Nanoporous CaCoO 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 CaCoO thin films by a sequential sputtering-annealing method. CaCoO 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 CaCoO. This phase transformation is accompanied by a volume contraction causing formation of nanopores in the film. The thermoelectric propoperties of the nanoporous CaCoO films can be altered by controlling the porosity. The lowest electrical resistivity is ∼7 mΩ cm, yielding a power factor of 2.32 × 10 WmK 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.