Enhancing Photoluminescence and Mobilities in WS₂ Monolayers with Oleic Acid Ligands

Abstract

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 dynamics that are largely trap free. In addition, field effect transistors show an increase in mobilities with the OA treatment. These results suggest that OA serves to passivate defect sites in the WS2 monolayers in a manner akin to the passivation of colloidal quantum dots with OA ligands. Our results open up a new pathway to passivate and tune defects in monolayer TMDs using simple “wet” chemistry techniques, allowing for trap-free electronic properties and bright neutral exciton and trion emission.

How defects influence the photoluminescence of TMDCs

“…Repairing and passivating the defects to heal the lattice structure could modify the mid-gap defect states, therefore effectively suppressing the defect-associated nonradiative recombination channels. For example, oleic acid (OA) can passivate defect sites, resulting in greatly enhanced PL emission with largely trap-free dynamics [74]. Sulfur vacancies can be powerfully healed by bis-(trifluoromethane) sulfonimide (TFSI) or poly(4-styrenesulfonate) (PSS) treatments [19,20].…”

Substrate-Dependent Exciton Diffusion and Annihilation in Chemically Treated MoS2 and WS2

“…In recent years, a variety of chemical and electrical approaches have been developed that can increase the QY of TMDs. [17][18][19][20][21][22] These treatments can increase QY at low excitation density, but exciton-exciton annihilation still often limits brightness at device-relevant exciton densities.…”

Exciton-to-trion conversion as a control mechanism for valley polarization in room-temperature monolayer WS2

“…The charge density of TMD monolayers can be controlled via electrostatic gating or chemical doping 10 , 11 , 14 – 20 . While electrostatic gating is a flexible technique that allows a continuous change of the charge density 10 , 11 , 14 , chemical doping provides a convenient alternative that requires no microfabrication and is well suited for achieving high doping levels 15 – 20 . Here, we study the valley polarization of excitons and trions in monolayer and show that chemical doping via aromatic anisole (methoxy-benzene) quenches the exciton photoluminescence and causes the spectrum to become dominated by trions with a strong valley polarization.…”