Excitons and trions in WSSe monolayers

The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloysopens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratioof 57/43 deposited on SiO2/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the 'parent' WS2 and WSe2 MLs, we assign the WSSe MLs to the ML family with the dark groundexciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane andout-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbiddencomplex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potentialfuture quantum applications of S-TMDs.