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Charting lattice thermal conductivity for inorganic crystals and discovering rare earth chalcogenides for thermoelectrics
Thermoelectric power generation represents a promising approach to utilize waste heat. The most effective thermoelectric materials exhibit low thermal conductivity κ. However, less than 5% out of about 105 synthesized inorganic materials are documented with their κ values, while for the remaining 95% κ values are missing and challenging to predict. In this work, by combining graph neural networks and random forest approaches, we predict the thermal conductivity of all known inorganic materials in the Inorganic Crystal Structure Database, and chart the structural chemistry of κ into extended van-Arkel triangles. Together with the newly developed κ map and our theoretical tool, we identify rare-earth chalcogenides as promising candidates, of which we measured ZT exceeding 1.0. We note that the κ chart can be further explored, and our computational and analytical tools are applicable generally for materials informatics.
Colloidal PbS nanoplatelets synthesized via cation exchange for electronic applications
In this work, we present a new synthetic approach to colloidal PbS nanoplatelets (NPLs) utilizing a cation exchange (CE) strategy starting from CuS NPLs synthesized via the hot-injection method. Whereas the thickness of the resulting CuS NPLs was fixed at approx. 5 nm, the lateral size could be tuned by varying the reaction conditions, such as time from 6 to 16 h, the reaction temperature (120 °C, 140 °C), and the amount of copper precursor. In a second step, Cu+ cations were replaced with Pb2+ ions within the crystal lattice via CE. While the shape and the size of parental CuS platelets were preserved, the crystal structure was rearranged from hexagonal covellite to PbS galena, accompanied by the fragmentation of the monocrystalline phase into polycrystalline one. Afterwards a halide mediated ligand exchange (LE) was carried out in order to remove insulating oleic acid residues from the PbS NPL surface and to form stable dispersions in polar organic solvents enabling thin-film fabrication. Both CE and LE processes were monitored by several characterization techniques. Furthermore, we measured the electrical conductivity of the resulting PbS NPL-based films before and after LE and compared the processing in ambient to inert atmosphere. Finally, we fabricated field-effect transistors with an on/off ratio of up to 60 and linear charge carrier mobility for holes of 0.02 cm2 V−1 s−1.