Bandgaps in free‐standing monolayer TiO₂: Ab initio diffusion quantum Monte Carlo study

Abstract

In this study, bandgap of the two‐dimensional (2D) TiO2, a transition‐metal oxide, was obtained by the diffusion Monte Carlo (DMC) method for the first time. This genuine 2D monolayer material that includes transition elements has been proven to be very challenging for reliable prediction of the bandgap. In order to improve the description of correlation effects, the DMC method was used to calculate the bandgaps of 2D‐TiO2 in α‐ and β‐phases, and the obtained results were compared with the density functional theory (DFT) calculations. The DMC results obtained from the infinite periodic superlattices predict ∆f of the 2D‐TiO2 in the α‐ and β‐phases to be about 5.54(4) and 5.69(2) eV, respectively, indicating that the available DFT results significantly underestimated the bandgaps of 2D‐TiO2 nanosheets. Considering the recently reported linear scaling relationship between the bandgap ∆f and the exciton binding energy ∆e of 2D semiconductors, (∆e ≈ 0.28∆f), the optical bandgap ∆o was further predicted to be ~3.99 eV for the α‐phase TiO2, and it is consistent with the experimental value (~3.84 eV). The successful application of the quantum Monte Carlo (QMC) method on the 2D‐TiO2 systems implies that QMC may serve as a feasible tool to investigate the electronic properties of the freestanding 2D transition‐metal oxides.