Optical tuning of exciton and trion emissions in monolayer phosphorene

Monolayer phosphorene provides a unique two-dimensional (2D) platform to investigate the fundamental dynamics of excitons and trions (charged excitons) in reduced dimensions. However, owing to its high instability, unambiguous identification of monolayer phosphorene has been elusive. Consequently, many important fundamental properties, such as exciton dynamics, remain underexplored. We report a rapid, noninvasive, and highly accurate approach based on optical interferometry to determine the layer number of phosphorene, and confirm the results with reliable photoluminescence measurements. Furthermore, we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range. Based on our measured optical gap and the previously measured electronic energy gap, we determined the exciton binding energy to be ∼0.3 eV for the monolayer phosphorene on SiO2/Si substrate, which agrees well with theoretical predictions. A huge trion binding energy of ∼100 meV was first observed in monolayer phosphorene, which is around five times higher than that in transition metal dichalcogenide (TMD) monolayer semiconductor, such as MoS2. The carrier lifetime of exciton emission in monolayer phosphorene was measured to be ∼220 ps, which is comparable to those in other 2D TMD semiconductors. Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene. An optical scheme for determining the number of monolayers on two-dimensional materials has been developed. Research into two-dimensional materials is thriving, but developing a way to identify a single monolayer has proved challenging. Now, Jiong Yang and co-workers have used phase-shifting interferometry to deduce the number of phosphorene layers. They then performed power-dependent photoluminescence measurements to determine various excitonic properties of a monolayer on a silicon oxide/silicon substrate. They obtained an exciton binding energy of about 0.3 electron volts, which agrees well with theoretical predictions. The researchers measured a carrier lifetime of approximately 220 picoseconds, which is comparable to that of the transition-metal dichalcogenides, another class of two-dimensional semiconductors. They also measured a trion binding energy of about 100 milli-electron volts, which is around five times higher than that of transition-metal dichalcogenides.