Order in disorder: oxygen vacancy driven electronic phase separation of LaNiO3-x epitaxial thin film surface investigated by scanning probe microscopy

Oxygen vacancies in nickelates are known to introduce a variety of emergent phenomena and are considered to significantly affect conductivity. Few studies have examined real-space evidence for oxygen vacancies on the surface, particularly using scanning probe microscopy. Understanding the surface composition, both structurally and chemically, is crucial for the application of nickelates, such as in electrochemical water splitting reactions as catalysts. In this study, we investigate a 20 nm epitaxial LaNiO3-x(LNO) film grown on SrTiO3 (STO) via pulsed laser deposition. We examine the surface of this film using scanning tunneling microscopy (STM) and reveal a complex surface morphology composed of densely packed crystalline nanometer-sized circular features with radius ranging from 10 to 20 nm. Scanning tunneling spectroscopy revealed an electronic inhomogeneity, or phase separation, in nanoscale islands of semi-conductive nature embedded within a metallic matrix. This change in the electronic density of the states was associated with increased concentration of oxygen vacancies. Further evidence for significant oxygen vacancies at the surface was inferred by examining scanning tunneling tip-induced surface degradation. The strong electric field between the tip and sample likely facilitates oxygen removal in a vacuum environment, increasing the formation of vacancies, which can lead to the breakdown of the crystal structure. This provides insight into the possible origin of the chemical surface transformation during the electrochemical water splitting reaction of this nickelate.