Control of electrical and optical properties of NdNiO₃ thin films via ionic liquid-gated electron doping

Rare-earth nickelates show diverse electrical, optical, and structural changes across their metal-insulator transition (MIT). Depending on the rare-earth ion involved, we get a range of temperatures across which this MIT can be observed. Achieving the ability to tune MIT around room temperature makes these quantum materials interesting for several applications. Carrier doping utilizing light atoms (like H or Li) is shown to electron-dope the Ni from its trivalent to divalent state, opening a band gap. This gives rise to colossal changes in resistance and optical constants. Being small, these doped atoms can be rapidly migrated in and out of the lattice, making it a high-speed tunable transition. Here, we will discuss electron doping of NdNiO₃ thin films using Li ions. We will focus on a large area, uniform, and reversible doping methods. As a case study, we have systematically studied the structural and optical properties of Li doped NdNiO₃ deposited on LaAlO₃ and Nb:SrTiO₃ substrates, calculating the doping profile along with other electrical and optical properties. Complex refractive index measured using variable-angle spectroscopic ellipsometry shows strong contrast between the metallic and insulating phases. The insulating phase has a refractive index of n~2 and low absorption in the visible and infrared spectrum. Further analysis of this complex refractive index for the insulating phase indicates a band gap of nearly 3-4 eV. The ability to tune such a large gap via electric fields opens new opportunities for nickelates in tunable photonics.