Synthesis, Structural Thermal Stability, and Thermoelectric Properties of Layered GaTe-based Single Crystals

Abstract

The emerging global energy crisis and climate change have intensified interest in clean and renewable energy sources, such as solar cells and thermoelectricity – one of the key ideas for addressing energy and climate challenges. Thermoelectric (TE) modules enable a direct conversion between waste thermal energy and useful electricity and vice versa. The abundant waste heat can come from various sources, e.g., combustion of fossil fuels or as a by-product of chemical reactions and nuclear decay, indicating a significant role in power generation as well as energy conversion of TE materials. In this century, layered materials have truly revolutionized TE research with outstanding properties as well. Particularly, the bonding heterogeneity between the intralayer and interlayer induces a significant anisotropy in mechanical, electrical, and thermal properties. By manipulating these anisotropic behaviors, it is easily enabling decouple and optimize the TE parameters, thereby achieving excellent performance. Likewise, gallium telluride (GaTe) owns a monoclinic layered structure with strong covalent in-plane and weak van der Waals (vdW) out-of-plane bonding. Each single-crystalline layer consists of Te-Ga-Ga-Te tetra layers (TLs), with only two-thirds of Ga-Ga bonds lying perpendicular to the layer and the rest belonging to the horizontal direction. Such a complex structure promises low lattice thermal conductivity - a key advantage for achieving high TE performance. Further, the presence of atomically thin layers causes two-dimensional electronic transport induced by the quantum confinement effect together with enhanced phonon scattering at the interface. On the other hand, the electronic band structure of GaTe has a coexistence of flat and dispersive valence bands, which are incredibly beneficial for obtaining a large Seebeck coefficient and good electrical conductivity, respectively. Moreover, existing theoretical studies also predicted an unexpecting potential for GaTe-based TE materials. Thus, it is highly desired to explore the TE performance of GaTe-based materials. In fact, the pristine GaTe exhibits the low electrical conductivity due to the intrinsically low carrier concentration of ~1015 cm-3. To achieving high performance, it is required to increase the carrier concentration of GaTe close to the optimal value, normally around 1019-1021 cm-3, depending on materials. To obtain this objective, we utilized bismuth (Bi) doping to enhance the TE performance of GaTe. In this thesis, we successfully prepared high-quality pristine GaTe and Bi-doped single crystals in a large size using a simple and effective “growth-from-the-melt” method, namely temperature gradient technique. As expected, Bi doping promoted to simultaneously increase the hole concentration and mobility up to 1.63 × 1017 cm-3 and 68.25 cm2 V-1 s-1, respectively, for BGT-4 samples. Our findings indicate the possibility of Bi doping in controlling the carrier concentration to increase the electrical and thermal properties of GaTe. From the practical point-of-view, thermal stability on structural and thermoelectric characteristics is an essential inherent property from a practical standpoint. Good thermal stability helps ensure the long-term endurance, reliability, and repeatability of high-temperature responsive devices. Good thermal stability helps guarantee the long-term endurance, repeatability, and reliability of high-temperature responsive devices. Therefore, we systematically attempted to explore the structural thermal stability of GaTe single crystals in various atmospheres at temperatures ranging from 300 to 1173 K. Furthermore, we also studied the influences of annealing temperature on structural properties using photoluminescence and Raman spectroscopy. Our results indicate that GaTe is thermally stable up to 700 K in Ar and even higher up to 935 K in N2 atmosphere due to the protective role of N2 adsorbed layers on GaTe surface at high temperatures. The physical adsorption of N2 molecules is originated from the weak electrostatic forces between quadrupole moments of N2 molecules and surface Te atoms in GaTe material. Moreover, heat treatment considerably increases Moreover, heat treatment considerably increases the crystalline quality of GaTe with the optimal annealing temperature of 673 K.