Electronic structures of air-exposed few-layer black phosphorus by optical spectroscopy

The electronic structure of few-layer (FL) black phosphorus (BP) sensitively depends on the sample thickness, strain, and doping. We show that it is also vulnerable to air exposure. The oxidation of BP caused by air exposure gives several optical signatures, including the broadening of resonance peaks and increased Stokes shift between infrared (IR) absorption and photoluminescence (PL) peaks. More importantly, air exposure causes blueshifts of all resonance peaks in IR absorption and PL spectra, with more prominent effects for thinner samples and higher-order subband transitions. Our study alludes to a convenient and exotic way for band-structure engineering of FL-BP through controllable air exposure or defect creation.