Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS₂

Abstract

Transition-metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quantitative nanometer-scale characterization of defects over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL) and electrical conductivity. In this work, we present conductive atomic force microscopy measurements which reveal nanometer-scale electronically active defects in chemical vapor deposition-grown WS monolayers with defect density varying from 2.3 × 10 cm to 4.5 × 10 cm. Comparing these defect density measurements with PL measurements across large areas (>20 μm distances) reveals a strong inverse relationship between WS PL intensity and defect density. We propose a model in which the observed electronically active defects serve as nonradiative recombination centers and obtain good agreement between the experiments and model.

Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition

“…-domain (P circ ~ 40%) of our h-WS 2 samples even at RT, in stark contrast to recent reports by other groups (with P circ = 0.2% ~ 32%) on monolayer WS 2 , [40][41][42] as summarized in Table S2. The PL peak for both domains was found to slightly blue-shift to 1.96 eV at a lower temperature (LT) 80 K. As exemplified in Figure 3g, the LT (80 K) PL intensity of the   detection at the -domain increased threefold relative to the corresponding RT signals, whereas the PL intensity of the   detection remained the same at 80 K. In particular, the DOP at the -domain approached nearly 90% at 80 K. In contrast, the LT (80K) PL spectra at the -domain revealed that the PL intensities for both the   and   detections increased tenfold relative to the corresponding PL spectra at RT, as exemplified by Figure 3h.…”

“…To understand the correlation between the defect density and PL intensity, we followed a similar analysis by Rosenberger et al 42 Specifically, we assumed that the defect-related non-radiative recombination occurred when excitons collided with defects and resulted in non-radiative recombination. For an exciton with an effective collision radius r traveling with a speed v, an area swept by the exciton over a time period t in the 2D sheet with a defect density n d would result in N collisions, where N = n d (2rvt).…”

Local Conduction in MoxW1–xSe2: The Role of Stacking Faults, Defects, and Alloying

“…These features have been observed for several TMDCs in both C-AFM and STM images. 29 , 33 , 40 , 66 They have been interpreted as metal-like defects and are induced by atom substitution/vacancy on the outermost TMDC layers. 33 A zoom-in between these defects shows a smooth and flat surface in both the topography and the current image (see Figure 2 b and its inset).…”

Oxidation of Monolayer WS2 in Ambient Is a Photoinduced Process

“…[26][27][28][29][30] Recent work counting individual defects using conductive AFM has demonstrated that this three-fold symmetric region has a higher defect density than surrounding areas of the single crystal. 56 Additionally, this three-fold symmetric region has previously been shown to be preferentially oxidized under high-power laser irradiation and suggests that it contains a higher density of defects. 26 The observation of increased defect density has not been limited to the threefold symmetric areas.…”