Highly conductive PdCoO₂ ultrathin films for transparent electrodes

We report on the successful synthesis of highly conductive PdCoO₂ ultrathin films on Al₂O₃ (0001) by pulsed laser deposition. The thin films grow along the c-axis of the layered delafossite structure of PdCoO₂, corresponding to the alternating stacking of conductive Pd layers and CoO₂ octahedra. The thickness-dependent transport measurement reveals that each Pd layer has a homogeneous sheet conductance as high as 5.5 mS in the samples thicker than the critical thickness of 2.1 nm. Even at the critical thickness, high conductivity exceeding 10⁴ S cm^-1 is achieved. Optical transmittance spectra exhibit high optical transparency of PdCoO₂ thin films particularly in the near-infrared region. The concomitant high values of electrical conductivity and optical transmittance make PdCoO₂ ultrathin films promising transparent electrodes for triangular-lattice-based materials.