Luminescence of black phosphorus films: Exfoliation-induced defects and confined excitations

Black phosphorus (BP) stands out from other two-dimensional (2D) materials by the wide amplitude of the band-gap energy $({E}_{g})$ that sweeps an optical window from visible to infrared wavelengths, depending on the layer thickness. This singularity made optical and excitonic properties of BP difficult to map. Here we report a comprehensive study of the intrinsic (i.e., measured at 4 K) optical properties of 79 passivated BP flakes obtained by mechanical exfoliation of thickness ranging from 4 to 700 nm. By following single- or multistamp exfoliation protocols and by combining micro-Raman and photoluminescence experiments, we demonstrate that the exfoliation step induces line like defects which open radiative recombination paths alternative to those of the crystalline bulk and that actually dominate the emission process. We also show that the evolution of the photoluminescence energy versus thickness follows an inverse square law. We relate this to a quantum well model whose validity is discussed and justified at intermediate thickness. Finally, we report that the emission energy of BP slabs placed in different 2D heterostructures is not significantly modulated by the dielectric environment.