Predicting Anomalous Quantum Confinement Effect in van der Waals Materials

Materials with van der Waals-bonding exhibit quantum confinement effect, in which the electronic bandgap of the three-dimensional (3D) form is lower than that of its two-dimensional (2D) counterpart. However, the possibility of an anomalous quantum confinement effect (AQCE) exists, where the bandgap trend is reversed. In this work, we computationally identify materials with AQCE. Using density functional theory (DFT), we compute ≈1000 OptB88vdW (semi-local functional), ≈50 HSE06 and ≈50 PBE0 (hybrid functional) bandgaps for bulk and their corresponding monolayers, in the JARVIS-DFT database. OptB88vdW identifies 65 AQCE materials, but the hybrid functionals only confirm such finding in 14 cases. Such materials are either hydroxides or oxide hydroxide compounds (AlOH₂, Mg(OH)₂, Mg₂H₂O₃, Ni(OH)₂, SrH₂O₃), Sb-halogen-chalcogenide compounds (SbSBr, SbSeI) or alkali-chalcogenides (RbLiS and RbLiSe). Electronic structure analysis shows that AQCE is often characterized by the lowering of the conduction band in the monolayer and related changes in the p_z electronic orbital contribution.