The oxides of platinum group metals are promising for future electronics and spintronics due to the delicate interplay of spin-orbit coupling and electron correlation energies. However, their synthesis as thin films remains challenging due to their low vapour pressures and low oxidation potentials. Here we show how epitaxial strain can be used as a control knob to enhance metal oxidation. Using Ir as an example, we demonstrate the use of epitaxial strain in engineering its oxidation chemistry, enabling phase-pure Ir or IrO2 films despite using identical growth conditions. The observations are explained using a density-functional-theory-based modified formation enthalpy framework, which highlights the important role of metal-substrate epitaxial strain in governing the oxide formation enthalpy. We also validate the generality of this principle by demonstrating epitaxial strain effect on Ru oxidation. The IrO2 films studied in our work further revealed quantum oscillations, attesting to the excellent film quality. The epitaxial strain approach we present could enable growth of oxide films of hard-to-oxidize elements using strain engineering.
Calculated formation enthalpy using DFT-GGA for bulk single crystal IrO2 and IrO2 when grown as an epitaxially strained film on various substrates for a fixed oxygen chemical potential. A modified definition as described in the main text (where formation enthalpy is defined with respect to epitaxially strained Ir as the reactant with oxygen) was used to calculate the formation enthalpies.
With increasing need for more energy efficient electronics and high-performance catalysts for a sustainable future, it has become imperative to devise new approaches to synthesize the desired materials with atomic level precision. In this work, we introduce epitaxial strain as a knob to control and enhance the oxidation of hard-to-oxidize metals to make high-quality metal oxide thin films which show unconventional electronic and magnetic properties, essential for next-generation sustainable technologies.
Using the EVO-50 UHV oxide MBE system equipped with a radio-frequency oxygen plasma system, custom low temperature effusion cell for the metal-organic Ir source, beam flux monitor and RHEED, we were able to grow high-quality IrO2 thin films and study the subtle effects of reactant fluxes and type of substrate on the resulting film composition.