Phase formation of manganese oxide thin films using pulsed laser deposition†
Manganese oxides have enabled a wide range of technologies including oxygen evolution catalysts, lithium ion batteries, and thermochemical water splitting. However, the variable oxidation state and rich polymorphism of manganese oxides make it difficult to find the processing conditions to target a particular phase of manganese oxide. Targeted synthesis requires a more complete understanding of the phase space and the impact of multiple processing variables on phase formation. Here, we demonstrate the impact of substrate temperature, total deposition pressure, partial pressure of oxygen, and target composition on the phase formation of manganese oxides grown using combinatorial pulsed laser deposition (PLD). Thin films were deposited from a MnO, Mn2O3 or MnO2 target onto amorphous glass substrates with a continuously varied temperature provided by a combinatorial heater. A combination of X-ray diffraction, Rutherford backscattering spectroscopy, and Raman and Fourier transform infrared (FTIR) spectroscopies were used to determine the phases present in the samples. The oxygen partial pressure was found to be the critical factor determining phase formation, while the total pressure, target composition, and substrate temperature have smaller and more complex effects on phase formation. Comparing the results of this work to the published temperature–pressure phase diagrams shows that the PLD thin films vary significantly from the expected equilibrium phases of either the bulk materials or nanoparticles. These results suggest that PLD provides a route to capture phases of manganese oxides at lower temperatures or higher oxygen pressures than those required for bulk synthesis or solution processing.