  We report scanning tunnelling microscopy (STM) and x-ray photoemission
spectroscopy (XPS) measurements on uncapped and SrTiO3 (STO) capped NdNiO2
realized by pulsed-laser deposition and 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
de-intercalation of the upper layers. A metallic surface and a full Ni1+
oxidation state is 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 4f 4L 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 non stoichiometric 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 in the properties of infinite-layer NdNiO2 thin
films prepared by the CaH2 topotactic reduction of perovskite NdNiO3 and
suggest methods to improve their surface quality.
