by about a factor of 4 at 723 K after replacing Cd with Mn, but the seebeck coefficient decreases only slightly from 356 to 289 μV/K, resulting in the significant increase of the power factor. Although the thermal conductivity increases with the doping content of Mn, the figure of merit (ZT) is still increased from 0.06 (x = 0) to 0.16 (x = 0.10) at 723 K, by a factor of 2.6. To explore the mechanisms behind the experimental results, we have performed an ab initio study on the Mn doping effect and find that the Fermi level of Cu2CdSnSe4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at the low level doping content. Optimizing the synthesis process and scaling Cu2Cd1−xMnxSnSe4 to nanoparticles may further improve the ZT value significantly by improving the electrical conductivity and enhancing the phonon scattering to decrease the thermal conductivity.

A lead-free SnTe compound shows good electrical properties but also high thermal conductivity, resulting in a low figure of merit ZT. We demonstrate a significant enhancement of the thermoelectric properties of SnTe by (Ge, Mn) co-doping. (Ge, Mn) co-doped samples (SnGe)MnTe with x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.2 were prepared for this investigation. The substitution of Ge for Sn in SnTe promotes the solubility of Mn in a SnTe-based phase up to 20 at%, which further enlarges the band gap and gives rise to enhanced valence band convergence as compared with Mn doping, leading to a notably increased Seebeck coefficient and a power factor. All alloys retain p-type conduction and hole carrier concentration increases with increasing Mn content. The solute Ge and Mn atoms as well as the second phase of Ge in a SnTe-based system enhance phonon scattering and thus reduce thermal conductivity. The synergistic role that Ge and Mn play in regulating the electron and phonon transport of SnTe yields a maximum figure of merit ZT of 1.22 at 873 K for the sample (SnGe)MnTe.

To achieve low thermal conductivity, polythiophene (PTh)/bismuth telluride (Bi 2 Te 3 ) nanocomposite has been prepared by spark plasma sintering using a mixture of nanosized Bi 2 Te 3 and PTh powders. Bi 2 Te 3 powder with sphericalshaped particles of 30 nm diameter and PTh nanosheet powder were first prepared by hydrothermal synthesis and chemical oxidation, respectively. X-ray diffraction analysis and scanning electron microscopy observations revealed that the hybrid composite consists of PTh nanosheets and spherical Bi 2 Te 3 . The organic PTh acts as an adhesive in the composite. Transport measurements showed that the PTh in the Bi 2 Te 3 matrix can reduce its thermal conductivity significantly, but also dramatically reduces its electrical conductivity. As a result, the figure of merit of the composite is lower than that of pure Bi 2 Te 3 prepared under the same conditions. The maximum value of ZT for the sample with 5% PTh (by weight) was 0.18 at 473 K, which is rather high compared with other polymer/inorganic thermoelectric material composites.