Low Temperature Nanoscale Oxygen-Ion Intercalation into Epitaxial MoO₂ Thin Films

In transition metal oxides (TMOs), lots of physical phenomena such as metal–insulator transitions (MIT), magnetism, and ferroelectricity are closely related to the amounts of oxygen contents. Thus, understanding surface oxidation process in TMOs and its effect are important for enhancing performances of modern electronic and electrochemical devices due to miniaturization of those devices. In this regard, MoO_2+<i>x</i> (0 ≤ <i>x</i> ≤ 1) is an interesting TMO, which shows MIT driven by the change of its oxygen content, i.e. metallic MoO₂ and insulating MoO₃. Hence, understanding thermally driven oxygen intercalation into MoO₂ is very important. In this work, we conducted <i>in situ</i> postannealing of as-grown epitaxial MoO₂ thin films at different temperatures in oxidative condition to investigate the thermal effect on oxygen ion intercalation and resultant MIT in MoO_2+<i>x</i>. Through the spectroscopic techniques such as spectroscopic ellipsometry and X-ray absorption spectroscopy, we observed that oxygen ions can intercalate into MoO₂ and trigger a phase transition in nanoscale at surprisingly low-temperature as low as 250 °C. In addition, after oxygen annealing at 350 °C, we find that both hybridization and interband transition energy between O 2p and Mo 4d t_2g are significantly shifted to low energy nearly 0.2 eV, which clearly supports that the electronic transition of MoO_2+<i>x</i> is predominantly driven by change of oxygen contents