All periodic density functional theory (DFT) calculations were performed with the Vienna Ab initio Simulation Package (VASP 5.4.4), utilizing the projector-augmented wave (PAW) method to describe the core electrons, while valence states were expanded by plane waves up to an energy cut-off of 400 eV. Geometries were pre-optimized (up to a force criterion of 5•10 -2 eV•Å -1 ) with the Perdew-Burke-Ernzerhof (PBE) generalizedgradient-approximation (GGA) exchange-correlation functional, followed by a fineoptimization using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional (with the range separation parameter of 0.2 Å -1 ) and 25% exact exchange. Van der Waals interactions were treated with Grimme's D3 dispersion correction scheme . Selfconsistency in the electronic structures were set to a limit of 10 -6 eV, utilizing a Gaussian smearing of 0.01 eV. Periodic images of the two-dimensional systems were separated by a vacuum layer of at least 20 Å, together with dipole corrections. Integrations in the reciprocal space were evaluated on a Monkhorst-Pack grid of 5x5x1 k-points.