Enhancing Photoluminescence and Mobilities in WS2 Monolayers with Oleic Acid Ligands

摘要

Many potential applications of monolayer transition metal dichalcogenides (TMDs) require both high photoluminescence (PL) yield and high electrical mobilities. However, the PL yield of as prepared TMD monolayers is low and believed to be limited by defect sites and uncontrolled doping. This has led to a large effort to develop chemical passivation methods to improve PL and mobilities. The most successful of these treatments is based on the nonoxidizing organic “superacid” bis(trifluoromethane)sulfonimide (TFSI) which has been shown to yield bright monolayers of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) but with trap-limited PL dynamics and no significant improvements in field effect mobilities. Here, using steady-state and time-resolved PL microscopy we demonstrate that treatment of WS2 monolayers with oleic acid (OA) can greatly enhance the PL yield, resulting in bright neutral exciton emission comparable to TFSI treated monolayers. At high excitation densities, the OA treatment allows for bright trion emission, which has not been demonstrated with previous chemical treatments. We show that unlike the TFSI treatment, the OA yields PL dynamicsthat are largely trap free. In addition, field effect transistorsshow an increase in mobilities with the OA treatment. These resultssuggest that OA serves to passivate defect sites in the WS2 monolayers in a manner akin to the passivation of colloidal quantumdots with OA ligands. Our results open up a new pathway to passivateand tune defects in monolayer TMDs using simple “wet”chemistry techniques, allowing for trap-free electronic propertiesand bright neutral exciton and trion emission.