(color online) Exciton brightening via surface defect engineering on monolayer phosphorene. (a) Schematic plot of free and localized excitons. (b) PL spectra of monolayer phosphorene on thermal oxide and PECVD oxide. (c) Excitation polarization PL results of localized exciton PL peak. (d) Emission polarization PL results of localized exciton PL peak. Figure reproduced from Ref. [49].