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

Abstract

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, MoO2+x (0 ≤ x ≤ 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 in situ 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 MoO2+x . 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 t2g are significantly shifted to low energy nearly 0.2 eV, which clearly supports that the electronic transition of MoO2+x is predominantly driven by change of oxygen contents.

Charge Density Depinning in Defective MoTe2 Transistor by Oxygen Intercalation

“…Subsequently, oxygen can react at the vacancy sites with Mo and occupy all vacancies in the inner layers. Thermal annealing in gas ambient at low pressure are frequently employed to facilitate intercalation of small gas molecules such as oxygen [ 31 ] and hydrogen [ 32 ] in 2D materials. Importantly, it should be very cautious that annealing only causes intercalation of oxygen in MoTe 2 without oxidizing it.…”

Stable Correlated 4d2 SrMoO3 Films Epitaxially Coated on Al2O3 for Electromagnetic Shielding and Transparent Conductors

“…This comparative experiment indicated that annealing in air may oxidize metallic SrMoO 3 into insulating SrMoO 4 above 450 °C, which was a considerably higher temperature than the 250 °C reported for oxygen intercalation from MoO 2 to MoO 3 films. [ 41 ] We propose that the oxygen diffusion barrier could preserve the properties of SrMoO 3 at high temperatures, as demonstrated by a six‐unit‐cell Ba 0.5 Sr 0.5 TiO 3 capping layer. [ 42 ] The thermal stability of correlated perovskite TCs is sufficient for most applications working below 400 °C, including military weapons, invisible circuitry, smart windows, infrared sensors, and transparent solar cells.…”

Effects of Sintering Processes on Microstructure Evolution, Crystallite, and Grain Growth of MoO2 Powder

“…MoO 2 thin films can be fabricated using a molybdenum target using spin coating, pulsed laser deposition, reactive sputtering, and thermal evaporation [18]. In the case of the sputtering process, to optimize the performances of the thin film, most researchers have studied the influences of sputtering parameters on the film properties, including the type of sputter power (radio-frequency (RF) magnetron reactive sputtering, direct current (DC) magnetron reactive sputtering), working pressure (oxygen partial pressure from 1.00 × 10 −3 mbar to 1.37 × 10 −3 mbar), base pressure, atmosphere (Ar-O 2 sputtering gas), sputter powder (electrical conductivity varying from 1.6 × 10 −5 S/cm to 3.22 S/cm), film thickness, and post-annealing treatment (oxygen annealing range from 250 to 350 • C) [19][20][21][22][23]. However, it is difficult to elucidate how the target influences the properties of various films, the sputtering process, and the correlations between film properties and target performance have not been well established.…”