  Materials with van der Waals-bonding are known to exhibit quantum confinement
effect, in which the electronic bandgap of the three-dimensional (3D)
realization of a material 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 for which such AQCE occurs. 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. Some of the AQCE systems identified through HSE06 and PBE0 are:
hydroxides or oxide hydroxide compounds (AlOH2, Mg(OH)2, Mg2H2O3, Ni(OH)2,
SrH2O3) as well as Sb-halogen-chalcogenide compounds (SbSBr, SbSeI) and
alkali-chalcogenides (RbLiS and RbLiSe). A detailed electronic structure
analysis, based on band-structure and projected density of states, shows AQCE
is often characterized by lowering of the conduction band in the monolayer and
corresponding changes in the pz electronic orbital contribution, with z being
the non-periodic direction in the 2D case. We believe our computational results
would spur the effort to validate the results experimentally and will have
impact on bandgap engineering applications based on low-dimensional materials.
