Paper AS-ThM11
Correlating Multiple Data Streams for Valence State Identification in Transition Metal Oxide during XPS Depth Profiling

Thursday, October 24, 2019, 11:20 am, Room A211

Transition metal oxides (TMO) of perovskite (ABO₃) structures exhibit a broad range of structural, compositional, and functional properties, which can be further tuned or even drastically transformed by means of judicious defect engineering. TMOs have a unique capability to incorporate large amount of oxygen defects owing to the multivalence nature of the transition metal cations, which can be directly probed by XPS. XPS is a surface sensitive technique. In order to study the deeper layers, depth profiling by Ar sputtering is often adopted. However, the sputtering process may change the valence state of the transition metal cations, and thus the data analysis affected by the data acquisition condition can be inaccurate or erroneous.

In this talk, using perovskite SrCrO₃ and its reduced structure, SrCrO_2.8, as model (SCO) materials, I will show how the XPS depth profiling data can reveal the valence state change and redox chemistry occurring in the deeper layers of the SCO thin films. In our experiments, the XPS Cr2p spectra suggested that a tensile strain applied by the substrate could stabilize the reduced SrCrO_2.8 structure near the interface (~10 nm) region. To examine whether the conclusion is correct, we compared SCO samples with different strain states and processing history. By correlating x-ray diffraction (XRD), transmission electron microscopy (TEM), time -of-flight secondary ion mass spectrometry (ToF-SIMS) data and density functional theory calculations, we achieved quantitative agreement and concluded that the Cr 2p spectra were not affected by the Ar beam during the XPS depth profiling process.