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.
Highlights
Transition metal dichalcogenides (TMDs) are a class of layered materials which have garnered intense research interest due to their unique optical, electronic, and catalytic properties.[1−3] TMD bulk crystals consist of monolayers bound by weak van der Waals interactions, which can be overcome via dry mechanical cleavage or via liquid phase exfoliation.[5,6]
We demonstrate that a simple long-chain acid, oleic acid (OA) can greatly enhance the PL of monolayer WS2 yielding bright neutral exciton emission comparable to TFSI
Λave 618.3 nm*→614.2 nm 618.4 nm*→617 nm Letter σλ 1.57 nm*→0.57 nm 2.16 nm*→1.29 nm Figure 2. (a−c) Raw PL spectra of pristine, OA, and TFSI-treated samples taken with 514 nm CW laser. (d) Excitation series derived from PL integrals from panels a−c for pristine, OA, and TFSI treated monolayers. (e) Ratio of PL integral to excitation intensity, that is, relative photoluminescence quantum efficiencies (PLQE) (γ) variation with excitation intensity for pristine, OA, and TFSI treated monolayers. (f) Ratio of ζ and neutral exciton (X) peaks fitted from OA treated sample PL spectra to show increasing ζ to neutral exciton PL integral with increasing laser excitation intensity, indicating the presence of trions at high laser power options for passivating and tuning the properties of monolayer
Summary
Still debated.[21,24−28] This has led to a number of treatments being proposed to enhance the PLQE of monolayer TMDs, most commonly MoS2 and WS2. Other studies have sought to preserve the intrinsic optical properties of TMDs via exfoliation onto hexagonal boron nitride (hBN)[32] or hBN encapsulation,[33,34] which isolates monolayers from doping and disorder induced by the underlying substrate (e.g., Si−SiO2) This has been shown to result in more uniformly distributed dominant neutral exciton PL with narrow homogeneous spectral line width free of substrate effects, which otherwise manifest as inhomogeneous contributions in monolayer TMD PL spectra.[33] Whereas hBN encapsulation improves overall optical quality, a large increase in PL at low excitation intensities has not been demonstrated. TMDs. Here, we demonstrate that a simple long-chain acid, oleic acid (OA) can greatly enhance the PL of monolayer WS2 yielding bright neutral exciton emission comparable to TFSI treated monolayers. WS2, draws parallels to the surface treatment of inorganic colloidal quantum dots, such as cadmium sulfide (CdS) and lead sulfide (PbS), where long-chain acids and in particular OA are used to passivate surfaces, and opens a range of
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