Room Temperature RF Sputtering of Mixed Ionic and Electronic Conductor Nd2Ni0.8Cu0.2O4+d films
Lowering the operating temperature of Solid Oxide Fuel Cells (SOFCs) is essential for improving durability and enabling large scale commercialization. Mixed ionic-electronic conductors (MIECs) of the Ruddlesden-Popper family, such as Nd$2$Ni${1-x}$Cu$x$O${4+\delta}$ (NNCO), offer attractive cathode properties due to their high oxygen transport and favourable defect chemistry. In this work, we investigate the fabrication of Nd$2$Ni${0.8}$Cu${0.2}$O${4+\delta}$ thin films using a room temperature RF sputtering process followed by moderate temperature annealing. To simplify deposition, a single stoichiometric target was employed, despite the compositional challenges posed by elements with different sputtering yields. We examine the effect of sputtering power density on phase formation and film stoichiometry through X-ray diffraction, Rutherford Backscattering Spectrometry, Energy Dispersive X-ray Spectroscopy, and temperature dependent resistivity measurements. Increasing the sputtering power density strongly reduces the presence of spurious phases and promotes stabilization of the desired n=1 Ruddlesden-Popper phase. Films deposited at 230 W (3.1 W cm$^{-2}$) exhibit a predominant NNCO structure, elemental ratios close to nominal composition, and resistivity values consistent with bulk materials. These findings demonstrate that high power sputtering combined with ex situ annealing enables the production of NNCO thin films suitable for SOFC cathodes. The results support the potential of PVD based approaches for scalable fabrication of advanced SOFC components.