Nanoporous Ca₃Co₄O₉ Thin Films for Transferable Thermoelectrics

The development of high-performance and transferable thin-film thermoelectric materials is important for low-power applications, e.g., to power wearable electronics, and for on-chip cooling. Nanoporous films offer an opportunity to improve thermoelectric performance by selectively scattering phonons without affecting electronic transport. Here, we report the growth of nanoporous Ca₃Co₄O₉ thin films by a sequential sputtering-annealing method. Ca₃Co₄O₉ is promising for its high Seebeck coefficient and good electrical conductivity and important for its nontoxicity, low cost, and abundance of its constituent raw materials. To grow nanoporous films, multilayered CaO/CoO films were deposited on sapphire and mica substrates by rf-magnetron reactive sputtering from elemental Ca and Co targets, followed by annealing at 700 °C to form the final phase of Ca₃Co₄O₉. This phase transformation is accompanied by a volume contraction causing formation of nanopores in the film. The thermoelectric propoperties of the nanoporous Ca₃Co₄O₉ films can be altered by controlling the porosity. The lowest electrical resistivity is ∼7 mΩ cm, yielding a power factor of 2.32 × 10^-4 Wm^-1K^-2 near room temperature. Furthermore, the films are transferable from the primary mica substrates to other arbitrary polymer platforms by simple dry transfer, which opens an opportunity of low-temperature use these materials.