Scanning tunneling microscopy and x-ray photoemission studies of NdNiO2 infinite-layer nickelates films

We report scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) measurements on uncapped and SrTiO₃ (STO) capped NdNiO₂ realized by pulsed-laser deposition and ex situ topotactic reduction process. We find that untreated NdNiO₂ surfaces are insulating and contain Ni mostly in a nominal Ni²⁺ oxidation state. Room temperature STM shows signatures of a striped-like topographic pattern, possibly compatible with recent reports of ordered oxygen vacancies in uncapped nickelates due to incomplete oxygen deintercalation of the upper layers. A metallic surface and a full Ni¹⁺ oxidation state are recovered by ultra--high vacuum annealing at 250 ^∘C, as shown by STM and XPS. STO-capped NdNiO₂ films, on the other hand, show Ni mostly in Ni¹⁺ configuration, but Nd 3d5/2 core-level spectra have a relevant contribution from ligand 4f⁴ states, suggesting the formation of a NdTiNiOₓ layer at the interface with the STO. By in situ unit cell by unit cell Ar-ion sputtering removal of the STO capping and of the nonstoichiometric interface layer, we were able to address the surface electronic properties of these samples, as shown by high-resolution valence-band photoemission spectroscopy. The results provide insights into the properties of infinite-layer NdNiO₂ thin films prepared by the ex situ CaH₂ topotactic reduction of perovskite NdNiO₃ and suggest methods to improve their surface quality.