Light-matter interactions and afterglow emission of 2D transition metal dichalcogenides

Light-matter interactions underlie the fundamental physics of two-dimensional (2D) transition metal dichalcogenides (TMDs). We show that both pure and alloyed 2D TMDs can absorb considerable optical energy and slowly emit it, which we attribute to spin-forbidden electron-flip transitions (i.e., phosphorescence). We measured an optical charging time of ∼20 s, indicating that photons are the energy source for this behavior, and extracted phosphorescence quantum efficiency (PQE) values reaching ∼3%. As W atoms are heavier than Mo atoms, Mo_1-x W _x S₂ (0 < x < 1) alloys demonstrated the heavy-atom effect, showing that a high concentration of W atoms presented a stronger afterglow signal. Our samples showed long-lasting room temperature afterglow, from several minutes to 1 h, and signal quenching in regions of low atomic order and when the TMDs exhibited enhanced interactions with ambient molecules. This understanding sheds light on the fundamental light-matter interactions of TMDs and, thereby, may inspire their integration into future optical applications.