Interfacial strain defines the self-organization of epitaxial MoO2 flakes and porous films on sapphire: experiments and modelling

TL;DR

This study combines experiments and modeling to understand how interfacial strain influences the self-organization and morphology of MoO2 flakes and porous films grown on sapphire substrates.

Contribution

It integrates density functional theory calculations with experimental growth to predict and verify epitaxial relationships and morphological evolution of MoO2 on sapphire.

Findings

DFT predicts preferred epitaxial relationships based on strain energy.

Experimental growth confirms predicted epitaxial relationships.

Interfacial strain drives self-organization from nanometer to micron scales.

Abstract

The epitaxy of MoO2 on c_plane sapphire substrates is examined. A theoretical approach, based on density functional theory calculations of the strain energy, allowed to predict the preferred layer/substrate epitaxial relationships. To test the results of these calculations, MoO2/(001) Al2O3 heterostructures were grown using the chemically_driven isothermal close space vapour transport technique. At the early stages of the growth, two kinds of morphologies were obtained, using the same growth parameters: lying and standing flakes. The composition and morphology, as well as the layer/substrate epitaxial relationships were determined for both kind of morphologies. Experimental epitaxial relationships coincide with those predicted by DFT calculation as the most favourable ones in terms of strain energy. For thicker films, the standing flakes evolve to form an epitaxial porous layer composed by coalesced epitaxial flakes. The interfacial strain between the sapphire substrate and MoO2 enables a self_organization from nanometer to micron scales between separated or coalesced flakes, depending on deposition condition.