Polaronic defects in monolayer CeO2: Quantum confinement effect and strain engineering

Abstract

We uncover the structure, stability and electronic properties of polaronic defects in monolayer (ML) CeO2 by means of first-principles calculations, with special attention paid to the quantum confinement effect induced by dimensionality reduction. Results show that polaron can be more stabilized in ML CeO2 than in the bulk, while formation of oxygen vacancy (Vo2+) and polaron-vacancy complexes [(Vo2+-1polaron)1+, (Vo2+-2polaron)0] turns to be more difficult. The polaronic defect states sit deeper in energy within the band gap of ML CeO2 compared to the bulk case. We further demonstrate that the epitaxial strain in ceria film, as normally exists when grown on metal substrate, plays a crucial role in regulating the defect energetics and electronic structures. In particular, the formation energies of polarons, Vo2+, (Vo2+-1polaron)1+ and (Vo2+-2polaron)0 generally decrease with tensile strain, leading to controllable defect concentration with strain and temperature. This study not only provides physical insights into the polaronic defects in ultrathin oxide films, but also sheds light on their potential technological applications in nanoelectronics, fuel cells and catalysts.