KPFM work function characterization of ongoing reduction and oxidation of transition metal oxide crystals on examples of SrTiO₃ and TiO nanowires

Abstract

Controlling the work function of transition metal oxides is of key importance towards future energy production and storage. As majority of applications comprise the use of heterostructures, the most suitable experimental technique is Kelvin Probe Force Microscopy, providing excellent energetic and lateral resolution. In this paper we demonstrate the possibility of the precise work function characterization using the example of artificially formed crystalline titanium monoxide TiO nanowires on strontium titanate SrTiO3 surfaces providing a sharp atomic interface. The measured value of 3.31(21) eV is the first experimental work function evidence for a cubic TiO phase, being additionally subjected to significant variations among different crystallographic facets. Despite the remarkable height of the formed TiO nanowires, FM-KPFM proved to be able to achieve high lateral resolution of 15 nm, which is close to the topographical limits. In this study we show also the unique possibility of obtaining conductivity and work function maps on the same area, by combining contact and non-contact atomic force microscopy. As most of real applications require ambient operating conditions, we have additionally checked the impact of air venting on the work function of the TiO/SrTiO3(100) heterostructure, proving the surface re-oxidation occurs and results in work function increases of 0.9 eV and 0.6 eV for SrTiO3 and TiO, respectively. In addition, the influence of physisorbed species was estimated to contribute 0.4 eV and 0.2 eV to the work function of both structures. The presented method of the KPFM (and LC-AFM) employment for the work function characterization of transition metal oxides may help to understand the reduction and oxidation impact on electronic properties, which is of high importance towards the development of effective sensing and catalytic devices.

Stability and Decomposition of Perovskite-Type Titanates upon High-Temperature Reduction

“…In the following, we focus on stoichiometry changes of the very surface of SrTiO 3 upon thermal reduction. It has been reported that annealing under reducing conditions leads to the evolution of a Ti‐rich surface with new reconstruction and eventually Ti 2 O 3 or TiO islands can form on the surface . In general, two different channels changing the chemical composition of the surface have to be taken into account, namely segregation and sublimation .…”