Tuning the electronic properties of ZnO nanofilms via strain-induced structural phase transformations and quantum confinement†

ZnO nanostructures have huge potential in a wide range of technologies, including photocatalysis, optoelectronics, and energy harvesting. ZnO commonly exhibits the wurtzite polymorphic phase (wz-ZnO) and is one of the few inorganic materials where nanoscale structural phase engineering has revealed alternative polymorphs. These structurally novel nanophases also have properties (e.g. mechanical, electronic) that differ from those of wz-ZnO, and thus may pave the way to new applications. Here, we follow a strain-induced transformation between the body centred cubic phase (BCT-ZnO) and the graphitic phase (g-ZnO), which has been experimentally demonstrated in ZnO nanowires. Using free-standing ZnO nanofilms as a reference nanosystem, we use density functional theory based calculations to follow the BCT-ZnO ↔ g-ZnO phase transformation relative to systematic changes in the in-plane biaxial strain and nanofilm thickness. Compressive strain favours the BCT-ZnO phase, whereas tensile strain induces the transformation to the g-ZnO phase. As the applications of nanoscale ZnO usually take advantage of its semiconducting nature, we mainly focus on the variance of the band gap and the character of the band edges. Our work features the use of Crystal Orbital Hamilton Population (COHP) analysis, which helps provide a uniquely detailed understanding of this complex nanosystem based on orbital overlap. We use this approach to reveal how strain and quantum confinement (through the nanofilm thickness) have distinct and significant effects on the structural and electronic properties of both BCT-ZnO and g-ZnO phases. The latter phase is particularly interesting as it involves a subtle competition between two structural sub-phases (the layered-ZnO and hex-ZnO phases). These two phases can be distinguished by their respective orbital overlap characteristics which, in turn, can be finely tuned by strain and thickness. We propose that the rich electronic properties of this nanosystem can be interpreted through a monolayer superlattice model in which localised surface states and more spatially delocalised quantum confined states compete. More generally, our work illustrates how the intricate interplay of strain, quantum confinement and structural phase transformations in an inorganic nanosystem can be analysed and understood through the use of COHP analysis of orbital overlap contributions.