Revealing the Competition between Defect‐Trapped Exciton and Band‐Edge Exciton Photoluminescence in Monolayer Hexagonal WS₂

Abstract

Monolayer transition‐metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD‐grown hexagonal WS2 monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect‐related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD‐grown hexagonal WS2 monolayer are revealed. The temperature‐dependent PL spectra show the correlation between the defect‐trapped and band‐edge exciton emission. The high‐resolution scanning transmission electron microscopy identifies the positive correlation between the density of WSx‐vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p‐ and n‐doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS2 monolayer, which provides important guidance in the regulation of electronic and optical properties of transition‐metal dichalcogenides via defect engineering.

Defect-Type-Dependent Carrier Lifetimes in Monolayer WS2 Films

“…The PL spectra of monolayer WS 2 treated by plasma exhibit only one peak around 2.0 eV, and the defect-induced PL peak is quite weak or even negligible. This result is consistent with the previous PL measurement at 295 K . It is indisputable that the defect state can lead to radiative recombination as shown in Figure c, but the defect-induced PL peak is more obviously to be detected at low temperature, as shown in Figure S4a–c.…”

“…The far left depicts the mechanism in the WS 2 monolayer with V S or O 2S defects. The unoccupied defect state acts as the electron trap, opening up both the radiative and nonradiative recombination pathways. − It is worth noting that at room temperature, there is sufficient thermal energy present to promote a portion of the trapped exciton population to the band edge, reducing the possibility of a defect-induced radiative recombination channel. , Therefore, in Figure c, the thinner arrow represents the radiative recombination channel introduced by the defect at room temperature. The nonradiative recombination mediated by defect states could occur at the same time as the band-edge recombination or in advance. , The right part depicts the mechanism in which an electron in the CBM directly recombines with a hole in the VB at the K point in both pristine and defective WS 2 monolayers.…”

The Hidden Flower in WS2 Flakes: A Combined Nanomechanical and Tip-Enhanced Raman Exploration

“…It was observed that the 2H phase shows a stronger PL compared to the 3R phase, indicating that the flower-like fine structure consists of the 3R phase . The PL quenching in this region could be due to defects or tungsten vacancies, which act as deep trap states, increasing the nonradiative recombination. ,, …”

“…41 The PL quenching in this region could be due to defects or tungsten vacancies, which act as deep trap states, increasing the nonradiative recombination. 38,39,41 Mechanical Properties. The preliminary AFM phase image in intermittent contact mode (see Figure 1b) gave indications that the domain formation is accompanied by a change in the mechanical properties of the WS 2 flake.…”

“…Increasing the exposure time of the flake to the sulfur leads to larger flakes. In general, the formation of the WS 2 crystals is quite complex and depends additionally on the generation of defects and lattice mismatches, which results in a core–shell growth , and screw dislocation. , In addition, hexagonally shaped flakes with a heterostructure of sulfur- and tungsten vacancies were reported. − Those vacancies were revealed by PL and Raman spectroscopy or labeling with a fluorophore . The understanding of the formation of such peculiar structures is therefore critical for the future application of these materials.…”