Sub-bandgap photoluminescence properties of multilayer h-BN-on-sapphire.

Two-dimensional hexagonal boron nitride (h-BN) materials have garnered increasing attention due to its ability of hosting intrinsic quantum point defects. This paper presents a photoluminescence (PL) mapping study related to sub-bandgap-level emission in bulk-like multilayer h-BN films. Spatial PL intensity distributions were carefully analyzed with 500 nm spatial resolution in terms of zero phonon line (ZPL) and phonon sideband (PSB) emission-peaks and their linewidths, thereby identifying the potential quantum point defects within the films. Two types of ZPL and PSB emissions were confirmed from the point defects located at the non-edge and edge of the films. Our statistical PL data from the non-edge- and edge-areas of the sample consistently reveal broad and narrow emissions, respectively. The measured optical properties of these defects and the associated ZPL peak shift and line broadening as a function of temperature between 77° and 300° K are qualitatively and quantitatively explained. Moreover, an enhancement of the photostable PL emission by at least a factor of ×3 is observed when our pristine h-BN was irradiated with a 532 nm laser.