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Engineering Metal Oxidation Using Epitaxial Strain

Result of the Month

(A) Schematic showing the epitaxial strain states for IrO2 films grown on (top-bottom) (001), (101) and (110) oriented TiO2 ¬substrates. (B) XRD 2θ-ω coupled scans for thin IrO2 films grown on different orientations of TiO2 ¬¬substrates. (C-E) Corresponding AFM images showing atomically smooth surfaces with root mean square roughness less than 3 Å. (F) XRD 2θ-ω coupled scans for thicker IrO2 films grown on different orientations of TiO2 ¬¬substrates. Corresponding AFM images showing (G) well-defined cracks along family of planes for IrO2 (001), (H) rod shaped Ir metal features on the surface of IrO2 (101), (I) a smooth surface for IrO2 (110) with (inset) showing spot-like surface Ir metal features observed in optical microscopy. The large epitaxial strain (~32 % along both directions) for Ir (111) on IrO2 (110) surface (as shown in Fig. S12-1) could have led to a disk-like shape whereas a lower lattice mismatch (~0.67 % along one direction) for Ir (111) on IrO2 (101) surface (as shown in Fig. S10-1) may have caused the islands to adopt a rod-like morphology.

Author: Sreejith Nair, Zhifei Yang, Dooyong Lee, Silu Guo, Jerzy T. Sadowski, Spencer Johnson, Abdul Saboor, Yan Li, Hua Zhou, Ryan B. Comes, Wencan Jin, K. Andre Mkhoyan, Anderson Janotti, Bharat Jalan Institute: ''1. Department of Chemical Engineering and Materials Science, University of Minnesota'' Nature Nanotechnology
URL: https://www.nature.com/nnano/
Date: 10/2023 Instruments: EVO 25/50 MBE

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.

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